Seasonal changes of chemical, isotopic and microbiological signatures in meltwater outflows of the West Greenland Ice Sheet

The faster rate of melting of the Greenland Ice Sheet and its implications for global hydrological and biogeochemical cycles has led to detailed studies of hydrological pathways and biogeochemical processes on top, within, and underneath the Ice Sheet. A challenge up to now is the identification of water sources and pathways and their seasonal development that can provide important information about glacier dynamics. Most studies are based on bulk glacier outflow and attempts that separate hydrographs in different water sources are often based on a combination of isotopic, geochemical, and hydrological information. These parameters can be indicative for meltwater sources as well as being indicative for fast and delayed flow. In this paper we present stable isotopes of water and strontium together with detailed chemical analysis and microbial counts. The data are used to evaluate the potential and limits of these parameters to delineate sources of water and solutes throughout a season and its implication for identifying and quantifying major biogeochemical processes. The study is performed at two terrestrial outflows of the West Greenland Ice Sheet: i) Thule on the Pituffik Peninsula (76°N, 68°W) and ii) Kangerlussuaq (Sondre Stromfjord, 67°N, 50°W). To identify primary meltwater sources fresh snow was collected along transects and depth profiles on the Ice Sheet catchment area and basal ice was collected from the border of the Ice Sheet. Supraglacial and bulk meltwater samples were collected throughout the season together with precipitation such as rain and snow. All samples were analyzed for isotopes, major and trace elements, organic and inorganic carbon and microbial counts. Stable isotope of waters collected on the surface of the Ice Sheet display an elevation and depth trend; however, there is overlap in the stable isotope ratios suggesting that isotopes alone may not be sufficient to identify meltwater source areas. Contrary, rain has unique isotope characteristics of rain each event allowing quantification of its contribution to bulk meltwater. Meltwater chemistry can be used to identify meltwater routing in and below the ice sheet and changes from sodium and chlorite dominated snowmelt water to calcium, magnesium and sulfate dominated water which is related to mineral weathering processes are seen throughout the season. In addition, strontium isotope ratios and stable isotopes of sulfate can be used to identify solute provenance areas and the contribution of microbial activity in solute sequestration, respectively.