Formation of trace element biogeochemical anomalies in surface soils; the role of biota

Understanding the mechanisms by which soil geochemical anomalies form is extremely relevant to exploration for buried mineral deposits. Most mechanisms proposed in the geological literature are based on abiotic processes such as physical transport, hydrodynamic dispersion, or immobilization along chemical (e.g. redox) gradients. However, trace elements could also become locally enriched in soils above ore bodies as a result of the activity of plants and soil organisms. In this work, we review literature on trace element cycling in arid and semi-arid terrestrial ecosystems, and develop a conceptual model of trace element biogeochemical cycling. This model incorporates the biogeochemical fluxes of elements expected to affect the formation of soil anomalies through biological means. A simplified mass-balance calculation, based on the conceptual model, was used to evaluate the likelihood of forming soil anomalies solely through plant uptake. The existence of soil geochemical anomalies depends on the balance between ecosystem net primary productivity and trace element losses through soil erosion and leaching. The calculation predicts that accumulation in soils should be favoured for more bioavailable elements, since the plant:soil concentration ratio affects the relative sizes of uptake and erosion fluxes. We suggest that soil geochemical anomalies are transient geological features, forming and dispersing as a result of the relative sizes of the accumulative and loss fluxes. Due to the complexity of trace element biogeochemical cycles, the simple mass-balance approach that we used is insufficient to account for the interaction between fluxes. This work provides the groundwork for a more complex model of trace element biogeochemical cycling that would include rate expressions and simulate the balance between multiple interacting fluxes.