The mysterious origins of England's Dark Age silver coins have finally been revealed, shedding light on trade between the nation and the rest of Europe.

Researchers discovered the coins - which are as much as 1,350 years old - had both Byzantine and later French origins as politics on the continent changed.

The new study, which has involved Oxford University, found the Byzantine bullion fuelled Europe's revolutionary adoption of silver coins in the mid-7th century, only to be overtaken a century later by silver from a mine in Charlemagne's France.

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The researchers say their findings could transform our understanding of Europe's economic and political development at the time.

Between 660 and 750 AD, Anglo-Saxon England witnessed a revival in trade involving a dramatic surge in the use of silver coins, breaking from a reliance on gold.

Around 7,000 of these silver 'pennies' have been recorded - around as many as we have for the rest of the entire Anglo-Saxon period between the 5th century and the battle of Hastings in 1066, when William of Normandy's triumph ushered in a new era of Norman rule.

For decades, experts have agonised over where the silver in these coins derived from.

The new study from the Universities of Cambridge, Oxford and Vrije Universiteit Amsterdam has finally solved the mystery by analysing the make-up of coins held by the Fitzwilliam Museum in Cambridge.

Dr Jane Kershaw, another co-author of the study from the University of Oxford, said: "These coins are among the first signs of a resurgence in the northern European economy since the end of the Roman Empire.

"They show deep international trade connections between what is now France, the Netherlands and England."

Rory Naismith, a Professor of Early Medieval English History at the University of Cambridge and co-author of the study, published in the journal Antiquity, explained: "There has been speculation that the silver came from Melle in France, or from an unknown mine, or that it could have been melted down church silver.

"But there wasn't any hard evidence to tell us one way or the other, so we set out to find it."

Previous research tested coins and artefacts from the silver mine at Melle in western France.

However, Prof Naismith and his colleagues analysed less-studied coins minted in England, the Netherlands, Belgium and northern France.

Nearly 50 coins from Cambridge's Fitzwilliam Museum, dating from 660 to 820 AD, were taken to a lab to trace their elements.

The coins were analysed by 'portable laser ablation' in which microscopic samples were collected onto Teflon filters for lead isotope analysis - a new technique pioneered by Vrije Universiteit.

Whilst the coins mostly contained silver, their proportion of gold, bismuth and other elements guided the researchers to the silver's previously unknown origins.

Oxford Mail: The mysterious origins of England's Dark Age silver coins have finally been revealed

In the 29 coins from the earlier period, between 660 and 750 AD, which were minted in England, the cross-border northwest European region of Frisia and France, the researchers found clear matches to 3rd and early 7th-century silver from the Byzantine Empire in the eastern Mediterranean.

The silver was homogenous in the coins and characterised by high gold values and a consistent isotopic range, with no distinguishable regional variations among them.

The researchers noted that no known European ore source matches the elemental and isotopic characteristics of these early silver coins; nor is there any meaningful overlap with late Western Roman silver coins or other objects.

"This was such an exciting discovery," Prof Naismith said.

"I proposed Byzantine origins a decade ago but couldn't prove it. Now we have the first archaeometric confirmation that Byzantine silver was the dominant source behind the great seventh-century surge in minting and trade around the North Sea."

Prof Naismith emphasised that this Byzantine silver must have entered Western Europe decades before it was melted down, as the late 7th century was a low point in trade and diplomatic contacts.

"Elites in England and Francia were almost certainly sitting on this silver already," he said.

"We have very famous examples of this: the silver bowls discovered at Sutton Hoo and the ornate silver objects in the Staffordshire Hoard."

Together, Sutton Hoo's Byzantine silver objects weigh just over 10kg and would have produced around 10,000 early pennies had they been melted down.

"These beautiful prestige objects would only have been melted down when a king or lord urgently needed lots of cash," Dr Kershaw said.

"Something big would have been happening, a big social change.

"This was quantitative easing: elites were liquidating resources and pouring more and more money into circulation.

"It would have had a big impact on people's lives.

"There would have been more thinking about money and more activity with money involving a far larger portion of society than before."

Prof Naismith cites a mixture of trade, diplomatic payments and Anglo-Saxon mercenaries serving in the Byzantine army as to how and why so much silver moved from the Byzantine Empire into Western Europe.

The new findings also raise questions about how and where silver was stored and why its owners suddenly decided to turn it into coins.

Analysing 20 coins from the later half of the period (750-820 AD), the researchers discovered that this silver was very different.

It now contained low levels of gold which is most characteristic of silver mined at Melle in western France - one of the oldest silver mines in the world.

Previously obtained radiocarbon data has shown that mining at Melle was particularly intense in the 8th and 9th centuries.

The researchers propose that Melle silver permeated regional silver stocks after 750 AD and was mixed with older, higher-gold stocks - including Byzantine silver.

In the coins minted closest to Melle, the proportion of gold was lowest at under 0.01 per cent, while furthest away, in northern and eastern Francia, this climbed to 1.5 per cent.

Though it was already known Melle was an important mine, it wasn't clear how quickly it became a major player in silver production.

The researchers argue King Charlemagne drove this very sudden and widespread surge in Melle silver as he took increasing control over how and where his kingdom's coins were made.

Management of silver supply went hand-in-hand with other changes introduced by Charlemagne, his son and his grandson - including changing the size and thickness of coins and marking their name or image on the coins.

Prof Naismith said: "We can now say more about the circumstances under which those coins were made and how the silver was being distributed within Charlemagne's Empire and beyond."

The study's findings give new context to Charlemagne's delicate diplomatic relations with King Offa of Mercia in England.

Like Charlemagne, Offa took an active role in the silver trade and currency management and both kings saw trade and politics as inseparable.

In a surviving letter sent to Offa in 796 AD, Charlemagne discussed trade in commodities as well as political exiles.

The pair also entered a trade embargo when a marriage negotiation turned sour.

"There was a lot of communication and tension between Charlemagne and Offa," Prof Naismith said.

"Offa wasn't in the same league, his kingdom was much smaller, he had less power over it, and he certainly didn't have as much silver.

"But he remained one of Europe's most powerful figures who was outside of Charlemagne's control.

"So they maintained a pretence of equality. Our findings add to a dynamic that England and France have had for a very long time.

"When commodities are only in certain places in limited quantities, questions of power and national interest will always come into play.

"In the early Middle Ages, this transcended borders and rulers weren't the only people involved.

"Merchants, churches and other wealthy people all had an interest.

"Rulers taking much more direct action was new for this period."