The Case for Carbon Dating

by Brian Dunning
November 25, 2025

This episode was sponsored by Michael Bigelow from Washington, Utah

Sponsor an episode!

You might not think that there is disbelief and distrust around the hardest of hard sciences, like atomic weights and arithmetic. But you would be mistaken. No matter how hard a science is, and no matter how thoroughly its conclusions are validated to the point of them becoming axiomatic, there are people strongly motivated to cast doubt and persuade the public that they are false — whether their motivation be ideological, religious, or otherwise. Today we're going to look at one such example: radiometric dating; more specifically, carbon-14 dating. Because it does have its critics. They come not from the realm of science, but from Young Earth Creationism. In their view, the Earth is 6,000 years old, and no older; because a fairly extreme interpretation of the Old Testament says so. And so, while respecting their religious views and their right to believe whatever they want, we're going to reexamine carbon dating and see if it actually is settled science, or if it needs to be discarded.

In 1650, Archbishop James Ussher published the Annals of the Old Testament, which detailed his study of Genesis and other sources to come up with the age of the Earth. There's a lot more to it than this, but the extra-short abridged version is that he started with the documented death of King Nebuchadnezzar II in 562 BCE. Nebuchadnezzar is a prominent figure in the Old Testament, so Ussher was able to use the names and ages of people who came before him to work backward, eventually going all the way back to the birth of Adam. Genesis 5 and 11 give the ages at which successive patriarchs fathered their sons from Adam through Noah through Abraham through to David and Solomon — a process which results in some fairly remarkable ages, such as Noah living to be 950 years old. From there Ussher used the Biblical lengths of the reigns of various kings, until he got up to Nebuchadnezzar. He added it all up and it came out to 3,442 years. Go back from there at the anchor point of Nebuchadnezzar's death, and you find that Adam was born on the sixth day, in 4004 BCE — a bit over 6,000 years ago.

Ussher was hardly the only Biblical scholar to do this. Many others have replicated his work, to the point that many Young Earth Creationists consider it to be as solid of a settled science as you and I would consider 1 + 1 = 2 to be. Consequently, anything that contradicts it — such as evidence of anything on Earth being older than 6,000 years — must be false. The problem faced by the Young Earthers is there is a lot of such evidence; many, many different lines of it.

Seafloor spreading at the Mid-Atlantic Ridge leaves stripes of geomagnetic lines that record reversals of the Earth's magnetic field, providing a record going back some 170 million years. Corals that grow today show about 365 daily growth lines within each annual growth ring, but fossil corals have been found with as many as 400 — because Earth spun faster long ago, and there were 400 days in a year some 350 million years ago. Ice cores in Greenland have been found with 123,000 continuous years of layers, and using calibration against other cores with overlapping date ranges, ice cores as old as six million years have been found in Antarctica. We can see the Andromeda Galaxy, though its light required 2.5 million years to get here. We can see the remnants of Supernova 1987A which exploded 168,000 light years away. Earth's interior cools at measurable rates as its heat seeps through the crust; to get to where we are today required billions of years of cooling.

However, it seems to be only radiometric dating at which Young Earthers direct their denialism. And even more specifically, at carbon-14 dating. Skeptoid episode #146 told the story of a Creationist who deliberately misused potassium-argon dating on the Mount St. Helens lava dome in an effort to disprove the validity of all radiometric dating, but other than that, it's always carbon dating that finds itself on the receiving end of Creationist ire. I expect the reason is that few people have heard of potassium-argon dating — or the approximately 13 other radiometric dating methods currently in use — but everyone's heard of carbon dating. So make that the boogeyman; it's good marketing.

They could use good marketing, because they're certainly not using good science. Noted Creationist Ken Ham continues his career-long attack on carbon-14. Just in the past year he has attacked the use of carbon dating on fossils that are supposedly "millions of years old" and on diamonds and coal. This tells us he's never so much as read the Wikipedia page on carbon-14, because it's only useful within a range of about 500-55,000 years ago. That's because carbon has a relatively fast decay rate. For a sample likely to be millions of years old, no geologist would use carbon dating. Instead they'd use something like potassium-argon dating, which is useful for rocks and minerals older than about 100,000 years up to several billion.

A very brief refresher on radiometric dating. Nearly everything in nature contains some amount of some radioactive elements: some of these are primordial, left over from the original formation of the Earth; some are cosmogenic, created when cosmic rays strike an atom's nucleus and alter it, like when one hits a nitrogen atom in our atmosphere and converts it into carbon-14. All radioactive elements decay at a measurable and stable rate. The rates are determined by fundamental nuclear forces and so cannot change. Some have tried to claim that conditions on the early Earth were different and so these rates may have been much faster, giving us the appearance of extreme age; but this is not so. Nuclear forces are unaffected by external factors such as temperature, pressure, or the chemical environment.

When an organism is living, the proportions of carbon isotopes in its body match the proportions in the atmosphere. But once it dies it stops taking in new carbon, so those proportions begin changing at this known rate as the carbon-14 decays into nitrogen-14. Measuring the proportion gives us the time since its death. Other radiometric dating methods are used for rocks and minerals; for example, potassium-argon dating measures how long since a volcanic rock was magma. Potassium-40 decays into argon-40 which escapes as gas from magma, but becomes trapped once the rock solidifies; and then the proportions can be measured.

One challenge with carbon-14 dating of organic matter is that the amount of carbon-14 in the atmosphere has not been constant over time. It's been subtly altered by changes in the Earth's magnetic field, by variations in solar activity, and by climate-driven changes in the carbon cycle worldwide. It was even altered by the era of atmospheric nuclear testing. But luckily, we have ample ways to calibrate it, because the planet has provided us many sequestered snapshots of organic material throughout the dating method's 55,000-year useful range.

One useful tool is lakebed sediments, and a great example of this is Japan's Lake Suigetsu. It's a lake with no connecting rivers and no life on the lake bottom, consequently the bottom sediments have been collecting undisturbed for millennia. Alternating layers of dark and light sediments called varves, representing summer and winter sediments, have provided a 76-meter core record representing 150,000 years. These layers provide an atmospheric carbon-14 record three times as old as carbon dating's useful range.

Half a world away, ice cores from the North Greenland Ice Core Project (NGRIP) stretch back nearly as far, 123,000 years — and align perfectly with the Lake Suigetsu record. Significantly, these cores also record the eruption of a volcano in Oregon some 7,700 years ago: Mt. Mazama, today known as Crater Lake — the remnants of that mighty blast. Mt. Mazama filled the atmosphere with tephra (volcanic dust), and that which landed on Greenland eventually made its way down into these ice cores; which we can match using not only the geochemistry and mineralogy of the tephra, but also Mt. Mazama's unique isotopic signature. The layer serves as one of many so-called isochrons: markers in the ancient record tied to specific known dates, to help us keep the record accurate all the way back through time.

As lava flowed down the slopes, it engulfed trees and burned them up, preserving their charcoal — carbonized organic matter which can, today, be carbon dated. Direct readings of the isotope ratios in the 1990s gave an age of 6,845 ±50 years, but once the calibration scales were applied, taking into account the changing amounts of carbon-14 in the atmosphere over the centuries, thanks to the many sources available, gave ages of 7,627 ±150 years — spot on. It was compelling proof that carbon dating not only works, but in concordance with all the calibration sources nature provides, can be spookily accurate.

Not far from Mt. Mazama, scattered about the Northern Great Basin, are at least six archaeological sites associated with the Klamath, Modoc, and Northern Paiute indigenous tribes. Found at all of these sites are the world's oldest footwear, sandals woven from shredded and twined sagebrush bark. Today we can carbon date these fibers and we find that the sandals, in their various locations, are anywhere from 10,400 to 9,100 years old. The Young Earthers would claim here that since no sandals could be older than 6,000 years, carbon dating must therefore be fatally flawed. But in this case, they've failed to account for just one small piece of data — data that covered these sandals.

The largest single concentration of the sandals was found at Fort Rock Cave, which gave the sandals their name we now know them by: The Fort Rock Sandals. Some 75 sandals were found here, most broken or worn, indicating this was likely a trash dump for discarded footwear. They were found via excavation by archeologists from the University of Oregon in 1938 — excavation which involved digging down to them through the nearly-solidified Mt. Mazama tephra that had buried them 7,700 years ago. The Fort Rock Sandals are unambiguously more than 6,000 years old.

And thus the story of carbon dating comes full circle. It might be easy to point at this episode and claim that it is flimsy as it relies only upon single, oddball pieces of information like a lake in Japan and an ice core in Greenland; but there are many, many alternate examples all around the world which we could have used as well; and each is supported by a dozen different lines of evidence. Radiometric dating is among our most robust and reliable tools. It is driven by fundamental atomic forces inside the nuclei of atoms that are, themselves, billions of years old; and form the basis of every material process that exists.

Carbon-14 dating: it works.

Special thanks to Michael Bigelow, who suggested this topic and provided copious amounts of research to get me started.

By Brian Dunning
Follow @BrianDunning