The Hydrology and Biogeochemistry of Cryoconite Holes in Taylor Valley, Antarctica: Evolution Over an Ablation Season

Cryoconite holes are small, cylindrical depressions present on the surface of glacier ablation zones throughout the world. Solar heating of low albedo organic and inorganic debris blown onto the ice surface forms a cryoconite hole, often water-filled and occasionally ice lidded, containing rock dust and organic matter. They are hydrologically connected by supraglacial and englacial flowpaths. Cryoconite holes in the McMurdo Dry Valleys, East Antarctica, are unique in their potential to remain ice-lidded and isolated from the surrounding drainage system for several years. In the polar desert of the Dry Valleys, availability of liquid water is a first order control on biological activity. Cryoconite holes are significant as a water and potential nutrient source to downstream ecosystems in this nutrient-poor environment. Here, we document the biogeochemical and hydrological evolution of cryoconite holes, predominantly on Canada Glacier, in Taylor Valley over an ablation season. Previous studies of Dry Valley cryoconite holes have been based on a limited number of samples taken at different times during the ablation season. This study attempts a comprehensive survey of the biogeochemical evolution of a larger suite of holes throughout an entire ablation season, from pre-thaw through to late season freeze. 160 cryoconite holes were sampled on 3 polar alpine glaciers in Taylor Valley during the 2005/6 ablation season. Samples were collected in both frozen and partially melted phases, and analysed for major ions, nutrients and dissolved organic carbon content. Samples of glacier ice and supraglacial runoff were also collected. Chemical trends in the composition of cryoconite holes, glacier ice and supraglacial runoff are used to make the following assertions about the hydrology and biogeochemical properties of cryoconite holes. The degree of hydrologic isolation is a key control on the chemical properties of the cryoconite hole, and on its potential as a solute source. When connected, the holes form a near-surface drainage system, providing at least 13% of glacier runoff in Taylor Valley. When disconnected, biochemical processes act to maximise nutrients in bioavailable phases, which are eventually flushed to downstream communities during decadal warm periods.