An international team of scientists led by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) have managed to open a new window into the climate history of the Arctic Ocean. Using unique sediment samples from the Lomonosov Ridge, the researchers found that six to ten million years ago the central Arctic was completely ice-free during summer and sea-surface temperature reached values of 4 to 9 degrees Celsius. In spring, autumn and winter, however, the ocean was covered by sea ice of variable extent, the scientists explain in the current issue of the journal Nature Communications. These new findings from the Arctic region provide new benchmarks for groundtruthing global climate reconstructions and modelling.The researchers had recovered these unique sediment samples during an expedition with Germany's research icebreaker RV Polarstern in summer of 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. In this context, 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. Until recently, only a very few cores representing such old sediments were available, and, thus, our knowledge of the Arctic climate and sea ice cover several millions of year ago is still very limited," Prof. Dr. Ruediger Stein, AWI geologist, expedition leader and lead author of the study, explains.