engleski

Tufas in the Krka river system (Croatia): a suitable climatic record?

Three types of recent carbonate precipitates from the Krka River, Croatia, were analysed: bulk tufa from 4 main cascades in a 34 km long section of the river flow through the Krka National Park, a laminar stromatolite-like incrustation formed in the tunnel diverting water to the hydroelectric power plant close to the lowest cascade and recent precipitates collected during winter, spring and summer period on different substrates. Stable isotope compositions of carbon and oxygen in the carbonates, as well as organic carbon in the sedimentary organic matter were determined and compared to those of water and dissolved inorganic carbon. The source of DIC was determined from its stable isotopic composition ; it corresponds to the dissolution of CO2 with delta 13-C between -19.5 and -23o/oo VPDB. Observed difference between measured and calculated temperature of carbonate precipitation showed that the higher the water temperature, the larger is the error in the estimated temperature of precipitation. Similar relationship between actual temperature of precipitation and the error in calculated temperature of travertine formation was observed in Quaternary travertine north of Rome (Italy), as described by Minssale et al. (2002). This implies that the climatic signals may be valid only in tufas or travertines precipitated at lower and relatively stable temperatures. In contrast to findings described in the literature by several authors, precipitation on artificial substrates did not occur in or close to the equilibrium conditions on any of the substrates used (glass, wood, copper, natural moss, plastic scouring pads). Although precipitated under same environmental conditions and overall water chemistry, the precipitates had different carbon and oxygen isotopic composition, as well as different delta 13-C of organic carbon. Obviously, different organisms colonised the substrates and the riverbed, and different substrates favoured colonisation by different organisms. It is supposed that the surface characteristics of substrates play an important role in availability of nucleation sites for carbonate crystallisation and isotope fractionation caused by precipitation/dissolution of metastable carbonate phases. The colonising biota, on the other hand, can critically change the microenvironments where precipitation occurs, affecting primarily the carbonate delta 13-C values.

tufa; carbon; oxygen; stable isotope

nije evidentirano