Guillermo Chong

Late Pliocene age for the Atacama Desert: Implications for the desertification of western South America

The Atacama Desert forms one of the major hyperarid deserts of the world. Previous studies suggest that desertification commenced at 14 Ma during global climate desiccation. Sedimentologic data from middle Miocene to upper Pliocene successions in the modern Atacama Desert indicate that a semiarid climate persisted from 8 to 3 Ma, punctuated by a phase of increased aridity at ca. 6 Ma. As such, hyperaridity did not commence until the late Pliocene. Implications are (1) that the rain shadow generated by the Andean Cordillera has had a minor influence on climate change, and (2) that the upwelling, north- flowing, cold Humboldt Current, although important in establishing the generally arid climate of western South America, did not control the shift to hyperaridity. The formation of the hyperarid Atacama Desert in the late Pliocene accompanied the development of the current phase of aridification in the Sahara and Namib Deserts and is attributed to a phase of global climate cooling.

150 million years of climatic stability: evidence from the Atacama Desert, northern Chile

The sedimentary succession in the Atacama Desert records deposition under an arid to semiarid climate from the late Jurassic (150 Ma) to the present day. Palaeomagnetic data indicate no significant latitudinal movement of this area since the late Jurassic. The present-day location of the Atacama within the dry subtropical climate belt is the principal cause of aridity. This situation is likely to have prevailed since the late Jurassic, supplemented by (1) the continentality effect (enhanced by the Gondwanan landmass), and (2) the presence offshore of a cold, upwelling current (from at least the early Cenozoic onwards and possibly earlier), resulting in conditions promoting climatic stability and desert development. Rapid and extreme climatic fluctuations during the Plio-Pleistocene were not sufficient to destabilize the climate within the Atacama. Comparison with other long-lived deserts (e.g. SW USA, Namib, Sahara and Australia) suggests that the Atacama is the oldest extant desert on Earth.

Distribution of prokaryotic genetic diversity in athalassohaline lakes of the Atacama Desert, Northern Chile

Athalassohaline lakes are inland saline aquatic environments with ionic proportions quite different from the dissolved salts in seawater. Prokaryotes inhabiting athalassohaline environments are poorly known and very few of such places have been surveyed for microbial diversity studies around the world. We analyzed the planktonic bacterial and archaeal assemblages inhabiting several of these evaporitic basins in a remote and vast area in northern Chile by PCR-denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene fragments. Most systems were springs and athalassohaline ponds in different saltflats of the Atacama Desert region, including Salar de Llamará (in the Central Depression), Salar de Atacama (in the Pre-Andean Depression) and Salar de Ascotán (in the Altiplano). Overall, we analyzed more than 25 samples from 19 different environments with strong gradients of altitude, qualitative ionic compositions and UV influence. Between 4 and 25 well-defined DGGE bands were detected for Bacteria in each sample, whereas Archaea ranged between 1 and 5. Predominant DGGE bands (defined by intensity and frequency of appearance) were excised from the gel and sequenced. Bacterial assemblages were dominated by the Cytophaga-Flavobacterium-Bacteroides (CFB) phylum and a few Proteobacteria. There was a tendency for increasing contribution of CFB with higher salinities and altitude. Thus, CFB accounted for the major fraction of band intensity in the Ascotán samples and for lower percentages in Atacama and Llamará. When the distribution of particular CFB sequences was examined, there were several relatives of Psychroflexus torquis substituting each other as salinity changed in Ascotán. Another set of CFB sequences, very distantly related to Cytophaga marinovorus, was abundant in both Llamará and Atacama at salinities lower than 7%. Archaeal assemblages were dominated by uncultured haloarchaea distantly related to cultured strains mostly obtained from thalassohaline environments. Most of the archaeal sequences did not have a close match with environmental 16S rRNA genes deposited in the database either. Therefore, athalassohaline environments are excellent sources of new microorganisms different from their counterparts in thalassohaline sites and useful tools to relate microbial genetic diversity and environmental characteristics such as changes in salinity (both qualitative and quantitative) and altitude.

Development of a continental forearc: A Cenozoic example from the Central Andes, northern Chile

In order to understand the response of a continental forearc to changes in subduction-zone geodynamics, we constructed a high-resolution chronostratigraphic cross section across the Central Andean forearc of northern Chile (21°–24°S). The tectono-stratigraphic development of the forearc differs from established models. No relationship was found between changes in rate of relative plate convergence and amount and style of deformation. Forearc response to continual compression since the Oligocene has been uplift and segmentation into discrete tectono-stratigraphic zones. From west to east, these zones are the extensional Coastal Cordillera, the extensional and/or transtensional Central depression, and the transpressional and/or compressional Precordillera-Preandean depression. Each area has recorded almost continuous sedimentation from Oligocene (?Eocene) time to the present day. Accommodation space has been generated by basin-margin uplift rather than active subsidence. We propose a model in which uplift of the leading edge of the South American plate is driven by subcrustal accretion of material removed at the trench by subduction erosion. Uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge. The tectono-stratigraphic complexity exhibited within the Central Andean forearc is likely to be representative of Cordilleran-type margins and would be difficult to recognize in an ancient continental forearc.

The Precordilleran fault system of Chuquicamata, Northern Chile: evidence for reversals along arc-parallel strike-slip faults

Abstract The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of Northern Chile, was the site of the Andean magmatic arc from the late Cretaceous to the Eocene-Oligocene boundary. Magmatism came to an end during the Incaic tectonic phase (38 Ma), which caused arc-normal shortening and the development of longitudinal dextral strike-slip faults (Precordilleran Fault System). This magmatic arc tectonism is also related to the formation of the Chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the Precordillera. Structural investigations in and around the Chuqicamata open-pit mine have shown that wrench tectonics determined the kinematics of the area. The NS-striking West Fissure, which separates a 35-Ma-old non-mineralised pluton to the west from a central late Paleozoic basement ridge containing the mineralization, became a sinistral fault along with other parallel faults in the area. The central part is separated from similar Paleozoic rocks to the east by the Messabi-Este fault and a narrow faulted and sheared syncline of Mesozoic-Cenozoic sediments. This fault bears structures indicating dextral movements, which probably are of an age that is similar to the mylonites (34.8 Ma) in the western pluton. The dextral movements preceded the sinistral shear. Thus, the fault system of Chuqicamata displays a reversal of arc-parallel shear movements. According to the orientation of quartz veins in the mineralized body, it is presumed that the sense of displacement of these strike-slip motions reversed, when mineralization started at about 32 Ma. During this time the stress field must have changed fundamentally. The Incaic Phase dextral transpression is supposed to have been induced by the oblique vector of plate motion. The following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling. As, however, the vector of plate motion remained unchanged during that time, oblique subduction cannot be used as an argument for arc-parallel sinistral shear movements.

Late Neogene lacustrine record and palaeogeography in the Quillagua–Llamara basin, Central Andean fore-arc (northern Chile)

Abstract The Cenozoic Quillagua–Llamara basin (northern Chile, Central Andes) is an asymmetrical, intramassif fore-arc basin with a relatively wide northern sector separated from a narrower southward extension by a basement threshold. The northern sector was characterised by a noticeable Oligocene?–late Neogene alluvial-fan and lacustrine dominated deposition which resulted in sequences up to 900 m thick, whereas the southern sector was often a bypass zone with thinner fluvial and lacustrine sediment accumulation. The basin infill includes two third-order alluvial–lacustrine unconformity-bounded units which include other higher-frequency (4th to 5th order) sequences. The evolution of the Late Miocene–Pliocene lacustrine episodes in the Quillagua–Llamara basin was not only controlled by the regional variations from arid to hyperarid palaeoclimate conditions, due to the combined influence of the Pacific high pressure cell, the rain shadow effect exerted by the rising Andes and the northward flowing cold oceanic currents, but also by: (a) the extensional tectonics and evolution and uplift of the fore-arc region which defined the location and size of the depocentres; (b) the resulting basement palaeorelief which affected sediment thickness and facies distribution during the late basin-infill episodes; and (c) the tectonic modifications of watersheds, water divides and drainage networks in the Precordillera which caused considerable changes of water income in the lacustrine systems. Understanding of this regional tectonosedimentary evolution is a necessary first step before analysing of the low- to high-order lacustrine sequence changes in the region. Lacustrine water supply was very sensitive to tectonics; even gentle tectonic tilting and uplifting in critical water-divide zones could result in changes in water balance in the lacustrine basins and trigger variations in the depositional record. The very conspicuous, lacustrine regime changes recorded in the Quillagua–Llamara basin infill cannot be considered in themselves conclusive proof of an exclusive climatic forcing, since they took place close to either major regional drainage changes or to gentle but noticeable tectonic reactivation in the fore-arc region.

Multiphase development of the Atacama Planation Surface recorded by cosmogenic 3He exposure ages: Implications for uplift and Cenozoic climate change in western South America

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Tectonic signatures in arid continental basins: an example from the Upper Miocene–Pleistocene, Calama Basin, Andean forearc, northern Chile

Abstract The Calama Basin, a transtensional basin situated within the Andean forearc, and its extension to the Pampa del Tamarugal (Central Depression), is filled with 700 m of sediments. Detailed sedimentary logging and 40Ar/39Ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick Upper Miocene–Pleistocene succession. Five facies associations, fluvio-lacustrine diatomite, palustrine carbonate, massive anhydrite, fluvial and alluvial fan, have been identified within this part of the basin-fill. The following events are recognised in the basin evolution: (1) deposition of diatomaceous lacustrine sediments in two separate depocentres in the Pampa del Tamarugal and the central Calama Basin, with alluvial fan accumulation along the basin margins at 8 Ma; (2) localised deformation within the central Calama Basin between 7 and 6 Ma but continuous sedimentation elsewhere; (3) palustrine carbonate deposition across the entire area from 6 to 3 Ma except in the Pampa del Tamarugal where diatomaceous sedimentation continued until 5 Ma before being replaced by palustrine carbonate deposition; (4) widespread folding after 3 Ma and entrenchment as the drainage system cut down to reach a new base level caused by the Rio Loa breaching the Coastal Cordillera and reaching the sea. Localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period. Comparison with similar age facies elsewhere in northern Chile has shown changes such as these to be localised and diachronous, indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting. The lacustrine deposits do not appear to be a reliable recorder of low-frequency climatic change. This is exemplified by the change from lacustrine to palustrine deposition at 6–5 Ma which would appear to record a drying of the climate. However, a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run-off.

Tapetes microbianos del Salar de Llamará, norte de Chile

Stratified photosynthetic microbial mats are described from the Salar de Llamara, a salt flat basin located in the Atacama desert of northern Chile. Microscopic and spectrophotometric techniques were used. The thickness of the photic zone of these communities spans 8 to 30 mm. This is probably due to the grain size and mineralogical composition of associated sediments. Three different types of mats were recognized. A first one was characterized by a green pigmented layer; a second with orange and green coloured layers, and the third with orange and green layers and an additional purple layer. At one sampling site, no pigmented layers were present. Sediments underlying the mats were white, but in one site, black coloured sediments were observed; this dark colour is probably the result of iron sulphide precipitation. Predominant microorganisms in the orange pigmented layers were diatoms and unicellular cyanobacteria, mainly from the Cyanothece and Synechococcus groups. Filamentous cyanobacteria Microleus sp. and Oscillatoria sp. were the most abundant in the green layer. When interstitial brines reached salinities between 12 and 33 %, no diatoms were observed, and the coccoidal cyanobacteria from the Synechococcus, Cyanothece and Gloeocapsa groups and genus Gloeobacter predominated over filamentous Cyanobacteria in the green layer. The purple layer was built primarily of anoxygenic phototrophic bacteria similar to cells of the genera Chromatium and Thiocapsa. Absorption spectra revealed that chlorophyll a is the most abundant pigment in most of analyzed samples. Integrated values of chlorophyll a and bacteriochlorophyll a reached values of up to 230 and 144 mg m -2 along all of the pigmented zone, respectively. Abundant non-photosynthetic microorganisms were found in the mats, including unidentified cocci and bacilli. Sulphate reducing bacteria were present in all the sampled mats.

Distribution of Microbial Arsenic Reduction, Oxidation and Extrusion Genes along a Wide Range of Environmental Arsenic Concentrations

The presence of the arsenic oxidation, reduction, and extrusion genes arsC, arrA, aioA, and acr3 was explored in a range of natural environments in northern Chile, with arsenic concentrations spanning six orders of magnitude. A combination of primers from the literature and newly designed primers were used to explore the presence of the arsC gene, coding for the reduction of As (V) to As (III) in one of the most common detoxification mechanisms. Enterobacterial related arsC genes appeared only in the environments with the lowest As concentration, while Firmicutes-like genes were present throughout the range of As concentrations. The arrA gene, involved in anaerobic respiration using As (V) as electron acceptor, was found in all the systems studied. The As (III) oxidation gene aioA and the As (III) transport gene acr3 were tracked with two primer sets each and they were also found to be spread through the As concentration gradient. Sediment samples had a higher number of arsenic related genes than water samples. Considering the results of the bacterial community composition available for these samples, the higher microbial phylogenetic diversity of microbes inhabiting the sediments may explain the increased number of genetic resources found to cope with arsenic. Overall, the environmental distribution of arsenic related genes suggests that the occurrence of different ArsC families provides different degrees of protection against arsenic as previously described in laboratory strains, and that the glutaredoxin (Grx)-linked arsenate reductases related to Enterobacteria do not confer enough arsenic resistance to live above certain levels of As concentrations.