Glacial erosion efficiency, early Holocene ice retreat rates, and interglacial exposure: new cosmogenic 10Be data from three sites in western Greenland

In order to decipher past behavior of the Greenland Ice Sheet, we quantified in situ cosmogenic 10Be concentrations in three types of samples: bedrock and boulder samples from outside of the present-day ice margin, and cobbles removed directly from glacial ice. To study subglacial erosion and develop a chronology for early Holocene ice retreat, we quantified 10Be in bedrock and boulder samples at two sites: Ilulissat (69°N) and Upernavik (73°N). At each site, we sampled a transect normal to the ice margin. Samples were collected at various locations along these transects, at high, medium, and low elevations. All 30 samples from Ilulissat had Holocene 10Be ages, suggesting that the ice in this area is warm-based and sufficiently erosive to remove most or all 10Be from previous exposure. Our 10Be ages show that deglaciation of the mainland began at ~9.1 ka BP and that the margin reached its current position ~6.5 ka BP. Ice retreat rates during this period were ~15 m yr-1. The 33 samples from Upernavik were notably different, as many had old 10Be ages indicative of cold-based, non-erosive ice that failed to strip away 10Be inherited from prior periods of exposure. While most low-elevation samples have Holocene ages, higher-elevation samples have considerable inheritance (up to 104 ka), indicating that these old terrains were repeatedly exposed and buried during at least several interglacial-glacial cycles. There is also considerable discordance between sample types, with bedrock samples having significantly higher 10Be concentrations than their corresponding boulders. The low-elevation boulder samples suggest that ice retreat occurred rapidly, at rates of ~180 m yr-1. We draw two conclusions from this data: first, subglacial erosion efficiency is dramatically different between these two sites, likely due to differences in basal conditions and water availability at the bed (a function of ice thickness, ice velocity, and/or air temperature). Second, ice retreat rates are also very different between the two sites, which may suggest that retreat occurs more slowly over land (Ilulissat) than it does over water (Upernavik). To study the exposure history of material inland of the present-day ice margin and make inferences about past ice sheet behavior, we quantified 10Be concentrations in icebound cobbles at three sites: Kangerlussuaq (67°N), Ilulissat (69°N) and Upernavik (73°N). We measured 10Be concentrations in 91 icebound cobbles, dispersed equally between the three sites. Most cobbles had low (but measurable) 10Be concentrations of ~600-1000 atoms g-1. A lesser portion of the cobbles had appreciably higher concentrations of up to ~5000 atoms g-1. These data suggest that some icebound cobbles have experienced up to several ka of exposure during warm periods, indicating that Greenland ice extent was smaller than present at one or multiple times.