Biomineralization of Pedogenic Carbonate at Multiple Scales in Space and Time 

Biomineralization is the process by which biological organisms make minerals. The process can be divided into two mechanisms. (1) Biologically controlled biomineralization that consists of internal tissue evolved to combine cations and anions into minerals, such as bone or shell. (2) Biologically induced biomineralization whereby an organism creates an external chemical environment conducive to mineral formation, such as the precipitation of calcite in the mucilaginous sheaths of fungal hyphae.At the microbial scale, thin section and electron microscopy in combination with both lab and field experiments provide evidence that the formation of pedogenic carbonate is not solely a physicochemical process, but is rather a biogeochemical process. On the other hand, a similar body of evidence using microscopy, laboratory, and field observations show no evidence of biomineralization. Likewise, at profile scale, biomineralization of pedogenic carbonate is not apparent with the exception of occasional rhizoliths. Instead, pedogenic carbonate reflects the depth of wetting and shows that progressively older soils often have progressively greater morphogenetic amounts of carbonate.At yet a broader scale, however, the biogeochemical nature of pedogenic carbonate again becomes apparent with the observation that the driest deserts which are largely barren of the effects of vegetation on soil moisture, CO2, pH, and available Ca2+ are also largely barren of pedogenic carbonate. This would be a third type of biomineralization—bioclimatically induced. This evidence leads to the hypothesis that pedogenic carbonate is facultatively induced by organisms when given the prerequisite geochemical environment of Ca2+, pH, CO2, and H2O, (i.e., removing vegetation removes carbonates). If so, it is another example of the importance of biology on soil formation occurring now and in the geological past.