engleski

SUBMERGED SPELEOTHEMS AND PHREATIC OVERGROWTHS ON SPELEOTHEMS (POS) AS INDICATORS OF RELATIVE SEA-LEVEL CHANGE ALONG THE EASTERN ADRIATIC COAST

We present the preliminary results of U-Th dating and XRD, XRF analyses of 3 submerged speleothems collected within the project SEALeveL (HRZZ-IP- 2019-04-9445) funded by Croatian Science Foundation. The main objective of the project is the study of algal rims and speleothems in order to reconstruct the relative sea-level changes and climate changes in different parts of the eastern Adriatic coast. One of the goals is to detect phreatic overgrowths on speleothems (POS), a secondary depositional structure (carbonate phreatic encrustation) which precipitates at the water table around pre-existing vadose speleothems if favourable geochemical conditions are acquired. So far, it has been used in sea-level research in only several locations in the world while most of the investigated sites are located in Mallorca (Onac et al. 2012). Recently, POS is presumed to be developed in eastern Adriatic coastal cave, offering a new tool for the study of relative sealevel change. Up to now, 2 presumably POS samples (SIP-1 and SIP-2) have been collected from Šipun Cave (Cavtat, southern Dalmatia) and one speleothem with biogenic overgrowth from Kravljačica pit (Kornat, northern Dalmatia) (Lončar et al. 2020). To determine the Mg content in the collected samples and thus provide a quick feedback on the POS, four points on the cross section of sample SIP2 were selected for XRF analysis and a thin section for the XRD analysis. The obtained results show only minimal values of Mg and do not indicate marine or brackish origin of the analysed carbonates. The most promising is the outer crust (spot sample SIP2-4). XRD was performed to determine whether the fabrics of the collected samples resemble those of POS found elsewhere. Most of the observed fabrics of SIP-2 do not resemble characteristic fabrics observed in known POS. The zoned crystals from the inner part of SIP-2 sample resemble some crystals observed in POS from Japan (Miklavič et al. 2018), but they are also very similar to diagenetic crystals observed in normal speleothems. U-Th dating was performed on 2 subsamples of SIP- 1, one subsample of SIP-2, one subsample of KRA- 1and a worm tube from KRA-1. Age of SIP-1 (6mm) is 479 394 ± 33 634 BP while the dating of SIP-1 (37mm) and SIP2 (10mm) showed that they are deposited beyond the U-Th method, meaning that they are older than 600 000 years or that the calcite was lightly altered. The U-Th age of KRA-1 stalactite is 436 013 ± 25 099 years BP, while the worm tube incrustated at KRA-1, taken from a few mm depth into the outer layer is 12 146 ± 4 479 years BP. As such, SIP-1 and KRA-1 represents one of the oldest dated speleothem in Croatia and further stable isotope analyses will be conducted in order to gain the paleoclimate records. 53 Research of speleothems with biogenic overgrowth, POS and related vadose speleothems, will contribute to increase the knowledge on relative sea-level change along the eastern Adriatic, obtained from different recent studies. Since the POS investigation is the first of that kind in the Adriatic, information on their morphology, conditions and time of deposition, will contribute to a better understanding of the POS as sea-level indicator and will allow comparison with results from other parts of the Mediterranean basin.

submerged speleothems, phreatic overgrowth, sea-level, eastern Adriatic, Croatia

nije evidentirano