CLUES TO ENIGMATIC MORAINE EXPOSURE AGES USING COSMOGENIC CARBON-14 EXPOSURE DATING

We use in situ produced carbon-14 (¹⁴C) to confirm the validity of beryllium-10 (¹⁰Be) exposure ages for alpine moraines sourced from the Grey Hunter Massif, Yukon Canada. Two families of moraines exist in the vicinity of Grey Hunter; an older, more-extensive group at valley mouths and a younger, less- extensive group confined to cirques. Based on previous work, we hypothesized that the more extensive moraines would be Last Glacial Maximum (LGM), and the less extensive moraines Lateglacial (e.g., circa Younger Dryas). Our ¹⁰Be ages from the more-extensive moraines, however, are older than LGM (median age of 34.5 ± 9.9 ka; n = 8), whereas the less-extensive moraines yield a median age of 17.9 ± 2.3 ka (n = 15). To assess whether the¹⁰Be ages faithfully track the timing of deglaciation or are affected by inheritance, we measured in situ ¹⁴C in six samples, three from each of the moraine groups.

Paired nuclide results for all samples are consistent with continuous exposure since deposition. The median in situ ¹⁴C exposure age for the less-extensive moraines is 11.4 ± 0.1 ka (n = 3), significantly younger than ¹⁰Be results and consistent with other Younger Dryas moraines in western Canada. We conclude that samples from the less-extensive moraine is affected by ¹⁰Be inheritance. Samples from the more-extensive moraine yield ages that are more difficult to interpret. Two ages are at secular equilibrium with respect to ¹⁴C production-decay systematics, while a third yields an age of 17.5 ± 0.8 ka. Because the isotope ratios indicate continuous exposure and concordance with the ¹⁰Be age for two of three samples, one interpretation is that the age of the moraines is at least 30 ka. Another interpretation, based on the one discordant age, involves ¹⁴C inheritance via rockfall saturated with ¹⁴C onto the glacier and transport until incorporation into the moraine.

Inheritance processes affecting the less-extensive moraines and probably also the more-extensive moraines may be due to limited glacial erosion resulting from both climatic conditions and limited glacial erosion processes. Unlike most of western Canada, the Gray Hunter massif was never covered by, and hence never eroded by, the Cordilleran ice sheet.