The efficacy of enhancing carbonate weathering for carbon dioxide sequestration

Abstract

Enhanced weathering is a geoengineering strategy aiming to remove atmospheric CO2 by increasing continental weathering rates, thereby increasing the delivery of atmospheric carbon (chiefly present as HCO3- in natural waters) to the oceans. Most enhanced weathering studies have focused on the capacity of silicate rocks (e.g. basalt) and minerals (e.g. olivine, Mg2SiO4 or wollastonite CaSiO3) have to remove atmospheric CO2. However, carbonate minerals (e.g. calcite, CaCO3) could additionally provide a rapid way to increase HCO3- export to the oceans on timescales relevant to human induced increases in atmospheric CO2. Recent work suggests 0.84 Gt C/yr (~15% of annual increases in atmospheric carbon) could be removed from the atmosphere through enhanced dissolution of calcite in high pCO2 soil waters, provided carbonic acid is the main dissolution agent. However, it is not clear whether the atmospheric CO2 dissolved in soil environments can be transported in solution to the oceans via rivers, which typically have lower [pCO2], hence lower calcite solubilities. This difference in calcite solubility between critical zone soils (where weathering occurs) and rivers (where HCO3- is transported) may lead to large amounts of secondary carbonate formation in rivers, releasing the CO2 consumed through dissolution. Here, we present a global modelling 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 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 is lost due to secondary calcite precipitation in rivers, and just 0.26 Gt of additional atmospheric C/yr 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 dissolution capacity of soils. These findings have implications for the efficiency of enhancing silicate weathering for ocean alkalinity enhancement, as secondary carbonate precipitation in rivers is not always considered.