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 f rom contrasting environments in the North Sea, Stations were located i n depositional areas on the southern shelf (German Eight) and on the n orth-eastern shelf-slope transition (Skagerrak) and in areas with no n et deposition in the southern North Sea. Poorly crystalline ferrihydri te and akageneite (extractable with 0.1 M HCl and 0.2 M NH4-oxalate) w ere identified in the fine sediment fraction (< 10 mu m) of surface sa mples at all locations. Evidence for the dominant role of these Fe oxi des in the binding of phosphorus in North Sea sediments was obtained f rom the good relationship of both the content of Fe-bound P and the li near 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 concentr ations of Fe2+ were similar to NH4-oxalate Fe/Fe-bound P ratios for su rface sediment at these locations, and were in the range known for syn thetic poorly crystalline Fe oxides. This suggests that pore water HPO 42- production at the time of core collection was dominated by release from poorly crystalline Fe oxides. In contrast, at the German Eight s tation, much higher HPO42- levels and a decoupling of pore water Fe2and HPO42- was observed, suggesting a larger contribution of mineraliz ation 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 crystallin e Fe oxides and Fe-bound P suggests that these Fe phases can act as bo th a temporary and permanent sink for P in continental margin sediment s.