engleski

Continuous 60-year stable isotopic and alkali element records in a modern laminated tufa (Jaruga hydroelectric power plant, river Krka, Croatia)

A continuous 60-year record (1938 – 1998) of stable isotope compositions of carbon and oxygen, as well as trace metal (Mg, Sr, Ba) concentrations in a laminated calcite crust precipitated in a short artificial tunnel on a relatively large, non-equilibrium groundwater-fed karstic river is presented. Chemical and isotopic records were compared to hydrometeorological data, available for the last 48 years of precipitation. An attempt was made to relate isotopic and geochemical variations in the crust to environmental parameters, such as temperature, precipitation and changes in vegetation cover, as well as to postdepositional recrystallisation of the older part of the crust. Isotopic composition of the crust is largely influenced by non-equilibrium precipitation, which favours incorporation of isotopically depleted C and O into the carbonate. Furthermore, because of the complicated hydrological situation, the stable oxygen isotope composition of water was not correlated with temperature. The 18O isotopic thermometers thus largely overestimated the measured precipitation temperatures. Temperatures calculated from Mg/Ca ratios of water and the carbonate match the δ 18O palaeotemperatures within ± 2.4°C in the older part of the crust, precipitated before the onset of industrial pollution of the river. It was demonstrated that the application of Mg palaeothermometry in natural systems, where the Mg/Ca ratio of water is influenced not only by the temperature, but also by other environmental parameters (precipitation, surface runoff, groundwater retention time, anthropogenic influences etc.) is subject to large uncertainty, reaching up to 10°C.

Laminated tufa ; Stable isotope ; Mg ; Palaeotemperature ; Uncertainty

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