Sedimentary <italic>n</italic>-alkanes record of precipitation D/H ratios in arid regions of the Tibetan Plateau

Abstract

The hydrogen isotope ratios of sedimentary leaf waxes have been investigated in various environments and are considered as a promising proxy for paleohydrological and paleoclimatic reconstruction. However, there is a growing concern regarding whether or not sedimentary leaf waxes can be used as an environmental proxy in extremely arid regions, where intensified evaporation strongly enriches the deuterium both in water and in leaf waxes. Here we present a record of the n -alkane D/H ratios of surface sediments from 21 lakes. The data were collected from 2012–2014. The n -alkanes extracted from lake surface sediments range from n C16 to n C35, with the most abundant compounds being n C23, n C25, n C29 and n C31. In addition, there is an evident odd-over-even preference for long-chain n -alkanes, with carbon preference index (CPI) values varying between 3.41 and 10.75, suggesting that the long-chain sedimentary n -alkanes mainly originated from terrestrial plants in arid regions on the Tibetan Plateau. The D/H ratios of the precipitation ( δ Dp) atall the lakes in this studywere calculated using the Online Isotopes in Precipitation Calculator. The OIPC calculates precipitation isotope ratios at a given site by combining an empirical model for isotopic trends related to latitude and altitude with detrended interpolation based on the isotope data from stations in the Global Network of Isotopes in Precipitation. The mean annual hydrogen isotopes ratios of precipitation ( δ Dann) at the lakes in this study vary from - 78‰ to - 115‰, with lower precipitation δ D values occurring in the southern TP and higher values in the northern TP and the Qaidam Basin. The δ D values for summer precipitation (July–September, δ Dsum) range from - 68‰ to - 126‰, with a similar range of variation for δ Dann. The sedimentary n -alkane D/H ratios are strongly correlated with the D/H ratios of annual precipitation ( r 2=0.84) and summer precipitation ( r 2=0.91), suggesting that the hydrogen isotope signal in sedimentary n –alkanes is mainly derived from precipitation, and that the n -alkane D/H ratios may reflect the D/H ratios of precipitation in arid regions on the Tibetan Plateau. Water is the ultimate source of hydrogen in plant tissues, and organic matter derived from terrestrial plants should record the isotopic ratio of soil water, which is ultimately recharged by precipitation. The δ D values of n -alkanes in lake surface sediments could thus be expected to be correlated with the δ D values of precipitation, on a large spatial scale. Furthermore, precipitation on the Tibetan Plateau mainly occurs from July to September, which accounts for ~70% during the year, and the n -alkanes of leaf wax are synthesized during the plant growing season. Therefore, it is unsurprising that the best correlation between D/H ratios of sedimentary leaf wax and precipitation occurs in the summer months. The apparent hydrogen isotopic fractionation between sedimentary n -alkanes and precipitation averages - 113‰, evidently smaller than that in relatively humid regions. A possible reason for the smaller apparent fractionation might be spatial differences in soil evaporation and plant transpiration. Mean annual precipitation amount and mean annual evaporation at the studied lakes is 207 and 2343 mm, respectively. However, in the humid areas, such as the southeastern TP, the mean annual precipitation amount and mean annual evaporation are 1298 and 795 mm, respectively. Due to the strong evaporation in arid regions, the δ D values of soil water and plant water are more isotopically-enriched relative to precipitation than those in humid regions. Overall, our results demonstrate that sedimentary n -alkanes can be used as a proxy indicator of variations in the isotopic composition of summer precipitation, in order to document past changes in summer monsoon intensity and hydrology.