Surface soil n-alkane molecular and δD distributions along a precipitation transect in northeastern China

Abstract

Hydrogen isotope compositions of leaf wax long-chain n-alkanes (δDalk) buried in soils have the potential to provide records of continental paleoclimatic and paleohydrological changes. However, different factors and processes influence the δDalk signature, making interpretation of the soil isotope record complicated. The relative humidity/aridity and vegetation types (C3 vs C4 plants) are two major factors responsible for the soil δDalk variations. In most cases, the interaction between relative humidity/aridity and temperature cannot be separated. Here, we analyzed the δDalk compositions in surface soils along the Northeast China Transect (NECT), where the mean annual precipitation (MAP) varies from ca. 120 to 815 mm, yet the mean annual temperature changes little, providing a setting to test the role of relative humidity/aridity in soil δDalk variations, independent of temperature changes. At the same time, the western part of the NECT, which is mainly a grassland environment, provides a test of the effects of C3 and C4 grasses on soil δDalk changes. In the semi-humid eastern NECT region, the δDalk values and their apparent fractionation relative to precipitation exhibit a strong negative correlation with the aridity index and a close relationship with soil water content. This finding supports the idea that the soil δDalk values are mainly influenced by relative aridity and that the impact of evapotranspiration, including soil evaporation and transpiration-induced 2H enrichment of woody plant leaf water, plays a vital role in shaping soil δDalk. In the semi-arid and arid western NECT region, the δDalk values and their apparent fractionations show no relationship with any environmental factors. However, in the westernmost area, soil δDalk values tend to show a negative relationship with the increasing proportion of C4 plants. This pattern probably indicates that in the arid area the soil δDalk values vary more closely with plant types than with the environmental variables. This study provides new insights into deciphering the relationship between soil δDalk values and relative humidity/aridity, and it affirms the applicability of δDalk in paleo-hydrologic reconstructions.