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Landscape and sediment processes in a proglacial , the Mittivakkat 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 , 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 near the valley mouth. The first recordings of the through the Mittivakkat Valley to the Sermilik Fjord at the mouth of 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 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-, 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 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 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 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 (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). shape, with the ice margin approximately following the The landforms and sediment process studies are all lo- outer limit of a present coastal delta as a consequence of cated in this proglacial valley between the Mittivakkat the deep water fronting it in the Sermilik Fjord (Nielsen, Glacier tongue and the Sermilik Fjord. 1994; Humlum, pers. com.). Evidence of a warm period around 600 AD found on a The Mittivakkat Glacier tongue covering18 km2 is the present nunatak located about 6 km upglacier from glacier largest single glacier in the complex transection type ice terminus indicate that the glaciated area here was less than and snow field of 31 km2. The glacier proper, which re- today (Hasholt, 2000), but so far this has not been con- sults from a merging of several glaciers, is a valley- firmed by observations at other localities around the pres- type glacier, approximately 5 km long and 2 km wide. ent ice margin. No large-scale Holocene neoglaciation is From its eastern border laying at 650 to 700 m a.s.l., the known in the study area, but during the Little Ice Age glacier flows westwards through a valley and terminates (LIA) cooling, which already started in Greenland by along a 2-km long ice front undulating north-south at about 1100 AD and lasted until about 1900 AD (Dans- about 200 m a.s.l., except in the extreme south where a gaard et al., 1975; Lamb, 1995), the Mittikvakkat Glacier narrow, steep reaches down to about 120 m advanced to the valley mouth, where terminal moraines a.s.l. (Figs 2A and 2B). At present the ELA is located exist (Christiansen et al., 1999). around 500 m a.s.l. and the maximum annual ice flow ve- Geologically, the area belongs to the Nagssugtoqidan locities are about 25 m. Radio-echo sounding indicates a Belt and the major bedrock is the Ammassalik Intrusive marked relief underneath the glacier with two major de- Complex with predominantly garnet granite gneiss and pressions that apparently have a strong influence on the some basalt inclusions (Friend & Nutman, 1989; Hansen subglacial drainage (Knudsen & Hasholt, 1999). Accord- & Kalsbeek, 1989). ingly, the Mittivakkat glacier is drained through two out- The present climate in the area is affected by the prox- lets located respectively in the northern and southern sec- imity of the Greenland Ice Sheet and the East Greenland tion of the western glacier terminus. The northern outlet Polar Sea Current which results in generally stable cold entering an ice-lake drains the northern part of the glacier weather. The mean annual air temperature is -1.7ºC indi- tongue, while the southern part of the glacier is drained by cating that the climate is low arctic and that the area is in a southern outlet from where water cascades over a cone the zone of discontinuous permafrost. The monthly mean of rocky debris and feeds the Mittivakkat stream. This temperature ranges from a minimum of -8.1ºC in Febru-

Geografisk Tidsskrift-Danish Journal of Geography 108(1) 99 Figure 3: (A) The proglacial Mittivakkat Valley with the braided Figure 3: (B) The present-day lower valley seen in 2007 from the stream flowing into the Sermilik Fjord seen from the Mittivakkat automatic camera station (Photo: B. Hasholt). On both photo- Glacier in 1958 (Photo: H. Valeur). graphs the arrow indicates the position of Rødhætte.

ary to a maximum of 6.4ºC in July. Due to the rather fre- Methodology quent passing of lows, the mean annual precipitation is relatively high and amounts to 984 mm water equivalent. Geomorphological mapping is used mainly to distinguish Data are from the Danish Meteorological Institute (DMI) the glacial and fluvial deposits in the proglacial valley. As station at the town Ammassalik, 15 km east of the Mitti- a basis for the geomorphological mapping a topographic vakkat area. Measurements of runoff and precipitation map, at a scale of 1:5000 with contours at 5-m intervals, within the study area, however, indicate higher annual and stereo-pairs of aerial photographs taken in August precipitation of the order 2000 mm (Hasholt, 1980). 1972 and 1981 were used. The photographs were carefully Mean monthly precipitation typically range between 47 studied by using a Zeiss Jena Interpretoscope and a geo- and 120 mm, although there is substantial inter-annual morphological map, at a scale of 1:5000, was then pro- variability (e.g. monthly precipitation for October ranges duced. This map was subsequently checked and com- between 35 and 470 mm). The maximum monthly pre- pleted in the field. cipitation occurs in October or November and the mini- Automatic digital photography was used to observe mum in June or July. Thus, the annual precipitation falls the fluvial conditions in particular during the snowmelt mainly as snow. The area is characterized by a glacio-ni- period and summer times (Christiansen, 2001). A weath- val hydrological regime; snow melting normally starts in erproofed box into which an automatic camera was built May and reaches a maximum in June or July, although was fixed on the rock surface 185 m above the eastern end rainstorms can cause short-lived peaks, while freezing up of the proglacial valley (Figure 3B). The photographed starts in October. The runoff from October until the end of area covers most of the proglacial valley. The camera was April is normally very low, reflecting the cold tempera- set up on 15th August 2001, and the camera watch was ture and the small amount of basal runoff from the glacier, programmed to take one photograph daily at noon. and can cause icing to develop. During winter, strong Geological sections, mainly along the steep-walled Foehn winds from NNW and NW (known by the Green- meltwater channels cut in the valley bottom, have been in- landic word piteraq; defined as winds above 100 km h-1) vestigated and sedimentary logs were recorded. The ap- can cause considerable snowdrift and generate runoff proaches presented by Miall (1977) and Krüger & Kjær from the snow surface. (1999) for detailed field description of fluvial deposits and glacial diamicts are adopted in this study to describe sur- face and near-surface sediments in the proglacial valley bottom. Clast fabric data were acquired from a small sub- horizontal area (25 x 25 cm) over a 10 cm vertical distance and using clasts only with an a:b axis ratio >1.5 and an a-

100 Geografisk Tidsskrift-Danish Journal of Geography 108(1) axis length between 0.6 and 6 cm (Kjær & Krüger, 1998). ley-Smith type sampler. The competence of the stream has Each fabric consists of 25 observations evaluated through been documented using tracer pebbles with built-in radio the three-dimensional eigenvector analysis (Mark, 1973; transmitters (Busskamp & Hasholt, 1996). Woodcook, 1977) using the program Spheristat for actual calculations. Clast fabric interpretation is validated and supported by contoured diagrams after Kamb (1959) using Morphology, sedimentology and recent processes two and four times the standard deviation to identify non- random distribution. Clast-morphological properties such Using the terminology proposed by Linton (1963), the as shape and roundness are recorded for samples of mini- Mittivakkat Valley is classified as an alpine- type valley, mum 50 clasts (Powers, 1953; Benn & Ballantyne, 1993). defined as a valley cut by a valley glacier emanating from The classification of glacial deposits largely follows that high ground. The glacially carved valley system is about summarized by Dreimanis (1988) and co-workers. 7 km long and shows a fall along the thalweg line of about To establish the volume of loose sediments deposited 500 m from the drainage divide to the fjord (Knudsen & in the valley and the coastal zone, the bedrock boundary Hasholt, 1999). Its present proglacial part, which is about below the sediment has been identified. The method em- 1.5 km long and has an open trough head, can roughly be ployed for measuring sediment thicknesses was geo-elec- divided into four sections (Figure 4): The upper first part tric sounding, using a Schlumberger set up. Because of lo- of the valley is narrow and V-shaped and cut the trough gistics, ground-penetrating radar was not available. The head, which is a 150 m high valley step representing an equipment used was an ABEM terrameter SAS 300C with important process threshold in the valley landscape. Melt- a coupled ABEM terrameter SAS2000 booster. A number water cascading from the glacier margin about 120 m a.s.l. of sounding lines were established where sufficient length cuts a series of small courses and continues could be obtained depending on topography and presence down a steep alluvial cone of cobbles, stones and boulders of water bodies. The soundings were carried out with a 1- onto the valley floor proper about 7 m a.s.l. Blocks that are m distance between the electrodes, supplied with 5-m and transported by the glacier and further by meltwater are de- in a single case 10-m distance between the electrodes. The posited in this cascade area. On the lower part of the cone, current was varied from 0.5 - 20 mA depending on the the violent stream runs between bars in a random pattern conditions. The location of the centres of the soundings and feeds the Mittivakkat stream. was determined using GPS. The interpretation of the The second valley section, which occupies the larger soundings is based on a modelling programme developed part of the valley train, has an asymmetric cross-profile at the Geophysical Department, University of Aarhus. The with a steep north-facing valley side and a more gently accuracy of the determination is by experience estimated sloping south-facing valley side (Figure 3B). The upper to 5 %, because of the lack of borehole observations. slopes have been modified by subaerial weathering and Water discharge has been measured with Ott current denudation processes, whereas the lower parts of the meters in a fixed cross section established 300 m behind glacially carved slopes are covered by scree deposits. This the outermost at a site free from tidal in- valley section, flat-bottomed and 200-400 m wide, is fluence. A stage/discharge relationship was established to mainly floored by fluvial sediments (Figure 4). Here, the provide a continuous record of discharge. Campbell SR50 Mittivakkat stream is a typical valley sandur stream with ultrasonic sensors connected to a CR10X data logger were one or two main water-channels, which branches out in a used. Water samples were taken manually with a depth-in- braided system bound by the shape of the tegrating water sampler (Nilsson, 1969). Sampling fre- valley bottom (Krigström 1962). It is normally braided quency has been daily when the station was manned. In into 2 to 5 branches with numerous intervening stream some periods when the station has been unmanned, water channel bars classified as longitudinal bars with point bars samples were collected using an ISCO automatic peri- in terms of Boothroyd & Ashley (1975) (Figs 3A and 3B). staltic pump water sampler. Partech transmissometers and Sediments on top of the bars vary from gravel to silt with OBS turbidity sensors also have been used to provide ad- ripples developed in fine sand and silt. In parts of the val- ditional information on the temporal variability of sus- ley the braiding is constrained by small remnants of pended sediment concentrations (Hasholt, 1993). Infor- moraines, or groups of boulders, or it is obstructed by mation on bedload transport has been obtained by meas- talus slopes of rock debris felt down especially from the uring the travel velocity of bed forms and by using a Hel- steep north-facing valley side which is most prone to frost

Geografisk Tidsskrift-Danish Journal of Geography 108(1) 101 N

Figure 4: Geomorphological map of the Mittivakkat Valley. Note the locations of study sites.

action. Talus is located in a narrow belt along the base of tive in mass-movement. Also along the north side of the this valley slope, but it has been observed that falling valley train, bank erosion takes place when braiding forces blocks may roll up to 20 m from the talus margin and into the stream channels against debris-flows or minor alluvial the stream area. cones fed by small adjoining tributaries (Figure 4). Although this part of the valley is presently a net sed- At places patches of ice-smoothed rock terrain fre- imentation area, recent bank-erosion is taking place at quently with marked roches moutonnées features with certain times of the year mainly in the bends around rem- abraded up-ice faces and quarried down-ice lee faces are nants of moraine or larger bars. During the spring break seen along the north side of the valley train (Figure 4). The up, the melt water in the beginning runs on top of flood roches moutonnée terrain is thinly draped by till. This till ice or the snowcover capping the valley bottom, but at is interpreted as being deposited in a subglacial environ- later stages the water sinks into tunnels and channels de- ment. Particularly diagnostic in substantiating this inter- veloped in the ice and snow. Such channels may reach pretation is the occurrence of small flutes and many bul- into sediment layers below and erode into them, creating let-nosed stones and boulders embedded in the till surface discontinuous stream channels. A striking example was as components of the till on topographic highs. These seen when a snow bridge blocked the main outlet; the wa- clasts showing a significant modification of their upper ter was forced to run across an otherwise abandoned part surfaces constitute small “roches moutonnées” morpholo- of the outwash rejuvenating an old stream channel. Pot- gies consistently parallel with the on ad- holes are also developed on the valley bottom during the jacent exposed bedrock surfaces (Krüger, 1984, 1994; spring break up, probably because ice in some of the Krüger & Kjær, 1999). In this subglacially formed terrain, stream channels has been covered with sediment during groups of bouldery material appear in topographic lows warm spells. The potholes are formed when buried ice but due to gravity sorting after melting of buried dead-ice. melts away. At present, there are no easily-discernible terminal The south-facing valley slope is less steep than the moraine ridges, or well-defined terrace levels in this part north-facing and has more loose sediments capping the of the valley. However, on a photograph taken in 1958 bedrock. In springtime minor debris flows have been ob- showing a section of the flat-bottomed valley floor almost served and in some places nivation processes are also ac- halfway between the 1933 and 1958 glacier-front position

102 Geografisk Tidsskrift-Danish Journal of Geography 108(1) a condensed stratigraphical column shown in Figure 6A. The lowest unit, which was exposed irregularly at the base of the sections, is a poorly sorted clast-supported, stony gravel facies with many clasts 20-30 cm in size. 75% of the clast content is subangular or subrounded. The share of well-rounded and rounded clasts amounts 16%. The RA value is 9% indicating that the content of angular and very angular clasts is very low (Benn & Ballantyne, 1993). The C40 value is 15%, indicating a low content of slabby and elongated shapes (Benn & Ballantyne, 1993). Upwards, follows a clast-paved, grey, massive, sandy-silty, matrix- supported, friable to firm diamict, 0.5-0.6 m thick, with a moderate content of clasts, outsized clasts of 15-20 cm in Figure 5: Photograph taken in 1958 showing remnants of a well- diameter occur. 62-69 % of the clast content is subangular defined end-moraine ridge located almost halfway between the or subrounded, while the share of well-rounded and 1933 and 1958 ice-front position (Photo: H. Valeur). rounded makes up 18%. The RA value is 13-20% and the C40 value is 10-11%. Two fabrics display very strong pre- it is possible to identify remnants of a well-defined end ferred orientation with low isotropy and very high degree moraine ridge surrounded by the braided stream (Figure of clustering (S1 values amount 0.79 and 0.87) and with 5). Now, meltwater erosion has reduced this moraine rem- the principal vectors in an E to ENE direction (73 and 90º). nant further. The mean fabric orientation (81º) lies very close to the val- The third valley section extends down-valley from the ley axis. Both fabrics show principal vectors with a shal- braided reach dominated by fluvial deposits and includes low upglacier plunge (3-10º). the terminal moraine, 2-4 m high, indicating the maxi- The lowest unit is interpreted as outwash representing mum extend of the glacier during the Little Ice Age (Fig- gravely bar deposits from the upper reach of an outwash ure 4). The outer frontal moraine is found 1.5 km from the fan deposited by high-energy streams from underneath the present ice margin. In this part of the valley, the Mitti- glacier (Boothroyd & Ashley, 1975; Miall, 1983). The di- vakkat stream is erosive and divides around remnants of amict unit overlying the is inter- ground moraine, small end moraines, 1-2 m high, a minor, preted as being deposited in a subglacial environment. winding , 1-2 m high, or outwash with abandoned Particular diagnostic in substantiating this interpretation meltwater channels. A great number of big boulders is is the relative high degree of compactness of the diamict, scattered all over the terrain most probably dropped by the the spatially consistent orientation of clast fabric being glacier when its terminus was located in this outer part of parallel with ice-flow direction and the occurrence of a the valley. Vegetation is well established in interstream ar- clast pavement on top of the diamict unit (Krüger & Kjær, eas. About 50 m upstream the gaging station, located 300 1999). The similarity in clast roundness and shape be- m behind the outer LIA moraine (Figure 4), the braided tween the diamict and the underlying outwash deposits Mittivakkat stream merges to a single, slightly winding suggests that the advancing glacier has picked up material river, 30 m wide. Downstream the gaging station, the from the substratum of outwash material and integrated it stream divides around an island which is part of an old ter- in the till deposit. minal moraine. Here, the rapids determine the upper limit In addition, a stratigraphical column representing a of tidal influence. After the rapids, the stream is 15-20 m stream-cut section located 100-200 m before the Mitti- wide and is confined between 3-4 m high steep banks cut vakkat stream passes through the outer LIA terminal into ground moraine, older outwash, or through the outer moraine is shown in Figure 6B. The lowest unit, more LIA terminal-moraine before entering the tidal delta flat. than 1.6 m thick, which was exposed at the base of the In this part of the Mittivakkat valley a few terrace levels steep section is a grey, massive, sandy-silty, matrix-sup- can be distinguished. ported, firm diamict with a moderate content of clasts. The sediment characteristics from two selected stream- Upwards, follows laminated silt, 25 cm thick, with a gra- cut sections directed transverse to the valley trend and lo- ditional contact to ripple cross laminated fine sand, 60 cm cated next to the gaging station (Figure 4) are presented in thick. The section is capped by a poorly sorted clast-sup-

Geografisk Tidsskrift-Danish Journal of Geography 108(1) 103 Figure 6: Data chart comprising a detailed description of two sedimentary successions (A and B) recognized in the Mittivakkat Valley. Partly after data chart developed by Krüger & Kjær (1999). Lithofacies code for sorted sediments after Miall (1977) and Eyles et al. (1983). ported, stony gravel facies, 0.5-0.6 m thick. No detailed through-fillings. The overlying glaciofluvial gravel de- investigations of this section have been done due to its posits are interpreted as outwash representing gravely bar very close proximity to the violent meltwater stream. deposits. The diamicts unit is very similar to that shown in Fig- The outermost LIA terminal moraine is the lowest ure 6A, and it is therefore interpreted as being deposited trace of moraine to be found in the proglacial valley. It in a subglacial environment. The laminated silt and fine curves slightly downvalley and consists of 3-4 well-de- sand units that cap the diamicts are interpreted as shallow fined parallel ridges strewn with boulders and with rela-

104 Geografisk Tidsskrift-Danish Journal of Geography 108(1) tively even crests. The seaward side is much steeper than The complex geology could result from the presence of a the up-valley slope. At three points the terminal moraine partly buried terminal moraine in this area. Sounding 14 is has been breached by meltwater streams, so the moraine located on the barrier offshore the tidal delta area. Direc- stands above the valley floor like slightly curved islands. tion of cable is 29º, L/2 is 125.89 and a = 1 and 5m. The The central and northern gap, however, is now dry, while sounding curve can be interpreted with a 2-layer model as the southern one is the scene of the most powerful section rather homogeneous sand probably saturated with saltwa- of the lower Mittivakkat stream which rushes through the ter. Depth to solid bed rock is 20 m, which is consistent gap. Wave-sapping, which takes place in autumn when the with other soundings from the delta area showing depth to fiord is depleted of drifting icebergs and before new sea- bed rock varying between 20 and 23 m. ice is formed, and river action have been responsible for a well-exposed section in the southern end of the central part of the LIA terminal moraine. The section suggests Sediment budget that the ridge system is formed by pushing of a pre-exist- ing basal till along the margin of the advancing glacier. The main contribution of material from basal glacial ero- Behind the outer moraine ridge system a number of small sion is 8000-18000 t of sediment per year ranging from moraine ridges which are partly terminal and partly lateral gravel to fine silt. Based on measured suspended load con- occur. They are recessional moraines formed by pushing centration it can be estimated that about one third of the and scraping of proglacial material and dumping of mate- eroded material is deposited in the valley and the rest goes rial from the steep ice front. to the sea (Hasholt & Mernild, 2006). However, on the The fourth valley section (Figure 4) extends some few tidal delta coastward transport expressed as the formation tens of meters to tidewater across outwash from the LIA of bars can take place (Nielsen, 1994). A sediment budget terminal moraine and ends in intertidal mudflats on the covering the transport/deposition system outside the gla- delta which probably rests on the drowned part of the out- cier can be divided into the valley proper and the coastal wash plain. On the basis of grain size, the outwash is iden- delta area. For each of them sediment fluxes and sediment tified as formed in a proximal proglacial environment storage can be calculated. (Boothroyd & Ashley, 1975; Krüger, 1994). The geomor- The solid bedrock is chosen as a base for storage. The phology and dynamics of the delta have been described by volume of the present glacier is estimated to equal the vol- Nielsen (1994). The temporary strong winds has relocated ume of maximum glacial erosion from the glacier area. some silt and sand and created small dunes and sand heaps The maximum erosion in the proglacial valley is deter- at the coast. mined as the volume of the valley from the top of the pres- ent valley sides and down to the level of bedrock deter- mined by the geoelectric soundings. Storage of sediment Sediment thicknesses is calculated as the present volume of sediment occupying the proglacial valley. This volume of sediment can be di- Three examples of a sounding are given to represent the vided into storage of older outwash deposits and recent thickness and the different types of deposits. Sounding 13 fluvial deposits. For comparison with measured transport is located on the recent floodplain covered with fine sand values, the valley erosion volumes in Table 1 have been and silt about 500 m upvalley from the LIA terminal multiplied with a density of 2.65 t m-3 for solid rock and a moraine. Direction of cable 84º, L/2 is 100.00 m a = 1 and dry bulk density of 1.6 t m-3 for deposited loose sedi- 5 m. The sounding curve is interpreted with a 3-layer ments (Hasholt, 2005). model. The upper silt rich layer is app. 2 m thick repre- The annual transport into the valley 5125 m3 is based senting recent fluvial deposits, below is a layer that can be on the range of measured annual load close to the present interpreted as comprising coarse grain material which outlet from the glacier. Estimates of net transfer through could be older outwash deposits. Solid bed rock is found the valley and resulting deposition are based on the differ- 18 m below the terrain surface, i.a. 15-16 m below present ence in sediment concentration measurements at the gla- sealevel. Sounding 12 was carried out on top of moraine cier outlet and at the ISCO island just before the stream is terrain 350 m upvalley from the LIA terminal moraine. flowing into the delta area. This sounding indicates a complex geological layering The erosion of the delta area cannot be determined, but and could not produce an accurate depth to the bed rock. the net deposition 7.7x 106 m3 (table 1) can be calculated

Geografisk Tidsskrift-Danish Journal of Geography 108(1) 105 Table 1: Sediment budget for the glacier-to-fjord system. Numbers to the right of the semicolon in the last column refer to values from the part of the glacier that drains to Lake Kuttuaq north of the main valley described here.

as sediment deposited above the solid bedrock in the pres- the narrow entrance of the valley and then spread out into ent delta area. Annual sediment input to the delta 5000 m3 a piedmont-shape, with the ice margin approximately fol- is therefore identical with the load out from the valley. lowing the outer limit of the present coastal delta as a con- The annual net output to the sea 2200-4500 m3 is calcu- sequence of the deep water fronting it in the Sermilik lated as the transport of wash load, defined as concentra- Fjord. A comparison between the total eroded volume of tion of suspended sediment in samples collected at the the valley system 2084x106 and the present annual glacial surface of the river. All the components of the sediment erosion ca. 7000 m3 clearly indicates that it must have budget are shown in Table 1. taken several 100 kyr cycles to create the valley. However, the erosion rate and pattern must have changed along with variations in sliding velocity and thickness of the down- Landscape development and discussion cutting glacier. In addition, in glacially carved troughs, overdeepened basins commonly form behind narrowings The main development of the Mittivakkat Valley system in the valley profile. This appears from the trough long is most probably the effects of pulsed erosion over multi- profile indicating at least one large-scale transverse rock ple glacier expansion and retreating cycles during the bar, namely the open trough head, separating two rock Pleistocene period. This evolution ended up with glacial basins (Figure 2B); a small one located below the termi- sculpturing by a late Weichselian glacier which passed nal part of the glacier behind the trough head and a large

106 Geografisk Tidsskrift-Danish Journal of Geography 108(1) one representing the main part of the proglacial valley and meltwater drainage routeways through the terminal terminating at a threshold of mostly buried solid rock at moraine were abandoned, and a simple, deep course was the narrow entrance of the valley (Figure 3B). The occur- cut into the outwash south of the terminal moraine. Dur- rence of a LIA ground moraine with a stratigraphy com- ing the onset of spring melts, however, when the deep bined of basal till resting on outwash deposits just behind meltwater channel is filled with snow and ice, the older the outermost terminal moraine suggests that the lowest routeways are rejuvenated. level of erosion in solid rock was already reached during In 1958, the glacier front had retreated to a position the late Weichselian glaciation, when the glacier advanced about 50 m up-valley from the bedrock knob unofficially to the Sermilik Fjord and eroded down to the present level named Rødhætte, but remnants of a terminal moraine was of solid rock below the modern delta flat. Along with the still visible 70-80 m down-valley from Rødhætte indicat- following glacier retreat in late Weichsel, the exposed val- ing a former re-advance phase, which has interrupted the ley must have acted as a sink where sedimentation of large general glacier retreat. Today this terminal moraine has quantities of glacio-fluvial sediments took place both in been changed significantly by fluvial erosion and deposi- the valley and on the delta platform. tion of sediments on the surrounding . After this event, the local finding of pieces of wood In 1970, the glacier front had retreated further back to 620 m a.s.l. indicates a relatively warmer period AD 600, the termination of the flat-bottomed valley train about 10 which was identical with the Viking Age warm period. m a.s.l. At this stage the valley still acted as a sink storing Whether the Mittivakkat Glacier complex disappeared up a significant part of the sediment released from the gla- completely during this period or small parts were left in cier by the melt water. Over the following 30 years, the high-lying botn features is unknown. glacier snout has retreated upslope through the steep and During the Little Ice Age, the glacier advanced again, narrow section of the valley, which is identical with the and eroded in the valley bottom fill of glacial and glacio- head of the proglacial Mittivakkat Valley. During this fluvial deposits, especially in the central part of the pres- stage of valley development, sediments were continu- ent-day proglacial valley basin. Around AD 1900 the gla- ously deposited in the flat-bottomed valley section. This is cier advance reached its maximum and produced the ter- confirmed partly by the Cs137 analyses, partly by the minal moraine-ridge system seen at the mouth of the val- monitoring of recent sediment transport respectively at ley. It is not possible to say if the location of the terminal the proximal and at the distal ends of the braided reach moraine is related to the existing transverse rock bar. The (Hasholt et al. 1992 and 2000; Busskamp & Hasholt, outwash plain beyond and the delta were fed by courses of 1996). The level in the braided river system in the present meltwater incised into the terminal moraine ridge and as a proglacial valley is a result of a net deposition during the consequence, the delta transgressed with a steep foreslope glacier retreat since 1933. Most recently, however, a re- further into the fiord (Nielsen, 1994). duction in the sediment transport to the delta has resulted During the following warm period up through the 20th in a degradation of the delta. The glacier front has re- century, the glacier retreated quite fast. The glacier left the treated to a position just behind the upper transverse rock terminal moraine and different patterns of smooth ground bar and an ice-dammed lake has formed. It is expected moraine and hummocky dead-ice moraine were exposed. that this lake will act as a sink which in turn will reduce In 1933, when the first observations of the glacier were further the amount of sediment delivered to the valley/ made, the ice front was located around 300 m up-valley delta system. from the LIA terminal moraines. The absence of ground The predicted future warming will most probably re- moraine remnants along the present valley sides suggests sult in a further decrease of the glacier, and new potential that the ground moraine has inclined very gently up-val- sink. Mapping of the glacier bed topography has shown, ley from the terminal moraine reflecting the shallow basin that a glacier retreat will expose new larger potential sink topography produced by glacial erosion. Along with the areas, which in turn will decrease the amount of sediment ice-front recession, meltwater has transformed the ex- reaching the proglacial valley and delta system. This may posed moraine terrain in the low-lying part of the valley to lead to increased water erosion near the valley mouth. At a valley sandur with a braided stream pattern; today present, bank erosion is already taking place in the area ground moraine is only seen in the most high-lying parts around ISCO Island west of the discharge gauging station of the valley floor immediately behind the terminal and in the meltwater channel near its mouth at the tidal moraine. During this period, the northern and central area. In addition, if the threshold formed by the old LIA

Geografisk Tidsskrift-Danish Journal of Geography 108(1) 107 terminal moraine disintegrates, it may lead to increased Acknowledgements backward erosion and creation of terraces along the val- ley train until a new stability is reached. The delta will The project was sponsored by the National Science Re- probably degrade further as indicated by the recent search Council and the Commission for Scientific Inves- coastal erosion which has removed the most southern part tigations in Greenland (KVUG) of the LIA terminal moraine. This process can be further enhanced if the climate change causes a sea level rise and decreasing ice cover in the fiord as well as a change in the References wind direction towards a more south westerly wind di- rection as indicated by observations. Aarseth, I. (1997): Western Norwegian fjord sediments: age, volume, stratigraphy, and role as temporary de- pository during glacial cycles. Marine Geology 143: Conclusions 39-53. Benn, D. & Ballantyne, C. K. (1993): The description and The work reported here from the proglacial valley of the representation of particle shape. Earth Surface Mittivakkat Glacier, permits the following conclusions: Processes and Landforms 18: 665-672. (1) The Mittivakkat Valley is basically an alpine-type val- Boothroyd, J. C. & Ashley, G. M. (1975): Process, bar ley, defined as a valley cut by a valley glacier emanating morphology and sedimentary structures on braided from high ground. (2) The main development of the Mit- outwash fans, northeastern Gulf of Alaska. Pp. 193- tivakkat Valley system is most probably the effects of 222 in: Jopling, A.V. & McDonald, B. C. (eds.): pulsed erosion over multiple glacier expansion and re- Glaciofluvial and glaciolacustrine sedimentation. So- treating cycles during the Pleistocene period. (3) Using a ciety of Economic Paleontologists and Mineralogists, conservative estimate of the total eroded valley volume Special Publication 23. and dividing it with the present measured annual sediment Busskamp, R. & Hasholt, B. (1996): Coarse bed load transport indicate that it must have taken several 100 k transport in a glacial valley, Sermilik, South East years to excavate the valley system. (4) The larger part of Greenland. Zeitschrift für Geomorphologie NF 40: the valley is floored by fluvial sediments and shows a typ- 349-358. ical valley sandur stream, the Mittivakkat stream, with Christiansen, H. H. (2001): Snow-cover depth, distribu- one or two main water-channels, which branches out in a tion and duration data from northeast Greenland ob- braided stream system with numerous intervening chan- tained by continuous automatic digital photography. nel bars. At present this part of the valley acts as a sink for Annals of 32: 102-108. the sediment released by glacial erosion. (5) In the outer Christiansen, H. H., Murray, A. S., Mejdahl, V. & Hum- part of the valley train, the Mittivakkat stream is erosive lum, O. (1999): Luminescence dating of Holocene ge- and divides around remnants of ground moraine, end- omorphic activity on Ammassalik Island, SE Green- moraines, and outwash with abandoned meltwater chan- land. Quaternary Geochronology 18: 191-205. nels. In this area some bank erosion is seen. During the Dansgaard, W., Johnsen, S. J., Reeh, N., Gundeestrup, N., melt period the blocking of the main stream bed with Clausen, H. B. & Hammer, C. U. (1975): Climate snow bridges may result in partly rejuvenation of older changes, Norsemen and modern man. Nature 255: 24- stream furrows. (6) The diminished input of sediment 28. from the valley to the delta area has caused a degredation Desloges, J. R., Gilbert, R., Nielsen, N., Christiansen, C., of the delta. (7) If the glacier terminus continues retreat- Rasch, M. & Øhlenschlæger, R. (2002): Glacimarine ing in the future, new sink areas may be created, as al- sedimentary environments in fiords of the Disko Bugt ready indicated by a new ice lake developed within the region, West Greenland. Quaternary Science Reviews last three years. (8) This will in turn further diminish the 21: 947-963. input of sediment fra the glacier to the valley and the delta Dreimanis, A. (1988): Tills: Their genetic terminology and lead to erosion of the treshold east of the LIA and classification. Pp. 147-169 in Goldthwait, R. P. moraines so that deep channels will be cut into the braided & Matsch, C. L. (eds.): Genetic Classification of part of the valley. Glacigenic Deposits. A. A. Balkema, Rotterdam. Eilertsen, R. (2002): Sedimentology and geophysical in-

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