Abstract
AbstractThe study of hydrothermal travertines contributes to the understanding of the interaction between physico‐chemical processes and microbial mats in carbonate precipitation. Three active travertine sites were investigated in Central Italy to characterise the types of carbonate precipitates and the associated microbial mats at varying physico‐chemical parameters. Carbonate precipitated fabrics at the decimetre to millimetre‐scale and microbial mat composition vary with decreasing water temperature: (a) at relatively higher temperature (55–44°C) calcite and aragonite crystals precipitate on microbial mats of Chloroflexi and sulphur‐oxidizing microbes forming filamentous streamer fabrics with sparse cyanobacteria, (b) at intermediate temperature (44–40°C), rafts, coated gas bubbles and dendrites are associated with Spirulina cyanobacteria and other filamentous and rod‐shaped cyanobacteria, (c) low temperature (34–33°C) laminated crusts and oncoids forming in a terraced slope system are associated with diverse Oscillatoriales and Nostocales filamentous cyanobacteria, Spirulina and diatoms. At the microscale, carbonate precipitates are similar in the three sites consisting of prismatic calcite crystals organised in radial rosettes or fibrous aragonite spherulites (40–300 µm in diameter), overlying or embedded in Extracellular Polymeric Substances. Clotted peloidal micrite dominates at temperatures <40°C, also encrusting filamentous microbes. Carbonates are associated with gypsum crystals; extracellular polymeric substances are enriched in silicon, aluminium, magnesium, calcium, phosphorous and sulphur; authigenic aluminium‐silicates form aggregates on Extracellular Polymeric Substances. This study confirms that microbial communities in hydrothermal settings vary as a function of water temperature. Carbonate precipitates at the microscale are similar in the three settings, despite different microbial communities, suggesting that travertine precipitation, driven by carbon dioxide degassing, is influenced by biofilm extracellular polymeric substances acting as a substrate for crystal nucleation (Extracellular Polymeric Substances‐mediated mineralization) and affecting the resultant fabric types, independently from specific microbial community composition and metabolism.
Highlights
Travertines are terrestrial carbonates precipitated in hydrothermal settings (Capezzuoli et al, 2014; Pedley, 1990), which thrive with microbial mats comprising a wide spectrum of thermophilic Archaea and Bacteria, including sulphide-oxidizing bacteria, sulphate-reducing bacteria, anoxygenic phototrophs, oxygenic photosynthetic cyanobacteria and eukaryotic algae (Farmer, 2000; Fouke et al, 2003; Konhauser, 2007; Pentecost, 2003, 2005)
Carbonate precipitated fabrics at the decimetre to millimetre-scale and microbial mat composition vary with decreasing water temperature: (a) at relatively higher temperature (55–4 4°C) calcite and aragonite crystals precipitate on microbial mats of Chloroflexi and sulphur-oxidizing microbes forming filamentous streamer fabrics with sparse cyanobacteria, (b) at intermediate temperature (44–40°C), rafts, coated gas bubbles and dendrites are associated with Spirulina cyanobacteria and other filamentous and rod-shaped cyanobacteria, (c) low temperature (34–33°C) laminated crusts and oncoids forming in a terraced slope system are associated with diverse Oscillatoriales and Nostocales filamentous cyanobacteria, Spirulina and diatoms
Carbonate precipitates at the microscale are similar in the three settings, despite different microbial communities, suggesting that travertine precipitation, driven by carbon dioxide degassing, is influenced by biofilm extracellular polymeric substances acting as a substrate for crystal nucleation (Extracellular Polymeric Substances-mediated mineralization) and affecting the resultant fabric types, independently from specific microbial community composition and metabolism
Summary
Travertines are terrestrial carbonates precipitated in hydrothermal settings (Capezzuoli et al, 2014; Pedley, 1990), which thrive with microbial mats comprising a wide spectrum of thermophilic Archaea and Bacteria, including sulphide-oxidizing bacteria, sulphate-reducing bacteria, anoxygenic phototrophs, oxygenic photosynthetic cyanobacteria and eukaryotic algae (Farmer, 2000; Fouke et al, 2003; Konhauser, 2007; Pentecost, 2003, 2005). The oldest reported terrestrial travertine deposits, attributed to deep-sourced carbon dioxide fluids at high temperature, are hosted in the Palaeoproterozoic (ca 2.2 Ga) Kuetsjärvi Sedimentary Formation (Pechenga Greenstone Belt; Fennoscandian Shield; Brasier, 2011; Melezhik & Fallick, 2001; Salminen et al, 2014). These dolomitic travertines and stromatolites might have formed through similar processes to present- day hydrothermal springs, but they do not contain traces of organic carbon (Salminen et al, 2014). The oldest siliceous deposits attributed to terrestrial thermal springs are known from the ca 3.5 Ga Dresser Formation (Pilbara, Western Australia) and were interpreted as putative geyser environments (Djokic et al, 2017)
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