Dr. Bruce W. Fouke

Biocomplexity and the Emergence of Terraced CaCO₃ Mineral Deposits during Microbe-Water-Mineral Interactions at Mammoth Hot Springs, Yellowstone National Park USA

Bruce W. Fouke, Department of Geology, Department of Microbiology, and Institute of Genomic Biology, University of Illinois Urbana-Champaign, 1301 W. Green Street, Urbana, IL 61801 USA (fouke@uiuc.edu)

A systems-scale study is underway to determine the extent to which microorganisms may influence the process of CaCO₃ mineral precipitation. This work is being done in collaboration with Nigel Goldenfeld (Illinois Physics) and Alison Murray (Desert Research Institute Microbiology). Integrated geology, microbiology, physics, hydrology, and chemistry analyses are underway at Mammoth Hot Springs (Fig. 1) to determine how the diversity and activity of specific living microbes and microbial communities influence the shape, morphology, and chemistry of carbonate mineral deposits (called travertine). Terraced travertine is universally observed in high-temperature and low-temperature springs in terrestrial, marine, and subterranean settings. Thus results will be applicable to a broad spectrum of paleoenvironments and will permit more accurate reconstruction of the history of ancient microbial life from the rock record.

Our initial work at Angel Terrace in the Mammoth Hot Springs complex established that spring drainage systems are composed of five distinct ecological zonations (termed depositional facies). Each facies has been identified based on the crystal form and chemistry of the travertine deposits, include the vent, apron channel, pond, proximal slope, and distal slope facies (Fig. 2). Water chemistry analyses, conducted within the depositional facies framework, indicates that the aqueous chemistry of the hot spring drainage system is "dominated" by CO₂ degassing and dropping temperature (e.g. Rayleigh-type fractionation calculations of spring water ?¹³C versus dissolved inorganic carbon (DIC). While these physical factors help drive the rapid precipitation of carbonate crystals to deposit travertine at rates as high as 5 mm/day, they are not the exclusive controls on precipitation. Significant biological controls have been identified with petrography (e.g. crystals entombing and preserving the shape of filamentous Aquificales bacteria; Fig. 3) and by quantitative subtraction of degassing and temperature effects on ?¹³C and ?¹⁸O isotopic fractionation. These robust disequilibrium signatures may be biologically mediated and systematically increase in magnitude with movement from the high (73°C) to the low (?25°C) temperature facies. Culture-independent 16S rRNA gene sequence screening revealed that 187 bacterial species representing 21 divisions live in the drainage system, and these bacteria exhibit a remarkable 90% partitioning between facies (Fig. 4). Thermodynamic saturation state of the spring water stops increasing at the pond facies.

Ongoing work includes: 1) performing in situ crystallization experiments to determine the form and chemistry of travertine deposited when the microbes have been filtered and UV-irradiated, sterilization techniques that leave many fundamental physical and chemical parameters of the spring water relatively unchanged (Figs 5); 2) documenting associations amongst water chemistry, calcite mineral deposition (crystal growth form, distribution, and chemistry) with microbial form, diversity and metabolic activity (Figs. 6 and 7); and 3) quantitative modeling of carbonate mineral precipitation to describe the combined effects of geological and biological processes (Fig. 8). Results suggest that travertine deposits record an integrated mixture of physical, chemical, and biological influences that can only be identified within the context of water cooling and degassing CO₂ along the spring drainage outflow (within the facies model). Abiotic processes are more influential in cm-to-m scale gross morphology of the mineral deposits, while biological signatures are best recorded in ultra-structural (micron-to-cm scale) crystal nucleation, growth morphology, and chemistry.