C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia) |
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| Authors: | S Lojen T Dolenec†‡ B Vokal‡ N Cukrov§ G Mihel&#;i&#;§ W Papesch¶ |
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| Institution: | Department of Environmental Sciences, J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia (E-mail: ); Department of Physical and Organic Chemistry, J. Stefan Institute, Ljubljana, Slovenia; Department of Geology, Faculty of Natural Sciences and Engineering, Ljubljana, Slovenia; Centre for Marine and Environmental Research, Rudjer Bo?kovi?Institute, Zagreb, Croatia; Austrian Research Centre, Seibersdorf Research GmbH, Vienna, Austria |
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| Abstract: | Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ13C) and oxygen (δ18O) in the carbonate and organic carbon (δ13Corg) were determined and compared with δ18O of water and δ13C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line (DIC]/DIC0]?1) vs. (δ13C‐DIC × (DIC]/DIC0])) ( Sayles & Curry, 1988 ). The δ13C value of added DIC was ?13·6‰, corresponding to the dissolution of CO2 with δ13C between ?19·5 and ?23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ13C and decreasing δ18O. The lack of covariation between δ13C and δ18O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ13C and δ18O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation. |
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| Keywords: | Croatia freshwater carbonate stable isotopes tufa |
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