Trend reversal of soil n-alkane Carbon Preference Index (CPI) along the precipitation gradient and its paleoclimatic implication

Abstract

Homologue distribution proxies of leaf wax n-alkanes (normal chain alkanes) have been widely used for chemotaxonomy and paleoclimate reconstruction, while modern calibrations of them reported contrasting conclusions in different climate settings, especially between the cold arid and other climate regions. To further explore n-alkane-abiotic factor correlation in cold arid regions, this study surveyed n-alkane patterns of soil and leaf wax of three plant species along confined temperature and precipitation gradients in the Junggar Basin of northwest China. Our results show that salinity has a possible strong constraint on the Average Chain Length (ACL) of soil n-alkanes in high salinity settings, while precipitation or water stress is the leading constraint on the Carbon Preference Index (CPI) of those in low salinity settings. Precipitation also shows strong control over CPI or ACL of leaf wax n-alkanes of different plant species. Our results confirmed that n-alkane patterns are sensitive to, and constrained by abiotic factors independently on levels of plant species and averaged biome. A compilation of soil CPI from transect studies across the hot-humid to cold-arid regions of China further revealed precipitation being the first-order constraint of soil CPI, and a CPI trend reversal pattern along the precipitation gradient, where CPI increases from arid to semiarid climate and then decreases toward humid climate. Further analysis of existing CPI records showed that this CPI trend reversal may help differentiate three climate states qualitatively, i.e. arid, semiarid-semihumid and humid climates, which could be a valuable complement to existing proxies for paleoclimate reconstruction.