Relative sea level change in Olbia Gulf (Sardinia, Italy), a historically important Mediterranean harbour

Abstract

Olbia Gulf, located in northeastern Sardinia, is bordered by granite, metamorphic rock, and limestones (Capo Figari and Tavolara Island). It has been an important merchant harbour for three thousand years. Tidal notches in limestone, considered one of the best sea level markers, are present between 5.1 and 8.6 m a.s.l. A fossil beach deposit on Tavolara Island is of Tyrrhenian age (MIS 5.5, 125 ka). Between 1999 and 2001, during the building of a new Olbia harbour tunnel, many shipwrecks hidden under recent fine sediments have been discovered. The archaeological excavation (380 x 20 m, to the surface of the carbonate bedrock at 4 m depth) was done by the Soprintendenza per i Beni Archeologici per le provincie di Sassari e Nuoro (Olbia branch). During the archaeological excavation campaign, thousands of ceramics, other materials which were not in situ, and 24 shipwrecks of different age and size were recovered. Eleven of these wrecks were sunk while they were still in the harbour during an attack by the Vandals in the middle of the fifth century. This attack determined the beginning of a sharp decline of Olbia, and gives a clear picture of one of the turning points of the Mediterranean cultural evolution: the end of the Roman Empire. Considering −1.4 m as the average altitude of the shipwrecks on the silty bottom of the Olbia Harbour and comparing this value with the predicted sea level curve from the Lambeck model calculated for northern Sardinia (−1 m ca. 1500 cal BP), it appears clear that the ships were at the margin of the harbour in water depth of less than 0.5 m. This confirms: a) the previous archaeological interpretation; and b) that the northern Sardinian coast was stable during the last millennia. Due to the tectonic stability of the coastal area, the evolution of the shoreline from the Bronze age (4 ka cal BP) up to 21.5 ka cal BP (LGM, last glacial maximum) is reconstructed.