engleski

The carbon transfer in Karst areas - an application to the study of environmental changes and paleoclimatic reconstruction

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.

karst; Postojna Cave; geochemistry; 13C; radiocarbon; stable isotopes; speleothem

Disertacija napravljena u okviru slovensko-francuskog doktorskog projekta (Co-tutelle) između Politehnike u Novoj Gorici, Slovenija (ko-mentor B.Obelić) i Université Paris-Sur iz Orsaya, Francuska (ko-mentor D.Genty).