Extended Data Fig. 3: Percentage of quartz-bearing rocks covered by ice as a function of time.

Glacier surface-mass balance was computed using a Positive-Degree-Day model with the annual average temperature at 2 000 m: present day = 3.4 °C, 23 ka = −5.6 °C, 43 ka = −3.6 °C and 59 ka = −3.9 °C (paleotemperatures derived from local δ¹⁸O SST, equation S1). Winter-summer amplitude is 16.8 °C and annual precipitations are 1500 mm.a⁻¹. Our model yields ELA of 3350 m (present), 1950 m (23 ka), 2260 m (43 ka) and 2200 m (59 ka) with an ablation gradient of 6 mm(w.e.).a⁻¹.m⁻¹. Weather data are from Meteo France (https://donneespubliques.meteofrance.fr), paleo temperatures calculated using S1 and the local lapse rate of 6.1 °C/km⁵⁰.