Suggestions for future improvements

The CAD method would gain considerable power if the AFT data were complemented by (U-Th)/He measurements as was done by Stock et al. [2007] for two catchments in the Sierra Nevada. Whereas the AFT CAD serves as a proxy for the PAZ curve, the (U-Th)/He CAD would be a proxy for the Partial Retention Zone (PRZ) curve. Because (U-Th)/He ages from the northern White Mountains have already been measured [Stockli et al., 2000], this would be relatively easy to do. Measurement uncertainities of single grain (U-Th)/He measurements are approximately normally distributed, and typically much smaller than single grain AFT measurement uncertainties. Unfortunately, this is only true for inclusion-free apatites. The vast majority of igneous apatites contain abundant $\alpha$-emitting mineral inclusions such as zircon and monazite, and the Pelissier Flats apatites are no exception to this. In basement studies, inclusion-free apatites are carefully selected under a binocular microscope. Not only would doing this for $\sim$100 detrital grains be very time consuming, it would create potentially biased samples. Therefore, detrital (U-Th)/He studies should also consider inclusion-bearing grains [Vermeesch et al., 2007].

Another approach to obtain more and better information from the AFT data is by artificially increasing the number of confined fission tracks through heavy-ion irradiation or exposure to $^{252}$Cf [e.g., Ohira et al., 1994]. Like the AFT ages, fission track lengths show a dependence on paleodepth (and thus elevation). At low and high paleodepths well above or below the paleo-PAZ, fission tracks are long, whereas in the PAZ, short tracks also exist. Each AFT age on Figure 1.a corresponds to a characteristic distribution of horizontally confined fission track (HCFT) lengths. Heavy ion irradiation or bombardment by $^{252}$Cf fission fragments provides pathways through which the etching acid can reach more HCFTs, up to a point where there is more than one HCFT per apatite grain, even for samples with low numbers of spontaneous fission tracks [ Ohira et al., 1994]. If the HCFT lengths and AFT ages of the same grains are measured, the provenance paleodepth distribution can be determined more reliably than by only using the AFT ages. The Marble Creek catchment would be the perfect test-case for this technique, because the HCFT-length distribution of the basement is known [ Stockli et al., 2000, 2003].