Evolution of layered density and microstructure in near‐surface firn around Dome Fuji, Antarctica

Abstract. To better understand the near‐surface evolution of polar firn in low accumulation areas (<30 mm w.e. yr⁻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 60 km around Dome Fuji, East Antarctica. The physical properties were measured at the intervals of ≤0.02 m over the top 10 m of the cores. The main findings are: (i) lack of significant density increase in the top ~4 m, (ii) lower density near the dome summit (~330 kg m⁻³) than the surrounding slope area (~355 kg m⁻³), (iii) developments of vertically elongated microstructure and its contrast between layers within the top ~3 m, (iv) more pronounced vertical elongation at sites and periods with lower accumulation rates than those with higher accumulation rates, (v) rapid decrease in SSA in the top ~3 m, and (vi) lower SSA at lower accumulation sites, but this trend is less pronounced than that of microstructural anisotropy. These observations can be explained by the 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 temperature gradient, determined by 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 ~100 km around Dome Fuji. These findings will help 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.