The disparity in the abundance of C4 plants estimated using the carbon isotopic composition of paleosol components

Long-term paleovegetational records from continental settings help in comprehending regional and global forcing on the abundance of C-3-C-4 plants in the past. The carbon isotopic composition of soil carbonates (delta C-13(SC)), soil organic matter (delta C-13(SOM)), organic matter occluded in soil carbonate nodules (delta C-13(NOM)) and biomarkers in paleosol organic matter (long-chain fatty acid; delta C-13(FAME)) are often used to estimate changes in the past-vegetational composition. However, it has been observed that depending on the type of proxy, the estimated abundance of C-3-C-4 plants varies, which can lead to uncertainty in paleovegetational records. Hence, the present study aims to comprehend the factors affecting the delta C-13(SC), delta C-13(SOM), delta C-13(NOM) and delta C-13(FAME) values within a paleosol. In this context, available delta C-13(SC), delta C-13(SOM), delta C-13(FAME) and newly measured delta C-13(NOM) values from the late Quaternary sequences of the Ganga Plain, India has been used. The abundance of C-4 plants calculated from the delta C-13(SC) and delta C-13(FAME) values is similar to 2% to 89% higher compared to the delta C-13(SOM) and delta C-13(NOM) values-based estimates. Even with a common source of organic matter, the delta C-13(SOM) values indicate a higher abundance of C-4 plants (similar to 2% to 50%) compared to the estimates from delta C-13(NOM) values. We suggest that the disparity is due to the variation in the response of proxies to perturbations in the paleovegetational regime, growing season condition, and isotopic fractionation during decomposition and incorporation of organic matter into the soil. For example, the organic matter is incorporated into the soil throughout the year and represents average annual biomass, whereas SC precipitates under warmer and often drier condition when the ratio of C-4 to C-3 plant respiration is higher. Additionally, preferential degradation of C-13 enriched labile compounds and C-4 plants derived organic matter may lower the delta C-13(NOM) values resulting in an under estimation of C-4 plants in a mixed C-3-C-4 environment. The higher abundance of C-4 plants estimated from the delta C-13(FAME) values is due to the isotopic fractionation (C-13 enrichment of 2 parts per thousand to 7 parts per thousand) during incorporation of plant-derived long-chain fatty acids into the soil. The disparity in the abundance of C-4 plants estimated from delta C-13(SOM) and delta C-13(NOM) values is due to the difference in the preservation of SOM and NOM. Contrary to the NOM (which is in a closed system), SOM in open system within the soil matrix is subjected to C-13 enrichment due to the long-term humification of organic matter. Various factors such as grain size and pedogenesis that are inherent to the depositional environment also control the delta C-13 values of paleosol components. Considering the uncertainties associated with the delta C-13 values of paleosol components, reporting the absolute abundance of C-4 plants would be uncertain. Therefore, we recommend presenting the relative change in abundance of C-3-C-4 plants during paleovegetational reconstruction.

Automated summary

The study reveals discrepancies in estimating the abundance of C-3-C-4 plants using different proxies due to variations in response to changes in vegetation and ecological conditions. Different proxies in paleosols exhibit diverse isotopic fractionation processes, leading to uncertainties in estimating plant abundances from soil components.