NEOGENE CLIMATE RECONSTRUCTION OF THE EASTERN HIMALAYAN REGION FROM A MULTIPROXY ORGANIC ANALYSIS OF THE SIWALIK DEPOSITS

The Neogene development of the South Asian monsoon system significantly influenced the Indian subcontinent’s climate. Both climate modeling and paleoclimate studies correlate the monsoon development with Tibetan Plateau growth. An increased Neogene exhumation of the Himalaya has also been linked with the monsoon induced weathering. The vegetation response to the onset of the monsoon is however not yet well understood. The modern day South Asian monsoon shows an East to West variation characterized by higher precipitation in the East compared to more arid condition of the West. This is due to influence of the Indian Summer Monsoon in East and the Westerlies in the West. Stable isotope records of pedogenic carbonates (δ¹³C and δ¹⁸O) and n-alkanes (δ¹³C and δD) from the Western Himalayan foreland deposits point to an overall switch from C₃ to C₄ plants in late Miocene – Pliocene time, coeval with the global vegetation change. The nature of this change is debated – between a sharp transitions from C₃ to C₄ versus a more gradual change involving replacement of C₃ by C₄ in more arid sub environments. The vegetation record in the East is however poorly constrained. Only paleo-vegetation records of the Eastern Himalayan region come from bulk carbon analysis of the foreland deposits and n-alkane δ¹³C record from the Bengal Fan sediment. These studies show contradictory record of lack of C₄ plants (bulk carbon record) versus a similar switch as the Western part (Bengal fan sediment). We present a new multiproxy climate reconstruction from the Eastern Siwalik deposits from the Tista Valley in West Bengal, India. Both δ¹³C and δD of n-alkanes from C₄ vegetation have higher values and these isotopic data can be used to reconstruct the vegetation signature. The δD of n-alkanes can be further used to reconstruct the precipitation δD when the enrichment factor is known. We used an independent vegetation proxy, lignin, from the same samples to constrain the type ofr plants and branched/isoprenoid tetraether index from Br-GDGTS to constrain the temperature during sediment deposition. Our dataset provide one of the first comprehensive climate records from the Eastern Siwaliks. It should be mentioned here that our study area is far west of the Shillong Plateau and therefore exhumation of the Plateau has no effect on the paleoclimate of this area.