Influence of Antarctic Bottom Water (AABW) and Monsoonal Activity in the Central Indian Ocean Over Past 5 Million Years: Benthic Foraminiferal Record at DSDP Site 238

Abstract: This study quantitatively analyzes Pliocene-Pleistocene benthic foraminifera from DSDP Site 238 in the central Indian Ocean. A Q-mode factor analysis of 28 most abundant species defines three significant faunal assemblages reflecting different environmental characteristics. The Cibicides wuellerstorfi-Oridorsalis umbonatus assemblage (Factor 1) represents well oxygenated bottom waters with active currents and intermediate to low supply of organic matter. The Uvigerina proboscidea assemblage (Factor 2) indicates a continuously high flux of organic matters to the sea floor in response to increased surface water productivity. The Nuttallides umbonifera assemblage (Factor 3) is associated with Antarctic Bottom Water (AABW) and reflects cold, oligotrophic, increased carbonate corrosiveness and high oxygen concentration. In most of the past 5 Ma (i.e. 5-4.2 Ma, 2.8-1.8 Ma and 1.2 Ma to Recent) the dominant occurrences of Cibicides wuellerstorfi-Oridorsalis umbonatus assemblage represent active bottom water currents with more ventilation and relatively low trophic levels reflecting the influence of NADW at abyssal depths in the central Indian Ocean. The early Late Pliocene (c. 3.6-2.8 Ma) and Early Pleistocene (c. 1.8-1.2 Ma) are two significant intervals of prominent faunal change, which indicate entirely different deep-sea conditions. The prominent occurrence of Nuttallides umbonifera assemblage during 3.6-2.8 Ma reflects cold, corrosive and well-oxygenated bottom waters possibly in response to increased flow of AABW towards the central Indian Ocean at abyssal depths. The Early Pleistocene (c. 1.8-1.2 Ma) is characterized by distinct occurrence of the Uvigerina proboscidea assemblage reflecting higher surface productivity. The widespread, intensified monsoon system during Early Pleistocene causes increased upwelling and higher surface productivity, which also enhanced the supply of organic carbon to the ocean floor.