PLEISTOCENE OCEANOGRAPHIC CHANGES INDICATED BY DEEP-SEA BENTHIC FORAMINIFERA IN THE NORTHERN INDIAN-OCEAN

An attempt has been made to understand the Pleistocene bottom water hi story in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera rec orded a few species show dominance with changing percent frequencies d uring most of the sequence. The dominant benthic foraminiferal assembl ages suggest that most of the Pleistocene bottom waters at site 219 an d Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate betwe en North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW). U vigerina proboscidea is the most dominant benthic foraminiferal specie s present during the Pleistocene at both the sites. A marked increase in the relative abundance of U. proboscidea along with less diverse an d equitable fauna during Early Pleistocene suggests a relative cooling , an intensified oceanic circulation and upwelling of nutrient rich bo ttom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increa sed winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relative ly warm and stable bottom waters as reflected by low abundance of U. p roboscidea and more diverse and equitable benthic fauna. The lower dep th range for the occurrence of Bulimina aculeata in the Indian Ocean i s around 2300 m, similar to that of many other areas. B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance of Stilosto mella lepidula occurs close to the Early/Late Pleistocene boundary.