engleski

Lokvarka Cave (Croatia) microclimate and dripwater settings: Implications for comprehensive speleothem paleoclimate records interpretation

There are many systematic studies which emphasize the importance of understanding cave atmosphere and hydrology in order to select suitable stalagmites for paleoclimate analyses. In order to collect data which are important for reliable and confident interpretation of Lokvarka Cave speleothems palaeoclimate records cave monitoring program (surface and cave air temperature, relative humidity, dripping rates, precipitation and drip water stable isotope composition) was established through one-year period (November 2014 – November 2015). Lokvarka Cave is situated in the Lokvarka stream blind valley side, in Gorski Kotar area, the narrowest part of the Dinaric mountain range, at an altitude of 780 m a.s.l. It is a complex cave which consists of three main levels formed in lower Jurassic limestone. Depth of the cave is 277 m and length is 1200 m. Broader area of the Lokvarka Cave is characterised by Cfb climate, with high values of annual precipitation (>3000 mm) while MAAT recorded during 1981-2014 period is 8.3°C. Monitoring sites were situated at the depth of 40 m and 50 m in respect to entrance elevation. Estimated thickness of limestone overlay is about 80-90 m. On both sites, data loggers for measurements of cave air temperature, relative humidity and drip rates were installed. External air temperature and relative humidity were measured for the purposes of comparison with cave air properties and composite monthly meteoric water samples were collected in the house yards 2.5 km from the cave, while daily precipitation data are provided by CHMS for the meteorological station Delnice (6 km from LOK). During the given monitoring period mean cave air temperature was in the range between 7.3 and 7.7 °C. Relative humidity of the cave air was between 99.9% and 100%. Mean annual air temperature and relative humidity in front of the cave were 9 °C and 85.1%. Both cave dripping sites display fracture- flow regime with fast response to rain events. Local meteoric water line δ2H = 7.31 × δ18O + 12.03, and d-excess of 17.9‰ suggests that monitored area is influenced by cool North Atlantic and warm and wet eastern Mediterranean air masses. δ18O and δ2H weighted means of collected drip water are close to the precipitation weighted means, but positioned slightly towards more positive part of LMWL, notwithstanding that the seepage of the water prevails in colder part of the year. Inter-annual δ18O variation of 8.6‰ in precipitation has been reduced to 1.3‰ and 0.3‰ at the drip sites which is in concordance with their fracture-flow drip regimes. Obtained values indicate that Lokvarka Cave has stable cave atmosphere suitable for equilibrium deposition of calcite in terms of constant air temperature and relative humidity and therefore is appropriate for further paleoclimate studies.

Lokvarka Cave; paleoclimate; Gorski kotar; variations of δ18O and δ2H in cave drip water; local meteoric water line

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