Earth Science 1949

Age Determinations by Radiocarbon Content: Checks with Samples of Known Age

James R. Arnold & Willard F. Libby

Living things take in carbon-14 from the air; at death it stops, and its steady decay counts the years since.

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In depth · the introduction

Every living thing is faintly radioactive in the same way — and the instant it dies, that tiny glow begins, very slowly, to fade. Read how far it has faded and you read how long ago it died.

The big idea

High in the sky, cosmic rays are constantly turning ordinary nitrogen into a rare, radioactive form of carbon called carbon-14. It drifts down, mixes into the air as carbon dioxide, and every plant breathes it in — and every animal that eats the plants takes it in too. So while you are alive, your body holds a fixed, tiny amount of carbon-14, kept topped up from the air.

When something dies, the topping-up stops. From that moment, the carbon-14 it contains slowly decays away at a perfectly steady, known pace — half of it gone every 5,730 years. Measure how much is left in an old bone, seed or scrap of cloth, and simple arithmetic tells you how long ago it stopped living.

Proving it on things we already dated

A clever idea is not enough; Libby had to show it gave the right answers. So he and James Arnold gathered objects whose ages were already known by other means — wood from the tombs of Egyptian pharaohs whose reigns historians had dated, and tree trunks whose age could be counted ring by ring. They measured the carbon-14 in each and predicted its age, then plotted prediction against the known truth. The points lined up. The radiocarbon age of wood from the tomb of the pharaoh Zoser, for example, landed right where 4,600-year-old wood should be. That graph — the “Curve of Knowns” — was the proof, and in 1960 Libby received the Nobel Prize in Chemistry for it.

Why it mattered

Before this, the deep human past had almost no dates at all. Archaeologists could say one layer was older than another, but not by how much, and never in actual years. Radiocarbon changed that overnight. Suddenly a charred grain from an ancient village, a flake of charcoal from a cave painting, a thread from a burial shroud could each be pinned to a year — anywhere on Earth, by the same universal clock. It rewrote the timeline of farming, migration and civilisation, and it remains the most widely used dating method in archaeology.

A way to picture it

Imagine that the day a tree is cut, its wood is handed a fixed pile of glowing embers that no one will ever refill, and that exactly half the embers wink out every 5,730 years. Walk into the room a long time later and count the embers still glowing: a full pile means it was just cut, half a pile means about 5,730 years, a quarter means twice that. The embers are the carbon-14, and counting their glow is exactly what a radiocarbon lab does.

Where it sits

Radiocarbon dating was born from the radioactivity that Curie (1898) and Rutherford (1911) discovered, and it is one branch of a larger family of atomic clocks. Its cousin, lead-isotope dating, let Patterson (1956) weigh the age of the whole Earth in billions of years; radiocarbon works at the human scale of centuries and millennia. Together they finally put real numbers on the deep time that Hutton (1788) and Lyell (1830) could only argue must exist.

The original document

Original source text

J. R. Arnold & W. F. Libby · Science 110(2869): 678–680 · 1949 · Institute for Nuclear Studies, University of Chicago

The problem (paraphrase)

Cosmic rays striking the upper atmosphere make a slow trickle of radioactive carbon-14, which mixes through the air as CO₂ and enters every living thing. While an organism lives it keeps replacing this carbon, so its tissues hold the same small radiocarbon fraction as the atmosphere. At death the exchange stops and the trapped ¹⁴C decays away at a fixed rate. The question this paper sets out to settle is not whether that should work in principle, but whether the radiocarbon ages it yields actually match the real ages of objects we already know.

The method (paraphrase)

Each sample's carbon is converted to a countable form and its residual ¹⁴C beta-activity is compared against contemporary wood, which stands for the living atmosphere. From the ratio of activities the age follows from radioactive decay, N = N₀e^(−λt), using the half-life Libby had measured (then 5568 ± 30 years). Because the surviving activity is tiny, the measurement demands heavy shielding and careful subtraction of background counts.

The known-age checks — the “Curve of Knowns”

The test samples were objects whose calendar ages were independently fixed by Egyptian history or by tree-ring counting. They included wood from the tombs of the pharaohs Zoser (Djoser) and Sneferu — historically about 2625 BC ± 75 — together with sequoia and Douglas-fir wood dated by dendrochronology and a piece of the funerary boat of Sesostris III. Plotting each predicted radiocarbon age against its known age, the points fell along the expected line: the Zoser–Sneferu wood, for instance, came out at about 2800 BC ± 250 against the historical 2625 BC ± 75.

The result (paraphrase)

Predicted and known ages agreed within the experimental scatter, across several thousand years. That agreement is the whole argument: a physical clock, calibrated only by nuclear decay, reproduced dates that historians and tree-rings had set by entirely separate means. With that, radiocarbon became a usable instrument, in principle reaching back tens of thousands of years.

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Institute for Nuclear Studies, University of Chicago · 1949