How ‘blue’ is your carbon? Sources of sedimentary shelf carbon in the North and Celtic Seas using n-alkane lipid tracers and geochemical tools

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