The Trouble With C12 C13 Ratios

Where Carbon Goes

Where Carbon Goes

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It is often asserted that we can measure the human contribution of CO2 to the air by looking at the ratio of C12 to C13. The theory is that plants absorb more C12 than C13 (by about 2%, not a big signature), so we can look at the air and know which came from plants and which came from volcanos and which came from fossil fuels, via us. Plants are ‘deficient’ in C13, and so, then, ought to be our fossil fuel derived CO2.

The implication is that since coal and oil were from plants, that “plant signature” means “human via fossil fuels”. But it just isn’t that simple. Take a look at the above chart. We are 5.5 and plants are putting 121.6 into the air each year (not counting ocean plants). There is a lot of carbon slopping back and forth between sinks and sources. Exactly how closely do we know the rate of soil evolution of CO2, for example?

Some problems:

How do we tell ancient carbon from fossil fuels from ancient carbon from black smokers at the bottom of the ocean? Do we know the isotope ratio of CO2 from mid-ocean ridges vs land volcanoes? (Land volcanoes are from subduction zones so I’d expect more rapid recycle of C from ocean sediment to yield a different isotopic ratio…) This is another example of things we assume we know, that are poorly explained.

In wandering off to learn more about C12 / C13 origins and ratios I ran into this gem. It does raise the interesting question: If human CO2 dropped dramatically during the great depression, where is the signature in the record?


I should acknowledge one imprecision in my description of Dr. Martin Hertzberg’s graph in my first column–”the smoothly rising curve of CO2″–which prompted several intemperate responses, charging that I couldn’t possibly expect CO2 or carbon levels to drop just because of a one-third cut in manmade CO2. Indeed, I should have written, “One could not even see a 1 part per million bump in the smoothly rising curve.” Even though such transitory influences as day and night or seasonal variations in photosynthesis cause clearly visible swings in the curve, the 30 percent drop between 1929 and 1932 caused not a ripple: empirical scientific evidence that the human contribution is in fact less than a fart in a hurricane, as Dr. Hertzberg says.

From the same article by ALEXANDER COCKBURN, with questions:

As for the alleged irrefutable evidence that people caused the last century’s CO2 increase, the “smoking gun” invoked by one of my critics, Dr. Michael Mann, and his fellow fearmongers at, the claim is based on the idea that the normal ratio of heavy to light carbon–that is, the carbon-13 isotope to the lighter carbon-12 isotope, is roughly 1 to 90 in the atmosphere, but in plants there’s a 2 percent lower C13/C12 ratio. So, observing that C13 in the atmosphere has been declining steadily though very slightly since 1850, they claim that this is due to man’s burning of fossil fuels, which are generally believed to be derived from fossilized plant matter.

OK, so both C12 and C13 are stable and they are looking for a ‘plant’ signature in burned fuel, not a nuclear decay signature. One Small Problem… C4 metabolism plants absorb more C13 than do C3 metabolism plants. Over the last 100 years we’ve planted one heck of a lot more grasses world wide than ever before. Grasses are often C4 metabolism…

Have they allowed for this? If so, how? I’m not sure how one would figure out the C4 vs C3 plant population ratio of the world, and certainly don’t see how you would figure out what it was 1,000,000 years ago.

On the naïve and scientifically silly assumption that the only way that plant-based carbon can get into the atmosphere is by people burning fuels, they exult that here indeed is the smoking gun: Decreases of C13 in the atmosphere mean that our sinful combustions are clearly identifiable as major contributors to the 100 ppm increase in CO2 since 1850.

This is misguided, simply because less than a thousandth of the plant-based carbon on earth is bound up in fossil fuel. The rest of the huge remaining tonnages of plant-based carbon are diffused through the oceans, the forests, the grasslands and the soil. In other words, everywhere. Obviously, lots of this C13-deficient carbon has the chance to oxidize into CO2 by paths other than people burning fuel, i.e., the huge amount of plant material that’s naturally eaten or decayed by the biosphere.

And as C4 plants have been sought out (they are more efficient, so more food per growth unit) we get more C13 in the plants. There are even efforts to transplant the C4 genes into C3 plants to get better yield. This would argue for more C13 being sequestered in soils over time as C4 plants have expanded. Have they examined the C12 vs C13 ratio changes in soils over time?

Perhaps even more significant, cold ocean waters absorb lightweight C12 preferentially, resulting in lots of C13-deficient carbon in the oceans. This low-C13 carbon most certainly would have been released massively into the atmosphere over the course of the world’s warming trend since 1850, when the Little Ice Age ended.

And would also argue that volcanic emissions from subduction zone volcanoes ought to be C13 deficient to the degree that ocean bottom ooze is being recycled. Has this been considered? Does C12:C13 ratio modulate with the level of volcanic activity?

All of these larger natural pathways for emitting low-C13 carbon into the atmosphere have been considerably accelerated by this same warming trend. So once again, the greenhousers have got it ass-backward. The 100 ppm increase in CO2 can’t be uniquely attributed to humans because at least as plausibly it could be the effect, not the cause, of the warming that started after the Little Ice Age denied by Dr. Michael “Hockey Stick” Mann.

It looks to me like there are very significant issues in trying to assert that C13:C12 ratio changes in the air can tell you anything about CO2 origin in fuel burning… And then there is the question of all the coal burning from natural coal seams (some of them started by lightning). Do we just accept that CO2 as ‘human’ even though we had nothing to do with it?

But at least we know the signature from oil and coal, right?


We get that the C12:C13 ratio is different in oils than in coals and varies in the source lipids from which oil is made. Oh dear. They are all different. That’s going to take some detailed accounting. What? We just burn it without doing the accounting? So how do we know what “our part” does to the C12:C13 ratio then?

Lipid fractions of organisms have consistently lower C13/C12 ratios than do the whole organisms. The average difference between nonlipid and lipid materials for all organisms studied is about 0.5% and ranges in individual species from as little as several hundredths to more than 1.5%. This suggests that petroleums and other noncoaly organic matter in ancient sediments are derived from lipids, or at least from certain components of the lipid fraction. In contrast, coal deposits apparently are derived from whole plants or from the cellulosic fraction of land plants, which is the major nonlipid constituent, of plant tissues.

Has the petroleum and coal from around the world been tested for differences in C12:C13 ratio? I’d expect significant variation based on the above. Is this allowed for in the attribution of atmospheric CO2 to fuel burning?

From a report on natural gas isotopes we get:

Bacteriogenic methane from Illinois generally has a C13 values in the range of -64 to -90% relative to the Peedee Belemnite ( PDB ) standard. The 11 samples from pipelines and storage reservoirs that have been analyzed have all had C13 values in the range of -40 to -46%.

Got that? Bacteria make methane with even less C13 in it than natural gas. So a little swamp gas can look like a whole lot of human generated C13 deficient CO2 once it air oxidizes.

Which seems to show that biological source methane can vary widely in C13 content and that pipeline gas is not the same signature as biological, coal, or petroleum. Has this be allowed for? If so, how? Frankly, given the biological origin variance and that bacteria are more deficient in C13 than natural gas (if I read that statistic right) I don’t see how it’s possible… There are giga-tons of bacteria in the world all emitting different ratios and we have no idea what they are. Great…

(I hand typed the above quote and there was what looked like maybe a sigma in front of the C13′s. Some day I’ll learn how to use Greek.)

It looks to me like there are more holes here than bucket… I don’t see how C12:C13 ratio can be reasonably used to make any clear assertion about where the CO2 in the air comes from.

How much Clathrate out gasses each year on the ocean bottoms? With what C12:C13 ratio? How much natural gas leaks from the ground? What are the ratios for bacteria produced methane from various ecosystems including ocean bottom? Are they all the same? How do you know? Since bacteria have been shown to eat oil and natural gas, how do you distinguish their CO2 from those eating wood? And from human sources?

How about the formation of carbonate deposits in the ocean? This article from Science Daily discusses a recent discovery that the whole history of C12 : C13 may be a bit broken since we don’t really know how carbonate formation tracks against atmospheric concentrations. Oops.

Notice that we are a tiny little rounding error on the surface ocean number (5.5 vs 1020) and even more nearly nothing compared to the deep oceans (5.5 vs 38,100) that we know almost nothing about.

IHMO, it is simply not possible to make sense of the C12:C13 ratio until a great deal more work is done. All the things we don’t know just make it a bald assertion without foundation.

UPDATE: per “fish rocks”. Seems that fin fish poo out carbonate rocks. Who knew? Seems we’re still learning things, large things, about the CO2 cycle…

So, speaking about “what we don’t know about the CO2 in the ocean” there is also this:

from January of 2009. A quote or two:

Fish ‘gut-rocks’ solve ocean puzzle

For decades marine scientists have been perplexed by the increase in alkalinity with depth in the top 1000 m of the ocean surface when chemistry suggests this should only take place lower down. But now, a team from the UK, US and Canada reckons excretion of a highly soluble form of carbonate from fish intestines could go some way to solving the mystery.

“Our most conservative estimates suggest three to 15% of the oceans’ carbonates come from fish, but this range could be up to three times higher,” said Rod Wilson of the University of Exeter, UK.

Notice the large ranges? 3% to 15% but maybe 3 x that…

Now what was that you were saying about “it had to come from people” based on some hypothetical ocean CO2 accounting? We haven’t a clue…

“We also know that fish carbonates differ considerably from those produced by plankton,” said Wilson. “Together these findings may help answer a long-standing puzzle facing marine chemists, but they also reveal limitations to our current understanding of the marine carbon cycle.”

The carbonate the fish excrete is high in magnesium and more soluble than the forms of carbonate produced by plankton. As a result it can dissolve at higher levels of the ocean.

Together with colleagues from the University of Miami, University of Ottawa in Canada, University of British Columbia, Canada, and the University of East Anglia, UK, Wilson estimated the total biomass of bony fish in the world’s oceans as between 812 million and 2050 million tonnes, leading to a total carbonate production of around 110 million tonnes.

Again with the 812 to 2050 … kind of a wide range, eh what? Yet we end up with a single nice 110 million tonnes answer… at least it has an ‘around’…

Now here’s a little thought for you:

We’ve reach “Peak Fish” some decade or two ago. (As of now some 30% of all fish eaten is aquacultured so nobody cared much about the Peak Fish crisis… The same kind of thing will happen for Peak Oil.) We are harvesting ocean fish at the fastest rate we can ever harvest them from the ocean. Do you think that maybe hauling billions of pounds of fish out of the ocean might reduce the quantity of carbonate pellets the fish in the ocean can excrete?

Do you think that might leave more CO2 in the ocean to outgas?

Do you think that might raise the CO2 level in the air?

Whether that it A Good Thing or A Bad Thing I’ll leave for another day…

And I wonder if anyone has any idea what the C12 to C13 ratio is in fish rocks for all the different combinations of fish and gut bacteria…

But we ‘know’ that the C12 : C13 ratio tells us the CO2 is from people…

“Edwards: Why the big secret? People are smart. They can handle it.
Kay: A person is smart. People are dumb, panicky dangerous animals and you know it. Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you’ll know tomorrow.”- Men In Black.

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About E.M.Smith

A technical managerial sort interested in things from Stonehenge to computer science. My present "hot buttons' are the mythology of Climate Change and ancient metrology; but things change...
This entry was posted in AGW Science and Background, Earth Sciences, Favorites and tagged , . Bookmark the permalink.

22 Responses to The Trouble With C12 C13 Ratios

  1. pyromancer76 says:

    E.M. Smith, I have thoroughly enjoyed your comments on What’sUpWithThat for many months and I have received an excellent education from you. I’m glad you are adding your own blog, too. Thanks for this exposition; I am working on “understanding CO2″ at present so I can manage both the scientific comments as well as those of the trolls.

    Please don’t let your own blog limit your commenting on WUWT. I love funny, intelligent, wise and tell-it-like-it-is. Thanks again.

  2. E.M.Smith says:


    I intend to continue participation on WUWT at about my usual rate (which does vary as ‘life happens’…). My purpose here is to simply let me put larger blocks of text together (with some pictures too!) and include an in depth dissection of GIStemp (that would not be appropriate to WUWT.)

    I’ll still put the same stuff on WUWT, but where before I’d feel like I had to limit or truncate some part, I’ll now include a link to here for ‘more’. Also, when a topic come around a few times, my first post on WUWT may become a page here so that later I can just say “link here” rather than cut/past or recreate the whole thing (like the 4 or 5 times I have gone through the “There is no energy shortage and there never will be” mantra…) It’s just going to be more efficient with everyone’s time; I hope.

    Basically, I’m not trying to build my own little empire; it’s more like I’m making an in depth extension to WUWT for some topics / opinions of mine and one or two added things (like the technology of Stonehenge and the fact that the Greeks an Minoans used the ‘foot’ measure just like the British Foot …)

    So: Welcome Aboard, and No Worries!

  3. Roger Sowell says:


    Echo the earlier sentiments. Nice to see that you have your own blog. Looking forward to your postings here, and as always on WUWT.


  4. e.m.smith says:



    It was astoundingly easy to set up the blog. I would recommend it to any poster who finds they repeat the same argument from time to time. Just click on the meta tag at the side for and sign up! Couldn’t have been easier.

    Now, for things like the GIStemp code, I can just put up a link to here rather than retyping the same stuff… Much easier.

  5. Roger Sowell says:

    Ed, I might set up a WordPress blog eventually, just to have the cool features like including graphs and such.

    I already have two blogs and a website to look after, though.,,


    Best to you,

    (I am saving rainwater as the drought seems serious this time!)

  6. aliunde says:

    I hand typed the above quote and there was what looked like maybe a sigma in front of the C13’s.

    I think you’re looking at a delta (could be confused for a sigma). Seems to be the standard when citing proxy isotopes.

  7. Bart says:

    Good post. Thanks.

    REPLY: [ Glad you like it! -ems ]

  8. You have effectively handled the C12:C13 argument, thanks for the information and considered thoughts.

  9. E.M.Smith says:

    This was posted in comments on a WUWT thread is is germane to this article, so I’m pasting a copy here to document it:

    David Middleton (13:39:50) :
    A comment on d13C excursions.

    A decline of δ13C in the instrumental record is often cited as an “anthropogenic fingerprint” on CO2. The logic being that fossil fuels contain very little 13C and therefore CO2 emissions from fossil fuels is diluting 13C relative to 12C.

    This line of reasoning is totally fallacious. δ13C declined sharply in the early Holocene. It pretty well declines whenever CO2 levels rise. Here is just one example of many…

    A distinct δ13C decline in organic lake sediments at the Pleistocene-Holocene transition in southern Sweden
    1 Department of Quaternary Geology, Lund University, Tornavägen 13, S-223 63 Lund, Sweden
    Copyright 1993 Collegium Boreas
    Values of δ13C obtained from conventional bulk sediment radiocarbon dates encompassing the Pleistocene Holocene boundary have been compiled and plotted against 14C age. In all. 286 lake sediment dates from southern Sweden in the range 8.000 to 13.000 BP have been evaluated. A significant decrease in δ13C values, initiated shortly before 10.000 RP and amounting to 5%, is distinguished. This change is accompanied by increased limnic productivity. decreased erosive input and increased organic carbon content of the sediments. A probable explanation for the δ13C decline in organic material is decreased importance of dissolution of silicates at the transition to the Holocene. During the Late Weichselian. extensive weathering of exposed minerogenic material with subsequent input of bicarbonate to the lake water may have caused a relative enrichment of 13C in dissolved inorganic carbon. Furthermore, the early Holocene increase in terrestrial vegetation cover probably led to an increased supply of 13C depleted carbon dioxide to the lake water by root respiration. Altered limnic vegetation, presumably towards increased production of phytoplankton. could also have contributed to the observed decreasing δ13C trend. The importance of these processes compared to other possible influencing factors. mainly endogenic carbonate production and changes in the global carbon cycle. is discussed.


    Here’s an example from early Holocene speleothems…

    Delta 13C values were high until 17.79 ka after which there was an abrupt decrease to 17.19 ka followed by a steady decline to a minimum at 10.97 ka. Then followed a general increase, suggesting a drying trend, to 3.23 ka followed by a further general decline. The abrupt decrease in δ-values after 17.79 ka probably corresponds to an increase in atmospheric CO2 concentration, biological activity and wetness at the end of the Last Glaciation…


    There is an overwhelming body of evidence that δ13C has declined when over all CO2 levels have risen. This was the case long before Colonel Drake drilled his first oil well near Titusville, PA in 1859.

    The thread in question is:

  10. gcb says:

    Got pointed here from a comment at WUWT – very useful and informative article, thank you. Again, I am frustrated at how trillions of dollars is being budgeted to fight “AGW” when we don’t even know how much we’re warming by, much less how much of that is anthropogenic.

  11. E.M.Smith says:

    The quantity of conclusions drawn from unsupported premisses in “climate science” is a bit extreme… Then you look at what does seem supported and find rampant holes.

    I find it odd that so many folks seem to have so much trouble admitting that we just don’t know a lot of important things about how the world works.

    (Posted from a cold and rainy California where on April 21st we’ve often had Tomatoes in the ground for a couple of weeks… and instead we’re having “winter storm warnings” for the mountains. I want that warming that was promised to me… I really really do… )

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  17. Another fact is that experimentally upwelling methane through a column of rock etc similar to the earth’s crust, causes that methane to become preferentially enriched in C12, C13 isotope being removed preferentially by chemical reaction further down.

    So, like it or not, using carbon isotopes to discriminate human from natural carbon has its problems.

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  20. assman says:

    I have a much more basic argument against C12/C13 that has never been addressed by anyone ever. Who cares about where the carbon dioxide came from. Its completely irrelevant.

    Imagine a bank account where me and my wife a vault where me and my wife are putting money in and taking money out. Lets say we mark each bill we put in the safe with our respective names however when we spend it we spend without regard for whose money it was. Also lets say my wife makes twice as much as I do. One year later we look in the vault after many transactions and find out that all the bills are marked with my initials. Does this mean that I am responsible for the money saved? No. It just means that for some odd reason (which should be scientifically explained) my bills were preferentially not spent whereas hers were.

    In carbon dioxide terms this would translate to, carbon from anthropogenic sources is not preferentially absorbed whereas natural carbon dioxide is. Why? I don’t know. But at the end of the day all that matters is the total amount put in – the total amount subtracted. If carbon dioxide were well-mixed that the anthrogenic amount should always be a small proportion in the atmosphere given that it is a small percentage of total terrestrial sources.

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