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New fossil finds as witnesses for fluctuations of Arctic sea ice cover during the past 30,000 years

Geoscientists have succeeded in reconstructing the ice conditions in the Fram Strait, a narrow passage between eastern Greenland and Spitsbergen, during the past 30,000 years.

They used a new research method. Based on fossilized algal remains in sediment cores, researchers from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association in collaboration with colleagues from the University Plymouth, Great Britain, worked out a uniform picture of the expansion of sea ice in this area that is particularly important for worldwide climate conditions.

The publication of "Variability of sea-ice conditions in the Fram Strait over the past 30,000 years" will be issued on Sunday October 25th in the online edition of the periodical Nature Geoscience.

The examination of a sediment core from the northern Fram Strait, the only deep-water connection between the central Arctic Ocean and the Atlantic Ocean, provides clues for extreme sea ice fluctuations during the past 30,000 years. "Our reconstructions of the various ice conditions show how drastic the Arctic reacts even to short-term climate fluctuations", explains Juliane Müller, geoscientist at the Alfred Wegener Institute.

By means of fossil organic molecular remains, so-called biomarkers which are contained in the layers of the sediment cores, the researchers were able to verify the dates when the Fram Strait was either ice-covered or ice-free. The biomarker IP25, a complex molecule produced by algae living in the sea ice, was found in varying concentrations in the sediment; it was used as an indicator for the ice cover. A second biomarker called brassicasterol which is produced by algae living in the open water was used as counterpart in the analyses. The presence of brassicasterol in the sediments indicates ice-free periods. The combination of these two parameters enables the researchers to reconstruct different ice conditions.

For a period of extremely cold climate conditions, the last glacial maximum of about 20,000 years ago, the absence of these two biomarkers indicates permanent ice cover in the northern Fram Strait. The lasting lack of light and nutrients under the thick ice shield minimized the growth of the ice algae. A short but significant warming of the climate about 15,000 years ago, the early Bølling, caused the Arctic sea ice to melt so far that the Fram Strait remained ice-free during the winter months. The ice marker IP25 is absent in the sediment layers of this period, while the content of brassicasterol is highly increased. The simultaneous occurrence of both biomarkers in sediments of the past 5,000 years, the late Holocene, shows that the strait was only ice-covered during the winter and spring months. This seasonal change between ice-covered and ice-free water surface therefore enabled the growth of both algal species.

The sharp decline of sea ice in the central Arctic with a dramatic minimum in the year 2007 caused alarm in many researchers. "Examinations on natural changes of sea ice extent in times when humans had no impact on the climate have become a focus of numerous international research projects in the Arctic", explains Prof. Dr. Rüdiger Stein, geoscientist at the Alfred Wegener Institute. Arctic sea ice plays an important role in the thermal balance of the oceans. The ice influences among other things the mechanisms of global ocean currents, a circulation propelled by differences in temperature and salt concentration. It is in particular responsible for the mild climate in Europe in the form of the Gulf Stream. One power source of this "heat pump" is located in the Fram Strait.

The Alfred Wegener Institute carries out research in the Arctic and Antarctic as well as in the high and mid latitude oceans. The institute coordinates German polar research and provides international science with important infrastructure, e.g. the research icebreaker Polarstern and research stations in the Arctic and Antarctic. The Alfred Wegener Institute is one of 16 research centres within the Helmholtz Association, Germany's largest scientific organization.

Margarete Pauls | idw
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