Rivers Draining Volcanic Rock Carry Less Carbon Than Soil Measurements Imply

A synthesis of natural volcanic watersheds published Aug. 26, 2026, in Nature finds that the dissolved chemistry carrying a carbon-removal benefit downstream is partly lost between the soil and the river, and concludes that assessments of basalt-based carbon removal have to account for what the watershed does in between.

The paper is by L. A. Derry of Cornell University and the Institut de Physique du Globe de Paris, K. Maher of Stanford University and O. A. Chadwick of the University of California, Santa Barbara. Enhanced weathering, the approach they assess, means spreading crushed basalt on farmland so that it dissolves, turning carbon dioxide into dissolved bicarbonate and carbonate, together called alkalinity, which rivers then carry to the ocean.

Data from basaltic catchments show alkalinity fluxes are attenuated on the way, reducing the amount exported. That attenuation is probably the result of secondary clay and carbonate minerals precipitating along underground flow paths and during river transport, locking up part of the alkalinity before it leaves the landscape.

From a compilation of watershed studies where atmospheric and hydrothermal inputs can be constrained, the researchers calculated a median carbon dioxide uptake in natural volcanic watersheds of 0.16 metric tons per hectare per year, rising to about 1.0 in the volcanically active Luzon and Bicol arcs of the Philippines. The authors note these figures may be biased high because hydrothermal inputs are hard to correct for and rivers tend to be sampled at low flow. They set the range against a carbon-removal potential of 4 to 12 tons per hectare per year proposed for some enhanced-weathering deployments that use far smaller amounts of basalt.

Natural weathering systems differ from engineered deployments, the authors note, and they present the compilation as an empirical baseline for watershed-scale alkalinity export rather than as a measurement of a treated field.

Their conclusion is that processes in the soil-to-bedrock layer known as the critical zone influence how efficiently that alkalinity is exported, implying the need to incorporate watershed processes into future assessments of enhanced-weathering carbon removal. The paper is open access.