influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field california summary soils formed in loess are evidence of both relict and buried landscapes developed on pliocene to latest pleistocene basalt flows of the cima volcanic field in the eastern mojave desert california the characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow four distinct phases of soil development are recognized phase 1 weakly developed soils on flows less than 0 18 m y old phase 2 strongly developed soils with thick argillic horizons on 0 18 0 7 m y old flows phase 3 strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0 7 to 1 1 m y old flows and phase 4 degraded soils with petrocalcic rubble on pliocene flows a critical aspect of the development of stage 1 soils is the evolution of a vesicular a horizon which profoundly affects the infiltration characteristics of the loess parent materials laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development slight reddening of the interiors of peds from vesicular a horizons of phase 1 soils and presence of weakly developed b horizons indicates a slight degree of in situ chemical alteration however clay and fe oxide contents of these soils show that these constituents as well as carbonates and soluble salts are incorporated as eolian dust in contrast to phase 1 soils chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in situ chemical alteration these data the abundant clay films and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist flow age data and soil stratigraphic evidence also indicate that several major loess deposition events occurred during the past 1 0 m y loess events are attributed to past changes in climate such as the pleistocene to holocene climatic change that periodically caused regional desiccation of pluvial lakes reduction of vegetational density and exposure of loose unconsolidated fine materials during times of warmer interglacial climates precipitation infiltrates to shallower depths than during glacial periods extensive saline playas which developed in the mojave desert during the holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous nongypsiferous argillic horizons of phase 2 and 3 soils

inertial instability and mesoscale convective systems part ii symmetric cisk in a baroclinic flow abstract numerous budget studies of organized persistent systems of convective clouds outside the tropics suggest that circulations of mesoscale proportions are important in supplying moisture to the convective clouds though the dynamical nature of mesoscale flows remains poorly defined in part i emanuel 1979 hereafter referred to as part i it is demonstrated that circulations resulting from symmetric instability of shear flows in rotating fluids have a fundamentally mesoscale character in that both ambient rotation and ageostrophic advection are necessary for instability the results of part i are here extended to include the effects of latent heat release in a conditionally unstable atmosphere using the formalism of the cisk approach it is found that the presence of shear parallel to the wave fronts introduces new wave cisk modes which are not strongly dependent on the specified vertical structure of the cumulus heating the new baroclinic modes are of mesoscale dimensions have growth rates pro

review mesoscale modelling in complex terrain abstract this review focuses on those aspects of mesoscale models which require special techniques and parameterizations when modelling flow over complex terrain the basic concepts and limitations of various approaches are discussed and applications of models to specific problems are presented

complex multiple cosmogenic nuclide concentration and histories in the arid rio lluta catchment northern chile terrestrial cosmogenic nuclide tcn concentrations measured in river sediments can be used to estimate catchment wide denudation rates by investigating multiple tcn the steadiness of sediment generation transport and depositional processes can be tested measurements of 10be 21ne and 26al from the hyper to semi arid rio lluta catchment northern chile yield average single denudation rates ranging from 12 to 75 m myr 1 throughout the catchment paired nuclide analysis reveals complex exposure histories for most of the samples and thus the single nuclide estimates do not exclusively represent catchment wide denudation rates the lower range of single nuclide denudation rates 12 17 m myr 1 established with the noble gas 21ne is in accordance with palaeodenudation rates derived from 21ne 10be and 26al 10be ratio analysis since this denudation rate range is measured throughout the system it is suggested that a headwater signal is transported downstream but modulated by a complex admixture of sediment that has been stored and buried at proximal hillslope or terrace deposits which are released during high discharge events that is best evidenced by the stable nuclide 21ne which preserves the nuclide concentration even during storage intervals the catchment wide single 21ne denudation rates and the palaeodenuation rates contrast with previous tcn derived erosion rates from bedrock exposures at hillslope interfluves by being at least one order of magnitude higher especially in the lower river course these results support earlier studies that identified a coupling of erosional processes in the western cordillera contrasting with decoupled processes in the western escarpment and in the coastal cordillera copyright 2008 john wiley sons ltd

uplift of the altiplano puna plateau a view from the west 1 the western flank of the central andean plateau is a crustal scale monoclinal fold expressed in the geomorphology and in the westward tilt of fore arc basin strata data from three fore arc basins quantify the magnitude and time of displacement of the plateau system relative to the fore arc from 18 30 s to 22 s there is a single monocline strand there other authors documented 2000 m 500 m of early and middle miocene structural relief growth across small scale monoclines and our data reveal 810 m 640 m of 11 5 ma relief growth and 400 m 170 m relief growth since 5 ma across a long wavelength monoclinal fold limb this structural relief growth since 11 ma approximates the topographic relief growth between the fore arc and the altiplano plateau from 22 s to 24 s there are two subparallel long wavelength monoclines structural relief on the east side of the fore arc increased by 2840 m 2510 m during 17 10 ma and by 2320 m 1050 m since 10 ma some part of this monoclinal limb rotation led to topographic uplift of the adjacent puna plateau rotation across the western monocline could have increased the altitude of the fore arc basins by 900 m since 6 ma and may result from intense heating of the eastern margin of the cold and strong atacama lithosphere block although the wavelengths of the monoclines vary along strike and through time the monoclinal style of deformation dominated the uplift history of the eastern fore arc of the central andes throughout the neogene

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

influence of strong gradients of wind and humidity on frontal ageostrophic circulations the impact is studied of strong gradients of wind and humidity on the ageostrophic circulation developed around a conceptual cold frontal system the simulated basic wind and temperature fields are chosen to best represent the frontal observations made during the fronts 87 field experiment and in particular to take account of the presence of strong low level jets the numerical solution of the diagnostic sawyer eliassen equation in both its adiabatic and diabatic forms permitted us to test the impact of the wind and humidity structure on the corresponding ageostrophic circulation in particular strong negative shear above the low level jet core enhanced subsiding motions on the warm side of the front which restricted the horizontal extent of the cloud cover on the other hand the presence of well marked horizontal humidity gradients aloft at the rear edge of the cloud cover enhanced the ascending motions ahead of the frontal surface creating locally a secondary maximum of upward motions the degree of stability within the prefrontal boundary layer was found to be of major importance with regard to the intensification of the upright convection near the leading edge of the front moreover the incorporation of a parametrization scheme representing the ekman pumping at the top of the boundary layer enhanced the diagnosed circulation ahead of the front a comparison with observations made during two case studies of fronts 87 seems to confirm these results

a vegetation history from the arid prepuna of northern chile 22 23 s over the last 13 500 years the quaternary paleoclimate of the central andes is poorly understood due to numerous discrepancies among the diverse proxy records that span this geographically and climatically complex region the exact timing duration and magnitude of wet and dry phases are seldom duplicated from one proxy type to another and there have been few opportunities to compare climatic records from the same proxy along environmental gradients vegetation histories from fossil rodent middens provide one such opportunity on the pacific slope of the andes we previously reported a vegetation history from the upper margin 2400 3000 m of the absolute desert in the central atacama desert of northern chile that record identified a distinct wet phase that peaked between 11 8 and 10 5 ka when steppe grasses and other upland elements expanded as much as 1000 m downslope and a secondary wet period during the middle to late holocene 7 1 3 5 ka the latter wet phase remains controversial and is not as readily apparent in our lowelevation midden record we thus sought to replicate both phases in a midden record from the mid elevations 3100 3300 m of the arid prepuna where slight precipitation increases would be amplified midden records from these elevations identify conditions wetter than today at 13 5 9 6 7 6 6 3 4 4 3 2 and possibly 1 8 1 2 ka dry phases occurred at 9 4 8 4 ka and possibly at ca 5 1 ka present floras and modern hyperarid conditions were established after 3 2 ka the records from the two elevational bands generally match with some important differences these differences could reflect both the discontinuous aspect of the midden record and the episodic nature of precipitation and plant establishment in this hyperarid desert 2003 elsevier science b v all rights reserved

importance of diabatic processes on ageostrophic circulations observed during the fronts 87 experiment the impact of condensation and evaporation on real cross frontal circulations is studied using data from the fronts 87 field experiment the sawyer eliassen equation is solved numerically in its primitive equation form for one intensive observing period of fronts 87 three different schemes of parametrization of condensation heating are used comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone in particular when a parametrization scheme for the low level moisture convergence is applied a cell of strong ascending motion is formed ahead of the frontal zone in good agreement with the observations further a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front as observed in regions where precipitation originating from the strong line convection evaporates in comparison with their adiabatic counterparts the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone

application of a linear spectral model to the study of amazonian squall lines during gte able 2b a linear nonhydrostatic spectral model is run with the basic state or large scale vertical profiles of temperature and wind observed prior to convective development along the northern coast of south america during the gte able 2b the model produces unstable modes with mesoscale wavelength and propagation speed comparable to observed amazonian squall lines several tests with different vertical profiles of low level winds lead to the conclusion that a shallow and or weak low level jet either does not produce a scale selection or if it does the selected mode is stationary indicating the absence of a propagating disturbance a 700 mbar jet of 13 m s with a 600 mbar wind speed greater or equal to 10 m s is enough to produce unstable modes with propagating features resembling those of observed amazonian squall lines however a deep layer of moderate winds about 10 m s may produce similar results even in the absence of a low level wind maximum the implications in terms of short term weather forecasting are discussed

greatly expanded tropical warm pool and weakened hadley circulation in the early pliocene the pliocene warm interval has been difficult to explain we reconstructed the latitudinal distribution of sea surface temperature around 4 million years ago during the early pliocene our reconstruction shows that the meridional temperature gradient between the equator and subtropics was greatly reduced implying a vast poleward expansion of the ocean tropical warm pool corroborating evidence indicates that the pacific temperature contrast between the equator and 32 n has evolved from 2 c 4 million years ago to 8 c today the meridional warm pool expansion evidently had enormous impacts on the pliocene climate including a slowdown of the atmospheric hadley circulation and el nino like conditions in the equatorial region ultimately sustaining a climate state with weak tropical sea surface temperature gradients may require additional mechanisms of ocean heat uptake such as enhanced ocean vertical mixing

assessing the age of relief growth in the andes of northern chile magneto polarity chronologies from neogene continental sections the andes of northern chile currently experience a phase of relief rejuvenation as indicated by valleys that are 1000 m dissected into pediplains however it has been unclear when this phase of relief enhancement started this paper discusses the use of palaeomagnetic chronologies from four sections in the taracapa region northern chile to assess this age the sections are located in distal and proximal positions sediment accumulation occurred between c 22 5 and 8 7 5 ma with a hiatus that possibly spans a time interval between c 19 5 and 11 ma the magnetic polarity chronologies suggest a preferred age between 8 0 and 7 5 ma for the time when relief growth started in proximal positions however alternative correlations suggest an age of 8 5 ma in addition the palaeomagnetic data reveal no rotation of the analysed strata suggesting a minimum age of c 22 5 ma for the tectonic block rotation south of the arica deflection

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

geomorphologic evidence for the late pliocene onset of hyperaridity in the atacama desert the atacama desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on earth but the duration of this aridity is poorly constrained we reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features alluvial fans hillslope soils soil chemistry river profiles in the southern portion of the present desert topographic reconstructions of a large watershed 11 000 km 2 show deep incision and sediment removal between the late miocene and the end of the pliocene and modest to negligible incision in post pliocene times these changes in incision suggest an 50 280 reduction in river discharge which should reflect corresponding changes in precipitation changes in the nature of hillslope soils in the atacama desert indicate that in the pliocene or earlier hillslopes were mantled with silicate derived soil this mantle was stripped off and locally deposited as alluvial fans late pliocene to early pleistocene that now block or otherwise cause a rearrangement of pliocene and earlier river channels finally the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late pliocene stripping suggesting a decline in annual precipitation of at least 125 mm yr 1 or more mean annual precipitation map is now 1 embedded in the long post pliocene era of salt accumulation there are a variety of features suggesting overland flow on hillslopes rills striped gravel deposits piping and water spouts and large infrequent storms that infiltrated gentle alluvial fans due to the depth of salt rich horizons despite evidence for episodes that punctuate the hyperaridity the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate chloride and nitrate the late pliocene cessation of many fluvial features is coincident with recent research on the tropical pacific which shows that the pacific was in a permanent el nino state until ca 2 2 ma at which time sea surface temperatures offshore of south america declined greatly relative to those of the western pacific in turn setting up the present el nino southern oscillation enso climate system these observations indicate that the latest period of aridity has been prolonged and largely continuous and it appears to have occurred in step with the onset of the enso climate system beginning 2 m y ago

landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 ma atacama desert chile the landscape of the hyperarid atacama desert in northern chile records extremely slow change on earth s surface disputed ages for the onset of hyperaridity range from the late paleogene through the pleistocene a long term paleoclimate record is recorded in a nonmarine basin whose fill is primarily alluvial strata for this setting the primary proxies for climate state are the mineralogical and chemical composition of soil which varies across a precipitation gradient and the landforms and deposits of alluvial fans during the most recent 15 million years five climate related landscape stages are resolved for the pampa del tamarugal sedimentary basin with each successively younger stage inset lower in the local topography than its predecessor the earliest landscape stage is expressed as a set of alluvial strata inherited from a time of arid or semi arid climate ca 14 12 ma the younger four landscape stages generated a composite long lasting exposure surface predominantly hyperarid conditions have persisted since ca 12 1 ma during which four intervals of arid to semi arid climate occurred each wet interval was short lived a million years or less whereas some of the hyperarid periods were lengthy 1 5 m y the hyperarid intervals were roughly 11 5 5 ma 4 5 4 ma 3 6 2 6 ma 2 2 1 ma and repeated intervals during the last 1 m y the onset of hyperaridity ca 12 ma likely reflects the growth of the andes mountains above a climate threshold in contrast sea surface temperature variability likely has controlled atacama paleoclimate changes since the late miocene

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

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

andean uplift and climate evolution in the southern atacama desert deduced from geomorphology and supergene alunite group minerals abstract supergene alunite group minerals from the late eocene el salvador porphyry cu district the el hueso epithermal gold deposit and the coya porphyry au prospect located in the precordillera of northern chile 26 to 26 30 lat s have been dated by the 40 ar 39 ar method and analyzed for stable isotopes these data support published geomorphologic and sedimentologic evidence suggesting that the precordillera in the southern atacama desert had been uplifted as early as the late eocene and thus significantly prior to the altiplano which attained its high elevation only in the late miocene the oldest supergene alunite from the damiana exotic deposit at el salvador was dated at 35 8 1 ma and yielded a d vsmow value of 74 which indicates elevations of the precordillera near el salvador of at least 3000 m in the late eocene in contrast miocene supergene alunite from el salvador el hueso and coya have less negative d signatures reaching values as high as 23 to 25 at el hueso and el salvador between about 8 2 and 14 ma late miocene to holocene supergene alunite jarosite and natroalunite ages are restricted to el hueso and coya located near 4000 m above sea level in the precordillera roughly 1000 m higher than the present elevation of el salvador the d values of samples younger than 5 ma vary between 57 and 97 the complex evolution of the d signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the central depression of the southern atacama desert after 15 ma which agrees well with published constraints from the atacama desert at 23 24 lat s our data also suggest that wetter climatic conditions than at present prevailed in the latest miocene and early pliocene in the precordillera the new and previously published age constraints for el salvador indicate that supergene mineralization at the damiana exotic cu deposit occurred periodically over 23 ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

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

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

migration velocity analysis based on common shot depth migration applied to the seismic data of the caribbean oceanic plateau summary in areas where the geology is characterized by strong vertical and lateral velocity gradients pre stack depth migration will produce the optimum subsurface image an accurate velocity model is required for an accurate pre stack depth migration we developed a velocity analysis based on recursive pre stack depth migration that gives better results than the traditional cmp velocity analysis when applied to data over the caribbean oceanic plateau first the shot records are migrated with a series of constant velocities for each identified geological layer the velocity that best flattens the event in common receiver gathers crg is picked a large number of velocity analyses are run along the seismic line in order to define the reflector geometry for each layer we obtain the depth and interval velocity in order to make a model down to that depth this model is then used as the input for the next layer velocity analysis finally when all the layers have been picked the entire line is depth migrated with the derived model the migration algorithm used is a common shot kirchhoff technique which uses the ray tracing method of vidale 1988 to compute the traveltimes needed to determine the operator shape we applied this migration velocity analysis technique to seismic profiles of the casis cruise to define the deep structure of the caribbean oceanic plateau we imaged a 2500 m deep reflection sub b beneath the top of the plateau b horizon which defines the top of a 4500 4700 m s 1 layer the images obtained with this method will help to define odp sites that will provide information about geochemical and petrological variations of the volcanic basement

remnants of a fossil alluvial fan landscape of miocene age in the atacama desert of northern chile using cosmogenic nuclide exposure age dating we have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the atacama desert in chile the oldest landforms we studied are extensive alluvial fans referred to as atacama gravels with the exception of samples collected in antarctica the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples as determined by cosmogenic nuclides analyzed to date the oldest cobble has a model surface exposure age of 9 myr based on combined measurements of cosmogenic 10be 26al and 21ne concentrations cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces maximum erosion rates for cobbles on alluvial surfaces are uniformly 0 1 m myr the survival of these gravels specifically and more generally the stability of landform features in this geographic area is made possible by the attainment of hyperarid conditions in the atacama desert resulting from global climatic cooling about 15 myr ago combined with the rain shadow effect caused by uplift of the central andes the landform features observed presently in the atacama desert are remarkably stable and despite the inevitable erosion that is detectable using cosmogenic nuclides undoubtedly bear considerable resemblance to conditions as they existed in the miocene over geologic time the atacama landscape is evolving in such a manner as to erode the higher bedrock ridges relative to the more stable but stratigraphically lower depositional surfaces through which clastic detritus now travels occasionally along the floors of incised drainage systems leaving the older permeable alluvial fan surfaces largely intact as widespread remnants of a miocene fossil landscape

changes in sea surface conditions in the equatorial pacific during the middle miocene pliocene as inferred from coccolith geochemistry abstract this study presents a new alkenone derived sea surface temperature sst record and 18onoelaerhabdaceae data of the 2 5 m carbonate fractions from the iodp site u1338 located in the eastern equatorial pacific eep over the miocene pliocene our data and those available from other sites of the same area show the establishment of a cold tongue during the early pliocene 4 4 3 6 ma sst and 18onoelaerhabdaceae time series indicate periods of significant salinity variations comparison with the 18obenthic curve from sediment cores of the equatorial pacific ocean allows us to distinguish between global changes and local surface salinity variations in the eep ice sheet growth and evaporation precipitation are then discussed as possible drivers of such changes along with the role of central american and the indonesian seaway restriction our data suggest a shallowing of the thermocline in the eep between 6 8 and 6 ma and its shoaling between 4 8 and 4 0 ma suggesting the appearance of the cold tongue steph et al 2010 the pliocene climate transition would therefore not be primarily driven by northern hemisphere glaciation

three dimensional mesoscale circulation within a convective post frontal system possible role of conditional symmetric instability for triggering convective motion the role of the conditional symmetric instability csi in the formation of convective systems in the cold airmass located at the rear of a secondary cold frontal system is evaluated this study is based on observations made on 4 june 1984 during the french experiment landes fronts 84 in particular the mesoscale three dimensional circulation deduced from doppler radar data within one of these convective systems is studied in detail and compared with that predicted by the csi theory in spite of the fact that on the one hand notable convective scale motion masks the mesoscale circulation and that on the other hand the synoptic scale circulation is poorly described this comparison shows that among the various theories involved at the mesoscale only csi gives a proper account of the observed mesoscale circulation

late quaternary hydrological and ecological changes in the hyperarid core of the northern atacama desert 21 s abstract the hyperarid core of the atacama desert possesses important reserves of fossil or ancient groundwater yet the extent and timing of past hydrologic change during the late quaternary is largely unknown in situ and or short distance transported leaf litter deposits abound along relict fluvial terraces inserted within four dry and unvegetated valleys that drain into the endorheic basin of pampa del tamarugal pdt 21 s 900 1000 m one of the largest and economically important aquifers in northern chile our exceptional archive offers the opportunity to evaluate the response of low elevation desert ecological and hydrological systems to late quaternary climate variability three repeated expansions of riparian wetland ecosystems and perennial rivers occurred along the southernmost pdt basin between 17 6 14 2 ka 12 1 11 4 ka and from 1 01 0 71 ka both early and late archaic archaeological artefact are present in clear association with our fossil riparian wetland assemblages which suggests that these palaeoenvironmental changes facilitated past human occupations in the hyperarid core of the atacama desert using modern analogues we estimate that these ecological and hydrological changes were triggered by a threefold increase in rainfall along the headwaters of what are presently inactive canyons comparisons with other regional palaeoclimatic records from the central andes indicate that these changes were synchronous with the widespread pluvial stages now termed the central andean pluvial event cape 17 5 14 2 ka and 13 8 9 7 ka in addition we summarize new evidence for perennial runoff riparian ecosystems and a major human settlement during the latest holocene our findings clearly show that local hydrological changes in the pdt were coupled with precipitation variability in the adjacent eastern highlands during the late quaternary the long term dynamics of low elevation desert ecological and hydrological systems are likely driven by changes in moisture sources with one source tied to the amazon region n ne mode and the other to the gran chaco region se mode we conclude by linking enso like variability and moisture variations over the gran chaco to the three major regional scale recharge events over the last 18 ka in the pdt basin we conclude by asserting that an important portion of the groundwater resources in the pdt is indeed fossil inherited from past pluvial events we recommend that the relationship between ancient recharge together with palaeoclimate records of past headwater rainfall fluctuations should be incorporated into future water balance models and evaluation of groundwater potential in northern chile

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

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

desert pavement dynamics numerical modeling and field based calibration desert pavements are widely used as a relative surface dating tool because they are progressively better developed on surfaces ranging from thousands to hundreds of thousands of years in age recent work however has highlighted the dynamic nature of pavements and undermined their use as surface age indicators quade 2001 proposed that latest pleistocene vegetation advances destroyed all mojave desert pavements above 400 m elevation making all such pavements holocene in age in an effort to reconcile young pavement evidence with their widespread use as pleistocene surface age indicators we developed a numerical model based on the classic conceptual model in which pavements co evolve with their underlying eolian epipedons over millennial timescales in this co evolutionary process fine grained eolian deposition and av horizon development within the eolian epipedon promotes surface clast motion and pavement development enhancing the eolian sediment trapping ability of the pavement in a positive feedback model results illustrate the multi scale nature of pavement dynamics pavements may require tens of thousands of years to fully develop from a newly abandoned alluvial surface but may heal over timescales of decades to centuries if a mature eolian epipedon is present as such there is no inconsistency between rapid pavement healing and a pleistocene age for the underlying alluvial surface to calibrate the model we conducted surficial geologic mapping and pavement sedimentological analysis on two desert piedmonts our study areas include both proximal and distal fan environments illustrating the role of parent material texture in controlling the mode of pavement formation using available geochronology our work provides a rigorous calibration of pavement formation rates in our study areas and provides evidence supporting the use of pavements as local relative surface age indicators over holocene to late pleistocene timescales copyright 2007 john wiley sons ltd

detecting lithology with advanced spaceborne thermal emission and reflection radiometer aster multispectral thermal infrared radiance at sensor data abstract the advanced spaceborne thermal emission and reflection radiometer aster aboard nasa s terra satellite measures multispectral thermal infrared tir emission from the earth s surface to space based on analysis of tir spectral properties of typical rocks on the earth several mineralogic indices including the quartz index qi carbonate index ci and mafic index mi for detecting mineralogic or chemical composition of quartzose carbonate and silicate rocks with aster tir data are proposed these indices are applied to the aster tir data scenes for selected study areas in china and australia the results show that aster tir can discriminate quartz and carbonate rocks as well as mafic ultramafic rocks even with atmospherically uncorrected radiance at sensor data lithologic interpretations agree well with published geologic data and field observations the mineralogic indices applied to aster tir provide one unified approach for lithologic mapping in arid and semi arid regions of the earth

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

late miocene to early pliocene paleohydrology and landscape evolution of northern chile 19 to 20 s abstract northern chile s forearc lies in the hyperarid atacama desert the northern portion 150 km of the forearc basin known as the central depression contains perennial streams that drain to the pacific ocean through canyons that are deeply incised within a pediplain however during the late miocene and early pliocene a lake existed in the western part of the central depression between 19 3 s and 19 7 s indicating a forearc geomorphic landscape and paleohydrologic regime distinct from the present one for approximately 2 ma this lake was large 375 km2 and deep up to 120 m the catchment area sourcing the lake covered an area of 2900 5200 km2 new 40ar 39ar ages of volcanic ashes intercalated with the lake deposits and the overlying fluvial deposits indicate that lacustrine diatomite rich deposition had begun by 10 86 0 04 ma and the final stage of incision of a canyon through the lake deposits is younger than 3 04 0 03 ma additional published age constraints demonstrate that the expanse of the lake shrunk shortly after 9 ma yet the water balance in the western sector of the basin permitted lacustrine diatomite deposition until more recently than 6 4 ma even after the sediment hydrology balance shifted from balanced fill to overfilled a near surface groundwater table led to accumulation of salt pan salar evaporites until more recently than 3 5 ma the existence of a large deep lake is consistent with but not conclusive evidence for a wetter than modern climate in the catchment region during the late miocene the ultimate demise of the paleolake system is attributed to a kilometer scale decline of the base level of drainages downstream of the paleolake this study takes advantage of the unique characteristic of the drainage profiles in the study region a spatial separation between areas of deep incision east and west of a prominent knickzone to evaluate three end member scenarios of landscape evolution that could explain the base level fall event s the sedimentological stratigraphic and geomorphological data best fit the interpretation that forearc tectonic uplift and tectonically driven coastal retreat both contributed to base level fall

tau migration and velocity analysis theory and synthetic examples prestack migration velocity analysis in the time domain reduces the velocity depth ambiguity usually hampering the performance of prestack depth migration velocity analysis in prestack migration velocity analysis we keep the interval velocity model and the output images in vertical time this allows us to avoid placing reflectors at erroneous depths during the velocity analysis process and thus avoid slowing down its convergence to the true velocity model using a 1d velocity update scheme the prestack migration velocity analysis performed well on synthetic data from a model with a complex near surface velocity accurate velocity information and images were obtained using this time domain method problems occurred only in resolving a thin layer where the low resolution and fold of the synthetic data made it practically impossible to estimate velocity accurately in this layer this 1d approach also provided us reasonable results for synthetic data from the marmousi model despite the complexity of

a study of mesoscale convective bands behind cold fronts part i mesoscale organization this paper reports observations made over the north atlantic of a system of convective cloud bands found within the cold air behind an occluded front the observations were made from an instrumented aircraft which additionally released a pattern of dropsondes measuring temperature relative humidity and wind as functions of pressure these data are analysed to establish some of the mesoscale cloud scale and microphysical properties of the system here in part i the mesoscale structure is described and compared with the predictions of various theoretical models the convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance one of the theoretical models considered conditional symmetric instability provides a piausible explanation of the mesoscale observations

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

the formation of multiple fronts using two dimensional semi geostrophic frontogenesis models and three dimensional semi geostrophic baroclinic wave models it is shown that atmospheric development in the presence of non uniformities in pre existing temperature distributions may be expected to lead to the frequent occurrence of multiple frontal structures in general such non uniformities in the temperature distribution will be important if they significantly change the ambient temperature gradient the frontal structures associated with the irregularities tend to be very shallow they are possible candidates for the triggering mechanism for frontal troughs well behind a cold front and for shallow rainbands ahead of such a front however it seems that a separate mechanism is still necessary for the production of the middle level banded structures that are frequently observed

a preliminary petrographic study of the chilean nitrates the nitrate deposits of the northern atacama desert occurin a narrow zone between the low lying pampa del tamarugal to the east and the hills of the coastal range to the west high purity nitrate ore occurs as stratiform seams about 20 cm thick present at depths of 3 7 m below thedesert surface the ore is hosted by a variety of lithologies and overlain by salt cemented regolith the ore is largely composed of nitratite sodanitre or chilean salt peter and halite with locally abundant humberstonite polyhalite and mirabilite other salts present include ca iodates and a variety of sulphate borate and chromate minerals textural relationships can be used to deduce paragenetic sequences for individual salt samples and these combined to produce a generalized paragenetic web for the nitrate ore these textural data can be combined with published solubility data to investigate the course of fluid evolution during ore genesis the multiplicity of paragenetic relationships within the ore reflects the derivation of precipitating fluids from a variety of sources westwards flowing andean groundwater coastal fogs occasional rainfall and andean derived surface floodwaters the unusual mineralogy of the nitrate ore reflects the extreme chemical evolution of the precipitating brines through multiple episodes of salt precipitation and remobilization during transport to the nitrate horizons the formation of high purity nitrate ore appears to be the result of multiple phases of dissolution reprecipitation and recrystallization that have separated the highly soluble nitrate salts from less soluble salts in the overlying profile salts have largely accumulated through displacive growth but some of the host silicate and carbonate lithologies have also undergonea small degree of salt replacement

migration velocity analysis accounting for the effects of lateral velocity variations the most common migration velocity analysis algorithms iterative profile migration focusing analysis and stack power analysis are based on restrictive subsurface assumptions that may cause the methods to break down in the presence of lateral velocity variations typically the subsurface is assumed to have either a constant velocity or at most a depth variable velocity recent innovations include the use of traveltime inversion philosophy to invert migration measurements of the curvature as a function of offset exhibited by an event following migration traveltime inversion makes few assumptions regarding the subsurface but is a rather unstable process thus an important question is under what conditions do the traditional methods break down and make the use of traveltime inversion methods mandatory marrying the common migration analysis with tomography results in a set of equations that while useful for generating updates from migration measurements are too complex for answering the above qu

mesoscale atmospheric dynamics and modeling of rainfall fields a nonuniform potential vorticity model for frontogenesis is used to study the effects of height dependent inhomogeneities in the potential temperature and potential vorticity fields at the mesoscale the model simulation results show that these irregularities induce in the vertical velocity field mesoscale patterns which have considerable vertical extent it is suggested that these mesoscale inhomogeneities may be possible causes for the high degree of organization of precipitation fields and the formation of middle level rainbands

andean uplift ocean cooling and atacama hyperaridity a climate modeling perspective abstract located along the west coast of subtropical south america the atacama desert features an extremely dry climate in sharp contrast with relatively wet conditions over the central andes and farther east the atacama s hyperaridity has been attributed to its subtropical location the cold waters over the adjacent southeast pacific ocean and the presence of the andes cordillera although geological evidence reveals less dry conditions in the remote past the timing of the arid to hyperarid transition is a matter of controversy several studies suggest that such transition occurred between 19 and 13 ma fostered by the concurrent uplift of the andes during the lower middle miocene other studies however suggest a much earlier 25 ma or later 2 1 ma transition associated with global or regional ocean cooling here we use plasim a simple global climate model to study the effect of the andean surface uplift and sea surface temperature changes upon atacama aridity in one set of experiments the continental topography was scaled by a factor ranging from 0 9 to 0 1 decreasing the height of the andes did not increase precipitation over the atacama region but rather decrease the precipitation over the central andes and the interior of the continent these results consistent with previous modeling studies suggest that the andean uplift was not an important ingredient in the onset of atacama hyperaridity even if both events overlapped in time a globally uniform sea surface warming does not result in a significant increase of precipitation over the atacama in contrast a regional warming over the subtropical southeast pacific very effectively increase precipitation along the west coast of south america by decreasing the subsidence and the dry advection at low levels it suggested that a reinforcement of the humboldt current since the late miocene and particularly during the pliocene pleistocene transition was crucial in the drying of the atacama desert that culminated with the present day hyperarid conditions

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

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

supergene enrichment of copper deposits since the onset of modern hyperaridity in the atacama desert chile supergene enrichment of cu deposits in the atacama desert has played a critical role in making this the prime cu producing province of the world previously this has been believed to have occurred exclusively over a long period from the middle eocene to the late miocene which ended when climatic conditions changed from arid to hyperarid here we report u series disequilibrium ages in atacamite bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the atacama desert 230th 234u ages of gypsum intergrown with atacamite in supergene veins from cu deposits cluster at 240 ka chuquicamata 130 ka mantos blancos spence and 80 ka mantos de la luna michilla when coupled with previous data these results indicate that supergene enrichment of cu deposits did not cease after the onset of hyperaridity we propose that supergene enrichment in the atacama region developed in two main stages the main phase caused by downward circulation of meteoric waters in a semi arid setting was active from 45 until 9 ma with a last pulse ca 5 ma in the southern atacama desert during this phase atacamite bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water this was followed by a second stage starting at 2 1 5 ma and continuing until at least the late pleistocene when deep formation waters derived from the basement passed up through and modified the pre existing supergene cu oxide minerals atacamite has then been preserved in the prevailing hyperarid climate

atmospheric origin of the devastating coupled air sea event in the east adriatic enhanced sea level oscillations with devastating effects occurred on 26 27 june 2003 in the east adriatic the phenomenon was recognized as a resonantly coupled air sea interaction where the sea wave is forced by the propagating atmospheric pressure disturbance this study examines the dynamics and predictability of the atmospheric component of the coupled system first the initiation amplification and maintenance of the system are examined the source of the perturbation is traced back to the alpine region where the potentially unstable air in the westerly current lifts over the alps resulting in convection and wave development the wave and convective activity apparently couple in a wave cisk manner and propagate southeastward down the eastern adriatic coast due to the potentially unstable air above the adriatic the system additionally amplifies along its track the dissipation of the system occurs after the landfall second the performance of the numerical model at the resolution that is similar to operational nwp model resolution is examined this severe event is highly susceptible to the details of the model configuration presumably due to the sensitivity in coupling of the wave and mcs thus although the model is able to reproduce and forecast this kind of event at a fairly coarse resolution the details of the spatial structure as well as the time of onset are questionable in this case the greatest sensitivity is primarily in the treatment of moist and convective processes

relevance of conditional symmetric instability in the interpretation of wide cold frontal rainbands a case study 20 may 1976 comparisons are made between the characteristics of rainbands observed in a frontal system using a c band doppler radar and dynamical mechanisms relevant on the mesoscale 100 km the orientation of these rainbands nearly aligned with the thermal wind suggests that symmetric instability may play a role in their formation this possible explanation of the precipitation is discussed

influence of the andes mountains on south american moisture transport convection and precipitation mountain ranges are known to have a first order control on mid latitude climate but previous studies have shown that the andes have little effect on the large scale circulation over south america we use a limited domain general circulation model regcm3 to evaluate the effect of the andes on regional scale atmospheric dynamics and precipitation we present experiments in which andean heights are specified at 250 m and 25 50 75 and 100 of their modern values our experiments indicate that the andes have a significant influence on moisture transport between the amazon basin and the central andes deep convective processes and precipitation over much of south america through mechanical forcing of the south american low level jet llj and topographic blocking of westerly flow from the pacific ocean when the andes are absent the llj is absent and moisture transport over the central andes is mainly northeastward as a result deep convection is suppressed and precipitation is low along the andes above 50 of the modern elevation a southward flowing llj develops along the eastern andean flanks and transports moisture from the tropics to the subtropics moisture drawn from the amazon basin provides the latent energy required to drive convection and precipitation along the andean front large northerly moisture flux and reduced low level convergence over the amazon basin leads to a reduction in precipitation over much of the basin our model results are largely consistent with proxy evidence of andean climate change and have implications for the timing and rate of andean surface uplift

evidence for active landscape evolution in the hyperarid atacama from multiple terrestrial cosmogenic nuclides the atacama desert is one of the driest places on earth multiple lines of evidence show that the atacama has been hyperarid since at least the late miocene among these are cosmic ray exposure ages indicating that individual clasts on some surfaces have been preserved for 9 ma and possibly since the oligocene although these remarkably old ages indicate slow landscape evolution it is not clear whether this pace is characteristic of the entire atacama or only of specific regions landforms or landscape elements to address this question we measured cosmogenic 10be 26al and 21ne from a wide variety of landscape elements in a transect across the central atacama where modern precipitation is at an extreme minimum but where the concentration of cosmogenic nuclides in stable landscape elements has not previously been recorded we find that the hyperarid core of the central atacama has substantially slower erosion rates than its eastern and western margins however even the driest part of this transect has erosion rates comparable to those of other deserts ranging from 0 2 0 4 m ma the most stable landscape elements are boulder fields with exposure ages of 1 5 2 6 ma the vast majority of samples in the central atacama desert however have cosmogenic nuclide concentrations corresponding to ages 5 ma documented elsewhere in the atacama were not found in our area and appear to be limited to exceptionally stable boulders or cobbles in either the northern or southern extremes of the atacama desert we suggest that the central atacama has been subject to episodic pliocene and pleistocene rainfall and geomorphic activity perhaps due to intrusion of pacific moisture

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

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

sedimentology based reconstructions of paleoclimate changes in the central andes in response to the uplift of the andes arica region between 19 and 21 s latitude northern chile we focus on the sedimentological record of the middle miocene to modern deposits in the andes of northern chile between 19 and 21 s these sediments deposited at the western escarpment of the central depression indicate successively more moisture on the western margin of the altiplano and the western cordillera where the sources are at the pacific coast 20 ma old exposure ages and salic gypsisols reflect an existing and ongoing hyperarid climate we interpret the increased divergence of climates between the coast and the altiplano as consequence of the andean rise to elevations higher than approximately 2 500 m a s l when the topography of the altiplano was sufficiently high and areally extensive to attract atlantic moisture accordingly the inferred general increase in run off was closely coupled with the uplift of the andes if the steady rise model applies in case that the rapid rise model for andean uplift is correct the inferred changes in sediment transport would have occurred independently of uplift requiring an alternative yet unknown driver

evolution of reg soils in southern israel and sinai abstract reg soils mostly camborthids and gypsiorthids cover some 15 of the negev and sinai deserts analytical data of seven profiles on depositional surfaces of increasing age in the negev show clear relationships between ages of surfaces and soil profile features exemplified in the development of the cambic salic and gypsic horizons the youngest soil a coarse desert alluvium soil typic torriorthent of dry wadi beds up to a few thousand years old has little profile differentiation and is generally non saline or slightly saline on the higher terraces connected with the lisan formation about 70 000 12 000 years b p clear profile differentiation and the beginning of development of cambic horizons can be discerned evident in their colour textural p differences the soils are already saline and somewhat gypsiferous both soils are not yet defined as regs because of their negligible or restricted profile differentiation reg soils on the older and higher geomorphic surfaces in the arava rift valley several hundred thousand years old have well differentiated profiles with cambic salic and gypsic horizons below their stony desert pavements typical reg soils have light coloured vesicular horizons over mellow very pale brown loam frequently with laminar structure almost completely stone free layers the reddish brown loamy to clay loam b horizons are very saline and show intensive salt weathering of gravels gypsic and petrogypsic horizons occur at depth such reg soils represent the stable surfaces and soils of deserts and were developed over a long period of desert weathering

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

late pleistocene human occupation of the hyperarid core in the atacama desert northern chile abstract few archeological sites in south america contain uncontroversial evidence for when the first peopling of the continent occurred largely ignored in this debate extreme environments are assumed either as barriers to this early wave of migration or without potential for past habitability here we report on a rare 12 13 ka human occupation from quebrada mani site qm12 a plantless near rainless landscape 1240 m asl and 85 km from the pacific ocean located in the hyperarid core of the atacama desert this location harbored wetlands and riparian woodlands that were fed by increased rainfall further east in the central andes during the latest pleistocene excavations at qm12 yielded a diverse cultural assemblage of lithics burned and cut bones marine gastropods pigments plant fibers and wooden artifacts alongside a prepared fireplace sixteen radiocarbon dates from site qm12 on charcoal marine shells animal dung plant remains and wood reveal that the occupation took place between 12 8 and 11 7 ka these results demonstrate that the atacama desert was not a barrier to early american settlement and dispersal and provide new clues for understanding the cultural complexity and diversity of the peopling of south america during the last glacial interglacial transition

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

radiocarbon reservoir effect and the timing of the late glacial early holocene humid phase in the atacama desert northern chile we revise substantially the regional chronology of lake level fluctuations from the late glacial early holocene humid phase along a high altitude transect 3500 to 4500 m between 18 s and 28 s in the southwestern altiplano of northern chile radiocarbon dates and 210pb profiles for limnic and terrestrial materials allow us to estimate and justify reservoir correction values for conventional 14c dates our chronology suggests that the latest pleistocene early holocene humid phase started between 13 000 and 12 000 14c yr b p and that maximum lake levels were reached between 10 800 and 9200 14c yr b p this is significantly younger than what has been established so far for the titicaca uyuni basin in bolivia the paleolakes disappeared sometime between 8400 and 8000 14c yr b p our revised chronology agrees with the regional history of human occupation and is broadly synchronous with vegetation changes in subtropical continental south america and with the onset of wetland expansion in the northern hemisphere tropics

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

pliocene summer sea surface temperature reconstruction using silicoflagellates from southern ocean odp site 1165 1 in the modern marine environment the silicoflagellate genus dictyocha is rare or absent south of the antarctic polar front apf the genus distephanus in contrast is dominant in sediments recovered from odp site 1165 1600 km south of the front however three intervals where dictyocha is abundant are interpreted to represent pliocene warm events comparison of our data with ciesielski and weaver s 1974 modern core top silicoflagellate relationship with sea surface temperature sst indicates that at site 1165 mean annual sst was approximately 5 c at 3 7 ma event i and approximately 4 c at 4 3 4 4 ma event ii and 4 55 4 8 ma event iii event i represents a 5 5 c warming and events ii and iii represents a 4 5 c warming relative to modern mean annual sst dictyocha is absent from other site 1165 pliocene intervals which suggests that cooler sst 2 c prevailed the warm events detected at site 1165 may represent times when north atlantic deep water production and ocean heat transport into the southern ocean exerted maximum influence

controls on supergene enrichment of porphyry copper deposits in the central andes a review and discussion the central andes host some of the world s largest porphyry copper deposits the economic viability of these deposits is dependent on the size and quality of their supergene enrichment blanket published models that have strongly influenced exploration policy suggest that supergene enrichment ceased at 14 ma due to an increase in aridity here we discuss these models using published geochronological geomorphological and geological data geochronological data indicate that supergene oxidation and enrichment has been active between 17 and 27 s across the forearc of northern chile and southern peru from 44 to 6 ma and on the bolivian altiplano and eastern cordillera of argentina from 11 ma to present there is evidence for cessation at 20 14 and 6 ma however a major problem is that as more geochronological data become available the age ranges and periods of enrichment increase this suggests that the full spectrum of enrichment ages may not have been sampled the relationship between supergene enrichment and the age of regional pediplain surface development is not well constrained only in two areas have surfaces related to enrichment been directly dated southern peru and south of 26 s in chile and suggest formation post 14 ma sedimentological data indicate that a fluctuating arid semi arid climate prevailed across the atacama desert until between 4 and 3 ma climatic conditions that are thought to be favourable for supergene enrichment the balance between uplift erosion burial and sufficient water supply to promote enrichment is complex this suggests that a simple model for controlling supergene enrichment is unlikely to be widely applicable in northern chile general models that involve climatic desiccation at 14 ma related to rainshadow development and or the presence of an ancestral cold upwelling humboldt current are not supported by the available geological evidence the integration of disparate sedimentological geomorphological and supergene age data will be required to fully understand the controls on and distribution of supergene oxidation and enrichment in the central andes

joint optimization of vertical component gravity and p wave first arrivals by simulated annealing abstractjoint seismic and gravity analyses of the san emidio geothermal field in the northwest basin and range province of nevada demonstrate that joint optimization changes interpretation outcomes the prior 0 3 0 5 km deep basin interpretation gives way to a deeper than 1 3 km basin model kirchoff prestack depth migrations reveal that joint optimization ameliorates shallow velocity artifacts flattening antiformal reflectors that could have been interpreted as folds furthermore joint optimization provides a clearer picture of the rangefront fault by increasing the depth of constrained velocities which improves reflector coherency at depth this technique provides new insight when applied to existing data sets and could replace the existing strategy of forward modeling to match gravity data we have achieved stable joint optimization through simulated annealing a global optimization algorithm that does not require an accurate initial model balancing the combined seismic gravity objective function i

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

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

applications of multiscale waveform inversion to marine data using a flooding technique and dynamic early arrival windows a recently developed time domain multiscale waveform tomography mwt method is applied to synthetic and field marine data although the mwt method was already applied to synthetic data the synthetic data application leads to a development of a hybrid method between waveform tomography and the salt flooding technique commonly use in subsalt imaging this hybrid method can overcome a convergence problem encountered by inversion with a traveltime velocity tomogram and successfully provides an accurate and highly resolved velocity tomogram for the 2d seg eage salt model in the application of mwt to the field data the inversion process is carried out using a multiscale method with a dynamic early arrival muting window to mitigate the local minima problem of waveform tomography and elastic effects with the modified mwt method reasonably accurate results as verified by comparison of migration images and common image gathers were obtained the hybrid method with the salt flooding technique is not used in this field data example because there is no salt in the subsurface according to our interpretation however we believe it is applicable to field data applications

influence of andean uplift on climate and paleoaltimetry estimates article i nfo article history recent elevation reconstructions of the andean plateau suggest a rapid 2 5 1 0 km rise of the central andes between 10 and 6 ma this rapid rise has been attributed to a catastrophic removal of a dense lithospheric mantle root beneath the andes however these findings are based on the assumption that climate did not change during deposition of paleoaltimetry proxies here we evaluate south american climate change due to andean uplift and its influence on interpretations of plateau elevation from climate sensitive paleoaltimetry data a series of experiments are presented using the regcm3 regional general circulation model rcm to characterize changes in andean precipitation amount surface temperature and wind direction vapor source as a function of changing plateau elevation results indicate that south american and andean climate changed significantly in response to plateau growth more specifically rising of the plateau results in up to a 900 mm increase in rainy season december january february precipitation over the plateau plateau uplift also results in a decrease in non adiabatic surface temperature of up to 6 5 c in addition to adiabatic cooling directly related to elevation change through the lapse rate finally the prevailing wind direction and the vapor source for precipitation switches from the south pacific ocean to the equatorial atlantic as plateau elevation increases above of its present day elevation taken together these changes in paleoclimate would have substantially depleted the oxygen isotopic concentration of paleoprecipitation through the cenozoic unless this climatic effect is taken into consideration paleoaltimetry reconstructions based on stable isotope methods may overestimate the rapid rise of the andes by up to several kilometers we conclude that some or all of the apparent rapid rise of the andean plateau from paleoaltimetry data could be an artifact of large changes in paleoclimate

the origin and distribution of salts on alluvial fans in the atacama desert northern chile the atacama desert has been predominantly hyperarid since the middle miocene combined ionic and x ray diffraction analysis shows that calcium sulphate is prevalent on three similar alluvial fans in salar basins across a transect of different environments in antofagasta province northern chile differences between the fans were largely due to the effect of local factors on salt input secondary redistribution and deposit preservation thus carbonate was notable in the high andes fan c and pre cordillera fan b whereas in the central valley fan a greater qualities of the more soluble salts chlorides and nitrates probably reflect the higher level of aridity and an oceanic source the camanchaca calcium sulphate distribution on the alluvial fans and on adjacent hilltops indicates an aeolian origin evidence from the pre cordillera fan b however suggests that salt input may have been episodic related to changes in conditions within the general aridity and possibly to stone pavement and soil development calcium sulphate sources range from an input in andean precipitation and the camanchaca to abundant regional evaporite deposits it is probable however that products of andean volcanism constitute the dominant primary source of calcium sulphate and that the salt has subsequently been distributed widely within the endoreic basins of the region by a combination of groundwater surface flow and the wind 1997 john wiley sons ltd

paleowetlands and regional climate change in the central atacama desert northern chile abstract widespread organic rich diatomaceous deposits are evidence for formerly wetter times along the margins of the central atacama desert one of the driest places on earth today we mapped and dated these paleowetland deposits at three presently waterless locations near salar de punta negra 24 5 s on the western slope of the andes elevated groundwater levels supported phreatic discharge into wetlands during two periods 15 900 to 13 800 and 12 700 to 9700 cal yr bp dense concentrations of lithic artifacts testify to the presence of paleoindians around the wetlands late in the second wet phase 11 000 9700 cal yr bp water tables dropped below the surface before 15 900 and since 8100 cal yr bp and briefly between 13 800 and 12 700 cal yr bp this temporal pattern is repeated with some slight differences in rodent middens from the study area in both paleowetland and rodent midden deposits north and south of the study area and in lake level fluctuations on the adjacent bolivian altiplano the regional synchroneity of these changes points to a strengthening of the south american monsoon which we term the central andean pluvial event in two distinct intervals 15 900 13 800 and 12 700 9700 cal yr bp probably induced by steepened sst gradients across the tropical pacific i e la nina like conditions

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