Hydrothermal iron-oxyhydroxide chimney mounds (iron mounds) have been discovered in a fishing port in Nagahama Bay, located on the southwest coast of Satsuma Iwo-Jima Island, south of Kyushu Island, Japan. In the fishing port, uncovered ∼1.0-m-high iron mounds in shallow waters formed under relatively calm conditions. Typically, the fishing port has orange-colored turbid waters that mix with outer ocean waters during high tide. Colloidal iron-oxyhydroxides form due to the oxidation of ferrous iron in hydrothermal waters (pH = 5.5; temperature = 55 °C) as they mix with seawater.The mounds are made of two types of material: hard, dark brown–orange, high-density material; and soft, brownish orange–yellow, low-density material. Computed tomography scans of the harder iron mound material revealed a cabbage-like structure consisting of micropipe structures with diameters of 2–5 mm. These micropipes have relatively hard walls made of iron oxyhydroxides (FeOH) and are identified as discharge pipes. Nucleic acid staining genetic sequencing and scanning electron microscope observations suggest that the mounds formed mainly from bacterial stalks with high concentrations of FeOH colloidal matter. In the harder parts of the mounds, these “fat stalks,” which contain oxyhydroxide colloidal aggregates, are entwined and concentrated. The softer material contains twisted stalk-like structures, which are coated with FeOH colloidal particles. Deoxyribonucleic acid (DNA) examination of the iron mounds revealed the presence of iron-oxidizing bacteria, especially at the mound surface.We estimate that the iron mounds accumulated at a rate of ∼1700 tons/1000 m2/yr. This is an order of magnitude higher than the rate of FeOH sedimentation via chemical precipitation of FeOH colloids within the fishing port. This suggests that biogenic activity, resulting in the production of entwined FeOH stalks, leads to the rapid accumulation of FeOH beds and that biogenic activity within the water mass rich in FeOH colloids is an efficient means of generating thick iron-rich sedimentary sequences. As such, we propose that some ancient iron formations may have also formed through the biogenic production of FeOH stalks rather than solely through chemical sedimentation in a water mass rich in FeOH colloids. It appears that these rapidly forming biogenic FeOH iron mounds, distributed over a wide area of ocean floor, are also relatively protected from erosion and diagenetic alteration (reduction). Previous studies have reported that ancient iron formations were commonly deposited in deeper environments via direct iron oxidation from the water column in a ferruginous ocean. However, there are several hydrothermal vent inflows preserved with FeOH that would have formed appropriate redox boundary conditions in an otherwise anoxic ocean. Under these conditions, iron mound mat-type sedimentary deposits might have formed and been well preserved and affected by early diagenesis where higher heat flow occurred in the Archean ocean. The FeOH mounds in Nagahama Bay provide an example of the iron formation sedimentary environment and important information for estimating the past depositional state of iron formations.

中文翻译：

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日本鹿儿岛萨摩硫磺岛浅半封闭海湾中氢氧化铁烟囱丘的热液形成

在位于日本九州岛南部萨摩硫磺岛西南海岸的长滨湾的一个渔港中发现了热液氧化铁-羟基氧化物烟囱丘（铁丘）。在渔港，在相对平静的条件下形成的浅水区发现了约 1.0 米高的铁丘。通常，渔港有橙色浑浊的海水，在涨潮时会与外部海水混合。由于亚铁在热水中（pH = 5.5；温度 = 55 °C）与海水混合时氧化，形成胶体羟基氧化铁。这些土丘由两种材料制成：坚硬、深棕橙色、高-密度材料；柔软的棕橙黄色低密度材料。对较硬的铁丘材料进行的计算机断层扫描显示出一种类似卷心菜的结构，由直径为 2-5 毫米的微管结构组成。这些微管具有由羟基氧化铁 (FeOH) 制成的相对较硬的壁，被确定为排放管。核酸染色基因测序和扫描电子显微镜观察表明，这些土丘主要由具有高浓度 FeOH 胶体物质的细菌茎形成。在土丘较硬的部分，这些含有羟基氧化物胶体聚集体的“脂肪茎”缠绕并浓缩。较软的材料包含扭曲的茎状结构，其上涂有 FeOH 胶体颗粒。铁丘的脱氧核糖核酸 (DNA) 检查显示存在铁氧化细菌，尤其是在土丘表面。我们估计铁丘的堆积速度约为 1700 吨/1000 平方米/年。这比通过在渔港内化学沉淀 FeOH 胶体的 FeOH 沉淀速率高一个数量级。这表明生物活动导致缠绕的 FeOH 茎的产生，导致 FeOH 床的快速积累，并且富含 FeOH 胶体的水团中的生物活动是产生厚铁沉积层序的有效手段。因此，我们提出一些古老的铁地层也可能是通过 FeOH 茎的生物生产形成的，而不仅仅是通过在富含 FeOH 胶体的水团中进行化学沉淀。这些快速形成的生物 FeOH 铁丘似乎分布在大面积的海底，也相对免受侵蚀和成岩改变（减少）。先前的研究报告称，古代铁层通常通过来自含铁海洋中水柱的铁直接氧化沉积在更深的环境中。然而，有几个热液喷口流入用 FeOH 保存，它们会在原本缺氧的海洋中形成适当的氧化还原边界条件。在这些条件下，铁丘垫型沉积矿床可能已经形成并保存完好，并受到早期成岩作用的影响，因为太古代海洋中发生了较高的热流。长滨湾的 FeOH 丘提供了铁形成沉积环境的一个例子，也是估计铁形成过去沉积状态的重要信息。先前的研究报告称，古代铁层通常通过来自含铁海洋中水柱的铁直接氧化沉积在更深的环境中。然而，有几个热液喷口流入用 FeOH 保存，它们会在原本缺氧的海洋中形成适当的氧化还原边界条件。在这些条件下，铁丘垫型沉积矿床可能已经形成并保存完好，并受到早期成岩作用的影响，因为太古代海洋中发生了较高的热流。长滨湾的 FeOH 丘提供了铁形成沉积环境的一个例子，也是估计铁形成过去沉积状态的重要信息。先前的研究报告称，古代铁层通常通过来自含铁海洋中水柱的铁直接氧化沉积在更深的环境中。然而，有几个热液喷口流入用 FeOH 保存，它们会在原本缺氧的海洋中形成适当的氧化还原边界条件。在这些条件下，铁丘垫型沉积矿床可能已经形成并保存完好，并受到早期成岩作用的影响，因为太古代海洋中发生了较高的热流。长滨湾的 FeOH 丘提供了铁形成沉积环境的一个例子，也是估计铁形成过去沉积状态的重要信息。