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

海洋缺氧事件是地球历史上碳循环在地质学上短时间尺度（约50万年）内发生重大变化的时期。这些事件的原因尚不清楚，尽管它们可能与大规模的火山二氧化碳脱气或大量甲烷的释放有关。海洋系统对这种碳释放的反应也没有得到很好的理解，尽管沉积物类型向缺氧沉积物的变化表明海洋状态非常不同，大量碳的埋藏表明这些时候的碳通量很高。除了本身是地球历史上迷人的时期外，OAE也特别令人感兴趣，因为它们可以提供有关碳系统对大量快速碳输入的自然反应的信息。已在OAE中测量了几个海洋同位素系统，以提供有关全球变化的信息。碳同位素表明碳循环的显著扰动;锶同位素表明大洋中部扩张和风化之间平衡的变化;锇同位素表明风化的变化;钼同位素表明全球海洋氧含量的信息。另一个有可能告诉我们重要变化的代理，但尚未应用于OAE，是Ca同位素。海水钙同位素收支受河流和洋中脊的输入和海洋碳酸盐的输出的影响。因此，正如一些作者所建议的那样，OAE期间洋中脊输出的变化应该会影响海水的Ca同位素组成（以及Sr同位素）。OAE期间海洋碳酸盐的埋藏较低（此时沉积物的碳酸盐含量变得不那么丰富）应该会导致海洋Ca同位素组成的系统性增加。在这项研究中，我们将测量海洋钙同位素组成的高分辨率记录，以评估，第一次，钙循环的变化与耳声发射相关的响应。我们将对下阿普第阶OAE（约1.2亿年前）进行这一研究，并将利用来自两个太平洋盖奥特顶部的快速沉积物，这些沉积物由非常纯的海洋碳酸盐组成。两个盖奥特的使用将使记录的比较，以评估他们如何代表全球钙同位素曲线，这将进一步评估有限的测量从陆地暴露从约克郡的箭石样品。OAE中产生的Ca同位素记录将与完全相同样品的现有C和Sr同位素测量相结合，有助于梳理这些事件期间发生的各种主要变化，并了解地球如何应对主要的碳循环扰动。

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.