The Tephrostratigraphy of Three, Late Quaternary, Mediterranean Marine Cores

Isochronous tephra layers provide the potential for the precise correlation of environmental records and, in the case of tephra layers of known age, for the importation of age estimates into sequences that lack independent ages, or for which the chronology is equivocal. This PhD project explored this potential for three important late Quaternary core sequences from the Mediterranean Sea: ODP975, which lies close to the Balearics in the western Mediterranean; LC21 in the Aegean Sea, close to Crete; and OPD967 in the far eastern Mediterranean, close to Cyprus. Each sequence was investigated for the presence of visible volcanic ash and cryptotephra layers. Very low amounts of volcanic glass shards were found in the ODP975 sequence, while 15 discrete tephra layers were found in core LC21 and 5 in the ODP967 sequence. These were geochemically analysed for constituent major and trace element ratios using EPMA-WDS, LA-ICP-MS and SIMS micro-analytical methods. Correlations of tephra layers were based on graphical comparison of the resulting geochemical data-sets to a developing data-base of the representative glass chemical compositions of European proximal and distal tephra deposits. The results reveal evidence for 19 separate volcanic eruptions spanning the last 166 ka, originating from Campania, Pantelleria, Santorini, Yali/Nisyros and Kos, and possibly also from Central Anatolia and Iceland. 12 of these eruptions were previously unknown and thus are here characterised for the first time.
The results add to the tephrostratigraphical record of the eastern Mediterranean, but also demonstrate that further progress requires some crucial procedural problems to be addressed first. Several chronologically distinct tephra layers have identical elemental abundances, which complicates their applicability as robust stratigraphic and chronological markers. In addition, proximal and distal deposits derived from the same eruption phase may have different geochemical attributes, and some evidence suggests a degree of chemical heterogeneity between different distal components of the same eruption. If confirmed, these findings have serious implications for assigning distal tephra layers to a contemporaneous proximal deposit, and hence impede the reliable transfer of age estimates obtained from proximal layers to distal tephra and environmental records.
These problems notwithstanding, the discovery of 12 previously unknown tephra layers demonstrates that the tephrostratigraphical record of the eastern Mediterranean is far from complete, and highlights the need for further research of this type. In particular, rhyolitic eruptions from Santorini are here shown to be more common than were previously thought. These findings therefore have importance for augmenting the history of volcanic activity in the region, especially as a contribution to understanding magma recharge rates and eruption frequencies and their relevance for developing more robust hazard assessments.