Spherulites in Calcrete Laminar Crusts: Biogenic CACO3 Precipitation as a Major Contributor to Crust Formation

Spherulites (calcitic fibro-radial spherulitic polycrystals) are a major component of calcite layers in Quaternary calcrete laminar crusts. To explain the formation of spherulites, petrographic and chemical studies were performed on Pleistocene calcrete laminar crusts, Holocene biological crusts, and laboratory (in vitro) cultures of cyanobacterial strains. Spherulites were found to be formed of acicular and radiating crystals, either smooth and regular-edged, or twisted and irregular-edged. Spherulites were composed of low-Mg calcite, the concentration of Mg in the spherulites increasing from nucleus to periphery. The shape and chemistry of the in vitro spherulites and laminar crust spherulites were identical. The structure, elementary crystal shape, and size of spherul tes can be used to differentiate them from Microcodium, with which they are commonly confused in the literature. Spherulite genesis can be linked to cyanobacterial activity involving photosynthetic uptake of HCO-3, leading to CaCO3 precipitation, the bicarbonate causing release of equivalent amounts of OH- in the mucilaginous sheath. The main steps in cyanobacteria biomineralization producing spherulites are: (1) uptake of CO2 and/or HCO-3 from the medium as the inorganic carbon source for photosynthesis, (2) release of OH- in the sheath, (3) carbonate' ion formation resulting from the reaction of OH- and HCO-3, followed by (4) CaCO3 precipitation. This process involves two steps in desert environments: during dry spells, the cyanobacterial mat calcifies, leading to spherulite production and the fo mation of thin, clear microsparitic layers. During wet periods, a thin sedimentary deposit forms, mixing micrite, detrital particles, and inherited or new tiny spherulites with the mucilaginous mat of cyanobacteria, which traps all of this material. An organo-micritic layer forms. During the following dry spell, the process is repeated and the laminar crust is formed. In conclusion, the presence of spherulites in the laminar crust is indicative of subaerial exposure, the crust being a terrestrial stromatolite.