Comparison of 14C and 230Th/234U Dating of Speleothems from Submarine Caves in the Adriatic Sea (Croatia)

Z metodama 14C oziroma U-Th smo datirali 16 sig iz sedmih podmorskih jam v Jadranu. Dve sigi smo datirali z obema metodama. Primernost metod za datacijo sig je različna zaradi različnih okoljskih pogojev, ki so vladali med izločanjem sige oziroma po zalitju jame. Obe metodi sta zanesljivi na dobro ohranjenih sigah. U-Th metoda je slabša, ko so karbonati onesnaženi z detritičnim materialom oz. preraščeni s plastjo morskega izvora . Vendar metoda U-Th z uporabo MC ICPMS analize, kjer potrebujemo le 100-300mg vzorca (namesto 30 g pri metodi 14C), omogoča boljšo časovno ločljivost datacij, kar je pri datiranju sig zelo pomembno.Among the 16 speleothems that were collected from 7 submarine caves and pits for the purpose of 14C and U-Th dating and reconstructing sea-level changes, two speleothems were dated by both methods. Different environmental conditions during the speleothem deposition and after the submergence resulted with different appropriateness for speleothem dating by these techniques. Well preserved speleothems gave reliable results by both methods, while U-Th method showed disadvantage in the case of carbonates contaminated with detrital material, as well as in the case of carbonate from marine overgrowth that covers the speleothems. However, U-Th method using MC ICPMS technique which requires only 100-300 mg of sample per analysis (instead of ca. 30 g for 14C conventional method), offers better age resolution that is essential for speleothem dating

Distribution of authigenic and allogenic fractions in recent lake sediment: Isotopic and chemical compositions

Recent sediments in Lake Kozjak, in the Plitvice Lakes National Park, Croatia, were used to study the impact of the material delivered to the lake sediment via Rječica Brook. Sediment cores, top 40 cm, were taken near the mouth of the Rječica Brook at three locations in the lake at different distances from the shore, water depth 2-9 m and analyzed using different isotopic and chemical methods. Sediment depth profiles of the following parameters were obtained: organic and carbonate fraction, C/N analyses of organic fraction, 14C activity (a14C) and d13C values of carbonate (d13Ccarb) and organic fraction (d13Corg) and d15N of total nitrogen. The results were compared with sediment core collected in the middle of Lake Kozjak, water depth 21 m. Distribution of organic matter, C/N values, d13Corg and d13Ccarb in sediment profiles showed strong influence of terrestrial organic and carbonate fraction run-off via Rječica Brook which decreased rapidly in the direction of the open lake. The sedimentation rate as well as the ratio of allogenic and authigenic fraction in sediments was estimated. Lower d13Corg values in surface sediments might be a consequence of the increase in algal productivity but might also be a contribution of fossil fuel combustion. The anthropogenicaly induced 14C in the atmosphere in 1963/64 is visible though shadowed in sediments

Some Geological Observations in Slovačka jama Cave (-1268m) in Velebit Mountain, Croatia

Tekom zadnjih treh speleoloških odprav je bila Slovaška jama raziskana do globine 1268 m. V njej je več rovov freatičnih oblik. Hipsometrično najvišji ostanki takih rovov so najbrž v zvezi s ponori iz časa, ko je nastajala uvala Veliki Lubenovac. Spodnji, recentni in subrecentni freatični horizonti v jamskem “dnu” so verjetno del kanalov podzemeljske reke Like, ki ponika na severovzhodni strani Velebita in teče skozenj v podmorske izvire v Jadranskem morju. Ostali rovi so predvsem brezna in meandri z vadoznimi oblikami. Analize izotopa 14C dokazujejo, da so najstarejše sige v obojih, v starih in v subrecentnih freatičnih rovih, medtem ko so mlajše sige v vadoznih meandrihDuring three speleological expeditions the Slovačka jama cave has been explored to the depth of 1268m. In the cave there are several channels with phreatic forms. The highest relict phreatic channel is probably due to sinkholes formed on the karst uvala Veliki Lubenovac at the time of uvala formation. Lower, recent and subrecent, phreatic horizons at the cave bottom are probably a part of the underground route of the Lika river which sinks on the north-eastern side of Velebit mountain and flows through the mountain to the submarine springs in the Adriatic sea. Other channels are mostly shafts and channels with meandering vadose forms. 14C isotope analysis of speleothems shows that the oldest are situated in both ancient and subrecent phreatic channels, while younger ones are found in the vadose meandering channel