Environmental science: Fresh water in the ice-age Arctic Ocean

The Arctic Ocean and adjacent Nordic seas were mostly filled with fresh water, rather than salt water, for intermittent periods during two recent glaciations, and were covered by a thick ice shelf, suggests a study published in Nature this week. These findings indicate that revisions of ancient sea-level reconstructions, based on previous estimates of freshwater levels, may be needed.

Understanding what the climate and environment in the Arctic region was like in the past can help us to predict how it might change in the future. Previous research has suggested that much of the Arctic Ocean may have been covered in an ice shelf in the past. However, evidence of such an ice shelf has been elusive, owing to difficulties interpreting samples from Arctic marine sediment cores.

Walter Geibert and colleagues show that analysis of isotopic thorium-230 (230Th) can be used to reconstruct past conditions in the Arctic Ocean. 230Th is produced from the decay of uranium in salt water and can be trapped in marine sediments, providing an indication of the level of salinity at a time point that is based on the depth of the marine sediment core. The authors report that 230Th was missing in multiple layers of sediment cores from the Arctic Ocean and Nordic seas, which suggests that no 230Th was produced at these time points, therefore indicating an absence of salt water. They propose that this region was filled with fresh water between 70,000–62,000 years ago and approximately 150,000–131,000 years ago, and that these changes occurred over relatively short periods of time.

The authors suggest that at the time when fresh water was present, the ice shelves extended into the Nordic seas, and might have dammed these bodies of water, preventing inflows of salty water from the Atlantic Ocean. They add that this process could provide a means for rapidly releasing fresh water into the ocean, something that sea-level reconstructions may need to account for. These results might spur a re-evaluation of how dramatically this region can change, notes Sharon Hoffmann in an accompanying News & Views.