Enhanced weathering is a carbon dioxide (CO2) mitigation
strategy that promises large scale atmospheric CO2removal.
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 CO2mineralization
site in Consett, Co. Durham, UK, where steel slags have been weathered
in a landscaped deposit for over 40 years. We provide new radiocarbon,
δ13C,87Sr/86Sr, 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 CaCO3deposited 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 (<3%) from silicate
minerals. We propose a novel method for quantifying carbon removal
rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned
sources of carbon being sequestered, and the proportion of carbon
being exported from the catchment to the oceans.