LIMITED GLACIAL EROSION BENEATH A LATE PLEISTOCENE ICE CAP, CHAJNANTOR PLATEAU, NORTHERN CHILE

A unique pattern in bedrock exposure ages is observed on the formerly glaciated Chajnantor Plateau (18°S-23°S) in the northern Andes, the site of a small ~250 km² ice cap during the Late Pleistocene. Surface exposure ages are oldest on the interior of the plateau and young towards the periphery of the former ice cap, suggestive of limited glacial erosion and the presence of an inherited nuclide inventory. The prevalence of limited glacial erosion and incomplete resetting is supported by discordant paired ¹⁰Be and ³⁶Cl exposure ages, with ³⁶Cl ages that are substantially older (~30-50%) than the ¹⁰Be age. We use additional in situ ¹⁰Be and ³⁶Cl measurements from striated bedrock surfaces across the Chajnantor Plateau alongside the existing cosmogenic dataset to reinterpret the glacial history on the plateau. We couple this cosmogenic dataset with simple numerical modeling to quantify the magnitude and pattern of subglacial erosion across the plateau and provide insight into local controls on subglacial erosion.

We calculated erosion depths for a range of different glacial histories constrained by measured nuclide inventories. Paired-nuclide samples converge toward similar inferred erosion depths under glacial histories that incorporate ~70 kyr of inheritance and deglaciation ages of 17-15 ka. These initial results suggest that the Chajnantor Ice Cap persisted until ca. 15 ka, ~5 ka longer than originally inferred through exposure ages on moraine boulders and bedrock. A deglaciation age of ca. 15 ka for the Chajnantor Plateau is consistent with the timing of regional deglaciation observed in glacial records across the arid Andes. Under this glacial history, inferred erosion depths across the plateau range from 0.74 ^+0.05/_-0.04 m (¹⁰Be) and 0.89 ^+0.11/_-0.08 m (³⁶Cl) to 2.53 ^+2.47/_-0.59 m (¹⁰Be) and 3.1 ^+1.32/_{-0.48 m} (³⁶Cl). Initial results from a 2D ice dynamics model suggests that the broad pattern of glacial erosion may be controlled by the topographic configuration of the plateau, whereby the volcanic peaks that ornament the plateau govern the convergence and divergence of glacial ice.