Volcanic activity driving rapid organic carbon burial during the Ordovician–Silurian transition

Abstract Volcanic activity plays a pivotal role in Earth’s material cycling and serves as a crucial mechanism in regulating atmospheric CO2 concentrations. Throughout the Ordovician–Silurian transition (OST), global volcanic activity was frequent, exerting substantial influences on paleoclimate, paleoceanographic changes, mass extinctions, and the formation of important hydrocarbon source rocks in the early Paleozoic era. Analysis using Y–Nb and Nb/Y–TiO2 diagrams, as well as assessments of bentonite layer count and thickness, reveal that volcanic ash deposited in the Yangtze Block (southern China) primarily originated from the reactivation of the convergent boundary between the Yangtze Block and Cathaysian Block to the southeast. Additionally, a portion derived from the collision of the South China Block with Gondwana and the subduction of the Paleo-Qinling oceanic crust toward the North China Block in the north. Geochemical analyses based on high-precision meter-scale and centimeter-to millimeter-scale sampling, considering Zr content, Hf content, and Zr/Cr ratio, indicate a decreasing trend in volcanic activity intensity from the bottom to the top of the stratigraphic sequence, aligning with the appearance frequency and thickness of bentonite layers. The global sea-level rise and volcanic activity during the OST considerably influenced paleo-environmental changes and sedimentary products, leading to the replacement of Katian carbonate platform deposits by the shales of the Wufeng-Longmaxi Formation and the transition from biosiliceous shales at the bottom to siliciclastic shales at the top. Proxies for volcanic activity, such as Hg content, demonstrated positive correlations with total organic carbon, Mo content, U/Th ratio, and V/Cr ratio, indicating volcanic control over primary productivity and the reducibility of the water body. Volcanic activity during the OST facilitated the import of substantial nutrients into the Yangtze Sea, promoting biological blooms that consumed dissolved oxygen and increased the reducibility of the water body, thereby affecting organic matter enrichment. The Wufeng-Longmaxi Formation in the Yangtze Block preserved ~4582.49 Gt of organic carbon, contributing to a global organic carbon burial of at least 16,131.14 Gt during the OST—the most extensive in the early Paleozoic. Volcanic activity during the OST emerged as a potential dominant factor triggering mass extinction and glaciation. The weathering of a substantial volume of silicate rocks during the OST sequestered ~1.72 × 103 Gt to 2.75 × 103 Gt of CO2, equivalent to depleting 2.39–3.47 times the present-day atmospheric carbon stock. Combined with the enhanced effect of the biological pump due to volcanic activity, the Hirnantian glaciation was initiated, resulting in a mass extinction.