Evidence for middle Eocene Arctic sea ice from diatoms and ice-rafted debris

The arrival of Arctic ice Sea ice has not always existed in the Arctic and indeed little is known about the sea-ice history of this climatically sensitive region. The recent discovery of ice-rafted debris in middle Eocene ocean sediments prompted suggestions that ice appeared in the Arctic about 46 million years ago, but it is important to distinguish between land-based glacial ice and sea ice as the source of the debris because each kind of ice has different climate implications. Now the analysis of an oceanic sediment core from the ACEX project reveals vast quantities of well-preserved fossils of the needle-like diatom Synedropsis spp., which relied on sea ice for its survival, at about 260 metres below the sea floor of the central Arctic. This, in combination with detailed grain analysis, suggests that sea ice was the dominant source of ice-rafted debris at around 47.5–45.5 million years ago. The results push back the first appearance of ice in the Arctic by 1.25 million years, and the first appearance of sea-ice diatoms by 16 million years. The presence of ice-rafted debris (IRD) in middle Eocene ocean sediments has previously been demonstrated, but it has been unclear whether the source of IRD was land-based glacial ice or sea ice, a distinction with important climate implications. The analysis of a sediment core from the ACEX project now reveals evidence that sea ice was the dominant source for IRD from 47.5 to 45.5 million years ago. Oceanic sediments from long cores drilled on the Lomonosov ridge, in the central Arctic 1 , contain ice-rafted debris (IRD) back to the middle Eocene epoch, prompting recent suggestions that ice appeared in the Arctic about 46 million years (Myr) ago 2 , 3 . However, because IRD can be transported by icebergs (derived from land-based ice) and also by sea ice 4 , IRD records 2 , 3 are restricted to providing a history of general ice-rafting only. It is critical to differentiate sea ice from glacial (land-based) ice as climate feedback mechanisms vary and global impacts differ between these systems: sea ice directly affects ocean–atmosphere exchanges 5 , whereas land-based ice affects sea level and consequently ocean acidity 6 . An earlier report 3 assumed that sea ice was prevalent in the middle Eocene Arctic on the basis of IRD, and although somewhat preliminary supportive evidence exists 2 , these data are neither comprehensive nor quantified. Here we show the presence of middle Eocene Arctic sea ice from an extraordin