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 modeling.
Bathymetric plot of the Lomonossov Ridge‘s western slope, where the unique sediment cores have been taken. Photo: Alfred-Wegener-Institute
The researchers had recovered these unique sediment samples during a Polarstern expedition 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. Rüdiger Stein, AWI geologist, expedition leader and lead author of the study, explains.
Taking first samples: Polarstern expedition leader Rüdiger Stein (orange clothes) and his team are sampling the unique sediment core in the so called geo-lab onboard the research ship Polarstern. Photo: Alfred-Wegener-Insti¬tut/ Audun Tholfsen, UoB
The scientists found an ideal place for recovering the sediment cores on the western slope of 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. We were also surprised about the wide-spread occurrence of these slide scars, which extend over a length of more than 300 kilometers, almost from the North Pole to the southern end of the ridge on the Siberian side," Rüdiger Stein explains.
Some scientists were of the opinion that the central Arctic Ocean was already covered with dense sea ice all year round six to ten million years ago – roughly to the same extent as today. The new research findings contradict this assumption. "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," says Rüdiger Stein.
"The first group of biomarkers is derived from carbonaceous algae that live in surface water, i.e. they need open water and, being plants, depend on light. 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," says Rüdiger Stein.
Furthermore, 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. Because these values are well above zero, this is a clear indication that ice-free conditions existed in the summer," says the scientist.
However, as 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. Rüdiger Stein: "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. 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."