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.
Geoff Sherrington says:
January 3, 2011 at 2:55 am
More for Geoff:
Re your van Hoof reference, can you point me please to a calibration with instrumentation (such as thermometry) and stomata properties?
and
So, we have some circular argument from where you point us. Using ice bubbles to calibrate stomata to calibrate ice bubbles?
I have no direct references about the influence of temperature, drought, etc… on stomata index level. I suppose these have their influences. My objections against stomata data is that the CO2 levels of which they are based on are not background CO2 levels, more variable and positively biased with unknown changes in local/regional variability and bias over time.
And as stomata are proxies, not direct data, they need to be calibrated: to direct atmospheric measurements when available, or ice cores if not. Not reverse: stomata data accuracy is quite rough (+/- 10 ppmv), compared to e.g. the Law Dome (10-year filtered data) accuracy of +/- 1.2 ppmv. And as you have referenced, they have a limited range: above and below that range, there is no effect anymore. Comparable to the limited response of tree rings to temperature changes…
So, I am not defending the stomata data: they give a good high frequency first approximation, but the ice core data are far more reliable, because that are direct measurements of background CO2 levels, be it smoothed over longer time periods: from a decade to 600 years for the youngest to oldest cores.
latitude says:
January 3, 2011 at 6:38 am
That’s the problem Lazy, you can’t calculate it.
Ice cores are full of microbes/bacteria that convert CO2 to methane.
That will lower CO2 levels, and it explains the reason the methane levels are elevated way above atmospheric methane levels – at least 10 times above.
When you look at the biological component of ice cores, it makes the whole science of trying to reconstruct CO2 levels from ice cores worthless
Latitude, you are confusing between mid-latitude (warm) ice cores and the (very) cold ice cores like most in Antarctica. The CH4 levels from the Vostok (and other) ice core(s) are available and show the same (even more spectacular) HS curve as CO2 and N2O: all since the start of the (agricultural and) industrial revolution. See:
http://www.pnas.org/content/94/16/8343.full
and
http://zipcodezoo.com/Trends/Trends%20in%20Atmospheric%20Methane.asp
Hu McCulloch says:
January 3, 2011 at 6:51 am
Nice to see you here (it is a bit calm at CA these days, I can imagine it that it takes a lot of one’s life to maintain a blog like this one or CA…).
I do agree with most of what you said, but some comment on:
Bubbagyro (1/1 @ur momisugly 6:56 AM) makes the very interesting point that CO2 may diffuse through ice.
and
Even if the net absorption/outgassing is small, this diffusion will continue to smooth the CO2 record long after the firn bubbles are sealed, and hence will make the ice core CO2 record look excessively flat.
The article shows that the total migration is lengthening the averaging of the CO2 levels by the “warm” Siple ice core from 22 years to 22.2 years at medium depth, up to a doubling (40 years) at deepest ice. The much colder Vostok ice core doesn’t show any flattening of the ratio between (gas phase) CO2 vs. the (ice phase) temperature proxies over each 100,000 interglacial/glacial transition. Thus there was no measurable CO2 migration.
Ferdinand Engelbeen says:
January 3, 2011 at 7:30 am
Latitude, you are confusing between mid-latitude (warm) ice cores and the (very) cold ice cores like most in Antarctica.
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No I’m not….
“Microbes” have been found in all depths, in all ice cores.
I know biology can get in the way of all these neat little formulas, and the tendency is to ignore it when you can’t explain it.
But it doesn’t change the truth.
Until someone can explain how many bacteria/microbes are present or even explain what they are doing, account for every temperature change and qualify their rate of metabolism at each temperature because ice does change temperature………….
Biology throws a monkey wrench into ice core temperature reconstructions, and until biology is explained, those reconstructions are worthless.
I’m thinking that in the absence of major vulcanism caused by colliding continental plates to replenish the CO2, the biosphere slowly slurps the CO2 up, converting it into fossil fuels and limestone. Eventually (as during the Carboniferous and Neogene), CO2 gets so low that the globe is vulnerable to ice ages. Perhaps restoring some of that fossil CO2 to the atmosphere (up to 500-1000 ppm) would prolong the Holocene indefinitely?
Just a suggestion, but no crazier than CAGW…
Hu McCulloch says:
January 3, 2011 at 9:09 am
Eventually (as during the Carboniferous and Neogene), CO2 gets so low that the globe is vulnerable to ice ages
===============================================
Hu, I don’t think there’s any correlation between CO2 and ice ages other than CO2 follows temperature changes. Which makes perfect sense. When temps warm metabolisms speed up, when temps fall metabolisms slow down. We forget that bacteria/microbes rule this planet.
I agree with you 100% on the CO2/carbon connection. The planet seems to need that carbon and is very good at using it.
If you consider that everything jump started when CO2 levels were high, temperatures went all over the place anyway, and CO2 levels have been consistently falling ever since.
The only real hockey stick is not Hansens, it’s the one that shows CO2 levels falling.
“”””” Joel Shore says:
January 1, 2011 at 7:48 pm
Ah, Dave…I just looked at the Law Dome data “””””
Joel, if there did exist such an out diffusion process; as has been postulated here, and evidently confirmed at Scripps, would not that lead to a situation where the very newest core samples would be expected to be most like the conditions at formation; and older samples would be expected to gradually fall further back from the formation conditions.
So the signature for such an effect would be a rising CO2 (sample) record for the last century or even a couple of centuries.
So your argument that the rising data in the cores since 1900 represents the rising atmospheric levels (which we know confidently from Mauna Loa only since 1957/8) , is not really any more robust than the model where the trapped gases simply out diffuse per Fick’s Law.
As an aside; in the early days of “Sampling Oscilloscope” technology; that being basically the early 1960s for commercially available instruments, it was believed that the rise time of the “scope” was limited by the length of the sampling pulse; so designers (at Tektronix, and Hewlett Packard) spent time trying to make ever shorter sampling gate drive pulses; using Tunnel Diodes, and the like. These efforts were hindered by the ability to make stable very low capacitance sample storage capacitors, so only small amounts of charge were collected during the short sampling pulse; so the early instruments were quite noisy; the noise being of the order of the square root of the number of electrons collected. (Shot Noise )
In the later 1960s it was discovered that in fact the rise time (inverse bandwidth) was not set by the width of the sampling pulse; but was instead determined by the time taken to switch the sampling gate off; in other words the fall time of the sampling gate drive signal. Short shutoff times were generally easier to achieve, than short pulse lengths.
This is pertinent to the ice core record; as the same phenomenon is in effect. The time taken to capture the atmospheric gas sample is not particularly significant; but the gate shutoff time is; that is the period where the air bubbles and the snow sample is being compacted to a degree, where the sample can be said to be truly encapsulated. And that period is evidently decades to perhaps centuries.
So of course fast spikes to higher CO2 levels are simply not going to be recorded, when it takes so long to close the barn door.
I’m generally skeptical of the value of the absolute levels of CO2 or Oxygen in the ice cores. The O2 in the ice is of no interest, since nobody knows when that H2O molecule was formed; it could be billions of years old; so only the O2 in the atmospheric samples has any proxy value.
But I’m not too concerned about the problem; because I don’t think CO2 is in much of a controlling position, as regards the global mean Temperature; compared to the negative feedback cooling effect due to clouds.
George E Smith says:
First of all, what the Scripps paper shows is that diffusion occurs but the rate is such that under perhaps the most extreme conditions (ice core in a warm place and the oldest part of the core) it could almost be comparable to other smoothing effects and otherwise it is much smaller. Second, it is not an “out diffusion” process…It is simply diffusing from higher concentration to lower concentration. Why would this result in a lowering of the CO2 in the core?
Again, I see no reason why the diffusion should work preferentially in one direction. What it would do, if it does anything on any length scale that matters, is smooth things. And, there is no evidence that it does that.
The supposed evidence presented in this post falls apart completely once one realizes that one isn’t just comparing different sampling rates but different time periods; the higher concentrations for the Law Dome core are simply due to sampling after the Industrial Age buildup began. Remove that data and the effect disappears. This is a classic case of correlation not meaning causation. David M.’s plot basically drops a lot of relevant information (the dependence of the measured CO2 levels on the time period that the core is measuring) and so detects an entirely spurious correlation between CO2 levels and sampling rate that is really just due to the dependence of CO2 levels on time.
It would be just as meaningful as if I took a pot of room temperature water and sat it out on the counter and measured its temperature every 10 minutes. Then, I put it on the stove and start measuring its temperature every minute. I plot the data like David has and say, “Look…The measured temperature seems to depend on the sampling rate because the temperature is higher on average when I measure every minute than when I measured every ten minutes!”
latitude says:
January 3, 2011 at 8:48 am
No I’m not….
“Microbes” have been found in all depths, in all ice cores.
Indeed, but at some depths more than other and some cores more than others. From the link you have read:
Sowers (10) reported peaks in the concentrations of N2O and d15N of N2O and dips in the values of d18O of N2O occurring at the same depths in Vostok ice where Abyzov et al. (1) found peaks in bacterial concentration and where Petit et al. (48) found peaks in dust concentration. The ice temperature at that depth is -40°C. The remarkable correlation strongly suggests that, during the penultimate glacial maximum 140,000 years ago, dust and microorganisms were windborne with greater than normal efficiency and deposited onto Antarctic ice.
The effect of this peak in dust and bacteria is a deficit of 0.1 ppmv CO2 over 140,000 years. If we may assume that the number of bacteria is directly related to dust deposits in the ice core, then the effect over time for the deepest useable part at a glacial maximum in the core is not more than 0.3 ppmv. Hardly a reason to reject the data.
The temperature data from the Vostok borehole should be here:
http://www.nature.com/nature/journal/v381/n6584/abs/381684a0.html
but I don’t want to pay the scandalous sum they ask for an over 10 years old article. An average of -40°C is often said, with the not used ultimate bottom layers above Lake Vostok at higher temperature.
Further, Antarctic coastal ice cores at higher temperature and far more dust/bacteria/salt/algue inclusions show very similar CO2, CH4, N2O,… levels for the same gas age. Thus not much influence from the microbes…
Ferdinand Engelbeen says:
January 3, 2011 at 11:17 am
Further, Antarctic coastal ice cores at higher temperature and far more dust/bacteria/salt/algue inclusions show very similar CO2, CH4, N2O,… levels for the same gas age. Thus not much influence from the microbes…
Ferdinand, thats interesting, because dust was the reason given for the redrawing of the Greenland ice cores so that they matched the Antarctic ice cores…
http://tallbloke.wordpress.com/2010/12/28/tom-van-hoof-historical-co2-records/
Any idea why that might be?
George E. Smith says:
January 3, 2011 at 10:04 am
Joel, if there did exist such an out diffusion process; as has been postulated here, and evidently confirmed at Scripps, would not that lead to a situation where the very newest core samples would be expected to be most like the conditions at formation; and older samples would be expected to gradually fall further back from the formation conditions.
So the signature for such an effect would be a rising CO2 (sample) record for the last century or even a couple of centuries.
Sorry George, but Joel is right: Fick’s Law and diffusion works only from high to low. The only possibility for the opposite flow is by reverse osmosis, which needs a lot of pressure difference (and the right porosity) which doesn’t exist between ice layers at 70 and 71 meter or 3000 and 3001 meter. Thus forget “out” diffusion. It doesn’t exist.
Moreover, if there was out diffusion, that would work for every gas, even better for the smaller ones: methane, Ar, O2, N2… than for CO2. Thus enriching the CO2 levels.
Further, I can only agree with Joel that the sampling rate and CO2 levels have nothing to do with each other: The oldest Siple Dome samples (Neftel, 1992) used two sampling methods: 3 samples, each covering 220 years (!) and a “newer” method, with 12 samples each covering 11-12 years. If you average the higher resolution samples, you will find the same CO2 levels for the same average gas age. See Neftel e.a.:
http://www.biokurs.de/treibhaus/180CO2/neftel82-85.pdf
Further the 3 youngest gas age samples overlap with the direct measurements in the atmosphere and all samples overlap with the Law Dome ice cores samplings, which have a sampling rate of 2-5 years in the overlapping years with similar CO2 levels…
Thus George (and David) there is no Sample Rate Problem, only a problem of a spurious correlation…
Ferdinand, you keep quoting that one paper like it’s the only game in town.
It’s not.
There’s a lot more out there that says bacteria are active in ice, that bacteria tend to be equally distributed in ice cores no matter where they are collected, etc
You can not draw a correlation between wind blown dust and bacterial populations simply because bacteria can reproduce in ice. Bacterial populations will flux because of that. The only correlation that can be assumed is that bacteria are attracted to the layers of dust because they are using that as food.
There is nothing else that can explain the high methane levels in ice cores.
And the simplest explanation is usually the right one.
Bacteria can live and grow in ice, bacteria will use CO2 and create methane.
tallbloke says:
January 3, 2011 at 11:48 am
Ferdinand, thats interesting, because dust was the reason given for the redrawing of the Greenland ice cores so that they matched the Antarctic ice cores…
http://tallbloke.wordpress.com/2010/12/28/tom-van-hoof-historical-co2-records/
Any idea why that might be?
The difference is in the type of dust: dust in Antarctica is mainly sea salt (composed of mostly NaCl but also carbonate and other salts), far more at coastal sites than at altitude far more inland. During glacials, there is far less water vapour/clouds/rain/snow and dust from far away (even sand) can easier reach the inland ice cores. The highest dust levels in the Vostok ice core are measured just before a new interglacial, when temperatures are at its lowest.
In the Greenland ice cores we have the same points (but in general a one order higher amount of dust than in coastal Antarctic cores) + an additional source: frequent volcanic eruptions from Iceland volcanoes. Iceland is at a hot spot and volanic ash is from the deep earth crust, quite toxic and very acidic (HCl, HF). When that is deposited in the layers of the Greenland ice core, that reacts over time with the sea carbonates and produces extra CO2 in situ…
Some references:
http://cat.inist.fr/?aModele=afficheN&cpsidt=3634884
http://europa.agu.org/?view=article&uri=/journals/jc/97JC00163.xml
latitude says:
January 3, 2011 at 12:35 pm
Ferdinand, you keep quoting that one paper like it’s the only game in town.
It’s not.
That one paper is a very good overview of the existing knowledge about bacterial life in ice cores.
There’s a lot more out there that says bacteria are active in ice, that bacteria tend to be equally distributed in ice cores no matter where they are collected, etc
That is certainly not true for the Vostok ice core.
You can not draw a correlation between wind blown dust and bacterial populations simply because bacteria can reproduce in ice. Bacterial populations will flux because of that. The only correlation that can be assumed is that bacteria are attracted to the layers of dust because they are using that as food.
Which means that the population around dust is (far) higher than at other places.
There is nothing else that can explain the high methane levels in ice cores.
And the simplest explanation is usually the right one.
I don’t know which ice core you mean: the methane levels in all Antarctic ice cores are very low: between 300 and 700 ppbv during glacials to interglacials. Since about 6,000 years ago that increased to 800 ppbv until about 1850. Since then the levels increased to currently 1900 ppbv in the atmosphere.
“”””” Ferdinand Engelbeen says:
January 3, 2011 at 11:56 am
George E. Smith says:
January 3, 2011 at 10:04 am
Joel, if there did exist such an out diffusion process; as has been postulated here, and evidently confirmed at Scripps, would not that lead to a situation where the very newest core samples would be expected to be most like the conditions at formation; and older samples would be expected to gradually fall further back from the formation conditions.
So the signature for such an effect would be a rising CO2 (sample) record for the last century or even a couple of centuries.
Sorry George, but Joel is right: Fick’s Law and diffusion works only from high to low. The only possibility for the opposite flow is by reverse osmosis, which needs a lot of pressure difference (and the right porosity) which doesn’t exist between ice layers at 70 and 71 meter or 3000 and 3001 meter. Thus forget “out” diffusion. It doesn’t exist. “””””
So you don’t like my usage of terms; so let me redefine; For OUT diffusion read IN diffusion; as in CO2 diffusing INTO THE ICE; which just co-incidently is diffusion OUT of the trapped air samples, INTO the cie walls around the air bubble; where rumor has it the CO2 abundance is much lower than in the air; hence the reason why Fick’s Law would be applicable; and my comment stands.
NO I WAS NOT MEANING DIFFUSION OF CO2 FROM THE TRAPPED POCKETS OUT INTO THE AMBIENT ATMOSPHERE.
“”””” Joel Shore says:
January 3, 2011 at 10:45 am
George E Smith says:
Joel, if there did exist such an out diffusion process; as has been postulated here, and evidently confirmed at Scripps, would not that lead to a situation where the very newest core samples would be expected to be most like the conditions at formation; and older samples would be expected to gradually fall further back from the formation conditions.
First of all, what the Scripps paper shows is that diffusion occurs but the rate is such that under perhaps the most extreme conditions (ice core in a warm place and the oldest part of the core) it could almost be comparable to other smoothing effects and otherwise it is much smaller. Second, it is not an “out diffusion” process…It is simply diffusing from higher concentration to lower concentration. Why would this result in a lowering of the CO2 in the core?
So the signature for such an effect would be a rising CO2 (sample) record for the last century or even a couple of centuries.
So your argument that the rising data in the cores since 1900 represents the rising atmospheric levels (which we know confidently from Mauna Loa only since 1957/8) , is not really any more robust than the model where the trapped gases simply out diffuse per Fick’s Law.
Again, I see no reason why the diffusion should work preferentially in one direction. What it would do, if it does anything on any length scale that matters, is smooth things. And, there is no evidence that it does that. “””””
Joel, see my response to Ferdinand. I’m quite prepared to accept that Fick’s Law deals with diffusion from a place of high concentration to a place of lower concentration. Have I told you of my past life of diffusing ZINC into GaAs0.6P0.4 in sealed ampoule. When using ZnAs2 as the zinc source, the diffusion is rapid and the doping level achieved is high.; but when a GaZn alloy source is used the diffusion is very slow, and much lower doping density is achieved in deeper layers. So I do understand Fick’s Law.
And when trapped air samples are entombed in a coffin of ice; which compressed from snow; that ice is depleted of CO2 by the segregation coefficient for CO2 at the freezing interface between liquid water and solid ice.
So the diffusion of CO2 OUT from the trapped CO2 containing air INTO the CO2 depleted Ice is driven by the concentration gradient from air into ice, and depends on the activation energy for whatever the ambient (ice) Temperature is.
I did NOT say the CO2 diffuses OUT to the ambient air (which conceivably could be of higher CO2 abundance.)
You have to read what I write; not what you think I wrote or meant. I write what I mean; except when I make a misteak.
George E. Smith says:
January 3, 2011 at 1:51 pm
So you don’t like my usage of terms; so let me redefine; For OUT diffusion read IN diffusion; as in CO2 diffusing INTO THE ICE; which just co-incidently is diffusion OUT of the trapped air samples, INTO the cie walls around the air bubble; where rumor has it the CO2 abundance is much lower than in the air; hence the reason why Fick’s Law would be applicable; and my comment stands.
Quite confusing, your use of IN and OUT, I must say…
Well, let’s take it that way:
– I don’t see any reason why the migration into the intercrystalline waterlike layer would start when all bubbles are closed, it would start at the very moment that the first snowflakes are sintering together (as far as it happens). Or if you really mean the ice itself, then already when the snowflakes were formed.
– The ice is itself is not a matrix that allows CO2 to be included. CO2 in many cases is measured in dry air by trapping water vapor over a cold trap: no measurable amount of CO2 is trapped in the matrix (we are not talking here about nanograms of dope!).
– three methods were/are used to measure CO2 from ice cores: the old method was by melting all ice and applying vacuum to remove CO2 from the water phase. That did show a deficit of a few ppmv compared with the usual method: crushing the ice at low temperature under vacuum and measuring CO2 in the air of the opened bubbles, after removing water vapour over a cold trap.
The third method is sublimating all ice under vacuum and cryogenic separation of all components. That gives the same result as for the second method in total CO2, but is used for isotope research, to avoid fractionation.
Thus even if there was some migration into the (intercrystalline) ice structure, it is unmeasurable, as the comparison between the three methods shows. By far not enough to explain the disappearance of a quarter of the total amount out of the bubbles.
Ferdinand says: That is certainly not true for the Vostok ice core.
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wrong
Almost all of the common bacteria that occupy permanently cold environments have been isolated from Vostok ice cores. Most of that work was with Vostok ice cores exclusively.
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Ferdinand says:Which means that the population around dust is (far) higher than at other places.
=================================================
Which means that bacteria have mobility in ice.
Which also means that you’re not looking at one type of bacteria that only feeds on one food.
The ice you’re looking at was not always that deep and not always that cold.
What happened to it before you got to it?
===================================================
Ferdinand says:I don’t know which ice core you mean:
===================================================
I’m talking about the ice cores that have elevated methane levels, the ones closest to the surface.
Where the ice would be exposed to temp changes, seep, etc
Where bacteria would be more able to produce methane.
My concerns about Ice cores and bacteria are this.
The oldest, deepest, ice cores show the highest CO2 levels.
As ice cores get newer, they are closer to the surface, and show lower CO2 levels.
That is exactly what goes on in another liquid environment that uses bacteria, marine sediments. Methane is produced at the aerobic/anaerobic interface.
Stepping back and looking at ice cores, not from atmospheric levels being captured, but from a purely biological stand, it can all be explained biologically.
David, I fear you have fallen into the same trap as another poster (where, I can’t recall). The signal you are finding is real. However, it is due to an unseen influence.
We all know that it takes a while for the firn to close off entirely. How long? At high-snow sites, it’s not long, decades to a century or so. At Vostok, which is a low-moisture “frozen desert”, it takes much longer.
So the difference between air age and ice age is critical at Vostok. To estimate the snowfall rate, the scientists use the changes in the d18O levels in the ice as a proxy for temperature. Higher temperature = more evaporation = more snowfall.
Unfortunately, higher temperatures also mean more CO2. So you end up with a correlation between CO2 and ice age. Since the general assumption is that ice age is not a function of CO2, this can mess up your numbers subtly but badly.
Second, if the diffusion rate were as high as you say, why do we find large differences in the CO2 content that are only a short distance apart? Difference of 3-5 ppmv per metre are common at Vostok, at an age of 300,000 years. If the ice can hold that differential for 300,ooo years, the diffusion rate must be really, really slow.
Let’s suppose, for example, that CO2 concentration in the first ice bubbles was actually 100 ppmv different from the second ice bubbles when they were were laid down. That seems possible.
For that to decay to 4 ppmv in 100,000 years, the decay rate would need to be 4 = 100 * X ^300000. Excel tells me that is .001% decline per metre per year.
Let us suppose that the actual CO2 was say 400 ppmv 300,000 years ago, and on average it has been 1,500 metres deep over that time. Let us say it has been slowly equalizing with a background CO2 level of 275 ppmv. How much diffusion would we expect?
Well, at .001% per metre per year, and 1500 metres on average, we’d expect a diffusion of .001% / 1,500 per year. If that were kept up for 300,000 years, we’d see a decay of … drum roll … wait for it … 0.2%.
I’m sorry, but your post doesn’t pass the math test. Ice can retain a 3 – 5 ppmv differential per metre for three hundred thousand years …
This diffusion issue is a valid and important point.
I believe the diffusion effect also essentially exists in the other proxies, too, for a similar reason – that the ages of the data points in different proxies, and even within a proxy, are all approximations that are then muddled together when averaging and homogenizing. High frequency extremes are lost when one peak in one proxy study is shifted slightly from the high frequency extremes of other studies or other proxies. Each peak is accepted as having a discrete and precise time factor, but this precision is only imaginary, since each C14 (and most other types of) data point has a fairly sizable imprecision. (Most efforts in challenging the methodologies seems to have been aimed at the amplitude of the data, not the time factor.) Mis-timed peaks are fairly analogous to David’s diffusion, since the timing of the extremes do not match up.
A primary residual of this is that high frequency extremes are reduced when multiple proxies are averaged. The averaging itself (as was noted on WUWT about 18 months ago) reduces the high frequency extremes. The time element/diffusion reduces them even further by essentially blending them.
@Willis,
1 meter ~ 1 year at Law Dome (late 19th to 20th centuries).
1 meter ~ 100 years at Vostok (during last Pleistocene glaciation).
1 meter ~ 1,000 years at Taylor Dome (during last Pleistocene glaciation).
3-5 ppmv/m at Vostok is about 3-5 ppmv/century. 3-5 ppmv/m at Taylor Dome can be 3-5 ppmv/millenium. 3-5 ppmv at Law Dome is about 3-5 ppmv/yr.
1 meter of “diffusion” at Law Dome is smoothing. 1 meter of “diffusion” at Taylor Dome (11-27kya) is a serious low-pass filter.
I’ve now included Vostok back to 157 kya and Taylor Dome (11-27 kya) to the graph…
CO2 v Sample Rate w/ Vostok
When I included the older data, I found that the exponential trend-line was not a good fit; a logarithmic trend-line fit quite well.
Then I plotted the GRIP (Greenland) data (8-40 kya) on the same graph…
CO2 v Sample Rate w/ Vostok and GRIP
The amplitude can be restored quite easily by a gain function. Surface consistent amplitude compensation processes are routinely used to boost the gain of seismic reflection data while preserving the relative amplitude relationships.
I think that it might also be possible to restore some of the higher frequency content as well through the use of deconvolution and other forms of wavelet processing commonly used to improve the resolution of seismic reflection data.
I’m going to run this idea by a friend of mine who runs the Dallas office of a seismic data processing company that specializes in this sort of thing. In many cases they can restore frequency content that traditionally was thought to have not been recorded.
David Middleton says:
January 4, 2011 at 1:45 am
1 meter ~ 1 year at Law Dome (late 19th to 20th centuries).
1 meter ~ 100 years at Vostok (during last Pleistocene glaciation).
1 meter ~ 1,000 years at Taylor Dome (during last Pleistocene glaciation).
3-5 ppmv/m at Vostok is about 3-5 ppmv/century. 3-5 ppmv/m at Taylor Dome can be 3-5 ppmv/millenium. 3-5 ppmv at Law Dome is about 3-5 ppmv/yr.
First, sorry David that I have made a mistake by misinterpreting your “sample” rate, as literal sampling, not m/years…
The interesting point is not the differences between different ice cores, but the changes in sampling rate within one core: Taylor Dome shows a sample rate starting as 10 years/meter and ends with 2,500 years/meter after 230,000 years at 550 meter depth.
Vostok starts with 20 years/meter and ends with 540 years/meter at 3310 meter depth.
Quite huge changes in sample rate with depth and quite huge differences between different ice cores.
The Vostok ice core goes back 420,000 years with ups and downs of 100 ppmv in four cycles, with decreasing sample rate with depth. The interesting point is that there is a quite good correlation between the (ice based) temperature proxy (dD and d18O) and the (gas based) CO2 levels over the four cycles. If there was even the slightest diffusion, that would be visible over time as a fading away of the ratio. As an extra, that would be accellerated by the sample rate, as any diffusion would have the same speed over smaller layers (at the same temperature), representing much broader time periods. This is not the case at all, the ratio remains the same over 420,000 years (and recently confirmed over 800,000 years).
And I like to see a sr/CO2 plot of the full 420,000 Vostok years (have no time now to do it myself)…
Further, if there was diffusion over the first millennia (the Holocene), it would be strange to find such low levels: diffusion averages the levels over time, but it doesn’t change the average. That means that either the real levels at closing time were (much) lower or there was little migration…
@ur momisugly Ferdinand,
If you plot that sample rate (I use m/yr rather than yr/m) against CO2 mixing ratio, you get a strong logarithmic correlation – Not just within individual cores; but across multiple cores (Law, Taylor, Vostok and GRIP) from the 20th century back to at least 157 kya (as far back as I’ve compiled sample rates)…
CO2 v Sample Rate w/ Vostok and GRIP
This is exactly the relationship that should exist if van Hoof’s theory that diffusion in the ice cores acts like a low pass filter is valid.
The data, at least, is available for free via http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html .
The assumptions behind the dating are crucial, but unfortunately are only discussed in the article.