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Tufa samples from the Krka River area in Croatia were dated by 14C and 230Th/234U methods. The study area is situated in the karst region of the southern Dinarides. 14C ages of 40 tufa samples collected at the waterfalls coincide with the early Holocene interglaciation, up to 6000 14C yr B.P. Comparison of conventional 14C dates of Holocene tufa and those of speleothems in the Dinaric Karst shows that speleothem formation started several thousand years earlier than tufa growth. Samples of old tufa deposits from the Krka River (17) and Plitvice Lakes (12) area and speleothem samples from caves in Dinaric Karst (5) yield 34 230Th/234U dates, most of which cluster around interglacial marine δ18O stage 5 (21). Eight of 13 older dates belong to interglacial stages 7 and 9; only 5 dates fall within stage 6 and 8 glaciations. 230Th/234U dates of speleothems and tufa samples from central Europe have indicated that these were formed preferentially during warm and humid interglacial and interstadial periods, and it appears that this is true of southern Croatia as well. Stable isotope (δ13C, δ18O) analyses of 40 tufa samples from the Krka River area were compared with stable isotope analyses of tufa from the Plitvice Lakes area. The δ13C values for both locations are similar and range from −10 to −6‰, indicating similar conditions of tufa formation. A systematic difference between the δ18O values of tufa in these two areas reflects the regional distribution of the oxygen composition of precipitation.

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A two-decade-long record of environmental isotopes (2H, 3H, 14C, 18O) in Croatia is presented and the data are statistically analyzed. The atmospheric 14C activity for the period before the actual measurements started is reconstructed by measuring tree rings from the clean-air sites, and the past tritium activity in precipitation is estimated by the correlation of our data with the Vienna station record. The long-term 3H record helped to determine a locally contaminated sampling site, and new clean sites are put into operation. The 14C data were fitted by an analytical function and the prediction of future levels is given assuming that the rate of the 14C releases remains constant. From the long-term stable isotope data record, the local meteoric water line and the temperature gradient of δ18O in precipitation are determined.

ABSTRACT. Samples of sediment cores from two lakes in the karst area of northwest Yugoslavia were analyzed. Both Lakes Kozjak and Prosce are in the Plitvice National Park, Central Croatia. 14C dating, sedlimentologic, seismic, and isotopic studies, and distribution of diatoms are presented.14C dating of lake marl revealed a uniforn sedimentation rate in Lake Prosce as opposed to Lake Kozjak. Both lake sediments belong to the Holocene period. 14C dating of lake sediment is in agreement with seismic profiles, sedimentologic analysis, and diatom frequency measurements both in an undisturbed as well as in a disturbed lake sediment. INTROI)UCTIONA comprehensive study of aquatic chemistry, hydrobiology, and hydrogeology of the Plitvice Lakes included coring of lake sediment and a subsequent systematic analysis of cores. Samples of cores taken every 5 to 50cm (depending on the type of analysis) were analyzed for pollen, diatom, and ostracode content, followed by sedimentologic, trace element, x-ray diffraction, stable isotope, and 14C analyses. The 14C, stable isotope, and diatom population analyses are presented here, along with a brief description of seismic and sedimentologic work.Lake Kozjak core was retrieved from 24m water depth. Even though a surprisingly thick lake sediment prevented the extraction of all core material overlaying bedrock, the first 12m of sediment gave very useful and, to a certain extent, unexpected data. Radiocarbon ages of an acoustically wellstratified top section indicated a fairly constant sedimentation rate of 0.8mm/a for the last 1800 years. Lake Prosce core was retrieved from 17.2m water depth and reached the clayey residual layer overlaying bedrock. Radiocarbon dating of lake sediment revealed a uniform sedimentation rate(1.4mm/a) as opposed to Lake Kozjak sediments. Both lake sediments belong to the Holocene.The x-ray diffraction analyses showed that the sediments consist of calcite with traces of dolomite, indicating that the sediments were formed by decomposition of dissolved calcium bicarbonate without any significant contribution from detrital limestone or dolomite. This was substantiated by b' 3C data, ranging between -8.5%o and -8.8%o, which is typical of sediments of biogenic origin. Geologic SettingAccording to Polsak (1979) the Plitvice Lakes are surrounded by Triassic, Jurassic, and Cretaceous beds. The whole area is gently folded and intersected by numerous faults. Two most prominent faults run in a north-

Results of measurements for 3 years (1981–1983) of 14C activity of dissolved inorganic carbon (DIG) in water samples from the Korana River, as well as that of recent tufa and aquatic plants, showed that 14C concentration increases from karst spring to the estuary. A model describing the increase of 14C activity was developed assuming that the increase is due to the exchange of the dissolved CO2 in stream water with atmospheric CO2 and to dissolution of CO2 from the decay of organic material and root respiration. It is possible to distinguish these two contributions by measuring the δ13C values of DIC in water. As expected, our data show that the exchange process between atmospheric CO2 and DIC dominates at rapids and waterfalls, while biologic contribution is much higher in lakes and along the lowland flow of the Korana River. Agreement between the calculated and the measured activities supports the proposed mechanisms of chemical and isotopic exchanges in stream waters.