2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 300-7
Presentation Time: 10:30 AM

USING CARBON ISOTOPES TO ASSESS BIOGENICITY OF MIXED-COMPOSITION ACCRETIONARY STRUCTURES FROM A THERMAL SPRING (LITTLE HOT CREEK, CALIFORNIA, USA)


DEMOTT, Laura M.1, PIAZZA, Olivia2, BERTRAN, Emma3, BONIS, Ben4, FRANTZ, Carie M.5, LOYD, Sean J.6, CORSETTI, Frank A.2 and SPEAR, John R.7, (1)Department of Geology, University of Illinois at Urbana-Champaign, Champaign, IL 61820, (2)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, (3)Harvard University, Cambridge, MA 02138, (4)University of Minnesota - Twin Cities, Minneapolis, MN 55455, (5)Applied Physics Laboratory, University of Washington, Seattle, WA 98105, (6)Department of Geological Sciences, California State University Fullerton, Fullerton, CA 92831, (7)Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887

The Long Valley Caldera is an actively volcanic, silicic caldera located on the eastern flank of the Sierra Nevada range in California. The region is known for the numerous geothermal features present, including hot springs and fumaroles. Little Hot Creek is a small stream fed by several hot springs located on the eastern edge of a resurgent dome in the central caldera. Spring waters emerge at ~80˚C, and flow down small channels until merging into the larger stream. These small channels are lined with shelf-like accretionary structures composed of calcium carbonate and silica, which precipitate at the air-water interface as well as below the water surface. Study of the composition and petrography of the structures (see Piazza et al., this meeting), spring water saturation states, and consideration of probable microbial metabolisms suggest that microbial activities could potentially influence precipitation. In order to further assess the biogenicity of these unique mixed-composition accretionary structures, evaluation of the δ13C composition of spring waters and minerals was conducted as part of a research project undertaken by the International Geobiology Course in June-July 2014. The δ13C composition of spring water TCO2 shows that spring waters are enriched by ~0.5‰ compared to an abiotic, equilibrium Rayleigh degassing model. Additionally, the δ13C values of the bulk carbonate of the structures are enriched 1-2‰ above equilibrium precipitation values from spring waters. These enrichments may result from kinetic effects associated with rapid degassing of CO2 from spring waters, rapid mineral precipitation rates, and/or from microbial carbon fixation. Geochemical modeling of the spring water evolution and the water-rock-microbe interactions present in the Little Hot Creek system lends insight into the processes affecting precipitation of these unusual accretionary structures.

Session No. 300

T135. New Developments in Microbialites (Posters)
Wednesday, 22 October 2014: 9:00 AM-6:30 PM
Exhibition Hall C (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.726
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