Search

Environmental and climatic changes are potentially recorded in recent terrestrial carbonates, such as speleothems or tufas. Stromatolite-like crusts have been the subject of great interest since they contain mm-size laminae, which are considered to represent annual climatic records. The origin of the light and dark layers has been attributed to seasonal variations in microbial assemblages and their activities, water chemistry and carbonate precipitation rates. In conclusion, both oxygen isotope, as well as geochemical records in the laminar crust to a certain extent reflect the temperature and other environmental conditions during the period of precipitation. However, the exact estimation of the precipitation temperature is not yet possible.

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 (d13C) and oxygen (d18O) in the carbonate and organic carbon (d13Corg) were determined and compared with d18O of water and d13C 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. (d13C-DIC × ([DIC]/[DIC0])) (Sayles & Curry, 1988). The d13C value of added DIC was -13&middot ; ; 6‰ , corresponding to the dissolution of CO2 with d13C between -19&middot ; ; 5 and -23&middot ; ; 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 d13C and decreasing d18O. The lack of covariation between d13C and d18O indicates that precipitation of calcite was not kinetically controlled for either of the elements. d13C and d18O 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.

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.

The isotopic values of carbon were determined in air, various soil components, different types of cave waters, and in the growth laminae of stalagmite. Outdoor atmospheric and cave air delta-13C and 14C concentrations were determined and compared to evaluate any influence of biogenic carbon inside the cave.

Karst terrain is complex and it is difficult to understand the processes that are in course in this system composed of distinctive hydrology and specific landforms. Slovenia is a classic Karst area with prevailing limestone and dolomite deposits. Several thousands of caves of different dimensions have been found in Slovenia. Postojna Cave was chosen for sample collection. Therefpre we expect this work to be a detailed study of carbon isotope transfer in the Karst, enabling us to see how the Karst can record environmental changes. This study was performed by measuring 13C and 14C contents in dofferent components formed in the Karst: surface air, soil CO2, organic matter, dripping water at different depths, cave air and recent cave deposits (stalagmites).

Environmental factors influencing speleothem formation such as air and water temperature as well as water properties at the surface and in the cave (pH, electrical conductivity, Ca2+, HCO3-, and Mg2+ ion concentrations and drip rates) were measured during an antire year in order to follow seasonal variations. The influence of the thickness of cave roof on water properties was also taken into consideration, and the difference between varius cave water types (pool, stalactite drip waters and fast-drip waters) was followed. Monthly water samples were collected at three locations in the cave and also from the river Pivka and sping of Močilnik. Rainwater samples were also collected and analyzed. From the results it was found that the temperature of cave waters is more or less constant during the year. Mean values of pH for karst waters are 7.8 +- 0.2. There is a good correlation between the chemical parameters and the different types of cave watres. The ratio Mg/Ca concentrations and the saturation index were determined, too.

Karst area constitute a large part of Slovenia, with several thousands of caves located in the limestone formations. The karstic caves provide valuable resources for reconstruction of environmental conditions on the continent in the past. This is possible due to the great stability of climatic conditions within a cave. Secondary minerals deposited in caves, known as speleothems, preserve records of long-term climatic and environmental changes at the site of their deposition and in the vicinity. The purity of speleothemsa and their chemical and physical stability make them exceptionally well suited for detailed geochemical and isotopic analysis. To understand the processes influencing the speleothem isotopic composition, monitoring of cave waters as well as springs and underground rivers is very imposrtant for at least one hydrological cycle. In this way we can observe the influence of seasonal changes on the cave waters feeding stalagmite growth laminae. Chemical and environmental parameters which influence speleothem formation are the temperature of air and water, as well as the properties of the water such as pH, electrical conductivity, Ca2+, HCO3-, and Mg2+ ion concentrations and drip rate. The thickness of the roof above the cave and the types of cave water may also determine the water properties. During one year monthly water samples were collected at three locations in Postojna Cave to characterize different types of cave waters (pool, fast and stalactite drip water), and also from the river Pivka and sping Močilnik. Rainwater samples were also collected and analysed. Dissolved CO2 and carbonate in cave seepage waters originate from various sources: atmospheric CO2, organic matter that decomposses in soil, CO2 from respiration of plants in the soil, and the dissolution of old layers of limestone. The isotopic composition of speleothem is directly linked with the isotopic composition of CO2 produced in the soil above the cave. Production of CO2 in the soil is connected to climatic changes and the isotopic composition of the CO2 is connected to the type of vegetation growing above the cave, according to the photosynthetic mechanism employed by the plants. The influence of human activity on carbon isotopic values of atmospheric CO2 due to fossil fuel combustion and nuclear experiments is also observed in speleothem carbonate composition. For this study, the isotopic values of carbon, hydrogen and oxygen isotopes were determined in air, various soil components, different types of cave waters, and in the growth laminae of a stlagmite. The growth rate of speleothems is party influenced by climatic variations. When speleothems grow in isotopic equilibrium with their parent seepage water, the isotopic composition of ancient speleothems gives a record of the paletemperatures of the cave in which these are formed. When the temperature in a cave is stable throughout the year, this temperature is equal to the mean annual temperature on the surface and so we can estimate the temperature above the cave for that year. By analysis of the stalagmite from Postojna cave we determined that the stalagmite deposition was not in isotopic equilibrium with its parent seepage water, thus making determination of paleotemperatures impossible.

U radu su prikazani rezultati praćenja radioaktivnog izotopa 14C u atmosferskom CO2, koja se na Institutu Rudjer Bošković provode od 1978. godine. Opažen je stalan pad srednje godišnje koncentracije 14C u atmosferskom CO2, uz određene sezonske varijacije tijekom godine. Uspoređene su mjesečne koncentracije 14C u CO2 sakupljenom na Medvednici (Puntijarka) i u Zagrebu (Institut R. Bošković). Značajne razlike opažene su tijekom zimskih mjeseci, kada je zbog izgaranja fosilnih goriva u gradu Zagrebu smanjena koncentracija 14C u odnosu na "čistu" atmosferu (Puntijarka). Prikazani su također rezultati mjerenja 14C aktivnosti u CO2 unutar i ispred Postojnske jame u Sloveniji.

Speleothems are secondary carbonate deposits which form often cave systems in karst and consist of several forms such as stalagmites, stalactites and flowstone. They are deposited from groundwaters with increased carbon dioxide concentration. This carbon dioxide is derived from the high partial pressure of CO2 in the soil atmosphere, generated by microbial processes and root respiration. Stalagmites and stalactites are formed by the degassing of ground waters from drip sources within caves, and flowstone is formed by the same mechanism, but in higher-discharge cave steeps and streams. The oxygen isotope composition of cave seepage water depends not only on the average value of precipitation, but also on evaporation processes occurring in the epikarst zone. The d13C values of speleothems can provide important paleoclimatic information because the carbon isotopic composition of seepage water is strongly influenced by the uptake of soil-CO2. The d13C values of the water will depend on whether the soil CO2 is derived from C3 type vegetation with the Calvin-Benson photosynthetic pathway or from C4 type vegetation with a Hatch-Slack photosynthetic pathway. Water samples were collected monthly in three different locations in the Postojna cave. The sampling locations were chosen to cover all types of cave water as well as varying positions relative to the thickness of the rock cover. Samples were collected also from the river Pivka and spring Močilnik. The results of a year-round measurements of d13C and d18O in karst waters will be presented.