A new experimental setup for studying the formation of phosphate binding iron oxides in marine sediments. Preliminary results

A new laboratory method is introduced to study the formation of phosphate b inding iron(III) oxides at the redox boundary in marine sediments. A sedime nt core is given a very well-defined oxic-anoxic interface by placing a 0.4 5 mum filter between the sediment and the overlying water. After a period o f 1 1/2 months the filter is covered with a layer of fresh iron oxides, for med by the oxidation of upward diffusing Fe2+ from the sediment pore water. The formed iron oxides are investigated by electron probe X-ray microanaly sis (EXPMA). With a sediment core from the brackish Baltic Sea the average molar composition of 788 formed particles is Fe1.00 +/-0.13P0.55 +/-0.06Ca0 .37 +/-0.04 plus unknown amounts of O, H and C. The results show that the p articles have a uniform composition, and that calcium plays an important ro le in the phosphate binding. The laboratory method is a useful supplement t o in situ sampling for studies of iron oxides.