The fine particles created by glacial comminution of carbonate minerals are susceptible to interaction with meltwater, and the isotopic composition of fine carbonate in glacial deposits has the potential of recording this interaction. Here, we focus on the carbonate fraction of glacial sediment and debris within basal ice at the Tsanfleuron Glacier, Switzerland. Size separates of basal ice debris and melt-out tills show a collinear isotopic trend interpreted as a mixing line between detritus (δ18OPDB = -5‰ and δ13OPDB = 0.6‰) and authigenic calcite, which in one case had δ18O values as light as -18.7‰ and δ13C values as light as -5.7‰. In most cases, the finer fractions (with calcite crystals as small as 0.1 μm) are enriched in the isotopically light component, which is interpreted to form largely by freezing in basal ice. The isotopic compositions of ice and included gases could be influenced by this process. Melt-out tills inherit the authigenic calcite; thus, carbonate-bearing till should no longer be regarded simply as clastic sediment.