The coupling control of biological precursors and environmental factors on β-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 β-carotane. Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments. However, the coupling control of biological precursors and environmental factors on the enrichment of β-carotane in the Fengcheng Fm. remains obscure. Based on a comprehensive investigation of the bulk and molecular geochemistry of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes, we proposed a new understanding of the biological precursors of β-carotane and elucidated the coupling control of biological precursors and environmental factors on the enrichment of β-carotane in the Fengcheng Fm. The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm. The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane, which is suggested by a good positive correlation between 7-+8- methyl heptadecanes/Cmax and β-carotane/Cmax in sedimentry rcoks and the predominance of cyanobacteria with abundant β-carotene in modern alkaline lakes. Land plants and algae do not significantly contribute to the enrichment of β-carotane, which is indicated by negative or weak correlations between terrigenous/aquatic ratio, C₁₉ tricyclic terpene/C₂₃ tricyclic terpene ratio, the concentration of C₂₇ sterane, and β-carotane/Cmax. The environmental facors such as paleoclimate, paleoredox, paleosalinity, and thermal maturity are the indirect factors that control the enrichment of β-carotane by affecting the precursor supply from cyanobacteria and the preservation conditions in the Fengcheng Fm.