Cave clue to 13,000 winters

Over 13,000 years ago a stalagmite began to grow in a cave in Oregon.

Each winter rainwater from the land above made its way through the cave's ceiling and dripped onto the floor. As each layer of the stalagmite formed, oxygen and carbon isotopes within these raindrops were captured and preserved inside the rock. 

Now, thousands of years later, a team led by Oxford University scientists is using the data locked inside this stalagmite to get a glimpse of the ancient winter climate of Western North America. 

The team's results, published this week in Nature Communications, show that in recent prehistory the region has seen rapid shifts between dry and warm and wet and cold periods. The findings hint at the importance of the Pacific Decadal Oscillation [PDO] – a pattern of climate variability that changes every 50-70 years – to this area.

'We picked Oregon because it's around this latitude where winter storms hit the West coast of North America, it is representative for an area stretching from California to British Columbia,' Vasile Ersek of Oxford University's Department of Earth Sciences, lead author of the report, told me. Water resources in the region are highly dependent on winter rainfall, without the winter rains the land is arid. 

'Most other ways of estimating past climate, like tree ring data, only tell us about summers, when plants are growing,' Vasile explains. 'This work gives us a unique insight into winter climate over thousands of years with an unprecedented combination of length, detail and dating accuracy.

'Moreover, because the cave is only around 70 km from the Pacific Ocean, and directly affected by processes occurring over the ocean, it also represents a record of past climate variability in the Eastern Pacific where detailed records of past climate are otherwise very hard to obtain.'

The stalagmite record suggests that there have been important variations in both rainfall and temperature (c.1 degree Celsius) over the last 13,000 years – with the region's climate switching between extreme dry-warm and wet-cold periods within just a few decades. 

'Whilst we can't directly relate these changes to the Pacific Decadal Oscillation the mechanisms involved do look similar,' comments Vasile. 'Getting a long-term perspective on these sorts of natural climate variations may help us to understand the potential for future loss of winter snow cover along the west coast, as well as what’s happening out in the Pacific to influence other cyclical climate events such as El Niño.'

But those hoping that this cave rock might tell us about man's influence on the climate will be disappointed; after bearing witness to so many winters its record-keeping stopped before the industrial age began.

Above: the stalagmite that recorded 13,000 winters.

A report of the research, entitled ‘Holocene winter climate variability in mid-latitude western North America’, is published in Nature Communications.