Geol. Soc. Am. Bull.http://doi.org/hzg (2012)
Today, the Atacama Desert in Chile is one of the driest places on Earth. A reconstruction of the desert's former fluvial systems shows that a decline in precipitation from at least 125 mm to less than 3 mm per year began about two million years ago.
Ronald Amundson of the University of California, Berkeley, and colleagues compiled geological maps and field observations of basin fill and soil characteristics to reconstruct the nature of the Atacama landscape before aridification. The ancient fluvial systems were characterized by at least episodic occurrences of vigorous deposition and erosion, with the average magnitude of Pliocene incision approximately ten times greater than that of the subsequent Quaternary. Furthermore, thick, bedrock-derived soils mantled the hillslopes during the Pliocene, whereas today's thin soil layer is composed primarily of dust and salts deposited from the atmosphere. Thick salt layers that formed after the main aridification provide evidence for extreme, but rare, rainfall events.
The timing of the aridification suggests that the uplift of the Andes was not the primary driver of the climate shift. Instead, the drying coincides with the intensification of upwelling in the eastern tropical Pacific Ocean, which cooled sea surface temperatures and reduced the amount of precipitation generated in the region.