The Preliminary Interpretation Of The Negative Excursion Event Of Inorganic Carbon Isotopes And The Mechanism Across The Ordovician-Silurian Boundary In The Middle And Upper Yangtze Region

The Hirnantian glaciation at the end-Ordovician, which lasted about 0.8 m.y.～1.3 m.y., is one of the most critical glaciation events in the Earth history. In this interval, a series of environmental changes and biological evolutions occurred, including a global cooling, a large-scale global sea level decline and the second devastated mass extinction since the Phanerozoic Eon. Meanwhile, a sharp carbon isotopic excursion was recorded in the Himantian stage all over the world, commonly appearing as a global positive excursion of both organic and inorganic carbon isotopes, which is called the HICE event. Nevertheless, the inorganic carbon isotopic variations of the Hirnantian stage in South China show a negative excursion that is different from the HICE event. So the interpretation of this negative excursion event of inorganic carbon isotopes throws light upon the perturbation of carbon reservoir, the change of oceanic chemistry and the mass extinction of the Yangtze Sea during the Ordovician-Silurian transition. Herein four typical sections across Ordovician-Silurian boundary at South China were studied and sampled, including the Wangjiawan Riverside Section, Huanghuachang Section and Miaoyaxiang Section in Hubei, as well as Guanyinqiao Section in Chongqing. In the studied area, the Himantian Stage sequentially outcrops the Upper Wufeng Formation comprising a set of black graptolite shale, the Kuanyinchiao Bed consisting of dark grey argillaceous limestone and carbonaceous mudstone, and the Lower Lungmachi Formation characterized as another set of black graptolite shale, in ascending order. As the main object of carbon and oxygen isotope meassurement, the Kuanyinchiao Bed is a set of shallow-water sediment dominated by argillaceous limestone less than lm in thickness bearing abundant Hirnantia fauna fossils, which recorded a notable sea level drop at the Himantian glacial maximum.Through high-resolution stable carbon and oxygen isotopic measurement and data from previous studies, high-resolution carbon and oxygen isotopic stratigraphy across Ordovician-Silurian Boundary in the Middle and Upper Yangtze region was reconstructed. All of the δ13Ccarb variations in these sections appear a prominent negative excursion in the the Kuanyinchiao Bed, with smaller and smaller magnitude of curve from shelf sections to intra-shelf sections. This distinct negative excursion indicates abrupt changes of sedimentary environment merely in the Yangtze Basin during the Kuanyinchiao period, revealing the injection of massive external light carbon (12C) component into the dissolved inorganic carbon reservoir (DIC) during the glacial maximum with a notable sea-level drop. The origin of massive 12C may relate to the aerobic and anaerobic oxidation of methane (AOM) and organic matter.Given multi-proxies such as the bathymetry, redox conditions, micro-lithofacies, sulfur isotope and organic carbon isotope, etc. into consideration, the evolution model of oceanic chemiscal regime and perturbation of DIC reservoir had also been established. From the Late Katian to the Early Hirnantian, the Yangtze Sea remained stratified with anoxic bottom water. When it came to the Middle Hirnantian, the sea level dropped dramatically with the rapid expansion of ice-sheet in the South Pole, leading to the dissociation of methane hydrate at the edge of shelf due to the decreasing hydrostatic pressure above. Then a plenty of methane gas was released to go through aerobic and anaerobic oxidation in both sediment and water column. This process, together with the oxidation of organic matter, reduced the concentration of oceanic sulfate, put enormous external light carbon flux into the oceanic DIC reservoir, changed the carbon isotope composition of seawater, and directly or indirectly promoted the migration and expansion of euxinic water in the deep. Finally, the glaciation ended at the late-Hirnantian, companied by the rise of sea level, the restability of hydrate system at the seafloor, and the termination of methane release and its AOM process. Thus, the composition of DIC reservoir and the hydrochemical stratification of the Yangtze Sea returned back to the pre-glaciation regime.