Carbon isotope and mercury anomalies evidence across the Ordovician-Silurian boundary of volcanism accelerating the global carbon cycle

Abstract

Late Ordovician–Early Silurian global volcanism triggered a series of interrelated changes in the lithosphere, atmosphere, hydrosphere, and biosphere. The process by which environmental effects such as silicate weathering, biological pumping, and atmospheric temperature changes caused by volcanic material input into the surface system cause fluctuations in the global carbon cycle is unclear. Analysis of global changes in Hg content, Hg/TOC ratios, and δ13Ccarb values during the Ordovician-Silurian transition, combined with volcanism indicators such as the development frequency of bentonite layers, Hg content, Zr content, Hf content, and other indicators, as well as Cu content, Mo content, TOC content, δ13Ccarb values, Sr/Cu, Sr/Ba, U/Th, V/Cr, V/Ni, and other sedimentary environment indicators in the Yangtze region, revealed that severe environmental damage caused by large-scale volcanism prevents biological populations from immediately beginning to recover after volcanism ceases, resulting in a certain environmental recovery period. This delay can occasionally result in a lag in biological flourishing caused by volcanism. Volcanism releases large amounts of light carbon, increasing the amount of 12C input received by the marine environment and sequestered in organic matter, resulting in a negative shift in the δ13Ccarb value of shale. Volcanism increased the amount of CO2 in the atmospheric carbon pool, intensified continental weathering, triggered the biological pump effect in the marine environment, and led to a significant increase in CO2 sequestration and initial organic carbon production. Carbon sequestration removes excess CO2 from the atmosphere, leading to a balanced state in the surface environment between the increase in CO2 caused by degassing and the consumption of CO2 caused by carbon sequestration. The high input rate of volcanic material into the environment accelerated the sedimentation rate and circulation rate of material in the sedimentary environment, which in turn accelerated the circulation rates of elements such as C, N, O, and P. Simultaneously, it caused carbon isotope fractionation, accelerating the sequestration of more 12C in the organic-rich shales of the Wufeng–Longmaxi Formation.