Foraminiferal shell-weight evidence for sedimentary calcite dissolution above the lysocline

Shell-weight measurements were carried out on planktonic foraminifera G. sacculifer and G. ruber specimens from coretop depth transects in the Atlantic, Indian and Pacific Ocean to investigate calcite dissolution above the lysocline. The results suggest that foraminifera deposited in sediments overlain by supersaturated bottom water undergo considerable dissolution at the sediment–water interface and that the calcite saturation state at the interface is considerably offset from that of bottom water. Also, the extent of exposure to undersaturated conditions at the interface is not constant. Rather, it increases towards the surface ocean, i.e. towards shallow marine sediments where the organic matter flux is expected to be higher. It is proposed that the benthic fluff layer at the sediment–water interface represents a zone of undersaturation through which the foraminifera pass prior to deeper burial, and that the residence time of foraminifera within this zone of intense organic matter respiration is long enough to result in significant decreases in shell weight.