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

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Julie Cosmidis, Shane O'Reilly, Eric T. Ellison, Katherine L. Crispin, David Diercks, Alexis S. Templeton


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.



Geochemistry, Geology, Sedimentology


CCS, Travertines, pisoids, CCS, iron oxidation, frutexites, Pisoids, Frutexites


Published: 2021-07-07 21:11

Last Updated: 2021-07-08 04:11


CC BY Attribution 4.0 International

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Conflict of interest statement:

Data Availability (Reason not available):
The data supporting the findings of this study are openly available in the supplied supplementary figures and tables.

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Comment #37 Julie Cosmidis @ 2021-07-09 17:16

Thanks a lot Valentin for the insightful comments and advice! Please let us know what you think of the paper after you read it.

Comment #36 Valentin Zuchuat @ 2021-07-09 16:50

Hei Julie, nice to see some people publishing on Crystal Geyser! I hope you got to enjoy the most incredible taco truck of all time of the whole world (I'll give alien life the chance to make better tacos, but that is going to be tough). I will give the manuscript a proper read over the weekend. You might want to consider adding a map of North America and Utah (both equally important, not just Utah) to Fig. 1, in order to show where Crystal Geyser actually is. I was lucky to there, so I know where it is, but for the sake of other readers, you might want to add such information to Fig. 1. You might also want to add a cross-section and mention that the CO2 is migrating along Little Grand Wash Fault (might also get your paper a little extra traction for the people working on that system/area). Cheers Valentin Zuchuat Postdoctoral Researcher University of Oslo Norway