The oldest recognized proxies for low atmospheric oxygen are massive iron-rich deposits. Following the rise of oxygen ∼2.4 billion years ago (Ga), massive iron formations (IFs) largely disappear from the geologic record, only to reappear in a pulse ∼1.88 Ga, which has been attributed to sea-level transgressions, changing ocean chemistry triggered by intense volcanism, or lowered atmospheric oxygen levels. The North American Gogebic Range is one of the rare records of this pulse and even more uniquely has exposures of both volcanics and IF, providing an ideal field locality to investigate triggers for this pulse of IF. To determine the environmental context and key factors driving IF deposition after the initial rise in oxygen, we made detailed observations of the stratigraphy and facies relationships and present updated mapping relationships of the Gogebic Range Ironwood Iron Formation and the Emperor Volcanics. This work expands existing mine datasets and logs to constrain variations in stratigraphy. Our results are the first to quantitatively constrain thickness variations along the entire Gogebic Range and tie them to syn-sedimentary faulting along listric normal faults and half grabens. Furthermore, our dataset suggests that initiation of intense syn-basinal volcanism linked to a large igneous province does not coincide with initial iron deposition, thus cannot be invoked as a causal trigger. Finally, the possibility of iron deposition in a shallow-water environment suggests that the post-GOE IF pulse may not reflect global marine transgressions, but instead a chemocline shallowing due to changing global atmospheric oxygen.

中文翻译：

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大氧化事件后大块铁层的新沉积框架

公认的最古老的低大气氧代用品是大量富含铁的矿床。随着大约 24 亿年前 (Ga) 氧的上升，大量铁层 (IF) 从地质记录中基本消失，只是以大约 1.88 Ga 的脉冲重新出现，这归因于海平面海侵，改变了海洋化学由强烈的火山活动或降低的大气氧含量引发。北美 Gogebic Range 是此脉冲的罕见记录之一，更独特的是同时具有火山和 IF 的暴露，为调查此 IF 脉冲的触发因素提供了理想的现场位置。为了确定氧气初始升高后驱动 IF 沉积的环境背景和关键因素，我们对地层和相关系进行了详细的观察，并提出了 Gogebic Range Ironwood Iron Formation 和皇帝火山的最新映射关系。这项工作扩展了现有的矿山数据集和日志，以限制地层变化。我们的结果是第一个定量限制沿整个 Gogebic 范围的厚度变化并将它们与沿列式正断层和半地堑的同沉积断层联系起来的结果。此外，我们的数据集表明，与大型火成岩省相关的强烈同盆地火山作用的开始与最初的铁沉积并不相符，因此不能作为因果触发。最后，浅水环境中铁沉积的可能性表明，后 GOE IF 脉冲可能无法反映全球海侵，