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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-

Systematic dating of tufa samples has been undertaken to establish a chronology of tufa deposits in Plitvice National Park, Yugoslavia. We conclude that tufa samples give reproducible data within the time span of ≈ 40,000 years up to recent. The 14C/12C ratio of carbon in the ambient biosphere and hydrosphere gives a detailed picture of the distribution of carbon isotopes in the investigated system. Despite the susceptibility of calcareous material to ambient conditions in the hydrosphere, the original 14C composition of tufa has not been substantially changed. A vertical profile which was cut in tufa during pathway construction showed that the tufa isochrones run vertically in the investigated area. The vertical isochrones and the 14C gradient were horizontal due to tufa build-up in Plitvice National Park which occurs on vertical escarpments that have been flooded. The relatively uniform radiocarbon content of most tufa deposits indicates redistribution of radiocarbon during the formation period. While detailed stratigraphy is often obscured, a general chronology of tufa deposits in the investigated area has been established.