Quantifying CO₂ Removal at Enhanced Weathering Sites: a Multiproxy Approach

Limited research exists on quantifying carbon removal ratesat field scale enhanced weathering sites. We present an initial frameworkfor quantifying carbon removal rates, which minimizes uncertainties. Enhanced weathering is a carbon dioxide (CO2) mitigationstrategy that promises large scale atmospheric CO2 removal.The main challenge associated with enhanced weathering is monitoring,reporting, and verifying (MRV) the amount of carbon removed as a resultof enhanced weathering reactions. Here, we study a CO2 mineralizationsite in Consett, Co. Durham, UK, where steel slags have been weatheredin a landscaped deposit for over 40 years. We provide new radiocarbon,& delta;C-13, Sr-87/Sr-86, and majorelement data in waters, calcite precipitates, and soils to quantifythe rate of carbon removal. We demonstrate that measuring the radiocarbonactivity of CaCO3 deposited in waters draining the slagdeposit provides a robust constraint on the carbon source being sequestered(80% from the atmosphere, 2 & sigma; = 8%) and use downstream alkalinitymeasurements to determine the proportion of carbon exported to theocean. The main phases dissolving in the slag are hydroxide minerals(e.g., portlandite) with minor contributions (<3%) from silicateminerals. We propose a novel method for quantifying carbon removalrates at enhanced weathering sites, which is a function of the radiocarbon-apportionedsources of carbon being sequestered, and the proportion of carbonbeing exported from the catchment to the oceans.