The term calcareous tufa, or freshwater travertine, is widely used in the scientific literature to describe carbonate deposits precipitated from cool groundwaters of meteoric origin enriched in CO₂ (carbon dioxide) by percolating through organic soils and, therefore, capable of attacking CaCO₃ (calcium carbonate) in limestone aquifers and dissolving it as Ca (HCO₃)₂ (calcium bicarbonate) according to the equation: [1]

Calcareous tufa deposits form from the degassing of carbon dioxide and related shifting of the above equation induced by flowing water turbulence and the photosynthetic process by vegetal organisms typical of aquatic environments such as bacteria, blue-green algae, and mosses whose remnants are usually present in the deposit structure [1,2,3] together with fossil fauna such as ostracods and mollusk shells [4,5]. Similar in origin to calcareous tufa are cave speleothems [6]. Carbonate deposits precipitated from geothermal waters highly enriched with concentrations of CO₂ are called thermogene travertines [6] or, more simply, travertines [7]. Calcareous tufa deposition has taken place in various environmental conditions since the earliest geological times [6], even though most deposits are referred to the Middle-Upper Pleistocene and Holocene ([8,9,10,11] and references therein).

The dissolution rate of CaCO₃ in water is very low [12]. However, if the solution includes some CO₂, CaCO₃ is easily dissolved as Ca(HCO₃)₂. The dissolved free carbon dioxide (not combined in the previous equation) is called equilibrium CO₂ [13]: With concentrations of dissolved CO₂ lower than the equilibrium values, precipitation of CaCO₃ will occur, while with higher concentrations, further dissolution of CaCO3 will be possible. The carbon dioxide concentration above the equilibrium value is called independent CO₂ [12].

The solubility of CaCO₃ in water directly depends on the partial pressure of CO₂ in the surrounding atmosphere [14,15]. It is very low in the open air but strongly increases in soils, where the partial pressure of CO₂ produced by biological processes and the decay of organic matter can attain values up to 1000 times higher than in the atmosphere [16]. The temperature also controls the CO₂ solubility: Water at 0 °C dissolves CO₂ about three times more than at 30 °C [12]. Then, the water reaches the phreatic zone where the only sources of additional CO₂, apart from a possible endogenous supply, is from the oxidation of minor amounts of transported organic matter or bacterial activity [17]. However, in such conditions, the total amount of CO₂ may be considered practically constant, but the relative amounts of free CO₂ (equilibrium plus independent CO₂) and combined CO₂ (to form CaCO₃ and Ca(HCO₃)₂₎ may change with variations of pressure and temperature. In a closed system, free CO₂ may also be derived from the mixture of solutions saturated with different concentrations of CaCO₃ [18].

Several factors may cause CaCO₃ precipitation [6,8]: Lower partial pressure of CO₂ at the groundwater emergence, increasing groundwater temperature at the emergence, consumption of CO₂ by aquatic plants, loss of dissolved CO₂ (degassing) induced by turbulence and pulverization of stream waters at waterfalls, breaks, and roughness reaches of the river profile, even at a great distance from the spring [19,20].

Calcareous tufa may be divided into two main groups: autochthonous tufa, deriving from in situ encrusted organisms, and allochthonous tufa, consisting of phytoclasts (encrusted fragments of plants) arenitic (microdetrital facies) and ruditic (macrodetrital facies) in size [21,22,23,24,25]. Based on the sedimentary facies, autochthonous tufa may be distinguished:

Allochthonous tufa deposits have a typical clastic texture with fragments of incrustations on vegetal organisms sometimes providing information (e.g., clast orientation, imbrication, etc.) about their transporting flow. Fragments with an irregularly laminated cortex of calcium carbonate, often characterized by a spheroidal to oblate shape and usually referred to as oncoids [26], are common components of allochthonous tufa deposited in streams, rivers, and lakes. Pedley [24] attributes the spheroid shapes of grains to high competence flow, the elongated shapes to slow flow, and the irregular shapes to calm waters.

Clastic fragments cemented by calcareous tufa are sometimes found inside terraced alluvial or slope deposits. They form mainly in the first stages of tufa deposition [27].

Following Choquette and Pray [28], the porosity of calcareous tufa limestone may be distinguished into non-fabric porosity (produced by fracturing, karstic dissolution, and burrowing invertebrates) and fabric porosity.

Depending upon the cohesion between the constituting crystals, calcareous tufa deposits range from soft and chalky to dense and highly indurated [6].

Tufa deposits are affected by meteoric diagenesis soon after deposition when exposed at the surface and by burial diagenesis when overlain by more recent thick sediments [6,25,29].

The principal changes caused by meteoric diagenesis are related to the dissolution/precipitation of calcium carbonate (void filling, cementation) induced by percolating rainwater or groundwater; other diagenetic effects are recrystallization, microbial micritization, bioturbation, oxidation of organic matter and sparmicritization, a term introduced by Kahle [30] to describe the etching action of microorganisms at or near the tufa surface [6]. Burial diagenetic effects resulting from increased lithostatic and hydrostatic pressure, heating, and the ingress of mineral-enriched solutions include compaction and porosity reduction resulting from further cementation, dissolution of the original fabric, sometimes with replacement by other minerals, and reactions between the original carbonate component and accessory minerals [6].

The original differences in porosity combined with those due to diagenesis make the permeability of tufa deposits extremely variable.

The deposition of calcareous tufa may give rise to construction landforms such as small mounds at springs and dams across the riverbeds or coatings of steep slopes, rough river beds, or swamp/lake bottoms generally lacking a recognizable shape [6,31,32]. These features are geologically not durable as the construction process can be interrupted, and landforms can be destroyed, in whole or in part, by erosion [6]. Bedding within the deposit, where present, is usually inclined and undulated and rarely horizontal; thin laminations resulting from daily/seasonal variations are often recognizable [6].

Slope deposits essentially consist of wedge-shaped, layered bodies of microhermal tufa locally passing to stromatolithic tufa with minor intercalations of phytoclastic tufa. Calcareous tufa systems may develop either along slopes forming wedge-shaped sedimentary bodies with the thickest accumulation downstream and transforming the original water flow into a system of hanging channels, low barrages, ponds, and terraces, or across rivers giving rise to dams with pools or larger basins on their backside [6,22].

Dams are the showiest construction bodies of calcareous tufa (Figure 4). They may reach heights up to several tens of meters in correspondence with breaks or obstructions of riverbeds that reduce erosion by flowing water, thus allowing CaCO₃ precipitation [6,33,34,35]. These features mainly consist of massive phytohermal tufa encrusted on a skeleton made of remnants of vegetal organisms. In addition to growing upward, the aggradation of tufa progrades onward, forming sub-vertical layers unconformably covering the earlier deposits, including those of the basin down valley (Figure 4) [21,24,36]. On the backside of dams, water basins form (ranging in size from small pools to vast lacustrine basins) whose bottom hosts tufa sands (deriving from dismantling tufa deposits upstream), phytoclastic tufa, and stromatolithic tufa, interspersed with clayey sediments and peaty layers (Figure 5) [1,21,24,33,34,37].

Dams and backside pools usually follow one another along the watercourse forming characteristic depositional systems (Figure 6) [6,21,24].

The growth of tufa dams occurs where the deposition rate of calcium carbonate from water is high enough to balance the streamflow erosion [38].

In correspondence with significantly high steps in the riverbed profile, dams often fail to grow due to the erosion exerted by rapid water flow, and the deposition of tufa mainly progresses downstream from the tufa dam, giving rise to a “cascade tufa” deposits (Figure 7) [6].

There is general agreement in referring the development of calcareous tufa to climatic causes [6,8,39,40,41].

Warm climates are believed to favor calcareous tufa formation due to higher concentrations of biogenic CO₂ in soils [6,8,14,39,40,41,42,43,44,45] enhancing the dissolution rates of CaCO₃ [12] and increasing photosynthetic activity by aquatic plants [1,8,25,46]. Conversely, cold climates are considered less favorable because of the reduced biological activity of soils and the lesser development of aquatic plants [6,45].

Humid climates are generally considered favorable for tufa deposition by allowing abundant water infiltration and emergence, enhancing the development of vegetation covers and related biogenic processes in the soils, and promoting the growth of aquatic plants. This is contrary to dry climates where there is a scarcity of rainwater and a consequent general reduction in water circulating in the ground and discharging at springs. However, delayed responses to climate aridification of deep aquifers reached by river incision may locally result in tufa deposition, even during dry periods [47,48]. 

An explanatory model for the alternating periods of tufa deposition/erosion during geological times refers to the variations of thermal gradient induced in the bedrock by significant climate changes [56 (https://www.mdpi.com/2076-3417/13/7/4410#B49- applsci-13-04410)]. Due to the low thermal conductivity of bedrock [57], major climatic changes to warmer conditions, such as the rapid increase in air temperature at the Late Pleistocene-Holocene transition, induce significant thermal contrasts between the surface and the ground and reverse thermal gradients in the deep limestone aquifers. With climate warming, the infiltration 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, causes relevant dissolution of CaCO₃ [14 ], higher than in normal conditions. At the emergence, because of the higher surface temperatures, the groundwater loses CO₂, becomes oversaturated with CaCO₃ and produces tufa deposition, even at a great distance from the spring, favored by the running water turbulence, photosynthetic activity of mosses and algae, and evaporation of spray droplets. Opposite effects, such as deposition of dissolved carbonate in the upper bedrock layers and the emergence of aggressive spring waters undersaturated with CaCO₃, are expected to occur with major climatic changes to cold conditions [11 ].

In all conditions, tectonics strongly influences tufa deposition by opening waterways in fractured rocks and giving rise to fault steps across rivers, thus favoring the growth of tufa dams [49].