Terrestrial cosmogenic nuclide dating
Figure: Quartz band on sliding surface bombarded by a cosmic ray and producing here the nuclide 10Be.
Earth is constantly bombarded with cosmic rays that are high-energy charged particles. These particles interact with atoms in atmospheric gases (and thereby producing northern lights) and the surface of Earth. When one of these particles strikes an atom it can dislodge protons and/or neutrons from that atom, producing a different element or a different isotope of the original element. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides. Using certain cosmogenic radionuclides, scientists can date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding. The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate. Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to cosmic rays. Although dating with this method is expensive and the entire process takes a long time, TCN dating has the advantage that the dateable material is produced by the rockslide event itself by exposing fresh material surfaces to the cosmic rays. With the accuracy of modern instruments general every surface older than about 1000 years can be dated and the uncertainty limits today lie below 10% of the age.
In NGU's projects on unstable rock slopes in Norway, TCN dating is used to date 1) rock-avalanche deposits and 2) sliding surfaces. Ages of rock avalanche deposits throughout Norway cluster in the first few thousand years after deglaciation, however ages throughout the entire Holocene have also been obtained. The principles of dating sliding surfaces are more explained in the publication on the Oppstadhornet slide on Otrøya. This sliding surface became active ca. 16.6 to 14.2 kyrs ago and past long-term displacement rates are in the order of 2 mm/yr. Displacements rates measured today by differential Global Navigation Systems Satellite Systems (GPS) indicate the same velocity suggesting that the rockslide has been moving nearly constantly over the past 14 thousand years. Results from other sliding surfaces are different and suggest accelerated displacement rates today.