V51C-0371: Columnar travertines: bio-influenced gen - esis, Porcelana Geysers, Northern Patagonia, Chil e Bárbara Ruiz-Velásquez [1]; Diego Morata [1]; Linda Daniele [1]; Beatriz Díez [2] [1] Department of Geology and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile [2] Department of Microbiology Pontificia Universidad Católica de Chile Porcelana Geysers are located on the slopes of Barranco Colorado volcano, southern Chile, and is characterized by having a lateral hydrothermal fluid transport and an im- portant CO2 content, having high gas exsolution rates on the surface at temperatures above 80°C. But it does not seem to be enough to explain the genesis of columnar travertines more than 2 meters high, considering that precipitation rates counteracts erosive rates due to rainy climate of the zone and the high slope gradient (>10 mm/h). The presence of thermophilic microorganisms revealed by EPS (exopolysaccharide) and microscopic textures together with the redox conditions of the hot springs waters, could explain the genesis of the exceptional travertine morphology found in Porcelana by a biological contribution. Abstract Introduction Travertine deposits are very common not only in marine environments but also they accumulate in continental depositional settings (Pentecost, 2005). Some morpholo- gies are usual to see them forming terraces, mounds, cascades and dams, and it is possible to find them associated to springs along fault systems. Southern Chile is a high favorability zone to find geothermal systems. However, travertine deposits in Los Lagos district are rare except for Porcelana Geysers, which present anomalous travertine pinnacles up to 2.5 m height (Fig. 1). Many factors could be contributing to the growth of these singular structures, physicochemical settings that control CO2 exsolution (high temperature groundwaters and bubbles formation) (Asta et al., 2017; Jones, 2017; Ladd and Ryan, 2016; Pentecost, 2005) and biological settings as microbial activity (Capezzuoli et al., 2014; Fouke, 2011; Fouke et al., 2003; Gao et al., 2013; Okumura et al., 2011; Okumura et al., 2013). In consequence, this study is relevant to consider not only physicochemical factors but also biological conditions to understand the different morphologies of travertine deposits and the fragile ecological balance existing in this zone. FONDAP Fondo de Financiamiento de Centros de Investigación en Áreas Prioritarias A1 A0 d18O ‰ (PDB) d13C ‰ (PDB) 0 -5 -25 -20 -4 -3 -2 -1 -15 -5 -10 1 2 3 4 5 6 7 8 9 10 11 Hydrothermal travertine Fluvial tufa Geological settings Porcelana Geysers are located in southern Chile, Los Lagos dis- trict. This area belongs to the Southern Volcanic Zone (Stern, 2004) and both volcanoes and springs are distributed along two mean fault systems: N-S trend Liquiñe-Ofqui Fault System (LOFS) and sinistral NW-SE trend Arc oblique Long-lived basement Fault System (ALFS). North Patagonian Batholith (NPB) and Bahía Mansa Metamorphic Complex (BMMC) are the mean lithological units (Duhart et al., 2001). Porcelana Geysers are located over Barranco Colorado Volcano slope, in the Peninsula of Huequi, over ALFS and LOFS structures therefore anomalous permeability is a possibility for this area. Travertines In the geysers area dthere are different geometries of travertines: smooth slopes, small cascades and waterfall in millimeter-scale and numerous pinnacles which vary from few centimeters to 2.5 m height. Also, there is abundant exopolysaccharide (EPS) on the travertine surface (cyanobacteria) (Mackenzie et al., 2013). The average rate of travertine precipitation is 0.5-0.8 cm/year, and in the spouters it is even higher (>1.5 cm/year). An inactive pinnacle (21 cm long and 10 cm diameter) was ob- tained. 4 layers was recognized according to their color (Fig. 2). Fi- brous aragonite (Fig. 3) and silica are the most present minerals in all of them except for the layer A1, where calcite and the silica (as tridymite) were found. An unidentified filamentous thermophilic organism was observed in layer A1 (Fig. 4). Negative δ13C and δ18O values were measured (Fig. 5). Figure 5. Isotopic ratios compared to hydrothermal travertine and tufa ratios (Della Porta, 2015). A0 corre- spond to EPS layer, and A1 to the outermost travertine layer Figure 2. Travertine pinnacle sampled. Figure 1. Travertine pinnacles form Porcelana Geysers. Hydrogeochemistry The hot waters are clasified as chloride waters with volcanic fluids input, which are mixing with groundwaters enriching boron and bromine (Ruiz and Morata, 2016) and fjord influence. The higher temperatures were recorded in the emerging waters of the pinnacles zone, measuring between 85.1 and 81.4°C, while the proximal and distal hot springs recorded 65,7°C and 53.4°C on average, respectively. All the waters present reducing conditions (from -284 to -71.1 mV, decreasing in the time). The values of partial pressure of CO2 in all hot water samples are near 0.0 bar (0,00371 – 0,06165 bar), all of them lower than pCO2 of CO2-enriched springs waters which use to be higher than 1 bar (Minissale et al. 2004). Saturation indices of arago- nite and calcite were calculated using LLNL databse (Appelo and Postma, 2005), which are very low in all waters, however only in the geysers they are positive (0.86 and 1.00 on average, respectively. Discussion and conclusion After carbonate precipitation three elements are generated: travertine deposit, depleted water and steam, thereby, joining their chemical compositions is possible to have an idea about the deep fluid that is generating these travertine pinnacles. Thus, is important to consider that the analyzed water correspond to the remaining fluid after first carbonate precipitation stage, which let us to suppose that the mean cause of high precipitation rate is a very high CO2 exsolution during the precipitation of travertine pinnacles. According to water chemistry, a volcanic origin of CO2 could be proposed. However, by observing the δ13C values these deposits cannot be classified as hydrothermal travertine, which use to have positive δ13C values (Della Porta, 2015). This means that CO2 ex- change from the hydrosphere to the atmosphere is happening possibly caused by high rainfall rates of the zone. Respect to microbial presence, all indicates that the microbial cells are present during carbonate precipitation, particularly in the pinnacles vents, be- cause the carbonate textures show dark microcrystalline bands and needles among radi- ating acicular aragonite crystals formed by rapid gas exsolution (Fig. 3), thus CO2 exsolu- tion occurs contemporaneous to the presence of microbial filaments (i.e. cyanobac- teria), which are being replaced by CaCO3 (Fouke, 2011). This microbial activity increas- es the carbonate precipitation rates (Fouke, 2011; Kandianis et al., 2008), contributing to high pinnacles formation. The unidentified filamentous thermophilic organism that was found in the outermost layer could represent a current activity of cyanobacteria, which could be dissolving and reprecipitating calcium carbonate. Figure 4. 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