Snowline depression in the tropics during the Last Glaciation

Abstract

Five primary methods have been used to reconstruct Pleistocene snowlines or equilibrium-line altitudes (ELAs) in the tropics (23.5°N–23.5°S) during the last glaciation, but each has inherent errors that limit the accuracy of the results. Additional potential errors in determining ELA depression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction of former glaciers, orographic effects, the presence of rockfall debris on glaciers, and calculation of regional ELA gradients. Eustatic sea-level lowering during the last glaciation is an additional factor influencing estimates of ELA depression (ΔELA). In cases where modern snowline lies above a mountain summit, only a minimum value for ΔELA can be obtained. At 12 tropical sites in Africa, the Americas (to 10°S latitude), and Pacific islands, estimates of average ΔELA range from 440 to 1400 m, but most fall in the range of 800–1000 m (mean ±1 σ=900±135 m). In a regional study of ELA depression in the southern tropical Andes (8–22°S), an average ΔELA of ca 920±250 m has been reported. Based on the assumption that glacier mass balance was controlled solely by ablation-season temperature, and assuming a full-glacial temperature lapse rate of −6°C/km, depression of mean annual temperature in glaciated alpine areas was ca 5.4±0.8°C. If adjusted for a sea-level fall of −120 m at the glacial maximum, this value is reduced to 4.7±0.8°C. The figure is based on the (unlikely) assumption that accumulation on alpine glaciers has been invariant; nevertheless, it is similar to values of temperature depresson (5–6.4°C) for the last glaciation obtained from various terrestrial sites, but contrasts with tropical sea-surface temperature estimates that are only 1–3°C cooler than present.