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doi:10.2204/iodp.proc.340.110.2013

Geochemistry

Samples for headspace analyses were taken from 49 cores throughout all three holes at this site (Holes U1400A–U1400C). In contrast to the two previous sites (U1398 and U1399) in the basin, not a single sample had a methane concentration >4.1 ppm, and the vast majority had concentrations <3 ppm. No higher hydrocarbons were detected in any of the samples.

A total of 78 samples were taken for X-ray diffraction (XRD) and carbonate analysis at Site U1400. In general, the XRD patterns are very similar to those obtained from Sites U1398 and U1399. However, subtle variations in the relative proportions of volcanic minerals are present, with quartz and Fe-Ti oxides being more abundant than those seen at the deeper sites (Fig. F5A). Clay minerals were identified in virtually every sample and are present in high abundance (typically smectite, kaolinite, and glauconite) in some samples (Fig. F5B). As is typical in sediment cores taken near volcanic islands, carbonate concentrations are highly variable and are lower in intervals with higher proportions of volcanic material (Fig. F6A; Table T2). Maximum carbonate concentrations are ~35 wt%, reflecting the greater proportion of terrestrial clay minerals and the absence of significant aragonite preservation. An unusual aspect of the organic carbon data is a general increase in concentrations with depth (Fig. F6B). Concentrations in the upper 250 mbsf are generally low (mostly <0.5 wt%) compared to previous sites, whereas approximately half the samples from below this depth contain higher organic carbon concentrations, with some samples reaching 0.8 wt%.

Pore water profiles are illustrated in Figure F7 and data are listed in Table T3. With the exception of the deepest sample from 427 mbsf, data can be readily interpreted in terms of a classic diagenetic profile driven by the oxidation of organic carbon (e.g., increasing alkalinity and ammonia and decreasing ΣS). Calcium and Mg data generally scatter near the seawater value with an increase in Ca and decrease in Mg in the deeper samples. Cl data show a steady increase in concentrations from the modern seawater value at the surface (560 mM) to 570 mM at 120 mbsf that likely reflects progressive hydration of altered volcanic material as it is altered to clay minerals. The chemical composition of the deepest sample deviates from this interpretation; in particular, it has a much lower ΣS concentration than the rest of the samples. This sample comes from much older sediment that appears to be separated from the younger overlying sediment by a hiatus of at least 1 m.y. (e.g., see “Paleontology and biostratigraphy”). The two deepest samples also contain the highest organic carbon concentrations. Hence, one interpretation of the data is that the pore water geochemistry of the deepest sample reflects a distinct diagenetic environment from that observed in the overlying sediment, which may either reflect changes in environmental conditions at this site over time or transport of the upper sediment to this area from a site of lower organic carbon deposition.

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