INVESTIGATING GLACIAL HISTORY AND LANDSCAPE CHRONOLOGY WITH COSMOGENIC 10BE AND 26AL IN THULE, NORTHWESTERN GREENLAND

To study the Pleistocene and Holocene history of glaciation in Thule, northwest Greenland, we measured cosmogenic ¹⁰Be and ²⁶Al in glacially deposited boulders.

The majority of the Thule landscape is covered by a dense, clay-rich diamict. We sampled boulders (n = 13) from this diamict in a 20 km transect stretching from the modern ice margin to the coast. Calculated ¹⁰Be ages range from 10.7-78.4 ka. There are two distinct age groups: the first, with an average of 10.8 ka, ranges in age from 10.7-10.9 ka (n = 3); the second more diffuse group, with an average of 25.0 ka, ranges in age from 18.9-29.7 ka (n = 9). A single older sample is 78.4 ka. Two-isotope (²⁶Al/¹⁰Be) ratios suggest that the Holocene-aged samples have been continuously exposed (ratios of ~7), while older samples have ratios as low as 5.3 that are indicative of prior exposure followed by burial. Modeled minimum limiting burial durations are several hundred ka.

A second diamict covers a small area of land (~20 km²) north of Thule, near the Harald Moltke Brae outlet glacier. This diamict has a sandy matrix and contains numerous sharp-crested moraines. Sampled boulders from these moraine crests (n = 15) have ¹⁰Be ages that range from 12.2-28.6 ka. The age distribution is similar to that on the outlying landscape, with a young pair of samples with ages of 12.2 and 12.4 ka (n = 2) and a second more diffuse group with an average of 23.2 ka and a range of ages from 16.8-28.6 ka (n = 13).

The presence of young cosmogenic ages suggests that regional deglaciation of the Thule landscape occurred ~10.8 ka (the average age of the youngest boulders in the clay-rich diamict). This also serves as a maximum limit for the age of the subsequent re-advance that deposited the sandy diamict and moraines. The large number of boulder ages clustered in the ~20-30 ka range is consistent with cosmogenic nuclides inherited from prior periods of exposure due to the presence of weakly-erosive, cold-based ice that would have buried but not eroded the boulders. However, the clustering of the ages below 30 ka may reflect a systematic process that limits the inherited nuclides each boulder can carry. For example, the boulders may have been plucked from bedrock during a glacial period with warm-based, erosive ice and then reworked but not eroded during subsequent glacial periods with cold-based ice.