Do redox-sensitive elements drive carbon dioxide release from high latitude rivers?

Permafrost is the rock, sediment or soil that remains at or below 0oC for more than two consecutive years. The permafrost zone occupies 24 % of the exposed land area in the Northern Hemisphere (Zhang et al., 2000). This high latitude region is warming twice as fast as the rest of Earth surface (Cohen et al., 2014) and amplified polar warming is thawing permafrost soils. These soils store a vast reservoir of organic carbon (OC) that has been locked away for thousands of years. Upon thaw, a portion of this carbon is transferred to the atmosphere as carbon dioxide (CO2) and methane (CH4), which threatens to further amplify Arctic warming and climate change, representing the permafrost carbon feedback loop (Schuur et al., 2015). Permafrost thaw and carbon release is of major importance to climate change mitigation (Natali et al., 2021) as outlined in the Special Report of the Intergovernmental Panel on Climate Change report in 2019 (Pörtner et al., 2019).

A portion of permafrost OC is transported into river waters. Approximately 40 % of this OC is converted to CO2 and released into the atmosphere during river transport (Cole et al., 2007). Therefore, Arctic River waters are an important source of CO2 to the atmosphere. Iron (Fe) and sulphur (S) are alternative electron acceptors that degrade OC in soil pore waters and river waters Permafrost soils contain a large stock of available (i.e., hosted within minerals) Fe and S. During mineral breakdown, Fe is released into soil pore waters as iron-organic carbon (Fe-OC) and sulphur-organic carbon (S-OC) complexes. These complexes are transported from soils to rivers where they undergo transformation via photo-reduction or microbial degradation and contribute to in-stream derived CO2 (e.g., Bowen et al., 2020). Ongoing permafrost thaw will likely increase the supply of Fe-OC and S-OC complexes to Arctic rivers. We hypothesize that the elevated supply of DOC and redox-sensitive elements will increase the CO2 released from river waters. To test this hypothesis, we seek to develop new tracers that can be directly coupled with CO2 gas release. The stable isotopes of iron (δ56Fe) and sulphur (δ34SSO4) offer an exciting, but hitherto unused, opportunity to quantify the redox reactions during organic carbon degradation in rivers.

In this project, the student will use iron (δ56Fe) and sulphate (δ34SSO4) isotopes coupled with CO2 gas measurements to quantify greenhouse gas emissions attributed to C-Fe and C-S reactions in permafrost-dominated Arctic rivers. This project will involve fieldwork in Alaska and the UK.

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Image Captions

An Arctic stream (Abisko, Sweden. Photo Credit: C Hirst)


To achieve the overarching aim, the project is constructed of the following stages:

Stage 1: Laboratory experiments will be carried out to determine the isotope composition of Fe-OC and S-OC complexes. These experiments will allow for an assessment of Fe and S isotope fractionation factors associated with organic carbon complexation. The approach will involve: i) co-precipitation of Fe and S standards with different OC compositions; ii) isolation of the Fe and S fractions using column chemistry; and iii) measuring the isotope composition of these complexes using multi-collector inductively coupled mass spectrometry (Neptune®).

Stage 2: Fieldwork will be carried out at a sub-Arctic field site (Moor House, UK) and an Arctic field site (Eight Mile Lake, Alaska). At both sites river water will be collected from a headwater stream. Collected water will be incubated in tanks under i) controlled conditions allowing for photo and microbial degradation; ii) light, sterile conditions to allow for photodegradation; iii) dark conditions to allow for microbial degradation; iv) spring flood conditions – recreated by adding a pulse of soil pore waters to the tank. For each tank, CO2 measurements will be collected using eosGP CO2 concentration sensors connected to CR1000 data loggers. Water will be sampled from each tank at regular hour-day intervals, filtered and refrigerated for subsequent analysis.

Stage 3: For experimental and field samples, the Fe and S fractions will be isolated using column chemistry; and the δ56Fe and δ34S values will be measured using multi-collector inductively coupled mass spectrometry (MC-ICPMS, Neptune®). Experimentally determined fractionation factors will support the interpretation of tank incubation-derived isotope values. The tank incubation-derived isotope values will be coupled with CO2 data in order to determine the role that redox-sensitive elements play in CO2 release from Arctic and sub-Arctic rivers.

Project Timeline

Year 1

Literature review, perform isotopic experiments, introduction to Fe and S isotope analysis, preparation for fieldwork and tank incubation tests.

Year 2

Fieldwork at Moor House, UK with tank incubations performed at the Department of Earth Sciences, Durham. Fieldwork at Eight Mile Lake, Alaska with tank incubations performed in the field cabin.

Year 3

Column chemistry to isolate Fe and S fractions. MC-ICPMS work to determine the δ56Fe and δ34S values on experimental and incubated samples. Data analysis, including coupling isotope compositions to CO2 measurements.

Year 3.5

Manuscript and thesis writing, submission and examination.

& Skills

This project will provide a fantastic platform to gain multidisciplinary training in field and experimental development and stable isotope geochemistry. The candidate will work closely with the supervisors throughout the PhD. Dr Catherine Hirst will oversee the direction, development and progress of the project and will provide expertise in experiment design and stable isotope geochemistry — including being a close mentor for the candidate’s career development. Dr Geoff Abbott will provide key experience in organic carbon characterization. Professor Fred Worrall will provide a wealth of knowledge on the carbon cycle in peatland environments. Dr Geoff Nowell and Dr Darren Gröcke will provide expertise on stable isotope geochemistry – the candidate will have tailored training on all instruments and become a proficient user of the multi collector-inductively coupled plasma mass spectrometer. Professor Sophie Opfergelt will provide key support regarding processes occurring in permafrost soils. The candidate will have access to extensive IAPETUS2-cohort and NERC training workshops.

The candidate will join a vibrant research community at Durham University, Newcastle University and Université Catholique de Louvain. In addition, the candidate will have the opportunity to present their results to, at least, one national and international research conference. Furthermore, the student will be encouraged to present the importance of peatlands to local UK and USA (Alaska) stakeholders about their importance in climate change. Together, these interactions will provide the candidate with opportunities to learn and practice skills in disseminating their research, networking with academics and communities and a greater understanding of climate change in the modern world.

References & further reading

Cohen, J., Screen, J.A., Furtado, J.C., Barlow, M., Whittleston, D., Coumou, D., Francis, J., Dethloff, K., Entekhabi, D., Overland, J. and Jones, J., 2014. Recent Arctic amplification and extreme mid-latitude weather. Nature geoscience, 7(9), pp.627-637.

Zhang, T., Heginbottom, J.A., Barry, R.G. and Brown, J., 2000. Further statistics on the distribution of permafrost and ground ice in the Northern Hemisphere. Polar geography, 24(2), pp.126-131.

Pörtner, H.O., Roberts, D.C., Masson-Delmotte, V., Zhai, P., Tignor, M., Poloczanska, E. and Weyer, N.M., 2019. The ocean and cryosphere in a changing climate. IPCC, 2019: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, 755 pp. https://doi.org/10.1017/9781009157964.

Schuur, E.A., McGuire, A.D., Schädel, C., Grosse, G., Harden, J.W., Hayes, D.J., Hugelius, G., Koven, C.D., Kuhry, P., Lawrence, D.M. and Natali, S.M., 2015. Climate change and the permafrost carbon feedback. Nature, 520(7546), pp.171-179.

Natali, S.M., Holdren, J.P., Rogers, B.M., Treharne, R., Duffy, P.B., Pomerance, R. and MacDonald, E., 2021. Permafrost carbon feedbacks threaten global climate goals. Proceedings of the National Academy of Sciences, 118(21), p.e2100163118.

Cole, J.J., Prairie, Y.T., Caraco, N.F., McDowell, W.H., Tranvik, L.J., Striegl, R.G., Duarte, C.M., Kortelainen, P., Downing, J.A., Middelburg, J.J. and Melack, J., 2007. Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget. Ecosystems, 10(1), pp.172-185.

Bowen, J.C., Ward, C.P., Kling, G.W. and Cory, R.M., 2020. Arctic amplification of global warming strengthened by sunlight oxidation of permafrost carbon to CO2. Geophysical Research Letters, 47(12), p.e2020GL087085.

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