Floristic and climatic reconstruction in the Indo-Burma region for the last 13,000 cal. yr: A palynological interpretation from the endangered wetlands of Assam,northeast India

Abstract

The present study deals with pollen analyses from two sedimentary sequences (280 and 150 cm deep) cored from wetlands in Deepor (Kamrup district) and Hasila (Goalpara district) in the western Brahmaputra flood plain of Assam. This region is a part of the Indo-Burma region and documents changes in vegetation, climate and alterations in the wetland level as well as of anthropogenic impacts as related to the fluctuating orientation of the southwest monsoon during the past 13,040 cal. yr. The presence of cool taxa, both trees and ferns, Schima, Ilex, Gleichenia dichotoma and Pteris pentaphylla, though present in small amounts, indicate cool and dry climatic conditions from 13,040 to 11,700 cal. BP. This is followed by a shift to warmer and moderately humid climate between about 8390–3450 cal. BP which coincides with the Holocene thermal maximum (8000–7000 yr BP) and could be attributed to an intensified southwest monsoon. A palynozone depicting less warm and humid climate was observed between 3450 and 2250 cal. BP because of the occurrence of low frequencies of Shorea and Terminalia along with marshy taxa. Climatic conditions were again improved from 2250 cal. BP onwards under warm and moderately humid climate owing to the proliferation of Sal and its associates and thereafter, between 1500 and 710 cal. BP, an increased humid climate is indicated by relatively high percentages of Shorea, Syzygium, Impatiens, Onagraceae and Myriophyllum indicating strong monsoonal conditions corresponding to Medieval Warm Period (MWP). Deterioration of the tropical mixed deciduous forest under a warm and relatively dry climatic regime has occurred since 710 cal. BP accompanied by accelerated human settlement, as shown by an abrupt increase in Poaceae > 45 µm along with typical ruderal pollen taxa.