Ephemeral formation of perennial sea ice in the Arctic Ocean during the middle Eocene

Sea ice in the Arctic Ocean is a key component of the modern climate system, but less is known about the evolution of Arctic sea ice throughout Earth's history, particularly in warmer climate states. Following early Palaeogene greenhouse conditions, seasonal sea ice in the Arctic developed during a period of relative cooling in the middle Eocene, some 47.5 million years ago. However, perennial sea ice has only been documented as recently as 18 million years ago. Here I document the provenance of iron grains in marine sediments from the central Arctic Ocean, and show that during several intervals, beginning about 44 million years ago, they were carried from distal Arctic shelf sources. The grains are too coarse to have been delivered by ocean currents or aeolian transport, and therefore must have been rafted by sea ice. Because grains entrained from the shelf sources would need to drift for more than one year to reach the depositional site, I conclude that sea ice must have persisted throughout the year. However, the presence of grains from these distal sources only occur in intervals of less than 100,000 years in the oldest part of the records, suggesting that perennial sea ice existed only ephemerally until 36.7 million years ago. © 2014 Macmillan Publishers Limited.