1. Palaeoecology, Institute of Environmental Biology, Utrecht University, Laboratory of Palaeobotany and Palynology, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
2. Royal Netherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, PO Box 59, 1790 AB, Den Burg, Texel, The Netherlands
3. Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK
4. Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
5. Department of Earth Sciences, Rice University, 6100 Main Street, Houston, Texas 77005, USA
6. Earth and Atmospheric Sciences Department and the Purdue Climate Change Research Center, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47906, USA
7. USGS National Center, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA
8. Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan
9. Bujak Research International, 105 North Park Drive, Blackpool, FY3 8NE, UK
10. Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany
11. Shell UK Exploration and Production, 1 Altens Farm Road, Nigg, Aberdeen, AB12 3FY, UK
12. School of Ocean & Earth Science, Southampton Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK
13. National Museum of Natural History, 'Naturalis', PO Box 9517, 2300 RA Leiden, The Netherlands
14. Department of Geology and Geochemistry, Stockholm University, Stockholm, SE-10691, Sweden
15. University of Rhode Island, Bay Campus, Narragansett, Rhode Island 02882, USA
16. Geological Sciences, Brown University, 324 Brook Street, PO Box 1846, Providence, Rhode Island 02912-1846, USA
17. Department de Géologie et Océanographie, Université Bordeaux 1, Avenue des facultés, c/o Bernei Housen, 33405 Talence Cedex, France
18. Department of Geophysics, CEREGE (CNRS)/University of Aix-Marseille 3, BP 80, 13545 Aix-en-Provence Cedex 4, France
19. Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden
20. Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
21. Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK
22. Graduate School of Oceanography, University of Rhode Island, Narragansett Bay Campus, Narragansett, Rhode Island 02882, USA
23. Norwegian Polar Institute, Polar Environmental Center, N-9296 Tromsø, Norway
24. Department of Earth Sciences, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, USA
25. British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
26. Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA
27. School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
28. Department of Geography and Environment, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, UK
29. Department of Geology, Paleontology & Geophysics, University of Padova, Via Giotto 1 I-35137 Padova, Italy
30. Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho 2-15, Yokosuka 237-0061, Japan
31. Department of Geology and Environmental Science, MSC 7703, James Madison University, Harrisonburg, Virginia 22807, USA
32. Institute of Life and Environmental Science, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan
33. Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aramaki, Aoba, Aoba-ku, Sendai City 980-8578, Japan
34. Institute of Geoscience, National Institute of Advanced Industrial Science, and Technology (Geological Survey of Japan) AIST, Tsukuba Central 7, Higashi-1-1-1, Tsukuba, Ibaraki 305-8567, Japan
35. Graduate School of Environmental Earth Science, Hokkaido University, Kita-10, Nishi-5, Kita-ku, Sapporo 060-0810, Japan
36. †A list of authors and affiliations appears at the end of the paper

Correspondence to: Henk Brinkhuis¹ Correspondence and requests for materials should be addressed to H.B. (Email: H.Brinkhuis@bio.uu.nl).

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 seas^{4, 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.

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.