The efficacy of enhancing carbonate weathering for carbon dioxide sequestration

• Published in: Frontiers in Climate
• Main Authors: Knapp, William J., Tipper, Edward T.
• Other Authors: Natural Environment Research Council
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Frontiers Media SA 2022
• Subjects: Carbonic acid
• Online Access: http://dx.doi.org/10.3389/fclim.2022.928215; https://www.frontiersin.org/articles/10.3389/fclim.2022.928215/full

Enhanced weathering is a geoengineering strategy aiming to increase continental weathering rates, thereby increasing the delivery of atmospheric carbon (as HCO 3 - ) to the oceans. Most enhanced weathering studies focus on the capacity of silicate rocks (e.g., basalt) and minerals (e.g., olivine, Mg 2 SiO 4 , or wollastonite CaSiO 3 ) to remove atmospheric CO 2 . However, carbonate minerals (e.g., calcite, CaCO 3 ) could provide an additional, rapid way to increase HCO 3 - export to the oceans. Recent studies suggest that 0.84 Gt C yr −1 could be removed from the atmosphere through the enhanced dissolution of calcite in soils, provided carbonic acid is the main dissolution agent. What is not clear is whether atmospheric CO 2 dissolved in soils can be transported by rivers, which typically have lower [pCO 2 ], to the oceans. This difference in calcite solubility between soils (where weathering occurs) and rivers (where HCO 3 - is transported) may lead to large amounts of secondary carbonate formation during transport, releasing the CO 2 consumed through dissolution. Here, we present a modeling study comparing the estimated soil dissolution capacity (SDC) in 149 of Earth's largest river basins, to the potential transport capacity of carbon (PTCC) in corresponding rivers. We find the SDC can only be exported to the oceans, without secondary carbonate precipitation, if rivers are in disequilibrium with respect to calcite (i.e., SIc = 1). In this instance, 0.92 Gt C yr −1 may be sequestered above background weathering rates, which is ~20% of annual increases in atmospheric carbon. If rivers are at equilibrium with calcite (i.e., SIc = 0), approximately two-thirds of the carbon dissolved in soil waters are lost due to calcite precipitation in rivers, and just 0.26 Gt of additional atmospheric C yr −1 can be transported to the oceans. Overall, the efficacy of enhanced carbonate weathering is a function of the capacity rivers have for transporting the products from carbonate weathering to the oceans, rather than the ...