long term atmospheric deposition as the source of nitrate and other salts in the atacama desert chile new evidence from mass independent oxygen isotopic compositions abstract isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the atacama desert in northern chile has shown anomalous 17 o enrichments in both minerals 17 o values of 14 21 in nitrate and 0 4 to 4 in sulfate are the most positive found in terrestrial minerals to date modeling of atmospheric processes indicates that the 17 o signatures are the result of photochemical reactions in the troposphere and stratosphere we conclude that the bulk of the nitrate sulfate and other soluble salts in some parts of the atacama desert must be the result of atmospheric deposition of particles produced by gas to particle conversion with minor but varying amounts from sea spray and local terrestrial sources flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200 000 to 2 0 m years during hyper arid conditions similar to those currently found in the atacama desert correlations between 17 o and 18 o in nitrate salts from the atacama desert and mojave desert california indicate varying fractions of microbial and photochemical end member sources the photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture biologic activity and nitrogen cycling these isotopic signatures have important implications for paleoclimate astrobiology and n cycling studies

isotopic evidence for the source of ca and s in soil gypsum anhydrite and calcite in the atacama desert chile abstract the origin of pedogenic salts in the atacama desert has long been debated possible salt sources include in situ weathering at the soil site local sources such as aerosols from the adjacent pacific ocean or salt encrusted playas salars and extra local atmospheric dust to identify the origin of ca and s in atacama soil salts we determined 34 s and 87 sr 86 sr values of soil gypsum anhydrite and 87 sr 86 sr values of soil calcite along three east west trending transects our results demonstrate the strong influence of marine aerosols on soil gypsum anhydrite development in areas where marine fog penetrates inland results from an east west transect located along a breach in the coastal cordillera show that most soils within 90 km of the coast and below 1300 m in elevation are influenced by marine aerosols and that soils within 50 km and below 800 m in elevation receive 50 of ca and s from marine aerosols 34 s values 14 and 87 sr 86 sr values 0 7083 in areas where the coastal cordillera is 1200 m in elevation however coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced most pedogenic salts from inland soils have 34 s values between 5 0 to 8 0 and 87 sr 86 sr ratios between 0 7070 and 0 7076 these values are similar to average 34 s and 87 sr 86 sr values of salts from local streams lakes and salars 5 4 2 34 s and 0 70749 0 00045 87 sr 86 sr in the andes and atacama suggesting extensive eolian reworking of salar salts onto the surrounding landscape ultimately salar salts are precipitated from evaporated ground water which has acquired its dissolved solutes from water rock interactions both high and low temperature along flowpaths from recharge areas in the andes therefore the main source for ca and s in gypsum anhydrite in non coastal soils is indirect and involves bedrock alteration not surficially on the hyperarid landscape but in the subsurface by ground water followed by eolian redistribution of ground water derived salar salts to soils the spatial distribution of high grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt supporting arguments for tropospheric nitrogen as the main source for soil nitrate

mid and late holocene limnogeology of laguna del negro francisco northern chile and its palaeoclimatic implications lacustrine sediments from the high elevation endorheic saline lake negro francisco 27 28 s 69 14 w 4125 m provide a detailed mid and late holocene environmental history of the southern chilean altiplano mineralogy chemical composition and sedimentary facies analyses of the deposits confirm the regional significance of mid holocene aridity between 6000 and 3800 bp precipitation rates were signifi cantly lower than the c 250 mm yr 1 of today fully arid conditions were interrupted by short term moist spells with an average return period of about 200 years effective moisture increased after 3800 bp and peaked between 3000 and 2600 bp and between 2200 and 1800 bp in two phases with conditions more humid than today comparison with regional palaeodata suggest that these two late holocene humid spells were most likely related to pacific moisture sources salinity levels increased again in laguna del negro francisco after 1800 bp the onset of modern climatic conditions was a stepwise nonlinear proce

potential and timescales for oxygen depletion in coastal upwelling systems a box model analysis a simple box model is used to examine oxygen depletion in an idealized ocean margin upwelling system near bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom upwelling supplied nutrients are consumed in the surface box and some surface particles sink to the bottom where they respire consuming oxygen steady states characterize the potential for hypoxic near bottom oxygen depletion this potential is greatest for faster sinking rates and largely independent of production timescales except in that faster production allows faster sinking timescales for oxygen depletion depend on upwelling and productivity differently however as oxygen depletion can only be reached in meaningfully short times when productivity is rapid hypoxia thus requires fast production to capture upwelled nutrients and fast sinking to deliver the respiration potential to model bottom waters combining timescales allows generalizations about tendencies toward hypoxia if timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing the steady state will generally be hypoxic and results indicate optimal timescales and conditions exist to generate hypoxia for example the timescale for approach to hypoxia lengthens with stronger upwelling since surface particle and nutrient are shunted off shelf in turn reducing subsurface respiration and oxygen depletion this suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia even as the oxygen levels in upwelled source waters decline

alluvial fan and playa sedimentation in an andean arid closed basin the pacencia group antofagasta province chile the mid tertiary pacencia group comprises over 2 km of continental sediments it was deposited in a half graben basin thought to have been active since early tertiary times conglomerates along the western basin margin were deposited as longitudinal bars rare transverse bars and channel fills with subsidiary debris flows all in a distal fan pebbly braided stream environment no proximal fan sediments are exposed petrographic and palaeocurrent analysis indicates these sediments were derived from cretaceous and possibly jurassic source areas to the northwest to the east within the basin a succession of sands silts muds thick evaporites and rare carbonates were deposited in playa sub environments saliferous muds central salt pan are overlain by interbedded red mudstones and thin high energy ribbon and sheet sands marginal dry mudflat and sandflat certain sand bodies carry stratiform cui agi au mineralization lakes existed at various stages and were fed by permanent fluvial systems the playa sediments were derived from crystalline source areas to the east proximal fan facies and sediment sources are now buried under the volcanic andes

holocene lacustrine deposition in the atacama altiplano facies models climate and tectonic forcing abstract we investigate the holocene sedimentary infilling of two high altitude 4000 m a s l lacustrine basins in the subtropical regions of the atacama altiplano central andes northern chile that today contain permanent shallow saline lakes laguna del negro francisco 27 s and laguna miscanti 23 s laguna miscanti was surveyed with multi frequency high resolution seismic techniques and sediment cores were retrieved from offshore areas of both lakes because of the geological location of these lacustrine basins within the active andean convergent margin and the presence of present and past climatic gradients in the altiplano our detailed sedimentologic mineralogic and geochemical study evaluates tectonic volcanic and climatic controls on lacustrine sedimentation in an arid environment both lakes originated by tectonic and volcanic activity during the pleistocene tectonic control of basin dynamics was clearly exerted during the early stages increased subsidence was responsible for alluvial fan activity in the laguna miscanti basin prior to the establishment of the lake and the negro francisco basin changed from exorheic to endorheic hydrogeologic conditions needed to maintain the two perennial saline lakes are largely controlled by the basins tectonic history the evolution of laguna del negro francisco comprises four successive stages 1 perennial saline carbonate sulfate lake 2 saline pan saline lake complex with a carbonate sulfate brine 3 a perennial brackish stage with high macrophyte chara productivity and mostly calcite formation and 4 a residual stage with halite precipitation the depositional history recorded in the miscanti core illustrates the evolution from an aragonite producing ephemeral saline lake to a macrophyte dominated perennial brackish lake with an aragonite saline pan saline lake complex and a calcite producing perennial brackish lake as intermediate stages the hydrologic budget of the lakes is a direct response to effective moisture precipitation evaporation fluctuations and therefore climate variability has played a major role in chemical composition and lake level changes climate forcing has controlled the depositional history of the lakes during the holocene our study illustrates different responses of lake systems to external climatic forcing especially with respect to cyclicity threshold dynamics and the importance of sedimentary recycling the main sedimentary sequences in both lakes correspond to cycles of increasing and decreasing effective moisture however this cyclicity is characterized by abrupt change rather than gradual evolution threshold dynamics modulated limnogeological changes and caused a stepwise evolution of the lacustrine systems laguna miscanti and laguna del negro francisco sedimentary sequences provide depositional models for small topographically closed brackish to saline lakes that developed on active margins and evolved under fluctuating but generally arid climatic conditions

antiquity of aridity in the chilean atacama desert the atacama desert of northern chile and southern peru has one of the longest histories of arid conditions known although most studies have focused on the hyper aridity prevalent since the miocene all terrestrial sediments in the region from the late triassic onwards also record evaporitic and thus arid climates supergene mineralization in the region did not develop under a more humid climate conducive to deep weathering but under arid conditions these processes may have been facilitated by hydrological changes during miocene uplift and drainage incision but were operational prior to the uplift similarly global cooling and changes to oceanic circulation in the post miocene period only accentuated existing conditions a whole regolith perspective is vital to understanding the history of aridity in the atacama desert and its relevance to arid zone morphogenesis regolith formation and supergene mineralization in particular the long history of aridity raises the possibility that supergene mineralisation under the appropriate conditions form in arid environments instead of requiring humid conditions

upper triassic to lower cretaceous continental and coastal saline lake evaporites inthe atacama region of northern chile evaporites within upper triassic to lower cretaceous sequences in the atacama region of northern chile are interpreted as the deposits of continental and coastal saline lakes halite casts and finely laminated calcareous evaporites intercalated with alluvial and fluvial sediments are probably playa lake deposits these continental evaporites have been recognized in upper triassic alluvial sediments cifuncho formation in upper triassic lower jurassic braided river deposits basal unit of the pan de azucar formation and within a sequence of lower cretaceous aeolian alluvial and playa lake mudflat sediments quebrada monardes formation lower cretaceous evaporites between marine limestones lautaro formation and continental redbeds quebrada monardes formation were probably deposited in coastal saline lagoons produced during a regionally extensive marine regression these sequences and other similar successions in northern chile provide a record of almost continuous evaporite deposition and hence of arid to semi arid conditions since upper triassic times these conditions were primarily the result of a constant latitudinal position within the subtropical zone a contributary factor was the geographical position of the area initially on the west coast of gondwanaland and subsequently on the coast of south america with cold northward flowing ocean currents and offshore winds

palynology of the last interglacial glacial cycle in midlatitudes of southern chile abstract pollen and spores in stratigraphic sections located between 40 and 42 s range in age from the holocene through much of the llanquihue glaciation to the last interglaciation chronology of the stratigraphy derives from some 35 14c ages and the age relations of llanquihue drift and related deposits q mode rotated principal components analysis of four key pollen records covering the last interglacial glacial cycle resulted in four leading components nothofagus dombeyi type gramineae weinmannia fitzroya type and myrtaceae analysis emphasizes interaction between the first two components loadings of gramineae during the interglaciation are high unlike the holocene weinmannia fitzroya type loadings prominent in the holocene are negligible during the interglaciation n dombeyi type is the primary component during llanquihue glaciation it becomes modified by increases of gramineae sometime after 31 000 and before 14 000 yr b p and of myrtaceae later the myrtaceae with weinmannia fitzroya type also registers some activity around 42 000 yr b p fluctuations in the belt of westerly winds reflecting changing meteorological conditions in polar latitudes are suggested by these data with the belt located farther south than it is today interglacial climate was much drier and warmer than during the holocene more northerly displacement of the belt obtained when climate was colder during llanquihue glaciation evidence from comparable latitudes in the southern hemisphere points toward a synchrony of major climatic events indicating harmonious fluctuations in the position of the westerlies

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern chile and southern california u s a natural surficial accumulations of nitrate rich salts in the atacama desert northern chile and in the death valley region of the mojave desert southern california are well known but despite many geologic and geochemical studies the origins of the nitrates have remained controversial n and o isotopes in nitrate and s isotopes in coexisting soluble sulfate were measured to determine if some proposed n sources could be supported or rejected and to determine if the isotopic signature of these natural deposits could be used to distinguish them from various types of anthropogenic nitrate contamination that might be found in desert groundwaters high grade calich a type nitrate deposits from both localities have 15n values that range from 5 to 5 but are mostly near 0 values of 15n near 0 are consistent with either bulk atmospheric n deposition or microbial n fixation as major sources of the n in the deposits 18o values of those desert nitrates with 15n near 0 range from about 31 to 50 v smow significantly higher than that of atmospheric o2 23 5 such high values of 18o are considered unlikely to result entirely from nitrification of reduced n but rather resemble those of modern atmospheric nitrate in precipitation from some other localities assuming that limited modern atmospheric isotope data are applicable to the deposits and allowing for nitrification of co deposited ammonium it is estimated that the fraction of the nitrate in the deposits that could be accounted for isotopically by atmospheric n deposition may be at least 20 and possibly as much as 100 34s values are less diagnostic but could also be consistent with atmospheric components in some of the soluble sulfates associated with the deposits the stable isotope data support the hypothesis that some high grade caliche type nitrate rich salt deposits in some of the earth s hyperarid deserts represent long term accumulations of atmospheric deposition possibly in the order of 104 yr for the death valley region 107 yr for the atacama desert in the relative absence of soil leaching or biologic cycling the combined n and o isotope signature of the nitrate in these deposits is significantly different from those of many other natural and anthropogenic sources of nitrate

paleohydrology of the laguna lejia north chilean altiplano and climatic implications for late glacial times paleoenvironmental and sedimentological data from laguna lejia 23 30 s 67 42 w 4325 m in the high altiplano of the chilean atacama desert indicate that climatic conditions during late glacial times were significantly wetter than today a water and energy budget model was used to simulate climatic scenarios that would have resulted in the observed changes in the water levels water surfaces water volumes and salinity during the time interval different climatic scenarios for the laguna lejia catchment include precipitation increases of over 100 up to 400 500 mm yr today 200 mm yr to account for the late glacial lake levels with shorelines 15 25 m higher than at present and a lake surface of 9 11 km2 compared to the present 2 km2 during the lake maximum about 13 500 11 300 yr b p finely laminated sediments consisting of mg calcite diatoms and sometimes gypsum were deposited the paleoenvironmental evidence indicates that north easterly wind direction prevailed and the summer rainfall resulted from a seasonal poleward shift of the tropical circulation the lake history of laguna lejia is representative for the altiplano area between 21 and 24 s

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

a 106ka paleoclimate record from drill core of the salar de atacama northern chile abstract a 100 m long salt core sqm 2005 from the salar de atacama northern chile 23 s 68 w a dry lake bed contains a 106 kyr paleoclimate record of hydrologic balances on the western slopes of the central andes of south america six u series disequilibrium dates range sequentially from 106 1 6 4 to 5 4 2 7 ka based on sedimentary structures and petrographic textures of salts and associated siliciclastic sediments interpretations of paleoenvironments and net hydrologic balance suggest that relatively wet periods saline lakes and expanded mudflats existed in the salar de atacama from 75 7 to 60 7 ka and from 53 4 to 15 3 ka with the wettest perennial lake interval from 26 7 to 16 5 ka short relatively wet periods also occurred in the holocene from 11 4 to 10 2 ka and from 6 2 to 3 5 ka these wet periods at salar de atacama correspond well with other late pleistocene climate records from the central andes that are a function of the net hydrologic budget the minchin tauca lake sequence from salar de uyuni bolivia is synchronous with the 53 4 15 3 ka saline lake wet period at salar de atacama the tauca phase coincided with the wettest perennial lake interval at salar de atacama from 26 7 to 16 5 ka the coipasa lake phase from uyuni was probably contemporaneous with the early holocene wet period at salar de atacama from 11 4 to 10 2 ka the early holocene wet interval at salar de atacama was also synchronous with the maximum holocene lake levels of the chilean altiplano lakes to the east and with grass rich rodent middens between 11 8 and 10 5 ka in age from the atacama basin the lake period at salar de atacama between 26 7 and 15 3 ka compares closely with low 18o values low anion concentrations and high accumulation rates from the sajama glacial ice core southwestern bolivia between 25 and 15 5 ka it is concluded that wet and dry phases were synchronous in the central andean plateau of southwestern bolivia and along the western flank of the andes of northern chile over the past 50 kyr this region which today receives its moisture from the amazon basin to the east probably also did so in the past the wet periods at salar de atacama are in phase with january insolation maxima at 15 s at 70 45 and 20 ka which suggests that past changes in summer insolation influenced precipitation patterns in the central andes however there are no major wet periods linked to the january insolation maximum at 93 ka and the present day extremely dry climate in the atacama desert is associated with a peak in summer insolation therefore there must be other important influences over precipitation in the central andes which modify the first order insolation controls

exploring for deeply covered mineral deposits formation of geochemical anomalies in northern chile by earthquake induced surface flooding of mineralized groundwaters northern chile is a major producer of copper from porphyry deposits most ores worked to date were exposed near the surface exploration is now focused on deposits along a basement unconformity buried under piedmont gravels deposited by ca 10 ma two recent discoveries the spence and gaby sur deposits are covered by gravel soils above these deposits have anomalies for elements present in saline mineralized groundwaters the anomalies occur above fracture zones in the gravels believed to represent the upward propagation of basement faults that guided emplacement of the porphyries we propose that during earthquakes in this seismically active region there was pumping of saline basement waters up the faults and fracture zones entraining mineralized groundwaters from the deposits after surface flooding and evaporation elements were redistributed by infrequent rains sampling shows that surface active cations such as cu 2 1 were adsorbed and retained in the top few centimeters of soil whereas nacl and porphyry indicator elements that dissolve as anions se re and as are not adsorbed and are removed to 40 cm depth given the 10 m y interval since the gravels were deposited and the hyperarid climate of the atacama desert anomalies may have formed by repeated episodes of seismically induced flooding company geochemical surveys show that such features are numerous in northern chile producing linear zones of saline soils plus other constituents of local groundwaters e g b and i where faults controlling these features intersect mineralization elements specific to the mineralization are also present saline zones may be outlined in the field by measuring the conductivity of soil plus water slurries sampling for full geochemical analyses can be limited to areas so defined anomalies may be generated along fault lines in other seismically active wetter climates but only for elements that are adsorbed on soil minerals although the context of the study is mineral exploration the process may be relevant to the formation of geochemical features of environmental interest above major faults

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

andean uplift and climate change sedimentological data indicate that a semi arid arid climate prevailed across the central andes from 15 ma to 4 ma between 4 and 3 ma a switch to hyperaridity occurred along the western margin of south america palaeoaltitude data suggest that a substantial proto central andean mountain range was in place between 15 and 9 ma these data support the idea that the andean rain shadow existed by 15 ma and that it reinforced the pre existing climatic regime rather than changing it the change to hyperaridity in western south america is attributed to a combination of global climate cooling and enhanced upwelling of the humboldt current generated by closure of the central american seaway between 3 5 and 3 ma and not to the andean rain shadow

groundwater sapping origin for the giant quebradas of northern chile northernmost chile is home to a well preserved disequilibrium landscape of great antiquity contrasting drainage patterns are developed on the western slope of the altiplano plateau the oldest of these patterns is a now relict parallel patterned drainage network in places a younger pattern comprising a series of deeply incised canyons or quebradas crosscuts the older parallel patterned network these canyons show strong evidence of a groundwater sapping origin we hypothesize that the initiation of the canyon network resulted from changes in the hydrological regime related to a drying out of climate of the forearc and to the uplift of the altiplano plateau

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments