Reconstructing lake evaporation history and the isotopic composition of precipitation by a coupled δ18O–δ2H biomarker approach

Abstract

Summary Over the past decades, δ 18 O and δ 2 H analyses of lacustrine sediments became an invaluable tool in paleohydrology and paleolimnology for reconstructing the isotopic composition of past lake water and precipitation. However, based on δ 18 O or δ 2 H records alone, it can be challenging to distinguish between changes of the precipitation signal and changes caused by evaporation. Here we propose a coupled δ 18 O– δ 2 H biomarker approach that provides the possibility to disentangle between these two factors. The isotopic composition of long chain n -alkanes ( n -C 25 , n -C 27 , n -C 29 , n -C 31 ) were analyzed in order to establish a 16 ka Late Glacial and Holocene δ 2 H record for the sediment archive of Lake Panch Pokhari in High Himalaya, Nepal. The δ 2 H n -alkane record generally corroborates a previously established δ 18 O sugar record reporting on high values characterizing the deglaciation and the Older and the Younger Dryas, and low values characterizing the Bolling and the Allerod periods. Since the investigated n- alkane and sugar biomarkers are considered to be primarily of aquatic origin, they were used to reconstruct the isotopic composition of lake water. The reconstructed deuterium excess of lake water ranges from +57‰ to −85‰ and is shown to serve as proxy for the evaporation history of Lake Panch Pokhari. Lake desiccation during the deglaciation, the Older Dryas and the Younger Dryas is affirmed by a multi-proxy approach using the Hydrogen Index (HI) and the carbon to nitrogen ratio (C/N) as additional proxies for lake sediment organic matter mineralization. Furthermore, the coupled δ 18 O and δ 2 H approach allows disentangling the lake water isotopic enrichment from variations of the isotopic composition of precipitation. The reconstructed 16 ka δ 18 O precipitation record of Lake Panch Pokhari is well in agreement with the δ 18 O records of Chinese speleothems and presumably reflects the Indian Summer Monsoon variability.