Surface exposure dating - Wikipedia
Advancements in cosmogenic 38Ar exposure dating of terrestrial rocks . in age and are located on surfaces suitable for cosmogenic radionuclide dating. Geant4-based simulation results are compared with the calculation of ACTIVIA and. Cosmogenic isotopes are created when elements in the atmosphere or earth are been exposed, but would not date the maximum age of the surface exposure. Abstract: In the last decades surface exposure dating using cosmogenic nuclides has emerged as a powerful tool in Quaternary The power of cosmogenic nuclide methods lies in 10Be from environmental materials and SIMS isotope .
Decay rates are given by the decay constants of the nuclides. These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age. The two most frequently measured cosmogenic nuclides are beryllium and aluminum These nuclides are particularly useful to geologists because they are produced when cosmic rays strike oxygen and siliconrespectively.
The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes.
As oxygen is also common in the atmosphere, the contribution to the beryllium concentration from material deposited rather than created in situ must be taken into account.
Each of these nuclides is produced at a different rate. Both can be used individually to date how long the material has been exposed at the surface. Because there are two radionuclides decaying, the ratio of concentrations of these two nuclides can be used without any other knowledge to determine an age at which the sample was buried past the production depth typically 2—10 meters.
Chlorine nuclides are also measured to date surface rocks. This isotope may be produced by cosmic ray spallation of calcium or potassium. It is also the most complex model, but this did not change the user interface. Using the scaling models of Desilets et al.
Default values for the relative SLHL production rate contributions of neutrons, slow and fast muons can be changed in the Settings menu Section 7.
The scaling models of DunaiDesilets et al. On the other hand, such models also offer the possibility to correct for secular variation of TCN production rates for short exposures. Instructions for doing so are given by Dunai and Desilets et al.
Compiling such a record is something for advanced users and falls beyond the scope of CosmoCalc. The scaling models of Pigati and Lifton and Lifton are accompanied by global datasets of magnetic field intensity, polar wander and solar activity and in principle, it would be possible to incorporate these datasets into CosmoCalc.
Therefore, researchers working with 14C, where secular variation of the magnetic field is really crucial, should use CosmoCalc only as an exploratory tool, and use the spreadsheets of Pigati and Lifton and Lifton et al. If these assumption are not fulfilled, the SLHL production rates must be multiplied by a second set of correction factors, quantifying the extent to which the cosmic rays were blocked.
Surface exposure dating
CosmoCalc implements three such correction factors: CosmoCalc follows the approach of Balco and Stone their Matlab function skyline. Sometimes, an exponent of 2. CosmoCalc treats this exponent as a variable, which can be changed in the Settings form Section 7. There is no restriction on the total number of measurements, provided they come in multiples of two. They must be integrated over the actual sample thickness and scaled to the surface production rates before an exposure age can be calculated.
Different reaction mechanisms are associated with different attenuation lengths. Gosse and Philips consider four kinds of thickness corrections, for spallogenic, thermal and epithermal neutrons, and muons. Because self-shielding corrections are generally small, CosmoCalc considers only the spallogenic neutron reactions: Neglecting the remaining pathways makes little difference, with the possible exception of 36Cl, because the latter can be strongly affected by thermal neutron fluxes, which are currently ignored by CosmoCalc.
These features are generally located at high latitudes or elevations, where snow cover poses a potential problem. The snow correction is similar to the self-shielding correction with the important difference that the former is highly variable with time.
CosmoCalc accomodates two types of banana plot: Equation 4 models TCN production by neutrons, slow and fast muons by a series of exponential approximations. The first term of the summation models TCN production by spallogenic neutron reactions, the second and third terms model slow muons and the last term approximates TCN production by fast muons.
The approach of Granger et al. For instance, neglecting muon production can be easily implemented by setting F1,F2 and F3 equal to zero in Equation 4. CosmoCalc uses Granger et al. Banana plots with non-zero muon contributions feature a characteristic cross-over of the steady-state and zero erosion lines which is absent when muons are neglected Figure 2.
However, in the presence of muons, the effective scaling factor Se may deviate from this value because the relative importance of the different production mechanisms changes as a function of age, erosion rate, elevation, latitude, sample thickness and snow cover.
The exact form of the function f S will be defined in Section 5. This means that, strictly speaking, the TCN concentrations should be multiplied by St prior to generating a banana plot. Because topographic shielding corrections are generally small, the systematic error caused by lumping St together with the other correction factors is very small.
In this case, the nuclide concentrations do not need to be pre-multiplied by St. By using the TCN production equation of Granger et al. TCN production by muons causes a characteristic cross-over of the zero-erosion and infinite exposure lines at low 10Be concentrations. The graphical output of CosmoCalc can easily be copied and pasted for editing in vector graphics software such as Adobe Illustrator or CorelDraw. The y-axis of the 26AlBe plot is logarithmic by default whereas the y-axis of the 21NeBe plot is linear.
Note that MS-Excel versions and only allows logarithmic tickmarks to have values in multiples of ten. Hopefully, this limitation will not be necessary in later versions of Excel.
CosmoCalc only propagates the analytical uncertainty of the measured TCN concentrations. No uncertainty is assigned to the production rate scaling factors, radioactive half-lives or other potentially ill-constrained quantities. On the banana plots, the user is offered the choice between error bars or -ellipses with the latter being the default. This causes the error ellipses to be rotated according to the following correlation coefficient: If only one nuclide was measured, we must assume values for two of these quantities in order to solve for the third.
If two nuclides were analysed of which at least one is radioactiveonly one assumption is needed. CosmoCalc is capable of both approaches. Note that in the case of two nuclides 26Al or 21Ne combined with 10Bethe assumption of zero burial can be verified on the banana plot. We somehow need to incorporate this scaling factor into the ingrowth equation Equation 4. This poses a problem because the scaling factor is a single number whereas Equation 4 explicitly makes the distinction between neutrons, slow and fast muons.
Granger and Smith avoid this problem by separately scaling the different production mechanisms: To ensure optimal flexibility and user-friendliness, CosmoCalc uses a slightly different approach.
As said before, some assumptions are needed to solve Equation 8. Statistical uncertainties are estimated by standard error propagation: If two nuclides have been measured with concentrations N1 and N2, sayonly one value must be assumed in order to solve for the remaining two. Other studies, however, intentionally target complex exposure histories, using radionuclides to date pre-exposure and burial e. It does not handle post-depositional nuclide production.
Because the stable nuclide is not affected by burial, it can be used to calculate the pre-exposure age, using Equation This age can then be used to calculate the burial age: The solution is then plugged into Equation 17using nuclide 1. Instead, CosmoCalc implements the two-dimensional version of the Newton-Raphson algorithm: Therefore, it can be more productive to solve each quantity separately instead of simultaneously.
Thus, using equations 11 and 13it is possible to estimate minimum exposure ages and maximum erosion rates e.