Speleothem growth and stable carbon isotopes as proxies of last glacial glacier coverage and thermo-dynamical states in the Alps

For more than a century, a major focus of Quaternary research has been the investigation of glacier advances and retreats during ice age cycles. The cradle for these studies are the European Alps. In recent years, glacier modelling permitted to produce long-term transient simulations of the European Alps glacier evolution. However, only sparse empirical data, e.g. geological reconstructions of glacier margins in the foreland of the Alps of the Last Glacial Maximum, are available to validate these simulations.

Speleothems from the Alps are a widespread palaeoclimate archive. They provide stable carbon isotope records, which can potentially inform about soil and vegetation conditions above a cave site but also about the lack of soil during times of glacier coverage. In addition, speleothem growth in cold, high-elevation cave sites during glacials are a strong indicator of temperatures in the soil-karst-cave system above the freezing point, which is only likely to occur if the cave is covered by a temperate glacier. We here utilise existing speleothem data (growth histories and stable carbon isotopes) from Alpine caves to infer glacier coverage and thermo-dynamical state during the last glacial cycle and to assess the compatibility with modelled reconstructions. We compare data from the last glacial cycle from multiple cave sites located at different elevations (785 to 2319 m a.s.l.) in the Alps with simulations obtained with the Parallel Ice Sheet Model (PISM). We find a general agreement between speleothem-derived soil presence or absence and modelled glacier coverage. However, the speleothem data provide evidence of temperate glacier coverage which is not shown by all of the PISM simulations. Our work demonstrates the value of speleothem-based reconstructions from the Alps as proxies for assessing performance of palaeo-ice flow models.