[A-CC27] Ice cores and paleoenvironmental modeling

[ACC27-06] Spatial distribution of vertical density and microstructure profiles near the surface of six firn cores collected around Dome Fuji, Antarctica

★Invited Papers

*Ryo Inoue¹, Shuji Fujita^1,2, Kenji Kawamura^1,2,3, Ikumi Oyabu^1,2, Fumio Nakazawa^1,2, Hideaki Motoyama^1,2, Teruo Aoki² (1.Graduate Institute for Advanced Studies, SOKENDAI, 2.National Institute of Polar Research, 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:firn, density, microstructure, Antarctica, Dome Fuji

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 of Dome Fuji, East Antarctica. The physical properties were measured at intervals of <0.02m over the top 10m of the cores. The main findings are (i) a lack of significant density increase in the top ~4 m, (ii) lower mean density near the dome summit (-330 kgm^-3) than in the surrounding slope area (-355 kgm^-3) in the top 1 m, (iii) developments of a 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) a rapid decrease in SSA in the top ~3 m, 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 ~100 km 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.