Abstract Iron (Fe) is an essential micronutrient and its availability controls primary production in large parts of the ocean. Further, the variability of oxygen minimum zones (OMZ) exerts a major control on the availability of Fe, concentrations of which are elevated off Namibia. Therefore, to understand the sources and processes involved in the cycling of Fe off Namibia, we determined major and trace element contents (Fe, Al, Si, K, P, organic carbon, Mn, Co, Zr) and the isotopic composition of Fe ( δ 56 Fe) in seven sediment cores along a transect from the shelf to the abyssal plain at ∼5 km water depth (Meteor cruise M76-1) as well as in the local lithogenic background (Namibian river bed sediments and dust). The depletion of Mn vs. the local lithogenic background (LLB) reflects the reducing character of sediments from the shelf to the lower slope. While depletion of Mn and enrichment of Co indicates the anoxic–sulfidic environment on the shelf, the enrichment of both elements in abyssal sediments documents highly oxidizing conditions there. Iron/Al ratios are elevated (0.6 to 1.6) and therefore equal to, or higher than the LLB. The highest Fe enrichment is seen on the shelf (core 1) and the shelf break (core 2) with a clear decrease towards the abyss. The bulk δ 56 Fe values range from +0.30 to −0.05‰ in the marine sediments, with an average of ∼0.08‰, which is slightly lighter than the δ 56 Fe of the LLB (0.16 to 0.21‰). Further, combined enrichments of K and Fe suggest the presence of detrital biotite (core 1) and authigenic glauconite (core 2), both of which are perfectly correlated with Zr/Al ratios reflecting the highly dynamic conditions of the shelf environment (0-300 m water depth). The absent large sedimentary Fe isotope fractionation in the sediments of this study compared to Peru sediments is likely due to a combination of the following: (i) a highly variable OMZ leading to random re-oxidation of any sediment-sourced Fe which prevents the export of larger amounts of dissolved Fe to the interior of the ocean, and which also prevents focused enrichments of Fe, and (ii) an enhanced dust flux, which frequently resupplies terrestrial Fe dampening any Fe isotope fractionation. This study is a snapshot on sedimentary Fe cycling in one area from the vast Namibian margin. Compared to the Peruvian margin, the data suggest little sedimentary Fe release, transport and redeposition. However, previous water column data suggest substantial Fe release. Hence, for a more subtle investigation of the Fe loss and re-supply a higher sampling resolution of multiple cores across the shelf and slope will be required.

中文翻译：

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纳米比亚本格拉上升流系统中的沉积铁循环

摘要 铁 (Fe) 是一种必需的微量营养素，其可用性控制着大部分海洋的初级生产。此外，氧最低区 (OMZ) 的可变性对铁的可用性施加了主要控制，铁的浓度在纳米比亚附近升高。因此，为了了解纳米比亚 Fe 循环的来源和过程，我们确定了主要和微量元素的含量（Fe、Al、Si、K、P、有机碳、Mn、Co、Zr）以及 Fe 的同位素组成( δ 56 Fe) 沿着从陆架到深海平原的横断面在约 5 公里水深（流星巡航 M76-1）以及当地岩性背景（纳米比亚河床沉积物和灰尘）中的七个沉积岩心中。Mn 的消耗与 局部成岩背景（LLB）反映了沉积物从陆架到下坡的还原特征。虽然 Mn 的消耗和 Co 的富集表明陆架上存在缺氧-硫化物环境，但深海沉积物中这两种元素的富集证明了那里的高度氧化条件。铁/铝比率升高（0.6 到 1.6），因此等于或高于 LLB。在架子（核心 1）和架子断裂（核心 2）上看到最高的 Fe 富集，向深渊明显减少。海洋沉积物中的大块 δ 56 Fe 值范围为 +0.30 至 -0.05‰，平均约为 0.08‰，略轻于 LLB 的 δ 56 Fe（0.16 至 0.21‰）。此外，K 和 Fe 的联合富集表明存在碎屑黑云母（核心 1）和自生海绿石（核心 2），这两者都与反映陆架环境（0-300 m 水深）的高动态条件的 Zr/Al 比率完全相关。与秘鲁沉积物相比，本研究的沉积物中没有大的沉积铁同位素分馏可能是由于以下原因的组合：(i) 高度可变的 OMZ 导致任何沉积物来源的铁的随机再氧化，从而阻止出口大量溶解的 Fe 进入海洋内部，这也阻止了 Fe 的集中富集，以及 (ii) 增强的尘埃通量，这通常会重新供应陆地 Fe，从而抑制任何 Fe 同位素分馏。这项研究是纳米比亚广阔边缘一个地区沉积铁循环的快照。与秘鲁边缘相比，数据表明沉积铁的释放、运输和再沉积很少。然而，先前的水柱数据表明铁释放量很大。因此，为了对 Fe 损失和重新供应进行更精细的调查，需要对架子和斜坡上的多个岩心进行更高的采样分辨率。