Phosphorus binding by poorly crystalline iron oxides in North Sea sediments

Differential X-ray powder diffraction (DXRD) and extraction procedures were used to characterize the iron oxides present in four sediments from contrasting environments in the North Sea. Stations were located in depositional areas on the southern shelf (German Bight) and on the north-eastern shelf-slope transition (Skagerrak) and in areas with no net deposition in the southern North Sea. Poorly crystalline ferrihydrite and akageneite (extractable with 0.1 M HCI and 0.2 M NH4-oxalate) were identified in the fine sediment fraction (< 10 μm) of surface samples at all locations. Evidence for the dominant role of these Fe oxides in the binding of phosphorus in North Sea sediments was obtained from the good relationship of both the content of Fe-bound P and the linear adsorption coefficient for phosphate with NH4-oxalate extractable Fe. A tight coupling of pore water Fe2+ and HPO42− was observed at 3 stations. Pore water Fe2+/HPO42− ratios at maximum pore water concentrations of Fe2+ were similar to NH4-oxalate Fe/Fe-bound P ratios for surface sediment at these locations, and were in the range known for synthetic poorly crystalline Fe oxides. This suggests that pore water HPO42− production at the time of core collection was dominated by release from poorly crystalline Fe oxides. In contrast, at the German Bight station, much higher HPO42− levels and a decoupling of pore water Fe2+ and HPO42− was observed, suggesting a larger contribution of mineralization of organic matter to pore water HPO42− than at the other sites. Solid phase P analyses indicate possible redistribution of Fe-bound P to another inorganic phase at depth at the Skagerrak station, but not at the other stations. The persistence with depth of poorly crystalline Fe oxides and Fe-bound P suggests that these Fe phases can act as both a temporary and permanent sink for P in continental margin sediments.