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. Past ocean temperatures and salinities can be approximated fromcombined stable oxygen isotopes ( δ18O ) and Mg ∕ Ca measurements in fossil foraminiferal tests withvarying success. To further refine this approach, we collected livingplanktic foraminifers by net sampling and pumping of sea surface water fromthe Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analysesof δ18O and Mg ∕ Ca in eight living plankticspecies (Globigerinoides sacculifer, Orbulina universa,Neogloboquadrina dutertrei, Pulleniatina obliquiloculata,Globorotalia menardii, Globorotalia ungulata,Globorotalia truncatulinoides and Globorotalia tumida) werecompared to measured in situ properties of the ambient seawater (temperature,salinity and δ18O seawater ) and fossil tests ofunderlying surface sediments. “Vital effects” such as symbiont activity andtest growth cause δ18O disequilibria with respect to theambient seawater and a large scatter in foraminiferal Mg ∕ Ca . Overall, ocean temperature is the most prominentenvironmental influence on δ18O calcite and Mg ∕ Ca . Enrichment of the heavier 18O isotope inliving specimens below the mixed layer and in fossil tests is clearlyrelated to lowered in situ temperatures and gametogenic calcification. Mg ∕ Ca -based temperature estimates ofG. sacculifer indicate seasonal maximum accumulation rates on theseafloor in early spring (March) at Caribbean stations and later in the year(May) in the Florida Straits, related to the respective mixed layertemperatures of ∼26 ∘ C. Notably, G. sacculiferreveals a weak positive linear relationship between foraminiferal derived δ18O seawater estimates and both measured in situ δ18O seawater and salinity. Our results affirm theapplicability of existing δ18O and Mg ∕ Ca calibrations for the reconstruction of past ocean temperatures and δ18O seawater reflecting salinity due to theconvincing accordance of proxy data in both living and fossil foraminifers,and in situ environmental parameters. Large vital effects and seasonallyvarying proxy signals, however, need to be taken into account.

biogeosciences

Mg ∕ Ca and &amp;lt;i xmlns=&amp;quot;&amp;quot;&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O in living planktic foraminifers from the Caribbean, Gulf of Mexico and Florida Straits