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doi:10.2204/iodp.proc.340.109.2013

Geochemistry

Samples for headspace analyses were taken from 28 depths throughout Hole U1399A. Methane concentrations were only a few parts per million in all but the deepest sample (276 mbsf), which yielded a methane concentration of 690 ppm. This sample is slightly deeper than the zone of high methane concentrations observed at Site U1398. Unfortunately, it was not possible to collect any deeper samples from Hole U1399B to confirm the presence of high methane concentrations at this site.

A total of 33 samples were taken for X-ray diffraction (XRD) and carbonate analysis. Overall, XRD patterns are very similar to those obtained from Site U1398. Calcium carbonate concentrations are highly variable and are lower in intervals with higher proportions of volcanic material (Fig. F3A; Table T2). Maximum calcium carbonate concentrations are ~35 wt%, reflecting the greater proportion of terrestrial clay minerals and the absence of significant aragonite preservation. Although there is considerable variation in organic carbon concentrations throughout the interval sampled, they generally decrease with depth. The upper 115 m of the interval contains several samples with organic carbon concentrations close to 1 wt%, whereas the maximum concentrations observed in the lower 155 m are ~0.7 wt% (Fig. F3B).

The pore water profiles illustrated in Figure F4 and listed in Table T3 are close to a classic deep-sea diagenetic sequence in the major ions. Total sulfur concentrations gradually decrease over the same interval in which alkalinity and ammonia show steady increases. The decline in S to <10 mM occurs at a greater depth than was observed at Site U1398, suggesting that diagenesis-driven oxidation of organic matter is less intense than it was at Site U1398. Calcium concentrations show a decrease with depth suggestive of secondary carbonate precipitation. Magnesium concentrations also decrease with depth, possibly because of uptake during alteration of volcanic material beneath the deepest sample taken from this site. The slight increase in chloride concentrations also reflects uptake of water into secondary mineral phases at depth.

The shapes of the pore water profiles suggests that most of the signal generated by the oxidation of organic carbon is taking place in the upper 60 m of the sediment column, so the change in organic carbon concentrations between the upper 115 m and the lower 155 m of the sediment column may reflect a change in the input of organic carbon to the system rather than changes in preservation. There is, however, a strong caveat to this hypothesis, as the percentage of volcanic material can also vary with depth, and this variation may act to cause variable dilution of a constant input flux of organic carbon to the sediment.

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