Oxygen and carbon isotopic composition and biogeographic distribution of planktonic foraminifera in the Indian Ocean

Abstract

Five planktonic foraminiferal assemblages were encountered along a north-south transect in the Indian Ocean with the following dominant species: Globorotalia truncatulinoides in the Southern Subtropical Assemblage; Globigerinoides ruber, Globigerinella aequilateralis and Globorotalia menardii in the Northern subtropical Assemblage; Globigerinoides sacculifer and Pulleniatina obliquiloculata in the Tropical Assemblage; Neogloboquadrina dutertrei and Globigerina bulloides in the Tropical Upwelling Assemblage; and Globigernoides conglobatus in the Tropical Arabian Sea Assemblage. Oxygen and carbon isotopic analyses were performed on three surface-dwelling species of planktonic foraminifera. In tropical and subtropical waters (22°–30°C) the δ 18O of Globigerinoides ruber and G. sacculifer varies with surface-water temperature and the slope of the isotope-temperature regression line is similar to that of Epstein et al. (1953) for marine molluscs. However, foraminiferal δ 18O values are significantly lighter than those of molluscs, indicating isotopic disequilibrium. Comparison between plankton tows taken within and below the mixed layer at the same stations shows that G. ruber has similar, low δ 18O values in both sets, suggesting that it secretes its shell within the mixed layer. By contrast, G. sacculifer has slightly higher δ 18O values below than within the mixed layer, because it continues to secrete calcite below the thermocline. No significant δ 18O variations with shell size were found. On the other hand, δ 13C values were strongly size-dependent, i.e., the smaller the size, the lighter the δ 13C. Comparison of living G. ruber and G. sacculifer with Recent specimens from the surface sediment shows that the living foraminifera are generally lighter in δ 18O than the fossils, supporting independent observations that the latter have secreted additional calcite during gametogenesis, when the organisms settle freely from the euphotic zone for about 12 h and pass through waters lower in temperature than the epipelagic waters where the foraminifera form the bulk of their shells.