North Atlantic warming during Dansgaard-Oeschger events synchronous with Antarctic warming and out-of-phase with Greenland climate

www.nature.com/scientificreports OPEN received: 10 July 2015 accepted: 05 January 2016 Published: 05 February 2016 North Atlantic warming during Dansgaard-Oeschger events synchronous with Antarctic warming and out-of-phase with Greenland climate Tine L. Rasmussen1, Erik Thomsen2 & Matthias Moros3 The precise reason for the differences and out-of-phase relationship between the abrupt DansgaardOeschger warmings in the Nordic seas and Greenland ice cores and the gradual warmings in the southcentral Atlantic and Antarctic ice cores is poorly understood. Termed the bipolar seesaw, the differences are apparently linked to perturbations in the ocean circulation pattern. Here we show that surface and intermediate-depth water south of Iceland warmed gradually synchronously with the Antarctic warming and out of phase with the abrupt warming of the Nordic seas and over Greenland. The hinge line between areas showing abrupt and gradual warming was close to the Greenland-Scotland Ridge and the marine system appears to be a ‘push-and-pull’ system rather than a seesaw system. ‘Pull’ during the warm interstadials, when convection in the Nordic seas was active; ‘push’ during the cold stadials, when convection stopped and warm water from the south-central Atlantic pushed northward gradually warming the North Atlantic and Nordic seas. The climate of last glacial period was extremely unstable and interrupted by about 24 distinct warming and cooling events. The events are generally termed Greenland interstadials and stadials1 or Dansgaard-Oeschger events (D-O) and they are most prominent in the Greenland ice core records, where they consist of an abrupt warming to warm interstadial conditions followed by a more gradual cooling and a rapid drop to very cold stadial conditions2. The events are also recorded in the Antarctic ice cores, but the amplitudes here are smaller and the warmings are gradual in contrast to the abrupt warmings in the Greenland cores. The D-O events in the northern and southern ice cores are furthermore out of phase or even in anti-phase3–5. Imprints of D-O events have widespread occurrences in the sediments and paleoceanographic records of the world oceans (Fig. 1). The strongest indications are from the North Atlantic and Nordic seas, where the imprints often resemble the pattern recorded in the Greenland ice cores with abrupt warmings and gradual coolings6,7 (Fig. 1). The primary cause for the climatic instability is accordingly attributed to changes in the rate of convection in the Nordic seas and North Atlantic, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC)4,8,9. At the beginning of the cold stadials, convection stopped or was severely reduced10–13. The result was a decrease in the northward transport of warm water and sudden cooling of the North Atlantic to very low temperatures and a warming of the South Atlantic4. Renewed convection at the beginning of the interstadials created the opposite effect. The out-of-phase relationship between the temperature fluctuations in the Greenland and Antarctic ice cores is often referred to as the bipolar seesaw or as a “southern lead” as the warmings seem to start earlier in the south than in the north4,5,8,14,15, although recent studies indicate that the actual temperature maxima occurred about 200 years earlier in Greenland than in Antarctica16. Paleodata indicate that the changes in sea surface temperatures (SST) in the southern and central Atlantic followed the gradual warming pattern from the Antarctic ice cores3,17–19. Recent studies suggest this pattern 1 CAGE- Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geology, UiT Arctic University of Norway, N-9037 Tromsø, Norway. 2Department of Geoscience, University of Aarhus, DK-8000 Aarhus C, Denmark. 3Leibniz Institute for Baltic Sea Research Warnemünde (IOW), D-18119 Rostock-Warnemünde, Germany. Correspondence and requests for materials should be addressed to T.L.R. (email: ) Scientific Reports | 6:20535 | DOI: 10.1038/srep20535 1 www.nature.com/scientificreports/ 40 W 70 N NGRIP Nordic seas IRD-belt Greenland Sea Greenland 60 N 15 Canada 16 27 Labrador Canada 11 Iceland 24 26 10 S-G. R. 8 23 25 7 18 2 rift D tic n tla h A 1 t r o N 17 Norway Norwegian Sea 13 14 50 N 40 E Fram Strait 12 Ice-covered during stadials Mainly symmetrical DO events Mainly asymmetrical DO events SO82-02GGC 60 N 0 Gulf Stream North Atlantic Drift (NAD) Cold surface currents 9 6 5 Ireland 4 England 3 1000 m Newfoundland 1000 m 40 N Gu eam lf Str 19 Spain Portugal 20 21 22 Figure 1. Map of the North Atlantic and Nordic seas showing location of core SO82-02GGC and examined published records. Modern major warm and cold surface currents are indicated. IRD-belt (ref. 21) is marked by darker blue color. Areas estimated to have been covered by sea ice during stadials outside of the IRD-belt are hatched. Records showing primarily abrupt warming at the stadial-interstadial transitions are marked by diamonds, while records showing primarily gradual warming are marked by circles. (The map was made using MapInfo Professional version 12 software, http://www.mapinfo.com/). continued all the way to the southern edge of the so-called IRD-belt20. This belt, which stretches across the Atlantic from Newfoundland to Ireland and Portugal, is characterized by glacial sediments containing distinct layers with abundant IRD reflecting periodical releases of huge numbers of icebergs from the Laurentide ice sheet6,21. These outbreaks, which are termed Heinrich events, are generally considered to be in phase with the larger and longer-lasting stadials in the Greenland ice cores6, although in the central northernmost Atlantic the arrival of icebergs may have lagged the beginning of the cold phase by several hundred years13. While is generally accepted that the overall difference between the D-O oscillations in the northern and southern hemispheres is caused by variability in the AMOC, there is no consensus regarding the various processes that might have affected this variability and on how they interplayed with each other. Numerous factors have been suggested including changes in the strength6,22–24 and location of the deep convection25, changes in the continental ice sheets26, melt water release9, variability of sea ice cover27, heat exchange between the ocean and the atmosphere, and atmospheric heat transport28. A major obstacle seems to be the scarcity of information from the North Atlantic between the IRD-belt and the Greenland-Scotland Ridge, where only a few studies have been carried out13,29–32. This area is important as it is close to the Nordic seas and Greenland ice cap and still represents the open Atlantic. Here we examine the configuration of D-O events 17–3 in core SO82-02GGC (SO2) taken at a water depth of 1730 m on the western side of the Reykjanes Ridge (Fig. 1). The core site is located north o (...truncated)