Variability of Indian monsoonal rainfall over the past 100 ka and its implication for C₃–C₄vegetational change

Abstract

Oxygen and carbon isotope ratios of soil carbonate and carbon isotope ratios of soil organic matter (SOM) separated from three cores, Kalpi, IITK and Firozpur, of the Ganga Plain, India are used to reconstruct past rainfall variations and their effect on ambient vegetation. The δ 18 O values of soil carbonate (δ 18 O SC ) analyzed from the cores range from −8.2 to −4.1‰. Using these variations in δ 18 O SC values we are able, for the first time, to show periodic change in rainfall amount between 100 and 18 ka with three peaks of higher monsoon at about 100, 40 and 25 ka. The estimation of rainfall variations using δ 18 O value of rainwater-amount effect suggests maximum decrease in rainfall intensity (~ 20%) during the last glacial maximum. The δ 13 C values of soil carbonate (δ 13 C SC ) and SOM (δ 13 C SOM ) range from −6.3 to + 1.6‰ and −28.9 to −19.4‰, respectively, implying varying proportions of C 3 and C 4 vegetations over the Ganga Plain during the last 100 ka. The comparison between monsoonal rainfall and atmospheric CO 2 with vegetation for the time period 84 to 18 ka indicate that relative abundances of C 3 and C 4 vegetations were mainly driven by variations in monsoonal rainfall.