3.5.7 Carbon Dating

This topic explains carbon dating — a method that uses the radioactive isotope carbon-14 to estimate the age of once-living things.

Definition

Carbon-14 is used to estimate the age of carbon-containing substances. This process is called dating (specifically, carbon dating or radiocarbon dating).

Simple explanation: Carbon dating tells us how long ago a living thing died by measuring how much carbon-14 is left in its remains.

Main Idea

Carbon atoms circulate between the oceans and living organisms at a rate much faster than they decay. As a result, the concentration of C-14 in all living things keeps on increasing (or remains constant, since it is continuously replenished).

After death, organisms no longer pick up C-14. The C-14 already present begins to decay without being replaced.

By comparing the activity of a sample (like a skull or jaw bone) with the activity of living tissues, we can estimate how long it has been since the organism died.

Core Concept

How Carbon Dating Works

Simple explanation: Living things constantly replace their carbon-14. When they die, replacement stops, and the carbon-14 begins to disappear at a known rate. By measuring how much is left, we can calculate the time since death.

Detailed Explanation

Carbon-14 in the Environment

• Carbon-14 is produced naturally in the atmosphere.
• It combines with oxygen to form radioactive carbon dioxide (¹⁴CO₂).
• Plants absorb this ¹⁴CO₂ through photosynthesis.
• Animals eat plants (or other animals), so they also take in C-14.
• All living things therefore contain a constant proportion of C-14 while they are alive.

What Happens at Death

When an organism dies:

• It stops taking in new carbon (no more eating, drinking, or photosynthesis).
• The C-14 already in its tissues begins to decay.
• It decays into nitrogen-14 by emitting a beta particle:

¹⁴₆C → ¹⁴₇N + ⁰_-1e

- The amount of C-14 decreases over time at a known rate.

How the Age Is Estimated

The half-life of carbon-14 is about 5,730 years.

• After 5,730 years → half of the C-14 remains.
• After 11,460 years → one-quarter remains.
• After 17,190 years → one-eighth remains.
• And so on.

By measuring how much C-14 is left in a sample, scientists can calculate how many half-lives have passed since the organism died.

Comparing Activity

The book describes the method as:

> "By comparing the activity of a sample of skull or jaw bones, with the activity of living tissues, we can estimate how long it has been since the organism died."

Simple explanation: A living tissue has a certain level of radioactivity from C-14. An ancient bone has less, because some C-14 has decayed. By comparing the two, we can estimate the age.

Intermediate Level

Why Carbon-14 Is Useful for Dating

• It is naturally present in all living things.
• It has a convenient half-life (5,730 years) — long enough to date ancient objects, short enough to give measurable changes over thousands of years.
• It decays at a known rate — so the age can be calculated precisely.
• It is continuously replenished in living things — so the starting amount is known.

What Can Be Dated

Carbon dating works on carbon-containing materials that were once alive:

• Wood
• Bone
• Cloth
• Paper
• Shells
• Peat

What Cannot Be Dated

• Rocks and minerals (they don't contain carbon from living things)
• Metals
• Objects older than about 50,000 years (too little C-14 remains)
• Objects younger than about 100 years (too little decay has occurred)

The Decay Process

Carbon-14 decays by beta emission:

¹⁴₆C → ¹⁴₇N + ⁰_-1e

- Carbon-14 (6 protons, 8 neutrons) → Nitrogen-14 (7 protons, 7 neutrons) + beta particle.

One neutron in the carbon nucleus turns into a proton, changing the element from carbon to nitrogen.

Assumptions of Carbon Dating

Carbon dating assumes:

• The production rate of C-14 in the atmosphere has been constant over time.
• The ratio of C-14 to C-12 in living things has been constant.
• The decay rate of C-14 has not changed.

Note: Scientists have made corrections for known variations (e.g., from nuclear testing or industrial effects).

Important Points

• - Carbon dating uses carbon-14 to estimate the age of carbon-containing substances.
• - C-14 is produced naturally in the atmosphere.
• - Living things constantly take in C-14 (through food, water, air).
• - At death, C-14 intake stops, and the existing C-14 begins to decay.
• - C-14 decays into nitrogen-14 by emitting a beta particle.
• - Half-life of C-14: about 5,730 years.
• - By comparing the activity of a sample with living tissue, the time since death can be estimated.
• - Useful range: up to about 50,000 years.
• - Works on once-living things (bone, wood, cloth).

Remember

• - Carbon dating = using C-14 to find age of once-living things.
• - C-14 is produced naturally; taken in by living things.
• - At death: intake stops, decay begins.
• - C-14 decays into N-14 (beta decay).
• - Half-life: ~5,730 years.
• - Method: compare activity of sample with living tissue.
• - Useful range: up to ~50,000 years.
• - Works on carbon-containing once-living materials.

Common Mistakes

• 1. Thinking carbon dating works for all objects. It only works for once-living, carbon-containing materials.
• 2. Thinking carbon dating gives an exact age. It gives an estimate with some uncertainty.
• 3. Thinking C-14 is stable. It is radioactive and decays over time.
• 4. Forgetting the half-life. C-14's half-life is about 5,730 years.
• 5. Thinking carbon dating works for rocks. Rocks don't contain C-14 from living things, so they cannot be dated this way.
• 6. Confusing C-14 with C-12. C-12 is stable; C-14 is radioactive and used for dating.

Quick Revision

• Carbon dating: uses carbon-14 to estimate the age of carbon-containing substances.
• C-14 is produced naturally in the atmosphere.
• Living things continuously take in C-14 through food, water, and air.
• At death, intake stops, and C-14 begins to decay.
• C-14 decays into nitrogen-14 by emitting a beta particle.
• Half-life of C-14: about 5,730 years.
• By comparing the activity of a sample with living tissue, scientists estimate the time since death.
• Useful for dating once-living things like bone, wood, and cloth.
• Useful range: up to about 50,000 years.

60-Second Revision

• Carbon dating = C-14 used to date once-living things.
• C-14 taken in while alive; decays after death.
• C-14 → N-14 (beta decay).
• Half-life: ~5,730 years.
• Method: compare activity with living tissue.
• Range: up to ~50,000 years.
• Warning: carbon dating works only on carbon-containing, once-living materials.