engleski

Isotope composition and morphological and structural properties of freshwater calcium carbonate from the Plitvice Lakes (Croatia)

14C activity (a14C), δ13C and δ18O of freshwater carbonates precipitated in the waters of the Plitvice Lakes, Croatia, were measured at different locations exhibiting different conditions of precipitation. In addition, structural and morphological properties of calcium carbonate precipitates were studied. X-ray diffraction indicated the calcite as the main fraction of the carbonates. For this study we collected 3 types of carbonates (CaCO3) precipitate in the water: 1) suspended CaCO3 precipitated in 4 sediment traps, 2-10 m below the surface in 3 lakes, 2) CaCO3 precipitated on porous plastic pads submerged in the water (surface water) at 7 locations with different water flow rates (0 to 0.7 m/s), and 3) surface lake sediments (top ~40 cm) at the bottom of 4 lakes. δ13Cof dissolved inorganic carbon (DIC) and δ18O of waters were also measured. Carbonates were collected in different seasons in period 2011 – 2013.a14C values showed different ranges for different types of carbonates. Suspended CaCO3 in sediment traps showed the highest a14C values (76.4 – 81.8 pMC) with increasing values in downstream flow, a14C of surface lake sediment varied from 71.6 to 81.0 pMC, while CaCO3 precipitated in the surface water showed significant variation from 10.3 to 82.0 pMC. δ13C values for all carbonates are generally in the range from -10 ‰ to -6 ‰ which is typical for freshwater carbonates, but the difference between 3 types of carbonates was obvious.δ13C values for CaCO3 in 4 sediment traps were very uniform, mean values from -8.5 ‰ to -7.5 ‰ showing stable conditions of carbonate precipitation in deep water.Difference between δ13CDIC and δ13Ccarb was between2 and 4 ‰, indicating partial equilibrium conditions of precipitation. Narrow range of δ13C values of surface lake sedimentsin 4 lakes (mean values -9.5 ‰ to –7.9 ‰) showed similar stable conditions of CaCO3 precipitation, also in deep water. δ13C values of CaCO3 precipitated in surface waters showed variations from -10 ‰ to -4 ‰ depending on CONT-O.05 INDEX AUTHORS 1 locations. At 2 out of 7 locations the higher δ13C indicated the mixing of authigenic CaCO3 and detrital CaCO3 of mineral origin. This was also indicated by a14C values in the same locations which were much lower (10 – 30 pMC) comparing with other 5 locations (70 – 80 pMC). δ18O values of surface water showed seasonal fluctuation (-10.9 to -9.8 ‰), while the δ18O values of CaCO3 showed more significant fluctuation depending on micro-location. Good correlation of δ18O of CaCO3 from 4 sediment traps with mean water temperature in period of precipitation was obtained. δ18Ovalues of water and CaCO3 were used for estimation of temperature of carbonate precipitation. The best agreement of estimated with the real/measured temperatures at all deep waters/traps was obtained by the approach of Leng and Marshall (2004). It was demonstrated that the formation and morphogenesis of calcium carbonate solids is governed by physicochemical properties occurring in water system. The results obtainedhighlight the importance of the recently reaffirmed mechanism based on the role of nanoscale processes in the formation of structurally and morphologically different calcite precipitates in natural environments. Thisresearch was conductedthrough the scientific projectsfinanced by the Plitvice National Park: Influence of environmental and climate changes on the biologically induced calcite precipitation in form of tufa and lake sediment in the Plitvice Lakes and Croatian Science Foundation, Project HRZZ-1623. References Leng, M. J. and Marshall, J. D., 2004.Palaeoclimate interpretation of stable isotope data from lake sediment archives. Quaternary Science Reviews 23, 811–831

freshwater carbonate ; stable isotope ; radiocarbon ; Plitvice Lakes

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