ORGANIC GEOCHEMICAL VARIABILITY OF DARK SNOW ON TWO ALASKAN GLACIER SURFACES

Glacier surfaces are darkening, which has key implications for the rate of global sea level rise. Originally, albedo decreases were believed to be an effect of solely inorganic particles from surrounding rock erosion or combustion pollution. However, recently that has been proven untrue. Yet, the relative inputs of inorganic to organic particles found on glacier surfaces and their compositional variability is not well known. Therefore, samples of supraglacial cryoconite material were collected from the Matanuska and Spencer Glacier in Alaska in August 2015. These samples were collected at different distances from the ice edge on the glacier in order to test for variability in the composition of cryoconite. Cryoconite is a dark substance found on the surface of glaciers that is believed to be a combination of organic material and dust or rock particles. In order to study the chemical composition of the cryoconite material, we quantified the n-alkanes. By quantifying the alkanes present, we can determine the relative contribution of biological or geologic inputs since fresh biomass is dominated by alkanes with odd chain lengths and ancient rocks that contain degraded organic material typically have an equal amount of odd and even chains. Alkanes were isolated from samples using solvent extraction and then purified using silica gel chromatography before being quantified and identified using gas chromatography mass spectrometry. For all six Matanuska cryoconite samples there was a higher relative abundance of odd alkanes than even alkanes, which indicates a higher abundance of organic material in the cryoconite than dust or rock particles. Yet, in the samples analyzed there was no observed relationship between the distance from the ice edge of the glacier and the Carbon Preference Index. This suggests that, for the Matanuska Glacier, there is no true relationship when comparing compositional variability on a single glacier with altitude or distance from ice edge, or such variation may be over larger scales. We plan to present results from the second Alaskan glacier during the conference as well. These results imply that when modeling the albedo changes of glaciers, the abundance of organic material in the cryoconite holes must also be included to accurately model the increasing melting rates seen on the surface of glaciers.