Identical stable isotopic signatures in Globigerinoides ruber morphotypes from northeastern Indian Ocean coretops

Globigerinoides ruber (white) is a mixed layer planktic foraminifera cosmopolitan to the tropical and subtropical oceans. G. ruber (white) has two principal morphotypes: sensu stricto (s.s.) and sensu lato (s.l.). Previous geochemical studies have suggested that differences in geochemistry (δ18O, Mg/Ca) across these morphs may arise from seasonal preferences (s.l.: cold/winter-biased) or distinct calcification depth habitats (s.s.: 0-25m; s.l.: 25-50 m). Contrary to this, recent studies have demonstrated that there are no systematic or significant geochemical offsets in coeval s.s.-s.l. δ18O and δ13C. However, no such study has been conducted in the northern Indian Ocean to determine the suitability of selective versus non-selective mixtures of G. ruber for paleoceanographic reconstructions. In this study, we report individual foraminiferal analyses (IFA) measurements of δ18O and δ13C on coevally occurring s.s.-s.l. G. ruber (white) morphotypes within core-tops and late Holocene samples at three sites from the northeastern Indian Ocean (cores: IODP Site U1446 - northwestern Bay of Bengal, MGS30 - eastern Bay of Bengal, and SK343 - Gulf of Mannar). We report a total of 362 IFA measurements of s.s. (n=201) and s.l. (n=161). Our observations and statistical approach confirm that there are no significant stable isotopic offsets in coeval s.s.-s.l. morphotypes of G. ruber in this region. Hence we suggest minimal biases for the use of non-selective mixtures of G. ruber morphotypes for paleoceanographic reconstructions of mixed-layer variability in the northern Indian Ocean, at least for Holocene samples, or for periods with similar biogeochemical and oceanographic conditions.