Variability of Indian monsoonal rainfall over the past 100 ka and its implication for C₃-C₄ vegetational 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 δ¹⁸O values of soil carbonate (δ¹⁸O_SC) analyzed from the cores range from -8.2 to -4.1‰. Using these variations in δ¹⁸O_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 δ¹⁸O value of rainwater-amount effect suggests maximum decrease in rainfall intensity (~ 20%) during the last glacial maximum. The δ¹³C values of soil carbonate (δ¹³C_SC) and SOM (δ¹³C_SOM) range from − 6.3 to + 1.6‰ and − 28.9 to − 19.4‰, respectively, implying varying proportions of C₃ and C₄ vegetations over the Ganga Plain during the last 100 ka. The comparison between monsoonal rainfall and atmospheric CO₂ with vegetation for the time period 84 to 18 ka indicate that relative abundances of C₃ and C₄ vegetations were mainly driven by variations in monsoonal rainfall.