Ice core changes numismatic history

19 January 2019

A new study has found ground-breaking evidence from an ice core in the Swiss-Italian Alps that proves the 7th century switch from gold to silver currencies in western Europe actually occurred a quarter of a century earlier than previously thought.

The findings, from the University of Nottingham and which are published in the journal Antiquity, will have major implications on the history of the European monetary system, and what we thought we knew about trade and the economy during this period. The long-standing dating of all events and archaeological remains linked to the old dating of the first silver coinage will also have to change in light of this study.

Gold coins were replaced by silver in the 7th century in two phases, assumed to have occurred in the 640s and then in the later 670s. The earlier transition has been verified from dating the rapid adulteration of gold coins with silver. The later shift seemed late in the context of the rapid development of trading towns from around 660.

The Nottingham University team analysed lead in Alpine ice-cores. Silver in north-western Europe was extracted from galena, lead-sulphide, releasing lead into the air that settled on ice in the Alps. The annual ice layers allow the shift to intensive silver production to be dated by using lead as a proxy for silver.

These layers reveal a rapid switch to silver coinage occurred soon after 660 and is thus likely to have been a stimulus for the development of the new trading emporia across north-western Europe at that time.

The lead peak dated to 640±10 at Colle Gnifetti occurred at a crucially important moment in the post-Roman monetary history of Western Europe. Late in the reign of Dagobert I (629–639), the gold content of the tremissis (one third of a gold solidus) coinage issued in the Frankish Merovingian kingdoms of Gaul declined substantially, due to the intentional addition of silver, and is unambiguously shown by the concurrent presence of significant amounts of lead in the coinage. Proton activation analysis indicated that coins struck by the mints of southern Gaul declined from 92–94 per cent gold to 31–61 per cent about 640.

The swiftness and extent of this change argue against a measure to compensate for the gradual depletion of gold content that might accompany ‘wear and tear’ and repeated recycling, or a diminishing gold supply. Instead, they suggest a conscious decision to increase the amount of gold, albeit alloyed with silver, available for striking. Expanding money supply may, in turn, hint at an increased demand for coinage, although the end of Byzantine gold subsidies to the Frankish kings after  600—exacerbated by declining or less favourable Mediterranean trade from the  620s—has also been suggested as the trigger for the sudden adulteration with silver of the Merovingian gold coinage. The Colle Gnifetti ice core confirms a surge in lead pollution in 640±10. This reflects lead-silver freshly mined and smelted to produce at least a proportion of this new coinage.

The second large seventh-century peak at 660±10 occurs when pure silver issues replaced gold-silver alloy coinage. Numismatists have previously estimated that the first silver coinage in the western Merovingian kingdoms was minted about 675 and  that a change in England from the use of debased Anglo-Saxon gold coins to silver replacements, known as sceattas, was contemporaneous. The new date based on the lead-peak fits a new chronology for dating Anglo-Saxon furnished graves, involving high-resolution carbon-dating, artefact seriations and Bayesian modelling, which dates the first sceattas (almost exclusively series B) from graves to 660.

The decision to shift to silver has been ascribed to Ebroin—the ‘mayor of the palace’—between about 658 and his death in 680. The new coins are mentioned in texts from 682. The revised chronology now suggests that the decision to shift to a purely silver coinage was taken by Ebroin very soon after he became mayor of the palace.