Carbon Recycling into the Deep Earth

Overview

Earth’s climate has been moderated by variations in atmospheric CO2 over much of geological history, reflecting the balance of supply to the atmosphere from volcanic degassing, metamorphism, and carbon oxidation and removal through silicate weathering, carbonate deposition, and the burial of organic material (Berner et al, 1981). On geological timescales, amongst the key fluxes affecting the carbon cycle is the recycling of sediments and oceanic lithosphere during subduction. Some of the subducted carbon is released to the atmosphere in arc volcanic gases, but that left is transferred to the deep mantle (Mason et al, 2016). The carbon in subducted material is concentrated in sediments, altered oceanic crust and serpentinised mantle rocks. Sediments account for some 30% of the subducted carbon, and at the present day the sediment that is being subducted at plate margins is dominated by terrigenous material. In these sediments carbon is present as carbonate, and organic carbon and minor carbonate in terrigenous sediments. The efficiency of transfer of carbon into arc volcanic gases or to the deep parts of the mantle, depends on the nature of the sediments and the fluids present in the sediment or adjacent mafic rock types (Kerrick & Connolly, 2001). For example, under oxidizing conditions carbonate destabilization may be associated with the loss of a carbonic fluid. In contrast, under reducing conditions carbon may transform to graphite (Galvez et al, 2013) that has a lower solubility under subduction conditions, providing a mechanism to retain carbon, and thereby enhance the transport of carbon to the deep Earth.

This project aims to quantify the mechanisms of carbon release and retention in sediments and mafic rocks, from the western European Alps, using a combination of field study, detailed petrography and the use of novel stable isotopes (e.g. Debret et al., 2016, Inglis et al., 2017). The focus will be on a well-characterised suite of metepelites (originally terrigenous sediments) and metacarbonates, in the in the Cottian Alps (Schistes Lustrés), similar sediments associated with the Zermatt-Saas ophiolite in Switzerland (Figs. 1), and others exposed at Lago di Cignana, Valtournenche, Italy.

These variably metamorphosed oceanic sediments were deposited in the Jurassic and Cretaceous, and subducted in the convergence that ultimately led to continental collision and the formation of the Alps. Subduction of the Tethyan ocean floor, initiated in the late Mesozoic, culminated in continental subduction at 60 Ma, followed by collision. These rocks represent an aggregate prograde P-T path, from greenschist to blueschist to eclogite facies, and offer the opportunity to characterize carbon mobility in a range of sedimentary rock types across a wide range of metamorphic conditions.

This project aims to reveal the efficiency of carbon release and retention from sediments during subduction. This information will then be used to inform models of how carbon fluxes are controlled by subduction, and how they may have varied over geological time, in response to different thermal states and modes of marine sedimentation.

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Image Captions

Carbonate-bearing metasediments in Zermatt, originally part of the terrigenous sedimentary cover of the Tethyean oceanic crust.

Methodology

The goal of this project is to quantify the extent and nature of carbon transfer from sediments during subduction. Achieved through: (i) fieldwork in the Alps to sample metasediments and mafic rocks from the key localities detailed above. Followed by (ii) petrological study to determine the P-T-fO2 and fluid conditions experienced by each sample, combined with (iii) isotope and elemental measurements on a bulk-rock and mineral scale to trace the extent and scale of carbon-bearing fluid mobility. This data will be used to determine the role of lithology and metamorphic conditions on the mobility of carbon-bearing fluids, the implications for arc degassing and recycling into the deep mantle, and serve to inform models of subduction zone cycling over the past 3 Ga, and the impact on the global carbon cycle.

Project Timeline

Year 1

Training in the measurement of metal stable isotopes isotopes, sample petrography; Preliminary isotope measurements of metasediments, fieldwork in the West European Alps; Completion of Year 1 Research Proposal and review

Year 2

Selection and characterisation of Alpine samples; continued petrography and isotope analysis of all samples; Further field sampling. Prepare research for presentation/publication; attend International geochemistry conference.

Year 3

Completion of isotope work and interpretation and modelling of data. Presentation at national/international conferences.

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

Training
& Skills

Training in the measurement of novel stable isotopes using high precision MC-ICP-MS and TIMs techniques at Durham, as well as geochemical sample characterisation.

Fieldwork in the French, Italian and Swiss Alps.

Interpretation and modelling of petrographic and isotope data to place new constraints on the efficiency of carbon transfer in subducted sediments, and implications for the carbon cycle.

Presentation of research at both national and international geochemistry conferences.

References & further reading

Berner et al. 1983, The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years. Am. J. Sci. 283, 641-683.

Debret et al. 2016, Isotopic evidence for iron mobility during subduction Geology 44, 215-218.

Galvez et al. 2013, Graphite formation by carbonate reduction during subduction, 2013. Nat. Geosci. 6, 473-477.

Inglis et al. 2017, The behaviour of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from western alpine ophiolites. G-cubed, 18, 2562-2579

Kerrick & Connolly, 2001, Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth’s mantle. Nature 411, 293-296.
Mason et al. 2017, Remobilization of crustal carbon may dominate volcanic arc emissions, Science. 357, 290-294.

Further Information

For further information please contact Kevin Burton (kevin.burton@durham.ac.uk)

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