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doi:10.2204/iodp.proc.340.107.2013

Geochemistry

Samples for headspace analyses were taken from 18 depths throughout Hole U1397A. Samples from the upper 76 mbsf tend to have slightly higher methane concentrations (3.1–4.4 ppm) than those from 94 to 266 mbsf (2.3–3.7 ppm). No higher hydrocarbons were detected.

A total of 44 samples were taken for X-ray diffraction (XRD) and carbonate analysis. Most of the samples collected for XRD are dominantly volcanic and contain mainly plagioclase, hornblende, quartz, and amphibole. The hornblende appears to be more alkali rich than that observed in samples from around Montserrat (e.g., Section 340-U1397-7H-7; Fig. F6), and there appear to be other types of amphibole present (e.g., cummingtonite). Clay minerals are ubiquitous throughout the hole and more abundant than in samples from Montserrat. The dominant clay minerals are smectite, kaolinite, and glauconite. A fourth mineralogic type has been defined for a series of six samples taken at the base of Hole U1397A (Cores 340-U1397A-32X through 35X). Samples from these cores contain high abundances of clay minerals, with smectite being particularly abundant; dolomite is also present in significant quantity (e.g., Section 340-U1397A-32X-1), possibly reflecting the presence of hydrothermal alteration products.

Carbonates are generally calcite with minor aragonite, reflecting the greater depth of the site. CaCO₃ concentrations are lower than they are in the sites around Montserrat, a difference which likely reflects the absence of any significant aragonite preservation; however, carbonate supply and/or greater dilution by noncarbonate sedimentation may also be involved (Fig. F7; Table T2).

Pore water alkalinity values increase with depth in the upper part of the core to reach a maximum of ~5.1 mM between 23 and 46 mbsf before declining to 3.9 mM in the deepest samples (164–182 mbsf), although one sample from 174 mbsf had an alkalinity of 5 mM (Fig. F8A). pH values vary between 7.3 and 8.2, but no consistent pattern is observable in the data. Ammonia concentrations are lower than those at comparable sites around Montserrat (Sites U1394 and U1395), which likely reflects the greater water depth at Site U1397 (Fig. F8B). The shape of the profile is similar to that of alkalinity, suggesting that both are dominated by diagenetic processes in the sediment column. Calcium concentrations decrease from the seawater value to 9.1 mM at 23 mbsf and then increase to ~11 mM in the deepest sample at 182 mbsf (Fig. F8C). Magnesium concentrations (which scatter around seawater value) (Fig. F8D) and potassium concentrations (which are higher in the upper 75 mbsf and show a weak trend to lower values between 164 and 182 mbsf) (Fig. F8E) suggest that alteration of volcanic matter does not play a dominant role in defining the major element pore water concentrations. ΣS concentrations decrease from near seawater value to 26.2 mM at 52.5 mbsf and then increase to near seawater concentration in the deepest samples (Fig. F8F), again suggesting that organic carbon–driven diagenesis is the major process in the upper part of the hole. Chloride concentrations are within the normal range (560–570 mM) expected for pore water obtained from squeezing carbonate-rich sediment and show a slight increase in the upper part of the hole (Table T3).

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