Radiocarbon Dating - Reliable but Misunderstood Dating Technique

How does the first and best-known archaeological dating technique work?

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Gerry McCormac and Paula Reimer at the CHRONO Centre, Queen's University Belfast
Gerry McCormac and Paula Reimer at the CHRONO Centre, Queen's University Belfast. ©Queen's University Belfast

Radiocarbon dating is one of the best known archaeological dating techniques available to scientists, and the many people in the general public have at least heard of it. But there are many misconceptions about how radiocarbon works and how reliable a technique it is.

Radiocarbon dating was invented in the 1950s by the American chemist Willard F. Libby and a few of his students at the University of Chicago: in 1960, he won a Nobel Prize in Chemistry for the invention.

It was the first absolute scientific method ever invented: that is to say, the technique was the first to allow a researcher to determine how long ago an organic object died, whether it is in context or not. Shy of a date stamp on an object, it is still the best and most accurate of dating techniques devised.

How Does Radiocarbon Work?

All living things exchange the gas Carbon 14 (C14) with the atmosphere around them—animals and plants exchange Carbon 14 with the atmosphere, fish and corals exchange carbon with dissolved C14 in the water. Throughout the life of an animal or plant, the amount of C14 is perfectly balanced with that of its surroundings. When an organism dies, that equilibrium is broken. The C14 in a dead organism slowly decays at a known rate: its "half life".

The half-life of an isotope like C14 is the time it takes for half of it to decay away: in C14, every 5,730 years, half of it is gone.

So, if you measure the amount of C14 in a dead organism, you can figure out how long ago it stopped exchanging carbon with its atmosphere. Given relatively pristine circumstances, a radiocarbon lab can measure the amount of radiocarbon accurately in a dead organism for as long as 50,000 years ago; after that, there's not enough C14 left to measure.

Tree Rings and Radiocarbon

There is a problem, however. Carbon in the atmosphere fluctuates with the strength of earth's magnetic field and solar activity. You have to know what the atmospheric carbon level (the radiocarbon 'reservoir') was like at the time of an organism's death, in order to be able to calculate how much time has passed since the organism died. What you need is a ruler, a reliable map to the reservoir: in other words, an organic set of objects that you can securely pin a date on, measure its C14 content and thus establish the baseline reservoir in a given year.

Fortunately, we do have an organic object that tracks carbon in the atmosphere on a yearly basis: tree rings. Trees maintain carbon 14 equilibrium in their growth rings—and trees produce a ring for every year they are alive. Although we don't have any 50,000-year-old trees, we do have overlapping tree ring sets back to 12,594 years. So, in other words, we have a pretty solid way to calibrate raw radiocarbon dates for the most recent 12,594 years of our planet's past.

But before that, only fragmentary data is available, making it very difficult to definitively date anything older than 13,000 years. Reliable estimates are possible, but with large +/- factors.

The Search for Calibrations

As you might imagine, scientists have been attempting to discover other organic objects that can be dated securely steadily since Libby's discovery. Other organic data sets examined have included varves (layers in sedimentary rock which were laid down annually and contain organic materials, deep ocean corals, speleothems (cave deposits), and volcanic tephras; but there are problems with each of these methods. Cave deposits and varves have the potential to include old soil carbon, and there are as-yet unresolved issues with fluctuating amounts of C14 in ocean corals.

Beginning in the 1990s, a coalition of researchers led by Paula J. Reimer of the CHRONO Centre for Climate, the Environment and Chronology, at Queen's University Belfast, began building an extensive dataset and calibration tool that they first called CALIB.

Since that time, CALIB, now renamed IntCal, has been refined several times--as of this writing (January 2017), the program is now called IntCal13. IntCal combines and reinforces data from tree-rings, ice-cores, tephra, corals, and speleothems to come up with a significantly improved calibration set for c14 dates between 12,000 and 50,000 years ago. The latest curves were ratified at the 21st International Radiocarbon Conference in July of 2012.

Lake Suigetsu, Japan

Within the last few years, a new potential source for further refining radiocarbon curves is Lake Suigetsu in Japan. Lake Suigetsu's annually formed sediments hold detailed information about environmental changes over the past 50,000 years, which radiocarbon specialist PJ Reimer believes will be as good as, and perhaps better than, samples cores from the Greenland Ice Sheet.

Researchers Bronk-Ramsay et al. report 808 AMS dates based on sediment varves measured by three different radiocarbon laboratories. The dates and corresponding environmental changes promise to make direct correlations between other key climate records, allowing researchers such as Reimer to finely calibrate radiocarbon dates between 12,500 to the practical limit of c14 dating of 52,800.

Constants and Limits

Reimer and colleagues point out that IntCal13 is just the latest in calibration sets, and further refinements are to be expected. For example, in IntCal09's calibration, they discovered evidence that during the Younger Dryas (12,550-12,900 cal BP), there was a shutdown or at least a steep reduction of the North Atlantic Deep Water formation, which was surely a reflection of climate change; they had to throw out data for that period from the North Atlantic and use a different dataset. We should see some interesting results in the very near future.

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