An Examination of Carbon Dating
It is accepted by many scientists, teachers, students, the media, and the average well-informed modern person, that billions of years ago our universe came into being by rapidly expanding and cooling, and some blob of matter congealed into our Earth, that millions of years ago life appeared and evolved, producing man hundreds of thousands of years ago, and that there is a great deal of incontrovertible scientific evidence to back this up. So much I have gathered in bits and pieces from books, lectures, TV shows, and casual conversations. Now, all else being equal, I would accept these things on their good authority. Indeed, many of the things I “know”—from the fact that man has been to the moon to the fact that platypuses are in Australia—I know only from these same sources. But when it comes to the origin of the universe, the Earth, and life, I face a predicament.
I am a Christian, and as such I believe that the Bible is the Word of God, written by men but inspired by the Holy Spirit, and therefore true and infallible. The Bible teaches, “In six days the LORD made heaven and earth, the sea and all that in them is” Exodus 20:11 (KJV), contrasting the idea of millions of years of evolution. In fact, along with other historical records, the genealogies and histories of the Bible can be used to construct timelines from “the Beginning” all the way up to well-established history. My own rough count puts the age of the Earth around 6,100 years. Even with a generous margin of error for miscounting and genealogical gaps—say plus or minus 2,000 years—that’s a long way from accounting for a few billion years. Crediting the Word of God above the word of man, thousands of Christians, including many Ph.D. scientists and Christian organizations reject the old-earth view and hold to a literally Biblical young-Earth creationism (1).
However, disregarding such a thorough and widely-accepted view of cosmic history is not a step to be taken lightly. The Biblical worldview itself implies that God gave man the ability to learn about Him and His creation through observation and reason; therefore science should be a valid way of learning about our world—and a variety of scientific disciplines, from geology to biology to astronomy, yield a fairly cohesive picture of the ancient earth. Many Christians, from some of my own family and teachers to the famous author C. S. Lewis, reconcile the discrepancy by reinterpreting key passages of Scripture in a less literal sense (2). After all, the Bible was not intended to be a scientific textbook; certain parts of it are clearly not meant to be read in the most literal sense; and Christians, like everyone else, are all too prone to misinterpreting our own doctrine.
Even so, as a thoughtful, rational, logical person, it does not sit well with me to reevaluate what I consider the primary source (the Bible) based solely on others’ (the scientific community’s) understanding of a secondary source (human observations). Is it really impossible to take Genesis at face value without discounting scientific inquiry? What is the nature of the scientific “proof” that the world is old? And can a creationist perspective offer a satisfactory, intellectually honest alternative? These are questions I must answer before I can rest satisfied in either position. Creation scientists claim that ‘old-earth’ scientists are essentially correct in their observations of the present world, but draw incorrect conclusions (evolution, the Big Bang, etc.) by presupposing that the world is old and came to exist by natural means (3). That is, no one is without bias. Old-earth Christians and secularists assume the world is old and see evidence that the world is old, while creationists assume the world is young and see evidence that the world is young. A full-fledged defense of either position is well beyond the scope of this paper; but as a case in point, let us consider one tool science has given us, and examine how definitive, rock-solid proof is much harder to come by than we tend to imagine.
One oft-referenced “proof” that the world is older than the Bible implies is carbon dating, an advanced scientific technique that is reported to provide absolute dates for things well more than 6,000 years old. As a creationist, I had always supposed that if carbon dating contradicts the Bible, carbon dating must be wrong, or based on faulty assumptions. But such an off-hand dismissal does not do credit to my position if I do not even know how carbon dating works. Here, I look into the technique of carbon dating, the assumptions that must be made in order to use it for dating objects, and the measure of uncertainty in its results. Can the raw data provided by carbon dating be reconciled to Biblical creation if viewed with Biblical assumptions? If not, what does that mean for the creationist perspective?
Overview of Carbon Dating
The measurement of radioactive carbon as a dating technique was first proposed by Libby, Anderson, and Arnold. The basic idea is as follows. The most common and stable form of carbon is 12C (6 protons + 6 neutrons), but in the upper atmosphere, cosmic rays colliding with atmospheric particles produce free neutrons, which in turn can interact with atmospheric nitrogen (14N, 7 protons + 7 neutrons) to produce the carbon isotope 14C (6 protons + 8 neutrons). This 14C enters the global carbon cycle, dissolved in the oceans as carbonate and bicarbonate ions, and also taken in by plants as carbon dioxide and thence consumed by animals (4). So, every living thing acquires some amount of 14C mixed in with its normal 12C—roughly 10-12 grams of 14C for every gram of 12C. Carbon-14 is unstable and decays by radioactive β emission back to stable 14N, but it is replenished in the atmosphere by more cosmic rays and thence distributed by the carbon cycle, so the supply of 14C is constantly renewed. But whenever a living thing dies, it is removed from the carbon cycle—its 14C is no longer replenished, and decays away (5).
Since its discovery by Becquerel in 1896, radioactive decay has been thoroughly studied and is well-characterized. The amount of a radioactive material is known to decrease exponentially with time. This type of decay is defined by a half-life, the amount of time required for some amount of radioactive material to decay to half that amount. The half-life of 14C is about 5,720 years, meaning that if we have a 10-gram lump of 14C now, in 5,720 years we would have a lump with 5 grams 14C and 5 grams 14N (6). This well-known decay pattern makes it possible to calculate, given a present amount of 14C, how much 14C there was at any point in the past.
Libby and co-workers proposed that by measuring the current amount of 14C in a once-living object, and estimating the amount of 14C in it when it died, one could calculate, to within a few hundred years, how long ago that object died (or was removed from the global carbon cycle). This, of course, requires accurate measures of the current and original 14C content. Measuring the amount of a radioactive substance is straightforward in theory, as the amount of radiation (measured as “counts” per unit time) is directly proportional to the amount of radioactive substance. But this can be difficult in practice, as the amount of 14C, and therefore, detectable radiation, is very low. Libby describes a method in which a sample to be dated is first processed to isolate the carbon and convert it into a convenient chemical form, then this refined carbon sample is placed in a sophisticated radiation detection device (7). The detector must be very sensitive to “see” the carbon radiation, but also shielded from any background radiation, as natural radiation sources could drown out the weak 14C radiation. But these technical difficulties can be overcome, and techniques for measuring 14C content have improved—in less than thirty years after Libby’s introduction of carbon dating, methods were reported for measuring radiation in carbon samples as small as a few milligrams, though they were expensive and time consuming (8). More recently, 14C content is also measured by atomic mass spectroscopy, which separates atoms by mass (14C being heavier than 12C), and it is possible to date samples with less than 0.03 mg of carbon (8).
But knowing the current carbon-14 content is not enough. It is also necessary to know the amount of carbon-14 present when the subject died (or was removed from the carbon cycle). Since this happened a long time in the past, there is no way to measure it, and it must be estimated with a series of assumptions and approximations. In their first paper on carbon dating, Libby, et al. presented measurements of 14C in modern samples of wood from around the world, showing that they had similar levels of radioactivity—on average 12.5±0.2 counts per minute per gram (± one standard deviation). They then used this average value as the original 14C content, assuming that the worldwide 14C distribution was the same in ancient times as today. But this assumption is not quite correct:
"But the amount of 14C produced in the atmosphere varies with the sun's solar activity and fluctuations in Earth's magnetic field. This means that the radiocarbon clock can race ahead or seemingly stop for up to 5 centuries. As a result, raw radiocarbon dates sometimes diverge from real calendar years by hundreds or even thousands of years. Thus researchers must calibrate the clock to account for these fluctuations, and that can be a challenge." (10)
International calibration curves have been developed for this purpose, based on fossilized trees and marine deposits. Such samples have “known” calendar ages—derived, for example, from tree ring chronologies. Carbon dating these samples then provides a “radiocarbon date” which is correlated to the “known” calendar age. Tree-ring data is used for up to about 12,000 years ago, and marine data beyond that. These calibrations are, of course, subject to their own assumptions and sources of error, and ongoing refinement of the calibrations can change measured “dates” by thousands of years (11). It is believed that such calibration methods can be extended to 50,000 years before present (12). Beyond that, too little 14C would remain to be detected—after all, 50,000 years is about nine 14C half-lives, so no more than (0.5)9 = 0.2% of the original 14C would remain.
Aside from the estimation of and adjustment for ancient atmospheric 14C fluctuations, other assumptions must be made for carbon dating to be useful. First, one must assume that 14C decay is truly exponential, with a constant half-life. This is not in doubt, as radioactivity is well-understood, but since we cannot observe 14C decay over periods comparable to its half-life, it should be kept in mind. Even the value of the 14C half-life is subject to some uncertainty, with measured values ranging from 5,500 to 6,000 years.
Other important assumptions are “the constancy of the cosmic radiation intensity and the possibility of obtaining unaltered samples” (13). The former is not strictly true, but attempts have been made to correct for this with calibrations, as discussed. The latter is more troubling. After thousands to tens of thousands of years, a sample is bound to experience changes to its carbon content. Simple contamination by other sources of carbon (younger or older) can ruin a radiocarbon date, and sophisticated “pretreatment” techniques have been developed which attempt to isolate carbon only from the original sample, in some cases attempting to pick out specific amino acids (14). There is also the possibility of carbon atoms in the sample being exchanged with others in the environment, the possibility of contaminating samples during pretreatment, and countless other potential problems to consider on a case-by-case basis. Last, it is necessary to assume that radiocarbon is, and has been, uniformly distributed among air, water, plants, and animals over the whole world, so that anything dying at a given time will have the same 14C content as any other thing dying at that time. Libby, et al. provided evidence that this is more or less the case in present times, but it is not strictly true (15). While it would be possible to characterize the distribution of 14C in present times, it is impossible to be certain what it was like thousands of years ago.
So we see that carbon dating not only requires sensitive and difficult measurements, but is also subject to various assumptions about the past, that may be reasonable and well-justified, but cannot be “proven” in the same experimental sense as the current carbon content can be.
Checking the Results
In the end, the only way to verify the accuracy of carbon dating is to use it to date objects of known age, and compare the radiocarbon date with the actual date. Arnold and Libby did just this with samples of wood obtained from archaeological sites and dated by archaeological and historical evidence, or in some cases by tree rings. The ages of their samples ranged from 1,000 to 5,000 years, and their radiocarbon dates agreed well—all but one within one standard deviation of the known age (the standard deviations were on the order of a few hundred years). Libby later reported difficulty in obtaining older historical samples:
"Arnold and I had our first shock when our advisers informed us that history extended back only for 5000 years. We had thought initially that we would be able to get samples all along the curve, back to 30,000 years before the present; we would put the points in, and then our work would be finished. You read statements to the effect that such and such a society or archeological site is 20,000 years old. We learned rather abruptly that these numbers, these ancient ages, are not known accurately; in fact, the earliest historical date that has been established with any real certainty is about the time of the 1st Dynasty in Egypt." (16)
Another early study by Flint and Deevey compared radiocarbon dates to dates determined by geologic stratigraphy (location in rock layers), reaching back to 20,000 years. Their results were “highly encouraging”, but did “not at once provide a complete and incontrovertible time-sequence” (17). More recently, studies have focused not on checking radiocarbon dates against known samples, but on using samples of known age to improve the calibration curves discussed above (18). While such calibrations allow carbon dates to be reconciled with and improved by other dating techniques, they rely upon the assumption that all the techniques involved are trustworthy; it is difficult, if not impossible, to verify any method one against an objective, absolute date.
Of course, there have also been documented failures of radiocarbon dating. Bird, et al. report radiocarbon dating results of charcoal in northern Australia in which radiocarbon dates were known to be incorrect below a depth of 150 cm, due to interaction with groundwater (19). There are several cases where parts of living or recently-deceased animals were carbon-dated to several thousand years ago, usually because the organisms had access to old, marine carbon sources (20). Also, many samples that other methods date to be hundreds of thousands to millions of years old — so that they should have no detectable 14C — such as fossil fuels and diamonds, have been found to contain 14C, implying that they are less than 50,000 years old (21).
Objections to Carbon Dating
So far, I have described carbon dating and some of the theory, measurements, and ideas that underlie it. Within the scientific community at large, it seems to be considered a well-established and reliable dating method, though subject to assumptions and sources of error, as is any indirect measurement. There is little doubt that we understand the physics of carbon-14 decay, can observe the present carbon-14 cycle, and can measure accurately the amount of carbon-14 in a sample; and even hard-core creationists do not argue with this empirical evidence (22). But the fundamental necessity of making certain assumptions about the ancient earth may leave room for reinterpretations of the raw data.
First, it should be noted that carbon dating cannot be used to prove that the Earth is millions of years old. As mentioned above, carbon dating is limited to about 50,000 years, as the radiation becomes too weak to measure with practicality. There are radiometric techniques that, in theory, can be used to date things millions of years old, by using much longer-lived radioactive species, such as uranium-238 (23). These methods also have implications for creationists, but a discussion of them is beyond the scope of this paper.
It is critical in carbon dating to know the amount of 14C present at the time of death. As discussed above, this is calculated by assuming that the amount of 14C in the atmosphere has been constant for tens of thousands of years, with corrections made based on ancient tree rings and marine sediments. Implicit in this is the idea that the overall rates of 14C production and disintegration are equal, so that the total amount stays more or less the same. And this is reasonable if the Earth has been exposed to cosmic radiation for a long period of time. But from a creationist perspective, the Earth is young enough (a little older than one half-life of 14C) that such a steady-state should not yet have been established, so that the rate of 14C formation in the atmosphere should exceed its overall rate of decay (24). This is perhaps the strongest alternative explanation young-earth scientist have to offer at present, and there has been some evidence that it is the case (25). If this is the case, ancient atmospheric 14C levels would be lower than predicted, making all samples appear to be older than they are in reality. Furthermore, in Genesis 6-8, the Bible states that there was a catastrophic flood that covered the whole Earth. Many creationists believe that the Earth’s atmosphere was different before the flood, further affecting ancient 14C levels.
But what about the calibration curves? Do they not account for changing 14C levels? These calibrations attempt to make small corrections to the model of a roughly constant 14C content, and likely do not even consider the drastic model change that would be required for a young-earth-consistent reevaluation. Moreover, recall that these calibrations were made by carbon dating samples of “known” ages, where those ages were determined by other dating methods. But as these methods themselves yield ages in the tens of thousands of years, they carry with them presuppositional assumptions similar to those that precede carbon dating analysis, and would likewise be questioned by proponents of a young Earth. No doubt they deserve a more thorough treatment, but once again, that is beyond the scope of this paper.
Finally, carbon dating, like any scientific method, is subject to personal bias (on both sides), and the fact that carbon dating is both checked by, and used to check, other dating methods can lead to age determinations that border on circular reasoning. Early in the history of carbon dating, Flint and Deevey observed:
"According to one’s point of view, it is refreshing or discouraging to recall the number of instances in which a direct challenge, offered by a radiocarbon date to a previously published stratigraphic assignment, has resulted in significant modification or withdrawal of published statements and cherished views. At the same time, in some of the same instances, acceptance of the radiocarbon dates requires a mental balancing of probabilities that is suspiciously close to special pleading. The new method, like older stratigraphic methods, is subject to the possibility of error, recognized and unrecognized, and at present it is impossible to be sure that any opinion about a date, favorable or unfavorable, is free from special pleading." (26)
Radiocarbon dating is a technique for measuring the “age” (length of time since removal from the carbon cycle) of certain samples, most often living organisms or carbonaceous marine sediments. It relies upon the measurement of carbon-14 content in the sample and estimation of atmospheric carbon-14 levels at the time of death. Within an old-Earth perspective, carbon-dating is well-established and reliable, and, with calibration, can be made to fit in well with other dating methods. But from the perspective of young-Earth creation, atmospheric 14C levels may have been vastly different in the recent past (~6,000 years ago at creation), and the dating methods against which radiocarbon dating is verified are themselves in question. While carbon dating is a viable and useful technique, it relies upon certain assumptions and perspectives. If these are called into question, the derived ages may be radically re-evaluated.
This is, in fact, a characteristic of all indirect measurement methods, especially prominent in sciences dealing with the distant past. The whole body of scientific data only extends back a few centuries, and any new information we gain must be based upon observations made in the present day. Using this data to gain a picture of the universe millions or billions of years ago is an exercise in extrapolation, and is only possible under certain assumptions. If the assumptions change, so does the picture of the past. We tend to think of knowledge, especially “scientific” knowledge, as fact—as if it were wholly founded upon objective experimental data and sound deductive reasoning—but a great deal of it can only be derived from inference and inductive reasoning, not providing the one conclusion inevitably drawn from the facts, but offering a possible solution that can explain all the facts. When facts are not directly verifiable, it is necessary to make simple generalizations and assumptions: whether it be that natural processes have, for millions of years past, carried on as they do today (as naturalists suppose), or that everything supernaturally came into existence fully formed (as Creationists suppose).
One might wonder why, if creation is a viable hypothesis, it has not produced a robust theory that can explain all the empirical evidence as well as, if not better than, the secular chronology. The answer is not necessarily that creationism is flawed, but possibly that, compared to secular science, is still a small and young movement. It has not had the time or funding to develop a comprehensive history. Neither is the secular chronology without its gaps and contradictions. The nature of science is to constantly evolve and (we hope) gradually approach the truth; it may be that a theory will be forthcoming to convince everyone. But I suspect that all our data will always be open to different interpretations. In the meantime, it is important for both groups to understand, appreciate, and critique each other’s arguments—after all, though we can’t all be right, we could all be wrong. One must keep in mind that one’s opponent’s position is often based not on ignorance or deceit, but a different perspective and different assumptions. All inference, and thus all models and theories, is predicated on and influenced by preexisting knowledge and beliefs. Humans are bound to come to different conclusions as long as we have different ideas about the nature of Reality, Knowledge, and Truth itself. And these are questions science can’t answer.
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