Burial of organic material in marine sediments represents a dominant natural mechanism of long-term carbon sequestration globally, but critical aspects of this carbon sink remain unresolved. Investigation of surface sediments led to the proposition that on average 10-20% of sedimentary organic carbon is stabilised and physically protected against microbial degradation through binding to reactive metal (e.g. iron and manganese) oxides. Here we examine the long-term efficiency of this rusty carbon sink by analysing the chemical composition of sediments and pore waters from four locations in the Barents Sea. Our findings show that the carbon-iron coupling persists below the uppermost, oxygenated sediment layer over thousands of years. We further propose that authigenic coprecipitation is not the dominant factor of the carbon-iron bounding in these Arctic shelf sediments and that a substantial fraction of the organic carbon is already bound to reactive iron prior deposition on the seafloor.

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and thus the controls on organic carbon cycling are far from understood. Here we coprecipitate carboxylic acids with iron (oxyhydr)oxide ferrihydrite and use near-edge X-ray absorption fine structure spectroscopy and wet chemical treatments to determine the relationship between sequestration mechanism and organic carbon stability against its release and chemical oxidative remineralisation. We show that organic carbon sequestration, stabilisation and persistence increase with an increasing number of carboxyl functional groups. We suggest that carboxyl-richness provides an important control on organic carbon preservation in the natural environment. Our work offers a mechanistic basis for understanding the stability and persistence of organic carbon in soils and sediments, which might be used to develop an overarching relationship between organic functional group-richness, mineral interactions and organic carbon preservation in the Earth system.

This is a repository copy of Experimental evaluation of the extractability of Fe-bound organic carbon in sediments as a function of carboxyl content.

The balance between degradation and preservation of sedimentary organic carbon (OC) is important for global carbon and oxygen cycles1. The relative importance of different mechanisms and environmental conditions contributing to marine sedimentary OC preservation, however, remains unclear2–8. Simple organic molecules can be geopolymerized into recalcitrant forms by means of the Maillard reaction5, although reaction kinetics at marine sedimentary temperatures are thought to be slow9,10. More recent work in terrestrial systems suggests that the reaction can be catalysed by manganese minerals11–13, but the potential for the promotion of geopolymerized OC formation at marine sedimentary temperatures is uncertain. Here we present incubation experiments and find that iron and manganese ions and minerals abiotically catalyse the Maillard reaction by up to two orders of magnitude at temperatures relevant to continental margins where most preservation occurs4. Furthermore, the chemical signature of the reaction products closely resembles dissolved and total OC found in continental margin sediments globally. With the aid of a pore-water model14, we estimate that iron- and manganese-catalysed transformation of simple organic molecules into complex macromolecules might generate on the order of approximately 4.1 Tg C yr−1 for preservation in marine sediments. In the context of perhaps only about 63 Tg C yr−1 variation in sedimentary organic preservation over the past 300 million years6, we propose that variable iron and manganese inputs to the ocean could exert a substantial but hitherto unexplored impact on global OC preservation over geological time.

Abstract. Association of organic carbon (OC) with reactive iron (FeR) represents an important mechanism by which OC is protected against remineralisation in soils and marine sediments. Recent studies indicate that the molecular structure of organic compounds and/or the identity of associated FeR phases exert a control on the ability of an OC–FeR complex to be extracted by the citrate–bicarbonate–dithionite (CBD) method. However, many variations of the CBD extraction are used, and these are often uncalibrated to each other, rendering comparisons of OC–FeR values extracted via the different methods impossible. Here, we created synthetic ferrihydrite samples coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. We altered some of the method parameters (reagent concentration, time of the extraction and sample preparation methods) and measured FeR recovery to determine which (if any) modifications affected the release of FeR from the synthetic sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method (the amount of Fe reduced by a fixed amount of dithionite) and find that the concentration of dithionite deployed can limit OC–FeR extractability for sediments with a high FeR content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing FeR. Moreover, we demonstrate that for synthetic OC–FeR samples dominated by ferrihydrite, freeze-drying samples can significantly reduce OC–FeR extractability; this appears to be less of an issue for natural marine sediments where natural ageing mechanisms may mimic the freeze-drying process for more stable Fe phases. While our study is not an all-inclusive method comparison and is not aimed at delivering the “perfect” extraction setup, our findings provide a collected summary of critical factors which influence the efficiency of the CBD extraction for OC–FeR. As such, we provide a platform from which OC–FeR values obtained under different methods can be interpreted and future studies of sediment carbon cycling can build upon.

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Abstract. Association of organic carbon (OC) with reactive iron (FeR) represents an important mechanism by which OC is protected against remineralisation in soils and marine sediments. Recent studies indicate that the molecular structure of organic compounds and/or the identity of associated FeR phases exerts a control on the ability of an OC-FeR complex to be extracted by the citrate-bicarbonate-dithionite (CBD) method. While many variations of this method exist in the literature, these are often uncalibrated to each other, rendering comparisons of OC-FeR values extracted by different method iterations impossible. Here, we created a synthetic ferrihdyrite sample coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. Method parameters (reagent concentration, time of the extraction and sample preparation methods) were altered and FeR recovery measured to determine which (if any) modifications resulted in the greatest release of FeR from the sediment sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method and find that the concentration of dithionite deployed can limit OC-FeR extractability for sediments with a high FeR content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing FeR. Finally, we demonstrate that for synthetic OC-FeR samples, the almost universal technique of freeze drying samples can significantly reduce OC-FeR extractability and we offer insight into how this may translate to environmental samples using Arctic Ocean sediments. These results provide a valuable perspective on how the efficiency of this extraction could be improved to provide a more accurate assessment of sediment OC-FeR content. Accurate determinations of OC-FeR in sediments and soils represents an important step in improving our understanding of, and ability to model, the global carbon cycle.

<p>Participation in offshore fieldwork is a frequent pathway into a successful career in marine science due to the unique opportunities for practical skills development provided by such field experiences. However, the ability to participate in scientific research cruises can be hindered for those from certain backgrounds, such as physically disabled scientists with mobility limitations, those with caring responsibilities who cannot spend extended periods of time away from home, and early career researchers from minority groups who may perceive the limited confines of a research ship as a hostile or unwelcoming environment. </p><p>Digital twinning is a new and rapidly developing area that describes how technologies and capabilities, including modelling, remote sensing, and linking shipboard equipment to shore visually in real-time, can be intertwined with traditional offshore operations to promote inclusivity and broaden the diversity of people involved in marine sciences. Here we will present preliminary results from our project that explores whether perceptions of fieldwork as a requirement for a career in marine science exist and whether jobs in marine science explicitly require these skills and experiences. Perceptions of fieldwork were evaluated through a series of questionnaires and semi-structured interviews with prospective marine scientists at undergraduate and PhD level. Additionally, we also conducted a systematic review of advertised vacancies in marine science to determine how perceived requirements for a career in marine science differed from the advertised required skills and experience.  Finally, we collated several case studies of effective use of digital twinning as a tool to enable those who cannot access offshore fieldwork to participate in scientific cruises. We aim to use these case studies to highlight the potential for digital twinning to act as a complimentary route into the field and act as an evidence base for continued investment in, and development of, new technologies to facilitate equitable and inclusive marine science.</p>