Temperature dominates leaf wax n-alkane carbon isotope composition of terrestrial C3 angiosperms in humid zones

The C₃ plant leaf wax δ¹³C (δ¹³C_alk) is commonly used to reconstruct past climates. However, our understanding of how precipitation and temperature influence terrestrial C₃ δ¹³C_alk is limited. To address this gap, we investigated the δ¹³C of n-C₂₉ alkane (δ¹³C₂₉) of C₃ terrestrial angiosperms in subtropical humid regions in China and compiled a global dataset. Results show a strong correlation between C₃ terrestrial angiosperms δ¹³C₂₉ and mean annual temperature in both Chinese subtropical humid regions (r = −0.94, p < 0.001) and global humid zones (r = −0.78, p < 0.001). The linear fit lines of the regional and global datasets have similar slopes (−0.3129 vs −0.2896) and intercepts (−30.82 vs −31.41), indicating consistent responses to air temperature in global humid zones. When considering only sites with temperature data from meteorological stations, the correlation coefficient increases to −0.96 (p < 0.001) for the global dataset. In contrast to humid zones, mean annual precipitation significantly correlates with C₃ terrestrial angiosperms δ¹³C₂₉ in arid zones (r = −0.75, p < 0.001), supporting the main controls of precipitation in these areas. These findings emphasize the impact of temperature on C₃ terrestrial δ¹³C_alk, particularly in humid zones, and highlight caution when interpreting δ¹³C_alk in geological records under changing hydroclimate conditions or unstable settings.