Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements

Metamorphism of stratified firn at Dome Fuji, Antarctica: A mechanism for local insolation modulation of gas transport conditions during bubble close off

“…For example, overburden pressures were estimated from the density profile in Figure 3a and they are higher than the atmospheric pressure by 0.45 MPa and 0.65 MPa at z s = 78 m and z c = 104 m, respectively. It has been presumed that the differences in pressure between closing/ closed air bubbles and air in open pores drive two processes: effusion of air from closing bubbles to open pores through a small opening [Bender, 2002] and permeation of gas molecules from closed bubbles to open pores through the ice lattice [Huber et al, 2006;Ikeda-Fukazawa et al, 2005;Severinghaus and Battle, 2006]. The former process, Dp-driven effusion, was proposed by Bender [2002] as a mechanism for gas fractionation.…”

“…In contrast, the latter process, Dp-driven permeation of gas molecules, causes both fractionation of gas molecules and a decrease in the air content of bubbles. Size-dependent fractionation occurs due to the preferential exclusion of small gases from shrinking bubbles by molecular diffusion through the ice lattice [Huber et al, 2006;Ikeda-Fukazawa et al, 2005;Severinghaus and Battle, 2006]. Thus, both accumulation of O 2 -enriched gas in the residual firn air and depletion of O 2 /N 2 ratio in air bubbles occur in the bubble formation zone.…”

“…: METAMORPHISM OF FIRN AT DOME FUJI of 10 1 years due to the upward diffusive flux of enriched firn gas through channels of open pores [Severinghaus and Battle, 2006;Ishijima et al, 2007;Kawamura et al, 2006;Sugawara et al, 2003] at a depth where the QMP peaks are preserved (Figure 9a). At both Vostok [Bender et al, 1994] and Dome Fuji (K. Kawamura, personal communication, 2008), the O 2 /N 2 ratio near the bottom of firn is higher than present atmosphere only by $0.3%, much smaller than at sites experiencing the lock-in effect (typically several %) [e.g., Huber et al, 2006;Severinghaus and Battle, 2006], suggesting that upward diffusive flux of O 2 -enriched gas is sufficiently large to allow escape through the surface.…”

“…This size-dependent fractionation occurs in response to the preferential exclusion of small gases from shrinking bubbles to the residual firn air (open pores) by molecular diffusion through the ice lattice driven by pressure differences [Huber et al, 2006;IkedaFukazawa et al, 2005;Severinghaus and Battle, 2006]. Fractionation of the O 2 /N 2 ratio also occurs between bubbles and clathrate hydrate crystals within the ice sheet [Bender et al, 1995;Ikeda-Fukazawa et al, 2001;Ikeda et al, 1999] and between clathrate hydrate crystals and the atmosphere during the storage of ice cores after drilling [Ikeda-Fukazawa et al, 2004;Ikeda-Fukazawa et al, 2005].…”

“…[3] During bubble close off near the bottom of the firn, firn air is progressively enriched with relatively small atoms and molecules such as He, Ne, O 2 , and Ar relative to N 2 with time (and depth), whereas larger gases such as Kr, CH 4 , CO 2 , and Xe are not fractionated [Huber et al, 2006;Severinghaus and Battle, 2006]. This size-dependent fractionation occurs in response to the preferential exclusion of small gases from shrinking bubbles to the residual firn air (open pores) by molecular diffusion through the ice lattice driven by pressure differences [Huber et al, 2006;IkedaFukazawa et al, 2005;Severinghaus and Battle, 2006].…”