IMPROVING MEDITERRANEAN TEPHRA STRATIGRAPHY: RESULTS FROM A LONG TERRESTRIAL SEQUENCE ON LESVOS ISLAND, GREECE

A 40.2m lake sediment core was recovered from Megali Limni on Lesvos Island, Greece. Six tephra layers have been identified and their chemical composition determined using electron probe microanalysis on single glass shards. The composition of the first tephra layer (1.8m depth) suggests an origin from the island of Santorini with its best corresponding eruption being the Cape Riva (or Y2) at cal. 18 ka BP. The second tephra layer (7.6m) originates from the Campanian area in Italy and corresponds to the most widespread eruption in the Mediterranean (the Y5 tephra layer), the Campanian Ignimbrite eruption at cal. 39 ka BP. The next two layers (10.5m and 12.0m) originate from the Island of Nisyros in Greece and correspond to the Upper Pumice and Lower Pumice eruptions, respectively. In the marine record of the Mediterranean Sea the chronological position of the 'Nisyros Ash' has been calculated by linear extrapolation of the sedimentation rate in the stratigraphy of the Y zone, between the Y2 and Y4 tephra layers, i.e. between 21 and 30 ka. The recovery of the Y5 tephra layer and the Nisyros Ash for the first time in the same sequence has important implications for establishing the chronological framework of the Nisyros eruptions. The fifth tephra layer (24.25m) corresponds to "the most distinctive tephra layer found in deep-sea cores" (Keller, 1981), correlated with the Green Tuff eruption (or Y6) on the island of Pantelleria at cal. 50 ka BP. Due to chemical weathering following the deposition of the last tephra layer (28.41m) its origin could only be determined qualitatively from the Hellenic Arc with no specific corresponding eruption found. At present, this core represents the longest distal tephra sequence found in a terrestrial setting in the eastern Mediterranean. The findings of, in particular, Campanian and Pantelleria ashes in this locality considerably extend the confirmed Northerly and Easterly distribution of these ash units. Studies based on Mediterranean deep-sea sediments, as well as continental areas, have been undertaken for at least three decades and today a detailed tephrostratigraphy with about thirty tephra markers exists. However, the identification and analysis of these six tephra layers in a single sequence has provided very important evidence for their stratigraphic position and chronology.