Habitats on Ice

Side view of cryoconite holes that have melted out. Canada Glacier, McMurdo Dry Valleys, Antarctica.
Andrew G. Fountain

Cryoconite (cold dust) holes are small ~10 cm diameter, water-filled, cylindrical holes (~10^3 cm) found in the glacier surface. Often, these holes contain algae. While common to glaciers globally, those in the Dry Valleys of Antarctica are covered by ice. Sand patches on the ice surface, blown on to the glacier by wind, melt into the ice. Once below the surface, the sand absorbs energy from the sun faster than it can be conducted through the ice and the sediment melts sufficient ice to form a subsurface pool of water. In some cases, these holes can be entirely disconnected from the atmosphere and from any other subsurface water supply.

The sediment is commonly inoculated with biologic material, some fraction (species and abundance) of which thrives in the near-freezing waters and limited nutrient supply. The holes also completely freeze in during the cold dark polar winter. In summer, the holes are oases for microbial life (mainly cyanobacteria) and biologically mediated chemical reactions on otherwise relatively inert ice surfaces of a glacier. Cryoconite holes may be thought of as natural terraria set into or entombed by ice. Those in isolation can develop unusual chemistries. Photosynthesis and hydrolysis of carbonates during the spring thaw can increase the pH to ~10.5, saturates the water with oxygen (160%) and calcium carbonate, and decrease pCO2. The electrical conductivity of the waters under these conditions is ~10^2 uS cm^-1. Ice melt, in contrast has a pH ~7 waters either somewhat depleted or at equilibrium with atmospheric gases, and EC ~1 uS cm^-1.

Over time the isolated cyroconite holes become connected hydrologically to the glacial runoff. Either the surface passageways connect to the hole, or the hole becomes cannibalized by the melt-out of larger basins at lower elevations on the glacier surface or when the hole is eventually advected to the edge of the glacier. These processes may happen slowly over time, or in pulse events initiated by episodes of unusually warm weather. In any case, the contents are returned to the valley floor. In the cycling of aeolian material from the valley floor to the glacier surface and back to the valley floor, the cryoconite holes act as a biologic filter for microbial organisms. Once the biota become incorporated into the holes, exposed to the biogeochemistry of the waters, and to the seasonal cycling of freeze-thaw conditions, only the fraction of biota that can withstand such an environment is recycled back to the valley ecosystem.

Fundamental questions include the interactions of competing biogeochemical processes that maintain and maximize microbial activity in these small habitats, how the water chemistry and sediment evolves with respect to the biologic activity, how do freezing and melting affect the speciation of nutrients, and, how the biology of the glaciers affect non glacial habitats down stream. Answers not only address the the fundamentals of nutrient exchange and ecosystem structure, but also addresses the possibility of life in cold environments such as 'Snowball earth', Mars, and icy planets elsewhere.

For further reading: 
Foreman, C. M., B. Sattler, J. A. Mikucki, D. L. Porazinska, and J. C. Priscu (2007), Metabolic activity and diversity of cryoconites in the Taylor Valley, Antarctica, J. Geophys. Res., 112, G04S32, doi:10.1029/2006JG000358.
Fountain, A. G., T. H. Nylen, M. Tranter, and E. Bagshaw, (2008), Temporal variations in physical and chemical features of cryoconite holes on Canada Glacier, McMurdo Dry Valleys, Antarctica, J. Geophys. Res., 113, G01S92, doi:10.1029/2007JG000430.
Tranter, M., A. G. Fountain, C. H. Fritsen, W. B. Lyons, J. C. Priscu, P. J. Statham, and K. A. Welch (2004), Extreme hydrochemical conditions in natural microcosms entombed within Antarctic ice, Hydrol. Processes, 18, 379-387.

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