Radiocarbon dating shells
Radiocarbon dating expert Associate Professor Fiona Petchey explains why her research focuses on shell artefacts in New Zealand and the Pacific.
Shell is a common material found on archaeological sites around New Zealand and the Pacific, but it is difficult to radiocarbon date because it takes up carbon from both the terrestrial and marine reservoirs. Fiona explains how she is seeking to improve the dating of these artefacts.
Questions for discussion
Why are shells problematic for radiocarbon dating experts?
How does Fiona identify shells related to human activity?
Why is she interested in these shells, and why are they of importance to people looking at the past in New Zealand?
Note: In this video, Fiona says it is important to “date something that is representative of the event that you wish to get an age on”. To understand what she means by ‘event’, watch the video Calibration curves and the challenges of C-14 radiocarbon dating.
Transcript
Associate Professor Fiona Petchey
I’m interested in the marine calibration curve, so I’m trying to find ways to interpret animals, shells, whatever that live in or eat food from both reservoirs. You know, say there was 20% marine influence on this shell and the rest is terrestrial influence. I’m trying to find ways to correct for that.
But by isolating those parts of shells that is only influenced by the marine environment, I am looking at the offset from the marine calibration curve, and I’m hindered by the possibility of a mixed signal from terrestrial material.
This is I think where most of my research differs from everybody else is that shells have been largely ignored from building any sort of global curve or any regional curves because they do take up carbon from many different sources. But then around New Zealand, the materials that are commonly used in developing marine calibration curves such as coral is quite rare.
A lot of the shells that I am interested in are estuarine shells. They’re living between the land and the sea and so they pick up carbon from both of these what we call reservoirs. I’m also interested in purely marine shell, which just live out in the ocean, and again, they give us very different pictures. So the ocean shells tell us about the carbon in the oceans at that time whereas the estuarine shells might tell us a little bit about the relationship between the ocean and the land and how things might have changed with the sea levels or whether there’s been deforestation and more sediment put into the estuaries.
Most of the shells that I am dating are associated with human activity. They’re usually food shells thrown away by the ancient inhabitants. And I’m interested in them because I can not only link the shells to the activity of those humans but they also tell me something about the environment at the time those humans gathered them.
The reason why we know that these shells are associated with human activity is that they are found in deposits that contain remnants of other human activities such as charcoal from fires, stone tools or other artefacts.
Shells can be used to make fish hooks– so manufacturing remnants. It can also be personal adornment – so jewellery – and also other food waste that you wouldn’t expect in a natural shell deposit such as the remains of bird bones or dog bones and sometimes even human bones.
One of the most important things in radiocarbon dating is to date something that is representative of the event that you wish to get an age on. Sometimes finding a shell in a food midden, it is possible that, during collection of that food material, we have picked up a dead shellfish that is not associated with that event. Whereas in the Pacific, dating things like human remains and other animals that definitely were brought by humans such as dogs, chickens and the Pacific rat, they tell you exactly about the event of the human arrival.
By using marine shellfish and potentially estuarine shellfish, I can build up a much bigger database of values that we’ve collected and I have a bigger amount of resource material available to me to do my research.
Acknowledgements
Fiona collecting coral samples in Tokelau; shell bands; shell excavation site; Mariana Islands shell artefact; Bank cutting showing shell midden and Fiona sampling peat. Dr Fiona Petchey. Shell samples, Petchey, F., Piper, P., Dabell, K., Brock, F., Turner, H., & Lam, T. (2022). Dating Thach Lac: cryptic caco3 diagenesis in archaeological food shells and implications for 14c. Radiocarbon, 64(5), 1093-1107. doi:10.1017/RDC.2022.63. Fiona collecting shell samples from estuary with Katy Anderson, and still of shells and bones from midden, Dr Louise Furey. Ocean and beach, Escaping Comfort Zone, CC BY 3.0. Shell adze blade, Épi, maker unknown (FE002707). Purchased 1954. Shell fish hook, Tuvalu, maker unknown (FE000439). Augustus Hamilton Collection. Purchased 1914. Kuri, Canis lupus familiaris, collected 1876, between “Waikava” & Mataura plains, Catlins, New Zealand (LM000828/1). Gift of Mr Anderson, 1876. All Te Papa, CC BY-NC-ND 4.0. Lapita shell jewellery, Patrick Nunn, CC BY-SA 4.0. Lapita dog jaw bones, Tiina Manne, Bruno David, Fiona Petchey, et al. How long have dogs been in Melanesia? New evidence from Caution Bay, south coast of Papua New Guinea, Journal of Archaeological Science: Reports, Volume 30, 2020. Archaeologist working on Lapita burial site, Oceanic Explorations: Lapita and Western Pacific Settlement, Chapter 14 The excavation, conservation and reconstruction of Lapita burial pots from the Teouma site, Efate, Central Vanuatu, Stuart Bedford, Matthew Spriggs, Ralph Regenvanu, Colin Macgregor, Takaronga Kuautonga and Michael Sietz. Terra Australis 26, 2007. CC BY-NC-ND 4.0. Plastic bag containing rat bones from several specimens. A70.603. Puke Ariki. Chicken bone impressions, Fiji. Elizabeth Shaw, Anthropology Photographic Archive (https://digitool.auckland.ac.nz/R/-?func=dbin-jump-full&object_id=536008&silo_library=GEN01), the Department of Anthropology, University of Auckland.
