Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier

Glacier beds may be host to a range of microbial communities, which drive oxic waters towards anoxia along certain 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. Increases in δ34S and δ18O of sulphate indicate that microbial sulphate reduction has resulted in significant decreases in sulphate concentration. The δ13C of the dissolved inorganic carbon (DIC) is isotopically light (δ13C=−8‰), which is consistent with the use of bedrock kerogen and/or the necromass of sulphide oxidising bacteria as organic substrates for the sulphate-reducing bacteria. Calculated rates of organic carbon mineralisation correspond to ∼10% of the total annual DIC flux of the glacial meltwaters. This microbial ecosystem is chemoautotrophically based, ultimately being sustained by the kerogen and/or bacterial necromass and sulphides in the bedrock. This work suggests that glacier beds can be refugia for life when climatic and/or atmospheric conditions are otherwise inclement and also supports the contention that microbial life is present in subglacial Lake Vostok.