We cannot carbon date samples after 1950 primarily because extensive atmospheric nuclear weapons testing in the late 1950s and early 1960s drastically altered the natural concentration of radiocarbon in the atmosphere, making traditional dating methods unreliable for this period.
Understanding Carbon Dating Basics
Carbon dating, also known as radiocarbon dating, is a scientific method used to determine the age of organic materials. It works on the principle that all living organisms absorb carbon-14 (¹⁴C), a radioactive isotope of carbon, from the atmosphere throughout their lives. When an organism dies, it stops absorbing ¹⁴C, and the existing ¹⁴C begins to decay at a predictable rate (its half-life is approximately 5,730 years). By measuring the remaining ¹⁴C in a sample and comparing it to the known atmospheric concentration at the time of death, scientists can estimate how long ago the organism died.
This method relies on a crucial assumption: that the natural concentration of ¹⁴C in the atmosphere has remained relatively constant over time. For samples older than 1950, this assumption generally holds true, allowing for accurate dating across tens of thousands of years. For example, a sample from 11,460 years ago might naturally produce a decay rate of 3.5 decays per gram per minute.
The "Bomb Pulse" Effect
The reason carbon dating becomes ineffective for samples after 1950 is due to what scientists call the "bomb pulse."
- Nuclear Testing Impact: Starting in the late 1950s and continuing into the early 1960s, numerous nations conducted atmospheric tests of nuclear weapons. These explosions released a tremendous amount of neutrons into the atmosphere.
- Artificial ¹⁴C Production: These neutrons reacted with nitrogen-14 atoms (¹⁴N) in the air, creating a significant amount of new carbon-14. This process artifically greatly increased the amount of radiocarbon in the atmosphere.
- Skewed Baseline: The natural balance of ¹⁴C was profoundly disrupted. The atmospheric ¹⁴C concentration more than doubled from its natural baseline. For instance, if a specific modern decay rate was, say, 14 decays per minute before the tests, it would have then shown more than double that rate afterwards.
- Dating Inaccuracy: Organisms living during and after this period absorbed this unnaturally high level of ¹⁴C. If you tried to carbon date a sample from, say, 1965, using the pre-1950 natural baseline, it would appear much younger than it actually is because it contains an artificially inflated amount of ¹⁴C. It becomes impossible to accurately determine its true age using standard radiocarbon dating techniques due to the unknown and non-natural starting concentration.
Why It Matters
The "bomb pulse" essentially reset or, more accurately, severely distorted the atmospheric ¹⁴C clock for recent times.
| Time Period | Atmospheric Carbon-14 Levels | Carbon Dating Applicability |
|---|---|---|
| Pre-1950 | Relatively stable (natural) | Effective for thousands of years |
| Post-1950 | Artificially elevated ("bomb pulse") | Inaccurate/Impractical for traditional dating |
While the bomb pulse poses a challenge for traditional carbon dating of very recent materials, it has also paradoxically enabled a new application: tracing environmental processes and studying recent historical events, as the artificially high ¹⁴C levels serve as a distinct marker for the post-1950 era. However, for determining age based on natural decay, the method is compromised for samples formed after this period.
