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Abstract: The Atacama Desert is one of the driest places on Earth. Multiple lines of evidence show that the Atacama has been hyperarid since at least the late Miocene, among these are cosmic-ray exposure ages indicating that individual clasts on some surfaces have been preserved for >9Ma and possibly since the Oligocene. Although these remarkably old ages indicate slow landscape evolution, it is not clear whether this pace is characteristic of the entire Atacama, or only of specific regions, landforms, or landscape elements. To address this question, we measured cosmogenic 10Be, 26Al, and 21Ne from a wide variety of landscape elements in a transect across the Central Atacama, where modern precipitation is at an extreme minimum, but where the concentration of cosmogenic nuclides in stable landscape elements has not previously been recorded. We find that the hyperarid core of the Central Atacama has substantially slower erosion rates than its eastern and western margins; however, even the driest part of this transect has erosion rates comparable to those of other deserts, ranging from 0.2–0.4m/Ma. The most stable landscape elements are boulder fields, with exposure ages of 1.5–2.6Ma. The vast majority of samples in the Central Atacama Desert, however, have cosmogenic nuclide concentrations corresponding to ages <1.2Ma, indicative of Pleistocene modification of almost the entire landscape. Furthermore, extreme boulder ages >5Ma documented elsewhere in the Atacama were not found in our area and appear to be limited to exceptionally stable boulders or cobbles in either the northern or southern extremes of the Atacama Desert. We suggest that the Central Atacama has been subject to episodic Pliocene and Pleistocene rainfall and geomorphic activity, perhaps due to intrusion of Pacific moisture. [Copyright &y& Elsevier]

Abstract: Varying ages from Triassic to Pliocene have been proposed for the onset of hyperaridity in the Atacama Desert. The exact timing for the initiation of hyperaridity is critical for determining potential causes, which range from regional effects of global cooling to Andean uplift above elevations conducive to extreme rain shadows. Analysis of the stable isotopic composition of lower Miocene–Quaternary (21–0.015Ma) palustrine and lacustrine carbonates in the Calama Basin reveals extreme changes in their oxygen and carbon isotopic composition during the Miocene. Limestone δ 18O values increased by ∼5‰ from middle to late Miocene, ranging from −5.5‰ at 12Ma to −1‰ at ∼6Ma. Carbon isotopic values increase by 9‰ over the Neogene, from average values of −3‰ at 21Ma to +3‰ at 12Ma, and reaching a maximum of +6‰ at 5Ma. The increase in oxygen isotopic values occurred over a time span in which the catchment area of the basin experienced significant uplift, causing the δ 18O value of precipitation to become more negative. We attribute the shift towards higher δ 18O values to enhanced evaporative enrichment both of soil water or snow prior to infiltration, and within shallow lakes or wetlands prior to carbonate precipitation. The large increase in δ 13C values was likely caused by a transition from a vegetated landscape influenced primarily by soil-respired CO2 to a landscape largely devoid of vegetation and influenced by atmospheric and volcanic CO2. Isotopic values of palustrine carbonates therefore indicate that hyperaridity commenced in the Calama Basin during the middle to late Miocene, in agreement with other paleoclimatic records from the basin. The cause for the onset of this climate change is thought to be due to the development of a strong Andean rain shadow associated with the uplift of the Andes to mean elevations >2km. [Copyright &y& Elsevier]

Abstract: Located along the west coast of subtropical South America, the Atacama Desert features an extremely dry climate in sharp contrast with relatively wet conditions over the Central Andes and farther east. The Atacama''s hyperaridity has been attributed to its subtropical location, the cold waters over the adjacent southeast Pacific Ocean and the presence of the Andes Cordillera. Although geological evidence reveals less dry conditions in the remote past, the timing of the arid-to-hyperarid transition is a matter of controversy. Several studies suggest that such transition occurred between 19 and 13Ma fostered by the concurrent uplift of the Andes during the lower–middle Miocene. Other studies, however, suggest a much earlier (∼25Ma) or later (2–1Ma) transition associated with global or regional ocean cooling. Here we use PLASIM, a simple global climate model, to study the effect of the Andean surface uplift and sea surface temperature changes upon Atacama aridity. In one set of experiments, the continental topography was scaled by a factor ranging from 0.9 to 0.1. Decreasing the height of the Andes did not increase precipitation over the Atacama region, but rather decrease the precipitation over the Central Andes and the interior of the continent. These results, consistent with previous modeling studies, suggest that the Andean uplift was not an important ingredient in the onset of Atacama hyperaridity, even if both events overlapped in time. A globally uniform sea surface warming does not result in a significant increase of precipitation over the Atacama. In contrast, a regional warming over the subtropical southeast Pacific very effectively increase precipitation along the west coast of South America by decreasing the subsidence and the dry advection at low levels. It suggested that a reinforcement of the Humboldt Current since the late Miocene and particularly during the Pliocene/Pleistocene transition was crucial in the drying of the Atacama Desert that culminated with the present day hyperarid conditions. [Copyright &y& Elsevier]

Abstract  Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the Atacama Desert. 230Th–234U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting, was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, longitudinal changes on the thin/thick-skinned tectonic styles of the Central Andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading of the orogen, and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the Peru–Chile trench. The search for paleo-climate records is therefore crucial for our understanding of the history of the Central Andes. In this paper, we focus on the Atacama Gravels, an extensive blanket of Miocene continental deposits filling a Neogene paleo-valley system along the southern Atacama Desert in northern Chile. An east–west transect, between Pedernales and Chañaral (26°30′S), enabled us to carry out a sedimentological and tectonic study of the Atacama Gravels, based on logging and field observations along the Rio Salado canyon. New 39Ar–40Ar ages obtained on intercalated and overlying ignimbrites date the beginning of the Atacama Gravels sedimentation at around the Oligocene–Miocene boundary and cessation of sedimentation in the Late Miocene. Thirteen lithofacies, included within five facies associations (A1 to A5) were identified. Depositional environments vary from proximal alluvial fan (A1, A2) in the Precordillera through ephemeral fluvial (A3, A4) to distal playa lake (A5) in the Coastal Cordillera. No evidences of synsedimentary deformation have been found, showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes, and climate change appears to be the dominant controlling factor of sediment preservation. A progressive change from semi-arid towards hyper-arid climatic conditions during the Miocene, led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleo-valley system formed during the Oligocene. [Copyright &y& Elsevier]