Tree rings and true dates

Section 5 of 5

Carbon dating works out an age from the share of carbon-14 a living thing started with. Tree rings show what that share was, year by year.

The share of carbon-14 in the air has wandered over time. When the Sun is more active, or Earth's magnetic field is stronger, fewer cosmic rays reach the air and less carbon-14 forms. A raw radiocarbon age can be out by hundreds of years, and by thousands for the oldest samples.

Counting rings

A tree adds one ring of new wood each year, and each ring keeps the carbon-14 of the air from the year it grew. Rings are wide in good years and narrow in poor ones, so the trees of one region share a pattern of wide and narrow rings. Matching that pattern links a living tree to old roof beams, and the beams to logs preserved in bogs and river gravel, in one unbroken count of years.

Matching tree rings link old wood into one long record Three strips of wood, each marked with its yearly rings, some wide and some narrow. The top strip, from a living tree, holds the newest rings. The middle strip, an old roof beam, starts further back, and its newest rings match the pattern of the living tree's oldest rings; a dashed frame joins the matching stretch. The bottom strip, a log from a bog, starts further back again, and its newest rings match the beam's oldest. An arrow along the bottom runs from older on the left to newer on the right. living tree roof beam log from a bog older newer
Rings from three pieces of wood, drawn as strips. Where the pattern of wide and narrow rings matches (dashed frames), the pieces grew in the same years, so the count of years carries on from one piece to the next.

Scientists have carbon-dated thousands of rings of known age. The results make a calibration curve, which turns a radiocarbon age into a calendar date. Counted tree rings reach back about 12,300 years, and older wood matched by its carbon-14 carries the tree-ring record to about 13,900 years. Beyond that, lake mud laid down in yearly layers, corals and cave stalagmites carry the curve on to 55,000 years. The curve in use today, IntCal20, was published in 2020.

Two famous dates

In September 1991 two walkers found a man's body melting out of the ice high in the Ötztal Alps, close to the border between Austria and Italy. Labs dated samples of his body and of grass found with him. He died between 3359 and 3105 BC, more than 5,000 years ago, and he is known as Ötzi.

The Shroud of Turin is a linen cloth 4.4 m long that carries the faint image of a man. Many Christians have honoured it for centuries as the burial cloth of Jesus. In 1988 labs in Oxford, Zurich and Tucson, Arizona, each dated a piece of one strip cut from the cloth. Their results, published in 1989, put the flax for the linen between AD 1260 and 1390. The cloth's written history begins in the 1350s, in the French village of Lirey.

Quick check

Why do labs use tree rings to turn a radiocarbon age into a calendar date?

That's the end of lesson 38

In this lesson you found out:

  • What isotopes are
  • How carbon-14 forms in the air and gets into living things
  • What a half-life is
  • How carbon dating finds the age of wood and bone, and how tree rings correct it
Start Lesson 39: Carbon in space All lessons