Refining dates for human habitation in the South Pacific
Associate Professor Fiona Petchey, Deputy Director of the University of Waikato Radiocarbon Dating Laboratory, is researching ways to refine the marine calibration curve used for converting carbon-14 values into calendar dates. Her team wants to help develop timelines that show how early peoples, including Māori, moved around and provide an understanding of natural and human impact on island ecosystems.
Sampling shell and charcoal
Dr Fiona Petchey (left) and C-14 pretreatment technician Katy Anderson on location at a Māori midden. Fiona is sampling shell and charcoal for radiocarbon dating.
The problem and why we need a solution
Early in the use of radiocarbon dating, scientists began to see discrepancies between measured ages from the radiocarbon dating process (the conventional radiocarbon age or CRA) and known historical dates for artefacts. These discrepancies were in part caused by the fact that the atmospheric C-14/C-12 ratio, which is a key element in calculating radiocarbon ages, has not been constant throughout history. As a result, calibration curves were developed to enable experts to infer calendar dates from CRAs.
Currently, three curves are available and the limitations of applying these have become apparent over the decades. Fluctuations in atmospheric and marine carbon are often unique to different geographical locations.
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?
Accurate dates are essential for scientists like those studying past climates. Some environmental scientists use shell and marine samples to look at past climates, environments and sea-level rise and fall.
Experts like archaeologists and anthropologists working to track the spread of people through the Pacific also need accurate dates to underpin their work. The study of human migration through Oceania and the South Pacific can provide insights into how early societies lived and adapte d to climat e changes, and it can enhance knowledge of wāhi tupuna (ancestral places).
Why shells?
Shells are a common material found in archaeological sites around the Pacific. Shellfish were a common food source – often discarded in large middens – but shells were also used to make decorative items and tools.
Shells present challenges for radiocarbon dating when interpreting a calendar age, so many researchers have avoided using this material. Fiona Petchey decided to take a closer look to see if there were ways to improve the calibration curve and offsets being applied to shells.
The challenges of dating shells
What makes shells so difficult to date using radiocarbon dating? Shells are found in freshwater, marine and estuarine environments. Within the estuarine environments, marine and terrestrial carbon mix, which means that neither a purely terrestrial (atmospheric) nor marine calibration curve will work.
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.
To obtain a calendar age, it is necessary to understand how the ratio of C-14 and C-12 in the atmosphere and ocean varies geographically and over time. In addition, changes in ocean circulation and upwelling change the ‘age’ of the water in different regions. Ocean upwelling is caused when winds blowing across the ocean surface push water away from an area and cooler, ‘older’, more nutrient-rich water rises from beneath the surface where it may have been stored for thousands of years. Shellfish absorb some of this ancient carbon to make their shells.
Carbon isotopes
The element carbon has three naturally occurring atomic forms – 12C, 13C and 14C. These forms, which differ only in the number of neutrons found in the nucleus, are called isotopes. 12C is the most abundant of the three and is a stable isotope.
Offsets in shell radiocarbon dates can also occur because of the ‘hard water effect’. This is when shellfish in limestone environments take up carbon from water that has drained through the limestone. Limestone forms from ancient calcium carbonate-based animals, and this ancient carbon can make shells living in these waters appear much older. Limestone landscapes and geology are common around Oceania, including in parts of New Zealand and Tongatapu as well as numerous smaller coral atolls.
Solving the shell issues
Fiona has approached her work from a number of different angles, including looking at combinations of different isotopes and dating shell growth rings.
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”?
To improve the chronologies of key settlement events, Fiona has selected samples from archaeological sites in the Mariana Islands and looked at different isotopes in the shells – not just carbon-14, but oxygen-18 and carbon-13. This approach was to see if she could detect a terrestrial signal in those shellfish that had lived in near-shore and estuarine environments. She hoped that, in an area where freshwater drained through limestone, a significant difference would be detected in the age of these estuarine shells compared with shellfish species that live further out in the ocean. Oxygen-18 can help to identify the temperature of the water, and carbon-13 identifies the source of the carbon input. By comparing all three isotopes, Fiona could separate shellfish influenced by cold marine water from those affected by warmer terrestrial water sources. She found errors of 300 years in the dates on some shells from the site studied.
As a scientist, Fiona had to look to see if she could reproduce the results at other sites. Looking at Tonga, she noticed a pattern. At specific times in the past, the magnitude of carbon-14 offset from the existing global marine calibration curve varied. Fiona plotted all the carbon-14 values and dates and identified that “something really strange or different” was happening between the ocean and the atmosphere between about 2,650 years ago and 2,350 years ago. This 300-year period is a significant time in the Pacific – it is when the Lapita people disappeared from the archaeological record, and soon after, a people termed ‘ancestral Polynesia society’ appeared.
Fiona then started to pull together radiocarbon dates from across the Pacific and could see patterns emerging at other times when significant cultural changes are apparent in the archaeological record.
Shell growth rings
Fiona is also investigating how shell growth rings might help to refine radiocarbon dates in the Pacific. Like trees, shells also put on growth rings that reflect the environment. As water conditions change due to tides and rainfall, the isotopes within these rings give us information about changing water conditions and potential sources of carbon that may influence the apparent age of the shell. However, the challenge of shell growth rings is that they’re very small.
Shell growth bands
Trees aren’t the only organism with growth rings – did you know shells also put down growth bands?
Learn why Associate Professor Fiona Petchey is investigating shell growth bands as part of her research to improve radiocarbon dating methods.
What next?
Fiona and her team of archaeologists and anthropologists have received a Marsden Fund grant to continue this research. Fiona and her team have turned their attention to New Zealand, where there are thousands of radiocarbon dates from different sites. Looking at the terrestrial and marine dates from these sites, Fiona has also recognised significant differences between the atmospheric calibration curve and the marine signal shortly after ancestral Polynesian settlers arrived in New Zealand.
Results from some of this research are detailed in the article Ancestral Māori adapted quickly in the face of rapid climate change.
Fiona is excited by the possibility of improving radiocarbon dates during this period to advance our understanding of the Polynesian settlement of the Pacific.
The Aotearoa New Zealand Radiocarbon Database upgrade
The Aotearoa New Zealand Radiocarbon Database (ANZRD) is an open online resource containing conventional radiocarbon ages measured on archaeological materials around New Zealand. The database was set up over 20 years ago but had not been updated since then.
Aotearoa New Zealand Radiocarbon Database
The Aotearoa New Zealand Radiocarbon Database is an online geospatial database for New Zealand archaeological radiocarbon dates. Information in this database comes from published sources, archived archaeological field reports and the New Zealand Archaeological Association records.
The need for consistent and accurate datasets for research and resource managemen t has grown, so a team, including Fiona, worked to upgrade the site. This work included adding new ages, verifying previous datasets and drawing up new protocols to ensure a robust information source.
The upgraded ANZRD contains more than 4,100 C-14 ages from over 1,650 different archaeological sites and some museum samples.
Related content
Learn more about the Lapita cultural complex and the work to uncover more about these early ancestors of many Pacific and Māori people.
Understand the relationship between calcium carbonate-based shells and limestone environments in Calcium carbonate biomineralisation and Limestone origins. Learn how different temperatures impact limestone formation in New Zealand limestone origins.
Watch a video where Fiona Petchey answers the question what is an isotope?.
Visit the Aotearoa New Zealand Radiocarbon Database (ANZRD). The database was upgraded in 2022.
Visit the Waikato Radiocarbon Dating Laboratory for more information about their research.
