Snowline depression in the tropics during the Last Glaciation

Five primary methods have been used to reconstruct Pleistocene snowlines or equilibrium-line altitudes (ELAs) in the tropics (23.5 degreesN-23.5 degre esS) during the last glaciation, but each has inherent errors that limit th e accuracy of the results. Additional potential errors in determining ELA d epression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction of former glaciers, orographic effects, the pre sence of rockfall debris on glaciers, and calculation of regional ELA gradi ents. Eustatic sea-level lowering during the last glaciation is an addition al factor influencing estimates of ELA depression (Delta ELA). In cases whe re modern snowline lies above a mountain summit, only a minimum value for D elta ELA can be obtained. At 12 tropical sites in Africa, the Americas (to 10 degreesS latitude), and Pacific islands, estimates of average Delta ELA range from 440 to 1400 m, but most fall in the range of 800-1000 m (mean +/ - 1 sigma = 900 +/- 135 m). In a regional study of ELA depression in the so uthern tropical Andes (8-22 degreesS), an average Delta ELA of ca. 920 +/- 250m has been reported. Based on the assumption that glacier mass balance w as controlled solely by ablation-season temperature, and assuming a full-gl acial temperature lapse rate of - 6 degreesC/km, depression of mean annual temperature in glaciated alpine areas was ca. 5.4 +/- 0.8 degreesC. If adju sted for a sea-level fall of - 120m at the glacial maximum, this value is r educed to 4.7 +/- 0.8 degreesC. The figure is based on the (unlikely) assum ption that accumulation on alpine glaciers has been invariant; nevertheless , it is similar to values of temperature depresson (5-6.4 degreesC) for the last glaciation obtained from various terrestrial sites, but contrasts wit h tropical sea-surface temperature estimates that are only 1-3 degreesC coo ler than present. (C) 2001 Elsevier Science Ltd. All rights reserved.