Rare Earth Elements and their Role in Assessing Soil Genesis.

            The rare earth elements (REE) are commonly defined as lanthanum (La) and the 14 elements comprising the Lanthanide series: cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm) ytterbium (Yb), lutetium (Lu). Many authors also include yttrium (Y). The abundances of the REE’s typically obey the Odden-Harkin rule; this states that an element with an even atomic number has a greater concentration than the next element in the Periodic Table. The REE’s typically exhibit trivalent oxidation states; however, europium may also occur as Eu²⁺ and cerium may occur as Ce⁴⁺. The REE’s ionic radii decrease on progression from La to Lu, which results in a slight but predictable change in their chemical affinity. The importance of the REE’s arises from there chemical similarity; such that, the trends in the REE abundances vary predictably with changes in parent materials, soil formation and human impact. Typically, the light REE (La to Sm) reside in trace minerals such as apatite, epidote and allanite, whereas the heavy REE (Gd to Lu) are associated with minerals such as zircon.  Investigations in North America, Africa, Asia, and Europe show that the REE are depleted in near-surface horizons and accumulate in deeper horizons or the regolith as clay-oxyhydroxide adsorbates or REE-phosphate precipitates, with the heavy REE accumulating in the deeper soil regions to a greater extent than the light REE. The degree of interhorizon transport has great potential to become an index of weather intensity. Additionally, discrete abundance changes in the Eu concentrations relative to the concentrations of Sm and Gd (Eu anomalies), have been shown to indicate parent material differences and mineral provenance. Cerium anomalies have been used to estimate the intensity or confirm the presence of oxidation-reduction soil processes.