Uranium–thorium dating

Uranium–thorium dating is a way scientists find out how old some things are. It started in the 1960s and has been used since the 1970s. This method helps find the age of materials made mostly of calcium carbonate, like cave formations called speleothem or pieces of coral.

Unlike some other dating methods, such as rubidium–strontium or uranium–lead dating, uranium–thorium dating looks at how much a radioactive material called thorium-230 has changed back to its parent material, uranium-234, inside the sample.

This technique uses the idea of secular equilibrium and relies on measuring radioactive isotopes. It’s a special kind of radiometric dating that helps scientists learn about the past.

Background

Thorium does not dissolve in water near the Earth's surface, so materials formed from this water usually do not contain thorium. However, uranium can dissolve in water, so materials formed from such water often contain small amounts of uranium. Over time, uranium-234 changes into thorium-230. Thorium-230 is also radioactive. Unlike some other systems, thorium-230 reaches a balance where the amount breaking down each year matches the amount being created.

History

In 1908, a geology professor named John Joly noticed that deep sediments had more radium than sediments closer to land. He thought the sediments were taking radium from seawater. Later, in 1942, scientists discovered that extra radium was linked to extra thorium (²³⁰Th). It wasn’t until the 1960s that scientists started using this method to date rocks like speleothems and travertines. By the late 1980s, better measuring tools helped improve the method, thanks to work by Larry Edwards. When a major book about uranium-234 was translated into English, more scientists began using uranium-thorium dating.

Methods

U-series dating is a group of ways to find out how old different materials are. Each way is named after special parts of atoms, called isotopes, that scientists measure. There are eight ways shown in the table below.

The ²³⁴U/²³⁸U method works because ²³⁴U dissolves more easily than ²³⁸U. When a ²³⁸U atom breaks apart by sending out an alpha ray, it moves from its usual place in a crystal. This creates a ²³⁴Th atom, which quickly changes into a ²³⁴U atom. After the uranium is placed somewhere, the amount of ²³⁴U compared to ²³⁸U goes back to a balanced state. This balance gets closer every 245,000 years.

Dating limits

Uranium–thorium dating helps us find out how old something is. It works best for things younger than about 500,000 years. This is because it measures how thorium-230 changes over time and how much thorium-230 and uranium-234 are in a sample. We also need to check the amount of uranium-234 compared to another type of uranium, called uranium-238.

Precision

U-Th dating is best for materials like stalagmites and some rocks. It is less reliable for bone or shell. Scientists use a tool called mass spectrometry to measure the age very precisely, within about ±1%. Older methods, like alpha counting, are less precise, with about ±5% accuracy. Mass spectrometry also needs smaller pieces of the sample.