Vulnerability of blue carbon stocks to disturbance in sediments with low burial efficiency

Abstract

Continental shelf sediments – particularly soft, muddy habitats - serve as significant reservoirs of organic carbon over glacial-interglacial timescales and are subject to disturbance, notably from mobile bottom fishing gear. The quantity and quality of accumulated organic carbon varies across shelf environments, with muds storing more organic carbon than other substrate types. For effective marine management, it has been recommended that the reactivity, or ‘quality’, of organic matter should be considered when assessing the vulnerability of sedimentary carbon to disturbance. If management interventions are to be made to protect vulnerable organic carbon reservoirs on the seafloor, then it is also important to consider the burial efficiency of carbon. In areas of rapid sediment accumulation, carbon will be moved from surface to geological reservoirs more quickly, thus facilitating carbon sequestration. In this study, we assess the quantity, quality, and accumulation rate of organic carbon in the muddy depocenter of the Fladen Ground, northern North Sea - an area of low active sediment accumulation, with both historic and ongoing bottom trawling. All sediment cores analysed in this study display upwards coarsening of surface sediments, consistent with sediment disturbance and winnowing. Southern Fladen Ground sediments are especially well sorted, depleted in total organic carbon, and enriched in calcium carbonate - patterns that align with intensified trawling. Our results show very low modern organic carbon accumulation rates (∼0.7 g C m⁻² yr⁻¹), low organic matter reactivity (18.5 % labile), and that modern trawl events can potentially disturb sediments and carbon accumulated over the last ∼2300 years. These results indicate that sediment and organic carbon accumulation rates are an important consideration when assessing the vulnerability of sedimentary carbon.