Salt-free Arctic seas
The cover shows Diamond beach and Jokulsarlon Glacier lagoon in Iceland. It is thought that much of the Arctic Ocean was once covered by an ice shelf, but clear evidence for this has proved to be elusive. In this week’s issue, Walter Geibert and his colleagues reveal results that suggest that in recent glacial periods the Arctic Ocean and adjacent Nordic seas were filled mostly with fresh water and covered by a thick ice shelf. The researchers analysed marine sediment cores for thorium-230, which is produced from the decay of uranium in salt water. They found that thorium-230 was missing from multiple layers in cores from the Arctic Ocean and Nordic seas, which they interpret to mean that there was no salt water present. The team suggests that the ice shelf effectively created a dam, separating these bodies of water from the Atlantic Ocean and filling the region with fresh water for two periods, 70,000–62,000 and 150,000–131,000 years ago.