Spatial variations in snowpack chemistry, isotopic composition of NO^-₃and nitrogen deposition from the ice sheet margin to the coast of western Greenland

The relative roles of anthropogenic nitrogen (N) deposition and climate change in causing ecological change in remote Arctic ecosystems, especially lakes, have been the subject of debate over the last decade. Some palaeoecological studies have cited isotopic signals (δ(¹⁵N)) preserved in lake sediments as evidence linking N deposition with ecological change, but a key limitation has been the lack of co-located data on both deposition input fluxes and isotopic composition of deposited nitrate (NO^-₃/. In Arctic lakes, including those in western Greenland, previous palaeolimnological studies have indicated a spatial variation in (δ¹⁵N) trends in lake sediments but data are lacking for deposition chemistry, input fluxes and stable isotope composition of NO^-₃. In the present study, snowpack chemistry, NO^-₃stable isotopes and net deposition fluxes for the largest ice-free region in Greenland were investigated to determine whether there are spatial gradients from the ice sheet margin to the coast linked to a gradient in precipitation. Late-season snowpack was sampled in March 2011 at eight locations within three lake catchments in each of three regions (ice sheet margin in the east, the central area near Kelly Ville and the coastal zone to the west). At the coast, snowpack accumulation averaged 181mm snow water equivalent (SWE) compared with 36mm SWE by the ice sheet. Coastal snowpack showed significantly greater concentrations of marine salts (NaC, Cl-, other major cations), ammonium (NH⁺₄; regional means 1.4.2.7 μmol L^-1), total and non-sea-salt sulfate (SO^2-₄; total 1.8.7.7, non-sea-salt 1.0.1.8 μmol L^-1/than the two inland regions. Nitrate (1.5. 2.4 μmol L^-1/showed significantly lower concentrations at the coast. Despite lower concentrations, higher precipitation at the coast results in greater net deposition for NO^-₃as well as NH⁺₄and non-sea-salt sulfate (nss-SO^2-₄/relative to the inland regions (lowest at Kelly Ville 6, 4 and 3; highest at coast 9, 17 and 11 mol ha-1 a-1 of NO^-₃, NH⁺₄and nss-SO^2-₄respectively). The (δ¹⁵N) of snowpack NO^-₃shows a significant decrease from inland regions (-5.7‰ at Kelly Ville) to the coast (-11.3 ‰). We attribute the spatial patterns of (δ¹⁵N) in western Greenland to post-depositional processing rather than differing sources because of (1) spatial relationships with precipitation and sublimation, (2) withincatchment isotopic differences between terrestrial snowpack and lake ice snowpack, and (3) similarities between fresh snow (rather than accumulated snowpack) at Kelly Ville and the coast. Hence the (δ¹⁵N) of coastal snowpack is most representative of snowfall in western Greenland, but after deposition the effects of photolysis, volatilization and sublimation lead to enrichment of the remaining snowpack with the greatest effect in inland areas of low precipitation and high sublimation losses.