Stable isotope ratios and speleothem chronology from a high-elevation alpine cave, southern San Juan Mountains, Colorado (USA): Evidence for substantial deglaciation as early as 13.5 ka

Deglaciation ages for valley glaciers from the last glacial maximum are difficult to obtain with precision. The unequal or non-uniform loss of glacial ice cover (spatial heterogeneity) during glacial retreat results in exposure of considerable land surface area prior to complete deglaciation. New U-series dates from an 11cm long stalagmite in a shallow, high elevation (3242 m) cave in the southern San Juan Mountains of southwestern Colorado (USA), indicate that substantial, high-elevation glacial ice ablation occurred prior to 13.5 ka. The stalagmite dates compare well with other deglaciation studies in central and southern Colorado that show deglaciation occurred between ca. 13 ka to 15 ka. Low δ13C values from 13.5 ka speleothem growth bands, relative to the δ13C value of the host carbonate, suggest that alpine plant and soil cover was established at this location, aspect, and elevation prior to 13.5 ka. Calcite luminescence in the 13.5 ka growth bands indicates the presence of humic and fulvic acids released by the roots of living plants and decomposition of vegetative matter. The carbon isotope ratios, calcite luminescence, and U-series dates suggest that the southern San Juan Mountains, at this high elevation site and southern aspect were substantially ice-free prior to 13.5 ka.