Neogene vegetation shift in the Nepalese Siwalik, Himalayas: A compound‐specific isotopic study of lipid biomarkers

Abstract

Abstract The late Neogene vegetation change of C4 plants replacing C3 plants is widely documented across the world. This vegetation shift has been particularly well‐documented in the Himalayan foreland based on δ13C isotopic data from palaeosol carbonates and bulk organic matter in the Siwalik sedimentary rocks of Pakistan, India and Nepal, showing asynchronous expansion of C4 plants between 8 and ~5 Ma, 9 and 6 Ma, and around 7 Ma, respectively. In this study, compound‐specific isotopic analysis of lipid biomarkers extracted from shale and palaeosols in the palaeomagnetically age‐constrained Nepalese Siwalik is utilized to better understand this vegetation shift. This is the first comprehensive lipid biomarker study in the Nepalese Siwalik, with new isotopic results from the previously undocumented Karnali River section. The δ13C n‐alkane (C27‐31) results from the Surai Khola section suggest C3 plants were dominant between 12 and 8.5 Ma. A stepwise expansion of C4 plants that started gradually at 8.5 Ma escalated rapidly after 5.4 Ma, so that by 5.2 Ma C4 vegetation dominated the landscape. This dramatic ecological shift at the Miocene‐Pliocene boundary was likely linked with an intriguing tectonic‐climate coupling in the Himalayan‐Tibetan region, prompting wetter summers and drier winters that drove C4 grass expansion. However, in the Karnali River section, the data show no clear sign of C4 plant expansion until 5.2 Ma (youngest sample age). In the two study locations separated laterally by ~200 km along tectonic‐strike, this different trend of vegetation change likely indicates local controls like river‐catchment influence. However, it is possible that, similar to the Surai Khola section, C4 plants dominated after 5.2 Ma in the Karnali River section. This study also suggests that the past vegetation makeup of an area is better reconstructed using isotopic signatures of molecular markers than of bulk organic matter or pedogenic nodules.

Keywords

• Carbon isotope
• Himalayan foreland
• Karnali River
• Siwalik
• Surai Khola