Salt-free Arctic seas
The cover shows Diamond Beach and Jokulsarlon Glacier Lagoon in Iceland. It is suspected that much of the Northern Ocean was once covered by an ice shelf, but clear evidence for this has been evasive. In this issue of this week, Walter Geibert and his colleagues reveal results that suggest that in recent ice ages, the Arctic Ocean and adjacent North Seas were mostly filled with fresh water and covered by a thick ice shelf. The researchers analyzed marine sediment cores for thorium-230, which is produced by the decay of uranium into salt water. They found that thorium-230 was missing in multiple layers in the core of the North Pole Sea and the North Sea, which they said meant no salt water was present. The team suggests that the ice shelf effectively created a dam, which separated these masses 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.