Quantifying CO 2 Removal at Enhanced Weathering Sites: a Multiproxy Approach

Enhanced weathering is a carbon dioxide (CO ) mitigation strategy that promises large scale atmospheric CO removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ C, Sr/ Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (