Carbon isotopic ratios of modern C3–C4 plants from the Gangetic Plain, India and its implications to paleovegetational reconstruction

Abstract

For the first time, carbon isotope ratios (δ13C) of plants and occluded carbon in phytoliths were analyzed from three locations of the middle (26.5°N, 80.3°E) and lower (22.3°N, 87.3°E and 22.9°N, 88.5°E) Gangetic Plain, India. The bulk δ13C values of C3 plants range from −32.6 to −19.2‰ with an average of −29.6±1.9‰ (n=76, 1σ) and for C4 plants values range from −16.6 to −10.4‰, averaging at −12.7±1.4‰ (n=45, 1σ). The measured average δ13C value of C3 plants is lower than globally reported dataset. Comparison of average δ13C values of C3 plants with mean annual precipitation (MAP) of the three locations show that lowering in δ13C value with increase in MAP. However, for C4 plants, increase in MAP resulted in higher δ13C values. These observations are similar with the earlier findings that the δ13C values in C3 plants are controlled by stomatal conductance and for C4 plants leakiness of carbon from bundle sheath cell played important role. The average δ13C value of C3 plants from the modern Gangetic Plain is lower compared to the observed δ13C value (~−26‰) of bulk soil organic matter (SOM) derived from contemporary C3 plants in the lower and middle Siwalik (ancestral Gangeitc plain). The enriched value in Siwalik SOM likely indicates contribution from C4 plants. This implies early appearance (at 11Ma) of C4 plants compared to previously reported timing. The δ13C values of occluded carbon in C3-derived phytoliths range from −33.9 to −24.5‰ with an average of −29.0±2.6‰ (n=26, 1σ) and that for C4-derived phytoliths range from −30.7 to −21.4‰, averaging at −26.1±2.6‰ (n=12, 1σ). Organic carbon (OC) concentrations in C3-phytoliths vary from 0.1 to 6.1%, averaging at 1.1±1.3% (n=26, 1σ) and in C4-phytoliths 0.1 to 3.8% with an average value of 1.1±1.4% (n=12, 1σ). Being protected from degradation, selective plantation with high OC in phytoliths will enhance terrestrial carbon sequestration.