Calibration curves and the challenges of C-14 radiocarbon dating

Director of the Radiocarbon Dating Laboratory at the University of Waikato Associate Professor Fiona Petchey talks about radiocarbon dating calibration curves and the challenges they present.

A calibration curve is a technique used to match a C-14 concentration value with samples of a known age (such as a tree ring) to determine a more accurate calendar age.

Questions for discussion

• What organic materials have been predominantly used to build up the atmospheric and marine calibration curves? Why were these materials used?

• Why is there a different calibration curve for marine samples and artefacts?

• Fiona says that one factor that enables radiocarbon labs to be able to build a calibration curve is funding. What do you think she means by this?

Transcript

Associate Professor Fiona Petchey

With radiocarbon dating, it’s not possible to date to a single year. So when I refer to ‘event’, I’m not talking about somebody turning up 2,003 years ago. I’m talking about a more general range – within 20, 30 perhaps even 50 years depending on the exact age – so the event of the arrival of humans on an island or a new activity like farming or horticulture starting up. So they’re not events that you can tie to a specific date, but they’re events that we can tie to a broader 20, 30, 40 year span.

When we measure the radiocarbon in a sample, what you actually get is just a C-14 concentration value. It has some relationship to age, but it does not give you an exact calendar age. A long time ago, scientists noticed that the production of C-14 changes over time. It’s affected by solar flares and the geomagnetic field of the Earth and a whole host of other things. So in order to actually get a calendar age from this raw C-14 value, we had to develop these things called calibration curves.

To build a calibration curve is a very time-consuming process. It takes thousands of radiocarbon dates and other specialists that do the dendrochronology – so the tree ring counting. It’s built up over time by those radiocarbon labs that have access to suitable material and also funding to be able to build a calibration curve.

And the way those are done is you get something really old like a kauri tree – every year, a tree will put on a new growth ring and we carbon date those rings and see how the C-14 content varies as we count back in time. And what scientists found was that there were these things called wiggles. That’s points where C-14 production is higher or lower rather than a steady state like physics predicted. So instead of having a straight diagonal line from today back into the past as you’d normally expect if production equalled the decay of C-14, we end up with this very, very wiggly line that deviates off.

So at this point in time, we have essentially three calibration curves – north atmospheric and southern atmospheric built from tree rings and a marine calibration curve which is built up from a wide range of different materials like corals, which again put on yearly growth, but also other things like sediment layers that deposit on a yearly basis.

This marine calibration curve relates to the whole world. And because there are regional differences right round the world from this global marine curve, we have to work out regional offsets and apply those to this calibration curve. And that’s where most of my work has come into play is looking at this global marine calibration curve and recognising that it does not work very well in the South Pacific.

C-14 production largely occurs in the upper atmosphere, and C-14 after it is produced gets taken up by plants and animals and absorbed into the ocean. Now the problem with the ocean is it’s such a big reservoir with multiple layers where C-14 actually gets dragged to the bottom and held down there for thousands of years before cycling back up to the top. As a result of this, if you date a twig today, you’ll get an age of zero, say. And if you date a living shellfish on the beach, it will be 500 years older than that twig, so that’s why we have two separate calibration curves.

Different groups are beginning to see not just regional offsets in the marine calibration zone but also within the atmospheric areas. So there’s a lot of work going on in different labs all around the world to look at minor offsets from these calibration curves that could have an impact on very precise dating efforts.

Acknowledgements

Vertical timeline, A new chronology for the Maori settlement of Aotearoa (NZ) and the potential role of climate change in demographic developments. Published by PNAS. Vol. 119 | No. 46, 15 Nov 2022. Tree being cut up for dendrochronology, fdcomite, CC BY-2.0. Tree biscuit with dendrochronolgical drill and core samples, Hannes Grobe/AWI, CC-BY-SA-2.5. Coral sample, WoVhuman Fauor, CC BY-SA 4.0. Animation of carbon cycle from What is the carbon cycle? NOAA’s National Ocean Service. Ocean circulation animation from The Great Thermohaline Circulation The Great Ocean Conveyer Belt. NASA.