Vegetational responses to monsoon variability during Late Holocene: Inferences based on carbon isotope and pollen record from the sedimentary sequence in Dzukou valley, NE India

Abstract

The study aims to identify regional vegetational changes during the late Holocene from the Dzukou valley in north-eastern (NE) India and the potential relationship of these vegetation dynamics with climatic shifts. The investigations utilize a combination of δ13Corg of organic matter, lipid biomarker (n-alkane) as well as pollen data derived from ~1.4 m sedimentary profile spanning the last ~3150 cal BP. The δ13Corg values during late Holocene range from −29.1‰ to −27.7‰ suggesting that organic carbon was derived exclusively from C3 vegetation. The profile is characterized by a homologous series of n-alkanes ranging in chain length from n-C18 to n-C35 with maxima at n-C29. The generated proxy data reveal three phases of climatic and vegetation shifts with the first phase from ~3150 cal BP to ~2300 cal BP being characterised by relatively high δ13Corg values and occupied by Pine-Oak forest indicating a comparatively dry climatic conditions. Subsequently, the second phase from ~2300 cal BP to ~1000 cal BP shows an increase in arboreal pollen (tree elements) and gradually decreasing trend of δ13Corg with a total shift of ~1.4‰ suggesting moist climatic conditions. During the third phase from ~1000 cal BP onwards, the precipitation declined till date as suggested by increasing trend in δ13Corg values and good recovery of Pinus-Oak forest pollens. This study provides significant insights into the use of δ13Corg and biomarkers along with pollen data to understand the influence of fluctuating rainfall (in a high rainfall zone) in altering the floristic composition within a region. Thus, the study has provided ground for further investigations to address issues related to biodiversity loss, extent of endemism in high rainfall zones dominated by single types (C3 and/or C4) of vegetation