Continental margins are an important external source of dissolved iron to the marine environment. However, the mechanisms responsible for the offshore transport of dissolved iron is impacted by the resulting iron speciation. We characterized the iron speciation in the Eastern Tropical North Pacific (ETNP) oxygen deficient zone (ODZ), including dissolved iron, organic iron-binding ligands, and reduced iron. Organic iron-binding ligands were measured using both competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) and liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) in order to explore the impact of organic ligands on dissolved iron (dFe_T) and iron(II) biogeochemistry in the region. Organic ligands were present in high concentrations (1.06–5.30 nmol L⁻¹) and exceeded dissolved iron concentrations (0.36–4.52 nmol L⁻¹) at all locations. Iron-binding strengths (${\mathrm{\text{log}}K}_{\mathit{\text{FeL}},\mathit{\text{Fe}}\prime }^{\mathit{\text{cond}}}$) ranged 11.22 to 12.75 and were elevated in the ODZ layer relative to the oxygenated water column. LC-ESI-MS revealed the presence of siderophores, or bacterially-produced organic ligands with high Fe-affinity, in all samples analyzed, suggesting these compounds may be produced by microbes in the ODZ despite high ambient dFe_T concentrations. This study is the first to characterize siderophores in an ODZ environment to date, and the three siderophores found (amphibactin B, synechobactin c9, synechobactin c10) could contribute to the observed elevated ${\mathrm{\text{log}}K}_{\mathit{\text{FeL}},\mathit{\text{Fe}}\prime }^{\mathit{\text{cond}}}$ of ligands in the ODZ. Comparative analysis of organic ligand ${\mathrm{\text{log}}K}_{\mathit{\text{FeL}},\mathit{\text{Fe}}\prime }^{\mathit{\text{cond}}}$ values in other low oxygen environments suggests that strong ligands, including siderophores, could be present in other low oxygen regions. In a simple model of the shelf-to-offshore iron transport mechanism, strong organic iron-binding ligands had a large impact on the longevity and transport of iron in the ODZ. These results suggest that organic ligand composition can have an impact on iron distributions in the ETNP ODZ and regulate the offshore transport of iron to the open ocean.

大陆边缘是海洋环境溶解铁的重要外部来源。然而，负责溶解铁的海上运输的机制受到由此产生的铁形态的影响。我们表征了东热带北太平洋 (ETNP) 缺氧区 (ODZ) 的铁形态，包括溶解铁、有机铁结合配体和还原铁。为了探索有机配体对溶解铁（dFe _{T )}和该地区的铁 (II) 生物地球化学。有机配体以高浓度 (1.06–5.30 nmol L^-1 ) 并在所有位置超过溶解铁浓度 (0.36–4.52 nmol L ^-1 )。铁结合强度（${\mathrm{\text{日志}}钾}_{\mathit{\text{铁电}},\mathit{\text{铁}}\prime }^{\mathit{\text{条件}}}$) 的范围为 11.22 到 12.75，并且在 ODZ 层中相对于含氧水柱升高。LC-ESI-MS 揭示了在所有分析的样品中都存在铁载体或细菌产生的具有高 Fe 亲和力的有机配体，这表明这些化合物可能由 ODZ 中的微生物产生，尽管环境 dFe _T浓度很高。这项研究是迄今为止首次在 ODZ 环境中表征铁载体，发现的三种铁载体（amphibactin B、synechobactin c9、synechobactin c10）可能有助于观察到的升高${\mathrm{\text{日志}}钾}_{\mathit{\text{铁电}},\mathit{\text{铁}}\prime }^{\mathit{\text{条件}}}$ODZ 中的配体。有机配体对比分析${\mathrm{\text{日志}}钾}_{\mathit{\text{铁电}},\mathit{\text{铁}}\prime }^{\mathit{\text{条件}}}$其他低氧环境中的值表明强配体，包括铁载体，可能存在于其他低氧区域。在架子到近海铁运输机制的简单模型中，强有机铁结合配体对 ODZ 中铁的寿命和运输有很大影响。这些结果表明，有机配体组成可以对 ETNP ODZ 中的铁分布产生影响，并调节铁向公海的海上运输。