Ice sheets, oceanic circulation & sea surface conditions: the NW European margin during the last 35,000 years
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The north-western European continental margin provides a key area for detailed investigations of changes within the inflow of Atlantic warm surface currents, fluctuations in continental ice sheets and the corresponding influence on sea surface conditions, such as (near) sea surface temperature and primary productivity. Knowledge of these fluctuations and forcing mechanisms is crucial to further our understanding of present day climate changes. This dissertation is principally based on continental slope cores, combined with shallow seismic lines and instrumentally measured data. The studied time interval is twofold, the last glacial cycle (35,000 - 15,000 years BP) and the mid- and late Holocene (the last 8000 years). During the last glacial cycle, the northwestern European margin was exposed to the advancing and retreating continental ice sheets, resulting in thick glacial, hemipelagic deposits on the continental slope that provide a continuous sedimentary archive of ice sheet build-up, maximum and decay. In contrast, the mid- and late Holocene encompassed relatively stable climatic conditions, with unusually high accumulation rates on the mid-Norwegian margin, allowing for a high-resolution analysis of the regional sea surface conditions. In addition, this dissertation presents a new automated procedure for counting ice rafted debris (IRD), providing a faster, more precise alternative method compared to the current manual counting technique utilised by numerous palaeo-environmental studies. The multi-proxy approach of this thesis was achieved through a set of different analytical techniques, ranging from X-ray fluorescence core scanning, to grain size analysis and foraminifera assemblage counts to numerical analysis and seismic interpretation. The complex ice-ocean interaction along the NW European continental margin during the last glacial cycle and the more recent oceanographic changes observed in the Holocene are chronologically constrained through numerous new and published radiocarbon and 210Pb/137Cs dated samples and the identification of Icelandic tephra events, which were used as input parameters for Bayesian age modelling. The main results of the sedimentological and micro-palaeontological analysis indicate, that the proposed confluence of the British-Irish and the Fennoscandian ice sheets during the last glacial cycle, was likely later and shorter than previously anticipated, between 25.5 and 18.7 ka BP. Furthermore, the data indicated that the Norwegian Channel Ice Stream might have only been active between 23.3 and 19 ka BP. Moreover, the results, supported through seismic interpretations from the central North Sea, showed evidence of a glacial outburst flood, connected to the break-up of the ice sheet confluence. The distal deposits of this event, an ultra-rapidly deposited meltwater plume dated to 18.7 ka BP, were additionally investigated through numerical modelling, indicating a significant influence of ultra-rapid sedimentation on the gas hydrate stability zone. Within the mid- and late Holocene, the results of this dissertation demonstrate a strong link between relative calcium content and primary production, suggesting that highresolution calcium data can be used as a relative productivity proxy within the southeastern Nordic Seas. Finally, cross-correlation efforts found that the primary production in the region is inversely linked to the North Atlantic Oscillation, which might indicate that this link can be used for NAO reconstructions.
Består avPaper I: Becker, L.W.M., Hjelstuen, B.O., Støren, E.W.N. and Sejrup, H.P., 2017. Automated counting of sand-sized particles in marine records. Sedimentology. 65(3):842-850. The article is available in the main thesis. The article is also available at: https://doi.org/10.1111/sed.12407
Paper II: Becker, L.W.M., Sejrup, H.P., Hjelstuen, B.O., Haflidason, H. and Dokken, M., 2017. Ocean-ice sheet interaction along the SE Nordic Seas margin from 35 - 15 ka BP. Marine Geology (GLANAM Special Issue). The article is available in the main thesis. The article is also available at: https://doi.org/10.1016/j.margeo.2017.09.003
Paper III: Hjelstuen, B.O., Sejrup, H.P., Valvik, E., Becker, L.W.M., In Press. Evidence of an ice-dammed lake outburst in the North Sea during the last deglaciation. Marine Geology (GLANAM Special Issue). The article is available in the main thesis. The article is also available at: https://doi.org/10.1016/j.margeo.2017.11.021
Paper IV: Karstens, J., Haflidason, H., Becker, L.W.M., Berndt, C., Rüpke, L., Planke, S., Liebetrau, V., Schmidt, M., Mienert, J., 2018. Glacigenic sedimentation pulses triggered post-glacial gas hydrate dissociation. 9:635. The article is available at: http://hdl.handle.net/1956/17688
Paper V: Becker, L.W.M., Sejrup, H.P., Hjelstuen, B., Haflidason, H., Kjennbakken, H., Werner, J., in prep. Holocene primary productivity fluctuations in the SE Nordic Seas. Full text not available in BORA.