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

Abstract

Oceanic sediments from long cores drilled on the Lomonosov ridge, in the central Arctic1, 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) ago2,3. However, because IRD can be transported by icebergs (derived from land-based ice) and also by sea ice4, IRD records2,3 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 exchanges5, whereas land-based ice affects sea level and consequently ocean acidity6. An earlier report3 assumed that sea ice was prevalent in the middle Eocene Arctic on the basis of IRD, and although somewhat preliminary supportive evidence exists2, these data are neither comprehensive nor quantified. Here we show the presence of middle Eocene Arctic sea ice from an extraordinary abundance of a group of sea-ice-dependent fossil diatoms (Synedropsis spp.). Analysis of quartz grain textural characteristics further supports sea ice as the dominant transporter of IRD at this time. Together with new information on cosmopolitan diatoms and existing IRD records2, our data strongly suggest a two-phase establishment of sea ice: initial episodic formation in marginal shelf areas 47.5 Myr ago, followed 0.5 Myr later by the onset of seasonally paced sea-ice formation in offshore areas of the central Arctic. Our data establish a 2-Myr record of sea ice, documenting the transition from a warm, ice-free3 environment to one dominated by winter sea ice at the start of the middle Eocene climatic cooling phase7.

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Figure 1: Idealized palaeogeography of the Arctic region for the early middle Eocene during the phase of biosilica production and preservation at the Lomonosov ridge (50–45 Myr ago).
Figure 2: SEM image of part of an aggregate of near-whole needle-shaped Synedropsis sp. valves.
Figure 3: IRD and sea-ice diatom abundance in the ACEX cores.
Figure 4: Results of IRD analysis in a short core interval.

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Acknowledgements

This research used samples and data provided by the Integrated Ocean Drilling Program (IODP). C.E.S. and N.K. acknowledge funding by VISTA (Norwegian Academy of Science and Letters and StatoilHydro) and the Research Council of Norway. We thank R. M. DeConto, M. P. Olney, S. M. Bohaty, A. Davies, M. A. Pearce, F. Sangiorgi, H. Brinkhuis, P. K. Bijl, J. Backman, J. Pike and A. E. S. Kemp for discussions. C.E.S. thanks StatoilHydro, Bergen for access to facilities, J. A. Barron, E. Fourtanier, I. Suto and J. Onodera for preliminary talks, and W. Hale for facilitating sample collection.

Author Contributions C.E.S. wrote the manuscript except for the sections on IRD and surface textures, which were written by K.S.J. and S.P. The manuscript incorporates comments by all authors, who also advised on its contents, structure and remit. Diatom analysis and light microscope imaging was undertaken by C.E.S.; statistical work on the surface textures was carried out by S.P.; R.B.P. and C.E.S. imaged Synedropsis spp. in the SEM at the NOCS; L.E.K. and K.S.J. undertook SEM analysis of surface textures; and SEM observations on Synedropsis spp. were also performed by R.W.J. and N.K.

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Correspondence to Catherine E. Stickley.

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Stickley, C., St John, K., Koç, N. et al. Evidence for middle Eocene Arctic sea ice from diatoms and ice-rafted debris. Nature 460, 376–379 (2009). https://doi.org/10.1038/nature08163

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