Rhenium isotopes to track carbon dioxide emissions by oxidative weathering

二氧化碳（CO2）作为温室气体在控制地球气候方面起着核心作用。大气中的二氧化碳浓度可以通过地球的碳循环来改变，碳循环在大气、植物和动物、海洋和岩石之间移动。近几十年来，由于人类通过燃烧化石燃料将碳从岩石转移到大气中，大气中的二氧化碳浓度一直在增加。然而，自然过程也可以改变碳向大气转移的速度。当含有大量有机碳的岩石受到风化作用时，就会发生这样的转移，这一过程会释放二氧化碳。这个过程可能在地球历史上有很大的变化，例如在造山时期。然而，我们目前缺乏可靠的工具来追踪这些变化，这限制了我们对控制大气中二氧化碳浓度自然变化的过程的理解，从而限制了我们准确预测未来碳循环将如何演变的能力。为了测量现代岩石有机质的氧化，我们可以直接追踪二氧化碳气体。这种方法可以提供一个局部的、短期的关于二氧化碳释放速率的观点。另一种方法是利用存在于岩石中的元素铼（Re）和有机碳。当岩石风化时，二氧化碳以气体的形式释放出来，而稀土则溶解在水中，由河流携带。通过这种方式，我们可以测量河流携带的稀土量，从而估算更大流域的二氧化碳排放量。不幸的是，要重建过去的风化和二氧化碳排放，我们不能直接使用这些技术。相反，回顾岩石中有机碳的风化和相关的二氧化碳排放，Re的同位素很有希望。这是因为风化可以改变释放到河水中的稀土同位素的比例，这反过来又有可能改变海水中全球稀土储量。最近由研究小组领导的分析地球化学的发展，意味着我们第一次能够测量河水中Re同位素的比例。我们未发表的新数据表明，河流中的Re同位素比率随着风化速率的增加而增加。这一观察结果有力地表明，Re同位素可以作为过去氧化风化和相关二氧化碳排放的代表。在这个项目中，我们将解决我们目前对Re同位素系统理解的基本限制，这些限制阻碍了它作为风化代理的当前应用。特别是，我们必须更深入地了解岩石、土壤、河流和世界上最大河流的土壤中反应的水之间的关系。与此同时，我们还必须测量从热液喷口到海洋的其他稀土来源以及来自主要海洋盆地的海水的稀土同位素。我们的建议是在两个具有相同分析能力的实验室之间进行独特的合作。这种合作使我们能够利用三个主要好处：(i)通过共享分析和方法的进步来确保数据的准确性；汇集一个多学科科学家小组的专门知识；（iii）通过在两个尖端实验室实现雄心勃勃的工作计划，最大限度地有效利用资源。该项目将首次对地球整体和输送到全球海洋的风化产物中的稀土同位素组成进行全面评估。在此过程中，我们将为Re同位素替代方法奠定基础，以量化和理解岩石风化过程中二氧化碳排放的过去变化，并解决当前气候模式在模拟过去、现在和未来碳循环变化轨迹方面的一个关键缺陷。

Carbon dioxide (CO2) plays a central role in controlling Earth's climate as a greenhouse gas. Atmospheric CO2 concentrations can be changed by Earth's carbon cycle, which moves carbon between the atmosphere, plant and animal life, the oceans and rocks. Atmospheric CO2 concentrations have been increasing in recent decades because of the human-induced transfer of carbon from rocks to the atmosphere by fossil fuel burning. However, natural processes can also change the rate of carbon transfer to the atmosphere. One such transfer occurs when rocks containing vast stores of organic carbon are exposed to weathering, a process that releases CO2. This process may have varied significantly over Earth's history, for instance during episodes of mountain building. However, we presently lack reliable tools to track these changes which limits our understanding of the processes controlling natural variability in atmospheric CO2 concentrations, and consequently our ability to accurately predict how the carbon cycle will evolve in the future. To measure the oxidation of rock organic matter in the modern day, we can track the CO2 gas directly. This approach can provide a local, short term view of the rates of CO2 release. Another approach has been developed which uses the element rhenium (Re) that is hosted in rocks alongside organic carbon. When rocks are weathered, the CO2 is released as a gas, while the Re becomes dissolved in water and is carried by rivers. In this way, we can measure how much Re rivers carry to estimate CO2 emissions over larger river basins. Unfortunately, to reconstruct weathering and CO2 emissions in the past, we cannot directly use these techniques. Instead, to look back in time at weathering of organic carbon in rocks and the associated CO2 emissions, the isotopes of Re hold much promise. This is because weathering could alter the ratio of Re isotopes released into river water, which in turn has the potential to change the global inventory of Re in seawater. Developments in analytical geochemistry, most recently led by the research team, mean that we have been able to measure the ratio of Re isotopes in river water for the first time. Our novel, unpublished data shows that the Re isotope ratio in rivers increases with weathering rate. This observation strongly suggests that Re isotopes could be used as a proxy of past oxidative weathering and associated CO2 emissions. In this project we will tackle the fundamental limitations of our present understanding of the Re isotope system, that hold back its current application as a weathering proxy. In particular, we must establish a deeper understanding of the relationship between Re isotope ratios in rocks, soils, the waters reacting in soils which feed rivers, and the largest rivers in the world. In parallel, we must also measure other sources of Re to the ocean from hydrothermal vents and the Re isotopes of seawater from the major ocean basins. Our proposal is planned as a unique collaboration between two laboratories with the same analytical capabilities. This collaboration allows us to capitalise on three major benefits: (i) To ensure data accuracy by sharing analytical and method advancements; (ii) To pool the expertise of a multi-disciplinary team of scientists; and (iii) To maximise the efficient use of resources by achieving an ambitious work programme across two cutting-edge laboratories. This project will produce the first complete assessment of the isotopic composition of Re in the bulk Earth and in weathering products being delivered to the global oceans. In doing so, we will lay the foundations for the Re isotope proxy to quantify and understand past changes in CO2 emission from rock weathering, and address a crucial shortcoming in the ability of state-of-the-art climate models to simulate the trajectory of carbon cycle changes in the past, present and future.