COMBINING UAV TERRAIN ANALYSIS AND IN SITU COSMOGENIC 10BE INVENTORIES TO DETERMINE LITTLE ICE AGE SUBGLACIAL ABRASION:QUARRYING RATIO AT JAKOBSHAVN FORE-FIELD

Glacial erosion is a fundamental earth-surface process, creating diagnostic bedrock sculpting and vast amounts of sediment. The rate that glaciers erode, and in particular the relative contributions of quarrying (i.e., plucking) versus abrasion to the overall glacial erosion budget, is constrained by very few empirical measurements. To address this, we generated a detailed terrain map using drone structure from motion (SfM) analysis, mapped ice flow indicator and surface textures, and collected 10 bedrock samples for cosmogenic nuclide analysis at a recently exposed bedrock surface fronting Jakobshavn Isbræ in western Greenland. We measured in situ cosmogenic ¹⁰Be along a lee cliff and floor surface from a bedrock step at the site of a now removed quarried block. Guided by our measured ¹⁰Be concentrations, we modeled the 3D cosmic flux to determine the geometry of the quarried block removed during an interval of Little Ice Age (LIA ~1800 C.E. - 2010 C.E.) ice cover. The best model fit to the measured in situ concentrations is achieved with a triangular block cross-section (vs a rectangular block), and an abrasion rate of ~ 0.1-0.2 mm/yr of the stoss surface above the block. The triangular profile of quarried block vs a rectangular block profile has implications on the volume of rock quarried and the mechanism of how the block was removed. By applying this geometry to other bedrock steps within an 15,000 m² tile of landscape mapped with SfM, we will estimate a range of quarried rock volume and abraded rock volume. Additionally, from our modeling and ice flow mapping, we find that blocks quarried by southward-flowing ice occurred during the recent LIA, whereas southwestward-flowing ice removed blocks during the earlier last glacial maximum (~ 21 ka - 7.5 ka). This constraint of ice flow to an ice overriding event helps prevent over-estimating quarrying rates attributed to LIA. Our results can be used to improve how subglacial erosion is estimated in ice sheet and glacier landscape evolution models.