New research published in the journal Nature provides evidence from Siberian caves on the essential role that summer sea ice in the Arctic Ocean plays in stabilising permafrost and its large store of carbon.

Permafrost is ground that remains frozen throughout the year and covers nearly a quarter of the Northern Hemisphere’s land. The frozen state of permafrost enables it to store large amounts of carbon; about twice as much as in the atmosphere. The rate and extent of future thawing of permafrost, and consequent release of its carbon, is hard to predict from modern observations alone.However, a new study has discovered a crucial past relationship between summer sea ice in the Arctic and permafrost. This has significant implications for the future as observations show Arctic sea ice has decreased in recent years.

A team of international researchers, including Dr Sebastian Breitenbach from Northumbria’s Department of Geography and Environmental Sciences, have found that times when permafrost melted in the past did not simply match up with times when the Earth was at its warmest. The new research relies on challenging fieldwork to discover and explore Siberian caves. Caves are a powerful recorder of periods when permafrost was absent in the past.

Why are caves important?

Stalagmites, stalactites and flowstones can only form when there is liquid water, and therefore do not form when overlying land is permanently frozen. The presence of stalagmites in caves under permafrosted land therefore demonstrates that there were periods when permafrost was absent in the past.

The researchers used new approaches to date the formation of stalagmites using the decay of natural uranium to lead.  This allows them to assess the timing of periods when permafrost was absent over the last one and a half million years. They found that stalagmites in Siberian caves grew intermittently from 1.5 million to 400,000 years before the present day and have not grown since, due to the overlying land being frozen.

The timing of stalagmite formation during the absence of permafrost does not relate simply to global temperatures in the past but was notably more common when the Arctic Ocean was free of summer sea-ice. The researchers say that several processes may lead to the relationship between Arctic sea-ice and permafrost. Read more.