The influence of microbial mats on travertine precipitation in active hydrothermal systems (Central Italy)

Abstract

The 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.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

“…Subsequently, the crystalline crusts with coated gas bubbles change into a microterraced (mm to cm-scale) system (Figure 3E), which forms cmwide pools containing clotted peloidal micrite dendrites, coated grains, and coated gas bubbles. Sparse botryoidal and spherulitic aragonite only occur in the travertines precipitated along the first 5 m adjacent to the vent where temperature is > 44°C (Della Porta et al, 2021).…”

“…Folk, 1994;Pentecost, 1995). Several studies have shown that travertine hydrothermal settings host diverse microbial mats as a function of water temperature and chemistry, including thermophilic archaea, sulfide-oxidizing bacteria, purple and green sulfur bacteria, filamentous green non-sulfur anoxygenic photoautotroph Chloroflexi, sulfate-reducing bacteria and cyanobacteria (Folk, 1993;Allen et al, 2000;Farmer, 2000;Pentecost, 2003, Pentecost, 2005Fouke, 2011;Della Porta et al, 2021).…”

“…Actively forming travertine deposits comprise a ca 10 m thick mound with a central vent and steeply dipping slopes at Bagni San Filippo (Della Porta et al, 2021). The central vent consists of a horizontal pool (1.five to two m wide) with three 20-40 cm wide circular vent orifices (Figure 3B).…”

“…In the case of Bagni San Filippo, travertine deposits form at the subaerial emergence of hydrothermal fluids at high temperatures, with carbonate precipitation primarily driven by physico-chemical processes such as CO 2 degassing (Folk, 1994;Pentecost, 2005;Capezzuoli et al, 2014;Della Porta, 2015;Della Porta et al, 2017a, Della Porta et al, 2017b, Della Porta et al, 2021). In the central Italian hydrothermal settings, primary carbonate precipitation is favored by sufficient Ca 2+ and CO 3 2− in thermal waters derived from decarbonation reactions with the Mesozoic carbonate substrate (Minissale, 2004).…”

“…Microbial mats are also ubiquitous in such high temperature environments and part of the travertine precipitates may be biologically influenced (cf. Dupraz et al, 2009), with EPS or other organic matter acting as a substrate for mineral precipitation (Folk, 1993(Folk, , 1994Chafetz and Guidry, 1999;Riding, 2008;Peng, 2012, Jones andPeng, 2014;Della Porta, 2015;Della Porta et al, 2017a, Della Porta et al, 2021. Although the existence of microbes may enhance CO 2 degassing, the influence of microbial biofilms and/or EPS on travertine mineralogies (calcite vs. aragonite) is difficult to assess, as it may change at scales <1 μm (Jones, 2017b).…”