Natural atmospheric CO2 mineralisation in unexpected places

Overview

Atmospheric CO2 naturally reacts with some rock types, particularly ultramafic igneous rocks. Ultramafic igneous rocks are composed of silicate minerals such as olivine which are rich in divalent metal cations like Mg and Ca (e.g. Oelkers et al., 2008). These minerals are relatively unstable at Earth surface conditions and weather easily, releasing Ca and Mg into solution in water, while consuming atmospheric CO2 (e.g. Clark et al., 1992; Falk et al., 2016). In ultramafic rocks, high pH in the natural waters results in the carbon hosted by carbonate ions (or bicarbonate depending on the pH) (e.g. Millero et al., 2006) which can react with dissolved Ca and Mg from the weathered ultramafic rocks to rapidly precipitate carbonate minerals such as calcite (e.g. Kelemen et al, 2011). The CO2 is therefore converted from a gas to a solid.

The generally held view is that only ultramafic igneous rocks will react with atmospheric CO2 this rapidly, and act as a rapid carbon capture and storage pathway. However, the supervisors have observed natural examples of CO2 mineralisation at Earth-surface conditions on other (i.e. not ultramafic) crystalline rock types with a wide range of component minerals. There are no biogenic or lithogenic CO2 sources at the locations of these natural examples, and pilot carbon isotope analysis indicates an atmospheric CO2 source. This challenges the received wisdom that only ultramafic rocks can rapidly mineralise atmospheric CO2 and stabilise the C in a mineral form at Earth-surface conditions. This project seeks to understand the factors which allow atmospheric CO2 to be mineralised through reaction with a wider-than-previously-known range of rock types.

In a time of rising atmospheric CO2 levels, we are faced with an urgent need to extend our knowledge of the mechanism of CO2 mineralisation. While we know that ultramafic igneous rocks will mineralise CO2, these rocks are very rare at the Earth’s surface (Amiotte Suchet et al., 2003). This study will determine the factors that lead to natural examples of rapid CO2 mineralisation by other crystalline rock types – hitherto undocumented – which cover at least 30% of the Earth’s land surface area. This advance in our understanding of the fundamentals of the CO2 mineralisation reaction could have implications for future carbon capture and storage measures as an approach to mitigating anthropogenically-induced climate change.

The overall aim of this project is to determine the physical, chemical, and environmental parameters which lead to natural mineral carbonation of mafic, intermediate and felsic crystalline rocks.

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Image Captions

Fig. 1: mineral carbonate deposit on intermediate gneiss at Badcall, an example natural atmospheric CO2 mineralisation site in NW Scotland

Methodology

This project will include an exciting mix of fieldwork and laboratory geochemical analysis. A range of sites with natural atmospheric CO2 mineralisation on igneous and metaigneous rock types in the Scottish Highlands have been identified (Fig. 1). Pilot analyses show the calcite deposits have a δ13C signature of ca. -5 ‰ to -14 ‰ indicating an atmospheric CO2 source. The PhD student will conduct a survey of these sites involving: mapping topography, geology and hydrology around sites; making in-situ measurements of parameters such as soil and water pH; and collecting rock and carbonate samples for laboratory analysis.
Samples will be subjected to an extensive suite of microanalyses (generally conducted at the University of Glasgow). Water, soil, carbonate and rock bulk and mineral chemistry will be analysed by AAS/ICP-MS, XRF and LA-ICP-MS to investigate a link between host rock chemistry and CO2 mineralisation. Carbon isotope signatures of the mineralised CO2 will be analysed by IRMS to confirm an atmospheric CO2 origin. Thin section petrography and XRD will determine mineralogy of the host rocks and carbonate deposits. Additionally, the student, with guidance from the supervisors, will apply for radiocarbon analysis of the carbonate deposits through the Nation Environmental Isotopes Facility access route; if successful this could determine the ages of CO2 mineralisation while also giving the student experience and training in writing funding proposals.
Field and laboratory data will be synthesised in a conceptual model, showing why mineral carbonation occurred at the case study sites. This will highlight the controlling parameters and show how this knowledge can contribute to optimising large-scale future deployment of CCSM.

Project Timeline

Year 1

• Getting started and understanding the project
• Literature review
• Scoping our field sites and fieldwork planning
• Fieldwork and sample collection

Year 2

• Processing and working up field data
• Sample preparation
• Laboratory analysis (petrography, XRD, carbon isotope analysis)

Year 3

• Laboratory analysis (bulk and mineral chemistry)
• Data processing and interpretation
• Development of conceptual model

Year 3.5

• Any final data collection/processing
• Thesis and paper writing

Training
& Skills

This project will equip the student a range of analytical and transferable skills which are desirable for careers in research or industry.
Research Methods
Fieldwork at the case study sites will be conducted with the supervisory team. Full training will be given in all of the laboratory techniques to be used in the project, mainly at the University of Glasgow but also in collaboration with some external facilities.
Researcher Development
Technical & personal skills development will be undertaken with guidance from doctoral advisors and within the framework of the DTP Researcher Development Statement. Researcher developmental training will be provided by IAPETUS2 and supplemented by the University of Glasgow. The School of Geographical and Earth Sciences at the University of Glasgow (GES) has a large research research student cohort (currently ~60 PhD students) that will provide peer-support throughout the research program. The scholar will participate in GES’s annual progression assessment and post-graduate research conference, providing an opportunity to present their research to postgraduates and staff within the School, and to also learn about the research conducted by their fellow postgraduate peers. Additionally, skills in NERC’s ‘most wanted’ list for PhD student training will be developed, including in multi-disciplinarity, data management, numeracy, and fieldwork, in addition to principles and practice of various other laboratory analytical techniques such as stable isotope geochemistry. Training and experience in national and international conference presentations, and preparation and submission of papers to international peer-reviewed journals will also be provided.

References & further reading

• Oelkers, E.H., et al. (2008). Elements, 4, 333-337
• Clark, I.D., et al. (1992). Geochimica et Cosmochimica Acta, 56, 2041-2050;
• Falk, E.S., et al. (2016). Geochimica et Cosmochimica Acta, 192, 1-28
• Millero, F.J., et al. (2006). Marine Chemistry, 100, 80-94.
• Kelemen, P., et al. (2011). Annual Review Of Earth And Planetary Sciences, 39, 93.
• Amiotte Suchet, P., et al. (2003). Global Biogeochemical Cycles, 17, 1038
• Most Wanted Postgraduate Skills. http://www.nerc.ac.uk/skills/postgrad/policy/skillsreview/2010/most-wanted-report.pdf

Further Information

Application procedure: For IAPETUS2 applications to the University of Glasgow please use the dedicated application portal: www.gla.ac.uk/ScholarshipApp (you will still need to submit your administrative details to the IAPETUS2 website as well).

Please contact the lead supervisor (john.macdonald.3@glasgow.ac.uk) to discuss the project.

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