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Waterfall tufa is widely distributed around the world, especially in tropical and subtropical karst areas. In these areas river water is generally supersaturated with respect to calcite, and the precipitation occurs mainly at waterfall and cascade sites. Development of waterfall tufa has been described as simply being the result of water turbulence. We believe, however, that three physical effects can lead to tufa deposition at waterfall sites: aeration, jet-flow, and low-pressure effects. The three physical effects are induced by two basic changes in the water: an accelerated flow velocity, and enlargement of the air-water interface area. These two changes increase the rate of CO2 outgassing and the SIc, so that a high degree of supersaturation is achieved, which then induces calcite precipitation. These "waterfall effects" have been simulated in laboratory and field experiments, and each of them can accelerate, or trigger, calcite precipitation. Field measurements of river water chemistry also show that tufa deposition occurred only at waterfall sites. In these experiments and observations, waterfall effects play the most important role in triggering and accelerating CO2 outgassing rates. Field and laboratory observations indicate that plants and evaporation also play important roles in tufa formation. Growth of algae and mosses on tufa surfaces can provide substrates for calcite nucleation and can trap detrital calcite, accelerating tufa deposition. However, the prerequisite for such deposition at waterfall sites is a high degree of supersaturation in river water, which is mainly caused by waterfall effects. Evaporation can lead to supersaturation in sprays and thin water films at a waterfall site and cause the precipitation of dissolved CaCO3, but the amount of such deposition is relatively small. |
