Investigation of Dating Technique

Cosmogenic nuclides are radioactive isotopes formed within the crystal lattice of a mineral after it is exposed to cosmic rays near the earth's surface (1 ). In other words, as rocks are uplifted or exposed to earth's surface by another means, they are exposed to high-energy cosmic rays from the sun. Some of the minerals within these rocks react to the cosmic rays to form the nuclides in a process referred to as a spallation reaction (2). The cosmic ray neutrons collide with certain elements in the rocks and sometimes have enough energy to fragment the nucleus of atoms within the minerals (2). These nuclides are often referred to as 'in-situ' , meaning that they were produced within the mineral structure. It is important to note that these reactions occur within the first three meters of the earth's surface, with fewer reactions taking place as the depth increases.

Common minerals used in cosmogenic nuclide analysis include but are not limited to ¹⁰Be, ²⁶Al, ³⁶Cl, ²¹Ne, ¹⁴C, and ³He. The nuclides are formed in very small amounts, but are created at varying rates due to the different strengths of the cosmogenic rays across the earth (1).

In order to determine the age, the composition of cosmogenic nuclides within the sample must be determined. But first, the sample itself needs to be appropriate. A rock that is rich in quartz is great to analyze, because quartz does not naturally contain ¹⁰Be or any other radioactive isotopes (2). To do the analysis, the nuclides of interest must be separated, which is an extensive process. For example, in a study measuring ¹⁰Be in quartz, the quartz must be isolated and cleaned. Unwanted elements from the resulting substance must be separated, which can take several weeks to complete. Then, an accelerator mass spectrometer is used to measure the concentration of the nuclides (1).

The most common features to sample using this technique are glacial boulders and glacially sculpted bedrock surfaces. Other features include river terraces, rockfalls, fault scarps, desert pavements, and marine platforms (1).

Because of the relatively long half-lives of many of these nuclides, cosmogenic nuclide analysis can be used to date rocks or landforms of a variety of ages, most accurately between 1,000 and 100,000 years but this range can be extended (1). Additionally, different isotopes have different half-lives (2). So, depending on what you'd like to date, you would have to pick an appropriate isotope. For example, ¹⁰Be has a half life of 1.39 × 10⁶ years while ¹⁴C has a half life of 5,730 years.

By sampling a variety of locations, we can get a sense of when the rocks were exposed to the cosmic rays. As previously stated, this technique is often used in areas that have been impacted by glaciation. Geologists are able to determine the extent of glacial ice, the approximate time at which the glacier receeded, and the approximate rate at which it receeded (2). Additionally, we can use information from cosmic nuclide analysis to create or interpret erosion models to see how erosion has impacted that landscape.