Cosmogenic nuclide inheritance in Little Ice Age moraines - A case study from Greenland

Abstract

Cosmogenic exposure dating is one of the most widely used methods to constrain the deglaciation history of former glaciated areas. In Greenland, more than 1000 cosmogenic 10 Be exposure ages ( 10 Be ages) have been published within the last two decades. However, a recurring problem is that many of these studies have reported variable amounts of nuclide inheritance making the 10 Be ages too old and difficult to assess without large datasets or independent age control. In this study, we test the accuracy of 10 Be dating of Holocene moraines using independent age constraints from threshold lake records. In Kangerlussuaq, West Greenland, the 10 Be ages of the Ørkendalen moraine system are highly clustered with a mean age of 6.8 ± 0.3 ka (no outliers). In contrast, the nearby Little Ice Age (LIA) moraine yields scattered 10 Be ages ranging from 2.5 to 0.1 ka but with a mean of 0.18 ± 0.06 ka after excluding outliers which coincides with independent age constraints from threshold lakes and boulder kill dates. At Gletscherlukket, Southeast Greenland, the 10 Be ages of the LIA moraine range from 10.2 to 1.6 ka with a mean of 1.9 ± 0.2 ka after excluding outliers. This is ~1.7 ka older than recorded in the proglacial threshold lakes and suggests that all samples from this site contain a significant amount of nuclide inheritance. Our results are consistent with other reports of skewed 10 Be age distributions in LIA re-advance moraines and it probably reflects nuclide inheritance from exposure during the Holocene Thermal Maximum when the glaciers in Greenland were inside the LIA extent. In contrast, there is no evidence of nuclide inheritance in the Ørkendalen moraines, most likely because the glacial erosion was more intense prior to the formation of the moraines i.e. sometime between the advance phase during Last Glacial Maximum position and the subsequent lateglacial and Holocene deglaciation. Our results highlight a potential pitfall related to dating re-advance moraines using cosmogenic exposure dating and we recommend using a multi-method dating approach.