Stable isotope analysis of Mt. Velebit karst hydrological system

Mt. Velebit is a 145 km long mountain in the Croatian Dinaric Karst lying between Adriatic Sea and the Licko and Gacko Polje fields. The massif is built of thick carbonate deposits from the upper Triassic, Jurassic and Cretaceous ages. In higher parts of the massif, the mentioned rocks are covered with younger Oligocene to Lower Miocene carbonate breccias, probably deposited on the flanks of tectonically uplifted areas during older tectonic phases. Owing to its position between Adriatic Sea and continental Lika region, as well as altitudes up to 1757 m, the mountain also serves as an important climatological border causing high precipitation rates (>3500 mm/y). A more than 2000 meters thick karst rock succession caused very deep karstification was controlled by Adriatic Sea level during Pleistocene and tectonic structures of the massif. The discovery of many vertical and deep caves in last decades (including the deepest and longest caves in Dinaric karst: 1431 m deep Lukina jama, 1321 m deep Slovacka jama and 1026 m deep Cave system Velebita) gives us the opportunity of gathering new insights on karst hydrology. We used stable isotopes of oxygen (18O, 16O) and hydrogen (2H, 1H) in water as environmental tracers to get an insight into the functioning of different karst subsystems of Mt. Velebit. Ratios of these isotopes (18O/16O, 2H/1H), and consequently δ18O and δ2H water values, depend on various factors: precipitation origin, re- evaporation, re-condensation, season, terrain altitude, etc. Therefore, the stable isotope composition of precipitation and karst groundwater may reveal information such as: mean recharge area elevation, importance of winter and summer recharge precipitation of a karst system and average duration of water retention in a system. We compared the stable isotope content of monthly precipitation samples with that of groundwater samples collected in caves, and spring water samples collected at the main discharge points of this complex karst system that belongs to the massif. The stable isotope measurements were conducted using the water equilibration technique performed on Delta plus XP isotope ratio mass spectrometer (Thermo Finnigan) in conjunction with HDOeq42/24 (IsoCal) equilibration unit and Dual Inlet system (Thermo Finnigan) peripherals.