Radium isotopes as a tracer of sediment‐water column exchange in the North Sea

Abstract

Sediment-water column exchange plays an important role in coastal biogeochemistry. We utilize short-lived radium isotopes ( 224 Ra and 223 Ra) to understand and quantify the dominant processes governing sediment-water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats and North Sea sediments as the dominant source. Pore waters show 100-fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment-water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation-based Ra flux estimates and the majority of previously reported Ra flux estimates for other regions. Ra-based estimates of benthic alkalinity fluxes compare well to observed values, and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.

The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

“…The A T fluxes reported here were obtained by monitoring the temporal evolution of A T in the overlying water of enclosed sediment incubations. In the same sediment incubations, Burt et al (2014) measured the efflux of the short-lived radium isotopes ( 224 Ra and 223 Ra) and subsequently estimated A T fluxes. To this end, these authors used the Ra-isotope data to estimate the overall water exchange rate between the pore water and the overlying water column.…”

“…This assumption might be wrong and could lead to an overestimation of the A T flux at Station 38. For a more detailed discussion on the correlation between both methods, see Burt et al (2014).…”

Looking beyond stratification: a model-based analysis of the biological drivers of oxygen deficiency in the North Sea

“…While the shallower southern parts are permanently well-mixed due to the strong influence of the M 2 tidal component (Otto et al, 1990), the deeper parts north of 54 • N reveal seasonal, mostly thermal stratification (e.g., Burt et al, 2014;Pingree et al, 1978;van Leeuwen et al, 2015). Seasonal haline stratification occurs to a lesser extent along the Norwegian coast.…”

“…The transition between these permanently mixed and seasonally stratified regions occurs gradually (Pingree et al, 1978). In consequence, even areas relatively near to the coast, which are affected by high riverine nutrient run-off, often reveal stratified conditions at subseasonal timescales (e.g., Burt et al, 2014).…”

Discovery and characterization of submarine groundwater discharge in the Siberian Arctic seas: a case study in the Buor-Khaya Gulf, Laptev Sea

“…Stratification factors, shown in Fig. 5, were calculated according to Burt et al (2014). The mixed layer depth is divided by the water column depth: a stratification factor of 1 indicates a fully mixed water column.…”