BG4.2 | Blue Carbon: The role of coastal and marine sedimentary organic carbon in the global carbon cycle.
EDI
Blue Carbon: The role of coastal and marine sedimentary organic carbon in the global carbon cycle.
Co-organized by OS3/SSP3
Convener: Craig SmeatonECSECS | Co-conveners: Ruth Parker, Sebastiaan van de Velde, Lucas PorzECSECS, Hannah MuirECSECS, Ed Garrett, Tania MaxwellECSECS
Orals
| Mon, 15 Apr, 08:30–12:25 (CEST)
 
Room 2.23
Posters on site
| Attendance Mon, 15 Apr, 16:15–18:00 (CEST) | Display Mon, 15 Apr, 14:00–18:00
 
Hall X1
Posters virtual
| Attendance Mon, 15 Apr, 14:00–15:45 (CEST) | Display Mon, 15 Apr, 08:30–18:00
 
vHall X1
Orals |
Mon, 08:30
Mon, 16:15
Mon, 14:00
Coastal and marine sedimentary systems are crucial components of the global carbon cycle and potentially play an important role in global climate regulation over varying timescales. Coastal vegetated habitats (classical Blue Carbon) such as seagrass, saltmarsh and mangroves, alongside marine sedimentary environments are estimated to trap and store globally significant quantities of carbon and potentially provide an important climate regulation service. Though these environments are clearly valuable both for carbon storage and climate change mitigation, these ecosystems are under growing natural and anthropogenic pressure with seagrass and saltmarsh extent decreasing annually and marine sediments being regularly disturbed (e.g., trawling, dredging).

These anthropogenic activities can modify sedimentary alkalinity generation and the burial efficiency of carbon, either through direct disturbance of the seafloor or indirectly by changing carbon supply, physical fields and/or ecosystem functions. These activities, their connection to the global carbon cycle, and implications for marine spatial management strategies are clearly significant. However, the magnitude of C release from such disturbance and what effect this has on the climate remains poorly quantified, hindering the development of policy and management.

To tackle the science questions and fill the policy needs in the field of Blue Carbon, we seek to bring together expertise from across the geosciences (e.g., ecology, biogeochemistry, sedimentology, minerology, spatial modelling). In this multidisciplinary session, we invite presentations from across these disciplines, scales (local, national, and/or global) and across study types (observational, experimental, modelling, and/or theoretical) to discuss recent advances in coastal and marine sedimentary carbon research.

Orals: Mon, 15 Apr | Room 2.23

The oral presentations are given in a hybrid format supported by a Zoom meeting featuring on-site and virtual presentations. The button to access the Zoom meeting appears just before the time block starts.
Chairpersons: Tania Maxwell, Ed Garrett, Craig Smeaton
Intertidal Blue Carbon Environments
08:30–08:40
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EGU24-3088
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BG4.2
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ECS
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On-site presentation
Nora Kainz, Franziska Raab, Andreas Kappler, and Prachi Joshi

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.

08:40–08:50
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EGU24-636
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BG4.2
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ECS
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Virtual presentation
Alice Puppin, Davide Tognin, Massimiliano Ghinassi, Andrea D'Alpaos, and Alvise Finotello

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.

08:50–09:00
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EGU24-7002
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BG4.2
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On-site presentation
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Nisa Novita, Adibtya Asyhari, Chandra Agung Septiadi Putra, Adi Gangga, Rasis Ritonga, Aji Anggoro, Topik Hidayat, Yiwei Yang, Allison Lewin, and Muhammad Ilman

Mangroves, as part of the blue carbon ecosystem, are considered a cost-effective nature-based solution pathway to help mitigate climate change and achieve the Paris Agreement’s aim to limit warming to 1.5˚C. The accurate quantification of greenhouse gas (GHG) emissions and carbon accounting has become a key challenge for policymakers and scientists addressing climate change.  Globally, Indonesia emits the highest potential CO2 emissions from soils in the mangrove ecosystems because of its high rates of mangrove losses in recent decades. Unfortunately, there are limited studies on carbon and GHG emissions from Indonesian mangroves. This study aims to quantify carbon loss due to mangrove conversion due to aquaculture development by combining carbon stocks and GHG emissions data located in Tabalar Muara Village, Berau, East Kalimantan, Indonesia. We collected data from aboveground (vegetation, downwood) and belowground (roots and soil) carbon stocks in five and three transects of mangrove forests and aquaculture ponds, respectively. Soil bulk density and carbon concentration in various soil depth intervals were also analyzed. In addition, we conducted three consecutive days of regular monthly monitoring of CO2 and CH4 fluxes associated with soil physicochemical properties in mangrove forests and aquaculture ponds from January – December 2023. Total ecosystem carbon stocks in mangrove forests and aquaculture ponds were 926 ± 20 and 658 ± 45 Mg C ha−1, respectively. Thus, it implies 984 Mg CO2 ha−1 of potential carbon loss during mangrove forest conversion to aquaculture ponds. Soil carbon stocks between 0 and 300 cm depth varied significantly, where carbon stock in aquaculture ponds (658 Mg C ha−1) was 18% lower than in mangrove forests (777 Mg C ha−1). Soil carbon dominates total ecosystem carbon stocks by up to 88% in mangrove forests.  For GHG fluxes, mangrove forests have six times higher heterotrophic CO2 emissions (79.44 ± 4.47 Mg CO2 ha-1 yr-1) compared to that from the aquaculture ponds (13.88 ± 0.88 Mg CO2 ha-1 yr-1). The annual total CH4 flux was 17 times higher in mangrove forests (7.72 ± 0.50 Mg CO2e ha-1 yr-1) than in aquaculture ponds  (0.46 ± 0.04 Mg CO2e ha-1 yr-1). The results of this research are useful to refine GHG emissions accounting on mangroves by providing higher Tier of emission factors to fulfill Indonesia’s Enhanced Nationally Determined Contributions.

How to cite: Novita, N., Asyhari, A., Putra, C. A. S., Gangga, A., Ritonga, R., Anggoro, A., Hidayat, T., Yang, Y., Lewin, A., and Ilman, M.: Carbon Stocks and Greenhouse Gas Emissions (CO2 and CH4) in Mangrove Forests and Aquaculture Ponds in East Kalimantan, Indonesia  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7002, https://doi.org/10.5194/egusphere-egu24-7002, 2024.

09:00–09:10
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EGU24-10923
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BG4.2
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ECS
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On-site presentation
Svenja Reents, Laura Hommes, Nele Schildt, Nicola Camillini, and Lasse Sander

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.

09:10–09:20
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EGU24-15773
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BG4.2
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On-site presentation
Christian Sanders and Faming Wang

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.

09:20–09:30
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EGU24-17164
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BG4.2
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On-site presentation
Gilad Antler, Soto Neta, and Gidon Winters

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, SO42-) 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.

09:30–09:40
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EGU24-863
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BG4.2
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ECS
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On-site presentation
Mayuri Rabha, Biswajit Roy, and Archana Singh

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.

Sedimentary Carbon Environments
09:40–09:50
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EGU24-5089
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BG4.2
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Highlight
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On-site presentation
Markus Diesing, Sarah Paradis, Henning Jensen, Terje Thorsnes, Lilja Bjarnadóttir, and Jochen Knies

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.

09:50–10:00
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EGU24-10812
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BG4.2
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ECS
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On-site presentation
Daniel Müller, Bo Liu, Moritz Holtappels, Walter Geibert, Susann Henkel, and Sabine Kasten

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.

10:00–10:10
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EGU24-19779
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BG4.2
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ECS
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On-site presentation
Anthony Grey, Brian Kelleher, Mark Chatting, Mike Long, Phoebe Walsh, Markus Diesing, and Mark Coughlan

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.

Coffee break
Chairpersons: Craig Smeaton, Hannah Muir, Lucas Porz
10:45–10:55
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EGU24-22145
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BG4.2
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On-site presentation
Claire Powell, Carolyn Graves, Franck Dal-Molin, Clement Garcia, Clare Hynes, Caroline Limpenny, Claire Mason, Paul Nelson, Craig Smeaton, and Ruth Parker

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.

10:55–11:05
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EGU24-16476
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BG4.2
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On-site presentation
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Ulrike Herzschuh, Josefine Weiß, Kathleen Stoof-Leichsenring, Lars Harms, Dirk Nuernberg, and Juliane Mueller

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.

11:05–11:15
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EGU24-16727
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BG4.2
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ECS
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On-site presentation
Ulrike Hanz, Bingbing Wei, Vera Fovonova, Lasse Sander, Robert Kopte, Henning Schröder, Sabine Kasten, and Moritz Holtappels

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.

11:15–11:25
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EGU24-17590
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BG4.2
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On-site presentation
Moritz Holtappels, Ute Daewel, Jannis Kuhlmann, Lucas Porz, Bettina Taylor, Klaus Wallmann, Wenyan Zhang, Nadja Ziebarth, and Sabine Kasten

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.

11:25–11:35
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EGU24-6380
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BG4.2
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On-site presentation
Mark Chatting, Markus Diesing, Anthony Grey, Brian Kelleher, and Mark Coughlan

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.

11:35–11:45
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EGU24-18005
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BG4.2
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On-site presentation
Naveenkumar Parameswaran, Ewa Bur­wicz-Galerne, Everardo Gonzalez, Klaus Wallmann, David Greenberg, and Malte Braack
Sediment accumulation rate is recognized as the primary parameter influencing the burial rate of organic carbon and other compounds in marine sediments. The prediction of a global map for burial rates is challenging due to the limited availability of measurements for total organic carbon (TOC) and sediment accumulation rates from the seafloor. Recent advancements in machine learning, including techniques such as K nearest Neighbours and Random Forests, have demonstrated promise in producing comprehensive predictions utilizing global maps of oceanic properties.
 
In this study, we introduce a sophisticated approach based on a newly developed deep neural network (DNN) model tailored for geospatial predictions. Employing few-shot learning techniques, such as the incorporation of prior physical knowledge into the model, along with strategies like multi-task learning and semi-supervised learning, enhances predictions amidst sparse data availability. Moreover, p​​​​​redictions of the global distribution of seafloor TOC and sediment accumulation rates presented here are coupled with uncertainty maps computed using Monte Carlo Dropout, a Bayesian approximation method that effectively inform about the degree of the model predictibility. With our results, we not only explore the global distribution of burial rates of organic carbon but also offer insights into the global carbon stocks in various marine regions.

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.

11:45–11:55
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EGU24-11348
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BG4.2
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ECS
|
On-site presentation
Guangnan Wu, Bingjie Yang, Klaas Nierop, Gert-Jan Reichart, Julia Gebert, and Peter Kraal

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.

11:55–12:05
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EGU24-22261
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BG4.2
|
ECS
|
On-site presentation
Hugo Woodward-Rowe, Frank Dal Molin, Ben Gregson, Claire Mason, Ruth Parker, and Natalie Hicks

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.

12:05–12:15
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EGU24-1793
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BG4.2
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On-site presentation
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Marija Sciberras, Sarah Paradis, Justin Tiano, Emil De Borger, Clare Bradshaw, Claudia Morys, Antonio Pusceddu, Claudia Ennas, Karline Soetaert, Pere Puig, and Pere Masque

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.

12:15–12:25
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EGU24-12461
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BG4.2
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ECS
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Highlight
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On-site presentation
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Sarah Paradis and Timothy I. Eglinton

Marine sediments play a crucial role in the global carbon cycle by acting as the gateway from short- to long-term reservoirs of both terrestrial and marine organic carbon (OC). To understand the spatiotemporal variability, sources, composition and reactivity of OC in marine sediments, a curated and harmonized database of OC content and associated parameters is needed, especially considering the logistical challenges and costs of retrieving samples at sea. This has prompted the development of the Modern Ocean Sediment Archive and Inventory of Carbon (MOSAIC) database. Here we present an updated version, MOSAIC v.2.0, which stores variables such as OC content, its isotopic composition (δ13C, Δ14C), sedimentological parameters (e.g., grain size, mineral surface area), and associated molecular signatures (e.g., lignin, fatty acid, alkane biomarkers). MOSAIC v.2.0 offers a broad spatiotemporal coverage, with data from more than 21’000 individual sediment cores collected since the 1950s, providing a key tool to assess the role of coastal and marine sedimentary organic carbon in the global carbon cycle. A new and interactive open-access web-interface to the database will be presented, along with Python and R packages that allow users to integrate MOSAIC in their data processing workflows. Finally, initial applications of MOSAIC (v.2.0) to understand spatial patterns in the geochemical composition of marine sediment on regional and global scales will also be illustrated.

How to cite: Paradis, S. and Eglinton, T. I.: Introducing the Modern Ocean Sedimentary Inventory and Archive of Carbon (MOSAIC v.2.0) database and its initial applications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12461, https://doi.org/10.5194/egusphere-egu24-12461, 2024.

Posters on site: Mon, 15 Apr, 16:15–18:00 | Hall X1

The posters scheduled for on-site presentation are only visible in the poster hall in Vienna. If authors uploaded their presentation files, these files are linked from the abstracts below, but only on the day of the poster session.
Display time: Mon, 15 Apr 14:00–Mon, 15 Apr 18:00
Chairpersons: Sebastiaan van de Velde, Craig Smeaton, Ruth Parker
BG 4.2 Posters - Blue Carbon Environments
X1.30
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EGU24-19812
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BG4.2
|
ECS
|
Clarisse Gösele and Peter Müller

Salt marshes are highly effective long-term carbon sinks. The capacity of these ecosystems to sequester carbon is controlled by the balance of plant primary production and microbial decomposition. Besides the input of litter, plants are able to excrete organic carbon into the soil by transporting recently fixed carbon compounds from the living roots into the surrounding soil. Despite playing an important role in the global carbon cycle, studies on root exudates in tidal wetlands are rare. This study reports findings on (1) the detection and (2) the stabilization of root exudates in a wetland plant-soil system. Conducting a 13CO2 pulse-labeling study, we (1) tested if Spartina anglica Hubb. supplies a relevant (i.e. detectable) flux of recently fixed carbon via root exudates to the soil environment. The biogeochemical conditions of a typical wetland were simulated by planting S. anglica, a dominant grass in large parts of the European salt marsh area, in waterlogged mesocosms. The aboveground biomass was labeled by acidifying 0.1 g of 13C-pure bicarbonate inside a cylindric transparent acrylic glass chamber. Labeling was conducted once daily over a period of ten days. Isotope-ratio mass-spectrometry was used to track the 13C label through different compartments of the plant-soil system, including leaves, roots and bulk soil. We found a rapid translocation of recently fixed carbon to belowground plants tissues. (2) Subsamples of the labeled bulk soil were used to study the decay and stabilization of root exudates in the soil. A full factorial pot experiment (labeled vs. unlabeled x vegetated vs. non-vegetated) was conducted, where a total of 12 mesocosms were sampled over approx. 15 months and the bulk soil was analyzed for its δ13C-signature. This long-term approach showed that root exudates stabilize in coastal wetland soils under anoxic soil conditions and thereby could play an important role in their carbon sequestration capacity.

How to cite: Gösele, C. and Müller, P.: Do root exudates stabilize in coastal wetland soils?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19812, https://doi.org/10.5194/egusphere-egu24-19812, 2024.

X1.31
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EGU24-11523
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BG4.2
|
ECS
|
|
Rebecca Dunn, Paul Hudson, and Ed Garrett

Coastal Blue carbon ecosystems offer a range of ecosystem services including, for example, carbon sequestration, feeding grounds for birds and flood defence. Therefore, the conservation and management of these ecosystems can act as a nature-based solution that contributes to multiple ecological and climate goals. Blue carbon ecosystems achieve this due to their multifaceted nature and how the communities surrounding these ecosystems interact with them. 

 

An example of blue carbon ecosystems can be found in Lindisfarne national nature reserve (NNR) located on the Northumberland coastline in the Northeast of England. The Lindisfarne NNR contains saltmarshes, seagrass meadows and sandflats in close proximity to each other. Moreover, Lindisfarne not only has value as a source of blue carbon but has further human value due to it being a destination for pilgrimages, bird watchers and holiday makers, as well containing a residential population with a small-scale fishery. Therefore, they represent locations of both natural and cultural capital.

 

The close proximity of multiple blue carbon ecosystems and various human interactions provides an opportunity for a significant comparative analysis of the above blue carbon ecosystems under similar socio-environmental conditions. We do so in terms of both their carbon sequestration potential as well as their socio-economic importance.

 

Through our comparative analysis we present two strands of work. The first is our findings regarding the estimated total sediment and vegetative carbon stock within the Lindisfarne NNR, including two seagrass meadows with contrasting sediment profiles. Additionally, we quantify the various pools of carbon storage: labile, refractory and organic; and the carbon accumulation rates of each studied ecosystem, using 210Pb and 137Cs dating analysed within a Bayesian framework. We also consider factors associated with carbon sequestration, such as vegetation coverage, surface elevation and sediment profile, in our analysis. In doing so this project will be one of the first to calculate carbon accumulation rates and carbon pools in UK seagrass meadows. The second strand of work is generated through a survey of the perceptions and values that people have towards coastal environments via discrete choice experiment survey. 

 

Taken together these results will provide insight into the design of coastal  management plans which focus on using the above blue carbon ecosystems as a tool for climate change mitigation through their long-term carbon sequestration as further contextualised through the priorities and values identified via the social survey.

How to cite: Dunn, R., Hudson, P., and Garrett, E.: Intertidal blue carbon ecosystems and their socio-economic value at Lindisfarne, northern England , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11523, https://doi.org/10.5194/egusphere-egu24-11523, 2024.

X1.32
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EGU24-10123
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BG4.2
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ECS
|
Highlight
Craig Smeaton, Cai Ladd, Ed Garrett, Martha Hall, Lucy Miller, Lucy McMahon, Glen Havelock, William Blake, Natasha Barlow, Martin Skov, Roland Gehrels, and William Austin

Saltmarshes play a key role in the coastal carbon cycle through the capture and storage of organic carbon. Assessments of both organic carbon (OC) stocks and rates of OC accumulation are vital for quantifying saltmarsh contributions to climate-change mitigation and for guiding efforts to protect and restore coastal ecosystems. Current assessments of the magnitude of the store and rate of OC accumulating in UK saltmarshes are based on a small and spatially limited dataset. To address this knowledge gap, we collected sediment cores to quantify the OC stored in the soil and biomass of 26 saltmarshes and estimate OC accumulation rates for 22 saltmarshes distributed around the UK.

Across the saltmarshes, the estimated average store is 11.55 ± 1.56 kg C m-2 with values ranging between 2.24 kg C m-2 and 40.51 kg C m-2. These saltmarshes accumulate OC at a rate of 110.88 ± 43.12 g C m-2 yr-1 with values ranging from 27.57 g C m-2 yr-1 to 343.68 g C m-2 yr-1. These highly variable OC stocks and accumulation rates are dependent on interlinked factors, including local geomorphology, organic carbon source, sediment type (mud vs sand), sediment supply, and relative sea-level history.

By upscaling these estimates to all UK saltmarshes, it is calculated that these systems currently store 5.20 ± 0.65 Mt of OC and accumulate 46563 ± 4353 tonnes of OC annually. The low OC accumulation rates indicate that UK saltmarshes have relatively low additional Greenhouse Gas (GHG) abatement potential, but that they contain significant stores of OC within the ecosystem. This highlights the crucial need for the protection and restoration of existing OC stores within UK saltmarshes, providing climate benefits several times more significant than the annual accumulation of OC in these ecosystems.

How to cite: Smeaton, C., Ladd, C., Garrett, E., Hall, M., Miller, L., McMahon, L., Havelock, G., Blake, W., Barlow, N., Skov, M., Gehrels, R., and Austin, W.: Carbon storage and accumulation across the United Kingdom’s saltmarsh habitat. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10123, https://doi.org/10.5194/egusphere-egu24-10123, 2024.

X1.33
|
EGU24-2594
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BG4.2
Linqiong Wang and Mengjie Zhu

Coastal tidal flats are intersection zones between terrestrial and marine environments and are considered repositories of pollutants from anthropogenic activities (e.g., fishery and aquaculture). Specifically, the extensive use of antibiotics in intertidal mudflat aquaculture area has substantially increased the dissemination risk of antibiotic resistance genes (ARGs). As hosts of ARGs, bacteria and virus exert vital effects on ARG dissemination. Thus, this study attempts to unravel the occurrence, dissemination, evolution, and driving mechanisms of ARGs associated with bacterial and viral communities using metagenomic sequencing in a typical intertidal mudflat. Abundant and diverse ARGs (22 types and 437 subtypes) were identified and those of ARGs were higher in spring than in autumn. It is worthy noted that virus occupied a more essential position than bacteria for ARGs dissemination through network analysis. Meanwhile, nitrogen exerted indirect effect on ARG profiles by shaping viral and bacterial diversity. According to the results of neutral and null models, deterministic processes dominated the ARG community assembly by controlling sediment nitrogen and antibiotics. Homogeneous and variable selection dominated phylogenetic turnover of ARG community, contributing 46.15% and 45.90% of the total processes, respectively. This study can hence theoretically support for the ARG pollution control and management in intertidal mudflat aquaculture area.

How to cite: Wang, L. and Zhu, M.: Unraveling antibiotic resistomes associated with bacterial and viral communities in intertidal mudflat aquaculture area, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2594, https://doi.org/10.5194/egusphere-egu24-2594, 2024.

BG 4.2 Posters - Sedimentary Carbon Environments
X1.34
|
EGU24-9181
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BG4.2
|
ECS
Bottom trawl fishing and dredging decrease marine CO2 sequestration by reducing natural alkalinity generation 
(withdrawn)
Sebastiaan van de Velde, Astrid Hylén, and Filip Meysman
X1.35
|
EGU24-5772
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BG4.2
|
ECS
Lucas Porz and Wenyan Zhang

There is ongoing debate on the influence of human activities on the marine carbon cycle, their potential impacts on climate and resulting implications for marine spatial management. In order to gauge the efficacy of different management options, we estimate the contributions of three direct human impacts on particulate organic carbon in the North Sea: material dumping, marine aggregate mining and bottom trawling. While dumping and mining are considered in a budgetary manner based on existing data, the impacts of bottom trawling are simulated using a high-resolution 3D ocean and sediment models. Several future scenarios of fishing closures and redistribution of effort are considered and their potential climate impacts discussed. Our results indicate that the impacts of sand mining are negligible, while both material dumping and bottom trawling can alter the sequestration capacity of carbon in the North Sea significantly. In the case of bottom trawling, we show that through targeted design of fishing closure zones, the impacts could be greatly reduced without the need to change the overall trawling effort.

How to cite: Porz, L. and Zhang, W.: Estimates of human influence on North Sea sedimentary carbon - Current impacts and future scenarios , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5772, https://doi.org/10.5194/egusphere-egu24-5772, 2024.

X1.36
|
EGU24-8634
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BG4.2
|
ECS
|
Phoebe Walsh, Mike Long, Anthony Grey, Rasmus Svendsen, and Mark Coughlan

Blue Carbon traditionally refers to carbon buried and stored in coastal or terrestrial environments such as mangrove forests and seagrass meadows. However, marine sediments, like clays and sands, found on continental shelves, are increasingly being recognised as important Blue Carbon settings and are being included in national marine management plans.  To fully understand the importance of such environments and its potential to mitigate against climate change, a thorough understanding of the quantification of the carbon stored is required. This is performed through the analysis of marine sediment cores. 


Marine sediment cores are typically extracted by forcing a PVC pipe into the seafloor through varied methodologies. During the extraction process, the method itself can have adverse effects  on the sediment, including causing changes in profile integrity resulting in a shortening of the core profile through consolidation. This can increase the dry bulk density of the sediment, which is an important parameter in calculating carbon stock, resulting in an overestimation of results. There is a paucity of studies regarding the impacts these effects can have on the quantification of Blue Carbon in marine sediments. 


In this study, a set of cores were gathered from the same geographical location using three different offshore coring techniques, namely: gravity coring, vibro coring and box coring. These techniques are standard in offshore site investigations. Samples from these cores were used to assess the extent of the impact of consolidation on quantifying carbon stock measurements in marine sediments through several geotechnical techniques. These include evaluating parameters that directly influence consolidation in marine sediments, such as moisture content, Atterberg limits and particle size. Additionally, compressibility measurements, through oedometer testing, can help elucidate to what degree compaction may have taken place. Carbon stocks are calculated using total organic carbon and loss on ignition measurements, which will be compared across profiles from different coring techniques. Similarly, accumulation rates calculated using gamma spectrometry (e.g. 210Pb) allow for comparison across core profiles. These tests were performed across the three offshore coring techniques to determine which method of core extraction is optimal for Blue Carbon quantification.

How to cite: Walsh, P., Long, M., Grey, A., Svendsen, R., and Coughlan, M.: Assessing the influence of consolidation in marine sediment cores for Blue Carbon quantification, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8634, https://doi.org/10.5194/egusphere-egu24-8634, 2024.

X1.37
|
EGU24-625
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BG4.2
|
ECS
|
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Hannah Muir, Jacqui Keenan, Rowan Henthorn, James Strong, David G. Reading, Peter Duncan, Martin W. Skov, Jan G. Hiddink, Richard K. F. Unsworth, Phillip E. Warwick, and Claire Evans

Temperate coastal ecosystems including seagrass, saltmarsh, and shelf-sea sediments are natural, long-term ‘blue carbon’ (BC) sinks, with the potential to be managed for carbon storage and sequestration. These BC hotspots could help offset unavoidable greenhouse gas emissions and contribute to nations' Net Zero ambitions. The Isle of Man, a self-governing British Crown Dependency situated in the Irish Sea, has territorial waters equivalent to approximately 85% of its total jurisdiction. The island's Government is actively developing a comprehensive BC management plan aimed at maximising carbon sequestration and restoring seabed biodiversity and ecosystem services.

To inform the management plan, sediment cores were collected from three major BC habitats around the Isle of Man: seagrass, saltmarsh, and shelf-sea sediments. The cores were analysed using elemental analysis and isotope ratio mass spectrometry to quantify organic and inorganic carbon stores. Radioanalytical methods were employed to measure radionuclides (137Cs, 210Pb, 241Am, and 210Po), which were used to determine sedimentation rates and subsequently carbon accumulation rates. Complementary analyses, including grain size analysis, X-ray fluorescence, X-ray scans, and high-resolution imagery, provide a holistic understanding of the chemical, physical, and biological attributes of the sedimentary cores, illuminating the processes influencing BC storage. Furthermore, side scan sonar, drop-down video, and drone imagery have been used to assess the extent of existing seagrass meadows, which is central to informing spatial-management strategies, particularly in the establishment of seagrass conservation zones.

Our findings will help to develop the first national BC inventory for the Isle of Man and set a precedent for co-designed, collaborative, evidence-informed approaches for the sustainable management of coastal ecosystems.

How to cite: Muir, H., Keenan, J., Henthorn, R., Strong, J., Reading, D. G., Duncan, P., Skov, M. W., Hiddink, J. G., Unsworth, R. K. F., Warwick, P. E., and Evans, C.: Developing the First National Blue Carbon Inventory for the Isle of Man, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-625, https://doi.org/10.5194/egusphere-egu24-625, 2024.

X1.38
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EGU24-5362
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BG4.2
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Highlight
Sophie Ward, James Scourse, Zoe Roseby, and Sarah Bradley

The ocean is known to be a vast and globally important carbon sink and there is an urgent need to better understand the role played by shelf sea sediments in the ocean carbon cycle. Organic carbon is preferentially stored in marine muds, the deposition of which has predominantly been dictated by bottom currents controlled by waves and tides. Through numerical modelling to predict carbon accumulation, alongside data mining and synthesis, we aim to make a first-order approximation of global carbon stocks in shelf sea sediments. We are developing new high resolution numerical models to simulate the timing and distribution of carbon-rich mud accumulations across global shelf seas. These models will reconstruct current flows near the seabed driven by the tides and waves, for the period since the Last Glacial Maximum (approximately 22,000 years ago). We incorporate dynamic palaeo-topographies from the latest regional glacial isostatic adjustment models, as well as adopting the novel approach of palaeo-wave modelling (forced by palaeo-wind datasets). The simulated hydrodynamics will be used to estimate where accumulation of fine sediments may occur on shelf seas. These synthetic maps of fine sediment deposits will be constrained and validated using observational data from sediment samples (e.g., data on sediment grain size and carbon content). Radiocarbon age-constrained samples from shelf sea sediment cores will then be used to test the validity of the model predictions for estimating accumulations of carbon-rich sediments over thousands of years. As this work considers potential mud accumulation – and carbon stocks - within muddy basin fills, it builds upon existing works which to date have primarily focused on surface sediment. This novel global inventory of shelf sea carbon stocks will inform global carbon budgets and protection- and restoration efforts of these globally significant blue carbon stocks.

How to cite: Ward, S., Scourse, J., Roseby, Z., and Bradley, S.: A global inventory of shelf sea carbon, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5362, https://doi.org/10.5194/egusphere-egu24-5362, 2024.

X1.39
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EGU24-14217
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BG4.2
Seung-Hee Kim, Dong-Hun Lee, Huitae Joo, Seok-Hyun Youn, and Young-Shin Go

We investigated physio-chemical properties, sedimentary bulk elements (C, N, S contents and heavy metals concentration), and isotopic compositions (δ13C, δ15N, 87Sr/86Sr) in the northwest Pacific marginal sea (Yellow Sea, East China Sea and East Sea; R/V Tamgu 3 and 9, February in 2019) to trace the distribution, origin, and reactivity of sedimentary organic matter (OM). Together with hydrodynamic influence (mainly freshwater input) near Yellow Sea and East China Sea, the spatial patterns of sedimentary bulk elements consider that there may be the potential accumulation of natural/anthropogenic derived-OMs transported from regional (Korea-China) river systems. Furthermore, δ13C and δ15N values in surface sediments may be characteristic of the mixture of autochthonous (e.g., algae)/allochthonous (e.g., C3 plant, soil, fertilizer) origins, suggesting the discriminative source contribution on the sedimentary OM distributions. Especially, compared to the isotopic end-members reported from Korean and China river estuaries, isotopic signatures of sedimentary OM may be regarded as the discriminative contribution of various terrestrial derived-origins within the Yellow Sea and East China Sea. Further, 87Sr/86Sr ratio indicated discriminative weathering impact and terrestrial origin OM transportation within the Korean and China river estuaries. Hence, with respect to the increase of anthropogenic activities near northwest Pacific marginal sea, the source tracing approach estimated via the multi-isotopic mapping may provide important insights for effectively understanding dramatic change of biogeochemical OM cycles from water column to sediments.

How to cite: Kim, S.-H., Lee, D.-H., Joo, H., Youn, S.-H., and Go, Y.-S.: Isotopic mapping of sedimentary organic matter in the northwestern Pacific marginal sea (Yellow Sea, East China Sea, East Sea), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14217, https://doi.org/10.5194/egusphere-egu24-14217, 2024.

X1.40
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EGU24-12798
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BG4.2
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ECS
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Stacey Felgate, Michel Kaiser, and Marija Sciberras

Marine sediments are a significant sink for anthropogenic carbon dioxide (CO2)1. Bottom trawl fisheries constitute the most widespread physical disturbance to carbon-rich seabed habitats2. Recent research has sparked concern that this disturbance can turn marine sediments into a large source of CO23, but this is subject to ongoing debate4,5,6. Uncertainties exist regarding the effect of bottom trawling on carbon sequestration, remineralisation, and storage. To address this, we conducted a systematic review and meta-analysis of the existing literature to assemble a comprehensive, up-to-date database looking at how demersal mobile fishing affects: (i) the amount and type of carbon found in benthic sediments; (iii) the geochemical, biological, and physical parameters which control the fate of benthic carbon; (iii) the magnitude and direction of benthic-pelagic carbon fluxes; and (iv) the geochemical, biological, and physical parameters which control the fate of resuspended carbon. Here we present methodological details alongside preliminary findings of the resultant meta-analysis. We highlight the parameters which carry the greatest and least uncertainties and suggest key knowledge gaps to help target future field and laboratory studies to help better constrain the effect of bottom trawling on the benthic-pelagic carbon fluxes and processing.

1. Atwood et al., 2020. Global patterns in marine sediment carbon stocks. Frontiers in Marine Science; 2. Hiddink et al., 2017. Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance. PNAS; 3. Sala et al., 2021. Protecting the global ocean for biodiversity, food and climate. Nature; 4. Hilborn and Kaiser, 2022. A path forward for analysing the impacts of marine protected areas. Nature; 5. Hiddink et al., 2023. Quantifying the carbon benefits of ending bottom trawling. Nature; 6. Atwood et al., 2023. Reply to: Quantifying the carbon benefits of ending bottom trawling. Nature.

How to cite: Felgate, S., Kaiser, M., and Sciberras, M.: Trawling Impacts on Benthic Carbon Sequestration, Storage, and Processing: A Systematic Review, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12798, https://doi.org/10.5194/egusphere-egu24-12798, 2024.

X1.41
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EGU24-18616
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BG4.2
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ECS
Blanca Ausín, Gina Bossert, Nicola Krake, Sarah Paradis, Negar Haghipour, Xavier Durrieu de Madron, Belén Alonso, and Timothy Eglinton

Regional studies on the origin and fate of organic carbon (OC) in marine sediments are scarce due to limited spatial data coverage and the complex interplay among biological, physicochemical, and geological processes that can influence OC content and geochemical signatures on different spatial scales. Yet, such studies are vital to constrain global carbon inventories for ocean sediments.

To shed light on the controls on the origin, distribution, and fate of sedimentary OC in continental margins and adjacent deep-sea basins, we investigate the geochemical and sedimentological characteristics of organic matter (OM) in the semi-enclosed Western Mediterranean Sea. Here, we analyze 149 core-top samples from the Western Mediterranean Sea and the adjacent Atlantic Ocean sector and explore the spatial distribution of OC content, OC-ẟ13C, OC-Δ14C, C/N, grain size, and mineral surface area, among others.

Most geochemical parameters depict a clear SW-NE gradient between the westernmost and the easternmost basins. This gradient reverses in the Gulf of Lions (NW Mediterranean). Thus, OC is younger and of primarily marine origin in samples from the Atlantic sector and the Alboran Sea (SW Mediterranean). In the Algerian Basin, the Balearic Sea, and the Algero-Provencal Basin the influence of terrestrial OC input increases towards the NE characterized by the presence of highly 13C- and 14C-depleted (aged) sedimentary OC. Finally, samples from the Gulf of Lions show a larger influence of fresh and young OC compared to other northeastern basins.

The interplay between marine primary productivity and delivery of terrestrial OC is the main factor that determines the observed gradient. Primary productivity decreases from the southwestern basins towards the NE and increases again northeasternmost basin, the Gulf of Lions. By contrast, the terrestrial OC carbon delivered by rivers and channeled to the deeper basin by canyons has an increasing influence on sedimentary OC toward the NE.

When explored from a sedimentological context, our results reveal that lateral transport of OC and OM protection by mineral surfaces potentially act as secondary controls on the OC fate in surface sediments of the Western Mediterranean Sea.

This integrated study contributes to a better knowledge of the interplay of biological, chemical, and hydrological factors that influence the amount and geochemical characteristics of sedimentary OC in the land-sea continuum and the deeper ocean, a fundamental consideration to constraining global carbon inventories.

How to cite: Ausín, B., Bossert, G., Krake, N., Paradis, S., Haghipour, N., Durrieu de Madron, X., Alonso, B., and Eglinton, T.: Towards a sedimentary organic carbon inventory of the Western Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18616, https://doi.org/10.5194/egusphere-egu24-18616, 2024.

X1.42
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EGU24-5777
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BG4.2
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ECS
Timo Spiegel, Andrew W. Dale, Nina Lenz, Mark Schmidt, Michael Fuhr, Habeeb Thanveer Kalapurakkal, Matthias Moros, Sebastian Lindhorst, Hendrik Wolschke, Sabine Kasten, Martin Butzin, Gesine Mollenhauer, Daniel Mueller, and Klaus Wallmann

Since industrial times, human and natural processes have affected the sediment system of the North Sea. As a substantial proportion of the suspended sediment in the North Sea is ultimately deposited in the Skagerrak, it offers a representative archive for reconstructing the temporal variability of the North Sea sediment system. However, little is known about how sedimentation rates in the Skagerrak may have changed over time. In this study, we present high-resolution age-depth models based on the natural radionuclide 210Pb and the anthropogenic time markers 137Cs, 14C and mercury to determine average sedimentation rates before and after the year 1963 at six stations in the Skagerrak. This year was selected because its age-depth relationship was clearly reflected by peak activities or concentrations in the sedimentary data of the time markers. The main result of this study is a consistent decrease in sedimentation rates at all stations. On average, sedimentation rates decreased from 0.36 to 0.15 cm yr-1, suggesting a substantial alteration of the North Sea sediment system. We tentatively discuss possible driving factors including a shift in the North Sea circulation pattern, increased sediment deposition in the Wadden Sea, and reduced sediment inputs into the North Sea due to coastal protection and river damming. In terms of the overall North Sea sediment cycle, these processes may outweigh the effects of sediment resuspension by human activities and storm events, as well as temperature, humidity and sea level rise caused by climate change.

How to cite: Spiegel, T., Dale, A. W., Lenz, N., Schmidt, M., Fuhr, M., Kalapurakkal, H. T., Moros, M., Lindhorst, S., Wolschke, H., Kasten, S., Butzin, M., Mollenhauer, G., Mueller, D., and Wallmann, K.: Sedimentation rate decrease in the Skagerrak and its implication for human and natural impacts in the North Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5777, https://doi.org/10.5194/egusphere-egu24-5777, 2024.

X1.43
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EGU24-11011
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BG4.2
William Austin, Rhiannon Grant, Craig Smeaton, and Mark Garnett

The burial of organic carbon (OC) in coastal and continental shelf sediments contributes to the regulation of atmospheric carbon dioxide on geological timescales and potentially mitigates present-day climate change. Major efforts are now underway to map and quantify the OC held in our continental shelf seas, including the first ever assessment of a national Exclusive Economic Zone (EEZ) sediment OC resource for the United Kingdom. When these OC-rich sediments are disturbed, either by natural or (increasingly) anthropogenic pressures, there is potential for significant quantities of carbon dioxide to be released to the water column and potentially the atmosphere.

The reactivity of sedimentary OC is generally defined as a susceptibility to decomposition, biotically or abiotically. Reactivity of OC in marine sediments determines the role the store plays in climate regulation and, equally, determines the vulnerability of the sedimentary OC store to disturbance both natural and anthropogenic. The recent development of the Carbon Reactivity Index (CRI) as a novel measure of OC reactivity based on thermogravimetric analysis has highlighted a wide continuum of OC reactivity in sediments across continental shelf seas. Here, we present new results that highlight the continuity of these processes across the land-ocean transition in the major estuaries (Clyde and Forth) of Scotland. Our results highlight new opportunities to integrate the protection of vulnerable sediment OC stores into objective marine management plans across coastal and shelf seas.

How to cite: Austin, W., Grant, R., Smeaton, C., and Garnett, M.: Source to Sea: The Climate Mitigation Significance of Shelf Sea Sedimentary Organic Carbon Stores, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11011, https://doi.org/10.5194/egusphere-egu24-11011, 2024.

Posters virtual: Mon, 15 Apr, 14:00–15:45 | vHall X1

The posters scheduled for virtual presentation are visible in Gather.Town. Attendees are asked to meet the authors during the scheduled attendance time for live video chats. If authors uploaded their presentation files, these files are also linked from the abstracts below, but only on the day of the poster session. The button to access Gather.Town appears just before the time block starts. Onsite attendees can also visit the virtual poster sessions at the PICO spot in the corresponding on-site poster hall (e.g. virtual posters of vHall X4 are visible at PICO spot 4).
Display time: Mon, 15 Apr 08:30–Mon, 15 Apr 18:00
Chairpersons: Craig Smeaton, Ruth Parker, Sebastiaan van de Velde
BG 4.2 Virtual Posters
vX1.1
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EGU24-9903
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BG4.2
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ECS
Satoko Owari, Marcelo Ketzer, Alexis Gilbert, Christian Stranne, Cheng Chang, and Changxun Yu

Methane is an important greenhouse gas, and global methane emission has been estimated separately from the perspective of anthropogenic and natural factors. However, in heavily populated semi-closed bays, methane emissions may be governed by both or even significantly amplified by human activities. One of the main factors mitigating methane emission from marine sediments to seawater and the atmosphere is the anaerobic oxidation of methane (AOM). The sulfate-rich zone acts as a barrier to methane release from the subseafloor because sulfate-dependent AOM removes sulfate and methane dissolved in interstitial water in a 1:1 molar ratio. Due to significant riverine inputs of freshwater and restricted water exchange, the seawater in some of the semi-closed bays is potentially fresher and has lower sulfate concentration, leading to a less effective AOM barrier for methane. Furthermore, the influx of nutrient-rich wastewater to densely populated semi-enclosed bays frequently leads to severe eutrophication, greatly enhancing biological productivity, anoxia, and the accumulation of organic-rich sediments in these systems. The objective of this study is to gain a deeper understanding of how the methane cycle is changed by anthropogenic activities in two case studies, with geochemical datasets collected from Tokyo Bay and the Baltic Sea, both known as heavily populated semi-closed bays.

We conducted sediment coring at the entrance of Tokyo Bay and offshore Stockholm in the Baltic Sea. Two cores (2.5 m in length) from Tokyo Bay and six cores (4 to 6 m in length) from the Baltic Sea were recovered, respectively. Organic matter in the surface of 1 m of sediment, which may have been strongly influenced by recent anthropogenic activities, showed 1.5 to 2% and 1.5 to 3.6% of total organic carbon (TOC) in Tokyo Bay and the Baltic Sea, respectively. These results indicate the Baltic Sea has a higher potential to generate more methane than the Tokyo Bay. The sulfate concentration at the seafloor was 27 mM in Tokyo Bay and 4 mM in the Baltic Sea and decreased with depth due to the AOM reaction reaching 0 mM at 2.5 mbsf in both bays. The thickness of the sulfate reduction zone was the same in both bays, even though they have a large difference in sulfate concentration in the bottom seawater. The iodine concentration, which has been used as a tracer for methane due to its close association with organic matter, increased with depth up to 74 µM at 2.5 mbsf in Tokyo Bay and 63 µM at 4.5 mbsf in the Baltic Sea. The iodine flux in Tokyo Bay was two times higher than in the Baltic Sea, indicating the possibility of strong methane flux from deeper sediments, which may not directly derive from Anthropocene organic-rich sediment. We will discuss and compare the details of the geochemical datasets in both Bays in the presentation.

How to cite: Owari, S., Ketzer, M., Gilbert, A., Stranne, C., Chang, C., and Yu, C.: Understanding the methane cycle in heavily populated semi-closed Bays: a case study of Tokyo Bay and the Baltic Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9903, https://doi.org/10.5194/egusphere-egu24-9903, 2024.