Aridity-controlled hydrogen isotope fractionation between soil n-alkanes and precipitation in China

Abstract

Leaf-wax n-alkane hydrogen isotope composition (δDalk) is widely applied as a proxy for paleohydroclimatic changes, but the factors controlling δDalk remain to be clarified. In this study, in order to determine the dominant controls, we measured δDalk in 54 surface soils along a southeast–northwest transect across a steep climatic gradient in China. δDalk is positively correlated with annual precipitation δD (δDpann) and better correlated with growing season (April–October) precipitation δD (δDpgs). However, the variability of δDpann (38‰) and δDpgs (35‰) does not explain all of the variance in δDalk (84‰). The apparent fractionation of hydrogen isotopes between n-alkanes and precipitation (εalk/p) is significantly correlated with variables related to aridity: relative humidity, rainfall amount, and aridity index (defined as the ratio of average precipitation to average potential evapotranspiration). This result demonstrates the important control of aridity on δDalk, which may result from the increasing soil evaporation and plant transpiration, as well as decreasing biosynthetic fractionation, in arid climates. We compiled published δDalk data from surface lake/soil sediments in China, and found that aridity effects are pronounced in regions with mean annual precipitation (MAP) < 800 mm. Therefore, we suggest that soil δDalk reflects precipitation δD modified by aridity, with a greater degree of modification in arid environments (e.g., MAP < 800 mm). Future work to quantitatively determine the effects of aridity on δDalk is needed to improve the reliability of the application of δDalk in paleoenvironmental studies.