Occurrence and environmental significance of long-chain alkenones in Tibetan Zabuye Salt Lake, S.W. China

Abstract

Long-chain alkenones, a group of sensitive organic molecular indicators of water temperature, have been rather extensively studied in marine environments. However they have never been systematically examined in lacustrine environments, despite reports of their occurrence in lake sediments. Here, we report on a recent study of long-chain alkenones in a sedimentary core from a high altitude (∼ 5400 m) salt lake, Zabuye Salt Lake (ZSL), Tibet. This is a critical location for global climate studies, especially of the atmospheric circulation of the North Hemisphere.C37–C39 methyl and/or ethyl alkenones, usually dominated by components with tetra- and tri-double bounds, are commonly the major components of the polar fraction of the extracted organic matter from most sections. Down-core (vertical) variation of alkenone indices, the measure of molecular unsaturation, is compared primarily with other environmental signals, including lithology, elemental and mineral compositions, and stable carbon isotopes of hydrocarbon biomarkers of this core. Down-core profiles of alkenone climatic indices (Uk 37and Uk 38) suggest ZSL had two warm periods, one during ∼ 20 – 30 ka (∼Jabula Interglacial Optimum)and another at 8 – 5 ka (Middle Holocene Optimum), and a severe cold period from ∼ 18 – 11 ka (Last Glacial Maximum). The proposed warmer intervals are generally characterized by higher contents of carbonate, organic carbon, alkenones and heavier δ13C values of n-alkanes.