Isotopic evidence for ecological and climate change in the richly fossiliferous Plio-Pleistocene Upper Siwalik deposits exposed around Chandigarh, India

Abstract

The Plio-Pleistocene Siwalik deposits around Chandigarh, well known for their great wealth of mammalian fossils, preserve a continuous record of calcareous paleosols ranging in age from ∼2.7 to ∼0.6 Ma. The carbon isotope composition (δ13C relative to Vienna Peedee belemnite [VPDB]) of 140 pedogenic carbonates from Ghaggar and Nadah sections and 13 gastropod samples from Nadah section record a landscape primarily occupied by C4 grasses during this interval. However, a mixed C3–C4 vegetation between 2.15 and 1.7 Ma and a progressive increase in the percentage of C3 plants from ∼1.1 to ∼0.6 Ma has been recorded. Most of the pedogenic carbonate δ18O values (VPDB) in the Ghaggar River Section range between −5.29 and −8.93‰ indicating the dominance of monsoon climate. These δ18O values exhibit somewhat consistent trend in relation to δ13C values suggesting influence of climate on the vegetation. At two instances ∼1.07 and ∼0.66 Ma, there is significant depletion in carbonate δ18O and δ13C values, suggesting monsoon intensification and a synchronous development of C3 vegetation. Pedogenic carbonate and gastropod δ18O values (VPDB) in the Nadah paleoswamp, also exhibit consistent trends in relation to δ13C values, indicating steady relationship between short-term climatic fluctuations in terms of warm-humid and cool-dry conditions and the abundance of C3/C4 biomass on the landscape nearly 1.8 Ma ago. The vegetation change on the Siwalik landscape during Early and Middle Pleistocene appears to have been influenced mainly by the regional tectonics, sedimentation patterns and monsoonal rains rather than global climate changes.