The Celtic Sea Shelf System Acts as a Persistent Source of Iron to the North Atlantic

Shelf sea sediments are an important source of dissolved iron (dFe) to the open ocean, yet at present this source is poorly understood. In particular, little is known about the fate and form of the dFe that enters the water column. Radium (Ra) builds up in sediment pore waters via radioactive decay of thorium and then, like dFe, is fluxed to the overlying water column via resuspension and diffusion. The half-lives of ²²⁴Ra, ²²³Ra and ²²⁸Ra can therefore be used to trace the path of water since it was last in contact with sediments and thus estimate a sediment flux of dFe to the adjacent ocean.

We determined dFe concentrations along with Ra isotope activities in waters overlying the Celtic Sea shelf break to depths of 2500 m. Distinct intermediate nepheloid layers (INLs) were identified in the Ra data and were associated with elevated dFe concentrations (> 1 nM). INLs were present in both December 2014 and April 2015 and persisted up to 40 km from the shelf break, with an estimated horizontal advective flux of 42 µmol dFe m^-2 d^-1 between 200-750 m in April. Furthermore, investigations into the soluble (sFe) and colloidal (cFe) fractions of dFe indicate that this flux comprised of 60-80 % cFe. The results indicate that oxic shelf sediments, which characterize the majority of shelf seas globally, represent a persistent and significant source of dFe to the open ocean.

In contrast to bottom and intermediate waters, cFe and sFe were depleted in the upper water column, indicating either biological uptake and/or scavenging. In April 2015 surface dFe concentrations were particularly low, ~0.1 nM compared to 0.2-0.3 nM in winter, due to drawdown during the spring bloom. This created the potential for iron limitation near the shelf break as elevated macronutrient concentrations were observed at; nitrate 8 µM, phosphate 0.5 µM and silicate 3 µM. This was a result of the on-shelf movement of iron depleted surface water.