STABLE ISOTOPES AND SPELEOTHEM CHRONOLOGY FROM A HIGH ALPINE CAVE, SOUTHERN SAN JUAN MOUNTAINS, CO: EVIDENCE FOR DEGLACIATION AS EARLY AS 13.5KA

An ~11cm tall stalagmite from a shallow, speleothem-poor, high elevation (10,636 ft; 3242 m) cave (near Molas Pass) in the southern San Juan Mountains (SSJM) was sampled for oxygen (O) and carbon (C) isotopes, and U-series dating. Long transects along continuous growth bands were sampled and used to constrain the age of the stalagmite at 13,446±170years. Speleothem growth bands provide irrefutable evidence of liquid water and non-freezing conditions in the shallow subsurface, indicating that the surface above the cave was ice-free at ~13.5ka. Previous studies suggest that the SSJM deglaciated at a rate of ~15m/year, becoming ice-free by ~12.3±1ka. New data from this study suggests that deglaciation likely occurred at an average rate of ~25m/year.

The bulk of the stalagmite (~86%) crystallized between 13,446±170 to 13,284±180, followed by a ~10k year growth hiatus; precipitation resumed ~2.7ka. Mean annual air temperature (MAT) correlates closely with air temperature in shallow caves; studies have shown that shallow caves respond within decades to major atmospheric temperature changes. O isotope variations in drip water are temperature dependent and reflect MAT. No significant variation in average δ¹⁸O values was observed between modern and 13.5ka growth bands (-10.0±0.1‰ vs -10.26±0.1‰ VPDB, respectively) suggesting that MATs were similar to modern MATs between 13.25 and 13.5ka. C isotope values principally reflect the C isotopic composition of soil CO₂ derived from oxidation of local organic matter and the isotopic composition of the bedrock. δ¹³C values in the growth bands are notably depleted relative to δ¹³C values in the host (Hermosa Group) carbonate, hence the δ¹³C values largely reflect soil CO₂ values and not bedrock values. δ¹³C values in the more recent growth bands are depleted by only ~1.5 ‰ relative to 13.5ka growth bands, suggesting that at least some alpine plant cover and soil cover must have been established at this location and elevation by ~13.5ka. The proxy δ values support the argument that the surface above the cave was ice-free.

The U-series and isotopic data from this study suggest that: (1) the area near Molas Pass was ice-free by ~13.5ka; (2) at least some alpine plant and soil cover had established by ~13.5ka; and, (3) deglaciation rate in the SSJM may have been as rapid as 25m/year.