Is basalt weathering a major mechanism for atmospheric CO<sub>2</sub> consumption?

Abstract

In the science of global change, a main focus of researchers who investigate the global carbon cycle is determining the fate of missing carbon sinks. Because atmospheric CO2 that is consumed by carbonate chemical weathering is thought to return to the atmosphere through the precipitation of carbonate, it is widely accepted that it is silicate weathering, rather than carbonate weathering, that constitutes the major mechanism of atmospheric CO2 consumption. In particular, the chemical weathering of basalt, which is a type of silicate rock, is considered to be an important “carbon sink” due to the CO2 drawdown that occurs during the basalt-carbonic acid reaction. However, the high CO2 consumption rates of basalt chemical weathering may also derive from the following four aspects, as identified in the extant literature. First, study areas with high carbon fluxes (CF) are usually ocean islands, volcanic arcs or situated in tropical regions, where precipitation is high, which results in large runoff depth. This may be one of the primary reasons for the high CF, since CF is equal to the product of the runoff depth and the concentration of bicarbonate. In addition, because of the presence of chemostatic behaviors of the concentration of bicarbonate, CF is mainly determined by runoff depth. In other words, high runoff depth will directly result in high CF. Furthermore, the interface between water and minerals will be enlarged, while the saturation state of water will be decreased, by the increase of runoff depth. Therefore, more minerals will take part in the dissolution, and the dissolving capacity of water will be increased. However, this kind of high CF results from high runoff depth, rather than basaltic properties. Second, the high concentration of dissolved inorganic carbon (DIC) may result from the chemical weathering of trace amounts of carbonate dispersed in silicate rock, rather than silicate minerals. This should be regarded as the contribution of carbonate chemical weathering, rather than silicate chemical weathering. Third, the reactions between exogenous acid and trace amounts of carbonate will contribute to DIC fluxes, during which no atmospheric CO2 is consumed. Fourth, the CO2 that participates in the rock chemical weathering in basalt watersheds may be from a deep-source, rather than the atmosphere or soil. If this is the case, the deep-source CO2 consumed by rock chemical weathering will be released into the atmosphere with the precipitation of carbonate in the oceans. In other words, the riverine DIC may become a carbon source instead of a carbon sink. Therefore, further research is needed to conclusively determine whether basalt chemical weathering constitutes a major consumption mechanism for atmospheric CO2.