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Pyrite Oxidation Drives Exceptionally High Weathering Rates and Geologic CO2 Release in Mountaintop‐Mined Landscapes

Weathering is the ultimate source of solutes for ecosystems, controls chemical denudation of landscapes, and drives the geologic carbon cycle. Mining and other land‐moving operations enhance physical weathering by bringing large volumes of shattered bedrock to the surface. Yet, the relative influence of these activities on chemical weathering remains poorly constrained. Here we show that catchments impacted by mountaintop removal coal mining have among the highest rates of chemical weathering ever reported. Mined catchments deliver more than 7,600 kg·ha−1·year−1 of dissolved solids downstream. The chemical signatures of these exceptionally high weathering rates reflect the product of sulfuric acid weathering of carbonate‐bearing rock, driven by the oxidation of pyritic materials. As this strong acid rapidly weathers surrounding carbonate materials, H+ ions are consumed and Ca2+, Mg2+, and HCO3− ions are exported to balance the elevated SO42− exports, generating alkaline mine drainage. The sulfate exports from pyrite oxidation in mountaintop‐mined catchments account for ~5–7% of global sulfate derived from pyrite, despite occupying less than 0.006% of total land area. Further, the suite of weathering reactions liberate 100–450 kg of rock‐derived C·ha−1·year−1 as CO2, with an additional 90–150 kg C·ha−1·year−1 of C released when HCO3− reaches the ocean. This rock C release contributes to the high carbon costs of coal combustion.