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Life Science Teachers: Take special note!

This is not yet an error in the mainstream press, but there is an error afoot, currently represented in the widely read slashdot, which I imagine will propagate. The purpose of this post is to alert you to this problem and prepare you for the occasion when you run into a wackaloon creationist waving their arms around and screaming “Carbon dating does not work! It’s been proven.” This story also has a Global Warming Denialism component.

What I’m going to do here is give you the basic facts, then the misinterpreted text.

We start with the basic facts.


The paper of interest is by Peter Swart and it is published in PNAS.

Carbon is one of the ‘building blocks’ of life, and thus, carbon atoms are incorporated into living tissues. But there are several isotopes of carbon, including Carbon 12 and Carbon 13. These two isotopes are stable … they do not decay radioactively like Carbon 14 does.

The differences between these different number’d carbons is the exact molecular weight. This is a difference that for many purposes probably does not matter. But in biological systems, there is a small bias in the use of each isotope (called “fractionation”) which results in biological tissues having a different ratio of 12C to 13C than the ‘wild’ carbon floating around, say, in the atmosphere.

The ratio also tells us, in turn, about organic activity. Carbon can be trapped in sediments available to geologists in a number of ways, and when you look at these sediments, you can tell how much of the Earth’s carbon is likely trapped in organic tissue (dead or alive). Inorganic carbon would mirror the organic trap, and organic carbon ratios would also be affected by what is already trapped, as new tissue uses the extant atmospheric carbon. For this reason, 12C/13C ratios have been used to infer important aspects of the global organic carbon cycle over time ranges of hundreds of millions of years.

The carbon used in such studies may come from deep sea sediment, and this is the best place to get the carbon in many ways: There is some chronological control, it is the output of a large, buffered, reasonably well understood system, other data comes out of the same sediments, and so on. But going back before something like 150 million years, there are no known/sampled sea floor sediments for this purpose. For this, geologists look to other sediments called platforms, which are also sea-features but formed under different conditions (I’m oversimplifying this part a lot), or other sediments.

It turns out that a study of these different depositional environments, in the paper by Swart, indicates that the two data sources behave differently and the non-ocean bottom deposits cannot be used as they previously were. As a result of this, our understanding of the history of the Earth’s carbon cycle has gone all topsy-turvy and now needs to be re-examined.

Science marches on. This assertion by Swart will be tested, challenged, and if he is wrong, tossed out or modified. At the same time, people will be working on reassessing the pre 150 mya record. There is a lot of work to do an if it is really true that the pre 150 mya record is borked, this means that we will soon be exposed to a new and different (presumably) understanding of early life on earth! Cool!

Now, look at the following statement from a report in Science Daily News:

…the history of organic material has been interpreted by geologists using the 13C/12C ratio of carbonates and organics, wherever these materials can be sampled and dated.

That statement is embedded in a description pretty similar to what I give you above. Note that the statement explicitly states that the carbon samples can be used where they can be dated. Not that the carbon samples are used to date anything. This is an important difference.

The following is the offending statement from today’s Slashdot News:

“New research funded by the National Science Foundation at the University of Miami is showing that carbon dating (the 13C/12C ratio used to infer age) in the ocean can only be trusted up to 150 million years ago. From the primary researcher, ‘This study is a major step in terms of rethinking how geologists interpret variations in the 13C/12C ratio throughout Earth’s history. If the approach does not work over the past 10 million years, then why would it work during older time periods? As a consequence of our findings, changes in 13C/12C records need to be reevaluated, conclusions regarding changes in the reservoirs of carbon will have to be reassessed, and some of the widely-held ideas regarding the elevation of CO2 during specific periods of the Earth’s geological history will have to be adjusted.’ While this research doesn’t necessarily throw carbon dating out the window, it should cause people to rethink so many theories about early life that revolved around ages of sediment in the oceans.”

Obviously, this writer gets it totally wrong. But more importantly, if you look at this text, it is clear that this is the writing of someone steeped in the creationist literature. This tells me that we are seeing the beginning of a disinformation campaign regarding this research. There are several clues in here that link this paragraph to the Creationist Conspiracy. For example, the mention of NSF funding in the first paragraph, followed by the phrase “carbon dating can ony be trusted up to 150 mya” is a clue.

Carbon dating is NOT what his research about. Carbon dating is different. Here is carbon dating in a nutshell: Organic tissue incorporate 12C, 13C and 14C. The 14C is radioactive and decays over time (into something that is not carbon) so tissue older than several tens of thousands of years has virtually none. If the sample is suitable for analysis, and has above a certain ratio of 14C to the other C’s, the ratio can indicate the time since death of the tissue-forming organism. There are caveats: Marine life and terrestrial life are different, for example. Carbon dating (aka radiocarbon dating) is pretty reliable, when the samples are good, up to 30 or 40 thousand years ago, and in some cases the method can be extended to just over 100 thousand years ago. Not even one million years, though. This is not a technique that is used, or could be used, in relation to the origin of life.

The middle of the Slashdot quote is mostly quotes of the Science Daly piece. The end, however, brings us back into creationist think. Here we learn that while this new research (remember, this was funded by the NSF!) does not totally throw carbon dating out the window (true, because it has absolutely nothing to do with carbon dating) it should cause people to rethink … as in understand that there is a controversy … so many theories about early life etc. etc. must be questioned. You get the idea.

Since this research calls into question currently used data on early periods of earth’s carbon cycle, there will also be those who claim that this means that the jury is still out on Global Warming. However, this is absurd. We could understand current climate and climate change with far less than the last 100 million years of a climate record. Regarding the relationship of this research and the origin of life: Again, most of the early evidence for the origin of life is based on fossil material dated in a way that has nothing to do with this. The overall pattern of life and the carbon cycle prior to 150 million years ago may be reassessed in relation to this research, but not the existence of life during this period.

ResearchBlogging.org
P. K. Swart (2008). Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0802841105

Comments

  1. #1 Larian LeQuella
    September 10, 2009

    Usually I would just say that science is hard for these idiots, and dismiss them, but wow. This is really not that hard a distinction to understand, yet we have this sort of asshattery going on? *sigh*

  2. #2 Charles
    September 10, 2009

    I wasn’t aware that radiocarbon dating can in some cases “be extended to just over 100 thousand years ago.” I thought the best we could get was about 80,000. But that’s what I heard about four or five years ago. Is this 100,000 years a recent development?

  3. #3 DrA
    September 10, 2009

    Carbon isotope ratio changes seem to trip up even biology majors, who whenever they see this immediately think of radiocarbon dating. There are graphites in excess of 3.5 billion years old that show a skewed carbon isotope ratio suggesting the skew was produced by metabolic processes, but their age was determined by other means, not by carbon dating. Yet, come exam time, a certain percentage of my class will tell me exactly the opposite.

  4. #4 VolcanoMan
    September 10, 2009

    I get this all the time from students who question the age of fossils, old sedimentary beds, and by extension, the age of the origin of life and the age of the Earth all based on a flawed understanding of C-14 dating. As far as I am aware, the usefulness of the technique is for samples between 100 and 50,000 years old, the former limit set by the inability to detect a change in C-14/C-12 in the sample vs. the atmosphere within the margin of errors involved, and the latter by the inability to detect C-14 at all (but perhaps more sensitive instrumentation can extend this further…not millions of years further, but further). Truth is, I don’t like carbon dating very much because I’m a rock guy and it doesn’t tell me about rocks (except recent lava flows that happened to burn vegetation and turn it to charcoal). In other areas of geology, like dating lake and wetland cores, I admit that it proves useful, but the fact is that there are variations in the concentration of C-14 in the atmospheric CO2 that plants metabolise, and modelling the supposed changes makes me nervous, like most modelling. Also, the error bars associated with C-14 dates make me cringe (I’m an analytical chemist by training), although the instrumentation is getting better. But if you need a rough-and-ready estimate of the age of a Stone Age hearth, it can be very useful. That’s more of an archaeology thing though, not really my area of interest.

    Now uranium dating, or potassium-argon dating, those have error bars, sure, but the samples are hundreds of millions of years old! What’s a few million years, plus or minus, when the number’s that large! Here’s where young-earth Creationism meets its ultimate destruction…there’s no getting around those zircons man, they absolutely slay any chance that the Earth is less than 4 billion years old, and the fact that they had to crystallise from a molten state adds half-a-billion to the tally. Is it that creationists don’t understand the damning evidence, or do they just live in blissful denial? Observation 1: Uranium turns into lead at a known half-life. Observation 2: If uranium is trapped in a crystal, the ratio between it and the particular radiogenic lead isotope it produces change according to the half-life, so you can calculate how long it took to produce the observed ratio.

    I oversimplify, but…duh?

  5. #5 Jim Thomerson
    September 10, 2009

    In historical geology class, in 1956, we got two full lectures on radioactive dating, with an emphasis on what can go wrong.

  6. #6 Greg Laden
    September 10, 2009

    When we were rebuilding a science archaeology lab at Harvard back when I was a grad student we had to get rid of the old C-14 dating machine because it was in the way.

    That was the original machine, with which the technique was invented, of course. The thing was COVERED with dust.

  7. #7 sinned34
    September 10, 2009

    Is it that creationists don’t understand the damning evidence, or do they just live in blissful denial?

    It’s a combination of blissful denial, ignorance of the evidence against their beliefs, and the demonizing of scientists as representatives of the devil. At least, that’s how I remember it being at the evangelical/pentacostal churches I used to frequent.

  8. #8 Joshua Zelinsky
    September 10, 2009

    This doesn’t seem to me like something written by someone as you put it “steeped in the creationist literature.” To the vast majority of the population anything related to old things and carbon is part of that whole vague “carbon dating.” Hanlon’s Razor may need to be applied here. Of course the actual YECs will use this to distort it and claim something is wrong with dating and all that, because that’s what they do. They would do that even if this were reported accurately.

  9. #9 Nathan Myers
    September 10, 2009

    VolcanoMan: Didn’t you know that radioactive decay rates have declined exponentially over the past 6Ky, causing events close to that boundary to appear spaced out over a much longer period? (I guess the energy released per decay event must have increased correspondingly, because otherwise everything would have cooked.)

    Can a creationist even consider a period of time as long as 150 My?

  10. #10 MadScientist
    September 10, 2009

    @VolcanoMan: The 14C dating is good to whatever contemporary period you want really. The usual source for 14C is cosmogenesis in the upper atmosphere – the isotope is generated by bombardment with very high energy particles (mainly) from the sun. Contemporary atmospheric values can be measured from air samples. Values from a few years ago (and also back to a few thousand years ago) can be established by cutting down trees. During the period of atmospheric nuclear tests there is the “bomb pulse” – a measurable increase in 14C (plus fairly large and easily measured amounts of other radioisotopes). There are many complicating factors so the estimated error is a few years going by 14C and checking against tree rings. 100yr as the earliest dateable is definitely not the case though.

  11. #11 Greg Laden
    September 10, 2009

    Actually, the atom bomb effect is huge, and really makes typical approaches to C14 dating difficult for the affected time period. But, the shift in carbon abundance owing to the bombs is itself a marker that works better than C14

    So the bombs converted Nitrogen to Carbon, and then scientists have converted the Carbon to Silver, as in Silver Lining.

  12. #12 VolcanoMan
    September 10, 2009

    MadScientist,

    The oldest tree known is a bit over 5,000 years old. And if you want dates within the past 100 years, Pb-210 is a much more reliable method. The bomb signature is great, but as you said, the other radioisotopes are there too, to establish a check against other dating methods. The key to using a method is establishing its limitations, and C-14 is useful for some purposes; however, it’s not the be-all-and-end-all of our arsenal, and the creationists would do well to recognise that the knowledge of the past scientists have assembled goes far beyond what we have learned with C-14.

  13. #13 Greg Laden
    September 10, 2009

    The oldest tree known is a bit over 5,000 years old.

    The oldest trees in the dendro time scale for various regions go back over 10,000 years. The trees don’t need to be alive, they just need to be the appropriate species, then linked to later ones and eventually living ones.

    The C14 calibration curves are somewhat context dependent (both environment and materials used because there are fractionation, context, and reservoir effects). They initially were based on trees, but that is not the only way to do it.

    There is no pragmatic limit on how far back C14 can be calibrated. Using trees, somewhat over 10K, but varves, speleothems, etc. can be linked into that as well. There is no theoretical reason that the entire time range of C14 can’t be calibrated.

    Currently, it is calibrated to somewhere between 25K and 30K. (see Reimer et al 2004 in Radiocarbon)

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