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Carbon isotope composition of dissolved inorganic carbon (DIC) was studied at springs, lakes and tributaries of the Plitvice Lakes. The Plitvice Lakes are a unique and complex karst lakes system consisting of 16 flow- through lakes connected by waterfalls and streams and characterized by intense calcium carbonate precipitation in the form of tufa and lake sediments. Two main springs have different 14C concentrations due to different geology and their variations were controlled by flow rates. Further downstream, at lakes to the Korana River that outflows from the lakes, the 14C activity and δ13C consistently increased down the flow. Carbon isotope composition from 2010- 2015 was compared with the values measured 30 years earlier. The variation in δ13CDIC was accounted for seasonal change, while 14C activity of surface water DIC decreased 7 – 12 % during the 30 years. Using a semi-empiric model, it is calculated that the downstream increase in the carbon isotope values is controlled by the exchange of DIC and atmospheric CO2 carbon and by introduction of decomposed biogenic carbon from the top soil organic meter of the lakes surrounding area in almost equal proportions.

Tritium presents a natural radioactive isotope which can be used as a tracer in hydrology. This paper presents the usage of tritium in hydrogeological research of the Zagreb aquifer, which presents the country`s strategic water reserves and is protected by the Republic of Croatia. In the research area, higher tritium content in surface water and groundwater is the result of releasing coolant water from the Nuclear power plant Krško. Tritium content was measured in 13 piezometers and one pumping well, in the wider area of the Petruševec and Kosnica well fields. It has been shown that tritium from cooling water is much more pronounced than its natural seasonal variability, and it can be very useful as a tracer. Also, results showed that the intensity and delay of responses in the aquifer depend on their distance from the Sava River and piezometer depth. Furthermore, the results suggest that variable hydrologic conditions can generate rapid change in the groundwater flow direction and affect tritium transport through the aquifer., Tricij je prirodni radioaktivni izotop koji se može koristiti kao traser u hidrologiji. Ovim radom prikazuje se korištenje tricija u hidrogeološkim istraživanjima zagrebačkoga vodonosnika koji je Republika Hrvatska zaštitila i uvrstila u strateške zalihe vode. Na istraživanome području povišen sadržaj tricija u površinskoj i podzemnoj vodi posljedica je ispuštanja rashladne vode iz nuklearne elektrane Krško u rijeku Savu. Sadržaj tricija mjeren je u 13 piezometara i u jednome zdencu na području crpilišta Petruševec i Kosnica. Pokazano je da je tricij podrijetlom iz rashladne vode naglašeniji od njegove sezonske varijabilnosti te može biti korišten kao traser. Također, rezultati su pokazali da intenzitet i zakašnjenje reakcije u vodonosniku izravno ovisi o udaljenosti od rijeke Save i dubini piezometra. Nadalje, rezultati upućuju na to da promjenjivi hidrološki uvjeti uzrokuju brze promjene u smjeru tečenja podzemne vode i utječu na transport tricija kroz vodonosnik.

Tufa is a fresh-water surface calcium carbonate deposit precipitated at or near ambient temperature, and commonly contains the remains of macro- and microphytes. Many Holocene tufas are found along the Zrmanja River, Dalmatian karst, Croatia. In this work we present radiocarbon dating results of older tufa that was found for the first time at the Zrmanja River near the Village of Sanaderi. Tufa outcrops were observed at different levels, between the river bed and up to 26 m above its present level. Radiocarbon dating of the carbonate fraction revealed ages from modern, at the river bed, up to 40 kBP ~20 m above its present level. These ages fit well with the hypothesis that the Zrmanja River had a previous surface connection with the Krka River, and changed its flow direction toward the Novigrad Sea approximately 40 kBP (Marine Isotope Stage 3). Radiocarbon AMS dating of tufa organic residue yielded a maximum conventional age of 17 kBP for the highest outcrop position indicating probable penetration of younger organic material to hollow tufa structures, as confirmed by radiocarbon analyses of humin extracted from the samples. Stable carbon isotope composition (δ13C) of the carbonate fraction of (−10.4 ± 0.6)‰ and (−9.7 ± 0.8)‰ for the Holocene and the older samples, respectively, indicate the autochthonous origin of the carbonate. The δ13C values of (−30.5 ± 0.3)‰ and (−29.6 ± 0.6)‰ for organic residue, having ages &lt, 500 BP and &gt, 5000 BP, respectively, suggest a unique carbon source for photosynthesis, mainly atmospheric CO2, with an indication of the Suess effect in δ13C during last centuries. The oxygen isotopic composition (δ18O) agrees well with deposition of tufa samples in two stages, the Holocene (−8.02 ± 0.72‰) and “old” (mainly MIS 3 and the beginning of MIS 2) (−6.89 ± 0.34‰), suggesting a ~4 °C lower temperature in MIS 3 compared to the current one.

During archeological excavations in the Lower Cerovačka Cave (Mt. Velebit, Croatia), the test trench penetrated to a depth of 1.8 m. An undisturbed sequence of sediments was exposed. Considering that caves represent highly efficient sediment traps it was possible to recognize changes in the depositional mechanisms during the Pleistocene–Holocene period. Using the multiproxy approach, the mineralogical, petrographic, and biostratigraphic characterization of the cave sediments was performed. Facies analysis revealed several stages in the development of the clastic filling of cave channels. Allochthonous origin of the sediment was assumed. Sedimentation took place under various conditions from pronounced cold and dry climate during Pleistocene stages in the base of the profile, to humid periods with anthropogenic influence during the Holocene at the very top of the profile. Although traditionally these sediments were believed to be of a Pleistocene age, here for the first time a stratigraphic calibration of the profile has been performed based on luminescence dating of detrital cave sediments and radiometric dating of speleothems.

The Zagreb aquifer is the main source of potable water for the inhabitants of the City of Zagreb and Zagreb County. It presents a strategic water reserve protected by the Republic of Croatia. All previous studies related to the definition of the groundwater–surface interaction in the study area have been made based on the isotopic composition of the Sava River from the location of the Domovinski Most bridge, which is located downstream of most pumping well fields. In 2019, a new monitoring station was established at the Podsusedski Most bridge, at the entrance of the Sava River into the Zagreb aquifer, approximately 23 km upstream of the Domovinski Most bridge. Within this research, water isotope data (δ2H, δ18O, deuterium excess) from both Sava River and groundwater sites were used along with hydrologic data to examine the extent to which hydrologic conditions affect the isotope signature and whether the interaction between groundwater and the Sava River causes a change in the isotopic composition of the Sava River. In addition, δ18O amplitudes were estimated for different time periods, as well as the mean residence time for the hydrological year 2019/2020. For that purpose, different statistical methods were applied to the new monthly data for six years for the Domovinski Most bridge and two years for the Podsusedski Most bridge. The δ18O amplitudes vary from 0.22 to 1.86 depending on the time interval and hydrological conditions, while the mean residence time for the hydrological year 2019/2020 was estimated to be about 2.5 months.

The application of tritium, 2H, and 18O in the characterization of the precipitation, groundwater, and surface and lake water of the Plitvice Lakes (PL), Croatia, over the 1979&ndash, 2019 period is presented. An increase in the mean annual air temperature of 0.06 &deg, C/year and in the annual precipitation amount of 10 mm/year is observed. The good correlation of the tritium activity concentration in the PL and Zagreb precipitation implies that the tritium data for Zagreb are applicable for the study of the PL area. The best local meteoric water line at PL was obtained by the reduced major axis regression (RMA) and precipitation-weighted ordinary least squares regression (PWLSR) approaches: &delta, 2HPWLSR = (7.97 &plusmn, 0.12) &delta, 18O + (13.8 &plusmn, 1.3). The higher deuterium excess at PL (14.0 &plusmn, 2.2 &permil, ) than that at Zagreb reflects the higher altitude and influence of the Mediterranean precipitation. The &delta, 2H in precipitation ranges from &minus, 132.4&permil, to &minus, 22.3&permil, and &delta, 18O from &minus, 18.3 &permil, 4.1&permil, The much narrower ranges in the groundwater (&lt, 1&permil, in &delta, 18O, &lt, 10&permil, 2H) indicate the good mixing of waters in aquifers and short mean residence times. The higher average &delta, 2H in all three karst springs observed after 2003 can be attributed to the increase in the mean air temperature. The mean &delta, 2H and &delta, 18O values in the surface and lake water increase downstream due to the evaporation of surface waters. There is no significant difference between the surface water line and the lake water line (2011&ndash, 2014). The stable isotope composition of the surface and lake waters reacts to extreme hydrological conditions.

The Aqueduct is one of the city landmarks of Skopje, Republic of North Macedonia. It was part of a water-supply system, with a total original length of about 10 km, while its surface remains are about 385 m long. The age of the Aqueduct is not known—several hypotheses place it to periods between the 6th and 16th centuries. Six mortar samples from different positions of the eastern façade were taken for radiocarbon (14C) dating. In order to extract only the carbon associated to the time of building, three strategies for sample preparation were used: (1) mechanical separation of lime lumps formed during mortar hardening (2) selection on the basis of particle size and the ability to suspend in water induced by ultrasonic shock, and (3) collection of two gas CO2 fractions produced from the same bulk in reaction with acid. Characterization of fractions was performed by isotopic carbon composition and FTIR-ATR analyses. The most plausible results were obtained from lime lump fractions that were dated in the timeframe of 15th to 17th century.

Six sediment cores, the top 30–40 cm, from two lakes of different sizes, Lake Prosce and Lake Kaluđerovac, situated in the karst region of the Plitvice Lakes system, Croatia, were studied by mineralogical and structural, C/N and total organic carbon (TOC) and isotopic analyses (a14C and δ13C of carbonate and organic fractions). The study of the two karst lakes where sediment composition is mainly calcite, showed that organic matter (OM) also plays an important role, and that both fractions analysed simultaneously give a good overview of the processes in the lakes. The morphological and structural properties of the calcite particles at micron and submicron levels indicate that the calcite mineral phase is formed by active authigenic precipitation in the studied lake systems. In the small Lake Kaluđerovac, the measured distributions of carbonate and OM fractions, C/N, TOC, and a14C and δ13C values of both fractions along the sediment depth showed similar values for all three cores, with small variations in each core indicating very homogenous precipitation of autochthonous calcite inside the lake, without allochthonous fraction. In the bigger Lake Prosce, the large difference in sediment composition of the three cores indicated different conditions of sediment precipitation inside the lake. Significant fractions of land-derived carbonate and OM were recognized by mineralogical, C/N, a14C and δ13C values in the shallow, coastal area indicating that the sediment was a mixture of allochthonous and autochthonous fractions. In contrast, the composition of the deep-water sediment in the same lake indicated in-situ calcite precipitation and aquatic OM produced in the lake. Sedimentation rates of 0.7 cm/yr and from 0.3 to 0.7 cm/yr for Lake Prosce and Lake Kaluđerovac, respectively, were determined based on the a14C of macrofossils. Extreme hydrological events in 1981 and 2010 were identified by disturbances in carbon isotopes distributions in sediment profiles. No evidence of anthropogenic influence in either lake on the local level was found, but the response to the global 14C contamination was observed by a14C peaks in both, carbonate and organic fractions. Increased bioproductivity in the recent decades was observed in Lake Prosce which can be correlated with a slight increase in lake water temperature in the last three decades. This process can be associated with enhanced eutrophication in Lake Prosce.

We present a new sea-level reconstruction for the past 1500 years based on biological indicators from the Central Adriatic islands. Biogenic littoral rims built by the coralline rhodophyte Lithophyllum byssoides were found on the particularly exposed sites on the rocky coasts of the islands of Vis, Ravnik and Bisevo in Croatia. The presence of thick and well-developed Lithophyllum rims, considered to be precise (± 10 cm) sea-level indicators, points directly to the rising sea-level environment. Biogenic rims were mapped, measured and sampled for 14C dating. The obtained results point to four phases of sea-level changes. The sea-level was near stable from around 550 till 770 cal AD, in the Dark Ages Cold Period (DACP), then during the Medieval Climate Anomaly (MCA) (770 till 1330 cal AD) the sea-level increased at a rate of 0.71 mm/yr. During the Little Ice Age (LIA) (1330 till 1640 cal AD) it was near stable again. Later, the sea-level started to rise at a much higher rate particularly during the Current Warm Period (CWP). These data were compared with local predictions derived from a glacio-hydro-isostatic models associated with the Last Glacial cycle. If the isostatic–eustatic component is separated, this area seems to have almost stable tectonic conditions during the past 1500 years. Our results show that the large algal rims most likely grew during near-stable sea-level conditions that occurred during two relatively colder periods in the past 1500 years. They also reveal that well-developed (up to 1.8 m wide) upper levels of algal rims were formed during ~ 300 years of stabilisation throughout the LIA.

Recent sediments in Lake Kozjak, in the Plitvice Lakes National Park, Croatia, were used to study the impact of the material delivered to the lake sediment via Rjecica Brook. Sediment cores, top 40 cm, were taken near the mouth of the Rjecica Brook at three locations in the lake at different distances from the shore, water depth 2-9 m and analyzed using different isotopic and chemical methods. Sediment depth profiles of the following parameters were obtained: organic and carbonate fraction, C/N analyses of organic fraction, 14C activity (a14C) and d13C values of carbonate (d13Ccarb) and organic fraction (d13Corg) and d15N of total nitrogen. The results were compared with sediment core collected in the middle of Lake Kozjak, water depth 21 m. Distribution of organic matter, C/N values, d13Corg and d13Ccarb in sediment profiles showed strong influence of terrestrial organic and carbonate fraction run-off via Rjecica Brook which decreased rapidly in the direction of the open lake. The sedimentation rate as well as the ratio of allogenic and authigenic fraction in sediments was estimated. Lower d13Corg values in surface sediments might be a consequence of the increase in algal productivity but might also be a contribution of fossil fuel combustion. The anthropogenicaly induced 14C in the atmosphere in 1963/64 is visible though shadowed in sediments.