Evidence for anoxic conditions in subglacial and proglacial environments

Glacial sediments may be host to a range of microbial communities, which drive oxic waters towards anoxia along hydrological flowpaths. Chemical and isotopic signatures in meltwaters from Finsterwalderbreen, a polythermal glacier on sedimentary bedrock in Svalbard, show clear evidence for anoxia at the glacier bed and in waterlogged proglacial sediments. Increases in ä34S and ä18O of sulphate in subglacial upwelling waters sampled in 1997 indicate the microbially-mediated reduction of sulphate. The ä13C of the dissolved inorganic carbon is isotopically light \{ä13C = -8 %\}, which is consistent with the use of bedrock kerogen and/or the necromass of sulphide oxidizing bacteria as organic substrates for the sulphate-reducing bacteria. Meltwaters sampled from the same outflow two years subsequently do not show evidence of sulphate reduction. Instead they are depleted in ä34S and ä18O of sulphate, suggesting that anoxic sulphide oxidation is a significant process and that subglacial flowpaths have become more oxygenated. Proglacial groundwaters are isotopically light with respect to ä34S and ä18O, demonstrating that anoxic sulphide oxidation is an important process here also. Variations in the isotopic composition of sulphate between years in the subglacial case and within a single summer in the proglacial case are linked to shifts in local hydrological conditions and drainage reorganisation.