The impact of ice layers on gas transport through firn at the North Greenland Eemian Ice Drilling (NEEM) site, Greenland

Abstract

Abstract. Typically, gas transport through firn is modeled in the context of an idealized firn column. However, in natural firn, imperfections are present, which can alter transport dynamics and therefore reduce the accuracy of reconstructed climate records. For example, ice layers have been found in several firn cores collected in the polar regions. Here, we examined the effects of two ice layers found in a NEEM, Greenland firn core on gas transport through the firn. These ice layers were found to have permeability values of 3.0 and 4.0 × 10 −10 m 2 , and are therefore not impermeable layers. However, the shallower ice layer was found to be significantly less permeable than the surrounding firn, and can therefore retard gas transport. Large closed bubbles were found in the deeper ice layer, which will have an altered gas composition than that expected because they were closed near the surface after the water phase was present. The bubbles in this layer represent 12 % of the expected closed porosity of this firn layer after the firn-ice transition depth is reached, and will therefore bias the future ice core gas record. The permeability and thickness of the ice layers at the North Greenland Eemian Ice Drilling (NEEM) site suggest that they do not disrupt the firn-air concentration profiles and that they do not need to be accounted for in gas transport models at NEEM.

Hydraulic Conductivity of a Firn Aquifer in Southeast Greenland

“…Further, ice layers can be permeable (Keegan et al, 2014). Humphrey et al (2012) describe meltwater bypassing ice layers in the percolation zone, a more similar setting to our work than Keegan et al (2014). Still the general decrease in hydraulic conductivity can be attributed to a gradual increase in density with depth, indicating an increase in ice, which may be more uniformly distributed as opposed to distributed in layers.…”

“…We attribute the difference in permeability across regions to the ice content represented in density profiles of the different locations where average density decreases from mountain glaciers to southeast Greenland firn aquifer to dry firn (Fountain, 1989;Adolph and Albert, 2014;Koenig et al, 2014). Although, Keegan et al (2014) and Adolph and Albert (2014), and Albert et al (2000) report vertical permeability, Keegan et al (2014) note that differences between lateral and vertical permeability are smaller than differences between vertical permeability of different layers (see Luciano and Albert, 2002), and are therefore adequate for a general comparison. Also, the horizontal and vertical permeability are within the same order of magnitude.…”

“…We can also compare permeability, which is only a function of the porous media, to sites where no aquifer exists. Permeability increases from mountain glaciers (∼10 −12 m 2 ) to the southeast Greenland firn aquifer (∼10 −12 to 10 −10 m 2 ) to dry firn (∼10 −10 m 2 ) (Oerter and Moser, 1982;Oerter et al, 1983;Fountain, 1989;Fountain and Walder, 1998;Schneider, 1999;Albert et al, 2000;Luciano and Albert, 2002;Adolph and Albert, 2014;Keegan Asterisk indicates estimates likely subject to turbulent flow effects caused by pumping.…”

“…This is likely because the horizontal flow induced by the slug test is controlled by the firn with the highest hydraulic conductivity within the screened interval of the piezometer. Further, ice layers can be permeable (Keegan et al, 2014). Humphrey et al (2012) describe meltwater bypassing ice layers in the percolation zone, a more similar setting to our work than Keegan et al (2014).…”

“…Hydraulic conductivity depends on properties of both the porous media (grain size, shape, distribution, and packing) and the fluid (viscosity and density). Firn permeability, which only depends on porous media properties, has been measured at various sites across Greenland using permeameters (Albert and Shultz, 2002;Adolph and Albert, 2014;Keegan et al, 2014) and Antarctica (Albert et al, 2000(Albert et al, , 2004. Hydraulic conductivity (K) is related to permeability (k) as…”