Morpho-tectonic control on the distribution of C3-C4 plants in the central Himalayan Siwaliks during Late Plio-Pleistocene

Abstract

The Siwalik deposits of the Himalayan foreland basin (HFB) preserve the Late Miocene-Pleistocene record of the Himalayan tectonics, monsoonal variation and the expansion of C4 plants. Previous vegetation reconstructions emphasized the Late Miocene expansion of C4 plants with an implicit assumption that the vegetation thrived in the floodplain of lowland rivers. The coarsening-upward sequence of the Siwalik Group suggests deposition in an alluvial fan setting in which the Lower and Middle Siwaliks are deposits of distal region whereas the Upper Siwaliks represents a proximal region of the fan. The modern alluvial fans forming in the Himalayan foothills show a significant difference in elevation (ca. 300 m) and vegetational composition between their proximal and distal regions. In the HFB, the elevation difference is expected to be more pronounced due to the surface uplift of Siwalik deposits along the Himalayan Frontal Thrust (HFT). The increased elevation would have affected the vegetation distribution in the Upper Siwaliks which implies that the vegetation in the proximal part of the fan might not represent lowland floodplain. However, the vegetation composition is less understood from the Upper Siwaliks region due to the scarcity of conventional proxies in these younger foreland deposits.In this context, δ13C values of bulk soil organic matter (SOM), n-alkane and n-alkanoic acid in the paleosols were measured from Late Mio-Pleistocene Siwaliks at Surai Khola (Nepal) to comprehend the impact of elevation on vegetation distribution in the HFB. The results suggest most commonly observed expansion of the C4 plants at ca. 7 Ma, and a unique second phase of expansion of C3 plants post-3 Ma. The higher δD values of n-alkane and n-alkanoic acid suggest that the climate was drier in the last 4 Myr; most likely driven by the onset of the Northern Hemisphere Glaciation (NHG). The growth of C3 plants was favored due to cool climatic conditions induced by a higher elevation in the proximal part of the fan. Water-bearing conglomerates in the Upper Siwaliks may have helped the C3 plants to thrive in a relatively drier climate. This multi-proxy paleovegetation reconstruction demonstrates a morpho-tectonic control on the abundance of C3-C4 plants in the Siwaliks with the possible influence of NHG.