THE TOARCIAN OCEANIC ANOXIC EVENT (T-OAE): AN ASSESSMENT OF GLOBAL CAUSES USING BELEMNITE C-ISOTOPE RECORDS

The Toarcian Oceanic Anoxic Event (T-OAE) is marked by large negative excursions (up to 7 permil PDB) in bulk carbonate, bulk organic carbon, and wood isotope records. These isotope anomalies have been attributed to two very different mechanisms. 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 watermass, 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.

Here we present C-isotope records from belemnites sampled from one locality in Yorkshire (England) and one in Dotternhausen (Germany), that can be used to assess which model best explains the observed changes in carbon isotopes. Both these sections are calibrated with high-resolution ammonite biostratigraphy and Sr-isotope stratigraphy. Our records of the carbon isotopic composition of belemnite calcite do not show the large negative C-isotope excursions shown by coeval records of carbon isotopes 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.

Based on 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, our 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 into the photic zone.