Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event

摘要

The Permian-Triassic Boundary event at 252.2 Ma marks the largest extinction of marine fauna in the Phanerozoic and there is a wide consensus that the extinction coincided with an intense oceanic anoxic event. The stratotype of the Changhsingian Stage, precisely constrained by the PTB Global Stratotype Section and Point (GSSP) and the GSSP for the Wuchiapingian-Changhsingian Boundary, both at Meishan in southern China, is well-documented in respect to geochronology and the pattern of extinction. Here we report secular trends in bulk isotopic parameters and lipid biomarkers in a core spanning 214 m of stratigraphic section across the PTB; and through the entire Changhsingian interval. Our analysis of these data, viewed in the context of relative sea level change and strontium isotopes, reveals distinct shifts in paleoenvironmental conditions and profound changes in plankton ecology well before and following the biological extinction event. Specifically, patterns of steroids and triterpenoids indicate a marine plankton community that was heavily dominated by bacteria during the late Wuchiapingian, middle Changhsingian and early Griesbachian stages. Secular trends in aromatic hydrocarbons diagnostic for anoxygenic green sulphur bacteria (Chlorobiaceae) identify periods when euxinic conditions extended into the photic zone during the entire Changhsingian stage. Here also, the delta N-15 of organic nitrogen progressively shifted from positive values around +2 or + 3%. to -1 parts per thousand coincident with a sharp negative excursion in delta C-13(org) and slightly postdating the sharp minimum in delta C-13 values of inorganic carbon that occurs at the top of Bed 24. These results, together the published chronology indicate that conditions unfavourable for aerobiosis existed in the marine photic zone at Meishan for 1.5 million years prior to the main phase of the biological extinction. The induction of marine euxinic conditions, worldwide, at the end of the Permian was likely a consequence of the aggregation of Pangea and the uplift, weathering and transport of nutrients to the ocean well in advance of the PTB. The protracted and widespread nature of the ensuing oceanic anoxic event suggests a causal association with the mass extinction.