Combined Sr-Nd isotopic and geochemical fingerprinting as a tool for identifying tephra layers: Application to deep-sea cores from Eastern Mediterranean Sea

Tephra layers from six deep-sea cores, recovered between the Ionian and the Aegean Seas, and covering a time span between ~ 102 and ~ 8.2 ka BP, were investigated with the aim of identifying their volcanic source. The stratigraphic position of each tephra layer defined by means of nannofossil biostratigraphy and occurrence of sapropel layers was integrated with a thorough geochemical characterization of glass shards based on major oxides and trace elements content, and Sr-Nd isotope ratios. Major oxides composition permitted their subdivision into three groups (K-trachyte, peralkaline rhyolite, andesite), confirmed by Principal Component Analysis on trace elements data. Primordial mantle-normalized trace elements distribution patterns allowed for precise identification of the geochemical affinity and geodynamical setting of each group. Although the Sr isotopic composition of some investigated tephras was deeply affected by seawater alteration as expected, the 143Nd/144Nd values discriminate well the three groups of tephras in combination with major oxides and trace elements data. Therefore, La/Yb and Th/Y vs. 143Nd/144Nd discrimination diagrams are proposed for identification of potential volcanic sources active in the 102-8.2 ka BP time span in the Eastern Mediterranean area for unknown tephras.The obtained results have permitted the attribution of six K-trachytic tephras to the Y-5 stratigraphic marker (Campanian Ignimbrite eruption of Campi Flegrei, Southern Italy, ~ 39 ka BP), and two pantelleritic (= peralkaline rhyolite) tephras to the Y-6 stratigraphic marker (Green Tuff eruption of Pantelleria Island, Sicily Channel, ~ 46 ka BP). The andesitic tephra, stratigraphically constrained between ~ 39 and ~ 83-102 ka BP, might correspond to the X-1 stratigraphic marker, found in several deep-sea cores of the Eastern Mediterranean Sea, although its attribution is still debated in the literature. On the basis of trace elements content, supported by Sr-Nd isotopic features, it is here hypothesized that this tephra could be related to a volcanic source located in the Aegean Sea area, probably the island of Santorini. More generally, this study demonstrates that the combination of Sr-Nd isotopic with major oxide and trace element geochemical fingerprinting on selected and purified glass shards is a very effective tool for identifying nature and source of doubtful tephra layers.

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