RELATIVE EQUILIBRIUM-LINE ALTITUDE AS AN INDICATOR OF GLACIER SENSITIVITY TO TEMPERATURE AND PRECIPITATION

The equilibrium-line altitude (ELA) of a glacier is a measure of mass balance, the altitude at which accumulation equals ablation. Greater accumulation drives ELA lower. Ablation depends on the total energy the glacier receives from insolation, which increases with altitude, and transfer of heat from the atmosphere, which decreases. Generally, ice loses mass via melting but even in sub-zero temperatures (T_air) up to 80 mm of ice may sublime. Inferring paleoclimate from glacial records require understanding of the dominant ablation processes. In regions with high precipitation (ppt) and moderate T_air, such as coastal North America, melting dominates ablation (>90%); a glacier must flow to lower altitudes and higher T_air before melting can balance the accumulation. Here, ELA varies ~linearly with T_air and ppt. In cold (subzero) arid regions with low water vapor pressure such as parts of The Andes or High Central Asia, little melting occurs, and sublimation may dominate ablation. Here, the ELA may be poorly defined since sublimation is largely independent of altitude, but glaciers will be restricted to high altitudes where little melting occurs. Therefore, the timing of maximum glaciations in cold arid regions may not coincide with the paleo-records of lowest T_air. If a mountain range near the melt-to-sublimation boundary transitions to cold, dry conditions as happened in MIS 2, its ELA chronosequence should parallel that of nearby ranges where melt dominates, except that the MIS 2 glaciers should be restricted to the highest altitudes. We mapped moraines and inferred paleoELAs across N–S and E–W climate transects in Central Asia to test this hypothesis. In addition to absolute values of ELAs we report a chronosequence of relative ELAs (ELA_max/ELA_min) for each valley. Mapping of the relative ELAs can reveal glacial asynchronism from region to region. Our ¹⁰Be ages for glacial boulders in six mountain ranges, in addition to published glacier chronologies, reveal that the lowest ELAs in the less arid regions did occur during MIS 2, suggesting that these glaciers were melt-dominated. In contrast, the lowest ELAs in hyperarid regions did not occur during MIS 2, suggesting that lower ppt “starved” these glaciers during the colder climates of MIS 2 and sublimation dominated their ablation.