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doi:10.2204/iodp.pr.321.2009

Results and highlights

Two major Neogene equatorial Pacific sediment sections were recovered during Expedition 321 by drilling seven holes at two sites (Holes U1337A–U1337D and Holes U1338A–U1338C). One additional hole (U1338D) was drilled at Site U1338 to provide sediment cores for the upcoming Bering Sea Expedition 323 to practice on. Expedition 320 drilled three other sites that also define Neogene sedimentation in the equatorial Pacific (Sites U1134–U1336). In this section, we synthesize our observations about the Neogene equatorial Pacific primarily from the Expedition 321 drilling but also include appropriate sites from Expedition 320. The cores recovered from Expedition 320/321 provide the raw material to address Neogene PEAT objectives for the equatorial Pacific megasplice. Shipboard research has provided the framework of studies to define the lithology, show continuity of the sediment section, and define the basic time framework. Shorebased work will refine the chronostratigraphy through orbital tuning and will measure proxies of surface and deep water change and paleoproductivity/carbon cycles to show how the modern equatorial Pacific developed as the icehouse world developed.

For the PEAT program, only Sites U1337 and U1338 recovered sedimentary sections of the late Miocene-Holocene with relatively high sedimentation rates and preserved carbonate (~15 m/m.y.) (Fig. F12). Sites further west and north on older crust (Sites U1331–U1336) suffer from low sedimentations rates or hiatuses in the younger section because they are farther from the modern equatorial productivity zone and on deeper (older) ocean crust where depth-dependent carbonate dissolution has become strong. Of the older sites, the best higher sedimentation-rate sediment sections for the middle Miocene through late Oligocene are found at Sites U1334–U1336.

One of the important accomplishments of Expedition 321 was to recover two continuous essentially complete Neogene sedimentary sections, Site U1338 from 0 to >17 Ma and Site U1337 from 0 to >23 Ma, just beyond the Oligocene/Miocene boundary. These represent the only complete Neogene sections in the equatorial Pacific, possibly for all the tropics, that have high enough sedimentation rates to resolve orbitally forced sediment cycles. Coring during DSDP Legs 8, 9, and 16 was accomplished with the rotary core barrel and the sediments were highly disturbed. During ODP Leg 85 at Site 574, the middle Miocene to present was double APC cored, but sedimentation rates were slow (~5 m/m.y.) from 0 to 10 Ma and the middle Miocene section was never spliced (Mayer et al., 1985). Below the middle Miocene, only one hole was drilled. Similarly, Site 572 had only a single hole drilled through the upper and middle Miocene interval (Mayer et al., 1985). Most Leg 138 sites were drilled on 10 Ma crust and did not contain significant middle and lower Miocene sediments (Mayer, Pisias, Janecek, et al., 1992). Site 846 was drilled on 17 Ma crust on the Nazca plate but experienced poor recovery below 12 Ma. In addition, Site 845 in the Guatemala Basin has a spliced sediment section to only ~12 Ma (Mayer, Pisias, Janecek, et al., 1992), even though it was located on 17 Ma crust. Even in the tropical Atlantic on the Ceara Rise, the middle and early Miocene were discontinuously cored and longer records were spliced between different sites (Shipboard Scientific Party, 1995; Raffi et al., 2006).

Neogene, Sites U1334–U1338

Neogene sediments of the tropical Pacific are primarily biogenic in origin (Figs. F13, F14). PEAT sites within the equatorial zone in the Neogene (Sites U1335–U1338) are all primarily composed of nannofossil oozes with secondary biosiliceous components. Aluminosilicates are only a small component of the sediments. Most clay is of aeolian origin and is typically in low abundance. Clays become more dominant ~5° north of the Equator, where biogenic carbonate and opal is being dissolved away and sedimentation rates are very low.

Within the equatorial zone (±2° of the Equator), diatoms, and to a lesser extent radiolarians, are a more important part of the sediments from the upper part of the middle Miocene to the present. An important diatom-rich interval occurs between 10 and 11.5 Ma. This interval is best developed at Site U1337 but can be correlated regionally. These events are discussed more fully in "Neogene biogenic events and cycles."

A short lower Miocene biogenic carbonate sediment section was found at the top of Site 1218 on 42 Ma crust (Lyle et al., 2002) below a radiolarian clay interval (Figs. F13, F14). Site 1218 is representative of the northern edge of Miocene carbonate deposition. Miocene carbonate sediments can only be found in piston cores to ~10°N (Riedel, 1967). At the PEAT sites, significant but slowly accumulating lower to middle Miocene carbonates were found at Sites U1334 and U1336. The upper parts of Sites 1218, U1334, and U1336 were all either biosiliceous clays or, in the case of Site U1336, a hiatus.

At Sites U1335, U1337, and U1338, sediments have experienced relatively little diagenesis. The lower sections have turned to chalk by compaction and because of mild increases in downcore temperature from geothermal heating (Fig. F11). The sediment columns of these sites are between 410 and 450 m thick, and at each site most of the sediment column was cored by APC. Site U1335 was cored by APC to 378 m core depth below seafloor, method A (CSF-A), whereas Site U1337 was cored by APC to 267 m CSF-A and Site U1338 was cored by APC to 414 m CSF-A, a new drilling record. The deep APC cores typically had little disturbance, much less than XCB cores taken at the same interval (Fig. F15). The excellent quality of the cores will make them useful for later high-resolution paleoceanographic studies. For the most part, shipboard correlators were able to splice together continuous sediment columns from the multiple holes drilled at each site, except for some difficulties in the upper to middle Miocene diatom-rich interval (see "Neogene biogenic events and cycles").

Site U1336 experienced significantly higher diagenesis than other PEAT sites, perhaps because of higher sediment temperatures and/or fluid flow. The basal 100 m of Site U1336 (upper–lower Oligocene) are barren of siliceous microfossils and have lithified to cherts and chalks even though the sediment column is only ~300 m thick. We speculate that faulting parallel to the Clipperton Fracture Zone—Site U1336 is located just north of the fracture zone—may have served to localize fluid flow in the vicinity of the site. Unfortunately, no sediment temperatures were taken during coring at this site.

New capabilities on the JOIDES Resolution

The shipboard laboratories have new capabilities that greatly improve the ability of shipboard scientists and later researchers to study the sediment column. One important capability is a much better capacity to integrate line-scan core images into descriptions and analyses. Figure F16 shows color images from different holes at Site U1337 and how they were joined into the Site U1337 splice. The color images are particularly useful to resolve color banding and cyclic behavior in the multicolored sediment intervals. In the upper parts of all the sites, it is common to find oxidized brown sediments that become green when Fe within the sediments is reduced (Lyle, 1983). Typically this feature has been described in the upper meter of the sediment column. Further west, where the PEAT sites are located, brown colors extend downward for tens of meters. Not all of the color information is stratigraphically useful—redox state and other diagenetic processes can overprint other sediment information. For example, the color change from green to yellow at the base of the Site U1338 sedimentary section cuts across stratigraphic correlations between different holes.

Dark-light color banding, whatever shade of color, is indicative of carbonate content in the equatorial Pacific. Darker layers usually have lower carbonate contents, higher magnetic susceptibility, higher natural gamma activity, and lower bulk density. Different scales of carbonate cycles are found in the core images. Because many of the low-carbonate events are chronostratigraphic, they can be used to correlate between cores. At the larger scale, the dark-light color banding can be used to correlate between sites.

Another new instrument on the JOIDES Resolution is the whole-round natural gamma detector. The instrument is much more sensitive than the ODP natural gamma detector and can be used as part of the normal core-flow measurements. Biogenic sediments of the equatorial Pacific are not ideal for natural gamma activity studies—they only have low levels of natural radioactivity. Nevertheless, the new detector was able to reliably detect natural gamma ray activity variations of <1 cps. Small natural gamma activity highs correlated with dark sediment layers. One of the objectives for the natural gamma ray measurement on cores is to produce a record that can be correlated with the wireline logging measurements. Unfortunately, the low natural gamma ray activity levels produced inconsistent peaks on repeat passes of the logging tool strings, with the exception of a few intervals.

The seafloor was one interval that consistently had high natural gamma ray activity (Fig. F17). Whereas counts at a distance from the seafloor were <5 cps, seafloor natural radioactivity levels were as high as 160 cps within 1 m of the top of the sediments. Natural gamma ray activity dropped to the typical low count a few meters below the surface. Spectral studies using hour-long counts on the surface cores revealed that natural gamma ray activity at the surface is being produced by high uranium levels, not thorium or potassium. It is not clear yet why uranium is being concentrated only near the sediment/water interface. Because the near-surface sediments have low organic carbon contents, uranium has not been concentrated by reduction and precipitation from seawater. Kunzendorf et al. (1983) instead argues that enrichments of uranium in tropical Pacific sediments result from coprecipitation of seawater uranium with Fe-oxides. This solution still begs the question as to why the level of Fe-oxides might be higher.

Neogene biogenic events and cycles

The Neogene has significant changes in temperature, glaciation, plankton community, and carbon cycle, including a significant warm interval at ~15–16 Ma, and multiple glaciations first in Antarctica and then in the Northern Hemisphere (Zachos et al., 2001a). There also have been important changes in the CCD. Changes in isotope composition and SST proxies can only be monitored by shore-based geochemical studies. However, shipboard monitoring of changes in plankton composition and relative abundance indicate that dissolution has taken place in the Neogene equatorial Pacific, achieving parts of objectives 2, 3, 4, and 7 listed in "Scientific objectives."

The eastern equatorial Pacific has times of known change in CCD (at ~18 and ~10 Ma) (Lyle et al., 1995; Lyle, 2003). There are also important intervals where biogenic fluxes have changed significantly (Farrell et al., 1995) and important time intervals of high diatom flux (Kemp and Baldauf, 1993). We were able to identify these intervals at Sites U1337 and U1338 and expect that further work will identify new intervals, thereby achieving objective 1 listed in "Scientific objectives."

Neogene chronostratigraphy

One of the major shipboard scientific efforts for every paleoceanographic drilling cruise is to place the depth column into a time framework (objective 3 in "Scientific objectives") (Fig. F12). During Expedition 321 we depended on shipboard biostratigraphy and magnetostratigraphy for the basic time framework. Shipboard biochronostratigraphy is based upon the PEAT timescale developed before the PEAT expeditions and will be refined postcruise. Depth positions of recognized biostratigraphic zones and microfossil datum levels will be listed in each of the site summaries.

Biostratigraphy

One of the highlights of Expedition 321 was the recovery of well-preserved and diverse assemblages of siliceous radiolarians and diatoms and calcareous planktonic foraminifers and nannofossils for the Neogene. Although the zonation and calibrations of each of these groups has been established and refined over the last 50 y, there are very few locations with both well-preserved and abundant siliceous and calcareous microfossils. Leg 199 established a well-defined radiolarian stratigraphy and calibration for the equatorial Pacific Ocean (Nigrini et al., 2006); unfortunately the planktonic foraminifers and calcareous nannofossils were sufficiently preserved to provide robust calibrations only for the Oligocene (Pälike et al., 2006b; Wade et al., 2007).

In spite of well-defined Neogene diatom stratigraphy and calibration for the low-latitude Pacific (Baldauf, 1985; Baldauf and Iwai, 1995; Barron, 1985), it is clear from Sites U1337 and U1338 that the presence of some of the diatoms used in the diatom stratigraphy is discontinuous in the expanded Miocene section at both drill sites. This has been noticed in sediments recovered during Leg 138 (Baldauf and Iwai, 1995). The discontinuous occurrences may reflect large variations in the abundances of these species with time and changing ecologic conditions. However, there is also the possibility that some of these intermittent disappearances of a species may reflect genetic changes in the lineage that give rise to either "iterative evolution" or changing ecological preferences (objective 7 in "Scientific objectives").

The diatom, radiolarian, calcareous nannofossil, and planktonic foraminifer stratigraphies at Sites U1337 and U1338 all generally agree, with some minor discrepancies. The fully integrated biostratigraphies coupled with the cyclostratigraphy and paleomagnetostratigraphy will lead to an integrated magneto-, astro-, and biochronology of all four microfossil groups for the equatorial Pacific Ocean (objective 3 in "Scientific objectives").

Magnetostratigraphy

Paleomagnetism and magnetostratigraphic studies are essential observations to obtain a well-intercalibrated Cenozoic megasplice and to constrain the tectonic motion of the Pacific plate (objectives 3 and 5 in "Scientific objectives"). Shipboard magnetostratigraphy (Fig. F18) was used as a Neogene chronostratigraphic tool. Unlike the Paleogene, significant parts of the sedimentary section have low magnetic intensity and do not retain a strong paleomagnetic signal. These intervals of weak magnetic intensity correlate in part to the "green" sediment intervals, but not entirely.

The magnetic polarity stratigraphy of the entire middle Eocene to Quaternary interval was resolved at the eight sites occupied during Expedition 320/321. The middle Eocene and Oligocene interval was well documented at Sites U1332 and U1333. At Site U1332, mean sedimentation rates in this interval range from 3 to 8 m/m.y., and at Site U1333 the mean sedimentation rates reach ~12 m/m.y. in the lower Oligocene. At Site U1334 the upper Oligocene and lower Miocene magnetic stratigraphy was well resolved with mean sedimentation rates of 12 m/m.y. in the lower part of the interval and 4 m/m.y. in the upper part. At Site U1335, the upper Miocene and Quaternary magnetic stratigraphy was well resolved at sedimentation rates of ~6 m/m.y., and below these sediments lies an interval of upper middle Miocene where the magnetic stratigraphy is not resolved because of weak magnetization intensities. This ~20 m long unresolved interval is underlain by sediments that carry a magnetic stratigraphy of part of the middle and lower Miocene, although the interval is interspersed with turbidites and evidence of reworking. At Site U1336, the magnetic stratigraphy of part of the middle and lower Miocene is resolved in the 0–80 m CSF-A interval at mean sedimentation rates of ~10 m/m.y.; however, below ~80 m CSF-A magnetization intensities are too low for resolvable magnetization directions. At Site U1337, although the magnetic polarity stratigraphy is interpretable for the upper Miocene to Quaternary, data quality is poor for the upper Miocene. At Site U1338, the magnetic stratigraphy is resolvable in three intervals: part of the Pliocene–Pleistocene (0–4 Ma), part of the late Miocene (8.7–11 Ma), and part of the middle Miocene (12.7–16 Ma). These three intervals at Site U1338 are separated by intervals of low magnetization intensities where the magnetic stratigraphy was not resolvable.

Biotic response to climatic change

The phytoplankton, zooplankton, and benthic foraminifers at these sites provide an excellent opportunity to document the biotic response to intervals of rapid climatic change in both the surface and deep ocean, addressing PEAT objectives 1, 2, 4, and 7 (see "Scientific objectives").

Planktonic foraminifer assemblages are generally well preserved. As with other microfossil groups, distinct changes in assemblage composition reflect preservational and evolutionary changes, fluctuations in the water column structure, and the position of each site relative to the paleoequator (Fig. F19). Globoquadrinids and dentoglobigerinids dominate the assemblages from the Oligocene to middle–late Miocene. This interval is also associated with extremely abundant and diverse paragloborotaliids. These taxa are considered to occupy the thermocline and prefer nutrient rich environments (Wade et al., 2007). The decrease in globoquadrinids and dentoglobigerinids is associated with an increase in Globigerinoides, which increases in abundance from the middle Miocene. Keeled globorotaliids dominate the Pleistocene assemblages. Peak abundances of Globigerinoides spp. occur within the late Miocene.

There appears to be a relationship between the diversity in the planktonic foraminifers and climatic change. At Site U1338, a high diversity of planktonic foraminifers are recorded in Zone M5, with many microperforate taxa (Globigerinita, Globigerinatella, Mutabella, and Tenuitella). The diversity of this group is particularly high through the early middle Miocene. These taxa occupied the warmer mixed layer (Pearson et al., 2001b; Majewski, 2003), and the high diversity may be related to higher SSTs and increased stratification associated with the Middle Miocene Climatic Optimum (MMCO).

Planktonic foraminifers, because they dissolve more easily than benthic foraminifers or nannofossils, are useful to evaluate the degree of dissolution in calcareous biogenic sediments (objective 1 in "Scientific objectives") (Fig. F20). Intervals of enhanced dissolution have been detected at Sites U1337 and U1338 and are associated with laminated diatom oozes between 11.5 and 9 Ma, the "carbonate crash" interval. Another short interval is barren of planktonic foraminifers and marked by a high benthic/planktonic foraminifer ratio at ~17 Ma at Site U1337.

Much like the planktonic foraminifers, calcareous nannofossil assemblages reflect global and regional oceanographic changes. Neogene placolith diversity peaks in the early to middle Miocene, during the MMCO. At ~14 Ma, nannofossil assemblages begin to fluctuate, indicating changes in productivity. At Site U1338 there are several instances when assemblages are dominated by very small (<3 μm) Dictyococcites species, here interpreted as bloomlike episodes. These events are also present at Site U1337, but in limited quantity, suggesting that Site U1338 was located under a more intense upwelling regime. Another indication that Site U1338 records a higher productivity signal is the exclusion of multiple taxa, such as the marker species Helicosphaera ampliaperta and Catinaster coalitus, both of which are rare but present at Site U1337.

Lower bathyal to abyssal benthic foraminifers occur throughout the early Miocene to Pleistocene in Sites U1337 and U1338 and show relatively good preservation. Marked variations in downcore abundance and assemblage composition relate to fundamental changes in global climate, major ice-volume fluctuations, and reorganizations in oceanic circulation during the Neogene. Organic flux–sensitive taxa track substantial changes in equatorial Pacific Ocean surface productivity throughout the latest Miocene to Pleistocene. Impoverished assemblages and high benthic/planktonic ratios prominently mark the late Miocene "carbonate crash" (Fig. F20), whereas diversified assemblages, including high numbers of epifaunal or near surface dwellers such as Cibicidoides, signal a marked improvement in deep ocean ventilation following middle Miocene high-latitude ice sheet expansion. Early to early middle Miocene assemblages exhibit significant fluctuations in abundance and diversity, hinting at major changes in upper ocean structure and deep water ventilation at the onset of and during the Miocene climate optimum.

Diatoms and diatom mat intervals in the Miocene and Pliocene

Diatoms are well represented at Sites U1337 and U1338 between ~0 and 18 Ma. Diatoms are a less common part of the equatorial Pacific plankton before this time. Based on smear slides, diatoms comprised <15% of the total plankton assemblage in the early Miocene but are often >25% of the plankton after 15 Ma (Fig. F21). In spite of minor differences in the qualitative composition of the assemblages, the diatom community preserved at Sites U1337 and U1338 mostly consists of Neogene species typical of the low-latitude eastern equatorial Pacific Ocean (Baldauf, 1985; Baldauf and Iwai, 1995; Barron, 1985).

A striking feature of Site U1337 sediments is the occurrence of thick sequences of laminated diatom oozes (diatom mats) (Fig. F22). These diatom mats are mainly composed of the needle-shaped diatom Thalassiothrix and are deposits that record unusual oceanographic conditions that repeated a few times in the eastern equatorial Pacific region in the middle to late Miocene and early Pliocene (Fig. F23) (Kemp and Baldauf, 1993). A diatom mat interval is made up of essentially pure diatoms and macroscopically has the appearance of layers of wet paper. One of the unsolved questions of Expedition 321 is why the mats were prominent at Site U1337, whereas at Site U1338 the same time intervals were marked by diatom-rich sediments but only rarely mats. Site U1338 was closer to the Equator during the mat-forming interval, being at the Equator during the major interval at 10.3–11.3 Ma, whereas Site U1337 was at 1°N.

Kemp and Baldauf (1993) proposed that the mats record extraordinarily high open ocean sedimentation rates and represent vast sinks of silica and carbon. However, at the coarse resolution of shipboard biostratigraphy we could detect no change in sedimentation rate through this interval and could easily correlate the bulk density profiles between Site U1337 with extensive mats to Site U1338 without them. Postcruise work will be important to determine how fast the diatom mats deposited.

Within the mat interval, the alternation between the Thalassiothrix and mixed-sediment laminae could reflect the periodic movement of the frontal zone in the eastern equatorial Pacific Ocean. An improved insight into Thalassiothrix mats from Site U1337 will be an important prerequisite for enhancing our understanding of the marine biogeochemical cycling and for assessing the impacts of rapid climate change on ocean export production in the low-latitude eastern Pacific Ocean (objectives 1, 2, 4, and 7 in "Scientific objectives").

Late Miocene carbonate crash and middle Miocene glaciation

Sites U1337 and U1338 recovered a full suite of Pleistocene to earliest Miocene calcareous and siliceous microfossils. All groups exhibit prominent downcore variations in preservation and abundance that appear related to major changes in primary production, export flux, and water-column and seafloor dissolution, thus reflecting fundamental changes in global climate. The carbonate crash, an extended period of low carbonate deposition widely recorded throughout the eastern Pacific Ocean at ~9–11 Ma (Lyle et al., 1995; Farrell et al., 1995), is strikingly expressed at Sites U1337 and U1338 by sharp decreases or the disappearance of planktonic foraminifers, high benthic to planktonic foraminifer ratios, and generally impoverished benthic foraminifer assemblages (Fig. F20). Calcareous and siliceous microfossils indicate that this prominent dissolution event was less intense and of shorter duration at Site U1338 than at Site U1337 (Fig. F20), probably because it is located on younger oceanic crust and in closer proximity to the Equator during this time window. Site U1338 was ~200 m shallower than Site U1338 and essentially at the Equator during the carbonate crash interval.

During episodic expansion of the Antarctic ice sheet at ~14–15 Ma, calcareous microfossils exhibit overall good preservation and relatively high diversity, suggesting a vigorous Pacific Ocean circulation and deep CCD. In contrast, the preceding prolonged period of global warmth, often referred to as the MMCO, appears marked by transient changes in CCD. Postcruise studies and integration of paleontological data will provide an opportunity to further investigate temporal and spatial variations in microfossil distribution across the equatorial Pacific Ocean and to unravel links with global climatic and oceanographic events during the Neogene (objectives 1 and 9 in "Scientific objectives").

Orbital cycles, physical properties, and site correlation

The equatorial Pacific is a classic "binary" sediment system, with variable amounts of biogenic calcium carbonate and biosiliceous sediment components but very little clay. It is also well known that carbonate contents of equatorial Pacific sediments can be estimated from the bulk density because the carbonates have lower porosity and higher grain density than biosiliceous sediments (Mayer, 1991). Consequently, physical property records contain meter-scale cyclicity that will ultimately be useful for orbital-tuning timescales (objective 3 in "Scientific objectives").

On longer depth scales, physical property records are useful to correlate between sites. Figure F24 shows the two high-resolution density logs at Sites U1337 and U1338 and illustrates the high degree of correlation between the two sites that are separated by ~600 km. Lines on the figure represent equivalent nannofossil biostratigraphic datum levels. The two records are aligned such that the top peaks line up. The depth scale for Site U1338 is compressed so that the bottom event also lines up. Density events can easily be correlated between the sites even though they are separated by >600 km.

Mayer et al. (1985) developed a seismic stratigraphy for the central Pacific at Site 574 during Leg 85. They noted that major seismic horizons were caused by density variations associated with low-carbonate intervals. They proposed that these intervals were chronostratigraphic because they were caused by paleoceanographic changes in deposition and/or dissolution of calcium carbonate.

Unfortunately, Mayer et al. (1985) did not have logs to measure in situ velocities. One of the important experiments of Expedition 321 was to use logging as well as cores to check this conclusion and to better constrain the age of the equatorial Pacific seismic horizons. For this reason, we planned a vertical seismic profile (VSP) experiment at one of the Neogene PEAT sites (objective 6 in "Scientific objectives"). We were able to run the VSP log at both Site U1337 (Fig. F25) and U1338. Figure F25 is an initial comparison between the Site 574 seismic stratigraphy of Mayer et al. (1985) and the initial results for Site U1337. The events appear to correlate in age, as would be predicted by Mayer et al. (1985). Site 574 is at essentially the same latitude as Site U1337 but >1000 km to the west. The length scale of the correlatable seismic horizons across the Pacific helps to define the length scale of the paleoceanographic events with which they are associated. Postcruise studies will focus on better defining the seismic stratigraphy at both Sites U1337 and U1338, allowing new tie points for the seismic stratigraphic study of the equatorial Pacific sediment bulge (Mitchell et al., 2003).

Cherts in the Neogene sediment section

Chert intervals were found at Sites U1336–U1338. At Site U1336, a chert interval was found in unlithified early Miocene sediments at ~130 m CSF-A. The section below 200 m CSF-A also had significant numbers of chert intervals and the carbonates had been turned to limestone, suggesting significant higher temperature diagenesis.

At Sites U1337 and U1338, only one cherty interval was found—what we referred to as the "baby chert" interval. At both sites the cherty interval hampered recovery around it, but it was easily penetrated by drilling. Interestingly, its position appears to be chronostratigraphic, at ~12 Ma in the upper middle Miocene (~240 m CSF-A at Site U1337 and ~282 m CSF-A at Site U1338). It was located in a diatom mat interval at Site U1337 and a diatom-rich interval at Site U1338. The interval was well-imaged by Formation MicroScanner (FMS) logging and shows that the chert interval is 40 cm thick at Site U1337 and about one-quarter of the way up from the base of the low resistivity diatom-rich interval (Fig. F26). At Site U1338 the baby chert interval was only 16 cm thick but also about one-quarter of the way through the low resistivity interval. The logs allowed us to estimate the amount of disturbed and missing section in the cored sediments—there is only a small loss of material (<1 m) around the chert.

At Site U1337 it was clear from interstitial water profiles that the baby chert interval is a barrier to diffusion. There were offsets in Li, Sr, and SO₄ in interstitial waters immediately below and above the baby chert interval. It is unclear why the chert formed in this interval alone at both sites. There were additional diatom-rich intervals below the baby chert that were unaffected, and the temperature gradients at both sites are not large. The more extensive chert layer at Site U1337 is actually formed ~30 m shallower (250 m CSF-A) than the interval at Site U1338 (280 m CSF-A). Postcruise research focused on diagenesis and fluid flow may provide insight into chert formation with these diatom-rich intervals.

Color banding and redox at the Neogene PEAT drill sites

The Expedition 320 scientific party noted a strong signal of magnetic susceptibility, brown-green color change, and Mn and Fe contents in interstitial waters at Sites U1334–U1336. The results are not as clear cut at Sites U1337 and U1338 (Fig. F27). All sites have significant dissolved manganese peaks in the upper sediment column. The low-latitude younger sites (U1335, U1337, and U1338) have the Mn peak very near the sediment/water interface, whereas the dissolved manganese peak in the older, more northerly sites is found tens of meters into the sediment column. The dissolved Mn peaks in Site U1337 and U1338 sediments are about a factor of 30 higher than those found in the other sites, however, perhaps because of higher water-column scavenging of Mn to the sediments. The dissolved-Fe peak does not clearly match the green intervals, especially at Site U1338. Although not plotted on the paleolatitude graph, green sediments appear when Sites U1337 and U1338 are near or within the equatorial zone, showing a sedimentary "memory" of the equatorial productivity zone.

Organic carbon and carbonate burial

Both total organic carbon (TOC) and CaCO₃ (estimated from evolved CO₂) were measured as part of the shipboard geochemical program during Expedition 321. TOC was measured by acidification of the bulk sediment to remove carbonate and analysis by the CHN analyzer during Expedition 321. Carbonate was measured using a coulometer with an acidification module. Trends in carbonate match with Neogene dissolution intervals, whereas TOC increases from low levels at the base of the sedimentary section to higher levels upcore. Postcruise research will focus upon measuring alkenones, other biomarkers, and productivity proxies to better understand the variations in TOC and productivity (objectives 2, 4, and 9 in "Scientific objectives").

Figure F28 shows the TOC and carbonate profiles versus age at Sites U1336–U1338. None of these sites were ever below the CCD, although Site U1337 briefly approached zero carbonate levels during the 9–11 Ma carbonate crash interval. We can date other carbonate minima between 16 and 18 Ma and 3 and 4 Ma. The 16–18 Ma minimum correlates with the early Miocene carbonate flux minimum identified by Lyle (2003) but is much better defined at Site U1337. It reaches its minimum at 17.2 Ma. The 3–4 Ma carbonate minimum is not well identified in Leg 138 sediments, perhaps because these sites are on younger, shallower crust.

TOC profiles identify a trend from lower to higher TOC from the base of the cores and small intervals of high TOC that were not resolved well at the very coarse shipboard sample interval. High TOC was found in high-biosiliceous intervals to levels as high as 0.7%. The distribution of high TOC deserves more study postcruise to determine how these short events have affected TOC burial fluxes. The base of each site had TOC contents of ~0.1% or below, whereas younger sediments have values of 0.2% or higher. Site U1336, which has a hiatus at 12 Ma, has low TOC values to the surface of the core. Earlier drilling did not find trends in TOC because the standard technique to measure TOC on ODP and DSDP legs (assigning the difference between total carbon and carbonate carbon to TOC) has very high errors in carbonate-rich sediments like the equatorial Pacific.

The longer TOC trend could be the result of age and long-term degradation or the strength of early diagenetic degradation, or it could result from changes in TOC particulate rain ("productivity") through time. Distinguishing between these alternatives will require postcruise study of mass accumulation rates and abundances of different biomarkers.

Site U1336

Two holes were cored at Site U1336 (proposed Site PEAT-5C; 7°42.067′N, 128°15.253′W; 4286 m water depth) 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 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 seafloor to 184.8 m (Cores 321-U1336A-1H through 21H) and 173.6 m (Cores 321-U1336B-1H through 20H). Nonmagnetic core barrels were used for Cores 321-U1336A-1H through 16H and Cores 321-U1336B-1H through 16H and steel barrels were used for all other cores. Two hard layers, one at ~121 m CSF-A (Cores 321-U1336A-14H and 321-U1336B-14H) and one at ~135 m CSF-A (Core 321-U1336B-16H) caused core loss and prevented the development of a continuous sediment section. XCB cores (321-U1336A-22X through 35X) were taken from 184.8 to 302.9 m CFS-A at Hole U1336A. We stopped coring before reaching the basement objective because of decreasing rates of penetration, relatively low recovery, and the possibility of obtaining a stratigraphically complete Miocene section.

At Site U1336, ~300 m of pelagic sediments are divided into three major lithologic units (Fig. F29). The sediments are composed mainly of nannofossil oozes, nannofossil chalks, and chert. The early to middle Miocene sedimentary sequence of Unit I (0–74.54 m CSF-A) contains more radiolarians, clay, foraminifers, and diatoms relative to the early Miocene to early Oligocene sediments below ~70 m CSF-A. Subtle changes in the relative proportions of these minor components produce meter-scale dark-light color cycles and two diatom rich layers. Numerous rounded fragments of pumice occur throughout this unit.

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. The oxidation-reduction reactions responsible for the observed vivid colors and pore water chemistry changes are likely fueled by enhanced availability of organic carbon relative to overlying and underlying sediments. Occasional thin chert layers were encountered below 120 m CSF-A in Unit II. Mainly broken chert fragments were recovered except for a small in situ chert fragment at 159.6 m CSF-A in Section 320-U1336B-18H-4, 106 cm. More abundant chert layers are common in the lower third of the recovered sequence.

Unit III (189.5–299.6 m CSF-A) was only recovered in Hole U1336A. The dominant lithologies of this unit 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-A. CaCO₃ contents remain >88% in the chalk layers. Igneous basement was not recovered at Site U1336.

All major microfossil groups have been 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 (Fig. F29). Calcareous nannofossils are moderately to poorly preserved throughout the succession. There appears to be a complete sequence of nannofossil zones from Zone NN6 (middle Miocene) through NP22 (lower Oligocene), except for Zone NN3, which could not be resolved. Planktonic foraminifers are present throughout the succession ranging from Zones N12 through O1. They are moderately well preserved in the Miocene and less well preserved in the Oligocene. The radiolarian stratigraphy at Site U1336 spans the interval from just above the RN6/RN5 boundary (middle Miocene) to the upper part of RP22 (upper Oligocene) at ~170 m CSF-A. Below this level the sediments are barren of radiolarians. Above this level the assemblages tend to have good to moderate preservation with intermittent intervals of good preservation in RN3 and RN4 (lower to middle Miocene). The downsection decrease in preservation and ultimate disappearance of the radiolarians below Core 320-U1336A-19H appears to be associated with dissolution and reprecipitation of the biogenic silica as intergranular cement and as chert.

Diatom stratigraphy in Hole U1336B spans the interval from just above the Cestodiscus peplum zone (middle Miocene) in Core 320-U1336B-1H to the lowermost part of the Crucidenticula nicobarica zone (upper lower Miocene) in Core 320-U1336B-7H. Below Sample 320-U1336B-7H-CC, the sediments are barren of diatoms. Above this level the valves tend to be mostly poorly preserved. Sample 320-U1336B-1H-CC contains the highest diversity with Cestodiscus pulchellus as dominant component, accompanied by Synedra jouseana and Thalassiosira yabei. Fragments of the large centric diatom Ethmodiscus are present in the upper part of Hole U1336B.

Benthic foraminifers are present throughout the section, although abundances are overall quite low. The preservation of tests is moderate in the upper part of Site U1336 (Sections 320-U1336A-1H-CC through 19H-CC, 8.22–170.63 m CSF-A, and 320-U1336B-1H-CC through 20H-CC, 1.68–174.01 m CSF-A) but deteriorates below this level. The Oligocene to middle Miocene benthic foraminifer assemblage is relatively diverse and indicates oligotrophic lower bathyal to abyssal paleodepths.

The Oligocene/Miocene boundary is placed between the bioevent base Paragloborotalia kugleri (23.0 Ma) and top Sphenolithus delphix (23.1 Ma). The base of planktonic foraminifer 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) and Samples 320-U1336B-16H-1, 52–54 cm, and 17H-3, 80–82 cm (137.72 m CSF-A). Calcareous nannofossil event top S. delphix is recognized between Samples 320-U1336A-17X-2, 90 cm, and 17X-4, 90 cm (145.9 m CSF-A), and between Section 320-U1336B-16H-CC and Sample 320-U1336B-17H-1, 150 cm (137.56 m CSF-A).

Paleomagnetic measurements were conducted on archive-half sections of 21 APC cores from Hole U1336A and 20 APC cores from Hole U1336B. Measurements of natural remanent magnetization (NRM) above ~80 m CSF-A in Holes U1336A and U1336B indicate moderate magnetization intensities (1 × 10^–3 A/m) with a patchy but generally weak viscous remanent magnetization (VRM) or isothermal remanent magnetization (IRM) coring overprint, and polarity reversal sequences are clearly recognized in general (Fig. F29). Demagnetization data from discrete samples above ~ 80 m CSF-A indicate that the characteristic remanent magnetization of the sediments is identified at the 10–20 mT demagnetization steps.

Below ~80 m CSF-A, a zone of diagenetic alteration involving dissolution of remanence carriers reduces remanence intensities after alternating-field (AF) demagnetization of 20 mT to values close to magnetometer noise level in the shipboard environment (~1 × 10^–5 A/m). In this zone, sediment magnetizations have been partly or entirely overprinted during the coring process and remanence inclinations are sometimes steep after AF demagnetization at peak fields of 20 mT. At ~130–140 m CSF-A (Cores 320-U1336A-15H through 16H and 320-U1336B-15H) and below ~160 m CSF-A (Cores 320-U1336A-19H through 21H and 320-U1336B-18H through 20H), polarity reversals are apparently present but the inclinations are steep (up to 80°), indicating that the drilling overprint has not been effectively removed during shipboard demagnetization.

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

A complete physical property program was conducted on whole cores, split cores, and discrete samples. Physical properties measurements on whole-round sections and samples from split cores reflect the differences among lithologies drilled at Site U1336 (Fig. F29). Nannofossil ooze with varying amounts of clay, radiolarians, and diatoms makes up lithologic Unit I and is characterized by high-amplitude and high-frequency variations in bulk density, magnetic susceptibility, natural gamma radiation (NGR), and color reflectance. Magnetic susceptibility is highest in Unit I, with values ranging from 5 × 10^–5 to 30 × 10^–5 SI. Natural gamma radiation is also high in this unit, with values to 56 cps near the seafloor. Wet-bulk densities are lowest in Unit I, with values ranging from 1.4 to 1.7 g/cm³. Porosity is highest in this interval, ranging from 65% to 80%. The grain density of most of the sediments of Unit I, as well as Units II and III, range from 2.6 to 2.9 g/cm³, reflecting the dominance of carbonate constituents at Site U1336. The sediment velocity in Unit I is low, averaging 1500 m/s. The color reflectance of Unit I is marked by luminance (L*) values that are slightly lower and more variable than values determined for sediments in Units II and III.

Below Unit I, a more uniform increase in wet bulk density and decrease in porosity in Units II and III reflect the increasing compaction of the sediments. A slight step increase in wet bulk density marks the transition between Units II and III. In Unit III wet bulk density and porosity average 1.9 g/cm³ and 51%, respectively. Magnetic susceptibility and NGR are low and nearly uniform in Units II and III. Magnetic susceptibility typically is below 5 × 10^–5 SI, and NGR is ~2 cps. Lower clay abundance in Unit II is marked by an increase in L* at the boundary between Units I and II. At 92 m CFS-A, within Unit II, sharp decreases in the a* and b* reflectance parameters mark the change in sediment color from pale yellow to greenish gray. One of the most pronounced changes in physical properties at Site U1336 is the sharp increase in velocity that accompanies the change from nannofossil ooze to nannofossil chalk at the boundary between Units II and III. The velocity at the base of Unit II is ~1700 m/s. Below 190 m CFS-A, in Unit III, the rate at which velocity increases with depth increases, ultimately reaching ~2200 m/s at 290 m CFS-A, near the base of Hole U1336A.

Special Task Multi Sensor Logger (STMSL) data were collected at 5 cm intervals from Hole U1336B and compared to the Whole-Round MultiSensor Logger WRMSL data obtained at 2.5 cm resolution from Hole U1336A during Expedition 320. Features in the magnetic susceptibility and gamma ray attenuation density are well aligned between Holes U1336A and U1336B down to a depth of ~94 m CCSF-A. Below 94 m CCSF-A the magnetic susceptibility signal drops to very low values but the density data are good enough to sustain a correlation to interval U1334B-14H-4, 122 cm. At this point (138.50 m CCSF-A) sediments recovered in both holes are disturbed.

Paleomagnetic reversals were used to calculate the average LSRs for the upper 74 m of the section at Site U1336 on the corrected CCSF-A depth scale. Below 74 m CSF-A only biostratigraphic datums were used to calculate the average LSRs. The LSR at Site U1336 decreases from 15 m/m.y. in the upper Oligocene to 12 m/m.y. in the lower Miocene and stays relatively constant at 9 m/m.y. in the remainder of the section.

A standard geochemical analysis of pore water and organic and inorganic properties was undertaken on Site U1336 sediments. Twenty two interstitial whole-round water samples from Hole 1336B were analyzed. Chlorinity values distinctly 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 CSF-A, with a pronounced decline to 1 mM by 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 values of ~1 μM by ~15 m CSF-A. Dissolved manganese has a broad peak in the depth range from ~25–120 m CSF-A, and dissolved iron appears then peaks below 100 m CSF-A. The increase of dissolved Fe occurs where Mn decreases downhole. Concentrations of dissolved silicate increase with depth from <400 to 800 μM.

Highlights

Miocene sedimentary section and cyclic sedimentations

One of the highlights from Site U1336 is the recovery of a 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 the variations of many petrophysical signals of physical properties including L*, b*, magnetic susceptibility, NGR, and gamma ray attenuation (GRA) bulk density. These high sedimentation–rate cyclically deposited sediments will facilitate the study of paleoceanographic processes at unprecedented resolution for the equatorial Pacific.

Oligocene–Miocene transitions and depth transects

Site U1336 was planned as part of 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 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. Biostratigraphy reveals that the Oligocene/Miocene boundary exists between 142.96 and 145.9 m CSF-A at Site U1336; this will allow the high-resolution study of this critical interval.

Geochemical front

Site U1336 recovered an interval of greenish gray carbonates that exhibit a distinct Mn increase and elevated Fe pore water concentrations with similar characteristics as geochemical alteration fronts at Sites U1334 and U1335. At Site U1336, this zone is ~200 m thick. The paleomagnetic signal is very weak in most parts of this section (80–160 m CSF-A). High amounts of dissolved Fe and Mn in pore water is probably caused by changes in the oxidation state in the sediments. The oxidation-reduction reactions are likely fueled by enhanced availability of organic carbon in overlying and underlying sediments. 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 paleolatitude 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. The radiolarians decrease in preservation downsection and disappear below Core 320-U1336A-19H. Instead, the sediments contain several chert fragments. Some inferred chert layers occur at ~120–140 m CSF-A and blocked APC penetration. Below ~190 m CSF-A, various colored chert layers and fragments occurred within the cores. The chert frequently contains foraminifer tests, reflecting 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 younger than 65 Ma, almost all cherts in the Pacific Ocean lie <150 m above basement. Although we did not recover 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 extend 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.

Site U1337

Science summary

The latest Oligocene through the middle Miocene appears to have been a time of relative warmth comparable to the latest Eocene. However, variability in the isotopic record of the early to middle Miocene is larger than that of the Eocene and may indicate more variability in climate and global ice volume. Site U1337 (proposed Site PEAT-7C; 3°50.009′N, 123°12.352′W; 4463 m water depth) was targeted to collect an early middle Miocene segment of the PEAT equatorial megasplice on ~24 Ma crust between the Galapagos and Clipperton Fracture Zones, ~390 km southeast of Site U1335. In conjunction with Sites U1335 and U1336, it was also designed to provide a latitudinal transect for early Miocene age slices. The recovered sediment column at Site U1337 represents a nearly complete and continuous Neogene sedimentary section.

Operations

Four holes were cored at Site U1337. At Hole U1337A, APC cores were taken from the seafloor to 195.5 m drilling depth below seafloor (DSF) (Cores 321-U1337A-1H through 21H). Nonmagnetic core barrels were used for all APC cores except for Core 321-U1337A-21H. FlexIt core orientation was conducted for all cores except 321-U1337A-1H. In addition, five successful advanced piston corer temperature tool (APCT-3) temperature measurements were taken with Cores 321-U1337A-5H, 7H, 9H, 11H, and 13H. XCB coring continued with Cores 321-U1337A-22X through 48X. The sediment/basement contact was recovered at the base of Core 321-U1337A-48X. Three logging strings (triple combination [triple combo], vertical seismic imager (VSI), and FMS-sonic) were deployed in Hole U1337A.

In Hole U1337B, APC cores were taken from the seafloor to 245.2 m DSF (Cores 321-U1337B-1H through 27H). Nonmagnetic core barrels were used through Core 321-U1337B-20H. The FlexIt core orientation system was deployed successfully for all but two APC cores (321-U1337B-17H and 18H). FlexIt and steel core barrels were used through Core 321-U1337B-27H. APCT-3 temperature measurements were obtained with Cores 321-U1337B-15H, 17H, and 19H. Coring continued with a single XCB core (321-U1337B-28X) to 251.9 m DSF; however, this barrel could not be recovered and Hole U1337B was abandoned prematurely.

Hole U1337C was cored to recover sections that were missing from Holes U1337A and U1337B. APC cores were taken from the seafloor to 11.4 m DSF (Cores 321-U1337C-1H through 2H) using nonmagnetic core barrels and the FlexIt core orientation system. A wash barrel (Core 321-U1337C-3W) was then deployed, and the hole was washed to a depth of 169.4 m DSF. APC coring resumed at that depth and continued through Core 321-U1337C-9H to 221.3 m DSF and then switched to steel core barrels. Coring with the XCB system continued with Cores 321-U1337C-10X through 33X. Basement was recovered in Core 321-U1337C-33X.

Hole U1337D was planned to target the few remaining areas that had yet to be fully recovered and to duplicate recovery through those sections of the formation already recovered to provide additional sample material. The most troublesome material encountered in the previous holes was the large diatom mats located directly above and below a hard ~0.4 m thick porcellanite (baby chert) layer. In Hole U1337D, APC cores were taken from the seafloor to 237.7 m DSF (Cores 321-U1337D-1H to 26H). Nonmagnetic core barrels were used through Core 321-U1337D-20H. The first XCB core (321-U1337D-27X) was designed to only core through the hard ~0.4 m thick baby chert layer. The APC was once again deployed and cored to 267.0 m DSF (Cores 321-U1337D-28H through 30H). At this point the XCB coring system was once again deployed for Cores 321-U1337D-31X through 49X to a total depth of 442.9 m DSF. The FlexIt core orientation system was deployed successfully with all APC cores. The Sediment Temperature Tool (SET) was deployed for the first time from the JOIDES Resolution after Core 321-U1337D-17X at 298.1 m DSF.

Lithostratigraphy

At Site U1337, latest Oligocene seafloor basalt is overlain by ~450 m of nannofossil and biosiliceous oozes and nannofossil chalks that are divided into four lithologic units (Fig. F30). The Pleistocene through uppermost Miocene sediments of Unit I are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil oozes, diatom oozes, and radiolarian oozes that alternate on meter scales with a general downsection increase in siliceous microfossils relative to nannofossils. Green and gray biosiliceous lithologies, interbedded on meter scales with white and light greenish gray nannofossil ooze, comprise the dominant sedimentary constituents in the uppermost Miocene to middle Miocene Unit II, which includes regular diatom mat deposits. Meter-scale color alternations in Units I and II are associated with variations in lithology and physical properties. However, similar to the common millimeter- and centimeter-scale color banding that do not mark compositional changes, they are likely associated with sediment redox conditions. White, pale yellow, and pale green nannofossil oozes and chalks dominate the sediments of middle Miocene to latest Oligocene age, although diatoms and radiolarians remain present in low abundances. Latest Oligocene seafloor basalt (Unit IV) was recovered at the base of the sedimentary section.

Biostratigraphy

All major microfossil groups have been found in the sediments recovered at Site U1337. Planktonic foraminifers at Site U1337 are rare to abundant with poor to good preservation throughout most of the succession but are absent or extremely rare in some intervals of the late Miocene and early Miocene. Biozones PT1b to O6 are recognized, with the exception of Zones PL4, M12, and M3 (Fig. F30). Calcareous nannofossils at Site U1337 are moderately to poorly preserved and some samples with high silica content are barren. Nannofossil Zones NN1 to NN21 are present, indicating an apparently complete sequence. The radiolarian stratigraphy at Site U1337 spans the interval from the uppermost part of Zone RN16-17 (upper Pleistocene) to RN1 (lower Miocene). The radiolarian assemblages of Pleistocene to upper Miocene age tend to have good preservation, whereas middle to lower Miocene assemblages show moderate preservation. In the lowermost part of the section, above the basement, sediments are barren of radiolarians. The high-resolution diatom stratigraphy at Site U1337 spans the interval from the Fragilariopsis (Pseudoeunotia) doliolus zone (upper Pleistocene) to the lowermost part of the Craspedodiscus elegans zone (lower Miocene). The diatom assemblage is generally well to moderately preserved throughout the recovered section; however, there are several intervals in which valve preservation becomes moderate to poor. The base of the sediment column is barren of diatoms. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal schemes generally agree, though some discrepancies occur in the lowest part of the core. Benthic foraminifers occur continuously throughout the succession recovered in Hole U1337A and show good to moderate preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths. Marked variations in downcore abundance and species distribution reflect major changes in global climate linked to fluctuations in ice volume and reorganization of Pacific circulation during the Neogene.

Stratigraphic correlation

Stratigraphic correlation provided a complete spliced record to ~220 m CCSF-A. Several gaps (perhaps three) were encountered over the next 50 m CCSF-A. Comparison of GRA density records with well logging density data suggest that no more than 1 m of section was lost in any of the gaps. Correlation between the holes was broken again several times between 440 m CCSF-A and basement at 490 m CCSF-A. Growth factor for the correlation was 1.12. The linear sedimentation rate decreases from ~21 m/m.y. in the middle Miocene to 17 m/m.y. in the late Miocene.

Paleomagnetism

Paleomagnetic measurements were conducted on archive-half sections of 20 APC cores and 14 XCB cores from Hole U1337A, 27 APC cores from Hole U1337B, 8 APC cores from Hole 1337C, and 30 APC cores from Hole 1337D. The FlexIt core orientation tool was deployed in conjunction with all APC cores, and we conclude that the FlexIt orientation data are generally reliable. Measurements of NRM above ~93 m CSF-A indicate moderate magnetization intensities (on the order of 10^–3 A/m) with a patchy but generally weak VRM or IRM coring overprint, and polarity reversal sequences from the Brunhes to the bottom of the Gilbert Chron are recognized. Below ~93 m CSF-A, remanence intensities after AF demagnetization of 20 mT are reduced to values close to magnetometer noise level in the shipboard environment (~1 × 10^–5 A/m). In this zone, sediment magnetizations have been partly overprinted during the coring process, and remanence inclinations are occasionally steep after AF demagnetization at peak fields of 20 mT. Nonetheless, polarity reversals are apparently recorded to ~200 m CSF-A and are provisionally correlated to the geomagnetic polarity timescale (GPTS) from to Chron C3An to C5n (~6–11 Ma) (Fig. F30). Magnetic polarity interpretation was impossible for APC cores taken with steel core barrels and XCB cores because of severe magnetic overprint during coring.

Physical properties

Physical property measurements on whole-round sections and samples from split cores display a strong lithology-dependent variation at Site U1337 (Fig. F30). Variations in the abundances of nannofossils, radiolarians, diatoms, and clay in Unit I account for high-amplitude, high-frequency variations of all physical properties. Intervals enriched in biogenic silica and clay generally display lower grain density and bulk density and higher porosity, magnetic susceptibility, and NGR. Velocity is generally directly related to bulk density; however, it is commonly higher in low-density siliceous-rich sediments than it is in more calcareous intervals. Wet bulk density is low in Unit I, ranging from 1.12 to 1.46 g/cm³. Porosity is as high as 92% in this unit. Velocity also is low, averaging 1525 m/s. The natural gamma record, as at previous Expedition 320/321 sites, is marked by an anomalously high near-surface peak (~65 cps). Magnetic susceptibility varies between 4 × 10^–5 and 18 × 10^–5 SI. The color of Unit I is characterized by the lowest L* and high and variable a* and b* values. Unit II is characterized by a continued high variability in grain density. Together, the grain density in Units I and II averages 2.51 g/cm³ and ranges from 2.17 to 2.85 g/cm³. All other physical properties display less variability in Unit II than in Unit I, reflecting a less variable lithology. Wet bulk density increases and porosity decreases with depth in Unit II; however, in Units II and III these trends are interrupted by low-density, high-porosity diatom- and radiolarian-rich intervals. Unit II is slightly lighter colored (lower L*) and distinctly more blue (lower a*) and green (lower b*) than Unit I. Unit III is characterized by more uniform physical properties that accompany the high and uniform carbonate composition of the unit. The nannofossil oozes and chalks of this unit are characterized by a uniform grain density that averages 2.67 g/cm³. The bulk density and porosity trends of Unit II continue in Unit III. The transition from ooze to chalk is marked by a change in gradient of these properties to a more rapid decrease in wet bulk density and an increase in porosity with depth. Wet bulk density and porosity at the base of the sediment section are 1.95 g/cm³ and 47%, respectively. The increase in velocity with depth also changes to a higher gradient in Unit III, with values increasing from 1510 m/s at ~340 m CSF-A to ~1800 m/s near the base of the hole. Magnetic susceptibility and NGR values remain low in Unit III but do vary in response to small changes in lithology. The sharp color change from greenish gray to pale yellow at ~410 m CSF-A is marked by a sharp increase in a* and b*. The change in color to pale brown chalk immediately above basement is marked by an increase in both a* and b* and a decrease in L*.

Downhole logging

Downhole logging consisted of deploying three tool strings in Hole U1337A. Two tool strings took downhole measurements of natural gamma ray radioactivity, bulk density, electrical resistivity, elastic wave velocity, and borehole resistivity images in the 77–442 m wireline log depth below seafloor (WSF) depth interval. The third tool string measured seismic waveforms in a VSP experiment in the 214–439 m WSF depth interval. Measurement depths were adjusted to match across different logging runs, obtaining a wireline log matched depth below seafloor (WMSF) depth scale. The downhole log measurements were used to define three logging units. Unit I (77–212 m WMSF) and Unit II (212–339 m WMSF) have average densities of ~1.3 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas Unit III (339–442 m WMSF) density increases with depth reaching 1.85 g/cm³ at the base of the hole (Fig. F31). Resistivity and P-wave velocity follow a pattern similar to that of density, suggesting that the major control on these physical properties are variations in sediment porosity. Natural gamma ray measurements are low throughout the logged interval (~5° gAPI), except for two pronounced peaks caused by uranium, one at the seafloor and the other at 240 m WMSF. The gamma ray peak at 240 m WMSF corresponds to the ~40 cm thick baby chert layer that has only been recovered as rubble in the cores but can be clearly identified in the downhole logs and borehole images as an interval of high density and resistivity. VSP logging measured arrival time of the seismic pulse from the sea surface at 16 stations. Together with the traveltime to the seafloor, VSP measurements are the basis for a traveltime-depth conversion that allows seismic reflectors to be correlated to stratigraphic events. Downhole temperature measurements and thermal conductivities of core samples were combined to estimate a geothermal gradient of 32.4°C/km and a heat flow of 28.4 mW/m² at Site U1337.

Geochemistry

A total of 85 interstitial water samples were collected from Hole U1337A, 49 using the whole-round squeezing approach across the entire hole and 36 in the upper 100 m by Rhizon sampling. Chloride ion concentration (not corrected for Br contribution) varies slightly with depth and ranges from 554 to 566 mM. Alkalinity increases slightly downhole from ~2.7 mM in the upper 100 m to values scattered around 3.8 mM below 300 m CSF-A. Sulfate concentrations vary between 26 and 29 mM, with slightly decreasing values with depth. An enormous dissolved manganese peak of ~150 mM at 13 m CSF-A is captured by the high-resolution interstitial water sampling. Dissolved iron is sporadically detectable in the upper 200 m and then increases to a peak of ~5 mM between 275 and 300 m CSF-A before becoming undetectable again below 400 m CSF-A. Nitrate concentrations are low and variable except for higher values at the top and bottom of the sediment column. These variations in nitrate, manganese, and iron reflect changes in redox chemistry that also manifest as changes in sediment color. The silicic acid (dissolved silicate) content of the interstitial waters is substantially greater than that of bottom waters (e.g., Peng et al., 1993) and increases with depth from ~700 μM in the uppermost sediments to peak at ~1200 μM at ~350 m CSF-A before decreasing to ~900 μM near the basaltic basement. Calcium carbonate and inorganic carbon concentrations were determined on 283 and 28 sediment samples from Holes U1337A and U1337B, respectively. Calcium carbonate concentrations vary greatly in the upper two lithologic units, ranging from 30% to 90%, reflecting the alternation between calcite and opal producers (Fig. F30). In lithologic Unit III calcium carbonate contents are generally high, scattered around 80%, but a distinctive decrease is observed between 350 and 400 m CCSF-A. The concentration of TOC was determined for 47 sediment samples from Holes U1337A and U1337B. In the upper 235 m CCSF-A, TOC content ranges between 0.10% and 0.34% except for the high value of 0.72% in the uppermost sample. TOC content increases at 44.00 m CCSF-A and in the interval from 87.28 to 108.59 m CCSF-A. Below 235 m CCSF-A, TOC values are generally <0.10%.

Shipboard geochemical analyses of interstitial water and bulk sediment samples reflect large variations in sediment composition resulting from shifts in carbonate versus opal primary production. The large-scale redox state and diagenetic processes of the sediment column are related to overall changes in sediment composition. Interstitial water chemistry is also influenced by the baby chert layer forming a diffusive boundary at ~240 m CSF-A and seawater circulation in the basement. The basement itself appears to exert little influence on the geochemistry of sediments and interstitial waters.

Highlights

Diatom mat deposition

Lithologic Unit II at Site U1337 is mostly composed of biosiliceous lithologies, notably diatoms. The abundance of diatoms in the middle and upper Miocene section at Site U1337 is much higher than encountered in any interval at Sites U1331–U1336. Several decimeter- to meter-scale intervals of diatom ooze are laminated, and smear slide analyses indicate that the diatom assemblage is composed primarily of pennate taxa, with abundant "needlelike" Thalassiothrix spp., indicating diatom mat deposition. The lowermost laminated diatom mat is in the upper portion of Unit III at ~15 Ma. Much larger intervals are present in Unit II at roughly 10 Ma and shorter intervals at ~4.5 Ma. Ages of laminated diatom mats at this site are similar to those found at Leg 138 sites farther to the east (Mayer, Pisias, Janecek, et al., 1992), which have been interpreted to reflect regional bursts of export silica production in the eastern equatorial Pacific (Kemp and Baldauf, 1993). No laminated diatom oozes were recorded during Expedition 320 at drill sites farther to the northwest; however, near 10 Ma at Site U1335, drilling recovered clayey diatom ooze and clayey radiolarian ooze containing no carbonate at all, suggesting that dissolution may also play an important role in the deposition of laminated diatom mats.

Oligocene–Miocene transition

The Oligocene/Miocene boundary was recovered in Holes U1337A, U1337C, and U1337D. In Hole U1337A, the Oligocene/Miocene boundary is estimated to fall between Samples 321-U1337A-48X-2, 85–87 cm, and -48X-3, 55 cm (445.56–446.75 m CSF-A; 490.92–492.11 CCSF-A). It occurs in white (2.5Y 8/1) nannofossil chalk with foraminifers, interbedded and heavily mottled with pale yellow (2.5Y 7/4) to very pale brown (10YR 7/4) nannofossil chalk. Abundant millimeter-scale dendritic manganese grains composed of manganese oxide occur throughout this interval. The lower 15 cm of the core catcher of this core represents basement. No prominent change in lithology, GRA bulk density, reflectance, or magnetic susceptibility is seen through the Oligocene–Miocene transition.

Neogene carbonate dissolution

The CCD of the Neogene is much more stable than that of the Eocene, but there are intervals of lower carbonate deposition at Site U1337 that probably represent significant changes of the Neogene CCD. In the early Miocene, a significant carbonate low reaches its minimum at ~17 Ma (340 m CSF-A in Hole U1337A), when the site was at a depth of ~3500 meters below sea level. This early Miocene interval marks a strong minimum at Site U1334 as well, on crust with a depth of ~4000 m at that time. Highly variable carbonate is also characteristic of the late/middle Miocene boundary, but the role of carbonate dissolution versus elevated deposition of biosilica needs to be determined.

Site U1338

Science summary

Site U1338 (proposed Site PEAT-8D; 2°30.469′N, 117°58.178′W; 4200 m water depth) was sited to collect an 3–18 Ma segment of the PEAT equatorial megasplice and is located on ~18 Ma crust just north of the Galapagos Fracture Zone, 324 nmi (600 km) southeast of Site U1337. A seamount (3.7 km water depth) with surrounding moat is found ~25 km to the north-northwest of Site U1338 at the downslope end of the survey area. Originally a site was chosen ~10 km from the seamount (proposed Site PEAT-8C). However, the alternate proposed site was selected and drilled uphill and further away from the seamount to avoid possible turbidites, as were found near seamounts during drilling of Expedition 320 Sites U1331 and U1335. The recovered sediment column at Site U1338 represents a nearly complete and continuous early Miocene to Holocene sedimentary section.

Operations

Four holes were cored at Site U1338. From Hole U1338A, APC cores were taken from the seafloor to 221.2 m DSF (Cores 321-U1338A-1H through 24H) using nonmagnetic core barrels and the FlexIt core orientation system installed. FlexIt and steel core barrels were used for Cores 321-U1338A-25H and 26H. In addition, five successful APCT-3 temperature measurements were taken with Cores 321-U1338A-5H, 7H, 9H, 11H, and 13H. XCB coring continued with Cores 321-U1338A-27X through 44X. A small piece of basement was recovered in the core catcher of Core 321-U1338A-44X.

From Hole U1337B, APC cores were taken from the seafloor to 188.1 m DSF (Cores 321-U1338B-1H through 20H) except for a short drilled interval of 2.5 m from 235.6 to 238.1 m DSF to adjust the core breaks. Nonmagnetic core barrels and the FlexIt core orientation system were used through Core 321-U1338B-20H. FlexIt and steel core barrels continued through Core 321-U1338B-42H to 387.4 m DSF. Coring continued with three XCB cores (321-U1338B-43X through 45X) to 416.1 m DSF. Basement contact was recovered in Core 321-U1338B-45X. Three logging strings (triple combo, VSI, and FMS-sonic) were deployed in Hole U1338B.

Hole U1338C was cored to recover sections that were missing from Holes U1338A and U1338B. APC cores were taken from the seafloor to 189.8 m DSF (Cores 321-U1338C-1H through 21H) using nonmagnetic core barrels and the FlexIt core orientation system. FlexIt and steel core barrels were used through Core 321-U1338C-44H to 396.9 m DSF. Coring continued through Core 321-U1338C-47H to a total depth of 414.4 m DSF, setting a new all time depth record for the APC.

Hole U1338D was primarily planned to recover a few "instructional" cores to be used during Expedition 323. Three APC cores were cut to 23.9 m DSF.

Lithostratigraphy

At Site U1338, ~415 m of nannofossil ooze and chalk with varying concentrations of diatoms and radiolarians overlie early Miocene seafloor basalt and are divided into four lithologic units (Fig. F32). Pleistocene through middle Pliocene sediments of Unit I are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze that alternate on a decimeter- to meter-scale. Light green and light gray nannofossil ooze with occasional darker intervals with abundant siliceous microfossils, notably diatoms, comprise the upper Miocene to middle Pliocene Unit II. Decimeter-, meter- and tens of meters–scale color alternations in Units I and II are associated with variations in lithology and physical properties. Some of these color changes, as well as common millimeter- and centimeter-scale color banding, are not associated with compositional changes and likely reflect variations in sediment redox state. White, pale yellow, light greenish gray, and very pale brown nannofossil oozes and chalks dominate Unit III of lower to upper Miocene, although slightly darker green and gray intervals with larger amounts of siliceous microfossils remain present. Lower Miocene seafloor basalt (Unit IV) was recovered at the base of the sedimentary section.

Biostratigraphy

All major microfossil groups have been found in the ~415 m thick succession of Holocene to lower Miocene sediment bulge recovered from Site U1338. Calcareous nannofossils at Site U1338 are in general moderately preserved, but there are some intervals in which the preservation is good or poor. Nannofossil Zones NN4 to NN21 are present, indicating an apparently complete sequence. Planktonic foraminifers vary from rare to abundant, with moderate to good preservation throughout most of the succession, but are absent or rare in a short interval in the late Miocene. Planktonic foraminifer Zones PT1b (late Pleistocene) to M2 (early Miocene) are documented, with the exception of Zones PL4, M12, and M6. The radiolarian stratigraphy spans the interval from the uppermost part of Zone RN16–17 (late Pleistocene) to the uppermost part of Zone RN3 (early Miocene). Radiolarian assemblages show good to moderate preservation except in the lowermost portion (early Miocene), which is barren of radiolarians. The high resolution diatom stratigraphy spans the interval from the Fragilariopsis (Pseudoeunotia) doliolus zone (late Pleistocene) to the lowermost part of the Craspedodiscus elegans zone (early Miocene). The diatom assemblage is generally well to moderately preserved throughout the recovered section; however, there are several intervals in which valve preservation becomes moderate to poor. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal schemes generally agree, with some inconsistencies (Fig. F32). Benthic foraminifers occur continuously throughout the succession recovered in Hole U1338A and show generally good preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths. Marked variations in downcore abundance and species distribution reflect major changes in global climate linked to fluctuations in ice volume and reorganization of Pacific Ocean circulation during the Neogene.

Stratigraphic correlation

Stratigraphic correlation provided a complete spliced record to a depth of ~260 m CCSF-A. Several gaps were seen between 280 and 360 m CCSF-A. Comparison of GRA density records with well logging density data suggests that no more than 1 m of section was lost in any of the gaps. Correlation between the holes was broken again several times between 435 m CCSF-A and basement at 460 m CCSF-A. Growth factor for the correlation was 1.11. The linear sedimentation rate decreases from ~29 m/m.y. in the Miocene to 13 m/m.y. in the Pliocene-Pleistocene.

Paleomagnetism

Paleomagnetic measurements were conducted on archive-half sections of 26 APC cores from Hole U1338A, 42 APC cores from Hole U1338B, and 47 APC cores from Hole U1338C. The FlexIt core orientation system was deployed in conjunction with all APC cores except for the deepest three cores of Hole U1338C, and we conclude that the FlexIt orientation data are generally reliable. NRM measurements indicate moderate magnetization intensities (on the order of 10^–3 A/m) for depth intervals 0–50, 280–225, and 295–395 m CSF-A. Polarity reversal sequences of these intervals are provisionally correlated to the Brunhes to the upper part of the Gilbert Chron (0 to ~4 Ma), Chron C4An to C5n (~9–11 Ma), and Chron C5r to C5Br (~12–16 Ma) of the GPTS, respectively (Fig. F32). Except for these intervals, remanence intensities after AF demagnetization of 20 mT are reduced to values close to magnetometer noise level in the shipboard environment (~1 × 10^–5 A/m). Magnetization directions are dispersed and not interpretable there. Sedimentation rates increase downcore from ~12 m/m.y. at the top to ~30 m/m.y. near the bottom.

Physical properties

Physical properties measurements on whole-round sections and samples from split cores display a variation strongly dependent on the relative abundance of biosiliceous and calcareous sediment components at Site U1338. As at Site U1337, intervals enriched in siliceous microfossils and clay generally display darker colors, lower grain density and bulk density, and higher porosity, magnetic susceptibility, and NGR. The variation of velocity is more complex in that it is dependent on both the wet bulk density and the sediment rigidity. These parameters vary independently with the variation in abundance of biosiliceous and calcareous components. The physical properties at Site U1338 also display cyclicity on multiple scales, a decimeter to meter scale and a scale with a spacing on the order of tens of meters.

Lithologic Unit I at Site U1338 is characterized by low wet bulk density that decreases from 1.4 g/cm³ near the seafloor to 1.2 g/cm³ at the base of the unit as a result of an increasing abundance of radiolarians and diatoms with depth. The grain density in Units I and II displays a greater variability than is found deeper at the site as a result of the greater variability in the abundance of biosiliceous and calcareous components. The average grain density for Units I and II is relatively low, at 2.59 g/cm³. The NGR signal at Site U1338 is characterized by a near-seafloor peak that is somewhat lower than those recorded at the other PEAT drill sites but extends deeper and is marked by a double peak. Spectral reflectance measurements show that Unit I is characterized by lower L* and higher a* and b* values in the upper 25 m of Unit I (Fig. F32). Below 25 m CSF-A, the sediment becomes lighter colored (L* increases) and more bluish green (a* and b* decrease).

Unit II is characterized by increasing wet bulk density with depth to ~175 m CSF-A. Below this depth, an increase in the abundance of siliceous microfossils produces a broad density minimum. Magnetic susceptibility and NGR signals are low in Unit II to the depth at which the biosiliceous material increases in abundance. The interval of the broad density minimum is characterized by higher magnetic susceptibility values that are roughly equal to those in the upper 25 m of Unit I. Unit II is lighter colored than Unit I (higher L*) and more blue (lower b*).

Unit III at Site U1338 is characterized by a higher and more uniform carbonate content and, as a result, more uniform physical properties. Wet bulk density increases from ~1.5 g/cm³ at the top of Unit III to 1.7 g/cm³ at the base of the unit. Grain density varies over a narrower range in Unit III than it does in Units I and II and displays an average (2.64 g/cm³) nearer to that of calcite. Velocity, which through much of Units I and II is close to the velocity of water, displays a regular increase in Unit III, from ~1620 m/s at the top to ~1820 m/s near the base of the unit. Velocity gradient increases near the base of Unit III accompanying the transition from nannofossil ooze to chalk. Magnetic susceptibility is low from the boundary between Units II and III, at ~245 m CSF-A, to 300 m CSF-A. Below 300 m CSF-A, susceptibility again increases to values comparable to those in the upper part of Unit I. NGR variability is lower in Unit III than in Unit II and remains uniformly low throughout the unit. Overall, Unit III is the lightest colored (highest L* values) unit at Site U1338. The transition from greenish gray to pale yellow is marked at ~385 m CSF-A by a shift to higher values of both a* and b*.

Downhole logging

Downhole logging of Hole U1338B began after the end of APC/XCB coring to a total depth of 416.1 m DSF. Three tool strings were deployed in Hole U1338B: a modified triple combo (that did not include a neutron porosity measurement), a FMS-sonic combination, and a VSI seismic tool with a Scintillation Gamma Ray (SGT-N) sonde. The modified triple combo and FMS-sonic tool strings took downhole measurements of natural gamma ray radioactivity, bulk density, electrical resistivity, elastic wave velocity, and borehole resistivity images in the 125–413 m WSF depth interval. The VSI seismic tool string measured seismic waveforms in a VSP experiment that covered the 189.5–414.5 m WSF depth interval. Measurement depths were adjusted to match across different logging runs, obtaining the WMSF depth scale.

Downhole log measurements were used to define three logging units: Unit I (139–244 m WMSF) and Unit II (244–380 m WMSF) have average densities of ~1.45 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas in Unit III (from 380 m WMSF) density increases with depth, reaching 1.7 g/cm³ at the base of the hole (Fig. F33). Resistivity and P-wave velocity follow a pattern similar to that of density throughout the logged interval, suggesting that the major control on these physical properties are variations in sediment porosity. Both resistivity and density measurements show a small-scale peak at 280 m WMSF. This peak at 280 m WMSF is clearly visible in the borehole resistivity images as a high-resistivity layer 16 cm thick, and it corresponds to a chert layer that has only been recovered as rubble in the cores. Natural gamma ray measurements are low throughout (~4° gAPI) but do show a pronounced high at the seafloor caused by a local increase in uranium concentration.

In the VSP experiment, the arrival time of a seismic pulse was measured from the sea surface at 14 stations. Together with the traveltime to the seafloor, the VSP measurements are the basis for a traveltime-depth conversion that allows seismic reflectors to be correlated to stratigraphic events. Downhole temperature measurements and thermal conductivities of core samples were combined to estimate a geothermal gradient of 34.4°C/km and a heat flow of 33.6 mW/m² at Site U1338.

Geochemistry

A total of 118 interstitial water samples were collected from Holes U1338A and U1338B, 43 using the whole-round squeezing approach and 75 by Rhizon sampling. Chloride ion concentration (not corrected for Br contribution) varies slightly with depth and is generally within 555 to 565 mM. Alkalinity increases slightly downhole from ~2.7 mM at the sediment–water interface to peak slightly above 4 mM at 140 m CSF-A. A monstrous dissolved manganese peak of 150 mM at 10 m CSF-A is captured by the high-resolution interstitial water sampling and is remarkably similar to that observed at Site U1337. These peaks are >10 times greater than the highest dissolved-manganese concentrations encountered during Expedition 320. Lithium concentrations decrease from ~26 μM at the surface to a minimum of ~3 μM at ~250 m CSF-A before increasing sharply with depth to seawater values at the base of the section. The interstitial water strontium profile is a mirror image to that of lithium except the decrease from the peak of 400 μM at 200 m CSF-A is punctuated by a sharp drop of >100 μM between ~260 and 290 m CSF-A. The lithium and strontium profiles indicate seawater circulation in the basement as their values tend toward seawater values near the basement.

Calcium carbonate concentrations range between 26% and 88% with substantial variability in the upper 273.31 m CCSF-A, corresponding to the alternation between calcite and opal production in the upper two lithologic units. Below 273.31 m CCSF-A (lithologic Unit III), calcium carbonate contents become generally high and stable between 66% and 91% compared with the upper part of the stratigraphic column (Fig. F32). In the upper ~230 m CCSF-A, TOC content is generally high and variable ranging between 0.09% and 0.46%, whereas below ~230 m CCSF-A, TOC content is <0.09%. Downhole TOC variability is most likely related to lithologic changes, with higher TOC being found in the more biosiliceous intervals.

Interstitial water and bulk sediment samples reflect large variations in sediment composition resulting from shifts in carbonate versus opal primary production. The large-scale redox state and diagenetic processes of the sediment column are related to overall changes in sediment composition. Interstitial water chemistry points to seawater circulation in the basement, although the basement itself appears to exert little influence on the geochemistry of the sediments and interstitial waters.

Highlights

Color changes, lithology, and redox state

Smear slide analyses and visual core descriptions show that many of the decimeter-, meter-, and tens of meters-scale color variations in lithologic Units I and II to some extent relate to changes in lithology (e.g., Fig. F32). We suspect, however, that some of these color variations, notably the transitions between pale green and pale yellow lithologies, are controlled by sediment redox state, similar to those recorded at Sites U1331–U1337 and earlier work in the equatorial Pacific Ocean (e.g., Lyle, 1983).

Magnetic susceptibility is relatively low in the light gray and light brown intervals in Unit I and most of Unit II (Fig. F32). A significant decrease in the intensity of the magnetic signal in Unit II suggests dissolution of magnetite resulting from intensified microbial Fe reduction. In the lower part of Unit III, a sharp downcore transition from green to yellow is not associated with any other lithologic change, does not occur at the same stratigraphic level between holes, and thus should not be considered as an equivalent time horizon. Although pore water Fe concentrations reach 6 to 7 μM/L in the green interval, Fe is absent below the transition to yellow and brown. Although some of this signal may be affected by seawater contamination during XCB drilling in Hole U1338A, all available information suggests that the lowermost color change represents a redox front.

Occurrence of diatom rich layers

Lithologic Unit II at Site U1338 is mainly composed of nannofossil ooze with relatively high abundances of biosiliceous components, notably diatoms (Fig. F32). The relative abundance of diatoms is lower than that at Site U1337, and the record lacks laminated diatom ooze intervals (diatom mats) such as those observed at Site U1337. However, centimeter to sometimes 1–2 m thick diatom nannofossil ooze layers containing abundant specimens of Thalassiothrix spp. are occasionally interbedded with nannofossil ooze (e.g., ~126.2–127.1 and ~231.8–234.3 m CSF-A in Hole U1338A and ~127.3–128.0 and ~233.8–234.8 m CSF-A in Hole U1338C). Units II and III also contain significant amounts of pyrite, particularly in diatom-rich intervals in Unit II (e.g., Cores 321-U1338B-14H, 19H through 21H, 26H, 28H, 29H, and 32H through 41H). In addition, the middle part of Unit III contains thin intervals of abundant pyrite-filled siliceous microfossils (e.g., intervals 321-U1338B-33H-4, 58–66 cm, and 35H-5, 76–82 cm). These diatom-rich layers, pyrite nodule occurrences, and pyrite-rich siliceous microfossil layers in Units II and III are associated with high TOC content, suggesting a relation between the abundance of diatoms in the sediments, sediment redox state, and the production or preservation of organic carbon.

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