Research Article

Deep abiotic weathering of pyrite

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Science  23 Oct 2020:
Vol. 370, Issue 6515, eabb8092
DOI: 10.1126/science.abb8092

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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.

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Structured Abstract

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.

Abstract

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.

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