Clay minerals and the stability of organic carbon in suspension along coastal to offshore transects

Particulate (POC) and dissolved organic carbon (DOC) concentration, clay mineral content and composition and the suspended particulate matter (SPM) concentration have been analyzed in water samples taken along transects from the high turbid nearshore to the low turbid offshore on the North Sea shelf. The suspended POC has been classified into a mineral-associated (POCmineral), a slowly degrading (POCslow) and a fresh fraction (POCfresh). The POCmineral has been estimated based on the clay mineral composition and on literature data of the mineral specific surface area per g and the OC content per specific surface area. It consists of organic molecules adsorbed onto mineral surfaces and is thereby the most refractory fraction. The POCfresh content (% of POCfresh in SPM) has been calculated using the semi-empirical POC-SPM model of Fettweis et al. (2022) and is intrinsically labile. The POCslow content is refractory with variable rates of degradation. The total POC content of the SPM was between 2 and 11%, from which about 0.3-6.6% (0.1-2.1%) was POCfresh in spring (resp., winter). The POCmineral content was between 0.4% and 1.1% and decreased towards the offshore, meaning that the POC offshore is less refractory than nearshore. The organic molecules adsorbed onto clay minerals, are dynamically exchanging with the DOC, and thus influencing its fate and concentration. However this process is not sufficient to explain the increasing POC/DOC ratio with increasing SPM concentration, which is further explained by primary production, advection and diffusion, density gradients and seabed erosion. Our results highlight the difficulty and the necessity of estimating the respective sample-POC and DOC concentrations, fluxes and fates along SPM concentration gradients in coastal zones. This is needed as organo-mineral interactions influence the vertical dynamics and horizontal transport of SPM and have an impact on particles and organic carbon fluxes.