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Science summary

Three holes were cored at Site U1331 (12°04.088′N, 142°09.708′W; 5116 mbsl), which is the northwesternmost site drilled during the PEAT program (Fig. F1; Table T1). In Hole U1331A, seafloor basalt is overlain by 187.02 m of pelagic sediment, containing radiolarian and nannofossil ooze with varying amounts of clay. The oldest sediment is of early Eocene age. For detailed coring activities, see "Operations."

The sediment column at Site U1331 has a strong resemblance to that of Site 1220 (Fig. F3) (Lyle, Wilson, Janecek, et al., 2002), but with noteworthy sharp erosive contacts concentrated within the upper two-thirds of the section (Fig. F4). A total of 7 m of Pleistocene–Pliocene clay overlies lower Oligocene to lower Eocene alternations of nannofossil ooze, radiolarian nannofossil ooze, and radiolarian oozes of varying clay and calcium carbonate content (see "Lithostratigraphy"), with a sharp lithologic change at the Eocene–Oligocene transition (~26 m core depth below seafloor [CSF]). The lowermost target interval in Hole U1331C is characterized by a ~20 m thick radiolarian ooze and porcellanite layer from 157 to 177 m CSF overlying a dominantly clayey radiolarian ooze and zeolitic clay with hydrothermal red staining from 177 to 187.5 m CSF deposited on top of a thin (187.6–188.5 m CSF in Hole U1331C) layer of calcareous ooze and zeolitic clay above the basalt. Some fine-grained basaltic fragments show fresh glassy chilled margins.

Carbonate content approaches 80% in the Oligocene nannofossil oozes in the upper part of the site and cycles between 0% and 40% in the middle Eocene section (see "Geochemistry"). A concentration of sharp erosive contacts occurs in the interval between 80 and 120 m CSF, with calcareous material dominating the basal portion of these contacts and then fining upward in grain size into radiolarian oozes. Rarely, the sediment above a sharp contact contains well-rounded clasts up to 1 cm in diameter (interval 320-U1331B-10H-6, 117–130 cm). The lithologic and stratigraphic characteristics of these sediments have been interpreted as gravity-driven deposits, possibly from nearby seamounts ~10 km to the south (Fig. F1B). Between ~177 and 187.2 m CSF, Cores 320-U1331A-22X, 320-U1331C-16H, and 17H achieved our site objective of recovering carbonate-bearing material from the time interval just following 52 Ma.

All major microfossil groups were found in sediments from Site U1331, and they provide a consistent and coherent biostratigraphic succession from basement to below the surficial clay layer. Nannofossils are common in the Oligocene and lower Eocene but sporadic in sediments from the upper Eocene because of dissolution. Middle Eocene sediments commonly contain calcareous nannofossils punctuated by several barren intervals, notably below Zone NP21 (radiolarian Zone RP19 equivalent), below Zone NP17, and between Zones NP15 and NP13 (radiolarian Zones RP12–RP8 equivalent). Radiolarians are common to abundant throughout the section. Radiolarian and nannofossil datums and their age determinations agree and range from nannofossil Zone NP12 in the basal sediment section (~51–53 m.y. before present) to Zones NP23/NP24 and radiolarian Zones RP8 just above basement through RP21 (late Oligocene, older than 25 Ma) in the uppermost section, below the Pleistocene clays. Both radiolarian and nannofossil assemblages contain reworked older components (deeper than ~50 m CSF) but within a coherent and ordered stratigraphy. Planktonic foraminifers are generally absent, except for sporadic occurrences often associated with sediment just above sharp lithologic contacts and also in the basal carbonate section (Zones E4/E5). Benthic foraminifers are generally rare and indicate lower bathyal to upper abyssal paleodepths. They are frequently found in the graded coarse sediment above the base of sharp contacts but indicate there is no apparent difference in the depth habitat between benthic foraminifers from just above sharp contacts and other parts of the section.

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

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

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

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

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

Wireline logging provided valuable information to constrain the interval of porcellanite formation within the borehole, and further analysis will aid in interpretations of carbonate content and lithologies. Integration with the seismic data will allow further improvements with the regional seismic interpretations. Data from Site U1331 indicate that the top of seismic Horizon P2 (Lyle et al., 2002) correlates with the top of the chert section. Downhole temperature measurements with the advanced piston corer temperature (APCT-3) tool, when combined with the thermal conductivity values obtained from the cores, indicate that Site U1331 had a thermal gradient of 13.4°C/km and a heat flow of 10.3 mW/m2. This is within the range of lower values in the global heat flow data set for the eastern Pacific but significantly lower than values obtained for ODP Sites 1218 and 1219.


Carbonate ooze in basal section

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

Stratigraphic integration

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

Eocene–Oligocene transition and depth transect

Site U1331 forms the oldest and deepest component of the PEAT depth transect 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 U1331 is estimated to have been ~4.2 km deep during the Eocene–Oligocene transition, ~800 m shallower than today. Sediments rapidly change from radiolarian ooze below the Eocene–Oligocene transition to nannofossil oozes above and will provide a tie point for calcium carbonate burial at ~5° paleolatitude.

Variations in the CCD

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

Age transect of seafloor basalt

At Site U1331 we recovered what appear to be fresh fragments of seafloor basalt, aged between 52 and 53 Ma as estimated from biostratigraphic results. This material will, when combined with other PEAT basalt samples, be of importance for the study of basalt alteration by seawater and paleomagnetic properties of oceanic crust.