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sur le développement des zones arides et semi-arides

Titre : Geochemical characteristics and microbial diversity of CO2-rich mound springs of the Tierra Amarilla anticline, New Mexico

Auteur : Cron, Brandi

Université de soutenance : University of New Mexico

Grade : Master of Science (MS) 2012

Résumé
This interdisciplinary research explores the water geochemistry, microbial heterogeneity, geochemical cycling, and paleohydrological longevity of a unique set of desert springs in north-central New Mexico. This series of travertine-depositing CO2-rich springs is located along the boundary zone between the Colorado Plateau and Rio Grande rift, on the southern extension of the Nacimiento fault at the Tierra Amarilla anticline (TA). Geochemical data on the spring waters show 3He/4He ratio of 0.17-0.20 RA which indicates the presence of mantle-derived helium in the groundwater. Carbon isotope values range from -4.6 to -8.1 per mil, suggesting that some of the CO2 is also deeply derived (endogenic). Based on an average of 8 samples, water chemistry modeling indicates CO2 is 5.49 ± 8 % from dissolved carbonate in the aquifer (Ccarb), 4.77 ± 9.16 % from organic derivation (Corg), and 89.74 ± 11.22 % from deep sources (Cendo). Stable isotope analysis of the waters suggests that these warm springs have a component perhaps related to distal effects of the Valles Caldera hydrothermal system with fluid transport up and along extensional faults networks near the boundary of the Rio Grande rift. Microbiological techniques were employed to investigate these waters and determine species diversity and groundwater microbiology. Microbiological species identified include Zetaproteobacteria, Vibrio diaztrophicus, and Algoriphagus sp. LYX05, and Candidatus ’Nitrotoga.’ The Zetaproteobacteria are also found in submarine seamount communities and Vibrio diaztrophicus are also found in other subsurface environments. 454-sequence analyses of Archaeal species determined that the phylotype Methanosarcinales (which is the dominate single phylotype found in Lost City hydrothermal field) is also found in the TA system. The presence of similar terrestrial communities in TA warm springs is interpreted to reflect endemic ecosystems that have evolved in ambient-temperature terrestrial springs because metabolic processes are similar to chemolithotrophs found in deep-sea vents. In order to link the metabolic environment of these springs to the geochemistry we calculated the chemical affinity for metabolically favorable reactions. Results show that oxidation of hydrogen, manganese, and hydrogen sulfide which are all metabolic reactions similar to those utilized by the Lost City Hydrothermal field communities are thermodynamically favorable in the springs at TA. Longevity of this spring system was investigated using U-series dating. The highest elevation extinct mounds ranges in age from 270 to 210 ka ; lower elevation mound systems range in age from 105-70 ka ; travertines around the active springs return ages of about 600 years. These data demonstrate that the microbial communities have evolved due to persistent flux of deeply sourced, warm, anoxic fluids through the neotectonically active Tierra Amarilla fault system over the last hundreds of thousands of years, with water flux (and episodic deposition of travertine) likely mediated by changing climate. Existing dates highlight travertine deposition during climate transitions from OIS 8-to-7 and 6-to-5, as well as Holocene changes.

Mots Clés : Geomicrobiology, geochemistry, molecular, Methanosarcinales, geothermal, Rio Grande rift

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