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Proc Natl Acad Sci U S A. 2016 Mar 1;113(9):2360-5. doi: 10.1073/pnas.1515080113. Epub 2016 Feb 16.

Marine anoxia and delayed Earth system recovery after the end-Permian extinction.

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Department of Geological Sciences, Stanford University, Stanford, CA 94305;
Department of Geological Sciences, Stanford University, Stanford, CA 94305;
Department of Geological Engineering, Middle East Technical University, 06531 Ankara, Turkey;
Department of Geosciences, The Pennsylvania State University, University Park, PA 16802;
Geosciences Department, Trinity University, San Antonio, TX 78212;
Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204;
College of Resource and Environment Engineering, Guizhou University, 550003 Guizhou, China.


Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and (238)U/(235)U isotopic compositions (δ(238)U) of Upper Permian-Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ(238)U across the end-Permian extinction horizon, from ∼3 ppm and -0.15‰ to ∼0.3 ppm and -0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelves-global biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.


Early Triassic; biogeochemical cycling; carbon isotopes; paleoredox; uranium isotopes

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