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Tufa formation at Zrmanja and Krupa rivers, Croatia

Presented study of Zrmanja River is part of the project Reconstruction of the Quaternary environment in Croatia using isotope methods financed by Croatian Science Foundation (2014-2018). The aim of the study is reconstruction of palaeoenvironment / paleoclimate settings from fresh-water carbonates/tufa and comparison with the similar areas in the Dinaric Karst. Zrmanja River is 69 km long, located in the Dinaric karst, the NW Dalmatian plateau, and flows into the Adriatic Sea. Zrmanja River and its tributary Krupa River are characterized by tufa precipitation in water stream forming tufa barriers. Results presented here cover analyses of chemical parameters of Zrmanja River important for authigenic carbonate precipitation, isotopic composition of water as well as isotopic analyses of fresh and old tufa deposits. Sampling of water covered: 9 locations at Zrmanja river, 3 at Krupa river and confluence of Krupa to Zrmanja river. In situ measurements revealed high seasonality of T (8.8 - 27.5°C), while pH shows a narrow span (7.38 - 8.34) independent of season. Concentrations of calcium, magnesium and bicarbonates showed neither seasonality nor downstream change. Calculated values of saturation index of CaCO3 from ≈1 at springs to 5 within the streams of both rivers indicate conditions favorable for tufa precipitation. Radiocarbon activities of dissolved inorganic carbon (a14CDIC) from Zrmanja river showed increase from ≈78 pMC at springs to ≈90 pMC downstream, and δ13CDIC values were quite uniform (-13.1 – (-11.4)). On the contrary, δ13CDIC from Krupa river showed downstream increase, while a14CDIC remained almost constant (≈88 pMC). a14C of recent tufa was 83 pMC (1516 BP), and old tufa deposits showed range of a14C from 75 pMC (≈1m above water level) to almost 0 pMC (≈30m above water level) indicated that the oldest tufa deposits could be older than 50000 years and should be dated by U-Th method. However, on the basis of presented results, it can be concluded that tufa precipitated intensively in the past, and the rivers were positioned at much higher elevation.

karst ; Zrmanja river ; Krupa river ; physico-chemical parameters ; tufa precipitation ; radiocarbon dating

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