The air content of Larsen Ice Shelf

Abstract

The air content of glacial firn determines the effect and attribution of observed changes in ice surface elevation, but is currently measurable only using labor‐intensive ground‐based techniques. Here a novel method is presented for using radar sounding measurements to decompose the total thickness of floating ice shelves into thicknesses of solid ice and firn air (or firn water). The method is applied to a 1997/98 airborne survey of Larsen Ice Shelf, revealing large spatial gradients in air content that are consistent with existing measurements and local meteorology. The gradients appear to be governed by meltwater‐induced firn densification. We find sufficient air in Larsen C Ice Shelf for increased densification to account for its previously observed surface lowering, and the rate of lowering superficially agrees with published trends in melting. This does not preclude a contribution to the lowering from oceanic basal melting, but a modern repeat of the survey could conclusively distinguish atmosphere‐led from ocean‐led change. The technique also holds promise for the calibration of firn‐density models, derivation of ice thickness from surface elevation measurements, and calculation of the sea‐level contribution of changes in grounded‐ice discharge.

Seabed topography beneath Larsen C Ice Shelf from seismic soundings

“…Integrating the density profiles produces a total firn-air content of 11.3 ± 1.3 m at the ice-front site, 12.9 ± 1.3 m at the Marmelon site and 14.4 ± 1.3 m at the Joerg site, compared with Holland et al's (2011) estimates of 11.0 ± 1.8 m, 10.0 ± 1.8 m, and 12.7 ± 1.8 m respectively, all of which agree within error bars. These results are consistent with a higher degree of melting of firn in the north than the south, as found in the borehole measurements of Nicholls et al (2012) and the firn air content variation presented by Holland et al (2011).…”

“…Satellite radar altimeter data indicate that surface lowering of Larsen C has been on-going since at least 1992 (Shepherd et al, 2003;Fricker and Padman, 2012). Several authors suggest that this thinning may be caused by a reduction in firn air, a manifestation of increased surface melting resulting from circumpolar air currents flowing over the peninsula and descending on the eastern side (Holland et al, 2011;Pritchard et al, 2012). Shepherd et al (2003) argue that a significant component of thinning must result from increased basal melting caused by warmer Weddell Sea Deep Waters observed at depths greater than the ice-shelf draft.…”

Oceanic and atmospheric forcing of Larsen C Ice-Shelf thinning

“…A spatial correspondence between ice-shelf collapses and mean atmospheric temperature suggests that atmospheric warming may have pushed some ice shelves beyond a thermal limit of viability (Morris and Vaughan, 2003); the northern edge of LCIS is at this limit. Observations of LCIS firn-air thickness confirm that there is sufficient firn air available for compaction, that lower firn air spatially corresponds with higher melting, and that the northward-intensified surface lowering spatially corresponds to areas of high melting and firn compaction (Holland et al, 2011;Trusel et al, 2013;Luckman et al, 2014). Modelled firn compaction entirely offset the lowering in one study of -2008, albeit with a high uncertainty.…”

“…In summary, there is a wealth of circumstantial evidence related to the lowering but no direct test of its origin. In this study we analyse repeated radio-echo sounding surveys of LCIS, applying a novel method to separate changes in ice thickness from changes in firn-air thickness (Holland et al, 2011). The method is presented in Sect.…”

Modeling the instantaneous response of glaciers after the collapse of the Larsen B Ice Shelf

“…Results were cross-checked against data from the 1997-1998 British Antarctic Survey airborne radio echo sounder survey , and a mean difference between the data sets of −0.87 m with a spread of about 1 m was obtained. The ice shelf thickness was computed from the surface elevation using a floatation criterion and a correction for the firn column air content obtained by Holland et al [2011].…”