Sedimentation rates and sediment chemistry in the South Indian basin

Sedimentation rates and sediment chemistry have been studied on a series of piston cores and trigger cores from the South Indian basin and the Southeast Indian ridge. Sedimentation rates were measured by α and γ spectrometry of ²³⁰Th excess in the sediment; general greement was observed between the two methods, and these data are generally consistent wih the rates determined from paleomagnetic reversal data. Continental slope and rise sediments are accumulating at 0.7-1.6 cm/10³ yr. The sedimentation rates generally decrease northward cross the South Indian basin but are locally high in biogenic oozes on and near the ridge; biogenic sediment accumulation rates are commonly 1-2 cm/10³ yr. The clay accumulation rates, estimated from Al₂O₃ analyses, decrease from 1.4 cm/10³ yr on the continental slope to 0.1 cm/10³ yr or less on the ridge. Ridge sediments have high Fe/(Al + Fe + Mn) and Mn/(Al + Fe + Mn) oxide ratios relative to basin sediments. Most of the uranium in the sediments is detrital in origin; however, ²³⁴U/²³⁸U activity ratios approaching the 1.15 value of sea water occur in ridge sediments, where the U/Al ratios are also highest. The increased U/Al ratios and uranium isotope ratios occur in biogenic sediments under the antarctic convergence, their occurrence implying possible removal of uranium from sea water by sediments rich in organic matter; one sample, however, has a ²³⁴U/²³⁸U ratio of 1.29 ± 0.07, which is suggestive of volcanic origin. There is a positive correlation between the excess ²³⁰Th of surface sediments and the Fe/(Al + Fe + Mn) oxide ratios. The correlation suggests that ²³⁰Th is being removed from sea water by or in association withFe (and Mn) colloids. Accumulation rates of Fe, Mn, and U in ridge sediments analyzed in this study are not significantly larger than those in nonridge sediment.