Quantifying CO₂ Removal at Enhanced Weathering Sites: a Multiproxy Approach

Abstract

Enhanced weathering is a carbon dioxide (CO2) mitigation strategy 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 result of enhanced weathering reactions. Here, we study a CO2 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, 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 CaCO3 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 (<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.

A review of measurement for quantification of carbon dioxide removal by enhanced weathering in soil

“…At high enough concentrations, TIC is relatively simple to quantify using calcimetry or thermogravimetric analysis and, if enough carbonate is generated, can be combined with techniques such as XRD analysis to characterize the carbonate mineralogy (Dudhaiya et al, 2019;Haque et al, 2019Haque et al, , 2020Haque et al, , 2023Khalidy et al, 2021). Additionally, the source of carbon and calcium can be quantified using isotopic tracers (see Section 2.5; radiogenic Sr. isotopes to confirm the source of Ca, and stable carbon and oxygen isotopes to confirm the source of carbon) to ensure the accumulation of TIC came from mineralization of silicate minerals (Knapp et al, 2023).…”

“…Catchment or watershed scale monitoring on rivers or channels have been applied to larger scales of deployment, providing integrated total system fluxes (Andrews and Taylor, 2019;Larkin et al, 2022;Knapp et al, 2023). Indeed, monitoring of dissolved loads at the riverine catchment scale is the primary method by which current global CO 2 removal via natural weathering is measured (Gaillardet et al, 1999b;Jacobson and Blum, 2003;Viers et al, 2007;Hartmann et al, 2014;Moon et al, 2014;Hilton and West, 2020) hinting at the scalability of this approach.…”

“…The advantage of TA is that it is a simple measurement to make with no specialist equipment needed, and is routinely measured during watershed monitoring by, for example, governmental bodies (Hartmann et al, 2014;USGS, 2019). Total alkalinity is a conventional parameter used to calculate the bicarbonate concentration of natural freshwaters at circum-neutral pH (Andrews et al, 2016;Knapp et al, 2023;Holzer et al, 2023a). Caution should be maintained, however, as TA can be affected by the presence of other accepting bases in addition to DIC.…”

“…Potentially δ 13 C, coupled with δ 18 O, can help separate sources of C and identify contributions from silicate weathering vs. carbonate weathering in fluids as well as pedogenic and lithogenic carbonates in soils (Cerling, 1984;Schulte et al, 2011). To date, δ 13 C and δ 18 O have been used to partition sources in carbonates at enhanced weathering sites (Knapp et al, 2023).…”

Initial Validation of a Soil-Based Mass-Balance Approach for Empirical Monitoring of Enhanced Rock Weathering Rates

“…To quantify weathering rates and/or initial CDR rates from ERW experiments, previous mesocosm and field studies have used measurements of soil inorganic carbon (e.g., refs , , , and ); the concentration of dissolved ions in porewaters and effluent waters, including cations such as Ca 2+ and Mg 2+ , as well as carbonate alkalinity (e.g., refs , , − , , , , and ); and Sr, Li, Mg, and C isotopic analysis of waters, soils, and rocks (e.g., refs , , , , and ). These data provide valuable insight into the rate of weathering of feedstocks and the fate of reaction products at different stages of transport from topsoil to the river–ocean system, and it is therefore important that such measurements be made for a representative range of ERW deployment scenarios.…”