Guest Post by David Middleton
In my first guest post (CO2: Ice Cores vs. Plant Stomata), we discussed the merits of ice cores vs. plant stomata as paleo-CO2 measurements. One of the key stomata papers I cited was Thomas van Hoof’s “Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis.” Van Hoof and his coauthors demonstrated that the Antarctic ice cores only reflected the low-frequency component of the CO2 “signal”…
It is well known that diffusion processes within the firn layer and the gradual enclosure of the air in the lock-in-zone of the ice lead to a reduced signal of the original atmospheric variability and may obscure high-frequency variations (e.g. Trudinger et al., 2003).
This “diffusion process” is primarily a function of snow accumulation rate. The higher the accumulation rate, the less diffusion and the higher frequency resolution. Compaction effects due to burial can also add to the diffusion process. Dr. Van Hoof’s paper presented strong correlative evidence that the plant stomata chronologies were capable of recording a much higher frequency CO2 signal than the ice cores. I was curious about whether or not any evidence of the diffusion effect on CO2 resolution could be identified in the ice cores through the correlation of CO2 mixing ratios and snow accumulation rates. NOAA’s paleoclimatology library does not include any accumulation rate data for Antarctic ice cores with published CO2 chronologies; but the accumulation rate can by approximated by calculating a sample rate.
I used data from two Antarctic ice cores (Law and Taylor Domes) over most of the Holocene (11 kya to the early 20th century) to compare the sample rate to the CO2 mixing ratio. The sample rates were calculated by simply dividing the sample depth interval by the ice age interval:
sr = [(zn – zn+1) / (tn – tn+1)]
Where sr = sample rate (m/yr), z = sample depth (m) and t = ice age (yr)
I then plotted the CO2 mixing ratio against the sample rate. Not surprisingly, there is an extremely strong correlation between the sample rate and the CO2 mixing ratio throughout the Holocene…
This makes it very clear that the low CO2 values in the Antarctic ice cores during the Holocene could easily be the result of diffusion and do not constitute valid evidence of a stable pre-industrial atmospheric CO2 level of ~275 ppmv.
References
Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. Journal of Geophysical Research 101:4115-4128.
Indermühle A., T.F. Stocker, F. Joos, H. Fischer, H.J. Smith, M. Wahlen, B. Deck, D. Mastroianni, J. Tschumi, T. Blunier, R. Meyer, B. Stauffer, 1999, Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. Nature 398, 121-126.
Van Hoof, T.B., K.A. Kaspers, F. Wagner, R.S.W. van de Wal, W. Kürchner, H. Vissher, 2005. Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis. Tellus (2005), 57B, 351–355.
bubbagyro says
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What this means is that 600 ppm in a bubble in ice with 300 ppm outside will deplete CO2 in the bubble until the outside concentration approaching 300 ppm is asymptotically reached. A concentration of 1200 ppm will diffuse twice as fast as 600, 2400 ppm 4 times as fast, etc. Twice the age, conversely, will have twice the time for diffusion, so old cores will be nearer outside equilibrium level.
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this is relevant but it ignores an important factor: the continuously increasing depth of snow. This increases the diffusion length and makes interchange with the atmosphere slower..
An interesting consequence of the ‘diffusion’ theory is that it could explain/be used to explain why CO2 rises lag temperature increases.
After all, diffusion in the ice core context should theoretically be skewed towards newer layers as opposed to older, more solidly frozen and compacted layers.
OFF-Topic: Hey Michael, are you still celebrating New Years, or are you just testing the moderators?
ON-Topic: Has anyone ever heard of any discoveries of pristine air samples perhaps trapped in a bubble inside an amber sample or some other well-preserved ancient compound? It seems to me that this may actually be possible. I know this is true because I saw it in Jurassic Park.
Another thing I have wondered about is whether anyone has tried to measure CO2 from *new* 5 or 10 year old ice in both the arctic and antarctic. This could be measured by a trusted scientist and then compared to recent CO2 samples from Hawaii. They had better match up or else, Houston, we’ve got a problem.
What do ya’ll think?
The layers aren’t annual per se. The sample rate is a proxy for snow/ice accumulation rate and compaction. These are both rather shallow cores (Talyor Dome 357 m, Law Dome 535 m). So the compaction effects should be very minor relative to the accumulation rates and would likely be very similar in the two cores. The vastly different accumulation rates would dominate the sample rate calculation as it pertains to resolution.
In the DE08 ice core from Law Dome, the depth interval from 83.1 m down to 197.55 m represents 100 years (1839-1939) of ice column. The average sample rate over that interval is 1.14 m/yr.
In the Taylor dome ice core, the comparable depth interval (86.75 m to 185.925 m) represents 2063 years (631 BC to 1560 AD) of ice column. The average sample rate over that interval is 0.05 m/yr.
The sample rate of Law Dome is more than 20 times that of Taylor Dome. Anyone who has aver processed or interpreted seismic reflection data would instantly recognize the implications of the vastly different sampling rates on the resolution differential between the two cores.
While the analysis is very intersting, I have my doubts about the shape of the above results. That is, if this is supposed to be a physical process of diffusion (some form of time-averaging of the CO2 signal).
For some type of averaging process, I’d expect the more heavily averaged signal (blue on the LHS of the above chart) to lie somewhere inside the range of the red ponts.
Instead, the points on the most extreme LHS are below the range on the RHS. And for that to happen, I assume there would need to be a process which removes CO2 from the ice (e.g. by chemical reaction).
The least we could then say would be that the term “diffusion” is confusing.
The CO2 is not removed from the snow. It diffuses throughout a depth interval in the snow as it accumulates and is eventually compacted into firn and then ice. This process is a well established fact.
Van Hoof et al., 2005 demonstrated that the diffusion process acts like a low pass filter on the CO2 “signal.”
I am using the sample rates in two Holocene-aged cores to demonstrate a strong correlation between CO2 mixing ratio and sample rate – A relationship that should exist if the diffusion process is behaving like a low pass filter on the CO2 “signal.”
There are no “N2” or “O2” data sets in the NOAA paleoclimatology library for either of these coring sites. The only results for a search of the “N” or “O2” variables were for cores in Greenland and Bolivia – There is no search option for “N2.” And only one data set actually included mixing ratios of N2 and O2 – GRIP Basal Ice Gas Concentrations.
Nice comments.
Of course, diffusion is both ways. What is important to realize is that equilibrium seeks the level outside. It has been less than 400 ppm for the last hundred years, so anything higher is seeking that low level, even though it was 20 times higher (according to stomata and other proxies) in the past.
Forget about the other gases: Fick’s Law is independent of other gases, only the CO2 matters. David: Fick’s Law is a law. Molecules do not accumulate; on the contrary they become more diluted, if the concentration along the diffusion path is lower.
My only take home message is that CO2 diffuses through ice. It is not trapped. There is no CO2 bubble “time machine”. We could argue all day about how much or how fast. Ice is even more complicated under pressure—it is heterogeneous, comprising faults and strata. It becomes compressed into polymerized structural features in a zig-zag pattern. The Scripps work proves that CO2 diffuses through normal ice, so that no method can measure what the initial amount was. There experiment just governed diffusion, not degassing. If the initial amount were higher than today, it will be measured closer to today’s ambient. If initial amount was lower (although we are at a CO2 starved state right now, plants would all die if we got much lower than 200 ppm, so if it were much lower in the past, we would not be here), then diffusion would be outside in.
And yes, an increase in O2 [ ] from photosynthesis being amplified 20-fold would result, especially at ground level where the increase would be much higher. This increment could positively affect animals metabolism, depending on the local amount.
bubbagyro says
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This is intuitively obvious to me, since the extreme size of dinosaurs 100 Mya required high CO2, parts per thousand, for plants to grow to supply them with food,; said plants could then generate the high oxygen levels the dinosaurs needed to maintain their metabolism at such high levels
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There would never be enough CO2 in the air to allow photosynthesis to [elevate] O2 by a significant amount.
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Yet, the amount of O2 in atmposphere during the early carboniferous period was supposed to be over 30%
http://en.wikipedia.org/wiki/Carboniferous
Should be “Their” experiment in my comment.
It is interesting to consider that a one millimeter diameter “bubble” in ice, if CO2 concentration were, say, one part per thousand (one thousand ppm), contains about a million billion molecules of CO2 (that is, a quadrillion)! Fick’s Law is statistical, and that many molecules racing around inside the bubble feverishly trying to get out should be conceptualized to appreciate how this works. At 250 degrees above absolute zero in ice, they are moving like crazy.
Yes, diffusion is related to mass. CO2 has about 50% more mass than oxygen or nitrogen, so if diffusion coefficients are equal for CO2 and (O2 or N2) [they are not], then diffusion for O2 or N2 will be 50% faster based only on mass. That is irrelevant, because they act independently of CO2 and each other, and the concentration outside for N2 and O2 is the same inside, so there is no net flux for either N2 or O2 for the wee bubble.
Diffusion occurs of course, but the question is how much? Consider the CO2 spikes during interglacials in the ice-cores. Wouldn’t those spikes have been gradually smoothed out farther down the ice-core, or at least show some indication of smoothing from significant diffusion? Graphs show distinct and sharp CO2 spikes during the interglacials all the way back at last 800k yrs.
Michael says
“The markets will front run the collapse. They seem to be starting to do that already. Buy Silver and Gold.”
Sound advice. Note, however, that it is the metal which is necessary. When markets collapse, paper claims to metal will collapse along with all else. Owning real metal is a cumbersome practice, unfortunately.
Dear Dave Middleton
Your formula take no account of the static compression of the overlying ice. It is therefore invalid.
Don’t forget that the density of the snow/firn/ice increases as you go down, so freshly trapped bolus of CO2 will find it easier to diffuse sideways and up. In those directions most of the diffusion is through the gas phase, while downwards, diffusion is increasingly through the liquid and solid phases. Thus, even if the gradient of CO2 concentration is steeper going down, the rate of diffusion in that direction will be reduced compared to what it would have been if snow density was constant in all directions.
beng says:
January 2, 2011 at 9:30 am
Right on.
Key concluding sentence from the paper by Scripps again*:
The good fit of the model to the data supports our proposition that the CO2 originally trapped in the melt layer has diffused through the ice for thousands of years.
*Journal of Glaciology, Vol. 54, No. 187, 2008
CO2 diffusion in polar ice: observations from naturally formed CO2 spikes in the Siple Dome (Antarctica) ice core
Jinho AHN,1,2 Melissa HEADLY,1 Martin WAHLEN,1 Edward J. BROOK,2 Paul A. MAYEWSKI,3 Kendrick C. TAYLOR4
1Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093-0225, USA E-mail: jinhoahn@gmail.com 2Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA 3Climate Change Institute, University of Maine, 303 Bryand Global Sciences Center, Orono, Maine 04469-5790, USA 4Desert Research Institute, University of Nevada, 2215 Raggio Parkway, Reno, Nevada 89512-1095, USA
And JT:
We don’t have enough of a handle on the variables to say “how much”, just simply that diffusion of CO2 happens.
Ah, Dave…I just looked at the Law Dome data ( ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt ) and I notice a little bit of a pattern in time…like the only data above 300 ppm is that dated after 1900 and, in fact, the data is not a scattering of points centered around a higher value…It is simply the march of CO2 levels upward in the last 100 years or so. The plot that you made completely obscures the very strong, steady dependence of the CO2 level on the mean air age in the Law Dome data! Since the Taylor core has such a low accumulation rate, it doesn’t have any data points in the post-industrial era!
All that you have shown here is that if you don’t have enough data points to have ANY in the industrial era then you don’t see the post-industrial rise and if you do have any in the industrial era then you see the industrial rise.
bubbagyro says:
Here’s a copy of that paper, so that we are all on the same page: http://ns.geocraft.com/WVFossils/Reference_Docs/CO2_diffusion_in_polar_ice_2008.pdf Reading that paper does not seem to lead to the conclusions you draw. In particular, they note:
(1) “Smoothing of the CO2 record by diffusion is one to two orders of magnitude smaller than the smoothing by diffusion in the firn at the depth of 287m (gas age = 2.74 kyr BP) in the Siple Dome ice, and so does not degrade the record.”
(2) At depths where the ice age is tens of thousands of years (they use 80,000 years as an example), the diffusion may be comparable to the diffusion in firn and hence it may have “an impact on the smoothing of CO2 records on a decadal scale”. However, they admit that “there are no decadal CO2 data for
ice that is 80 kyr old”. Hence, it would seem that their conclusion about smoothing is, at least at the moment, basically academic.
(3) They also note factors that make the diffusion less of an issue for other ice cores: “Ice cores from colder sites than Siple Dome would experience slower CO2 diffusion in deep ice. The formation of clathrate ice (bubble-free ice) at depths from 500 to 1200m (25–65 kyr) at other Antarctic cold-drilling sites (Vostok, Dome Fuji and EPICA Dome C) is expected to result in highly reduced gas diffusion (Salamatin and others, 1998).”
Perhaps your misreading of the paper stems from the fact that you are reversing how the timescales work. You seem to think their talk about what happens on years to decades mean much more diffusion will happen over longer times. However, what they are doing is looking at 80,000 year old ice and saying, “The amount of diffusion in this ice might be enough for the CO2 to migrate enough to smooth out the CO2 data over a time period of years to a decade or so.” Do you understand the difference?
Not really. That paper concludes that it is at worst of marginal significance (i.e., it might in the worst case cause a comparable smoothing as the other known effects) and at best of no significance.
The equation calculates a sample rate, not the accumulation rate.
From the text of the post:
I can’t differentiate how much of the diffusion is due to accumulation rate and how much is due to compaction.
For this particular exercise, I chose two relatively shallow cores. The compaction effects should be about the same in both cores.
From one of my previous comments:
Can the equation then be used in reverse to estimate what the co2 levels would have actually been once mixing etc… are accounted for? Would be very interesting to see the results, even if only indicative!
bubbagyro says
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What this means is that 600 ppm in a bubble in ice with 300 ppm outside will deplete CO2 in the bubble until the outside concentration approaching 300 ppm is asymptotically reached. A concentration of 1200 ppm will diffuse twice as fast as 600, 2400 ppm 4 times as fast, etc. Twice the age, conversely, will have twice the time for diffusion, so old cores will be nearer outside equilibrium level
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So are you saying that CO2 in deep ice is in equilibrium with the atmosphere?
David Middleton says
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The CO2 is not removed from the snow. It diffuses throughout a depth interval in the snow as it accumulates and is eventually compacted into firn and then ice. This process is a well established fact.
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I don’t doubt the diffusion thing. What I am trying to figure out is your suggestion that the low-pass filter effect is reducing the measured concentration of CO2.
The low-pass filter would reduce both up and down extremes of CO2 concentration and force the extreme values towards a time average of the concentration. The averaging would not bias the concentration to higher values which appears to be what you are suggesting.
So I am guessing that I am not understanding some aspect if you explanation.
Wow, nearly missed this posting, as it was released while the previous discussion of ice cores still is in it’s final state…
To begin with: sample rate indeed has something to do with accumulation, but as the technique of sample analyses evolves, smaller and smaller samples are needed. Thus the relation is not 1:1, but sampling frequency increases for the more recent ice cores. That makes that:
This makes it very clear that the low CO2 values in the Antarctic ice cores during the Holocene could easily be the result of diffusion and do not constitute valid evidence of a stable pre-industrial atmospheric CO2 level of ~275 ppmv.
Is a little premature, as the correlation between sample rate and/or accumulation rate with CO2 levels is mostly spurious (as good as the ice age – gas age “correction” by J.J. Drake is based on a completely spurious correlation).
What happens in the ice core with CO2 during the firn densification was measured in the Law Dome ice cores by Etheridge e.a.:
http://www.agu.org/pubs/crossref/1996/95JD03410.shtml
unfortunately behind a paywall.
Migration of all molecules takes place in all directions, faster in the upper part of the firn, slower with depth, as the pores become smaller. That makes that:
1. the heavier isotopes and heavier molecules are enriched (1-2%) by graviation with depth. That is corrected for, based on the measured 15N/14N enriching.
2. the gas composition at bubble closing depth is a mix of several years. This averaging depends of the diffusion speed and the years necessary to reach the depth where vertical diffusion stops (mostly slightly above closing depth, due to more dense winter layers).
Below the vertical diffusion stop depth, the compositions doesn’t change anymore and air samples in situ taken from the still open pores at bubble closing depth and from the bubbles in the ice cores (via the normal route of relaxation and transport, crushing under vacuum over a cold trap) have the same CO2 level.
In the case of the Law Dome ice cores, the composition of the air at closing depth was only 10 ppmv CO2 below the CO2 level of the atmosphere. As that happened in the period that direct measurements were already done for decades at the South Pole, one could determine that the average age of the air was about 7 years older than at the surface (with a theoretical averaging based on diffusion speed of about a decade). That makes that the top meters of the ice cores and the direct measurements have an overlap of about 20 years. See the overview here:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/law_dome_overlap.jpg
As one can see, the ice cores CO2 simply follows the atmospheric CO2 levels, be it smoothed over several years and is somewhat older (until much older), depending of the accumulation rate.
About diffusion during densification in the firn:
As already said by others, diffusion happens always from the highest level to the lowest level (in fact both ways, but faster from the highest level than reverse), as long as the pores are wide enough. Thus it is impossible to find 290 ppmv in the ice cores, if the atmosphere was higher than that for a period longer than the gas age averaging. Thus if one measures 290 ppmv in an ice core, either that was the average of a quite stable period with 290 ppmv in the atmosphere or it is the average of (large) swings over and below 290 ppmv. Thus any proxy that shows higher or lower levels over a period long enough to be detected in the ice core(s) averaging is biased. That includes historical measurements over land and stomata data…
The (theoretical) smoothing of a peak of CO2, if that happens in a short period, can be seen here for the Law Dome ice core, together with a lot of other interesting items:
http://courses.washington.edu/proxies/GHG.pdf
where does the snow that falls on the ice pack come from ? Rain drops, snap frozen, from near zero will have huge amounts of CO2 inside:-
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html
These data sets are very interesting in thinking about Keeling Ar/N2 ratios. Argon is completely inert and N2, though part of the biotic cycle, is in huge excess.
From the slope of the Ar and N2 solubility curves one can see that the atmospheric ratio of Ar/N2 should change in a way which would give us a real temperature readings. If ocean temperatures were to rise from 0 to 20 degrees the solubility of N2 in water drops to 67% initial , whereas the solubility of Ar would drop to 56% initial. So the ration of Ar/N2 should give you the average temperature of the oceans between 0 and 20 degrees.
This is why I asked about Ar and N2 in ice core data.
As Tim Folkerts already said, the diffusion (theoretically) calculated from migration in the Siple ice core is very small. Here a comment on the same point I have made on the other ice core discussion:
The migration of CO2 in ice cores was theoretically calculated by looking at the migration in remelted layers of a “warm” (-24°C) ice core, Siple Dome:
http://catalogue.nla.gov.au/Record/3773250
The extra smoothing of the gas age averaging caused by migration is about 2 orders of magnitude less than the averaging itself. For the Siple Dome ice core, the average gas resolution is about 22 years. Migration increases that with about 0.2 years.
At lowest depth, the layers become thinner and migration may relatively increase and give a doubling of the resolution to some 40 years.
The Vostok and Dome C ice cores are much older at depth, but also much colder (-40°C), which means less water (no water layer at all at -32°C, as long as no salt inclusions are involved). But there is a simple proof that migration doesn’t play a role at Vostok and Dome C:
The Vostok (and Dome C) ice cores show a quite nice relationship between the temperature proxy (dD and d18O) and CO2 levels, be it with a lag. Nevertheless, the ratio between CO2 and temperature is about 8 ppmv/°C between interglacials and glacial periods over the past 420,000 years (recently confirmed for the full 800,000 years in Dome C). If there was some migration, the ratio would fade over time for each 100,000 years period further back. That is not the case. Thus there is little migration of CO2 in the Vostok and Dome C ice cores…
I’ve wondered how accurate those ice core Co2 levels really are…..
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Microbes can survive ‘deep freeze’ for 100,000 years
Microbes can survive trapped inside ice crystals, under 3 kilometers of snow, for more than 100,000 years, a new study suggests. The study bolsters the case that life may exist on distant, icy worlds in our own solar system
They say a tiny film of liquid water forms spontaneously around the microbe. Oxygen, hydrogen, methane and many other gases will then diffuse to this film from air bubbles nearby, providing the microbe with sufficient food to survive
http://www.newscientist.com/article/dn12752-microbes-can-survive-deep-freeze-for-100000-years.html
fhsiv says:
January 1, 2011 at 6:46 pm
Is the pattern of CO2 concentration versus depth/age the same in other ice cores? Does the CO2 level out at the same concentration? Is this what the warmers are relying on as evidence of a lower and stable pre-industrial CO2 concentration?
There are huge differences with depth, simply because there are huge differences in accumulation rate. Despite that, it is possible to calculate (and measure for the high accumulation ice cores) the average age of the air at closing depth. Gas composition from extreme different ice cores (temperature, salt/dust inclusions) is the same (within 5 ppmv) for the same gas age in overlapping periods. Here for the past 1,000 years:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/antarctic_cores_001kyr.jpg
Other processes play little role in Antarctica (more in Greenland ice cores): some bacteria use CO2 for DNA repair to survive the Vostok temperatures (0.1 ppmv!), clathrate formation plays a role, but clathrates are effectively destroyed under vacuum (at measurement time)…