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doi:10.2204/iodp.proc.320321.109.2010

Science summary

The latest Oligocene through the middle Miocene appears to have been a time of relative warmth comparable to the latest Eocene. However, variability in the isotopic record of the early to middle Miocene is larger than that of the Eocene and may indicate more variability in climate and global ice volume. Site U1337 (proposed Site PEAT-7C; 3°50.009′N, 123°12.352′W; 4463 mbsl) (Fig. F1; Table T1) was targeted to collect an early middle Miocene segment of the PEAT equatorial megasplice on ~24 Ma crust between the Galapagos and Clipperton Fracture Zones, ~390 km southeast of Site U1335. In conjunction with Sites U1335 and U1336, it was also designed to provide a latitudinal transect for early Miocene age slices. The recovered sediment column at Site U1337 represents a nearly complete and continuous Neogene sedimentary section.

Operations

Four holes were cored at Site U1337. In Hole U1337A, advanced piston corer (APC) cores were taken from the seafloor to 195.5 m drilling depth below seafloor (DSF) (Cores 321-U1337A-1H through 21H). Nonmagnetic core barrels were used for all APC cores except for Core 321-U1337A-21H. FlexIt core orientation was conducted for all cores except Core 321-U1337A-1H. In addition, five successful advanced piston corer temperature tool (APCT-3) temperature measurements were taken with Cores 321-U1337A-5H, 7H, 9H, 11H, and 13H. Extended core barrel (XCB) coring continued with Cores 321-U1337A-22X through 48X. The sediment/basement contact was recovered at the base of Core 321-U1337A-48X. Three logging strings (triple combination [triple combo], Formation MicroScanner [FMS]-sonic, and Versatile Seismic Imager [VSI]) were deployed in Hole U1337A.

In Hole U1337B, APC cores were taken from the seafloor to 245.2 m DSF (Cores 321-U1337B-1H through 27H). Nonmagnetic core barrels were used through Core 321-U1337B-20H. The FlexIt core orientation tool was deployed successfully for all but two APC cores (321-U1337B-17H and 18H). FlexIt and steel core barrels were used through Core 321-U1337B-27H. APCT-3 measurements were obtained with Cores 321-U1337B-15H, 17H, and 19H. Coring continued with a single XCB core (321-U1337B-28X) to 251.9 m DSF; however, this barrel could not be recovered and Hole U1337B was abandoned prematurely.

Hole U1337C was cored to recover sections that were missing from Holes U1337A and U1337B. APC cores were taken from the seafloor to 11.4 m DSF (Cores 321-U1337C-1H through 2H) using nonmagnetic core barrels and the FlexIt core orientation tool. A wash barrel (Core 321-U1337C-3W) was then deployed, and the hole was washed to 169.4 m DSF. APC coring resumed at that depth and continued through Core 321-U1337C-9H to 221.3 m DSF and then switched to steel core barrels. Coring with the XCB system continued with Cores 321-U1337C-10X through 33X. Basement was recovered in Core 321-U1337C-33X.

Hole U1337D was planned to target the few remaining areas that had yet to be fully recovered and to duplicate recovery through those sections of the formation already recovered to provide additional sample material. The most troublesome material encountered in the previous holes was the large diatom mats located directly above and below a hard ~0.4 m thick porcellanite layer. In Hole U1337D, APC cores were taken from the seafloor to 237.7 m DSF (Cores 321-U1337D-1H to 26H). Nonmagnetic core barrels were used through Core 321-U1337D-20H. The first XCB core (321-U1337D-27X) was designed to only core through the hard ~0.4 m thick porcellanite layer. The APC was once again deployed and cored to 267.0 m DSF (Cores 321-U1337D-28H through 30H). At this point the XCB coring system was once again deployed for Cores 321-U1337D-31X through 49X to a total depth of 442.9 m DSF. The FlexIt core orientation tool was deployed successfully with all APC cores. The Sediment Temperature Tool (SET) was deployed for the first time from the R/V JOIDES Resolution after Core 321-U1337D-17X at 298.1 m DSF.

Lithostratigraphy

At Site U1337, latest Oligocene seafloor basalt is overlain by ~450 m of nannofossil and biosiliceous oozes and nannofossil chalks that are divided into four lithologic units (Fig. F3). The Pleistocene through uppermost Miocene sediments of lithologic Unit I are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil oozes, diatom oozes, and radiolarian oozes that alternate on meter scales with a general downsection increase in siliceous microfossils relative to nannofossils. The uppermost Miocene to middle Miocene lithologic Unit II is composed of green and gray biosiliceous sediments interbedded on meter scales with white and light greenish gray nannofossil ooze. Laminated decimeter-thick diatom ooze ("mat") deposits are commonly found in the unit. Meter-scale color variability in both lithologic Units I and II are associated with variations in lithology and physical properties. However, there is also millimeter- and centimeter-scale color banding that is also likely a response to redox conditions. White, pale yellow, and pale green nannofossil oozes and chalks with low abundances of silica fossils dominate the Unit III sediments of middle Miocene to latest Oligocene age. Seafloor basalt (lithologic Unit IV) was recovered at the base of the sedimentary section, dated as latest Oligocene.

Biostratigraphy

All major microfossil groups occur in the sediments recovered at Site U1337. Planktonic foraminifers at Site U1337 are rare to abundant with poor to good preservation throughout most of the succession but are absent or extremely rare in some intervals of the upper Miocene and lower Miocene. Biozones PT1b to O6 are recognized, with the exception of Zones PL4, M12, and M3 (Fig. F3). Calcareous nannofossils at Site U1337 are moderately to poorly preserved and some samples with high silica content are barren. Nannofossil Zones NN1 to NN21 are present, indicating an apparently complete sequence. The radiolarian stratigraphy at Site U1337 spans the interval from the uppermost part of Zones RN16–RN17 (upper Pleistocene) to RN1 (lower Miocene). The radiolarian assemblages of Pleistocene to upper Miocene age tend to have good preservation, whereas middle to lower Miocene assemblages show moderate preservation. In the lowermost part of the section, above the basement, sediments are barren of radiolarians. The high-resolution diatom stratigraphy at Site U1337 spans the interval from the Fragilariopsis (Pseudoeunotia) doliolus Zone (upper Pleistocene) to the lowermost part of the Craspedodiscus elegans Zone (lower Miocene). The diatom assemblage is generally well to moderately preserved throughout the recovered section; however, in several intervals valve preservation becomes moderate to poor. The base of the sediment column is barren of diatoms. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal schemes generally agree, though some discrepancies occur in the lowest part of the core. Benthic foraminifers occur continuously throughout the succession recovered in Hole U1337A and show good to moderate preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths.

Stratigraphic correlation

Stratigraphic correlation provided a complete spliced record to ~220 m core composite depth below seafloor (CCSF-A; see "Core composite depth scale" in the "Methods" chapter). Several gaps were encountered over the next 50 m CCSF-A. Comparison of gamma ray attenuation (GRA) density records with well logging density data suggest that no more than 1 m of section was lost in any of the gaps. Correlation between the holes was broken again several times between 440 m CCSF-A and basement at 490 m CCSF-A. Growth factor for the correlation was 1.12. The linear sedimentation rate decreases from ~21 m/m.y. in the middle Miocene to 17 m/m.y. in the late Miocene.

Paleomagnetism

Paleomagnetic measurements were conducted on archive-half sections of 20 APC cores and 14 XCB cores from Hole U1337A, 27 APC cores from Hole U1337B, 8 APC cores from Hole U1337C, and 30 APC cores from Hole U1337D. The FlexIt core orientation tool was deployed in conjunction with all APC cores, and we conclude that the FlexIt orientation data are generally reliable. Measurements of natural remanent magnetization (NRM) above ~93 m core depth below seafloor (CSF) indicate moderate magnetization intensities (on the order of 10^–3 A/m) with a patchy but generally weak viscous remanent magnetization (VRM) or isothermal remanent magnetization (IRM) drilling overprint, and polarity reversal sequences from Chrons C1n to C3r (0 to ~6 Ma) are recognized. Below ~93 m CSF, remanent intensities after alternating-field (AF) demagnetization of 20 mT are reduced to values close to magnetometer noise level in the shipboard environment (~2 × 10^–5 A/m). In this zone, sediment magnetizations have been partly overprinted during the coring process, and remanent inclinations are occasionally steep after AF demagnetization at a peak field of 20 mT. Nonetheless, polarity reversals are apparently recorded to ~200 m CSF and are provisionally correlated to the geomagnetic polarity timescale (GPTS) from Chrons C3An to C5n (~6–11 Ma) (Fig. F3). Magnetic polarity interpretation was impossible for APC cores taken with steel core barrels and XCB cores because of severe magnetic overprint during coring.

Physical properties

Physical property measurements on whole-round sections and samples from split cores display a strong lithology-dependent variation at Site U1337 (Fig. F3). A complete physical property program was conducted on whole cores, split cores, and discrete samples. Whole-Round Multisensor Logger (WRMSL) (GRA bulk density, magnetic susceptibility, P-wave velocity, and electrical noncontact resistivity), thermal conductivity, and natural gamma radiation (NGR) measurements comprised the whole-core measurements. Compressional wave velocity measurements on split cores and moisture and density (MAD) analyses on discrete core samples were made at a frequency of 1 per undisturbed section in Cores 321-U1337A-1H through 48X. Compressional wave velocities were measured toward the bottom of sections. MAD analyses were located 10 cm downsection from carbonate analyses (see "Geochemistry"). Lastly, the Section Half Multisensor Logger (SHMSL) was used to measure spectral reflectance on archive-half sections. Variations in the abundances of nannofossils, radiolarians, diatoms, and clay in lithologic Unit I account for high-amplitude, high-frequency variations of all physical properties. Intervals enriched in biogenic silica and clay generally display lower grain density and bulk density and higher porosity, magnetic susceptibility, and NGR. Velocity is generally directly related to bulk density; however, it is commonly higher in low-density siliceous-rich sediments than it is in more calcareous intervals. Wet bulk density is low in Unit I, ranging from 1.12 to 1.46 g/cm³. Porosity is as high as 92% in this unit. Velocity also is low, averaging 1525 m/s. The natural gamma record, as at previous sites, is marked by an unusually high near-surface peak (~65 counts per second [cps]). Magnetic susceptibility varies between 4 × 10^–5 and 18 × 10^–5 SI. The color of Unit I is characterized by the lowest L* and high and variable a* and b* values. Lithologic Unit II is characterized by a continued high variability in grain density. Together, the grain density in Units I and II averages 2.51 g/cm³ and ranges from 2.17 to 2.85 g/cm³. All other physical properties display less variability in Unit II than in Unit I, reflecting a less variable lithology. Wet bulk density increases and porosity decreases with depth in Unit II; however, in Units II and III these trends are interrupted by low-density high-porosity diatom- and radiolarian-rich intervals. Unit II is slightly lighter colored (lower L*) and distinctly more blue (lower a*) and green (lower b*) than Unit I. Unit III is characterized by more uniform physical properties that accompany the high and uniform carbonate composition of the unit. The nannofossil oozes and chalks of this unit are characterized by a uniform grain density that averages 2.67 g/cm³. The bulk density and porosity trends of Unit II continue in Unit III. The transition from ooze to chalk is marked by a change in gradient of these properties to a more rapid decrease in wet bulk density and an increase in porosity with depth. Wet bulk density and porosity at the base of the sediment section are 1.95 g/cm³ and 47%, respectively. The increase in velocity with depth also changes to a higher gradient in Unit III, with values increasing from 1510 m/s at ~340 m CSF to ~1800 m/s near the base of the hole. Magnetic susceptibility and NGR values remain low in Unit III but do vary in response to small changes in lithology. The sharp color change from greenish gray to pale yellow at ~410 m CSF is marked by a sharp increase in a* and b*. The change in color to pale brown chalk immediately above basement is marked by an increase in both a* and b* and a decrease in L*.

Downhole logging

Three downhole logging tool strings were deployed in Hole U1337A. Two tool strings took measurements of NGR radioactivity, bulk density, electrical resistivity, elastic wave velocity, and borehole resistivity images in the 77–442 m wireline log depth below seafloor (WSF) depth interval. The third tool string measured seismic waveforms in a vertical seismic profile (VSP) experiment in the 214–439 m WSF depth interval. Measurement depths were adjusted to match across different logging runs, obtaining a wireline log matched depth below seafloor (WMSF) depth scale. The downhole log measurements were used to define three logging units. Unit 1 (77–212 m WMSF) and Unit 2 (212–339 m WMSF) have average densities of ~1.3 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas Unit III (339–442 m WMSF) density increases with depth, reaching 1.85 g/cm³ at the base of the hole (Fig. F4). Resistivity and P-wave velocity follow a pattern similar to that of density, suggesting that the major control on these physical properties are variations in sediment porosity. NGR measurements are low throughout the logged interval (~5 gAPI), except for two pronounced peaks caused by uranium, one at the seafloor and the other at 240 m WMSF. The gamma ray peak at 240 m WMSF corresponds to the ~40 cm thick pocellanite layer that has only been recovered as rubble in the cores but can be clearly identified in the downhole logs and borehole images as an interval of high density and resistivity. VSP logging measured arrival time of the seismic pulse from the sea surface at 16 stations. Together with the traveltime to the seafloor, VSP measurements are the basis for a traveltime-depth conversion that allows seismic reflectors to be correlated to stratigraphic events. Downhole temperature measurements and thermal conductivities of core samples were combined to estimate a geothermal gradient of 32.4°C/km and a heat flow of 28.4 mW/m² at Site U1337.

Geochemistry

A total of 85 interstitial water samples were collected from Hole U1337A, 49 using the whole-round squeezing approach across the entire hole and 36 in the upper 100 m by Rhizon sampling. Alkalinity increases slightly downhole from ~2.7 mM in the upper 100 m to values scattered around 3.8 mM below 300 m CSF. Sulfate concentrations vary between 26 and 29 mM, with slightly decreasing values with depth. A dissolved manganese peak of ~150 µM at 13  m CSF is captured by the high-resolution interstitial water sampling. Dissolved iron is sporadically detectable in the upper 200 m and then increases to a peak of ~5 µM between 275 and 300 m CSF before becoming undetectable again below 400 m CSF. These variations in manganese and iron reflect changes in redox chemistry that also manifest as changes in sediment color. Calcium carbonate and inorganic carbon concentrations were determined on 283 and 28 sediment samples from Holes U1337A and U1337B, respectively. Calcium carbonate contents vary greatly in the upper two lithologic units, ranging from 30 to 90 wt% (Fig. F3). In lithologic Unit III calcium carbonate contents are generally high, scattered around 80 wt%, but a distinctive decrease is observed between 350 and 400 m CCSF-A. In the upper 235 m CCSF-A, total organic carbon (TOC) content ranges between 0.10 and 0.34 wt% except for the high value of 0.72 wt% in the uppermost sample. TOC content increases at 44.00 m CCSF-A and in the interval from 87.28 to 108.59 m CCSF-A. Below 235 m CCSF-A, TOC values are generally <0.10 wt%.

Shipboard geochemical analyses of interstitial water and bulk sediment samples reflect large variations in sediment composition resulting from shifts in carbonate versus opal production. The large-scale redox state and diagenetic processes of the sediment column are related to overall changes in sediment composition. Interstitial water chemistry is also influenced by the porcellanite layer forming a barrier to diffusion at ~240 m CSF and by seawater circulation in the basement. The basement itself appears to exert little influence on the geochemistry of sediments and interstitial waters.

Highlights

Diatom mat deposition

Lithologic Unit II at Site U1337 is mostly composed of biosiliceous lithologies, notably diatoms. The abundance of diatoms in the middle and upper Miocene section at Site U1337 is much higher than encountered in any interval at Sites U1331–U1336. Several decimeter- to meter-scale intervals of diatom ooze are laminated, and smear slide analyses indicate that the diatom assemblage is composed primarily of pennate taxa, with abundant "needlelike" Thalassiothrix spp., indicating diatom mat deposition. The lowermost laminated diatom mat is in the upper portion of Unit III at ~15 Ma. Much thicker intervals are present in Unit II at roughly 10 Ma and shorter intervals at ~4.5 Ma. Ages of laminated diatom mats at this site are similar to those found at Ocean Drilling Program (ODP) Leg 138 sites farther to the east (Mayer, Pisias, Janecek, et al., 1992), which have been interpreted to reflect regional bursts of silica export in the eastern equatorial Pacific (Kemp and Baldauf, 1993). No laminated diatom oozes were recorded during Expedition 320 at drill sites farther to the northwest.

Oligocene–Miocene transition

The Oligocene/Miocene boundary was recovered in Holes U1337A, U1337C, and U1337D. In Hole U1337A, the Oligocene/Miocene boundary is estimated to fall between Samples 321-U1337A-48X-2, 85–87 cm, and 48X-3, 55 cm (445.56–446.75 m CSF; 490.92–492.11 CCSF-A). It occurs in white (2.5Y 8/1) nannofossil chalk with foraminifers, interbedded and heavily mottled with pale yellow (2.5Y 7/4) to very pale brown (10YR 7/4) nannofossil chalk. Abundant millimeter-scale dendritic manganese oxide grains occur throughout this interval. The lower 15 cm of the core catcher of Core 321-U1337A-48X is basaltic basement. No prominent change in lithology, GRA bulk density, reflectance, or magnetic susceptibility is seen through the Oligocene–Miocene transition.

Neogene carbonate dissolution

The CCD of the Neogene is much more stable than that of the Eocene, but there are intervals of lower carbonate deposition at Site U1337 that probably represent significant changes of the Neogene CCD. In the early Miocene, a significant carbonate low reaches its minimum at ~17 Ma (340 m CSF in Hole U1337A), when the site was at a depth of ~3500 meters below sea level. This early Miocene interval marks a significant carbonate minimum at Site U1334 as well, on crust with a depth of ~4000 m at that time. Highly variable carbonate is also characteristic of the late/middle Miocene boundary interval, but the role of carbonate dissolution versus elevated deposition of biosilica needs to be determined.

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