The relative importance of grain size and mineral weatherability for enhanced rock weathering rates: a comparison of glacial rock flour and basaltic feedstocks

There is currently no consensus in the literature as to whether the mineralogical composition and theoretical weatherability or grain size and surface area of feedstock materials used for carbon capture via enhanced rock weathering are stronger determinants of weathering rates of silicate minerals applied to agricultural soils. Felsic source rocks have typically been discounted for enhanced rock weathering in favor of more easily weatherable mafic and ultramafic rocks. However, previous work has indicated that Greenlandic glacial rock flour, a potential feedstock with an exceedingly fine grain size (d50 = 2.6 µm) but a felsic composition, can weather at sufficiently rapid rates to be effective for carbon capture and improving crop yields through the release of nutrients during weathering. Here we present initial experimental results comparing the use of Greenlandic glacial rock flour and several sources of basaltic material as feedstocks for enhanced rock weathering. Two field trials installed in Ghana and South Carolina demonstrate the varying effects of these materials on maize yield, and two flow-through laboratory experiments, one with plants and one without, assess the differences in alkalinity generation and cation release between these feedstocks over time. Among the tested basalts, chemical composition seems to be a stronger driver of weathering rates than differences in grain size. However, none were as finely ground as the glacial rock flour, which was found to weather at a rate comparable to or, in some cases, higher than the basaltic materials, despite being composed of less chemically reactive minerals. These results suggest that mineral weatherability is an important predictor of weathering rates, but with a large enough difference in grain size the amount of surface area available for reaction can be equally important.