Scientific background

Along the presently ice-free Northeast Greenland continental margin, upland plateaus and coastal lowlands are often strongly weathered, in contrast to fjords and valleys, which are characterised by fresh-looking surfaces of glacial erosion. Based on the degree of weathering and on the lack of identified deposits from the last glaciation, it has been suggested that the areas between fjords were ice-free during the Last Glacial Maximum (LGM), ca. 20,000 years ago (Funder and Hjort, 1973, Hjort, 1981). During the past decades considerable effort has been put into both onshore and offshore investigations of the Late Quaternary glacial history of Northeast Greenland. One large step was taken through the ESF programme “Polar North Atlantic Margins” (PONAM), with most of the investigations focusing on Jameson Land (figure 1c). This work led to the reconstruction of outlet glaciers restricted to the fjords during the LGM, leaving interfjord areas free from ice (e.g. Möller et al. 1994, Funder et al. 1998). However, the concept of a restricted LGM glaciation in Northeast Greenland has recently been challenged by marine geological st6udies, suggesting grounded ice on the shelf at this time (Evans et al. 2002, Ó Cofaigh et al., 2004).

To examine these minimum versus maximum concepts, fieldwork has been carried out along the Northeast Greenland coast since the summer of 2003 as part of a PhD project at Lund University (Hjort et al., 2005, Alexanderson and Håkansson, 2006). Field sites are shown in figure 1b.

Was LGM ice indeed more extensive than in earlier reconstructions, covering the presently ice-free coastal areas and the shelf, or was it only the outlet glaciers from the main fjords that inundated parts of the shelf, leaving the adjacent coast and shelf areas with only limited glaciation? Our main aim is to test these two conflicting hypotheses of icesheet extent by using cosmogenic exposure dating along the differentially weathered Northeast Greenland continental margin.

Cosmogenic exposure dating

The analysis of cosmogenic isotopes like ¹⁰Be and 26^Al provides a measure of the time a rock surface has been exposed to cosmic radiation. When cosmic rays interact with the nucleus of an atom, cosmogenic isotopes can be produced (Gosse and Phillips, 2001). Most of this production takes place in the atmosphere, but it also occurs in the uppermost few meters of the lithosphere. When accumulated within a rock, two processes act to reduce the amount of these isotopes: radioactive decay and erosion of the rock surface. The accumulated amounts of cosmogenic isotopes can be used as a measure for the time elapsed since the last deglaciation. This assumption is valid if the ice once covering the sampled rock surface eroded away enough material to erase the accumulation from earlier ice-free periods. However this is not always the case. If the erosion was not sufficient to achieve “zeroing” then the rock will be left with a nuclide inheritance from earlier exposures, which will make the apparent age of our sample older than it is. By using two isotopes with different half-lives (¹⁰Be and ²⁶Al) it is possible to conclude whether a sampled surface has been “zeroed” or not. Thus the use of multiple cosmogenic isotopes not only provides a means of dating former ice-margins but also indicates whether the ice cover was eroding its substrate or not.

Jameson Land 2006

The Jameson Land peninsula (70–71°N) is situated on the northern side of the Scoresby Sund fjord, a major drainage path for the eastern part of the Greenland ice sheet (figure 1). Jameson Land is ice-free today, but is partly covered by glacial deposits containing rocks of a western provenance, which indicate that the Greenland Ice Sheet has earlier advanced across the peninsula.

The fieldwork of 2006, following up on work done within the same project in 2004 and 2005, was carried out during four weeks in August, when we revisited two of the field areas from 2005 (Alexanderson and Håkansson, 2006). From small base camps we made daily excursions and 2–3 day hikes sampling boulders and bedrock, studying landforms and digging into sediment exposures.

Area 1: the Central Plateau (3–14 August)

The central plateau of Jameson Land (400–550 m a.s.l.) is covered with ground moraine that has previously been mapped using satellite imagery and aerial photographs and is referred to as the “Older Drift” (Ronnert and Nyborg, 1994, figure 1c). Earlier studies suggest that this drift pre-dates the Weichselian (last glacial cycle) and originates from the last extensive ice sheet advance over Jameson Land (Möller et al., 1994). The western boundary of the “Older Drift” has been interpreted as a result of postglacial erosion.

We walked along this sediment boundary to ground-truth the previous mapping efforts and we can confirm the interpretation of the boundary as an erosional boundary. Additionally, samples for cosmogenic exposure dating were collected from the central plateau with the aim of documenting further at what time the ice sheet last covered central Jameson Land.

Area 2: Fynselv (16–31 august)

In the area around Fynselv the bedrock is exposed as weathered sandstone remnants more or less eroded by glacial meltwater (picture 2). Erratic boulders and patches of ground moraine are found, especially in lower parts of the terrain. The patchy distribution of glacial deposits and the pattern of melt water erosion in combination with the softness of the sandstone led to the interpretation that the weathered rock formations were formed by a combination of subglacial erosion and postglacial weathering (Hjort and Salvigsen, 1991).

During the field season of 2006 rock samples were collected from glacial deposits (erratics on bedrock and ground moraine) and from weathered bedrock surfaces with different imprints of meltwater erosion (picture 1). In addition clay mineral samples were collected from weathering pits to test whether the weathering remnants of the Jameson Land sandstone have formed since the last deglaciation, as earlier anticipated, or if they are much older than that.

Preliminary results

At present 27 boulder samples from Jameson Land have been dated, giving ¹⁰Be-ages between 8 000–250,000 years. These dates fall within one older group of pre-Weichselian ages and roughly three groups of Weichselian ages (around ca. 14,000, 35,000 and 70,000 years). Due to the large spread of ages and the complexity of the method a larger dataset is needed before we can draw any firm conclusions about the timing of ice sheet advances over Jameson land. However more ¹⁰Be-ages from Jameson Land and the field sites north thereof are pending.

Jason Briner at the Department of Geology in Buffalo, NY, USA is acknowledged for letting me use his cosmogenic isotope laboratory. I also thank my PhD advisors Christian Hjort, Svante Björck, Per Möller and Johan Kleman.