Carbonate polymorphism controlled by microbial iron redox dynamics at a natural CO2 leakage site (Crystal Geyser, Utah)

Crystal Geyser (Utah, USA) is a CO2-rich low-temperature geyser that is studied as a natural analog for CO2 leakage from carbon capture and storage (CCS) sites. In order to better constrain the biogeochemical processes influencing CaCO3 precipitation at geological CO2 escape sites, we characterized fast-forming iron-rich calcium carbonate pisoids and travertines precipitating from the fluids expelled by the geyser. The pisoids, located within a few meters from the vent, are composed of concentric layers of aragonite and calcite. Calcite layers contain abundant ferrihydrite shrubs in which iron is encasing bacterial forms. The aragonite layers contain less abundant and finely dispersed iron, present either as iron-oxide microspherules or iron adsorbed to organic matter dispersed within the carbonate matrix. We propose that carbonate polymorphism in the pisoids is mostly controlled by local fluctuations of the iron redox state of the fluids from which they form, caused by episodic blooms of iron-oxidizing bacteria. Indeed, the waters expelled by Crystal Geyser contain >200 µM dissolved iron (Fe2+), a known inhibitor of calcite growth. The calcite layers of the pisoids may record episodes of intense microbial iron oxidation, consistent with observations of iron-oxide rich biofilms thriving in the rimstone pools around the geyser and previous metagenomic analyses showing abundant neutrophilic, microaerophilic iron-oxidizing bacteria in vent water. In turn, aragonite layers of the pisoids likely precipitate from Fe2+-rich waters, registering periods of less intense iron oxidation. Separately, CaCO3 polymorphism in the travertines, where calcite and aragonite precipitate concurrently, is not controlled by iron dynamics, but may be locally influenced by the presence of microbial biofilms. This study documents for the first time an influence of microbial iron oxidation on CaCO3 polymorphism in the environment, and informs our understanding of carbonate formation at CO2 leakage sites and in CCS contexts.