A tufa is a freshwater and open-air carbonatewhich is potentially important for the reconstruction ofQuaternary terrestrial environments. Ubiquitous occurrence oftufas is the prominent feature of the Atetsu Limestone Plateau,Okayama Prefecture, southwest Japan; the 36 locations of thetufas, including 23 newly found, make up the largest numberamong the limestone areas in Japan. Analyses of waterchemistry indicate that the most critical chemical condition fortufa deposition is a saturation index for calcite larger than 0.5.The water acquires such supersaturating firstly by dissolvingCaCO3 in an underground water system, and then by degassingCO2 after issuing from a spring. Effective CO2-degassing isachieved by turbulent flowing over a certain distance. In theAtetsu Limestone Plateau, the latter hydrological condition ismet for the tufa-depositing waters by their flowing down onmore than 20m high steep slope. These waters generally appearsat the lithological (or thrust) boundaries between limestone andother rock type. Whether or not the water appears is stronglycontrolled by the relation between the lithologic boundaries,topography, and underground prerequisites. This partly explainsthe ubiquitous occurrence of tufas in the plateau.
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