Geological Society of America

T343 World of Lakes (Posters)

Wednesday, 30 October 2013
9:00 AM - 6:30 PM
Hall D

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

Paper number: T343-1
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Prabhat C. Neupane Presenting Author
University of New Orleans, New Orleans, LA
M. Royhan Gani
University of New Orleans, New Orleans, LA
Nahid D. Gani
Western Kentucky University, Bowling Green, LA
Yongsong Hunag
Brown University, Providence, RI

Abstract

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.