Particulate organic carbon dynamics of a depositional area in a high-energy shelf environment

The marine carbon pump can sequester CO₂ from the atmosphere in marine sediments. Much of the carbon is taken up from the atmosphere in productive coastal waters, whereas carbon deposition often takes place in deeper areas. Tidal- and wave activity in shallow waters are producing a high energy environment where constant resuspension counteracts sinking and prevents accumulation of organic matter at the seafloor, a process that is reflected by the ubiquitous presence of non-accumulating sands covering more than 50% of the shelf areas. Nevertheless, in some shallow coastal areas, we find organic matter accumulation even under high energy conditions. One example is a 500km² region in the German North Sea, called the Helgoland Mud Area, where local hydrodynamic conditions cause the trapping of suspended particulate matter (SPM) and subsequent sedimentation. In this study we describe the particle transport dynamics over a diurnal tidal cycle, observed via a benthic lander deployment, repeat CTDs and analysis of reactivity and isotopic composition of the particulate organic matter (POM). Driven by tidal currents, we found SPM concentrations in the bottom water fluctuating between 35 and 130 mg/l, resulting in a total amount of suspended particles within the water column of up to 400 g /m². The constant resuspension and thus remineralisation of associated POM led to a one order of magnitude decreased carbon specific mineralization rate, compared to the upper water column. From eddy covariance measurements, the SPM resuspension flux was calculated and counteracting SPM sinking velocities of around 6 x 10^-4 m/s were derived. Interestingly, the POM background under stagnant current conditions showed more terrestrial d¹³C values compared to the resuspended POM during strong current conditions, suggesting distinct particle size classes and transport conditions for marine and terrestrial POM, respectively. The locally observed resuspension dynamics helps to understand the larger hydrodynamic regime that controls sediment accumulation, and ultimately the carbon sequestration in the area.