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Isotope records in submarine speleothems from the Adriatic coast, Croatia

Maa Suri¹, Nada Horvatini², Axel Suckow³, Mladen Jurai⁴ and Jadranka Barei²

¹ Department of Geography, University of Zadar, Tudmanova 24 i, 23000 Zadar, Croatia, msuric{at}unizd.hr
² Radiocarbon and Tritium Laboratory, Ruer Bokovi Institute, Bijenika 54, P.O. Box 180,10002 Zagreb, Croatia,nada.horvatincic{at}irb.hr, jbaresic{at}irb.hr
³ Leibniz Institute for Applied Geosciences, Geochronology and Isotope Hydrology (S3), Stilleweg 2, D-30655 Hannover, Germany. Now at the Isotope Hydrology Laboratory, International Atomic Energy Agency, Wagramer Strasse 5, A-1400 Vienna, Austria, Axel.Suckow{at}IAEA.org.
⁴ Department of Geology, Faculty of Science, University of Zagreb, Zvonimirova 8, 10000 Zagreb, Croatia, mjuracic{at}geol.pmf.hr

Isotope studies, using ¹⁴C dating, ¹³C and ¹⁸O measurements, were performed at eight speleothems taken from three submerged caves situated along the eastern Adriatic coast, Croatia. The speleothems were taken from 17 m to 38.5 m depth below mean sea level. The samples consist of four stalagmites and four stalactites in position of growth, covered with marine biogenic overgrowth, and the length of speleothems ranges from ~80 mm to ~190 mm.

The youngest (surface) and the oldest (base) layers of speleothems were radiocarbon dated and the ¹⁴C ages range from 21,600 cal B.P. to 37,000 yr B.P. During that period the global sea level was more than 40 m below the recent one, so presently submerged objects were under the subaerial conditions necessary for speleothem deposition. ¹⁴C ages of the youngest layer range from 21,600 to 32,200 cal B.P. for different submerged speleothems. This indicates the time when the speleothem growth ceased, most probably due to flooding of the cave with either fresh or brackish water. Speleothem growth during the Last Glacial Maximum (30–19 kyr ago) and different time of growth cessation for the different speleothem samples suggest that climate change was not the reason for cessation of deposition.

Samples for ¹³C and ¹⁸O measurements were taken from six submerged speleothems with sampling distances of ca. 5–10 mm from the surface to the base of speleothems. Most of the ¹³C values are in the range from –10.5 to –8.5, with few exceptions to –6. These values are typical for Dinaric karst, and very different values for marine biogenic overgrowth indicate that no isotopic exchange took place during the submerged period. ¹⁸O values range from –6.7 to –4.1. A weak correlation between ¹³C and ¹⁸O values indicates possible kinetic isotope fractionation during the calcite precipitation. If the ¹⁸O record is interpreted as climatic signal, it suggests similar climatic conditions for the late Pleistocene and the Holocene, especially no significant differences in temperature and/or moisture transport.

Key Words: Karst • Submarine speleothem • ¹⁴C dating • ¹³C • ¹⁸O • Climate change • Adriatic Sea • Croatia