C4 plant expansion in the Ganga Plain during the last glacial cycle: Insights from isotopic composition of vascular plant biomarkers

This study aims at reconstructing the paleo-vegetation and soil organic matter (SOM) dynamics in the Ganga Plain during 80-20 ka time interval. We studied a sediment core (Kalpi, 50 m long) from south part of the Ganga Plain. We measured the concentration of n-alkanoic acids and n-alkanes and the δ13C values of long chain saturated fatty acids extracted from the Ganga Plain paleosols. The abundance and distribution of n-alkanes and fatty acids reveal that the SOM in the Ganga Plain sediments are composed of a complex mixture of petrogenic, microbial and vascular plant inputs. The isotopic composition of long chain fatty acids suggests that SOM in the Ganga Plain sediments was formed by partial degradation of a Himalayan riverine organic carbon component followed by addition of the local Ganga Plain organic matter. In addition, the Ganga Plain paleo-vegetation history can be reconstructed using the δ13C values of selected long chain fatty acids. The δ13C values of C32 fatty acid (δ13C32:0) suggests mixed C3-C4 plants over the Ganga Plain between 80-20 ka and a rapid doubling in C4 plants abundance at around 45 ka. Comparison with a set of paleo-environmental proxies suggests that C4 plant expansion was closely controlled by hydrological conditions in the Ganga Plain.