Late Quaternary carbonate preservation in the Andaman Sea versus the Central Indian Basin: A test of dissolution under diverse oceanographic settings in the Indian Ocean

Abstract

Late Quaternary carbonate dissolution was evaluated by using planktonic foraminifera indices perfect test ratio (PTR) of Goborotalia menardii, G. menardii fragmentation index (MFI), percentage of total resistant species (RSP) and sediment calcium carbonate (CaCO3) content in five sediment cores collected from two diverse oceanographic settings of the Indian Ocean. The cores collected from a depth range of 2000 and 4250 m of the Andaman Sea, a marginal sea in the Northern Indian Ocean and a seamount rise in the Central Indian Basin (CIB) in the abyssal Indian Ocean depict the intensity of selective carbonate dissolution in the glacial-interglacial cycles of the late Quaternary period. The assemblages from the Andaman Sea show little or no dissolution (deepest core AAS 11, water depth: ~3000 m), whereas cores collected from the CIB shows moderate to very high dissolution (SVBC 37, water depth of ~4000 m and core BC 37, water depth of ~4250 m). Excellent carbonate preservation in the Andaman Sea is reflected in the high order of species diversity consisting of thirty-five species. In contrast, selective dissolution in the CIB resulted in poor diversity with just about twenty-three species. The well-preserved planktonic foraminifera assemblages provide a better paleoceanographic record in the Andaman Sea. The carbonate dissolution proxies in the Andaman Sea point towards excellent preservation during Marine Isotope Stage 2 (MIS 2), especially during the deglacial period (Heinrich event 1) attributed to enhanced ventilation and deepening of carbonate compensation depth (CCD). Cores from the CIB demonstrate that the dissolution does not uniformly modify an assemblage due to the differential susceptibility of each species. Selective dissolution is an important issue in interpreting foraminiferal assemblages for climatic and environmental changes. The study reveals that the relationship between G. menardii abundances and its fragments in a single core is an effective parameter to understand the dissolution as its ratio is not affected by climatic or related environmental changes. Utmost care should be taken while using species variation as an evidence for climatic or related environment change from the CIB. The present investigation observes that the MFI can trace deep-sea carbonate dissolution quantitatively at both CIB and the Andaman Sea locations.