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Edited by Paul F. Hoffman, University of Victoria, Victoria, British Columbia, Canada, and approved January 8, 2016 (received for review August 1, 2015)
Significance
The end-Permian mass extinction not only decimated taxonomic diversity but also disrupted the functioning of global ecosystems and the stability of biogeochemical cycles. Explaining the 5-million-year delay between the mass extinction and Earth system recovery remains a fundamental challenge in both the Earth and biological sciences. We use coupled records of uranium concentrations and isotopic compositions to constrain global marine redox conditions across the end-Permian extinction horizon and through the subsequent 17 million years of Earth system recovery. Our finding that the trajectory of biological and biogeochemical recovery corresponds to variations in an ocean characterized by extensive, shallow marine anoxia provides, to our knowledge, the first unified explanation for these observations.
Abstract
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 238U/235U isotopic compositions (δ238U) 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 δ238U 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.
Footnotes
- ↵1To whom correspondence should be addressed. Email: kvlau{at}stanford.edu.
↵2Present address: ExxonMobil Upstream Research Company, Houston TX 77389.
Author contributions: K.V.L., K.M., and J.L.P. designed research; K.V.L. performed research; K.V.L., K.M., L.R.K., and J.L.P. analyzed data; K.V.L., K.M., D.A., L.R.K., D.J.L., J.C.S.-T., K.L.W., and J.L.P. wrote the paper; D.A., B.M.K., D.J.L., and M.Y. provided samples and stratigraphic data; L.R.K. contributed to data interpretation and modeling; and J.C.S.-T. contributed to data interpretation.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1515080113/-/DCSupplemental.
Freely available online through the PNAS open access option.
Anoxia and recovery from end-Permian extinction
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- Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction
- Decoupled taxonomic and ecological recoveries from the Permo-Triassic extinction
- Abrupt global-ocean anoxia during the Late Ordovician-early Silurian detected using uranium isotopes of marine carbonates
- Extensive marine anoxia during the terminal Ediacaran Period
- Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction
- Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2
- A model for the decrease in amplitude of carbon isotope excursions across the Phanerozoic
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