First relative sea-level index points obtained from phreatic overgrowths on speleothems (POS) in the Kvarner area, Croatia

Abstract

<p>Presently, 272 partly or completely submerged caves and pits have been recorded along the Croatian’s eastern Adriatic Sea coast. Marine conditions are found in 153 caves, anchialine in 84, whereas the others, related to submarine springs are still unclassified caves. Speleothems are found in more than 140 caves, providing potential records for reconstructing the Quaternary relative sea-level changes. Recently, phreatic overgrowths on speleothems (POS) have been discovered in the Kvarner region (eastern coast of Lošinj island). POS are carbonate incrustations that deposit around submerged speleothems and cave walls in coastal caves, where groundwater level is controlled by sea level. Since the POS precipitate at the very top of the water column, i.e. at the water-air interface, their occurrence is limited to a very narrow band of the tide-induced groundwater level fluctuation. POS can be precisely dated with U-Th as well as with radiocarbon method, and their occurrence in caves guarantees protection from weathering. The combination of their narrow occurrence, datability, and preservation makes POS excellent sea-level indicators, which have been so far reported only from five locations across the globe (Onac et al., 2012).</p><p>A POS recovered from ~1 m below the surface in Medvjeđa špilja Cave was dated by U-Th method to ~3000 years, indicating a relative sea-level stability for a few hundred years ~1 m below modern sea level. The obtained results were compared with available GIA models and other studied RSL indicators in the broader surrounding area. We expect that the collected POS data along with the information obtained from algal rims will allow a precise reconstruction of the relative sea-level changes over longer periods of time. Increasing the number of sea-level index points obtained from different markers can further contribute to our understanding on the mechanisms driving sea-level changes (climate change, hydro- and glacio-isostatic adjustment, and tectonics) along the Croatian coast.</p><p>The research of POS along the eastern Adriatic is conducted within the SEALeveL project (HRZZ-IP-2019-04-9445) funded by Croatian Science Foundation.</p>