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 (&lt;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 &quot;Oldest-Ice&quot; cores.为深入理解极低积累区（<30毫米水当量·年⁻¹）内极地粒雪（firn）的近地表演化过程，我们对南极东富士穹顶（Dome Fuji）周边60千米范围内采集的6根粒雪岩芯的物理性质展开了研究。所测物理性质包括：(1) 密度；(2) 由冰基质与孔隙空间构成的微观结构各向异性，该各向异性通过张量相对介电常数测得；(3) 比表面积（specific surface area, SSA）。上述物理性质沿岩芯顶部10米段以数厘米或更小的分辨率连续测定。 本次测定的主要结果如下：(1) 岩芯顶部约4米范围内密度无显著升高；(2) 平坦穹顶顶部附近的密度（约330 kg·m⁻³）低于周边斜坡区域（约355 kg·m⁻³）；(3) 顶部约3米范围内出现垂直拉长型微观结构，且不同层位间该结构存在差异；(4) 南部测点以及低积累时期的垂直拉长现象更为显著；(5) 顶部约3米范围内比表面积快速降低；(6) 南部测点的比表面积更低，但该趋势不如微观结构各向异性的趋势明显。 上述观测结果可通过以下机制解释：风力作用决定了地表的物理性质，同时太阳辐射引发的地表加热效应，会驱动表层数米范围内由垂直温度梯度下的水汽输运所引发的变质作用（即温度梯度变质作用，temperature gradient metamorphism, TGM）。TGM的强度取决于粒雪层处于温度梯度环境下的持续时长，而该时长由积累速率决定：持续时长越长，微观结构的垂直拉长程度越高，比表面积则越低。由地表风力再分布作用决定的密度与比表面积初始差异，同样会促进垂直拉长型微观结构的层状发育，这可能是因为TGM的作用模式取决于密度与晶粒尺寸。 综上，本研究强调了富士穹顶周边近地表区域物理性质存在显著的空间与垂直差异。上述研究结果对于深入理解深层粒雪的后续致密化过程与气体信号形成机制，以及对比现有深冰芯与未来的‘最古老冰（Oldest-Ice）’岩芯的气体信号均具有重要意义。