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We have examined the distributions of phosphorus and iron in sediments from well-oxygenated environments on the Atlantic Canadian and the Portuguese continental margins and from the anoxic region of the Chesapeake Bay. The measurements include total, citrate-dithionite-bicarbonate (CDB) extractable, ascorbate extractable, and dissolved P and Fe; acid volatile sulfide; and pyrite. A surface layer (varying in thickness between 2 and 4 cm) enriched in P and Fe was revealed by both the CDB and the ascorbate extractions in all sediments except those from the Chesapeake Bay. The amount of phosphate extracted by the two reagents was similar, but more iron was extracted by the CDB reagent, probably because of its ability to dissolve crystalline iron oxides. Within the Feand P-enriched surface layer, the Fe : P ratio in the ascorbate extract varied within a narrow range (6-14), as did the soluble-reactive phosphate (SRP) concentration (5-16 PM), suggesting that SRP is in sorption equilibrium with the solid phase. Our data are consistent with a dynamic cycling of P and strong interactions between the cycles of P, Fe, and sulfur in many marine environments. The reductive dissolution of amorphous Fe during burial and the formation of pyrite diminish the capacity of the sediment to sequester P, and only a portion of the P that arrives at the sediment-water interface actually gets buried with the sediment.There is much interest in the ability of sediments to sequester and bury phosphorus because of the effects on the oceanic P budget (Froelich et al. 1982;Howarth et al. 1995; Ruttenberg and Berner 1993) and on the chemistry of the ocean and the atmosphere throughout geologic time (Broecker 1982; Van Cappellen and Ingall 1996). On shorter time scales, P sequestration by sediments affects the trophic state of lakes and the productivity of estuaries (Nixon 198 1; Carace et al. 1990).Both the amount and the form of P sequestered by sediments are affected by diagenesis. Near the sediment-water interface, where most of the freshly deposited organic matter is decomposed, phosphate is rapidly remineralized and released to the sediment pore water from which it can readily escape to the overlying water. Consequently, only a portion 1 Present address: Universite de Bordeaux I,

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