Speleothem sulphate and trace elements constrain Alpine glacial inception across the Marine Isotope Stage 11/10 transition

Abstract. Alpine glacier histories beyond the Last Glacial Maximum are poorly resolved, impeding our ability to test how mountain glaciers respond to dynamic forcings on orbital to sub‑orbital timescales. This gap may be addressed through proxy records from alpine caves, in which subglacial speleothem growth is facilitated by recharge beneath temperate glaciers through sulphide-bearing epikarst. However, the use of conventional stable isotopes of carbon (δ¹³C) and oxygen (δ¹⁸O) in speleothem calcite yields ambiguities when interpreting environmental transitions and ice cover during glacial inception and retreat. Herein, we present a multiproxy speleothem record from Betten Cave (Melchsee Frutt, central Swiss Alps) spanning 415–360 ka that integrates sulphate stable isotopes (δ³⁴S_SO4, δ¹⁸O_SO4) and trace‑element geochemistry with calcite δ¹³C–δ¹⁸O to diagnose redox state, sulphide‑oxidation pathways, and hydrological reorganization across the Marine Isotope Stage 11/10 glacial inception. Three environmental phases are identified from this dataset, marking the transition from (1) a vegetated and soil-covered montane valley to (2) a soil-limited periglacial setting, hydrologically influenced by glacier advance through the adjacent valley (~2,000 m a.s.l.), to (3) subglacial speleothem growth, in which a temperature glacier covered the lowest elevation of the cave system (~1,700 m a.s.l.). After 402 ± 4 ka, a long-term decrease in the isotopic offset between water and aqueous sulphate (Δδ¹⁸O_SO4−H2O) records progressive oxygen limitation concomitant with glacier thickening and advance over the cave site by 372 ± 3 ka. Coeval peaks in cations and redox‑sensitive transition metals reflect the enhanced delivery of glacially comminuted detritus and coupled Fe–Mn redox cycling within the subglacial karst system. These results directly link sulphate oxygen- and sulphur-isotope systematics and trace‑element fingerprints to glacier dynamics, providing geochronologically precise benchmarks and a transferable framework for reconstructing mountain glacier behaviour where geomorphic records are incomplete.