Oxygen Isotope Values of Planktic Foraminifera: A Tool for the Reconstruction of Surface Water Stratification

Abstract

The δ18Ocalcite composition of planktic foraminiferal shells in 59 samples collected from surface sediments between 34°S (south-eastern Subtropical Gyre) and 56°S (Antarctic Zone of the Antarctic Circumpolar Current) and additionally from 20°S (Walvis Ridge) in the South Atlantic Ocean documents the apparent calcification depth of 24 planktic foraminiferal taxa. Based on the δ18Ocalcite signal of their shells, four depth related groups of foraminifera are distinguished, calcifying in a depth between 0 and 50 m (Sphaeroidinella dehiscens, Pulleniatina obliquiloculata, Orbulina bilobata, Globigerinoides sacculifer (with and without sac), Globigerinella aequilateralis, Globigerinoides ruber (white), Globigerina bulloides); 0 and 200 m (itGlobigerinoides conglobatus, Orbulina universa, Neogloboquadrina dutertrei, Turborotalita quinqueloba, Neogloboquadrina pachyderma} (dextral and sinistral)); 100 and 250 m (Globorotalia tumida, Globorotalia menardii, Globigerinella calida, Globorotalia inflata); and deeper than 250 m (Globorotalia hirsuta, Globorotalia scitula, Globorotalia crassaformis, Globorotalia truncatulinoides (dextral and sinistral)). We propose a method to reconstruct temperature gradients between various depths in the water column, but most specifically between 30 m and 250 m, using the δ18Ocalcite composition of foraminiferal calcite. Changes in past sea-surface water stratification in the temperate southern South Atlantic Ocean and in the Antarctic Circumpolar Current can be reconstructed in particular with G bulloides, G truncatulinoides andG inflata. Modern temperature gradients between 30 m and 250 m water depth are reconstructed accurately within a range between 1.7 and 9.4 °C with a correlation coefficient of 0.77 and a standard deviation of ±1.3 °C.