In the meso-oligotrophic Bay of Biscay, a diminishing downward organic matter flux with depth is accompanied by an important decrease of the live foraminiferal density. Although bottom water oxygenation is not directly influenced by organic matter input, the oxygenation of interstitial waters and the primary redox fronts do change in response to variations of the organic matter flux. The occurrence of deep and intermediate infaunal taxa can be linked to fundamental redox fronts and putative associated bacterial consortia. Our data are in agreement with the TROXmodel, which explains the benthic foraminiferal microhabitat as a function of organic flux and benthic ecosystem oxygenation. Both the depth of the principle redox fronts and the microhabitat of deep infaunal species show important increases with depth. At the deepest oligotrophic stations, deep infaunal faunas become relatively poor. Therefore, the exported flux of organic matter appears to be the main parameter controlling the composition and the vertical distribution of benthic foraminiferal faunas below the sediment-water interface. The oxygenation of pore waters plays only a minor role. A species-level adaptation of the TROX-model is presented for the Bay of Biscay. r

We applied laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) as a new and precise technique for measuring trace elements in benthic foraminifera (Hoeglundina elegans). With this technique, trace element concentrations were accurately measured without the elaborate cleaning required in standard methods. Contaminated coatings are easily excluded during spatially resolved analysis. Application of this new technique allowed us to calibrate the trace elements incorporated in single tests of living (when sampled) benthic foraminifera to ambient seawater temperature (Mg and Sr) and redox conditions (Mn) for the first time. Incorporation of Ba showed a more complex pattern that cannot be explained by a direct correlation to water column concentration.

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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