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CGC-12 The challenge of the Younger Dryas?
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Ocean-atmosphere flickering ended the younger dryas cold period
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Jostein Bakke, University of Bergen (Norway)
Øyvind Lie, University of Bergen (Norway)
Einar Heegaard, University of Bergen (Norway)
Trond Dokken, University of Bergen (Norway)
Gerald Haug, ETH Zurich (Switzerland)
Peter Dulski, GeoForschungsZentrum Potsdam (Germany)
Hilary Birksq, University of Bergen (Norway)
Atle Nesje, Univeristy of Bergen (Norway)
Svein Olaf Dahl, University of Bergen (Norway)
Trygve Nilsen, University of Bergen (Norway)
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Multi-proxy reconstructions from a sub-annually resolved sediment record from Lake Krkenes in western Norway and a marine core from the North Atlantic show that the late Younger Dryas (YD) period experienced large amplitudes in summer climate variability. This "flickering" could be explained by rapid changes in heat-exchange involving sea-ice/ocean current adjustments due to the strong extra-tropical thermohaline gradient present during the YD. The subdecadal to interannual time scales of the flickering apparent in the Lake Krkenes record also suggest the involvement of the atmosphere. We propose that periodic break up of the sea-ice cover in the eastern North Atlantic freed the westerlies from their previously imposed zonal track, allowing ocean-atmosphere heat convection. Feedback mechanisms, driven by the increased fresh melt-water, repeatedly brought the climate in the North Atlantic region back to the YD cold mode. Finally the positive forcing overran the negative feedbacks and the abrupt transition into the Holocene occurred. The basis for the study is analyses of a 1.4 m core of undisturbed sediments from Lake Krkenes that spans the Allerød-YD-Holocene transitions. The age-depth relationship is based on three 'fixed' points at the transitions into (12,896 yr BP (yr BP = yr BP b2k) and out of the Younger Dryas (11,703 yr BP) in addition to the Vedde tephra horizon; 12,171 yr BP). Altogether 96 radiocarbon dates are reported from the lake sediments. Geochemical analyses were carried out with the Micro X-ray fluorescence core scanner at GFZ, Potsdam . The 11 geochemical elements (for every 60m) and six physical sediment parameters (every 0.5 cm) were measured at the same depths, making an exact relative chronology within the section. The increased mass turnover of the Krkenes cirque glacier is detected by increased Ti concentration and elemental variability in the lake sediments. The increased flux of fresh melt-water to the ocean resulted in more extensive sea ice which pushed the jet south once more, thus re-establishing the stadial state. Thus we see the variability in the late YD as a rapid oscillation between a persistent stadial state with North Atlantic ice cover and brief incursions of the Gulf Stream until the system finally switched to the latter state at the onset of the Holocene. We propose that this mechanism may be important for other rapid events such as the DO events, where similar variability in climate proxies is apparent during the latter part of the stadials.
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