A synthesis of hydrogen isotope variability and its hydrologica significance at the Qinghai-Tibetan Plateau

摘要

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 biosyn-thetic fractionation on 6D values of n-alkanes. We measured hydrogen isotope values (8D) of n-alkanes in recent sediment and plant samples as well as 8D 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 5D values of n-alkanes, 8D of alkane n-C_23 record the lake water isotope composition and track the evaporative enrichment of the lake system. In contrast, 8D of alkane n-C_29 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 C_23 and C_29 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 A8DC_23_C_29 values. This relationship is more pronounced in regions with mean annual precipitation below 350 mm. Even in contrasting environments, the application of the 5D 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.