Late Quaternary Glaciation of the Tropical Andes

Abstract

The effects of climate change are intrinsic features of Earth’s landscapes, and South America is no exception. Abundant evidence bears witness to the changes that have shaped the continent over time—from the glacial tillites inherited from late Paleozoic Gondwana to recent terrigenous sediments and life forms trapped in alluvial, lacustrine, and nearby marine deposits. Preeminent among this evidence are the landforms and sediments derived from the late Cenozoic glaciations of the Andes, which have been the focus of so much recent and ongoing research. Because South America has long been a mainly tropical and subtropical continent, most of it escaped the direct effects of these glaciations. Nevertheless, portions of the continent extend sufficiently far poleward and rise high enough to attract snowfall and promote glaciers today. Glaciers were more emphatically present during Pliocene and Pleistocene cold stages, and it is their legacies that provide information about the changing environments of those times, and more especially of the past 30,000 years. There is evidence for glaciation in the tropical and extratropical Andes as early as Pliocene time (Clapperton, 1993). In southern South America, along the eastern side of the Patagonian Andes, Mercer (1976) dated a series of basalts interbedded with glacial tills that suggest multiple glacial advances after ~3.6 Ma (million years before present). In the La Paz Valley, Bolivia, volcanic ashes dated by K/Ar (potassium/argon) methods are interbedded with glacial tills indicative of at least two phases of glaciation in the late Pliocene, at 3.27 and 2.20 Ma (Clapperton, 1979, 1993). This evidence for early glaciation in disparate parts of the Andes indicates that portions of the cordillera were high enough and climatic variations were great enough in the Pliocene for glaciers to form long before the cold episodes of the Pleistocene. Glacial deposits in Ecuador, Perú, and Bolivia provide evidence for climate variability in tropical South America in the recent geological past. In the late Pleistocene, glacier equilibrium-line altitudes were as much as 1,200 m lower than they are today on the eastern slopes of the Andes, indicative of a significant depression in mean annual temperature in the tropics at maximum glaciation (e.g., Klein et al., 1999).