Vegetated coastal wetlands – comprising mangroves, salt marshes, and seagrass meadows – play an important role in the global carbon cycle due to their high sequestration rates of carbon (annual organic carbon burial rate 114-131 Tg C). The decomposition of organic carbon by microorganisms in these ecosystems causes greenhouse gas releases such as carbon dioxide (CO₂) and methane (CH₄). Understanding the rate and extent of microbially mediated greenhouse gas formation from coastal wetlands under current climate conditions is needed to predict greenhouse gas fluxes from these ecosystems with future climate change. Here, we investigate the processes that control the microbial decomposition of organic carbon at the Wadden Sea, northern Germany. Our preliminary field and laboratory results indicate that the degradation of organic carbon is not limited by the availability of electron acceptors such as sulfate, but rather by the concentrations and composition of the organic carbon itself. The objective of this project was therefore to test how the microbially mediated degradation of organic carbon and thus greenhouse gas fluxes change as a consequence of organic carbon input to the sediment. To do this, we conducted a field experiment in which we injected two different organic carbon sources separately into the sediment of the Wadden Sea and measured greenhouse gas fluxes over the course of six weeks. We choose acetate as a relatively labile organic carbon source and humic acids (purchased from the International Humic Substance Society) as a recalcitrant source. The in situ experiment was performed at two locations with differing tidal influence: (i) tidal flats, which are inundated twice a day during high tide, and (ii) pioneer marshes, which are inundated twice a month during spring tide. In addition to flux measurements, porewater, and sediment were sampled and used to study geochemical processes. For both marsh zones, an enhanced CO₂ flux was measured for the plots where labile organic carbon was injected relative to control plots in which no organic carbon was added. However, the addition of the recalcitrant organic carbon only caused an increase in the CO₂ flux in the tidal flat. Porewater data showed that the addition of the labile organic carbon promoted iron(III) reduction, especially in the pioneer marsh, while for the tidal flat, enhanced sulfate reduction was observed for both organic carbon sources. Overall, a significantly higher CO₂ flux was measured from plots enriched with labile organic carbon. The gained knowledge is important in the context of predicting how such an ecosystem reacts to an additional input of organic matter e.g., caused by eutrophication or mobilization of organic matter. Furthermore, it is also relevant for estimating the extent and rate of greenhouse gas fluxes from these ecosystems.

How to cite: Kainz, N., Raab, F., Kappler, A., and Joshi, P.: Effect of addition of organic carbon on greenhouse gas release and subsurface biogeochemistry in salt marshes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3088, https://doi.org/10.5194/egusphere-egu24-3088, 2024.

Tidal marshes, recognized as “Blue Carbon ecosystems” for their high carbon sequestration rates, owe their carbon storage potential to primary production and rapid surface accretion driven by complex feedbacks among hydrodynamic, morphological, and biological processes. Tidal channel networks cutting through tidal wetlands exert a first-order control on ecogeomorphological dynamics by critically controlling fluxes of nutrients, sediments, and particulate matter. Although these networks have been traditionally seen as stable features, recent studies have shown that they are in fact highly dynamic systems. In particular, lateral channel migration, coupled with high drainage density, leads to frequent channel abandonment through meander cutoffs and channel piracies (i.e., stream captures). These processes significantly impact sediment dynamics, since reduced flow velocities within abandoned channels promote particle settling and channel infill, thereby providing ideal conditions for rapid organic matter deposition and trapping.

To characterize the depositional processes occurring in abandoned tidal channels and investigate their role in blue carbon sequestration and storage, we analysed several sediment cores retrieved from abandoned tidal channels in the microtidal Venice Lagoon, Italy. Cores were sampled every 5 cm for soil dry bulk density, organic matter, and organic carbon content. Organic matter content was estimated as the difference in weight before and after the Loss-On-Ignition (LOI), while organic carbon was directly measured using an elemental analyser. Sedimentary facies analyses allowed for identifying the deposits accumulated during the abandonment phase, while aerial and satellite image analyses facilitated the evaluation of the temporal evolution of the channel infill process, enabling the estimation of the related infill rate. Combining infill rate and organic carbon density, we estimated the carbon accumulation potential of abandoned tidal channels, as well as its variability, comparing it to surrounding marshes.

Preliminary results show that even if channel fill deposits are characterized by slightly lower organic matter content relative to marsh deposits, they feature significantly higher carbon accumulation rates owing to higher sediment deposition rates. These findings suggest that abandoned tidal channels could represent key hotspots for blue carbon accumulation. Consequently, a better understanding of depositional processes and carbon accumulation in abandoned tidal channels, as well as their characteristic spatiotemporal dynamics, can critically enhance the assessment of blue carbon sequestration and stock in coastal wetlands, providing crucial insights for effective conservation and restoration strategies.

How to cite: Puppin, A., Tognin, D., Ghinassi, M., D'Alpaos, A., and Finotello, A.: Abandoned tidal channels as hotspots of Blue Carbon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-636, https://doi.org/10.5194/egusphere-egu24-636, 2024.

Vegetated coastal ecosystems of the Wadden Sea, such as salt marshes and seagrass meadows, are important habitats and provide various ecosystem services. Efforts to protect and restore these valuable coastal systems are currently paralleled with an interest to better understand and quantify their carbon sequestration potential. Intertidal seagrass meadows comprise an area of more than 20,000 ha in the Wadden Sea, which might lead to the assumption that significant amounts of carbon are stored in these ecosystems. At present, however, very little data exists on carbon storage and dynamics in seagrass environments in the German Wadden Sea. Seagrasses declined massively about a century ago, but over the last decades underwent a process of unassisted recovery in the Wadden Sea of Schleswig-Holstein (northern Germany). Nevertheless, the two seagrass species, Zostera marina and Zostera noltii, are still mostly absent in similar coastal environments in western Germany and in The Netherlands – providing an example for the potential to enhance both ecosystem quality and carbon storage capacity.

In an ongoing study, we investigate characteristics of the tidal landscapes (e.g., surface elevation and geomorphology, adjacent landscape elements, and anthropogenic structures) and specific habitat properties (biomass productivity, sediment characteristics, hydrodynamic conditions, pore-water nutrients) at seven seagrass sites, located on tidal flats along dike-forelands of the North Sea coast of Schleswig-Holstein, northern Germany.

The aims are to (A) deliver a first assessment of the current carbon stocks, (B) quantify intra-site differences in realized seagrass habitats, and (C) understand differences in parameters that locally drive or inhibit the long-term build-up of carbon in seagrass meadows and their associated sedimentary systems. In this presentation, we will show and discuss our first preliminary results and interpretations.

How to cite: Reents, S., Hommes, L., Schildt, N., Camillini, N., and Sander, L.: Landscape properties, habitat dynamics, and the carbon storage potential of seagrass meadows in the Wadden Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10923, https://doi.org/10.5194/egusphere-egu24-10923, 2024.

Coastal blue carbon habitats perform many important environmental functions, including long-term carbon storage. These carbon storage estimates are typically limited to the sediments within specific types of coastal vegetation. However, recent studies have shown that large fluxes of organic carbon originating from traditional and non-traditional blue carbon systems are being buried along the margins and intertidal mudflats. For example, our recent study in China showed that over 75% of the blue carbon burial occurred in unvegetated tidal flats. Further, in Brazil, organic carbon burial rates along mudflats were found to be almost 3 times greater than the flux from within coastal vegetated systems. The implications of this research are that there is an underestimation of carbon burial from blue carbon systems as a result of the large burial rates along adjacent unvegetated regions.

How to cite: Sanders, C. and Wang, F.: Blue carbon burial along intertidal mudflats, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15773, https://doi.org/10.5194/egusphere-egu24-15773, 2024.

Seagrasses are marine flowering plants that play an important role in mitigating climate change by carbon sequestration. While only covering 0.2% of the ocean floor, seagrasses store over 15% of accumulated global carbon in the ocean’s sediments. The oxidizing microenvironments around their roots create strong and complex redox gradients which greatly affect microbial carbon mineralization rates in marine sediments. Despite seagrasses’ enormous ecological services as habitat and climate regulators, they are rapidly degrading around the world at alarming rates. Therefore, understanding the chemical changes and feedback that occur in sediments following the disappearance of seagrasses holds ecological importance.

We incubated different compartments of the tropical seagrass Halophila stipulacea (old and young leaves, rhizomes, or roots) with two sediment types from the northern tip of the Gulf of Aqaba. We measured the chemical changes in major ions (DIC, Fe²⁺, H₂S, SO₄^2-) and sulfur isotope ratios in sulfate within the water. We used these measurements to calculate the remineralization rate of each seagrass compartment. Our results aid us in predicting the potential effects of H. stipulacea disappearance on key microbial processes in the marine environment.

We show that the rhizomes had the fastest decomposition rates, followed by the young leaves, roots, and old leaves. This indicates the preservation potential of belowground biomass. Moreover, high hydrogen sulfide concentrations were only detected in the slurries containing rhizomes and young leaves. High sulfide concentrations can lead to enhanced seagrass mortality and a positive feedback loop where seagrass loss generates sulfide, leading to more seagrass loss. These results emphasize the importance of a deeper understanding of biogeochemical pathways following seagrass disappearance.

How to cite: Antler, G., Neta, S., and Winters, G.: The effect of anaerobic remineralization of the seagrass Halophila stipulacea on porewater biogeochemistry in the Gulf of Aqaba, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17164, https://doi.org/10.5194/egusphere-egu24-17164, 2024.

The rapidly warming Arctic, exceeding global averages, experiences heightened macroalgal growth in high Arctic fjords due to rising seawater temperatures and reduced sea ice. However, uncertainties surround the consequences of climate-induced changes in the carbon cycle resulting from extensive macroalgal growth and increased carbon flux dynamics in these fjords. This study examines the fate of macroalgal-derived fatty acids (Saturated Fatty Acid: SFA, Monounsaturated Fatty Acids: MUFA and Polyunsaturated Fatty Acids: PUFA) across Kongsfjorden and Krossfjorden (Ny-Alesund) in response to Arctic amplification. For this study, dominant brown, green, and red macroalgal species (n=20), along with sediment samples (n=18) across the fjords, were collected during summer 2022. Brown algae dominated with the highest average fatty acid concentration 435.72 ±534.14 μg/g, while red and green algae had lower concentrations 72.84 ±52.75 μg/g and 90.25 ± 84.67 μg/g, respectively. Brown algae exhibited a concentration trend of SFA>MUFA>PUFA, while green and red showed SFA>PUFA>MUFA. The primary PUFA in these algae were n-C₁₈ and n-C₂₀, and filamentous growth forms exhibited higher levels compared to thallus or short/dwarf forms in green and red algae. However, brown algae, except for the genus Chorda, did not exhibit clear trends for these compounds. The distinct phylogenetic position of brown algae from red and green algae likely accounts for these divergent patterns. The filamentous form having the highest concentration of fatty acids could result from increased resistance to degradation, attributed to their minimized surface-to-volume ratio. Macroalgal species outside their natural habitat (ex-situ) had higher PUFA, MUFA, and SFA concentrations, likely due to unfavourable conditions of growth in intertidal regions, suggesting enhanced adaptation for growth across the arctic fjords. While in the sediments, a significant (~50%) reduction in the PUFA and MUFA fraction concerning SFA was observed. The transport of the algal material was more towards the outer fjord and was possibly favoured by glacial melting and runoff activities. The decrease in fatty acids derived from algae, coupled with the presence of iso^- and anteiso^- branched-chain fatty acids, implies limited residence and faster turnover of algal matter into intermediate metabolites by microorganisms, possibly bacteria. Such observation suggests a potential release of carbon fluxes into the atmosphere through degradation of lipids, and contributing to a negative trend in the macroalgal-induced carbon storage in fjords.

How to cite: Rabha, M., Roy, B., and Singh, A.: Exploring Macroalgal Carbon Dynamics in a Changing Climate of Arctic Fjords, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-863, https://doi.org/10.5194/egusphere-egu24-863, 2024.

To keep the global average temperature rise well below 2°C requires drastic emission reductions and a removal of carbon dioxide (CO₂) from the atmosphere. It has been suggested that part of the CO₂ removal could be achieved by nature itself, if ecosystems that remove substantial amounts of carbon from the atmosphere are protected, managed, or restored. In the marine environment, the focus has so far been placed on coastal ecosystems with rooted vegetation (saltmarshes, mangroves, and seagrass beds), as they sequester carbon at high rates, are threatened by human activities and are amenable to management. Collectively, these are called actionable Blue Carbon ecosystems. More recently, other ecosystems such as marine sediments have been put forward, but these are currently considered emerging Blue Carbon ecosystems, as knowledge gaps do not allow us to decide yet, whether they are actionable or not. To help close some of the existing knowledge gaps we applied machine learning to spatially predict the amount of organic carbon that is stored in sediments of the Norwegian continental margin and the rates at which it is accumulated. We found that Norway has 100 times more organic carbon stored in its surficial (0 – 0.1 m sediment depth) seabed sediments than in its vegetated coastal ecosystems (0 – 1 m sediment depth). Rates of organic carbon accumulation vary spatially and are highest in depressions of the continental shelf that were carved out by glaciers during the last ice age. These so-called glacial troughs are found on the formerly glaciated continental margins of North America, Eurasia, south America, and Antarctica, covering an area ten times larger than that we mapped and might be important places of organic carbon accumulation globally. To improve our estimates of how much carbon accumulates in marine sediments at a global scale will require a) data on organic carbon content, dry bulk density and sediment accumulation rates of sufficient quality and quantity, b) relevant predictor variables of global coverage and sufficient resolution, and c) predictive spatial models that consider the complex nature of continental margins, where centres of organic carbon accumulation and cycling might be found in close proximity to each other. Based on improved global estimates of organic carbon stocks, accumulation rates and the release of CO₂ due to anthropogenic disturbance (demersal fisheries, seafloor cables, offshore wind farms, deep-sea mining etc.) it should be possible to decide whether marine sediments can be considered actionable Blue Carbon ecosystems.

How to cite: Diesing, M., Paradis, S., Jensen, H., Thorsnes, T., Bjarnadóttir, L., and Knies, J.: Substantial amounts of organic carbon are accumulated and stored in surface sediments of the Norwegian continental margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5089, https://doi.org/10.5194/egusphere-egu24-5089, 2024.

Fine-grained coastal and continental margin sediments are the largest permanent sink for carbon on our planet. They are typically rich in organic matter and often characterised by high sedimentation rates – favouring the burial of carbon. The ultimate processes that control the preservation of organic matter (OM) and its burial to deeper sediment layers are the different aerobic and anaerobic microbial OM mineralisation pathways that occur in surface sediments. In order to assess the rates and pathways of OM degradation in fine-grained sediments of the North Sea, we have chosen the Helgoland Mud Area (HMA), which represents the most important mud depocenter in the German Bight. The HMA is located at water depths between 11 and 27 m and covers an area of about 500 km² southeast of the island of Helgoland. We present a high spatial and vertical resolution pore-water dataset for the HMA of surface sediments retrieved using a multi-corer (MUC). This dataset includes oxygen profiles, pore-water profiles of sulfate, sulfide, nitrate, ammonia, dissolved iron, dissolved manganese, dissolved inorganic carbon and its stable carbon isotopic composition. A full diagenetic model for the uppermost 25 cm of the sediments was applied to estimate the rates of the different OM mineralisation pathways and the respective diffusive fluxes towards and across the sediment-water interface. The organic carbon burial flux and organic matter mineralisation rates range from 2.6 to 9.9 mmol m^-2 d^-1 and 1.9 to 9.1 mmol m^-2 d^-1, respectively. The highest remineralisation rates are attributed to aerobic respiration and account for up to 86 % of total OM mineralisation in the investigated surface sediments. Sulfate reduction is shown to be the second-most important mineralisation pathway of OM in the study area – except for three sites which are characterised by iron reduction and denitrification as the dominant mineralisation process after aerobic respiration. These results will be discussed in the context of the different depositional conditions, variations in particulate organic carbon (POC) accumulation and POC origin across the HMA.

How to cite: Müller, D., Liu, B., Holtappels, M., Geibert, W., Henkel, S., and Kasten, S.: Rates and pathways of organic carbon mineralisation in different sedimentary environments of the Helgoland Mud Area, SE German Bight, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10812, https://doi.org/10.5194/egusphere-egu24-10812, 2024.

Globally, continental shelf environments, and the marine sediments therein, have been recognised as having significant roles to play in the sequestration, cycling and storage of. Recently, shelf sediments have been identified as the largest, but most uncertain, stock of carbon stored on the continental shelf, citing a lack of empirical data. Moreover, seabeds are coming under increased pressure through anthropogenic impacts, such as offshore renewable energy development, trawling and dredging, and climate change effects. To fully understand, and effectively manage the seabed in terms of maximising this Blue Carbon potential requires a thorough understanding of carbon cycling in the marine environment over time, physical processes at the seafloor and high-quality spatial mapping. The QUEST project scope aims to conduct a multidisciplinary research programme to qualify, quantify and elucidate the provenance of carbon stocks in offshore marine sediments in Irelands EEZ. Furthermore, the research will examine and characterise threats to the stability of Blue Carbon in these settings and support the development of long-term management strategies. This programme will comprise spatial predictive modelling along with offshore surveying and sampling, laboratory analysis and hydrodynamic modelling, with past and new data to deliver comprehensive geochemical, geological, geotechnical, environmental and morphodynamical assessments of Blue Carbon ‘hotspots’ in the Irish offshore, as identified in the National Marine Planning Framework.

To date, this programme has worked with a variety of stakeholders to collate historical data relevant as predictors for sediment OC and generated new geochemical data sets through analysis of legacy sediment samples. The combined data sets have been used to produce spatial predictive modelling maps providing preliminary baselines for sediment OC stocks in Ireland’s EEZ.

Spatial mapping has identified knowledge gaps in the spatial extent and resolution of available sediment data. Additionally, the data collation and mapping exercise has highlighted a sparsity of physio-geochemical data essential for the accurate estimation and upscaling of OC stocks including OC content, Bulk density, and grain size analysis. Likewise, the paucity of available data extends to deeper sediments, consequently inhibiting the determination of OC stocks to the desired standard of 1 meter depth in assessment of BC ecosystems.  The first off-shore sampling survey for QUEST was completed in October 2023 providing a selection of grab (surface ~10cm depth), gravity (1-2m depth ), Vibro- ( 2-5m) and box core ( 10-25cm) samples from a series of near to off-shore transects from Ireland’s East coast encompassing the high sedimentary region of the Western Irish Sea Mudbelt (WISMB). Initial work carried out on cores has generated data describing updated OC stocks for Irish Sea sediments up to a 1m depth. Furthermore, data sets have been used to produce a region-specific conversion equation to calculate OC content using values attained from Loss on ignition analysis (LOI). This conversion factor has been applied to convert historical data LOI data for spatial predictive modelling.

How to cite: Grey, A., Kelleher, B., Chatting, M., Long, M., Walsh, P., Diesing, M., and Coughlan, M.: The QUANTIFICATION, CHARACTERISATION, SOURCE AND FATE OF PAST AND PRESENT CARBON STORAGE IN COASTAL AND OFFSHORE SEDIMENTS FOR EFFECTIVE MARINE MANAGEMENT (QUEST), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19779, https://doi.org/10.5194/egusphere-egu24-19779, 2024.

Understanding the capacity of marine sediments to store and sequester atmospheric carbon is an essential first step in assessing the possibilities for the management of these stores, including management of pressure such as bottom contacting fisheries, and addressing policy questions such as their potential as nature-based solutions to climate change. Using a toolbox of complimentary techniques for determining carbon abundance, provenance and reactivity, accumulation rates and vulnerability we have analysed a total of 18 sediment cores taken from sites across the North Sea during February and December 2021. Additionally, we have preliminary results from an additional 40 cores taken from a range of sites across the North Sea in June 2023, including across trawling gradients.

We present results from our toolbox approach, measuring the carbon stock and sequestration for the cores using a suite of complimentary analyses: from novel techniques such as alkane biomarkers and thermogravimetric analysis (TGA), to radiometric determination of sedimentation rates by lead-210 and stable carbon isotopes (δ¹³C) in bulk organic carbon, to the more routine techniques such as particle size distribution (PSA), organic & inorganic carbon and nitrogen, porosity, chlorophyll/phaeopigment, and black carbon. We show how viewing the results together can increase the understanding of how carbon is processed in the seabed at a regional scale, and how this can inform where management measures would be most appropriately applied.

How to cite: Powell, C., Graves, C., Dal-Molin, F., Garcia, C., Hynes, C., Limpenny, C., Mason, C., Nelson, P., Smeaton, C., and Parker, R.: A toolbox approach to measuring carbon stocks and sequestration in the North Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22145, https://doi.org/10.5194/egusphere-egu24-22145, 2024.

Marine sediments contain abundant organic matter which forms a major carbon sink. About one third of it originates from land plants. The main source taxa and source region as well as the large-scale translocation are hitherto poorly understood, mainly because we lack a proxy that can identify the source taxa with high taxonomic resolution. Here, we investigate the land plant component of sedimentary ancient DNA in six globally distributed marine sediment cores as a proxy for the terrestrial organic matter quantity and preservation as well as the source taxa. The spatial and temporal plant composition reflects mainly the vegetation composition and dynamics from the nearby continents as revealed by pollen records. However, we also find indications of a global north-to-south translocation of organic matter. The plant composition shows that upland vegetation is strongly underrepresented compared to riverine and coastal sources and there is a high contribution from mosses and ferns, particularly at high latitudes during the Holocene. We find that plant matter has a higher share and is better preserved in samples from the Late Glacial, which is characterized by high runoff and mineral load. Our results suggest that plant DNA in marine sediments may provide the missing proxy that links terrestrial plant sources to marine sedimentary carbon sinks. This represents the basis of how climate change and land-use change translates into carbon-sink dynamics and also informs about natural carbon-capture solutions.

How to cite: Herzschuh, U., Weiß, J., Stoof-Leichsenring, K., Harms, L., Nuernberg, D., and Mueller, J.: Sedimentary ancient DNA connects land carbon sources and marine carbon sinks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16476, https://doi.org/10.5194/egusphere-egu24-16476, 2024.

The marine carbon pump can sequester CO₂ from the atmosphere in marine sediments. Much of the carbon is taken up from the atmosphere in productive coastal waters, whereas carbon deposition often takes place in deeper areas. Tidal- and wave activity in shallow waters are producing a high energy environment where constant resuspension counteracts sinking and prevents accumulation of organic matter at the seafloor, a process that is reflected by the ubiquitous presence of non-accumulating sands covering more than 50% of the shelf areas. Nevertheless, in some shallow coastal areas, we find organic matter accumulation even under high energy conditions. One example is a 500km² region in the German North Sea, called the Helgoland Mud Area, where local hydrodynamic conditions cause the trapping of suspended particulate matter (SPM) and subsequent sedimentation. In this study we describe the particle transport dynamics over a diurnal tidal cycle, observed via a benthic lander deployment, repeat CTDs and analysis of reactivity and isotopic composition of the particulate organic matter (POM). Driven by tidal currents, we found SPM concentrations in the bottom water fluctuating between 35 and 130 mg/l, resulting in a total amount of suspended particles within the water column of up to 400 g /m². The constant resuspension and thus remineralisation of associated POM led to a one order of magnitude decreased carbon specific mineralization rate, compared to the upper water column. From eddy covariance measurements, the SPM resuspension flux was calculated and counteracting SPM sinking velocities of around 6 x 10^-4 m/s were derived. Interestingly, the POM background under stagnant current conditions showed more terrestrial d¹³C values compared to the resuspended POM during strong current conditions, suggesting distinct particle size classes and transport conditions for marine and terrestrial POM, respectively. The locally observed resuspension dynamics helps to understand the larger hydrodynamic regime that controls sediment accumulation, and ultimately the carbon sequestration in the area. 

How to cite: Hanz, U., Wei, B., Fovonova, V., Sander, L., Kopte, R., Schröder, H., Kasten, S., and Holtappels, M.: Particulate organic carbon dynamics of a depositional area in a high-energy shelf environment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16727, https://doi.org/10.5194/egusphere-egu24-16727, 2024.

Shelf sediments represent one of the largest natural carbon sinks on earth. At the same time, shelf regions are increasingly affected by human activity, disturbing sediment reservoirs directly by bottom trawling and offshore construction, or altering the carbon supply by changing river discharge, sediment management and the trophic status and food web of the sea. As global warming progresses, the sedimentary carbon sinks are becoming increasingly important in climate change mitigation measures. Thus, there is a need for in-depth knowledge of both, the dynamics and vulnerabilities of the sedimentary carbon sinks, as well as the legal and political options to protect their sequestration efficiency from human disturbances. Here we report on the transdisciplinary research project APOC, which addresses the Anthropogenic impacts on the cycling of Particulate Organic Carbon in the North Sea. Important results of the project include the quantification of sedimentation rates in the accumulation areas of the German Bight and the Skagerrak, assessing the factors that enhance organic carbon storage and the determination of the sources of deposited carbon. As major anthropogenic disturbances, the effects of bottom trawling and wind farm construction on benthic carbon storage were investigated and assessed. Bottom trawling in particular was significantly decreasing the benthic carbon storage due to a multitude of coupled physical and ecological effects. However, at the environmental policy level, it became clear that sedimentary deposits are not sufficiently recognized as valuable carbon sinks, although their storage capacity is believed to be much higher than that of blue carbon ecosystems at similar latitudes. While the project was staffed mainly with natural scientists, important expertise in environmental policies was provided by the marine protection office of the BUND, one of the largest environmental NGOs in Germany. As a fully-fledged project partner, BUND made it possible to recognize the relevance of the various project focal points for the environmental policy arena throughout the entire project. In turn, the policy experts were able to distribute the latest scientific findings to the relevant political decision makers. In effect, the transdisciplinary cooperation within the project not only produced valuable scientific results, but also numerous expert briefings for environmental policy at all levels, from local authorities to the EU Parliament, emphasizing the importance of protecting sedimentary carbon sinks for climate change mitigation measures. Key to this outcome was the continuous exchange of scientific findings and practical environmental policy knowledge, which kept all participants focused on the societal relevant objectives that were originally pursued with the project funding.

How to cite: Holtappels, M., Daewel, U., Kuhlmann, J., Porz, L., Taylor, B., Wallmann, K., Zhang, W., Ziebarth, N., and Kasten, S.: Assessing the sedimentary carbon sink for climate change mitigation - the need for transdisciplinary cooperation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17590, https://doi.org/10.5194/egusphere-egu24-17590, 2024.

The recent “30 by 30” global initiative to protect 30% of the world’s land and ocean by 2030 has increased the need for marine spatial planning decisions to be grounded in locally relevant empirical evidence. Continental shelves play a key role in the cycling of carbon, where marine sediments can act as an important sink of organic carbon (OC). As a result, marine sediments storing carbon have attracted recent scientific attention to elucidate the amount of OC stored and mechanisms influencing its sequestration. Spatial models of marine sediment OC stocks have previously been developed and provide preliminary estimates of standing stocks over wide geographical scales. However, these broad-scale predictions are derived from models of broad scale environmental regimes, which makes them unlikely to capture local spatial variations in environmental conditions and subsequently local variations in OC, reducing their utility for local scale marine spatial planning decisions. This study aims to determine whether legacy data could be used to produce local scale spatial predictions of OC relevant for policy makers. To achieve this aim, local scale predictors relevant for OC were produced/sourced in order to predict local-scale marine sediment OC in the Irish Sea. Legacy data of bottom water temperature (BWT) and bottom water salinity (BWS) measurements were used to bias correct and downscale global models of BWT and BWS. Recently developed high resolution sediment properties (% mud, % sand and % gravel) and locally developed Sediment Mobility and Sediment Disturbance Indices (SMI and SDI, respectively) were also sourced as potential predictors. Public-good, environmental consultancy and government agency repositories were also searched for OC-content data. A Random Forest model was trained to predict OC-content on localised predictors as well as previously identified important predictors of marine sediment OC. The outputs from the localised model were compared to one that was trained on broad-scale predictors to determine the change in model performance and utility for making local scale predictions. This study highlights the value of legacy data in contributing to locally refined spatial predictions of OC-content relevant for marine spatial planning decisions.

How to cite: Chatting, M., Diesing, M., Grey, A., Kelleher, B., and Coughlan, M.: Locally refined spatial predictions of marine sediment carbon stocks from legacy data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6380, https://doi.org/10.5194/egusphere-egu24-6380, 2024.

How to cite: Parameswaran, N., Bur­wicz-Galerne, E., Gonzalez, E., Wallmann, K., Greenberg, D., and Braack, M.: Predicting Global Seafloor Organic Carbon Burial Rates: A Deep Learning Approach with Uncertainty Quantification, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18005, https://doi.org/10.5194/egusphere-egu24-18005, 2024.

Estuaries are highly active biogeochemical environments at the land-sea interface. They release approximately 0.25 Pg C y⁻¹ on a global scale, which is equivalent to 17% of the total oceanic uptake despite occupying an area that is only 0.03% of the global oceans (Li et al., 2023). This disproportionate impact underlines the importance of understanding the processing of riverine and coastal carbon in estuarine systems. This processing, particularly the breakdown to form the greenhouse gases CO₂ and CH₄, is controlled by the properties (e.g. source and composition) of organic matter (OM) and the depositional conditions. In this respect, harbors are profoundly human-impacted estuaries that continuously supply vast quantities of organic-rich dredged sediment, the environmental footprint of which is of prime concern for sustainable coastal and port management. While sources and composition are essential parameters with respect to CO₂ (and CH₄) generation, these are challenging to determine in the dynamic setting of a harbor with strong tidal influence. Here, we use detailed organic chemical analyses to investigate how OM composition and depositional conditions control the release of greenhouse gases from (dredged) Port of Rotterdam sediment.

The Port of Rotterdam (PoR) is located in the Rhine-Meuse estuary, with sediment and OM composition controlled by the interaction between river, sea, and human activities. During a sampling campaign in 2021, both bulk surface sediments and intact sediment cores were collected at different geographical locations throughout the harbor. A general west-to-east gradient of marine influence was presented, which coincided with the changes of organic carbon and nitrogen content and their isotope abundance. The macromolecular organic matter (MOM) was isolated and analyzed with pyrolysis-GC-MS, revealing it to be of mixed terrestrial, marine, and potential anthropogenic origins. Particularly, the abundance of terrestrial pyrolytic biomolecules (e.g. guaiacols, syringols, polysaccharides) decreased as the depositional environment became increasingly marine. Despite of OM composition changes along the salinity gradient, similar organic matter degradation rates were measured in short-term (8-hour) whole-core incubations at two sites with contrasting bulk OM signatures. This was likely attributed to the rapid degradation of fresh OM at the sediment surface. In comparison, 6-week aerobic incubation suggested marine sediments possessed a larger labile carbon pool than riverine sediments. Our results indicated that PoR sediments are characterized by large spatial variability in OM quantity and quality, further determining the carbon stock and stability. OM source seems to play a crucial role in influencing the carbon stability. Considerable attention still needs to be given to link OM characterization and degradability. However, OM degradation results from OM properties governing degradability in combination with environmental conditions (e.g. electron acceptors, microbial activities, and temperature). This was witnessed by a significantly larger benthic methane efflux in riverine sediment than marine sediment. Besides, the relative significance of OM composition influencing on degradation also depends on the timescale of interest. Nevertheless, the spatial heterogeneity in OM stability between different depositional environments highlights the need for applying ‘carbon-sensitive’ management of sediments in relation to their reactive carbon fraction when exposed to human pressure and climate change.

How to cite: Wu, G., Yang, B., Nierop, K., Reichart, G.-J., Gebert, J., and Kraal, P.: Organic carbon burial and degradation in estuarine sediments in Europe's largest port area (Port of Rotterdam, The Netherlands), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11348, https://doi.org/10.5194/egusphere-egu24-11348, 2024.

Continental shelf sediments contain significant carbon stocks while being increasingly subject to anthropogenic pressures such as trawling, oil and gas extraction and more recently the introduction of offshore wind farms. Despite extensive research on the effect of man-made structures on the marine environment, there remains a research gap regarding their effect on shelf sediment carbon storage through their installation, operation and following decommissioning. This is the first study to explicitly study sediment carbon dynamics surrounding man-made structures. This talk presents carbon data from two decommissioned oil and gas platforms (Northwest Hutton and Miller) in the North Sea, from sediment cores taken at increasing distances away from the decommissioned sites. Understanding the carbon dynamics includes presenting the carbon stocks, sediment composition, and carbon accumulations rates using radiochemistry techniques. This research is important for determining the role of MMS on carbon dynamics, and has implications for decommissioning practice across the North Sea.

How to cite: Woodward-Rowe, H., Dal Molin, F., Gregson, B., Mason, C., Parker, R., and Hicks, N.: The effect of man-made structures on sedimentary blue carbon dynamics , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22261, https://doi.org/10.5194/egusphere-egu24-22261, 2024.

Marine sediments represent a hot spot of ecosystem services, but their integrity is increasingly put at risk by anthropogenic disturbance, most notably by demersal fisheries. The need for global action to minimize the impacts of destructive fishing techniques on the marine environment is urgent. The urgency to act, however, needs to be met with caution, as scientists are pushed for action, global predictions of trawling impacts are tempting, yet poor validation and oversimplified assumptions can lead to large uncertainties. We visit the scientific literature on trawl studies to map out current evidence from the literature and report on a global meta-analysis to quantify the effects of demersal fishing on sedimentary and biogeochemical properties. 

Studies examining the direct impacts of bottom fishing revealed significant reductions in total organic carbon (TOC; -10%), chlorophyll-a (Chl-a, -10%), phaeopigments (-21%) and proteins (-24%), and largest impact was detected on surficial sediment (0-2 cm). Implications of methodological biases as a result of inappropriate sampling in trawl studies and the importance of context-dependency for effect size is flagged up. Environmental parameters such as bottom current velocity and surface primary productivity significantly influenced both the direction and magnitude of fishing effects. We highlight where the lack of evidence lies that might create bias in regional and global models that require empirical data for validation. The objective is to summarize current knowledge and to direct future studies towards more robust analysis of the impacts of bottom trawling, which will provide a basis of sound advice to fisheries managers and policy makers.

How to cite: Sciberras, M., Paradis, S., Tiano, J., De Borger, E., Bradshaw, C., Morys, C., Pusceddu, A., Ennas, C., Soetaert, K., Puig, P., and Masque, P.: Impacts of demersal fishing on sedimentary organic matter: a global meta-analysis., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1793, https://doi.org/10.5194/egusphere-egu24-1793, 2024.