CO2-RICH SPRINGS AND TRAVERTINES OF THE ROCKY MOUNTAIN REGION: MANTLE HELIUM ASSOCIATED WITH THE ASPEN ANOMALY AND GEOMICROBIOLOGY OF "CONTINENTAL SMOKERS”

We report geochemical and microbial results from a suite of hot and cold springs, and CO₂ geysers in traverses across the Aspen anomaly.  The Aspen anomaly is a low-velocity mantle domain coinciding with the intersection of the Rio Grande rift and the Colorado Mineral belt.  Quaternary travertine and active CO₂-rich springs in the study region reflect a long-lived interaction of neotectonics, deeply-sourced (endogenic) fluids and the surficial hydrologic regime.  He isotope and water chemistry data show mixing between surface water recharge and inputs from deep crustal and mantle sources. Endogenic components have high ³He/⁴He, high CO₂, and high salinity compared to recharge waters; they also exhibit elevated trace element concentrations (e.g., F, Li, As, U). Spatial correlation of helium isotope data with mantle velocity structure determined from tomographic studies suggest heterogeneous mantle degassing, with highest ³He/⁴He in surface waters (hence highest mantle helium contribution) corresponding to regions of lowest mantle velocity. Gas compositions are dominated by CO₂ (up to 99 %).  Aqueous geochemical modeling indicates that considerable external carbon is present (up to 0.02 mol/L; external carbon = HCO₃-(Ca+Mg) with an adjustment for possible gypsum contribution to the calcium term). Hydrogen and reduced sulfur are present, as well as elevated He (low N₂/He) and helium isotope values as high as 2.17 R_A (R_A= air ³He/⁴He) indicating a mantle input. CO₂/³He ranges from 10⁷ to 10¹², and d¹³C values range from -2 to -10‰. These springs represent a flux of deeply-sourced carbon to the atmosphere (at 1800 l/s, Glenwood Springs emits 10⁹ mol C/yr). High CO₂ abundance, hydrogen, and a suite of redox-sensitive trace components offer a chemically-rich setting for a diverse microbial community. These systems are geochemically analogous to the chemolithotrophic microbial ecosystems associated with mid-ocean ridges (hence “continental smokers”).  Analysis of microbial community small sub-unit ribosomal RNA genes by DNA sequencing is underway to test for differences among the sites and the presence of microorganisms utilizing similar metabolic pathways to those found in oceanic hydrothermal settings.