Oxygen and carbon isotope studies in recent foraminifera from the southwest Indian ocean

Abstract

Eighteen species of planktonic foraminifera have been analyzed for their oxygen and carbon isotopic composition in five Recent samples of deep-sea sediment from the southwest Indian Ocean; one sample of glacial age and one mid-Holocene sample were also studied. On the basis of oxygen isotopic composition three groups are recognized. Species in the first group ( Globigerinoides ruber, G. sacculifer and G. conglobatus; G. Globigerina rubescens and Globigerinita glutinata) calcity in the near-surface Tropical Water, so that the oxygen isotopic composition of their test carbonate may be used to indicate surface temperature. Species in the second group ( Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Orbulina universa, Globigerinella siphonifera and Sphaeroidinella dehiscens) are associated with the sub-surface high-salinity Subtropical Water, so that their oxygen isotope composition indicates trends in the temperature of this water mass. The third group (the species of Globorotalia) calcity in the deeper Central Water. The average oxygen isotopic composition of each Globorotalia species is more or less constant over the range studied and does not reflect the surface temperature trend. The carbon isotopic composition of three species ( Globigerina rubescens, Globigerinoides ruber and Globigerinita glutinata indicate departure from isotopic equilibrium by at least 3% 0. Among the remaining species the variation of carbon isotopic composition with depth (where depth is inferred from the temperature estimated from oxygen isotopic composition) implies that N. dutertrei, P. obliquiloculata and G. siphonifera occupy the shallow subsurface oxygen minimum, while the deeper-dwelling globorotaliids approach the deeper oxygen minimum. Hence it is possible, despite scatter among the data, to discern the pattern of oxygen content with depth in the overlying water masses from an examination of oxygen and carbon isotopic composition among foraminiferal species in the sediment. This promises to be an exciting new tool for palaeo-oceanographic investigations.