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Late Quaternary Ice Sheet History and Dynamics in Central and Southern Scandinavia Timothy F

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Late Quaternary Ice Sheet History and Dynamics in Central and Southern Scandinavia Timothy F Dissertations from the Department of Physical Geography and Quaternary Geology No 22 Late Quaternary ice sheet history and dynamics in central and southern Scandinavia Timothy F. Johnsen Doctoral Thesis in Quaternary Geology at Stockholm University, Sweden 2010 Late Quaternary ice sheet history and dynamics in central and southern Scandinavia Timothy F. Johnsen Doctoral Dissertation 2010 Department of Physical Geography and Quaternary Geology Stockholm University To Mike © Timothy F. Johnsen ISBN: 978-91-7447-068-0 ISSN: 1653-7211 Paper I: © Swedish Society for Anthropology and Geography Paper II: © The Boreas Collegium Layout: Timothy F. Johnsen (except for papers I and II) Cover photo: View from Handöl, Sweden to north-northeast over the shoreline of Lake Ånnsjön and to Mt. Åreskutan at background right (Timothy F. Johnsen, May 2005) Printed in Sweden by PrintCenter US-AB Late Quaternary ice sheet history and dynamics in central and southern Scandinavia Doctoral dissertation 2010 Timothy F. Johnsen Department of Physical Geography and Quaternary Geology Stockholm University Abstract Recent work suggests an emerging new paradigm for the Scandinavian ice sheet (SIS); one of a dynamically fluctuating ice sheet. This doctoral research project explicitly examines the history and dynamics of the SIS at four sites within Sweden and Norway, and provides results covering different time periods of glacial his- tory. Two relatively new dating techniques are used to constrain the ice sheet history: the optically stimulated luminescence (OSL) dating technique and the terrestrial cosmogenic nuclide (TCN) exposure dating tech- nique. OSL dating of interstadial sediments in central Sweden and central Norway indicate ice-free conditions during times when it was previously inferred the sites were occupied by the SIS. Specifically, the SIS was absent or restricted to the mountains for at least part of Marine Isotope Stage 3 around 52 to 36 kyr ago. Inland portions of Norway were ice-free during part of the Last Glacial Maximum around 25 to 20 kyr ago. Consistent TCN exposure ages of boulders from the Vimmerby moraine in southern Sweden, and their compatibility with previous estimates for the timing of deglaciation based on radiocarbon dating and varve chronology, indicate that the southern margin of the SIS was at the Vimmerby moraine ~14 kyr ago. In central Sweden, consistent TCN ages for boulders on the summit of Mt. Åreskutan and for the earlier deglaciated highest elevation moraine related to the SIS in Sweden agree with previous estimates for the timing of deglaciation around 10 ka ago. These results indicate rapid decay of the SIS during deglaciation. Unusually old radiocarbon ages of tree remains previously studied from Mt. Åreskutan are rejected on the basis of incompatibility with consistent TCN ages for deglaciation, and incompatibility with established paleoecological and paleoglaciological reconstructions. Altogether this research conducted in different areas, covering different time periods, and using compara- tive geochronological methods demonstrates that the SIS was highly dynamic and sensitive to environmental change. Keywords: Scandinavian ice sheet, ice sheet dynamics, luminescence dating, cosmogenic exposure dating, geochronology, moraine, interstadial, deglaciation, nunatak Late Quaternary ice sheet history and dynamics in central and southern Scandinavia Timothy F. Johnsen Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden This doctoral thesis consists of a summary and four appended papers. The papers are listed below and are referred to as Paper I-IV in the summary. Paper I: Johnsen, T.F., Alexanderson, H., Fabel, D., Freeman, S.P.H.T. 2009. New 10Be cosmogenic ages from the Vimmerby moraine confirm the timing of Scandinavian Ice Sheet deglaciation in southern Sweden. Geografiska Annaler: Series A, Physical Geography, 91: 113–120. – Reprinted with permission of the Swed- ish Society for Anthropology and Geography. Paper II: Alexanderson, H., Johnsen, T., Murray, A.S. 2010. Re-dating the Pilgrimstad Interstadial with OSL: a warmer climate and a smaller ice sheet during the Swedish Middle Weichselian (MIS 3)? Boreas, 39: 367–376. – Reprinted with permission of The Boreas Collegium. Paper III: Johnsen, T.F., Olsen, L., Murray, A., Submitted. OSL ages in central Norway confirm a MIS 2 interstadial (25-20 ka) and a dynamic Scandinavian ice sheet. Quaternary Science Reviews. Paper IV: Johnsen, T.F., Fabel, D., Stroeven, A. High-elevation cosmogenic nuclide dating of the last de- glaciation in the central Swedish mountains: implications for the timing of tree establishment. Manuscript. 5 Timothy F. Johnsen Introduction millions of humans in coastal regions (IPCC 2007). These processes along with others related to global Ice sheets are a crucial component of the function- climate change remind us of how we as a species ing of the Earth system (Oerlemans and van der are linked to the activities of ice sheets that in turn Veen 1984). Their large size displaces vast areas of at least partially reflect our own activity, and that plants and animals (Robertsson 1994, Hewitt 2000) our fate is tied to how the Earth and its ice sheets and changes the albedo and climate of the Earth and climate will behave. Despite intensive scien- (Manabe and Broccoli 1985, Ruddiman 2003). tific efforts it remains difficult to accurately predict Tremendous quantities of water from the oceans the magnitude and rate of changes for the future, are stored on land as ice, causing global sea levels and this uncertainty is directly tied to our under- to lower over 100 metres, and their massive weight standing of the how the Earth system is operating depresses the surface of the Earth hundreds of and has operated. Answering important questions metres which leads to flooding of coastal areas and about the future climate and conditions on Earth the diversion of rivers (Lambeck and Chappell are inextricably linked to our understanding of how 2001). Immense quantities of meltwater can be the Earth has operated in the past. Thus, by under- discharged, disrupting ocean circulation and the standing the dynamics of past ice sheets, as in this climate system and resulting in sudden sea level doctoral research project, we will better predict rise (Fairbanks 1989). And, huge areas of the land- how modern ice sheets will respond to climate scape are altered and shaped by the erosional and change and affect society. depositional activity of ice sheets (Lundqvist During the Quaternary Period multiple glaci- 2002). In addition, our own past is closely linked to ations of varying spatial extent occurred in Scandi- that of ice sheets and ice dynamics. Modern hu- navia starting in the Early Quaternary (Mangerud mans evolved during the Late Quaternary, a period et al. 1996, Kleman et al. 2008) and with the first characterized by glaciations and rapid climatic and glaciation reaching the shelf-edge occurring ~1.1 environmental shifts resulting in great changes in Ma (Sejrup et al. 2000). As ice sheets are effective the distribution of organisms. The present genetic agents of erosion the best evidence for glaciations structure of populations, species and communities is from the most recent glaciation, the Weichselian, has been mainly formed by Quaternary ice ages spanning from ~117 to 11.7 ka, and the reconstruc- (Hewitt 2000). tion of glacial history prior to the Last Glacial In recent time dramatic changes in the margins Maximum (LGM) is in many cases difficult and of the Greenland and Antarctic ice sheets have ambiguous (Fig. 1 and 2). Numerous deposits and occurred including rapid but episodic glacier accel- landforms related to the last deglaciation dominate eration and thinning from their marine-terminating the landscape (Fredén 2002) while reconstruction sectors (Shepherd and Wingham 2007): e.g., the of earlier ice sheet activity mostly relies on discov- collapse of sections of the Larsen Ice Shelf in Ant- ery and study of terrestrial sediments or landforms arctica (Rott et al. 1996), and loss of about 100 Gt that have managed to survive being overrun by the yr-1 of mass from the the Greenland ice sheet Scandinavian ice sheet (SIS; e.g., Lagerbäck 1988, (Shepherd and Wingham 2007). The rate of mod- Robertsson and García Ambrosiani 1992, Kleman ern changes of ice sheets is occurring faster than and Stroeven 1997, Olsen et al. 2001b, Hättestrand many scientists anticipated, and have made it easier and Stroeven 2002, Lundqvist and Robertsson to imagine dynamic glacier activity for the past. As 2002, Heyman and Hättestrand 2006, Lokrantz and well, a major change in our understanding of the Sohlenius 2006), marine sediments (e.g., Sejrup et dynamics of ice sheets occurred when present and al. 1994, Baumann et al. 1995, Vorren and Laberg past ice streams were recognized for their impor- 1997), and ice sheet modelling (e.g., Holmlund and tance in the mass balance of ice sheets and dy- Fastook 1995, Kleman et al. 1997, Lambeck et al. namic behaviour (e.g., Bentley 1987, Stokes and 1998, Boulton et al. 2001, Siegert et al. 2001, Clark 2001, Alley et al. 2004). Decay of modern Charbit et al. 2002, Näslund et al. 2003). With the ice sheets along with global climate warming is set exception of the last deglaciation, the timing of ice to cause global sea levels to rise considerably dur- sheet advances and retreats is based mainly on ing this century and potentially displace tens of correlation to ‘global’ continuous records such as 6 Late Quaternary ice sheet history and dynamics in central and southern Scandinavia western margin of the ice sheet was highly dy- namic with multiple ice-free periods during the last 55 ka including around the LGM (Olsen et al. 2001a,b, 2002). Large moraine systems from the southeast portion of the ice sheet may be younger than previous estimates (Rinterknecht et al. 2006). There were possibly ice-free conditions around the LGM in southern Sweden (Alexanderson and Murray 2007) and southern Norway (Bøe et al.
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