Pleistocene Climatic Changes as Deduced from a Pollen Analysis of Site 717 Cores

Abstract

Pollen from marine sediment has been used to the study of monsoon fluctuations in the Arabian Sea (Van Campo et al., 1982; Van Campo, 1986) and off the northwest African coast (Hooghiemstra, 1988). These studies reveal major changes in the monsoon cycle since the last interglacial period. However, fluctuations before 120 k.y. B.P. remain unclear. In this report, we document a dramatic climatic change that occurred around the beginning of the middle Pleistocene in the Bay of Bengal, based mainly on the results of pollen analysis from ODP Site 717 cores. ODP Site 717 cores were recovered from the tip of the Bengal deep sea fan. The site (0°55'S, 81°23'E) is located about 700 km south of Sri Lanka (Fig. 1). Alam (1989) distinguishes two major phases of sedimentation within the Bengal Basin and pointed out that the second major phase started in the middle Miocene. It is generally assumed that sedimentation of the Bengal deep sea fan was strongly influenced by the uplift and erosion of the Himalayas. Sedimentation of palynomorphs was also influenced by river transport of the Ganges, Brahmaputra, and Irrawaddy Rivers. The Bengal Basin is characterized by a very pronounced monsoon circulation system, with a northeast monsoon in winter and a southwest monsoon in summer (Figs. 1,2). These winds are largely responsible for the transportation of windborne pollen in the area. Surface water circulation in the Bengal Bay and Andaman Sea is also dependent on the monsoonal wind patterns. During the northeast (winter) monsoon, a North Equatorial Current appears, while during the southwest (summer) monsoon, a Southwest Monsoon Current develops (Figs. 1, 2). These water currents influence the transportation and deposition of pollen and spores in the sediments on the ocean floor (Figs. 1,2). Due to the scarcity of wind-pollinated flowers, the very rich flora of the tropical rain forest is poorly preserved in the marine sediments. Mangrove pollen is, however, well preserved, because the major palynomorphs are transported by river and surface water currents. The distribution of mangrove swamps in the coastal area of the Bengal Bay and Andaman Sea is shown in Figure 1.