Almost a quarter of the land in the Northern Hemisphere, amounting to just over 23 million square kilometres, is covered in permafrost – soil, sediment and rock that freezes for years. Vast expanses of permafrost can be found in Alaska, Siberia and the Canadian Arctic, where persistent freezing temperatures have kept carbon, in the form of decaying pieces of plants and animals, locked in the soil.
Scientists estimate that more than 1,400 gigatons of carbon are trapped in Earth's permafrost. As global temperatures rise and permafrost melts, this frozen reservoir could escape into the atmosphere in the form of carbon dioxide and methane, significantly amplifying climate change. However, little is known about the stability of permafrost, not only today, but also in the past.
Now, MIT geologists together with those from Boston College are leading research in which they have reconstructed the history of permafrost over the last 1.5 million years. The authors of the study, just published in the journal Science Advances, analyzed cave deposits in places in western Canada and found evidence that between 1.5 million and 400,000 years ago, permafrost was prone to thawing, even in the high latitudes of the Arctic and at temperatures not much warmer than today. Since then, however, the thawing of permafrost has been limited to the subarctic regions, becoming 'stabilised'. The results suggest that the planet's permafrost shifted to a more 'perennial' state over the past 400,000 years and has been less susceptible to melting since then.
But this phenomenon has a consequence: in this more stable state, the permafrost has probably been able to retain much of the carbon it has accumulated during this time, having little opportunity to gradually release it, which in a context of global warming like the current one can be a big problem. "The stability of the last 400,000 years may work against us, as it has allowed carbon to steadily accumulate in the permafrost during this time. Melting could now lead to substantially greater carbon releases into the atmosphere than in the past,” explains co-author David McGee, an associate professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT.
Accumulated heating
Earth warming episodes in the past occurred in interglacial periods or periods between global ice ages. These 'brief' windows (geologically speaking) can warm the permafrost enough for it to thaw. Signs of ancient permafrost thawing can be seen in stalagmites and other mineral deposits that remain as water moves through the ground and enters caves. These caves, particularly in the high latitudes of the Arctic, are often remote and difficult to access and, as a result, little is known about the history of permafrost and its past stability in warm climates.
However, in 2013, researchers at the University of Oxford were able to sample cave deposits from some locations in Siberia; Their analysis suggested that permafrost thawing was widespread throughout Siberia before 400,000 years ago. Since then, the results showed a very narrow range of permafrost thawing. The authors wondered whether the trend toward more stable permafrost was global, and sought to conduct similar studies in Canada to reconstruct the history of permafrost in those places. Thus, they took samples from the Rocky Mountains of southern Canada, Nahanni National Park in the Northwest Territories and northern Yukon.
In total, the team obtained 74 samples of speleothems, portions of stalagmites, stalactites and stones, from at least five caves in each region, representing various depths, geometries and glacial histories. Each cave sampled was located on exposed slopes that were likely the first parts of the permafrost landscape to thaw with warming.
The samples were sent to MIT, where McGee and his team used precise geochronology techniques to determine the ages of the layers in each sample, each layer reflecting a period of permafrost thawing. "Each speleothem was deposited over time like stacked traffic cones," says McGee. "We started with the outermost and youngest layers to date, the most recent time the permafrost thawed."
Arctic change
McGee and his colleagues used uranium/thorium geochronology techniques to date the layers of each speleothem. The dating technique is based on the natural decay process of uranium to its 'daughter' isotope, thorium 230, and the fact that uranium is soluble in water, while thorium is not. "In the rocks above the cave, as the waters percolate, they accumulate uranium and leave behind thorium," explains McGee. «Once the water reaches the surface of the stalagmite and precipitates at time zero, there is uranium and no thorium. “Then gradually the uranium decays and produces thorium.”
The team extracted small amounts of each sample and dissolved them through several chemical steps to isolate uranium and thorium. They then passed the two elements through a mass spectrometer to measure their quantities, the ratio of which they used to calculate the age of a given layer. From their analysis, the researchers noted that samples collected from the Yukon and sites further north had samples no younger than 400,000 years old, suggesting that permafrost thawing has not occurred at those sites since then.
"There may have been a shallow thaw, but in terms of all the rock above the cave melting, that hasn't happened for the last 400,000 years, and was much more common before that," McGee says.
The results suggest that Earth's permafrost was much less stable before 400,000 years ago and was more prone to thawing, even during interglacial periods, when atmospheric temperature and carbon dioxide levels were on par with modern levels. as demonstrated by the other study referring to the Siberian permafrost. "Seeing this evidence of a much less stable Arctic before 400,000 years ago suggests that even under similar conditions, the Arctic may be a very different place," concludes McGee. "It raises questions for me about what caused the Arctic to change to this more stable condition, and what mechanisms can cause it to change again."