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Landscape and Sediment Processes in a Proglacial Valley, the Mittivakkat Glacier Area, Southeast Greenland

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Landscape and Sediment Processes in a Proglacial Valley, the Mittivakkat Glacier Area, Southeast Greenland Kopi fra DBC Webarkiv Kopi af: Landscape and sediment processes in a proglacial valley, the Mittivakkat Glacier area, Southeast Greenland Dette materiale er lagret i henhold til aftale mellem DBC og udgiveren. www.dbc.dk e-mail: [email protected] Landscape and sediment processes in a proglacial valley, the Mittivakkat Glacier area, Southeast Greenland Bent Hasholt, Johannes Krüger & Lilian Skjernaa Abstract During the Little Ice Age (LIA), the present proglacial Mittivakkat 106 m3 of sediment has been deposited in the valley. The estimated Valley, stretching 1.5 km ENE-WSW from the terminus of the Mitti- volume of glaciofluvial sediments deposited in the valley and delta is vakkat Glacier tongue to a delta terminating in the Sermilik Fjord, 9.9 x 106 m3. Southeast Greenland, was transgressed by the glacier as indicated by Recent sediment transport from the glacier basin of 18.4 km2 a terminal moraine near the valley mouth. The first recordings of the through the Mittivakkat Valley to the Sermilik Fjord at the mouth of glacier terminus in 1933 show a frontal retreat of about 300 m since the valley has been monitored. The annual average is around 7,000 the Little Ice Age (LIA). Since then, the glacier has retreated another m-3. The investigations confirm that a slight net deposition takes 1200 m leaving a valley train characterized by steep slopes flanking place in the upper part of the valley, showing that the proglacial val- a 150-300 m wide flat-bottomed, sediment-floored trough. The larger ley will trap more of the sediment released by glacial erosion if the part of the valley is floored by fluvial sediments and shows a typical Mittivakkat Glacier continues to retreat. Thus, the investigation valley sandur stream, the Mittivakkat stream, with one or two main demonstrates the close interaction between the different morpholog- water-channels, which branches out in a braided stream system with ical zones in a proglacial sediment transport system. numerous intervening channel bars. In the outer part of the valley train, the Mittivakkat stream is erosive and divides around remnants Keywords of ground moraine, end-moraines, and outwash with abandoned Ammassalik Island, Greenland, Mittivakkat Glacier, sediment meltwater channels. In this area some bank erosion is seen, but reju- processes, sediment budget. venation of older stream channels takes place during spring time when snowbridges cover the recent outlet and force the meltwater to Bent Hasholt (Corresponding author) follow the older channels at a higher level. Most recently, the seaward Johannes Krüger side of the outermost end-moraine ridge has been eroded by the sea, Lilian Skjernaa probably because the delta area is degrading. This could be a result Department of Geography & Geology, University of Copenhagen, of a decreased delivery of glaciofluvial sediments, due to the sink ef- Geocenter Copenhagen, Denmark. fect of the proglacial valley, but also because of less sea ice during E-mail: [email protected] summer periods and a more recent shift in wind directions during open water periods towards south and west. Geoelectric measure- ments (Schlumberger) along a number of traverses show sediment Geografisk Tidsskrift-Danish Journal of Geography thicknesses in the valley trough of 6-23 m, which suggest that 2.2 x 108(1):97-110, 2008 The first scientific investigations of glaciers in the Sermi- climate, and the effects of a glacier burst were described lik area, Southeast Greenland, were made by the geologist by Valeur (1959). In 1970, a permanent field station, the K. Milthers in 1933. He took photographs and carried out Sermilik Station, was established close to the mouth of the ablation measurements at several glaciers including the high-relief type proglacial valley of the Mittivakkat Glac- Mittivakkat Glacier on the Ammassalik Island (65°41´N, ier, and a glaciological and hydrological monitoring pro- 37°48´W). In 1958, the Mittivakkat Glacier was selected gram was initiated by the then Institute of Geography, as representing an East-Greenlandic glacier type in a University of Copenhagen (Fristrup, 1970; Hasholt, 1976, study of Greenlandic glaciers by Fristrup (1960) as a con- 1980). Since 1990s, the focus of the research has shifted tribution to the International Geophysical Year (IGY). towards an integrated study of climate-landscape interac- Among others, the runoff from the glacier, its response to tions (e.g. Jakobsen, 1990; Hasholt & Walling, 1992; Geografisk Tidsskrift-Danish Journal of Geography 108(1) 97 Nielsen, 1994; Busskamp & Hasholt, 1996; Christiansen et al., 1999; Knudsen & Hasholt, 1999; Hasholt et al., 2000; Hasholt, 2005; Mernild, 2006; Mernild et al., 2006). These investigations show that the front of the Mittivakkat Glacier has retreated continuously about 1200 m since 1933. In 1958 the ice front was located around 450 m behind the 1933 position. The exposed proglacial valley, the Mittivakkat Valley, plays an impor- tant role as a conduit for sediment transport from the gla- cier in the mountain area to the Sermilik Fjord at the mouth of the valley. As a consequence of the present gla- cier retreat, the coastal delta at the mouth of the Mitti- vakkat Valley shows evidence of degradation, because of decreasing fluvial transport capacity in the proglacial val- ley (Busskamp & Hasholt, 1996). Thus, the glacier-to- Figure 1: The Mittivakkat Glacier area, South East Greenland, with the location of the geomorphological map. fjord system may tentatively be considered as a land sys- tem where variations in the glacial activity initiated by processes in the proglacial valley for the understanding of climate change, may change the role of the proglacial val- the glacial valley landsystem development during glacial ley from a contributor of previously loosened sediment, and deglacial phases, (3) to provide quantitative data on to a pure conveyor and further to a sink trapping part of the relative size of source and sink areas, and compare the sediment eroded by the presently retreating glacier. sediment deposits with present transportation rates as in- Previous investigations within Norwegian and Green- put for models on sediment budgets. landic fjords have resulted in increased understanding of fjord sediment fills (Syvitski & Shaw, 1995; Sejrup et al., 1996; Aarseth, 1997; Gilbert et al., 1998, 2002; Desloges Setting et al., 2002; Møller et al., 2006). However, sediment fills and the role of proglacial valleys in the glacier-to-fjord The Mittivakkat area, dominated by the warm-based Mit- system have attracted only minor attention (Vanderburgh tivakkat Glacier complex, is located on the west coast of & Roberts, 1996; Eilertsen, 2002). An ongoing Norwe- the Ammassalik Island, East Greenland (Figure 1). It is a gian project: Past and current valley-to-fjord sediment high-relief Alpine type area (Sugden, 1974) with moun- transport hosted by the Geological Survey of Norway, fo- tains peaking nearly 1000 m a.s.l. east of the Mittivakkat cuses on a comprehensive understanding of the sedimen- Glacier, where the highest mountain, Vegas Fjeld, culmi- tation in a glacially carved valley and its development nates 1096 m a.s.l. Westwards towards the Sermilik Fjord, from deglaciation to its modern stage targeting a transect mountains rise up to 300-400 m a.s.l. The whole area is from Jostedalsbreen to Nordfjorden, western Norway governed by fissure-valley topography with numerous (Lyså et al., 2006). narrow and deep valleys running predominantly NE-SW The present paper reports results from part of the Mit- and NNE-SSW. The valleys were glacially carved during tivakkat Valley project hosted by the Department of Ge- the Quaternary, but the initial tectonic control on the land- ography and Geology, University of Copenhagen, aiming scape is still apparent (Christiansen et al., 1999). During to establish qualitative and quantitative models for Arctic the last glacial maximum, 18 14C ka ago, the Mittivakkat valley-to-fjord sedimentary systems. This approach can area was located about 100 km within the south-eastern be used for calculating and estimating glacial erosion margin of the Greenland Ice Sheet (Funder & Hansen, rates and rates of denudation, transportation, and sedi- 1996). Deglaciation by calving on the shelf and in the ma- mentation in glaciated valley landsystems (Larsen & jor inlets, primarily caused by rising sea level, started at c. Mangerud, 1981; Svendsen et al., 1989; Hasholt, 2005). 13.6 ka BP. The deglaciation on land from about 11-10 ka The specific objectives are: (1) to describe the mor- BP was very rapid, and in the study area the present ice phology and sedimentology of the proglacial valley as a margin positions of the Greenland Ice Sheet were already part of the valley-to-fjord landscape, (2) to provide basic reached by 7-8 ka BP (Funder & Hansen, 1996; Chris- knowledge on erosion, transport, and sedimentary tiansen et al., 1999). The main evolution of the Mitti- 98 Geografisk Tidsskrift-Danish Journal of Geography 108(1) Figure 2: (A) The upper Mittivakkat Valley with the Mittivakkat Figure 2: (B) The present-day upper valley and the retreated gla- Glacier in the background seen in 1958 (Photo: H. Valeur). cier seen in 2006. (Photo: B. Hasholt). On both photographs the arrow indicates the position of a very characteristic bedrock knob known as Rødhætte. vakkat Valley, however, is probably the effects of pulsed stream flows in the proglacial Mittivakkat Valley stretch- erosion over multiple glacier expansion and retreating cy- ing 1.5 km ENE-WSW from the glacier terminus to a delta cles. This development ended up with glacial sculpturing terminating in the Sermilik Fjord. The proglacial valley is by a late Weichselian glacier which passed the narrow en- characterized by steep slopes flanking a 150-300 m wide trance of the valley and then spread out into a piedmont- flat-bottomed, sediment-floored trough (Figs 3A and 3B).
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