Stable isotopic and mineralogical investigations of an arid Quaternary lacustrine palaeoenvironment, Western Qaidam, China

Abstract

Stable isotope analyses on carbonates from lake evaporites collected from the non-marine western Qaidam basin yield a positive excursion from Pliocene to Quaternary times. At Dafeng Shan, the Quaternary sequences are composed of alternating layers of celestine/dolomite and aragonite/calcite/barite with distinct isotopic compositions. The sequence described at Dafeng Shan formed in a low energy, hypersaline lacustrine environment as indicated by the microstructures and evaporitic minerals as well as by the absence of lithoclasts. The peloids, ooids and oncoids described are related to microbial activities in saline lake. The oxygen isotopic composition of the carbonates vary between +34.4 and +39.8 (SMOW), representing the heaviest values measured until now. The d18 O and the d34 S isotopic composition of the celestine range between 20.1 to 22.3 (SMOW) and +19 to +22 (CDT) respectively, suggesting sulfur recycling via sulfide oxidation. The carbon isotopic compositions of the carbonates show a large negative excursion of up to -30. The microstructures, mineralogy and isotopic compositions as well as the geological context suggest oxidation of methane from a deep source.