Sea ice and millennial-scale climate variability in the Nordic seas 90 kyr ago to present

ARTICLE Received 27 Oct 2015 | Accepted 15 Jun 2016 | Published 26 Jul 2016 DOI: 10.1038/ncomms12247 OPEN Sea ice and millennial-scale climate variability in the Nordic seas 90 kyr ago to present Ulrike Hoff1, Tine L. Rasmussen1, Ruediger Stein2,3, Mohamed M. Ezat1,4 & Kirsten Fahl2 In the light of rapidly diminishing sea ice cover in the Arctic during the present atmospheric warming, it is imperative to study the distribution of sea ice in the past in relation to rapid climate change. Here we focus on glacial millennial-scale climatic events (Dansgaard/ Oeschger events) using the sea ice proxy IP25 in combination with phytoplankton proxy data and quantification of diatom species in a record from the southeast Norwegian Sea. We demonstrate that expansion and retreat of sea ice varies consistently in pace with the rapid climate changes 90 kyr ago to present. Sea ice retreats abruptly at the start of warm interstadials, but spreads rapidly during cooling phases of the interstadials and becomes near perennial and perennial during cold stadials and Heinrich events, respectively. Low-salinity surface water and the sea ice edge spreads to the Greenland–Scotland Ridge, and during the largest Heinrich events, probably far into the Atlantic Ocean. 1 CAGE—Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geology, UiT, The Arctic University of Norway, NO-9037 Tromsø, Norway. 2 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, D-27568 Bremerhaven, Germany. 3 Department of Geosciences (FB5), Klagenfurter Strasse 4, University of Bremen, 28359 Bremen, Germany. 4 Department of Geology, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt. Correspondence and requests for materials should be addressed to U.H. (email: ). NATURE COMMUNICATIONS | 7:12247 | DOI: 10.1038/ncomms12247 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12247 D ansgaard/Oeschger (D/O) events in Greenland ice cores consist of warm interstadial (IS) and cold stadial events1 and are strongly imprinted in sediments from the northern North Atlantic region and Nordic seas2,3. In general the warming to insterstadial conditions was abrupt as seen in Greenland ice cores and marine records. The warm conditions were followed by gradual cooling called the insterstadial transitional cooling phase, and a rapid transition to cold stadial conditions. Larger and/or longer-lasting stadials correlate with North Atlantic Heinrich events (H-events)2, where numerous icebergs were released from the Laurentide ice sheet and melting over the North Atlantic region in the so-called Ruddiman belt4,5 (Fig. 1). Even though D/O events have been extensively studied, changes in sea ice cover have only been inferred by indirect evidence for presence or absence of sea ice (for example, deposition patterns of ice-rafted debris, oxygen isotope records and palaeo-temperature reconstructions) (Supplementary Fig. 1 and Supplementary Table 1). The Nordic seas are characterized by northward inflow of warm, saline Atlantic surface Water (North Atlantic Current, Northwest Atlantic Current, North Atlantic surface Water, Faroe Current) and southward outflow of cold Polar surface Water (East Greenland Current and East Icelandic Current)6 (Fig. 1). In the Fram Strait, the Atlantic Water continues its flow below the sea ice-covered Polar surface Water as an intermediate water mass7. In the central part of the Nordic seas cooling and sinking of the salty surface water during the winter months generate cold deep overflows over the Greenland–Scotland Ridge into the North Atlantic6,7. The inflow of Atlantic surface Water is the major source of heat to the Arctic and Nordic seas, and it is generally agreed that changes in ocean circulation and sea ice cover has played a major role in the control of past millennial-scale climate changes of the glacial D/O events2,8,9. The Atlantic surface Water is ice-free throughout the year, while the East Greenland Current is covered by drifting nearperennial sea ice. In the central parts of the Nordic seas, mixing of Atlantic Water with Polar Water forms the zone of Arctic surface Water, which is located between the Arctic and Polar fronts6 (Fig. 1). The Arctic surface Water is seasonally sea ice covered and comprises the marginal ice zone (MIZ). The location of the MIZ and the Arctic and the Polar fronts changes with the seasons and on inter-annual and longer-time scales10. In the East Greenland Current behind the Polar front productivity is very low, while intermediate to high productivity is found in the ice-free zone of Atlantic surface Water. The highest seasonal 80 °N °N 80 75 °N Svalbard MSM5/5–712–2 er PF °N AF NGRIP 70 °N 75 Fram Strait at ce °N w 70 rfa 65 °N ic ct Ar Greenland °N 65 55 GreenlandScotland Ridge Iceland IF F °N Faroe Islands °N AC C EG 60 Nw °N Nordic seas EIC 60 su FC Norwegian Sea Norway °N 55 JM11-FI-19PC ENAM93–21 FC Atlantic Ocean DW NA Scotland NA C 45 °W °E 40 °W 25 35 Ruddiman belt 30° W NAW °W 25° W 20° W 15° W °E 20 Ireland 10° W 5° W 0° 5° E 10° E 15° E Figure 1 | Map of the Nordic seas. Location of studied sediment core JM11-FI-19PC (yellow star) along with nearby core ENAM93-21/MD95-2009 (refs 3,66–68) (magenta coloured circle) and core MSM5/5-712-217 (black star) from the Svalbard margin, discussed in the text, are marked. Bathymetry from GEBCO 2014 grid (http://www.gebco.net/). Major surface (solid and dashed black lines) and bottom currents (dotted black lines), locations of Arctic Front (AF) including the Iceland-Faroe front (IFF)) and Polar Front (PF) (dashed white lines) are indicated together with the modern location of summer sea ice limit (shaded white area with drift ice), and the location of the Arctic surface water (shaded area with diagonal lines), as well as the Ruddiman belt (shaded area with waveshaped lines). EGC, East Greenland Current; EIC, East Icelandic Current; FC, Faroe Current; NAC, North Atlantic Current; NADW, North Atlantic Deep Water; NAW, North Atlantic Water; NwAC, Northwest Atlantic Current; NGRIP, North Grip ice core (orange triangle). Scale bar, 500 km. 2 NATURE COMMUNICATIONS | 7:12247 | DOI: 10.1038/ncomms12247 | www.nature.com/naturecommunications ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12247 productivity occurs at the frontal areas and in the MIZ11,12. The positions of the Arctic and Polar fronts and the degree of sea ice cover thus depend on the distribution of the major surface water masses in the Nordic seas. A recent study showed that in the Arctic Ocean, the flow of Atlantic Water has a direct impact on sea ice distribution13. Previous studies of a C25 isoprenoid lipid (IP25) synthesized mainly by diatoms have shown its potential as a valuable new proxy for the reconstruction of the presence of seasonal sea ice14–19. IP25 reportedly is produced by a few sea ice d (...truncated)