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Is there a need for reinterpretation of the relative sea-level change on the Eastern Adriatic coast (Croatia)?

Rocky coast with low tidal range, diverse coastal biocenosis, repeatedly submerged karstified bedrock (with caves and speleothems within them), as well as an early establishment of the civilization with numerous facilities from the Roman Age, provide exceptional potential for the reconstruction of sea-level changes along the Eastern Adriatic. Yet, the reconstruction appears to be difficult and delicate due to the complex tectonics of Adria (Apulian) microplate, located between converging African and Eurasian plates. Eastern Adriatic coast and islands, excluding Istria, belong to the External Dinarides – fold- and-thrust belt (part of Alpine orogenic system) considered as the detached and backthrust upper crust of the Adria, highly deformed during subduction towards NE (Korbar, 2009). Such structural fabric, i.e. tectonically active region, is not suitable for the eustatic sea-level reconstruction ; on the contrary, global sea-level curve, together with the related glacio-hydro- isostatic models, is used in order to reconstruct tectonics. North Adriatic is generally known as a subsiding region although that usually relates only to the northernmost part which includes Gulf of Venice, Gulf of Trieste and western Istrian coast. But, following the common division of the Adriatic Sea (Northern, Central and Southern), northern part encompasses much wider region, including Kvarner area, which might have had completely different tectonic history according to recent research (Surić et al., 2009). In this study, two submerged stalagmites (-14.5 m and -18 m) from Krk Island displayed several growth phases and associated hiati. According to U-Th ages of spelean calcite, and mineral assemblage characteristic for marine regression phases found within the growth hiati, it appears that these speleothems recorded high sea-level double peak within MIS 5a as recorded on the uplifted island of Barbados at ~84 and ~77 ka BP (e.g. Potter & Lambeck, 2003 ; Radtke & Schellmann, 2005). Considering present speleothems' depth, sea level should have been higher than -18.8 m from ~90 ka to ~82 ka and from ~77 ka to ~64 ka, and higher than -14.5 m between ~87 to ~82 ka, but compared to the global eustatic sea level curve (Lambeck & Chappell, 2001) with much lower sea level during the MIS 5a double peak, apparently the speleothems were uplifted for approximately 13-18 m during the last ~75-85 ka. Therefore, we can assume a long term regional tectonic uplift with an average rate of ~0.15 to 0.25 mm/a during that period with episodical subsidence events generated probably by collision of Adria microplate with European continent. Complex tectonic activity of this area with upward trend was also revealed by recent GPS measurements (Altiner et al., 2006). In light of this interpretation, several local Late Pleistocene- Holocene sea-level indicators (inland marine deposits, marine terraces, tidal notches) to which 0.6 mm/a subsidence rate was ascribed earlier, could be reinterpreted. However, aforementioned interpretation relies on one (Lambeck & Chappell, 2001) of several predicted global sea-level curves. Additionally, it is well-known that MIS 5a highstands positions recorded worldwide range from –30 m to +10 m relative to the present sea level (Coyne et al., 2007), due to the different isostatic response of the Earth (Potter & Lambeck 2003). Western Mediterranean i.e. Mallorca, with MIS 5a sea level ~1 m higher than present (Dorale et al., 2010) share similar glacio-hydro-isostatic properties regarding GIA with North Adriatic, and it is not implausible that the same value of MIS 5a eustatic sea-level should be taken into consideration when reconstructing complex tectonics of NE Adriatic. References: Altiner, Y., Marjanović, M., Medved, M., Rasić, LJ., 2006. Active deformation of the Northern Adriatic region: result from the CRODYN geodynamical experiment. In: Pinter, N. et al. (Eds.), The Adria Microplate: GPS Geodesy, Tectonics and Hazards. Springer, Dordrecht, pp. 257-268. Coyne, M.K., Jones, B., Ford, D., 2007. Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies. Quat. Sci. Rev. 26 (3-4), 536-559. Dorale, J.A., Onac, B.P., Fornós, J.J., Ginés, J., Ginés, A., Tuccimei, P., Peate, D.W., 2010. Sea- level highstand 81, 000 years ago in Mallorca. Science 327, 860-863. Korbar, T., 2009. Orogenic evolution of the External Dinarides in the NE Adriatic region: a model constrained by tectonostratigraphy of Upper Cretaceous to Paleogene carbonates.– Earth Science Reviews, 96/4, 296–312. Lambeck, K., Chappell, J., 2001. Sea level change through the Last Glacial Cycle. Science, 292, 679- 686. Potter, E.K., Lambeck, K., 2003. Reconcilation of sea-level observations in the West North Atlantic during the last glacial cycle. Earth Planet. Sci. Lett. 217 (1-2), 171-181. Radtke, U., Schellmann, G., 2005. Timing and magnitude of sea level change during MIS 5 derived from Barbados coral reef terraces: A critical literature review and new data. J. Coastal Res. 42, 52-62. Surić, M., Richards, D.A., Hoffmann, D.L., Tibljaš, D., Juračić, M., 2009. Sea-level change during MIS 5a based on submerged speleothems from the eastern Adriatic Sea (Croatia), Marine Geology, 262, 62-67.

: tectonics; MIS 5a; submerged speleothems; Eastern Adriatic

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