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

For the first time, carbon isotope ratios (delta C-13) of plants and occluded carbon in phytoliths were analyzed from three locations of the middle (26.5 N, 803 E) and lower (22.3 degrees N, 873 degrees E and 22.9 degrees N, 88.5 degrees E) Gangetic Plain, India. The bulk delta C-13 values of C-3 plants range from 32.6 to 192%0 with an average of 29.6 1.9%0 (n = 76, lo) and for C-4 plants values range from -16.6 to -10.4%0, averaging at -12.7 +/- 1.4%0 (n = 45, 1 sigma). The measured average delta C-13 value of C-3 plants is lower than globally reported dataset Comparison of average delta C-13 values of C-3 plants with mean annual precipitation (MAP) of the three locations show that lowering in delta C-13 value with increase in MAP. However, for C-4 plants, increase in MAP resulted in higher delta C-13 values. These observations are similar with the earlier findings that the 613C values in C-3 plants are controlled by stomatal conductance and for C-4 plants leakiness of carbon from bundle sheath cell played important role. The average delta C-13 value of C-3 plants from the modern Gangetic Plain is lower compared to the observed delta C-13 value (- 26%) of bulk soil organic matter (SOM) derived from contemporary C-3 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 11 Ma) of C-4 plants compared to previously reported timing. The 813C values of occluded carbon in C3-derived phytoliths range from -33.9 to -24.5%0 with an average of 29.0 +/- 2.6%0 (n = 26, la) and that for C-4-derived phytoliths range from -30.7 to -21.4%0, averaging at 26.1 +/- 2.6%0 (n = 12, lo). Organic carbon (OC) concentrations in C-3-phytoliths vary from 0.1 to 6.1%, averaging at 1.1 +/- 13% (n = 26, 1 sigma) and in C-4-phytoliths 0.1 to 3.8% with an average value of 1.1 +/- 1.4% (n = 1 sigma, 1 sigma). Being protected from degradation, selective plantation with high OC in phytoliths will enhance terrestrial carbon sequestration. (C) 2015 Elsevier B.V. All rights reserved.