Abiotic and biotic processes controlling travertine deposition: Insights from eight hot springs in Japan
Fumito Shiraishi, Yusaku Hanzawa, Yuki Nakamura, Yuki Eno, Asayo Morikawa, Rafael França de Mattos, Jiro Asada, Leonardo Fadel Cury, Anelize Manuela Bahniuk
Abstract
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 CO 2 concentration and increases in CO 3 2− 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 CaCO 3 precipitation and photosynthetic induction/inhibition of CaCO 3 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 CaCO 3 saturation state; instead, they contributed to creating pore spaces and trap/bind suspended particles. Downstream CO 2 degassing increased the CaCO 3 saturation state by shifting carbonate chemical equilibrium and caused abiotic CaCO 3 precipitation. Suspended particles trapped by cyanobacteria increased the surface area for crystal growth to further accelerate precipitation. The contribution of photosynthesis‐induced CaCO 3 precipitation was low because of several factors, including variable cyanobacteria populations and photosynthetic inhibition of CaCO 3 precipitation. The average contributions of photosynthesis‐induced CaCO 3 precipitation, Ca 2+ 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 SO 4 2− and Mg 2+ , much more than with Mg/Ca ratio and water temperature, suggesting their importance for controlling CaCO 3 polymorphs in travertines.