Glacial and iceberg sediments contain nanoparticulates of schwert-mannite, ferrihydrite, and goethite formed where pyrite was oxidized by dissolved oxygen in aqueous subglacial environments. Schwertmannite, typically found in acid mine drainage, only forms at low pH by the oxidative weathering of pyrite. Stoichiometric models show that these conditions can be created in closed-system microenvironments containing at least 10−4 M dissolved oxygen, where volume ratios of water/pyrite are ~105 to 106. Ferrihydrite is the only product at higher and lower volume ratios. In these microenvironments the oxidation of pyrite to form schwertmannite is possible in several decades. Schwertmannite and ferrihydrite are metastable in contact with water and are transformed to goethite in <100 yr unless preserved in ice. The presence of these nanoparticulates demonstrates the existence of transient, acidic, and oxic aqueous microenvironments where enhanced biochemical and/or geochemical activity occurs beneath glaciers and ice sheets.