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doi:10.2204/iodp.proc.331.105.2011

Geochemistry

Interstitial water

Five whole-round sections were processed for interstitial water at Site C0015 (Table T6). These sections were collected from immediately above or below microbiology-dedicated whole-round sections and were processed at ambient laboratory temperatures.

Only two cores were collected at Site C0015, so the deepest sample comes from only 8.8 mbsf. Four of the five samples have major element compositions close to those of seawater; the fifth has lower chloride but retains major ion ratios that are similar to seawater and so may have been inadvertently diluted during sample handling and processing (Figs. F8, F9, F10). Although minor and trace species do not necessarily follow this same pattern, their departures from seawater values could be driven by any number of in situ geochemical processes or sampling and analytical artifacts, so we have not interpreted those differences. Dissolved silica is likely enriched over bottom water concentrations, but this could be an artifact of processing temperature (Fig. F10). As only one sample had detectable ammonium levels (Table T6), we did not plot those data.

Whereas the uppermost 9 mbsf of sediment resembles seawater at Site C0015, this was not the case at Sites C0013 and C0014, which showed a strong hydrothermal influence even at this shallow depth.

Headspace gas analysis

Low concentrations of methane were detected in all samples at Site C0015, and ethane was not detected (Tables T7, T8). Methane at this site (Fig. F11) is much lower than at Sites C0013 and C0014, suggesting that a robust zone of microbial methanogenesis is not present at the shallow depths sampled (0–6.5 mbsf), in contrast with some of the other sites.

Hydrogen concentrations are low and variable (Table T7). Hydrogen was detected at 1.34, 2.75, 5.57, and 6.50 mbsf (Fig. F11) but was undetectable at the other depths sampled. In contrast to Sites C0013, C0014, and C0017, Site C0015 does not appear at present to support a robust microbial community, nor does it have any significant hydrothermal input.

Sediment carbon, nitrogen, and sulfur composition

Calcium carbonate (CaCO₃) calculated from inorganic carbon concentration ranges from 0.07 to 1.3 wt% in four of the six samples analyzed (Fig. F12; Table T9). The other two samples (from Sections 331-C0015B-1H-1 and 1H-3) have 21.4 and 38.7 wt% carbonate. The first is an oxidized brown mud with abundant foraminifers, and the second is a calcareous gravel containing numerous coral fragments (see “Lithostratigraphy”).

Total organic carbon (TOC) ranges from 0.013 to 0.33 wt% (Fig. F13). Total nitrogen (TN) is below detection (<0.001 wt%) in four samples and low in the other two. Total sulfur (TS) is also low, ranging from about 0.02 to 0.07 wt% except for one sample at 0.5 wt%. The two horizons with higher carbonate also have higher TOC, consistent with a biogenic origin (from foraminifers and from corals). The one sample with higher sulfur (from Section 331-C0015B-1H-3) is a calcareous gravel, which also contains fine-grained pyrite; TS is higher than TOC in this sample, consistent with some of the sulfur being diagenetic.

The generally low TOC indicates that both deposition and in situ production of organic matter are small. Sediment at Site C0015 is typical of pelagic sediment in this region and shows little or no influence of hydrothermal activity.

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