The central Arctic was completely ice-free during summers some six to 10 million years ago and the sea-surface temperature reached four to nine degrees Celsius, a new study has found. In spring, autumn and winter, however, the ocean was covered by sea ice of variable extent, researchers said. The scientists recovered sediment cores from the Lomonosov Ridge, a large undersea mountain range in the central Arctic. "This slope must have experienced gigantic recurring landslides in the past, which resulted in the exhumation of more than 500-metre thick ancient sediment and rock formations," said Ruediger Stein, said from Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) in Germany. "We were also surprised about the wide-spread occurrence of these slide scars, which extend over a length of more than 300 kilometres, almost from the North Pole to the southern end of the ridge on the Siberian side," said Stein, expedition leader and lead author of the study. Within a two-days coring action, researchers took 18 sediment cores from the Lomonosov Ridge. 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. "Our data clearly indicate that six to ten million years ago, the North Pole and the entire central Arctic Ocean must in fact have been ice-free in the summer," said Stein. This statement is based on studies of organic compounds that were produced by certain organisms that lived in the Arctic Ocean at that time and that have been preserved in the sediment deposits. "The first group of biomarkers is derived from carbonaceous algae that live in surface water, ie they need open water and, being plants, depend on light," said Stein. "Since in the central Arctic Ocean sunlight is only available during the spring and summer months and is pitch-dark at all other times, the data derived from these carbonaceous algae provide us with information about the surface water conditions during the summer period," he said. These carbonaceous algae produce different biomarker compounds depending on the water temperature. "These molecules allowed us to estimate that the surface water temperature of the Arctic Ocean was approximately 4 to 9 degrees Celsius in the late Miocene," Stein said. The second group of biomarkers shows, the Arctic Ocean was not ice-free all year round. It is formed by specific diatoms that live in the Arctic sea ice. "The seasonal ice cover around the North Pole must have been similar to that in the Arctic marginal seas today," Stein said. The study was published in the journal Nature Communications.