Cosmogenic nuclides are isotopes that are radioactive enough to have all decayed by now, even if they were at once present on Earth, but which are present nevertheless.
The fact that they are indeed present is because they are being continually created on Earth by cosmic rays from outer space impinging and reacting with other isotopes on Earth to produce these so-called Cosmogenic nuclides. Cosmogenic Nuclides can be split into two varieties, meteoric cosmogenic nuclei produced by primary cosmic rays (mainly protons and alpha particles) interacting with atoms in the Earths' (mainly upper) atmosphere, and terrestrial cosmogenic nuclei produced by the cascade of secondary particles (mainly neutrons and muons) [which are themselves created by primary cosmic rays when they hit the upper atmosphere] interacting with atoms on the surface of Earth.
I collected the samples in the field in 20: Having collected the samples in the field and received funding to run them, I went up to the Scottish Universities Environmental Reserach Centre (SUERC) laboratories in East Kilbride to start the process!
As 10-Beryllium and 26-Aluminium preferentially build up in quartz, the aim of the first week was to crush down the samples and extract as pure quartz as we could.
Ages were determined at two hominid localities from the Chad Basin in the Djurab Desert (Northern Chad).
In the Koro Toro fossiliferous area, KT 12 locality (16°00′N, 18°53′E) was the site of discovery of (Toumaï).
The base of the mapped sections consists of a well developed, thick, aeolian facies (8). U.) is composed of poorly cemented sand and argillaceous sandstone alternation characterized by dense networks of root tubules/root molds (palaeosols) and termite nests (9, 10). The uniform stratigraphy at the TM localities allowed us to use absolute ages from both TM 266, where Toumaï was discovered, and TM 254 to assign an age to Toumaï.
Be-10 falls to the Earth's surface, where it binds tightly to sediment particles in non-acidic soils over the life-span of those soils.
Cosmic rays, originating from outer space, bring rare cosmogenic nuclide isotopes (I am using Aluminium) to the Earth’s surface, where they build up in exposed rock surfaces at known rates.
The total concentration of these isotopes in a rock surface therefore represents the length of time that the surface has been exposed to the atmosphere.
Some stable cosmogenic nuclides are also produced, for instance helium-3 and neon-21, but some of these nuclei also have other terrestrial origins.
These two nuclides, and three other radioactive terrestrial cosmogenic nuclides (beryllium-10, chlorine-36 and aluminium-26) are the most commonly used cosmogenic nuclides in geological rock dating, where they accumulate within rock which overlays the Earths surface. Thus glacial erratics and some meteor crators can be dated this way.