Burial efficiency of phosphorus and the geochemistry of iron in continental margin sediments

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 Fe- and 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 mu M), 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.