| 19 Dec 2025
Firn microstructure variations under extreme metamorphism at Allan Hills Blue Ice Area, Antarctica
Abstract. The spatial variability of firn properties, including microstructure, is critical for mass-balance estimates and ice-core interpretations. This includes local and regional variations in firn microstructure, which influence compaction and air movement in the firn. Some of the oldest ice cores are located in blue-ice areas and originated from low-accumulation areas. Surface processes, including small variations in surface conditions and topography, may cause significant alteration of the firn microstructure and bulk firn properties in low-accumulation areas, as firn may interact with surface processes for long periods of time (e.g., hundreds of years). Here, we examine the impact of very low-accumulation rates (A = O(10-3 - 10-2 m a-1)) on firn microstructure using micro-computed tomography data from a firn core (PICO2303) retrieved in the accumulation zone at Allan Hills Blue Ice Area (BIA), Antarctica during the 2023–2024 NSF-COLDEX field season. First, we analyze microstructural parameters such as total porosity, density, specific surface area, degree of anisotropy, and structure thickness, as well as 2D and 3D reconstructed digital datasets. Subsequently, we compare our observations with a firn core from Dadic et al. (2015) that we refer to here as AHMIF, and located ∼9 km downslope. In the PICO2303 core, we find 1) faceted grains, 2) wind crusts, 3) vertically oriented networks, and 4) specific surface area of 1.5 m2 kg-1 greater and density of 205 kg m-3 lower below the surface compared to those measured in the AHMIF core. We propose that these characteristics are a result of differences in both surface processes (e.g., formation and decay of sastrugi) and subsurface evolution (e.g., depth-hoar formation) that likely occur at varying degrees. We model surface snow density as a function of wind speed, which is approximated from a katabatic wind model. We find modeled surface density closely predict data at AHMIF and PICO2303 with errors of 8.2 % and 0.4 %, respectively, which suggests that core differences are influenced by spatial variability of surface wind and topography. Broadly, the differences between AHMIF and PICO2303 may impact near-surface air movement with resultant implications on post-depositional modification of stable water-isotope ratios and gas exchange.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
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