Sea ice survived the Arctic summer when global temperatures were warmer than today, only a few million years after a period of extreme warmth known as the Palaeocene-Eocene Thermal Maximum. According to a study published online this week in Nature Geoscience, year-round sea ice cover was ephemeral until about 36.7 million years ago, suggesting that less of a temperature change may be required to switch from ice-free summers to all-year sea ice (and back) than previously thought.
Using geochemical analyses, Dennis Darby determined the source of ancient sedimentary iron grains found in a sediment core in the central Arctic Ocean, which are so coarse that they must have been transported by sea ice. He reconstructed travel times from the grains’ source regions on the shelf to their final deposition site: if transport of the sea-ice bound grains took longer than a year, sea ice must have survived the Arctic summer, he argues. In the oldest part of the record, grains from distant sources occur in bursts, suggesting ephemeral year-round sea ice.
In an accompanying News and Views article, Catherine Stickley writes that “exploring the transition between perennial and seasonal sea ice may help us to understand what we can expect when the Arctic becomes ice-free in future summers.”