# Encyclopedia of Geoarchaeology

2017 Edition
| Editors: Allan S. Gilbert

# Cosmogenic isotopic dating

Reference work entry
DOI: https://doi.org/10.1007/978-1-4020-4409-0_40

## Definition

Calculating the age of rock and sediment surfaces according to their concentration of cosmogenic isotopes.

Cosmogenic isotopes, such as 26Al, 10Be, 21Ne, 36Cl, 14C, and 3H, are produced in the atmosphere as meteoric nuclides and at the surface of the Earth as in situ terrestrial cosmogenic nuclides, or TCNs, by interaction between cosmic particles and target atoms (Gosse and Phillips, 2001; Dunai, 2010). The production of TCN depends on geographic location and altitude.

Cosmic particles are attenuated so that they produce cosmogenic isotopes only within the upper several meters of the Earth’s crust. Therefore, the concentration of TCN in rock or sediment is a good indication that it spent time close to or at the surface.

The concentration of TCN in a sample can be interpreted in two end-member ways (Bierman, 1994):
1. 1.
Representing a constant erosion rate (E) over a long time (t ≈ ∞)
$$N = \frac{P}{{\lambda} + \frac{E}{\Lambda/\rho}}$$
(1)

2. 2.
Representing exposure time at the surface and assuming E ≈ 0
$$N = \frac{P}{\lambda }\left( {1 - {e^{ - \lambda t}}} \right)$$
(2)
where
• N = concentration in atoms g−1 quartz

• t = exposure time in years

• λ = decay constant (yr−1)

• E = erosion rate in cm yr−1

• P = production rate at depth in atoms year−1 g−1 quartz

• Λ = attenuation length (g cm−2)

• ρ = overburden rock density (g cm−3)

The choice of calculating an erosion rate or exposure age depends on the context. Relevant examples are the calculation of exposure ages of relict surfaces, such as glacially carved surfaces, or of boulders that have experienced very little erosion. On the other hand, erosion rate calculations are suitable for landforms that are continuously being eroded due to tectonic uplift.

Quartz is commonly used in TCN dating. It is widely available in rocks and sediments, its chemistry is simple, and the analytical extraction of TCN is relatively straightforward.

Cosmogenic isotopes can also be used to date the age at which a sediment was buried, for example, in an alluvial terrace, glacial till, or in caves (Granger, 2006). The 26Al/10Be ratio in buried quartz grains, which were previously exposed, will decrease at an exponential rate due to the different half-lives of the two isotopes:
$$\frac{{{N_{26}}}}{{{N_{10}}}} = {\left( {\frac{{{N_{26}}}}{{{N_{10}}}}} \right)_0}{e^{ - {t_{burial}}\left( {\frac{1}{{{\tau _{26}}}} - \frac{1}{{{\tau _{10}}}}} \right)}}$$
(3)
where
• N26, N10 = measured concentrations of 26Al and 10Be (atoms g−1 quartz)

• (N26/N10)0 = initial 26Al/10Be at burial

• tburial = time since burial

• τ26, τ10 = mean lives of 26Al and 10Be (year) (τ = t1/2/ln2)

This application of cosmogenic isotopic dating has been used to date buried sediments and artifacts in prehistoric sites such as Sterkfontein (Partridge et al., 2003) and Wonderwerk Cave (Matmon et al., 2012) in South Africa.

### Bibliography

1. Bierman, P. R., 1994. Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution: a review from the geomorphic perspective. Journal of Geophysical Research: Solid Earth (1978–2012), 99(B7), 13885–13896.
2. Dunai, T. J., 2010. Cosmogenic Nuclides: Principles, Concepts and Applications in the Earth Surface Sciences. Cambridge: Cambridge University Press.
3. Gosse, J. C., and Phillips, F. M., 2001. Terrestrial in situ cosmogenic nuclides: theory and application. Quaternary Science Reviews, 20(14), 1475–1560.
4. Granger, D. E., 2006. A review of burial dating methods using 26Al and 10Be. In Siame, L., Bourlès, D. L., and Brown, E. T. (eds.), In Situ-Produced Cosmogenic Nuclides and Quantification of Geological Processes. Boulder: Geological Society of America. GSA Special Paper 415, pp. 1–16.Google Scholar
5. Matmon, A., Ron, H., Chazan, M., Porat, N., and Horwitz, L. K., 2012. Reconstructing the history of sediment deposition in caves: a case study from Wonderwerk Cave, South Africa. Geological Society of America Bulletin, 124(3–4), 611–625.
6. Partridge, T. C., Granger, D. E., Caffee, M. W., and Clarke, R. J., 2003. Lower Pliocene hominid remains from Sterkfontein. Science, 300(5619), 607–612.