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doi:10.2204/iodp.proc.340.108.2013

Geochemistry

Samples for headspace analyses were taken from 23 depths throughout Hole U1398A. Methane concentrations were only a few parts per million in the upper 180 mbsf and typical of those observed at previous sites. Below this depth, however, methane concentrations increased rapidly to a maximum value of 4700 ppm at 240 mbsf (Fig. F3). Despite these elevated methane levels, concentrations of the higher hydrocarbons remained close to detection limits, with between 0.4 and 1.4 ppm ethane and ethene measured in seven samples.

A total of 46 samples were taken for X-ray diffraction (XRD) and carbonate analysis. The only significant difference between the XRD patterns obtained at Site U1398 and those found at previous sites is that the deeper samples contain larger peaks for clay and quartz (Fig. F4).

Calcium carbonate concentrations are highly variable and are lower in intervals with higher proportions of volcanic material (Fig. F5; Table T2). Maximum concentrations are ~35 wt%, which partly reflects the greater proportion of terrestrial clay minerals and the absence of significant aragonite preservation.

Because of the coarse-grained nature of the sediment, it was not possible to take any pore water samples in the upper 60 mbsf at the site. In addition, one sample (from Section 340-U1398A-27X-1) contained a high proportion of coarse-grained material and was likely contaminated to some extent by circulating drill fluid (seawater). Hence, the uppermost pore water data come from Section 340-U1398A-9H-3 at a depth of 62.5 mbsf (Table T3). At this point pore water alkalinity values are close to 10 mM and remain at this level to 150 mbsf. Alkalinity then gradually falls to 5.9 mM by 254 mbsf (Fig. F6A). We note the lower value for the sample at ~187 mbsf and that concentrations are also comparatively low for Cl and NH₄ (as compared to the surrounding fluids), and we suspect that this sample suffered from contamination during drilling. Ammonium concentrations increase steadily from 0.8 mM in the shallowest sample to ~1.6 mM in the deepest sample (Fig. F6B). Calcium concentrations decrease from 4.8 mM at 62.5 mbsf to 3.9 mM at 103 mbsf and then show an increase to 10.9 mM in the deepest sample (Fig. F6C). Magnesium concentrations show a gradual decrease from 51 mM in the shallowest sample to 44.4 mM at the base of the hole (Fig. F6D). Chloride concentrations show a consistent increase from close to the seawater value in the shallowest sample to 674 mM at 254 mbsf (Fig. F6E). Even the shallowest sample has ΣS concentrations that are less than half the seawater level, and ΣS falls to values near the detection limit by ~160 mbsf (Fig. F6F). Overall, data are consistent with diagenesis driven by oxidation of slightly higher levels of organic carbon than those seen in previous sites, coupled with a lesser contribution to the pore water profiles from alteration of volcanic material. The increase in chloride with depth may reflect hydration of clay minerals.

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