Subglacially precipitated carbonate crusts (SPCCs) formed on bedrock and till boulder surfaces adjacent to the Barnes Ice Cap (BIC), central Baffin Island, Arctic Canada, act as unique archives of Laurentide Ice Sheet basal conditions. Uranium-series dating of these features reveals that carbonate precipitation from subglacial meltwater occurred during the Last Glacial Maximum (LGM), requiring warm-based ice in the region at that time. However, the preservation of fragile SPCCs is unlikely beneath erosive warm-based ice, suggesting that the transition to subsequent cold-based conditions took place shortly after the LGM, and glacial erosion in the region occurred dominantly prior to the LGM. The oxygen isotopic composition of the meltwater from which the SPCCs precipitated is indistinguishable from that of the debris-rich BIC basal ice (δ18O –24‰ referenced to Vienna standard mean ocean water), but distinct from that of the overlying white Pleistocene ice (δ18O ∼–35‰), demonstrating that SPCCs are reliable archives of the isotopic composition of only the basal ice of past ice sheets.

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