The sensitivity of cosmogenic radionuclide analysis to soil bulk density: implications for soil formation rates

Improving our knowledge of soil formation is critical so that we can better understand the first‐order controls on soil thickness and more effectively inform land‐management decisions. Cosmogenic radionuclide analysis has allowed soil scientists to more accurately constrain the rates at which soils form from bedrock. In such analysis, the concentration of an isotope, such as Beryllium‐10, is measured from a sample of bedrock. Because this concentration is partly governed by the lowering of the bedrock‐soil interface, a cosmogenic depth‐profile model can be fitted to infer the bedrock and surface lowering rates compatible with the measured concentrations. Given that the bedrock‐soil interface is shielded by soil, the cosmic rays responsible for the in‐situ production of the radionuclide are attenuated, with attenuation rates dependent on the density profile of this soil. Many studies have assumed that soil bulk density is either equal to that of the bedrock or constant with depth. The failure to acknowledge the variations in soil bulk density means that cosmogenically derived soil formation rates previously published may be under‐ or overestimates. Here, we deploy a new model called “CoSOILcal” to a global compilation of cosmogenic analyses of soil formation and, by making use of estimated bulk density profiles, recalculate rates of soil formation to assess the sensitivity to this important parameter. We found that where a soil mantle >0.25 m overlies the soil‐bedrock interface, accounting for the soil bulk density profile brings about a significantly slower rate of soil formation than that previously published. Moreover, the impact of using bulk density profiles on cosmogenically derived soil formation rates increases as soil thickens. These findings call into question the accuracy of our existing soil formation knowledge and we suggest that future cosmogenic radionuclide analysis must consider the bulk density profile of the overlying soil