New insights into factors controlling the extraordinarily high organic matter accumulation across the Ordovician−Silurian transition, South China

The Ordovician−Silurian (O−S) transition was marked by widespread deposition of organic-rich shales worldwide. However, the mechanism of organic matter accumulation remains debated. Previous studies mainly focus on primary productivity versus preservation conditions. Here, we emphasize the mechanisms driving accumulation of the extraordinarily high organic matter (EHOM) with total organic carbon (TOC) content over 3.0 wt%. In this study, the Wufeng-Lungmachi shale formations of two sections across the O−S transition on the northern margin of the Yangtze Shelf, South China, were investigated for TOC, elemental geochemistry, petrology of barite and pyrite concretions, and pyrite sulfur isotopes. Our results reveal two step-wise increases of TOC content identified as two episodes of EHOM accumulation. The first episode of EHOM enrichment in the Upper Ordovician was likely triggered by increased nutrient input from hydrothermal activity related to volcanism and probably upwelling events. The second episode, marked by large-scale EHOM enrichment in the Lower Silurian, was driven by large nutrient supply from enhanced continental weathering following the Hirnantian deglaciation and from enhanced phosphorus recycling under widespread ocean euxinia. These findings suggest that geological events acted as the primary triggers for the EHOM enrichment, and could sustain EHOM accumulation over longer time scales by affecting environmental factors.