Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 6 2007 10.5194/bg-4-957-2007 http://www.biogeosciences.net/4/957/2007/ http://www.biogeosciences.net/4/957/2007/bg-4-957-2007.html http://www.biogeosciences.net/4/957/2007/bg-4-957-2007.pdf 957 973 2007-11-12 Early diagenetic overprint in Caribbean sediment cores and its effect on the geochemical composition of planktonic foraminifera M. Regenberg D. Nürnberg J. Schönfeld G.-J. Reichart Leibniz Institute of Marine Sciences IFM-GEOMAR, Kiel, Germany Faculty of Earth Sciences, Utrecht University, The Netherlands now at: Institute of Geosciences, Christian-Albrechts-University, Kiel, Germany 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 >6 mmol/mol only occur, when high-magnesium calcite has dissolved within the sediment. 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