Controls of water salinity on biological diversity and productivity in the Late Paleozoic alkaline lake, NW Junggar Basin, NW China

Alkaline lake is one of the most productive ecosystems on Earth, commonly characterized by massive organic matter accumulation. However, the primary biological precursors of organic matters and the controls on their accumulation in the ancient alkaline lakes remain poorly understood. Here we use petrology and organic geochemistry analysis of the Upper Paleozoic Fengcheng Formation of the Halaalate area in the Junggar Basin, NW China, to study the biological diversity and controls of water salinity on primary productivity in the ancient alkaline lakes. Two depocenters have been identified in the Halaalate area: a proximal depocenter close to the boundary mountains and a distal depocenter far away from source areas. The results show that water salinity was much larger for the first and second members of Fengcheng Formation (FC1 and FC2) compared to the third member (FC3), and the distal depocenter had more saline lake water than the proximal depocenter. Abundant primary producers have been identified to be flourishing in the low-salinity alkaline lakes, such as cyanobacteria, dinoflagellates and green algae, whereas only a special haloalkaliphilic green alga can survive in the hypersaline alkaline lakes. Therefore, the low-salinity alkaline lakes are characterized by a higher primary productivity and can deposit mudstones containing richer organic matters compared to the high-salinity ones. This study suggests that water salinity is the major factor controlling the biomass and biodiversity of ancient alkaline lakes and mudstones deposited in the low-salinity alkaline lakes are more promising for oil exploration.