Michael Bender Todd Sowers and Edward Brook by The National Academy of Sciences

PNAS is available online at http://www.pnas.org.

ABSTRACT Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic (last 200 yr) to glacial/interglacial (hundreds of thousands of years). In this paper, we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere. We then summarize concentration-time records for CO2 and CH4 over the last 200 yr. Finally, we summarize concentration-time records for CO2 and CH4 during the last two glacial-interglacial cycles, and their relation to records of global climate change.

Ice crystals near the surface of a glacier are compressed by the continual addition of snow at the surface. As the ice crystals travel down into the glacier, they grow and reorient themselves into a closer packing. Density rises, open porosity decreases, and by some depth between 40 and 120 m, the crystals are sintered together into an impermeable mass ("glacial ice") in which about 10% of the volume is composed of isolated bubbles. The gas trapped in these bubbles is close in composition to contemporaneous air and allows us to reconstruct changes in atmospheric chemistry on three important time scales: the last 200 years (relating to anthropogenic change), the Holocene (last 10,000 years), and the last several hundred thousand years (relating to glacial-interglacial cycles). In this paper we discuss the gas trapping process and summarize data on the changes in the greenhouse gas concentrations of air over the three time scales of interest.

Physics of Gases in Glaciers

The density of ice at the surface of an ice sheet is typically 0.3-0.35 g cm-3; the corresponding porosity is 62-67%. Settling and packing cause the density to rise rapidly to about 0.55 g cm-3 by a depth of 10-30 m. Below, recrystallization and other processes drive a somewhat slower increase in density, which continues until individual crystals are fused together into an impermeable mass of glacial ice (1). At the "bubble closeoff depth," about 10-15% of the volume is air, and the density is about 0.81-0.84 g cm-3 (2, 3). The "firn" is the zone of porous snow and ice above the closeoff depth, and the depth interval in which bubbles close is termed the "firn-ice transition." Below the transition, densification continues by the compression of bubbles due to hydrostatic pressure.

When the snow accumulation rate at the surface of an ice sheet is greater than about 4 cm yr-1 (expressed as the ice-equivalent thickness of annual layers), discrete seasonal layers of snow are preserved that have characteristic physical and chemical properties. Wintertime layers initially have higher densities than summertime layers. This density contrast is maintained during the densification process (Fig. 1). The firn-ice transition occurs at the same density for wintertime and summertime layers, but wintertime layers attain the closeoff density at a shallower depth. Consequently, there is an interval of about 10 m in which wintertime layers are more extensively sealed than summertime layers. In this interval, permeable and impermeable layers alternate in the ice sheet (3).

There are three regimes of gas transport in the firn (4, 5.) The uppermost layer, which appears to extend down to about 10-m depth at some sites where firn air has been sampled and analyzed, is affected by convective mixing driven by surface wind stress. Underlying the convective zone is the "stagnant air column," in which transport is by molecular and atomic diffusion only. Diffusivities of gases are typically about 1 m2 day-1 at 10-m depth. Below, they decrease with increasing density (5), due to a combination of lower porosity and higher tortuosity (the latter factor accounts for the extra distances gas atoms and molecules must travel as they wind their way through the ice crystals to move from one depth to another).

The diffusivity of the firn is such that air at the base of the stagnant column today has a "Co2 age" ranging from about 6 yr for the GISP2 core (central Greenland) to about 40 years at Vostok (East Antarctica). In point of fact, however, air in firn at a given depth is not of a single age. The composition of firn air is convoluted by a number of processes. Air in the convective zone responds instantaneously to changes in atmospheric chemistry. These changes then propagate down into the stagnant column by molecular or atomic diffusion. Schwander et al. (6) calculated that the age of maximum abundance at the base of the firn is about 0.65-zt2/D, where zt is the height of the stagnant column and D is the free air diffusivity of the gas at the ambient temperature and pressure of the site. A small fraction of the gas is younger, and there is a long tail to older ages.

The diffusivity of an element or compound decreases with increasing mass and increasing atomic or molecular diameter. Thus each element or compound diffuses at a different rate, and each isotope of a compound diffuses at a different rate. In consequence, the covariation between the composition of one gas and another (e.g., CO2 and CH4) in firn is different from their historical covariation in air. The isotopic composition of a gas (e.g., CO2) in firn air also varies with the concentration of that gas in a way that is different from the historical relationship. The concentrations of gases and isotopes that diffuse most rapidly will be closest to their current atmospheric concentrations. Because light isotopes diffuse more rapidly, the concentration of a gas in firn air will be more depleted in heavy isotopes than was the atmosphere at the time it had the same concentration as a firn air sample. Differential diffusivity is a first-order effect that must be taken into account when interpreting data on the concentration and isotopic composition of gases in firn air and ice cores (7).

Abbreviation: kyr, thousands of years.

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