A 10 ka ice core from Tödi (Switzerland) reveals recent surface loss and revises Alpine age–altitude relationships

Abstract. Understanding whether the current decline of Alpine glaciers is unprecedented requires direct evidence of ice persistence through past warm periods. While maximum glacier extents are well constrained, for example through landmarks (moraines), minimum Holocene ice extents—particularly at high elevations—remain poorly documented. Cold, low-accumulation summit glaciers can preserve very old ice, yet these archives are increasingly threatened by recent atmospheric warming. Here we present the first ice core record from the summit glacier of Tödi at 3565 m a.s.l. (Swiss Alps). Two ice cores drilled to bedrock (~20 m) were absolute dated with multiple radionuclides (²¹⁰Pb, ³H, ³⁹Ar, ¹⁴C) complemented by glaciological observations and age–depth modelling. Based on evidence for recent surface ablation, we determined that the ice surface at the time of drilling (2023) dates to 1960 ± 1 CE. An exceptionally high-resolution radiocarbon dataset reveals stratigraphically consistent preservation of basal ice dating to ~10 ka cal BP, confirming the persistence of early Holocene ice. The Tödi basal age questions the previously suggested relationship in timing of Alpine ice-free conditions around 6 ka ago at altitudes of about 3500 m a.s.l.. Instead, our results indicate that long-term ice persistence is not controlled by elevation alone, but rather dependent on local glaciological factors. Further, our results imply that other cold-based high-altitude glaciers in the Alps may preserve older ice than currently recognized and underscore both the scientific value and increasing vulnerability of these disappearing climate archives.