This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Spreading finely crushed basalt across farmland has been promoted as a relatively simple way to remove large quantities of carbon dioxide from the atmosphere. But a new study led by Cornell researchers suggests the real-world potential of this method, known as enhanced weathering, may be far smaller than widely cited estimates.
The study, "Critical Zone Processes Limit Alkalinity Export from Natural Basaltic Systems," published in Nature, draws on observations of natural volcanic landscapes to examine what happens after basalt begins to weather. The researchers found that chemical reactions and limited water movement can substantially reduce the amount of weathering-generated alkalinity that leaves soils, moves through rivers and ultimately reaches the ocean. That alkalinity helps convert carbon dioxide into forms that can remain stored for long periods.
The study challenges a central assumption underlying many projections for enhanced weathering: that carbon captured during the initial dissolution of basalt in soil can largely be counted as durable carbon removal. "You obviously can buffer some CO2 emissions with this kind of process, but much less than people have hoped," said lead author Louis Derry, professor in the Department of Earth and Atmospheric Sciences in the Cornell Duffield College of Engineering. "The idea that we're going to get gigaton levels of CO2 reduction is not going to happen." Co-authors are professors Kate Maher of Stanford University and Oliver Chadwick of the University of California, Santa Barbara.
Natural volcanic watersheds composed largely or entirely of basalt generally export carbon at rates far below those projected for agricultural enhanced weathering, according to the study. Median estimates compiled by the researchers range from about 0.22 metric tons of CO2 per hectare annually to roughly 1 metric ton in highly active volcanic regions of the Philippines. Those rates are substantially below projections for some enhanced weathering deployments, even though agricultural amendments typically add only a small percentage of basalt to existing soils.
The difference, the researchers argue, comes partly from focusing too narrowly on what happens immediately after basalt is added to soil. Many enhanced weathering assessments estimate CO2 removal from the disappearance of calcium, magnesium and other elements from crushed basalt in the upper 10 to 30 centimeters of soil. But dissolving the rock is only the first step.
As water carries those weathering products downward through soil, groundwater and streams, it passes through the "critical zone"—the reactive layer where rock, soil, water, air and living organisms interact. Along the way, newly formed clays, oxides and carbonate minerals can capture calcium and magnesium or generate acidity that consumes some of the alkalinity created by weathering. In some Icelandic environments, previous research indicates that secondary minerals retain 30% to 67% of calcium and 45% to 81% of magnesium entering the critical zone from silicate weathering.
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