Spatiotemporal evolution of a Late Paleozoic alkaline lake in the Junggar Basin, China

Abstract

Alkaline lakes are considered some of the most extreme aquatic environments on Earth and are thus of significance to our understanding of the biogeochemistry of extreme environments. However, the environmental evolution of these lakes (especially in ancient times) temporally and spatially is still poorly understood due to the unavailability of suitable samples. Here we present a case study of the Late Paleozoic Fengcheng Formation in the Junggar Basin, NW China, based on organic (biomarker) as well as inorganic (elemental) geochemical approaches. Results show that the Fengcheng Formation was deposited in a non-marine environment, as illustrated by the absence of 24-n-propylcholestanes and 24-isopropylcholestanes combined with the established fossil and sodium-carbonate evaporite records (wegscheiderite and trona). In detail, based on the biomarker assemblages in general, the Fengcheng Formation formed in an anoxic, hypersaline, stratified lacustrine environment. Temporally, the lower part of the formation (the lower two members F1 and F2) was formed under stable anoxic and stratified conditions with higher salinity, while the upper part (the third member F3) had higher freshwater influx, which reduced the salinity and resulted in relatively higher terrestrial organic matter input. Spatially, the central region of the alkaline lake was influenced by hydrothermal springs, while the marginal region was more affected by volcanic activities resulting in clay-rich and suboxic environments during F3. Our work suggests that the spatiotemporal evolution of alkaline lakes is multi-stage and heterogeneous, with hydrothermal and volcanic influence under restricted conditions and a dry climate.