Correcting the curve
Associate Professor Fiona Petchey is involved in research projects that aim to refine radiocarbon dating of marine samples and artefacts in New Zealand and the South Pacific. She is focused on refining regional offsets for the marine calibration curve to enable radiocarbon experts to infer more accurate calendar dates from the C-14 concentration value (conventional radiocarbon age or CRA).
Jargon alert
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.
An atmospheric calibration curve is used for artefacts whose origins are on the land.
A marine calibration curve is used for artefacts whose origins are in the sea.
Questions for discussion
Fiona talks about the human inclination to see patterns – how did she assure her data was showing a pattern and it wasn’t just her seeing a pattern where there wasn’t one?
What does Fiona mean when she says “incorrect interpretation of the timing of some of those things will have a domino effect all the way down the line”?
Transcript
Associate Professor Fiona Petchey
A lot of my work has been involved in looking at why C-14 doesn’t seem to work in a particular situation. I started to look at shellfish and limestone environments and how they were taking up carbon from the limestone instead of just the marine environment. Limestone can be 50,000 years old, it can be 100,000 years old. So if a shellfish is incorporating the carbon from limestone into their shell, then you’re adding ancient carbon into an animal that’s living, and it skews the age. A lot of the islands in the Pacific have got limestone cores to them, so I wanted to know if I could correct for this.
I’m basically taking shells and terrestrial material – whether it is a bird bone or a piece of charcoal from the same context – and I’m then trying to work out the marine correction from that. So it’s that 500-year separation that is assumed between atmospheric and marine. And in some points in time, that correction actually reduces to only 100 years separation, and in some places, it could be 6 or 700 years. So it fluctuates. And I’m trying to get a greater understanding and develop a correction back through time. So I have to go out and find a site that’s 200 years old, a site with material that’s 300 years old, a site that’s 500 years old and so on to see how that offset changes back through time.
I was looking for patterns in the stable isotopes and the C-14 age to see if there was an offset occurring between shells that I thought were entirely marine and those that were partly estuarine. The patterns that I’ve seen in the marine calibration curve tell us that our interpretation of some marine radiocarbon dates has been wrong. It doesn’t make the raw radiocarbon values incorrect – they’re still perfectly fine – but how we’ve interpreted that result and turned it into a calendar age has been wrong.
People generally see patterns all around us every single day. So when I started to see offsets in my archaeological dates, I needed to check, so I did about 20 dates on that one particular site just to prove that I wasn’t fooling myself and there was actually a pattern here, and then did the same sort of thing in Tongatapu to see if I could predict, based on my theory, which shells were going to give me the right age or the wrong age.
I’m interested in humans and the expansion of human beings through the South Pacific, how they adapted to the environments that they came across, and so incorrect interpretation of the timing of some of those things will have a domino effect all the way down the line. So I want to get a much tighter picture of what actually happened through the Pacific. Many of these events are way beyond the stories and the genealogies and all those sort of things that you can get from the people that still live in these places.
I’m specifically interested in sites that are round about 600 years old, because that is shortly after Māori ancestors got to New Zealand. And it’s the start of the Little Ice Age, and that climatic change appears to have made a major offset between the terrestrial C-14 signal and marine C-14 signal.
Acknowledgements
Photos of limestone rubble and beach, Dr Steve Hood. South Tarawa Atoll, Government of Kiribati, CC BY 3.0. Satellite image of Atafu Atoll, Tokelau, NASA. Boat above coral reef, Fondation Tara Océan, CC BY 3.0. Underwater coral reef, Hideaway Holidays, CC BY 3.0. Fiona collecting coral samples in Tokelau, Dr Fiona Petchey. Graphs and 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. Table of Charcoal and shell radiocarbon dates from Unai Bapot Area showing pattern, Petchey, F., & Clark, G. (2021). Clarifying the age of initial settlement horizon in the Mariana Islands and the impact of hard water: a response to Carson (2020). Radiocarbon, 63(3), 905-913. doi:10.1017/RDC.2021.2.
