The Toarcian Oceanic Anoxic Event in a shelf environment (Eastern Tethys): Implications for weathering and redox conditions

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) is characterized by significant environmental perturbation and climatic instabilities due to a substantial release of greenhouse gasses. Several investigations of the T-OAE were conducted based on reliable and well-developed integration of multi-proxy approaches, revealing the global distribution of this event in the Panthalassic, Boreal, and Western Tethyan regions. However, a few marine shelf T-OAE records are reported from the Eastern Tethys. This study presents high-resolution organic and inorganic and isotope geochemistry, mineralogical, and sedimentological analyses from the marine Suobucha section of the Qiangtang Basin to assess the role of continental weathering and paleoceanographic conditions, including paleoredox and paleoproductivity conditions, and factors that govern organic carbon accumulation during the T-OAE in the Eastern Tethys. The carbon isotope profile of Suobucha displays a long-term negative excursion characterizing the Jenkyns Event, which is recognized in geographically widespread sedimentary archives indicating its global nature. Continental weathering was intensified at the onset of the Jenkyns Event. Increased values of fluvial detrital proxies (i.e., Si/Al and Ti/Al) and the occurrence of coarser-grained sediments (i.e., silty mudstones) at Suobucha indicated enhanced terrigenous input at the onset of the Jenkyns Event, which was driven by accelerated continental weathering at this time. The lower Toarcian interval in the marine shelf environment of the Qiangtang Basin is characterized by fully oxidizing conditions intermittent with minor phases of dysoxic setting, especially during the Jenkyns Event interval. Given that a stratified water column with anoxic bottom water occurred in the proximal and more restricted lagoonal area (i.e., the Bilong Co area), redox conditions were spatially variable and controlled by basin hydrography and water depth during the Jenkyns Event in the Qiangtang Basin. Oxygen-enriched conditions and low marine bioproductivity were responsible for the low organic carbon burial within the Suobucha section.