Pinpointing Earth-System Thresholds for Anoxia with new Reconstructions of the Cretaceous Hothouse

由于人为营养物质的输入和二氧化碳的释放，地球海洋中的氧气水平正在迅速下降，这对海洋生态系统的影响很难预测。白垩纪(6600万至1.45亿年前)见证了许多极端海洋缺氧事件(OAEs)--有些是区域性的，有些是全球性的--被认为是由火山活动和二氧化碳释放的脉冲引起的。如果我们知道火山作用触发这些白垩纪海洋环境事件的确切机制，以及反馈和边界条件的作用，它们可以提供关于地球系统潜在引爆点所在位置的重要信息。然而，目前我们对白垩纪的气候、大气二氧化碳和碳循环的估计充其量是定性的，这阻碍了OAEs成为有用的类似物。有了PetrArch，我将通过将新的和经过验证的地球化学代理档案与尖端的地球系统模型相结合来纠正这一点。具体地说，我将：a)校准放射虫硅壳中的硼同位素，作为海洋pH和大气二氧化碳的新替代档案，这将为古气候学开辟新的前沿；b)将这一新的放射虫数据与对碳酸盐微型化石的新测量相结合，以重建白垩纪的碳循环；以及c)使用地球系统模型，调整到这些数据，以测试是什么确切的生物地球化学反馈和气候压力使白垩纪地球陷入严重的，有时是全球的海洋缺氧。通过探测大陆形态和大气O2和CO2水平的敏感性，我将确定在多大程度上缺氧是温室世界二氧化碳上升的必然结果，以及白垩纪在多大程度上预先适应了缺氧。最后，我将综合过去地质上的这些教训，准确地指出我们必须在多大程度上扰乱今天的地球系统--它的气候、海洋生态系统结构和地球化学循环--才能在未来几个世纪导致OAE。

Oxygen levels in Earth's oceans are dropping fast due to anthropogenic nutrient input and CO2 release, and the consequences of this for marine ecosystems are difficult to predict. The Cretaceous Period (66-145 million years ago) witnessed numerous extreme ocean anoxic events (OAEs) - some regional, some global in scale - that are thought to have been caused by pulses of volcanism and CO2 release. If we knew the exact mechanisms by which volcanism triggered these Cretaceous OAEs, and the role of feedbacks and boundary conditions, they could provide vital information as to where potential tipping points in the Earth system lie. Presently, however, our estimates of climate, atmospheric CO2 and carbon cycling in the Cretaceous are qualitative at best, preventing OAEs from being useful analogues. With PETRARCH, I will rectify this, by combining both new and proven geochemical proxy archives with cutting-edge Earth system modelling. Specifically, I will: a) calibrate boron isotopes in the silica shells of radiolarians as a new proxy archive for ocean pH and atmospheric CO2, that will open up new frontiers for palaeoclimatology, b) combine this new radiolarian data with new measurements of carbonate microfossils to reconstruct the Cretaceous carbon cycle, and c) use Earth System modelling, tuned to these data, to test what exact biogeochemical feedbacks and climate forcings tipped the Cretaceous Earth into profound, and sometimes global, ocean anoxia. By probing the sensitivity to continental configuration and starting atmospheric O2 and CO2 levels, I will determine to what extent anoxia is an inevitable consequence of CO2 rise in a greenhouse world, and to what extent the Cretaceous was preconditioned to anoxia. Finally, I will synthesise these lessons from the geological past to pinpoint exactly how much we would have to perturb today's Earth system - its climate, marine ecosystem structure and geochemical cycles - to cause an OAE in the coming centuries.