Toarcian oceanic anoxic event: An assessment of global causes using belemnite C isotope records

Authors

  • B. van de Schootbrugge,

    1. Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA
    2. Department of Geological Sciences, Wright Labs, Rutgers, The State University of New Jersey, Piscataway, New Jersey, USA
    3. Now at Institute of Geology and Paleontology, Johann Wolfgang Goethe University, Frankfurt am Main, Germany.
    Search for more papers by this author
  • J. M. McArthur,

    1. Department of Earth Sciences, University College London, London, UK
    Search for more papers by this author
  • T. R. Bailey,

    1. Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA
    2. Now at National Museums and Galleries of Wales, Cardiff, UK.
    Search for more papers by this author
  • Y. Rosenthal,

    1. Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA
    2. Department of Geological Sciences, Wright Labs, Rutgers, The State University of New Jersey, Piscataway, New Jersey, USA
    Search for more papers by this author
  • J. D. Wright,

    1. Department of Geological Sciences, Wright Labs, Rutgers, The State University of New Jersey, Piscataway, New Jersey, USA
    Search for more papers by this author
  • K. G. Miller

    1. Department of Geological Sciences, Wright Labs, Rutgers, The State University of New Jersey, Piscataway, New Jersey, USA
    Search for more papers by this author

Abstract

[1] Two hypotheses have been proposed to explain simultaneous large negative excursions (up to 7‰ PeeDee belemnite) in bulk carbonate (δ13Ccarb) and organic carbon isotope records (δ13Corg) from black shales marking the Toarcian oceanic anoxic event (T-OAE). The first explanation envisions recycling of dissolved inorganic carbon (DIC) with a light isotopic signature into the photic zone from the lower levels of a salinity-stratified water mass, essentially requiring a regional paleoceanographic driver of the carbon cycle. The second involves the rapid and massive dissociation of methane from gas hydrates that effectively renders the T-OAE a global perturbation of the carbon cycle. We present C isotope records from belemnites (δ13Cbel) sampled from two localities, calibrated with high-resolution ammonite biostratigraphy and Sr isotope stratigraphy, in Yorkshire (England) and Dotternhausen (Germany), that can be used to assess which model best explains the observed changes in carbon isotopes. Our records of the δ13C composition of belemnite calcite do not show the large negative C isotope excursions shown by coeval records of δ13C in sedimentary organic matter or bulk sedimentary carbonate. It follows that isotopically light carbon cannot have dominated the ocean-atmosphere carbon reservoir during the Toarcian OAE, as would be required were the methane release hypothesis correct. On the basis of an evaluation of available carbon isotope records we discuss a model in which the recycling of DIC from the deeper levels of a stratified water body, and shallowing of anoxic conditions into the photic zone, can explain all isotopic profiles. In particular, the model accounts for the higher C isotope values of belemnites that are characteristic of open ocean, well-mixed conditions, and the lower C isotope values of neritic phytoplankton communities that recorded the degree of density stratification and shallowing of anoxia in the photic zone.

Ancillary