Response of δD values of sedimentary n-alkanes to variations in source water isotope signals and climate proxies at lake Nam Co, Tibetan Plateau

Compound-specific δD values of sedimentary n-alkanes from Nam Co were compared with oxygen isotope values from three different ice cores of the Tibetan Plateau and other climate proxies including temperature and solar radiation. This enabled estimation of the importance of source water and of environmental factors at the ecosystem scale on variations of the isotopic composition of sedimentary n-alkanes. The ice core records from Dasuopu, Puruogangri and Rongbuk glacier of the past 1000 years recording the source water isotopes show a continuous deuterium enrichment indicating a warming trend or decreased precipitation. Sedimentary n-alkane δD values also represent the same general tendency to enrichment in heavier isotopes. However, the correlation to the δ18O values derived from the ice record is very weak. The δD values of n-alkanes agree with known climatic events such as the Little Ice Age and Medieval Warm Period showing depleted and enriched δD values, respectively. The δD values of the n-alkanes were best related to environmental drivers including temperature and solar radiation. The results suggest that δD values of sedimentary n-alkanes mainly record evapotranspiration and relative humidity that strongly influence δD values at the plant level, while variability of the source signal is less important.