A multiproxy speleothem-based approach to reconstructing alpine glaciation beyond the limits of geomorphological evidence

The sensitivity of alpine glaciation to climatic warming is underscored by global reductions in ice mass over the last century and accelerated losses observed in recent decades. Given the dependence of local populations, infrastructure, and ecosystems on reliable snowpack and meltwater supply, the forecasted demise of many mountain glaciers critically motivates investigations into the dynamics underlying glacial retreat and advance under past boundary conditions. However, the timing and extent of glacial limits are progressively less well constrained through geologic time, particularly in mountain regions, due to the inevitable loss of geomorphic indicators through subsequent glacial advance and erosion. Secondary mineral deposits in some alpine caves offer a unique solution to glacial reconstructions, because warm-based ice cover allows the cave system to remain unfrozen with active speleothem growth. Importantly, the loss of soil cover and presence of glacial ice induces an abrupt switch from a carbonic-acid to a sulfuric-acid dominated system in caves hosted by impure carbonate rocks, which can be detected through geochemical analysis of radiometrically dated speleothems. While this category of ‘subglacial speleothems’ has long been described, only recently has a thorough multiproxy approach been developed and tested, which can robustly identify the timing and dynamics of glaciation over alpine cave sites. In addition to key changes in the stable-isotope composition of carbon (δ¹³C) and oxygen (δ¹⁸O), the dominance of sulfuric-acid dissolution leads to an increase in speleothem sulfate, often by more than an order of magnitude, which may be accompanied by a decrease in the δ³⁴S of sulfate due to enhanced sulfide oxidation. Glacial weathering processes are captured by certain trace elements in speleothem calcite, whereas the redox evolution of infiltrating waters (reflecting the hydrological balance of surface and subglacial meltwaters) is reflected in the oxygen-isotope composition of speleothem sulfate. Finally, U-series dating (U-Th and U-Pb) allows for accurate geochronological constraints, sometimes with per mil precision, throughout the Quaternary and beyond. Herein, we present case studies from the European Alps and Western Caucasus that successfully document the advance and retreat of warm-based glaciers without reference to surficial deposits and landforms. We further discuss uncertainties associated with the site-specific behavior of geochemical proxies that may limit their application to some glacial reconstructions.