Despite the undeniable effect of iron on shaping patterns of ocean productivity, the relative importance of the different sources of this limiting nutrient to the ocean is still under debate. Although global estimates indicate that the benthic input of iron to the oceans is significant, most studies have investigated continental margins exposed to either upwelling or large riverine inputs, environments that are not representative of the majority of the oceans. Additionally, the number of studies that report dissolved iron concentrations in continental slope sediments is limited, despite the fact that these regions between the shelf edge and the continental rise make up >5% of the sedimentary surface area of the global ocean. The sedimentary flux of iron has traditionally been considered negligible due to the rapid oxidation of Fe²⁺ in oxic waters and poor solubility of the Fe(III) product. The recent realization that ferric iron may be stabilized in solution by organic ligands during oxidation near the sediment-water interface suggests that a significant fraction of the dissolved iron pool may be present under the form organic-Fe(III) complexes that could eventually reach the overlying waters. In this study, the speciation and biogeochemical importance of iron was determined in intact sediment cores along a transect across the entire continental margin near Cape Lookout, North Carolina, a region not dominated by upwelling or riverine inputs that is representative of most passive continental margins. Rates of diffusive oxygen uptake (DOU) and maximum diffusive fluxes of both dissolved Fe²⁺and organic-Fe(III) complexes decreased from the coastal zone to the continental shelf, remained low on the shelf and the upper continental slope, but rebounded to reach a maximum in mid-slope sediments where concentrations of Fe(III) oxides were the highest along the transect. In turn, DOU decreased and dissolved iron was below detection in lower-slope sediments, indicating that mid-slope sediments represent depocenters where Fe(III) oxides and organic matter may accumulate. Pore water sulfate and sulfide concentrations as well as separate sediment incubations confirmed that sulfate reduction does not greatly influence the cycling of iron in these sediments. The production of dissolved organic-Fe(III) in these continental margin sediments is likely regulated by a combination of aerobic oxidation in the presence of natural organic ligands near the sediment-water interface, dissimilatory iron reduction, or chemical oxidation of Fe(II) complexed to natural organic ligands. Fluxes of Fe²⁺ and organic-Fe(III) complexes across the sediment-water interface were not observed. However, diffusive fluxes of Fe²⁺ and organic-Fe(III) complexes into the oxic zone of these sediments (<1 cm from the sediment-water interface) and production of dissolved Fe(III) in sediment slurry incubations suggest that complexation of Fe(III) in these sediments may contribute to the stabilization and potential transport of dissolved iron into oxygenated deep ocean waters. Extrapolation to the global ocean suggests that mid-slope depocenters contribute considerably to the iron inventory of the ocean, thus warranting the need for measurement of benthic iron fluxes and dissolved iron speciation in these environments.

尽管铁对海洋生产力的形成方式具有不可否认的作用，但这种限制养分的不同来源对海洋的相对重要性仍在争论中。尽管全球估算表明，向海洋的底栖铁输入量很大，但大多数研究已调查了上升流或大型河道输入所暴露的大陆边缘，而这些环境并不代表大多数海洋。此外，尽管大陆架边缘与大陆上升之间的这些区域构成全球海洋沉积表面积的5％以上，但报告大陆斜坡沉积物中溶解铁浓度的研究数量有限。传统上，由于Fe的快速氧化，铁的沉积通量可以忽略不计在含氧水中为²⁺，并且Fe（III）产物的溶解度较差。最近的认识是，在沉积物-水界面附近的氧化过程中，三价铁可能会被有机配体稳定化，这表明大部分溶解的铁池可能以有机-铁（III）配合物的形式存在，最终可能到达上覆水域。在这项研究中，确定了北卡罗来纳州Cape Lookout附近整个大陆边缘横断面上完整的沉积物芯中铁的形态和生物地球化学重要性。溶解的Fe ²⁺的扩散氧气吸收率（DOU）和最大扩散通量有机-Fe（III）的配合物从沿海地区到大陆架减少，在大陆架和大陆上坡保持较低水平，但在中斜坡沉积物中以Fe（III）氧化物浓度最高的反弹，达到最大值。沿断面最高。反过来，在低坡度沉积物中DOU降低并且铁含量低于检测范围，表明中坡沉积物代表沉积中心，Fe（III）氧化物和有机物可能在其中沉积。孔隙水中硫酸盐和硫化物的浓度以及单独的沉积物孵育证明，硫酸盐的还原不会极大地影响这些沉积物中铁的​​循环。这些大陆边缘沉积物中溶解的有机铁（III）的产生可能受到沉积物-水界面附近天然有机配体的有氧氧化，异化铁还原或铁（II）的化学氧化的组合的调节。与天然有机配体络合。铁的助焊剂没有观察到跨越沉积物-水界面的²⁺和有机-Fe（III）配合物。然而，Fe ²⁺和有机-Fe（III）络合物扩散进入这些沉积物的氧化区（距沉积物-水界面<1 cm）的扩散通量和在沉积物淤浆培养中产生的溶解的Fe（III）表明，络合物的络合这些沉积物中的Fe（III）可能有助于溶解的铁稳定并潜在地输送到氧化的深海水中。外推到全球海洋表明，中坡沉积中心对海洋铁储量有很大贡献，因此需要在这些环境中测量底铁通量和溶解铁形态。