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doi:10.2204/iodp.proc.320321.109.2010

Geochemistry

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 a chert layer forming a diffusive boundary at ~240 m CSF and seawater circulation in the basement. The basement itself appears to exert little influence on the geochemistry of the sediments and interstitial waters.

Sediment gases sampling and analysis

A total of 45 headspace gas samples were taken from Hole U1337A at a frequency of one sample per core as part of the routine environmental protection and safety monitoring program. The concentration of methane (C₁) in the samples ranged between 0.9 and 1.3 ppmv. No hydrocarbon gases higher than C₁ were detected.

Interstitial water sampling and chemistry

A total of 85 interstitial water samples were collected from Hole U1337A, 49 using the whole-round squeezing method across the entire hole and 36 in the upper 100 m CSF by Rhizon sampling (Table T17). Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. Rhizon samplers were inserted into the middle of the whole round through holes predrilled in the end cap at the bottom of sections. This allowed Rhizon samples to be taken adjacent to squeezed whole-round samples and facilitates direct comparison of the two interstitial water sampling techniques in the same hole. Analyses of squeezed and Rhizon-sampled interstitial waters were conducted during the same analytical sessions, and the data from adjacent Rhizon and squeeze samples agree, within the measurement uncertainties, for virtually all analytes (Table T17; Fig. F24). Notable exceptions are the pH values, which may have increased during the time Rhizon sample splits waited to be measured, and iron, which is higher in some squeezed samples.

Chloride ion concentration (not corrected for Br– contribution) varies slightly with depth and ranges from 554 to 566 mM (Fig. F24). Chloride values increase from 554 to 565 mM in the upper section with a peak between 50 and 100 m CSF, potentially reflecting the more saline bottom waters of the Last Glacial Maximum (e.g., Adkins and Schrag, 2003). However, the precision of the potassium chromate indicator technique is poor compared to the observed variation in chloride, and a potentiometric titration technique (Adkins and Schrag, 2003) would be required to reveal more detail. Alkalinity increases slightly downhole from ~2.7 mM in the upper 100 m CSF 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. An enormous dissolved manganese peak of nearly 150 µM at 13 m CSF is captured by the high-resolution interstitial water sampling. Below this, manganese values remain very high at ~100 μM until 100 m CSF, where values decrease in a stepwise manner to barely detectable concentrations at 250 m CSF. Dissolved iron is sporadically detectable in the upper 200 m CSF 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, which also manifest as changes in sediment color. For example, below 412 m CSF in Hole U1337A a sharp transition from pale green to yellow sediments occurs (see "Lithostratigraphy") and the dissolved iron concentration drops below detection. Such tandem changes in sediment color and interstitial water chemistry are well documented for the uppermost sediments where iron hosted in smectites is reduced (Lyle, 1983). However, to find such a redox front in a reversed position, extending from the basement upward, is surprising and suggests seawater has circulated through the basement for some time.

The silicic acid (dissolved silicate) content of the interstitial waters is substantially greater than that of bottom waters (e.g., Peng et al., 1993) and increases with depth from ~700 µM in the uppermost sediments to peak at ~1200 µM at ~350 m CSF before decreasing to ~900 µM near the basaltic basement. This strong enrichment in silicic acid almost certainly originates from the dissolution and diagenesis of biogenic silica. Interestingly, the downcore pattern of silicic acid does not correspond directly with the occurrence of the biosiliceous sediments, which are mostly concentrated between 93.35 and 214 m CSF in lithologic Unit II (see "Lithostratigraphy"). Furthermore, no change in silicic acid was found around the chert layer at ~240 m CSF. Instead, the maximum silicic acid concentrations occur in the carbonate-rich sediments of lithologic Unit III below the ooze to chalk transition (see "Lithostratigraphy").

Calcium concentrations display very little variation downcore, with values increasing from a seawater value (~10.5 mM) to a maximum of 13 mM between 300 and 350 m CSF before decreasing to seawater values in the bottom of the hole (Fig. F24). Magnesium and potassium concentrations deviate even less from seawater values, but a slight decrease with depth is evident. As the alteration of basaltic basement exchanges potassium and magnesium for calcium (e.g., Geiskes, 1981), the lack of strong gradients in these elements suggests the basement is no longer reactive at this site.

Lithium concentrations decrease from ~26 µM at the surface to ~17 µM at 150 m CSF. Below this, lithium values are relatively constant until 240 m CSF, where a dramatic decrease to ~7 µM is observed. From 250 to 340 m CSF lithium concentrations are low, fluctuating between 5 and 10 µM, but then increase with depth to almost seawater values near the basement. The interstitial water strontium profile is a mirror image to that of lithium with a slight increase with depth to 240 m CSF, where a sharp increase to ~180 µM occurs. This maximum in strontium extends for the next ~100 m downhole before values decrease toward seawaterlike values near the basement. The lithium and strontium profiles reveal a strong barrier to diffusion at 240 m CSF where a chert/porcellanite layer was found (see "Lithostratigraphy" and "Downhole measurements"). A distinctive decrease in dissolved sulfate also occurs below this chert layer, whereas other tracers vary little across this physical boundary. Below the chert layer a source of strontium to the pore waters exists, which is most likely the dissolution and recrystallization of biogenic calcite (e.g., Baker et al., 1982). Conversely, there seems to be a sink for dissolved lithium below the chert layer, but the nature of this sink remains unknown. The lithium and strontium profiles, along with the calcium, magnesium, and silicic acid profiles, all indicate seawater circulation in the basement, as their values tend toward seawater values near the basement.

The concentration of barium is below the detection limit (0.8 µM in an undiluted sample) in all but four samples, which is not surprising given the high dissolved sulfate concentrations throughout the sediments. Boron concentrations range between 440 and 400 µM, showing a slight decrease from top to basement.

Bulk sediment geochemistry

Major and minor elements

At Site U1337, bulk sediment samples at a frequency of one per core were analyzed for sulfur, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, barium, copper, chromium, scandium, strontium, vanadium, yttrium, and ziconium (Table T18).

The aluminum, magnesium, iron, and barium contents of the bulk sediment all follow a similar pattern, decreasing from a maximum in the upper 100 m CSF to a low at ~250 m CSF before a distinctive peak at ~350 m CSF, followed by a further decrease toward the base of the sediment column. The calcium and strontium content of the bulk sediments effectively mirrors that of the aluminum group, indicating an important role played by dilution with carbonate. The SiO₂ content of the bulk sediment is distinctive, with a broad peak between 40% to 50% in the uppermost 100 m CSF, a smaller second peak between 150 and 250 m CSF, and a third peak of ~30% SiO₂ centered around 350 m CSF. Between 190 and 250 m CSF three samples have SiO₂ contents >50% and one sample is as SiO₂ rich as 70% (Fig. F25).

Sedimentary inorganic and organic carbon

Calcium carbonate and inorganic carbon concentrations were determined on 283 sediment samples from Hole U1337A and 28 from Hole U1337B (Table T19; Fig. F26). Cores 321-U1337A-22X through 25X were recovered by XCB, and as discontinuity and core disturbance was expected in this interval, additional samples were taken from Hole U1337B. Calcium carbonate concentrations vary greatly in the upper two lithologic units (see "Lithostratigraphy"), from 30 to 90 wt%, reflecting the alternation between calcite and opal. 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.

The concentration of TOC was determined for 47 sediment samples from Holes U1337A and U1337B (Table T19; Fig. F26). In the upper 235 m CCSF-A, 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%; however, transient increases to 0.74 and 0.17 wt% are observed in samples from 267.46 and 441.08 m CCSF-A, respectively. Such downhole variability of TOC content is most likely related to lithology with higher TOC being found in the more siliceous rich intervals.

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