Tracing a Geochemical Reaction from Nanometers to Kilometers: Pyrite Oxidation

Abstract:

Weathering partially controls concentrations of important gases in the atmosphere including CO₂ and O₂. For example, pyrite, the most common iron sulfide in Earth's crust, oxidizes rapidly today at Earth’s surface, drawing oxygen out of the atmosphere. Currently, we have little ability to model weathering accurately because our approach to understanding such mineral-water-gas reactions is to study separate parts of different systems at different spatial scales from that of electron microscopy to seismic mapping to integration of riverine chemistry across watersheds. Once measured, we have no way to put these observations into one model that crosses scales. We have used a critical zone observatory to create such a model that crosses scales from nanometers to tens of kilometers in documenting how pyrite oxidizes. Our study site is a catchment developed on weathering grey shale that contains trace pyrite that oxidizes pseudomorphically to different iron oxides. When a pyrite grain is imaged under transmission electron microscopy, the thickness of the oxidation front of the pyrite is about 100 nm. When shale matrix is imaged under scanning electron microscopy, the thickness of the oxidation front is approximately 1mm. Finally, when imaged based on bulk chemical analysis at the scale of a borehole, the front varies between 1 and 16 m in thickness. At this scale, the front also somewhat parallels the land surface although with lower relief. The rate-limiting step for oxidation of pyrite grains is diffusion of oxygen from fractures through the shale matrix but the rate of oxidation is limited by fracturing and erosion at watershed scale. Differences in reactions around the pyrite cause small mineralogical differences from one shale to the next, but the cross-scale model of oxidation appears consistent across 12 orders of magnitude of spatial scale and 3 orders of magnitude of erosion rate.