Holocene variations in atmospheric circulation in the North Atlantic region reconstructed from lake sediments
MetadataVis full innførsel
Holocene variations in atmospheric circulation in the North Atlantic region have been reconstructed, based on three individual lake sediment studies from sites along the coast of Norway and Svalbard. This thesis contributes with new palaeoclimatic reconstructions revealing variability in wind and precipitation patterns in the northeastern North Atlantic. In Paper I, we present a new record of Holocene glacier variability of Ålfotbreen ice cap in western Norway. By applying a novel approach of calibrating lake sediments with instrumental glacier mass-balance measurements we are able to extend glacier mass-balance variability as reflected in equilibrium-line altitude (ELA) changes for the last 1400 years. Our data suggest that deglaciation of Ålfotbreen occurred ~9700 cal yr BP, and the ice cap was subsequently absent or very small until a short-lived glacier event is seen in the lake sediments ~8200 cal yr BP. The ice cap was most likely completely melted until a new glacier event occurred around ~5300 cal yr BP. Ålfotbreen was thereafter absent (or very small) until the onset of the Neoglacial period ~1400 cal yr BP. The Little Ice Age (LIA) ~650-50 cal yr BP was the largest glacier advance of Ålfotbreen since deglaciation, with a maximum extent at ~400-200 cal yr BP, when the ELA was lowered approximately 200 m relative to today. The late onset of the Neoglacial at Ålfotbreen is suggested to be a result of its low altitude relative to the regional ELA. Further, we apply a known relationship between summer temperature and ELA variations at 10 glaciers in Norway (including Ålfotbreen) to reconstruct winter precipitation during the last 1400 years. In Paper II, we present a lake record from lake Hakluytvatnet at Amsterdamøya island, the northwesternmost island on Svalbard. The lake sediment archive reveals large environmental changes that have taken place at Hakluytvatnet since the Late Glacial, as detected by multi-proxy analyses including physical sediment properties and diatom analysis. A robust chronology has been established for the lake sediment core through 28 AMS radiocarbon (14C) ages, and this gives an exceptionally wellconstrained age control for a lake at this latitude (79.5°N) that is not varved. The sedimentary archive recorded the last ~13,000 years of climate change, and is the first lake record going back to the Late Glacial at this site. Our findings indicate that a local glacier was present during the Younger Dryas (YD), and we estimate YD equilibrium-line altitude (ELA) lowering. Further, we construct a new time-series reflecting precipitation-based detrital sediments entering Hakluytvatnet (i.e., runoff) covering the period from ~5000-1300 cal yr BP. We discuss our runoff record and the internal productivity of the lake towards a record of varying sea ice extent in the Fram Strait acting as a moisture source area for Hakluytvatnet. In Paper III, a late-Holocene record of storminess in Arctic Norway is reconstructed from aeolian sediment input into the coastal lake Måvatnet, Andøya island. The study site is situated at the extreme west coast of Arctic Norway; a sensitive location for changes in North Atlantic westerly winds. Through a novel approach, combining monitoring of wind-blown lake sedimentation in sediment traps with multi-proxy analyses of lake sediments we quantify the input of wind-blown sand from a westfacing beach acting as source area into lake Måvatnet during the late-Holocene. We further assess the validity of this record to represent variations in the strength of the westerlies (i.e., storminess). The high-resolution record reveals an abrupt increase in storminess synchronously with the onset of the Little Ice Age (LIA), ca. 600 cal yr BP, coeval with increased winter precipitation at Ålfotbreen (Paper I) and a strengthening of the persistent low-pressure west of Iceland (Icelandic Low) that exerts a strong effect on North Atlantic storm tracks. Further, the timing of the onset of the LIA along the coast of Norway appears to be linked to the dynamics of the large-scale atmospheric circulation systems in the North Atlantic.
Består avGjerde, M., Bakke, J., Vasskog, K., Nesje, A., Hormes, A. (2016). Holocene glacier variability and Neoglacial hydroclimate at Ålfotbreen, western Norway. Quaternary Science Reviews, 133. pp 28-47. The article is available at: http://hdl.handle.net/1956/11246
Paper II: Gjerde, M. Bakke, J., D’Andrea, W., Balascio, N.S., Bradley, R.S., Vasskog, K., Ólafsdóttir, S., Røthe, T.O., Perren, B., Hormes, A.: Holocene multi-proxy environmental reconstruction from lake Hakluytvatnet, Amsterdamøya Island, Svalbard (79.5°N). Quaternary Science Reviews, 183. pp 164-176. The article is available at: http://hdl.handle.net/1956/17808
Paper III: Gjerde, M., Bakke, J.: Increased storminess at Andøya (Arctic Norway) during the Little Ice Age reconstructed from lake sediments. Full text available in the thesis.