LATE NEOGENE VEGETATION SHIFT AND HYDROLOGIC CHANGES IN THE NEPAL HIMALAYA FROM COMPOUND-SPECIFIC ISOTOPE STUDY

Late Neogene global expansion of C₄ grasses replacing C₃ trees is well documented. Although a drop in atmospheric _pCO₂ levels was advocated for this vegetation shift, recent studies have argued for other causes, including changes in precipitation pattern, seasonal temperature, and fire frequency. For a deeper understanding of this vegetation shift in the Siwalik succession of Nepal, we deployed compound-specific isotope analysis of biomarkers preserved in floodplain shale and paleosols. This analysis includes δD (precipitation proxy) and δ¹³C (vegetation proxy) from n-alkyl lipids (both n-alkanes and n-alkanoic acids) of leaf wax, and branched glycerol dialkyl glycerol tetraethers (GDGTs) (mean-annual air temperature proxy) derived from soil bacteria. Organic-rich shale and paleosol samples were collected from the Siwalik strata in Surai Khola and Karnali River sections. The ages of studied successions that range from 15 to 3 Ma were constrained by correlating strata with established paleomagnetic age data of the region.

Here, we present new results of compound-specific isotope analysis of the Nepal Himalaya. δ¹³C values of C₂₇ n-alkanes show that C₄ grasses started to dominate the landscape since 6.5 Ma, and by 5.2 Ma there was a clear predominance of C₄ plants. This timing of vegetation shift broadly matches with the previous vegetation change studies of the Siwalik rocks. δD values of C₂₇ n-alkanes indicate gradual intensification of precipitation since 9 Ma and an abrupt (monsoonal) intensification at 5.2 Ma, the timing of which also matches with an abrupt increase in mean-annual air temperature as interpreted from GDGT-derived paleotemperature data. These results indicate that 5.2 Ma could be an important time for drastic paleoecological and paleoclimate changes in the Nepal Siwalik, which is being further investigated.