Mio–Pliocene monsoonal record from Himalayan foreland basin (Indian Siwalik) and its relation to vegetational change

Carbon and oxygen isotope ratios of soil carbonate nodules and carbon isotope ratio of associated organic matter were measured from three Indian Siwalik stratigraphic successions in order to reconstruct vegetational history and change in contemporaneous precipitation. δ13C values of soil carbonates show that, from 10.5 to 6 Ma, the vegetation was C3 type and around ∼6 Ma C4 grasses appeared. The δ18O variations of soil carbonates suggest that the monsoon system intensified, with one probable peak at around 10.5 Ma and a clear onset at 6 Ma, with peak at 5.5 Ma. After 5.5 Ma, monsoon strength decreased and attained the modern-day condition with minor fluctuations, which is supported by marine proxy of upwelling in the Arabian Sea and sedimentary morphology in Siwalik. The covariation between δ18O and δ13C data suggests that a change in precipitation pattern was partly responsible for expansion of C4 grasses. In a mixed C3–C4 environment estimation of abundance of C3 and C4 plants using soil carbonate in one case, and residual soil organic matter in another, may differ. This can be explained by assuming that the plant-respired CO2 is the main contributor of carbon in soil carbonate and may have different isotopic composition from that of the residual organic matter in the soil. In addition, the abundance estimate of C3–C4 plants also shows variation with time, probably caused by change in growing season conditions through time.