Comparison of isotopic composition of different forms of calcite precipitated from fresh water

In the karst area of NW Yugoslavia calcite is precipitated from fresh water in the form of speleothems in karst caves, as lacustrine deposits in river beds (tufa or travertine), and as lake sediments. The isotopic compositions of these forms of calcite were compared to determine conditions for calcite precipitation, i.e. whether precipitation takes place under isotopic equilibrium or not. The content of stable isotopes of carbon (13C) and oxygen (18O), as well as radiocarbon (14C) activity, were measured in calcite samples. Speleothems formed under various physicochemical conditions were collected in the Slovenian karst area, while most tufa samples and lake sediments were collected in the Plitvice Lakes area, Croatia. The δ18O of investigated young speleothems varies from -5 per mille to -7 per mille PDB and that of recent tufa and lake sediment from -8.5 to -10 per mille. The observed variations of δ13C content are much higher: δ13C of speleothems varies between -6.5 and -11 per mille, and that of tufa between -10 and -2 per mille PDB. However, the most frequent δ13C values of speleothems and tufa samples are very similar, indicating similar conditions of calcite formation. Higher δ13C values can be explained by the escape of isotopically lighter CO2 from water and by exchange with atmospheric CO2. It was observed that δ18O and δ13C values were higher for tufa deposits that were precipitated at waterfalls than those of sediments or tufa formed in lakes or under steady water flow conditions. More pronounced differences between tufa and speleothems were found in oxygen isotopic composition. The δ18O values of tufa are significantly more negative than the δ18O of speleothems owing to the different isotopic composition of water from which the calcite was precipitated. Tufa is precipitated from water with mean annual δ18O values of -10.7 per mille SMOW, while speleothems are formed from water with mean annual 6180 of -8.6 per mille (samples from caves Mackovca, Planina Postojna and Predjama) and -7.2 per mille (Divaca cave). These results prove that the oxygen isotopic composition of calcite formed under equilibrium conditions is governed by the oxygen isotopic composition of the water and by the temperature at which calcite formation occurs. The temperature of recent calcite formation was calculated from Ref. [I]: 1000 Ln α = 2.78 x 106 / T2 - 3.4, where α is (1000 + δ18Ocalcite) / (1000 + δ18Owater). The calculated temperatures of speleothem formation are 0.5 to 3.2 deg C lower than the measured temperatures in caves, because speleothems were formed in a colder period of the year (mostly from September to December). Lake sediment calcite was precipitated under isotopic equilibrium from water having a saturation index higher than 3. This was proved by comparing the δ18C values in sediment with those in dissolved inorganic carbon in water. The temperature of tufa and lake sediment precipitation was also determined. It turned out that the calculated temperature is higher than the average water temperature measured for several past years. Hence, the sediment was precipitated during the warmer period of the year. The δ18O value is very uniform throughout the investigated sediment depth, indicating that no significant climatic changes occurred during the sedimentation process, which lasted approximately 8000 years in Lake Prosce. Radiocarbon dating of tufa showed two age groups: the Holocene tufa up to 6000 years old and the older tufa with an apparent 14C age of 20,000 years or more. It was proved by the 230Th/234U method that the true age of tufa was about 120 000 years, i.e. the periods of tufa growth coincided with warm and humid interglacials. The δ18O values of tufa of various ages are very similar showing that both groups of tufa were formed under similar climatic conditions, supposing that the isotopic composition of meteoric water was the same as during t he Holocene