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

摘要

Surface soil samples collected over a high spatial resolution in eastern China were analyzed for carbon isotope composition (delta C-13) 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 delta C-13 values are significantly correlated and show similar trends in spatial variation. The spatial distribution of delta C-13 shows less negative values in the mid-latitudes between 31A degrees N and 40A degrees 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, delta C-13 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 delta C-13 of a soil samples remains relatively constant. Our data demonstrate that in eastern China, soil delta C-13 composition of both TOC and long-chain n-alkanes is effective indicators of C-3/C-4 ratios of the prevailing vegetation. This work suggests that -22aEuro degrees and -32aEuro degrees are good estimate end members for the weighted mean delta C-13 values of long-chain n-alkanes (C-27, C-29 and C-31 n-alkanes) from soils under dominant C-4 or C-3 vegetation, allowing us to reconstruct paleovegetation trends.