engleski

Isotopic characteristics of tufa in Krka River, Croatia : relation to climate conditions

Three types of recent carbonate precipitates from the Krka River, Croatia, were analyzed: bulk tufa from 4 main cascades in a 37 km long section of the river flow through the Krka National Park, laminar stromatolite-like incrustation formed in a tunnel of a hydroelectric power plant at the lowest cascade and recent precipitates collected on artificial substrates through winter, spring and summer season. Hydrochemical (Ca2+, Mg2+, total alkalinity, pH and temperature) and isotopic (18O, ; D, ; 13C of dissolved inorganic carbon) parameters of the river water were observed through a 2-year period to explore the conditions of precipitation of recent carbonate. The river is predominantly recharged by groundwater, which influences the isotopic composition and some chemical parameters of river water. Downstream changes in 18O of the river water are mostly due to evaporation. The source of dissolved inorganic carbon, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC0] – 1) vs. (13C-DIC ([DIC]/[DIC0])) (Sayles and Curry, 1988). The 13C value of added DIC was estimated to be -13.57‰ , which corresponds to the dissolution of CO2 with 13C between - 19.47 and – 22.97‰ V-PDB. Bulk tufa precipitates collected in the riverbed show similar 13C and 18O values as ancient tufa precipitates from other locations in Croatia. They were precipitated under non-equilibrium conditions for both carbon and oxygen. The difference between equilibrium and observed temperature of precipitation vs. actual temperature of precipitation shows a linear relationship (r2 = 0.97), indicating that the higher the temperature, the larger is the error in estimated temperature of precipitation, which makes 18O parameter useless for determination of climate conditions of tufa precipitation in the studied area. Laminar crust comprising a continuous record of last 40 years, which was precipitated on the ceiling of a concrete tunnel from spraying water, shows a consistent trend of increasing 13C and decreasing 18O. Lack of co-variation between 13C ad 18O indicates that precipitation of calcite was not kinetically controlled for either of the elements, however, it is difficult to determine whether the dark and light lamina originate from biological activities or different rate of precipitation in summer and winter period. Precipitation on artificial substrates occurred in non-equilibrium conditions, showing no consistent seasonal trends or relation to the substrate.

tufa; Krka River; Isotopic characteristics

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