Learning from creationists; radiocarbon dating
Radiocarbon dating only takes us back some 50,000 years. This makes it a much smaller threat to Young Earth creationists than, say, lead-uranium dating, which takes us back billions of years. So why do creationists single it out for attack? Because there are indeed problems with the most simple-minded application of the method, and it does not matter to the creationist that these problems have long since been solved. Creationists think, and argue, more like lawyers than like scientists. In the courtroom, changing your story under cross-examination will destroy your credibility, and yet this is what scientists do all the time. Scientists accept that even the most well-established findings are subject to revision and refinement; lawyers, like theologians, seek certainty whether the data justify it or not.
This post is derived from a talk given by my friend Leonard Sym to Glasgow Skeptics in the Pub, and like Leonard I will follow Rapoport’s rules for debate, which specify that one should first summarise one’s opponents’ position in terms they would accept, next, list points of agreement, then point out what one has learnt from one’sopponents, and only at that stage embark on criticism.
I start with a simplified summary of the principles behind radiocarbon dating, without which the discussion would be meaningless. Most radiometric methods depend on measuring the amount of a parent radioactive isotope present in the sample, and the amount of the daughter into which it decays. Add up the amount of parent still present and the amount of daughter, and that gives you the amount of parent present initially. If you know the rate constant for the decay, you now have enough information to work out how long has passed since the parent material was put in place. You can determine the rate constant by taking a known amount of parent, and counting the number of decays per second, as measured with a Geiger counter or a more reliable and up-to-date instrument such as fluorescence counter.
In the case of radiocarbon dating, the parent is carbon-14 and the daughter is nitrogen-14, which is lost from the sample. So we can’t find the initial amount of parent in the way described above, because we don’t know the amount of daughter. This seems like a dead end, until we remember where carbon-14 comes from. Carbon-14 is formed in the upper atmosphere by the effects of cosmic ray bombardment on nitrogen, is rapidly converted to carbon-14 dioxide, and then mingles with the rest of the CO2 in the atmosphere (see Figure). If we assume a steady rate of bombardment, that means we will have a steady rate of production of carbon-14, and a steady state abundance of carbon-14 in the atmosphere, where the amount decaying each year is equal to the amount being formed.
Now consider what happens during the life of an organism, and after its death. As long as it is alive and metabolising, it will exchange carbon with its environment, taking it in directly as carbon dioxide by photosynthesis (for a plant) or indirectly as food (for an animal). At this stage, the proportion of carbon present as carbon-14 will be directly dependent on that in the atmosphere. But as soon as it stops metabolising, it stops exchanging, and the proportion present starts decaying according to the radioactive decay law, with a halflife of 5730 years. So it looks as if we can just use the proportion present in the atmosphere right now as a measure of the initial proportion, and compare it with the proportion remaining.
So far, so good. Now let me list the creationists’ objections:
1) As in all radiometric dating, the decay rate is assumed to be constant. What if this isn’t true?
2) The production rate is assumed constant. But this is unrealistic, since the intensity of cosmic ray bombardment is known to change over time
3) For 150 years, and especially in the last 50 years, we have been adding carbon dioxide from fossil fuels to the atmosphere, diluting the radiocarbon since all the radiocarbon in the fossil fuels will have long since decayed
4) Considerable amounts of carbon-14 were added to the atmosphere by nuclear testing in the 1950s, further undermining the assumptions
5) What if carbon-14 is less readily taken up than carbon-12 by plants? Won’t this undermine the reasoning?
6) We can check the method by applying it to materials whose age we know, but this will only serve where we have a good historical record, and this record only goes back, at best, some 5000 years
7) The Genesis flood, which in Young Earth accounts is responsible for the formation of our fossil fuel deposits, would have further upset the clock by burying huge amounts of carbon-12. Moreover, it could have been associated with an increase in the rate of carbon-14 production, making pre-flood specimens look much older than they really are.
With the exception of the first and last, all these objections have some degree of plausibility, but unfortunately for the creationists they have all long since been answered, many of the answers being set out by Willard F. Libby, inventor of the method, in his 1955 book on the subject.
1) Radiometric decay constants are just not the kind of thing that could change, unless everything else changes at the same time. We have known since the work of George Gamow in 1928 that radioactive decay is what is known as a quantum mechanical tunnelling effect, and that its rates depends on such things as the strength of nuclear and electrical forces, the mass of fundamental particles, and Planck’s Constant h, which gives the scale for all quantum mechanical phenomena. If any of these had been different, we would not have had the same kind of physics and chemistry that we have today. But we know from their structure that ancient rocks were formed under the same rules as we have today, because they contain the same kinds of elements combined to make the same kinds of minerals. The creationists have published theoretical curves for changes in decay constants, but these have no basis in science, and are generated merely to make observations fit the biblical timeline.
2) From the outset, radiocarbon dating has relied on calibration, using objects for which dates were known from historical records, then tree ring counting extending back 10,000 years. This method works because all but the outermost layers of a tree are metabolically inert, and out of circulation. The most recent calibration comes from organic debris in varves (annual layers of sediment) deposited in a lake (Lake Suigetsu, in Japan) that happens to be free of turbulent inflows. This has made possible the establishment of a calibration curve going back 52,800 years.
3) and (4) There have indeed been major disruptions since 1950, but no one uses radiocarbon dating for such recent material. The situation in 1950 is regarded as a baseline, material from that year is the standard for comparison, and 1950 is the “present” in conventional dating of “years before present”
5) It will surprise many people to learn that plants really do take up carbon-14 less readily than carbon-12. One of the lies you were probably told at school is that all isotopes of the same element have precisely the same chemical properties. This is not true, and generally speaking, heavier isotopes are slightly more sluggish in their chemical reactions. this gives rise to the process known as isotopic fractionation.
This effect has been measured for photosynthesis. In addition to very small amounts of radiocarbon, atmospheric carbon dioxide contains roughly 1% of the stable isotope carbon-13, the remainder being carbon-12. Carbon dioxide in plants is, as expected, slightly depleted in carbon-13 relative to carbon-12, and the effect is far from trivial; around 27 thousands of carbon-13 abundance for most kinds of plant. We expect the effect to be twice as large for carbon-14, which, using the known 5730 year halflife of carbon-14, corresponds to 435 years; not trivial when dating historical artefacts. However, exactly the same effect will apply to the material used to set up the calibration, and the errors will systematically cancel out. Ideally, the fractional abundance of carbon-13 should be measured, as well as that of carbon-12, to calibrate out any minor fractionation effects, and this is less arduous than it sounds because nowadays carbon-14 abundance is measured by direct counting in a mass spectrometer rather than, as in the original studies, indirectly inferred from sample radioactivity.
For plants, it is straightforward to match like with like. Not so for material derived from animals, where the total amount of isotopic fractionation will depend on their diets, and also on what they have been eating.
6) This objection would have had some force in 1946, when the method was newly developed. However, as already explained, we now have direct calibration back to 52,800 years before present, beyond which the amount of remaining carbon-14 is so small that using current techniques the method becomes useless.
7) This is pure special pleading. If carbon-12 had been buried in the flood, the appropriate amount of carbon-14 would have been buried with it. And the ideas of a changed rate of production or decay have been dealt with under (1) and (2) above.
There are other “objections” based on the obvious fact that organisms like cave water snails, alive right now but deriving their carbon from limestone, will have radiocarbon apparent ages measured in thousands of years. I have discussed this before. And marine specimens will always contain less carbon-14 than terrestrial specimens of the same age, because of the time it takes for mixing between the atmosphere and surface waters, and again between surface waters and the depths.
So how should we respond to the self-styled “creation scientist”? The first, and most difficult, thing is to realise that he is been perfectly sincere. He is certain that his reading of the Bible is true; but the facts of geology are also true; and it is therefore his mission to create an account that reconciles the two. If this means the mountains must have skipped like rams, then that is how they must have skipped. He will feel no more absurd at this point, than the cosmologist feels in invoking a time when our Universe was smaller than a tennis ball and its temperature was trillions of degrees. Counter-arguments will be dismissed as so many minor anomalies that will no doubt be explained away in due course. If the creationist repeats long-refuted claims, that is because he believes that there are refutations of the refutation, even if he cannot immediately call them to mind, or does not have time to explain them properly. He will remember the weaknesses of his opponents’ arguments, and attack them, while suppressing the recollection of their strengths, and in the process he will create, and then triumphantly destroy, a series of straw men. You and I of course would never do such things, but your friends might when it comes to defending emotionally precious but logically fragile beliefs; consider, for example, what passes for political discussion in your favourite pub or chatroom.
And what does this mean for debating with creationists? Simply this: don’t do it. Such a debate, unlike a discussion between people willing to learn from each other, is a zero-sum game. He will project simplicity, sincerity, and certainty, and when you come to reply, you will sound as if you are making excuses. He will present anomalies (did I mention those 2000-year-old water snails?), and when you explain the special circumstances, you will be the one who seems guilty of special pleading. His followers will end up confirmed in their convictions, as will yours, and those in the middle will come away confirmed in their own initial conviction that there are two sides to the story, both worth hearing. Which there aren’t.
But does that mean that we can learn nothing from the creationists? Not at all. In terms of Rappaport’s rules of debate, the scientific community had already come up with arguments (1) through (6), and taken the necessary countermeasures, and so cannot be said to have learnt from the creationists. But both Leonard and I have learnt a great deal from examining the creationist claims. Be smart, and learn from everyone.*
1] It is of course necessary to eliminate errors caused by the movement of material, or the presence of daughter in the initial material. There are standard techniques for doing this, for instance by measuring non-radiogenic isotopes of the daughter material, and, these days, by microsampling of single crystalline grains
2] Even if it is not lost as N2 gas, it will be undetectable against the background of organic nitrogen compounds already present
*Ben Zomah, Mishnah Pirkei Avot 4a
** extra credit for spotting (a) the misleading labelling in the diagram (h/t John Gribbin), (b) the reference in the text to Psalm 114
There are other creationist objections to radiocarbon dating, based on sample contamination or simple misinterpretation of data, but these have been discussed elsewhere and need not detain us.
Lecture scene from Glasgow Skeptics in the Pub Facebook page. Atmospheric carbon-14 diagram public domain, by Hokanomono via Wikipedia. North Ronaldsay sheep by Liz Burke, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=3499298. Radiocarbon cycle schematic from ANU Radiocarbon Dating Lab materials
Posted on March 25, 2016, in Creationism, Religion, Science and tagged carbon isotopes, carbon-14 dating, isotopic fractionation, Lake Suigetsu, photosynthesis, Radiocarbon dating, Radiometric dating, Willard Libby. Bookmark the permalink. 7 Comments.