Extended Data Figure 2: Two-stage exposure histories fit to ¹⁰Be and ²⁶Al measurements.

In a and d, the red and blue boxes represent ¹⁰Be (n = 5; red) and ²⁶Al (n = 2; 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 Tables 2 and 3) on nuclide concentrations. The thick black lines are nuclide concentrations predicted for each core segment by the best-fitting parameters of each model. b and e show the exposure history implied by the best-fitting parameters of each model compared to the LR04 oxygen isotope stack from ref. 30. Red bars represent periods of surface exposure and blue bars periods of cover by the ice sheet. c and f show observed nuclide concentrations compared to model predictions for samples in which both ¹⁰Be and ²⁶Al were measured, normalized to production rates at their respective depths implied by each model, on a two-nuclide diagram²⁹. Red ellipses are 68% confidence regions for the nuclide concentrations including measurement uncertainties only, and black dots are nuclide concentrations predicted by best-fitting model parameters. The solid black lines show the simple exposure region; darker dashed lines are isolines of burial in increments of 1 Myr, and lighter dotted lines are isolines of exposure time in increments of 0.1 Myr. a, b and c show the fit of model 1, the simplest possible model that fits the data, which includes a single period of surface exposure, a single period of burial, 350 g cm⁻² of cover thickness above the bedrock surface during exposure, and zero surface erosion. In c, nuclide concentrations are normalized to production rates at sample depths below this additional cover thickness. This model provides a good fit to the measurements. The exposure history implied by the best-fitting parameters for model 1 (280-kyr exposure, 1.1 Myr burial) provides a maximum limiting constraint on the length of time the present ice sheet has been continuously present at the core site. d, e and f show the fit of model 1B, which includes a long period of continuous exposure with steady surface erosion, a single period of burial, and zero additional cover thickness above the bedrock surface. In f, nuclide concentrations are normalized to production rates at sample depths below the bedrock surface. This model cannot be adequately fitted to the observations.