Rhenium isotopes to track carbon dioxide emissions by oxidative weathering

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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 Our novel, unpublished data shows that the Re isotope ratio in rivers increases with weathering rate.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.