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

Surface soil samples collected over a high spatial resolution in eastern China were analyzed for carbon isotope composition (δ ¹³C) 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 δ ¹³C values are significantly correlated and show similar trends in spatial variation. The spatial distribution of δ ¹³C 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, δ ¹³C 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 δ ¹³C of a soil samples remains relatively constant. Our data demonstrate that in eastern China, soil δ ¹³C 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 δ ¹³C values of long-chain n-alkanes (C₂₇, C₂₉ and C₃₁ n-alkanes) from soils under dominant C₄ or C₃ vegetation, allowing us to reconstruct paleovegetation trends. Supported by the Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. KZCX3-SW-152), National Natural Science Foundation of China (Grant No. 40331009), the Research Foundation of the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Grant No. OGL-200604), and the NSFC National Innovative Research Team Project (Grant No. 40721061)