Rock Weathering Can't Fully Draw Down CO2, Study Finds

Atmospheric carbon dioxide (CO₂) drawdown from the chemical weathering of rocks peaks across a narrow range of moderate erosion rates, according to a new study. The findings provide new insights into the constraints of weathering-mediated CO₂ drawdown and help to resolve conflicting data on the impact of uplift and erosion on the carbon cycle. The chemical weathering of rocks on Earth's surface, in part, mediates the concentration of CO₂ in the atmosphere and, thereby, the planet's climate. Understanding the constraints of this process is critical to modeling Earth's evolution over geologic time scales and evaluating Earth's systemic response to natural and anthropogenic impacts. Since chemical weathering rates are largely controlled by the exposure of fresh minerals to the surface, mountain building and subsequent erosion are thought to play a crucial role in modulating atmospheric CO₂. However, the relationship between erosion and chemical weathering remains poorly understood. Previous studies have produced conflicting results and shown that increased weathering does not always result in the drawdown of CO₂. To evaluate the effect of erosion on chemical weathering, Aaron Bufe and colleagues compiled and analyzed four datasets that report water chemistry from small streams that sample wide erosion rate gradients with well-constrained climate controls. According to the findings, CO₂ drawdown from rock weathering peaks across a narrow window of erosion rates – 0.06 to 0.11 millimeters per year – which is much lower than the most active mountain ranges. Below these rates, weathering is limited by a lack of newly exposed minerals. Above these rates, CO₂ emissions from carbonate weathering by sulfuric acid contribute to the atmospheric CO₂ budget. Thus, Bufe et al. show that while landscapes with moderate uplift and erosion rates may bolster CO₂ drawdown in some locations, more active regions decrease drawdown or even contribute CO₂ to the atmosphere. The findings help to reconcile conflicting observations on the impact of chemical weathering on atmospheric CO₂.