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Quantifying Climate and Glacier Mass Balance in North Norway During the Younger Dryas ⁎ Brice R

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Quantifying Climate and Glacier Mass Balance in North Norway During the Younger Dryas ⁎ Brice R Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 307–330 www.elsevier.com/locate/palaeo Quantifying climate and glacier mass balance in north Norway during the Younger Dryas ⁎ Brice R. Rea a, , David J.A. Evans b a Geography and Environment, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UF, UK b Department of Geography, University of Durham, South Road, Durham DH1 3LE, UK Received 28 March 2006; received in revised form 6 October 2006; accepted 16 October 2006 Abstract Øksfjordjøkelen is located at ∼70° N on the Troms–Finnmark border in North Norway. During the Younger Dryas, it was decoupled from and sat just beyond the margin of the Scandinavian Ice Sheet. At this time the major fjords in Troms and Finnmark were ice-free with outlet glaciers from the icefield filling a number of smaller side valleys. Only one outlet from the icefield, Sörfjorddalen, is temporally well-constrained by 14C dating and association with the Main Shoreline (associated with a period of minimal crustal rebound dated to the Younger Dryas). Sörfjorddalen is reconstructed using a valley centre-line iterative model and assuming a no-slip basal boundary condition. This assumption of cold-based ice is supported by the geomorphological evidence of angular bouldery fronto-lateral moraines formed during the Younger Dryas. The equilibrium line altitude for the Sörfjorddalen is calculated using both the Balance Ratio and Accumulation Area Ratio methods, and this is used to constrain the snout positions (generally to mapped moraines) of the other outlets. This approach assumes similarity of mass balance gradients and geometries of the outlet glaciers which is supported by present-day symmetry of the icefield. This method is extremely useful in such environments where dateable material is often difficult, if not impossible, to find. Some margins terminated in deep water where bathymetry was lacking, making calving quantification problematic with subsequent impacts on equilibrium line altitudes poorly constrained. These deep-water terminating snouts were discounted from subsequent palaeo-climate reconstructions. An empirical equilibrium line altitude temperature-precipitation relationship was used to define limits of climate change required to sustain the reconstructed icefield. Palaeo-precipitation estimates were refined using a palaeo-temperature estimate for the Younger Dryas from Andøya. Calculations of ice flux through the equilibrium line altitude were used to further constrain the mass balance characteristics of the reconstructed icefield and these suggest similarities with ice masses found in the northern (Nordaustlandet) regions of Svalbard. © 2006 Elsevier B.V. All rights reserved. Keywords: Scandinavian Ice Sheet; Equilibrium line altitude; ELA; Balance ratio; Accumulation area ratio; Palaeoclimate; Younger Dryas; Mass balance; Plateau icefield; Mass balance gradient 1. Introduction Øksfjordjøkelen is a plateau icefield located on the Troms–Finnmark border in, North Norway (Fig. 1), a ⁎ Corresponding author. geographical position significant to understanding both E-mail address: [email protected] (B.R. Rea). past and present and small-scale and large-scale climate 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.010 308 B.R. Rea, D.J.A. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 307–330 Fig. 1. Troms–Finnmark regions of North Norway, location of Øksfjordjøkelen, and the margin of the Scandinavian Ice Sheet (thick black line) during the Younger Dryas (from Sollid et al., 1973). Meterological stations at Loppa and Kvænangen are also shown. dynamics. It is in a region currently affected by the sitions are defined by a combination of 14C dating, North Atlantic Oscillation, the Arctic Oscillation, the association with marine limits and an assumed com- Polar Front and the North Atlantic Drift. The aim of this monality of ELAs (Evans et al., 2002). Using a widely paper is to reconstruct the Younger Dryas (YD) con- applied temperature-precipitation relationship climate at figuration and ice dynamics of Øksfjordjøkelen which at the ELA is defined for a number of outlet glaciers. Ice this time had decoupled from the Scandinavian Ice flux through the ELA is calculated and in combination Sheet (SIS), the margin of which lay some 10 km further with reconstructed accumulation and ablation gradients to the south (Sollid et al., 1973), with the main fjords allows quantitative estimates of mass balance to be being for the most part ice-free (Evans et al., 2002). Due made. Based on these estimates climatic conditions on to its size Øksfjordjøkelen is likely to have reacted the north-western margin of the Scandinavian Ice Sheet “rapidly” to the YD cooling and so the reconstructed YD are assessed. icefield may be assumed to be in equilibrium with its contemporary climate. The lack of organic material in 2. Background sedimentary deposits associated with the former limits of Øksfjordjøkelen, hampers the construction of chro- The fjords and plateaux of the Bergsfjord Peninsula nological control for all margins of the icefield. This were fully submerged beneath ice during the LGM (Rea paper presents an approach that utilises a single, well et al., 1996), and as deglaciation proceeded, the ice sheet dated moraine to approximate the ELA (equilibrium line margin retreated up the fjords producing regional mo- altitude) for the icefield during the YD. The ELA is raine systems during still stands or readvances (Ander- assumed to represent an integration of the local climate, sen, 1965; Sollid et al., 1973). The sea level history and mainly temperature and precipitation (Benn and Evans, ice retreat pattern in Finnmark and western Troms re- 1998), and not assumed to vary significantly over the gions have been investigated respectively by Sollid et al. icefield, though glacier hypsometry may significantly (1973) and Andersen (1965, 1968). Evans et al. (2002) alter the ELA (Benn and Lehmkuhl, 2000). The icefield have summarised and integrated these regional data sets, is reconstructed using a perfect-plasticity approximation placing them in context for the Bergsfjord Peninsula. for ten outlet glaciers that drain the plateau. Snout po- From oldest to youngest these are the L11 (15–14 ka 14C B.R. Rea, D.J.A. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 307–330 309 BP), Outer Porsanger, Skarpnes (12.5 ka 14C BP), icefield landsystem (Rea et al., 1998; Rea and Evans, Tromso–Lyngen (12–10 ka 14C BP), Stordal (9–10 ka 2003) is summarised briefly here. Plateau summits of 14C BP) and Post-Stordal (Evans et al., 2002). Land- north Norway are characterized by a blockfield cover forms of relevance to this study are moraines, marine (Whalley et al., 1981; Gellatly et al., 1988; Rea et al., limits and associated deltas from the Tromsø–Lyngen 1996). Above outlet valley heads there may be marginal substage (T–LS) taken to be synonymous with the meltwater channels, bedrock erosional forms, such as Younger Dryas (Andersen, 1968; Sollid et al., 1973). The roches moutonnées and striae, and exposed bedrock T–LS is manifest in many localities in the region as a stripped of its former blockfield cover (Rea and Whalley, marine platform and shoreline notch (Main Shoreline). 1994; Rea et al., 1996). On the larger plateaux, low The shoreline is developed only outside and cross cuts amplitude and discontinuous moraines document the re- the distal faces of many T–LS moraines, indicating cession of Little Ice Age maximum glaciers (Gellatly synchroneity of the features (Marthinussen, 1960, 1962). et al., 1989; Whalley et al., 1995). In valleys with The T–LS configuration of Øksfjordjøkelen was extensive rock faces the largest accumulations of glacial characterized by an expanded version of the present day sediments occur as lateral and latero-frontal moraines plateau icefield from which outlet lobes descended into (Rea et al., 1998) formed from passively transported surrounding valleys and fjord heads. The plateau supraglacial and englacial rock avalanche/rock fall Fig. 2. (a) Glacial geomorphology in outlet valleys from Øksfjordjøkelen related to the Tromsö–Lyngen Substage (Evans et al., 2002). (b) View along Isfjorden towards the southern margin of Øksfjordjøkelen, showing the plateau, the plateau icefield and the outlet of Isfjordjøkelen and its reconstituted glacier produced by ice avalanching down the precipitous fjord head. 310 B.R. Rea, D.J.A. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 307–330 Fig. 2 (continued). material. Some subglacially-derived material appears in moraines. At lower altitudes in the outlet valleys the moraines and in valley bottoms as a patchy till cover. bouldery latero-frontal moraines, shorelines and deltas Where outlet glaciers terminated in the fjords, ice mar- document the former limits of extended outlet glaciers. ginal locations are marked by Gilbert-type deltas. 3.2. Tromsö–Lyngen substage (T–LS) 3. Field evidence for Younger Dryas plateau icefield coverage Fig. 2 presents a simplified map illustrating the moraines and raised marine landforms mapped around 3.1. Present-day Øksfjordjøkelen (Evans et al., 2002). Local moraines and associated raised marine features (Evans et al., Øksfjordjøkelen currently covers ca. 40 km2, with 2002) have been temporally well-constrained using six outlet glaciers draining into surrounding valleys regional isobase maps (Marthinussen, 1960). During the (Fig. 2a). The altitudinal range of
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