DECIPHERING THE MODERN CALCIFICATION DEPTH OF GLOBIGERINA BULLOIDES IN THE SOUTHWESTERN INDIAN OCEAN FROM ITS OXYGEN ISOTOPIC COMPOSITION

The water depth at which the planktic foraminiferal species Globigerina bulloides calcifies its shell has been deciphered based on the oxygen isotopic composition of its shells in surface sediment samples collected from the southwestern Indian Ocean. The seawater temperature for water depths ranging from 0−200 m was estimated from G. bulloides δ 18 O by using various paleotemperature equations. Out of several paleotemperature equations, the one given by Epstein and others (1953) provided the best-fit results. Similarly, the estimated seawater salinity was calculated using the seawater δ 18 O calculated from G. bulloides δ 18 O, and measured seawater temperature taken from the Levitus database. A comparison of seawater salinity and temperature estimated from G. bulloides δ 18 O, with the measured seawater salinity and temperature at different water depths, shows that the calcification depth of G. bulloides varies latitudinally. From the equator to ~15°S, the calcification depth of G. bulloides was inferred to be ~75 m. Between 15°S and ~43°S, the estimated seawater temperature matched well with the seawater temperature at ~200 m water depth, indicating that the G. bulloides calcification depth was comparatively deeper in this region. Farther south of ~43°S, however, the calcification depth appears to be considerably shallower than in the northern part of the study area. The comparison also showed that irrespective of latitudinal region, the estimated seawater temperature matches well with the seawater temperature during the austral spring season suggesting that G. bulloides is abundant at that time. The findings will help in paleoclimatic reconstruction studies based on characteristics of G. bulloides.