Accelerated carbon dioxide release from sedimentary rocks in a warming world

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences


Rocks contain a vast store of carbon locked in the ancient remains of plants and animals. In the upper metre of Earth's surface alone, there is estimated to be more than 1000 billion tonnes of carbon in rocks, which is similar to the amount of carbon found in plants alive today and more than is present in the atmosphere. Weathering processes, which break down rocks into pieces and result in chemical reactions, can release this carbon stored in rocks back to the atmosphere as carbon dioxide (CO2), a greenhouse gas that controls global temperature.

Until recently, the stores of carbon in rocks were considered to be relatively unreactive, and thus viewed as a stable store of carbon. However, a new method developed by the team has directly measured CO2 emissions from rock weathering for the first time. This work shows that weathering can release CO2 rapidly, and that the rates of CO2 release increase with temperature. In addition, there is evidence that in mountain locations, the retreat of glaciers can expose these ancient carbon-containing minerals to weathering processes. This provides another link between climate warming and release of CO2 from rocks to the atmosphere.

These recent findings lead us to question how the Earth's long term carbon cycle operates. Traditionally, chemical weathering of rocks is viewed as a CO2 sink via the weathering of silicate minerals. This process increases CO2 drawdown as temperature warms - a so called "negative feedback". However, it appears that weathering of sedimentary rocks acts in the opposite direction, raising fundamental questions on what controls the trajectory of atmospheric CO2 over Earth's history. In addition, the release of CO2 to the atmosphere from rock weathering may increase over the coming century. While the global fluxes are modest compared to fossil fuel CO2 emissions, they are currently very poorly constrained and will eat into our remaining carbon budget.

This ambitious proposal will establish how CO2 emissions from sedimentary rock weathering respond to climate change. To do this, we will use the newly-devised methods to quantify CO2 release and assess how it changes with temperature. We will focus on areas of sedimentary rock exposure caused by melting glaciers. These areas, and rates of exposure over time, will be assessed in terms of the exposed rock types, elevations and land surface form.

Having selected the best field areas to sample, we will use a combination of two novel approaches to quantify weathering of sedimentary rocks and CO2 release. Direct CO2 measurements will be made where rocks are exposed. Radiocarbon will be used to fingerprint the sources of CO2. In parallel, larger areas will be investigated by measuring the chemistry of streams and rivers, using novel trace element and isotope proxies to identify reactions and quantify CO2 fluxes. When combined with experiments in the laboratory, we will be able to assess how rock weathering is enhanced by climate change.

We will use models to explore how this process may have controlled CO2 in the past, and also assess how the weathering of sedimentary rocks may impact the net carbon budget over the coming century. Only with this information of how the natural carbon cycle will respond can we best establish the most effective and feasible solutions to help mitigate the impacts of rising atmospheric CO2 concentrations.


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