Dry climatic oscillations in Epiatlantic and Subboreal represent probably the most catastrophic climatic events in Holocene. Climatic changes are well recorded in accumulations of calcareous tufa deposited by some karstic springs. During dry periods horizons of humic soils were formed instead of tufa. A calcareous tufa mound of the size of about 40,000 m3 was deposited in Svaty´ Jan pod Skalou (30 km SW of Prague) during Holocene by a large karstic spring which now discharges at its base. The spring is characterised by a discharge close to 20 L/s, and virtually constant temperature (11.3 to 11.6 °C), which is slightly higher than the mean annual temperature in this area and indicates deeper circulation of karstic water. The middle part of Holocene is developed as an approximately 13 m thick complex formed by various types of calcareous tufa with interbedded fossil soils and debris layers.
Four independent methods were applied to date individual layers. Five carbonate samples covering the whole stratigraphic extent known were dated by 14C and by U-series methods. In addition, 3 samples of charcoal from the upper part of the profile were dated by AMS. The upper part of the profile was dated also archaeologically. As of today, radiocarbon data of carbonates and charcoal and the archaeological dating of pottery are available.
All 14C dates of carbonates were corrected for their lower initial radiocarbon content using a value of 83 % activity of modern carbon measured in dissolved bicarbonate of the spring water. Tree-ring calibration was introduced to both carbonate and charcoal 14C dates. The agreement of both sets of radiocarbon dates from the upper part of the profile and of archaeological dating was good. In the middle part of the profile the radiocarbon age of charcoal is about 800 years higher compared to the radiocarbon age of carbonate. The charcoal was either produced by combustion of older wood (alternatively, charcoal pieces could have been redeposited by water or human activity), or the initial radiocarbon content of carbonate varied with time. Regardless of these uncertainties it is clear that the carbonate deposition in Svaty Jan pod Skalou covers the range from about 8000 to 2500 BP.
d18O values of carbonate are from -7.3 to -8.3 PDB, d13C values vary from -7.9 to -10.5 . Small variability of both d13C and d18O values, the character of tufa accumulation and high spring discharge indicate that carbonate may have been deposited under near-equilibrium conditions and isotopic composition of carbonate may thus reflect climatic changes. The constant spring temperature suggests that changes in oxygen isotope composition of carbonate were controlled mainly by changes in isotopic composition of water. Assuming a long-term temperature gradient of d18O in meteoric water of 0.6 /oC (Rozanski et al., 1993), the measured differences in d18O values of tufa correspond to a temperature changes of about 1.5 oC.
Isotope data obtained for calcareous tufa at Svaty´ Jan pod Skalou, Czech Republic, were compared with data for calcareous tufa deposits from southern Poland (Pazdur et al., 1988) . The d18O values of carbonate from the Czech profile are about 1 higher than those from the Polish profiles but the variability of about 1 in d18O values of carbonates is similar in all profiles. In southern Poland, lower d18O values (lower mean annual temperature) were found for 2200 ys BP, 5500 ys BP and 6300 ys BP; higher d18O values (higher temperature) were found for 6000 ys BP and 6500 ys BP. Considering the uncertainty in radiocarbon data in the carbonate material, it is possible to view the trends in d18O values from the calcareous tufa deposit in Svaty´ Jan pod Skalou and in southern Poland as consistent.
This work was supported by IAEA in Vienna (RC No.7119) and by Czech Grant Agency in Prague (205/95/1392).
Pazdur, A., Pazdur, M.F., Starkel, L. & Szulc, J., Quaternary Research 30, 177-189 (1988).
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