Ice in the Swiss Alps began to be contaminated with lead between AD 640 and 660. That suggests Europeans had begun producing large quantities of silver 鈥� releasing lead as a side-effect 鈥� 25 years earlier than we thought, which could help explain the rise of powerful towns like London.
Historians have long known that north-west and central Europe largely abandoned gold coins for silver ones in the 600s, but exactly when and why has been unclear.
鈥� had dated it on the basis of coinage,鈥� says Christopher Loveluck of the University of Nottingham in the UK. They thought the switch happened around 675 or 680.
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Loveluck is part of a team that is analysing an ice core drilled from the Colle Gnifetti glacier in the Swiss Alps. New layers of ice were laid down each year, so the core provides a year-by-year record of environmental conditions.
The team tracked when lead appeared in the record, because lead is a proxy for silver mining. 鈥淪ilver in north-west Europe is found in galena, which is a lead-sulphide ore,鈥� says Loveluck. 鈥淪ilver is always the minor product.鈥� The process of extracting the silver releases tiny particles of lead, which are carried away by the wind. Some ended up in the ice, revealing when the currency switch happened.
An economic recovery
鈥淚t actually occurred in two phases,鈥� says Loveluck. 鈥淭he first is in 640, when numismatists had already picked out that the gold coinage had been adulterated by a lot of silver.鈥� But instead of the final switch occurring in 675 or 680, the team found a second big spike in lead levels in 660, suggesting it was in this year that there was a wholesale change to silver currency.
鈥淚t takes the total shift to silver back about 20 to 25 years,鈥� says Loveluck. That might seem like a minor change, but it actually means the history now makes more sense.
The move to silver probably helped the economy grow, because silver coins have a lower face value and are more useful for small commercial exchanges. 鈥淎 gold coin might buy you three or four cows, but you might not want that, you might want something smaller,鈥� says Loveluck. 鈥淎 silver coin is more useful.鈥�
After 200 years of disorder following the collapse of the Western Roman Empire, the first large port towns began appearing in the late 600s. This was confusing if the currency switch happened after these towns began to grow. 鈥淢ajor port towns like London, under the old chronology, were only starting to use silver coinage at a point when they were already developed,鈥� says Loveluck. 鈥淲hat were they doing before that?鈥�
If people switched to silver earlier, it implies that the new currency helped to drive the rise of these towns.
The team also used reconstructions of wind patterns to identify where the silver was mined: Melle, in western France. Silver mining was a big industry there in the 800s and 900s, and it now seems it got an early start.
Antiquity
