Extended Data Figure 1: ¹⁰Be and ²⁶Al concentrations compared to production rates.

¹⁰Be (n = 5) and ²⁶Al (n = 2) concentration–depth profiles compared to the depth dependence of nuclide production rates (including both spallogenic and muon production). In both a and b, the red and blue boxes represent ¹⁰Be (red) and ²⁶Al (blue) measurements. The vertical dimension of each box represents a distinct segment of core, and vertical lines connect multiple core segments that were amalgamated for each ¹⁰Be or ²⁶Al analysis. In most cases amalgamated segments were adjacent to each other, but in some cases (for example, the uppermost two core segments), they were separated by gaps. The width of the boxes shows measurement uncertainty (1σ; see Extended Data Table 1) on nuclide concentrations. In a, we attempt to fit the observations by assuming that the bedrock surface is the land surface, the erosion rate is zero, and by allowing the duration of a single period of exposure to vary. For this exercise we treat ¹⁰Be and ²⁶Al separately, that is, the predictions are not forced to obey the production ratio. The continuous thin black lines show predicted nuclide concentrations for this model, and the discontinuous, darker, black bars show predicted nuclide concentrations averaged over depth ranges corresponding to each analysis. The black bars, therefore, are the model predictions that we compare to the measurements. This model cannot be fitted to the data, because observed nuclide concentrations do not decrease as rapidly as they would if the bedrock surface were the land surface during exposure. In b, we perform the same fitting exercise, but also include a thickness of shielding mass above the bedrock surface as an additional fitting parameter. This removes the systematic misfit shown in a and makes it possible to fit the observations. A cover thickness of 350 g cm⁻² best fits the observations.