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doi:10.2204/iodp.proc.320321.107.2010

Science summary

Two holes were cored at Site U1335 (5°18.735′N, 126°17.002′W; 4327.5 mbsl) (Fig. F1; Table T1), 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]" in the "Expedition 320/321 summary" chapter) 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, seafloor basalt is overlain by ~414 m of pelagic sediment. The oldest sediment is of late Oligocene age (26 Ma).

The sedimentary sequence at Site U1335 is divided into two major lithologic units (see "Lithostratigraphy") . The topmost ~64 m thick lithologic Unit I comprises an alternating sequence of earliest late Miocene to Pleistocene calcareous nannofossil, diatom, radiolarian, and foraminifer oozes. The topmost sediment of Unit I is younger than the Pleistocene/Pliocene boundary (see "Biostratigraphy") as recognized by the top of planktonic foraminifer Globigerinoides fistulosus (between Samples 320-U1335A-1H-CC and 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. F3, F4). One of the prominent features of Unit II is the presence of at least 49 described beds (2–176 cm thick) of nannofossil foraminifer ooze that have sharp basal boundaries, many of which are irregular and some of which are inclined. These beds are interpreted as gravity flow deposits from the nearby seamounts and represent ~2% of the total sediment recovered.

Holes U1335A and U1335B provided high-quality advanced piston corer (APC)-cored sediments from the mudline to ~341 and 378 m core depth below seafloor (CSF), respectively (Cores 320-U1335A-36H and 320-U1335B-41H). At the time it was recovered, the APC-cored interval from Hole U1335B represented 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 extended core barrel (XCB) cutting shoe. XCB coring advanced to ~420 m drilling depth below seafloor (DSF) through lower Miocene and upper 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 "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 lower Miocene sections from the equatorial Pacific to date (~320 m cored depth from the lowermost to uppermost Miocene).

At Site U1335, carbonate content fluctuates between 12 and 87 wt% within Unit I (see "Geochemistry") (Fig. F5), presumably reflecting the close proximity of the seafloor to the lysocline. With the exception of the depth interval from 140 to 220 m CSF, the remainder of Unit II exhibits uniformly high calcium carbonate content between 80 and 90 wt%. From ~150 to 210 m CSF (approximately equivalent to Cores 320-U1335A-16H through 22H), carbonate content cycles between ~50 and 90 wt% and corresponds to a change in dominant sediment color from light greenish gray to tan, displaying higher magnetic susceptibility values up to 25 x 10–5 SI.

A series of upper Oligocene through upper middle Miocene cores (320-U1335A-8H through 40X) were recovered with distinct colors ranging from light grayish green to light blue (see "Lithostratigraphy"), similar but much thicker in total stratigraphic thickness (~70–170 and ~200–350 m) than those observed at Site U1334 (see the "Site U1334" chapter). The colored carbonate oozes have extremely low magnetic susceptibilities that complicated a confident stratigraphic correlation. These colored oozes have lost almost their entire magnetic susceptibility signal from ~70 to ~105 m CSF and below ~210 m CSF (Figs. F4, F6, F7). Similar colored cores have previously been described for DSDP Sites 78 and 79 (Hays et al., 1972).

All major microfossil groups occur in sediments from Site U1335, representing a complete biostratigraphic succession at the shipboard sample resolution level of Pleistocene to uppermost Oligocene sediments, including a thick sequence of lower Miocene nannofossil ooze and chalk (see "Biostratigraphy"). 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 Zone NP25 (upper Oligocene) above basaltic basement through Zone NN20 (Pleistocene). Planktonic foraminifers are present throughout the succession and are moderately to well preserved. Recognized planktonic foraminifer zones range from Zone PT1a (Pleistocene) to Zone 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 was fully recovered in Cores 320-U1335A-37X and 320-U1335B-38H as approximated by the planktonic foraminifer datum base of Paragloborotalia kugleri between Samples 320-U1335A-37X-4, 136–138 cm, and 37X-CC (midpoint = 348.6 m CSF), in good agreement with the calcareous nannofossil event top of Sphenolithus delphix at 349.7 m CSF between Samples 320-U1335A-37X-6, 50 cm, and 37X-CC. The oldest sediment overlying seafloor basalt has been assigned to calcareous nannofossil Zone NP25 (24.4–26.8 Ma).

Sedimentation rates, as derived from the magneto- and biostratigraphic age determinations (see "Stratigraphic correlation and composite section"), 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 obvious hiatus at the shipboard biostratigraphic resolution, although some condensed horizons exist (e.g., near the early/middle Miocene boundary and in the early late Miocene; see "Biostratigraphy"). The presence of all major fossil groups as well as a detailed and partly 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 Whole-Round Multisensor Logger (WRMSL) measurements of magnetic susceptibility, bulk density, and P-wave velocity; natural gamma radiation (NGR); and measurements of color reflectance, followed by discrete measurements of moisture and density (MAD) properties, sound velocities, and thermal conductivity in Hole U1335A. All track data vary throughout the section, allowing a detailed correlation among holes, with the exception of a low magnetic susceptibility signal within an interval extending slightly above and below the light greenish gray tinted cores of Unit II (see "Lithostratigraphy" for exact color definitions), between ~70 and 110 and ~210 and ~380 m CSF. Magnetic susceptibility varies between 5 x 10^–5 and 20 x 10^–5 SI in the upper parts of Unit I and then increases to ~25 x 10^–5 SI toward the lower part 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) and then fall to values around –1 x 10^–5 SI near 70 m CSF. Between ~110 and 150 m CSF, magnetic susceptibility values increase slightly and become highly variable (0 to 10 x 10^–5 SI). Magnetic susceptibility values are higher in the interval from 160 to 200 m CSF, coinciding with an observed decrease in Fe reduction (see "Lithostratigraphy"). Below 200 m CSF, the magnetic susceptibility signature is dominantly diamagnetic, with values close to zero. Magnetic susceptibility values slightly increase again in the basal 20 m of Unit II (below ~400 m CSF). NGR is elevated at the surface sediment (~73 counts per second [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. P-wave velocities are between 1460 and 1490 m/s in Unit I and the upper part of Unit II and then increase to >1500 m/s. Slightly below the ooze–chalk transition near 345 m CSF, velocities increase significantly, reaching 1600–1750 m/s at the bottom of Unit II. This partly explains the thicker sediment section than was expected from seismic data prior to coring (~60 m thicker). Bulk density and grain density increase with depth, with an increase in wet bulk density from 1.2 to 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% to 90% in Unit I to 50%–60% at ~300 m CSF in Unit II. Ephemeral whole-round samples were collected at ~96, ~196, and ~305 m CSF 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 (see "Stratigraphic correlation and composite 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). Features in magnetic susceptibility and gamma ray attenuation (GRA) density are well aligned down to a depth of 337 m CSF (Hole 1335A) and 344 m CSF (Hole U1335B), corresponding to ~398 m core composite depth below seafloor (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; 398.15 m CCSF-A). Stratigraphic correlation between individual holes indicates a growth factor (ratio between the CCSF-A and CSF 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 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 and below ~210 m CSF. We could not obtain any reliable paleomagnetic directions from this interval because the magnetic intensity after 20 mT alternating field (AF) demagnetization is on the order of 10^–5 A/m, which is comparable to the noise level of the superconducting rock magnetometer (see "Paleomagnetism"). 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, 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 declination. When combined with biostratigraphic age constraints (see "Biostratigraphy"), the data allow a continuous magnetostratigraphy from Chrons C1n (0–0.781 Ma) to C5n.2n (9.987–11.040 Ma) from 0 to 65.95 m CSF in Hole U1335A and from Chrons C1n to C5r.1n (11.118–11.154 Ma) from 0 to 66.225 m CSF in Hole U1335B. Below the bottom of the first magnetic low zone (~70–110 m CSF), magnetostratigraphy is again interpretable downhole: from Chrons C5Br (15.160–15.974 Ma) to C6n (18.748–19.722 Ma) from 155.35 to 208.40 m CSF in Hole U1335A and from Chrons C5AAn (13.015–13.183 Ma) to C5Er (18.524–18.748 Ma) from 107.95 to 202.60 m CSF 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 analyses of pore water and organic and inorganic sediment properties was undertaken on samples 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, respectively. Dissolved iron also shows three peaks, with concentrations up to 6 µM at ~6 m CSF, between 90 and 170 m CSF, and between 190 and 370 m CSF. 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 (see "Lithostratigraphy"). Lithium concentrations decrease from ~26 µM at the sediment surface to 5 µM at ~300 m CSF, 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, followed by a decrease toward basement. Calcium carbonate, inorganic carbon (IC), and total carbon (TC) contents were determined on sediment samples from Hole U1335A (Fig. F5). CaCO₃ contents ranged between 13 and 96 wt%. In the uppermost ~67 m, carbonate contents range from 12 to 87 wt%, and concentrations are then consistently high (~72–96 wt%) between 67 and 157 m CSF and below 222 m CSF. Carbonate contents vary more widely (between 37 and 89 wt%) from 157 to 222 m CSF. Total organic carbon (TOC) concentrations were determined by acidification and are generally low.

Wireline logging was not conducted at Site U1335. Five downhole temperature measurements were conducted in Hole U1335B with the advanced piston corer temperature tool (APCT-3) 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 (see "Geochemistry").

Highlights

Highly expanded Miocene sedimentary section

One of the highlights from Site U1335 is the recovery of a thick Miocene carbonate-dominated section from the central equatorial Pacific, one of the high-priority objectives of the PEAT program. The early Miocene (7.1 m.y. duration) is captured in ~190 m of sediment, corresponding to a sedimentation rate of 27 m/m.y. The middle Miocene (4.4 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 component, 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 calcium carbonate compensation depth (CCD) throughout this transition and during 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 site. Physical property data from Site U1335 provide an important contribution toward the Cenozoic megasplice, connecting with younger sediments from ODP Leg 138 (e.g., 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 light greenish gray carbonates that show a distinct peak in dissolved Fe concentrations, characteristic of a geochemical alteration front. At Site U1335, this zone is similar to but much thicker in total stratigraphic thickness (~70–170 and ~200–350 m CSF) than that observed at Site U1334 (~50 m; see the "Site U1334" chapter). 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 (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 (see "Geochemistry"), which have also been observed at 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 thick) of nannofossil foraminifer ooze that have sharp basal boundaries, many of which are irregular and some of which are inclined. These beds are interpreted as gravity flow deposits from the nearby seamounts and represent ~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). 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. F1B) situated ~15–20 km northeast and southeast of Site U1335, with present summit water depths that are 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.

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