How Trustworthy is Carbon Dating?

How Trustworthy is Carbon Dating?

Would you trust a dating technique that said living mollusks had shells 2,300 years old, or worse, 27,000 years? What if that same technique yielded dates for Triassic wood (when the dinosaurs lived) at 34,000 years and dated millions-of-years-old coal, oil, and even diamonds at less than 100,000 years? Even with these weird––and challenging from an old-earth perspective––results, radiocarbon (or, carbon-14) dating remains one of the best tools for determining the ages of things that lived from 500 to 50,000 years ago.

First developed by W. F. Libby and others in 1949, radiocarbon dating revolutionized archaeology––and other scientific fields––by establishing robust dates for organic materials of a biological origin like wood, bone, or shell. Carbon-14 (14C) is a naturally occurring radioisotope of carbon and is found in trace amounts on Earth. It is produced in Earth’s atmosphere as cosmic rays hit nitrogen molecules and is then absorbed from the air by plants, which then pass it on to animals in the food chain.

Carbon-14 decays to nitrogen-14 by emitting an electron and a neutrino, and it does so with a half-life of 5,730 years. Thus, if one started with 1,024 atoms of carbon-14, after 5,730 years, only 512 would remain. After 11,460 years (two half-lives), only 256 atoms are left. After ten half-lives (or 57,300 years), less than one-thousandth of the original amount remains.

Years 14C Atoms Remaining
0 0 1,024
1 5,730 512
2 11,460 256
10 57,300 1

All life requires carbon and, chemically speaking, carbon-14 acts just like the far-more-abundant carbon-12. Any living thing will incorporate carbon-14 into its body until it dies, after which no new carbon-14 enters and any previously incorporated amount decays as described. Given this knowledge, to extract a radiocarbon age for an organism one needs to know the initial and final amounts of carbon-14 as well as the half-life.

Although conceptually simple, difficulties and complications arise when trying to measure these three parameters, and a misstep anywhere usually leads to an incorrect date. Consequently, scientists go to great lengths to ensure good measurements.

Measuring the Half-life

Typically scientists measure the number of decays from samples of carbon dioxide gas containing a known fraction of carbon-14.

Because radioactive decay occurs in the nucleus of carbon atoms, decay rates are almost impervious to any environmental conditions such as corrosive chemicals, temperature, and pressure. Yet scientists have measured changes in a handful of decay rates. Does this mean radiocarbon dating can yield false results? Two points need emphasis: first, the changes are small—less than one percent—variations in a half-life. Second, the changes are well-understood and accounted for in scientific theories of nuclear behavior.

Changing the decay rates by any significant amount requires altering the laws of physics—something both physicists and biblical authors agree doesn’t happen. Therefore, scientists confidently believe that nuclear decay rates are steady over time.

Determining the Initial Amount

Finding the initial amount of carbon-14 poses the most significant complications for extracting accurate radiocarbon dates. Normally, scientists measure the ratio of carbon-14 to carbon-12 as 14C/12C, because this ratio will be constant for a steady cosmic ray flux (inflow). However, two significant factors cause changes to this ratio in living organisms. First, astronomical and human activities affect the amount of carbon-14 produced in the atmosphere. Second, not all organisms draw carbon-14 directly from this atmospheric reservoir.

Astronomical Effects. Fluctuations in the strength and size of the Earth’s and Sun’s magnetic field affect the number of cosmic rays hitting Earth’s atmosphere and, consequently, the 14C/12C ratio. Left unaccounted, these fluctuations introduce errors in radiocarbon ages that grow with the age. Fortunately, scientists can correct these errors using other techniques for calibration. The most helpful calibration technique utilizes tree ring data to ensure accurate dates for the last 12,000 years and foraminifera and corals to extend the calibration out to 50,000 years.

The strong correlation between carbon-14 and tree ring data over the past 12,000 years validates the general reliability of the two techniques. Anyone wishing to argue otherwise faces two hurdles. First, they must explain why the technique is unreliable. Second, they must explain how a technique as straightforward as counting tree rings correlates so well with an “unreliable” method. A more reasonable conclusion is that the correlation between carbon-14 and tree ring data, in conjunction with other dating methods like varves, provides strong evidence for an Earth older than 6,000 to 10,000 years.

Human Effects. The most relevant human factors affecting carbon-14 production stem from burning fossil fuels (starting in the 1890s) and nuclear bomb testing (starting in the 1940s). However, these errors do not affect anything dating over 150 years old. Fossil fuels like coal and oil formed from the remains of organisms that died millions of years ago. Consequently, all the original carbon-14 these organisms absorbed from the atmosphere has already decayed away. (Although ancient fossil fuel formations will have some carbon-14 because they contain nitrogen and usually are exposed to Earth’s low levels of underground radiation.) Burning these fossil fuels adds a large amount of carbon to the atmosphere, but only in the form of carbon-12. So, burning fossil fuels makes the 14C/12C ratio smaller.

In contrast, the blasts from nuclear bombs radiate abundant nitrogen in the atmosphere, producing significant amounts of carbon-14. Thus, nuclear bomb testing increased the 14C/12C ratio—at one point to twice its preindustrial value.

Reservoir Effects. Because cosmic rays only produce carbon-14 in the atmosphere, not all living organisms incorporate the same initial concentration. Consider three examples.

Trees absorb carbon dioxide directly from the atmosphere (one reservoir). Thus, radiocarbon dates for tree remains require no reservoir corrections and provide the standard for radiocarbon dating.

Seals breathe air from the atmosphere. However, their main food supply is seafood that draws its carbon from underwater sources (a second reservoir) not in direct contact with the atmosphere. It takes some time for atmospheric carbon to mix deeper into the ocean, furthering the carbon-14 decay. This leads to a smaller 14C/12C ratio than in land animals and plants and, unless corrected, older radiocarbon dates.

Shellfish living in a lake surrounded by limestone show a more dramatic (third) reservoir effect. Like oil, limestone forms from the remains of long-dead organisms that are severely depleted of carbon-14. Weathering of the limestone into the lake (where shellfish will incorporate the carbon into shells) dilutes the 14C/12 ratio and, unless corrected, will lead to unduly old ages.

Scientists know of, and correct for, many other reservoir effects as they extract radiocarbon ages. The most important point to remember is that researchers must carefully study each sample to know its general history and environment in order to correct for the many influences on the initial 14C/12C ratio incorporated by the sample.

Measuring the Final Amount

The simplest measurement to understand (and avoid) potential problems is determining the final amount of carbon-14 left in an object. After realizing that radiocarbon dating will likely not work beyond 50,000 years (ten times the half-life), one must avoid sample contamination with other carbon sources (such as cloth contamination in the Shroud of Turin). Scientists have developed general pretreatment procedures to remove many of these contaminants, but again, each sample must be treated individually to account for the exposure to different contamination processes.

While radiocarbon dating is not a trivial task, in sixty years scientists have developed a remarkable understanding of all its subtleties and complexities. This progress supplies a powerful tool for constructing accurate histories of organisms and their surroundings over the last 50,000 years. Any Christian concerned about the historicity of the Bible will welcome radiocarbon dating because it offers the potential to provide more evidence that the Bible accurately describes the record of humanity.