Calculation of cosmogenic radionuclide burial ages: a comparison of two models

Two published cosmogenic radionuclide (CRN) ²⁶Al/¹⁰Be burial age calculation methods developed to correct for post-depositional production of nuclides in settings with low sediment overburden are compared. The advantages and limitations of simple (ISO; [1], [2]) and inverse modelling (INV, [3]) isochrons are investigated.

The studied dataset originates from the gravel of a Danube terrace in the Central Vienna Basin (Austria) [4]., where two horizons (5.5 m and at 11.8 m subsurface depth) were sampled. Each sample set contained 6 quartz or quartzite cobbles.

The advantage of ISO is that it is uninfluenced by changes in sample depth over time. However, the initial ²⁶Al/¹⁰Be ratio is fixed and no pre- and post-burial denudation rates can be calculated. In addition to age, INV models source and sink denudation rates, but assumes constant depth over burial time.

For correct application of ISO and INV outliers, must be excluded. The robustness of both methods is tested by systematically including or excluding data points (bootstrapping) to estimate the dependence of numerical ages on sample selection either in the field, or during outlier identification.

For outlier identification the traditional method of data exclusion of points above or below the isochron line is used. In addition, a new way is introduced here: the post-burial production is calculated using the modelled burial age and denudation rate and compared to the measured inventories of ¹⁰Be and ²⁶Al. If the fraction of post-burial production is equal or higher compared to the measured inventory and its ratio is considerably different for the two isotopes from the same sample, the datapoint is invalid.

In addition, the influence of each sample on the modelled burial age, tested by bootsrapping, is used to exclude samples with a large effect on the age.

The resulting ages at both levels using ISO and INV agree within errors with ISO being systematically slightly younger. The importance of outlier removal is stressed by the fact that inclusion of all samples results in a considerably older age of the stratigraphically higher level compared to the underlying one.   When outliers are excluded, burial ages of the two sampled horizons overlap within uncertainty, suggesting one single deposition event for the whole sediment package.

Interestingly, when the entire dataset is merged, both methods provide similar ages regardless of the outliers being excluded or kept in. This demonstrates that a larger sample number increases the robustness of a dataset considerably and decreases the sensitivity of either method to potential outliers.

In summary, both ISO and INV are robust ways of CRN burial age determination, provided that model presumptions are not violated and outliers are excluded or the sample number large enough to overprint the influence of outliers.

Funding: NKFIH FK124807; OMAA 90ou17; OMAA 98ou17.

 

References

[1] Balco, G., Rovey, C.W., 2008. American Journal of Science 308(10), 1083-1114.

[2] Erlanger, E.D., et al., 2012. Geology 40(11), 1019-1022.

[3] Pappu, S. et al., 2011. Science, 331(6024), 1596-1599.

[4] Ruszkiczay-Rüdiger, Zs. et al., 2021. Journal of Radioanalytical and Nuclear Chemistry, 329(3), 1523-1536.