Spatial distribution of vertical density and microstructure profiles in near-surface firn around Dome Fuji, Antarctica

Abstract

To better understand the near-surface evolution of polar firn in low-Accumulation areas (<30gmmgw.e.gyr-1), we investigated the physical properties-density, microstructural anisotropy of ice matrix and pore space, and specific surface area (SSA)-of six firn cores collected within 60gkm of Dome Fuji, East Antarctica. The physical properties were measured at intervals of ≤0.02gm over the top 10gm of the cores. The main findings are (i) a lack of significant density increase in the top g1/44gm, (ii) lower mean density near the dome summit (g1/4330gkggm-3) than in the surrounding slope area (g1/4355gkggm-3) in the top 1gm, (iii) developments of a vertically elongated microstructure and its contrast between layers within the top g1/43gm, (iv) more pronounced vertical elongation at sites and periods with lower accumulation rates than those with higher accumulation rates, (v) a rapid decrease in SSA in the top g1/43gm, and (vi) lower SSA at lower-Accumulation sites, but this latter trend is less pronounced than that of microstructural anisotropy. These observations can be explained by a combination of the initial physical properties on the surface set by wind conditions and the metamorphism driven by water vapor transport through the firn column under a strong vertical temperature gradient (temperature gradient metamorphism, TGM). The magnitude of TGM depends on the duration of firn layers under the temperature gradient, determined by the accumulation rate; longer exposure causes a more vertically elongated microstructure and lower SSA. Overall, we highlight the significant spatial variability in the near-surface physical properties over the scale of g1/4100gkm around Dome Fuji. These findings will help us better understand the densification over the whole firn column and the gas-Trapping process in deep firn and possible difference in them between existing deep ice cores and the upcoming "Oldest-Ice"cores collected tens of kilometers apart.