IODP

doi:10.2204/iodp.pr.320.2009

Results and highlights

During Expedition 320, 16 holes at 6 sites (Holes U1331A–U1331C, U1332A–U1332C, U1333A–U1333C, U1334A–U1334C, U1335A, U1335B, U1336A, and U1336B) (Table T1) were cored as part of the PEAT program. By drilling a series of sites that follow the position of the paleoequator and a limited latitudinal and depth transect, as outlined below, we recovered cores that allow us to address the combined PEAT objectives (see "Scientific objectives").

Lithologies recovered and stratigraphic summary

During Expedition 320 sediments similar in lithology to previous DSDP and ODP expeditions to the central equatorial Pacific region were recovered (e.g., Lyle, Wilson, Janecek, et al., 2002). Figure F12 summarizes the lithostratigraphy of the northwest–southeast transect of sites drilled during Expedition 320 together with the sedimentary sequence from Site 1218, which is included in the PEAT flow line strategy (see "Scientific objectives, introduction, and background"). In this figure, the Eocene sequence (green shading) thins from north to south, pinching out between Sites U1335 and U1336 where the basement is of early Oligocene age. In contrast, the Miocene sequence (yellow shading) thins substantially from south to north whereas the Oligocene sequence (blue shading) is thickest in the middle of the transect (Sites U1334 and U1335) and thins both north and south. Sediments of early Eocene age are only present at Site U1331, and sediments of Pliocene–Pleistocene age are only present at Sites U1331, U1334, and U1335. The thickness of sediments from different parts of the age transect is compatible with that expected from our drilling strategy (see "Site selection strategy and site targets").

Five main lithologies are present in these sites:

  1. Surface clays;

  2. Nannofossil oozes and chalks;

  3. Radiolarian ooze and radiolarite;

  4. Porcellanite and chert; and

  5. Basal chalks, limestones, and clay.

Surface clays are only present from Site U1334 to the north and in strata of post-Oligocene age. This pattern reflects the accumulation of dominantly aeolian-derived fine-grained sediments below the CCD as the northward motion of the Pacific plate transports the sites out of the equatorial upwelling zone of high production and the underlying crust subsides with age.

Nannofossil oozes and chalks are the main lithology of the Oligocene throughout the Expedition 320 transect. To the south, where Sites U1335 and U1336 have not yet subsided below the present CCD, nannofossil ooze is also the main lithology in strata of Miocene age. North of these two sites, nannofossil ooze is also present but in decreasing importance moving northwestward as basement age becomes progressively older. At the northernmost end of the transect (Site U1331), nannofossil ooze is primarily restricted to a short interval of middle Eocene age and is broadly correlative to carbonate accumulation event (CAE) 3 (Lyle et al., 2005).

Radiolarian ooze is present at all sites (Fig. F12) except for Site 1218, but it is a far more abundant lithology in the north (Sites U1331 and U1332), where it is the main lithology of the Eocene, than in the south (Sites U1334–U1336).

Porcellanite and chert are diagenetic in origin, but they are sufficiently regional in extent in the equatorial Pacific to be considered individually. Porcellanite is a major lithology in sediments of early and early middle Eocene age toward the northern end of the Expedition 320 transect (especially Sites U1331 and U1332), where it is associated with thin clay horizons that are interbedded with radiolarian ooze. Chert is a major lithology in sediments of early and early late Oligocene age at Site U1336, where it proved a major impediment to recovery of a complete section.

At nearly all sites drilled on the PEAT transect, the recovered basal sediments overlying basalt are calcareous in lithology, indicating that, at the onset of sediment accumulation at these sites, the seafloor lay above the local CCD. This result is in keeping with the Expedition 320 rationale of drilling a flow line of sites on crust of decreasing age (southeast to northwest) to recover stratigraphic "windows" of calcareous sediments overlying contemporaneously young crust prior to its subsidence below the CCD. The only site on the Expedition 320 transect where calcareous sediments were not recovered overlying basement is Site U1332, where the basal sediment unit is primarily zeolite clay of middle Eocene age. This result indicates that the crust at Site U1332 lay below the CCD, even at the point of its formation (~50 Ma), pointing to a very shallow CCD (<2700 m) at this time (see "Cenozoic CCD in the equatorial Pacific").

The combined results of Leg 199 and the PEAT program (Fig. F13) will potentially allow us to decipher paleoceanographic and paleoclimatic changes within a latitudinal and depth transect in the equatorial Pacific Ocean. Intervals of interest include the EECO (Zachos et al., 2001a; Lyle, Wilson, Janecek, et al., 2002), the MECO (Bohaty and Zachos, 2003; Bohaty et al., 2009), the middle through late Eocene CAE events (Lyle et al., 2005), the Eocene–Oligocene transition (Coxall et al., 2005), the late Oligocene warming (see supplementary material in Pälike et al., 2006b), the Oligocene–Miocene transition (Zachos et al., 2001b; Pälike et al., 2006a), and the middle Miocene glaciation intensification event (Holbourn et al., 2005).

Sediments of early Eocene age in the Pacific equatorial age transect are characterized by the absence of calcareous sediments and the presence of clay, cherts, and porcellanite. In general, sediments of middle Eocene age from the equatorial Pacific transect are dominated by radiolarian ooze, radiolarite, and clay, but carbonate-rich intervals also occur and appear correlative between Expedition 320 sites and Sites 1218 and 1219, where the CAE events were originally defined. Sediments of middle Eocene age at Site U1333 are carbonate rich compared to those at Site 1218, an observation that cannot be explained by their assumed relative paleodepths during this interval. The Eocene–Oligocene transition is characterized by a major lithologic change from Eocene radiolarian oozes to Oligocene nannofossil oozes at ODP Sites 1218–1221 and Sites U1331–U1333. At Site U1334, sediments of latest Eocene age are more carbonate rich than at any of the other sites.

The Oligocene–Miocene transition occurs within a succession of pale and dark nannofossil ooze cycles at all sites where it was recovered except for Site 1220, where it is characterized by a transition from Oligocene radiolarian ooze to Miocene clay. Clay- and diatom-rich sediments characterize the middle to late Miocene interval.

Cenozoic CCD in the equatorial Pacific

One of the primary objectives of the PEAT program was to detail the nature and changes of the CCD throughout the Cenozoic in the paleoequatorial Pacific (Objective 1 in "Scientific objectives"), with potential links to organic matter deposition (Olivarez Lyle and Lyle, 2005). The choice of drilling locations, targeting positions on the paleoequator to track carbonate preservation during crustal subsidence throughout time (Fig. F14), followed the initial work on DSDP sites by van Andel (1975). This first reconstruction of the Cenozoic CCD was augmented by additional results from Leg 199 (Lyle, Wilson, Janecek, et al., 2002; Rea and Lyle, 2005). One of the significant contributions of Leg 199 drilling was the latitudinal mapping of CCD variations with time. During the Eocene, a generally shallow CCD appeared to be deeper outside the zone ±4° from the equator, opposite to the pattern established during the Neogene (Lyle, 2003). The PEAT cores will allow us to refine our knowledge of temporal and spatial variation in sediment accumulation rates resulting from plate movement, varying biologic productivity at the equatorial divergence, and carbonate preservation (Fig. F14). The shipboard sampling program allowed for >1000 determinations of CaCO3 concentrations, approximately one every section from one hole of each site. Shipboard results reveal the carbonate accumulation events of Lyle et al. (2005) as sharp carbonate concentration fluctuations at ~44, 41, 39, and 36 Ma across Sites U1331–U1334 and 1218, followed by a sharp transition into much higher carbonate accumulation rates from the Eocene into the Oligocene. Results from Expedition 320 reveal a complex latitudinal pattern where Sites U1331, U1332, and U1334 track the equatorial CCD that matches well the signal observed from Site 1218, but Site U1333, which is slightly north of the equatorial zone during the Eocene–Oligocene transition, shows significantly more carbonate accumulation.

The early Eocene equatorial CCD is much shallower than previously thought, as shown by results from Site U1332, where we did not recover any carbonate in the basal sediment section above basement, in contrast to Site U1331, which is only ~2 m.y. older. Our estimated CCD at ~49 Ma is only ~3000 m paleodepth. Surprisingly, Expedition 320 results also show a shallower CCD than previously known during the late Oligocene, perhaps 300 m shallower in the time interval between 23 and 27 Ma. This shallower CCD, at a paleodepth of ~4.5 km, and associated reduced carbonate fluxes to the seafloor could be linked to the gradual late Oligocene cooling, first fully recovered at Site 1218 (supplementary fig. 3 in Pälike et al., 2006b). The design of our drilling locations in combination with existing data will allow us to generate a 3-D view of CCD evolution during the Cenozoic during postcruise research.

Biostratigraphy and preservation

Biostratigraphic integration

A virtually complete composite section with biogenic sediments spanning 51 m.y. from the upper Pleistocene to the lower Eocene was recovered during Expedition 320 (Objectives 2, 3, 4, and 7 in "Scientific objectives"). The youngest record during the last 12 m.y. (late middle Miocene to recent) was well preserved at Site U1335 but elsewhere is only present as a thin (5–10 m) section of noncalcareous brown clay. Biostratigraphic records spanning the middle Miocene through lower Eocene are composed of nannofossil and radiolarian oozes as two major biogenic components. At Sites U1331 and U1335 turbidite beds containing reworked microfossils were present, with mixing most obvious at Site U1331. At the shipboard biostratigraphic resolution, all drilled sites contribute apparently continuous successions to this composite section and stratigraphic highlights include multiple recoveries of a complete Eocene–Oligocene transition at Sites U1331–U1334 and Oligocene/Miocene boundaries at Sites U1332–U1336. These sections provide excellent records of biotic response to rapid environmental change in the principal phytoplanktonic and zooplankton groups as well as benthic foraminifers.

Dissolution and microfossil preservation

The preservation of carbonate microfossils varies dramatically throughout the succession as a result of biotic production and export rates, water-column and seafloor dissolution, and other processes. The strength of dissolution reflects the depths of the drilling sites, which are all presently bathed at >4.3 km water depth, whereas the amount of dissolution strongly depends on the paleodepth (subsidence) history at each site and fluctuations of the CCD on a regional and basin-wide scale. The dissolution effect is greatest in the oldest successions at Sites U1331–U1333 (Figs. F14, F15). The Eocene equatorial CCD has been estimated at a depth shallower than ~3.5 km (Lyle et al. 2005), with short-term CCD fluctuations occurring during the middle to late Eocene based on preservation of calcareous microfossils and calcium carbonate records at Sites U1331–U1334. The most striking CCD change has been recorded close to the Eocene–Oligocene transition where the sediments change from radiolarian-dominated Eocene sediments to Oligocene nannofossil oozes. The depth transect of these sites indicates a deepening of at least 1 km over this short time interval. The recovery of carbonate-rich Oligocene successions at all sites is evidence for a considerably deeper CCD (>4.5 km water depth) throughout this interval (see Fig. F14).

A compilation of semiquantitative estimates of preservation and abundance of calcareous microfossils reveals a strong coupling of the fossil records with paleodepth history and the CCD at these drilling sites. The carbonate dissolution effect is strikingly different between microfossil groups (Fig. F15). Planktonic foraminifers are the most sensitive to dissolution, and well-preserved specimens were found in sediments if carbonate contents exceeded at least 60%–70%. At the deepest Site U1331, planktonic foraminifers are only present during carbonate maxima in the Oligocene, middle Eocene, and early Eocene ages. At Site U1332 they were present only in the carbonate-rich Oligocene nannofossil oozes (Fig. F15). At Sites U1334 and U1335 high-carbonate sediments (80%–90%) contain abundant and well-preserved planktonic foraminifers.

Calcareous nannofossils and benthic foraminifers are less susceptible to dissolution than planktonic foraminifers and closely track the presence or absence of carbonates in the sediments. The preservation of both groups varies with carbonate content, but the preservation of calcareous nannofossils varies even in sediments with low carbonate content that are barren of planktonic foraminifers. Poor preservation of specimens is observed in sediments of 5%–25% carbonate contents, moderate preservation in sediments of 30%–70%, and good benthic foraminifer and moderate to good nannofossil preservation in sediments with >75% CaCO3.

Within the nannofossil assemblages, however, certain taxa are never present in these sediments, such as holococcoliths. The relatively robust holococcolith Zygrhablithus bijugatus was only recorded in one or two samples from Sites U1335 and U1336, and other taxa show distributions that are more similar to those of planktonic foraminifers, such as the long-ranging heterococcolith genus Helicosphaera (Fig. F15).

Basal carbonates

Eocene basal carbonate sediments (nannofossil oozes with foraminifers) were recovered on top of basaltic basement at Sites U1331–U1334 (Fig. F16). The existence of carbonates suggests that paleodepths at these sites were maintained at ~2.75 km above the shallow Eocene CCD during the early to middle Eocene. At all four sites these carbonate intervals are thin and their lower parts are lithified to limestones, probably because of the combined influence of hydrothermal fluid processes and overburden. These sediments contain slightly diagenetically modified calcareous microfossils and are barren of siliceous microfossils. Sites where Eocene sediments were recovered were located within 2° latitude of the paleoequator at the time of first sediment accumulation, so evidence of equatorial upwelling might be expected in the assemblages. The absence of siliceous microfossils is likely a result of dissolution associated with hydrothermal flow of the crust (Moore, 2008a).

Productivity indicators

Shipboard analyses of quantitative microfossil faunal assemblages allow only preliminary speculations on potential microfossil-based productivity indicators; quantitative work will be required to follow up on these initial observations and fully address Objective 2 of the PEAT program (see "Scientific objectives"). Calcareous nannofossil assemblages at these lowest stratigraphic levels are not strikingly different from younger examples. At several sites, however, the common presence of taxa that are considered to be higher productivity (or cooler water) indicators is suggestive of an upwelling signal (e.g., common Chiasmolithus and small reticulofenestrids [Reticulofenestra minuta] at Sites U1333 and U1334). At all sites sphenoliths are also common at these levels, and although some representatives of this group are considered to be oligotrophs (e.g., Gibbs et al., 2004), certain species clearly display more opportunistic behavior, which explains their abundant presence here (e.g., Wade and Bown, 2006; Dunkley Jones et al., 2008).

The absence or relative rarity of the warm-water oligotrophic discoasters in the lowest parts of Sites U1331, U1333, and U1334 is also suggestive of higher productivity surface waters, at least for the sediments immediately overlying basement. Discoasters are actually common in the lowest sediments at Site U1331, and this either represents selective concentration due to the dissolution of less robust taxa or indicates that these species (Discoaster deflandrei and Discoaster lodoensis) were adapted to more eutrophic paleoenvironments. Planktonic foraminifer assemblages in the basal carbonates at Sites U1333 and U1334 are dominated by relatively robust taxa: subbotinids, parasubbotinids, and paragloborotalids. These genera are thought to occupy a (sub)thermocline habitat (Wade et al., 2007; Sexton et al., 2006) and are often associated with high-productivity environments (Wade et al., 2007), an association consistent with both sites being situated in the equatorial upwelling region. However, planktonic foraminifer assemblages at Sites U1333 and U1334 may also be biased toward these more robust taxa by the effects of dissolution. Detailed assemblage study is required to elucidate the relative contributions of calcium carbonate dissolution and the true paleocological signal.

Radiolarian stratigraphy

The radiolarian stratigraphy in sediments recovered during Expedition 320 (Table T2) span the zones from RN14 (lower Pleistocene) to RP10 (lower middle Eocene) and provide the highest shipboard biostratigraphic resolution for most sections within the Eocene. The preservation of assemblages is generally good with only a few scattered intervals of moderate to poor preservation. Nigrini et al. (2006) took a comprehensive approach toward establishing ages for all radiolarian datums recovered at Leg 199 sites. In so doing, they produced age estimates for >300 radiolarian datums, greatly enhancing our ability to date the Cenozoic section in the tropical Pacific. However, several of these calibrations need to be checked and/or refined. In addition, some of the datums appear to be more reliable than others, and this needs to be further evaluated. In some cases, variation in the levels of first and last appearances of species may be caused by variation in taxonomic interpretation, but more often these variations are due either to real differences in the ranges of species at different locations in the tropical Pacific or to the extremely low abundance of certain species in samples from a given site. Thus, taxonomic difficulty, abundance, and preservation all figure into how well a species serves as a stratigraphic marker.

A few levels were not adequately covered by Leg 199 sites. For the upper Miocene we have relied on the radiolarian studies of Leg 138 in the far eastern tropical Pacific (Moore, 1995). In their studies of Leg 199 sites, Kamikuri et al. (2005) and Funakawa et al. (2006) added substantially to our understanding of the radiolarian assemblage transitions at the Oligocene/Miocene and Eocene/Oligocene boundaries. However, these studies focused on the statistical changes in the faunal assemblages as a whole and made no wholesale attempt to recalibrate first and last appearance datums. During Expedition 320 we have been able to add to the stratigraphic control in the lower part of the middle Eocene collected at Site U1331 and test the usefulness of individual datums at the sites drilled. The full integrated biostratigraphies will address our third objective (see "Scientific objectives").

Diatom distribution from smear slide analysis

One of the great achievements of the PEAT program has been the recovery of all major fossil groups, including diatoms. The diatom biostratigraphy of sediments from Expedition 320 will be conducted postcruise, although initial descriptions of diatom abundance were made during lithostratigraphic smear slide analyses. Sediments from Expedition 320 with diatoms are typically less abundant in smear slides than radiolarians, nannofossils, clay, and foraminifers. Nevertheless, a site-to-site compilation of the smear slide data generated during this expedition and from Site 1218 reveals noticeable stratigraphic changes in diatom abundance relative to these other main lithologic constituents (0%–50%) (Fig. F17). In general, diatoms are quantitatively more important in Miocene sediments than in sediments of Oligocene and Eocene age at these equatorial Pacific sites. Noticeable peaks in diatom abundance occur superimposed on this secular pattern, with the most sustained high abundances (up to >25%) observed across the early/middle Miocene boundary (Sites U1335 and U1336). Diatom abundances of up to 25% occur over shorter durations in sediments from the Eocene–early Oligocene (Sites U1332, U1333, and 1218), late Oligocene (Site U1333), early late Miocene (Site U1335), and middle late Pliocene (Site U1336). Sediments of middle Eocene age at these sites are, in comparison with overlying sediments, poor in diatoms with a single minor abundance peak documented in radiolarian Zone RP15 at two sites (U1331 and 1218).

Paleomagnetism

Paleomagnetism and magnetostratigraphic studies are important observations needed to fulfill the expedition objectives of obtaining a well-intercalibrated Cenozoic megasplice and constraining the Pacific plate tectonic motion (Objectives 3 and 5 in "Scientific objectives"). Results obtained so far indicate that the sediments recovered will provide one of Expedition 320's lasting legacies toward addressing these objectives in a comprehensive fashion: shipboard paleomagnetic results were obtained from 56,222 intervals measured along ~2000 split-core sections and from detailed progressive alternating-field (AF) and thermal demagnetization of 411 small discrete samples (Fig. F18). These data indicate that a useful magnetic signal (characteristic remanent magnetization [ChRM]) is preserved in most APC cores after removal of the drilling-induced overprint by partial AF demagnetization at 20 mT. Exceptions were mainly limited to intervals affected by reduction diagenesis (70–110 and 210–410 m core depth below seafloor, method A [CSF-A; when using method A, core expansion lengths overlap if longer than 9.5 m and are not scaled], at Site U1335 and 80–160 m CSF-A at Site U1336), which have very low to even negative (diamagnetic) magnetic susceptibilities and retain little or no remanent magnetization.

Cleaned paleomagnetic data were characterized by shallow inclinations, consistent with the sites being near the paleoequator, and by 180° alternations in declinations downhole, reflecting magnetic polarity zones (magnetozones). These qualities, along with demagnetization results from discrete samples, indicate that the ChRM is the primary depositional magnetization. Magnetostratigraphies at each site were thus constructed by correlating the distinct declination alternations with the geomagnetic polarity timescale (GPTS) (Fig. F18).

In total, these magnetostratigraphies yield 803 dates ranging from 51.743 Ma (base of Chron 23n.2n at Site U1331) to the present (Chron C1n; 0–0.783 Ma at Site U1335). In addition, 83 short polarity intervals were observed that might correspond to cryptochrons or geomagnetic excursions. At these short events and at the geomagnetic reversals, magnetization intensities are low, as would be expected if the sediments are accurately recording the past paleomagnetic field intensity. Analysis of paleomagnetic directions over stable polarity intervals (full chrons) indicates the long-term record provides paleolatitude information that will aid in refining the Pacific apparent polar wander path and directional dispersion information for studying geomagnetic secular variation (Objective 5 in "Scientific objectives"). Thus, besides providing ages for the six sites, the high-quality paleomagnetic records have the potential to resolve long- and short-term geomagnetic field variability and provide important plate kinematic constraints.

The Cenozoic megasplice and stratigraphic correlation

One of the prime objectives for the PEAT program was a detailed intercalibration of bio-, magneto-, and chemostratigraphic records for the Cenozoic from the early Eocene to the present within an astronomically age-calibrated framework (Objective 3 in "Scientific objectives"). The PEAT program was designed to incorporate results from Leg 138 for the younger Neogene part and Leg 199 for time intervals in the Eocene (Pälike et al., 2008) (Fig. F2). Expedition 320 shipboard results indicate that we can achieve this objective, based on the observation that even decimeter-scale features in the sedimentary record from the drilled sites can be correlated over large distances across the Pacific seafloor (Fig. F9) (Pälike et al., 2005). The PEAT program will leave a long-lasting legacy for the detailed intercalibration of all major fossil groups, a detailed magnetostratigraphy with >800 dated reversals, and sedimentary cycles that can be calibrated across large distances in the Pacific Ocean. Figure F19 demonstrates that a Cenozoic megasplice can be constructed from the material recovered and spliced onto previous Leg 138 and 199 sites. Physical property data that proxy calcium carbonate oscillations at Sites U1331 and U1332 show a remarkable match with those from Site 1220, which also has an excellent magnetostratigraphy. Similarly, Sites U1333 and U1334 can be spliced to Site 1218, providing a coherent and integrated record of large-scale Pacific sedimentation patterns of biogenic material from the Eocene through the Miocene and younger (Fig. F20). Such stratigraphic correlation makes possible the study of sedimentation patterns and mass accumulation rates at orbital resolution. The material recovered will also allow us to verify existing calibrations (e.g., Pälike et al., 2006b) and further extend these up into the Miocene and down into the Eocene.

We can exemplify the approach toward the Cenozoic by constructing a preliminary multisite splice of gamma ray attenuation (GRA) bulk density (Fig. F21). This record of GRA bulk density from tropical Pacific sediments encompasses a major part of the Cenozoic, stretching from the upper Eocene to the recent, using data from Legs 138 and 199 and the PEAT program.

Sedimentation rates

A study of paleoceanographic processes and variations of mass accumulation rates across the PEAT latitudinal transect and its evolution over time depends on a detailed knowledge of sedimentation rates. The integrated bio- and magnetostratigraphies obtained for all Expedition 320 sites will allow us to fully exploit and understand the complex interplay of productivity, dissolution, and spatial biogenic sedimentation patterns, which leave their imprint in the sedimentation rates recorded at different drill sites. Depending on the crustal subsidence and age for each site, sedimentation rates vary from site to site over time (Figs. F22, F23).

Our results reveal the change in linear sedimentation rates (LSRs) in both the latitudinal and age transect components of the PEAT program. LSRs of the middle Eocene are extremely high, frequently >10 m/m.y. and with a maximum of 18 m/m.y. at Site U1331. Rates at Sites U1332 and U1333 are similar (8–6 m/m.y.). LSRs of the late Eocene decrease to 3.5–6 m/m.y. at Sites U1331–U1333. The highest LSR peaks (>20 m/m.y.) exist in the early to late Oligocene section at Sites U1333 and 1334 and in the early Miocene for Sites U1336 and U1335. LSRs decrease uphole to <10 m/m.y. during the middle to late Miocene. LSRs also increase from the west (Site U1331) to the east (Site U1336), reflecting the relative age, depth, and latitudinal position of the sites. LSRs frequently show high rates >20 m/m.y in the east (Sites U1334–U1336) but rarely exceed 15 m/m.y. in the western sites. The LSRs of Sites 1218 and 1219 are also reflected at Site U1334 but show slightly lower values during the early Miocene (15–20 m.y.). By combining the available data from Leg 199 and Expeditions 320 and 321, we will obtain a continuous history of sedimentation rates in the equatorial Pacific region for the past 55 m.y.

Middle Eocene Climatic Optimum

A complete downhole transition of the MECO event was recovered at Sites U1331–U1333 (Bohaty and Zachos, 2003; Bohaty et al., 2009) (Fig. F24). Based on bio- and magnetostratigraphic datums, the MECO event (40–41 Ma) occurs between magnetic Chrons C18n.1n and C18r and falls into the radiolarian Zone RP15 to lowermost RP16. Bohaty et al. (2009) revised the position of peak middle Eocene warming at 40.0 Ma in Chron C18n.2n. At Site U1333 the lithostratigraphy of the MECO event is characterized by an alternating sequence of nannofossil ooze and radiolarian nannofossil ooze interrupted by an interval of radiolarian clay as thick as 4.2 m (Fig. F24). The MECO event at Site U1332 is marked by an alternating sequence of nannofossil ooze, radiolarian nannofossil ooze, radiolarian ooze, and clayey radiolarian ooze. Site U1331 lithostratigraphy also shows an alternating sequence of nannofossil radiolarian ooze, radiolarian nannofossil ooze, and nannofossil ooze. However, at Site U1331 the interval is interrupted by coarse-grained gravity flows. These lithostratigraphic results for the MECO event are similar to those obtained from Sites 1219–1221; at Site 1222 the interval is dominated by clay (Lyle, Wilson, Janecek, et al., 2002).

Eocene–Oligocene transition

Lithology

An Eocene–Oligocene transition was recovered at four sites drilled during Expedition 320 (Sites U1331–U1334). The Eocene/Oligocene boundary as formally defined cannot be identified at these sites because of the absence of the planktonic foraminifer biostratigraphic marker Hantkenina. Magnetostratigraphy from APC-cored intervals and biostratigraphy (radiolarians and nannofossils) provide excellent age control, however, with the Eocene/Oligocene boundary falling just below the Magnetochron C13n/C13r reversal, near the middle of Biozone NP21, and just above the Biozone RP20/RP19 boundary. (Fig. F25).

Sites U1331–U1334 capture the lithostratigraphy of the Eocene–Oligocene transition in the equatorial Pacific Ocean in a depth transect from ~3600 to 4300 m paleowater depth (~34 Ma) (Fig. F25). At each site, a downhole transition takes place from white to pale brown nannofossil ooze of earliest Oligocene age to much darker brown sediments of Eocene age. At the deep end of the depth transect where the Eocene–Oligocene transition is least expanded (Site U1331), the transition is sharp (over a ~5 cm thick interval) into homogeneous dark brown clayey radiolarian ooze. At the shallow end of the transect (Site U1334), where the correlative section is much more expanded, it is less sharp and takes place through dark clayey nannofossil chalk to alternations of dark nannofossil chalk and even darker clayey nannofossil chalk. At Site U1332 the lithologic transition is through radiolarian nannofossil ooze to radiolarian ooze with clay; at Site U1333 the transition is through radiolarian ooze to alternations of radiolarian nannofossil ooze with clay and clayey radiolarian ooze. At Sites U1332–U1334 prominent ~50 to 100 cm thick beds of particularly dark clays or radiolarian clays are noticeable (Fig. F25). At all sites, the line-scan core images reveal stepwise downhole transitions in sediment color (Fig. F25). Associated pronounced downhole stepwise increases occur in magnetic susceptibility, a*, and b*, together with pronounced downhole deceases in GRA bulk density, L*, and CaCO3 content.

The lithostratigraphy of the Eocene–Oligocene transition from Expedition 320 sites is remarkably consistent with both the expedition rationale for drilling these sites and Leg 199 results and will allow the study of the early history of Cenozoic glaciation and CCD behavior (see Coxall et al., 2005; Pälike et al., 2006b) across a depth transect. Two major lithostratigraphic results from the Eocene–Oligocene transition from Expedition 320 are unexpected and also demand evaluation:

  1. The discovery that the section at Site U1333 (paleowater depth = ~34 Ma; ~4000 m) is more carbonate rich than Site 1218 (paleowater depth = ~34 Ma; ~3850 m), perhaps a paleolatitudinal signal given that this site is at the northern edge of the latitudinal transect of PEAT sites during the Eocene–Oligocene; and

  2. The recovery in the most expanded section (Site U1334) of darker more clay rich and yet more calcareous lithologies than at either Site U1333 or 1218 (Figs. F25, F26).

The multiplicity of datums throughout a major part of the Cenozoic section provided by the Nigrini et al. (2006) calibration offsets any taxonomic, abundance, or preservation problems that were encountered with a few of the species. One interval, however, has proven to be particularly troublesome: the Eocene/Oligocene boundary. This part of the section not only shows substantial turnover in the radiolarian fauna (Fig. F27) (Funakawa et al., 2006), it is often represented by a hiatus and is associated with reworked older Eocene radiolarians being deposited in uppermost Eocene and lower Oligocene sediments. Thus, the upper appearance limit of many Eocene species is problematic. A further complication in establishing the true age of first and last appearances of these lower Oligocene and upper Eocene species arises from the impact of missing sections on establishing a paleomagnetic stratigraphy. With part of the section missing, chron boundaries can be truncated, giving an inaccurate estimate of the age of the sediment marking that boundary. Finally, the sharp change in the CCD across the Eocene/Oligocene boundary often makes it difficult to correlate the section recovered at one site to that of another site in the same region (Fig. F28). Of all the means of making regional correlations across the Eocene/Oligocene boundary, paleomagnetic chron boundaries appear to be the most reliable (Fig. F28).

Through the many papers on the material collected during Leg 199, we have come to appreciate more fully the true stratigraphic nature of the Eocene/Oligocene boundary. From the study and comparison of these sections we have been able to identify those sites that appear to have the most complete record across this boundary. From Leg 199, Site 1218 appears to provide the most complete stratigraphic record. From this expedition, a complete boundary section appears to have been recovered at Sites U1333 and U1334; however, only Site U1333 has paleomagnetic control.

In an effort to make a detailed correlation of these three sites, we compared their magnetic susceptibility records and, where available, the paleomagnetic stratigraphy (Fig. F28). This comparison is revealing in many ways. Although Site 1218 and U1334 magnetic susceptibility records look quite similar superficially, when compared in detail they are substantially different. The two-step change in magnetic susceptibility (as well as in other geochemical variables) at the base of Chron C13n is common to Sites 1218 and U1334, as well as to Site U1333. This is one of the first indications that the Eocene/Oligocene boundary section at a site is relatively complete (e.g., Coxall et al., 2005). By simple "peak counting," the maximum in magnetic susceptibility at the top of Chron 15n is also fairly easily identifiable at Site U1334 (as well as at Site U1333). However, the broad, major peak in magnetic susceptibility seen at Site 1218 is only partially represented at Site U1334, with only the younger part of this broad maximum seen at Site U1334. It is not seen at all at Site U1333. A sharp minimum in magnetic susceptibility just below the top of Chron C17n.1n at Site 1218 can also be seen, along with similar minima at Sites U1334 and U1333. Finally, the minimum in magnetic susceptibility just below the broad maximum near the top of Chron 18n.1n is associated with the first appearance of Calocyclas turris at Site 1218, as well as at Sites U1334 and U1333.

The magnetic susceptibility record from Site U1333 looks very dissimilar to those of the two sites drilled on younger basement (Fig. F28). It looks more similar to those records from Sites 1219 and 1220 that were drilled on 56 Ma crust (Fig. F29). These differences and similarities exist even though Site U1334 has fairly good preservation of calcium carbonate in the upper Eocene section, whereas at Site 1218 carbonate preservation is relatively poor and at Sites 1219, 1220, and U1333 carbonate is only occasionally present. Thus, it appears that the high-amplitude excursions in magnetic susceptibility records near the top of Chron 16n.1n are found only in sections deposited on crust that is only a few million years older than this chron (~35.5 Ma).

One would hope that the biostratigraphic record of the Eocene/Oligocene boundary region in the more complete sites would reveal a consistent record of faunal turnover. This may be true for the first appearances of species in the lower Oligocene; however, the position of the last occurrences of Eocene species within the uppermost Eocene seem to indicate that the reworking of older fossils into younger sections is common, even if there are no apparent breaks in the sections. For example, the position of the last occurrence of Cryptocarpium azyx at Site 1218 is considerably higher in the section than those seen at Sites U1333 and U1334 (Fig. F28). The first appearance of species in these sections is somewhat more reliable (Fig. F28), but it is the very nature of such large faunal turnovers as seen near the Eocene/Oligocene boundary that most of the biostratigraphic datums involve extinctions. Only a very detailed sampling of the section and a more quantitative analysis of the fauna can reveal when the presence of a species in a sample is likely to indicate reworking of older sediments.

Just as the rapid extinction of many Eocene species is a dramatic illustration of the impact of climate change on the planktonic fauna, the recovery from this event is also of interest. A few radiolarian species survived the transition from the warm Eocene to the cooler Oligocene (Funakawa et al., 2006), but the rapid appearance of new species did not occur until several hundred thousand years after the Eocene/Oligocene boundary at 33.7 Ma. Two lower Oligocene marker species (Lithocyclia crux and Theocyrtis tuberosa) first appear near 33.4 Ma (recalibrated age), followed by Dorcadospyris pseudopapilio, Dorcadospyris quadripes, and Centroboytris gravida near 33.0 Ma (recalibrated age) (Fig. F27). Abundant diatoms are found in the coarse fraction slides alongside the radiolarians starting in the uppermost Eocene at the same level as the first (older) step in the magnetic susceptibility record (Fig. F27), and the species makeup of this flora changes rapidly upsection. Thus, it appears that there is an increase in the productivity of larger diatoms associated with the Eocene–Oligocene transition and that the changeover of the species makeup of the flora occurs quickly in the diatoms compared to the radiolarians. Radiolarians allow the correlation of Eocene/Oligocene boundary sections between different sites without relying directly on the lithostratigraphy (Fig. F30).

Oligocene–Miocene transition

At the end of the Oligocene a significant multimillion year–long rise in the oxygen isotope record (Lear et al., 2004) is closely followed by a relatively short, sharp increase in oxygen isotope values that has been interpreted as a major glacial episode (Mi-1) (Zachos et al., 1997, 2001a, 2001b; Pälike et al., 2006a, 2006b) and correlated to a pronounced drop in sea level (Miller et al., 1991). This event is very close to the Oligocene/Miocene boundary and has now been astronomically age calibrated in several ocean basins (Shackleton et al., 2000; Billups et al., 2004; Pälike et al., 2006a). Although there are clear periodic isotopic signals indicating major changes in ice volume, ocean temperatures, and/or ocean structure, this biostratigraphic boundary has always been somewhat of an enigma. Unlike the major changes in the isotopic stratigraphy, the biostratigraphies of the planktonic microfossils show very little change across this boundary. In fact, it is one of the most difficult epoch boundaries to pick using solely microfossil biostratigraphies.

At Sites 1218 and 1219 this interval was well recovered; however, carbonate preservation still presented a problem for foraminifer stratigraphy. Both sites were deep and well within the lysocline, making the application of temperature proxies such as Mg/Ca ratios in foraminifer tests more difficult (Lear et al., 2008). At the time Miocene–Oligocene sediments were deposited, Site 1218 already resided on 18 m.y. old crust and was ~4.1 km deep. Site 1219 was on ~34 Ma crust and was ~4.5 km deep (Lyle, Wilson, Janecek et al., 2002). A relative increase in large diatoms near this boundary in the siliceous coarse fraction suggests increased productivity; however, detailed, high-resolution flux rates across this interval have yet to be determined.

Complete sequences to the biozone and mangetochron level of the Oligocene–Miocene transition were recovered at Sites U1332–U1336 (Fig. F31), providing an excellent integrated stratigraphy. Sites U1332–U1334 display unambiguous magnetostratigraphy coherent with biostratigraphy and a distinct record of alternations in sediment constituents and physical properties. Because of Fe reduction, late Oligocene and early Miocene sediments from Sites U1335 and U1336 do not retain a sufficiently strong magnetic intensity to allow retrieval of a reliable shipboard magnetostratigraphy, but good biostratigraphic control is available. The northwesternmost Site U1331 (Fig. F1) does not record the Oligocene–Miocene transition. The Oligocene/Miocene boundary is defined by the first occurrence of the planktonic foraminifer Paragloborotalia kugleri (23.0 Ma) and is approximated well by the short-lived (~100 k.y.) calcareous nannofossil Sphenolithus delphix (23.1–23.2 Ma) just below Chron C6Cn.2n. The Oligocene–Miocene transition in Expedition 320 sediments is characterized by alternations of nannofossil- and radiolarian/clay-dominated intervals upsection of the Oligocene/Miocene boundary at Sites U1332 and U1333 and by subtle light–dark color alternations of nannofossil ooze at Sites U1334 and U1336 (Fig. F31).

Miocene

Carbonate-bearing sediments were recovered primarily at Sites U1335 and U1336 during Expedition 320. At Site U1335, the early and middle Miocene are expanded and sedimentation rates are as high as 20–30 m/m.y., allowing us to achieve our Miocene specific objectives (see "Latest Oligocene–earliest Miocene [Site U1335; 26 Ma crust]," "Miocene [Site U1337; 24 Ma crust]," and "Middle Miocene [Site U1338; ~18 Ma crust]"). The Miocene and younger periods form the prime focus for Expedition 321.

Equatorial productivity and redox-related color changes

Relatively white nannofossil oozes are the backdrop for the vivid color changes observed midsection at Sites U1334–U1336. Sediment color shifts from brown/very pale brown to light greenish gray and white. At each of the three sites, the shift in color is illustrated by a steplike drop in b* reflectance (yellow–blue) and a near complete loss of magnetic susceptibility (Fig. F32). Dissolved Fe concentrations in pore fluids increase at least sixfold in the zone of greenish gray sediments. Dissolved Mn concentrations increase at least fivefold in peaks just shallower than the dissolved Fe peaks, consistent with less reducing suboxic diagenesis. The link between sediment color and suboxic diagenesis is clearest at Site U1335, where the light greenish gray color is interrupted by a small interval of very pale brown before returning to greenish gray again. That very pale brown interval corresponds to a pronounced dip in dissolved Fe and a small increase in dissolved Mn. Together with the loss of magnetic properties, increases in dissolved Fe concentrations and changes in sediment color indicate intensified microbial Fe reduction, perhaps fueled by higher organic carbon accumulation rates across this interval. The intensification of suboxia at depth is largely controlled by site location with respect to the core of the equatorial upwelling system (Figs. F6, F32A). The greenish gray coloration is restricted to the time interval when each site was located south of 3°N. This pattern of geographic control on organic matter deposition and sediment diagenesis is supported by sediment color change observations in three additional DSDP sites (78, 79, and 574) (Fig. F32B). Further postcruise research will establish to what extent geochemical proxies of organic matter productivity, burial, and degradation contribute to the observed patterns, following Olivarez Lyle and Lyle (2005).

Geochemistry

Bulk sediment geochemistry: sedimentary carbon

A high-priority objective of the PEAT program is to reveal the history of CCD fluctuations during the Cenozoic. The findings resulting from >1000 coulometric carbonate measurements (Fig. F33) are described in "Cenozoic CCD in the equatorial Pacific" (Fig. F14). Calcium carbonate, inorganic carbon (IC), and total carbon (TC) concentrations were determined on sediment samples from every hole at Sites U1331–U1335 and Hole U1336A (TC was not determined in Hole U1336A during Expedition 320 and will instead be determined during Expedition 321).

We measured TC and total organic carbon (TOC) at a similar sample resolution and determined very low TOC concentrations, as previously found during Leg 199 (Lyle, Wilson, Janecek, et al., 2002; Olivarez Lyle and Lyle, 2005). TOC concentrations were determined separately by a difference method and by an acidification method for Site U1331. However, we concluded that the TOC concentrations determined by the normal difference method might be overestimates in high-percent CaCO3 and very low percent TOC sediments because they were determined as a small difference between two numbers comparable in magnitude. Therefore, TOC analyses were performed only by the acidification method, in which TOC was determined by using carbonate-free sediments after treatment by acidification for Sites U1332–U1335. Using this acidification technique, we reduced the detection limit for TOC measurements to 0.03 wt%. TOC concentrations in sediments determined by this method are very low throughout the sediment column and near or below the detection limit for samples (below 0.04%) from Sites U1331–U1335 (Figs. F34, F35). TOC concentrations tend to be slightly higher at those depths where CaCO3 concentrations are low at Sites U1332–U1335. The maximum TOC value determined is 0.18% in surface sediments from Site U1332. Despite the very low TOC values across the PEAT sediments recovered, postcruise research will be able to measure biomarkers and alkenones from some of the more organic rich sediments, addressing Objectives 2, 4, and 9 (see "Scientific objectives").

Interstitial water profiles: Si, Sr, and Li

Dissolved silicate increases with depth at each of the Expedition 320 sites to values as high as 1000 μM, at or near saturation with biogenic silica (Fig. F36). The increases with depth are generally larger for those sites with larger thermal gradients and higher heat flow, reflecting a temperature dependence of biogenic silica dissolution. In addition, one site shows a pronounced decrease in the depth zone of the Eocene/Oligocene boundary (Site U1332). Although chert was present at some of the sites, no pronounced decreases in dissolved silicate were found.

Interstitial water geochemistry profiles of the different sites drilled during Expedition 320 show considerable differences in respect to dissolved Sr2+ concentrations (Fig. F37). Whereas Site U1331 shows little variability with depth, Sites U1332–U1336 reveal increasing variability of Sr2+ concentrations. At Sites U1331 and U1332, Sr2+ shows mainly concentrations around seawater values; Sr2+ increases at Sites U1333 and U1334 up to ~110 μM, at Site U1335 up to 250 μM, and at Site U1336 up to 430 μM. Site U1335 is characterized by a pronounced increase in Sr2+ with depth followed by a strong decrease toward basement to seawaterlike concentrations. A similar pattern is revealed for Sites 1333 and 1334, but it is considerably less pronounced and developed. This pattern indicates the influence of carbonate diagenesis and or recrystallization, releasing Sr2+ to the pore fluid at intermediate depth, and the flow of relatively unaltered seawater through basement and diffusion between end-members. Strontium at Site U1336 is characterized by a steady increase in concentration with depth, indicating, together with the increase in Ca2+ and decrease in Mg2+, the influence of a fluid that reacted with the basement. The limited variability of Sr2+ at Sites U1331 and U1332 might be related to the relatively thin sediment thickness preventing the establishment of large gradients.

Similar to Sr2+, Li+ shows considerable differences between the different sites of Expedition 320, with the least variability at Sites U1331–U1333 and increasing depletion of Li+ in the pore fluid at Sites U1334 (down to 15 μM), U1336 (down to 7 μM), and U1335 (down to 4 μM) at intermediate depth (Fig. F38). Near basement, Li+ increases again toward seawaterlike values. Profiles of Li+ in the pore fluid indicate diagenetic reactions in the sediments consuming Li, possibly low-temperature clay alteration. Li concentrations similar to seawater values are compatible with observations of Sr, suggesting the flow of relatively unaltered seawater through the oceanic crust.

Seismic stratigraphy and integration

Objective 6 of the PEAT program (see "Scientific objectives") is to establish the age and lithologic origin of the seismic reflections previously identified in the eastern equatorial Pacific and make use of the high level of correlation between tropical sedimentary sections and existing seismic stratigraphy to develop a more complete model of equatorial circulation and sedimentation (Lyle et al., 2002a, 2006). We achieved this objective by calculating synthetic seismograms made from bulk density and sonic velocity data from core material and downhole logs.

The known depths and two-way traveltimes (TWTs) to the seafloor and basement provided an initial depth to TWT model and average sonic velocity for the sediment cover. The depth to TWT model was then adjusted to bring the reflectors in the synthetic seismogram into line with the corresponding reflectors in the seismic section. Typically, only a small number of adjustments were needed to give a good match. An example from Site U1331 is given in Figure F39.

Downhole logging at two sites represents the petrographic signals between radiolarian and nannofossil ooze using density, conductivity, and magnetic susceptibility, which are more useful in understanding the lithology in the poor recovery intervals composed of chert and porcellanite. In addition, a hard lithified limestone altered by hydrothermal activity occurred within 10–20 m above basement at most drilling sites. Correlation between seismic profiles and drilling is important in understanding an estimation of basement age and thickness of sedimentary sequences. At Site U1335 the thickness is estimated to be 361 m, but 420 m of sediment was recovered above the basement. Thus, our results contribute to the drilling strategy of the next PEAT Expedition 321 and future drilling in the equatorial Pacific.

MORB age transect

An age transect of sediments on top of basaltic basement in the equatorial Pacific region was recovered during Expedition 320. Basement ages represent eastward younging from Site U1333 to U1336 (Fig. F16). Basement basalts are covered by hard lithified limestones spanning from early Eocene through early Miocene age except at Site U1332, where zeolitic clays were recovered above basalt. These clays yielded poorly preserved calcareous nannofossils, and the position of the clays may be due to hydrothermal alteration. At Site U1331, however, a typical calcareous ooze with high carbonate contents containing abundant planktonic foraminifers and calcareous nannofossils was recovered as observed at Site 1221. Basement basalts are highly altered with a spherulithic texture, whereas ferromagnesian minerals (mainly clinopyroxene) are replaced by chlorite. Thin section analysis indicates a sparsely phyric basalt mainly composed of phenocryst plagioclase.

Chert and porcellanite occurrence

Sites U1331–U1333 and U1336 recovered chert and porcellanite (Fig. F40). Cherts are characterized by their hardness and highly silicified matrix in which sediments are cemented with microcrystalline quartz. Primary pore spaces (e.g., within chambers of foraminifers and radiolarians) are sometimes filled with chalcedony. Porcellanites are silicified to a lesser extent and are richer in clay minerals than the cherts. Porcellanite is much more abundant than chert at Sites U1331–U1333, whereas Site U1336 contains mostly chert. Porcellanite-bearing intervals at Sites U1331–U1333 correspond to a time interval between early Eocene and early middle Eocene (~42 Ma), roughly coincident with the chert-rich intervals at Sites 1220–1222 recovered during Leg 199 (Fig. F40). At Sites U1331 and U1332, porcellanite layers are associated with thin clay horizons interbedded with radiolarian ooze. Oligocene chert layers from Site U1336 show various colors such as greenish gray, dark gray, pink, and black and contain abundant foraminifers that are occasionally replaced with microcrystalline quartz and pyrite.

Porcellanites are interbedded with radiolarian ooze and nannofossil ooze in the early middle Eocene interval at Sites U1332 and U1333 (Fig. F40). Cherts are also interbedded with nannofossil ooze and chalk in the Oligocene interval at Site U1336. In stratigraphic intervals from the early Eocene through earliest middle Eocene at Sites U1331 and U1332, the presence and original structures of porcellanite are not clear because of poor core recovery.

At the base of the sections recovered there was always an interval in which all biogenic silica had been dissolved and which was barren of radiolarians. This "silica free zone" (SFZ) is usually not thickÔÇ"only 7 to 16 m at most of the sites drilled (Table T2). This is well within the usual 1 to 40 m thickness of the SFZ described by Moore (2008a) for Pacific open-ocean sections recovered by scientific ocean drilling. Moore (2008a) associated this dissolution zone with the circulation of hydrothermal waters in the upper oceanic crust and related the silica removed by this process to the ultimate formation of cherts. Thus, the final site drilled, Site U1336, stands out as very unusual. It has a SFZ of almost 130 m, with abundant chert stringers and lithified sediments below ~170 m in the section. This suggests that hydrothermal waters may actually invade well up into the section near this site, possibly along more permeable, small-offset faults.

Heat flow measurements

Geothermal gradient and heat flow values were determined at Sites U1331–U1335 from in situ temperature measurements made using the advanced piston corer temperature tool (APCT-3). At least four APCT-3 deployments were made per site, to a maximum depth of 106 m. Bottom water temperature was also determined for each site, with values ranging from 1.44° to 1.47°C. Linear geothermal gradients obtained from these data ranged widely from 7.4°C/km at Site U1335 to 75.0°C/km at Site U1332.

Heat flow was calculated according to the Bullard method, to be consistent with the Leg 199 analyses and the synthesis of ODP heat flow data by Pribnow et al. (2000). Similar to the geothermal gradients, Site U1335 had the lowest heat flow, 6.9 mW/m2, and Site U1332 the highest, 70.7 mW/m2 (Fig. F41). These heat flows are within the range of values in the eastern equatorial Pacific heat flow data set maintained by the International Heat Flow Commission (Pollack et al., 1993).

The wide range of values emphasizes the possibility that local crustal hydrothermal circulation is strongly influencing heat flow values. Results from the PEAT expeditions will also provide heat flow information in an area of the Pacific Ocean that has only a sparse existing heat flow data set.

Gravity flow deposits

Throughout the sedimentary section drilled at Sites U1331 and U1335, sharp irregular contacts are present between lithologies (Fig. F42). Many are recognizable by distinct changes in color. Sharp contacts, occasionally associated with an erosional basis, are often overlain by coarser grained, more carbonate rich (including planktonic and benthic foraminifers), and/or opaque-coated sediments than those below the contact. At Site U1335 angular basalt fragments, pyritized foraminifers and/or radiolarians, and fish teeth are often present at the base of the gravity flows. The overlying sediments fine uphole and in some cases show cross or parallel laminations in the middle of the bed. These features indicate that the erosional contacts and their overlying coarse sediments at both Sites U1331 and U1335 are the product of mass flow events, typically turbidity currents. Commonly, turbidite thickness is between 2 and 25 cm, with the maximum thickness of 176 cm found at Site U1335. The total thickness of the identified turbidites at Site U1335 occupies at least 2% of the recovered sediment. The provenance of the inferred turbidites observed at Site U1331 is unknown, but their typically calcareous composition points to a source that lay above the CCD at the time that the reworked sediments were originally deposited, possibly a seamount lying a few kilometers south of the drilled location. A similar provenance of turbidites is proposed for Site U1335, where two seamounts lie ~15–20 km to the northeast and southeast of the drilled location.

Site U1331

Three holes were cored at Site U1331 (12°04.088′N, 142°09.708′W; 5116 m water depth), which is the northwesternmost site drilled during the PEAT program (Fig. F43). At Site U1331, Eocene age seafloor basalt is overlain by 187.2 m of pelagic sediment, comprising radiolarian and nannofossil ooze with varying amounts of clay. For detailed coring activities, see "Site U1331" in "Operations."

The sediment column at Site U1331 has a strong resemblance to that of Site 1220 (Fig. F44) (Lyle, Wilson, Janecek, et al., 2002) but with noteworthy sharp erosive contacts concentrated within the upper two-thirds of the section (Fig. F45). A total of 7 m of Pleistocene–Pliocene clay overlies lower Oligocene to early Eocene alternations of nannofossil ooze, radiolarian nannofossil ooze, and radiolarian oozes of varying clay and calcium carbonate content, with a sharp lithologic change at the Eocene–Oligocene transition (~26 m CSF-A). The lowermost target interval is characterized by a ~20 m thick radiolarian ooze and porcellanite layer from 157 to 177 m CSF-A, overlying a sometimes clayey radiolarian ooze and zeolitic clays with hydrothermal red staining from 177 to 187.5 m CSF-A, deposited on top of a thin (187.6–188.5 m CSF-A in Hole U1331C) layer of calcareous ooze and zeolitic clay above the basalt. Some of the fine-grained basaltic fragments show fresh glassy chilled margins.

Carbonate content approaches 80% in the Oligocene nannofossil oozes in the upper part of the site and cycles between 0% and 40% in the middle Eocene section. There is a concentration of sharp erosive contacts in the interval between 80 and 120 m CSF-A, with calcareous material dominating the basal portion of these contacts and then fining upward in grain size into radiolarian oozes. Rarely, the sediment above a sharp contact contains well-rounded clasts up to 1 cm in diameter (interval 320-U1331B-10H-6, 117–130 cm). The lithologic and stratigraphic characteristics of these sediments have been interpreted as gravity-driven deposits, possibly from nearby seamounts ~10 km to the south (Fig. F43B). Between ~177 and 188.5 m CSF-A, Cores 320-U1331A-22X, 320-U1331C-16H, and 17H achieved our site objective of recovering carbonate-bearing material from the time interval just following 52 Ma.

All major microfossil groups were found in sediments from Site U1331, and they provide a consistent and coherent biostratigraphic succession from basement to below the surficial clay layer. Nannofossils are common in the Oligocene and lower Eocene but sporadic in sediments from the upper Eocene, probably because of dissolution. Middle Eocene sediments commonly contain calcareous nannofossils punctuated by several barren intervals, notably below Zone NP21 (radiolarian Zone RP19 equivalent), below NP17, and between NP15 and NP13 (radiolarian Zones RP12–RP8 equivalent). Radiolarians are common to abundant throughout the section. Radiolarian and nannofossil datums and their age determinations agree and range from nannofossil Zone NP12 in the basal sediment section (~51–53 m.y. before present) to NP23/24 and radiolarian Zones RP8 just above basement through RP21 (late Oligocene, older than 25 Ma) in the uppermost section, below the Pleistocene clays. Both radiolarian and nannofossil assemblages contain reworked, older components (deeper than ~50 m CSF-A) but within a coherent and ordered stratigraphy. Planktonic foraminifers are generally absent, except for sporadic samples often associated with sediment just above sharp lithologic contacts and also in the basal carbonate section (provisionally Zones E4/E5). Benthic foraminifers are generally rare and indicate lower bathyal to upper abyssal paleodepths. They are also frequently found in the graded coarse sediment above the base of sharp contacts but suggest there is no apparent difference in the depth habitat between benthic foraminifers from just above sharp contacts and other parts of the section. Diatoms are observed throughout the column and, once analyzed by specialists not onboard during Expedition 320, will contribute to the stratigraphic integration of the PEAT program.

Apparent sedimentation rates, as implied by the biostratigraphic age determinations and aided by magnetostratigraphic polarity interpretations, vary throughout the section. The radiolarian-rich section below ~80 m CSF-A and basement was deposited at an average rate of 10 m/m.y., whereas the late middle Eocene to Oligocene section was deposited at a rate of ~4 m/m.y., with an apparent inflection between 60–80 m CSF-A. The porcellanite horizon spans a time interval of ~2–3 m.y. The presence of all major fossil groups as well as a detailed magnetostratigraphy will allow us to achieve one of the main PEAT objectives to arrive at an integrated Cenozoic stratigraphy and age calibration (e.g., Pälike et al., 2006b).

A full physical property program was run on cores from all three holes, comprising Whole-Round Multisensor Logger (WRMSL) measurements of magnetic susceptibility, bulk density, P-wave velocity, noncontact resistivity, and natural gamma radiation (NGR), followed by discrete measurements of color reflectance, index moisture and density properties, sound velocities, and thermal conductivity. Bulk density measurements show a marked increase in the carbonate-rich Oligocene section. Magnetic susceptibility measurements are variable throughout the section, allowing a detailed correlation between different holes and picking out sharp contacts and clay layers by increased susceptibilities. NGR measurements are elevated by an order of magnitude in the surfacial clay layer and reach 130 counts per second (cps) at the seafloor, dropping to <5 cps below 30 m CSF-A. Porosity values are generally high in radiolarian-rich sediments (80%) and decrease within the Oligocene carbonate section. Carbonate content is positively correlated with thermal conductivity. Discrete physical property measurements will prove useful to calibrate WRMSL velocity and density estimates and generally agree with WRMSL estimates, once appropriate correction factors are included for the core liner. Discrete velocities are significantly higher (50–100 m/s) than track measurements in the direction perpendicular to the split plane of the core section (x-axis), which is likely an artefact.

Using whole-round magnetic susceptibility measurements, Holes U1331A–U1331C can be spliced to form a continuous section to at least 140 m CSF-A or 150 m core composite depth, method A (CCSF-A; when using method A, core expansion lengths are appended if longer than 9.5 m), with no apparent gaps. Core expansion is ~15%. It is possible that Hole U1331C cores can provide an additional spliced section to the top of the porcellanite interval at ~157 m CSF-A. Below 149 m CCSF-A, it was only possible to tentatively correlate features in the track data down to Core 320-U1331A-17X for a total composite section of ~172 m CCSF-A.

A full range of paleomagnetic analyses was conducted on cores and samples from Site U1331. Our aims are to determine the magnetostratigraphy and study geomagnetic field behavior, environmental magnetism, and Pacific plate paleogeography. Shipboard analyses conducted so far suggest that a useful magnetic signal is preserved in almost all APC-cored intervals. Preliminary comparison of biostratigraphic data and changes in magnetic paleodeclinations suggest the recovery of Oligocene magnetochrons to the base of the middle Eocene (Chron C21n; ~47 Ma). Paleomagnetic directions from discrete samples agree well with those from split-core results.

A standard shipboard suite of geochemical analysis of pore water and organic and inorganic properties was conducted on sediments from Site U1331, including a pilot study of high-resolution "Rhizon" pore water sampling, which does not require the cutting of core whole rounds for squeezing. Carbonate coulometry yielded carbonate concentrations of ~80% in the Oligocene nannofossil ooze and sporadic horizons with up to 40% CaCO3 in the middle Eocene radiolarian-rich oozes. Preliminary calcium carbonate determinations from the white, hydrothermally stained sediments just above basement (whole-round Sample 320-U1331C-17H-4, 83–84 cm) yielded low values of only 2%–3% CaCO3. Alkalinity values range between 2.5 and 3 mM throughout the section. Additional ephemeral samples were taken for shore-based microbiology and permeability studies.

Wireline logging provided valuable information to constrain the interval of chert or porcellanite formation within the borehole, and further analysis will aid in interpretations of carbonate content and lithologies. Integration with the seismic data will allow further improvements with the regional seismic interpretations. Data from Site U1331 indicate that the top of seismic Horizon "P2" (Lyle et al., 2002a) correlates with the top of the chert section. Downhole temperature measurements with the APCT-3 tool, when combined with the thermal conductivity values obtained from the cores, indicate that Site U1331 had a heat flow of ~10.4 mW/m2 and a thermal gradient of 13.4°C/km. This is within the range of the lower values in the global heat flow data set for the eastern Pacific but significantly lower than values obtained for Sites 1218 and 1219.

Highlights

Carbonate ooze in basal section

At Site U1331 we recovered a 1.2 m thick interval (lithologic Unit IV) of calcareous ooze with concretions and reddish color streaks, achieving one of the objectives for this site. The nannofossil ooze recovered in Section 320-U1331A-22X-CC contains a moderately to poorly preserved assemblage of early Eocene planktonic foraminifers (planktonic foraminifer Zone E5). This moderately preserved assemblage was also observed in the basal section of Hole U1331C.

Stratigraphic integration

One of the primary objectives of the PEAT science program is the integration of different stratigraphic methodologies and tools. Site U1331 contains almost all major fossil groups (nannofossils, radiolarians, foraminifers, and diatoms), as well as an excellent magnetostratigraphy. The possibility of a cycle-by-cycle match between Sites U1331 and 1220 has been demonstrated using magnetic susceptibility and bulk density data, providing additional stratigraphic tie points and verification of the completeness of the stratigraphic section on a regional scale. Thus, Site U1331 will help us to achieve an integrated stratigraphy for the Cenozoic Pacific Ocean.

Eocene–Oligocene transition and depth transect

Site U1331 forms the oldest and deepest component of the PEAT depth transect, which will allow the study of critical intervals (such as the Eocene–Oligocene transition; see Coxall et al., 2005) and variations of the equatorial CCD. Site U1331 is estimated to have been ~4.2 km deep during the Eocene–Oligocene transition, ~800 m shallower than today. Sediments rapidly change from radiolarian ooze below the transition to nannofossil oozes above and will provide a tie point for calcium carbonate burial at ~5° paleolatitude.

Variations in the CCD

Site U1331 will provide important constraints for variations and depth of the CCD from the early Eocene to the late Oligocene. This site shows increased carbonate content and much increased mass accumulation rates approximately from the middle of Magnetochron C18r to the base of C19r during the middle Eocene and can be correlated to an interval of enhanced carbonate burial that was previously documented by Lyle et al. (2005) at Leg 199 cores.

Age transect of seafloor basalt

At Site U1331 we recovered what appear to be fresh fragments of seafloor basalt, aged between 52 and 53 Ma. This material will, when combined with other PEAT basalt samples, provide important sample material for the study of, for example, seawater alteration of basalt and paleomagnetic studies.

Site U1332

Three holes were cored at Site U1332 (11°54.722′N, 141°02.743′W; 4924 m water depth) (Fig. F46), which is the second northwesternmost site drilled during the PEAT program. At Site U1332 Eocene age seafloor basalt is overlain by 150.4 m of pelagic sediment, comprising radiolarian and nannofossil ooze with varying amounts of clay and zeolitic clay. Hole U1332A provided high-quality and high-recovery APC-cored sediments from the mudline to 125.9 meters below seafloor (mbsf) (Core 320-U1332-14H), where we encountered chert and after which we switched to the extended core barrel (XCB) cutting shoe. XCB coring advanced to 152.4 m drilling depth below seafloor (DSF) through a ~10 m thick porcellanite-rich interval with reduced recovery. In the basal section, we recovered a short, ~3.8 m long interval of barren very dense and stiff clay above basalt, ~10 m shallower than predicted from the seismic profile, in Core 320-U1332A-18X. Basement was reached at 152.4 m CSF-A. For detailed coring activities, see "Site U1332" in "Operations."

The sediment column at Site U1332 has a strong resemblance to that of Site 1220 (Lyle, Wilson, Janecek, et al., 2002). The uppermost 17.7 m of section is a late Miocene to Pleistocene–Pliocene clay, with varying amounts of radiolarians and zeolite minerals, overlying ~130 m of Oligocene to middle Eocene nannofossil and radiolarian ooze with porcellanite deep in the section. A thin ~3 m thick unit of middle Eocene zeolite clay bearing small chert nodules was recovered at the base of the sedimentary sequence, above basaltic basement. The sedimentary sequence at Site U1332 was divided into five major lithologies (Fig. F47).

The upper stratigraphy at Site U1332 has a strong resemblance to that of Site U1331, but without the sharp erosive contacts described at Site U1331. Several meters of white to beige-colored Pleistocene–Pleiocene clay (lithologic Unit I) overlie lower Miocene to lowermost Oligocene nannofossil ooze (Units II and III). There is a sharp lithologic change at the Eocene–Oligocene transition (multiple recovery due to slumping within Cores 320-U1332A-8H, 9H, 320-U1332B-9H, and 320-U1332C-9H) to alternating radiolarian ooze with nannofossils and nannofossil ooze (Subunit IVa). The lithology then gradationally changes downhole into radiolarian ooze with nannofossils and clay intercalated with the sporadic presence of chert (Subunit IVb) and a basal cherty interval (Subunit IVc, down to at least 138 m CSF-A). Lithologic Unit V, below the chert horizon and between ~138 to at least ~147 m CSF-A, is composed of very dark grayish brown to black clay, very dark grayish brown to black zeolite clay, and chert. The sediments directly above basaltic basement are partially lithified. Basalt is designated as lithologic Unit VI, at ~150 m CSF-A.

Carbonate content approaches 85% in Unit III within the Oligocene nannofossil oozes and cycles between 0% and 40%–60% in the middle Eocene section (Unit IV) (Fig. F48). All major microfossil groups were found in sediments from Site U1332 and provide a consistent, coherent, and high-resolution biostratigraphic succession from basement to the top of Unit II. Calcareous nannofossils are abundant and moderately well preserved in the Oligocene and poor to moderately well preserved in the Miocene and Eocene. Most of middle Eocene sediments commonly contain nannofossils, with several barren intervals. Radiolarians are common to abundant throughout most of the section, apart from the lowermost sediment section above basalt, and are well preserved in the Eocene. Radiolarian and nannofossil datums and zonal determinations agree, ranging from nannofossil Zone NP13/14 in the basal dark clay section (~48.4–50.7 m.y. before present, Ma) to NN1 and radiolarian Zones RP13 above basement through RN1 (lowermost Miocene, ~22.3 Ma) below the upper Pliocene–Pleistocene clay cover in Core 320-U1332A-3H (Fig. F48). Planktonic foraminifers are generally rare throughout the Oligocene but are absent in the Miocene and Eocene. Benthic foraminifers are present through most of the section but are rare in Miocene and Eocene sediments. They indicate lower bathyal to abyssal paleodepths. Diatoms were observed throughout the column but will have to await analysis by specialists not onboard Expedition 320. Apparent sedimentation rates, as implied by biostratigraphic age determinations, vary throughout the section and are ~5 m/m.y. in the Eocene section and ~2.5 m/m.y. in the Oligocene, with two prominent hiatuses in the Miocene and between the Miocene and younger sediments. The presence of all major fossil groups as well as a detailed and well-resolved magnetostratigraphy will allow us to achieve one of the main PEAT objectives, to arrive at an integrated Cenozoic stratigraphy and age calibration (e.g., Pälike et al., 2006b) for major parts of the Oligocene and Eocene.

Magnetostratigraphic studies as well as high-resolution biostratigraphy and stratigraphic correlation determined that a ~4 m interval from the base of Core 320- U1332A-8H was repeated in the top of Core 320-U1332A-9H, which comprises Magnetochron C13n, and the lowermost Oligocene. This repetition also occurs in Cores 320-U1332B-8H and 9H and within Core 320-U1334C-9H. The lithologic succession from the lower occurrence of Chron C13n downhole as well as from the upper occurrence of Chron C13n uphole both appear complete and continuous, and hence Site U1332 achieved the fortuitous feat of recovering the complete Eocene–Oligocene transition four times and the upper part of Magnetochron C13n five times at a triple-cored site.

A full physical property program was run on cores from all three holes, comprising WRMSL measurements of magnetic susceptibility, bulk density, P-wave velocity, noncontact resistivity, and NGR, followed by discrete measurements of color reflectance, index moisture and density properties, sound velocities, and thermal conductivity. Bulk density measurements show a marked increase in the carbonate-rich Oligocene section, as well as in carbonate-bearing horizons in the Eocene (CAE cycles; Lyle et al., 2005). Magnetic susceptibility measurements are variable throughout the section, allowing a detailed correlation between holes. NGR measurements are elevated by an order of magnitude in the surfacial clay layer. Porosity values are generally high in the radiolarian-rich sediments (85%) and decrease in the Oligocene and Eocene carbonate section, which also shows higher thermal conductivity values of ~0.9 to 1.2 W/(m·K), compared with ~0.8 W/(m·K) in the radiolarian oozes and surficial clay.

Stratigraphic correlation allowed us to obtain a composite section to ~125.5 m CSF-A near the top of the cherty interval in Hole U1332A, equivalent to a composite depth of ~140 m CCSF-A. The overall core expansion (growth factor), which is caused by core expansion and calculated by the ratio between the CCSF-A and CSF-A (formerly meters composite depth [mcd] and mbsf) depth scales, is ~10%. The tops of APC cores were often affected by ~3 m heave that occurred during operations at Site U1332. Stratigraphic correlation supports the biostratigraphic, paleomagnetic, and sedimentologic description of a repeated sequence, possibly due to slumping, spanning the Eocene–Oligocene transition.

A full range of paleomagnetic analyses was conducted on cores and samples from Site U1332 and resulted in a spectacularly well resolved magnetostratigraphy. Shipboard analyses suggest that a useful magnetic signal is preserved in all APC-cored intervals and that it was possible to remove the drilling-induced steep inclination overprint after demagnetization. Comparison of biostratigraphic data and changes in magnetic paleodeclinations suggests the recovery of magnetic reversals C1n/C1r.1r to C2An.3n/C2Ar above a hiatus and then a continuous sequence of magnetic reversals from C5En/C5Er (18.52 Ma) in the Miocene at ~12.95 m CSF-A (interval 320-U1332C-2H-4, 95 cm) to C19r/C20n (42.54 Ma) at interval 320-U1332A-14H-5, 80 cm. Magnetostratigraphic interpretation supports the presence of a slump through multiple recovery (five times) of parts of Chron C13n in a triple-cored sequence. Paleomagnetic directions from discrete samples agree well with those from split-core results.

A standard shipboard suite of geochemical analysis of pore water and organic and inorganic properties was conducted on sediments from Site U1332. Alkalinity values increase from ~2.2 to 3.4 mM downsection, and Sr2+ increases from ~80 to ~110 μM. H4SiO4 remains relatively stable between 400 and 600 μM above 90 m depth in the Oligocene nannofossil oozes but increases to 800–1000 μM in the Eocene silica-rich radiolarian oozes, approaching opal solubility values. Carbonate coulometry yielded carbonate concentrations of ~85% in the Oligocene nannofossil ooze and horizons with up to 60% CaCO3 in the middle Eocene radiolarian-rich oozes. TOC concentrations were measured both by difference between TC and total IC as well as by using an acidification method. Using the acidification method, TOC values were <0.3% for all measured samples. The top ~5 m shows values of 0.2% TOC. Between ~40 and 70 m CSF-A the measurements indicate TOC below the detection limit of 0.03%. Between ~90 and 150 m CSF-A three peaks reach ~0.2% to 0.27% TOC. We conducted a high-resolution Rhizon pore water experiment across the prominent alkalinity trough around 40 m CSF-A, which highlighted differences between squeezed and Rhizon-sampled pore waters. Additional ephemeral samples were taken for shore-based microbiology and permeability studies.

Wireline logging provided valuable information to constrain the interval of chert formation within the borehole. Downhole NGR, density, and magnetic susceptibility logs provide important constraints on the poorly recovered lithologies below and between cherty horizons. The logging data document the presence of two thin chert or porcellanite horizons at ~126 and 130 m wireline log depth below seafloor (WSF) and an ~14 m thick interval of increased magnetic susceptibility, reduced conductivity, and enhanced density and photoelectric factor that appears to be the dark and dense clays and zeolitic clays above basement, rather than carbonate. Integration with the seismic data will allow further improvements with the regional seismic interpretations. Data from Site U1332 indicate that the top of seismic Horizon P2 (Lyle et al., 2002a) correlates with the top of the chert section, just as it did for Site U1331. No Formation MicroScanner (FMS) data were collected, as it was not possible to retrieve the "Paleo-" triple-combo tool string back into the bottom-hole assembly. Eight downhole temperature measurements were conducted in Holes U1332B and U1332C with the APCT-3 tool. Three of these yielded good data; the other measurements were impaired by strong, sometimes >3 m heave during operations in Hole U1332B.

Downhole temperature measurements, when combined with the thermal conductivity values obtained from the cores, indicate that Site U1332 had a heat flow of ~70.6 mW/m2 and a thermal gradient of 75.6°C/km. This is significantly higher than the values obtained for Site U1331 but comparable to values obtained for Sites 1218 and 1219.

Highlights

Shallow early Eocene CCD

Coring at Site U1332 was designed to capture a very short period of time (~2 m.y.) at ~50 Ma during which this site was thought to be located above the very shallow Eocene CCD (~3.3 km) (Lyle, Wilson, Janecek, et al., 2002; Rea and Lyle, 2005) just after the EECO (Zachos et al., 2001a). Unlike Site U1331, at Site U1332 we cored a ~10 m thick section of dense and dark brown clays, zeolite clays, and chert above basement. This finding will provide important new constraints on the depth of the CCD at ~48–50 Ma at the paleoequator, indicating that the CCD was shallower than previously thought.

Stratigraphic integration

One of the primary objectives of the PEAT science program is the integration of different stratigraphic methodologies and tools. Site U1332 contains all major fossil groups (nannofossils, radiolarians, foraminifers, and diatoms), as well as an excellent magnetostratigraphy and composite depth correlation, which can be tied to nearby Leg 199 sites (e.g., Site 1220) by way of physical property variations. The possibility of a cycle-by-cycle match between Sites U1332 and 1220 has been demonstrated using magnetic susceptibility and bulk density data, providing additional stratigraphic tie points and a verification of the completeness of the stratigraphic section on a regional scale. Thus, Site U1332 will help us to achieve an integrated stratigraphy for the Cenozoic Pacific Ocean, ranging from the Miocene to the middle Eocene.

Eocene–Oligocene and Oligocene–Miocene transitions and depth transects

Site U1332 forms the second oldest and deepest component of the PEAT depth transect, which will allow the study of critical intervals (such as the Eocene–Oligocene transition; see Coxall et al., 2005) and variations of the equatorial CCD. Site U1332 is estimated to have been ~4 km deep during the Eocene–Oligocene transition, ~1 km shallower than today and 200 m shallower at that time than Site U1331. Sediments rapidly change from radiolarian ooze below the transition into nannofossil oozes above, and unlike Site U1331, Site U1332 also contains carbonate-bearing sediments across the Oligocene–Miocene transition (e.g., Zachos et al., 2001b). For the Eocene–Oligocene transition, Site U1332 will provide a tie point for calcium carbonate burial at ~4° to 5° paleolatitude.

Variations in the CCD

Site U1332 has provided important constraints for variations and depth of the CCD from the early Eocene to the late Miocene. This site shows increased carbonate content and much increased mass accumulation rates approaching 200 mg CaCO3/cm2/k.y. around the middle of Magnetochron C18r to the base of C19r during the middle Eocene and can be correlated to an interval of enhanced carbonate burial that was previously documented by Lyle et al. (2005) at Site 199 cores. The high early Oligocene CaCO3 concentrations decrease significantly in sediments younger than ~27 Ma. By ~22 Ma, in the early Miocene, carbonate was no longer preserved. This is presumably related to Site U1332 sinking below the prevalent CCD and coincides with a CCD shoaling event between ~20 and 15.5 Ma described by Lyle (2003).

Formation of porcellanite

Together with Site U1331, Site U1332 provides important new information on the formation of porcellanite and chert. Coring has shown that the top of the porcellanite-rich interval is mapped by seismic Horizon P2 (Lyle et al., 2002a). In lithologic Subunit IVc, layers and pebbles of very dark brown partially to well-lithified mudstones, often layered or even laminated, are observed within alternating sequences of nannofossil ooze and radiolarian ooze of late to late middle Eocene age. In hand-specimen, the partially lithified mudstones are particularly rich in clay and show evidence of partial secondary silicification. Pieces of porcellanite contain clay minerals, microcrystalline quartz, opaques, and calcite, as well as biogenic shells and fragments from radiolarians and foraminifers. Sediments from Sites U1331 and U1332 appear to document the silicification process in clay-rich horizons near basement, which will likely extend the findings of Moore (2008b).

Age transect of seafloor basalt

At Site U1332 we recovered what appear to be fresh fragments of seafloor basalt, aged between 49 and 50 Ma. This material will, when combined with other PEAT basalt samples, provide important sample material for the study of seawater alteration of basalt.

Site U1333

Three holes were cored at Site U1333 (10°30.996′N, 138°25.159′W; 4853 m water depth) (Fig. F49). At Site U1333, Eocene age seafloor basalt is overlain by ~183 m of pelagic sediment, dominated by nannofossil and radiolarian ooze with varying amounts of clay (Fig. F50).

In Hole U1333A, APC-cored sediments were recovered from ~3 m below the mudline (~4850 m water depth) to 95 m CSF-A (Core 320-U1333A-10H). XCB coring advanced to 184.1 m DSF through an ~60 m thick sequence of lowermost Oligocene carbonate oozes and nannofossil-bearing Eocene sediments. Near the basal section, we recovered a 30 cm long interval of lithified carbonate in Core 320-U1333A-20X. The following Core 21X contained a dolostone basalt breccia. A 6 cm piece of basalt was recovered in Core 320-U1333A-22X.

Coring in Hole U1333B started 5 m shallower than in Hole U1333A to recover the mudline and to span the core gaps from the first hole. A total of 7.73 m of carbonate-bearing ooze overlain by a few meters of clay were recovered in Core 320-U1333B-1H. Because the cores recovered from Hole U1333A showed no significant porcellanite or chert layers, we used the APC drillover strategy in Hole U1333B to obtain APC cores across and below the Eocene–Oligocene transition to 162.7 m CSF-A. We then XCB cored to basement and a total depth of 180.3 m CSF-A.

Hole U1333C was designed to provide stratigraphic overlap and confirm stratigraphic correlations made between Holes U1333A and U1333B. APC coring in Hole U1333C started 2.75 m shallower than Hole U1333B and reached to 163.2 m CSF-A before we had to switch to XCB coring. No downhole logging was conducted at Site U1333.

The sediment column at Site U1333 has a strong resemblance to that of Site 1218 (Lyle, Wilson, Janecek, et al., 2002) but with notably more carbonate-bearing sediments in the Eocene portion. The ~183 m of pelagic sediments has been divided into four major lithologic units (Fig. F51). Unit I is ~7 m thick and contains an alternating sequence of clay, clayey radiolarian ooze, radiolarian clay, clayey nannofossil ooze, and nannofossil ooze from the early Miocene period. Unit II is ~112 m thick and composed of alternating very pale brown nannofossil ooze and yellowish brown nannofossil ooze with radiolarians of early Miocene to latest Eocene age. Unit III is ~60 m thick and composed of Eocene biogenic sediments comprising clayey nannofossil ooze, nannofossil radiolarian ooze, nannofossil ooze, radiolarian nannofossil ooze, and porcellanite of latest Eocene to middle Eocene age (Unit III). Unit III is divided into two subunits, based on the absence (Subunit IIIa) or presence (Subunit IIIb) of porcellanite. Porcellanite is a third lithology in Unit III between ~168 and 174 m CSF-A. Unit IV is a ~3.3 m thin unit of lithified carbonate (partly dolostone) and dolomitized nannofossil ooze, overlying basalt of Eocene age (Unit V).

All major microfossil groups were found in sediments from Site U1333 and provide a consistent, coherent, and high-resolution biostratigraphic succession from basement to the top of lithologic Unit II. Shipboard biostratigraphy indicates that sediments recovered at Site U1333 span a near-continuous succession from around the lower Miocene boundary to the middle Eocene. Radiolarians are common and well preserved in the Eocene succession but less well preserved in the Oligocene sediments. A complete sequence of radiolarian zones from RN2 to RP14 (middle Eocene) was described. Initial assessment of the radiolarian assemblages across the Eocene/Oligocene boundary interval indicates a significant loss of diversity through this apparently complete succession. Although a few species from the Eocene carry through to the Oligocene, only one stratigraphic marker species (Lithocyclia angusta) first appears near the Eocene/Oligocene boundary. Calcareous nannofossils are present and moderately to well preserved through most of the succession, although there are some short barren intervals in the middle to upper Eocene. The succession spans a complete sequence of nannofossil zones from lower Miocene Zone NN1 to middle Eocene Zone NP15. The Oligocene/Miocene boundary is bracketed by the base of Sphenolithus disbelemnos in Sample 320-U1333A-2H-5, 70 cm (16.20 m CSF-A), and the presence of rare S. delphix in Section 320-U1332A-2H-CC (9.57 m CSF-A). Discoasters are very rare in basal assemblages, indicative of a eutrophic environment and consistent with the paleolatitude of this site in the early middle Eocene within the equatorial upwelling zone. Planktonic foraminifers are relatively abundant and well preserved from the lowest part of the Miocene to the lower Oligocene. Oligocene fauna is characterized by the common presence of Catapsydrax spp., Dentoglobigerina spp., and Paragloborotalia spp. In contrast, upper Eocene sediments contain poorly preserved specimens or are barren of planktonic foraminifers. Preservation and abundance slightly increased in some intervals of the middle Eocene, which is recognized by the presence of acarininids and clavigerinellids. The absence of the genera Globigerinatheka and Morozovella makes precise age determination of individual samples problematic. The high abundance of Clavigerinella spp. has been linked to high-productivity environments, consistent with the paleogeographic location of this site. Benthic foraminifers were almost continuously present and indicate lower bathyal to abyssal depths. Oligocene fauna is characterized by calcareous hyaline forms, such as Nuttallides umbonifer, Oridorsalis umbonatus, and Cibicidoides mundulus. Nuttallides truempyi and O. umbonatus often dominate the Eocene fauna. Benthic foraminifers are present through most of the section apart from an interval in the middle Eocene equivalent to radiolarian Zone RP16. They indicate lower bathyal to abyssal paleodepths. Diatoms were observed throughout the column but will have to await analysis by specialists not onboard Expedition 320.

Sedimentation rates at Site U1333 are ~6 m/m.y. in the upper sediment column from the early Miocene to the late Oligocene. In the early Oligocene, linear sedimentation rates increased to ~12 m/m.y. Between ~31 Ma (earliest Oligocene) and the earliest late Eocene they are ~4 m/m.y., increasing slightly in the middle Eocene section (~39–45 Ma) to ~5 m/m.y.

Paleomagnetic results from measurements made along split-core sections and on small discrete samples from Site U1333 provide a well-resolved magnetostratigraphy. Shipboard analyses suggest that a useful magnetic signal is preserved in most APC-cored intervals after removal of the drilling-induced overprint by partial AF demagnetization at 20 mT. The overprint was nearly absent in those cores collected in nonmagnetic core barrels at Site U1333, whereas it was quite prominent for cores recovered in standard steel core barrels. Paleomagnetic directions from discrete samples agree well with those from split cores, confirming that AF demagnetization at 20 mT is generally sufficient to resolve the primary paleomagnetic direction regardless of which type of core barrel was used. Cleaned paleomagnetic data provide a series of distinct ~180° alternations in the declination and subtle changes in inclination, which, when combined with biostratigraphic age constraints, allow a continuous magnetostratigraphy to be constructed that correlates well with the geomagnetic polarity timescale. The magnetostratigraphic record extends from the base of Chron C6n (19.722 Ma) at 1.7 m CSF-A in Hole U1333C to the top of Chron C20r (43.789 Ma) at 161.6 m CSF-A in Hole U1333C. Highlights include very high quality paleomagnetic data across Chrons C13r and C13n, which span the latest Eocene and earliest Oligocene, and a newly recognized cryptochron within Chron 18n.1n.

Geochemistry results indicate that samples from the uppermost ~4 m of Site U1333 have modest CaCO3 concentrations of 26%–69% with frequent variations between 58% and up to 93% in the interval between 4 and 35 m CSF-A. Carbonate concentrations are consistently high (75.5%–96%) from 35 to 111 m CSF-A, whereas in the Eocene (between 111 and 171 m CSF-A) CaCO3 concentrations vary rapidly between <1% and 74%. The lowermost lithified carbonate rocks between 173 and 180 m CSF-A have high CaCO3 concentrations between 76% and 90%. TOC concentrations, as determined using an acidification method, are generally very low or below the detection limit (<0.1%, apart from samples in the top most 5 m, which reached ~0.17%). Pore water alkalinity values are never elevated, but alkalinity and dissolved strontium values are somewhat higher near the Eocene–Oligocene transition; these are generally consistent with carbonate dissolution or recrystallization processes. Dissolved silica increases with depth, with values always <1000 μM.

A full physical property program was run on cores from Holes U1333A–U1333C comprising WRMSL measurements of magnetic susceptibility, bulk density, P-wave velocity, noncontact resistivity, NGR, and measurements of color reflectance, followed by discrete measurements of moisture and density properties, sound velocities, and thermal conductivity on Hole U1333A cores only. All track data show variability throughout the section, allowing a detailed correlation between holes primarily using magnetic susceptibility and density (magnetic susceptibility varies around 24 × 10–5 SI in radiolarian ooze–dominated sections and ~3 × 10–5 SI in more carbonate rich intervals). Magnetic susceptibility values gradually increase uphole. NGR measurements are elevated by an order of magnitude in the uppermost clays and increase near the lower Oligocene at ~115 m CSF-A (from 5 to 8 cps). P-wave velocity gradually increases downhole as we move from carbonate- to radiolarian-dominated successions. P-wave velocity generally varies between 1490 and 1560 m/s depending on lithology, with lower velocities corresponding more to carbonate-rich sections. Bulk density and grain density show a marked decrease at ~112 m CSF-A (~1.704 to 1.313 g/cm3 in bulk density), where carbonate content decreases rapidly. Porosity values are generally high in the radiolarian-rich sediments (80%) and decrease in the carbonate-rich section (~60%). Thermal conductivity measurements are increased in carbonate-rich intervals and range from ~0.8 W/(m·K) in lithologic Unit I to 1.2–1.3 W/(m·K) in lithologic Unit II.

Stratigraphic correlation indicated that a composite section was recovered to ~130 m CSF-A in the upper Eocene, equivalent to a composite depth of ~150 m CCSF-A. For Site U1333, a growth factor of 15% is estimated from the ratio between the CCSF-A and CSF-A (formerly mcd and mbsf) depth scales. Stratigraphic correlation with Site 1218 suggests a complete stratigraphic section in the Oligocene to uppermost Eocene interval.

Five formation temperature measurements were conducted in Hole U1333B with the APCT-3 tool. These temperature measurements, when combined with thermal conductivity values obtained from the cores, indicate that Site U1333 has a heat flow of ~42.3 mW/m2 and a thermal gradient of 37.9°C/km.

Highlights

High carbonate fluctuations in middle Eocene sediments

Coring at Site U1333 was designed to capture a time period when the CCD was slightly deeper within the middle Eocene interval that showed prominent fluctuations of carbonate content (Lyle et al., 2005). This interval occurs during the cooling that took place after the EECO (Zachos et al. 2001a) and before the Eocene–Oligocene transition (e.g., Coxall et al., 2005). Unlike Site 1218, Site U1333 sediments show carbonate concentrations >75% in this interval at a deeper water depth and apparently coeval with the CCD cycles described by Lyle et al. (2005). Basal lithologic Unit IV recovered partially lithified carbonates.

MECO, Eocene–Oligocene and Oligocene–Miocene transitions, and depth transects

Site U1333 forms the third oldest and deepest component of the PEAT depth transect and can be directly compared with Site 1218, which will allow the study of critical intervals (such as the Eocene–Oligocene transition; see Coxall et al., 2005) and variations of the equatorial CCD. Site U1333 is estimated to have been ~3.8 km deep during the Eocene–Oligocene transition, ~1 km shallower than today and 200 m shallower at that time than Site U1332. Carbonate content in these sediments does not change as rapidly as at the deeper and older Sites U1332 and U1333. Some of these sediments appear to be Eocene–Oligocene transition sediments that are suitable for paleoceanographic studies using carbonate-based geochemical proxies and thus are an improvement over Site 1218. Of note, Site U1333 also contains high carbonate content–bearing sediments around the MECO event (Bohaty and Zachos, 2003; Bohaty et al., 2009), allowing a detailed study of the sequence of events linking carbonate preservation cycles (Lyle et al., 2005) with climatic oscillations.

Carbonate-bearing sediments across the Oligocene–Miocene transition were also recovered at Site U1333 (e.g., Zachos et al., 2001b), adding important data to the study of this time interval in the context of the PEAT Oligocene/Miocene depth transect.

Age transect of seafloor basalt

At Site U1333 we recovered what appear to be fresh fragments of seafloor basalt, aged between 45 and 46 Ma. This material will, when combined with other PEAT basalt samples, provide important sample material for the study of seawater alteration of basalt.

Site U1334

Three holes were cored at Site U1334 (7°59.998′N, 131°58.408′W; 4799 m water depth; Fig. F52), targeting the events bracketing the Eocene–Oligocene transition as part of an investigation of the wider Cenozoic climatic evolution (e.g., Zachos et al., 2001a) and providing data toward a depth transect across the Oligocene (see "Eocene/Oligocene Boundary [Site U1334; 38 Ma crust]") that will allow exploitation and verification of a previous astronomical age calibration from Site 1218 (Pälike et al., 2006b).

Site U1334 is in the center of the PEAT program, ~100 km north of the Clipperton Fracture Zone and ~380 km southeast of the previously drilled Site 1218. At Site U1334, late middle Eocene age (38 Ma) seafloor basalt is overlain by ~285 m of pelagic sediment.

The topmost ~47 m thick lithologic Unit I comprises a 15 m thick interval of brown radiolarian clay overlying ~32 m of alternating radiolarian clay and nannofossil ooze. The uppermost section (320-U1334A-1H-CC) is of late Miocene age (radiolarian Zone RN7; ~8.5 Ma). Below, Unit II comprises a ~200 m thick succession of upper Miocene to Oligocene nannofossil ooze and chalk above a ~35 m thick sequence of late Eocene age nannofossil chalk, radiolarite, and claystone (Unit III). Basal lithologic Unit IV (~1 m CSF-A) consists of middle Eocene intercalated micritic chalk and limestone on basalt (Figs. F53, F54).

Holes U1334A–U1334C provided high-quality APC-cored sediments from the mudline to ~210 m CSF-A (Cores 320-U1334A-22H, 320-U1334B-22H, and 320-U1334C-22H). Below this depth we encountered increasingly stiffer and harder sediment, after which we switched to the XCB cutting shoe. XCB coring advanced to 288.5 m DSF through lower Oligocene and Eocene sediments with high recovery. In the basal section, an intercalated unit of basalt and hard micritic chalk and limestone below a 10–20 m thick basal section of nannofossil ooze and chalk was recovered in Core 320-U1334A-32X. For detailed coring activities, see "Site U1334" in "Operations."

The sediment column at Site U1334 has a strong resemblance to that of Site 1218 (Lyle, Wilson, Janecek, et al., 2002) but with a thinner uppermost clay layer and higher Oligocene and Eocene sedimentation rates, as well as higher carbonate content in the middle and late Eocene sections, as was planned for this site.

Carbonate content exceeds 92% in the upper lower Miocene section below Section 320-U1334A-5H-3 and remains high throughout the Oligocene. Eocene sediments still contain considerable amounts of carbonate, and nannofossil ooze and chalk are dominant lithologies apart from several short less carbonate rich intervals (e.g., Section 320-U1334A-28X-3). In the middle Eocene section, carbonate content cycles between ~40% and 85% (Fig. F55), with higher values encountered toward the basal part of the Eocene section. Two short intervals in the late Eocene (~249 to ~257 m CSF-A) exhibit carbonate content of <20%.

A series of middle Oligocene cores (Cores 320-U1334A-16H through 21H) were recovered that had very distinct colors ranging from light grayish green to light blue. These uniquely colored carbonate oozes exhibit extremely low magnetic susceptibilities that complicated a confident stratigraphic correlation. These colored oozes lost almost their entire magnetic susceptibility signal from ~145 to ~215 m CSF-A (Figs. F54, F56). Similar colored cores have previously been described for DSDP Sites 78 and 79 (Hays et al., 1972).

The Eocene–Oligocene transition at Site U1334 is much more expanded than at Sites U1331–U1333 and even Site 1218. The Eocene–Oligocene transition was encountered at ~250 m CSF-A and fully recovered in Cores 320-U1334A-27X and 320-U1334B-26X; Hole U1334C was used to fill small stratigraphic gaps. The Oligocene–Miocene transition was fully recovered in all three holes in Cores 320-U1334A-10H (based on magnetostratigraphy, the boundary is at interval 320-U1334A-10H-6, 98 cm), 320-U1334B-10H (top of Section 2), and 320-U1334C-10H.

All major microfossil groups were found in sediments from Site U1334 and provide a consistent, coherent, and high-resolution biostratigraphic succession spanning a near-continuous sequence from the middle Miocene to the uppermost middle Eocene. The uppermost 12 m of radiolarian clay is barren of calcareous microfossils but contain radiolarians of middle Miocene age, similar to the site survey piston Core RR0306-08JC (Lyle et al., 2006). Nannofossil ooze and radiolarian clays are present in the Miocene and Eocene parts of the section, with nannofossil ooze dominant in the thick Oligocene section. Radiolarians are present through most of the section, apart from the lowermost cores, and are well preserved in the Eocene. They provide a coherent, high-resolution biochronology and indicate a complete sequence of radiolarian zones from RN7 (upper Miocene) to RP17 (uppermost middle Eocene). Calcareous nannofossils are present and moderately to well preserved through most of the succession, and there appears to be a complete sequence of nannofossil zones from NN6 (middle Miocene) to NP17 (uppermost middle Eocene), providing a minimum age estimate for basaltic basement of 37 Ma. In the Eocene, the base of Chiasmolithus oamaruensis is determined in Sample 320-U1334A-30X-1, 66 cm, and the top of Chiasmolithus grandis in Sample 320-U1334-30X-2, 74 cm. Intriguingly, both species are mid- to high-latitude taxa (Wei and Wise, 1989) and are present only rarely and sporadically at Site U1334. Planktonic foraminifers are present through most of the succession and are relatively abundant and well preserved from the lower Miocene to the lower Oligocene. The lower Miocene is characterized by the presence of Dentoglobigerina spp., Paragloborotalia siakensis–mayeri, P. kugleri, and Paragloborotalia pseudokugleri. Oligocene sediments contain Catapsydrax spp., Paragloborotalia opima-nana, and characteristic Dentoglobigerina spp. The preservation and abundance of planktonic foraminifers is more variable in the middle Miocene and upper Eocene/lowermost Oligocene. No Eocene/Oligocene boundary marker hantkeninids were identified. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths.

Apparent sedimentation rates, as implied by magneto- and biostratigraphic age determinations, vary throughout the section and are ~4 m/m.y. in the topmost sediment cover, vary between ~12 and 14 m/m.y. in the early Miocene through late early Oligocene section, increase to ~24 m/m.y. in the early Oligocene, and are ~8 m/m.y. in the late Eocene. There is no apparent hiatus at the shipboard biostratigraphic resolution. The presence of all major fossil groups as well as a detailed and well-resolved magnetostratigraphy will allow us to achieve one of the main PEAT objectives of arriving at an integrated Cenozoic stratigraphy and age calibration for major parts of the Miocene, Oligocene, and Eocene.

A full physical property program was run on cores from Site U1334C. This program comprises WRMSL measurements of magnetic susceptibility, bulk density, P-wave velocity, noncontact resistivity, NGR, and measurements of color reflectance, followed by discrete measurements of moisture and density properties, sound velocities, and thermal conductivity on Hole U1334A. All track data are variable throughout the section, allowing a detailed correlation between different holes, with the exception of a very low susceptibility signal within an interval extending slightly above and below the light greenish gray tinted cores of Unit II, between ~140 and 210 m CSF-A. Magnetic susceptibility varies between 10 × 10–5 and 40 × 10–5 SI in Unit I, oscillates around 5 × 10–5 to 10 × 10–5 SI above the colored sediments, and then drops to near zero and negative values, returning to values around 10 × 10–5 SI in the lower part of Unit II and Subunit IIIa. NGR slightly increases at the Eocene/Oligocene boundary at ~246 m CSF-A (from 4 to 7 cps). P-wave velocity remains continuous through the upper 150 m of sediment (varying around 1500 m/s) but increases rapidly below the ooze/chalk boundary to ~1600 m/s. This explains the slightly thicker sediment section than was expected from seismic data prior to coring (~20 m thicker). For Hole U1334B, no P-wave velocity WRMSL data were collected between ~125 and 240 m CSF-A to allow for a more timely stratigraphic correlation of cores within the iron reduction–dominated colored cores with the GRA instrument. Bulk density and grain density increase gradually with carbonate content to ~204 m CSF-A to a maximum of ~1.8 g/cm3 and then show stepped decreases in the lower part of this succession. Ephemeral whole-round samples were collected at ~50 and ~165 m for shore-based studies of sediment permeability.

WRMSL data were used to achieve stratigraphic correlation between holes at Site U1334. Magnetic susceptibility was initially the main parameter used for real-time correlation, as a second loop of the susceptibility meter is mounted on the Special Task Multisensor Logger (STMSL); the second bulk density instrument on this track was not working. In the very low (negative) susceptibility interval between ~145 and ~198 m CSF-A (Cores 320-U1334A-16H through 21H), the magnetic signal was not useful for correlation, and we measured the corresponding cores from Hole U1334B out of sequence to establish the amount of core overlap using bulk density. The coring effort in Hole U1334C was successful at covering gaps between cores at this site to ~111 m CCSF-A, as well as from 250 to 335 m CCSF-A, almost to the bottom of the section. The correlation was challenging between the three holes at Site U1334 in the greenish–light gray interval (Cores 320-U1334A-15H through 22H, 320-U1334B-14B through 22H, and 320-U1334C-14H through 22H) and in the bottom 80 m of the section, where XCB coring compromised the GRA density variations that would otherwise help stratigraphic correlation. Visual inspection, comparison with core imagery, and biostratigraphic datums were used to establish and verify hole-to-hole correlation where track data lacked clearly identifiable features. Stratigraphic correlation between individual holes indicates a growth factor (ratio between the CCSF-A and CSF-A depth scales) of ~16%. Stratigraphic correlation resulted in a complete splice through the Eocene–Oligocene transition almost to basement (~38 Ma).

A full range of paleomagnetic analyses was conducted on 66 APC cores and 188 discrete paleomagnetic samples from Site U1334 for the APC-cored section of Site U1334 (upper ~209 m). Unlike Sites U1331 and U1332, the drilling overprint was generally weak for Site U1334 cores, but only for those collected with the nonmagnetic core barrel (Cores 320-U1334A-1H through 16H, 320-U1334B-1H through 15H, and 320-U1334C-1H through 15H). In contrast, those cores collected with the steel core barrels are highly overprinted to the extent that the overprint is so severe that even demagnetization at 20 mT is only partially able to remove it. This extreme overprint notably degrades the paleomagnetic declination data as can be noted by their higher variability, which makes polarity determination much more difficult in the intervals collected with steel core barrels. The problem is exacerbated by the decay in the intensity (and susceptibility), which occurs at ~135 m CSF-A in all three holes as a result of apparent reduction diagenesis. Even within the highly reduced interval, an interpretable signal was present prior to switching to steel core barrels. Susceptibility in the upper 45 m of Hole U1334A averages ~18 × 10–5 SI (volume normalized) and decreases to a mean of 6 × 10–5 SI from 45 to 135 m CSF-A. A notable low occurs from ~142 to 204 m CSF-A, where the average susceptibility is 0.6 × 10–5 SI. This low interval is associated with a change in sediment color from yellowish tan to very light green, blue, and gray at ~140 m CSF-A and another abrupt change to reddish brown tones at ~205 m CSF-A, which corresponds to middle early Oligocene (~30 Ma). Just below 205 m susceptibility steps up to an average of 5 × 10–5 SI and then increases again across the Eocene/Oligocene boundary (~245 m) to an average of 18 × 10–5 SI. The magnetostratigraphy in Hole U1334A has been interpreted from the top of Chron 11r (29.957 Ma), which occurs ~55 cm below the top of Section 320-U1334C-21H-4 (~195 m CSF-A), through the base of C3n.4n (5.235 Ma) in Core 320-U1334A-1H. Magnetic reversals have also been interpreted from C1n through C2r.1r in the upper ~2 m of Core 320-U1334A-1H.

A standard shipboard suite of geochemical analysis of pore water and organic and inorganic properties was undertaken on sediments from Site U1334. We also conducted a high-resolution (1 per section) Rhizon pore water investigation across the interval's middle Oligocene cores (320-U1334A-16H through 21H) that exhibited colored sediments. Site U1334A is marked by alkalinities between 3 and 4 mM throughout. The most striking features in the interstitial water geochemistry are a dissolved manganese peak from ~20 to ~240 m CSF-A with a maximum of ~6 μM at ~110 m CSF-A and a dissolved iron peak as high as >15 μM centered at 165 m CSF-A. The depth range of the dissolved iron peak, indicative of iron oxide reduction, coincides with the colorful interval seen in the lithology and with the interval of low magnetic susceptibilities (~140–205 m CSF-A). Sulfate results indicate limited sulfate reduction. Calcium carbonate contents are low in the uppermost ~35 m of Site U1334, and initial results indicate high calcium carbonate contents below the uppermost clay layer.

Wireline logging was attempted in Hole U1334C with a redesigned tool string configuration after the loss of equipment at Site U1332. However, this attempt had to be abandoned after the logging winch failed when the tool was on its way down the drill pipe.

Five downhole temperature measurements were conducted in Hole U1334B with the APCT-3 tool and reveal a thermal gradient of 33°C/km. Temperature data combined with whole-round core temperature conductivity measurements indicate the heat flow is 31.6 mW/m2 at this site. This is somewhat lower than values obtained for the nearest site (1218). Seafloor temperature is ~1.5°C.

Highlights

Eocene–Oligocene and Oligocene–Miocene transitions and depth transects

Site U1334 was planned as the youngest and shallowest component of the PEAT Eocene–Oligocene depth transect, which will allow the study of critical intervals (such as the Eocene–Oligocene transition; see Coxall et al., 2005) and variations of the equatorial CCD. Site U1334 is estimated to have been ~3.5 km deep during the Eocene–Oligocene transition, ~1.3 km shallower than today and 800 m shallower at that time than Site U1333. Unlike previously drilled sites, the dominant lithology is still nannofossil ooze and chalk below the Eocene–Oligocene transition, with significant amounts of carbonate present, which will allow us to achieve the prime objective for this site. The Eocene–Oligocene transition, which was cored multiple times at Site U1334, has much higher sedimentation rates than Site 1218. The remaining Oligocene is also much more expanded than at Site 1218, with better preservation of planktonic foraminifers over a longer time interval, allowing a more detailed study of the Oligocene climate system. Site U1334 also contains carbonate-bearing sediments across the Oligocene–Miocene transition (e.g., Zachos et al., 2001b; Pälike et al., 2006a). Physical property data from Site U1334 can be correlated cycle by cycle to Site 1218, allowing correlation to a previously astronomically calibrated site for the Oligocene.

Geochemical front

At Site U1334 we recovered a ~50 m thick interval of multicolored carbonates that show a distinct Mn increase and elevated Fe pore water concentrations, characteristic of a geochemical alteration front. A detailed Rhizon pore water sampling program will provide insights into limited sulfate reduction processes. A similar but much thicker alteration zone is also observed at Site U1335 and provides the opportunity to study organic matter degradation while these sites migrate from south to north through the equatorial belts of high productivity.

Age transect of seafloor basalt

At Site U1334 we recovered what appear to be fresh fragments of seafloor basalt, aged ~38 Ma. This material will, when combined with other PEAT basalt samples, provide important sample material for the study of seawater alteration of basalt.

Site U1335

Two holes were cored at Site U1335 (5°18.735′N, 126°17.002′W; 4327.5 m water depth) (Fig. F57), targeting paleoceanographic events in the late Oligocene and into the early and middle Miocene, including and focusing on the climatically significant Oligocene–Miocene transition and the recovery from the Mi-1 glaciation event (Zachos et al., 2001b; Pälike et al., 2006a) and the expansion of the East Antarctic cryosphere (Holbourn et al., 2005). Site U1335 also provides data toward a depth transect across the latest Oligocene and Miocene (see "Latest Oligocene–earliest Miocene [Site U1335; 26 Ma crust]") that will allow exploitation and verification of a previous astronomical age calibration from Site 1218 (Pälike et al., 2006b).

Site U1335 (~26 Ma crust) is situated halfway between Site U1336 ~340 km toward the northwest and Site U1337 ~390 km toward the southeast, ~250 km south of the Clipperton Fracture Zone (Lyle et al., 2006). At Site U1335, late Oligocene age (26 Ma) seafloor basalt is overlain by ~420 m of pelagic sediment.

The sedimentary sequence at Site U1335 is divided into two major lithologic units. The topmost ~64 m thick lithologic Unit I comprises an alternating sequence of earliest late Miocene to Pleistocene calcareous nannofossils, diatoms, radiolarians, and foraminifer oozes. The topmost sediment of Unit I is younger than the Pleistocene/Pliocene boundary as recognized by the top of planktonic foraminifer Globigerinoides fistulosus (between Section 320-U1335A-1H-CC and interval 320-U1335A-2H-2, 104–106 cm) and then follows a continuous biostratigraphic succession to the early late Miocene. Below, lithologic Unit II comprises a ~350 m thick succession of late Miocene to late Oligocene (calcareous nannofossil Zone NP25) nannofossil ooze and chalk overlying basalt (lithologic Unit III) (Figs. F58, F59). One of the prominent features of Unit II is the presence of at least 49 described beds (2–176 cm thickness) of nannofossil foraminifer ooze that have sharp basal boundaries, many of which are irregular and some of which are inclined, interpreted as gravity flow deposits from the nearby seamounts and representing ~2% of the total sediment recovered.

Holes U1335A and U1335B provided high-quality APC-cored sediments from the mudline to ~341 and 378 m CSF-A, respectively (Cores 320-U1335A-36H and 320-U1335B-41H). The APC-cored interval from Hole U1335B represents the second deepest APC-cored depth in ODP and IODP history. Below this depth we encountered stiffer and harder sediment, after which we switched to the XCB cutting shoe. XCB coring advanced to ~420 m DSF through early Miocene and late Oligocene sediments with high recovery. In the basal section, Core 320-U1335B-46X recovered pieces of basalt up to 10 cm in length with a glassy rim and overlain by nannofossil chalks of Unit II. For detailed coring activities, see "Site U1335" in "Operations."

The sediment column at Site U1335 represents the youngest end-member drilled during Expedition 320 and provides one of the most stratigraphically complete and expanded early Miocene sections from the equatorial Pacific to date (~320 m cored depth from the earliest to latest Miocene).

At Site U1335 carbonate content fluctuates between 12% and 87% within Unit I (Fig. F60), presumably reflecting the close proximity of the seafloor to the lysocline. With the exception of the depth interval from 140 to 220 m CSF-A, the remainder of Unit II exhibits uniformly high calcium carbonate content between 80% and 90%. From ~150 to 210 m CSF-A (approximately equivalent to Cores 320-U1335A-16H through 22H), carbonate content cycles between ~50% and 90% and corresponds to a change in dominant sediment color from light greenish gray to tan, displaying higher magnetic susceptibility values up to 25 × 10–5 SI.

A series of late Oligocene through late middle Miocene cores (320-U1335A-8H through 40X) were recovered with distinct colors ranging from light grayish green to light blue, similar but much thicker in total stratigraphic thickness (~70–170 m and ~200–350 m) than those observed at Site U1334. The uniquely colored carbonate oozes exhibit extremely low magnetic susceptibilities that complicated a confident stratigraphic correlation. These colored oozes lost almost their entire magnetic susceptibility signal from ~70 to ~105 m CSF-A and below ~210 m CSF-A (Figs. F59, F61, F62). Similar colored cores have previously been described for Sites 78 and 79 (Hays et al., 1972).

All major microfossil groups were found in sediments from Site U1335, representing a complete biostratigraphic succession at the shipboard sample resolution level of Pleistocene to latest Oligocene sediments, including a thick sequence of lower Miocene nannofossil ooze and chalk. Radiolarians are present through most of the section apart from the basal 3 m of nannofossil chalk. They provide a coherent high-resolution biochronology through a complete sequence of radiolarian zones from RN14 (Pleistocene) to RP21 (upper Oligocene). Calcareous nannofossils are present and moderately to well preserved through most of the succession, representing the complete sequence from NP25 (upper Oligocene) above basaltic basement through NN20 (Pleistocene). Planktonic foraminifers are present throughout the succession and are moderately to well preserved. Recognized planktonic foraminifer zones range from PT1a (Pleistocene) to O6 (upper Oligocene). Nannofossil, radiolarian, and planktonic foraminifer datums are in good agreement. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths. The Oligocene–Miocene transition at Site U1335 was encountered at ~350 m and fully recovered in Cores 320-U1335A-37X and 320-U1335B-38H as defined by the planktonic foraminifer datum base of P. kugleri between Samples 320-U1335A-37X-4, 136–138 cm, and 37X-CC (midpoint = 348.6 m CSF-A), in good agreement with the calcareous nannofossil event top of S. delphix at 349.7 m CSF-A between Sample 320-U1335A-37X-6, 50 cm, and Section 37X-CC. The oldest sediment overlying seafloor basalt has been zoned within calcareous nannofossil Zone NP25 (24.4–26.8 Ma).

Apparent sedimentation rates, as implied by the magneto- and biostratigraphic age determinations, vary throughout the section and are ~6 m/m.y. in the late to middle Miocene to recent sediment cover, ~17 m/m.y. in the middle early Miocene, and as high as ~25 m/m.y. throughout the late Oligocene and early Miocene. There is no apparent hiatus at the shipboard biostratigraphic resolution, although some condensed horizons are apparent (e.g., near the early/middle Miocene boundary and in the early late Miocene). The presence of all major fossil groups as well as a detailed and well-resolved magnetostratigraphy will allow us to achieve one of the main PEAT objectives of arriving at an integrated Cenozoic stratigraphy and age calibration for the Miocene and late Oligocene.

A full physical property program was run on cores from Site U1335. This program comprises WRMSL measurements of magnetic susceptibility, bulk density, P-wave velocity, NGR, and measurements of color reflectance, followed by discrete measurements of moisture and density properties, sound velocities, and thermal conductivity on Hole U1335A. All track data are variable throughout the section, allowing a detailed correlation between different holes, with the exception of a low susceptibility signal within an interval extending slightly above and below the light greenish gray tinted cores of Unit II, between ~70 and 110 and ~200 and ~380 m CSF-A. Magnetic susceptibility varies between 5 × 10–5 and 20 × 10–5 SI in the upper parts of Unit I and then increases to ~25 × 10–5 SI toward the lower portion of Unit I, coinciding with the presence of clayey radiolarian ooze within the major lithology of nannofossil ooze. Magnetic susceptibility values decrease at the top of Unit II (~64 m CSF-A) and then fall to values around –1 × 10–5 SI near 70 m CSF-A. Between ~110 and 150 m CSF-A, magnetic susceptibility values increase slightly and become highly variable (0 to 10 × 10–5 SI). Magnetic susceptibility values are higher in the interval from 160 to 200 m CSF-A, coinciding with an observed decrease in Fe reduction. Below 200 m CSF-A, the magnetic susceptibility signature is largely diamagnetic, with values close to zero. Magnetic susceptibility values slightly increase again in the basal 20 m of Unit II (below ~400 m CSF-A). NGR is elevated at the surface sediment (~73 cps) but low throughout the rest of the sedimentary column. P-wave velocities from the WRMSL agree with discrete velocity measurements and reflect key lithologic transitions, particularly the ooze to chalk transition near ~220 m CSF-A. P-wave velocities are between 1460 and 1490 m/s in Unit I and the upper portion of Unit II and then increase to >1500 m/s. Slightly below the ooze–chalk transition near 345 m CSF-A, velocities increase significantly, reaching 1600–1750 m/s at the bottom of Unit II. This partly explains the thicker sediment section than expected from seismic data prior to coring (~60 m thicker). Bulk density and grain density increase with depth, with a decrease in wet bulk density from 1.2–1.6 g/cm3 in Unit I to ~1.7 g/cm3 at the top of Unit II and ~1.8 g/cm3 in the basal part of the section. Sediment porosity ranges from 70%–90% in Unit I to 50%–60% at ~300 m CSF-A in Unit II. Ephemeral whole-round samples were collected at ~96, ~196, and ~305 m CSF-A for shore-based studies of sediment permeability.

The coring effort in Holes U1335A and U1335B was successful at covering stratigraphic gaps between cores at this site from the surface throughout most of the APC-cored section, with the exception of a gap (~1 m) at the bottom of Core 320-U1335A-16H due to flow-in (~146.40–151.46 m CSF-A). Features in magnetic susceptibility and GRA density are well aligned down to a depth of 337 m CSF-A (Hole 1335A) and 344 m CSF-A (Hole U1335B), corresponding to ~398 m CCSF-A. Between ~230 and ~398 m CCSF-A, GRA density data allowed confident alignment of cores despite very low magnetic susceptibility values. The section below ~398 m CCSF-A was mostly XCB cores, lacked clearly identifiable features, and therefore had to be appended to the splice. A single spliced record was assembled for the aligned cores down to Section 320-U1335B-37H-6 (343.76 m CSF-A; 398.15 m CCSF-A). Stratigraphic correlation between individual holes indicates a growth factor (ratio between the CCSF-A and CSF-A depth scales) of ~16%. Stratigraphic correlation resulted in a complete splice through the Eocene–Oligocene transition almost to basement (~38 Ma).

A full range of paleomagnetic analyses was conducted on 78 archive halves and 257 discrete paleomagnetic samples from Site U1335 for the APC-cored section (upper ~378 m). The most prominent feature of the records is the magnetic intensity and susceptibility low that occurs between ~70 and 110 m CSF-A and below ~210 m CSF-A. We could not obtain any reliable paleomagnetic directions from this interval because the magnetic intensity after 20 mT AF demagnetization is in the order of 10–5 A/m, which is comparable to the noise level of the superconducting rock magnetometer. Except for these low magnetic intensity intervals, we found distinct declination reversals at 20 mT demagnetizations. The drilling overprint was generally weak when nonmagnetic core barrels were used (Cores 320-U1335A-1H through 16H and 320-U1335B-1H through 19H). In contrast, those cores collected with the steel core barrels are highly overprinted. Except for the low magnetic intensity interval, the cleaned paleomagnetic data provide a series of distinct ~180° alternations in the declination. When combined with biostratigraphic age constraints, the data allow a continuous magnetostratigraphy from Chron C1n (0–0.781 Ma) to C5n.2n (9.987–11.040 Ma) from 0 to 65.95 m CSF-A in Hole U1335A and from Chron C1n to C5r.1n (11.118–11.154 Ma) from 0 to 66.225 m CSF-A in Hole U1335B. Below the bottom of the first magnetic low zone (~70–110 m CSF-A), magnetostratigraphy is again interpretable downhole: from Chron C5Br (15.160–15.974 Ma) to C6n (18.748–19.722 Ma) from 155.35 to 208.40 m CSF-A in Hole U1335A and from Chron C5AAn (13.015–13.183 Ma) to C5Er (18.524–18.748 Ma) from 107.95 to 202.60 m CSF-A in Hole U1335B. The highlights of the magnetostratigraphy at Site U1335 are the identifications of (1) a previously observed cryptochron (C5Dr-1n) in two holes and (2) 40 potential geomagnetic excursions (10 of which are recorded in both holes).

A standard shipboard suite of geochemical analysis of pore water and organic and inorganic properties was undertaken on sediments from Site U1335. Site U1335 is marked by alkalinities between 2.5 and 4.3 mM throughout, sulfate concentrations between 23 and 28 mM, and dissolved phosphate concentrations of ~2 μM in the shallowest sample, decreasing to ~0.5 μM in the uppermost ~50 m. The most striking features in the interstitial water geochemistry are three dissolved manganese peaks with concentrations of up to 44, 13, and 5 μM at ~0–40, 50–80, and 150–210 m CSF-A, respectively. Dissolved iron also shows three peaks, with concentrations up to 6 μM at ~6 m CSF-A, between 90 and 170 m CSF-A, and between 190 and 370 m CSF-A. Minima in dissolved Fe correspond to elevated Mn concentrations. The alternating pattern of dissolved Mn and Fe correspond well to apparent color changes in the sediment column. Lithium concentrations decrease from ~26 μM at the sediment surface to 5 μM at ~300 m CSF-A, below which Li concentrations increase strongly to ~32 μM. The Sr concentration profile mirrors that of Li, with concentrations ranging between 82 and 250 μM. Sr values increase from the top to 200 m CSF-A, followed by a decrease toward basement. Calcium carbonate, IC, and TC concentrations were determined on sediment samples from Hole U1335A (Fig. F60). CaCO3 concentrations ranged between 13% and 96%. In the uppermost ~67 m, carbonate concentration ranges from 12% to 87%, and concentrations are then consistently high (~72%–96%) between 67 and 157 m CSF-A and below 222 m CSF-A. Concentrations vary more widely (between 37% and 89%) from 157 to 222 m CSF-A. TOC concentrations were determined by acidification, with a range from below the detection limit to 0.08%. TOC is significantly higher in the uppermost ~57 m and at ~220 m CSF-A (0.08% and 0.04%, respectively), corresponding to intervals with lower carbonate concentrations.

Wireline logging was not conducted at Site U1335. Five downhole temperature measurements were conducted in Hole U1335B with the APCT-3 tool and reveal a thermal gradient of 7.5°C/km. Temperature data combined with whole-round core temperature conductivity measurements indicate the heat flow is 7 mW/m2 at this site. This is much lower than values obtained for any of the other Expedition 320 sites and would suggest recirculation of seawater through basement, consistent with some of the interstitial pore water results.

Highlights

Highly expanded Miocene sedimentary section

One of the highlights from Site U1335 is the recovery of a very thick Miocene carbonate-dominated section from the central equatorial Pacific, one of the high-priority objectives of the PEAT program. The early Miocene (~7 m.y. duration) is captured in ~190 m of sediment, corresponding to a sedimentation rate of 27 m/m.y. The middle Miocene (4.5 m.y. duration) is recovered in ~95 m sediment, with a sedimentation rate of ~21 m/m.y. The sedimentation rate from the late Oligocene into the Miocene is just under 20 m/m.y. These high sedimentation rates will facilitate the study of paleoceanographic processes at unprecedented resolution for the equatorial Pacific.

Oligocene–Miocene transition and depth transects

Site U1335 was planned as the youngest and shallowest component of the PEAT Oligocene–Miocene depth transect, which will allow the study of critical intervals (such as the Mi-1 glacial inception; see Zachos et al., 2001b; Pälike et al., 2006a) and variations of the equatorial CCD throughout this transition as well as the latest Oligocene and early Miocene. Site U1335 is estimated to have been ~3.3 km deep during the Oligocene–Miocene transition, ~1.5 km shallower than today. The dominant lithologies are nannofossil ooze and chalk, with better preservation of calcareous microfossils than any other site drilled during Expedition 320, which will allow us to achieve the prime objective for this coring site. Physical property data from Site U1335 provide an important contribution toward the Cenozoic megasplice, connecting with younger sediments from Leg 138 (e.g., ODP Site 850) and older sediments from Leg 199 (Site 1218), allowing the generation of astronomically calibrated datums and isotope stratigraphies from the Miocene into the Eocene.

Geochemical front

At Site U1335 we recovered an interval of multicolored carbonates that show a distinct Mn increase and elevated Fe pore water concentrations, characteristic of a geochemical alteration front. At Site U1335, this zone is similar but much thicker in total stratigraphic thickness (~70–170 and ~200–350 m CSF-A) than that observed at Site U1334 (~50 m). Although the paleomagnetic signal was lost in most parts of this section, sediments recovered will provide the opportunity to study organic matter degradation while these sites migrated from south to north through the equatorial belts of high productivity. Paleolatitudinal reconstructions show that these characteristic geochemical alteration fronts can be mapped to similar equatorial positions between Sites U1334 and U1335, roughly between the Equator and ~2°N. One feature of interest at Site U1335 is the observation that the multicolored interval of sediments is interrupted between ~170 and 200 m CSF-A (Cores 320-U1335A-18H through 20H), again showing higher magnetic susceptibility values. It remains to be established whether this interruption in the geochemical alteration front is related to the shape and position of the equatorial high-productivity zone or instead is the result of reduced sedimentation rates during this time (late early Miocene). Interstitial pore water profiles provide additional important information about the redox chemical processes operating in this zone, which have also been observed at DSDP Sites 78, 79, and 574 (e.g., Hays et al., 1972).

Gravity flow deposits

One of the prominent features of Unit II is the presence of at least 49 described beds (2–176 cm thickness) of nannofossil foraminifer ooze that have sharp basal boundaries, many of which are irregular and some of which are inclined, interpreted as gravity flow deposits from the nearby seamounts and representing ~2% of the total sediment recovered. Their grain size fines upward from medium sand to silt and they are often darker colored than immediately overlying deposits and instantly recognizable by their coarser texture. Angular basalt fragments (<1 mm), fish teeth, and pyritized foraminifers and radiolarians were also found within the basal parts of these beds, of which at least three show parallel or cross laminations in their upper or middle part. These beds, interpreted as gravity flow deposits, are present with an approximate frequency of one or two beds per core. The abundance and thickness of these beds is highest within Cores 320-U1335-21H through 37X (189.4–350.1 m CSF-A). No gravity flow deposits were observed in Cores 320-U1335A-3H through 8H. The provenance of these deposits, as indicated by the observed basalt fragments, is inferred to be the nearby seamounts (Fig. F57B) situated ~15–20 km toward the northeast and southeast of Site U1335, with a present summit water depth that is 400–600 m shallower than Site U1335. Initial indications are that these gravity flow deposits, unlike those observed at Site U1331, might not be very erosive and therefore essentially add to the sediment column rather than removing large sections of geological time. The high sedimentation rates at Site U1335 will allow paleoceanographic studies to avoid the generally thin layers of gravity flows.

Age transect of seafloor basalt

At Site U1335 we recovered what appear to be fresh fragments of seafloor basalt with an age of ~26 Ma, as inferred by the oldest biostratigraphic datums from the sediment above. This material will, when combined with other PEAT basalt samples, provide important sample material for the study of seawater alteration of basalt.

Site U1336

Two holes were cored at Site U1336 (7°42.067′N, 128°15.253′W; 4286 m water depth) (Fig. F63), targeting paleoceanographic events in the late Oligocene and into the Miocene, including a focus on the Oligocene–Miocene transition and the recovery of the Mi-1 glaciation event (Zachos et al., 2001b; Pälike et al., 2006a). In conjunction with Sites U1335 and U1337 it was also designed to provide a latitudinal transect for early Miocene age slices. Site U1336 provides data toward a depth transect across the late Oligocene and Miocene (see "Oligocene [Site U1336; ~32 Ma crust]") that allow us to verify and apply a previous astronomical age calibration from Site 1218 (Pälike et al., 2006b).

At Site U1336, APC cores were taken from the mudline to 184.8 m CSF-A (Cores 320-U1336A-1H through 21H) and 173.6 m CSF-A (Cores 320-U1336B-1H through 20H). Nonmagnetic core barrels were used for Cores 320-U1336A-1H through 16H and 320-U1336B-1H through 16H, and steel barrels were used for all other cores. A hard layer at 120–140 m CSF-A prevented us from achieving a full stroke during coring (Cores 320-1336A-14H and 320-1336B-16H). XCB cores (320-U1336A-22X through 35X) were taken from 184.8 to 302.9 m CSF-A in Hole U1336A. We stopped coring before reaching the basement objective because of the decreasing rates of penetration, the relatively low recovery, and the possibility of obtaining a stratigraphically complete Miocene section.

The sedimentary section at Site U1336 is composed of an ~300 m thick nannofossil ooze and chalks of middle Miocene through early Oligocene age. They are divided into three lithologic units (Figs. F64, F65). Unit I (0–74.54 m CSF-A) consists of Miocene nannofossil ooze with varying amounts of radiolarians, foraminifers, diatoms, and clay as minor constituents. Physical properties, including magnetic susceptibility, b* and L* reflectance, and GRA bulk density, all represent higher amplitude variability throughout Unit I. CaCO3 contents are also variable, ranging between 48% and 90% (Fig. F65).

Unit II (74.50–189.50 m CSF-A) is dominated by nannofossil ooze. Sediment color changes occur downhole from pale yellow to light greenish gray at 92 m CSF-A. Below this boundary, the color of Unit II alternates between light greenish gray and white to 184.80 m CSF-A. Greenish gray millimeter-scale color bands are present below 120.86 m CSF-A. CaCO3 contents are relatively constant in Unit II and typically >85%.

The dominant lithologies of Unit III (189.5–299.6 m CSF-A) are light greenish gray and white nannofossil chalk with light greenish gray millimeter-scale color banding and chert layers. The chert shows many different colors including black, dark greenish gray, very dark greenish gray, dark gray, olive-yellow, dark brown, and pink. The Unit II–III transition is identified by the uppermost common occurrence of chert. Below 289 m CSF-A nannofossil chalk contains increasing amounts of micrite and the cherts vary in color. The lowermost cherts are olive-yellow, then pink, and finally dark brown at the base. The chalk changes color to white below 298.54 m CSF. CaCO3 contents remain >88% in the chalk layers. Igneous basement was not recovered at Site U1336. Lithologic descriptions of this site are based only on sediments recovered in Hole U1336A. A second hole (U1336B) was cored to 174.01 m CSF-A at this site during Expedition 320. Cores from Hole U1336B were left onboard to be split and described during Expedition 321.

Light–dark cycles in the nannofossil oozes of Unit I are associated with variations in the relative amounts of accessory lithologic components within the nannofossil oozes, including clay, radiolarians, and diatoms (Fig. F64) along with higher amplitude variations in the physical properties, including L*, b*, magnetic susceptibility, and GRA bulk density.

Light greenish gray sediments were recovered just as they were at Sites U1334 and U1335 and persist for ~250 m before shifting to white and pink. Magnetic susceptibility drops to near zero throughout the light greenish gray interval. Unlike other sites drilled during Expedition 320, discrete millimeter- to centimeter-scale color bands frequently occur within the interval where Fe reduction has occurred in Units II and III.

All major microfossil groups were found in sediments from Site U1336, representing a complete biostratigraphic succession at the shipboard sample resolution level of middle Miocene to early Oligocene sediments. They provide a coherent high-resolution biochronology through a complete sequence. Calcareous nannofossils are moderately well preserved throughout the succession, and there appears to be a complete sequence of nannofossil zones from NN6 (middle Miocene) to NP22 (lower Oligocene). Planktonic foraminifers are present throughout the succession, ranging from Zones M9 through O1. They are abundant and well preserved in the Miocene and less well preserved in the Oligocene. The radiolarian biostratigraphy at Site U1336 spans the interval from just above the RN6/RN5 boundary (middle Miocene) in Core 320-U1336A-1H to the uppermost part of RP22 (upper Oligocene) in Core 320-U1336A-19H (Section 320U1335-19H-CC; ~170.3 m CSF-A). Below this level the sediments are barren of radiolarians. Above this level the assemblages tend to have moderate preservation, with intermittent intervals of good preservation in RN3 and RN4 (lower to middle Miocene). This downsection decrease in preservation and ultimate disappearance of the radiolarians below ~170 m CSF-A appears to be associated with the dissolution of biogenic silica. However, the nannofossil, radiolarian, and planktonic foraminifer datums are in good agreement except for the dissolution interval of radiolarians. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths.

The Oligocene/Miocene boundary marker of the base of P. kugleri (23.0 Ma) occurs between Section 320-U1336A-16H-CC and Sample 320-U1336A-17H-2, 38–40 cm (142.96 m CSF-A), whereas calcareous nannofossil event top of S. delphix is recognized at 145.9 m CSF-A between Samples 320-U1336A-17X-2, 90 cm, and 17X-4, 90 cm.

Paleomagnetic measurements were conducted on archive-half sections of 21 APC cores from Hole U1336A and the magnetic susceptibilities and masses of 138 discrete samples. NRM measurements above ~80 m CSF-A in Hole U1336A indicate moderate magnetization (1 × 10–3 A/m) with a patchy but generally weak viscous isothermal remanent magnetic (IRM) coring overprint. Between ~80 and ~160 m CSF-A is a zone of diagenetic alteration within the greenish gray core interval in which sediments effectively have no remanence or have been entirely overprinted during the coring process. Below ~160 m CSF-A polarity reversals are present, but the inclinations are steep (up to 80°), indicating that the drilling overprint was not effectively removed during demagnetization.

The magnetozones in the upper ~80 m CSF-A correlate well with the biostratigraphic framework. At the top of Hole U1336A, magnetozones in Cores 320-U1336A-1H and 2H tentatively correlate with the interval from the base of Chron C5Ar.3r to the base of C5ABn. Core 320-U1336A-3H contains one polarity interval, corresponding to polarity Chrons C5ACn and C5ADn, but Chron C5ACr was not identified. Core 320-U1336A-4H includes Chrons C5ADr and C5Bn, and the top of Chron C5Br occurs at the base of Core 320-U1336A-4H and terminates at the base of Core 5H. Below Chron C5Br the correlation with the GPTS is relatively unambiguous through Core 320-U1336A-9H, which contains the upper portions of Chron C6n. Between ~80 and ~160 m CSF-A the magnetization of the sediment deteriorates below analytical noise level within the greenish gray core interval as observed at Site U1335. Below ~160 m CSF-A a reversal pattern is discernible and tentatively correlates with Chrons C6Cr through C7An.

Biostratigraphic datums and magnetostratigraphy results allow the calculation of average LSRs that are 9 m/m.y. for the upper 74 m of the section on the corrected CCSF-A depth scale. Site U1336 LSRs increase from 12 m/m.y. in the lower Miocene to 15 m/m.y. in the Oligocene. There are no apparent hiatuses in the shipboard biostratigraphic resolution.

A complete physical property program was conducted on whole cores, split cores, and discrete samples. Hole U1336B was analyzed only using the WRMSL and NGR detector. Physical property data for Hole U1336B have not been filtered for drilling disturbances.

Magnetic susceptibility measurements correlate well with the major differences in lithology of Site U1336. Magnetic susceptibility values are highest in Unit I with high amplitude and frequency variations from 5 × 10–5 to 30 × 10–5 SI. In Unit II, they decrease from ~10 × 10–5 to near 0 SI. There is a slight increase in the amplitude and frequency of the variation in Unit III. The highest NGRs are present at the seafloor (~56 cps), rapidly decrease with depth in Unit I, and then are very low (2 cps) in Unit II.

Velocities are between 1480 and 1500 m/s in Unit I and between 1480 and 1530 m/s in Unit II. At ~176 m CSF-A velocities begin to increase rapidly into Unit III, from 1530 to ~1960 m/s at 208 m CSF-A. The remainder of Unit III shows high-frequency and high-amplitude variation averaging 1910 m/s in this more lithified interval.

Wet bulk density is lowest in Unit I (1.4–1.7 g/cm3), which contains a large amount of clay, radiolarians, and diatoms, and then increases slightly and is more uniform (~1.7 g/cm3) in Unit II. In Unit III wet bulk density becomes variable deeper than 180 m CSF-A, averaging 1.9 g/cm3. Grain density variations match changes in lithology. At the top of the section, grain density is 3.0 g/cm3 and falls rapidly, reflecting clay-rich sediments that grade rapidly into nannofossil ooze. Grain density averages 2.7 g/cm3 for most of the succession (varying from 2.6 to 3.0 g/cm3); this reflects the dominance of carbonate material through the succession (grain density of calcite = 2.70 g/cm3). Porosity is highest in Unit I, varying from 65% to 80%, and decreases gradually toward the base of Unit II to 55%–60% at ~184 m CSF-A. In Unit III, porosity ranges from 45% to 60%.

Thermal conductivity decreases from 1.2 to 1 W/(m·K) through Unit I, with a minimum conductivity of 0.91 W/(m·K) at 31 m CSF-A. Units II and III values increase from 1 to 1.4 W/(m·K). Below 183 m CSF-A thermal conductivity decreases because of increased core disturbance by XCB coring.

Spectral reflectance corresponds to pronounced lithologic and diagenetic changes. In Unit I, L* values are lowest (50%–80%) and display high-amplitude variations because of minor lithologic changes. Unit II values are higher and more uniform, and Unit III values are slightly lower and more variable. Variations in b* (blue–yellow) decrease abruptly below 92 m CSF-A.

Hole U1336B cores were not split during Expedition 320; the position of coring disturbances is unknown and the construction of a spliced section for sampling purposes had to be postponed. Magnetic susceptibility and GRA density were used for correlating Holes U1336A and U1336B. Features in these data are well aligned between Holes U1336A (84 m CSF-A) and U1336B (84 m CSF-A) to a depth of ~94 m CCSF-A. Below 94 m CCSF-A the correlation between the two holes becomes challenging without additional split-core data, and a single spliced record was not assembled at this point. A growth factor of 1.13 is calculated by linear regression for the top 94 m CCSF-A of Site U1336, indicating a 13% increase in CCSF-A relative to CSF-A depth.

A standard geochemical analysis of pore water and organic and inorganic properties was undertaken on sediments from Site U1336. A total of 22 whole-round interstitial water samples from Hole U1336B were analyzed. Chlorinity values show a distinct increase from ~555 to ~570 mM in the uppermost 40 m CSF-A, potentially reflecting the boundary condition change from the more saline ocean at the Last Glacial Maximum to the present. Alkalinity is relatively constant at values >2.5 mM in the upper 110 m, with a pronounced decline to 1 mM at 170 m CSF-A. Sulfate concentrations decrease with depth to values as low as 22 mM. Dissolved phosphate concentrations are ~5 μM at ~9 m CSF-A, decreasing to ~1 μM at ~15 m CSF-A. Dissolved manganese and iron have broad peaks in the depth range from ~25 to 120 m CSF-A and below 100 m CSF-A. An increase of iron mirrors a decrease in manganese. Concentrations of dissolved silicate increase with depth from <400 to 800 μM but do not reach saturation with biogenic opal.

Highlights

Miocene sedimentary section and cyclic sedimentations

One of the highlights from Site U1336 is the recovery of a very thick Miocene carbonate section from the central equatorial Pacific, one of the high-priority objectives of the PEAT program. We recovered the complete early Miocene sequence (~9 m.y. duration) in a ~110 m thick section with a sedimentation rate of 12 m/m.y. and the middle Miocene sequence (4.5 m.y. duration) in a ~45 m thick with a sedimentation rate of ~21 m/m.y. These high sedimentation rates will facilitate the study of paleoceanographic processes at unprecedented resolution for the equatorial Pacific.

The obvious variations of both color and biogenic composition within nannofossil oozes represent cyclically changing fluctuations of CCD and upwelling intensity during the middle Miocene through early Miocene. The variable lithology also results in variations of many petrophysical signals of physical properties including L*, b*, magnetic susceptibility, and GRA bulk density. These high sedimentation rate and cyclic sediments will facilitate the study of paleoceanographic processes at unprecedented resolution for the equatorial Pacific.

Oligocene–Miocene transition and depth transects

Site U1336 was planned as a latitudinal transect for early Miocene age slices and the PEAT Oligocene–Miocene depth transect in conjunction with Sites U1335 and U1337. The Miocene sequence at these sites includes the critical intervals of the Mi-1 glaciation and middle Miocene ice sheet expansion (Holbourn et al., 2005; Zachos et al., 2001b; Pälike et al., 2006a). The dominant lithologies of nannofossil ooze and chalk at Sites U1336 and U1335, with better preservation of calcareous microfossils than any other site drilled during Expedition 320, will allow us to achieve the prime objective for this coring site.

The Oligocene–Miocene transition in Hole U1336A occurs in very homogeneous nannofossil ooze within the alternations of white and light greenish gray ooze. The same alternating sequence is observed above the Oligocene–Miocene transition at Site U1334 (see "Site U1334"). The biostratigraphy reveals that the Oligocene/Miocene boundary exists between 142.96 and 145.9 m CSF-A at Site U1336; that will allow high-resolution study of this critical interval.

Geochemical front

At Site U1336 we recovered an interval of greenish gray carbonates that show a distinct Mn increase and elevated Fe pore water concentrations with characteristics similar to geochemical alteration fronts at Sites U1334 and U1335. At Site U1336, this zone is ~200 m thick. The paleomagnetic signal was very weak in most parts of this section (80–160 m CSF-A). High Fe and Mn pore water concentrations may be related to changes in the oxidation state in the sediments. The oxidation-reduction reactions are likely fueled by the enhanced availability of organic carbon overlying and underlying the sediment zone. This site may provide the opportunity to study organic matter degradation.

Site U1336 migrated from south to north through the equatorial belt of high productivity. Based on paleolatitudinal reconstructions, these geochemical alteration fronts can be mapped to similar equatorial positions between Sites U1334 and U1335, roughly between the Equator and ~4°N.

Chert formation in the early Oligocene

The sequence at Site U1336 includes barren intervals of radiolarian fossils and many thin intercalated chert layers and fragments. Radiolarians decrease in preservation downsection and disappear below Core 320-U1336A-19H. Instead, the sediments contain several chert fragments. Some inferred chert layers are present at ~120–140 m CSF-A and prevented the APC penetration. Below ~190 m CSF-A, various colored chert layers and fragments are present in the cores. The chert frequently contains foraminifer tests, reflecting the diagenetic process of dissolution and reprecipitation of the biogenic silica.

The dissolution of biogenic silica is the source of porcellanite and chert, and on crust <65 Ma in age almost all cherts in the Pacific lie <150 m above basement. Although we have not recovered basement rocks at this site, the sediments became hard, lithified limestones and the drilled section is probably close to basement. The dissolution of silica in the basal sedimentary section is likely associated with the circulation of warm hydrothermal waters in the upper oceanic crust that extends into the lower sediments where they are cut by fractures and faults (Moore, 2008a, 2008b). This site will provide information on chert formation in the equatorial Pacific regions.