Calcareous Tufa: Deposition and Erosion during Geological Times

Abstract

There is a general agreement in referring the deposition of calcareous tufa to climatic causes. Warm climates are believed to favor calcareous tufa formation due to higher concentrations of biogenic CO2 in soils, enhancing the dissolution rates of CaCO3 and the broader development of aquatic plants that remove CO2 from spring waters. Conversely, cold climates are considered less favorable because of the reduced biological activity of soils and the lesser development of aquatic plants. Dry climates are also considered unfavorable to the deposition of calcareous tufa due to scarcity of rainwater and the consequent reduction of water circulating in the ground and spring discharge contrary to humid climates, which, besides allowing abundant water infiltration and emergence, favor the spreading of vegetation cover, the development of biogenic processes in the soils, and the growth of aquatic plants. An additional factor controlling calcareous tufa deposition may be the temperature difference between the ground surface and the aquifer in connection with major climatic changes due to the low thermal conductivity of the limestone bedrock. With climate warming, the infiltrating water, made highly acidic when crossing the soil due to the elevated partial pressure of biogenic CO2 present therein, percolating through the progressively colder levels of the aquifer, induces a relevant dissolution of CaCO3, definitely higher than in normal conditions. At emergence, because of the higher surface temperatures, running water turbulence, photosynthetic activity of mosses and algae, and evaporation of spray droplets, the groundwater loses CO2, becoming oversaturated with CaCO3 and causing tufa deposition, even at a great distance from the spring. Opposite effects, such as the deposition of dissolved carbonate in the upper bedrock layers and the emergence of spring waters undersaturated with CaCO3, capable of further dissolution, are expected to occur with major climatic changes to cold conditions. This model appears to be confirmed by the deposition/erosion stages of calcareous tufa, which repeatedly occurred during the Holocene and the late Pleistocene in different parts of the world.