Pollen-inferred vegetation vis-a-vis climate dynamics since Late Quaternary from western Assam, Northeast India: Signal of global climatic events

摘要

Palynological observation of one 3.4 m deep sedimentary profile cored from Chayagaon swamp, Kamrup District, Western Brahmaputra flood plain of Assam, Northeast India, has revealed climate and vegetation succession since the Late Quaternary. The contemporary pollen/vegetation relationship in and around the swamp can be employed as background information for the palaeoclimate studies. Persistent fluvial activity during 14,895-12,450 cal BP may be responsible for the paucity of pollen and spores, with fluvial marker taxa such as Ludwigia octavalvis, Mimosa pudica, Pleospora, Glomus and Botryococcus along with frequent Pseudoschizia. Later, fluvial activity was succeeded by tropical tree savanna under cool and dry climate with weakened monsoon precipitation between 12,450-10,810 cal BP, corresponding to the Younger Dryas. Between 10,810-7680 cal BP, relatively less cool and dry climate prevailed with inception of tropical mixed deciduous arboreals including Syzygium cumini, Dillenia pentagyna and Lagerstroemia parviflora. Human activity was witnessed and some sort of incipient cereal-based pastoral practice commenced during this phase. This phase is followed by fluvial activity between 7680-6780 cal BP, as indicated by reduced frequency of pollen and spores along with frequent fluvial marker taxa. The fluvial activity was succeeded by enrichment of tropical mixed deciduous forest laden with the invasion of arboreal taxa including S. cumini, Terminalia bellirica, Mimusops elengi and D. pentagyna along with a relative increase of marshy and aquatic taxa. The warm and moderately humid climatic regime between 6780-1950 cal BP is well matched with the peak period of Holocene Climatic Optimum (7000 BP) signifying an increase in monsoon precipitation. The size of the swamp increased comparatively along with a relative decrease in agricultural practice. During 1950-989 cal BP, the establishment of tropical mixed deciduous forest occurred with further proliferation of deciduous tree taxa under increased warm and humid climate, corresponding to the Medieval Warm Period. The swamp level rose relatively, along with an abrupt decline in cereal and other cultivation taxa during this phase. Deterioration of the tropical mixed deciduous forest under a warm and relatively dry climatic regime since 989 cal BP was accompanied by accelerated human settlement, as shown by an abrupt increase in cereal pollen along with typical ruderal pollen taxa. Relative increases in Melastoma, Ziziphus and Areca catechu combined with decreased arboreal Shorea, Terminalia, Lagerstroemia, Dillenia and Emblica imply forest clearance in this phase.