Hydrogen and carbon isotopes of plant leaf wax n-alkanes over the entire Loess Plateau of China: Implications for paloehydroclimatic reconstructions

Leaf wax n-alkanes have been widely used to indicate past changes of hydroclimates. However, the responses of hydrogen (δ²H_wax) and carbon isotopes (δ¹³C_wax) of leaf wax n-alkanes to hydroclimates still remain unclear. In this study, we systematically investigated leaf wax n-alkane compounds such as n-alkanes, δ²H_wax, and δ¹³C_wax values in modern plants throughout the spatial Chinese Loess Plateau (CLP; ∼ 640, 000 km²) across different seasons (spring, summer, autumn), and analyzed the relationships between δ²H_wax and δ¹³C_wax values and hydroclimatic factors. Our results showed that, over the regional-scale CLP, the effects of plant types on leaf wax n-alkanes were prominent, but the seasonal differences became muted. These finding indicates that plant type effects should be considered, but seasonal variations might be ignored, when leaf wax n-alkanes were used to reconstruct paleohydroclimates on the CLP. Additionally, the isotopic apparent fractionations (ε_app values for δ²H_wax; ε_wax/bulk values for δ¹³C_wax) also showed consistent plant type effects, highlighting differences of ε_app values between dicots and monocots and discrepancies of ε_wax/bulk values between C₃ and C₄ plants. Considering plant type effects, δ²H_wax and δ¹³C_wax values from the CLP were consistent with expectations from the global contexts, and they well responded to hydroclimatic factors (temperature and precipitation). The δ²H_wax values in both dicots and monocots likely responded mainly to temperature, but δ¹³C_wax values in C₃ plants primarily reflected precipitation whereas δ¹³C_wax values in C₄ plants primarily reflected temperature. These findings indicate that δ²H_wax and δ¹³C_wax values have great potentials as proxies for paleotemperature and paleoprecipitation on the CLP and globally.