Researchers working in limestone formations in a far-removed Scottish cave have discovered five stalagmites that have recorded three millennia of climatic variations. The work, published in Scientific Reports last week, provides the environmental context for some major human migration events in Europe.
Uamh an Tartair, or Roaring Cave, in northwest Scotland, north of Ullapool, is a shallow cave beneath a blanket of peat that’s been accumulating over 4,000 years. Rainfall levels in the area correspond closely with a climate phenomenon called the North Atlantic Oscillation (NAO). During its positive phase, northern Europe receives higher amounts of precipitation.
A team led by Andy Baker from the University of New South Wales analyzed five of Roaring Cave’s stalagmites, the deposits that rise up from the cave floor. (Stalactites are the ones that drip downwards from the roof of the cave.) The upward growth rate of stalagmites is highly sensitive to rainfall: The water table in the overlying peat affects soil CO2 production and concentration, limestone dissolution, and the resulting stalagmite calcite precipitation. In other words, the more water in the peat, the more slowly the stalagmites grow.
“We painstakingly measured the thickness of each annual growth ring in five stalagmites taken from the cave, including one that provides a continuous annual record spanning more than 1,800 years,” Baker says in a statement. By overlapping the five stalagmites they were able to get a record of climate at the cave from about 1000 BC to 2000 AD.
Baker with part of a stalagmite from Roaring Cave in Scotland. Mediakoo/UNSW
This study is the first to use a compilation of cave measurements to track NAO changes: high stalagmite growth rates typically reflect warm and dry conditions, while low rates indicate cool and moist conditions.
Their 3,000-year record found persistently low growth rates that confirm how the oscillation index was in an unusually prolonged positive phase during the Medieval Warm Period between 1080 and 1430. During that anomalous period, Scotland got increased rain, while the western Mediterranean saw drier conditions.
“Our results also reveal there was another persistent positive phase between 290 and 550, which coincides with the decline of Rome and a period of intensified human migration in southern Europe during the Dark Ages,” Baker adds. “This was followed by a persistent negative phase between 600 and 900, which may have provided warm and dry conditions in northwestern Europe that made it suitable for westward expansion by the Vikings.”