A recent publication in the journal Radiocarbon reports on a new calibration technique for radiocarbon dates between 12,000 and 50,000 years ago. The new calibration promises to shed some light on some of our most puzzling questions to date. Let me explain.
Radiocarbon dating is one of the best known archaeological dating techniques available to scientists, and the general public by and large has heard of it at least. But there are a lot of misconceptions about how radiocarbon works and how reliable a technique it is; this article will attempt to clear them up, and provide a way to explain why the news in Radiocarbon is so exciting.
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 the 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 up to 50,000 years ago; after that there's not enough C14 left to measure.
- Read more on Radiocarbon Dating
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 years—dendrochronology, or tree-ring dating. 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.
- Read more about calibrating radiocarbon
The Search for Calibrations
As you might imagine, scientists have been attempting to discover organic objects that can be dated securely pretty steadily for the past fifty years. Other organic data sets looked at have included varves, which are 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 currents.
In the 2009 Radiocarbon article, a coalition of researchers led by Paula J. Reimer of the CHRONO Centre for Climate, the Environment and Chronology, School of Geography, Archaeology and Paleoecology, Queen's University Belfast, report on the extensive dataset that they used to develop a new radiocarbon calibration called IntCal09. IntCal09 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 new curves were ratified at the 20th International Radiocarbon Conference in June 2009.
Answers and More Questions
There are many questions that archaeologists would like answered that fall into the 12,000-50,000 year period. Among them are:
- When were our oldest domesticate relationships established (dogs and rice)?
- When did the Neanderthals die out?
- When did humans arrive in the Americas?
Most importantly, for today's researchers, will be the ability to study in more precise detail the impacts of previous climate change.
Reimer and colleagues point out that this is just the latest in calibration sets, and further refinements are to be expected. For example, they've 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. It's an interesting paper, and we should see some interesting results in the very near future.
Lake Suigetsu, Japan
In 2012, a lake in Japan was reported to have the potential to further finetune radiocarbon dating. Lake Suigetsu's annually formed sediments holding 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, the Greenland Ice Cores.
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.
Sources and Further Information
Bronk Ramsey C, Staff RA, Bryant CL, Brock F, Kitagawa H, Van der Plicht J, Schlolaut G, Marshall MH, Brauer A, Lamb HF et al. 2012. A complete terrestrial radiocarbon record for 11.2 to 52.8 kyr B.P. Science 338:370-374.
Reimer PJ. 2012. Refining the radiocarbon time scale. Science 338:337-338.
Reimer P, Baillie M, Bard E, Bayliss A, Beck J, Blackwell PG, Bronk Ramsey C, Buck C, Burr G, Edwards R et al. 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP. Radiocarbon 51(4):1111-1150.
