Abstract
Abstract. Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age–gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the pore space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of interconnected pore space. The low-accumulation sites, with generally coarse grains, close off at lower open porosities (~<10%) than the open porosity (~>10%) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.
Highlights
As an archive for past atmospheric composition, the polar ice sheets play an important role in understanding climate change, both natural and anthropogenic in origin
In this paper, using laboratory measurements on firn cores retrieved from Antarctica, we explore the role of microstructure including pore structure on the nature of layers in deep firn and the range of depths where pore close-off occurs
This study aims to better understand the role firn microstructure plays in controlling gas transport in deep firn and to investigate the validity of using density to predict gas transport in polar firn, including pore closeoff, by comparing firn structure from two Antarctic sites, West Antarctic Ice Sheet (WAIS) Divide and Megadunes, which are both sites that do not experience melt but that have very different local climates
Summary
As an archive for past atmospheric composition, the polar ice sheets play an important role in understanding climate change, both natural and anthropogenic in origin. The surface of the Greenland and Antarctic ice sheets is covered in a 60–120 m-thick layer of firn, multiyear snow that undergoes further metamorphism with depth until it becomes solid ice at the firn–ice transition. Knowing the depth at which air can no longer exchange with the atmosphere is pivotal for determining the gas age–ice age difference. At present-day polar sites, most studies have focused on the bulk properties of the firn, and not until the recent past have studies started to include the layered nature of the firn (Albert et al, 2004; Courville et al, 2010; Freitag et al, 2004; Fujita et al, 2009; Horhold et al, 2011)
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