CONSTRAINTS ON THE HOLOCENE EXTENTS OF THE SOUTHWESTERN MARGIN OF THE GREENLAND ICE SHEET, KANGERLUSSUAQ, GREENLAND

Borehole records from atop the Greenland Ice Sheet (GrIS) indicate muted temperature fluctuations during the Holocene compared to those of the last glacial period. During this time, however, the sensitive, low-elevation margins of the GrIS experienced major fluctuations. Determining the extent and timing of these marginal changes is important for understanding the response of the GrIS to both warmer and colder climate conditions and for calibrating ice sheet models.

The Kangerlussuaq region (67.0°N, 50.7°W) of southwestern Greenland hosts an excellent geomorphic record of deglaciation of the GrIS subsequent to the last ice age. Sets of moraines trending north-south mark the eastward retreat of the GrIS from the continental shelf, near Sisimiut, to the present day ice margin. Three of these moraine systems exist between Kangerlussuaq and the present day ice margin. They are, from oldest to youngest, the Keglen, the Ørkendalen and the late Holocene moraines. The ages of the Keglen and Ørkendalen moraines have been previously constrained using minimum and maximum-limiting radiocarbon ages. However, prior to this study, they have not been directly dated. Historical records have been used to date at least part of the late Holocene moraines to 1880 AD, during the culmination of the Little Ice Age. We applied surface exposure dating using the cosmogenic nuclide ¹⁰Be to assign direct ages to these moraines and to determine if we could accurately date the late Holocene moraines using ¹⁰Be dating.

Between 2009 and 2011, we collected ¹⁰Be samples from boulders on the Keglen, Ørkendalen and historical moraines as well as bedrock and boulder samples from atop Mount Keglen and other bedrock highs in the region. These ¹⁰Be ages constrain the extent and thickness of the southwestern margin of the GrIS during the Holocene and provide the first direct ages on deglaciation in the region. We present an age for the Ørkendalen moraines of 6.8 ± 0.3 ka, which is identical to that put forth by van Tatenhove et al. (1996). The Ørkendalen moraines are located just outboard of the late Holocene moraines, indicating that the GrIS was smaller than present between 6.8 ka and 1880 AD. Other ¹⁰Be ages are forthcoming. These ages provide important constraints on the extents of the GrIS during the Holocene.