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doi:10.2204/iodp.proc.320321.106.2010

Science summary

Three holes were cored at Site U1334 (7°59.998′N, 131°58.408′W; 4799 mbsl) (Fig. F1), targeting the events bracketing the Eocene–Oligocene transition as part of an investigation of the wider Cenozoic climatic evolution (e.g., Zachos et al., 2001) and providing data toward a depth transect across the Oligocene (see "Eocene/Oligocene Boundary [Site U1334; 38 Ma crust]" in the "Expedition summary" chapter) that will allow exploitation and verification of a previous astronomical age calibration from Site 1218 (Pälike et al., 2006a).

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

The topmost ~47 m thick lithologic Unit I contains a 15 m thick interval of brown radiolarian clay overlying ~32 m of alternating radiolarian clay and nannofossil ooze. The uppermost section (320-U1334A-1H-CC) is of late Miocene age (radiolarian Zone RN7; ~8.5 Ma). Below, Unit II comprises a ~200 m thick succession of upper Miocene to Oligocene nannofossil ooze and chalk above a ~35 m thick sequence of upper Eocene nannofossil chalk, radiolarite, and claystone (Unit III). Basal lithologic Unit IV (~1 m thick) consists of middle Eocene intercalated micritic chalk and limestone on basalt (Figs. F4, F5).

Holes U1334A–U1334C provided high-quality APC-cored sediments from the mudline to ~210 m core depth below seafloor (CSF) (Cores 320-U1334A-22H, 320-U1334B-22H, and 320-U1334C-22H). Below this depth we encountered increasingly stiffer and harder sediment, after which we switched to the extended core barrel (XCB) cutting shoe. XCB coring advanced to 288.5 m drilling depth below seafloor (DSF) through lower Oligocene and Eocene sediments with high recovery. At the base of the holes, an intercalated unit of basalt and hard micritic chalk and limestone occurs below a 10–20 m thick section of nannofossil ooze and chalk. For detailed coring activities, see "Operations."

Carbonate content exceeds 92 wt% in the upper lower Miocene below Section 320-U1334A-5H-3 and remains high throughout the Oligocene. Eocene sediments still contain considerable amounts of carbonate, and nannofossil ooze and chalk are dominant lithologies apart from several short less carbonate rich intervals (e.g., Section 320-U1334A-28X-3). In the middle Eocene, carbonate content cycles between ~40 and 85 wt% (Fig. F6), with higher values encountered toward the basal part of the Eocene section. Two short intervals in the upper Eocene (~249 to ~257 m CSF) exhibit carbonate content of <20 wt%.

A series of middle Oligocene cores (Cores 320-U1334A-16H through 21H) were recovered that had very distinct colors ranging from light grayish green to light blue. These uniquely colored carbonate oozes exhibit extremely low magnetic susceptibilities that complicated stratigraphic correlation. These oozes have lost almost their entire magnetic susceptibility signal from ~145 to ~215 m CSF (Figs. F5, F7). Similar colored cores have previously been described for DSDP Sites 78 and 79 (Hays et al., 1972).

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

All major microfossil groups occur in sediments from Site U1334 and provide a consistent, coherent, and high-resolution biostratigraphic succession spanning a near-continuous sequence from the middle Miocene to the uppermost middle Eocene. The uppermost 12 m of radiolarian clay is barren of calcareous microfossils but contains radiolarians of middle Miocene age, similar to the site survey piston Core RR0306-08JC (Lyle et al., 2006). Nannofossil ooze and radiolarian clays are present in the Miocene and Eocene parts of the section, with nannofossil ooze dominant in the thick Oligocene section. Radiolarians are present through most of the section, apart from the lowermost cores, and are well preserved in the Eocene. They provide a coherent high-resolution biochronology and indicate a complete sequence of radiolarian zones from RN7 (upper Miocene) to RP17 (uppermost middle Eocene). Calcareous nannofossils are present and moderately to well preserved through most of the succession, and there appears to be a complete sequence of nannofossil zones from NN6 (middle Miocene) to NP17 (uppermost middle Eocene), providing a minimum age estimate for basaltic basement of 38 Ma. In the Eocene, the base of Chiasmolithus oamaruensis is determined in Sample 320-U1334A-30X-1, 66 cm, and the top of Chiasmolithus grandis in Sample 320-U1334-30X-2, 74 cm. Intriguingly, both species are mid- to high-latitude taxa (Wei and Wise, 1989) and are present only rarely and sporadically at Site U1334. Planktonic foraminifers are present through most of the succession and are relatively abundant and well preserved from the lower Miocene to the lower Oligocene. The lower Miocene is characterized by the presence of Dentoglobigerina spp., Paragloborotalia siakensis–mayeri, Paragloborotalia kugleri, and Paragloborotalia pseudokugleri. Oligocene sediments contain Catapsydrax spp., Paragloborotalia opima-nana, and characteristic Dentoglobigerina spp. Preservation and abundance of planktonic foraminifers is more variable in the middle Miocene and upper Eocene/lowermost Oligocene. No Eocene/Oligocene boundary marker hantkeninids were identified. Benthic foraminifers are present through most of the section and indicate lower bathyal to abyssal paleodepths.

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

A full physical property program was run on cores from Site U1334. This program comprises Whole-Round Multisensor Logger (WRMSL) measurements of magnetic susceptibility, bulk density, and P-wave velocity, along with 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 U1334A. All track data are variable throughout the section, allowing a detailed correlation between different holes, with the exception of a very low magnetic susceptibility signal within an interval extending slightly above and below the light greenish gray tinted cores of Unit II, between ~140 and 205 m CSF. Magnetic susceptibility varies between 10 × 10^–5 and 40 × 10^–5 SI in Unit I, oscillates around 5 × 10^–5 to 10 × 10^–5 SI above the colored sediments, and then drops to near zero and negative values, returning to values around 10 × 10^–5 SI in the lower part of Unit II and Subunit IIIa. NGR increases slightly at the Eocene/Oligocene boundary at ~246 m CSF (from 4 to 7 counts per second [cps]). P-wave velocity remains uniform through the upper 150 m of sediment (varying around 1500 m/s) but increases rapidly below the ooze/chalk boundary to ~1600 m/s. This explains the slightly thicker sediment section than was expected from seismic data prior to coring (~20 m thicker). For Hole U1334B, no P-wave velocity WRMSL data were collected between ~125 and 240 m CSF to allow for a more timely stratigraphic correlation of cores within the iron reduction–dominated colored cores with the gamma ray attenuation (GRA) instrument. Bulk density and grain density increase gradually with carbonate content to ~204 m CSF to a maximum of ~1.8 g/cm³ and then show stepped decreases in the lower part of this succession. Ephemeral whole-round samples were collected at ~50 and ~165 m for shore-based studies of sediment permeability.

WRMSL data were used to achieve stratigraphic correlation among holes at Site U1334. Magnetic susceptibility was initially the main parameter used for real-time correlation, as a second loop of the susceptibility meter is mounted on the Special Task Multisensor Logger (STMSL); the second bulk density instrument on this track was not working. In the very low (sometimes negative) magnetic susceptibility interval between ~140 and ~205 m CSF (Cores 320-U1334A-16H through 21H), the signal was not useful for correlation, and we measured the corresponding cores from Hole U1334B out of sequence to establish the amount of core overlap using bulk density. The coring effort in Hole U1334C was successful at covering gaps between cores at this site to ~111 m core composite depth below seafloor (CCSF-A; see "Core composite depth scale" in the "Methods" chapter), as well as from 250 to 335 m CCSF-A, almost to the bottom of the section. The correlation was challenging between the three holes at Site U1334 in the greenish–light gray interval (Cores 320-U1334A-15H through 22H, 320-U1334B-14B through 22H, and 320-U1334C-14H through 22H) and in the bottom 80 m of the section, where XCB coring compromised the GRA density variations that would otherwise help stratigraphic correlation. Visual inspection, comparison with core imagery, and biostratigraphic datums were used to establish and verify hole to hole correlation where track data lacked clearly identifiable features. Stratigraphic correlation between individual holes indicates a growth factor (ratio between the CCSF-A and CSF depth scales) of ~16%. Stratigraphic correlation resulted in a complete splice through the Eocene–Oligocene transition almost to basement (~38 Ma).

A full range of paleomagnetic analyses was conducted on 66 advanced piston corer (APC) cores and 188 discrete samples from Site U1334 for the APC-cored section of Site U1334 (upper ~209 m). Unlike Sites U1331 and U1332, the drilling overprint was generally weak for Site U1334 cores, but only for those collected with nonmagnetic core barrels (Cores 320-U1334A-1H through 16H, 320-U1334B-1H through 15H, and 320-U1334C-1H through 15H). In contrast, those cores collected with steel core barrels are highly overprinted. The overprint is so severe that even demagnetization at 20 mT only partially removes it. This extreme overprint notably degrades the paleomagnetic declination data, as can be noted by their higher variability, which makes polarity determination much more difficult in the intervals collected with steel core barrels. The problem is exacerbated by the decay in the intensity (and magnetic susceptibility) that occurs at ~135 m CSF in all three holes as a result of reduction diagenesis. Even within the highly reduced interval, an interpretable signal was present prior to switching to steel core barrels. Magnetic susceptibility in the upper 45 m of Hole U1334A averages ~18 × 10^–5 SI (volume normalized) and decreases to a mean of 6 × 10^–5 SI from 45 to 135 m CSF. A notable low occurs from ~140 to 204 m CSF, where the average susceptibility is 0.6 × 10^–5 SI. This low interval is associated with a change in sediment color from yellowish tan to very light green, blue, and gray at ~143 m CSF and another abrupt change to reddish brown tones at ~192 m CSF, which corresponds to middle early Oligocene (~30 Ma). Just below 205 m, magnetic susceptibility steps up to an average of 5 × 10^–5 SI and then increases again across the Eocene/Oligocene boundary (~245 m) to an average of 18 × 10^–5 SI. The magnetostratigraphy in Hole U1334A has been interpreted from the top of Chron 11r (29.957 Ma), which occurs ~55 cm below the top of Section 320-U1334C-21H-4 (~195 m CSF) through the base of Chron C3n.4n (5.235 Ma) in Core 320-U1334A-1H (~4 m CSF). The youngest sediments recovered are in the upper ~2 m of Core 320-U1334A-1H, which record Chrons C1n through C2r.1r.

A standard shipboard suite of geochemical analyses of pore water and organic and inorganic sediment properties was undertaken on samples from Site U1334. We also conducted a high-resolution (one per section) Rhizon pore water investigation across the interval's middle Oligocene cores (320-U1334C-16H through 21H) that exhibited colored sediments. Site U1334A is marked by alkalinities between 3 and 4 mM throughout. The most striking features in the interstitial water geochemistry are a dissolved manganese peak from ~20 to ~240 m CSF with a maximum of ~6 µM at ~110 m CSF and a dissolved iron peak as high as >15 µM centered at 165 m CSF. The depth range of the dissolved iron peak, indicative of iron oxide reduction, coincides with the colorful interval seen in the lithology and with the interval of low magnetic susceptibilities (~140–205 m CSF). Sulfate results indicate limited sulfate reduction. Calcium carbonate contents are low in the uppermost ~35 m of Site U1334, and calcium carbonate contents are generally high (~80 wt%) below the uppermost clay layer.

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

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

Highlights

Eocene–Oligocene and Oligocene–Miocene transitions and depth transects

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

Geochemical front

At Site U1334 we recovered a ~50 m thick interval of light greenish gray carbonates that show a distinct peak in dissolved Fe concentrations, characteristic of a geochemical alteration front. A similar but much thicker alteration zone is also observed at IODP Site U1335 and provides the opportunity to study organic matter degradation while these sites migrate from south to north through the equatorial belts of high productivity.

Age transect of seafloor basalt

At Site U1334 we recovered what appear to be fresh fragments of seafloor basalt, aged ~38 Ma as estimated from biostratigraphic results. 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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