Early Holocene Indian summer monsoon and its impact on vegetation in the Central Himalaya: Insight from δD and δ~(13)C values of leaf wax lipid

摘要

Increase in rainfall during the early Holocene has been reported from different regions of Indian subcontinent, although the magnitude shows spatial variability with lower change in higher rainfall regions and vice versa. While the early-Holocene rainfall records are available from lowland areas of the Indian sub-continent, the record is poorly documented from the high-altitude Central Himalayan region where orography plays an important role in the spatial variability in the rainfall pattern. For the first time, delta D-C29, delta C-13(C29) values of alkane and delta C-13 values of organic matter from a relict lake of Benital area in the Central Himalaya have been used to reconstruct monsoonal rainfall and contemporary vegetation for last 10.5 ka with an emphasis on the early Holocene. The delta D-C29 values suggest that the early Holocene was characterized by a wet phase at ca. 9 ka with 25% higher rainfall compared to present while the middle-late-Holocene was relatively arid. The estimated increase in monsoonal rainfall for the early Holocene is the highest compared to the previous report from elsewhere. The magnitude of early-Holocene rainfall estimated in this study suggests that in addition to erosion of the bed-rocks at the head-water region, increased stream power reworked the older floodplain and resulted in 2.3 times higher sediment discharged into the Ganges-Brahmaputra (GB) delta compared to the present discharge. The delta C-13(C29) values of the lake sediments suggest a transition from woody to non-woody plant assemblages occurred at 7 ka. The poor correlation between delta D-C29 and delta C-13(C29) values suggests at higher rainfall, the delta C-13(C29) values of catchment vegetation were less responsive.