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doi:10.2204/iodp.proc.320321.104.2010

Science summary

Three holes were cored at Site U1332 (11°54.722′N, 141°02.743′W; 4924 mbsl) (Fig. F1; Table T1), which is the second northwesternmost site drilled during the PEAT program. At Site U1332, seafloor basalt is overlain by ~150 m of pelagic sediment, containing radiolarian and nannofossil ooze with varying amounts of clay and zeolitic clay. The oldest sediment is of earliest middle Eocene age. Hole U1332A provided high-quality and high-recovery advanced piston corer (APC)-cored sediments from the mudline to 125.9 m core depth below seafloor (CSF) (Core 320-U1332A-14H), where we encountered porcellanite and chert and switched to the extended core barrel (XCB) cutting shoe. XCB coring advanced to 152.4 m drilling depth below seafloor (DSF) through a ~10 m thick porcellanite-rich interval with reduced recovery. In the basal section, we recovered a short, ~3.8 m long interval of barren very dense and stiff clay above basalt, ~10 m shallower than predicted from the seismic profile, in Core 320-U1332A-18X. Basement was reached at 152.4 m CSF. For detailed coring activities, see "Operations."

The uppermost 17.7 m consists of upper Miocene to Pleistocene–Pliocene clay, with varying amounts of radiolarians and zeolite minerals, overlying ~130 m of Oligocene to middle Eocene nannofossil and radiolarian ooze with porcellanite deep in the section. A thin ~3 m thick unit of middle Eocene zeolite clay bearing small porcellanite and chert nodules was recovered at the base of the sedimentary sequence, above basaltic basement. The sedimentary sequence at Site U1332 was divided into five major lithologies (Fig. F3).

The upper stratigraphy at Site U1332 has a strong resemblance to that of Site U1331 but without the sharp erosive contacts described at Site U1331. Several meters of white to beige-colored Pleistocene–Pliocene clay (lithologic Unit I) overlie lower Miocene to lowermost Oligocene nannofossil ooze (Units II and III). There is a sharp lithologic change at the Eocene–Oligocene transition to alternating radiolarian ooze with nannofossils and nannofossil ooze (Subunit IVa). The lithology gradationally changes downhole into a dominance of radiolarian nannofossil ooze and nannofossil radiolarian ooze (Subunit IVb) and then into an interval of alternating radiolarian ooze, radiolarian nannofossil ooze, and nannofossil radiolarian ooze with porcellanite layers (Subunit IVc). Lithologic Unit V is composed of very dark grayish brown to black clay, very dark grayish brown to black zeolite clay, and chert. The sediments directly above basaltic basement are partially lithified. Basalt is designated as lithologic Unit VI, at ~150 m CSF.

Carbonate content approaches 85 wt% in Unit III within the Oligocene nannofossil oozes and cycles between 0% and 40–60 wt% in the middle Eocene section (Unit IV) (Fig. F4). All major microfossil groups were found in sediments from Site U1332 and provide a consistent, coherent, and high-resolution biostratigraphic succession from basement to the top of Unit II. Calcareous nannofossils are abundant and moderately well preserved in the Oligocene and poor to moderately well preserved in the Miocene and Eocene. Most middle Eocene sediments commonly contain nannofossils; however, there are several barren intervals. Radiolarians are common to abundant throughout most of the section, apart from the lowermost sediment section above basalt. Radiolarians are well preserved in the Eocene and moderately well preserved in the Oligocene to lower Miocene section above. Radiolarian and nannofossil datums and zonal determinations agree, ranging from nannofossil Zones NP13/NP14 in the basal dark clay section (~48.4–50.7 Ma) to Zone NN1 and radiolarian Zones RP13 above basement through RN1 (lowermost Miocene, ~22.3 Ma) below the upper Pliocene–Pleistocene clay cover in Core 320-U1332A-3H (Fig. F4). Planktonic foraminifers are generally rare throughout the Oligocene but are absent in the Miocene and Eocene. Benthic foraminifers are present through most of the section but are rare in Miocene and Eocene sediments. They indicate lower bathyal to abyssal paleodepths. Sedimentation rates, as implied by biostratigraphic age determinations, vary throughout the section and are ~5 m/m.y. in the Eocene section and ~2.5 m/m.y. in the Oligocene, with two prominent hiatuses in the Miocene and between the Miocene and younger sediments. 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, to arrive at an integrated Cenozoic stratigraphy and age calibration (e.g., Pälike et al., 2006) for major parts of the Oligocene and Eocene.

Magnetostratigraphic studies as well as high-resolution biostratigraphy and stratigraphic correlation determined that a 4 m interval from the base of Core 320-U1332A-8H was repeated in the top of Core 9H, which comprises Chron C13n and the lowermost Oligocene. This repetition also occurs in Cores 320-U1332B-8H and 9H and within Core 320-U1332C-9H. The lithologic succession from the lower occurrence of Chron C13n downhole as well as from the upper occurrence of Chron C13n uphole both appear complete and continuous; hence Site U1332 achieved the fortuitous feat of recovering the complete Eocene–Oligocene transition four times and the upper part of Chron C13n five times at a triple-cored site. A likely explanation for this is the widespread occurrence of a slumped interval.

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 show a marked increase in the carbonate-rich Oligocene section, as well as in carbonate-bearing horizons in the Eocene (carbonate accumulation event [CAE] cycles; Lyle et al., 2005). Magnetic susceptibility is variable throughout the section, allowing a detailed correlation among holes. NGR measurements are elevated by an order of magnitude in the surficial clay layer. Porosity values are generally high in the radiolarian-rich sediments (85%) and decrease in the Oligocene and Eocene carbonate section, which also shows higher thermal conductivity values of ~0.9 to 1.2 W/(m·K), compared with ~0.8 W/(m·K) in the radiolarian oozes and surficial clay.

Stratigraphic correlation allowed us to obtain a complete section to ~125.5 m CSF near the top of the porcellanite interval in Hole U1332A, equivalent to a composite depth of ~140 m core composite depth below seafloor (CCSF-A) (see "Core composite depth scale" in the "Methods" chapter). The overall core expansion (growth factor), which is calculated by the ratio between the CCSF-A and CSF (formerly meters composite depth [mcd] and meters below seafloor [mbsf]) depth scales, is ~10%. The tops of APC cores were often affected by ~3 m heave that occurred during operations at Site U1332. Stratigraphic correlation supports the biostratigraphic, paleomagnetic, and sedimentologic description of a repeated sequence, possibly due to slumping, spanning the Eocene–Oligocene transition.

A full range of paleomagnetic analyses was conducted on cores and samples from Site U1332 and resulted in a well-resolved magnetostratigraphy. Shipboard analyses suggest that a useful magnetic signal is preserved in all APC-cored intervals and that it was possible to remove the drilling-induced steep inclination overprint by alternating-field demagnetization. Comparison of biostratigraphic data and changes in magnetic paleodeclinations suggests the recovery of magnetic reversals Chrons C1n/C1r.1r to C2An.3n/C2Ar above a hiatus and then a continuous sequence of magnetic reversals from Chrons C5En/C5Er (18.52 Ma) in the Miocene at ~12.95 m CSF (interval 320-U1332C-2H-4, 95 cm) to C19r/C20n (42.54 Ma) at interval 320-U1332A-14H-5, 80 cm. Magnetostratigraphic interpretation supports the presence of a slump through multiple recovery (five times) of parts of Chron C13n in a triple-cored sequence. 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 U1332. Alkalinity values increase from ~2.2 to 3.4 mM downsection, and Sr²⁺ increases from ~80 to ~110 µM. H₄SiO₄ remains relatively stable between 400 and 600 µM above 90 m depth in the Oligocene nannofossil oozes but increases to 800–1000 µM in the Eocene silica-rich radiolarian oozes. Carbonate coulometry yielded carbonate contents of ~85 wt% in the Oligocene nannofossil ooze and horizons with up to 60 wt% CaCO₃ in the middle Eocene radiolarian-rich oozes. Total organic carbon (TOC) contents were measured both by difference between total carbon (TC) and total inorganic carbon (IC) as well as by using an acidification method. Using the acidification method, TOC values were <0.3 wt% for all measured samples. The top ~5 m shows values of 0.18–0.17 wt% TOC. Between ~40 and 70 m CSF the measurements indicate TOC below the detection limit of 0.03 wt%, and downhole from this, values are generally low. We conducted a high-resolution Rhizon pore water experiment across an alkalinity trough around 40 m CSF, which highlighted comparisons between squeezed and Rhizon-sampled pore waters. Additional ephemeral samples were taken for shore-based microbiology and permeability studies.

Wireline logging provided valuable information to constrain the interval of porcellanite and chert formation within the borehole. Downhole NGR, density, and magnetic susceptibility logs provide important constraints on the poorly recovered lithologies below and between porcellanite-bearing horizons. The logging data document the presence of two thin porcellanite horizons at ~126 and 130 m wireline log depth below seafloor (WSF) and an ~14 m thick interval of increased magnetic susceptibility, reduced conductivity, and enhanced density and photoelectric factor that appears to be the dark and dense clays and zeolitic clays above basement, rather than carbonate. Integration with the seismic data will allow further improvements with the regional seismic interpretations. Data from Site U1332 indicate that the top of seismic Horizon P2 (Lyle et al., 2002) correlates with the top of the porcellanite section, just as it did for Site U1331. No Formation MicroScanner (FMS) data were collected, as it was not possible to retrieve the "paleo-" triple-combination (triple combo) tool string back into the bottom-hole assembly (BHA). Eight downhole temperature measurements were conducted in Holes U1332B and U1332C with the advanced piston corer temperature (APCT-3) tool. Three of these yielded good data; the other measurements were impaired by strong, sometimes >3 m heave during operations in Hole U1332B.

Downhole temperature measurements, when combined with the thermal conductivity values obtained from the cores, indicate that Site U1332 has a heat flow of 70.7 mW/m² and a thermal gradient of 75.0°C/km. This is significantly higher than the values obtained for Site U1331 but comparable to values obtained for Sites 1218 and 1219.

Highlights

Shallow early Eocene CCD

Coring at Site U1332 was designed to capture a very short period of time (~2 m.y.) at ~50 Ma during which this site was thought to be located above the very shallow Eocene CCD (~3.3 km) (Lyle, Wilson, Janecek, et al., 2002; Rea and Lyle, 2005) just after the EECO (Zachos et al., 2001). Unlike Site U1331, at Site U1332 we cored a ~10 m thick section of dense and dark brown clays, zeolite clays, and chert above basement, although relatively common nannofossils were present in the lowermost samples from Hole U1332B. This finding will provide important new constraints on the depth of the CCD at ~48–50 Ma at the paleoequator, indicating that the CCD was shallower than previously thought.

Stratigraphic integration

One of the primary objectives of the PEAT science program is the integration of different stratigraphic methodologies and tools. Site U1332 contains all major fossil groups (nannofossils, radiolarians, foraminifers, and diatoms), as well as an excellent magnetostratigraphy and composite depth correlation, which can be tied to nearby Leg 199 sites (e.g., Site 1220) by way of physical property variations. The possibility of a cycle-by-cycle match between Sites U1332 and 1220 has been demonstrated using magnetic susceptibility and bulk density data, providing additional stratigraphic tie points and a verification of the completeness of the stratigraphic section on a regional scale. Thus, Site U1332 will help us to achieve an integrated stratigraphy for the Cenozoic Pacific Ocean, ranging from the Miocene to the middle Eocene.

Eocene–Oligocene and Oligocene–Miocene transitions and depth transects

Site U1332 forms the second oldest and deepest component of the PEAT depth transect, 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 U1332 is estimated to have been ~4 km deep during the Eocene–Oligocene transition, ~1 km shallower than today and 200 m shallower at that time than Site U1331. Sediments rapidly change from radiolarian ooze below the transition into nannofossil oozes above, and unlike Site U1331, Site U1332 also contains carbonate-bearing sediments across the Oligocene–Miocene transition. For the Eocene–Oligocene transition, Site U1332 will provide a tie point for calcium carbonate burial at ~4° to 5° paleolatitude.

Variations in the CCD

Site U1332 has provided important constraints for variations and depth of the CCD from the early Eocene to the late Miocene. This site shows increased carbonate content and much increased mass accumulation rates approaching 200 mg CaCO₃/cm²/k.y. around the middle of Chron C18r to the base of Chron C19r during the middle Eocene, which can be correlated to an interval of enhanced carbonate burial that was previously documented by Lyle et al. (2005) in Leg 199 cores. The early Oligocene high CaCO₃ concentrations decrease significantly in sediments younger than ~27 Ma. By ~22 Ma, in the early Miocene, carbonate was no longer preserved. This is presumably related to Site U1332 sinking below the prevalent CCD and coincides with a CCD shoaling event between ~20 and 15.5 Ma described by Lyle (2003).

Formation of porcellanite and chert

Together with Site U1331, Site U1332 provides important new information on the formation of porcellanite and chert. Coring has shown that the top of the porcellanite-rich interval is mapped by seismic Horizon P2 (Lyle et al., 2002). In lithologic Subunit IVc, layers and pebbles of very dark brown partially to well-lithified mudstones, often layered or even laminated, are observed within alternating sequences of nannofossil ooze and radiolarian ooze of late to late middle Eocene age. In hand specimen, the partially lithified mudstones are particularly rich in clay and show evidence of partial secondary silicification. Pieces of porcellanite contain clay minerals, microcrystalline quartz, opaques, and calcite, as well as biogenic shells and fragments from radiolarians and foraminifers. Sediments from Sites U1331 and U1332 appear to document the silicification process in clay-rich horizons near basement, which will likely extend the findings of Moore (2008).

Age transect of seafloor basalt

At Site U1332 we recovered what appear to be fresh fragments of seafloor basalt, aged between 49 and 50 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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