Late Ordovician-Early Silurian global volcanism triggers biological pumping in the Yangtze region driving ocean and climate change

Volcanic activity plays a pivotal role in Earth’s material cycling and serves as a crucial mechanism in regulating atmospheric CO₂ concentrations. During the Late Ordovician–Early Silurian, 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. In this study, Hg content, Zr content, Hf content, and Zr/Cr were used to identify volcanic activity; Cu content, Mo content, TOC content, and carbon isotopes were used to determine primary productivity; U/Th, V/Cr, V/(V+Ni), and Ni/Co were used to analyze the redox conditions of the sedimentary environment; chemical index of alteration, Sr content, and Sr/Cu were used to discriminate paleoclimate; and Sr/Ba to discriminate paleosalinity. In the Katian in the Yangtze region, the water body was highly reducing, and at the beginning of the Rhuddanian, the maximum values of all redox indicators appeared, with the maximum values of U/Th reaching 7.99, V/Cr reaching 25.68, V/(V+Ni) reaching 0.89, and Ni/Co reaching 25.15, which meant that the water body was in the strongest period of reductivity at this time. In the middle and late Rhuddanian, U/Th, V/Cr, V/(V+Ni), and Ni/Co all showed a decreasing trend, indicating that the reductivity of the water body gradually weakened. The trend in marine water's reducibility paralleled that of primary productivity, as indicated by Cu, Mo, and TOC content and the δ¹³C value increasing from the Katian to the beginning of the Rhuddanian, and then starting to decrease, and reached their maximum values at the beginning of the Rhuddanian. Additionally, the frequency and thickness of the bentonite layers were gradually decreasing and thinning from the Wufeng Formation to the Longmaxi Formation, and indicators of volcanic activity intensity, such as Zr content and Hf content, and Zr/Cr ratio exhibited an overall declining trend from the bottom to the top, aligning with the pattern of volcanic activity and the evolution of the sedimentary environment in the Late Ordovician–Early Silurian. The weathering process of volcanic rocks and volcanic ash brought huge amounts of P to the ocean during the Late Ordovician-Early Silurian, accompanied by inputs of N, Fe, Zn, and other vital elements necessary for biological growth and development, triggering the flourishing of marine organisms in the Yangtze Sea, with a rapid increase in biomass and consumption of more oceanic and atmospheric CO₂. The original organic carbon sequestered in the Wufeng-Longmaxi Formation in the Yangtze region is about 4582.493 Gt, and the global total original organic carbon sequestered during this period is at least 16131.135 Gt. Volcanic activity enhanced the biological pumping effect, which resulted in the largest organic carbon sequestration in the Early Paleozoic.