Multi-System and Compound-Specific Isotopic Study of Neogene Vegetation and Climate Changes in the Siwalik Strata, Nepal Himalaya

Abstract:

Despite many studies, causes of the late Neogene vegetation and climate change in the Siwalik succession deposited in the Himalayan foreland basin are still controversial. To render plausible mechanisms of C₄ grass expansion replacing C₃ trees, we applied compound specific isotope analysis of lipid biomarkers preserved in mudstones and paleosols of the Nepal Siwalik. We investigate δ¹³C (vegetation proxy), δD (precipitation proxy) and brGDGTs (mean annual air temperature proxy) of the sedimentary strata deposited in a continental fluvial environment. Samples were collected from various river sections of the Nepal Siwalik to document temporal as well as lateral (along east-west tectonic-strike) variations in vegetation and climate shift. Published paleomagnetic ages of the region provides age constrain of the studied deposits, which range in age from 16 Ma to 2 Ma.

This is the first study that provides compound-specific isotopic data and paleotemperatures of the Siwalik strata in the region. As shown by δ¹³C values, C₄ vegetation (grasses) likely started to expand around 6.5 Ma and became highly dominated in 5.2 Ma. Increased precipitation, likely due to monsoonal intensification, is recorded in δD data around this interval of vegetation shift. brGDGTs data revealed an intriguing cyclic (~2 Ma cycle) variation of paleotemperatures. Integration and further analyses of these key proxy data are ongoing.

Key words: monsoon, Nepal Siwalik, late Neogene, vegetation and climate shift, paleotemperature.