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Algal Rims And Phreatic Overgrowths On Speleothems (POS) As Indicators Of Relative Sea Level Change Along The Eastern Adriatic Coast

We present the research of algal rims and phreatic overgrowths on speleothems (POS) as indicators of relative sea level change along the Eastern Adriatic coast which will be conducted within the project SEALeveL (HRZZ-IP- 2019-04- 9445) funded by Croatian Science Foundation. Lithophyllum byssoides, a fixed biological indicator is a coralline algae belonging to the order Corallinales of the Phylum Rodhophyta, and is characterized by a hard thallus formed by calcareous deposits contained within the cell walls (Rovere et al., 2015). Coralline algae are widespread along the world’s rocky shores. Under favourable conditions, Lithophyllum byssoides build reef like bioconstructions just above the biological mean sea-level which can be 14C dated. Rims of coralline algae have been largely used in the Mediterranean (Laborel, 1987) and have proven to be a good proxy of past sea levels in microtidal environments (Laborel et al., 1994, Laborel and Laborel- Faivre et al., 2013, Faivre et al., 2019a) because their living range is constrained around sea level. It creates the highest biogenic build-up in the Mediterranean (Rovere et al., 2015). The study of Lithophyllum rims as sea-level markers along the eastern Adriatic coast has started ten years ago with the aim of restraining the error bars and obtaining more accurate results. Research conducted on the islands of Vis, Ravnik and Biševo (Central Adriatic) (Faivre et al., 2013) as well as along the coast of Istria (Northern Adriatic) (Faivre et al., 2019a) revealed algal rims as possible high resolution relative sea-level marker. The accuracy of the marker has been much improved with the knowledge of the Lithophyllum byssoides marine radiocarbon reservoir age (MRE) (Faivre et al., 2019b). The MRE has been determined on algal samples of known age (ten samples from the Mediterranean and one from the Atlantic) obtained from Museums collections of the Natural History Museum (Museum National d’Histoire Naturelle) in Paris, Naturhistorisches Museum in Vienna and Universitat Hamburg, Biozentrum Klein Flottbek und Botanischer Garten in Hamburg. Results of the studies of algal rims (Faivre et al., 2013, 2019a) show that large rims most likely grew during near- stable sea-level conditions that occurred during relatively colder periods over the past 2000 years. They also indicate that best- developed upper levels of the rims formed during 300 years of stabilization throughout the Little Ice Age period. Those periods of stabilization could be directly related to the morphogenesis of tidal notches on the studied areas, providing thus the possibility of firm relative sea-level reconstructions. Croatia is abundant with submerged or partially submerged caves with brackish water (Surić et al., 2010b). Thus far, detailed reconstructions of the Pleistocene and Holocene relative the sea- level changes along the eastern Adriatic, were done based on data obtained from U-Th dating and stable isotope analyses of the submerged speleothems/biogenic encrustations (Surić, 2002 ; Surić et al. 2004, 2005a, 2005b, 2009, 2010a). These studies encouraged detailed research of cave sediments in order to reconstruct climatic conditions and paleoenvironmental changes during the Quaternary (Lončar, 2012 ; Lončar et al., 2017, 2018 ; Surić et al., 2017a, 2017b, 2018). Research provided insight into environmental change recorded in stalagmites from the late Pleistocene (119 ka) to late Holocene (1.5 ka) (Lončar, 2012). Holocene is characterised by numerous δ18O and δ13C fluctuations reflecting sudden changes in the environment (Lončar, 2012 ; Rudzka et al., 2012 ; Lončar et al., 2017, 2018). Recent diving explorations presumed POS to be deposited in eastern Adriatic coastal caves, offering a new tool for studying relative sea- level changes. POS is a secondary depositional structure (carbonate phreatic encrustation) which precipitates at the water table around pre- existing vadose speleothems if favorable 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, Tuccimei et al. 2006, 2019, 2012 ; Vesica et al., 2000). Calcite sample collected from Šipun cave (Dubrovnik area) show pre- existing vadose stalactite (a soda straw) around which we assumed POS precipitated, while some inner parts of the speleothem are richer in Mg. Further analyses should confirm the phreatic overgrowth while U-Th dating will give the age of its formation. 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. We expect that new data from both markers will enable the development of longer and more precise curves of relative sea-level change along the eastern Adriatic coast. Moreover, increasing the number of sea-level index points obtained from different markers can contribute to the understanding of the driving mechanisms: climate change, hydro- and glacio-isostatic adjustments, and tectonics. Acknowledgement This research was supported by Croatian Science Foundation project HRZZ-IP-2019- 04-9445 – Relative sea-level change and climate change along the eastern Adriatic coast – SEALeveL.

algal rims ; speleothems ; sea level ; eastern Adriatic

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