Formula omitted] and [formula omitted] ratios, interstitial water chemistry and diagenesis in deep-sea carbonate sediments of the Ontong Java Plateau

Abstract

Interstitial waters and sediments from DSDP sites 288 and 289 contain information on the chemistry and diagenesis of carbonate in deep-sea sediments and on the role of volcanic matter alteration processes. Sr Ca ratios are species dependent in unaltered foraminifera from site 289 and atom ratios (1.2–1.6 × 10 −3) exceed those predicted by distribution coefficent data (~0.4 × 10 −3). During diagenesis Sr Ca ratios of carbonates decrease and reach the theoretical distribution at a depth which is identical to the depth of Sr isotopic equilibration, where 87Sr 86Sr ratios of interstitial waters and carbonates converge. Mg Ca ratios in the carbonates do not increase with depth as found in some other DSDP sites, possibly because of diagenetic re-equilibration with interstitial waters showing decreasing Mg 2+/Ca 2+ ratios with depth due to Ca input and Mg removal by alteration of volcanic matter. Interstitial 18O 16O ratios increase with depth at site 289 to δ 18 O = 0.67%. (SMOW), reflecting carbonate recrystallization at elevated temperatures ( $ ̆ = 20°C), the first recorded evidence of this effect in interstitial waters. Interstitial Sr 2+ concentrations reach high levels, up to 1 mM, chiefly because of carbonate recrystallization. However, 87Sr 86Sr ratios decrease from 0.7092 to less than 0.7078, lower than for contemporaneous sea water, showing that there is a volcanic input of strontium at depth. This volcanic component is recorded in the Sr isotopic composition of recrystallized calcites. Isotopic compositions of the unrecrystallized calcites suggests that the rate of increase of the 87Sr 86Sr ratio of sea water with time has been faster since 3 my ago than in the preceding 13 my.