Not all continental shelf seafloor is the same: detailed sediment characterization dramatically reduces estimates of organic carbon standing stock

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

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