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doi:10.2204/iodp.proc.320321.110.2010

Science summary

Site U1338 (proposed Site PEAT-8D; 2°30.469′N, 117°58.178′W; 4200 mbsl) was sited to collect a 3–18 Ma segment of the PEAT equatorial megasplice and is located on ~18 Ma crust just north of the Galapagos Fracture Zone, 324 nmi (600 km) southeast of Site U1337 . A seamount (3.7 km water depth) with surrounding moat is found ~25 km north-northwest of Site U1338 at the downslope end of the survey area. Originally a site was chosen ~10 km from the seamount (proposed Site PEAT-8C). However, the alternate proposed site was selected and drilled uphill and further away from the seamount to avoid possible turbidites, as were found near seamounts during drilling of Expedition 320 Sites U1331 and U1335. The recovered sediment column at Site U1338 represents a nearly complete and continuous lower Miocene to Holocene sedimentary section.

Operations

Four holes were cored at Site U1338. From Hole U1338A, advanced piston corer (APC) cores were taken from the seafloor to 221.2 m drilling depth below seafloor (DSF) (Cores 321-U1338A-1H through 24H) using nonmagnetic core barrels and the FlexIt core orientation tool installed. FlexIt and steel core barrels were used for Cores 321-U1338A-25H and 26H. In addition, five successful advanced piston corer temperature tool (APCT-3) temperature measurements were taken with Cores 321-U1338A-5H, 7H, 9H, 11H, and 13H. Extended core barrel (XCB) coring continued with Cores 321-U1338A-27X through 44X. A small piece of basement was recovered in the core catcher of Core 321-U1338A-44X.

From Hole U1338B, APC cores were taken from the seafloor to 188.1 m DSF (Cores 321-U1338B-1H through 20H) except for a short drilled interval of 2.5 m from 235.6 to 238.1 m DSF to adjust the core breaks. Nonmagnetic core barrels and the FlexIt core orientation tool were used through Core 321-U1338B-20H. FlexIt and steel core barrels continued through Core 321-U1338B-42H to 387.4 m DSF. Coring continued with three XCB cores (321-U1338B-43X through 45X) to 416.1 m DSF. Basement contact was recovered in Core 321-U1338B-45X. Three logging strings (triple combination [triple combo], Formation MicroScanner [FMS]-Sonic, and Versatile Sonic Imager [VSI]) were deployed in Hole U1338B.

Hole U1338C was cored to recover sections that were missing from Holes U1338A and U1338B. APC cores were taken from the seafloor to 189.8 m DSF (Cores 321-U1338C-1H through 21H) using nonmagnetic core barrels and the FlexIt core orientation tool. FlexIt and steel core barrels were used through Core 321-U1338C-44H to 396.9 m DSF. Coring continued through Core 321-U1338C-47H to a total depth of 414.4 m DSF, which, at the time, set a new all time depth record for the APC.

Hole U1338D was primarily planned to recover a few "instructional" cores to be used during Expedition 323. Three APC cores were cut to 23.9 m DSF.

Lithostratigraphy

At Site U1338, ~415 m of nannofossil ooze and chalk with varying concentrations of diatoms and radiolarians overlie the seafloor basalt and are divided into three lithologic units (Fig. F3). Pleistocene through middle Pliocene sediments of Unit I are characterized by multicolored (various hues of white, brown, green, and gray) nannofossil ooze, diatom nannofossil ooze, and radiolarian nannofossil ooze that alternate on a decimeter to meter scale. Light green and light gray nannofossil ooze with occasional darker intervals with abundant siliceous microfossils, notably diatoms, comprise the upper Miocene to middle Pliocene Unit II. Decimeter-, meter- and tens of meters–scale color alternations in Units I and II are associated with variations in lithology and physical properties. Some of these color changes, as well as common millimeter- and centimeter-scale color banding, are not associated with compositional changes and likely reflect variations in sediment redox state. White, pale yellow, light greenish gray, and very pale brown nannofossil oozes and chalks dominate Unit III of the lower to upper Miocene, although slightly darker green and gray intervals with larger amounts of siliceous microfossils remain present. Seafloor basalt (Unit IV) was recovered at the base of the sedimentary section, overlain by lower Miocene sediments.

Biostratigraphy

All major microfossil groups have been found in the ~415 m thick succession of Holocene to lower Miocene sediment bulge recovered from Site U1338. Calcareous nannofossils at Site U1338 are in general moderately preserved, but there are some intervals in which the preservation is good or poor. Nannofossil Zones NN4 to NN21 are present, indicating an apparently complete sequence. Planktonic foraminifers vary from rare to abundant, with moderate to good preservation throughout most of the succession, but are absent or rare in a short interval in the upper Miocene. Planktonic foraminifer Zones PT1b (upper Pleistocene) to M2 (lower Miocene) are documented, with the exception of Zones PL4, M12, and M6. The radiolarian stratigraphy spans the interval from the uppermost part of Zones RN16–RN17 (upper Pleistocene) to the uppermost part of Zone RN3 (lower Miocene). Radiolarian assemblages show good to moderate preservation except in the lowermost portion (lower Miocene), which is barren of radiolarians. The high resolution diatom stratigraphy 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, there are several intervals in which valve preservation becomes moderate to poor. The nannofossil, foraminifer, radiolarian, and diatom datums and zonal schemes generally agree, with some inconsistencies (Fig. F3). Benthic foraminifers occur continuously throughout the succession recovered in Hole U1338A and show generally good preservation. The overall assemblage composition indicates lower bathyal to abyssal paleodepths.

Stratigraphic correlation

Stratigraphic correlation provided a complete spliced record to a depth of ~260 m core composite depth below seafloor (CCSF-A) (see "Core composite depth scale" in the "Methods" chapter). Several gaps were seen between 280 and 360 m CCSF-A. Comparison of gamma ray attenuation (GRA) density records with well logging density data suggests that no more than 1 m of section was lost in any of the gaps. Correlation between the holes became difficult again several times between 435 m CCSF-A and basement at 460 m CCSF-A. The growth factor for the correlation was 1.11. The linear sedimentation rate decreases from ~29 m/m.y. in the Miocene to 13 m/m.y. in the Pliocene–Pleistocene.

Paleomagnetism

Paleomagnetic measurements were conducted on archive-half sections of 26 APC cores from Hole U1338A, 42 APC cores from Hole U1338B, and 47 APC cores from Hole U1338C. The FlexIt core orientation tool was deployed in conjunction with all APC cores except for the deepest three cores of Hole U1338C, and we conclude that the FlexIt orientation data are generally reliable. Natural remanent magnetization (NRM) measurements indicate moderate magnetization intensities (on the order of 10^–3 A/m) for depth intervals 0–50, 280–225, and 295–395 m core depth below seafloor (CSF). Polarity reversal sequences of these intervals are provisionally correlated to Chrons C1n to C2Ar (0 to ~4 Ma), Chrons C4An to C5n (~9–11 Ma), and Chrons C5r to C5Br (~12–16 Ma) of the geomagnetic polarity timescale (GPTS), respectively (Fig. F3). Except for these intervals, remanent magnetic 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). Magnetization directions are dispersed and not interpretable there.

Physical properties

A complete physical property program was conducted on whole cores, split cores, and discrete samples. Whole-Round Multisensor Core 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 one per undisturbed section. 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. Physical property measurements on whole-round sections and samples from split cores display a variation strongly dependent on the relative abundance of biosiliceous and calcareous sediment components at Site U1338. As at Site U1337, intervals enriched in siliceous microfossils and clay generally display darker colors, lower grain density and bulk density, and higher porosity, magnetic susceptibility, and NGR. The variation of velocity is more complex in that it is dependent on both the wet bulk density and the sediment rigidity. These parameters vary independently with the variation in abundance of biosiliceous and calcareous components. The physical properties at Site U1338 also display cyclicity on multiple scales, a decimeter to meter scale and a scale with a spacing on the order of tens of meters.

Lithologic Unit I at Site U1338 is characterized by low wet bulk density that decreases from 1.4 g/cm³ near the seafloor to 1.2 g/cm³ at the base of the unit as a result of an increasing abundance of radiolarians and diatoms with depth. The grain density in Units I and II displays a greater variability than is found deeper at the site as a result of the greater variability in the abundance of biosiliceous and calcareous components. The average grain density for Units I and II is relatively low, at 2.59 g/cm³. The NGR signal at Site U1338 is characterized by a near-seafloor peak that is somewhat lower than those recorded at the other PEAT drill sites but extends deeper and is marked by a double peak. Spectral reflectance measurements show that Unit I is characterized by lower L* and higher a* and b* values in the upper 25 m of Unit I (Fig. F3). Below 25 m CSF, the sediment becomes lighter colored (L* increases) and more bluish green (a* and b* decrease).

Unit II is characterized by increasing wet bulk density with depth to ~175 m CSF. Below this depth, an increase in the abundance of siliceous microfossils produces a broad density minimum. Magnetic susceptibility and NGR signals are low in Unit II to the depth at which the biosiliceous material increases in abundance. The interval of the broad density minimum is characterized by higher magnetic susceptibility values that are roughly equal to those in the upper 25 m of Unit I. Unit II is lighter colored than Unit I (higher L*) and more blue (lower b*).

Unit III at Site U1338 is characterized by a higher and more uniform carbonate content and, as a result, more uniform physical properties. Wet bulk density increases from ~1.5 g/cm³ at the top of Unit III to 1.7 g/cm³ at the base of the unit. Grain density varies over a narrower range in Unit III than it does in Units I and II and displays an average (2.64 g/cm³) nearer to that of calcite. Velocity, which through much of Units I and II is close to the velocity of water, displays a regular increase in Unit III, from ~1620 m/s at the top to ~1820 m/s near the base of the unit. Velocity gradient increases near the base of Unit III accompanying the transition from nannofossil ooze to chalk. Magnetic susceptibility is low from the boundary between Units II and III, at ~245 m CSF, to 300 m CSF. Below 300 m CSF, susceptibility again increases to values comparable to those in the upper part of Unit I. NGR variability is lower in Unit III than in Unit II and remains uniformly low throughout the unit. Overall, Unit III is the lightest colored (highest L* values) unit at Site U1338. The transition from greenish gray to pale yellow is marked at ~385 m CSF by a shift to higher values of both a* and b*.

Downhole logging

Three downhole logging tool strings were deployed in Hole U1338B: a modified triple combo (that did not include a neutron porosity measurement), an FMS-sonic combination, and a VSI seismic tool with a Scintillation Gamma Ray (SGT-N) sonde. The modified triple combo and FMS-sonic tool strings took downhole measurements of natural gamma ray radioactivity, bulk density, electrical resistivity, elastic wave velocity, and borehole resistivity images in the 125–413 m wireline log depth below seafloor (WSF) depth interval. The VSI seismic tool string measured seismic waveforms in a vertical seismic profile (VSP) experiment that covered the 189.5–414.5 m WSF depth interval. Measurement depths were adjusted to match across different logging runs, obtaining the wireline log matched depth below seafloor (WMSF) depth scale.

Downhole log measurements were used to define three logging units: Unit I (139–244 m WMSF) and Unit II (244–380 m WMSF) have average densities of ~1.45 and ~1.6 g/cm³, respectively, that do not show any trend with depth, whereas in Unit III (from 380 m WMSF) density increases with depth, reaching 1.7 g/cm³ at the base of the hole (Fig. F4). Resistivity and P-wave velocity follow a pattern similar to that of density throughout the logged interval, suggesting that the major control on these physical properties are variations in sediment porosity. Both resistivity and density measurements show a small-scale peak at 280 m WMSF. This peak at 280 m WMSF is clearly visible in the borehole resistivity images as a high-resistivity layer 16 cm thick, and it corresponds to a chert layer that has only been recovered as rubble in the cores. Natural gamma ray measurements are low throughout (~4 gAPI) but do show a pronounced high at the seafloor caused by a local increase in uranium concentration.

In the VSP experiment, the arrival time of a seismic pulse was measured from the sea surface at 14 stations. Together with the traveltime to the seafloor, the 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 34.4°C/km and a heat flow of 33.6 mW/m² at Site U1338.

Geochemistry

A standard shipboard suite of geochemical analyses of pore water and organic and inorganic sediment properties was undertaken on samples from Site U1338. Alkalinity increases slightly downhole from ~2.7 mM at the sediment/water interface to peak slightly above 4 mM at 140 m CSF. A large dissolved manganese peak of 150 mM at 10 m CSF is captured by the high-resolution interstitial water sampling and is remarkably similar to that observed at Site U1337. These peaks are >10 times larger than the highest dissolved manganese concentrations encountered during Expedition 320. Lithium concentrations decrease from ~26 µM at the surface to a minimum of ~3 µM at ~250 m CSF before increasing sharply with depth to seawater values at the base of the section. The interstitial water strontium profile is a mirror image to that of lithium except the decrease from the peak of 400 µM at 200 m CSF is punctuated by a sharp drop of >100 µM between ~260 and 290 m CSF. The lithium and strontium profiles indicate seawater circulation in the basement as their values tend toward seawater values near the basement.

Calcium carbonate contents range between 26 and 88 wt% with substantial variability in the upper 273.31 m CCSF-A, corresponding to the alternation between calcite and opal production in the upper two lithologic units. Below 273.31 m CCSF-A (lithologic Unit III), calcium carbonate contents become relatively high and stable between 66 and 91 wt% compared with the upper part of the stratigraphic column (Fig. F3). In the upper ~230 m CCSF-A, total organic carbon (TOC) content is generally high and variable ranging between 0.09 and 0.46 wt%, whereas below ~230 m CCSF-A, TOC content is <0.09 wt%. Downhole TOC variability is most likely related to lithologic changes, with higher TOC being found in the more biosiliceous intervals.

Interstitial water and bulk sediment geochemistry reflect large variations in sediment composition resulting from shifts between carbonate and opal dominance. The large-scale redox state and diagenetic processes of the sediment column are related to overall changes in sediment composition. Interstitial water chemistry points to seawater circulation in the basement, although the basement itself appears to exert little influence on the geochemistry of the sediments and interstitial waters.

Highlights

Color changes, lithology, and redox state

Smear slide analyses and visual core descriptions show that many of the decimeter-, meter-, and tens of meters–scale color variations in lithologic Units I and II to some extent relate to changes in lithology (e.g., Fig. F3). We suspect, however, that some of these color variations, notably the transitions between pale green and pale yellow lithologies, are controlled by sediment redox state, similar to those recorded at Sites U1331–U1337 and earlier work in the equatorial Pacific Ocean (e.g., Lyle, 1983).

Magnetic susceptibility is moderately low in the light gray and light brown intervals in Unit I (Fig. F3). A significant decrease in magnetic susceptibility in Unit II suggests dissolution of magnetite resulting from intensified microbial Fe reduction. In the lower part of Unit III, a sharp downcore transition from green to yellow is not associated with any other lithologic change, does not occur at the same stratigraphic level between holes, and thus should not be considered as an equivalent time horizon. Pore water Fe concentrations reach 6 to 7 µM in the green interval, and Fe is absent below the transition to yellow and brown. Although some of this signal may be affected by seawater contamination during XCB drilling, all available information suggests that the lowermost color change represents a redox front.

Occurrence of diatom-rich layers

Lithologic Unit II at Site U1338 is mainly composed of nannofossil ooze with relatively high abundances of biosiliceous components, notably diatoms (Fig. F3). The relative abundance of diatoms is lower than that at Site U1337, and the record lacks laminated diatom ooze intervals (diatom mats) such as those observed at Site U1337. However, centimeter to sometimes 1–2 m thick diatom nannofossil ooze layers containing abundant specimens of Thalassiothrix spp. are occasionally interbedded with nannofossil ooze (e.g., ~126.2–127.1 and ~231.8–234.3 m CSF in Hole U1338A and ~127.3–128.0 and ~233.8–234.8 m CSF in Hole U1338C). Units II and III also contain significant amounts of pyrite, particularly in diatom-rich intervals in Unit II (e.g., Cores 321-U1338B-14H, 19H through 21H, 26H, 28H, 29H, and 32H through 41H). In addition, the middle part of Unit III contains thin intervals of abundant pyrite-filled siliceous microfossils (e.g., intervals 321-U1338B-33H-4, 58–66 cm, and 35H-5, 76–82 cm). These diatom-rich layers, pyrite nodule occurrences, and pyrite-rich siliceous microfossil layers in Units II and III are associated with high TOC content, suggesting a relation between the abundance of diatoms in the sediments, sediment redox state, and the export or preservation of organic carbon.

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