A KEY ROLE FOR IRON-BOUND PHOSPHORUS IN AUTHIGENIC APATITE FORMATION IN NORTH-ATLANTIC CONTINENTAL PLATFORM SEDIMENTS

A combination of pore water and solid phase analysis was used to deter mine whether authigenic carbonate fluorapatite (CFA) is currently form ing in the sediment at two locations (OMEX I and II) on the North Atla ntic continental platform Goban Spur (southwest of Ireland). Results o f selective P extractions suggest that an early diagenetic redistribut ion of Fe-bound P to an authigenic P phase may be occurring at both st ations. A steady-state diagenetic model describing the depth profiles of pore water HPO42- and three solid phase forms of P (organic P, Fe-b ound P and authigenic P) was developed and applied to the data of stat ion OMEX-I. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe- bound P and pore water HPO42- at the redox interface can create condit ions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ-formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HPO42- diffusion are compensated for by sorption of HPO42- released from organic matter to Fe oxides in the oxidized su rface sediment. Fe-bound P thus acts as an intermediate between organi c P and CFA. CFA can account for between 25 and 70% of the total buria l flux of reactive P at station OMEX-I and thus may act as an importan t sink for P in this low sedimentation, continental margin environment .