TERRESTRIAL COSMOGENIC NUCLIDE DEPTH PROFILES USED TO INFER CHANGES IN HOLOCENE GLACIER COVER, VINTAGE PEAK, SOUTHERN COAST MOUNTAINS, BRITISH COLUMBIA

Most glaciers in western Canada reached their greatest Holocene extents during 1200-1850 CE, a period referred to as the “Little Ice Age” (LIA). Glacier expansion throughout the Holocene destroyed or obscured terrestrial evidence that could be used to resolve Holocene glacier activity at many alpine sites in western Canada. The production of rare isotopes on the Earth’s surface by bombardment of high-energy particles rapidly decreases with depth. Shielding by ice or snow will shift the concentration vs depth curve, changing both the surface nuclide concentration and steepness of the profile. We test if cosmogenic nuclide depth profiles can be used to determine unique burial/exposure histories resulting from intermittent cover by snow and ice. We report paired cosmogenic nuclide concentrations from four bedrock cores collected in the vicinity of a <0.05 km² glacier that flanks Vintage Peak (N 50.25°, W 124.30°). We obtained the 35-50-cm long cores using a gas-powered rock drill; three pairs of cores originate from recently deglaciated bedrock and one set of cores are from bedrock beyond the inferred Holocene extent of the glacier. We chose sites where the influence of erosion was minimized, including a broad ridgeline and weathered bedrock surfaces with preserved striations interpreted to reflect an absence of plucking. We measured in situ cosmogenic ¹⁴C at 5-10 cm intervals and include paired ¹⁴C/¹⁰Be measurements from the upper 5 cm of each core. ¹⁴C/¹⁰Be ratios indicate all sample sites have experienced 3 to 6 kyr of apparent snow and/or glacier cover since the LGM, with sites within LIA limits averaging ~5 kyr burial. Additionally, we obtained ¹⁰Be ages from nine moraine boulders and one erratic just beyond the inferred late Holocene moraine. These ages respectively yielded a median exposure age of 705 ± 219 a (n=9; median ± IQR) and 11,653 ± 207 a (n=1; ± analytical error). Our preliminary results suggest LIA moraine stabilization on Vintage Peak occurred at 1100-1530 CE. Our results imply an early LIA age for the outermost moraines that front glaciers on Vintage Peak. Future work will use model simulations to determine the glacier and snow depth conditions and erosion rates that best reproduce the observed cosmogenic nuclide depth profiles.