Paleoceanographic significance of Globigerinoides ruber (white) morphotypes from the Andaman Sea

Abstract

Globigerinoides ruber (white), a tropical to sub-tropical planktonic foraminiferal species which dwells in the mixed layer, has garnered significant attention of paleoceanographic investigations. Recent studies on G. ruber (w) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) have inferred differences in seasonal calcification depths and offsets in isotopic and elemental data. To decipher if the morphotype abundances vary in response to changes in Indian Summer Monsoon (ISM), two well-dated sediment cores from the Andaman Sea, a core monsoon area in the northeastern Indian Ocean, are investigated. The abundance variations of G. ruber s.s. are very similar to total G. ruber (w), whereas G. ruber s.l. is very sensitive to the hydrologic changes linked to the ISM variability. Increased abundances of G. ruber s.l. were observed during periods of weak stratification and the resultant increase in productivity. The last glacial maximum (LGM), deglacial, Younger Dryas (YD) and late Holocene periods with weak ISM are characterized by a high abundance of G. ruber s.l. Conversely, this morphotype abundance decreased from 30 to 23 ka, Bølling/Allerød (B/A), early and mid-Holocene when the ISM was enhanced. The large variation of G. ruber morphotypes in the Andaman Sea is attributed to the selective preference of nutrient-rich water which varied with water column changes (salinity-controlled stratification) associated with ISM evolution. The elevated abundances of G. ruber s.l. during the glacial period closely match with the available records from other regions, possibly making them potential climatic markers.