engleski

Altitudinal and seasonal isotope variation in meteoric and cave drip water in north Dalmatia (Croatia) – importance for the palaeoenvironmental studies

Palaeoenvironmental studies commonly rely on proxy records from different sources, one of which are speleothems – secondary carbonate deposits precipitated from the dripping groundwater within the cave environment. In order to ascribe isotopic signal in speleothems to particular past events, it is necessary to understand the hydrological behaviour and chemical variations of the water on its path from the vapour source, via precipitation, through the soil, towards the cave drip sites. Water's spatially and temporally dependant variations have been recently monitored in north Dalmatia (Croatia) at three locations with different climate settings and altitude: Strašna peć Cave on Dugi otok Island (74 m a.s.l.), Manita peć Cave in Velika Paklenica canyon (Velebit Mt, 570 m a.s.l.) and Spilja u Zubu Buljme Cave (Velebit Mt, 1250 m a.s.l.). We conducted: i) 2-year monitoring of (micro)climate elements (air temperature /T/ and relative humidity /RH/) inside and outside the caves ; ii) 1-year collecting of the meteoric and cave drip water on monthly basis for the stable isotope (δ18O, δ2H) analyses, and iii) 1-year drip intensity monitoring to estimate the correlations to the rain events. Isotopic composition of the precipitation showed expected altitude effect (δ18O decrease with increased altitude), which substantially attenuated in the cave water. In spite of geographical position which suggests that the major atmospheric influence would be of the Mediterranean origin, overview of the isotopic imprint from the surrounding continental and coastal GNIP (Global Network for Isotopes in Precipitation) stations displays more complex situation, due to the Adriatic Sea deeply indented into European continent and near-coast orographic structures. Local meteoric water lines significantly differ from the global meteoric water line both by the intercept and the slope, showing mutual influence of the Atlantic and Mediterranean air masses, being similar in the low- and mid-altitudes, but quite different in the highest region. Isotopically homogenized water that enters the cave is one of the prerequisites when planning speleothem-based palaeoenvironmental studies, and regardless of the drip intensity and response to the rain events, that condition was fulfilled at all drip sites. Along with typically stable cave environment (constant T and RH), it was rational to expect carbonate deposition in isotopic equilibrium with drip water, which would ensure that only cave temperature affected isotopic fractionation and the isotopic signal would be reliable in terms of palaeoclimate variations. Sadly, Spilja u Zubu Buljme had adversely high T variations (>5 °C), but Manita peć Cave and Strašna peć Cave apparently offer a good potential for further palaeoenvironmental studies.

meteoric water; cave dripwater; isotopic composition; palaeoenvironment; Croatia

Projekt Sveučilišta u Zadru 60200 (2012-2015): Rekonstrukcija regionalnih paleoklimatskih promjena – zapisi iz siga sjeverne Dalmacije