The pathway

Enhanced weathering (EW) accelerates a chemical reaction that the Earth already runs at geological pace: the dissolution of silicate rocks by carbonic acid in rainwater. When fast-weathering rocks like basalt, olivine, or wollastonite are crushed to fine particles and spread — usually on cropland, sometimes in rivers, forests, or mine pits — the surface area available for reaction increases by orders of magnitude. CO₂ dissolved in soil water reacts with the minerals, producing dissolved bicarbonate ions that drain through soils to groundwater and eventually the ocean, where the carbon is stored on timescales of 10,000 to 100,000+ years. That long-tailed durability is why EW sits alongside direct air capture and mineralization in most “durable CDR” portfolios, even though it borrows infrastructure (quarries, ag spreaders) from existing industries.

How it works

The core reaction for a calcium-bearing silicate like wollastonite is:

CaSiO₃ + 2CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻ + SiO₂

Two moles of CO₂ are consumed per mole of divalent cation released — though field-realized ratios are lower because of side reactions, cation exchange on clays, and downstream re-precipitation of carbonates. Rate is governed by mineral surface area, soil moisture, temperature, pH, and the partial pressure of CO₂ in soil gas, which is itself elevated by root respiration. Beerling et al. (2020) laid out the per-hectare potential on global cropland; Kantzas et al. (2022) estimated that the UK alone could draw down ~6–30 Mt CO₂/yr by 2050 with basalt amendments at agronomically realistic application rates.

Two parameters dominate the techno-economics: grinding energy (which scales roughly with the inverse of particle diameter) and feedstock chemistry. Lewis et al. (2021) characterized how basalt mineralogy — olivine and pyroxene content, glass fraction — controls both capture potential and the release of plant nutrients and trace metals like Ni and Cr. Olivine weathers faster but carries higher trace-metal loads; basalt is slower but more agronomically benign. Seawater kinetics, relevant to ocean-deposition pathways, were measured by Rimstidt et al. (2022). The geological backdrop — why silicate weathering is Earth’s long-term thermostat in the first place — is reviewed in Brantley et al. (2023).

Who’s doing it

The supplier set has consolidated around cropland spreading, with adjacent variants in rivers, forests, and wastewater.

  • Lithos — US cropland basalt at quarry-byproduct cost, ML-driven soil sampling for measurement.
  • UNDO — UK and now North American basalt and wollastonite; states it has spread >313,000 tonnes of rock across ~400 farms.
  • Terradot — Stanford spinout operating in Brazil’s tropical soils; acquired olivine-focused Eion in 2026.
  • InPlanet — Brazilian basalt on tropical farmland, with emphasis on co-located weathering measurement.
  • Alt Carbon — Indian basalt (Rajmahal Traps) applied to tea and rice; one of the few EW operators sourcing from a flood basalt province at scale.
  • CarbonRun — Nova Scotia, river alkalinity enhancement with limestone in acidified salmon rivers; not strictly silicate weathering but mechanistically adjacent.
  • CREW Carbon — limestone dosing inside municipal wastewater plants, an engineered variant that bypasses soil heterogeneity entirely.
  • Silicate — Ireland-based, uses milled returned concrete rather than mined rock, sidestepping quarrying emissions.
  • Rock Flour Company — harvests naturally ultra-fine glacial flour from Greenlandic fjords, avoiding the grinding-energy penalty.

Smaller operators (Carbonaught, Silica, AEROC, ClimeRock, Tropicarbon, reverce, Goal 300, Tambora) are largely regional cropland plays. Syntopa and RubiscoBlack are pursuing microbial acceleration; Aquarry targets post-mining pit lakes; Carbony is testing managed forests.

Durability and the measurement problem

The thermodynamic case for durability is solid: bicarbonate stored in the ocean has a residence time on the order of 10⁵ years. The empirical case is harder. Field-measured CO₂ removal lags the stoichiometric ceiling because (1) some released cations adsorb onto clay exchange sites rather than charge-balancing bicarbonate, (2) bicarbonate can re-precipitate as soil carbonate in alkaline soils, losing half the captured CO₂, and (3) lateral transport from field to ocean introduces uncertainty about how much bicarbonate actually reaches long-term storage versus outgassing en route.

There are roughly three measurement, reporting, and verification (MRV) schools. Solid-phase methods track cation depletion in soil cores (slow, noisy at low application rates). Aqueous-phase methods sample soil pore water, tile drains, or streams for alkalinity (faster signal, but extrapolation to long-term storage requires hydrological modelling). Reactive transport models bridge the two. The major registries — Isometric, Puro.earth, and Cascade — have published EW protocols, but they disagree on counting rules: when to credit (at dissolution? at the river? at the ocean?), what counterfactual to use, and how to handle the trace-metal risk that Dupla et al. (2023) and others have flagged on long-term cropland deployment.

What to watch over the next year

  • Field MRV convergence. Whether the major registries land on a consistent crediting point, and whether direct field measurements from operators like reverce, Lithos, and UNDO show the bicarbonate flux that l