North Pole was once ice-free in summer: Study

London: Nearly six-10 million years ago, the central Arctic was completely ice-free during summer and sea-surface temperature reached values of four to nine degrees Celsius, an international team of scientists has revealed.

They used unique sediment samples from the Lomonosov Ridge -- a large undersea mountain range in the central Arctic, while travelling on board Germany's research icebreaker RV Polarstern in 2014. 

"The Arctic sea ice is a very critical and sensitive component in the global climate system. It is therefore important to better understand the processes controlling present and past changes in sea ice," said study lead author Ruediger Stein from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI).

"One of our expedition's aims was to recover long sediment cores from the central Arctic, that can be used to reconstruct the history of the ocean's sea ice cover throughout the past 50 million years," Stein added.

Although the recovered sediment cores were only four to eight metres long, one of them turned out to be precisely one of those climate archives that the scientists had been looking for a long time. 

"With the help of certain microfossils, so-called dinoflagellates, we were able to unambiguously establish that the lower part of this core consists of approximately six to eight million-year-old sediments, thereby tracing its geological history back to the late Miocene," Stein said.

With the help of so-called "climate indicators or proxies", the scientists were able to to reconstruct the climate conditions in the central Arctic Ocean for a time period for which only very vague and contradictory information was available. 

The study, published in the journal Nature Communications, however, said that in spring, autumn and winter the ocean was covered by sea ice of variable extent.

"By combining our data records on surface water temperature and on sea ice, we are now able to prove for the first time that six to ten million years ago, the central Arctic Ocean was ice-free in the summer," Stein said. 

"In the spring and the preceding winter, on the other hand, the ocean was covered by sea ice. The seasonal ice cover around the North Pole must have been similar to that in the Arctic marginal seas today," he noted.