TIMING OF MOUNTAIN GLACIER MAXIMA IN THE TROPICAL ANDES AND THE NORTHERN HEMISPHERE: UNEXPECTED SYNCHRONEITY

Mountain glacier chronologies from low and middle latitudes in the Northern and Southern Hemispheres are surprisingly similar, given the inversely related insolation patterns on opposite sides of the equator. Mountain glaciers in the Himalayas, the Olympic Mountains of Washington in the U.S., and the tropical Andes of Peru and Bolivia reached their local last glacial maxima ~35-30 ka, well in advance of the global ice volume maximum as inferred from the marine oxygen isotope record. Glacial maxima in the Himalayas and Olympics have been linked to increased moisture delivery during insolation highs that increased storminess. Insolation maxima in the Northern Hemisphere coincide with insolation minima in the Southern Hemisphere, where the Peruvian and Bolivian Andes are located. In the Andes, insolation maxima are associated with reduced ablation on glaciers because increased insolation enhances convection and cloudiness so that receipt of solar energy at the ice surface falls. Based solely on this observation, we would expect mountain glaciers in the Andes to have reached their last maxima during the last insolation maximum, which occurred ~21 ka, rather than during the penultimate insolation minimum ~32 ka, as they actually did. Invoking a global reduction in temperature to explain the synchroneity of glacial maxima between mountain glaciers in the Northern and Southern Hemispheres ~35-30 ka conflicts with the widely reproduced record of global ice volume. The timing of glacial advances in the tropical Andes is similar to the timing of Heinrich events in the North Atlantic during the latter part of the last glacial cycle. Heinrich events have been credited with producing steeper meridional temperature gradients and enhanced trade wind circulation, resulting in southward displacement of the intertropical convergence zone (ITCZ) and increased moisture delivery to tropical South America. The early local last glacial maximum in the tropical Andes may thus be more closely linked to ice discharge events that occurred in the North Atlantic than to the direct effects of insolation changes.