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doi:10.2204/iodp.proc.320321.108.2010

Geochemistry

Sediment gases sampling and analysis

Headspace gas samples were taken at a frequency of one sample per core in Hole U1336A as part of the routine environmental protection and safety monitoring program. All headspace sample analyses resulted in nondetectable levels of methane (C₁; <2 ppmv), with no higher hydrocarbons.

Interstitial water sampling and chemistry

Twenty-two interstitial water samples were collected from Hole U1336B (Table T16) in the APC-cored interval using the whole-round squeezing method. Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Methods" chapter. Chlorinity varies from ~555 mM in the shallowest sample to ~565–570 mM in deeper samples (Fig. F18). Chlorinity values have a distinct increase from ~555 to ~570 mM in the uppermost 40 m CSF, potentially reflecting the change from the more saline ocean at the Last Glacial Maximum to the present (Adkins and Schrag, 2003). Alkalinity is relatively constant at >2.5 mM in the upper 110 m CSF, with a pronounced decline to 1 mM by 170 m CSF. Sulfate concentrations decrease with depth to values of 22 mM. Dissolved phosphate concentrations are ~5 µM at ~9 m CSF, decreasing to ~1 µM by ~15 m CSF. Dissolved manganese has a broad peak to >5 µM from ~25 to 120 m CSF and then returns to ~1.8 µM in the deepest four samples. Below 100 m CSF, dissolved iron increases coincident with manganese decline, with concentrations reaching >10 µM below 130 m CSF.

Because of the relatively high sulfate concentrations, dissolved Ba concentrations are below detection (0.5 µM). Concentrations of dissolved silicate increase with depth from <400 to 800 µM.

Calcium and magnesium concentrations increase and decrease with depth, respectively, with calcium values from 10 to 29 mM and magnesium values from 50 to 28 mM (Fig. F18). This is indicative of diffusional control of Mg and Ca profiles between seawater at the top and the reaction zone of fluid and basalt in the underlying basement (e.g., Gieskes, 1981).

Lithium concentrations decrease from ~26 µM at the surface to <10 µM below 130 m CSF, and potassium concentrations decrease with depth to values as low as 8 mM. Strontium concentrations increase from near-seawater values in the shallowest sample to >400 µM in the deepest samples, the highest values seen during this expedition. Boron concentrations range between 440 and 490 µM, showing a slight decrease downcore.

Bulk sediment geochemistry

Calcium carbonate (CaCO₃) and inorganic carbon concentrations were determined on sediment samples from Hole U1336A (Table T17; Fig. F19), and calcium carbonate, inorganic carbon, and total organic carbon (TOC) concentrations were determined on 39 sediment samples from Hole U1336B. TOC was not run on samples from Hole U1336A because samples from Hole U1336A were accidently sent to the Gulf Coast Repository during the Expedition 320/321 port call (Table T17; Fig. F19). Calcium carbonate concentrations ranged between 48 and 96 wt%. In the uppermost ~90 m CSF, CaCO₃ concentrations vary between 48 and 95 wt%, with a distinct low at 48 m CSF. Below 90 m CSF, CaCO₃ concentrations are consistently high (77 to 96 wt%). Variations in CaCO₃ concentrations correspond to lithostratigraphic observations (see "Lithostratigraphy"). TOC content is generally low throughout the sediment column, ranging between 0.05 and 0.08 wt%; however, relatively high TOC concentrations (0.10 to 0.30 wt%) are observed at 0.71 and 51.52 m CSF and in the interval between 108.51 and 135.22 m CSF.

In Hole U1336B, bulk sediment samples were analyzed at a frequency of one per core for silicon, aluminum, iron, manganese, magnesium, calcium, sodium, potassium, titanium, phosphorus, barium, copper, chromium, scandium, strontium, vanadium, yttrium, and zirconium (Table T18). Concentrations of noncarbonate components like SiO₂, Al₂O₃, and Fe₂O₃ display similar patterns, with the highest values occurring in the uppermost sediments and decreasing to a minimum between 100 and 140 m CSF before a small peak at the base of the section. Ca (carbonate) content displays the opposite pattern, indicating the important role of carbonate dilution in determining the bulk sediment composition at this site.

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