The CO2 Kink; Firn to Ice Transition

Guest Post by Renee Hannon

Introduction

This post examines CO₂ data collected in Antarctic firn and its journey as firn transitions to ice where CO₂ is eventually trapped in bubbles. Atmospheric gases within the firn and trapped in bubbles are smoothed due to gas mixing processes with depth and time. The bubble trapping zone, also known as the Lock-in-Zone (LIZ), is a mysterious thin interval where CO₂ concentrations decrease significantly with depth creating a kink in CO₂ concentrations.

Properties of Compacting Firn and Transition to Ice

Firn is the intermediate stage of snow transforming into ice and ranges from 50 to over 100 meters thick in the Antarctic. The compaction of snow results in systematic trends of increasing density and decreasing pore space due to the weight of overlying layers with depth. Density increases with depth from a surface value of 340 kg m-3 to the density of ice which is 918 kg m-3. The rate of firn compaction is controlled by snow accumulation and temperature. High accumulation and warmer temperature ice core sites such as Law Dome compact faster than low snow accumulation and colder sites like Vostok. The firn to ice processes are summarized below and Bender, 1997 provides a detailed description of the processes.

Figure 1 is an illustration of Law Dome firn to ice transition and density and porosity properties. The diffusion zone, DZ, where sintering (compaction to a solid without melting) occurs lasts until density reaches about 0.8 g/cm3 and open porosity is about 10%. Open pores dominate as channels allow for gas diffusion and vertical mixing with atmospheric gases to occur.

Figure 1: Antarctic Law Dome firn and ice properties with depth. CO2 measurements in Firn and ice on left hand side (Rubino, 2019). Density, open pores, and closed pores on right hand side (Fourteau, 2020). Cartoon after Raynaud, 1993.

Around 0.8 g/cm3 density the open pores begin to close forming bubbles that trap atmospheric gases. Gases no longer easily diffuse and mix with the atmosphere. This zone is referred to as the Lock-in-Zone or LIZ. The bubble LIZ is defined by a Lock-in-depth, LID, at the top and by a close-off-depth, COD, at the base. The LID shows a rapid increase in closed pores in a distinct step-like fashion. The COD at the LIZ base is defined by the last sample taken in the firn; however, the closed porosity profile shows a gradational base due to pore compression.

Table 1 summarizes the firn diffusion and lock-in-zone properties. The diffusion zone thickness varies across the Antarctic and is related to temperatures and accumulation rates as shown in Figure 2. West Antarctic and peripheral East Antarctic sites have thinner gas diffusion zones than low accumulation sites such as South Pole, Dome C, and Vostok. In contrast, the bubble LIZ is a thin zone at all sites that is approximately 10 meters thick. This significant zone represents a barrier in the firn where vertical diffusion of gases becomes inhibited and gas trapping occurs. Closed pores in higher temperature and accumulation sites show more scatter in the data due to preservation of summer and winter layer properties.

Table 1: Antarctic firn properties summarized for the diffusive zone and the bubble LIZ.

Figure 2: Thickness of the firn diffusion (DZ) and bubble lock-in zones (LIZ) at various Antarctic sites.

Gas mixing in the diffusion zone is well understood and modeled. However, gas movement processes within the LIZ are controversial and not well quantified (Fourteau, 2019). Vertical gas diffusion essentially has ceased due to the dominance of closed pores. However, slower gas movement continues by dispersion and lateral mixing (Bruiner, 2018). Mitchell, 2015 suggests that Darcy’s Law of bulk fluid flow may apply, and others have suggested a percolation theory or eddy diffusion (Buizert, 2012).

A CO₂ Kink is Present with Depth

A kink in CO₂ concentrations with depth is notable in firn gas profiles shown in Figure 3. At the lock-in-depth, CO₂ measurements begin to show a more rapid decrease with depth over about 10 meters. The LID is frequently determined by a change in slope of CO₂, also seen as a “kink”. It is the result of gas moving from the gas diffusive zone and being trapped in bubbles. In the diffusive zone, CO₂ concentrations decrease slowly with depth and gas is aging according to diffusive mixing with shallower gases. Therefore, the gas is younger than the ice age. In the LIZ, bubble enclosure reduces vertical mixing of gases. CO₂ values drop quickly because the gas in the few open pores no longer communicates with the overlying atmosphere. This younger trapped gas is now aging with the ice (Battle, 2011; Trudinger, 2002).

Figure 3: CO₂ concentrations with depth in the firn shown on left graph. Sample dates shown in years on top. Right graph shows CO₂ data normalized to 1996 atmospheric concentrations. LID noted where CO₂ concentrations show a change in slope. Data from Battle, 2011 and Rubino, 2019.

The thickness of the diffusion zone and onset of the LID is variable across the Antarctic as shown in Figures 2 and 3. High accumulation ice core sites generally have a shallower LID than low accumulation sites. The LID at Law Dome DSS site is only 40 meters deep with ice accumulation rates of 16 cm per year. The LID at the South Pole site is 115 meters deep where accumulation rates are only 8 cm ice per year. One exception is the DE08-02 site which has the highest ice accumulation rates of 120 cm per year, however, the LID is deeper than both DSS and WAIS.

CO₂ decreases linearly by an average of 2.3 ppm per 10 meters in the diffusion zone shown in Figure 4. The similar slopes of CO₂ decreasing with depth are not surprising considering vertical gas diffusion occurs throughout this zone.

Figure 4: Linear trends of CO₂ with depth in the diffusion zone shown on the left graph. Linear trends of CO₂ with depth within the LIZ shown on the right graph. The table shows CO₂ ppm decreases per meter for the DZ and LIZ. Data from Battle, 2011 and Rubino, 2019.

However, it is surprising that the slope of decreasing CO₂ within the bubble lock-in-zone between the various sites is similar regardless of temperature and accumulation. The average decrease in CO₂ is 36 ppm over the 10-meter-thick bubble LIZ demonstrated by Figure 4. In theory, gases are now modeled to be aging with ice within the LIZ. This means that CO₂ in the South Pole ages much slower than the CO₂ decrease at DSSW20K and WAIS even though the CO₂ changes in the bubble LIZ are practically identical. Mixing of gas in the bubble LIZ is not well quantified according to Fourteau, 2019. He states that gas within the LIZ may continue to age slightly less rapidly than the surrounding ice due to expulsion of air/gas driven by compaction of the firn.

The CO₂ Kink is Present in Time as well

Figure 5 shows Antarctic Firn CO₂ data both in depth and gas age. Also noted on the gas age graph is whether the CO₂ data is measured from the atmosphere, firn DZ or bubble LIZ. There is overlap when combining different sites. Note the presence of a kink still exists around 1960.

The CO₂ kink in gas age occurs when going from predominately atmospheric and diffusion zone CO₂ measurements to predominately bubble LIZ CO₂ measurements. The bubble LIZ which is only 10 meters thick makes up a significant portion of over 80 years on the gas age graph. Underlying trends shows CO₂ decreases 13 ppm per decade from 2000 to about 1960, and then slows to 3 ppm per decade from 1960 to 1900. Also, note where CO₂ is flat from 1940 to about 1960. Several authors such as Trudinger, 2002, and MacFarling, 2006, discuss that the 1940 peak is reduced due to firn smoothing of gases and the true atmospheric variation is larger.

Figure 5: Firn diffusion zone and LIZ CO2 measurements in depth on the left graph. The firn sites are pinned at the CO2 kink for direct comparison. The x-y axis represents South Pole data. Right graph shows CO2 plotted as gas age according to Rubino,
2019 and Battle, 2011.

The CO₂ kink is unlikely to be a real atmospheric signal. The CO₂ kink in depth is a result of the diffusive mixing of gases in the firn versus trapped gas in bubbles now aging with ice in the LIZ. This CO₂ kink is still apparent in 1960.

Atmospheric CO₂ is Smoothed within the Firn

Many authors have documented gas smoothing in the firn layer due to vertical gas diffusion and gradual bubble close-off during the transition from firn to ice (Trudinger, 2002; Spahni, 2003; MacFarling, 2006; Joos and Spahni, 2008; Ahn, 2012; Fourteau, 2019; Rubino, 2019). Gas concentrations measured during firn densification are an average of atmospheric concentrations that range from 10 years at high accumulation sites like DE08-2 to hundreds of years at low accumulation sites such as Dome C and Vostok. Fourteau shows the measured rate of change of CO₂ in ice bubbles can be three times lower than the actual atmospheric rate of change. Even though firn models can reproduce the measured gas profiles, the gas age distributions can differ substantially according to Buizert, 2012. He discovers that the mean age and distribution width of gases in the firn were found to differ among the models by up to 25% in low accumulation sites.

As atmospheric CO₂ passes from firn to ice, it is altered due to gas mixing processes and compaction as discussed above. Most CO₂ graphs are presented using a simple splice of modern atmospheric CO₂ measurements onto Antarctic ice CO₂ data. The necessary corrections for attenuation of CO₂ in ice due to gas mixing and depth of burial are not applied or even noted. By neglecting these corrections, resulting plots are misleading and amplify the difference between modern and older ice core CO₂ measurements such as this one on Scripps’ website. Beware of the CO₂ Kink, it is not a real atmospheric signal, it is an artifact.

Acknowledgements: Special thanks to Donald Ince and Andy May for reviewing and editing this article.

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