Geochemistry of Periplatform Carbonate Sediments, Leg 115, Site 716 (Maldives Archipelago, Indian Ocean)

Abstract

Site 716 is a continuous sequence (upper Miocene to Holocene) of periplatform oozes and chalks from the Maldives Ridge, Indian Ocean. Mineralogical and geochemical studies of these carbonate sediments indicate that submarine burial diagenesis has played an important role in the induration of sediments at this site. Metastable carbonates, high-magnesium calcite (HMC) and aragonite, convert to low-magnesium calcite (LMC) rapidly, within 1.1 and 6.0 Ma, respectively. Strontium concentrations in carbonate decrease with depth as the result of the burial diagenesis of calcium carbonate, primarily aragonite, with excess strontium being expelled into pore waters. The formation of celestite at depth indicates that sufficient diagenesis of carbonate sediments has occurred to saturate or supersaturate pore waters with respect to this authigenic mineral. Sodium also decreases monotonically with depth as a result of the burial diagenesis of calcium carbonate. Magnesium and carbon and oxygen isotopic curves are remarkably similar. Carbon isotopic compositions record inputs of 13C-enriched components from shallow carbonate banks. Magnesium concentrations vary widely, recording enhanced episodes of cementation by LMC with slightly elevated magnesium contents. Positive shifts in oxygen isotopic composition also record episodes of cementation during burial diagenesis. Intervals with increased accumulation rates of metastable components have undergone more rapid diagenesis than intervals with predominately pelagic deposition.