Anna Jentzen

Mg / Ca and δ 18 O in living planktic foraminifers from the Caribbean, Gulf of Mexico and Florida Straits

Past ocean temperatures and salinities can be approximated from combined stable oxygen isotopes (δ18O) and Mg /Ca measurements in fossil foraminiferal tests with varying success. To further refine this approach, we collected living planktic foraminifers by net sampling and pumping of sea surface water from the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analyses of δ18O and Mg /Ca in eight living planktic species (Globigerinoides sacculifer, Orbulina universa, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii, Globorotalia ungulata, Globorotalia truncatulinoides and Globorotalia tumida) were compared to measured in situ properties of the ambient seawater (temperature, salinity and δOseawater) and fossil tests of underlying surface sediments. “Vital effects” such as symbiont activity and test growth cause δ18O disequilibria with respect to the ambient seawater and a large scatter in foraminiferal Mg /Ca. Overall, ocean temperature is the most prominent environmental influence on δOcalcite and Mg /Ca. Enrichment of the heavier 18O isotope in living specimens below the mixed layer and in fossil tests is clearly related to lowered in situ temperatures and gametogenic calcification. Mg /Ca-based temperature estimates of G. sacculifer indicate seasonal maximum accumulation rates on the seafloor in early spring (March) at Caribbean stations and later in the year (May) in the Florida Straits, related to the respective mixed layer temperatures of ∼ 26 C. Notably, G. sacculifer reveals a weak positive linear relationship between foraminiferal derived δOseawater estimates and both measured in situ δOseawater and salinity. Our results affirm the applicability of existing δ18O and Mg /Ca calibrations for the reconstruction of past ocean temperatures and δOseawater reflecting salinity due to the convincing accordance of proxy data in both living and fossil foraminifers, and in situ environmental parameters. Large vital effects and seasonally varying proxy signals, however, need to be taken into account.