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Algal rims as markers of late Holocene relative sea-level change and climate change in the northeastern Adriatic (Istria, Croatia)

One of the best sea-level indicators on rocky coasts in the microtidal environment are bio- constructions formed by the alga Lithophyllum byssoides, called Lithophyllum rims. Their vertical precision (±10 cm) comes from the restricted environmental conditions of the alga as their living range is constrained around sea level. The alga thus creates the highest biogenic build-up in the Mediterranean. Consequently, the latest research on relative sea-level change along the eastern Adriatic has been centred on algal rims. Here we present new precise relative sea-level (RSL) reconstruction for the past 1500 yr based on the study of four algal rims on the Istrian peninsula. The chronology is based on 47 radiocarbon dates which were accurately calibrated in order to provide precise relative sea-level curve, to relate sea level changes to periods of climate changes and to try to distinguish land-level changes in the studied area. The RSL reconstruction was 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 data from the algal rims enabled the distinction of four phases of relative sea- level (RSL) changes. The rims from the southern Istrian coast show that RSL was almost stable during the Early Middle Ages. After AD 1000, during the Medieval Climate Anomaly (MCA) the RSL rise at a rate of 0.8 mm/y. The following Little Ice Age period, LIA I interval, is again characterised by the relative sea level stability which allows the rims at the southern coast to reach the width of 40 to 80 cm at its highest parts. After AD 1600 on the southern coast the major rim building phase is stopped. On the other side, along the eastern coast of Istria the rim building favourable conditions are achieved only after AD 1400 but the growth is very limited as favourable periods are obviously very short. Between AD 1600 and 1750, during the colder LIA II interval, algal rims do not form. From the second part of the 19th century the RSL rise again. Following the correction for estimated land- level changes during the MCA the sea level rise at a rate of 0.5 mm/y assumed to be a consequence of the Medieval warmth. This increased RSL rise during the MCA was already previously observed in the Central Adriatic. Furthermore, L. byssoides δ18O records show that those periods of different sea-level changes are generally consistent with changes in temperature and with periods of rapid climate changes. This research was supported by the Croatian Science Foundation (project no. HRZZ-IP-11- 2013-1623, Reconstruction of the Quaternary environment in Croatia using isotope methods – REQUENCRIM) and by University of Zagreb Grants.

sea-level change ; climate change ; algal rims ; Istria ; Croatia

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