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Biotic cluesThese 3-micrometer-wide needle-shaped fossils of marine algae from the genus Synedropsis indicate that sea ice first formed in the Arctic Ocean at least 47.5 million years ago.C. Stickley/University of Tromsø, Richard Pearce/National Oceanographic Centre, Southampton

Fossils of ice-dependent algae reveal that Arctic sea ice, which is today very much an endangered species, formed at least 47.5 million years ago, about 1.5 million years earlier than previously recognized.

Sediments on the bottom of the Arctic Ocean often contain pebbles and mineral grains carried to sea by icebergs or by ice that formed in shallow water along the coast. Previous analyses of such ice-rafted debris, such as physical studies of the shape and surface texture of those bits of rock, suggest that sea ice first graced the Arctic Ocean, at least seasonally, about 46 million years ago, says Catherine Stickley, a micropaleontologist at the University of Tromsø in Norway. However, new analyses of fossils in the sediments push back the date ice first appeared, Stickley and her colleagues report in the July 16 Nature.

The researchers examined sediments drilled from a site on a submarine ridge about 250 kilometers from the North Pole, where waters today are about 1,300 meters deep. The critical clue betraying the presence of ancient sea ice — fossils of marine algae whose modern cousins depend on sea ice as habitat — are found in sediments deposited about 47.5 million years ago, says Stickley. Remains of those needle-shaped microorganisms, part of the Synedropsis genus, make up as much as 61 percent of the fossils in those sediments.

The team’s findings provide new details about the timing and pace of global cooling that was occurring during an era when atmospheric concentrations of carbon dioxide were declining, Stickley notes.

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