A cold laboratory hyperspectral imaging system to map grain size and ice layer distributions in firn cores

Abstract. The Greenland and Antarctic ice sheets are covered in a thick layer of porous firn. Knowledge of firn structure improves our understanding of ice sheet mass balance, supra- and englacial hydrology, and ice core paleoclimate records. While macroscale firn properties, such as firn density, are relatively easy to measure in the field or lab, more intensive measurements of grain-scale properties are necessary to reduce uncertainty in remote sensing observations of mass balance, model meltwater infiltration, and constrain ice age – gas age differences in ice cores. Additionally, as the duration and extent of surface melting increases, refreezing meltwater will greatly alter firn structure. Field observations of firn grain size and ice layer stratigraphy are required to inform and validate physical models that simulate the ice sheet-wide evolution of the firn layer. However, visually measuring grain size and ice layer distributions is tedious, time-consuming, and subjective. Here we demonstrate a method to systematically map firn core grain size and ice layer stratigraphy using a near-infrared hyperspectral imager (NIR-HSI; 900–1700 nm). We scanned 14 firn cores spanning ∼1000 km across western Greenland’s percolation zone with the NIR-HSI mounted on a linear translation stage in a cold laboratory. We leverage the relationship between ice grain size and near-infrared absorption to retrieve effective grain radii by inverting measured reflectance to produce high-resolution (0.4 mm) maps of grain size and ice layer stratigraphy. We show the NIR-HSI reproduces visually-identified ice layer stratigraphy and infiltration ice content across all cores. Effective grain sizes change synchronously with traditionally-measured grain radii with depth, although effective grains in each core are 1.5x larger on average, which can be explained by firn grain geometry. To demonstrate the utility of the firn stratigraphic maps produced by the NIR-HSI, we track the 2012 melt event across the transect and assess its impact on deep firn structure by quantifying changes to infiltration ice content and grain size. These results indicate that NIR-HSI firn core analysis is a robust technique that can document deep and long-lasting changes to the firn column from meltwater percolation, while quickly and accurately providing detailed firn stratigraphy datasets necessary for firn research applications.