Carbon sequestration processes in the rusty carbon sink

Carbon is sequestered in soils and sediments via mineral associations. Over 20% of organic carbon in sediments is directly bound to reactive iron mineral phases, giving rise to the so-called ‘rusty carbon sink’ (Lalonde et al. 2012). In a world of global environmental change, we need to understand how the rusty carbon sink reacts to changing environmental conditions such as ocean acidification, increasing temperatures, sea level rise, or enhanced rainfall and drought events.

The global biogeochemical iron cycle controls the carbon cycle (Raiswell and Canfield 2012). For example, iron is a limiting nutrient for phytoplankton growth in large areas of the ocean (Moore et al. 2013; Tagliabue et al. 2017), and more than 20% of organic carbon sequestered in sediments are directly bound to reactive iron mineral phases (Lalonde et al. 2012). While some organic compounds such as citrate are specifically excreted by microorganisms to reduce and dissolve iron oxides, other organic compounds such as humic substances have no such effect (Braunschweig et al. 2014). Instead, when binding to the mineral surface, they appear to result in mutual stabilization of both the reactive iron mineral and the organic carbon compound: the organic compounds are protected against microbial processing and redox processes and the reactive iron minerals are protected against mineral transformations. Diagenetic processes (increased heat and pressure) eventually lead to dehydration and reduction of the reactive iron minerals while the carbon compounds are oxidized into carbonate (Posth et al. 2013). However, even under diagenetic conditions, a greater presence of organic compounds slows down the mineral transformation (Schröder et al. 2016). To understand this stabilization is important in particular in light of global environmental change: Where can we enhance carbon storage through such processes and where are existing carbon sinks at risk through changing environmental conditions (e.g. ocean acidification and warming temperatures).

Reactive iron minerals tend to be metastable and it is hypothesized that reactive iron minerals and certain organic compounds mutually stabilize each other (e.g. Schröder et al. 2016): the organic compounds are protected against microbial processing and redox processes and the reactive iron minerals are protected against mineral transformations. In this project, we aim to understand how the mutual stabilization works. What types of organic compounds and functional groups enhance this stabilization (Hu et al. 2023; Zhao et al. 2023)? Are different kinds of reactive iron minerals more efficient in preserving organic carbon that way than others? What environmental factors increase or decrease this stabilization?

In this project, we will investigate sediments collected from different coastal and marine settings and compare those to experimental setups in the laboratory, in which we will investigate reaction pathways of iron minerals and organic compounds. Specifically, we will study mineral transformations via Mössbauer spectroscopy, using the stable 57Fe isotopes as a trace to understand reaction pathways (Notini et al. 2023), and use advanced methods to identify the organic compounds binding to these minerals.

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Image Captions

View over Loch Creran, a fjord-like sea loch in Scotland. Fjords are efficient carbon sinks yet the role of iron-carbon interactions is unclear. Image credit: Paul Birrell, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=9166148


Sediments from different coastal and marine settings are available via sediments cores collected during cruises around the British Isles, which are stored at Stirling. Fresh sediments will be collected in dedicated field trips to estuaries and fjord-like sea lochs in Scotland. Iron mineralogy and organic compounds in these samples will be compared to experimental setups.

We will use Mössbauer spectroscopy at the University of Stirling’s Mössbauer Spectroscopy laboratory for Earth and Environment (MoSEE). Mössbauer spectroscopy is a powerful tool to investigate iron minerals (Gütlich and Schröder 2012; Gütlich et al. 2012), however, it is sensitive to the isotope 57Fe only. While 57Fe occurs with a natural abundance of ~2% of iron, we can synthesize iron minerals with pure 56Fe (invisible to Mössbauer spectroscopy) and add 57Fe to follow the reactions of only that selected iron pool. This way we can investigate different reaction scenarios and environmental conditions (Notini et al. 2023), and compare our experimental results with natural samples collected from diverse environmental settings where different conditions prevail.

Besides Mössbauer spectroscopy we will study particle size, geochemical composition, and mineralogy via Scanning Electron Microscopy (SEM), X-Ray Fluorescence (XRF), and Inductivley-Coupled Plasma Mass Spectrometry (ICP-MS) at Stirling. Transmission Electron Microscopy, X-Rau Diffraction (XRD), and Raman spectroscopy will be carried out at the University of Glasgow.

We will use chemical sequential extractions to quantify the amount of organic carbon associated with reactive iron minerals at Stirling. Organic carbon compounds will be extracted using Accelerated Solvent Extraction and investigated for their composition, functional groups, and origin using a range of techniques, including Raman spectroscopy, Gas Chromatography Mass Spectrometry (GC-MS), Liquid Chromatography Mass Spectrometry (LC-MS), and compound-specific isotope analyses (GC-IRMS) in the Biomarkers for Environmental and Climate Science at the University of Glasgow. There is also an opportunity to compare recovery and characterisation of organic compounds between different techniques (i.e., chemical and solvent extraction versus thermogravimetric analysis).

Project Timeline

Year 1

– Review literature on organic carbon preservation in different coastal and marine settings (e.g. estuaries, fjords, shelf sediments etc.).
– Identify suitable sediment cores representing different coastal and marine settings.
– Geochemical and mineralogical characterisation of of cores.
– Synthesis of reactive iron minerals with 57Fe and 56Fe isotopes.
– Mineral transformation experimental set-up.
– National conference participation (e.g. MASTS or SAGES ASM).

Year 2

– chemical sequential extractions of cores to quantify organic carbon bound to iron minerals.
– organic carbon and biomarker analysis of core samples.
– Mössbauer spectroscopy of mineral transformation experiments
– field trip to Scottish sea loch to collect fresh sediment samples.
– revise experimental set up based on results from first set of experiments and iron and organic carbon analyses of cores.
– National conference participation.

Year 3

– Analysis of samples collected during field trip.
– Analyses of revised experiments.
– submission of first paper.
– International conference participation (e.g. Goldschmidt conference).

Year 3.5

– Write-up of thesis and publications.

& Skills

This is a multi-disciplinary project in the field of biogeochemistry. You will gain a broad set of field and laboratory skills. Bespoke training in advanced techniques for mineralogical nanoparticle analysis and organic carbon and biomarker analysis will be delivered at the Universities of Stirling and Glasgow. You will have the opportunity to participate in workshops organized through Iapetus, the Scottish Alliance for Geoscience, Environment and Society (SAGES), and the Marine Alliance for Science and Technology for Scotland (MASTS).

References & further reading

Braunschweig, J., C. Klier, C. Schröder, M. Händel, J. Bosch, K.U. Totsche, and R.U. Meckenstock (2014), Citrate influences microbial Fe hydroxide reduction via a dissolution-disaggregation mechanism, Geochimica et Cosmochimica Acta 139, 434–446, http://dx.doi.org/10.1016/j.gca.2014.05.006.

Gütlich, P. and C. Schröder (2012), Mössbauer Spectroscopy. In: Methods in Physical Chemistry, edited by R. Schäfer and P.C. Schmidt, Wiley-VCH, pp. 351-389, http://dx.doi.org/10.1002/9783527636839.ch11.

P. Gütlich , C. Schröder, and V. Schünemann (2012), Mössbauer Spectroscopy – An indispensable tool in solid state research, Spectroscopy Europe 24(4), 21-32, https://www.spectroscopyeurope.com/article/m%C3%B6ssbauer-spectroscopy%E2%80%94-indispensable-tool-solid-state-research.

Hu L, Ji Y, Zhao B, Liu X, Du J, Liang Y, Yao P. (2023). The effect of iron on the preservation of organic carbon in marine sediments and its implications for carbon sequestration. Science China Earth Sciences, 66, https://doi.org/10.1007/s11430-023-1139-9.

Lalonde et al (2012) Preservation of organic matter in sediments promoted by iron. Nature 483, 198–200, doi:10.1038/nature10855.

Moore et al (2013) Processes and patterns of oceanic nutrient limitation. Nature Geoscience 6, 701–710, doi:10.1038/NGEO1765.

Notini et al. (2023), A New Approach for Investigating Iron Mineral Transformations in Soils and Sediments Using 57Fe-Labeled Minerals and 57Fe Mössbauer Spectroscopy, Environmental Science & Technology in press, https://doi.org/10.1021/acs.est.3c00434.

Posth, N.R. I. Köhler, E. Swanner, C. Schröder, E. Wellmann, B. Binder, K. O. Konhauser, U. Neumann, C. Berthold, M. Nowak, and A. Kappler (2013), Simulating Precambrian banded iron formation diagenesis, Chemical Geology, 362, 66-73, http://dx.doi.org/10.1016/j.chemgeo.2013.05.031.

Raiswell and Canfield (2012) The iron biogeochemical cycle past and present. Geochem Perspect 1, 1–220, doi:10.7185/geochempersp.1.1.

Schröder, C., I. Köhler, F.L.L. Muller, A.I. Chumakov, I. Kupenko, Rudolf Rüffer, and A. Kappler (2016), The biogeochemical iron cycle and astrobiology, Hyperfine Interactions 237, 85, http://dx.doi.org/10.1007/s10751-016-1289-2.

Tagliabue et al (2017) The integral role of iron in ocean biogeochemistry. Nature 543, 5 1-59, doi:10.1038/nature21058.

Zhao, B., P. Yao, T.S. Bianchi, X. Wang, M.R. Shields, C. Schröder, and Z. Yu (2023), Preferential preservation of pre-aged terrestrial organic carbon by reactive iron in estuarine particles and coastal sediments of a large river-dominated estuary, Geochimica et Cosmochimica Acta 345, 34–49, https://doi.org/10.1016/j.gca.2023.01.023.

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