engleski

Microclimate and dripwater hydrology in Nova Grgosova Cave (Croatia) - results from one-year monitoring for paleoclimate study

Nova Grgosova cave is located in Otruševec village in Samobor hills fluviokarst unit (NW Croatia). The cave was discovered in 2004 during construction works. Before that, there was no natural entrance to the cave. Few years after the discovery, due to the speleothem richness, it became a show cave. It is 97 m long and 14 m deep simple cave developed in intensively karstified Neogene lithothamnium limestone. It consists of two chambers connected with narrow, partially artificially widened passages. According to the geomorphological position, passage morphology and recorded fluvial deposits, in the past it was a ponor of a nearby stream. Considering hydrological zoning important for speleothem deposition, microclimate and dripwater hydrology and chemistry, the cave today extends in epikarst zone. The entrance is at 240 m a. s. l. and limestone overlay above the cave is 5-10 m thick. The area has temperate humid climate with warm summers (Cfb type after Köppen-Geiger climate classification system). Mean annual air temperature in Samobor climatological station (ca. 3 km to the SE) for the 1981-2013 period was 11.2 °C. The highest mean monthly temperature occurs in July (21.7 °C), and the lowest one in January (0.5 °C). The mean annual precipitation is 1076 mm. Most of the precipitation occurs during the summer and early autumn (September 116 mm). The lowest amounts occur during the winter (February 57.4 mm).The monitoring program, a precondition for the speleothem-based palaeoenvironmental study took place from November 2014 to November 2015 and included high- resolution logging of surface and cave air temperature, relative humidity and drip intensity. Mean annual air temperature and relative humidity in front of the cave were 11.4 °C and 82.5% and in the cave 11.2 °C and 99.9%. In the cave, three drip sites located at the distances of less than 7 m, showed quite different discharge regime illustrating characteristic triple-porosity (conduits, fracture and matrix). According to flow regime characterized by the relationship between mean discharge and discharge variability, two of them were classified as seepage and one as seasonal drip class. Isotopic composition of the meteoric precipitation (δ18O and δ2H values and their relationship) reveals the vapor source region. Nova Grgosova cave local meteoric water line (δ2H = 7.1 × δ18O + 7.9) lies above the global meteoric water line (δ2H = 8 × δ18O + 10), and precipitation δ18O-δ2H pairs plot between eastern Mediterranean MWL and western Mediterranean MWL, but closer to the western one. It shows the influence of transitional zone with mixing influence of cooler North Atlantic and warmer and wetter eastern Mediterranean air masses. Seasonal amplitude of cave precipitation δ18O is 0.3-0.6‰ and δ2H 1.0-2.7‰ in drip water. Mean weighted δ18O and δ2H values are slightly more negative than the precipitation means suggesting prevalence of the winter precipitation. Longer measurements should verify these preliminary results, but it seems that this cave is promising site for paleoenvironmental reconstruction.

microclimate; hydrology; stable isotopes; Nova Grgosova Cave; Croatia

Projekt HRZZ-a: Reconstruction of Quaternary Environments in Croatia using Isotope Methods (2014- 2018)