Abstract
This study provides the first contemporaneous measurements of the concentration and speciation of dissolved nitrogen (N) in snow, meltwater and ice during the onset of the ablation season at a site within the Dark Zone of the Greenland Ice Sheet. The decaying, partially leached snow pack near S6 in south-west Greenland produced meltwater with relatively constant nitrate (NO3−) concentrations, approximating the snow pack average (1.1 µM). By contrast, ammonium (NH4+) (0–∼ 4 µM) and dissolved organic nitrogen (DON) (0–∼ 3 µM) concentrations were more variable, and sometimes higher than the average snow pack concentrations of 1.0 and 0.4 µM, respectively. This variability could be the result of microbial uptake and production within the melting snow pack. We observed pooled meltwater at the snow-ice interface that appeared to scavenge DON (∼1–8 µM) and possibly NO3− (∼1–2 µM) from the underlying ice, whose initial surface was a continuum of superimposed ice and weathering crust from the previous season. The shallow meteoric ice (∼10 cm–1 m) beneath the snow pack had high concentrations of DON and NH4+(6.5 and 2.6 µM, respectively), while NO3− concentrations were similar to the snow pack (1.1 µM). The absence of NH4+ in the snowmelt traversing the snow-ice interface may also point to microbial activity occurring at this boundary layer. Melt modelling indicated the presence of liquid meltwater at the snow-ice interface and that at least 10 cm of the surface ice below the snow pack was at 0°C. Solar radiation transmitted through the thin snow pack to the ice surface is absorbed by pigmented glacier algae causing melt of the surrounding ice, allowing the possibly of photosynthesis to begin under the thinning snowpack in these micro-melt environments. Hence, we conjecture that glacier algal blooms can commence before the snow pack has completely melted, occuring at a time when meltwaters are enhanced in nutrients scavenged from the snowpack, superimposed ice and the remnants of the weathering crust from the previous year.
Highlights
The “Dark Zone” along the western margin of the Greenland Ice Sheet (GrIS) is an annually occurring area of dark surface ice, up to ~ 50 km wide and some 20–30 km from the ice margin, with an albedo as low as 0.2 (Wientjes and Oerlemans, 2010; Box, et al, 2012; Stroeve, et al, 2013; Cook, et al, 2020)
It may well be that the surface ice was a mixture of both superimposed ice and weathering crust, which better explains the relative concentrations of NO3−, NH4+ and dissolved organic nitrogen (DON) concentrations in the snow pack, shallow surface ice and meteoric ice
This study provides the first dataset on the concentration and speciation of dissolved N in snowmelt within the Dark Zone of the GrIS
Summary
The “Dark Zone” along the western margin of the Greenland Ice Sheet (GrIS) is an annually occurring area of dark surface ice, up to ~ 50 km wide and some 20–30 km from the ice margin, with an albedo as low as 0.2 (Wientjes and Oerlemans, 2010; Box, et al, 2012; Stroeve, et al, 2013; Cook, et al, 2020). The ionic pulse occurs when solutes fractionate into snowmelt at higher concentrations than are present in the overall parent snow cover (Davies, et al, 1982; Hewitt, et al, 1991; Harrington and Bales, 1998), with the initial 30% of meltwater often containing ~50–80% of the total snow pack solute load (Johannessen, et al, 1975; Johannessen and Henriksen, 1978; Cragin, et al, 1996). This arises because solutes are excluded from ice crystal interiors onto their surfaces during snow metamorphism over winter. The meltwater which refreezes at the base of the snow pack above the cold ice, referred to as superimposed ice, is often rich in solutes (Bales, et al, 1990)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
