Paired-cosmogenic nuclide paleoaltimetry

Abstract

Goal – The reconstruction of past topographies remains challenging and only a few methods allow accurate determination of past surface elevations. We propose here a new technique for deriving paleo-elevations, in which multiple cosmogenic nuclides are measured in the same geological sample exposed at the Earth's surface. This method relies on the altitude dependence of the cosmogenic nuclides' production rates combined with the radioactive decays of nuclides with different half-lives.Theory – The position of the two cosmogenic nuclide exposure curves (26Al/10Be vs10Be or 10Be/21Ne vs10Be) depends on the altitude of exposure. If the studied surfaces have been exposed for sufficiently long durations (>500 ka), or have been affected by low erosion rates (<1 mMa−1), measurement of two cosmogenic nuclides with different half-lives thus allow accurate elevations to be determined with a reasonable uncertainty (<1000 m at 1σ). For shorter exposure durations, the method is able to constrain minimum elevations. The main advantage of the method is that it is only slightly sensitive to erosion: even if the preservation state of the surface is unknown, the bias on the computed elevation remains lower than 1500 m in most cases. The approach can also be applied to previously exposed surfaces that have subsequently been buried, in order to reconstruct the paleo-elevation of a given surface over time ranges of ∼0 to 8 Ma (using the 26Al–10Be pair) and ∼0 to 12 Ma (using the 10Be–21Ne pair).Data comparison – We tested the method using the multiple cosmogenic nuclides dataset available for the western arid tropical Andes. The altitudes computed using the cosmogenic nuclide concentrations agree within uncertainties with the reported sampling altitudes over a range of 0 to more than 4000 m, illustrating the applicability of the method. Altitudes computed under the assumptions of continuous exposure or steady state erosion yields best fits that are statistically in agreement and close to the 1:1 line for both the 26Al–10Be and the 21Ne–10Be dataset. The 21Ne–10Be inventories in samples that have been exposed for more than 5 Ma yield elevations that are several hundreds of meters below their present-day elevations (∼1000 m). This may result from a post 10 Ma uplift of the West Andes, or from an unrecognized exposure underwater, or below a soil cover.Implications – This study may also have implications in other fields that rely on multiple cosmogenic nuclide measurements. The same approach might notably be used to compute the depth of exposure of samples located below the rock surface or underwater. This study may also help to improve the accuracy of the common burial dating method that uses multiple radioactive cosmogenic nuclides. For long pre-burial exposures (>500 ka), or low erosion rates (<1 mMa−1), the values of the pre-burial nuclides ratios indeed depend strongly on the altitude of exposure. It may be important to consider the pre-burial altitude of exposure in order to calculate accurate burial ages.