Gradual onset of anoxia across the Permian–Triassic Boundary in Svalbard, Norway

Abstract

The Permian–Triassic extinction event is considered to be the most devastating environmental crisis of the Phanerozoic. With many coinciding factors involved, the role of marine anoxia during the extinction is still poorly understood. In this study a boreal Permian–Triassic Boundary (PTB) section from Svalbard, Norway has been investigated with the aim of better understanding the timing and nature of local marine anoxia onset and extinction events across the PTB. The section comprises the Kapp Starostin and Vikinghøgda formations; δ13Corg values indicate the PTB is located within the lower part of the Vikinghøgda Formation. Lag deposits at the top of the Kapp Starostin Formation indicate a marine hiatus and ensuing transgression during the Late Permian shortly before the extinction event, implying concurrence of major ecological changes and sea-level rise. Pyrite framboid size distributions and total organic carbon (TOC) were used to evaluate bottom water oxygen conditions, and show that changes in bottom water redox conditions and extinction are clearly linked. Oxic to dysoxic bottom water conditions prevailed during deposition of Kapp Starostin Formation sediments and changed to anoxic to euxinic conditions above the formation boundary. The onset of anoxia is not abrupt but rather shows a gradual increase within the Kapp Starostin Formation during the Late Permian. The tipping point where bottom waters reach a long-term state of anoxia to euxinia coincides with the final extinction event, though changes in biotic assemblages at the top of the Kapp Starostin Formation indicate a marine ecosystem crisis prior to this. Oxygen depletion in the boreal region, as seen in our study section and correlating to Greenland and Arctic Canada, seems to be consistently more severe than in lower latitude settings.