Stable isotopic studies of palaeosol sediment from Upper Siwalik of Himachal Himalaya: evidence for high monsoonal intensity during late Miocene?

摘要

Palaeosols from sedimentary section exposed 2800 m at Haripur–Kolar on Somb River in Himachal Himalaya were studied for stable isotopes of carbon, oxygen, and hydrogen along with Sr isotopic ratios. Stable isotopic studies of middle and upper Siwalik sections from Pakistan and Nepal region have been used for inferring the palaeoclimate of the region during Early and Late Miocene time. The aim of our work was to extend this type of study to Himachal Himalaya, India. Carbon isotopes in palaeosol carbonate nodule and associated organic matter indicate an abrupt transient of C_3 vegetation dominance around 4 Ma. This transient disrupted the general trend of C4 ecological dominance observed from about 8 to 4 Ma in this region of Asian subcontinent. We attribute this observation with the higher monsoonal intensity, which also is supported by different archives from the region (G. bulloides abundance from DSDP core from the Arabian Sea and grass pollens abundances on land). We propose that greater moisture availability around 4 Ma due to higher monsoonal activity caused an abrupt transient of C_3 dominance in this region. The proposition is further supported by our data on δ~(18)O of carbonates and δD of clay, showing depletion around 4 Ma. Observed high ~(87)Sr/~(86)Sr ratios in carbonates around the same time period suggest increased weathering and, in turn, higher rainfall activity.