The two main provinces of Pleistocene tephra eruption in the Mediterranean Sea are the eastern half of the Hellenic arc, represented by calc-alkalic and weakly alkalic material, and the Neapolitan area, represented by high-K2O material.
Of twenty distinctive ash layers that have been identified in Pleistocene deep-sea cores of the eastern Mediterranean, one is calc-alkalic ash that originated in the Minoan eruption of Santorini at about 3500 B.P., and another is trachytic ash that originated in the Citara-Serrara eruption on Ischia Island at about 25,000 B.P. Rb, Zr, and Y content has been analyzed in the upper and the lower ash from both the land and deep-sea cores and in three tephra deposits from the east Hellenic volcanoes of Nisyros, Yali, and Kos.
In addition to the K2O/SiO2 ratio, values of the trace elements Rb, Zr, and Y versus SiO2 in the ash layers can be used in correlating the ash layers and in tracing the source of ash. All relative K2O, Rb, Zr, and Y contents can be used to trace the source of ash to one of the two main provinces of tephra eruption. Zr and Y are most useful for identifying individual volcanoes as sources.
Good agreement of chemical analyses of correlatable samples of the two ash layers from land and deep-sea cores argues against major alteration of the glass shards or a measurable cation exchange between glass shards and sea water on the time scale of 25,000 yr.