Paleohydrology and Paleoecology of the Neogene Siwalik rocks, Nepalese Himalaya using multi-proxy lipid biomarker isotopic study

This study deploys compound-specific multi-proxy isotopic study of lipid biomarkers to understand Neogene climatic and ecological variabilities in the Himalayan foreland. The investigation of compound-specific carbon and hydrogen isotopes along with glycerol dialkyl glycerol tetraether (GDGT) is the first of its kind for the Nepalese Siwalik. A total of 49 mudstone (and some paleosol) samples were collected from the paleomagnetically age-constrained Siwalik strata in the Surai Khola and Karnali River sections. δ13C results suggest a domination of C3 trees between 12 and 8.5 Ma, and a stepwise expansion of C4 grasses starting gradually at 8.5 Ma and culminating rapidly around 5.5 Ma. δD results show an overall gradual increase in rainfall since 12 Ma, with a rapid intensification around 5.5 Ma. The negative correlation between rainfall and GDGT-derived paleotemperature prior to 5.5 Ma indicates that the region experienced higher rainfalls during colder periods and vice versa. We propose that this negative correlation could be related to the strong presence of mid-latitude westerlies in the region because of the subdued Himalayas, when summer monsoon winds were weaker, that brought enhanced winter-precipitation particularly during colder periods. After 5.5 Ma, our data show a conspicuous positive correlation between rainfall and annual temperature, indicating the onset of modern-style seasonality in rainfall in the Indian subcontinent, which generates more rainfall during summer than during winter. Notably, this initiation of the Indian monsoon around 5.5 Ma favored the dominance of C4 grasses over C3 trees that is reflected in our δ13C data