Palynological record of tropical rain forest vegetation and sea level fluctuations since 140 ka from sediment core, south-eastern Arabian sea.

Abstract

Palyno-chronological study of a 552 cm deep sediment core from the south-eastern Arabian Sea covers a time span of ~ 140 ka. The age estimate is based on oxygen isotopic data of planktic foraminifera. Six zones were identified on the basis of palynology covering partially MIS-6 and MIS-5 to MIS-1 (Marine Isotopic Stages). The interglacial period (MIS-5) shows high rate of sedimentation corresponding to high percentage of terrestrial rain forest pollen. The palynological study reveals highstand and intense rain forest vegetation during MIS 5a which corresponds to the similar earlier records of estuarine ecosystem on land in Kerala. The relative sea level rise and fall since ~ 140 ka have been inferred through quantitative estimate of marine versus terrestrial palynomorphs. The terrigenous non-pollen forms comprised abundant Neogene extinct nanoplanktons (Discoaster spp.) until MIS-3 which was transported from the Varkala Formation (highlands of the Kerala coast). The pollen assemblage is comparable to the present day flora in Kerala except Ongokea gore which is now present in South Africa and therefore, the provenance of pollen recorded in the studied core is Western Ghats of India. High percentage of planktic foraminifera and dinoflagellate cysts throughout the core indicates high productivity that suggests upwelling induced by seasonal reversals in the monsoon winds even during the glacial periods. Botryococcus remains and poaceous leaf cuticles dominated by the Pooideae group in the marine realm also point towards active winter monsoon. Considerable fragmentation of rain forest took place post 74 ka and during the LGM as the sea retreated to its maximum which resulted into the vast exposure of land favoring the expanse of grassland with pockets of rain forest “plant refugia” on land. Neogene rain forest flora recorded earlier from the Varkala Formation and the present record of its existence since MIS-6 in the region provide an understanding that the monsoon circulation over southern India was similar since the Middle Miocene Climate Optimum and the flora in the South-Western Ghats is a fossil ecosystem and a relic of the Tertiary period.