Getting rid of fool's gold Pyrite, also called fool's gold, is an iron sulfide mineral that is very commonly found in rock but is almost nonexistent in sediments today. Pyrite oxidizes quickly and is a major source of sulfur to the ocean, but it is also a proxy for the oxygen content historically in Earth's atmosphere. Gu et al. conducted a set of detailed observations of the pyrite oxidation process in a shale unit. The authors found that erosion tied to fracturing is just as important as the oxygen content for the dissolution process. They developed a model that helps determine the conditions in Earth's past for which pyrite might have been stable and the role of microorganisms in the oxidation process. Science, this issue p. eabb8092 Subsurface fractures play an unexpected role in oxidizing pyrite. INTRODUCTION Oxidative weathering of pyrite, the most abundant sulfide mineral in Earth’s crust, is coupled to the biogeochemical cycles of sulfur, oxygen, carbon, and iron. Pyrite oxidation is key to these cycles because of its high reactivity with oxygen. Before the Great Oxidation Event (GOE), atmospheric oxygen concentrations were low on early Earth and pyrite was exposed at Earth’s surface, allowing erosion into sediments that were preserved in river deposits. Today, it oxidizes at depth in most rocks and is often not exposed at the land surface. To understand pyrite weathering through geologic time, researchers extrapolate the reaction kinetics based on studies from the laboratory or in acid mine drainage. Such work has emphasized the important role of microorganisms in catalyzing pyrite oxidation. But to interpret the oxidation rates of pyrite on early Earth requires knowledge of the rate-limiting step of the oxidation as it occurs naturally in rocks. RATIONALE We investigated the oxidation of pyrite in micrometer-sized grains, in centimeter-sized rock fragments, and in meter-scale boreholes at a small, well-studied catchment in a critical-zone observatory. Our goal was to determine the reaction mechanism of pyrite weathering in rocks as it occurs today. The slow-eroding catchment is underlain by shale, the most common rock type exposed on Earth. We determined weathering profiles of pyrite through chemical and microscopic analysis. RESULTS At the ridgelines of the shale watershed, most pyrite oxidation occurs within a 1-m-thick reaction zone ∼16 m below land surface, just above the depth of water table fluctuation. This is the reaction front at the borehole scale. Only limited oxidation occurs in halos around a few fractures at deeper depths. Above the depth where pyrite is 100% oxidized in all boreholes, rock fracture density and porosity are generally higher than below. However, the narrow parts of pore openings called pore throats remain small enough in oxidizing shale to limit access of microorganisms to the pyrite surface. During oxidation, iron oxides pseudomorphically replace the pyrite grains. High-resolution transmission electron microscopy (TEM) reveals that the oxidation front at grain scale is defined by a sharp interface between pyrite and an iron (oxyhydr)oxide (Fh) that is either ferrihydrite or feroxyhyte. This Fh then transforms into a banded structure of iron oxides that ultimately alter to goethite in outer layers. This complex oxidative transformation progresses inward from fractures when observed at clast scale. CONCLUSION Under today’s atmosphere, pyrite oxidation, rate-limited by diffusion of oxygen at the grain scale, is regulated by fracturing at clast scale. As pyrite is oxidized at borehole scale before reaching the land surface in most landscapes today, the oxidation rate is controlled by the movement of pyrite upward, which is in turn limited by the rate of erosion. Comparisons of shale landscapes with different erosion rates reveal that fracture spacing varies with erosion rate, so this suggests that fracture spacing may couple the landscape-scale to grain-scale rates. Microbial acceleration of oxidation globally today is unlikely in low-porosity rocks because pyrite oxidation usually occurs at depth, where pore throats limit access, as observed here for shales. Before the GOE, the rate of pyrite oxidation was instead controlled by the slower reaction kinetics in the presence of lower atmospheric oxygen concentrations. At that time, therefore, pyrite was exposed at the land surface, where microbial interaction could have accelerated the oxidation and acidified the landscape, as suggested by others. Our work highlights the importance of fracturing and erosion in addition to atmospheric oxygen as a control on the reactivity of this ubiquitous iron sulfide. Schematic depiction of oxidative weathering of pyrite in rocks buried at meters depth. Pyrite oxidation was studied from the molecular (TEM) scale of the pyrite―Fe oxide interface through clast and borehole scales to extrapolate to landscapes. The rate of oxidation of pyrite, limited at grain scale by oxygen diffusion through the shale matrix, is regulated at larger scales by fracturing and erosion. Pyrite is a ubiquitous iron sulfide mineral that is oxidized by trace oxygen. The mineral has been largely absent from global sediments since the rise in oxygen concentration in Earth’s early atmosphere. We analyzed weathering in shale, the most common rock exposed at Earth’s surface, with chemical and microscopic analysis. By looking across scales from 10−9 to 102 meters, we determined the factors that control pyrite oxidation. Under the atmosphere today, pyrite oxidation is rate-limited by diffusion of oxygen to the grain surface and regulated by large-scale erosion and clast-scale fracturing. We determined that neither iron- nor sulfur-oxidizing microorganisms control global pyrite weathering fluxes despite their ability to catalyze the reaction. This multiscale picture emphasizes that fracturing and erosion are as important as atmospheric oxygen in limiting pyrite reactivity over Earth’s history.

中文翻译：

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黄铁矿深度非生物风化

摆脱愚人金 黄铁矿，也称为愚人金，是一种硫化铁矿物，在岩石中很常见，但在今天的沉积物中几乎不存在。黄铁矿氧化迅速，是海洋中硫的主要来源，但它也是历史上地球大气中氧含量的代表。顾等人。对页岩单元中的黄铁矿氧化过程进行了一系列详细观察。作者发现，与压裂相关的侵蚀与溶解过程中的氧含量一样重要。他们开发了一个模型，有助于确定地球过去黄铁矿可能稳定的条件以及微生物在氧化过程中的作用。科学，这个问题 p。eabb8092 地下裂缝在氧化黄铁矿方面起着意想不到的作用。简介 黄铁矿是地壳中含量最丰富的硫化物矿物，其氧化风化作用与硫、氧、碳和铁的生物地球化学循环有关。黄铁矿氧化是这些循环的关键，因为它与氧的反应性高。在大氧化事件 (GOE) 之前，早期地球上的大气氧浓度很低，黄铁矿暴露在地球表面，使河流沉积物中保存的沉积物受到侵蚀。今天，它在大多数岩石的深处氧化，通常不会暴露在地表。为了了解黄铁矿在地质时间中的风化作用，研究人员根据实验室或酸性矿山排水中的研究推断出反应动力学。这些工作强调了微生物在催化黄铁矿氧化中的重要作用。但是要解释早期地球上黄铁矿的氧化速率，需要了解氧化的限速步骤，因为它在岩石中自然发生。基本原理 我们在临界区天文台的一个经过充分研究的小型集水区调查了微米级颗粒、厘米级岩石碎片和米级钻孔中黄铁矿的氧化。我们的目标是确定当今岩石中黄铁矿风化的反应机制。缓慢侵蚀的集水区下方是页岩，这是地球上最常见的岩石类型。我们通过化学和显微分析确定了黄铁矿的风化特征。结果 在页岩流域的山脊线，大部分黄铁矿氧化发生在地表以下 16 m 厚的 1 m 厚反应带内，刚好在地下水位波动深度之上。这是钻孔尺度的反应前沿。在更深的一些裂缝周围的晕圈中仅发生有限的氧化。在所有钻孔中黄铁矿被100%氧化的深度以上，岩石裂缝密度和孔隙度一般高于以下。然而，被称为孔喉的孔隙开口的狭窄部分在氧化页岩时仍然足够小，以限制微生物进入黄铁矿表面。在氧化过程中，氧化铁假晶地取代了黄铁矿颗粒。高分辨率透射电子显微镜 (TEM) 显示，晶粒尺度的氧化前沿是由黄铁矿和铁（羟基）氧化物（Fh）之间的尖锐界面定义的，铁（羟基）氧化物（Fh）是水铁矿或水铁矿。然后，该 Fh 转变为氧化铁的带状结构，最终在外层变为针铁矿。当在碎屑尺度上观察时，这种复杂的氧化转化从裂缝向内进展。结论在今天的大气下，黄铁矿氧化受氧在晶粒尺度扩散的速率限制，受碎屑尺度的压裂调节。由于黄铁矿在今天的大多数景观中在到达地表之前在钻孔尺度上被氧化，氧化速率受黄铁矿向上运动的控制，而后者又受到侵蚀速率的限制。对不同侵蚀速率的页岩景观的比较表明，裂缝间距随侵蚀速率而变化，这表明裂缝间距可能将景观尺度与颗粒尺度速率耦合。今天全球微生物加速氧化在低孔隙度岩石中是不可能的，因为黄铁矿氧化通常发生在深度，在那里孔喉限制了进入，正如这里对页岩所观察到的那样。在 GOE 之前，黄铁矿氧化的速率是由在较低大气氧浓度存在下较慢的反应动力学控制的。因此，当时黄铁矿暴露在地表，正如其他人所建议的那样，微生物相互作用可能会加速氧化并使景观酸化。我们的工作强调了压裂和侵蚀以及大气氧作为控制这种无处不在的硫化铁反应性的重要性。埋藏在米深的岩石中黄铁矿氧化风化的示意图。从黄铁矿-Fe 氧化物界面的分子 (TEM) 尺度通过碎屑和钻孔尺度外推到景观，研究了黄铁矿氧化。黄铁矿的氧化速率，通过页岩基质的氧扩散在颗粒尺度上受到限制，在更大尺度上受到压裂和侵蚀的调节。黄铁矿是一种无处不在的硫化铁矿物，可被微量氧氧化。自从地球早期大气中氧浓度升高以来，全球沉积物中基本上没有这种矿物质。我们通过化学和微观分析分析了页岩中的风化作用，这是地球表面最常见的岩石。通过查看 10-9 到 102 米的尺度，我们确定了控制黄铁矿氧化的因素。在今天的大气下，黄铁矿氧化受氧扩散到晶粒表面的速率限制，并受大规模侵蚀和碎屑破裂的调节。我们确定，尽管铁氧化微生物和硫氧化微生物都有催化反应的能力，但它们都不能控制全球黄铁矿风化通量。这张多尺度图片强调，在地球历史上，压裂和侵蚀与大气氧在限制黄铁矿反应性方面同样重要。