The role of physical erosion in the weathering of fossil organic carbon: An investigation using the trace element rhenium

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The store of carbon in rock as fossil organic matter contains approximately 20,000 times more carbon than is presently in the atmosphere as the greenhouse gas carbon dioxide. Organic matter has been buried throughout Earth history, countering carbon input from volcanic degassing and playing a role in moderating global climate. However, this fossil organic carbon can be oxidised when rocks are exposed to chemical weathering at the surface and ancient carbon input into the modern environment. It is known that human activities are rapidly increasing the rate of this process by burning fossil fuels. However, the natural rate at which this occurs and how it varies throughout the globe is not well known. Perhaps more importantly, the controls on this carbon transfer from rock to atmosphere are poorly constrained. For example, it is not clear how this carbon source might evolve with changing environmental conditions (e.g. precipitation patterns) on land over the coming decades. This proposal addresses these issues, aiming to better understand how Earth's carbon cycle has functioned in the past, and to predict future changes. This research will quantify the rate of fossil organic carbon weathering in river catchments in the French and Swiss Alps, and the mountains of Taiwan. These rivers have been selected because their physical erosion rate, climatic setting and bedrock geology are all well known and vary between catchments. These are the factors that are thought to be important for fossil organic carbon weathering and the project will test their specific role on this carbon transfer. To achieve these aims, the products of fossil organic carbon weathering will be tracked by determining the geochemical composition of river water and sediment. Specifically, the element rhenium (Re) will be used. Re is bound within organic matter in rocks and so when fossil organic carbon is exposed to weathering Re is dissolved into water, contributing to the dissolved load of a river. The source of Re will be confirmed by comparing its concentration in the river water with other elements released from rocks during chemical weathering. Additional information will be acquired from measuring the solid residue of weathering in soils and river sediments. The Re flux in a river will be calculated using measurements of water discharge and this will be converted into a fossil organic carbon weathering rate using the constraint placed on the source of Re. The fossil organic carbon weathering rate in the studied catchments will be compared to their physical erosion rate, climate (runoff and runoff variability) and bedrock geology. This will provide, for the first time, an empirical link between the rate at which this carbon is transferred from rock and the controlling parameters. As such, it will be possible to extrapolate the findings over a much larger scale, where these parameters are known but the geochemistry of the river water has not been studied. In addition, it will be possible to assess how this carbon source may evolve in response to changes in environmental conditions over the coming decades. It will also allow the balance between fossil organic carbon weathering and organic carbon burial to be more vigorously assessed over longer, geological time periods.