Surface exposure dating
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. 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.
This sliding surface became active ca. 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. This time range covers the entire Quaternary and Pliocene hence it has occupied a significant role among the tools of Quaternary geochronology.
Two stable noble gas nuclides are also important, the 3 He and the 21 Ne. Radioactive nuclides reach their secular equilibrium after half-lives, which defines the applicability range of the method. See more about the method in: Gosse and Phillips ; Dunai ; Granger et al. Exposure age of a rock is the time elapsed since it has been exposed to cosmic irradiation.
The measured TCN concentration is representative of the exposure age of the studied landform 1 if the formation of the landform was instantaneous and 2 if no surface denudation or 3 sediment accumulation has occurred since its formation. Glacial landforms, fluvial terraces and lava flows are among the most frequent targets of exposure age determination. In case of steady erosion TCN concentration within the rock is approaching a secular equilibrium.
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The faster is the denudation the lower is the equilibrium level. Accordingly, on a surface of long term steady erosion TCN concentrations are suitable for the determination of the surface denudation rate. The method is suitable for the quantification of surface denudation rates on uncovered or soil mantled surfaces and for the determination of average erosion rates of entire drainage basins. In contrast with exposure age and denudation rate determinations, burial age dating is based on the radioactive decay of cosmogenic nuclides. Those rocks and sediments are suitable for burial dating which once were exposed to cosmic irradiation, but have been buried since then.
The time of burial shielding from cosmic rays can be determined using cosmogenic nuclide-pairs with different half-lives.
River terraces show an up-warped pattern approaching the axis of the TR documenting differential uplift rates along the river. Numerical age determination of the terraces is essential for the determination of the incision rate of the Danube and connected uplift rate of the TR.
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As a continuation of this work exposure age and denudation rate determination of aggradational terraces of the Danube using cosmogenic 10 Be depth profiles revealed that the onset of the incision of the Danube was probably triggered by the mid-Pleistocene climate transition between 1. CosmoCalc was developed independently from these other tools, except for its topographic shielding correction function, which was translated into VBA from the Matlab code of Balco and Stone The reader is strongly encouraged to try these other programs.
Because geomagnetic field fluctuations and thermal neutron reactions are ignored, results for 36 Cl and 14 C may be inaccurate. CosmoCalc can be used as an exploratory tool for these nuclides, but for more accurate results, CHLOE or the spreadsheet of Lifton et al. TCN concentration, half-life and production rate. There exist only a few calibration sites where TCN production rates are accurately known thanks to the availability of independent age constraints e.
These production rates are only valid for the specific conditions latitude, elevation, age of each particular calibration site.
To apply the TCN method to other field settings, the production rates must be scaled to a common reference at sea level and high latitude SLHL. Although several efforts have been made to directly measure TCN production rate scaling with latitude and elevation using artificial H 2 O and SiO 2 targets Nishiizumi et al. The oldest and still most widely used scaling model is that of Lal Stone noted that this approximation is not valid in certain areas, such as Antarctica and Iceland.
To avoid the systematic errors caused by the standard atmosphere model, Stone recast the polynomial equations of Lal in terms of air pressure instead of elevation.
Terrestrial cosmogenic nuclide dating
A second improvement of the Stone model is the independent scaling of TCN production by slow negative muons Heisinger et al. In spite of this added complexity, the CosmoCalc interface for Stone scaling factors is identical to that for Lal scaling: The scaling factors of Stone can be different for different nuclides, because the importance of muons depends on the nuclide of interest.
Because most TCN production rate calibration sites are not located at SLHL, it is crucially important to scale the production rates using the same method as the unknown sample. This is exactly what CosmoCalc does when the user selects a nuclide from the scroll-down menu of the scaling-form. The program comes with a set of default calibration sites, but these can be changed.
Also the relative importance of the production pathways neutrons, slow and fast muons can be changed Section 7. Behind the scaling models of both Lal and Stone lies an extensive database of neutron monitor measurements, ordered according to geomagnetic latitude. To avoid this approximation, Dunai ordered the neutron monitor data according to geomagnetic inclination, which also represents the non-dipole field.
Just like Stone , also Dunai incorporates separate muon scaling and atmospheric effects. Using CosmoCalc it is very easy to convert air pressure to elevation or atmospheric depth and back Section 6. Ultimately, both geomagnetic latitude and inclination are merely proxies for a more fundamental physical quantity: Ordering the neutron monitor data according to this parameter results in yet another set of scaling factors.
Unfortunately, at least three different methods for calculating R c exist in the literature. Dunai used a database of horizontal magnetic field intensities and inclinations to estimate the cutoff-rigidity of an equivalent axial dipole field, for which an approximate analytical solution exists.