Maša Surić

Modern C, O, and H isotope composition of speleothem and dripwater from Modrič Cave, eastern Adriatic coast (Croatia)

Modric Cave is a shallow horizontal cave situated in the middle of the eastern Adriatic coast (Croatia). The cave entrance is located 120 m from the coast at an altitude of 32 m above sea level, and due to its position on the SW slope of the Dinaridic mountain range, a Mediterranean climatic influence is dominant. Due to the stable environmental conditions [(15.6 ± 0.1) °C] Modric Cave was recognized as a potential site for detailed palaeoclimatic studies. Isotope analyses of modern carbonate speleothems, rain and dripwater were conducted in order to evaluate the isotopic equilibrium conditions. The δ18O composition of rain and cave seepage waters shows an absence of kinetic isotopic fractionation within the epikarst zone, whereas the relation between δ13C and δ18O in modern carbonate samples and dripwater suggests the isotopic equilibrium conditions during the carbonate deposition. These results contribute to a better understanding of the present-day isotopic composition and they provide a basis for interpretation of speleothem- derived palaeoclimatic records.

Isotope records in submarine speleothems from the Adriatic coast, Croatia

Isotope studies, using 14C dating, [sigma]13C and [sigma]18O measurements, were performed at eight speleothems taken from three submerged caves situated along the eastern Adriatic coast, Croatia. The speleothems were taken from 17 m to 38.5 m depth below mean sea level. The samples consist of four stalagmites and four stalactites in position of growth, covered with marine biogenic overgrowth, and the length of speleothems ranges from ~80 mm to ~190 mm. The youngest (surface) and the oldest (base) layers of speleothems were radiocarbon dated and the 14C ages range from 21,600 cal B.P. to 37,000 yr B.P. During that period the global sea level was more than 40 m below the recent one, so presently submerged objects were under the subaerial conditions necessary for speleothem deposition. 14C ages of the youngest layer range from 21,600 to 32,200 cal B.P. for different submerged speleothems. This indicates the time when the speleothem growth ceased, most probably due to flooding of the cave with either fresh or brackish water. Speleothem growth during the Last Glacial Maximum (30-19 kyr ago) and different time of growth cessation for the different speleothem samples suggest that climate change was not the reason for cessation of deposition. Samples for [sigma]13C and [sigma]18O measurements were taken from six submerged speleothems with sampling distances of ca. 5-10 mm from the surface to the base of speleothems. Most of the [sigma]13C values are in the range from -10.5%o to -8.5%o, with few exceptions to -6%o. These values are typical for Dinaric karst, and very different values for marine biogenic overgrowth indicate that no isotopic exchange took place during the submerged period. [sigma]18O values range from -6.7%o to -4.1%o. A weak correlation between [sigma]13C and [sigma]18O values indicates possible kinetic isotope fractionation during the calcite precipitation. If the [sigma]18O record is interpreted as climatic signal, it suggests similar climatic conditions for the late Pleistocene and the Holocene, especially no significant differences in temperature and/or moisture transport.

Influence of submarine groundwater discharge on seawater properties in Rovanjska-Modrič karst region (Croatia)

Coastal karst aquifers are dynamic hydrogeological systems with substantial hydrological, geomorphological and ecological impacts on the marine and coastal karst ecosystems, mostly induced by the submarine groundwater discharge (SGD). This paper presents complex hydrogeological settings of the Rovanjska-Modrič coastal karst region on the eastern Adriatic coast characterized by: (1) high temporal and spatial variations in seawater properties; (2) significantly reshaped coastal bottom morphology, and (3) plethora of associated biocenoses from fresh water, brackish, marine and cave assemblages. With the aim of resolving spatiotemporal interconnections of SGD and marine environment changes, we conducted field prospection and seasonal measurements of different water properties on- and off-shore. Obtained results pointed to: (1) the composite recharge system consisting of numerous sinking rivers in the distant hinterland (with different climate settings) and local precipitation input; (2) specific discharge settings highly governed by a complicated karst conduits network, the hydrological situation in the hinterland and local meteorological conditions, i.e. one system that permanently feeds coastal and probably submarine springs, while the other operates periodically; (3) disturbed winter isothermy and three-layered summer stratification of the seawater column, and (4) slightly changed trophic settings. All of that is the consequence of extreme heterogeneity of the coastal karst aquifer, characteristic for the eastern Adriatic coast.

Groundwater quality in selected caves in Ravni kotari (Croatia)

Complex mechanisms of pollution transport in karst underground and the fact that contaminated cave water can be a health hazard if used for human consumption, imposes the need for monitoring the cave water quality. In order to determine groundwater quality, water samples were collected in four caves in Ravni kotari (Croatia) and analyzed for microbiological and chemical indicators of pollution. Microbiological analyses showed increased concentration of various coliform bacteria at all four locations. Probable sources of pollution are inadequately built septic tanks in nearby villages and bat guano in caves with bat colonies. Chemical analyses indicated values of possible pollutants to be much lower than the permitted values, which reflects the fact that agriculture, as a frequent source of chemical pollution in rural regions, is not as developed as it used to be, thus posing no serious threat to the groundwater quality.