Intrinsic Flaws of Element Enrichment Factors (EFs) in Environmental Geochemistry

Abstract

The popular concept of enrichment factors (EFs) as a means of identifying and quantifying human interference with global element cycles is evaluated. It is demonstrated that the concept of normalizing element concentrations to an average total crust value is of doubtful merit, for theoretical considerations alone. Serious flaws with EFs include the variable composition of the Earth's crust at any given point compared to the global average, the natural fractionation of elements during their transfer from the crust to the atmosphere through processes such as weathering and winnowing of fines, and the differential solubility of minerals in the weak chemical digestions generally used in environmental studies. Furthermore the impact of biogeochemical processes is neglected. A biomonitoring survey of atmospheric chemistry over a 1.5 million km2 area in northern Europe is used to show that EFs are governed here by completely different processes than anthropogenic activities. As a simple means to assess the maximum possible interference of human activities with global element reservoirs, the mining interference factor (MIF = (total tonnage mined annually/total tonnage in upper 2 km of crust) × 100%) is introduced. The elements C ≫ Br > Cu > Sb ≫ Ag > Cd > Pb show the highest MIFs, while Ga, Cs, Ge, K, In, I, and Rb show very low MIFs. We recommend indiscriminate usage of EFs in the scientific literature be abandoned and more large-scale surveys be supported internationally to collect quality, statistically significant environmental data. Only thus can human interference with natural biogeochemical cycles be properly documented.