2.3. Geologic, geomorphologic, and climatic settings of tropical western South America
2.3.4. Geography and climate of coastal Peru and the Atacama Desert
Rocks characteristic of continental crust are exposed along the coast of southern Peru and northern Chile. The Coastal Cordillera is made up of upper Jurassic to Cretaceous tholeiitic rocks, plutonites and marine backarc strata of Mesozoic age (Pinto et al., 2007). The Central Depression is a forearc depression, at ~1,000 m elevation that lies between the Coastal Cordillera and the western mountain front and is underlain by subhorizontal Oligocene to Recent alluvial, colluvial and volcanoclastic deposits (Allmendinger et al., 1997; Pinto et al., 2007). A line of Miocene to Recent stratovolcanoes overlying older ignimbrite sheets marks the Western Cordillera, the modern magmatic front. The Altiplano surface is covered by several large salars, Quaternary fill, and, locally, Late Oligocene to Recent volcanic rocks, including immense Late Miocene to Pliocene ignimbrite centers at the southern end of the plateau. Sparse exposures of the underlying basement consist of Ordovician and Cretaceous rocks ignimbrites (Allmendinger et al., 1997; Pinto et al., 2007).
2.3.4. Geography and climate of coastal Peru and the Atacama Desert
South of 6°30’ S the Andes reach almost all the way to the coast, restricting the coastal lowlands to a narrow strip, which is often no wider than 10 km. This pattern continues, with minor exceptions, to Chile. Several physiographic provinces exist in the study area (Figure 2.7).
From west to east, they are 1) Coastal Cordillera, 2) Central Depression, 3) western mountain front, 4) Western Cordillera, and 5) Altiplano. The Coastal Cordillera, less than 20 km wide, lies between the Pacific Ocean and the Central Depression and has peaks as high as 2,500 m and an average elevation of ~1,500 m that wanes toward the north, where it is less than 1,000 m at 18.5°S. Smoothed hills and shallow valleys form it. The deepest canyons of South America were cut by the Cotahuasi, Ocoña, and Colca rivers at 15°–16°S. Although their valley heads currently impinge beyond the Quaternary volcanic arc into the semi-‐arid Altiplano (rainfall 250–350 mm/yr), these canyons occur in a region where rainfall is <100–200 mm/yr (Thouret et al., 2007).
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Figure 2. 7 Morphostructural units in the orocline of the Central Andes, from Pinto et al. (2004).
To the west, the Coastal Cordillera is cut by a subvertical escarpment (“Coastal Escarpment”) of 1,000 m high. To the east, the altitude diminishes towards the Central Depression and the deposits filling the Central Depression overlap the basement rocks. The Central Depression extends parallel to the continental margin, bracketed between the Coastal Cordillera and the Precordillera. It corresponds to a forearc continental basin with a flat top surface at altitudes from 500 to 1,000 m in the west to 1,900 to 2,300 m in the east (Allmendinger et al., 1997;
Pinto et al., 2007). The Central Depression is void of hydrologic processes, and is where the Atacama’s hyper-‐arid conditions occur. The surface of the northern part of Central Depression has been locally deeply dissected by quebradas (canyons), in some cases up to 1.5 km deep, but typically less than 1 km (Mortimer, 1980). Their incision postdates deposition of the Late Tertiary valley-‐fill sediments of the Central Depression succession. Only the trunk drainages of the quebradas contain actively flowing or intermittent streams. Some tributaries are relict and contain no evidence of recent flow or even of relict fluvial sediments along their floors, and may have been shaped, at least partly, by groundwater sapping (Hoke et al., 2004). The western mountain front connects the high elevations of the Western Cordillera and Altiplano to the Central Depression. The Western Cordillera is a magmatic arc that consists of widely spaced volcanic peaks superimposed on a 4,000 to 5,500 m plateau. The Altiplano province is a
Study area: The Eastern tropical and subtropical Pacific.
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250 km wide, 3,700 m high plateau with internal drainage. The eastern limit of the plateau is marked by the high topography of the Eastern Cordillera (Allmendinger et al., 1997; Pinto et al., 2007).
Figure 2. 8 (A) Location map showing present-‐day climatic zones of western South America, from Hartley and Chong
(2002). (B) Digital Elevation Map of the subtropical Andes showing precipitation seasonality in the Atacama Desert and key sites, from Betancourt et al. (2000). Approximate elevations are >4,000 m (blue), 4,000 to 3,500 m (pink), 3,500 to 3,000 m, 3,500 to 2,500 m (brown), 2,500 to 1,000 (yellow), and, 1,000 m (green). Broad areas of pink denote the Bolivian/Peruvian Altiplano.
All parts of the study area below 3,500 m elevation are subject to the hyperarid and arid conditions of the Atacama and Peruvian Coastal Desert, which extends from the Ecuador-‐Peru border (3°30’S) to la Serena (30°S), Chile (Figures 2.8a, b). The Atacama Desert is one the driest locations in the world and harbors no vegetation. A primary cause for Atacama aridity is its location at the eastern boundary of the subtropical Pacific. In that region, large-‐scale atmospheric subsidence of the Hadley circulation significantly reduces precipitation (Houston, 2006) and maintains a surface anticyclone over the southeast Pacific that hinders the arrival of mid-‐latitude disturbances (Garreaud et al., 2010). The subtropical anticyclone drives equatorward winds along the coast that, in turn, foster the transport of cold waters from higher latitudes (I.e., the Peru Current), forcing upwelling of deep waters, that inhibits the moisture capacity of onshore winds creating a persistent inversion that traps any Pacific moisture below 1,000 m.a.s.l., and that also leads to the formation of a persistent deck of stratus clouds (see section 2.1). These factors result in a marked regional cooling of the lower troposphere that is compensated by enhanced subsidence along the Atacama coast (E.g.
Takahashi and Battisti, 2007) further drying this area. The proximity of the Andes Cordillera has been regarded as an additional factor for the dryness of the Atacama. The effect of the Andes on continental precipitation was discussed on section 2.1. This mountain range supposedly restricts the moisture advection from the interior of the continent thus producing a rain shadow effect that is reflected in the marked east–west rainfall gradient (Houston and Hartley, 2003).
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Figure 2. 9 . Schematic chronology of the Andes cordillera paleoelevation, from Garreaud et al., (2010), proposed
onset of Atacama hyperaridity (different sources indicated in inset), presence of Antarctic ice sheets and global deep-‐sea oxygen and carbonate isotopes reflecting cooling of the deep ocean and changes in ice volumen, and some key biotic events off north-‐Central Chile.
In the central and northern Atacama Desert >80% of the mean annual precipitation occurs in the summer months (December–March). Absolute precipitation amounts depend on elevation and distance from the crest of the Andes, which controls rainout from spillover storms. The analysis of meteorological station data by Houston and Hartley (2003) shows that rainfall is strongly dependent on elevation in this area. At 1,000 m, in the Central Depression, rainfall is
<50 mm/yr. Between 1,000–2,000 m above sea level is less than 50 mm, generally falling in Austral winter and decreasing with latitude in the coastal desert and at elevations >3,500 m, rainfall is ~150 mm/yr. The high peaks of the Western Cordillera (5,000–6,000 m) do not have station data but are believed to have annual precipitation in excess of 200 mm/yr.
Temperature and precipitation vary with both latitude and elevation within the Atacama Desert. Mean annual temperatures range between 10° and 16°C (mainly varying with elevation and proximity to the coast). Marine fog, which gives rise to light drizzling rain (garúa) in coastal areas, is the most important source of water for native plants and biological soil crusts in the Atacama Desert, but inland incursion of this fog, as well as formation of inland radiation fogs, depends on elevation and topographic connection to the coast (E.g. Cereceda et al., 2002;
Cáceres et al., 2007). Thus biological systems such as soil bacteria, hypolithic cyanobacteria, lichens and even cacti that rely on fog moisture will be more abundant along the coast.
The El Niño events affect interannual precipitation variability on the Altiplano, especially during Austral Summer (Aceituno, 1988). Precipitation during Austral Winter (JJA) shows no relationship with the extremes of the Southern Oscillation. The Eastern and Western Cordilleras, however, exhibit different levels of sensitivity to El Niño events (Garreaud, 1999;
Vuille et al., 2000). High-‐altitude westerly wind anomalies that inhibit convection over the western edge of the Altiplano characterize El Niño years, causing sustained dry conditions by
Study area: The Eastern tropical and subtropical Pacific.
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limiting moist air advection from the eastern Cordillera across the Altiplano (Garreaud, 1999).
Conversely, a southward displacement of the Bolivian High and enhanced easterly circulation that produces greater advection and increased precipitation characterized La Niña years (Vuille, 1999). In the western coastal regions of the Americas, an El Niño event is associated with is first recorded by Late Triassic–Early Jurassic (pre-‐Sinemurian) evaporites (Clarke, 2006). Late Jurassic evaporites occur at modern latitudes of 21°–35°S, which are close to the Quaternary distribution of 19°–27°S, indicating that there has been little latitudinal shift of South America and the climatic arid zone over this period, although the Jurassic zone was twice as wide as the Quaternary (Clarke, 2006). Consistent with the nearly fixed latitudinal position of the South American continent during the last 150 Ma (E.g. Beck et al., 2000; Clarke, 2006) it is generally accepted that stable arid/semiarid conditions (≤50 mm/year) have prevailed over the Atacama region at least since 150 Ma ago despite extreme fluctuations in climate during the Plio-‐ subsidence, respectively. However numerical simulations of the climate system (Garreaud et al., 2010) suggest that SST cooling off Chile/Peru since the Late Miocene and, especially, during the Pliocene/Pleistocene transition, very effectively resulted in a drying over the Atacama Desert, either gradual or stepwise, that culminated with the present day hyperarid conditions.
Conversely, the Andean surface uplift had little, if any, effect on the lack of precipitation and
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moisture over the Atacama, regardless of the age of uplift (Garreaud et al., 2010).