Ancient rocks boost case for mini ice age linked to fall of Rome

Rocks carried from Greenland to Iceland by icebergs add to evidence that the climate in Europe got a lot colder for a century or two starting around AD 540 – a “mini ice age” that may have played a role in the fall of the Roman Empire.

This cold period in the northern hemisphere, which has been previously indicated by studies of tree rings and sediment cores, has been linked to a number of historical events around the world, from the collapse of the Northern Wei dynasty in China to the decline of the city state of Teotihuacan in Central America. The Plague of Justinian, which affected the Eastern Roman Empire following the fall of the Western Roman Empire in 476, may also have been caused in part by a colder climate.

Christopher Spencer at Queen’s University in Kingston, Canada, and his colleagues decided to explore a beach on the west coast of Iceland because satellite photos showed it was light-coloured rather than the basalt black of almost all other beaches in Iceland.

That lighter colour turned out to be due to the presence of lots of shells, but while walking in the area, Spencer spotted cobble-sized granite stones. For him, it was instantly clear these rocks weren’t from Iceland. “It’s a little bit embarrassing how easy it was to make the discovery,” he says.

Sure enough, an analysis of the rocks confirmed they came from many different locations in Greenland, which is around 300 kilometres away from Iceland at the closest points. That means the rocks Spencer found must have been carried by icebergs that calved from glaciers in Greenland and got washed up on the beach.

The layer of the beach in which the Greenland rocks are present has previously been dated to around AD 500 to AD 700, says Spencer. While icebergs from Greenland still reach this area occasionally, Greenland rocks haven’t been found in any other layers of the beach.

So, the discovery shows that large numbers of Greenland icebergs were washing up on this beach during the period in which this layer formed. This suggests that due to colder conditions, Greenland’s glaciers grew larger and calved many more icebergs during this period, says Spencer.

This ties in neatly with evidence for a colder period sometimes called the Late Antique Little Ice Age. The cause of this event is unclear – some think it was triggered by volcanoes, others by fragments of a comet striking Earth. Spencer thinks it was simply down to orbital changes affecting how much of the sun’s heat reaches Earth.

While the extent to which the climate contributed to events such as the fall of Rome remains debated, there is growing evidence that climate changes shaped the fate of many civilisations.