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

Abstract

Two hypotheses have been proposed to explain simultaneous large negative excursions (up to 7‰ PeeDee belemnite) in bulk carbonate (δ 13 C carb) and organic carbon isotope records (δ 13 C org) 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 (δ 13 C bel) 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 δ 13 C composition of belemnite calcite do not show the large negative C isotope excursions shown by coeval records of δ 13 C 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.