GLACIAL SEDIMENTS AND LANDSCAPE CHRONOLOGY IN THULE, NORTHWESTERN GREENLAND

To study the glacial history of Thule, northwestern Greenland, we conducted surficial geologic mapping, excavated soil pits and stratigraphic sections, sampled boulders for cosmogenic nuclide analysis, and collected shells from marine deposits for radiocarbon dating.

The Thule land surface is covered by at least two till units. The till covering the majority of the landscape is dense, has a clay-rich matrix, and contains permafrost features such as sorted circles. Boulders in the clay-rich till show evidence for long-lived subaerial weathering. Preliminary minimum limiting ¹⁰Be exposure ages from boulders at the surface of this till are 21.7, 26.4, and 28.3 ka. A second till unit covers a small area of land (~20 km²) north of Thule in a bowl-shaped depression adjacent to a large fjord and outlet glacier. This till unit has a sandy matrix, a hummocky surface, and contains numerous moraines; boulders contained within it have fresh surfaces and no permafrost land surface features are visible. Preliminary minimum limiting ¹⁰Be exposure ages from boulders in this unit are 11.7 and 26.9 ka. Excavation of stratigraphic sections in stream channels near the moraines indicates that the sandy till overlies shell-rich marine sediments.

Based on the spread of exposure ages, we hypothesize that boulders in the clay-rich till contain inherited ¹⁰Be. They may have been deposited during an earlier glaciation and then preserved beneath cold-based ice. The sandy till likely represents a later period of ice advance, perhaps one that was predominately confined to the fjords since its spatial extent is limited. This later re-advance may have incorporated both fresh and reworked rock material, leading to the disparate exposure ages. Since the sandy till overlies marine sediments, the re-advance may post-date the Last Glacial Period and the marine high-stand.

Future work will focus on obtaining more detailed landscape chronology. We will perform multi-isotope cosmogenic analysis of boulders in both till units in order to study exposure/burial history and to determine the timing of the re-advance that deposited the sandy till. We will also use radiocarbon dating of marine shells to provide a maximum limiting estimate for the age of the re-advance.