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Establishing high resolution geochronology using algal rims in relative-sea level studies – Examples from the eastern Adriatic coast

Among fixed biological indicators, bio- constructions formed by the alga Lithophyllum byssoides, are one of the best sea-level indicators on rocky coasts. Their vertical precision (±10 cm) in microtidal environment comes from the restricted environmental conditions of the alga as its living range is constrained around sea level. In order to approach the 14C reservoir effect which could have an important influence on the final 14C results we engaged different studies of marine radiocarbon reservoir effects (MRE) of the alga and shells in the Adriatic and in the other parts of the Mediterranean using samples of known age from museum collections. The corrections of MRE and local R are fundamental particularly for the short, late Holocene sea- level and palaeoenvironmental chronologies. Here we present examples of precise relative sea-level (RSL) reconstructions for the past 3000 yrs based on the studies of Lithophyllum rims from Istrian peninsula in the northern, and from Lopud Island in the southern Adriatic. The chronologies were based on 47 (in Istria) and 23 (at Lopud Island) radiocarbon dates which were accurately calibrated in order to provide precise relative sea-level curves, to relate sea-level changes to periods of climate changes and to try to distinguish land- level changes in the studied area. The RSL reconstructions were quantitatively analysed using an error in-variables integrated Gaussian process (EIV-IGP) model to identify sea-level trends with full consideration of the available uncertainty. The high-resolution geochronology obtained at Lopud Island allows us to distinguish the effects of coseismic movements on RSL histories what confirmed the precision of L. byssoides bioconstructions not only in the studies of relative sea-level variations but also in the studies of palaeoearthquakes in seismotectonically active areas. Also, it confirmed the importance of precise determination of marine radiocarbon reservoir age of the alga as the geomorphologic consequences of known AD 1667 Dubrovnik earthquake could be accurately determined. Consequently, the study of algal rims could also allow assessment of palaeoseismicity. 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.

Geochronology ; 14C, algal rims ; Adriatic

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