Volcanic impact on terrestrial sulphur cycling during the Carboniferous‒Permian in an alkaline lake in the Junggar Basin, NW China

The sulphur (S) cycle is important for determining paleoenvironmental evolution and organic matter enrichment. Compared with research on marine facies, studies on the terrestrial sulphur cycle and its relationship with key geological events, such as volcanic activity or hydrothermal fluids, are more limited. The Fengcheng Formation in the Mahu Sag of the Junggar Basin in northwestern China, which deposited approximately 360m during the Carboniferous to early Permian in an alkaline lake, is an ideal research object for studying the relationship between the terrestrial sulphur cycle and geological events. Therefore, in this work, we identified volcanic activity during the deposition of the Fengcheng Formation and established a link between volcanic activity and the lacustrine alkaline carbon‒sulphur cycle during the Carboniferous‒Permian through petrologic, geochemical, and geophysical data from the MY1 Well in the Mahu Sag. The results revealed that (1) multiple volcanic episodes occurred during the deposition of the Fengcheng Formation, as evidenced by high mercury (Hg) concentrations, high Hg/S ratios, increased sulphate concentrations and large negative pyrite sulphur isotope (δ³⁴S_py) values (ranging to -20.52‰); (2) long-term ferruginous bottom water conditions may have been conducive to the preservation of organic matter; however, sulphate from volcanic activity promoted bacterial sulphate reduction, resulting in intermittent alternating euxinic conditions, as evidenced by iron speciation, molybdenum concentrations, and framboid and euhedral pyrite morphologies, which may have resulted in some consumption of organic matter; and (3) after volcanic activity, the sulphate in the lake water was depleted, and the bottom water system gradually closed and was continuously enriched with δ³⁴S_py. Therefore, volcanic activity appears to have been the key factor controlling the sulphur cycle and organic matter enrichment through increased sulphate fluctuations in the oldest alkaline lake during the deposition of the Fengcheng Formation. This study sheds new light on the sulphur cycle of ancient alkaline lakes and can serve as a reference for organic matter enrichment under different mechanisms in shale.