Choosing a suitable εw-p by distinguishing the dominant plant sources in sediment records using a new Pta index and estimating the paleo-δDp spatial distribution in China

Abstract

The hydrogen isotopic composition of plant wax (δDwax) inherits the isotopic values of the source water and isotopic fractionation that take place in lipid biosynthesis. The δDwax values of n-alkanes or n-acids in sediment records are considered to be effective paleo-δD for precipitation proxies for different timescales. However, the vegetation effect also contributes substantially to the apparent fractionation values (εw-p) between the precipitation and plant. Therefore, a method for evaluating the dominant plant source in the sediment records is key for reconstructing a reliable paleo-δDp record. In this study, based on field data from the extremely arid Qaidam Basin and the semi-arid central Chinese Loess Plateau (CLP), a plant type index (Pta), based on the traditional Paq and n-alkane C31/C29 index, was used to evaluate the dominant plant type that contributed to the sediments. In the arid and semi-arid region, the Pta values of the aquatic plants were <90, while the shrub Pta ranged from 90 to 180 and the grass Pta was >180. A test on contemporary soils and surface lake sediments also showed that the Pta index could estimate the regional vegetation distribution in modern environments. We also evaluated the dominant n-alkane source for four sediment records in China during the last millennium and reconstructed a regional paleo-δDp by choosing a different εw-p. The estimated paleo-δDp values showed a good correlation with the modern regional summer δDp. We suggest that a large number of field studies are needed to improve the Pta index for quantitatively calculating the precise ratios for various plant sources. Such data would play a key role in improving hydrogen isotopic hydroclimatic research.