Volume 590 Issue 7844, Feb 4, 2021

Salt-free Arctic seas

The cover shows Diamond Beach and Jokulsarlon Glacier Lagoon in Iceland. Much of the Arctic Ocean is thought to have once been covered by an ice shelf, but clear evidence for this has proved elusive. In this week’s issue, Walter Geibert and his colleagues reveal results suggesting that in recent ice ages, the Arctic Ocean and adjacent Nordic seas were largely filled with fresh water and covered with a thick ice shelf. The researchers analyzed marine sediment cores for thorium-230, which is produced from the decay of uranium in salt water. They found that thorium-230 was missing in multiple layers in cores from the Arctic Ocean and the Nordic seas, which, they say, means no salt water was present. The team suggests that the ice shelf actually created a dam that separated these bodies of water from the Atlantic Ocean and filled the region with fresh water for two periods, 70,000-62,000 and 150,000-131,000 years ago.

Cover photo: Aleksandar Tomic / Alamy.