Paired terrestrial cosmogenic nuclides support the mapping of relict polythermal terrains under the Keewatin Ice Divide, central mainland Nunavut, Canada

Understanding basal thermal regimes of paleo-ice sheets is fundamental to provide constraints on ice sheet dynamics and geometry, subglacial hydrology and sediment fluxes, landscape evolution, and long-term climate responses. While various methods have characterized glacial terrains from a range of subglacial conditions, the relationship between landsystem mapping and terrestrial cosmogenic nuclides (TCNs) remains largely unexplored. Here, we investigate a continuum of relict polythermal landsystems and surrounding warm-based terrains mapped in areas formerly covered by the Keewatin Ice Divide by utilizing paired ¹⁰Be and ²⁶Al cosmogenic nuclide analyses on bedrock, boulder, and till samples in the eastern Keewatin region of Nunavut, northern Canada. Exposure ages in low-elevation warm-based landscapes cluster at 8.2 ± 0.6 ka and align with the timing of deglaciation (7.7 ± 1 ka) indicating relatively high (deep) glacial erosion under dominantly warm-based conditions. Moderate nuclide abundances and apparent mean ages of 23.5 ± 4.6 ka (surface tills) and 21.8 ± 2.1 ka (bedrock trimlines at marine limit) in relict intermediate landsystems extending across the ice divide migration zone above 200 m elevation reflect moderate erosion during transient cold-based and warm-based conditions. High nuclide abundances and apparent mean ¹⁰Be ages of 53.7 ± 4.6 ka (bedrock and boulder) in relict cold-based landsystems, predominant where the ice divide was anchored in the uplands above 400 m elevation, are consistent with low erosion and persistent cold-based conditions partially preserving surfaces weathered prior to the last glaciation. The apparent exposure ages in the relict terrains, significantly exceeding expected deglacial ages, and the relatively long periods of burial derived from ²⁶Al/¹⁰Be ratios, indicate widespread TCN inheritance and reflect differential erosion under polythermal basal ice regimes as inferred from landsystem mapping. Our findings emphasize the importance of geomorphological mapping and landsystem analysis to inform the TCN sampling plan and highlight the need for caution in interpreting exposure ages within predominantly warm-based glacial landscapes in a core region of the Laurentide Ice Sheet where relict weathered terrains are recognized.