S7 - - CD 2- OO/l S10200001 Nova Gorica Polytechnic, Slovenia AN APPLICATION TO THE STUDY OF ENVIRONMENTAL CHANGES AND PALEOCLIWATIC RECONSTRUCTION Doctoral thesis Barbara Vokal Nova Gorica, November 1999 © 9 PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK CIP - Katalozni zapis o publikaciji Narodna in univerzitetna knjiznica, Ljubljana 550.4:551.44(043.3) VOKAL, Barbara The carbon transfer in karst areas - an application to the study of environmental changes and paleoclimatic reconstruction : doctoral thesis / Barbara Vokal. - Nova Gorica : Polytechnic, 1999 ISBN 961-6311-03-4 104048384 This doctoral thesis was supported by the Ministry of Science and Technology of Slovenia. Co- operation with the French University Paris-Sud was supported by the Ministry of Foreign Affairs of France. ACKNOWLEDGEMENTS The author is grateful to the supervisors Dr. Ivan Kobal, Dr. Bogomil Obelic and Dr. Dominique Genty for their guidance in the formulation of this doctoral work. I wish to acknowledge and thank the members of my committee for their evaluation of my doctoral thesis: Prof. Dr. Tadej Dolenec, Prof. Dr. Andrej Kranjc, Prof. Dr. Nikola Kallay and Prof. Dr. Laurent Dever. Permission for my experimental work in Postojna cave was granted by Mr. Alojz Cernigoj. For assistance during sampling in the cave I am grateful to Mr. Stanislav Glaiar, Mr. Vilijem Cerne and Mr. Stojan 2igon, who also helped me with the sample measurements. Thanks also to Dr. Sonja Lojen, Dr. Nives Ogrinc and Mag. Polona Vreda who helped me with the sample measurements. Many thanks go to Dr. Ede Hertelendi and Mrs. Magdolna Sandor for their help with the measurements and in the development of this doctoral work. Mr. Mark Massault was of great assistance in preparation of samples and measurements with Accelerator Mass Spectrometry. Mr. Dan Doctor corrected the English grammar of my manuscript and I am grateful to him. Thanks to my parents for offering me moral support throughout my work on this thesis. Tomaz, thank you for everything. SUMMARY Karst areas 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 speleothems 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 important 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 2+ 2+ of the water such as pH, electrical conductivity, Ca , HCO3" and Mg 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 spring Mo6ilnik. Rainwater samples were also collected and analysed. Dissolved CO2 and carbonate in cave seepage waters originate from various sources: atmospheric CO2, organic matter that decomposes in soil, CO2 from respiration of plants in the soil, and the dissolution of old layers of limestone. The isotopic composition of speleothems 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 stalagmite. The growth rate of speleothems is partly influenced by climatic variations. When speleothems grow in isotopic equilibrium with their parent seepage waters, the isotopic composition of ancient speleothems gives a record of the paleotemperatures 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 paleotemperature impossible. This research represents a significant contribution to the improvement of knowledge about the karstic world, especially regarding the influence of climatic and environmental factors on karstic phenomena. This is the first interdisciplinary investigation in the Postojna karst system of this kind, combining the determination of environmental factors together with chemical and isotopic measurements. This research enabled the determination of the processes of carbon transfer from the surface to the cave, where speleothems are formed. This research was performed in co-operation with scientists from the Rudjer Boskovi6 Institute, Zagreb, Croatia and the University of Paris-Sud, Orsay, France. CONTENTS Introduction 5 Theory 8 The carbon in the nature 8 Carbonate equilibria 10 Isotope study 15 Stable isotopes 15 Radioactive isotopes 19 The formation of speleothems 22 Speleothem growth rate theory 25 Speleothems and climate 29 Modelling 29 Site description 33 Methods 37 Sampling 37 Measurements 41 Results and discussion 44 Chemical measurements 44 Temperature 44 pH 45 Calcium and bicarbonate ion concentrations and electrical conductivity 46 Mg/Ca ratio 57 Saturation index 60 Growth rate 62 Isotope measurements 65 Atmosphere 65 SoilCO2 65 Soil organic matter 67 Water. 69 Speleothems 76 Modelling 79 Conclusions 82 References 85 INTRODUCTION The word karst can be traced back to pre-lndo-European origins (Gams, 1973, 1981). It stems from karra, meaning stone, and its derivatives are found in many languages of Europe and the Middle East. In Slovenia the word evolved via kars to kras, which in addition to meaning stony, barren ground is also a regional name for a district on the Slovenian/Italian border. This district is sometimes referred to as the "classical karst', being the type locality where its natural characteristic first received intensive scientific investigations. Caves in Karst region provide unique and potentially valuable resources for the reconstruction of environmental conditions on the continent over the last few hundreds of thousands to millions of years of earth history, mostly due to the great stability of climatic conditions within a cave. Several thousands of caves of different dimensions and with prevailing limestone and dolomite deposits have been found in Slovenia (Habic, 1982). In the karst region of the underground Ljubljanica River, the Postojna Karst represents the most speleologically interesting region. Since its discovery in 1818, several works more or less completely present the history of exploration and research in the Postojna Cave system (Gospodaric, 1976; Kranjc, 1997). With the exception of chronostratographic measurements performed in the last 20 years, some employing 14C and 23OTh/234U dating methods, little work has been done on isotopic investigations of deposited calcite (Brodar, 1952; Brodar, 1956; Gospodaric, 1972; Gospodaric, 1977; Gospodaric, 1981; Gospodaric, 1985; Urbane, et. al, 1985; Gospodaric, 1988; Zupan, 1991; Zupan-Hajna, 1993). Karst terrain is complex, and it is difficult to understand the processes that are occurring in such systems composed of distinctive hydrology and specific landforms. Karst landforms are produced as a result of combined rock solubility and well developed secondary porosity. Secondary minerals deposited in caves, known as speleothems, consist of several forms such as stalagmites, stalactites and flowstones. The purity and chemical and physical stability of the speleothems make them exceptionally well suited for detailed geochemical and isotopic analysis. Speleothems preserve a record of long-term climatic and environmental changes at the site of their deposition and within the regional vicinity. The oxygen isotopic composition of speleothems ideally reflects the isotopic composition of the regional annual precipitation at the time of deposition. However, it is difficult to reconstruct the paleoclimatic conditions based solely upon oxygen because the oxygen isotopic content of dripping water in the past is not known. (Yonge et al., 1985; Ford and Williams, 1989; Hendy, 1971; Issar era/., 1991; Talma and Vogel, 1992). Speleothems are also of great importance for Quaternary studies, because they may