PALEOCLIMATE ANALYSIS OF THE NEOGENE EASTERN HIMALAYAN SIWALIK DEPOSITS THROUGH BIOMARKER PROXIES

The Neogene Siwalik Group deposits of the Himalayan foreland basin provide critical information on monsoon establishment, ecological changes, and Himalayan tectonics. Stable isotope and biomarker analysis of the western Siwalik deposits from Pakistan and Nepal point to a transition from C3 to C4-dominated ecology at ~9-7 Ma. In-situ deposits from the central Siwalik Basin and transported sediment from the Bengal Fan indicate that a similar switch may have occurred a few million years later in this region. This Neogene C3-C4 switch was not documented in the few studies from the far-eastern Siwalik locations. Thus, this is an important area to study as proper identification of paleovegetation can provide critical information on the timing of the establishment and strengthening of the Indian Summer Monsoon (ISM) and the timing of possible marine incursions, as recently proposed. Our study uses biomarkers extracted from sedimentary rocks along the Siji River in Arunachal Pradesh, India.

We analyzed δ¹³C_org, total organic carbon (TOC), n-alkane distributions, and compound-specific Hydrogen (δD_cs) and Carbon (δ¹³C_cs) values of n-alkanes to reconstruct the paleovegetation. Samples come from the Lower Siwalik Dafla (n=8), Middle Siwalik Subansiri (n=7), and the Upper Siwalik Siji (n=14) and Kimin (n=4) formations. The n-alkane distribution of the Dafla and Siji formations (recently proposed as marine deposits) show bimodal distributions – as would be expected in a marine setting. However, the δ¹³C_org, TOC, and isotope values show a pronounced change between the lower two and upper two formations. In the lower formations (Dafla and Subansiri), average TOC is 0.199% and 0.411%(wt), δ¹³C_org averages –25.95‰ and –25.19‰, respectively, and δD_cs values range from –173.5‰ to –121.4‰. A shift towards lower TOC, more positive δ¹³C_org and δD_cs occur in the two upper formations (Siji and Kimin) with average TOC of 0.065% and 0.13%(wt), δ¹³C_org values of –23.75‰ and –23.26‰, respectively, and δD_cs values ranging from –169.5‰ to –7.8‰. The timing of this shift is contemporaneous with similar δ¹³C_org shifts of 2-3‰ reported in paleoclimate reconstructions from other parts of the Siwalik Group. This change is potentially linked to the development of strong seasonality and the onset of ISM.