The Trouble With C12 C13 Ratios

Where Carbon Goes

Full size image

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?

From: http://www.thenation.com/doc/20070611/cockburn

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 realclimate.com, 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?

From: http://www.springerlink.com/content/f5272856220314nk/

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:

http://environmentalresearchweb.org/cws/article/research/37370

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.