Abiotic and biotic processes controlling travertine deposition: Insights from eight hot springs in Japan

Abstract

Contributions of abiotic and biotic processes on travertine deposition are still not well‐understood due to technical difficulties, despite that the travertines draw attention as analogues for ancient microbial carbonates and oil reservoirs. To evaluate their contributions, this study examined eight hot springs in Japan. Water chemistry analyses showed common downstream trends: a decrease in CO2 concentration and increases in CO32− concentration and pH. Mineralogical analysis showed that the constituent minerals of travertines at six hot springs were both calcite and aragonite, while one was just calcite and another only aragonite. Microscopic observations of travertine surfaces indicated the dominance of cyanobacteria secreting extracellular polymeric substances without a detectable amount of carboxyl groups. Small particles were sometimes entangled/covered by these cyanobacteria. Microelectrode measurements showed the occurrence of abiotic CaCO3 precipitation and photosynthetic induction/inhibition of CaCO3 precipitation, the extent of which was different at each site. By integrating these results, the contributions of abiotic and biotic processes were evaluated. Cyanobacteria inhabiting travertine surfaces were generally not calcified regardless of an ambient high CaCO3 saturation state; instead, they contributed to creating pore spaces and trap/bind suspended particles. Downstream CO2 degassing increased the CaCO3 saturation state by shifting carbonate chemical equilibrium and caused abiotic CaCO3 precipitation. Suspended particles trapped by cyanobacteria increased the surface area for crystal growth to further accelerate precipitation. The contribution of photosynthesis‐induced CaCO3 precipitation was low because of several factors, including variable cyanobacteria populations and photosynthetic inhibition of CaCO3 precipitation. The average contributions of photosynthesis‐induced CaCO3 precipitation, Ca2+ adsorption and abiotic precipitation in the eight hot springs were 16%, 3% and 81%, respectively, indicating predominance of the abiotic process for travertine deposition. Mineralogical composition of travertines significantly correlated with concentrations of SO42− and Mg2+, much more than with Mg/Ca ratio and water temperature, suggesting their importance for controlling CaCO3 polymorphs in travertines.

Factors controlling the geometry of travertine mounds: Insights from Heinitang (China)

“…Distinguishing equilibrium‐controlled degassing and precipitation‐controlled degassing is difficult in hot spring systems, because travertine deposition and Ca 2+ concentration decrease often occur concurrently with pH elevating, instead of after it (e.g. Amundson & Kelly, 1987; Chafetz et al ., 1991; Hammer et al ., 2005; Okumura et al ., 2012; Sugihara et al ., 2016; Shiraishi et al ., 2022). Fortunately, both equilibrium‐controlled degassing and precipitation‐controlled degassing would cause p CO 2 decline and our simulations have illustrated the fast precipitation lines and the slowest precipitation lines at all p CO 2 levels (Fig.…”

“…existing excessive ) (e.g. Pentecost, 1995, 2005; Özkul et al ., 2013; Renaut et al ., 2013; Shiraishi et al ., 2022). The influence of Ca 2+ concentrations has been revealed by our simulations (Fig.…”

“…Additionally, microbial biofilms can provide crystal nucleating sites (e.g. Pedley & Rogerson, 2010; Shiraishi et al ., 2019; Della Porta et al ., 2021) and trap and bind suspended crystals (Shiraishi et al ., 2019, 2022). In spring‐related carbonate systems, the contributions from microbial metabolic processes and trapping/binding of suspended crystals by micro‐organisms are generally low, but crystal nucleation on organic matter might strongly affect carbonate deposition (e.g.…”

“…In spring‐related carbonate systems, the contributions from microbial metabolic processes and trapping/binding of suspended crystals by micro‐organisms are generally low, but crystal nucleation on organic matter might strongly affect carbonate deposition (e.g. Merz‐Preiß & Riding, 1999; Pentecost & Coletta, 2007; Arp et al ., 2010; Della Porta et al ., 2021; Shiraishi et al ., 2022). In the present case, some microbial boundstone exists, but the main lithofacies, crystalline crust, is presumed abiotic (e.g.…”

“…Thus, considering that waters from travertine‐depositing springs commonly show Ca 2+ / ratios <0.5 (for example, ‐excessive) (e.g. Pentecost, 2005; Özkul et al ., 2013; Kano et al ., 2019; Shiraishi et al ., 2022), the end of travertine precipitation is generally controlled by the available Ca 2+ concentration in spring water. Therefore, calcium concentration variations along the flow path can help us to understand the lateral extent of travertine mounds.…”