Traces of Holocene tsunamis across the Sound of Lefkada, NW Greece

This paper gives evidence of multiple tsunami inundation of the Sound of Lefkada (NW Greece) since the mid-Holocene based on the analysis of sediment cores by means of geomorphological, sedimentological, geochemical, micromorphological and micropalaeontological methods. Layers of sand, gravel and shell debris, mostly unsorted, were found intersecting autochthonous lagoonal muds of the sheltered and quiescent inner-sound environment They are further characterized by erosional unconformities at their base, rip-up clasts from the underlying sediments, fining upward sequences and an upward increase in sorting. The coarse grained high energy deposits include macro- and microfaunal remains typical of open-marine, partly even deep-water conditions which underlines their allochthonous character. Several distinct event layers indicate multiple tsunami passage across the sound. Earth resistivity measurements and vibracore transects revealed that the entire sound has been affected by catastrophic wave events. Thin sections of tsunami-influenced sediments found in lateral parts of the sound show a mixture of marine, lagoonal and terrigenous material. The inner Sound of Lefkada, well protected against storms, thus represents an excellent trap for tsunamigenic deposits. A preliminary local tsunami geochronology is based on 9 radiocarbon dates and diagnostic ceramic fragments. Several early tsunami impacts hit the sound between the 6th and 3rd millennium BC. Younger events seem to be consistent with tsunami landfalls that hit adjacent areas around 1000 cal BC, 395-247 cal BC as well as in Roman and medieval times. Vibracore data document an isthmus-like shallow-water environment which existed in the central sound and which was repeatedly inundated by tsunami wave action. Multiple tsunami passage eroded a natural channel which, we suggests is the precursor of the famous waterway excavated by the Corinthians in the 7th century BC. Based on historical data, it is concluded that the navigable channel across the sound was repeatedly choked with sediments by tsunamigenic inundation during the following centuries. This study reveals a high tsunami risk for the Sound of Lefkada with at least one strong event in every 500-1000 years. (C) 2009 Published by Elsevier B.V.