The densification of layered polar firn

Abstract

[1] High-resolution density profiles of 16 firn cores from Greenland and Antarctica are investigated in order to improve our understanding of the densification of layered polar firn. A vertical resolution of 1-5 mm enables us to study the detailed densification processes and the evolution of the layering and the resulting variability in density with increasing depth. The densification of layered firn is important for the process of air enclosure in ice and is connected with the observed formation of a nondiffusive zone. Our findings show the following. (1) Mean density profiles, obtained from high-resolution measurements, only partly show clear transitions in densification rate at densities of 550, 730, or 820-840 kg/m 3 , as they are commonly used in literature. (2) The density variability, induced by the layering, shows a similar pattern at all sites: high variabilities at the surface, a rapid drop to a relative minimum in variability at mean density of 600-650 kg/m 3 , followed by a second relative maximum. (3) This leads to increased variability at densities of the firn-ice transition for most of the sites. (4) The variability at the surface decreases with increasing mean annual temperature and accumulation rate, whereas the variability at the firn-ice transition increases. We can exclude a change in local climate conditions as an explanation for the density variability since the firn cores in this study cover a broad range in mean annual temperature, accumulation rate, and age. Overall, high-resolution density profiles deliver a more complex picture of compaction of polar firn as a layered granular medium than has been obtained from mean density profiles in the past.

On the impact of impurities on the densification of polar firn

“…The observed mean density profiles agree well with the density profiles predicted by the firn densification model most commonly used in the literature (Herron and Langway, 1980). However, the analysis of the density variability revealed a paradoxical behavior (Freitag et al, 2004;Gerland et al, 1999;Hörhold et al, 2011). The variability in density first decreases with depth and time, but unlike in a homogeneous medium, it increases again in the middle of the firn column.…”

“…Firn models usually predict that seasonal variations in density decrease and fade out slowly with depth in the later stage of densification (Zwally and Li, 2002). This behavior, however, is opposite to the observations (Hörhold et al, 2011), which show an increase in density variability with depth. This observation leads to the hypothesis of this study that densification might be influenced by impurities.…”

“…Freitag et al, 2004;Gerland et al, 1999;Hörhold et al, 2011). Firn is extremely stratified with layer thicknesses of less than 10 cm.…”

“…In a recent study Hörhold et al (2011) compiled high-resolution density records of 17 firn cores from Greenland and Antarctica, nearly all reaching the firn-ice transition. The observed mean density profiles agree well with the density profiles predicted by the firn densification model most commonly used in the literature (Herron and Langway, 1980).…”

Ice shelf structure derived from dispersion curve analysis of ambient seismic noise, Ross Ice Shelf, Antarctica

“…The variations are up to ±80 kg m −3 at shallow depths, decreasing rapidly within the first 20-30 m (e.g. Freitag et al 2004;Hörhold et al 2011). Density and associated velocity variations on the mm-scale are thin compared to seismic wavelengths of tens of metres in the studied frequency band.…”