C4 plants decline in the Himalayan basin since the last glacial maximum

Abstract

The Bengal Fan turbiditic system, supplied by the Ganga–Brahmaputra river system, provides an integrated record of Himalayan system erosion. Thanks to minor inputs of marine organic matter and almost complete preservation of riverine organic carbon, organic matter buried in Bengal Fan sediments is a proxy of Himalayan basin paleo-vegetation. The active channel-levee system of the middle fan documents the last 19 ka and allows the reconstruction of vegetation change in the Himalayan basin since the Last Glacial Maximum (LGM). We measured δ¹³C of organic carbon (C_org) in order to track changes in the proportions of C3 and C4 plants in the Himalayan basin. From LGM to mid-Holocene, δ¹³C of bulk C_org shifts of 3–4‰ towards more negative values. Relative abundance of individual n-alkanes reveals that terrestrial higher plant inputs represent a dominant fraction of C_org buried in Bengal Fan sediments. δ13C of higher plants biomarkers mimic that of bulk C_org showing that the later can be used as continental paleo-vegetation proxy. δ¹³C negative shift from LGM to mid-Holocene, mostly indicates the transition from a dominant input of C4 plants to a dominant input of C3 plants and therefore reveals that C4 plants were more abundant in the basin under glacial conditions. Vegetation repartition in the basin simulated using the CARAIB dynamic vegetation model is consistent with C_org data. The model indicates a dominance of C4 plants in the Gangetic plain during the LGM while eastern part of the basin remains dominated by C3 plants. The comparison between our data and proxies of regional paleo-climate suggests that the large decline of C4 plants after the LGM was due to combined increase of atmospheric CO₂ and humidity levels. Integrated record of Himalayan basin paleo-vegetation suggests more arid conditions during the LGM than during the mid-Holocene and agrees with reconstructions of the monsoon indicating stronger SW monsoon during interglacial and stronger NE monsoon during glacial periods.