Rapid Adiustments of the Western Part of the Scandinavian Lee Sheet During the Mid and Late Weichselian - a New Model
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NORWEGIAN JOURNAl Of GEOLOGY Rapid changes of the Scandinavian lee Sheet 93 Rapid adiustments of the western part of the Scandinavian lee Sheet during the Mid and late Weichselian - a new model Lars Olsen, Harald Sveian & Bjørn Bergstrøm Olsen, Sveian, H. Bergstrøm, B.: Rapid adjustments of the western part of the Scandinavian ice sheet during the Mid- and Late Weichselian L., & a new model. Norsk Geologisk TidsskriftVol. 81, pp.93-118. Tr ondheim 200 ISSN 0029-196X. l. Regional Quaternary stratigraphy, fo ssil content (marine mollusc shells, dinocysts, pollen, etc.), some palaeomagnetic data, and more than 200 datings, mostly AMS-14C datings of organic-bearing tills and sub-till waterlain sediments from the northern, central and southern parts of Nor way are the basis fo r construction of nine glaciation curves from the inland to the coast and shelf, and fo r interpretation of the palaeodimate. The results show rapid shifts between glacial and interstadial conditions in semi-cydes of five thousand to seven thousand years in the interval fromc. 40-45 to 10 (14C) BP. We describe these glacial variations in a new model which retlects rapid and rythmic glacier tluctuations. The condusi ka ka ons with regard to num ber and size ( extent) of the glacial and interstadial events are based on stratigraphy, whereas the timing and rapidity of events are based on dates. these basis data are presented more thoroughly in an accompanying pa per ( this volume). The interstadials are named All the Hattfjelldal interstadial I (30-39 BP ) and Il BP), and the Tr ofors interstadial (17-21 BP). Previously reported interstadials are ka (24-27 ka ka extended in this study to include larger inland areas, indicating extreme tluctuations several times both in the extent and volume of the ice. Consi derable ice retreat with very extensive ice-free areas in several parts of Norway during the last two interstadials (c. 24-27 and c. 17-21 BP) have ka not been reported before, except fo r preliminary short notices from our studies. The stratigraphical record includes many indications of high pre Holocene relative sea-levels, suggesting a considerable glacial isostatic depression of western Scandinavia during the interstadials. We suggest that, in addition to precipitation, the mountainous fjord and valley topography, glacial isostasy and relative sea-level changes were probably more important fo r the size of the glacial tluctuationsthan the air-temperature changes. Lars Olsen, Harald Sveian Bjørn Bergstrøm, Geological Surveyof Norway, N0-7491 Trondheim, Norway. & Corresponding author: Lars Olsen (e-mail: [email protected]) Keywords: Quaternary stratigraphy, AMS-14C da ting, palaeodimate, ice-sheet tluctuations, Scandinavia,Mid- and Late Weichselian lntroduction a 'Heinrich' event, reflecting abrupt iceberg rafting episo des of the North American Laurentide ice sheet and During the last two to three decades, most climate increased carbonate content in the sediments. Both the models for the Last Glacial Maximum (LGM, c.15-25 ka Greenland ice record and the North Atlantic records indi BP) rely on CLIMAP and COHMAP data (CLIMAP cate that each 'Heinrich' event and its correlative glacial 1976, COHMAP 1988, Lowe & Walker 1997), and thus on event is followed by an abrupt warming and subsequent a relatively continuous ice cover in Scandinavia and gradual cooling that together constitute a 'Bond' cycle extensive adjacent perennial sea-ice cover in the Norwe (Lehman 1993). Similar trends with variations in ice-raf gian Sea during most of the interval 40-15 ka BP. Proxy ted debris (IRD) and carbonate content are documented climatic records from terrestrial and deep-sea sediments from the Norwegian Sea, indicating rapid shifts in glacial and ice cores reported during the last decade seriously conditions also along the western margins of the Scand challenge these views. It is reported that seasonally apen inavian ice sheet (Baumann et al. 1995). waters existed at !east three times in this period, at c. 19, A major ice retreat, with ice-free areas reaching far 25 and 30-35 ka BP (!'C), along the Norwegian coast and into the inland of southern Norway c. 32-37 ka BP, indi even north to the Fram Strait in the Polar North Atlantic cated by 14C-dates on organic-bearing sub-till sediments, Ocean (Vorren et al. 1988, Mangerud 1991a, b, c, Hebbeln was reported by Thoresen & Bergersen (1983) and Raka et al. 1994, Valen et al. 1996). The climatic record recon engen et al. (1993a). Thermoluminescence (TL) dating structed for the last glacial cycle from the Summit ice co re of 37-40 ka (calendar years) BP of aeolian sand, also on Greenland (Dansgaard et al. 1993), shows rapid shifts from inland areas of southern Norway (Bergersen et al. on a millenial scale in atmospheric temperature that 1991), gives further evidence of considerable Mid match the variations in the surfaceocean temperatures of Weichselian ice retreat. The published record showing the North Atlantic, including the variations during the similar glacier fluctuations at c. 30-40 ka BP in northern LGM interval (Bond et al. 1993). This climatic variability Norway includes both U/Th-dating of speleothems from is quasi-cydic, starting with a temperature minimum and inland caves in Nordland (Lauritzen 1991), TL-dating of 94 l. Olsen et al. NORWEGIAN JOURNAL OF GEOLOGY A BARENTSSEA � �� �;1 Finnmark l • NORTH SEA Fig. l: A) Map with stratigraphical sites (dots) used in this study. Also indicated are sites with comparable information (open circles) from other published or unpublished sources, the western margin of the Scandinavian ice sheet during the Weichsel maximum (LGM) and the Younger Dryas (YD) Stadial, transects fo r the glaciation curves (Fig. 17), and stratigraphicalsites (letters and closed circles with crosslines) referred to in the text;see also Fig. l from Olsen et al. (this volume). Positions of the major ice-stream channels - the Dj uprenna, Vestfjorden, Trænadjupet, Sklinnadjupet and the Norwegian Channel ice streams - are alsoindicated (afterOttesen et al. 2000). B) Inset map shows the topography of Fennoscandia (Finland - No rway - Sweden) and adjacentareas. Light colour!shading indicate higher elevations. NORWEGIAN JOURNAL OF GEOLOGY Rapid changes of the Scandinavian lee Sheet 95 glaciotluvial sand and AMS-14C dating of organic-bea functionedas effective calving channels during ice-stream ring sub-till sediments from inland areas of Finnmark retreat and disintegration. It is likely that the firstsignifi (Olsen 1988, Olsen et al. 1996). The present study con cant hindrance to rapid ice retreat in the North Sea area firms and extends these results, as well as previously would have been the shallower areas of the Kattegat reported interstadials from the coastal areas (Sandnes, towards the 'outlet' of the Baltic Sea basin. Considerable Ålesund, Arnøya, Hamnsund and Andøya interstadials), ice-retreat along the Norwegian coast may therefore have both geographically and with regard to time. occurred without a major contemporary ice retreat in The aim of this article is to amplify the background the Baltic Sea region (Fig. l). data and present a 2-D reconstruction of ice-sheet variati Glacial erosion was more pronounced in most parts of ons in Norway during the last glaciation (40-15 ka BP) the mainland of Norway than on the adjacent shelves and based on mapping of the regional Quatemary strati in the other peripheral zones of the area covered by the graphy (Fig.1), AMS-14C dating of organic-bearing sedi Scandinavian ice-sheet. Thus, each major glacial advance ments, shell dates and same palaeomagnetic data. These has tended to 'dean' the surface of older glacial and non results indicate glacial advances reaching across the coast glacial deposits, leaving only the youngest set of deposits to the continental shelf and interstadials with ice retreats on the ground when the ice melted. However, same excep reaching to the inland areas in the period 40-15 ka BP tions to this general trend exist, e.g. lang and complex (Olsen 1993a, b, 1995a, b, 1997a, b). These intervals sediment sequences ('sediment traps') in particular topo appear to correlate fairly well with the North Atlantic and graphical positions. This contrasts with what is found in Norwegian Sea 'Bond cycles: and describe strong cyclicity. most peripheral (ice marginal) areas, such as in Germany Another important element in this cyclic pattem is the and Poland, where deposition generally has taken place highly productive surface water (HP-zones) indicative of with little erosion of older sediments. Resedimentation apen water conditions in the Polar North Atlantic (Dok and erosion in these areas, however, may have been strong ken & Hald 1996). Together allthese factors are thought to enough to recirculate old soil remnants, which is conside describe strong cyclicity with ice growth mainly during red to be of low risk in most of Norway. This is important the last part of the HP-zones, and iceberg production and for the approach adopted in this article. ice retreat indicated by the IRD sediment horizons (Olsen Same of the tributary valleys and topographical dep 1997a), on which we have based our conceptual model. ressions in Norway make efficient sediment traps during During these studies traces indicative of marine envi major ice advances, and are considered well suited to the ronments have been found at several localities in sub-till identification and study of ice-dammed lake sediments. sediments situated much higher than the postglacial Our approach was, first to identify from regional Quater marine limit in these areas (e.g. Olsen & Grøsfjeld 1999). nary geologicalmaps potential localitieswith preservedgla This evidence, together with all the utilized methods and ciolacustrine or other fine-grained sediments covered by basis data are presented more thoroughly in an accom till,and then to select those with few or no traces of oxida panying paper (Olsen et al., this volume).