Comparison of the carbon isotope composition of total organic carbon and long-chain n-alkanes from surface soils in eastern China and their significance

Abstract

Surface soil samples collected over a high spatial resolution in eastern China were analyzed for carbon isotope composition (δ13C) of total organic carbon (TOC) and higher plant-derived long-chain n-alkanes, with the latter reported as weighted mean values. The two sets of δ13C values are significantly correlated and show similar trends in spatial variation. The spatial distribution of δ13C shows less negative values in the mid-latitudes between 31°N and 40°N and more negative ones at higher and lower latitudes. This is consistent with previously reported carbon isotope data from surface soil phytoliths in the same region and suggests that the mid-latitude area provides relatively favorable growing conditions for C4 plants. Furthermore, δ13C values of both TOC and long-chain n-alkanes from 12 surface soil samples collected from a small grassland in north China displayed similar carbon isotope values and the difference between paired δ13C of a soil samples remains relatively constant. Our data demonstrate that in eastern China, soil δ13C composition of both TOC and long-chain n-alkanes is effective indicators of C3/C4 ratios of the prevailing vegetation. This work suggests that −22‰ and −32‰ are good estimate end members for the weighted mean δ13C values of long-chain n-alkanes (C27, C29 and C31n-alkanes) from soils under dominant C4 or C3 vegetation, allowing us to reconstruct paleovegetation trends.