Rech et al. (2006) describe a variety of palaeosol characteristics from Miocene sections present along the southwest margin of the Calama Basin in the Atacama Desert of northern Chile. They identified a change in soil character from a calcic vertisol to an extremely mature gypsisol with pedogenic nitrate. The change in paleosol character is interpreted to record a decrease in precipitation related to a climatic shift from a semi-arid to hyper-arid climate some time between 19 and 13 Ma. This change is considered to be due to uplift of the Andes to an approximate elevation of 2000 m, creating an orographic barrier that prevented moisture from the South American summer monsoon entering the Atacama.
While it is likely that the Miocene gypsic paleosols do record a climate change, sedimentological and stratigraphic evidence from sections within the Calama Basin and elsewhere in the Atacama suggest that the significance and lateral extent of this change is limited and cannot be used to constrain Andean uplift, rain shadow development, and climate change. For example, sections within the Calama Basin that directly overlie the gypsic paleosols east and north of the sections studied by Rech et al. contain up to 55 m of alluvial conglomerate and sandstone (referred to as the Chiquinaputo Formation, Marinovic and Lahsen, 1984; May et al., 1999, 2005). These alluvial sediments are laterally equivalent to up to 85 m of palustrine limestones that are developed across much of the central part of the Calama Basin. The age of these sediments is constrained by inter-bedded ashes as being 8–3 Ma (May et al., 2005). The presence of alluvial gravels and sandstones conformably overlying the gypsic soils indicates that precipitation must have fallen directly on the eastern flank of the Calama Basin between 8 and 3 Ma.
Additional evidence for post-13 Ma alluvial activity on the west slope of the Andes and the Precordillera is provided by the sedimentary record throughout northern Chile (Hartley and Chong, 2002). In the Pampa del Tamarugal, alluvial and lacustrine sedimentation of the Quillagua Formation commenced between 7.3 and 6.3 Ma (Kiefer et al., 1997; Sáez et al., 1999). These include extensive alluvial fan deposits that were derived from catchments in the Precordillera. Sedimentation along the north western margin of the Salar de Atacama Basin commenced unconformably over early Miocene sediments around 10 Ma (Mpodozis et al., 2000). Importantly, in all of these examples, alluvial sediments were derived from adjacent catchments and deposited along basin flanks. Sedimentation was not restricted to valley systems, and groundwater recharge can be ruled out as a significant depositional process.
The study of Rech et al. (2006) cannot be used to infer a permanent climatic shift to hyperaridity generated by the rain shadow effect asso ciated with Andean uplift, as: 1) the paleosols are aerially restricted, 2) they are overlain by a significant thickness of alluvial sediment deposited under a ‘wetter’ climate, and 3) sedimentological and stratigraphic data from adjacent basins indicate significant post-13 Ma alluvial activity, all of which indicate precipitation in catchments on the west flank of the Andes. Paleosol characteristics should not be used in isolation to constrain Andean uplift as they need to be integrated with other sedimentological and stratigraphic data in order to derive conclusions that can be applied to the whole of the Atacama. Additionally, in the Southern Hemisphere, a mountain range is not required to create hyperarid conditions on the western margin of a continent; however, such conditions are present beneath the downgoing limb of the Hadley cell (e.g., the hyperarid coastline of Namibia).