Rare earth and trace element geochemistry of termite mounds in central and northeastern Namibia: Mechanisms for micro-nutrient accumulation

Rare earth element (REE) and trace element concentrations of ten termite mounds and adjacent topsoil from central and northeastern Namibia were used to investigate the processes underlying the alteration of soil chemical and physical properties by termites (Macrotermes spp.). Accumulation of micro-nutrients in the mounds was of particular interest because of the ecological implications of the enhanced availability of these scarce elements. The absolute concentrations of REE and trace elements, including nine micro-nutrients (B, Fe, Mn, Ni, Cu, Zn, Se, Mo and Cd), were greater in termite mounds compared to topsoil, suggesting a possible external supply of enriched materials or accumulation of in situ weathering products of the underlying bedrock. Similarities between the chondrite normalized REE patterns of the samples and those of the average upper continental crust (UCC) and Post-Archaean Average Australian Shale compositions indicated that the mounds and topsoil at each site originated from the same host bedrock. Coherent behavior between two incompatible elements (Th and La) in both topsoil and mound samples suggested that the two sets of samples were exposed to two different weathering processes. The relatively low Zr/Sc ratios, fine particle sizes and the persistence of positive Eu anomalies of the mounds relative to UCC were interpreted as reflecting (1) a lack of grain sorting effects and (2) active transport of materials rich in primary clay minerals and trace elements from the deeper regolith to the surface by termites. Lower permeability of mound materials may also prevent leaching of primary minerals and trace elements. Titanium and Zr enrichment in most mound samples (8 samples) was attributed to the decomposition of biotite by Macrotermes with the concomitant release of Ti and Zr into the mounds, suggesting the termites' ability to transform soil minerals and enhance chemical weathering. This was further corroborated by a substantial increase in Th/U ratios in most mounds relative to topsoil. Because of the oxidizing conditions of the mounds, Ti and Zr are likely to be adsorbed onto Fe-oxides. Positive Ce anomalies relative to UCC and the high pH values in the mounds indicate calcrete aggradation or mound aeration as a result of burrowing activities of the termites and evaporation from mounds. The impacts of the changes in environmental conditions and the termite activities on the mound geochemistry are highlighted by heavy REE (HREE) as well as Fe and Mn retentions in the mounds. Heavy REE and light REE (LREE) are likely to coprecipitate with carbonate complexes and Fe- and Mn-oxyhydroxides, and in the process, scavenge a series of micro-nutrients such as Zn, Cu, Cd and Co. A substantial enrichment of Se in mounds, despite the oxidizing conditions of the mounds, could be attributed to active transport of Se-enriched materials from deep soil horizons. The study demonstrated the suitability of REE and trace element geochemistry for assessing the influence of Macrotermes on the physico-chemical properties of semi-arid soils.