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doi:10.2204/iodp.proc.310.105.2007

Geochemistry

All geochemical pore water data are presented in tables allsitesIW.xls and IWsaturation.xls in “Supplementary Material.”

pH, alkalinity, ammonia, chloride, and sulfate

pH and alkalinity values of Maraa western transect pore waters are essentially that of Tahitian seawater with the exception of the slightly lower pH and alkalinity values found in Hole M0005D samples (Fig. F79A, F79B). Sample 310-M0007B-5R-1, 16 cm, yielded an anomalously high alkalinity of 10.3 mmol/L even by repeated measurements of different dilutions. Ammonia was only detectable in two samples from Hole M0007B (Fig. F79C). Chloride concentrations in Maraa western transect pore waters do not significantly deviate from seawater values, precluding any significant influence of freshwater on interstitial water (IW) geochemistry (Fig. F79D). Sulfate concentrations of all samples are similar to seawater.

Mg, K, Ca, and Sr

Concentrations of both Mg and K in Maraa western transect pore waters do not vary significantly from seawater concentrations (Fig. F79E, F79F). Most of those samples exhibit seawater-like Ca concentrations except those of Hole M0007B, which are slightly depleted (Fig. F79G). Sr concentrations in these pore waters are similar to seawater, apart from significant elevations in Hole M0005D samples (Fig. F79H). This could indicate some aragonite dissolution occurring in Hole M0005D material, resulting from the lower pH values of Hole M0005D pore waters.

Li, P, Mn, Fe, and Ba

Li is enriched relative to seawater in IW samples from the shallowest depths at Maraa western transect sites (Fig. F79I). This indicates a sedimentary source of Li to the pore waters at these depths. Similarly, P, Mn, and Ba concentrations are elevated in the uppermost IW samples (Fig. F79J–F79L), suggesting all these trace elements share a common source at these depths in that area. This source may result from the dissolution of phosphatic-iron-manganese crusts associated with microbialites (Camoin et al., 2006). Fe was only detected in Section 310-M0006A-3R-1 (1.32 mbsf; 82.90 mbsl). In contrast, at Maraa western transect sites both Mn and Ba display enrichments with no associated changes in the Li or P of these samples (Fig. F79I–F79L). This indicates a separate source for Mn and Ba at depth in the Maraa western transect sections.

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