Multiproxy records of vegetation and monsoon variability from the lacustrine sediments of eastern Ganga Plain since 1350 A.D.

Abstract

The lacustrine sediments of 90 cm thick profile of Nikahari Tal (lake), Deoria district in the Ganga Plain were analyzed for multiproxy study using pollen analysis, sedimentology and AMS radiocarbon date to deduce short term climate and vegetational changes since about 1350 A.D. (600 BP). The presence of arboreal and dominance of herbaceous vegetation along with aquatic elements and fresh water alga Botryococcus from the basal part suggests the prior existence of lake and open grass land vegetation in the region. However, the scantiness of tree and shrubby taxa along with higher percentage of clay in the beginning indicates relatively warm and less humid climate than today. The presence of Cerealia with other culture pollen taxa indicates prevalence of agrarian practice in the region. Substantial increase in the tree taxa of Holoptelea integrifolia, Madhuca indica, Acacia, Emblica officinalis, Syzygium cumini and Myrtaceae along with the increase in shrubs, marshy and aquatic elements suggest the expansion of forest grove within the grassland as a consequence of enhanced precipitation during ∼1420–1620 A.D. Similar trends are observed by higher percentage of silt content owing to increased surface runoff in response to enhanced precipitation and high water budget under strong Indian Summer Monsoon (ISM) during above period. A highly variable vegetation scenario existed during 1620–1900 A.D. as a consequence of fluctuating climatic conditions. The variable percentages of silt and clay indicate the increase and decrease in the surface runoff so as the water column and energy in the lake, in direct response to enhanced and weak ISM respectively. During 1620–1700 A.D., the arboreals became less diversified and a decline in the diversity of terrestrial herbs is also witnessed, which is also corroborated by exceptionally high percentages of clay elucidating stagnant low-energy ponding condition under weak ISM during the Little Ice Age (LIA). During the time interval of 1700–1800 A.D., considerable improvement in the floristics has been noticed due to climate amelioration. From 1800 A.D. onwards, the increasing frequency trends of arboreal and aquatic taxa along with relatively increased silt content suggests moderately warm and humid climate. The well-sorted sediment explains the consistency in the water level and energy of the lake.Pollen analysis coupled with sediment grain parameters indicate intense ISM from 1400 to 1600 A.D. and 1850 A.D. onwards and weak monsoon around 1600–1750 A.D. The signals of weak monsoon around 1650, 1770 and 1850 A.D. are synchronous with the Little Ice Age.