Background and aim

A number of natural processes interact to cause sea level changes. The sea level position is at every point of time and space determined by the volume and shape of the oceans, redistribution of water within the ocean basins and vertical movements of the coastal areas. These parameters are driven by internal Earth processes such as movement in the mantle and flexure or movement in the crust as well as by external processes like ice sheet growth and decay, sediment deposition in the oceans basins (infilling) and on a smaller scale also temperature changes of the ocean water. These various processes act on different temporal and spatial scales, thus creating a complex pattern of local sea level changes.

During the transition from the Last Glacial Maximum (LGM) to the Holocene period, large and rapid climatic changes caused most of the northern Hemisphere’s ice sheets to decay. Although the Greenland ice sheet experienced considerable reduction in its size during this global deglaciation, it is today the only remaining ice sheet in the northern Hemisphere.

The southern part of Greenland is a key area in understanding the interplay between the different Earth system components affecting climate. In this area, many of these Earth system components meet and interact with each other in a complex manner. Studying the area’s glacial history will give insights into palaeoclimatic and sea level changes and subsequent environmental responses. Knowledge in regard to these changes is highly relevant to predictions of future change in the global climate.

The changing climate caused complete or partial disappearance of the large continental ice sheets following the Last Glacial Maximum (LGM, ca. 22 ka BP). Significant vertical uplift of the formerly ice-covered areas occurred due to glacial-isostatic adjustment, which has resulted in relative sea-level changes. Sea-level  changes outside ice-covered areas are largely caused by the redistribution of water between continents and ocean basins as a consequence of glacial growth and decay. Observations of relative sea-level fall since the LGM until the present day from formerly glaciated regions help to reconstruct local ice-volume changes and their retreat/growth history.

A recent research project investigating sealevel changes and modelling glacial-isostatic adjustment showed that earlier ice sheet models underestimate the ice thickness and/or the extent of ice sheets in southern Greenland during the LGM (Bennike et al., 2002, Sparrenbom, 2006).

Several ice sheet scenarios were tested and the results suggest: that the margin of the Greenland ice sheet extended all the way out to the shelf edge during the LGM; that ice sheet recession began early (ca. 22 000 cal. years BP); and that the ice-margin recession proceeded quickly (Sparrenbom, 2006). By 12 000 cal. years BP, the ice margin was inland of the present-day outer coast and by 10 500 cal. years BP it had reached the present margin. The ice sheet was smaller than at present from 10 500 cal. years BP and reached a minimum at 30 km inland of the present-day margin at 9 000 cal. years BP. The neo-glacial re-advance started before 6 500 cal. years BP and the present-day margin was reached by 5 500 cal. year BP. The ice thickness in central southern Greenland during the LGM must have been of the order of 4 000–4 500 m (Sparrenbom, 2006).

Recent data collected in the area around Bredefjord (Sparrenbom et al., 2006a) could, however, imply that the ice sheet was thinner in this specific area and that the fjord acted as a major drainage path for the southern part of the Greenland Ice Sheet. Bredefjord is also located in southwest Greenland, and in some places reaches depths over 600 m. The Bredefjord region appears to be the key area for the drainage of the ice sheet in the south but only few data were available before the expedition carried out in the summer of 2007.

This project aims to reconstruct the relative sea-level history of the Bredefjord area (see map in figure 1). This will enable us to model the evolution of the southern part of the Greenland Ice Sheet and the ice sheet drainage mechanisms. To do this, we have cored and retrieved sediment sequences from eleven lakes and three marine basins.

Fieldwork

To obtain records of past relative sea level changes, we have cored sediments in small local basins, i.e. lakes, lagoons and bays with appropriate thresholds (see picture3 for a view of sites Ka3 and Ka4). During the course of their development, the studied basins should have been isolated/uplifted from the sea and/or inundated by seawater. Basins situated below the past marine limit have been isolated as a consequence of land uplift following deglaciation after LGM. Basins at low altitude have, in their fairly recent history, been submerged. This late relative sea level rise is a consequence of several processes: a mid-late Holocene Greenlandic glacial growth, decay of the American inland ice sheet and/or ocean warming (water expansion) and melting of non-polar ice. Different kinds of evidence for a late glacial and early Holocene regression and a late Holocene submergence have already been found at several sites from different parts of Greenland (Sparrenbom et al., 2006a and 2006b, Bennike et al., 2002, Long et al., 1999 and Kuijpers et al., 1999).

During three weeks in August we worked in the Tasiusaq–Kangerlua area in the inner part of Bredefjord. Our first “camp” was established in the youth hostel in Tasiusaq and our second camp at the farmhouse in Kangerlua. Using a Russian corer, multiple core sediment sequences were collected from a total of fourteen basins at altitudes between -5.5 m and 54 m. Two large Zodiacs and, on a number of occasions, a speedboat, were used for reconnaissance work in the area and for the transportation of coring equipment and people to the different coring sites (picture 1). The bathymetry of the basins was investigated from a Zodiac with an echo sounder in order to find suitable sites for coring and to estimate threshold depths for the submarine basins. Heights above sea level were measured with differential GPS. A small specially designed Zodiac, in which a funnel hole has been constructed through the bottom, was used as a coring platform. Transport on land was mainly by foot, car and tractor.

Preliminary results

Based on preliminary interpretations of the fourteen multiple core sediment sequences collected, ten or eleven sequences show isolation contacts with marine sediments in the bottom and with an upward transition into brackish and freshwater deposits. This is visible to the naked eye in terms of sediment colours, grain-size, and macrofossils. Generally, the colours shift from greenish grey in the marine segment, to more blackish dark-brown signifying brackish conditions, and to lighter brown colours in the fresh water depositional environment (picture 2). Both textures and macrofossils reveal the same story and in the marine sediments we find marine fossils, for example marine algae and molluscs, while the lake deposited sediments contain remains of freshwater algae and freshwater bryozoans. Our first results from the XRF core scanning and macrofossil analyses are shown from site Ta1 in figure 2. The 14C-analyses show that this site was isolated at ca. 8 900 cal. years BP, which is younger than earlier results from the area have suggested (cf. Sparrenbom et al., 2006).

In addition, two of the ten isolation sequences show at least one transgression close to the top of the sedimentary column. To establish the course of events properly, the sediments from the other sites are currently being analysed in detail in the laboratory and more macrofossils are being selected for 14C dating. Three of the sites will be presented as two Master theses (D. Fredh and L. Randsalu) in early 2008. The remaining analyses will be completed in the winter-spring of 2008 and will then be used for the ensuing geophysical modelling.