Quantifying CO₂ Removal at Enhanced Weathering Sites: a Multiproxy Approach.

Area of Science:

• Geochemistry
• Environmental Science
• Climate Change Mitigation

Background:

• Enhanced weathering is a key strategy for atmospheric carbon dioxide (CO2) removal.
• Monitoring, reporting, and verifying (MRV) CO2 removal is a significant challenge for enhanced weathering.
• Steel slags offer a unique, long-term site for studying enhanced weathering processes.

Purpose of the Study:

• To quantify carbon removal rates at a 40-year-old steel slag weathering site.
• To determine the sources of sequestered carbon using geochemical tracers.
• To develop a novel method for MRV of enhanced weathering CO2 removal.

Main Methods:

• Analysis of radiocarbon, stable isotopes (δ13C, 87Sr/86Sr), and major elements in water, calcite, and soil.
• Radiocarbon dating of carbonate precipitates to identify carbon sources.
• Downstream alkalinity measurements to assess carbon export.

Main Results:

• Radiocarbon analysis indicates 80% of sequestered carbon originates from the atmosphere.
• Hydroxide minerals in slag are the primary dissolving phases, with minor silicate contributions.
• A novel quantification method for carbon removal rates was developed.

Conclusions:

• Radiocarbon measurements provide robust source apportionment for sequestered carbon.
• Alkalinity measurements help determine oceanic carbon export proportions.
• The proposed method enhances the MRV of enhanced weathering CO2 removal efficacy.