GEOCHEMISTRY OF THE SNOWDON ROCK GLACIER OUTFLOW STREAM: SOUTHERN SAN JUAN MOUNTAINS, CO

Snowdon Rock Glacier (SRG), located in the southern San Juan Mountains (SJM), Colorado (262738mE, 4177303mN, Z13, elevation 3550 m to 3700 m, length 0.43 km), is a tongue shaped, ice-cored rock glacier with a well-developed (centimeter – to meter – sized) debris cover. The Precambrian Uncompahgre Quartzite is the principle source of rock debris for the SRG. During the multi-decade long drought in the southern SJM the SRG has sustained a perennial outflow stream. Three large oval collapse pits (up to 25 m deep, 30 m wide, 40 m long) are prominent surface morphological features that suggest possible degradation of internal ice. The purpose of this study was to gather geochemical outflow data to determine whether these data could be used as an indicator of ice-core degradation over the course of one field season. Tritium, hydrogen and oxygen isotopes, and water chemistry data, collected from March 2013 to November 2013, were used to distinguish between older ice core melt water, and younger melt water from rock debris interstices. Data collected during 2013 was likely impacted by an extended monsoon season and early snow fall. Outflow water samples were collected ~46 meters down gradient from the SRG terminus (elevation, 3512.5 m).

Tritium concentration (TU) values during spring run-off (6.73±0.11 TU) were similar to late season values (6.24±0.11 TU). The presence of tritium (half-life, 12.32 yrs) in the late season water sample indicates (for this study season) that older, core ice was not a significant contributor to the outflow. Hydrogen and oxygen isotopes were collected monthly (and bi-monthly) to check for isotopic depletions during the summer (another possible indicator of ice core melting). However, none of the isotopic values recorded during the summer were depleted, similarly suggesting minimal input from colder, core ice. Neither the tritium nor the stable isotope data, for this study season, indicate that the SRG ice core is degrading.

Samples collected for water chemistry (Ca²⁺, Mg²⁺, SiO₂, Na⁺, Cl^-, SO₄^2-, K⁺, Si, HCO₃^-, Alkali CO₃^2- and Alkali OH^-) were used to monitor seasonal variation in outflow water. Minimal changes were observed in outflow water chemistry; water quality of the outflow was not impacted by alpine soil and biological processes, erosion, or runoff, at the sample location, during this field season.