The carbon-cycle response to an ocean-anoxic-event derived from calcium isotopes

Ocean anoxic events are periods in Earth's history when large changes in the carbon cycle occurred on geologically short timescales (about 0.5 million years). The causes of these events are not well known, though they may relate to massive volcanic degassing of CO2, or to release of large amounts of methane. The response of the ocean system to such carbon release is also not well understood, although changes in sediment type to anoxic sediment indicate a very different ocean state and the burial of large amounts of carbon indicates high carbon fluxes at these times. In addition to being fascinating periods of Earth history in their own right, OAEs are also of particular interest because they may provide information about the natural response of the carbon system to large and rapid input of carbon. Several marine-isotope systems have been measured across OAEs to provide information about global change. C isotopes indicate significant perturbation to carbon cycling; Sr isotopes, change in the balance between mid-ocean spreading and weathering; Os isotopes, changes in weathering; and Mo isotopes, information about the oxygen content of the global ocean. Another proxy with potential to tell us about important changes, but one that has not yet been applied to OAEs, is that of Ca isotopes. The Ca-isotope budget of seawater is governed by inputs from rivers and mid-ocean-ridges, and by outputs to marine carbonates. Changes in mid-ocean-ridge output during OAEs, as suggested by some authors, should therefore influence the Ca isotope composition of seawater (as well as the Sr isotopes). And the lower burial of marine carbonates during OAEs (when sediments become less carbonate rich) should lead to a systematic increase in the ocean Ca-isotope composition. In this research, we will measure high-resolution records of ocean Ca isotope composition to assess, for the first time, the response of the Ca cycle to the changes associated with OAEs. We will do this for the Lower Aptian OAE (about 120 million years ago), and will make use of rapidly depositing sediments from the tops of two Pacific guyots that consists of very pure marine carbonates. The use of two guyots will enable comparison of the records to assess how well they represent the global Ca isotope curve, and this will be further assessed with limited measurements of belemnite samples from terrestrial exposure from Yorkshire. The resulting Ca isotope records across an OAE will, in conjunction with existing C and Sr isotope measurements on exactly the same samples, help to tease apart the various major changes that occur during these events, and to understand how the Earth responds to major carbon-cycle perturbations.