Environmental tipping points during supercontinent breakup

在整个地球历史中，超级大陆（联合的大陆块）在板块构造的重复循环中聚集和分裂。有人认为，这种“超大陆循环”驱动了海洋-大气系统的化学反应，进而影响了地球的气候。然而，这种联系在很大程度上是定性的，导致地球历史转折点的详细机制、反馈和化学通量仍有待量化。了解这些过程是如何相互关联的需要地球化学，气候和构造建模工具和专业知识的整合。我们建议将现有的火山风化模型（NE/K00543X/1）与悉尼和阿德莱德的世界领先团队开发的动态板块模型相结合，量化超大陆分裂的地球化学足迹。具体来说，我们将考虑Rodinia在~ 7.5亿年前（Ma）和Gondwana在~180 Ma的分裂。这两个事件的前兆都是一系列明显相似的事件，包括火山活动和风化作用的加剧，但导致了根本不同的气候反应。罗迪尼亚板块的分裂最终导致了持续数千万年的“雪球地球”冰川作用，而冈瓦纳板块的分裂则导致了白垩纪温室世界的出现。我们假设，超大陆分裂的气候结果是由产生的大陆或板块拓扑结构的排列决定的。超大陆的分裂给地球系统带来了混乱。大的火成岩省释放出大量的二氧化碳和气溶胶，将热量困在大气中，使地球变暖。破裂还会加剧化学风化，也就是说，雨水与岩石发生反应，将溶解的元素通过河流冲进海洋。这是因为陆地的撕裂产生了更多的海道，导致更大比例的大陆陆地靠近海洋。我们的工作表明，形成新的洋中脊，这是破裂的一个组成部分，导致了新鲜火山岩的强烈风化。当河流和火山将钙等元素冲入海水时，这些元素与二氧化碳结合形成CaCO3，最终降低大气中的二氧化碳水平，并可能导致净冷却效应。靠近赤道的大陆比靠近两极的大陆更容易受到化学风化的影响，因此板块的拓扑结构是气候变化的强大驱动力。正如这个例子所示，某些过程或状态的微小变化可能会使地球进入一个长期存在的温室或冰窖阶段，这取决于背景地质条件的组合（即“蝴蝶效应”）。我们目前由nerc资助的研究表明，火山灰扩散和山脊火山作用在影响地球碳循环和海洋生物泵方面发挥了重要作用。再加上火山释放的气体，这些反馈可能足够强大，足以破坏气候系统的稳定，并使反应朝任何一个方向倾斜。我们的模型将使用蒙特卡罗方法来解决系统复杂性，并考虑地质条件和通量的不确定性。由于大陆“解压缩”强烈影响大陆和海洋地壳的化学通量，将我们的模拟与动态板块运动模型相结合将大大减少风化通量估算的不确定性。贝叶斯模型将使我们能够解卷积火山构造过程速率与环境变化的地球化学代用物之间的联系。再加上对地球系统模型的质疑，这将使我们更好地了解气候强迫，从而建立一个框架，以理解超大陆分裂期间的“临界点”。虽然雪球地球和白垩纪通常被认为是两极对立的，但这两个时期的化学风化作用可能分别刺激了复杂生命的出现和浮游生物的辐射，它们在调节海洋化学方面起着至关重要的作用。

Throughout Earth history, supercontinents (united continental landmasses) assemble and break up in a repeating cycle of plate tectonics. It has been suggested that this 'supercontinent cycle' drives the chemistry of the ocean-atmosphere system, and in turn influences Earth's climate. However, this association is largely qualitative and the detailed mechanisms, feedbacks and chemical fluxes leading up to turning points in Earth history remain to be quantified. Understanding how these processes are interrelated requires integration of geochemical, climate and tectonic modelling tools and expertise. We propose to integrate our existing volcanic weathering models (NE/K00543X/1) with dynamic plate models developed by world-leading groups at Sydney and Adelaide, to quantify the geochemical footprint of supercontinent breakup.Specifically, we will consider the breakup of Rodinia ~750 million years ago (Ma) and Gondwana at ~180 Ma. Both events were heralded by an apparently similar chain of events, including intensified volcanism and weathering, but resulted in fundamentally different climatic responses. Rodinia breakup culminated in a 'Snowball Earth' glaciation lasting tens of millions of years, whereas Gondwana breakup gave rise to the Cretaceous greenhouse world. We hypothesise that the climatic outcome of supercontinent breakup is governed to a first order by the arrangement of the resulting continents, or plate topology. Supercontinent breakup spells chaos for the Earth system. Large Igneous Provinces release vast amounts of CO2 and aerosols, trapping heat inside the atmosphere making the planet warmer. Breakup also brings intensified chemical weathering, that is, rainwater reacting with rocks to flush dissolved elements via rivers into the oceans. This occurs because tearing up a landmass produces more seaways, resulting in closer proximity to the oceans for a higher proportion of the continental landmass. Our work has shown that forming new mid-ocean-ridges, an integral part of breakup, leads to intense weathering of fresh volcanic rocks. When rivers and volcanoes flush elements such as calcium into seawater, these combine with CO2 to form CaCO3, ultimately reducing atmospheric CO2 levels and potentially causing a net cooling effect. Continents near the equator are more prone to chemical weathering than those near the poles, so plate topology is a powerful driver of climate.As illustrated by this example, slight changes in certain processes or states could tip the Earth into a long-lived greenhouse, or an icehouse phase, depending on a combination of background geological conditions (i.e. a 'butterfly effect'). Our current NERC-funded research demonstrates a major role for volcanic ash dispersal and ridge volcanism in affecting the Earth's carbon cycle and marine biological pump. Coupled with volcanic outgassing, these feedbacks might be sufficiently powerful to destabilise the climate system and tip the response in either direction.Our models will use a Monte Carlo approach to resolve system complexity, and account for uncertainties in geological conditions and fluxes. As continental 'unzipping' strongly influences chemical fluxes from the continents and ocean crust, combining our simulations with dynamic plate motion models will significantly reduce uncertainties in weathering flux estimates. Bayesian models will allow us to deconvolve links between rates of volcano-tectonic processes and geochemical proxies of environmental change. Together with interrogation of Earth system models, this will allow us to better understand climate forcings, and thereby develop a framework for understanding 'tipping points' during supercontinent breakup. Although Snowball Earth and the Cretaceous world are often regarded as polar opposites, chemical weathering during these periods likely stimulated, respectively, the rise of complex life and radiation of planktonic organisms, which play a crucial role in regulating ocean chemistry.