Geomicrobiology of Meltwater From the Western Margin of the Greenland Ice Sheet

Subglacial environments are cold, dark, and possess a range of redox conditions. These environments are gaining attention in global biogeochemical cycles as to their role in releasing bioavailable micronutrients such as Fe and the production of greenhouse gases. However, there is uncertainty about how the microbial communities interact with lithology and mediate geochemical reactions under glacial conditions. We examined the microbial communities and their influence on elemental cycling in two glacial environments along the western Greenland Ice Sheet margin: Thule in the north (76ºN, 68ºW) and Kangerlussuaq in the south (67ºN, 51ºW). The north is dominated by supraglacial melting with considerable contribution from the periglacial environment; the south has a well-developed subglacial drainage system. The lithology is sedimentary rocks in the north and crystalline rocks in the south and this difference was reflected in the geochemistry of the drainages. Runoff in the north was oxygen saturated throughout the season. A change from Na and Cl dominance in spring to Ca and SO4 and overall increase in solute concentration marked a stronger contribution from active layer thawing. In the south, waters were undersaturated in oxygen at times, presumably due to biological and chemical sinks of subglacial origin. The meltwater here was dominated by HCO3, SO4 and Ca. In subglacial outflows Fe (oxyhydr)oxide concentrations increased with decreasing oxygen concentration suggesting their formation under oxygen limiting conditions. The high abundance of sulfate implies oxidation of iron sulfides which is consistent with inverse modeling of subglacial weathering processes under anoxic conditions. Meltwater in both locations transported reactive particulate iron which in the north consisted mainly of Fe oxides while Fe(oxyhydr)oxides dominated in the south. DNA and RNA signatures indicate microbial phylotypes that are active in iron reduction, sulfidic mineral weathering, sulfur oxidation, and sulfide reduction supporting the assumption that FeS2 oxidation and formation of particulate reactive Fe(oxyhydr)oxide is microbial mediated. Our data suggest the presence of active microbially iron and sulfur cycling in ice-covered and ice-free zones near the margin of the Greenland Ice Sheet