engleski

Characterization of the cave environments for the palaeoclimate studies – from Dugi otok Island to Velebit Mt (Croatia)

Speleothems have been recognized as excellent archives of palaeoenvironmental changes since they form from the solution that carries information from the atmosphere in its temperature dependent isotopical composition. In order to assess eligibility of the particular cave and cave sediments for reconstruction of the palaeoenvironments at the different altitudes, we analysed cave microclimate settings and isotopic composition of precipitation and cave dripwater along the transection from Dalmatian islands to Velebit Mt peaks (Croatia). We selected Strašna peć Cave on Dugi otok Island (70 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, 1305 m a.s.l.). The aim was to determine cave microclimate conditions and to characterize the hydrological behavior of drip sites which fed the stalagmites already taken for analyses from Manita peć Cave, and of those predicted for future analyses from Strašna peć Cave and Spilja u Zubu Buljme Cave. Due to the topographic locations of the caves in epikarst and shallow vadose zone near the summits, recharge areas of all three caves are limited, and source of the cave drip water is restricted to local precipitation without any influence of mixing with the groundwater aquifer. Because of the relatively shallow overburden, infiltration elevation i.e. altitude of the catchment, is just slightly above the elevation of the cave. Modification of the isotopic composition can occur during transition through the soil zone and epikarst, but in our case of bare karst, soil (sparse terra rossa) is negligible, and overlaying bedrock is 2-80 m thick. The lowermost Strašna peć Cave (SP), although relatively small, shows quite stable microclimatic conditions in its farthermost part, having Tav= 10.8 °C (ampl. 1.8 °C), and RH = 100%. However, it stands out with significant aberration from the mean annual surface air temperature (T= 16.4 °C). Although we have only one year data set, this difference could be attributed to the general cave morphology (descending passage suitable for colder air trap, long enough passage that separates entrance part from the back chamber where measurements have been performed and fissure air flux during colder winter winds). Dripwater isotopic values cluster along the local meteoric water line (LMWL) δ2H = 6.82 x δ18O + 6.86 which has lower slope than the global meteoric water line (GMWL δ2H = 8 x δ18O + 10) due to the enhanced evaporation during the warm season. Drip intensity at the site chosen for future study showed stable discharge with no significant response to rain events. Mid-altitude Manita peć Cave (MP) has Tav= 8.8 °C (ampl. 1.7 °C), and RH = 100% in the distant part of the cave where two stalagmites (MP2 and MP3) have been collected. Rainwater isotopic composition, defined by LMWL δ2H = 6.61 x δ18O + 4.92 resembles the Strašna peć Cave record, both by the slope and intercept, in spite of the altitude difference of 500 m. In epikarst zone dripwater isotopic signal is attenuated (rainwater δ18OMPR amplitude of 6‰ is reduced to dripwater δ18OMPW amplitudes of 0.9-1.6‰ for different sites), but the average δ18OMPW (-7.1‰) value is controlled by altitude effect when compared to average δ18OSPW (-6.4‰). Recorded drip intensities showed different hydrological regimes: stalagmite MP2 had relatively low and quite stable discharge rates with only weak response to rain events, while stalagmite MP3 site shows a fracture flow response whit drip rate highly correlated with rain events. The highest Spilja u Zubu Buljme Cave (ZB) significantly differs from previous caves by lower temperature Tav= 4.0 °C, but also by quite large amplitude of 5.3 °C, and relative humidity which occasionally decreases to 95%. The reasons for those microclimate settings are the highest altitude, high tertiary porosity that sustains ventilation and ascending passage suitable for seasonal warm air accumulation. Nevertheless, dripwater δ18OZBW signal does not vary a lot (δ18OZBW = -8.2 – -7.4‰) unlike the rainwater composition with δ18OZBR = -13.4 – -5.9‰. The LMWL δ2H = 7.83 x δ18O + 14.45 has the slope of the GMWL, but the interception deviates to the higher value that are characteristic for the water vapour deriving from Mediterranean area. In accordance with the expected altitude effect, isotopic composition of rainwater becomes lighter with increased altitude being: δ18OSPR = -6.4‰, δ18OMPR = -6.9‰ and δ18OZBR = -9.2‰. Average gradient is ∆δ18O/100m = 0.2‰, but with different lower (∆δ18O/100m = -0.1‰) and higher parts (∆δ18O/100m = 0.32‰). As for the groundwater, values of δ18OSPW = -6.4‰, δ18OMPW = -7.1‰ and δ18OZBW = 7.8‰, give the gradient of ∆δ18O/100m = -0.1‰. Deuterium excess (d = δ2H - 8 x δ18O) controlled by the kinetic fractionation associated with evaporation helps to define the source and trajectories of the atmospheric moisture. Obtained values of dZB = 16.0‰, dMP = 14.6‰ and dSP = 14.5‰ fit between Atlantic (10‰) and Mediterranean (22‰) values, indicating both continental and maritime influences. Cave air temperatures do not follow the external trend since, due to the morphology, the remote parts of the caves with stable cave conditions act as the "pockets" for the cold (in descending chamber) or warm air (in ascending chamber) throughout the year. Cave air temperature variations are governed by conduction during the warm season and by advection during the colder part of the year when ventilation occurs. Unlike two lower caves (Strašna peć and Manita peć), the highest one (Spilja u Zubu Buljme) does not fulfil requirement of low- amplitude temperature variations. Moreover, in spite of well attenuated dripwater signal, investigation on equilibrium condition and palaeoclimate records will not continue in this cave due to the lack of recent carbonate precipitation and no adequate flowstone formations. Strašna peć and Manita peć caves appeared to be suitable study sites regarding cave climate and drip hydrological settings, and it is likely that speleothem calcite has been precipitated in isotopic equilibrium with dripwater, providing the reliable records of past events.

cave climate ; speleothems ; dripwater ; isotopic composition ; altitude gradient ; palaeoclimate ; Croatia

Projekt 60200 Sveučilišta u Zadru: Rekonstrukcija regionalnih paleoklimatskih promjena – zapisi iz siga sjeverne Dalmacije. Voditeljica: Maša Surić