Gases in ice cores

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 glacial͞interglacial (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 CO 2 and CH 4 over the last 200 yr. Finally, we summarize concentration-time records for CO 2 and CH 4 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.

Microbial metabolism directly affects trace gases in (sub) polar snowpacks

“…In addition, the distance between the chambers and tarp edge reduced the impact of wind-driven horizontal transport and mixing of atmospheric air with pore spaces in the snowpack [1,34].…”

“…As more snow is deposited onto the surface of the snowpack, older snow layers compress eventually into ice, encasing small samples of the atmosphere existing over and within the snow at the time of deposition. This simple mechanism of glacial formation was described in the 1990s [1], and has been presented as a justification to use greenhouse gases (CO 2 , CH 4 ) entrapped in glacial ice as a proxy for atmospheric compositions (and hence, climate conditions) back in time. This same logic has been used to justify the quantification of shorter-lived, more reactive trace gases in ice cores including methyl bromide [2] and methyl chloride [3,4].…”

“…Concurrently, snow is readily transparent to a range of UV-visible light spectra, which is known to drive substantial photochemical reactions, including methyl halide production [35]. The quasi-liquid layer on the surface of snow particles incorporates complex chemical reactions and provides limited habitat for microbial life [31], while seasonality drives snow-pack thinning and expansion [1], and longer time frames lead to compression, consolidation and removal from atmospheric influence [1]. Nearby and sub-snowpack soils can also influence snowpack air chemistry through diffusion/ advection from local biological sources/sinks with access to more favourable environments [36,37].…”

Constraining variable density of ice shelves using wide-angle radar measurements

“…Knowledge of the depth-density profile and its spatial and temporal variability is important for a number of applications: (i) to determine the age difference of enclosed air bubbles and the surrounding ice in ice cores (Bender et al, 1997); (ii) to determine the depth and the cumulative mass above radar reflectors in order to map surface mass balance with radar (Waddington et al, 2007;Eisen et al, 2008); (iii) to interpret the seasonality of surface elevation changes (Zwally and Jun, 2002;Ligtenberg et al, 2014) in terms of surface mass balance, firn compaction, and dynamic thin-…”

Investigation of a deep ice core from the Elbrus western plateau, the Caucasus, Russia

“…5), close to the critical density across the whole depth interval, is unlikely to be a systematic error in measurements and needs further investigation. Future research should account for the ice flow characterization and the possible effects related to the "intervening depth interval" (the alternation of layers which have already reached the close-off density, with layers that are still permeable) due to seasonal (Bender et al, 1997) or wind-induced (due to seasonal differences in wind speeds) snow density variability at high accumulation sites. Unlike polar ice cores where the "intervening depth interval" is just a fraction of the whole length of the ice core (Bender et al, 1997), the measured bulk density in the Elbrus ice core spans a wide interval between 800 and 915 kg m −3 towards the bottom of the glacier (Fig.…”