The subtidal seabed sedimentary habitats in the UK’s English waters contain between 80 and 100 Mt of organic carbon (OC) and so have a significant climate change mitigation potential (as Blue Carbon). Although it is known that OC input (amount and rate) and composition (source and reactivity), sediment type and environmental setting (e.g., temperature, oxygen) control both amount of OC stock and OC burial rates, the regional understanding of the links between these controls, their spatial variability remains poorly understood.  

Much of the UK’s shelf seabed organic carbon stock is under pressure from physical disturbances by various human activities, significantly trawling as well as climate forcing processes and temperature increases. These pressures promote changes in OC status and climate regulation service provision (storage and burial), although the net direction of change is highly uncertain. Management of human activities, including protection or restoration of seabed areas containing OC, may therefore provide a significant Nature-based Solution (NbS) to climate change itself.

We present an exemplar case study in the North Sea which aimed to identify and explore regions of OC climate regulation service provision and draw together evidence strands to support management decisions and measures to allow optimisation of sedimentary OC (‘Blue Carbon’) to mitigate climate change.

Three key aspects and related questions are addressed:

• Organic carbon storage and burial in space across the shelf: what drives variability, and what is the role of OC composition, source and reactivity?
• Present pressures on seabed carbon: Where is OC storage under pressure? By how much? What is the potential impact?
• Policy and management: How can this evidence base inform management interventions and measures, including protection and recovery measures which promote the potential of the seabed climate change regulation service (blue carbon)?

Within each element insights into the developing underpinning datasets, future approaches needed, and remaining knowledge gaps and priorities will be presented.

How to cite: Parker, R., Aldridge, J., Brown, L., Clare, D., Dal Molin, F., Garcia, C., Hughes, D., Hynes, C., Mason, C., Martinez, R., McEwan, R., Powell, C., Proctor, W., and Graves, C.: Assessing seabed carbon storage and sequestration (Blue Carbon): response to pressures and management interventions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9573, https://doi.org/10.5194/egusphere-egu25-9573, 2025.

The North and Celtic Seas are influenced by both natural and anthropogenic inputs of organic carbon. Allochthonous sources include terrestrial carbon, delivered via riverine and atmospheric pathways, while autochthonous sources include in-situ production of marine phytoplankton and other primary producers. Ultimately the sediment acts as sink and storage for deposited organic carbon. However, gaps remain around our knowledge of the provenance of this carbon, which is important for accurate carbon budgets and the move towards Net Zero, and for understanding how carbon may behave in the environment. Carbon composition differs with source, whether marine, terrestrial or anthropogenic, and this can influence the reactivity of that carbon. Labile, fresh carbon will be more easily degraded whilst refractory carbon will be more stable and long lasting. We used a molecular marker approach employing n-alkanes as carbon tracers to help unravel the complex sources of sedimentary organic carbon. Sediment cores were taken between 2021 - 2024 over numerous surveys encompassing different regions of the North and Celtic Seas, including nearshore, coastal and offshore locations, with differing sediment types ranging from sandy through to muddy. We analysed a suite of organic compounds, including n-alkanes and other biomarkers, polycyclic aromatic hydrocarbons (PAHs), and black carbon, to probe natural and anthropogenic carbon sources. Our results showed that organic carbon provenance varied in both space and time (depth). Nearshore and coastal sites generally had higher levels of terrestrial rather than marine carbon, however appreciable terrestrial inputs were also observed at various offshore locations, including at depth. Elsewhere offshore sites were dominated by marine and/or mixed sources, whilst anthropogenic inputs were observed at nearshore sites, and at coastal sites with finer sediment, but also offshore especially in the vicinity human oil and gas activity. Our study provides insights into different carbon sources, areas of accumulation and storage, as well as post depositional changes. This work moves us towards a classification of marine-terrestrial-anthropogenic sites in the North and Celtic Seas, which informs and supports marine management strategies including the climate mitigation potential of the English seabed.

How to cite: Hynes, C., Powell, C., Dal Molin, F., Mason, C., and Parker, R.: How ‘blue’ is your carbon? Sources of sedimentary shelf carbon in the North and Celtic Seas using n-alkane lipid tracers and geochemical tools, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21148, https://doi.org/10.5194/egusphere-egu25-21148, 2025.

Continental shelf sediments contain some of the largest stocks of organic carbon (OC) on Earth and critically influence the global carbon cycle. Quantifying how much OC continental shelves store and determining its residence time is key to assess how the ocean carbon cycle will be altered by climate change and anthropogenic perturbations of the seabed. Spatial variations in terrestrial carbon stocks are well studied and mapped at high resolution, but our knowledge of the distribution of marine OC in different seafloor settings is still very limited, particularly in dynamic and spatially variable shelf environments. This lack of knowledge reduces our ability to understand and predict how much and for how long the ocean sequesters CO₂.

In this study, we use high-resolution multibeam echosounder (MBES) data from the Eastern Shore Islands offshore Nova Scotia (Canada), combined with OC measurements from discrete samples, to assess the distribution of OC content in seafloor sediments. We derive four different spatial estimates of organic carbon stock: (i) OC density estimates scaled to the entire study region assuming a homogenous seafloor, (ii) interpolation of OC density estimates using empirical Bayesian kriging, (iii) OC density estimates scaled to areas of soft substrate estimated using a high-resolution classified substrate map, and (iv) empirical Bayesian regression kriging of OC density within areas of estimated soft sediment only. These four distinct spatial models yielded dramatically different estimates of standing stock of OC in our study area of 223 km²: 80,901, 58,406, 16,437 and 6,475 t of OC, respectively. Our study demonstrates that high-resolution mapping is critically important for improved estimates of OC stocks on continental shelves and for the identification of carbon hotspots that need to be considered in seabed management and climate mitigation strategies. These results will be discussed in the larger context of OC storage on the Atlantic Canadian Shelf.

How to cite: Kienast, M., Brenan, C., Maselli, V., Algar, C., Misiuk, B., and Brown, C.: Not all continental shelf seafloor is the same: detailed sediment characterization dramatically reduces estimates of organic carbon standing stock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13070, https://doi.org/10.5194/egusphere-egu25-13070, 2025.

Coastal sediments act as a huge reservoir of sedimentary organic carbon (SOC). Yet, the processes governing the long-term burial and preservation of this type of coastal organic carbon are complex and depend on the interplay between several factors, such as sedimentation rates, regional climatic conditions, post-depositional biological activity, and degradation. All these factors can significantly vary spatio-temporally within local micro-environmental conditions. The present study aims to understand the preservation of organic matter from two distinct climate zones of tropical India, the humid eastern lower Gangetic floodplain containing mangroves (average annual precipitation ranging ~1200–1600 mm) and the dry western Kachchh basin, which is essentially a salt flat (average annual precipitation ranging ~ 200–400 mm). At both places, sedimentation rates, total organic carbon (TOC), separated labile and refractory fractions of SOC (through chemical oxidation method), and their stable carbon isotopic (δ¹³C) compositions have been compared. The available data, along with the results from the present study, show that the sedimentation rate patterns through the Holocene are comparable at both study sites, showing higher rates (~0.4–0.6 cm/yr) up till Mid-Holocene which decreases to <0.05 cm/yr during the Late Holocene. The TOC ranges from 0.1 to 1.0% in Kachchh (average ~0.4%) and 0.2 to 2.1% in the lower Ganges floodplain (average ~0.6%). However, when the bulk and refractory fractions of SOC are compared, they reveal distinct patterns. The Gangetic floodplain exhibits a difference in the bulk and refractory δ13C values as it contains more labile and partially decomposed organic matter. In contrast, the Kachchh sediment shows consistent δ13C values in both the bulk and refractory fractions, indicating negligible presence of labile or decomposed organic matter. These findings suggest that in arid climates, the SOC is predominantly deposited in oxidized conditions, thus comprising mainly the refractory fraction of organic matter. At the same time, in humid environments, the SOC includes a mixture of labile and partially decomposed organic matter. This comparative study provides an example of how climatic variability plays a critical role in shaping SOC characteristics in otherwise similar depositional settings and emphasizes the importance of studying both labile and refractory fractions of SOC for reconstructing past climates.

How to cite: Ram, F.: Nature of organic matter preservation in coastal sediments: Insights from humid to arid climatic regimes of India., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5189, https://doi.org/10.5194/egusphere-egu25-5189, 2025.

Bioturbation by benthic and sediment-dwelling species significantly influences organic carbon preservation, accounting for 4% of the variation in sediment mixing depth. Yet our understanding of how different species, communities or functional bioturbation roles influence water-sediment carbon fluxes or contribute to long-term carbon burial is poorly understood. This is a key knowledge gap for assessing the impacts that seabed disturbance, such as bottom trawling, may have on carbon processes or for determining the potential carbon benefits of better seabed protection.  

As part of the Convex Seascape Survey, we have been investigating the effect of faunally mediated sediment mixing and burrow ventilation on water-sediment carbon processes and associated nutrient fluxes. In a series of mesocosm experiments, we assembled 17 macrofaunal invertebrate species in monoculture and in a three-species mixture. In all experiments, individuals were collected using a van veen grab from the Firth of Clyde or Loch Etive, Scotland, and were placed in sediment-filled mesocosm aquaria (6 x 6 x 23 cm) with overlying seawater. Fluorescent sediment particles were added after 24 hours to quantify sediment reworking, and experiments were maintained for up to 10 days. Using Carbon-13 labelled algae, we quantified the movement of particulate organic carbon into the sediment. Overlying water parameters (e.g., pH, DIC, oxygen consumption) and sediment mixing and burrow ventilation activities were measured to assess inter- and intra-species contributions to nutrient cycling and carbon flux.

Our data reveal that different groups of sediment mixers (predominantly deep burrowers versus surficial modifiers) perform distinct functional roles in the cycling of nutrients and carbon. We find, for example, that deep burrowers and active bioturbators promote higher levels of nitrite (NO₂⁻) and nitrate (NO₃⁻) release into the overlying water. This suggests that their burrow formation and ventilation enhance microbial nitrification, converting ammonium into nitrite and then nitrate. In contrast, surficial modifiers were associated with elevated levels of phosphorus and ammonium in the overlying water. This pattern likely reflects the dominance of ammonification, where organic matter decomposition releases ammonium and remineralisation releases phosphorous in surface layers with moderate oxygenation. Dissolved inorganic carbon release and concomitant alkalinity changes produced by individuals in our studies are species-specific and can be quite pronounced in relation to bioturbation function. The amount of particulate organic carbon redistributed by bioturbation is also species-specific and our labelled algae both help understand this redistribution of carbon in the surface sediments beyond more traditional methods of measuring bioturbation but also call into scrutiny categorical methods of grouping bioturbating organisms.

These findings highlight that functional traits of bioturbating animals matter more than taxonomic species identity in regulating carbon fluxes and burial at the water-sediment interface. They reveal a divergence of roles across sediment depths, emphasising the potential for functional loss and ecosystem degradation from depth-specific disturbances like bottom trawling.

How to cite: Porter, A., Godbold, J., Kitidis, V., Solan, M., and Lewis, C.: From Sediment to Sequestration: Linking Bioturbation to Carbon Cycling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20490, https://doi.org/10.5194/egusphere-egu25-20490, 2025.

Physical disturbance of the seafloor related to bottom trawling has led to widespread concern within the scientific community, as it may cause long-lasting impacts to benthic ecosystems, sediment composition, and biogeochemical cycling. Although trawling-induced effects on benthic habitats have been widely studied, the effects on sediment carbon mineralization and carbon stocks on the seabed remain unclear. Most of the catch obtained by bottom trawling is harvested from productive continental shelves, which play an important role in carbon sequestration and burial. Parts of the Baltic Sea have been trawled between one to ten times annually and thus provide a prime locality to study trawling related effects on benthic ecosystems and on the geochemical system. This study investigates the effects of trawling on carbon mineralization within the Bornholm Basin of the southern Baltic Sea, which has been consistently trawled until 2019. This was achieved through on-site sampling and analysis of porewater geochemistry, solid-phase sediment profiles, benthic fluxes and macrofaunal abundances sampled in the summer of 2023.

Comparisons between 4 study sites, paired into high-and low-trawled pairs based on fishing intensity data, revealed greater differences in measured physical and chemical properties than those between high- and low-trawled locality pairs, suggesting that environmental variability significantly influences carbon mineralization. However, high-trawled areas generally exhibited higher concentrations of TOC (total organic carbon) in their sediments as well as higher DIC (dissolved inorganic carbon) and nutrients in their porewaters. The higher porewater concentrations are likely a result of reduced advective transport and macrofaunal-related mixing, as a consequence of lower bioturbation and bioirrigation, whereby mineralization end-products (NH₄⁺, PO₄^3- and DIC) accumulate in porewaters due to slow upward diffusive transport to the sediment surface in comparison to low-trawled localities. Chlorophyll concentrations at the sediment surface from recently settled organic material from the spring phytoplankton bloom were higher at high-trawled localities and likely associated with the decreased macrofauna biomass and grazing activity. It is concluded that biogeochemical ecosystem functions in once-trawled sediment biotopes are only partially reconstituted after 4 years, with lasting differences in macrofaunal benthic community structure and bioturbation potential.    

How to cite: Golda, C., Brüchert, V., and Bradshaw, C.: Impact of benthic trawling on carbon mineralization in continental shelf sediments of the Bornholm Basin, southern Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4432, https://doi.org/10.5194/egusphere-egu25-4432, 2025.

Recent reviews have highlighted that the effect of bottom trawling on sediment carbon content and biogeochemistry varies depending on environmental conditions, the type of sediment (e.g. mud vs sand) and the chemical nature of the organic carbon. However, most data are from analyses of total carbon or organic matter and from bulk surface sediments (top 2 cm) rather than deeper sediment profiles, limiting our ability to interpret these results in terms of the biogeochemical processes involved in carbon degradation and burial.

The Kattegat is one of the most heavily trawled seas in the world with some parts being swept by fishing gear up to 15 times per year. However, fishing effort is patchy and there is also a marine protected area where bottom trawling is forbidden; the resulting large range in trawling disturbance makes the Kattegat an ideal site for studying potential impacts on the seafloor.

We analysed sediments across the trawling gradient, in 1cm-layers downcore, and determined the relative effect of trawling intensity and environmental variables (e.g. bottom water oxygen, sediment grain size) and biological variables (e.g. bioturbation) on sediment carbon content, reactivity and degradation rates. In general, environmental factors and physical properties of the sediment appeared to be the strongest explanatory variables, with trawling intensity being less important. The best explanatory variables also varied depending on the sediment depth analysed, potentially due to the relative importance of seasonal inputs of fresh organic carbon at the sediment surface, mixing depth and type of bioturbation and penetration depth of the trawls.

How to cite: Bradshaw, C., Blomqvist, M., Sköld, M., Ennas, C., Seidel, L., Maciute, A., and Pusceddu, A.: Relative effects of bottom trawling and environmental factors on sedimentary carbon properties and degradation in the heavily trawled Kattegat, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10289, https://doi.org/10.5194/egusphere-egu25-10289, 2025.

Continental margin sediments are a major hotspot for organic carbon burial and play a vital role in the carbon cycle. Disturbance of sedimentary organic carbon by human activities such as mobile bottom fishing might lead to (i) reductions of the organic carbon stocks, (ii) impacts on carbon cycling, primary productivity and biodiversity and (iii) ocean acidification and atmospheric CO₂ emissions. Spatially explicit studies that have been conducted to inform marine management have so far looked at organic carbon stocks that have already been affected by mobile bottom fishing. Here, we focus instead on areas on the Norwegian continental margin that have not been fished previously, based on fishing data covering the years 2009 – 2020. We estimate that the surface sediment layer (0 – 2 cm) in unfished areas covering 765,600 km² contains 139.2 Tg of organic carbon. Based on data from a meta-analysis of demersal fishing impacts on organic carbon density, we estimate that 16.4 Tg (1.8 – 29.6 Tg) of organic carbon might be vulnerable to mobile bottom fishing. Of this, approximately one third is currently located in existing area-based protection measures. Additional protection could be guided by hotspots of vulnerable organic carbon, which are found in the Barents Sea. We argue that the protection of vulnerable organic carbon that is at high risk of being lost in areas becoming accessible to fishing due to sea ice retreat in the northern Barents Sea should be prioritised over easing pressure on already impacted organic carbon stocks.

How to cite: Diesing, M., Sciberras, M., Thorsnes, T., Bjarnadóttir, L. R., and Moe, Ø. G.: Mapping vulnerable organic carbon in Norway’s continental margin sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3024, https://doi.org/10.5194/egusphere-egu25-3024, 2025.

Coastal seas play a critical role in removing carbon from the atmosphere and sequestering it in marine sediments, partially offsetting anthropogenic greenhouse gas emissions. The North Sea, despite its shallow depth, intense tidal mixing, and increasing anthropogenic pressures, stores over a million tonnes of organic carbon (OC) annually in the deep Norwegian Trench, and exports OC to the Skagerrak Strait. However, the rapid pace of global climate change is disrupting biogeochemical cycles, including carbon dynamics in coastal seas.

In recent decades, the North Sea has become a hotspot for offshore wind farm (OWF) construction. Their hard substrates are colonized by filter feeders (e.g., blue mussels), which filter OC particles from the water column and biodeposit them onto the seabed, creating localized areas of carbon-enriched sediments near OWFs.

As part of the JPI Climate & Oceans project CE2COAST, which aims to evaluate pressures on coastal seas and their ecosystem services under a changing climate, we developed a high-resolution coupled hydrodynamic-wave-sediment-biogeochemical-diagenetic model. This model covers the North Sea, utilizing a 5x5 km resolution horizontal grid and a vertical grid comprising 30 water column layers and 26 sediment layers. It integrates pelagic and benthic biogeochemical processes, simulating sedimentary fluxes and solute diffusion at the sediment-water interface. OWFs are represented in the model as surface areas suitable for bivalve colonization, based on their location and turbine density. Changes to sediment properties that affect OC resuspension are incorporated to represent the retention of deposited OC by local ecosystems. The model has been calibrated and validated using available physical and biogeochemical data for both pelagic and sedimentary environments.

The model is used to assess the combined effects of climate change and OWFs on biogeochemical cycling, with a focus on carbon cycling and sequestration. Simulations were conducted for both current (1993–2023) and future climates (up to 2100) under the IPCC SSP370 “upper-middle” scenario. Scenarios for OWF construction were based on plans for 2035 and assumed constant until the end of the century. The model was forced by a regional atmospheric model (MAR), driven by outputs from the MPI climate model and at the lateral open boundaries by outputs from the NorESM2 Earth System model.

Key findings from comparisons between present conditions and projections include higher remineralization rates of organic carbon in both the water column and upper sediment layers, along with enhanced conditions for phytoplankton carbon fixation via photosynthesis. While increased primary production offsets higher remineralization rates in the water column, OWF construction along the European coast significantly alters traditional carbon transport pathways. Instead of being transported to the Skagerrak Strait and the Norwegian Trench, more organic carbon is retained in the shallow European shelf. This enhances the North Sea’s capacity to sequester OC in the medium term but raises concerns about its fate following the OWF decommissioning and the removal of hard substrates.

This study highlights the complex interplay of physical and biogeochemical processes in the North Sea and emphasizes the importance of coupled modeling approaches to predict future changes in carbon storage under a changing climate and increasing anthropogenic influence.

How to cite: Ivanov, E., Grailet, J.-F., and Grégoire, M.: Carbon Storage in North Sea Sediments: Impacts of Climate Change and Offshore Wind Farms Revealed by Coupled Modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18790, https://doi.org/10.5194/egusphere-egu25-18790, 2025.

As global warming progresses, 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 and restore their natural carbon sequestration efficiency. Here, we report on the transdisciplinary research project APOC, which addressed 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 burial/storage and the determination of the sources and reactivity 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 and protected as valuable natural 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 conservation 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 environmental policy committees. In effect, the transdisciplinary cooperation within the project not only produced valuable scientific results, but also numerous expert briefings on environmental policy at all levels, from local authorities to the EU Parliament, emphasizing the importance of protecting natural fine-grained 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. The results of the project APOC can contribute directly to the new EU Nature Restoration Law adopted in June 2024. To this end, measures to strengthen and protect ecosystems both on land and in national coastal waters are to be introduced on 20 % of land and marine areas across the EU, including restoration of at least 30 % of important habitat types in poor condition by 2030.

How to cite: Kasten, S., Holtappels, M., Müller, D., Wallmann, K., Porz, L., Daewel, U., Zhang, W., Kuhlmann, J., Taylor, B., and Zeibarth, N.: Assessing, protecting and restoring the natural carbon storage capacity of marine sediments – the need for enhanced transdisciplinary dialogue and cooperation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21441, https://doi.org/10.5194/egusphere-egu25-21441, 2025.