The weathering of sedimentary rocks has profound effects on the Earth system. The chemical breakdown of silicate minerals can remove carbon dioxide (CO2) from the atmosphere, and over geological time acts as a feedback that helps to stabilise long-term climate. However, there is growing recognition that the oxidative weathering of shales (Fig. 1, 2) may in fact release more CO2 than is consumed by silicate weathering. This happens by two main pathways: i) the oxidation of organic matter contained within rocks; and ii) the dissolution of carbonate minerals by sulphuric acid (formed from sulphide oxidation). The global fluxes of these CO2 emissions remain poorly constrained (Hilton et al., 2014; Burke et al., 2018) and we require more information on how these reactions may have changed over geological timescales, and how they may be modified over the coming century.
Metals and their isotope systems can offer new insights. For oxidative weathering reactions, the elements Vanadium (V) and Rhenium (Re) are of particlular interest, as they are known to be enriched in organic matter within sedimentary rocks, but may also be hosted in sulphide and silicate minerals. However, measurements of their concentration and flux in rivers still remain sparse (Shiller and Mao, 2000). The isotope systems of these metals can offer new insight because used in tandem with other elements (e.g. V, Re alongside Sulphur and Molybdenum), they can better track the dissolution of phases with distinct isotope compositions and/or better determine the weathering processes and pathways that lead to CO2 release.
Significant advances in the measurement precision of stable isotope systems (e.g. Prytulak et al., 2016) mean that we are now in a position to measure a range of isotope systems of elements derived from oxidative weathering reactions in shales, soils and natural river water samples. While the CO2 emissions can be hard to track over large scales, element fluxes and their isotope composition provide a tool to quantify them (Hilton et al., 2014). However, to date, there are only a handful of V isotope measurements from river water, and no published Re isotope measurements.
In addition to using these isotope systems to track modern weathering processes, this research is relevant to tracing past ocean chemistry. For example, to use metal isotopes to provide quantitative constraint on redox conditions in the past, it is clear that we need to better understand isotope fractionation during weathering and erosion.
This project will aim to:
– Determine trace metal fluxes in well-studied catchments where oxidative weathering processes operate.
– Establish the controls on the isotope composition of river waters.
– Provide the first riverine isotope measurements of novel isotope systems (Re, V) to assess the surficial cycling of these important trace metals
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An example of shale, rich in sulfide minerals and organic matter, whose weathering can release CO2. This project will help track this process by developing and applying novel isotopic tools. Photo – R.G.Hilton