Oxygen isotopic-hydrographic relationships among recent planktonic Foraminifera from the Indian Ocean

Abstract

The stable isotopic composition of fossil Foraminifera has become an increasingly important tool in palaeoecological and stratigraphic studies of marine sediments^1–3, prompting several studies of the factors governing the isotopic composition of living and recent Foraminifera. Many studies have documented the good agreement between the inferred depth of growth from oxygen isotopic data^4–8 and the depth stratification found in plankton tows^9–11. Although results have suggested disequilibrium in some species^12–14, especially among the benthic species^15–16, recent data from Sargasso Sea plankton tows^17–19 and Indian Ocean core top sediments^20,21 suggest that oxygen isotopic equilibrium is nearly achieved in many ecologically important species. In the present study, oxygen isotopic data on as many as eight species and subspecies of planktonic Foraminifera from 52 Indian Ocean core top samples permit a comprehensive comparison of the isotopic–temperature trends across the major océanographie regions of the Indian Ocean (Table 1). Several shallow-dwelling species (Globigerinoides ruber, G. sacculifer and Orbulina universa) were used as a measure of the upper 50–100 m of the water column (the mixed surface layer). Several deep-dwelling species (G. truncatulinoides (left and right coiling varieties), G. inflata, G. scitula and G. hirsuta) were used for comparison with hydrographie data from 200 to 300m depths. The results show that the isotopic difference between the shallow and deep-dwelling species is directly related to the temperature difference between the surface waters and the depth of the permanent thermocline. This isotopic–temperature relationship suggests that isotopic analyses of species which inhabit different water depths may be used to determine the thermal gradient in the upper water column during the Cenozoic.