Novel isotope systems to track CO2 release during rock weathering

Biogeochemical Cycles

IAP2-19-028

Overview

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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Image Captions

Shale weathering can release CO2 to the atmosphere, and here we aim to better track this using novel isotope systems

Methodology

The student will use a combination of cutting-edge techniques and unique sample sets to achieve the research objectives.

Part of the work will be based on an extensive river sample set of river water and sediments are held by the research team in Durham, and will be used as part of this project.

Depending on the skills and interests of the applicant, additional fieldwork can be built into the project. Potential locations for fieldwork include: the East River, Colorado; the Mackenzie River basin, Canada; rivers of Iceland; and/or Peruvian Andes

The student will use a wide range of cutting-edge laboratory methods:

• Sample purification by column chemistry
• Trace metal concentration analysis by ICP-MS
• Stable isotope analysis by MC-ICP-MS.

Project Timeline

Year 1

Literature review and compilation of published river trace element datasets; receive training column chemistry, clean laboratory procedures and solid and water sample preparation; receive training on isotope analysis by MC-ICP-MS; undertaking a field season to collect new materials in May-July Year 1; write/ defend Research Proposal

Year 2

Sample and data processing, main geochemical approaches (ICP-MS, MC-ICP-MS); further develop writing skills and manuscript preparation for publication.

Year 3

Synthesise field and modelling datasets; attend international conferences; publication and thesis writing;

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Specialist training will cover field sampling methods for trace metal analyses.

S/he will also be trained on cutting-edge geochemical methods, including: i) major ion analysis by ion chromatography; ii) measurement trace metal concentration; iii) non-traditional stable isotope measurements by MC-ICP-MS.

The supervisory team has the necessary expertise to train the student in these specialist skills, all supported by a dedicated team of technicians in Durham. In addition to receiving regular supervisory meetings and support at Durham (in partnership with St Andrews), the student will also be enrolled in a graduate training programme at Durham University and through IAPETUS-specific training, gaining a range of transferable skills relevant to completion of the PhD and developing a career path, including writing research proposals and giving oral presentations. S/he will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment. The student will attend and contribute to the programme of regular departmental seminars and paper reading groups on a wide range of topics, to support the development of a well-rounded scientist. S/he will attend national and international conferences (e.g. AGU, EGU, Goldschmidt), networking events and outreach activities, developing networks for feedback and future employment.

References & further reading

Burke, A. et al., 2018, Sulfur isotopes in rivers: insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle. Earth and Planetary Science Letters. 496:168-177.

Hilton, R. G., Gaillardet, J., Calmels, D. & Birck, J.-L. Geological respiration of a mountain belt revealed by the trace element rhenium. Earth and Planetary Science Letters. 403:27-36.

Prytulak, J., Sossi, P.A., Halliday, A.N., Plank, T., Savage, P.S. & Woodhead, J.D (2016). Stable vanadium isotopes as a redox proxy in magmatic systems?. Geochemical Perspectives Letters. 3(1): 75-84.

Shiller, A.M. and Mao L., 2000 Dissolved Vanadium in rivers: effects of silicate weathering. Chemical Geology. 165, 13-22.

Further Information

Prof. Robert Hilton, Durham University, 0044191 33 41970, r.g.hilton@durham.ac.uk

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