Basic Knowledge of Geochemical Processes

Kano, Akihiro; Okumura, Takahiro; Takashima, Chizuru; Shiraishi, Fumito

doi:10.1007/978-981-13-1337-0_2

Cited by 1 publication

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“…In general, the major controlling factors of CaCO 3 polymorphs in travertines (mainly calcite and aragonite) are considered as the Mg/Ca ratio of water (aragonite formation is favourable when Mg/Ca >1) and the water temperature (aragonite formation is favourable above ca 40 to 45°C), although there are many exceptions (e.g. Özkul et al, 1990;Jones, 2017a;Kano et al, 2019d;and references therein). Other suggested factors include pH, pCO 2 , Ω, cations other than Mg 2+ , organic matter and biofilm EPS; however, none of them can solely explain calcite and aragonite precipitation (Jones, 2017a).…”

Section: Physicochemical Parameters and Caco 3 Polymorphsmentioning

confidence: 99%

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

et al. 2021

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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.

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Section: Physicochemical Parameters and Caco 3 Polymorphsmentioning

confidence: 99%