Paleoenvironmental Controls on Organic Matter Enrichment in Marine–Continental Transitional Shales of the Upper Carboniferous Yanghugou Formation, Western Ordos Basin

Abstract

Organic matter enrichment in marine–continental transitional shales is influenced by volcanism, continental weathering, and paleoceanographic evolution, but their interactions during the Late Paleozoic Ice Age (LPIA) remain unclear. Mineralogical, elemental, and organic geochemical data from the Upper Carboniferous Yanghugou Formation, western Ordos Basin, were used to reconstruct volcanic input, weathering intensity, productivity, redox conditions, and organic carbon accumulation. High Hg/TOC, Zr/Al2O3, and Zr/Cr ratios indicate strong volcanic input during early deposition, followed by an upward decline. Chemical Index of Alteration values of 76.5–91.5 increase upward, indicating intensified chemical weathering under warmer and more humid conditions. Increasing Corg/P, MoEF, and UEF values record a shift from oxic–suboxic to predominantly anoxic bottom waters. The weak relationship between productivity proxies and TOC suggests that volcanic fertilization was not the main control on organic matter enrichment. Instead, negative δ13Corg values, high Al2O3 contents, organic matter–clay associations, and reducing conditions indicate that enhanced terrestrial organic matter input and improved preservation jointly promoted organic carbon accumulation. Volcanism likely intensified climatic warming and weathering, whereas subsequent organic carbon burial contributed to atmospheric CO2 drawdown and climatic feedbacks during the LPIA.