## 1 Introduction  
Tufa is a result of calcium carbonate precipitation under cool, ambient temperature in freshwater saturated in CaCO3. Therefore, tufa is found in fluvial, lacustrine, and palustrine environments [1]. Tufas are highly porous, poorly bedded [2], and composed of calcite precipitated from calcium bicarbonate water derived from the dissolution of carbonate rocks [3]. Tufa can be deposited both at springs and downstream at rapids or waterfalls, depending on the rate of CO\({}_{2}\) release. The first case occurs when dissolved CO\({}_{2}\) in groundwater, in equilibrium with soil CO\({}_{2}\) partial pressure, outgasses due to the lower partial pressure of CO\({}_{2}\) in the atmosphere. In the second case, there is a sharp increase in the outgassing rate due to the increased surface area between water and air caused by turbulence [4]. Tufa deposits usually contain abundant remains of micro- and macrophytes, invertebrates, and bacteria [5, 6, 7, 8] and are characterized by a low or medium deposition rate (up to a few mm per year). Tufa deposition is related to specific, clearly defined geological, geomorphological, and environmental conditions, which, together, represent synonyms for \"healthy\" and preserved natural environments [9]. The distribution of tufa deposits depends on the availability of meteoric water and on local and climatic conditions. Since tufa often occurs at waterfalls, its distribution is also related to the microclimate in their immediate vicinity, which is characterized by cooler temperatures and higher humidity than in the surrounding area. Tufa deposits are a useful tool for deciphering long-term climatic changes during the Quaternary period, as they can be accurately dated using a variety of dating techniques [10]. The potential of fluvial tufa deposits as a valuable archive for paleoenvironmental and paleoclimate reconstructions has been investigated in several studies that focused on tufa oxygen and carbon isotopes and geochemical proxies [4, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20].  
The most used carbonate in paleoenvironmental reconstructions is calcite, as its precipitation is accompanied by the fractionation of stable carbon and oxygen isotopes [21]. A large number of tufa deposits is recorded on the territory of Serbia, mainly in eastern Serbia, but also in western Serbia, within the Dinaric Karst. They are located south of the Danube River and Sava River, usually at altitudes higher than 500 m.a.s.l. The bibliography of tufa accumulations in Serbia is insufficient. Most of the papers have been published in local languages and journals and are, therefore, not accessible to a wide geological community. In recent decades, more attention has been paid to individual smaller deposits in the context of environmental protection and geological heritage. Nevertheless, many authors have dealt with similar tufa deposits in the region [17; 20; 22; 23; 24; 25; 26; 27; 28; 29; 30]. The importance of tufa deposits is unambiguous. As it precipitates relatively fast, in specific conditions, it can serve as an applicable tool for the comparison of environmental conditions and regional correlations with similar deposits. Tufa deposits in Serbia were important in medieval time, as they were frequently used as a building material. However, nowadays, most of these deposits are under protection and exploitation is forbidden. In this work, tufa deposits from two sites were studied--tufa of the Sopotnica River Basin (further: SRB) and tufa of the Gostilje River Basin (further: GRB). Both sites include active and fossil tufa accumulations. The main goal of this study is to distinguish and describe different tufa facies and to determine environmental conditions based on stable isotope analysis. We also give a comparison with other tufa deposits in the region, as well as in Europe.