Somewhere in Ōwhiro Bay, Wellington, on the 22nd of June 2025, a person was doing what New Zealanders do best on a sunny winter’s day. Gardening. Unremarkably, peacefully, gloriously ordinary gardening. And then, the not ordinary arrived from te pō ki te ao mārama (from the darkness to the light). Human remains, right there in the dirt (the quiet anonymity of this burial — undocumented, unprotected, found by chance — speaks to a broader issue explored here in verse).
The police arrived. The property was locked down. A forensic team established a crime scene perimeter, because that is the sensible, correct thing to do. And then, nine months later, in March 2026, the results came back. No crime. No missing person. These were the remains of a person who lived sometime during the 1600s to 1700s, three to four hundred years ago. Likely a tupuna Māori (Māori ancestor) from pre-European contact times. As is protocol, local mana whenua are now caring for this tupuna who will or has been reinterred at a designated location. Haere, moe mai rā, okioki tonu i Hawaiki pāmaomao.
But here is the question I want you to sit on today. How do we know that? How does a scientist look at a fragment of bone (or other organic remains) and confidently say, “this person died roughly three hundred and fifty years ago”? The answer is radiocarbon dating and it is, without exaggeration, one of the most elegant pieces of science ever applied to the human past (radiocarbon dating was instrumental in establishing that Māori first arrived in Aotearoa between 1250 and 1300 CE — find out more about how we know this). This article explains the scientific marvel that is radiocarbon dating, how it works, how scientists calibrate for variation in atmospheric radiocarbon and what we can date with it.
The Universe is Doing Chemistry At Us, Constantly
Let us begin at the very beginning. Cosmic rays — high-energy particles streaming in from outer space — are constantly bombarding our atmosphere. When they strike nitrogen and oxygen atoms in the upper atmosphere, a cascade of tiny nuclear reactions occurs. One of those reactions converts nitrogen-14 into carbon-14 (¹⁴C), a radioactive isotope of carbon.
Now, carbon comes in three flavours. Carbon-12 (¹²C) and carbon-13 (¹³C) are stable — they will sit there, utterly unbothered, for the rest of the universe’s existence and not change into another state on their own. Carbon-14, on the other hand, is as unstable and anxious as me when trying to parallel park with passengers in the car. It wants to and cannot help but slowly decay back into nitrogen-14.
But here is the crucial bit. Before it decays, carbon-14 binds with oxygen in the atmosphere to form carbon dioxide (CO₂). Plants take in that CO₂ through photosynthesis. Animals eat the plants. Other animals eat those animals. And just like that, radioactive carbon-14 finds its way into every living thing on Earth, including you, right now, as you read this. You are mildly radioactive despite your country’s position on nuclear energy!
While organisms are alive, they continuously replenish their carbon-14 by eating, breathing, and photosynthesising. The ratio of carbon-14 to carbon-12 in a living thing stays roughly constant because it mirrors the ratio in the atmosphere. But when an organism dies? That’s when things get interesting.
Death Starts the Clock
The moment something dies, it stops taking in carbon dioxide or eating food. It stops replenishing its carbon. The unstable carbon-14 built up when the organism was alive, left to its own devices, ticks away, decaying back into nitrogen-14 at a slow, steady and utterly predictable rate. Meanwhile, the amount of carbon-12 does not decay and remains stable in the material of the organism.
The rate of carbon-14 decay is described by its radioactive half-life, which is 5,730 years. That means that after 5,730 years, half of the carbon-14 in any sample will have decayed away. After another 5,730 years or after 2 half-lives totalling 11,460 years, half of that remaining amount will have gone. And so on, halving every 5,730 years, in a beautifully regular exponential decline.
The useful range of radiocarbon dating extends back approximately 50,000 years or 8.7 half-lives. Before that point, the amount of carbon-14 remaining in a sample is too small to measure accurately, and other techniques, like potassium-argon dating and luminescence dating take over. At the other end of the timeline, nuclear weapons testing in the mid-twentieth century injected enormous quantities of artificial radiocarbon into the atmosphere, requiring a separate “bomb curve” calibration for anything that died after roughly 1950 (Hua et al. 2022).
This predictability is the foundation of radiocarbon dating. By measuring the ratio of carbon-14 to carbon-12 remaining in an organic sample and comparing it to the ratio present in living organisms, we can calculate how long ago that organism died. This is the carbon clock, and it was first developed by American chemist Willard Libby and his team at the University of Chicago in 1949 (Libby 1955)— a discovery so significant it earned Libby the Nobel Prize in Chemistry in 1960.
Now, the truly mind-bending thing? The proportion of carbon-14 to carbon-12 in the atmosphere (where it’s most abundant) is approximately one part per trillion. One. In. One. Trillion. If you counted one number per second, it would take you around 31,700 years to count to a trillion. Radiocarbon scientists are finding and counting that one-in-a-trillion atom. Amazing!
But Wait — the Clock is a Little Wobbly
Here is where it gets delightfully complicated, because good science rarely lets you off easily. Libby’s original method assumed that the amount of carbon-14 in the atmosphere had been constant through time. Yeah, nah, turns out it hasn’t been.
The production of carbon-14 in the atmosphere fluctuates based on changes in solar activity, Earth’s magnetic field, and other factors. This means that the ratio of carbon-14 to carbon-12 in a living organism in 800 AD was slightly different to that ratio in 1200 AD or 200 BC. If we do not account for this, our dates will be systematically off.
The solution is calibration. Dendrochronologists (tree ring scientists) realised that trees preserve a perfect natural archive. Each ring in a tree represents one calendar year (you counted them with me in the video!), so by radiocarbon dating individual or sets of rings from long-lived trees and overlapping sequences, researchers built long-term records of past atmospheric carbon-14. These have been extended further using other archives, including corals, cave deposits (speleothems) and lake sediments, creating calibration curves that now stretch back over 55,000 years.
The current standard calibration curves used by archaeologists globally are IntCal20 for the Northern Hemisphere and SHCal20 for the Southern Hemisphere (Reimer et al. 2020; Hogg et al. 2020). Because we are in Aotearoa New Zealand, dates derived from terrestrial samples here are calibrated against SHCal20; a critical but regular task of New Zealand archaeologists.
Why Your Radiocarbon Date is a Range, Not a Year
I want to address something that confuses people. Why do radiocarbon dates always come out as a range? Why can’t the lab just tell me the year the organism died?
The calibration curve is the culprit (and some errors associated with the initial dating process that we don’t go into here). As we know, atmospheric carbon-14 has fluctuated through time. Those fluctuations correspond to wiggles in the carbon-14 dating calibration curves, and boy can they be wiggly! Some periods have sections of the curve that are clean and diagonal (meaning the atmospheric carbon-14 barely changed for decades and the resulting amount of carbon-14 in a sample can be predicted more precisely), and some have multiple peaks and troughs in close succession. When your sample’s measured carbon-14 ratio corresponds to one of these problematic wiggly sections, the resulting calibrated date spans a wide range, which can be hundreds of years wide.
This is not vagueness. This is honesty. The archaeologist and the radiocarbon lab are telling you precisely what the data can and cannot resolve. Although the precise dates of the person found in Ōwhiro Bay have not been reported, the broad range of 1600s–1700s reflects the presence of several wiggles in the calibration curve for this period. It places that person in pre-European contact time to very early contact by European explorers and whalers. Having precise (and accurate) dates is essential for many research questions and for iwi and hapū identifying and repatriating ancestral remains.
What Can We Actually Date?
Basically, any organic material, with some important caveats. The most common materials in New Zealand archaeology include:
Charcoal preserves well in archaeological deposits and usually date reliably. A concern to be aware is the “old wood effect”, whereby if a tree was several hundred years old before it was burnt, the charcoal could give a date significantly older than the actual event you are trying to date (think: burning driftwood on a beach versus branches you have pulled off a tree).
Shell is trickier. Marine organisms do not get their carbon from the atmosphere. Instead, they get it from the ocean, where carbon-14 takes time to circulate down from the surface. This means marine carbon is typically “older” (has a lower proportion of carbon-14 to carbon-12) than atmospheric carbon of the same age. Shell dates require calibration against the Marine20 curve (Heaton et al. 2020) and may also need a local marine reservoir correction (ΔR) specific to the region. In Aotearoa, this matters a great deal for sites with abundant shell midden (the most commonly recorded archaeological feature in Aotearoa).
Bone, hair, and other human or animal remains can all be dated, and are frequently used in cases like the Ōwhiro Bay discovery. Bone collagen is the preferred fraction for dating, and the quality of preservation significantly affects the reliability of results.
Back to the Garden
So. A Wellington gardener. A winter afternoon. Human remains long buried and undisturbed. Nine months of waiting, and then — clarity. Radiocarbon dating told us that the likely tupuna Māori found at Ōwhiro Bay lived in the 1600s to 1700s (Radiocarbon dating has similarly helped us understand early Māori agricultural life at Ahuahu Great Mercury Island — read about what was recently uncovered there). It told us this through cosmic rays creating carbon-14 from nitrogen atoms in the upper atmosphere; through that carbon entering the food web and into every living cell of that ancestor’s body; through the steady, predictable decay of carbon-14 from the moment of death; and through calibration against Southern Hemisphere tree rings and other archives painstakingly assembled over decades of scientific collaboration.
This is not a rare or unusual case. Across Aotearoa, when building projects, erosion events, and yes, gardens disturb burials and other archaeological deposits, radiocarbon dating is one of the first tools called upon (but radiocarbon dating is just one part of the archaeological toolkit — find out how archaeologists decide where to investigate in the first place). It is the workhorse of archaeological chronologies, not just here, but anywhere on Earth where human activity occurred within the last fifty thousand years.
It is, in my genuinely and unashamedly nerdy opinion, one of humanity’s greatest scientific achievements. A Nobel Prize’s worth of elegance, buried in the ratio between two isotopes of carbon.
Ngā mihi maioha ki a koe i tāu aroaro nei. Thank you for reading this far. Want to go deeper? The key references for this post are below. If you found this useful, please share it with a teacher, a student, a curious person, or anyone who has ever found something unexpected in their garden. And if this has sparked deeper inspiration, here are seven ways you can engage with archaeology in Aotearoa.
Subscribe on YouTube and Instagram at thepastbeforeus for long and short form videos on archaeology and cultural heritage in Aotearoa New Zealand, and come back to this website for the companion articles for the deeper dives and references.
Tēnā koutou, tēnā koutou, tēnā tātou katoa.
Nā Isaac McIvor
Key References
- Heaton TJ, Köhler P, Butzin M, et al. Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP). Radiocarbon. 2020;62(4):779-820. doi:10.1017/RDC.2020.68
https://www.cambridge.org/core/journals/radiocarbon/article/marine20the-marine-radiocarbon-age-calibration-curve-055000-cal-bp/B3013899914A3198D4B884B7B6E5CE66 - Hogg, A.G. et al. (2020). SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP. Radiocarbon 62(4): 759–778. https://www.cambridge.org/core/journals/radiocarbon/article/shcal20-southern-hemisphere-calibration-055000-years-cal-bp/2C20CF55E1251FD2822B009EB795C080
- Hua, Q. et al. (2022). Atmospheric Radiocarbon for the Period 1950–2019. Radiocarbon 64(4): 723–745. https://www.cambridge.org/core/journals/radiocarbon/article/atmospheric-radiocarbon-for-the-period-19502019/28F8E4DD4BA3F32266541445EC3F64C8
- Libby, W.F. (1955). Radiocarbon Dating. University of Chicago Press.
- Reimer, P.J. et al. (2020). The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve. Radiocarbon 62(4): 725–757. https://www.cambridge.org/core/journals/radiocarbon/article/intcal20-northern-hemisphere-radiocarbon-age-calibration-curve-055-cal-kbp/83257B63DC3AF9CFA6243F59D7503EFF
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