Hydrography in the Nordic Seas During Dansgaard-Oeschger Events 8-5
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The modern rapidity of Arctic sea ice loss and rise in Arctic air and ocean temperatures is pushing research to understand mechanisms for abrupt climate fluctuations. Resolving the mechanisms of past abrupt changes is a key tool for illuminating potential future reorganization of the climate system. A series of large and abrupt climate fluctuations, first discovered in Greenland ice cores and characterized by sudden warmings followed by more gradual coolings, referred to as Dansgaard- Oeschger events, punctuated the last glacial period. These fluctuations are thought to be linked to changes in the hydrography of the Nordic Seas. In order to draw parallels to the modern changes taking place in the Arctic, this thesis studies the reorganization of the hydrography in the Nordic Seas during Dansgaard-Oeschger events 8-5 (40-30 ka b2k). This thesis aims to resolve the role that the Nordic Seas hydrography played in the climate fluctuations of the Dansgaard-Oeschger events. To do this, geochemical analysis of foraminiferal CaCO3 and its trace and major elements, alongside oxygen and carbon isotopes of the same foraminiferal species are used to reconstruct a picture of the hydrographical conditions in the Nordic Seas. Two sediment cores are implemented; one from the Denmark Strait, and the other from the Faroe-Shetland Channel, to capture the inflow and outflow regions to the Nordic Seas. A combination of near surface and intermediate water proxies are used to reconstruct the water column in both these regions. The combined results of research Papers 1-3 highlight a Nordic Seas that reorganized itself as sea ice retreated and expanded in time with the Dansgaard-Oeschger cycles. Colder air temperatures and expanded sea ice cover are linked to a well-stratified cold and fresh surface underlain by a homogenized warm intermediate water of Atlantic origin across the entire Nordic Seas basin. When warm air temperatures and reduced sea ice cover are present, the eastern and western Nordic Seas are decoupled. Disintegration of the surface stratification and increased mixing occurs in the eastern Nordic Seas. In the western Nordic Seas where there is a continued presence of the sea ice cover, stratification is strong. However, both the eastern and western Nordic Seas experience similar intermediate water temperatures of cooled Atlantic Originating Water. The vertical stability of the water column changes through the fluctuations, with potential implications for our understanding of the stability of the modern Arctic Ocean and its sea ice cover.
Består avPaper I: Sessford, E.G., Tisserand, A.A., Risebrobakken, B., Andersson, C., Dokken, T., and Jansen, E. 2018: High resolution benthic Mg/Ca temperature record of the intermediate water in the Denmark Strait across DO stadial-interstadial cycles. Paleoceanography and Paleoclimatology, 33 (11), 1169-1185. The article is available in the main thesis. The article is also available at: https://doi.org/10.1029/2018PA003370
Paper II: Sessford, E.G., Jensen, M.F., Tisserand, A.A., Muschtiello, F., Dokken, T., Nisancioglu, K.H., Jansen, E. Coherent fluctuations in intermediate water temperature off the coast of Greenland and Norway during Dansgaard- Oeschger events. The article is not available in BORA. The article is available at: https://doi.org/10.1016/j.quascirev.2019.105887
Paper III: Sessford, E.G., Wille, S.B., Tisserand, A.A., Jansen, E. Surface and deep-water interactions in the Nordic Seas during Greenland Stadial 9 to Greenland Interstadial 8. The article is not available in BORA.
Paper IV: Sessford, E.G., Tisserand, A.A. and Chalk, T.B. The relationship between carbonate ion saturation and Mg/Ca temperature reconstructions for the benthic species C. neoteretis. The article is not available in BORA.