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Neogene Climate Change and Uplift in the Atacama Desert, Chile

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Neogene Climate Change and Uplift in the Atacama Desert, Chile Neogene climate change and uplift in the Atacama Desert, Chile Jason A. Rech ⎤ ⎥ Department of Geology, Miami University, Oxford, Ohio 45056, USA Brian S. Currie ⎦ Greg Michalski Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA Angela M. Cowan Department of Geology, Miami University, Oxford, Ohio 45056, USA ABSTRACT elevations above ϳ2800 m, but do not cause The relationship between Andean uplift and extreme desiccation of the west coast of rainfall in the central Atacama. South America is important for understanding the interplay between climate and tectonics The Calama Basin is located on the eastern in the Central Andes, yet it is poorly understood. Here we use soil morphological char- margin of the Atacama, ϳ150 km from the acteristics, salt chemistry, and mass independent fractionation anomalies (⌬17O values) in Pacific Coast at elevations between 2200 and dated paleosols to reconstruct a middle Miocene climatic transition from semiaridity to 3500 m (Fig. 1). Precipitation in the center of extreme hyperaridity in the Atacama Desert. Paleosols along the southeastern margin of the basin (2200 m) is ϳ4 mm/yr, whereas the Calama Basin change from calcic Vertisols with root traces, slickensides, and gleyed along the eastern margin (3350 m) precipita- horizons to an extremely mature salic Gypsisol with pedogenic nitrate. We interpret this tion is ϳ50 mm/yr. transition, which occurred between 19 and 13 Ma, to represent a change in precipitation from Ͼ200 mm/yr to Ͻ20 mm/yr. This drastic reduction in precipitation likely resulted PALEOSOLS IN THE CALAMA BASIN from uplift of the Central Andes to elevations Ͼ2 km; the uplift blocked moisture from We examined Miocene strata and Quater- the South American summer monsoon from entering the Atacama. The mid-Miocene Gyp- nary landforms along the southeastern margin sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the Cal- of the Calama Basin for evidence of pedogen- ama Basin, significantly higher than modern nitrate soils, which occur below ϳ2500 m. esis. Miocene gypcretes were first reported in Modern and Quaternary soils in this elevation zone contain soil carbonate and lack ped- this region by Hartley and May (1998). We ogenic gypsum and nitrate. We infer that Ͼ900 m of local surface uplift over the past 10 identified Miocene and Quaternary paleosols m.y. displaced these nitrate paleosols relative to modern nitrate soils and caused a return developed on substrates of alluvial fan and to wetter conditions in the Calama Basin by decreasing local air temperatures and creating flood-plain deposits, and basement bedrock. an orographic barrier to Pacific air masses. Keywords: Atacama Desert, Andes, paleosols, Calama Basin, soil nitrate. INTRODUCTION maintained through the existence of a strong The extreme aridity of the Atacama Desert orographic rain shadow, the mid-Miocene ini- in northern Chile has been associated with the tiation of hyperaridity has implications for the uplift of the Central Andes and the subsequent uplift history of the Andes. blocking of moisture from the Amazon during the mid-Miocene (e.g., Alpers and Brimhall, STUDY AREA 1988; Sillitoe and McKee, 1996). Recent pa- The Atacama Desert is between the central leoclimatic studies from the Atacama, how- Andes and Pacific Ocean in northern Chile ever, have questioned this relationship. Hart- (Fig. 1). Several factors produce the extreme ley and Chong (2002) suggested a late hyperaridity of the Atacama today, including Pliocene age for the initiation of hyperaridity the Andean rain-shadow effect, the coastal and argued that desiccation of the west coast of South America was unrelated to Andean temperature inversion, and the latitudinal po- uplift. Dunai et al. (2005) proposed that hy- sition of this region (Houston and Hartley, peraridity commenced during the late Oligo- 2003). The hyperarid core of the Atacama re- Ͻ cene (ca. 25 Ma) and that this aridity could ceives 3 mm/yr precipitation, does not sup- have been a significant contributor to the re- port vascular plants, and contains soils with gional uplift in the Cordillera, as proposed by high concentrations of soluble salts, including Lamb and Davis (2003). Hoke et al. (2004), sulfates, chlorides, and nitrates. We use the however, identified a major shift toward more term extreme hyperaridity to refer to compa- arid conditions ca. 10 Ma and suggested that rable climatic conditions in the geologic desiccation at that time was related to the up- record. lift of the Altiplano plateau. In order to deter- Occasional precipitation events in the Ata- mine the relation and potential feedbacks be- cama generally result from Pacific air masses tween Central Andean uplift and climate that migrate northward from the westerly pre- change, we need records of Neogene precipi- cipitation belt. Along the eastern margin of the Figure 1. Location of Atacama Desert, Cal- tation and elevation. Here we present evidence Atacama (ϳ2500 m), precipitation is Ͼ20 ama Basin, and soil nitrate mines in north- from paleosols in the Calama Basin for ex- mm/yr and is associated with the South Amer- ern Chile. Red circles (1–5) identify loca- ican summer monsoon (SASM). SASM air tions of stratigraphic sections (Fig. 2). treme hyperaridity by ca. 12 Ma along the Shaded relief digital elevation model was eastern margin of the Atacama Desert. As ex- masses spill over the central Andes and gen- created by NASA/JPL/NIMA with data from treme hyperaridity in the Atacama today is erate precipitation on the eastern Atacama at Shuttle Radar Topography Mission. ᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2006; v. 34; no. 9; p. 761–764; doi: 10.1130/G22444.1; 3 figures; Data Repository item 2006159. 761 cm across and 2 m deep. These fractures are generally filled with eolian silts and sands as well as salts, and are similar to fractures in modern Atacama salic soils (Ericksen, 1981). Petrographic analysis of the Bym horizon shows a complex history of precipitation and dissolution of pedogenic salts, also analogous to modern soils in the Atacama. Between 1.5 and 3.5 m depth in the salic Gypsisol, and in some locations superimposed on top of older paleosols, is a salic horizon that contains as much as 2.5% NO3, 0.5% Cl, and trace amounts of perchlorate (Fig. 3). Soils on top of Quaternary landforms along the southeastern margin of the Calama Basin are Calcisols that contain pronounced calcic or petrocalcic horizons (stage II to stage IV de- velopment within ϳ25 cm of the surface), but lack argillic, gypsic, and salic horizons. These soils generally occur on the surface of alluvial Figure 2. Stratigraphic sections of Neogene paleosols from southeastern margin of Calama fans and fluvial terraces. Basin. Bym—petrogypsic horizon. DISCUSSION All paleosols were classified using the system oxides and organic matter. At the Rı´o Seco Calcic Vertisols with gleyed horizons and of Mack et al. (1993). Four classes of paleo- locality, calcic Argillisols occur in a gravelly root traces form in poorly drained, vegetated sols are present. Middle-upper Miocene strata parent material. These paleosols have diffuse flood plains with seasonal precipitation. These contain calcic Vertisols and calcic Argillisols, calcic horizons (stage I) and argillic horizons soils do not occur in the Atacama today, but which are overlain by a thick, well-developed composed of montmorillonite and illite. are present to the south in central Chile (ϳlat salic Gypsisol in all locations examined. Cal- A well-developed salic Gypsisol, hereafter 28Њ–34ЊS). The calcic Argillisols, with a cisols are present within Quaternary deposits defined as the Barros Arana geosol, is pre- coarse-grained parent material, form in basin and geomorphic surfaces. served across the southeastern margin of the margin environments (alluvial fan and baja- The oldest paleosols we investigated are in- Calama Basin (Figs. 1 and 2). This paleosol das) where the water table is lower. terbedded in the lower-middle Miocene El Loa formed on top of basement bedrock (Paleozoic The Barros Arana geosol formed across the Formation (ca. 20 Ma). These paleosols are volcanics) at the El Hotel and Agua de la Teca southeastern margin of the Calama Basin, on exposed at the Barros Arana and Rı´o Seco lo- localities, and in Miocene alluvial fan deposits top of pediments, alluvial fans, and bajadas. Ͼ calities (Fig. 2) and consist of calcic Vertisols at other localities (Fig. 2). The Gypsisol is 3 This paleosol was originally described by and calcic Argillisols, respectively. The calcic m thick and contains a 1.5–6-m-thick petro- Hartley and May (1998, p. 361), who inter- ϳ Vertisols are 1 m thick and contain red ar- gypsic horizon (Bym). The petrogypsic hori- preted it as ‘‘a subsurface crust formed by a Ͼ 2 gillic horizons with montmorillonite and illite zon has a high bulk density, 2.0 g/cm , and combination of hydromorphic and illuvial pro- ϳ clay minerals. Argillic horizons have angular contains 10–45 wt% SO4,or 20%–90% cesses and subject to periodic exhumation and blocky structure with clay skins, slickensides, gypsum (Fig. 3). Sulfate concentrations are weathering.’’ This interpretation was based and pseudoanticlinal fractures. These soils highest at the top of the paleosol profile, primarily on evidence of surficial exposure also have soil carbonate (stage II; 1–3 cm nod- where clasts commonly float in a gypsum ma- (salt fractures) and hydromorphic precipitation ules), gleyed mottling and gleyed horizons (5– trix. The paleosol has large v-shaped salt frac- of gypsum (poikilitic textures). Poikilitic tex- 15 cm thick), and root traces with manganese tures, or sand dikes, that are as much as 35 tures, however, are common in salic soils in the Atacama, most of which form in regions with low water tables and are not influenced by hydromorphic processes.
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