Precipitation and Lake Water Evaporation Recorded by Terrestrial and Aquatic n-Alkane δ2H Isotopes in Lake Khar Nuur, Mongolia

The compound-specific hydrogen isotopic composition (delta H-2) of n-alkanes is a valuable proxy to investigate hydrological conditions in lake sediments. While terrestrial n-alkanes reflect the isotopic signal of the local precipitation, aquatic n-alkanes incorporate the isotopic signal of the lake's water, which can be strongly modulated by evaporative enrichment. So far, the spatial distribution of the terrestrial and aquatic delta H-2 signal within lakes have not systematically been investigated. Here, we present compound-specific delta H-2 results of terrestrial (delta H-2(C31)) and aquatic (delta H-2(C23)) n-alkanes of surface sediment samples from Lake Khar Nuur, a semi-arid and high-altitude lake in the Mongolian Altai, and additionally investigate the delta H-2 signal of topsoils from the catchment. Our results show that the majority of the n-alkane delta H-2 values from the catchment topsoils correspond well with modeled local growing season precipitation (JJAS). However, few samples in the northern catchment show more positive delta H-2 values possibly due to increased evapo(transpi)ration by southward exposition and shallower soils there. The only small variability of delta H-2(C31) in the surface sediments is in the range of most topsoils delta H-2 from the catchment, and thus, well reflects local growing season precipitation. delta H-2(C23) in surface sediment samples from the central and deepest parts of the lake, that is, the lake's sediment accumulation zones, shows distinctly more positive delta H-2(C23) values due to evaporative lake water enrichment. Consequently, Delta(aq-terr), which is the isotopic offset between delta H-2(C23) and delta H-2(C31), indicates distinct lake water enrichment in the lake's accumulation zones and is a valuable proxy to investigate past hydrological changes.

Automated summary

This study systematically investigates the spatial distribution of terrestrial and aquatic hydrogen isotopic signals in lake sediments, providing valuable insights into hydrological conditions. The results reveal the influence of water availability on the isotopic composition of n-alkanes and suggest that the isotopic offset between terrestrial and aquatic n-alkanes can be used as a proxy for past hydrological changes.