Physical properties (density, relative permittivity, dielectric anisotropy, and SSA) in the top 10 m of six cores drilled around Dome Fuji, Antarctica

To better understand the near-surface evolution of polar firn in very low accumulation areas (<30 mm w.e. yr-1), we investigated the physical properties of six firn cores collected within 60 km around Dome Fuji, East Antarctica. The properties include (i) density, (ii) anisotropy in microstructure composed of ice matrix and pore space detected through tensorial relative permittivity, and (iii) specific surface area (SSA). These properties were continuously measured at resolutions of a few centimeters or less along the top 10 m of the cores. The main findings from our measurements are: (i) a lack of significant density increase in the top ~4 m, (ii) lower density near the flat dome summit (~330 kg m-3) compared to 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 southern sites and during periods with lower accumulation, (v) a rapid decrease in SSA in the top ~3 m, and (vi) lower SSA at the southern sites, but this trend is less clear than that of the microstructural anisotropy. These observations may be explained by wind to set physical properties on the surface and the effects of surface heating from solar radiation, which leads to metamorphism driven by water vapor transport through the vertical temperature gradient within the top few meters (known as temperature gradient metamorphism, TGM). The magnitude of TGM depends on the duration of firn layers under temperature gradient, which is determined by accumulation rate; a longer duration results in more vertically elongated microstructure and lower SSA. The initial variability in density and SSA, determined by wind-driven redistribution at the surface, also contributes to the layered development of vertically elongated microstructure, probably because the manner of TGM depends on density and grain size. Overall, we underline the noticeable variability in the physical properties near the surface around Dome Fuji. These findings are crucial for a better understanding of subsequent firn densification and gas signal formation in deep firn, and when comparing gas signals between existing deep ice cores and coming "Oldest-Ice" cores.