Latitudinal variations of CPI values of long-chain n-alkanes in surface soils: Evidence for CPI as a proxy of aridity

Abstract

Carbon preference index (CPI) of long-chain n-alkanes preserved in surface soil increases gradually from southeastern China to the north margin of Loess Plateau. Along this latitudinal transect, the CPI value correlates to relative humidity, precipitation, and temperature with a negative linear relationship, respectively, whereas the correlation of CPI to temperature is relatively weak. In the Wuyi, Shennongjia, and Tianshan Mountains, CPI values do not change systemically with altitude increasing (or temperature decreasing). However, mean value of CPI for the individual mountain increases in turn from the humid mountain to the arid. These results jointly suggest that aridity (or humidity) is a dominate climate factor in altering soil CPI value. High CPI values of geological records therefore indicate the arid paleoclimate. Though long-chain n-alkanes in soil are derived mainly from leaf wax of terrestrial vascular plants, the regular latitudinal variations of soil CPI might not be caused by the change of vegetation. We speculate that increased long-chain n-alkanes from microbes and/or enhanced biodegradation in the humid climate lead to the decrease of soil CPI.