Carbon isotopic composition of lignin biomarkers: Evidence of grassland over the Gangetic plain during LGM

Abstract

Carbon isotope (δ13C) compositions of total organic carbon (TOC) as well as several extracted higher plant derived lignin biomarkers, along with the recovery of grass phytoliths, provide the first direct evidence of existence of extensive grass land (C4) vegetation over the Ganges deltaic plain during the Last Glacial Maximum (LGM). The earliest Holocene (∼9ka) is marked by pronounced 13C depletion in the lignin phenols coinciding with rapid sea level rise, intensified monsoon, and replacement of grassland vegetation by mangroves. Another C4 invasion phase is identified during the late Holocene (∼6–2ka). Both the LGM and late Holocene C4 phases coincide with the well known monsoon minima. The study suggests that change in pCO2 might not be the singular driver of vegetation change as proposed recently and the climate models must take the monsoon like mega-climate processes into account for reconstructing the glacial biomes.