par Placitu, Silvia
Président du jury Regnier, Pierre
Promoteur Bonneville, Steeve
Publication Non publié, 2025-02-07
Thèse de doctorat
Résumé : The burial of organic carbon (OC) in aquatic sediments is a critical component of global carbon and oxygen cycles and intricately linked to the Earth’s climate. Over 90% of sedimentary OC is found associated with mineral matrix, limiting microbial accessibility and enabling long-term storage. Reactive iron phases (FeR) directly bind about 15-20% of the OC in sediments, forming the so-called ‘’rusty carbon sink’’. Yet, the mechanisms driving these associations remain unclear. This thesis addresses this knowledge gap through field and experimental work in coastal sediments, exploring the preferential association of specific OC compounds and the influence of sedimentary parameters (e.g., oxygen regimes, Fe, and OC content and composition) on the formation and stability of the rusty carbon sink.Chapter 2 examines OC-Fe associations among three Swedish fjords presenting contrasting oxygenation regimes. We found that the oxygenation regimes, the intensity of benthic iron cycling have minimal influence on the amount of OC bound to Fe. The absence of correlation with the sediment parameters investigated, combined with the highest OC-Fe concentrations in fjords with riverine input, suggest that at least a portion of these associations originates from allochthonous, terrestrial sources. Even though these fjords are hotspots of OC burial, the rusty carbon sink plays a modest role, with burial being controlled predominantly by high sediment accumulation rates.Chapter 3 investigates the role of physical protection in sediments from the anoxic West Gotland Basin (WGB) in the Baltic Sea, which exhibit very high OC reactivity rates, in line with those reported for oxic areas. We found very high OC loadings and low OC-Fe associations, indicating that these sediments receive large supply of OC relative to the lithogenic particles input, far exceeding the potential of physical protection. As a result, OC reactivity is high, even though bottom waters are anoxic. Thus, the lack of OC-Fe associations and saturation of physical protection most likely contribute to the unexpectedly high OC reactivity observed in the WGB sediment, suggesting that anoxia does not always lead to high OC burial efficiencies.Chapter 4 explores the in-situ formation and dynamics of OC-Fe associations in a freshwater mudflat in the Scheldt estuary. We buried ferrihydrite 2-lines and 6-lines-coated sands and quartz sands over 18 months, following the temporal and spatial dynamics of Fe and OC, combining bulk geochemical, isotope and micro-spectroscopy analysis. The Fe content decreased with sediment depth and time due to active microbial iron reduction, while OC accumulated over time, especially above the sediment-interface. In the FeR-coated sands, adsorption was the main binding mechanism during the initial months whereas coprecipitation gained prominence over time. Surprisingly, quartz sands also accumulated over time OC and Fe through coprecipitation. Most of the OC bound is thermally labile, enriched in aliphatic functional groups, although above or at the sediment-interface, thermally recalcitrant and refractory OC constitute up to 45% of total OC. This refractory pool is enriched in aromatic compounds and exhibits distinctive C-XANES spectra, characteristic of black carbon-like compounds. Overall, these findings emphasize the contrasted nature and pathways of OC-Fe associations in estuarine sediment and their role in stabilizing OC.This thesis highlights the site-specific biogeochemical complexity of the rusty carbon sink. Future research should target underexplored regions, such as high-latitude fjords or riverine sediments and the development of improved extraction methods, combined with the use of more refined spectroscopic techniques to enhance our understanding of the rusty carbon sink variability and significance.

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