Early diagenetic overprint in Caribbean sediment cores and its effect on the geochemical composition of planktonic foraminifera

Abstract

Early diagenetic features are noticed in the vicinity of carbonate platforms. Planktonic foraminifera of two tropical Atlantic deep-sea sediment cores show the strict relation between micro-scale euhydral crystallites of inorganic precipitates, higher oxygen isotope values and Mg/Ca ratios, and lower Sr/Ca ratios than expected for their pelagic environment in the time interval of ∼100 000-550 000 calendar years before present. Laser ablation Mg/Ca (Sr/Ca) of crystallite-bearing foraminiferal chamber walls revealed 4-6 times elevated (2-3 times depleted) ratios, when ablating the diagenetic overgrowth. Crystalline overgrowth in proportion of 10-20% are estimated to cause the observed geochemical alteration. The extent of foraminiferal Mg/Ca alteration, moreover, seems to be controlled by the composition of the bulk sediment, especially the content of high-magnesium calcite. Anomalous ratios of >6mmol/mol only occur, when high-magnesium calcite has dissolved within the sediment. The older parts (back to ∼800 kyrs) of the records are characterized by similar trends of Mg/Ca and Sr/Ca. We discuss possible scenarios to accommodate the obtained geochemical information.