Carbonate dissolution in Recent sediments of the eastern equatorial Indian Ocean: Preservation patterns and carbonate loss above the lysocline

Abstract

Multiple dissolution indices from 45 sediment samples show that the foraminiferal lysocline is near 3800 m in the eastern equatorial Indian Ocean, close to the saturation horizon with respect to calcite in the water column. Below the lysocline, the increase in foraminiferal fragmentation, decrease in absolute abundance and progressive modification of planktic foraminiferal assemblages with depth is accompanied by an increasing rate of loss of carbonate from the sediments. Above the lysocline, some dissolution must occur in order to produce the dramatic reduction in foraminiferal numbers (up to 60%) and the pattern of increased fragmentation with depth. However, the amount of carbonate actually lost from supralysoclinal sediments reaches at most 20–30% of the total at the depth of the lysocline. The principal driving force behind supralysoclinal dissolution is most likely the oxidation of organic matter incorporated into the sediments. Dissolution above the lysocline has a greater apparent impact on relative preservation in the coarse ( > 150 μ m ) fraction than it does on total carbonate loss because of redistribution of carbonate to the finer size fractions as fragments. Though the effects of dissolution on foraminiferal preservation are clearly visible at all depths, the lysocline itself does appear to mark the local onset of large-scale loss of dissolved carbonate from the sediments. Such an interpretation is broadly consistent with models regarding the saturation state of the overlying waters as the principal driving force behind the preservation or dissolution of Recent carbonate sediments on the sea floor.