A team of international scientists, led by researchers from the University of Southampton, has traced strange, out-of-place rocks in the jagged cliffs of Iceland’s west coast to Greenland, carried across the sea by icebergs during a sudden and severe cooling event in the 6th century AD.

Known as the Late Antique Little Ice Age, this centuries-long chill followed a series of powerful volcanic eruptions. It also coincided with the last days of the Roman Empire.

“When it comes to the fall of the Roman Empire, this climate shift may have been the straw that broke the camel’s back,” said Tom Gernon, a professor of Earth science at the University of Southampton and co-author of the new study.

A Tale in Stones

The research began with the first observations of these puzzling rocks. Along a raised beach terrace in Iceland, scientists encountered rocks that didn’t belong there, geologically speaking. Iceland’s landscape is dominated by dark basalt from its volcanic heart — but these stones were different.

“We knew these rocks seemed somewhat out of place because the rock types are unlike anything found in Iceland today, but we didn’t know where they came from,” said Dr. Christopher Spencer, lead author and associate professor at Queen’s University in Kingston, Ontario.

To solve the mystery, the team crushed the rocks into fragments and extracted microscopic zircon crystals. These crystals, smaller than a pen tip, act like time capsules. Their age and chemistry offer a precise fingerprint of the land they formed in.

“The combination of age and chemical composition allows us to fingerprint currently exposed regions of the Earth’s surface, much like is done in forensics,” Dr. Spencer explained.

The zircon signatures matched rocks from across Greenland — some formed nearly 3 billion years ago. The newest rocks are 500 million years old. This geological diversity in the Greenland rocks suggested a glacial origin: ice scraping across ancient terrain. It picked up cobbles (rounded rocks) and pebbles from many regions before dumping them into the ocean.

“This is the first direct evidence of icebergs carrying large Greenlandic cobbles to Iceland,” Dr. Spencer said.

The Straw that Broke the Camel’s Back?

The timing of the rocks’ arrival is key. The scientists dated them to the 7th century, during a period of widespread iceberg activity known as the Bond 1 event. This aligns with historical records of a dramatic climate shift that darkened skies, stunted crops, and chilled much of the Northern Hemisphere.

The suspected trigger is a trio of massive volcanic eruptions around 536 AD that spewed ash into the atmosphere, blocking sunlight and sending temperatures plunging. We have long known that in the year 536, there was no summer. For about 15 months, the sun seemed to shine only dimly, unnerving people worldwide. Nothing like it had been seen in thousands of years. This violent sequence of eruptions triggered what is now called the ‘Late Antique Little Ice Age,’ when much colder temperatures endured for at least 150 years.

That cooling likely played out in waves. As glaciers expanded, more icebergs calved into the ocean. Ocean currents shifted. Landmasses slowly rebounded after the last great ice sheets retreated, raising beaches where the Greenlandic rocks came to rest.

This icy upheaval had ripple effects far beyond the Arctic.

One of Many Culprits

Historians have long debated what led to the fall of the Roman Empire — barbarian invasions, political instability, or even plagues. But climate, too, may have played a role. The cooling period would have disrupted agriculture, weakened armies, and fueled migrations. The all-important annual Nile flood became erratic. Droughts and severe cold spells became more common. 

In the last decades of the fourth century and the first decades of the fifth century, the empire suffered a series of military defeats unlike anything in its history — at the hands of the Goths. But the Goths, in turn, were prodded to move against the Romans because of an incursion into Europe of Huns, from central Asia. Megadroughts may have made these warrior nomads desperate for greener pastures.

Of course, climate was not the sole cause of Rome’s fall, but this study adds weight to the idea that nature gave history a shove. The Roman Empire had already fractured by the 6th century. Despite Emperor Justinian’s efforts to rebuild the empire (he recaptured Italy and Northern Africa), the fate of Rome was sealed.

The new findings appeared in the journal Geology.