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

a major holocene enso anomaly during the medieval period 1 here we present a high resolution marine el nino flood record from peru a period of extreme drought without strong flooding occurred from a d 800 1250 anomalous precipitation patterns characterized the entire indo pacific enso domain with dry events in the northern arabian sea and the mid latitudes of both americas coinciding with wet periods in the atlantic cariaco basin the occurrence of contemporaneous moisture anomalies in other archives in the enso region highlights the role of el nino strength in global climate evolution during the late medieval period when temperature reconstructions show a rather heterogeneous pattern

an assessment of stratigraphic completeness in climate sensitive closed basin lake sediments salar de atacama chile abstract dated subsurface cores 40 m 100 m and 200 m in length from the salar de atacama chile record changes in climate and tectonics over the past 325 ka these cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change the same basic facies efflorescent halite crust and cement subaerial halite and chevron halite saline lake occur in each core the dominant facies is efflorescent halite formed in a subaerial environment like that in the atacama basin today thick well preserved efflorescent crusts in the salar de atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines similar pedogenically formed evaporites may be pre served in the geologic record the n s trending salar fault system has downdropped the eastern block of the salar de atacama core 2002 relative to the western block cores 2005 and 2031 relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium series ages from core 2005 major faulting along the salar fault system occurred between 16 5 and 5 4 ka with offset of 26 5 m and faulting rates of 2 4 m kyr aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting although stratigraphic units in core 2002 shifted down 30 m over the past 60 ka the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores faulting along the salar fault system despite significant offset did not alter the basic stratigraphic record temporal completeness of the stratigraphic record within the limits of the age dates and their errors of greater than 3 ka was examined qualitatively by comparing long term sedimentation rates 0 6 0 9 m kyr and shorter term sedimentation rates 0 3 3 6 m kyr the similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of salar de atacama sediments at least at the 10 kyr scale although hiatuses long periods of subaerial exposure and dissolution of evaporites may have occurred the main climate fluctuations at salar de atacama over the last 100 ka are recorded in all three subsurface cores paleoclimate records obtained from such cores are therefore representative of basin wide climate change and are valid for regional paleoclimate reconstructions

application of strontium isotopes to understanding the hydrology and paleohydrology of the altiplano bolivia peru abstract strontium concentrations and strontium isotopic ratios were measured in natural waters and carbonate sediments from throughout the bolivian and peruvian altiplano in order to improve hydrologic and paleohydrologic mass balances with the ultimate goal of better understanding the paleoclimatic history of the central andes rivers flowing into lake titicaca have a wide range of strontium isotopic ratios that exhibit spatial patterns consistent with the lithologies of the different drainage basins because of the limited exchange of water between the two main sub basins of lake titicaca lago grande and lago huinaimarca and between the sub basins of lago huinaimarca there are significant differences in strontium isotopic ratios between the sub basins calculated elemental balances of strontium in lake titicaca are in reasonable agreement with previously published budgets of water and major elements however the strontium isotopic budget indicates that the lake is not in isotopic steady state this also implies that the major element budgets are unlikely to be in steady state lake titicaca had a higher than modern strontium isotopic ratio during the early and middle holocene elevated values persisted in lago grande until at least 2000 cal yr bp consistent with other evidence that modern hydrologic conditions namely overflow were not established until that time an isotopic budget calculated for late pleistocene paleolake tauca in the central altiplano suggests that between 70 and 83 of its riverine inputs were derived from lake titicaca overflow this calculated flow represents about a 30 fold increase over the average discharge of the modern rio desaguadero at desaguadero or a seven fold increase over its average discharge into lago poopo the strontium isotopic budget if complete precludes the possibility that decreased evaporation alone could have accounted for the existence of paleolake tauca

geomorphic evidence for post 10 ma uplift of the western flank of the central andes 18 30 22 s 1 the western andean mountain front forms the western edge of the central andean plateau between 18 5 and 22 s latitude the mountain front has 3000 m of relief over 50 km horizontal distance that has developed in the absence of major local neogene deformation models of the evolution of the plateau as well as paleoaltimetry estimates all call for continued large magnitude uplift of the plateau surface into the late miocene i e younger than 10 ma longitudinal river profiles from 20 catchments that drain the western andean mountain front contain several streams with knickpoint bounded segments that we use to reconstruct the history of post 10 ma surface uplift of the western flank of the central andean plateau the generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to pacific ocean capture eustatic change or climate change as causes for creating the knickpoint bounded stream segments observed minor valley filling alluvial gravels intercalated with the 5 4 ma carcote ignimbrite suggest uplift related river incision was well under way by 5 4 ma the maximum age of river incision is provided by the regionally extensive approximately 10 ma el diablo altos de pica paleosurface the river profiles reveal that relative surface uplift of at least1 km occurred after 10 ma

the stratigraphic record of changing hyperaridity in the atacama desert over the last 10 ma new radiometric and magnetostratigraphic data from quillagua and calama basins atacama desert indicate that the stratigraphic record over the last 10 ma includes two hiatuses lasting approximately 2 and 4 million years respectively these sedimentary gaps are thought to represent prolonged periods of hyperaridity in the region with absence of sediment production and accumulation in the central depressions their remarkable synchrony with antarctic and patagonian glacial stages humboldt cold current enhancement and cold upwelling waters lead us to suggest long term climate forcing higher frequency climate orbital precession and eccentricity forcing is thought to control the sequential arrangement of the lacustrine units deposited at times of lower aridity hyperaridity trends appear to be modulated by the activity of the south american summer monsoon which drives precipitation along the high altitude areas to the east of atacama this precipitation increase combined with the eastward enlargement of the regional drainage during the late pleistocene enabled water transfer from these high altitude areas to the low lying closed quillagua basin and resulted in the deposition of the last widespread saline lacustrine deposits in this depression before its drainage was open to the pacific ocean

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

water chemistry and isotope study of streams and springs in northern chile abstract the relationship between the stable isotopic and chemical composition of rain spring and stream water was studied in the high andes of northern chile the isotopic pattern of the springs is mainly a reflection of the altitude of their recharge areas however processes that occur during snow melting seem to play a major role in the high altitude springs the streams show patterns similar to the springs in the higher part of the basins but their isotopic composition is modified along the river course due mainly to evaporation the difference in chemical composition between the springs at high and intermediate elevations reflects the duration of water rock interaction the best parameter to trace this length of travelling time is the na k ratio which increases as a result of removal of k into alteration minerals k silicates the na cl ratio increases with salinity in springs and decreases in stream water this reflects the different sources of salt whereas the main source of salts in spring water is the dissolution of silicate and sulfide minerals halite accumulations close to the surface is the main salt source in the streams during floods the stream water chemical characteristics change na cl increases with decreased salinity and na k decreases

re evaluation of mid holocene deposits at quebrada puripica northern chile duringthe middle holocene 8 3 ka wetland deposits accumulated in areas with emerg ent water tables in the central atacama desert 22 24 s producinga stratig raphic unit unit c that can be mapped and correlated across different basins and geomorphic settings wherever mapped unit c is located between 6 and 30 m above modern wetlands and includes thick sequences of diatomite and organic mats the origin depositional environment and paleoclimatic significance of unit c is controversial and currently under debate grosjean science 292 2001 2391a suggests the unit developed under a regime of falling lake and ground water levels whereas evidence presented here suggests that unit c formed during a period of rising ground water levels and increased vegetation cover the debate is embedded in broader discussions about geomorphic processes in ground water fed streams as well as the history and forcingof climate variability in the central andes the central atacama and andes are remote with few opportunities for different researchers to examine the same site one exception is a sequence of mid holocene deposits at the confluence of quebrada seca and r o puripica in northern chile 23 s grosjean et al quat res 48 1997 239 246 initially suggested the deposits accumulated in temporary lake basins which formed when recurringdebris flows from a side canyon quebrada seca dammed the main channel r o puripica duringa period of drought and reduced stream flow upon our visit to the site we found evidence that clearly demonstrates the deposits were not formed in temporary lakes but rather were deposited in a wetland environment disagreement remains about the climatic interpretation of wetland deposits grosjean science 292 2001 2391a now suggests that local ground water levels rise and wetland deposits aggrade in deep canyon systems such as r o puripica when stream power and channel erosion is reduced duringprolong ed dry spells however sedimentological evidence and the presence of unit c in many depositional environments not just deep canyons indicate that it formed during a period of higher regional ground water levels that were sustained by enhanced precipitation and recharge in the high andes 2003 elsevier science b v all rights reserved

87sr 86sr in recent accumulations of calcium sulfate on landscapes of hyperarid settings a bimodal altitudinal dependence for northern chile 19 5 s 21 5 s an elevation dependent relationship of the 87sr 86sr ratio of holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting in the atacama desert of northern chile gypsum and anhydrite of multiple origins exist widely on superficial materials that originated during the last 10 000 years an important source of calcium sulfate is from offshore generated stratocumulus clouds that are advected onto the continent where they generate fog that transfers water droplets to the ground surface which upon evaporation leaves calcium sulfate crystals meteorological measurements of the cloud base and top altitudes average 400 m and 1100 m above sea level masl respectively the seawater ratio of 87sr 86sr 0 70917 is distinctively higher than that reported for weathered mean andean rock less than 0 70750 samples of 28 modern surface salt accumulations for locations between 200 and 2950 masl and between 19 30 and 21 30 s verify that 87sr 86sr varies as a function of site altitude sites below 1075 masl and above 225 masl display calcium sulfate 87sr 86sr of mean value 0 70807 0 00004 while the ratio outside this altitudinal domain is 0 70746 0 00010 thus the 87sr 86sr ratio of holocene salt accumulations differentiates two altitudinal domains

groundwater recharge through an alluvial fan in the atacama desert northern chile mechanisms magnitudes and causes the chacarilla fan in the atacama desert is one of several formed in the late miocene at the foot of the pre andean cordillera overlying the large complex pampa tamarugal aquifer contained in the continental clastic sediments of the fore arc basin the pampa tamarugal aquifer is a strategic source of water for northern chile but there is continuing doubt over the resource magnitude and recharge during january 2000 a 1 in 4 year storm in the andes delivered a 34 million m3 flash flood to the fan apex where c 70 percolated to the underlying aquifers groundwater recharge through the fan is calculated to be a minimum of 200 l s or 6 of the long term catchment rainfall these figures are supported by hydrochemical data that suggest that recharge may be 9 of long term rainfall isotopic data suggest groundwater less than 50 years old is transmitted westward through the permeable sheetflood sediments of the fan overlying the main aquifer analysis of this and other events shows that the hydrological system is non linear with positive feedback the magnitude of groundwater recharge is dependent on climatic variations antecedent soil moisture storage and changes in channel characteristics long term declines in groundwater level may partly result from climatic fluctuations and the causes of such fluctuations are discussed copyright 2002 john wiley sons ltd

in stream wetland deposits megadroughts and cultural change in the northern atacama desert chile a key concern regarding current and future climate change is the possibility of sustained droughts that can have profound impacts on societies as such multiple paleoclimatic proxies are needed to identify megadroughts the synoptic climatology responsible for these droughts and their impacts on past and future societies in the hyperarid atacama desert of northern chile many streams are characterized by perennial flow and support dense in stream wetlands these streams possess sequences of wetland deposits as fluvial terraces that record past changes in the water table we mapped and radiocarbon dated a well preserved sequence of in stream wetland deposits along a 4 3 km reach of the rio san salvador in the calama basin to determine the relationship between regional climate change and the incision of in stream wetlands the rio san salvador supported dense wetlands from 11 1 to 9 8 6 4 to 3 5 2 8 to 1 3 and 1 0 to 0 5 ka and incised at the end of each of these intervals comparison with other in stream wetland sequences in the atacama desert and with regional paleoclimatic archives indicates that in stream wetlands responded similarly to climatic changes by incising during periods of extended drought at 9 8 3 5 1 3 and 0 5 ka

cooling of the southern high latitudes during the medieval period and its effect on enso abstract a high resolution multiproxy study performed on a marine record from se pacific off southern south america was used to reconstruct past regional environmental changes and their relation to global climate particularly to el nino southern oscillation enso phenomenon during the last 2200 years our results suggest a sustained northward shift in the position of the zonal systems i e the southern westerly wind belt and the antarctic circumpolar current which occurred between 1300 and 750 yr bp the synchrony of the latitudinal shift with cooling in antarctica and reduced enso activity observed in several marine and terrestrial archives across south america suggests a causal link between enso and the proposed displacement of the zonal systems this shift might have acted as a positive feedback to more la nina like conditions between 1300 and 750 yr bp by steepening the hemispheric and tropical pacific zonal sea surface temperature gradient this scenario further suggests different boundary conditions for enso before 1300 and after 750 yr bp

neogene climate change and uplift in the atacama desert chile the relationship between andean uplift and extreme desiccation of the west coast of south america is important for understanding the interplay between climate and tectonics in the central andes yet it is poorly understood here we use soil morphological char acteristics salt chemistry and mass independent fractionation anomalies 17 o values in dated paleosols to reconstruct a middle miocene climatic transition from semiaridity to extreme hyperaridity in the atacama desert paleosols along the southeastern margin of the calama basin change from calcic vertisols with root traces slickensides and gleyed horizons to an extremely mature salic gypsisol with pedogenic nitrate we interpret this transition which occurred between 19 and 13 ma to represent a change in precipitation from 200 mm yr to 20 mm yr this drastic reduction in precipitation likely resulted from uplift of the central andes to elevations 2 km the uplift blocked moisture from the south american summer monsoon from entering the atacama the mid miocene gyp sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the cal ama basin significantly higher than modern nitrate soils which occur below 2500 m modern and quaternary soils in this elevation zone contain soil carbonate and lack ped ogenic gypsum and nitrate we infer that 900 m of local surface uplift over the past 10 m y displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the calama basin by decreasing local air temperatures and creating an orographic barrier to pacific air masses

stratospheric sulfate from el chichon and the mystery volcano stratospheric sulfate was collected by high altitude aircraft and balloons to assess the impacts of el chichon and an unidentified volcano on the stratosphere the mystery volcano placed about 0 85 tg of sulfate in the northern hemisphere stratosphere el chichon injected about 7 6 tg sulfate into the global stratosphere

lake level chronology on the southern bolivian altiplano 18 23 s during late glacial time and the early holocene stratigraphic analyses of outcrops shorelines and diatoms from the southern bolivian altiplano uyuni coipasa basin reveal two major lacustrine phases during the late glacial period and the early holocene based on a chronology established by radiocarbon and u th control a comparison of14c and230th 234u ages shows that during times of high lake level radiocarbon ages are valid however during low water periods 14c ages must be corrected for a reservoir effect the lacustrine tauca phase started a little before 16 00014c yr b p and the lake level reached its maximum between 13 000 and 12 00014c yr b p a dry event ticana occurred after ca 12 000 and before 950014c yr b p a moderate lacustrine oscillation coipasa event occurred between ca 9500 and 850014c yr b p using a reservoir corrected conventional14c chronology comparisons between the lake level chronology in the uyuni coipasa basin and data from other southern tropical areas of south america suggest that the lacustrine evolution may reflect large scale climatic changes

geochronology and stratigraphy of late pleistocene lake cycles on the southern bolivian altiplano implications for causes of tropical climate change large paleolakes 33 000 60 000 km2 that once occupied the high altitude poopo coipasa and uyuni basins in southern bolivia 18 22 s provide evidence of major changes in low latitude moisture in these now dry or oligosaline basins extensive natural exposure reveals evidence for two deep lake and several minor lake cycles over the past 120 k y fifty three new u th and 87 new 14c dates provide a chronologic framework for changes in lake level deposits from the ouki deep lake cycle are extensively exposed in the poopo basin but no deep lakes are apparent in the record between 98 and 18 1 ka the ouki lake cycle was 80 m deep and nineteen u th dates place this deep lake cycle between 120 and 98 ka shallow lakes were present in the terminal uyuni basin between 95 and 80 ka salinas lake cycle at ca 46 inca huasi lake cycle and between 24 and 20 5 ka sajsi lake cycle the tauca deep lake cycle occurred between 18 1 and 14 1 ka resulting in the deepest 140 m and largest lake in the basin over the past 120 ka multiple 14c and u th dates constrain the highest stand of lake tauca along a topographically conspicuous shoreline between 16 4 and 14 1 ka a probable post tauca lake cycle the coipasa produced a 55 m deep lake that is tentatively dated between 13 and 11 ka we suggest that paleolakes on the bolivian altiplano expanded in response to increased moisture in the amazon and enhanced transport of that moisture onto the altiplano by strengthened trade winds or southward displacement of the intertropical convergence zone itcz pole to equator sea surface temperature sst and atmospheric gradients may have influenced the position of the itcz affecting moisture balance over the altiplano and at other locations in the amazon basin links between the position of the itcz and the ca 23 ka precessional solar cycle have been postulated march insolation over the altiplano is a relatively good fit to our lake record but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire amazon basin temperature may influence altiplano lake levels indirectly as potentially dry glacial periods in the amazon basin are linked to dry conditions on the altiplano intensification of the trade winds associated with la nina like conditions currently brings increased precipitation on the altiplano and deep lake development during the tauca lake cycle coincided with apparently intense and persistent la nina like conditions in the central pacific this suggests that sst gradients in the pacific are also a major influence on deep lake development on the altiplano

perennial stream discharge in the hyperarid atacama desert of northern chile during the latest pleistocene abstract a large fraction of the vital groundwater in the atacama desert of northern chile is likely composed of fossil or ancient reserves that receive little or no recharge in today s hyperarid climate here we present evidence for latest pleistocene perennial streamflow in canyons from the hyperarid core of the atacama desert in northern chile fluvial terraces in the pampa del tamarugal pdt basin 21 s contain widespread fossil wood in situ roots and well preserved leaf litter deposits indicative of perennial surface flow currently absent in these channels nineteen radiocarbon dates on these deposits from four separate drainages within this endorheic basin indicate ages from 16 380 to 13 740 cal yr bp synchronous with paleolake tauca on the bolivian altiplano and other regional evidence for wetter conditions during the latest pleistocene groundwater fed riparian ecosystems and associated fluvial deposits abound today in the absence of direct rainfall in northern atacama canyons with perennial discharge our relict riparian ecosystems from the pdt basin are indicative of conditions similar to these northern canyons given that discharge was higher than present during this time we propose that these deposits represent the most important groundwater recharge events of the last 18 000 years a lesser recharge event occurred during the holocene when phreatophytic trees also grew in these drainages between 1 070 and 700 cal yr bp during the medieval climatic anomaly taken together our evidence lends further support for gradient changes in the equatorial pacific as a major driver of hydrologic change in the atacama on both centennial and millennial time scales central andes hyperaridity riparian ecosystems late quaternary fluvial terraces

late quaternary vegetation and climate history of a perennial river canyon in the rio salado basin 22 s of northern chile abstract plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the rio salado northern chile yield a unique record of vegetation and climate over the past 22 000 cal yr bp presence of low elevation prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5 c and may have been near modern at 16 270 cal yr bp displacements in the lower limits of andean steppe and puna taxa indicate that mean annual rainfall was twice modern at 17 520 16 270 cal yr bp this pluvial event coincides with infilling of paleolake tauca on the bolivian altiplano increased enso activity inferred from a marine core near lima abrupt deglaciation in southern chile and heinrich event 1 moderate to large increases in precipitation also occurred at 11 770 9550 central atacama pluvial event 7330 6720 3490 2320 and at 800 cal yr bp desiccation occurred at 14 180 8910 8640 and 4865 cal yr bp compared to other midden sites in the region early holocene desiccation seems to have happened progressively earlier farther south emerging trends from the cumulative midden record in the central atacama agree at millennial timescales with improved paleolake chronologies for the bolivian altiplano implying common forcing through changes in equatorial pacific sea surface temperature gradients

isotope hydrology of groundwaters in the pampa del tamarugal chile the water resources in northern chile are extremely scarce and development is limited by this this paper discusses the isotope hydrology of groundwaters in the pampa del tamarugal an area of extreme aridity rainfall 1 mm yr whose groundwater resources are mined for the town of iquique as well as for industrial and agricultural purposes the aim of the project was to obtain information on modern recharge and to delineate if possible recharge environments to obtain the necessary background information a precipitation survey in the high andes as well as spring and surface water studies were carried out the results show that a well defined meteoric water line exists where 2h 7 8 18o 10 3 and altitude effects depend on air mass movements and cannot be defined without a broader regional and detailed sampling programme however it is still possible to assign maximum altitudes of recharge to springs in the andes and at the eastern border of the pampa del tamarugal comparison of these data with groundwater compositions show that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation a dependence of individual groundwater systems on specific quebradas river valleys is recognized however low 14c activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old some minor subsurface recharge does occur at the foot of the andes especially at pica where high altitude waters discharge but even there groundwater appears to be a diminishing resource

cenozoic climate change as a possible cause for the rise of the andes causal links between the rise of a large mountain range and climate have often been considered to work in one direction with significant uplift provoking climate change here we propose a mechanism by which cenozoic climate change could have caused the rise of the andes based on considerations of the force balance in the south american lithosphere we suggest that the height of and tectonics in the andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands and shear stresses in the subduction zone depend on the amount of subducted sediments we propose that the dynamics of subduction and mountain building in this region are controlled by the processes of erosion and sediment deposition and ultimately climate in central south america climate controlled sediment starvation would then cause high shear stress focusing the plate boundary stresses that support the high andes

late quaternary paleohydrology of the central atacama desert lat 22 24 s chile in northern chile precipitation in the high andes 3500 m recharges groundwater systems that flow down the pacific slope and feed large aquifers in the hyperarid atacama desert wetlands which are often found along the base of the andes mark locations where the water table intersects the land surface at these locations paleo wetland deposits which are present as terraces between 3 and 20 m above modern wetlands record past water table heights along the andean front and are used to reconstruct changes in groundwater discharge paleo wetland deposits in the central atacama desert lat 22 24 s record an episode 15 4 9 ka of high water tables followed by an episode 8 3 ka of moderately high water tables elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the andes the concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations this interpretation is supported by close agreement with other paleoclimatic records in the region the periods of greater groundwater discharge were separated by episodes 9 8 and 3 0 ka of significant groundwater lowering and stream incision implying greatly diminished discharge the central atacama and andes lat 22 24 s receive precipitation mainly from moist air masses transported from the amazon basin by the south american summer monsoon sasm increases in groundwater recharge are therefore thought to reflect an increase in the frequency and or moisture content of sasm air masses crossing the andes fluctuations in sasm precipitation have previously been linked to summer insolation in the southern hemisphere the wettest period in the central atacama 15 4 9 ka however coincides with a minimum in austral summer insolation at 10 ka suggesting that regional summer insolation is not a dominant influence on the sasm instead intensification of the sasm may be linked to extraregional forcings such as the walker circulation

the great atacama flood of 2001 and its implications for andean hydrology in february 2001 widespread flooding occurred throughout the atacama desert of northern chile and southern peru it was particularly severe in the rio loa basin where roads and bridges were disrupted and the town of calama inundated the instantaneous peak flow in the rio salado a tributary of the rio loa reached 310 m3 s 1 an order of magnitude higher than any previously recorded event the flood is estimated to have a return period of 100 200 years and is shown to have been caused by intense long duration rainfall in the western cordillera associated with la nina the surface water response is typical of arid areas and highly dependent on antecedent conditions but is quite different in perennial and ephemeral catchments ephemeral flood flows suffer high transmission losses recharging phreatic aquifers perennial rivers have lower runoff coefficients but baseflow levels remained high after the event for several months due to bank storage rebound and interflow extremely high energies of 3000 w m 2 were generated by the floods in the cordillera becoming less in the precordillera and downstream erosion and sediment transport were consequently highest in the upper and middle reaches of the rivers with mixed erosion deposition in the lowest reach the new insights gained from the interpretation and quantification of this event have important implications for palaeoenvironmental analysis hazard management water resource evaluation and the palaeohydrological evolution of the andes copyright 2005 john wiley sons ltd

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

rates and geochemical processes of soil and salt crust formation in salars of the atacama desert chile abstract the hyperarid atacama desert contains numerous local basins with surficial salt crusts known as salars where evaporation of shallow groundwater drives the major soil processes we examine chemical and isotopic profiles in two soils of differing ages from the salar llamara to determine the geochemical processes involved in their formation evaporation which provides salts to the soils through mineral precipitation decreases with increasing salt crust thickness and average 0 03 mm m 2 d 1 over geological time frames salt distribution varies predictably with depth and soil age with the most soluble compounds concentrated nearest to the land surface indicating the direction of fluid flow 34s values of mineral sulfate tend to decrease with decreasing soil depth following a pattern indicative of rayleigh like fractionation as solute rich waters migrate toward the land surface 13c and 18o values of carbonate suggest that the uppermost halite layers which contain very small amounts of carbonate have a strong biological signature in contrast carbonate rich layers deeper in the profiles consist of largely unmodified lacustrine carbonate that formed in highly evaporitic lake conditions the continuous upward evaporation of water and dissolved solutes creates a rugged and physically dynamic halite crust composed of rounded salt nodules the crust undergoes deliquescence as atmospheric relative humidity rises from marine air intrusions and we found that the halite nodules on the surface of the salar llamara are nearly always at or above deliquescence relative humidity the interiors of these nodules are therefore able to buffer the large diurnal changes in atmospheric relative humidity allowing for the survival of halophilic microbial communities in an otherwise very dry environment radiocarbon measurements of occluded organic c in the surface crusts indicate that c cycling occurs at differing rates depending on local micrometeorological conditions and that a given salt crust feature may persist for thousands of years once formed

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

more than ten million years of hyper aridity recorded in the atacama gravels abstract the atacama desert s hyper aridity is closely linked to the development of world class copper and nitrate iodine ores and to regional tectonics and global paleoclimate changes in the cenozoic era the timing when the hyper aridity commenced remains controversial with proposed ages ranging from late oligocene to pleistocene in this study we provide an independent constraint on the initiation of atacama hyper aridity utilizing a 100 m deep profile within the atacama gravels and underneath porphyry copper deposit in spence northern chile the overall high concentration of sulfate up to 10 wt and a multimodal distribution of water soluble salt sulfates chlorides and nitrates indicate multiple generations of sedimentation and salt accumulation events under semi arid to hyper arid climate conditions the multiple sulfate isotope compositions 17o 18o 34s of the upper section 15 0 to 34 5 m are close to those of modern hyperarid surface sulfates while the lower section 34 5 to 65 m displays a depth dependent isotope trend that is best interpreted as marking a period of climate change from semi arid to hyper arid when these data are combined with new chronological 40ar 39ar dates obtained from a volcanic ash layer at depth of 28 0 m our results show that hyper arid condition in the atacama desert was prevailing at least prior to 9 47 ma and may go back as old as the middle miocene

oligocene miocene age of aridity in the atacama desert revealed by exposure dating of erosion sensitive landforms the age of onset of hyperaridity in the atacama desert chile which is needed to validate geological and climatological concepts has been heretofore uncertain measurement of cosmogenic 21 ne in clasts from erosion sensitive sediment surfaces in northern chile shows that these surfaces have been barely affected by erosion since 25 ma surface exposure ages of sediment clasts give replicate values at 25 20 and 14 ma and individual values at 37 and 9 ma predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on earth our findings are compatible with the hypothesis that the onset of aridity in the atacama desert could be the reason for rather than the consequence of uplift of the high andes

prosopis sp tree ring oxygen and carbon isotope record of regional scale hydroclimate variability during the last 9500 years in the atacama desert abstract the hyper arid core of the atacama desert is one of the most persistently arid environments in the world yet there is evidence of significant hydroclimate variability during the holocene the timing of regional scale pluvial or humid periods has important implications for understanding the drivers of south american climate variability as well as the development of agriculture by prehistoric cultures here we present oxygen 18o and carbon 13c isotope values from ancient prosopis sp tree ring cellulose as a record relative humidity rh and water availability via intrinsic water use efficiency over century long intervals throughout the past 9 5 ka in the northern atacama desert 19 21 s these data provide the first high resolution ca annual evidence for decreasing water availability and increasing climate variability in the atacama from the early to late holocene intrinsic water use efficiency steadily increases over the holocene indicating a lowering of the water table and a reduction in the availability of groundwater resources in the region as water availability decreases 18o based estimates of rh show increased variance beginning 4 9 ka this change in rh coincides with the onset of modern day central andean atmospheric circulation patterns and previously documented humid periods in the southern altiplano 22 24 s subannual tree ring isotope analysis of modern and early holocene 8 3 ka trees show a similar seasonal cycle in carbon isotope values that correlates with modern annual changes in temperature however the early holocene interseasonal variability is greater than modern seasonality 18o values of the atacama tree rings are highly correlated with those of the sajama ice core from the bolivian andes demonstrating a teleconnection between high altitude andean precipitation and water availability in the atacama desert these reconstructions together provide a record of holocene water availability and ephemeral recharge related to changes in enso and south american monsoon

pleistocene recycling of copper at a porphyry system atacama desert chile cu isotope evidence we present cu isotope data of hypogene and supergene minerals from the late paleocene spence cu mo porphyry in the atacama desert of northern chile chalcopyrite displays a restricted range of 65cu values within the values reported for primary porphyry cu sulfides 0 28 to 0 34 n 6 supergene chalcocite samples show heavier and remarkably homogeneous 65cu values between 3 91 and 3 95 n 6 consistent with previous models of cu leaching and enrichment in porphyry systems secondary cu minerals from the oxide zone show a wider range of composition varying from 1 28 and 1 37 for chrysocolla n 6 to very light cu isotope signatures reported for atacamite between 5 72 to 6 77 n 17 these data suggest redox cycling of cu during supergene enrichment of the spence cu deposit characterized by a first stage of supergene chalcocite formation from acidic isotopically heavy leach fluids of meteoric origin down flowing in a semi arid climate 44 to 15 9 ma reworking of the initial supergene copper assemblage during the pleistocene by rising neutral and chlorine rich deep formation waters under well established hyper arid climate conditions lead to the formation of atacamite with extremely fractionated cu compositions essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter climatic conditions occurring in the latest pleistocene

surface vitrification caused by natural fires in late pleistocene wetlands of the atacama desert we describe extended occurrences of unusual silicate glass surface layers from the atacama desert chile these glasses found near the town of pica at four localities separated by up to 70 km are neither fulgurites nor volcanic glasses nor metallurgical slags related to anthropic activity but show close similarities to other glasses that have been previously attributed to large airbursts created by meteoroids entering the earth s atmosphere the glasses are restricted to specific late pleistocene terrains paleo wetlands and soils rich in organic matter with sio2 rich plant remains salts and carbonates 14c dating and paleomagnetic data indicate that the glasses were formed during at least two distinct periods this rules out the hypothesis of a single large airburst as the cause of surface melting instead burning of organic rich soils in dried out grassy wetlands during climate oscillations between wet and dry periods can account for the formation of the pica glasses large oases did indeed form in the hyperarid atacama desert due to elevated groundwater tables and increased surface discharge during the central andean pluvial event roughly coeval with the mystery interval and younger dryas finally we discuss the implications of our results for the other surface glasses previously attributed to extraterrestrial events

sources sinks and long term cycling of iodine in the hyperarid atacama continental margin abstract the atacama region in northern chile hosts the driest desert on earth and is the world s premier iodine production province the origin of iodine enrichment in atacama is controversial and fundamentally different processes have been invoked over the years that involve marine eolian and more recently deep sedimentary fluid and groundwater sources as a result of the very limited geochemical iodine data in atacama and the western south american margin the origin of iodine enrichment in this region still remains elusive in this study we present a comprehensive survey of iodine concentrations and isotopic ratios 129i i of different reservoirs in the atacama desert of northern chile including nitrate soils supergene copper deposits marine sedimentary rocks geothermal fluids groundwater and meteoric water nitrate soils along the eastern slope of the coastal cordillera are found to have mean iodine concentrations of at least three orders of magnitude higher than the mean crustal abundances of 0 12 ppm with a mean concentration of 700 ppm soils above giant copper deposits in the central depression are also highly enriched in iodine 100 s of ppm range and cu iodide and iodate minerals occur in the supergene enrichment zones of some of these deposits further east in the precordillera jurassic sedimentary shales and limestones show above background iodine concentrations the latter averaging 50 ppm in the southern portion of the study area the highest iodine concentrations in fluids were measured in groundwater below nitrate soils in the coastal range 3 5 10 ppm and in geothermal waters 1 3 ppm along the volcanic arc although highly variable the iodine isotopic ratios 129i i of jurassic marine sedimentary rocks 300 600 10 15 nitrate soils 150 1500 10 15 and waters 215 10 15 are consistently low

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