When ice disappeared from the Northern Atlantic

The sudden events that caused the Northern Hemisphere to warm up during the last Ice Age, have been accompanied by an equally rapid decrease in the extension of sea ice in the Northern Atlantic. These are the findings of a study on paleoclimate published in the Proceedings of the National Academy of Sciences (Pnas) journal and carried out by an international research team coordinated by the University of Bergen (Norway), in which the National Research Council’s Institute for Polar Studies (CNR-Isp) and Ca’ Foscari University of Venice also took part. 

“Between 10 and 100 thousand years ago, during the last glacial period, the Northern Hemisphere was completely white due to the big ice sheets that enveloped the northern continents and to the vast amount of sea ice that covered the Northern seas” - explains Andrea Spolaor, CNR-Isp researcher and one of the authors of the study. “Yet, the cold glacial climate has been interrupted more than once by a series of events that caused a significant and sudden spike in temperatures up to 16°C on the Greenlandish ice sheet, known as Dansgaard-Oeschger events.” 

The causes behind these warming episodes, already discovered in the mid-80’s by analysing Greenland ice cores, are still an object of debate, even though the publication of this research has contributed to a better understanding of the subject. As a matter of fact, the results of the study indicate that the marked decrease in the extension of sea ice could have taken place during a span of 250 years or less, simultaneous to the beginning of a mixing process in the oceanic layers of Northern Atlantic, therefore causing an intense amount of heat to be released and the subsequent atmospheric warming. 

“While the Northern Atlantic was rapidly losing its ice cover, the heat coming from ocean waters was being transmitted to the atmosphere above, thus leading to the amplification of the climate warming events underway” states Niccolò Maffezzoli, Marie Curie Fellow at Ca’ Foscari University of Venice and co-author of the study. 

In this research work, the team has combined climate data from sea sediment cores and ice cores for the first time ever in the Arctic region.

“Our Norwegian colleagues have analysed the two sediment cores extracted in the Norwegian Sea, while in the Ca’ Foscari and Cnr-Isp labs we measured the concentration of bromine and sodium in the Greenland ice core from Renland. These two elements are very sensitive to the presence of seasonal sea ice in the Northern Atlantic Ocean, in particular the area between Norway and Greenland” -continues Maffezzoli. 

“Bromine is an especially important indicator of the extension of seasonal sea ice, because it is emitted from sea ice surface in the atmosphere during the Arctic spring and it then deposits on the ice cap. These seasonal “bromine explosions” revealed by the ice cores allowed us to reconstruct the dynamics of sea ice over the past millennia”.

“The collected data has then been aligned through the identification, in all ice cores, of different layers of tephra, volcanic ash from past Icelandic eruption, that allowed for temporal synchronization” - concludes Spolaor. “Our study has highlighted the usefulness of carrying out climate reconstructions that combine ocean sediment and ice cores, providing a deeper understanding of the past variations of sea ice in Northern seas”