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doi:10.2204/iodp.proc.340.106.2013

Geochemistry

Samples for headspace analyses were taken from 15 depths throughout Hole U1396A. The uppermost sample (from Section 340-U1396A-1H-3) has a methane concentration of 3.6 ppm, but all other samples have levels between 2.1 and 2.6 ppm. No higher hydrocarbons were detected.

A total of 30 samples were taken for X-ray diffraction (XRD) and carbonate analysis. No pure volcanic samples were collected from Hole U1396A. In contrast to the previous sites, high Mg-calcite was only present in one section (340-U1396A-6H-2). All other carbonate-rich layers contain pure calcite and aragonite in variable proportions, with calcite generally dominant. Clay minerals are ubiquitous throughout the hole, with smectite and kaolinite present in all samples and glauconite commonly identified (e.g., Section 340-U1396A-9H-2; Fig. F4A). Halloysite may be present in some samples, but the spectra are not clear enough to be certain. The bulge beneath the main peaks in some of the volcanic-rich samples (e.g., Section 340-U1396A-12H-5; Fig. F4B) suggests that significant amounts of volcanic glass are present in some tephra layers.

CaCO₃ and organic carbon abundances reflect mixtures of carbonate-rich hemipelagic sediments with carbonate-free and organic carbon–free volcanogenic material delivered by ash fallout. The highest CaCO₃ concentration was only 72 wt%, with most samples having much lower levels, suggesting that volcanic material is dispersed throughout the core even where it is not visible to the naked eye (Fig. F5; Table T2).

Samples for pore water extraction were taken from every core from Hole U1396C (Fig. F6; Table T3). Alkalinity values are generally low (<1.5 mM) throughout the hole (Fig. F6A). pH values are generally lower (7.6–7.1) than those at Sites U1394 and U1395, but no consistent pattern is observable in the data. Ammonia concentrations are much lower than those at the previous two sites (Fig. F6B). These differences may be related to the fact that Site U1396 is located at shallow depths on a basement high, where strong bottom currents have been observed on previous research cruises to the area. This effect tends to winnow out the finer grained, more reactive organic matter and leads to less intense organic carbon–driven diagenesis. Calcium concentrations are consistently higher than bottom water values in samples from throughout the hole (Fig. F6C), and magnesium concentrations are consistently lower than seawater concentrations (Fig. F6D). These features are commonly seen in deep sediment pore water as a result of alteration of basaltic glass and suggest that many of the tephra layers observed in the core may have a basaltic composition. This hypothesis is supported by the potassium concentrations which are also lower than seawater (Fig. F6E). Overall, ΣS concentrations are slightly depleted relative to seawater sulfate values but do not show a particularly distinct trend with depth. Chloride concentrations fluctuate within the normal range (550–570 mM) expected for pore water obtained from squeezing carbonate-rich sediment (Table T3).

Three samples were taken for shore-based microbiological analyses after the piston core had been preloaded with a microsphere bag to assess potential surface contamination. However, the bag failed to burst at the bottom of the hole. A total of 20 g of sediment was taken for shore-based microbiological activity measurements and stored at 0°C. A total of 4 g of sediment was taken for shore-based RNA analysis and stored at –80°C.

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