Inclusions in Ice Layers of an Ice Core, Greenland

Recent global warming with polar amplification increases snowmelt in Greenland ice sheet especially after 2000. Many ice cores are drilled in the accumulation area to reconstruct paleoenvironment. On percolation zone in the accumulation area, ice layers are formed by snowmelt and refreezing processes. However, ice layers may disturb the reconstruction because of post depositional effect of proxies by snowmelt and refreezing processes. So, it is important to know how and where the proxies are existed and relocated in the ice layers for future ice core study. The 60 m SIGMA-A core drilled in northwestern Greenland ice sheet in 2017 (78 0306N, 67 3742W, 1490 m a.s.l, -20C of annual average temperature) covers past 100 years and has two thick ice layers formed in 2006 and 2012. We observed size, shape, location and elemental compositions of inclusions preserved in the thick ice layers of SIGMA-A ice core. As a result, huge inclusions more than 100 m in diameter were found in the ice layers by microscope observation, those are never observed in ice cores without melting. They are categorized into two groups; particle-like inclusions detected in ice grains, and string-like inclusions in grain boundaries. The string-like inclusions are particularly large in size, ranging from 0.1 to 1 mm in diameter. By Raman spectroscopy analyses, particle-like inclusions in ice grains tend to contain CaSO4 and Na2SO4 while string-like inclusions in grain boundaries contain liquid SO42-. By Scanning Electron Microscope Energy Dispersive X-Ray Spectroscopy analyses, both particle-like and string-like inclusions mainly consist of Na, S, Cl, or Ca elements. The mole fractions of the four elements (Na, S, Cl, Ca) correlated with the surface areas of inclusions, respectively. The fraction of Na and Cl converge to 0.5 while the fraction of S and Ca decrease markedly in the string-like inclusions with huge surface areas. These results suggest that huge string-like inclusions are mainly consisted of liquid phase of Na+ and Cl- with small amount of SO42-. The liquid phase brine of Na+ and Cl- with mm scale in diameter become a proxy of a large amount of refreezing ice in deeper ice core and may be water and ion impurities pathway when the ice layer melts further global warming.