Anker Weidick

Present and past climate control on fjord glaciations in Greenland: Implications for IRD‐deposition in the sea

Calving of icebergs is the dominant ablation mechanism for large outlet glaciers from the Greenland ice sheet except in northernmost Greenland where bottom melting from floating glaciers dominates. This difference is controlled by present climate conditions. Glacial geological evidence indicates that the transition between the associated types of fjord-glaciations moved north-south in response to past climate change. In cold periods, local melt-out of debris from the bottom of an increasing number of floating glaciers reduces the potential for iceberg transport of IRD. Thus, the marine IRD signal of Greenland origin is not a simple cold climate signal. Our findings are discussed in the context of the ongoing debate about the kind of ice transporting IRD - icebergs or sea ice.

Sea ice and the stability of north and northeast Greenland floating glaciers

Abstract The interaction between sea ice and glaciers has been studied for the floating tongue of Nioghalvfjerdsfjorden glacier, northeast Greenland (79°30’N, 22° W). Information from glacial geological studies, expedition reports, aerial photographs and satellite imagery is used to document the glacier front position and fast-ice conditions on millennial to decadal time-scales. The studies indicate that the stability of the floating glacier margin is dependent on the presence of a protecting fast-ice cover in front of the glacier. In periods with a permanent fast-ice cover, no calving occurs, but after fast-ice break-up the glacier responds with a large calving activity, whereby several years of accumulated glacier-ice flux suddenly breaks away. Climate-induced changes of sea-ice conditions in the Arctic Ocean with seasonal break-up of the near-shore fast ice could lead to disintegration of the floating glaciers. The present dominant mass loss by bottom melting would then to a large extent be taken over by grounding-line calving of icebergs. The local influx of fresh water from the north Greenland glaciers to the sea would be reduced and the local iceberg production would increase.

The recession of the Inland Ice margin during the Holocene climatic optimum in the Jakobshavn Isfjord area of West Greenland

Recent subsurface mapping of parts of the Greenland Inland Ice margin in the region of Jakobshavn Isbrae indicates that the fjord system in the period of at least 2700–4700 calendar yr B.P. was more ice free than at present, and that the front of the glacier was at least 15 km behind the present position. The 14C-datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for Greenlands first settlers, the Sarqaq people, who arrived in the region about 4000 yr ago to find hunting grounds 10–20% larger than the present.

Holocene boreal molluscs in Greenland — palaeoceanographic implications

Abstract Boreal molluscs, now extinct in Greenland waters, occur as subfossils in West Greenland between lat. 65°30′ and 68°30′N. A number of new localities for these warmth demanding molluscs are described, and the list of species now comprises the gastropod Emarqinula fissura and the bivalvalves Heteranomia squamula, Modiolus, Arctica islandica, Panopea norvegica, and Zirphaea crispata. During their time the molluscs formed an isolated northern outpost, located in the area which — as a result of the complex oceanic circulation — today has Greenlands highest ocean surface temperatures. The occurrences are 14C dated to the interval 8400–4900 yr B.P., and summer surface temperatures were 1–3°C higher than now. For its beginning this period coincides with similar “marine optimal periods” in East Greenland and on the Baffin Island shelf, whereas it took 1000 years before the beginning of warmer than present temperatures on land. On the basis of this evidence it is concluded that the rise of sea surface temperatures was afforded by a decrease in inflow of cold polar water and/or increase in inflow of warm atlantic water into Davis Strait, rather than climatic change.

A catastrophic break-up of the front of Jakobshavn Isbræ, West Greenland, 2002/03

Jakobshavn Isbr (also called Ilulissat Isbr or Sermeq Kujalleq) is situated on the west coast of Greenland at 69‡100 N, 49‡450 W, and is known for its size, high rate of movement and large calving rate into the 50 km long Jakobshavn Isfjord (also called Ilulissat Isfjord or Kangia). This 1000m deep fjord is separated from Disko Bay and Davis Strait by a threshold at its mouth, the ‘‘Iceberg Bank’’ with depths of 200^300m, where the larger icebergs run aground. Changes of the glacier front are known from several descriptions since l850 (e.g. Weidick and others, 1990; Echelmeyer and others, 1991; Sohn and others, 1998). Between 1850 and about 1950 the front gradually retreated from a Neoglacial maximum position at 24 km from the mouth of the fjord to 50 km from this locality, and since 1950 it has maintained a rather stable positionwith annual fluctuations of the front, the winter position being about 2^4 km in front of the summer position. This half-century of stability now seems to be ending after a recent major disintegration event. In early spring 2003, the glacier front retreated approximately 11km, and it is now located close to the estimated grounding line. A series of satellite images covering the years 2001^03 were used to document the retreat of the glacier tongue. The results of this image analysis are shown as dates in Figure1.The images were retrieved from the Landsat 7 satellite and geo-referenced (projection: Universal Transverse Mercator; datum: World Geodetic System 1984 ellipsoidal elevation (WGS84); zone: 22 North). The pixel resolution of each image is relatively poor, with a pixel size of only 150m by 150m. Although the estimated error for each determination of the glacier terminus position is therefore 220m, this is adequate for our purposes (i.e. the initiation of the recent recession). The difference between the winter and summer positions of the glacier front for 2001/02 was determined to 4 km (Fig. 1). This fluctuation between summer retreat and winter advance of the glacier front is somewhat larger than observations during the past half-century have indicated (e.g. Sohn and others,1998). In Figure 1 the advanced winter position for 4 May 2001 is taken as a reference line; the winter position for 4 March 2002 is almost identical. However, the winter position for 2003 (23 and 30 March) was not reached and is located approximately 2 km behind the normal summer position; the floating glacier front is clearly disintegrating. The break-up of the front continued during spring and early summer 2003, and on l7 May 2003 the front was 11km behind the reference position, close to the grounding zone.The retreat of the glacier front is significant and it seems unlikely that the previous winter positions of the Jakobshavn Isbr front will be reached during winter 2003/04. To the above can be added some observations made during a visit to the glacier and the ‘‘Iceberg Bank’’ on 2 and 5 August 2003. On the ‘‘Iceberg Bank’’ an unusually large number of detached parts of the glacier front were observed, which are easily distinguishable by their pinnacled, dark surface comparable to the glacier front surface. In August 2003 the fjord was densely packed with ice, and the retracted position of the glacier front to a position near the grounding zone was evident.The glacier front itself was not clearly defined, but appears as a gradation between the ice stream proper and densely packed fragments of the front in the fjord. It appears that the recession shown by the Landsat scene of 17 May 2003 has continued. In August 2003 the northern part of the glacier front was located at the easternmost tip of the wedge of land known as Nunatarsuaq, which suggests a further recession of 1^2 km since May 2003. A recession of the samemagnitude seems to have occurred at the southern part of the front (cf. Fig. 2) in this period. The glacier front is presently poorly defined, a marked change from the situation during the stable period 1950^ 2000 when the glacier front was usually clearly discernible from the fjord and its icebergs, but comparable to descriptions from the recessional period 1850^1950, when there were difficulties in pinpointing the exact position of the front (cf. Engell,1904). The recession of the front has detached a large ice-filled Journal of Glaciology,Vol.50, No. 168, 2004

Johan Dahl Land, south Greenland: the end of a 20th century glacier expansion

The Qajuuttap Sermia glacier system north of Johan Dahl Land, on the southern slope of the Greenland ice sheet (the inland ice), has shown continuous advancing behaviour from approximately 1940 until approximately 2000. This contrasts with neighbouring sectors of the inland ice to the west and southeast, where the ice sheet has shown a more ‘normal’ trend of recession and thinning of the margin since about 1850-1890, and that has continued throughout the last half of the 20th century. The Qajuuttap Sermia glacier system has also attracted interest for its hydropower potential, and detailed investigations were carried out in 1977-1983. This article summarises the fluctuations of the Qajuuttap Sermia glacier system, and documents the demonstrable advance of the individual glaciers of the system from the early 1940s until the early years of the present millennium. The cause of the advance seems likely to be related to variations in precipitation over the southernmost parts of the inland ice, with the highlands north of Johan Dahl Land directly influencing the passage of low pressure systems moving both northwards along the west coast and northeastwards along the east coast of Greenland.