Extreme hydrochemical conditions in natural microcosms entombed within Antarctic ice

Cryoconite holes are near-vertical tubes that form in the surface of glaciers when solar-heated debris melts into the ice. Those that form in the McMurdo Dry Valleys of Antarctica are distinctive, in that they have ice lids and are closed to the atmosphere for periods of years to decades. Photoautotrophs. and heterotrophs grow within this closed environment, perturbing the poorly buffered water chemistry, yet maintaining the potential for photosynthesis. Microbial excretion and decomposition of organic matter produces dissolved organic carbon (DOC): dissolved inorganic carbon ratios of similar to1:2. Much of the dissolved nitrogen pool (80-100%) exists as dissolved organic nitrogen (DON). The DON: DOC ratio is similar to1:11 (mol/mol), typical of organic particulate material at the Earth's surface. The combination of photoautotrophy, heterotrophy and weak chemical buffering within these microcosms promotes values of pH,pCO(2), O-2 saturation and percentage total dissolved nitrogen as DON that reach 10-99, 10(-7.6) atm, 160% and 100% respectively, which are a unique combination among the surface waters on Earth. These ice-sealed cryoconite holes could be important analogues of refugia on Snowball Earth and other icy planets. Copyright (C) 2004 John Wiley Sons, Ltd.