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doi:10.2204/iodp.proc.340.105.2013

Geochemistry

Samples for headspace analyses were taken from 21 depths throughout Hole U1395A; none were taken from cores that consisted largely of coarse-grained material. With the exception of one sample (from Section 340-U1395A-9X-1) that had a methane concentration of 10.3 ppm, all other samples had levels between 2.1 and 3.1 ppm. No higher hydrocarbons were detected.

A total of 44 samples were taken for X-ray diffraction (XRD) and carbonate analysis. XRD results fall into three groups. Samples from pelagic sediment intervals predominantly contain calcite and high-Mg calcite ± aragonite, together with minor volcanic phases (mostly plagioclase with lesser orthopyroxene and hornblende). The specific clay minerals present in each interval cannot generally be distinguished from the XRD patterns, but kaolinite, smectite, and glauconite were detected in several of the samples and generally become more abundant below 100 mbsf (Fig. F4). Samples from volcanic-rich horizons contain dominant plagioclase and lesser amounts of orthopyroxene and hornblende, along with minor carbonate (Table T2).

CaCO₃ weight percent is much higher in the largely pelagic section in the upper part of the hole than in the largely volcanic turbidites (i.e., values greater than ~20% CaCO₃ have an average value of ~55%), but the presence of significant quantities CaCO₃ in many of the volcanic turbidite sections indicates a significant biogenic component (Fig. F5A). CaCO₃ content is higher (≤88 wt%) in the semilithified sediments toward the base of the hole. Organic carbon concentrations in the pelagic sections are similar to those expected in this area (average = ~1 wt%) and are much lower in the turbidite sections (Fig. F5B). Organic carbon concentrations also increase toward the base of the hole (≤1.5 wt%). The ratio of organic carbon to CaCO₃, however, remains relatively constant in the hemipelagic intervals throughout the hole, suggesting that concentration increases simply reflect less dilution by volcanic material at depth (Table T2).

It is not possible to collect meaningful pore water data from permeable turbidite horizons, so all data are from zones dominated by hemipelagic sediments. Alkalinity values increased from 3.5 mM in the uppermost two sections (340-U1395A-1H-3, 4.4 mbsf, and 2H-3, 10.4 mbsf) to a maximum value of 7.5 mM at 29.3 mbsf (Section 4H-3), before decreasing to relatively constant values of 5.4 mM from 72.7 mbsf to the deepest sample at 195 mbsf (Fig. F6A). pH values are more variable than those observed in Hole U1394B, but no consistent pattern is observable in the data, suggesting they may contain some analytical artifacts. Ammonia concentrations increase from 0.01 mM in the uppermost sample to ~1.1 mM at 72.7 mbsf and only increase gradually to a value of 1.2 mM at 185 mbsf (Fig. F6B). Dissolved calcium concentrations are generally lower than bottom water values in samples from the uppermost 106 mbsf of Hole U1395A but show an increase to higher values approaching 15 mM in the deepest samples (Fig. F6C). Although these samples come from semilithified sediment, the absence of concomitant alkalinity anomalies suggests that the high calcium concentrations are not simply related to squeezing artifacts. Interestingly, magnesium concentrations show a marked decrease at the same depth as the high calcium levels (Fig. F6D). These features are commonly seen in deep sediment pore water as a result of alteration of basaltic glass. Hence, it is possible that the covariation in calcium and magnesium concentrations in the deeper section of Hole U1395B may reflect a higher basaltic component in the volcanogenic material at this horizon. Sodium concentrations do not show a clear trend with depth, but potassium concentrations show a decrease with depth that is consistent with the reaction of pore water with volcanic material (Fig. F6E). Overall, ΣS concentrations decrease slightly below the shallowest data point to near-constant values of 23–25 mM. Chloride concentrations show a slight increase below 30 mbsf to a value of ~570 mM, which is expected for pore water obtained from squeezing carbonate sediments (Table T3).

Three samples were taken for microbiological analysis after the piston core had been preloaded with a microsphere bag to assess potential surface contamination. 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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