The coupling control of biological precursors and environmental factors on 0-carotane enrichment in alkaline lacustrine source rocks: A case study from the Fengcheng formation in the western Junggar Basin, NW China

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation (Fm.) are typically alkaline lacustrine source rocks, which are typified by impressively abundant 0-carotane. Abundant 0carotane has been well acknowledged as an effective indicator of biological sources or depositional environments. However, the specific biological sources of 0-carotane and the coupling control of biological sources and environmental factors on the enrichment of 0-carotane in the Fengcheng Fm. remains obscure. Based on a comprehensive investigation of the bulk, molecular geochemistry, and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes, we proposed a new understanding of the biological precursors of 0-carotane and elucidated the enrichment mechanism of 0-carotane in the Fengcheng Fm. The results show that the biological precursors crucially control the enrichment of 0-carotane in the Fengcheng Fm. The haloalkaliphilic cyanobacteria are the primary biological sources of 0-carotane, which is suggested by a good positive correlation between the 2-methylhopane index, 7- & thorn; 8- methyl heptadecanes/Cmax, C29%, and 0-carotane/Cmax in sedimentary rocks and the predominance of cyanobacteria with abundant 0-carotene in modern alkaline lakes. The enrichment of 0-carotane requires the reducing condition, and the paleoredox state that affects the enrichment of 0-carotane appears to have a threshold. The paleoclimate conditions do not considerably impact the enrichment of 0-carotane, but they have some influence on the water's paleosalinity by affecting evaporation and precipitation. While it does not directly affect the enrichment of 0-carotane in the Fengcheng Fm., paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions. (c) 2024 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).