Stalagmite and stalactite deposits in Siberian and Mongolian caves have revealed the most accurate permafrost history yet, suggesting that a global 1.5°C temperature rise could trigger a widespread thaw. “The finding shows how vulnerable the permafrost is,” said Anton Vaks from the University of Oxford. “Russian gas facilities in north-western Siberia are located close to the boundary of the continuous permafrost and rely on it as hard ground. Thawing of the permafrost may cause damage both to Russia, as well as its gas trade partners, like the European Union. The melting permafrost may also release part of the organic carbon currently trapped in it as greenhouse gases, CO2 and methane, enhancing global warming.”
Anton first used cave deposits’ power to study climate history during his PhD to build a 350,000 year record for the northern margin of the Saharan-Arabian Desert. “Stalagmites and stalactites grow only when rain or snowmelt water seep into the cave through the ceiling,” Anton explained. “Therefore each layer of growth of stalagmites and stalactites records a humid event in the desert.” He realised that these deposits’ ability to track water flow could equally measure melting of previously permanently frozen soil known as permafrost. “Cave deposits cannot grow when the rock above the cave is frozen,” Anton said. “Thus, each growth layer in a stalagmite forms during warm periods, whereas growth breaks represent cold periods with permafrost. Past periods that were warmer than now are especially important, because they can show what may happen to the permafrost in the future warmer world.”
Dating with confidence
Other scientists had realised this in the 20th century, and had used similar methods on cave deposits in Canada to try and build records. To date layers in the deposits they wanted to use the fact that uranium’s radioactive decay produces one form – or isotope – of the element thorium, called thorium-230. Uranium dissolves in water, but thorium-230 doesn’t, meaning that there is none in the deposits when they first form. But as time passes, uranium’s decay means thorium-230 appears in the deposit, and the ratio between the elements can date different layers. However the method was so new when researchers tried to use it before that it was hard to be confident in its results. “That finally discouraged scientists from continuing,” Anton told me. “Today uranium-thorium dating is very accurate, enabling us to perform the study as we did.”
To work out how far south permafrost reached through history, Anton needed several caves containing deposits falling roughly on a north-south line. To find caves in Russia he asked for help from the Arabica Caving Club and the Institute of the Earth’s Crust, Russian Academy of Sciences, both in Irkutsk, Siberia. For caves in Mongolia, he called on the Institute of Geography, Mongolian Academy of Sciences. Together this team, and scientists from the Swiss Federal Institute of Technology, ETH Zurich, collected 36 samples from six caves, and did 111 uranium-thorium dating tests. Their results have just been published in top research journal Science.
Hundreds of millennia
In the southern caves of Siberia stalactites and stalagmites grew during each of five previously-identified warm periods, called interglacials, over the last 500,000 years. “In the northernmost cave, located in continuous permafrost, but close to its boundary, stalactites grew during a short episode about 400,000 years ago, called interglacial Marine Isotope Stage (MIS) 11,” Anton said. “Then, global average temperatures were 1.5°C higher than present. No younger stalagmites and stalactites were found in this cave, indicating continuous permafrost conditions until now.” That fits with other researchers’ work showing that during MIS-11 the ice cap in southern Greenland melted and was replaced by forest. His team also found deposits in a cave in the Gobi Desert that grew during MIS 11, showing that this area was wetter at this warmer time, which also agrees with previous research.
The researchers now hope to find more northerly caves, and also hope to reach even further into the past. “We plan to date the growth periods of stalactites and stalagmites more than 500,000 years old,” Anton says. “That will help us find how permafrost responded to even warmer climates in the past, like during the Pliocene epoch, when the temperatures were 2.5-4°C higher than present.”
A. Vaks, O. S. Gutareva, S. F. M. Breitenbach, E. Avirmed, A. J. Mason, A. L. Thomas, A. V. Osinzev, A. M. Kononov, G. M. Henderson (2013). Speleothems Reveal 500,000-Year History of Siberian Permafrost Science