Hydrogen isotope ratios of sedimentary biomarkers are known to record the climatic variability in terrestrial and marine environments. However, there is still a lack of calibration studies that can quantitatively retrace the driving forces, especially at the Tibetan Plateau. Here, we elaborate the actual influence of environmental parameters such as temperature, evapotranspiration, salinity and biosynthetic fractionation on δD values of n-alkanes.
We measured hydrogen isotope values (δD) of n-alkanes in recent sediment and plant samples as well as δD values of different water sources (lake and inflow water, precipitation as well as leaf, root and soil water) from six Tibetan lakes along a 10°-spanning longitudinal transect covering an aridity gradient. As expected, the deuterium record can be used to distinguish two water pools at the Plateau: (I) precipitation water, which supplies water for the lake inflow and terrestrial plants, and (II) enriched lake water having a clear evaporative signal. Based on significant correlations of the source water and δD values of n-alkanes, δD of alkane n-C23 record the lake water isotope composition and track the evaporative enrichment of the lake system. In contrast, δD of alkane n-C29 can be used to retrace the isotope composition of the inflow displaying the integrated rainfall signal in the vegetation period modified by soil and leaf water evaporation. While temperature changes are less pronounced across the Tibetan transect, the isotopic difference between C23 and C29 could potentially be used as a proxy to reconstruct effective moisture (precipitation minus evaporation). Areas with lower precipitation amounts and higher evaporation rates are characterized by higher ΔδDC23–C29 values. This relationship is more pronounced in regions with mean annual precipitation below 350 mm. Even in contrasting environments, the application of the δD proxies is very promising. Combining other Tibetan studies, it is possible to present a comprehensive picture of the usage of compound-specific hydrogen isotopes of n-alkanes on the Tibetan Plateau.