Hanks and Finkel attempt to discredit our cosmogenic nuclide profile dating of a gravel-capped terrace at Bluff, Utah, by suggesting that the gravels could have been deposited in two stages separated by a hiatus of nearly a million years. The cosmogenic nuclide data alone cannot eliminate this possibility. Nor can the data exclude an arbitrarily complex history, if just the right amount of erosion occurred and new material was added with just the right amount of inheritance. However, our cosmogenic nuclide profile unambiguously indicates the minimum age of the base of the deposit. In addition, all of our data and field observations are consistent with the sediments being deposited in a single episode that occurred quickly with respect to radioactive decay of 26Al (i.e., <105 yr). In the absence of evidence to the contrary, we therefore conclude that a single depositional episode is the most likely explanation (Wolkowinsky and Granger, 2004).
We would like to elaborate first by reviewing the capabilities and weaknesses of dating sediments with cosmogenic nuclide profiles, and second by discussing the absence of evidence for multistage deposition at the Bluff terrace.
When analyzing cosmogenic nuclides in sediments from a profile, it is important to realize what can and cannot be determined. As shown by Granger and Smith (2000), sediments at depths >5–10 m have 26Al/10Be ratios that depend strongly on time since deposition, and weakly on the erosion rate of the deposit. The 26Al and 10Be concentrations at shallower depths depend only weakly on depositional age, but are sensitive to the erosion rate of the past few meters of sediment removed. The erosion rate determined from the upper part of the profile can then be used to refine the burial age determined for the lower part of the profile. A cosmogenic nuclide profile indicates with certainty only the minimum age of the lowermost sediments. It is therefore essential that the cosmogenic data be interpreted in the context of field observations.
Given the possibility, however remote, that sediment erosion and deposition can occur and leave no cosmogenic trace, we must ask if there is any evidence for multiple erosion and deposition episodes at the Bluff terrace. Hanks and Finkel offer four potential lines of evidence, none of which we find convincing.
1. Hanks and Finkel suggest that a two-stage deposition model improves the data fit. Examination of the model fits to the data (rather than their idealized smooth curves) indicates that both our single-stage model and their two-stage model agree with the data to well within analytical uncertainty. There is no part of the data that is not accounted for by our simpler model, and thus no compelling reason to invoke a more complex model.
2. Hanks and Finkel identify a “significant unconformity” in the profile, based upon a photograph exchanged through e-mail (Fig. 1). The gravel/sand transition in Figure 1 is simply the top of a local sand lens, hardly evidence for a million-year erosional unconformity as Hanks and Finkel suggest.
3. Inherited cosmogenic nuclide concentrations vary significantly among samples in the profile. We believe that this is due primarily to variation in the source area, where quartz pebbles are derived from a high-relief, partially glaciated mountain range. Part of the variation could also reflect production as the sediments accumulated on the terrace over thousands of years. Such variability does not seriously affect our burial ages.
4. Hanks and Finkel extrapolate our erosion rate of 14 m/m.y. to infer an original terrace thickness of 31 m, a value they consider unlikely. However, the cosmogenic erosion rate reflects only the recent past, and may not necessarily be extrapolated for the life of the terrace. Moreover, it is certainly possible that thick gravel deposits can accumulate during a single glacial cycle; terrace sediments elsewhere in the region have thicknesses ranging up to 40 m (Patton et al., 1991).
Given that both cosmogenic nuclide data and field evidence are consistent with a single stage of deposition, it is difficult to justify invoking a million-year hiatus in sediment deposition. We therefore maintain our conclusion (as stated in our paper) that although such a scenario is conceivable, we consider it unlikely.
We thank M. Caffee for discussion and for joining us in the field.