On the impact of impurities on the densification of polar firn

Abstract

Editor: P. DeMenocalKeywords: polar firn density densification impurity density variability Understanding polar firn densification is crucial for reconstructing the age of greenhouse gas concentrations extracted from ice cores, and for the interpretation of air in ice as a dating tool or as a climate proxy. Firn densification is generally modeled as a steady burial and sintering process of defined layers, where the structure of the layering is maintained along the whole firn and ice column. However, available high-resolution density data, as well as firn air samples, question this picture and point to a lack of understanding of firn densification. Based on analysis of high-resolution density and calcium concentration records from Antarctic and Greenland ice cores, we show for the first time that also impurities may have a significant impact on the densification. Analysis of firn cores shows a correlation between density and the calcium ion (Ca++) concentration, and this correlation increases with depth. The existence of this relationship is independent of the local climatic conditions at the core sites analyzed. The strong positive correlation between the density and the logarithm of Ca++ concentration indicates that impurities induce softening and lead to faster densification over a wide range of concentrations. In one core, the impurity effect manifests itself so strongly that the density develops a seasonal cycle closely following the seasonal cycle of Ca++. Our results clearly show that the structure of the firn layering changes with depth and suggest that the increased variability in density observed in deep firn, recently described as a universal feature of polar firn, may arise from the influence of Ca++ and/or other impurities. The impurity effect is likely to have direct implications on our understanding of glacial firn densification and on glacial gas age estimates.

Pore morphology of polar firn around closure revealed by X-ray tomography

“…It also focuses on differentiating polar sites, or relating microstructure to deformation mechanisms. However, despite giving useful information for densification modeling (e.g., Hörhold et al, 2011Hörhold et al, , 2012, these X-ray tomography studies do not provide an unambiguous prediction of LIZ and COD. To that end, the recent work of Schaller et al (2017) uses X-ray tomography to scan very large volumes of firn from three different polar sites.…”

Constraining variable density of ice shelves using wide-angle radar measurements

“…The precise nature of this densification depends on a number of local factors that also vary temporally (Arthern et al, 2010), including surface density and stratification (Hörhold et al, 2011), surface mass balance and temperature (e.g., Herron and Langway, 1980), as well as dynamic recrystallization and the strain regime. Recent studies also highlight the role of microstructure (Gregory et al, 2014) and impurities (Hörhold et al, 2012;Freitag et al, 2013a, b).…”

Densification of layered firn in the ice sheet at Dome Fuji, Antarctica

“…Implication for the local insolation modulation of gas transport conditions during bubble close-off Fujita and others (2009) hypothesized that the strength of the textual effects determines the duration for firn-to-ice transition and so determines the degree of lower O 2 /N 2 ratio and smaller TAC. More recently, based on the empirical correlations between Ca 2+ concentrations and densification enhancement, Hörhold and others (2012) suggested that seasonal density-layering loses its initial stratigraphic information in the top 10-15 m of the firn column, and therefore a direct line of influence of the local radiation balance on the surface snow density cannot be the ultimate reason for the observed O 2 /N 2 fractionation at close-off depth. In the present study, based on the data and the explanations, we hypothesize as follows: (1) density-layering does not lose its initial textural information in the shallow portion (10-15 m in the Hörhold and others (2012) cases and 0-30 m at DF) of the firn column, though some ions provide additional features to the initial density-layering.…”