Project 4: "Thera Ashes"

Abstract

In the framework of the research program „Synchronization of Civilizations in the Eastern Mediterranean Region in the 2nd Millenium B.C.“ instrumental neutron activation analysis (INAA) is used to determine up to 30 elements in pumice from archaeological excavations to reveal their specific volcanic origin. The widespread pumiceous products of several eruptions in the Aegean region were used as abrasive tools and were therefore popular trade objects. A remarkable quantity of pumiceand pumiceous tephra (several km3) was produced by the „Minoan eruption“ of Thera (Santorini), which is assumed to have happened in 1627 B.C. Thus the discovery of the primary fallout of „Minoan“ tephra in archaeologically stratified locations can be used as a relative time mark. Additionally, pumice lumps used as abrasive can serve for dating by first appearance.
Essential to an identification of the primary volcanic source is the knowledge that pumices from the Aegean region can easily be distinguished by their trace element distribution patterns, as previous work has shown. The elements Al, Ba, Ca, Ce, Co, Cr, Cs, Dy, Eu, Fe, Hf, K, La, Lu, Mn, Na, Nd, Rb, Sb, Sc, Sm, Ta, Tb, Th, Ti, U, V, Yb, Zn and Zr were determined in hundreds of pumice samples from excavations in Egypt, Palestine, Cyprus, Turkey and Greece. A reliable identification of the samples is achieved by comparing these results to the database compiled in previous studies.

Introduction
During the Quaternary, including prehistoric and historical times, the Hellenic Arc is a terrain of extensive volcanism.

Fig. 1: The "Aegean" and the Eastern Mediterranean Region.

One of the main centers of explosive eruptions is located on Thera (Santorini) and the eruption of the Thera volcano in late Minoan time (about 1600 – 1300 B.C.) is considered to have been the most significant Aegean explosive volcanism during the late Holocene.

Fig. 2: The quarry at Fira.

The precise date of the „Minoan eruption“ is the object of intense debates, most recent scientific results strongly support a date about 1627. Thera is situated on the South Hellenic Island Arc, which is a consequence of tectonic activity involving the Eurasian, Turkish and African plates.
The same geologic situation led also to young, Quaternary volcanic activity on other islands of the Arc, like Milos, Nisyros, Giali and Kos.

Fig. 3: Giali.

Why is volcanology important for chronological purposes?

• The huge explosive character of this activity, like the Minoan eruption, and the large amount of erupted material (several km3) produced in a very short time span provides earth scientists and archaeologists with an important time-synchronous stratigraphic marker horizon of tephra that allows correlation of sites across entire regions.
• Additionally „Minoan“ pumice lumps, discovered in archaeologically stratified locations also provide an opportunity for "dating by first appearance".

Volcanic products

• Volcanic ash is a fine grained eruption product (grain size <2 mm), which consists in case of explosive eruptions mainly of silicate glass shards. This so called “tephra” is distributed over large distances (>1000 km) depending on the meteorologic situation.

Fig. 4: Kos: Layer of the Plinian phase of the Kos Plateau Tuff eruption.

• Pumice is a vesicular, light coloured volcanic rock with a mainly glassy matrix. It is formed by mostly explosive eruptions of viscous magmas rich in gas and has a low specific mass due to its highly vesicular structure. These physical properties are the reasons why pumice has been used since ancient times as an abrasive tool. Pumice floats on water and is transported by marine currents and wind over large distances. As a consequence of widespread alluvial deposition pumice has been a popular trade object since ancient times and lumps were found in archaeological context. An identification of such findings is not only useful for chronological purposes, as mentioned above, but also as an indicator for trading relations.

Fig. 5: Overview: Excavation at Tell el-Dab'a.

Fig. 6: Excavation: Used pumice.

Fig. 6a: Pumice stone.

Essential to an identification of the specific volcanic origin of such findings is the distinction of all possible different volcanic centres in the Aegean region. Therefore previous studies were necessary to establish a data basis, to reveal the often very subtle differences in the chemical compositions of Aegean pumices. Based on the knowledge that accurate trace element data obtained by Instrumental Neutron Activation Analysis (INAA) can ensure confident assignment of discoveries of pumice to their primary volcanic source, many different pumice samples found in archaeological context during excavations were investigated.

Fig.7. Archaeological excavations where pumice has been found

Additionally, soil samples from Turkey, Cyprus, Syria, Israel and Egypt are being investigated by microscopical methods to reveal the presence of tephra layers. After identification of a tephra deposit different separation techniques are applied to obtain a pure volcanic glass fraction ready for analysis. But it is very important to be aware of the fact that tephra particles could also be abrasive remnants and not only natural deposits. Additional previous studies have revealed, that the compositions of tephra layers may change with the distance from the vent due to the progressive loss of the coarser, phenocrysts containing (crystal-bearing) fraction.

Conclusions

The distribution patterns of a large number of elements obtained by INAA allow the differentiation of several groups of pumice deposits in the Aegean region.

Characteristic pairs of elements enable a simple and fast method for a safe classification and relation of pumice of unknown origin. Comparison of typical compositions of Aegean pumices and their variability to samples from archaeological excavations in Egypt of unknown origin are presented in following graphics.

Fig. 8: Chart: Ratio between Ta-Eu.

Fig. 9: Chart: Ratio between Th-Hf.

The whole chemical fingerprint of one pumice lump from archaeological excavations in Tell-el-Dab`a (Nile Delta, Egypt) in comparison to pumice from OCB (Upper Caldera Pumice), Nisyros is shown. The concentrations are normalized to the mean concentrations in "Minoan pumice" (“oberer Bimsstein”, Bo) from Thera.

Fig. 10: Diagram: Bo norm.

The use of chemical fingerprinting for chronological purposes, like „dating by first appearance“ of pumices can be extended to the fine ash layers of the Minoan eruption, an important time-synchronous stratigraphic marker horizon (e.g. discovered in Rhodes, Kos, Gölhisar Gölü, Black Sea and other locations) and allows therefore the correlation of sites across entire regions.

Acknowledgements
This work was supported by the European Commission Project Nr. 98/034065 and by the Austrian „Fonds zur Förderung wissenschaftlicher Forschung“, Project Nr. 13151-SPR. The Project „Thera Ashes“ is supported also as a part of the Austrian Spezialforschungsbereich SCIEM 2000 „Synchronization of Civilizations in the Eastern Mediterranean in the Second Millenium B.C.“.

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For further information please contact Prof. Dr. Max Bichler, Abt. Radiochemie; Atominstitut der Österreichischen Universitäten, Stadionallee 2, A-1020 Vienna, Austria
Santorini Decade Volcano hompage:
http://www.geo.aau.dk/palstrat/tom/santorini_homepage/santorini.htm