Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Ajai Kumar Rai ¹, M. S. Srinivasan ², Abhayanand Singh Maurya ¹

Affiliations
1 Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad 211 002, IN
2 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN

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 assembiages reflecting different environmental characteristics. The Cibicides wuellerstorfi-Oridorsalis umbonatus assemblage (Factor I ) represents well oxygenatcd bottom watcrs 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 Lo increased flow of AABW towards the central Indian Ocean at abyssal depths. The Early Plcistocene (c. 1.8-1.2 Ma) is characterized by distinct occurrence of the Uvigerina proboscidea assemblage reflecting higher surface produc6vity. 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.

Keywords

Benthic foraminiferal record, Antarctic Bottom Water, Monsoonal activity, DSDP Site 238, Central Indian Ocean.

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Ajai Kumar Rai ¹, Abhayanad Singh Maurya ¹

Affiliations
1 Department of Earth and Planetary Sciences, Nehru Science Center, University of Allahabad, Allahabad - 21 1 002, IN

Abstract

In the present work, late Oligocene to latest Miocene (-28 6 5 Ma) Deep-Sea benthic foraminifeia(over 149 μm size fraction) were studied from ODP sites 760A and 761B on Wombat Plateau in the eastern lndian Ocean to understand the faunal diversity pattern. The samples were commonly examined at about 0 75m intelva at both the sites. The bentic foraminifeia species diversity was measured in terms of Shannon-Weaver Index [H(S)] Hurlbe lts Diversity Index (S₁₀₀), Alpha Index (α) and Equltability (Ε), all of which show almost similar trends with few exceptions. The Miocene section at both the sites show marked fluctuations in the values of various diversity indices of Beep-Sea benthic foraminiteral assemblages. Distinctly low vatues of diversity indices across the Oligocene-Miocene transition correspond well with the higher values of benthic δ¹⁸0 and δ¹³c reflecting relatively cold, nutrient depleted, young bottom waters which possibly indicate the influence of Antarctic Circumpolar Current (ACC) in this region Faunal diversity reached to maximum values at both the sites near the end of Miocene climatic optimum at about 15 Ma reflecting warm bottom wateis with low trophic levels. The low bottom water temperature, expansion of East Antarctic Ice Sheet (EATS) and possibly higher trophic level appear to be responsible for less diverse fauna during the middle Miocene 1 e -14Ma and younger. Further, decrease in the values of diversity during latest Miocene (-7 5-6 5 Ma) is suggested to be due to increased surface water productivity in response to the intensified monsoonal activity in the eastern Indian Ocean.

Keywords

ODP, Miocene, Benthic Foraminifera, Diversity, Paleoceanography, Wombat Plateau Eastern Indian Ocean.

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