Assessing the Biological Contribution to Mineralized Cap Formation in the Little Hot Creek Hot Spring System

FLOYD, JAMES G.; BEELER, SCOTT R.; MORS, R. AGUSTIN; KRAUS, EMILY A.; GEOBIOLOGY 2016; PIAZZA, OLIVIA; FRANTZ, CARIE M.; LOYD, SEAN J. ; BERELSON, WILLIAM; STEVENSON, BRADLEY S.; MARENCO, P. J. ; SPEAR, JOHN R.; CORSETTI, FRANK A.

San Francisco, California

Congreso; AGU 2016 Fall Meeting; 2016

American Geophysical Union

Hot spring environments exhibit unique redox/physical gradients that may create favorable conditions for the presence of life and commonly contain mineral precipitates that could provide a geologic archive of such ecosystems on Earth and potentially other planets.However, it is critical to discern biologic from abiotic formation mechanisms if hot spring-associated minerals are to be used as biosignatures. The study of modern hot spring environments where mineral formation can be directly observed is necessary to better interpret the biogenicity of ancient/extraterrestrial examples. Little Hot Creek (LHC), a hot spring located in the Long Valley Caldera, California, contains mineral precipitates composed of a carbonate base covered with amorphous silica and minor carbonate in close association with microbial mats/biofilms.Geological, geochemical, and microbiological techniques were integrated to investigate the role of biology in mineral formation at LHC. Geochemical measurements indicate that the waters of the spring are near equilibrium with respect to carbonate and undersaturated with respect to silica, implying additional processes are necessary to initiate cap formation. Geochemical modeling, integrating elemental and isotopic data from hot spring water and mineral precipitates, indicate that the abiotic processes of degassing and evaporation drive mineral formation at LHC, without microbial involvement. However, petrographic analysis of LHC caps revealed microbial microfabrics within silica mineral phases, despite the fact that microbial metabolism was not required for mineral precipitation. Our results show that microorganisms in hot spring environments can shape mineral precipitates even in the absence of a control on authigenesis, highlighting the need for structural as well as geochemical investigation in similar systems.