The Influence of Clay Mineralogy on Carbon Stabilisation in Ireland’s Shelf Sediments: Past and Present

Traditionally, Blue Carbon habitats included coastal and intertidal settings such as salt marshes, seagrass meadows and mangrove forests. However, the sub-tidal sediments of continental shelves are now also recognised as a globally significant carbon sink due to the sheer volume and expanse, with increasing focus placed on qualifying organic carbon quality (or reactivity) and quantifying organic carbon stock. The ultimate sink for Blue Carbon in the marine environment is in seafloor sediments, however, once deposited, the fate of this carbon is variable. Many environmental factors are at play in the stabilisation, transformation, and/or liberation of carbon from sediments. Clay minerals and Fe-(hydr)oxides have been shown to play instrumental roles in OC preservation and transformation in marine sediments. Building on previous carbon-stock evaluations performed in the area, an investigation into the role that mineralogy plays in Blue Carbon storage and stability in Irish offshore continental sediments is outlined.

 

Focus is placed on vibrocores to a depth of >5m below the seafloor from mud-dominant, depositional areas, to assess how carbon storage, source and stability has changed throughout modern sediments and the Holocene. Bulk physical and geochemical characterisation of sediment are determined, including particle size analysis (PSA), % total OC (TOC), total nitrogen (TN) and elemental concentrations generated by X-ray fluorescence (XRF). The relationship between PSA, TOC, and total specific surface area (TSSA) is examined to help elucidate the role of mineralogy in carbon storage. From selected depths, the <2μm sediment fraction is separated from the bulk by centrifugation. Clay mineral identification is conducted by X-ray diffraction (XRD) on the <2μm fraction. Organic matter characterisation through pyrolysis-gas chromatography-mass spectrometry is conducted on the same depths as those selected for clay mineral XRD. Sedimentation rates for the uppermost horizons are estimated using gamma spectrometry (e.g. ²¹⁰Pb). Radionuclide profiles suggest minimal downcore sediment mixing has occurred at the study site, with sedimentation rates remaining relatively stable in the upper 1m if the sediment column. TOC concentrations range from 0.8-1.4w t%, with values stabilising below 2m and remaining relatively consistent to 5.5m, indicating sustained sediment OC storage throughout the sediment profile. C:N molar ratios suggest organic matter is primarily of marine source, however increased C:N ratios are observed in the upper 30cm of the sediment profile, coinciding with increased heavy metal concentrations (Zn, Pb, Cu). Preliminary Bulk XRD results indicate clay-rich mineral assemblage, consistent with enhanced organic matter preservation in fine-grained sediments.

 

These results highlight the strong control that sedimentary environment and mineralogical composition exert on the capacity of continental shelf sediments to store and stabilise organic carbon. The predominance of marine-derived organic matter suggests that offshore shelf sediments play a significant role in recycling and preserving oceanic primary production, while the elevated C:N ratios and metal concentrations in surface sediments point to recent anthropogenic influences on carbon inputs and sediment geochemistry. As such, the preservation of mud-dominated shelf environments should be considered in marine spatial planning and climate mitigation strategies, given their potential to function as stable, long-term carbon sinks under future environmental change.