Oxygenation as a driver of the Great Ordovician Biodiversification Event

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The largest radiation of Phanerozoic marine animal life quadrupled genus-level diversity towards the end of the Ordovician Period about 450 million years ago. A leading hypothesis for this Great Ordovician Biodiversification Event is that cooling of the Ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups, such as corals. A complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate, but direct constraints on atmospheric O2 levels remain elusive. Here, we use high-resolution paired bulk carbonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the Ordovician radiation. These results can be used to reconstruct atmospheric O2 levels based on the O2-dependent fractionation of carbon isotopes by photosynthesis. We find a strong temporal link between the Great Ordovician Biodiversification Event and rising O2 concentrations, a pattern that is corroborated by O2 models that use traditional carbon–sulfur mass balance. We conclude that that oxygen levels probably played an important role in regulating early Palaeozoic biodiversity levels, even after the Cambrian Explosion.

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J. Houghton is thanked for valuable discussions in improving earlier versions of this paper. This paper is a contribution to IGCP Projects 591 and 653. Funding was provided in part by the Evolving Earth Foundation (CTE), a Geological Society of America Graduate Student Research Grant (CTE), a Paleontological Society Student Research Grant (CTE) and NSF Grants EAR-0819832 and EAR-0745452 (M.R.S.).

Author information


  1. Department of Geological and Environmental Sciences, Appalachian State University, Boone, NC, USA

    • Cole T. Edwards
  2. School of Earth Sciences, The Ohio State University, Columbus, OH, USA

    • Matthew R. Saltzman
  3. Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, USA

    • Dana L. Royer
  4. Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA

    • David A. Fike


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This project was conceived by C.T.E. and M.R.S. with input from D.L.R. and D.A.F. Isotopic data preparation and analysis was done by C.T.E. Modelling was conducted by C.T.E. with input from D.L.R. The manuscript was developed by C.T.E. and received equal contributions from all authors on editing the final manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Cole T. Edwards.

Electronic supplementary material

  1. Supplementary Information

    Supplementary discussion and figures

  2. Supplementary Table 1

    Binned isotope data used for GEOCARB and model results of O2 and CO2

  3. Supplementary Table 2

    Isotope data and atmospheric O2 using the photosynthetic fractionation effect approach

  4. Supplementary Table 3

    New δ13C and δ34S data used in the photosynthetic fractionation effect and GEOCARB models

  5. Supplementary Table 4

    Taxonomic data used to construct the biodiversity curve