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Episodic fresh surface waters in the Eocene Arctic Ocean


It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions1 that characterized the early Palaeogene period (55–45 Myr ago) probably induced an intensified hydrological cycle2 with precipitation exceeding evaporation at high latitudes3. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an 800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas4,5,6,7 probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea8. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from 10 °C to 13 °C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas.

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Figure 1: The early Eocene Arctic basin, site locations, and geographic distribution of the Azolla pulse in adjacent basins.
Figure 2: Core recovery, chronology, palynological, physical properties and geochemical data across the Azolla phase of holes 302-4A and 913B.
Figure 3: Azolla mega- and microspores from Hole 302-4A-11X.

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H.B. thanks the Netherlands Organization for Scientific Research (NWO) and Utrecht University for enabling participation in the ACEX expedition. A.S. thanks the Utrecht Biogeology Centre for funding. M.H. thanks the Purdue Research Foundation for funding. This research used samples and data provided by the Integrated Ocean Drilling Program (IODP). We thank L. Bik, T. Brain, S. Gibbons, P. Goggin, N. Holloway, J. van Tongeren, N. Welters and M. Woltering for technical support and L.J. Lourens, H. Nohr-Hansen, M. Pagani, C.E. Stickley, G.L. Williams and J.C. Zachos for discussions. Author Contributions H.B., A.S., F.S. and J.S.E. carried out the palynology, S.S. and J.S.S.D. the TEX86 and BIT analyses, M.E.C., J.v.d.B. and H.v.K.-v.C. the palaeobotany, J.O. and K.T. generated siliceous microfossil numbers, while R.S. generated the TOC data. M.H. carried out the comparison to climate models. J.B. and K.M. were co-chiefs on the ACEX. J.P.B. and H.B. compiled the unpublished industrial palynological records. All authors contributed to writing the paper.

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Correspondence to Henk Brinkhuis.

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Brinkhuis, H., Schouten, S., Collinson, M. et al. Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441, 606–609 (2006).

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