How deep is deep enough: disambiguating the acceptable depth for cosmogenic isochron burial dating

Cosmogenic ¹⁰Be/²⁶Al isochron burial dating is an effective way to accurately determine the depositional time of fluvial sediments that were shallowly buried without requiring any information on post-burial nuclide production and time-dependent burial history. The apparent differences in measured ¹⁰Be and ²⁶Al concentrations among a sample assemblage, which derives from the differential erosion in provenance, plays a crucial role in building the isochron. While, when the burial depth is too shallow and/or the amounts of post-burial produced nuclides constitute a main part of total nuclide inventories in samples, the concentration differences among samples would be overwhelmed by the analytical uncertainties of measured ¹⁰Be and ²⁶Al concentrations in samples, especially for the old sediments, and it thus leads to a large uncertainty in obtained burial age or destroys totally the isochron. As a result, in the situation of shallow outcrops, a compromise needs to be done between time consumption to dig few meters for sampling and the attainable dating precision. Furthermore, in the cases of shallowly deposited river terraces, especially in the southeastern margin of Tibetan Plateau where the surface erosion is fast and sediments preserved are commonly thin, there is no alternative deeper-buried sediments, and the dilemma between dating but without acceptable uncertainty and not dating has to face. Therefore, the question of how deep is deep enough has to be weighed when sampling. Here, we proposed that the sampling depth of sediments in isochron burial dating should be a flexible standard depending on the isochronicity of the sample assemblage, the predicted burial duration, local geomorphic conditions, and the probably analyzed uncertainties. We try to plot the correlation of predicted age uncertainties with sampling depth, burial ages, as well as the geomorphic parameters, to disambiguate the acceptable depth in the isochron dating for the required dating precision.