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doi:10.2204/iodp.proc.320321.105.2010

Science summary

Three holes were cored at Site U1333 (10°30.996′N, 138°25.159′W; 4853 mbsl) (Fig. F1; Table T1). At Site U1333, seafloor basalt is overlain by ~183 m of pelagic sediment, dominated by nannofossil and radiolarian ooze with varying amounts of clay (Fig. F3). The oldest sediment is of early middle Eocene age.

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

Coring in Hole U1333B started 5 m shallower than in Hole U1333A to recover the mudline and to span the core gaps from the first hole. A total of 7.73 m of carbonate-bearing ooze overlain by a few meters of clay were recovered in Core 320-U1333B-1H. Although the cores recovered from Hole U1333A showed significant porcellanite layers, we used the APC drillover strategy in Hole U1333B to obtain APC cores across and below the Eocene–Oligocene transition to 162.7 m CSF. We then XCB cored to basement and a total depth of 180.3 m CSF.

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

The sediment column at Site U1333 has a strong resemblance to that of Site 1218 (Lyle, Wilson, Janecek, et al., 2002) but with notably more carbonate-bearing sediments in the Eocene portion. The ~183 m of pelagic sediments has been divided into four major lithologic units (Fig. F4; Table T2). Unit I is ~7 m thick and contains an alternating sequence of clay, clayey radiolarian ooze, radiolarian clay, clayey nannofossil ooze, and nannofossil ooze of early Miocene age. Unit II is ~112 m thick and composed of alternating very pale brown nannofossil ooze and yellowish brown nannofossil ooze with radiolarians of early Miocene to latest Eocene age. Unit III is ~60 m thick and composed of Eocene biogenic sediments comprising clayey nannofossil ooze, nannofossil radiolarian ooze, nannofossil ooze, radiolarian nannofossil ooze, and porcellanite of latest Eocene to middle Eocene age (Unit III). Unit III is divided into two subunits, based on the absence (Subunit IIIa) or presence (Subunit IIIb) of porcellanite, which occurs between ~168 and 174 m CSF. Unit IV is a thin unit (~3.3 m) of lithified carbonate (partly limestone) and nannofossil ooze, overlying basalt (Unit V).

All major microfossil groups were found in sediments from Site U1333 and provide a consistent, coherent, and high-resolution biostratigraphic succession from basement to the top of lithologic Unit II. Shipboard biostratigraphy indicates that sediments recovered at Site U1333 span a near-continuous succession from around the lower Miocene boundary to the middle Eocene. Radiolarians are common and well preserved in the Eocene succession but less well preserved in the Oligocene sediments. A complete sequence of radiolarian zones from RN2 to RP14 (middle Eocene) was described. Initial assessment of the radiolarian assemblages across the Eocene/Oligocene boundary interval indicates a significant loss of diversity through this apparently complete succession. Although a few species from the Eocene carry through to the Oligocene, only one stratigraphic marker species (Lithocyclia angusta) first appears near the Eocene/Oligocene boundary. Calcareous nannofossils are present and moderately to well preserved through most of the succession, although there are some short barren intervals in the middle to upper Eocene. The succession spans a complete sequence of nannofossil zones from lower Miocene Zone NN1 to middle Eocene Zone NP15. The Oligocene/Miocene boundary is bracketed by the base of Sphenolithus disbelemnos in Sample 320-U1333A-2H-5, 70 cm (16.20 m CSF), and the presence of rare Sphenolithus delphix in Sample 320-U1332A-2H-CC (9.57 m CSF). Discoasters are very rare in basal assemblages, indicative of a eutrophic environment and consistent with the paleolatitude of this site in the early middle Eocene within the equatorial upwelling zone. Planktonic foraminifers are relatively abundant and well preserved from the lowest part of the Miocene to the lower Oligocene. Oligocene fauna is characterized by the common presence of Catapsydrax spp., Dentoglobigerina spp., and Paragloborotalia spp. In contrast, upper Eocene sediments contain poorly preserved specimens or are barren of planktonic foraminifers. Preservation and abundance slightly increased in some intervals of the middle Eocene, which is recognized by the presence of acarininids and clavigerinellids. The absence of the genera Globigerinatheka and Morozovella makes precise age determination of individual samples problematic. High abundances of Clavigerinella spp. have been linked to high-productivity environments, consistent with the paleogeographic location of this site (Coxall et al., 2007). Benthic foraminifers were almost continuously present and indicate lower bathyal to abyssal depths. Oligocene fauna is characterized by calcareous hyaline forms, such as Nuttallides umbonifer, Oridorsalis umbonatus, and Cibicidoides mundulus. Nuttallides truempyi and O. umbonatus often dominate the Eocene fauna. Benthic foraminifers are present through most of the section apart from an interval in the middle Eocene equivalent to radiolarian Zone RP16. They indicate lower bathyal to abyssal paleodepths.

Sedimentation rates at Site U1333 are ~5 m/m.y. in the middle Eocene section (~39–45 Ma), and ~4 m/m.y. between the early late Eocene and the early Oligocene (~31 Ma). In the early Oligocene, sedimentation rates increase to ~12 m/m.y. and then reach ~6 m/m.y. from the late Oligocene to the early Miocene in the upper sediment column.

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

Geochemistry results indicate that samples from the upper part of Site U1333 have modest CaCO₃ contents of 26–69 wt% between 0 and 4 m and have frequent variations between 58 wt% and up to 93 wt% in the interval between 4 and 35 m CSF. Calcium carbonate contents are consistently high (75.5–96 wt%) from 35 to 111 m CSF, whereas in the Eocene (between 111 and 171 m CSF) CaCO₃ contents vary abruptly between <1 and 74 wt%. The lowermost lithified carbonate rocks between 173 and 180 m CSF have high CaCO₃ contents between 76 and 90 wt%. TOC content, as determined by the acidification method, is generally very low. Pore water alkalinity values are never elevated, but alkalinity and dissolved strontium values are somewhat higher near the Eocene–Oligocene transition; these are generally consistent with carbonate dissolution or recrystallization processes. Dissolved silica increases with depth, with values always <1000 µM.

A full physical property program was run on cores from Holes U1333A–U1333C comprising 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 properties, sound velocities, and thermal conductivity on Hole U1333A cores only. All track data show variability throughout the section, allowing a detailed correlation among holes primarily using magnetic susceptibility and density (magnetic susceptibility varies around 24 x 10^–5 SI in radiolarian ooze–dominated sections and ~3 x 10^–5 SI in more carbonate rich intervals). Magnetic susceptibility values gradually increase uphole. NGR measurements are elevated by an order of magnitude in the uppermost clays and increase near the lower Oligocene at ~115 m CSF (from 5 to 8 counts per second [cps]). P-wave velocity gradually increases downhole as we move from carbonate- to radiolarian-dominated successions. P-wave velocity generally varies between 1490 and 1560 m/s depending on lithology, with lower velocities corresponding more to carbonate-rich sections. Bulk density and grain density show a marked decrease at ~112 m CSF (~1.70 to 1.31 g/cm³ in bulk density), where carbonate content decreases rapidly. Porosity values are generally high in the radiolarian-rich sediments (80%) and decrease in the carbonate-rich section (~60%). Thermal conductivity measurements are increased in carbonate-rich intervals and range from ~0.8 W/(m·K) in lithologic Unit I to 1.2–1.3 W/(m·K) in lithologic Unit II.

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

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

Highlights

High carbonate fluctuations in middle Eocene sediments

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

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

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

Carbonate-bearing sediments across the Oligocene–Miocene transition were also recovered at Site U1333, adding important data to the study of this time interval in the context of the PEAT Oligocene/Miocene depth transect.

Age transect of seafloor basalt

At Site U1333 we recovered what appear to be fresh fragments of seafloor basalt overlain by sediments aged 45 to 46 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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