Influence of the Late Ordovician-Early Silurian Paleoenvironment and Related Geological Processes on the Organic Matter Accumulation and Carbon Isotope Excursion

Although previous studies have shown that the paleoenvironment and geological processes contributed to the organic matter accumulation (OMA) and carbon isotope excursions (CIEs) during the late Ordovician-early Silurian, the dominated controlling factor for the OMA and the origins of CIEs still remains unclear due to complex interaction between various paleoenvironmental factors and geological processes. Therefore, based on the elemental geochemistry of the Wufeng-Longmaxi Formation shales in the upper Yangtze Platform, we analyzed the late Ordovician-early Silurian paleoenvironment and related geological processes, and further explored the origin of the OMA and CIEs. As a result, the Wufeng-Longmaxi Formation shale was divided into four stages. During Stage 1 (late Katian, similar to 447.62-444.50 Ma), local tectonic and volcanic activities controlled the paleoproductivity and redox conditions, facilitating the OMA. By contrast, the productivity of the surface water and the anoxic bottom water were mainly controlled by the global climate after the Hirnantian glaciation, which contributed to the OMA during Stage 2 (early Rhuddanian, similar to 444.50-441.00 Ma). The decreasing sea level and rapid uplifting of Xuefeng and Qianzhong Uplifts resulted in the organic matter depletion during Stages 3 (late Rhuddanian, similar to 441.00-440.80 Ma) and 4 (Aeronian, similar to 440.80-439.21 Ma). Besides, the release of C-12-enriched carbon reservoirs triggered by volcanic activities and C-13-enrichment caused by the OMA regulated the carbon cycling: the negative CIE may be the result of light carbon emissions, such as the reactivated organic matter and mantle derived carbon, and the Hirnantian CIE event is jointly controlled by the weakened carbon emission effect and the OMA.