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

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 delta C-13(org) of organic matter, lipid biomarker (n-alkane) as well as pollen data derived from similar to 1.4 m sedimentary profile spanning the last similar to 3150 cal BP. The delta C-13(org) values during late Holocene range from -29.1 parts per thousand to -27.7 parts per thousand suggesting that organic carbon was derived exclusively from C-3 vegetation. The profile is characterized by a homologous series of n-alkanes ranging in chain length from n-C-18 to n-C-35 with maxima at n-C-29. The generated proxy data reveal three phases of climatic and vegetation shifts with the first phase from similar to 3150 cal BP to similar to 2300 cal BP being characterised by relatively high delta C-13(org) values and occupied by Pine-Oak forest indicating a comparatively dry climatic conditions. Subsequently, the second phase from similar to 2300 cal BP to similar to 1000 cal BP shows an increase in arboreal pollen (tree elements) and gradually decreasing trend of delta C-13(org) with a total shift of similar to 1.4 parts per thousand suggesting moist climatic conditions. During the third phase from similar to 1000 cal BP onwards, the precipitation declined fill date as suggested by increasing trend in delta C-13(org) values and good recovery of Pinus-Oak forest pollens. This study provides significant insights into the use of delta C-13(org), 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 (C-3 and/or C-4) of vegetation.