Novel proxy constraints on subglacial speleothem growth in the Northern Alps bounding the MIS-11 Interglacial

Late Pleistocene climate of the European Alps was characterized by orbitally forced, high-magnitude oscillations in temperature and glacial ice extent. Beyond the Last Glacial Maximum, however, the geographic extent of continental glaciation is notably difficult to constrain, due to the erosion and reworking of associated surficial deposits. Subglacial speleothem growth occurs when warm-based ice sheets cover karst terrain, providing a thermal buffer to ground temperature and a source of liquid water infiltration. In place of carbonic-acid dissolution from the soil zone, the oxidation of sulfide minerals provides a source of acidity to facilitate carbonate dissolution and vadose-zone precipitation. The proxy identification of subglacial processes can therefore serve to constrain ice-sheet evolution from absolutely dated speleothems, but these techniques have yet to be systematically developed. Herein we present a novel composite record of climatic change across MIS-12, -11, and -10 from three stalagmites in Klaus Cramer Cave, a high-elevation site located in the northern Alps of western Austria. Stable-isotope values of oxygen (carbon) are low (high) during glacial episodes that bound the MIS-11 interglacial. When warm-based ice is likely to be present above the cave, δ¹³C exceeds +4‰, signaling that sulfuric-acid dissolution became dominant in the epikarst. To investigate this process further, we measured δ³⁴S and δ¹⁸O in speleothem sulfate, which confirm that pyrite was the primary sulfur source and elucidate redox conditions in both subglacial and soil-dominated systems. Glacial periods also exhibit abrupt and dramatic contrasts to MIS-11 with regard to major- and trace-element concentrations, including a ~20-fold increase in sulfur concomitant with elevated Mg and Sr. This pattern is consistent with a marked increase in prior calcite precipitation associated with sulfuric-acid dissolution that would have elevated initial Ca²⁺ in the system. Finally, we assess trace elements in the context of provenance analysis as a potential indicator of enhanced glacial weathering at the ice-rock interface. Collectively, this suite of geochemical proxies can identify precisely when warm-based ice advanced or retreated across the specific location and elevation of Klaus Cramer Cave in the total absence of evidence from conventional glacial geomorphology.