Compound specific D/H ratios and molecular distributions of higher plant leaf waxes as novel paleoenvironmental indicators in the Chinese Loess Plateau

We measured D/H ratios of individual long chain leaf wax n-alkanes isolated from a loess profile spanning the last 130 ka from Xifeng, China. The δD values show large variation (−140 to −200‰) for the past 130 ka and are highly correlated with changes in magnetic susceptibility (MS), an accepted proxy for past monsoon circulation in the Chinese Loess Plateau. Peaks in high δD values (−140 to −160‰) coincide with low MS values for loess sections formed under relatively dry conditions (marine isotope stages, MISs, 2 and 4), whereas peaks in low δD values (−190 to −200‰) coincide with high MS values for paleosol sections formed under relatively wet conditions (MISs 1, 3 and 5). We attribute the isotopic variations to the combined effects of varying aridity, which controls the soil and leaf water evaporation, and temperature and monsoon intensity, which alter the D/H ratios of precipitation. We also discovered a strong negative correlation between the molecular distributions (ratios of C31/C29 and C31/C27n-alkanes) and the δD values of n-alkanes, based on studies of modern plants and the Xifeng loess sequence. The complex relationship between MS and monsoon circulation has been difficult to understand mechanistically. However, the climatic controls on the D/H ratios of individual long chain n-alkanes and C31/C29n-alkane ratios can be readily demonstrated and calibrated from modern plants and surface soils. Therefore, these new molecular and isotopic proxies have potential as quantitative proxies for paleoenvironmental reconstruction in the Chinese Loess Plateau.