Exposure dating is used to determine the age of geologically formed surfaces, such as glacially polished bedrock, moraines, river terraces, landslides, or lava flows. A key requirement is that the surface has remained largely stable since its formation and has not been significantly modified by erosion or shielded from direct cosmic radiation by temporary cover, such as snow or soil. Alternatively, erosion rates or periods of burial must be constrained through independent observations. A good understanding of the geomorphological setting and careful, detailed sampling are essential, as errors cannot be corrected analytically at a later stage.

Suitable materials include quartz (¹⁰Be, ¹⁴C, ²¹Ne, ²⁶Al), feldspar (³⁶Cl), carbonates (³⁶Cl), olivine and pyroxene (³He, ²¹Ne), basalt (³⁶Cl) and zircon (^{78, 81}Kr). The maximum dating range is approximately four half-lives, corresponding to about 5.6 Ma for ¹⁰Be, 2.8 Ma for ²⁶Al and 1.2 Ma for ³⁶Cl. For stable noble gases (³He, ²¹Ne, ⁷⁸Kr), no such upper limit exists; in exceptionally stable landscapes, such as the Atacama Desert or Antarctica, exposure ages of several tens of millions of years can be determined.

Different decay rates of cosmogenic radionuclides make it possible to determine how long formerly exposed sediments were shielded from cosmic radiation by burial, provided that the initial nuclide ratio is known. Typical applications include sediments washed into caves, sediments at the bottom of thick deposits, and clastic lacustrine sediments.

For terrestrial samples, the nuclide pair ²⁶Al/¹⁰Be in quartz is most commonly used, as its production-rate ratio is known and generally approximately constant. The suitable age range for ²⁶Al/¹⁰Be is typically a few hundred thousand to a few million years. More recently, the nuclide pair ⁸¹Kr/⁷⁸Kr in zircon has also become available for the age range of approximately 50 ka to 1 Ma.

The concentration of cosmogenic nuclides in Earth surface materials depends on their erosion rate. Therefore, representative sediments from a catchment can be used to estimate the mean erosion rate. This requires erosion to be long-lasting and relatively uniform. For this application, ¹⁰Be in quartz is most commonly used.

Here, “long-lasting” refers to the time required to remove a few metres of rock.
“Uniform” means that the erosion rate remains largely constant over this period.

Beryllium has one stable isotope (⁹Be) and two cosmogenic radionuclides (⁷Be, T_1/2 = 53 days; ¹⁰Be, T_1/2 = 1.39 Ma). Due to its long half-life, ¹⁰Be is particularly well suited for geological applications throughout the Quaternary and Pliocene. It is mainly produced in near-surface rocks and minerals by spallation reactions on oxygen.

Because ¹⁰Be is also produced in the atmosphere, separating the atmospheric component from the in situ-produced component is a key step in sample preparation. Therefore, quartz is predominantly used as the target mineral for in-situ applications, as the atmospheric component is difficult to remove quantitatively from other minerals. The atmospheric component, known as meteoric ¹⁰Be , can in turn be used to date fine-grained sediments, covering an age range of approximately 1 to 14 Ma.

Descriptions of ¹⁰Be sample preparation and measurements in Cologne, as well as comparisons with other laboratories, can be found in:

Neon has three stable isotopes, ²¹Ne, ²²Ne and ²⁰Ne, all of which are produced by spallation reactions on silicon, magnesium, and aluminium. To quantify Earth surface processes, the concentration of the rarest neon isotope, ²¹Ne, is determined; it accounts for only about 0.3% of atmospheric neon. The simultaneous measurement of ²⁰Ne and ²²Ne allows the cosmogenic component, ²¹Ne_cos, to be determined accurately.

Quartz is the most commonly used target mineral because it is abundant, resistant to weathering, and chemically easy to process; silicon is the only target nuclide. Cosmogenic neon is particularly well suited for exposure dating of old surfaces (>1 Ma). In combination with ¹⁰Be, ²¹Ne can also be used for burial dating of old sediments beyond the range of ²⁶Al/¹⁰Be.

Descriptions of ²¹Ne sample preparation and measurements in Cologne, as well as comparisons with other laboratories, can be found in:

Aluminium has one stable isotope (²⁷Al) and one cosmogenic radionuclide (²⁶Al, T_1/2 = 700 ka). In geological applications, ²⁶Al is frequently used together with ¹⁰Be, as this combination allows complex exposure histories to be resolved. Together with ¹⁰Be, this nuclide also forms the basis of classical burial dating.

²⁶Al is produced in near-surface silicates mainly by spallation reactions on silicon. Significant atmospheric production does not occur. Quartz is used almost exclusively for measurements, as high concentrations of stable aluminium (>100 ppm), which are common in other silicates, complicate AMS analysis. Determining the ²⁶Al concentration also requires precise measurement of stable ²⁷Al.

Descriptions of ²⁶Al measurements in Cologne, as well as comparisons with other laboratories, can be found in:

Chlorine has two stable isotopes (³⁵Cl and ³⁷Cl) and one radionuclide (³⁶Cl, T_1/2 = 300 ka). ³⁶Cl is produced in near-surface rocks and minerals mainly by spallation reactions on calcium and potassium, as well as on iron and titanium. In chlorine-bearing samples, production by thermal neutrons also plays an important role.

Because the flux of thermal neutrons is sensitive to environmental conditions such as moisture and is therefore difficult to predict, samples should ideally contain little chlorine. A particular advantage of ³⁶Cl is that it is the only cosmogenic nuclide that can be routinely measured in carbonates. During sample preparation, in addition to the removal of chlorine-bearing phases by mineral separation and chemical leaching, the quantitative removal of sulfur is particularly important because ³⁶S interferes with AMS measurements.

Descriptions of ³⁶Cl sample preparation and measurements in Cologne, as well as comparisons with other laboratories, can be found in:

Krypton is the only noble gas that possesses both stable isotopes (⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³Kr, ⁸⁴Kr, ⁸⁶Kr) and radioisotopes (⁸¹Kr, T½ = 229 ka, ⁸⁵Kr, T½ = 10.7 a). Furthermore, ⁷⁸Kr, apart from its cosmogenic and atmospheric components, is free from geochemical interference (a significant difference from ³He or ²¹Ne). All isotopes are formed via spallation reactions involving rubidium, strontium, yttrium, zirconium and niobium.

The simultaneous measurement of all krypton isotopes allows the precise determination of cosmogenic ⁷⁸Kr_cos; ⁸¹Kr is exclusively of cosmogenic origin. Possible target minerals include, for example, zircon, xenotime and columbite. Cosmogenic ⁷⁸Kr enables the determination of palaeo-erosion rates in sediments of any age. The ⁸¹Kr/⁷⁸Kr pair is suitable for burial age dating between approximately 50 ka and ~1 Ma. 

The methodological foundations for terrestrial in-situ applications of cosmogenic krypton were developed in Cologne.

Information on the methodology, krypton sample preparation and measurements in Cologne can be found in:

Together with other anthropogenic radionuclides, such as ⁹⁰Sr and ¹³⁷Cs, plutonium was released during atmospheric nuclear weapons testing in the 1950s and early 1960s and deposited worldwide as fallout. The use of fallout plutonium in soil science is broadly comparable to that of ¹³⁷Cs, but offers several advantages.

Plutonium is not widely dispersed during nuclear reactor accidents, such as Chernobyl. As a result, the bomb-fallout Pu signal in Europe has remained unaltered, whereas this is not the case for ¹³⁷Cs. AMS-based determination of Pu isotope abundances also enables both Pu-source identification and the analysis of very small sample sizes.

Descriptions of plutonium sample preparation and measurements in Cologne, as well as comparisons with other laboratories, can be found in: