Effect of lake evaporation on δD values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)

Compound-specific hydrogen isotope ratio values of lacustrine n-alkanes from two contrasting ecosystems, the semi-arid to arid Nam Co, Central Tibet and the humid Holzmaar, Germany, were compared in order to assess whether or not these environmental conditions are recorded in the isotopic signatures of biomarkers. Increased evaporation of lake water at Nam Co is recorded by the n-alkanes of aquatic origin. Hence, isotopic enrichment results in a difference between terrestrial and aquatic n-alkanes in the opposite direction (∼−68‰) from that known for humid climate conditions (∼+30‰) predominating at Holzmaar. Based on this isotopic difference between terrestrial and aquatic n-alkanes, evaporation to inflow ratio (E/I) values were estimated and suggest that the isotopic difference is indicative of the general hydroclimatic characteristics of a lake system. Moreover, the comparison with E/I values calculated with actual stable water isotope data showed that the δD values of aquatic and terrestrial lacustrine n-alkanes serve as a proxy for the relative isotopic differences between lake and inflow waters and can be used to assess the proportion of water undergoing evaporation relative to the inflow, and thus to reconstruct the lake water balance in the past.