late miocene holocene canyon incision in the western altiplano northern chile tectonic or climatic forcing abstract major fluvial incision 600 1000 m affecting the coastal cordillera and central depression of northern chile is analysed to evaluate supposed coeval uplift of the altiplano and or climatic changes in the atacama desert the timing of the beginning of incision is constrained by the age of deposition of the central depression top in the north 18 19 s this top corresponds to fluvial gravels accumulated between 11 9 0 6 ma and 8 3 0 5 ma which are genetically related to semiarid climate and to an eastward poorly dissected parallel drainage network that developed between 15 0 0 6 and 11 2 0 6 ma thus gravel deposition ended at 11 9 11 2 ma to the south 19 20 s the central depression top corresponds to c 6 ma alluvial deposits stratigraphically determined canyon ages and knickzone locations indicate that southward dissection began later and or developed under a regime of lower erosion capacity owing to drier climate vertical incision rate evolution is compatible with eastward knickzone migration dissection required a considerable altitude difference between ancient and present day river base levels which was achieved predominantly by basin infill on an already partially elevated bedrock therefore subsequent incision would have been triggered by local semiarid climatic periods rather than by contemporaneous surface uplift exoreic canyons occur when climatic conditions in the catchments are arid semiarid whereas endoreism is developed when these conditions in catchments are hyperarid

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

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

moored wind temperature and current observations made during coastal ocean dynamics experiments 1 and 2 over the northern california continental shelf and upper slope moored observations of winds currents and temperature made over the northern california shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the coastal ocean dynamics experiment code are described in both field experiments code 1 and code 2 the onset of the upwelling season is triggered by the strengthening of the north pacific high off northern california and signalled by the presence of water colder than 9 c near the bottom over the shelf vertical temperature stratification over the shelf is reduced during active upwelling local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction and the mean wind stress is directed equatorward response of alongshelf currents to wind forcing is similar at all depths over the shelf the response of the cross shelf current is more complex in the surface layer offshelf flow is significantly and positively correlated with the upwelling favorable alongshelf wind stress beneath this layer a weak return flow exists the thickness of the upper mixed layer varies greatly reaching 60 m during upwelling events thus direct wind forcing affects a substantial fraction of the water over the shelf low frequency wind stress current and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure the most energetic mode typically accounts for 75 or more of the variance of wind stress and temperature over the middle and inner shelf the most energetic current mode accounts for 90 or more of the variance is dominated by he alongshelf component and veers to the left with depth the low frequency alongshelf flow fluctuations are quasi barotropic with some vertical shear but no reversal with depth vertical shear in alongshelf current and the cross shelf density gradient obey the thermal wind relation the simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross shelf transects in code 2 volume transport over the shelf is often nearly two dimensional but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope correlation of low frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf where a 15 cm 1 alongshelf flow lags a 1 dyn cm 2 alongshelf wind stress by the order of 10 hours currents over the upper slope are not significantly correlated with local wind alongshelf transport over the shelf is highly correlated with local wind stress explaining 80 of the variance in the low frequency transport during code 2 volume transport is best correlated with wind stress approximately 30 km to the south winds in the code area during spring and summer are characterized by strong and persistent upwelling favorable winds interrupted by shorter periods of weak winds regression analysis indicates that in the absence of wind a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the code area an upwelling favorable wind counters this jet over the shelf so that the alongshore flow is everywhere equator ward during strong wind events since the maximum response to wind forcing is found at midshelf and near surface the mean current over the upwelling season is strongly sheared in both the vertical and cross shelf directions mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward

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

circulation in the santa maria basin california during 1998 1 moored observations and numerical model results are used to describe the circulation during 1998 in santa maria basin smb and santa barbara channel sbc located to the northwest and southeast respectively of a bend in the southern california coastline singular value decomposition svd analysis of observed wind stress and currents yields a first mode that represents large scale alongshore flow variability that appears to be wind driven and a second mode which mostly represents flow variability in sbc and appears coherent with wind stress shear there a three dimensional numerical ocean circulation model is used to simulate the flow the model derived surface heat flux is most similar to observation derived values in summer and least similar in winter and more similar in smb than in sbc empirical orthogonal function eof analysis of the coriolis and pressure gradient terms for modeled near surface currents supports the hypothesis that the large scale flow variability of the first mode is wind driven it further suggests that an alongshore pressure gradient drives the poleward flow along sbc s northern shelf wind stress curl of which the shear along sbc is a part or its gradient may govern such a pressure gradient although this could not be ascertained modeled and observed currents are similar in their directions but both the annual mean and variance tend to be smaller for modeled flow svd analysis shows that modeled and observed flows have similar modes of variability remotely driven flow may play a role in smb especially in summer

characteristic patterns of the circulation in the santa barbara channel the subtidal near surface circulation in the santa barbara channel sbc and on the shelf north of point conception is described based on observations obtained during the 3 year period from 1993 to 1995 near surface currents in the channel are a superposition of a larger than sbc scale flow and a cyclonic circulation of variable intensity located inside the channel on seasonal timescales the larger than sbc scale flow near the surface is equatorward in spring and poleward from summer through winter the increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near surface temperatures and synthetic subsurface pressures ssps the flow reverses in late spring simultaneously with the increase in the along channel ssp difference and months before wind stress has reached its peak the period during which the cyclonic circulation within the sbc is strongest coincides with the period of strongest poleward flow through the eastern entrance a synoptic description of the circulation in the sbc is presented in terms of six characteristic patterns labeled upwelling relaxation cyclonic propagating cyclones flood east and flood west an analysis of the 5 and 45 m currents into empirical orthogonal functions eofs isolates 50 53 of the 5 m 45 m low frequency current variance into three two modes combining these modes with the mean current fields the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near surface currents from late spring through fall the two largest 5 m current modes produce a repeating pattern of circulation in which the sequence of four states namely upwelling cyclonic relaxation and quiescent is traversed roughly every 16 days in addition to the large scale cyclonicity in the central sbc smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0 06 m s 1 the patterns described here develop in response to the wind stress and the along channel ssp gradient

objective maps of near surface flow states near point conception california abstract objective streamfunction and velocity potential maps of three commonly observed flow states near point conception california are derived from moored current meter and drifter observations the states are defined using current meter data drifter data are sorted by flow state and averaged within spatial bins to resolve their spatial structures spatial correlations of drifter data show the along shelf and cross shelf decorrelation distances are approximately 45 and 25 km respectively this information is used to make the objective streamfunction and velocity potential maps the total velocity is represented as the sum of the rotational nondivergent streamfunction component and the irrotational divergent velocity potential component the velocity derived from the streamfunction is much stronger than that derived from the velocity potential the streamfunction is sufficiently well resolved to make meaningful vorticity maps the velocity potential indicates a source term in the western santa bar

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

near surface trajectories off central and southern california the near surface circulation in the santa barbara channel and off the coast of central and southern california is described based on 20 releases of drifters drogued 1 m beneath the surface from 12 sites within the channel at bimonthly intervals this description includes small scale features of the circulation which are not part of descriptions based on moored observations or of the statistics of the drifter releases the eventual fate of drifters at long time intervals compared to the residence time in the channel about 7 days is also included in the channel the trajectories document a persistent cyclonic circulation with a typical recirculation period between 3 and 5 days in the spring currents near the mainland are weaker than near the channel islands and the overall flow is toward the southeast trajectories document the possibility for water parcels to leave the channel through the interisland passes in the late fall and winter a poleward flow with velocities often exceeding 0 5 m s 1 is confined within 20 km of the mainland between these two seasons the cyclonic tendency is enhanced although most of the drifters eventually migrate westward the trajectories of drifters released at the same time from sites only 20 km apart can be remarkably different once the drifters migrate out of the channel their trajectories can be grouped into a few patterns in spring and summer drifters tend to remain in the southern california bight their trajectories often remain close over extended periods as if they were caught in convergence zones in fall the drifters often are caught in a poleward current

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

diurnal period internal waves near point conception california diurnal period internal waves were observed near point conception california using an array of moorings extending 120 km along the inner shelf the waves have an along shelf coherence scale of at least 50 km and appear to propagate nearly straight onshore wave amplitudes vary over time depending on thermal stratification and the amplitude of the diurnal sea breeze oscillation barotropic tides and vorticity over the mid shelf are not correlated with internal wave amplitude large amplitude internal waves with supercritical froude numbers are observed in mid summer although such waves may drive vertical mixing and cross shelf transport of passive particles there is no significant correlation between wave amplitude and invertebrate settlement in the santa barbara channel

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

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

a view of the 1993 1994 california current based on surface drifters floats and remotely sensed data near surface drifters were deployed in the california current system during the period 1993 1994 in order to study mean and eddy currents the deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5 grid eddy variability which tended to be isotropic was higher near the coast standard deviations around 0 2 m s 1 and decreased offshore down to 0 1 m s 1 seasonal mean surface flow west of california was southward during all seasons if a davidson current existed during 1993 1995 it was probably either short lived or only found closer to the coast than our study area i e within 100 150 km of shore in some cases drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior we note a tendency for individual eddies which can last for up to 3 5 months to drift westward at rates of o 5 km day 1 not all eddies in the region are surface intensified there is clear evidence for at least the occasional presence of a thin meandering southward jet in the region that can be 1000 km or more long this jet can be identified with that found in the coastal transition zone program in association with cool filaments the jet appears to become broader and slower south of point conception we hypothesize on the basis of others results as well as ours that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the northward undercurrent

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

rise of the central andean coast by earthquakes straddling the moho coastlines above subduction zones often uplift over geological timescales analysis of landscape evolution and seismicity at the central andean margin suggests that earthquakes on the deep plate interface help build up coastal topography

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

large scale response to atmospheric forcing of shelf currents and coastal sea level off the west coast of north america may july 1981 and 1982 the response of alongshore currents v over the continental shelf and of coastal sea level along the pacific coast of north america to atmospheric forcing on large alongshore scales 100 2000 km during may july 1981 coastal ocean dynamics experiment code 1 and may july 1982 code 2 has been studied using sea level and wind measurements from coastal stations and buoys between 33 n and 48 n and using current measurements from five locations between 35 n and 48 n characteristics of the large scale response of v and to fluctuations in the alongshore component of the wind stress varied with latitude and with year in relation to variations in in 1981 large scale fluctuations were coherent within but not between separate domains north and south of 42 n in 1982 large scale coherent fluctuations were found only in a northern domain that extended much farther south to near 37 n accordingly in 1981 alongshore current v fluctuations from 39 n code were coherent with v from 35 n while in 1982 v fluctuations from 39 n were coherent with those north to 48 n in 1981 the northern domain fluctuations were forced by between 42 n and 45 n with southern domain forcing at 35 36 n in 1982 northern domain fluctuations were forced by over a region of much larger along shore extent 37 48 n high values of space lagged coherences between v and y y is the alongshore coordinate indicate that v at 39 n was strongly wind driven maximum squared coherences of v versus y are found with to the south by 100 300 km for low frequencies 0 23 cpd in 1981 0 14 cpd in 1982 and with from near local stations at higher indicating the importance of remote forcing at low frequencies and local forcing at high frequencies the response of v to y at 35 n and 43 n was more complicated but evidence for strong wind forcing is found in several frequency bands for v at 43 n during 1981 and 1982 and at 35 n during 1981 at 48 n v was forced in 1982 for 0 18 cpd by from locations 300 1400 km to the south

direct observations of atmospheric boundary layer response to sst variations associated with tropical instability waves over the eastern equatorial pacific tropical instability waves tiws with a typical wavelength of 1000 km and period of 30 days cause the equatorial front to meander and result in sst variations on the order of 18 28c vertical soundings of temperature humidity and wind velocity were obtained on board a japanese research vessel which sailed through three fully developed sst waves from 1408 to 1108w along 28n during 21 28 september 1999 a strong temperature inversion is observed throughout the cruise along 28n capping the planetary boundary layer pbl that is 1 1 5 km deep temperature response to tiw induced sst changes penetrates the whole depth of the pbl in response to an sst increase air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height as a result this temperature dipole is associated with little tiw signal in the observed sea level pressure slp the cruise mean vertical profiles show a speed maximum at 400 500 m for both zonal and meridional velocities sst based composite profiles of zonal wind velocity show weakened intensified vertical shear within the pbl that is consistent with enhanced reduced vertical mixing causing surface wind to accelerate decelerate over warm cold ssts taken together the temperature and wind soundings indicate the dominance of the vertical mixing over the slp driving mechanism based on the authors measurements a physical interpretation of the widely used pbl model proposed by lindzen and nigam is presented

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

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

fault related folding drainage network evolution and valley incision during the neogene in the andean precordillera of northern chile abstract we describe neogene drainage network development before during and after regional folding in the precordillera of the andes of northern chile 18 19 s the drainage network developed on an extensive regular erosional surface formed on oligocene miocene ignimbrites oxaya formation the folding affecting the rocks of the precordillera is represented by the oxaya anticline oa which trends n to nw is 50 km long and 30 km wide the fold is gentle west vergent and has resulted in an uplift of the hinge zone by up to 850 m it formed by the propagation on its western border of a subvertical east dipping blind reverse fault ausipar fault af the folding occurred in the middle late miocene essentially between 11 7 0 7 and 10 7 0 3 ma its maximum duration was 2 my two phases of drainage development are recognised in the precordillera an initial lower middle miocene 19 12 ma parallel drainage network is well preserved and synchronous with fluvial and lacustrine sedimentation in the central depression the network was poorly structured and attained maximum incision of to 250 50 m the minimum long term rate of incision was 36 7 m my folding generated the progressive abandonment and fossilisation of the initial parallel drainage network which became trellissed and concentrated in a few deeply incised valleys in the anticline hinge the lluta and azapa main valleys present a total incision of 1650 50 and 1605 50 m respectively whereas the cardones valley presents 595 50 m during folding river incision at the fold hinge was equal to or greater than the amplitude of the folding which is 790 50 m in the lluta valley and 715 50 m in the azapa valley consequently the long term rate of synfolding incision was much greater than 395 25 lluta valley and 358 25 m my azapa valley the cardones valley was fossilised approximately in the middle of the folding after folding 11 to 0 ma the incision in the lluta and azapa rivers was 610 100 and 640 100 m and with a rate of 56 9 and 58 9 m my respectively the postfolding incision rate is approximately twice than of the prefolding incision rate this implies an acceleration of valley incision since the late miocene which is considered as consequence of regional uplift of the andean forearc

geochronologic and stratigraphic constraints on canyon incision and miocene uplift of the central andes in peru the deepest valleys of the andes have been cut in southern peru by the rios cotahuasi ocona and colca majes these canyons are late miocene landforms based on a new ignimbrite stratigraphy supported by 42 new 40ar 39ar age determinations obtained on plateau forming and valley filling ignimbrites between 19 and 13 ma a gently sloping surface bevelling the clastic wedge southeast of the developing mountain front was mantled by widespread ignimbrites after 13 ma this paleosurface was tilted up from 2 2 km a s l at the mountain front to 4 3 km a s l at the base of the pliocene and pleistocene volcanoes that crown the southwestern edge of the altiplano the canyons incised this topography after 9 ma while the dated base of younger ignimbrite valley fills suggests that these canyons had been cut down to near their present depths as early as 3 8 ma by 1 4 ma however the canyons had been almost completely refilled by 1 3 km thick unwelded pyroclastic deposits which were subsequently eroded valley incision since 9 ma at an average rate of 0 2 mm yr 1 is the response to topographic uplift after 13 ma combined with increasing runoff due to a wetter climate recorded after 7 ma although long term aridity generated an imbalance between high long term uplift rates and low plateau denudation rates the combination of aridity and volcanism still promoted canyon incision because episodic volcanic fills maintained a cycle of catastrophic debris avalanches and subsequent dam breakouts

statistical aspects of surface drifter observations of circulation in the santa barbara channel argos tracked drifters are used to study the near surface circulation in the santa barbara channel the mean consists of a cyclonic cell in the western santa barbara channel with weaker flow in the eastern channel drifter mean velocities agree well with record means from near surface current meters at the eastern entrance to the channel drifter velocities are biased toward outflow eastward velocity conditions drifter variability at synoptic and seasonal scales shows a tendency for upwelling and eastward flow in spring a strong cyclonic circulation in summer poleward relaxation in fall and weak variable circulation in winter drifter estimates of eddy stress divergence indicate advective terms play a secondary role in the mean surface momentum balance lagrangian time and space scales are about 1 day and under 10 km respectively the mismatch between lagrangian and eulerian timescales indicates advective terms are important to the fluctuating circulation

families of miocene monterey crude oil seep and tarball samples coastal california biomarker and stable carbon isotope ratios were used to infer the age lithology organic matter input and depositional environment of the source rocks for 388 samples of produced crude oil seep oil and tarballs to better assess their origins and distributions in coastal california these samples were used to construct a chemometric multivariate statistical decision tree to classify 288 additional samples the results identify three tribes of 13c rich oil samples inferred to originate from thermally mature equivalents of the clayey siliceous carbonaceous marl and lower calcareous siliceous members of the monterey formation at naples beach near santa barbara an attempt to correlate these families to rock extracts from these members in the nearby cost continental offshore stratigraphic test ocs cal 78 164 well failed at least in part because the rocks are thermally immature geochemical similarities among the oil tribes and their widespread distribution support the prograding margin model or the banktop slope basin model instead of the ridge and basin model for the deposition of the monterey formation tribe 1 contains four oil families having geochemical traits of clay rich marine shale source rock deposited under suboxic conditions with substantial higher plant input tribe 2 contains four oil families with traits intermediate between tribes 1 and 3 except for abundant 28 30 bisnorhopane indicating suboxic to anoxic marine marl source rock with hemipelagic input tribe 3 contains five oil families with traits of distal marine carbonate source rock deposited under anoxic conditions with pelagic but little or no higher plant input tribes 1 and 2 occur mainly south of point conception in paleogeographic settings where deep burial of the monterey source rock favored petroleum generation from all three members or their equivalents in this area oil from the clayey siliceous and carbonaceous marl members tribes 1 and 2 may overwhelm that from the lower calcareous siliceous member tribe 3 because the latter is thinner and less oil prone than the overlying members tribe 3 occurs mainly north of point conception where shallow burial caused preferential generation from the underlying lower calcareous siliceous member or another unit with similar characteristics in a test of the decision tree 10 tarball samples collected from beaches in monterey and san mateo counties in early 2007 were found to originate from natural seeps representing different organofacies of monterey formation source rock instead from one anthropogenic pollution event the seeps apparently became more active because of increased storm activity

regional model simulations of marine boundary layer clouds over the southeast pacific off south america part ii sensitivity experiments abstract the sensitivity of a regional climate model to physical parameterizations and model resolution is investigated in terms of its simulation of boundary layer stratocumulus scu clouds over the southeast pacific specifically the physical schemes being tested include shallow cumulus convection subgrid vertical mixing cloud droplet number concentration cdnc and drizzle as described in part i the model with standard settings captures the major features of the boundary layer in the region including a well mixed marine boundary layer a capping temperature inversion scu clouds and the boundary layer regime transition from the well mixed layer near the coast of south america to a decoupled cloud layer over warmer water to the west turning off the shallow cumulus parameterization results in a dramatic increase in the simulated scu clouds while the boundary layer structure becomes unrealistic losing the decoupled regime over warm water with reduced penetrative mixing at the top of shallow cu

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

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

trench parallel shortening in the northern chilean forearc tectonic and climatic implications in the central andes the frictional coupling between south america and the subducting nazca plate occurs beneath the coastal cordillera of northern chile one of the most distinctive characteristics of the coastal cordillera is a suite of ew topographic scarps located between 19 and 21 6 s latitude these scarps are associated with predominantly south dipping reverse faults that have almost pure dip slip and produce shortening parallel to the plate boundary limited geochronology as well as more regional studies indicate that the scarps formed during the late miocene and early pliocene though some show activity extending into the quaternary in several areas the scarps dammed local drainages producing internally drained basins that accumulated evaporites this relationship indicates that the coastal cordillera was probably moister during the late miocene and pliocene than it is today and also indicates that the coastal cordillera has been significantly uplifted or that the coastal escarpment of northern chile has advanced significantly eastward since the pliocene the limited latitudinal extent of the ew scarps and their location symmetrically about the axis of topographic and wadati benioff zone symmetry suggest that they owe their origin to the concave seaward shape of the continental margin due to prior formation of the bolivian orocline

circulation in the point conception santa barbara channel region during spring and early summer 1983 fifteen current moorings were deployed around the periphery of santa barbara channel and over the continental shelf to the west off point conception data from these moorings have been used to describe the regional circulation and to address problems of wind forcing and the spatial coherency scales for current fluctuations the mean circulation in santa barbara channel was dominated by a westward jet which occupied the northwestern portion of the channel current and temperature fluctuations were generally well correlated among all locations around the channel and time lags suggested cyclonic propagation of events around the channel the time fluctuations were dominated by a relatively few low frequency events it was impossible to determine exactly either the driving mechanisms for these events or their relationships to regional flow the flow regime was considerably different off point conception than in the channel locally wind driven upwelling was observed in both the mean and fluctuating flows and the time scales of current variability i e spectral shapes mimicked those of the local winds a dramatic change in the regime off point conception occurred in late may before the transition currents and sea levels had been well correlated in time with both local and remote winds after late may the relation to fluctuating winds was less clear and the alongshore structure of sea level variability had changed radically

renewed uplift of the central andes forearc revealed by coastal evolution during the quaternary most of the pacific coast of the central andes between 15 s and 30 s displays a wide a couple of kilometres planar feature gently dipping oceanwards and backed by a cliff this morphology usually of marine orgin is called rasa and argues for a recent and spatially continuous uplift of the margin over the 1 500 km long coastal region we describe the cliff foot is found at a similar elevation 110 m amsl all over the studied area with the exception of peninsulas such as the mejillones peninsula the compilation of published chronological data and the extrapolation of re appraised uplift rates provide evidence for a common cliff foot age of around 400 ka i e marine isotopic stage mis 11 this together with other geological constraints indicates a quaternary renewal of uplift in the central andes forearc after a late pliocene quiescence or subsidence

large scale structure of the spring transition in the coastal ocean off western north america past measurements off the coast of central oregon and washington have shown that the rapid change from northward monthly mean winter winds to southward summer winds forces a spring transition of the coastal ocean sea levels and temperatures drop and mean surface currents shift from northward to southward current and water temperature data from 35 n to 48 n from 1981 and 1982 and sea level and wind stress data from 1971 1975 and 1980 1983 show the transition to have a large alongshore scale typically 500 to 2000 km the large scale wind stress appears to be the forcing mechanism at latitudes north of approximately 37 n south of 37 n sea level usually falls more gradually before the northern transition event both wind and sea level events generally progress from south to north over a 3 to 10 day period but this is not always true several aspects of the spring transition reflect coastal trapped wave dynamics previous studies at 45 n found persistent vertical shear of the southward summer current associated with a cross shelf density gradient during 1982 the shear and the density front develop over the shelf break immediately after the transition at 43 n and to the south but they are much less persistent than at 48 n the stronger winds between 38 n and 42 n and the narrower shelf result in an offshore displacement of the density front and vertical shear past the shelf break leaving the water over the shelf less stratified and more subject to barotropic reversals of the current than that farther north where the front stays closer to the coast

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

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

seismic structure and tectonics of the continental wedge overlying the source region of the iquique mw8 1 2014 earthquake abstract on april 1 2014 a large earthquake mw 8 1 ruptured the central part of a historic seismic gap in northern chile in order to study the relationship between the co seismic rupture characteristics and the crustal structure of the subduction zone we processed a trench perpendicular seismic reflection profile acquired across the zone of maximum slip and generated a p wave velocity model the results show a frontal prism in the continental wedge characterized by low velocities that increase rapidly towards the shore and acted as a barrier for trench ward propagation of aftershocks landward a transition zone with increasing upper crust velocity 4 5 km s concentrates most of the aftershocks in addition a trench ward dipping set of fault zones is observed along the continental wedge associated to the iquique forearc basin formation 1 5 km thick at the depocenter on this profile we identify three stratigraphic units within the basin a landward tilt and thickness increase is detected in each stratigraphic unit along with growth strata and domino structures suggesting landward migration of syn extensional deformation in response to basal subduction erosion by extrapolating our results to the plate boundary and based on published focal mechanisms of intra crustal seismicity we find a strong spatial correlation between the iquique basin and the highest slip area for the 2014 earthquake suggesting long term extensional deformation due to coseismic tensional stresses

slow cenozoic uplift of the western andean cordillera indicated by cosmogenic 3he in alluvial boulders from the pacific planation surface to determine the mechanisms responsible for mountain belt growth it is important to accurately establish the timing of surface uplift here we exploit the altitude control on the production rate of in situ cosmogenic nuclides to test the hypothesis that the andes was uplifted in the late miocene high concentrations of in situ cosmogenic 3he 3hecos have previously been measured in alluvial boulders on the western flank of the central andes northern chile evenstar et al 2009 these are consistent with deposition soon after formation of the surface 13 14 ma we have modeled the accumulation of cosmogenic 3he in several different surface uplift scenarios and compared them to the measured concentrations the measured 3hecos concentrations are too high to be produced by late miocene uplift and imply that the western flank of the andean cordillera attained a substantial part of its current elevation prior to 14 myr ago

characteristic patterns of shelf circulation at the boundary between central and southern california 1 the coastal circulation in the santa barbara channel sbc and the southern central california shelf is described in terms of three characteristic flow patterns the upwelling pattern consists of a prevailing equatorward flow at the surface and at 45 m depth except in the area immediately adjacent to the mainland coast in the sbc where the prevailing cyclonic circulation is strong enough to reverse the equatorward tendency and the flow is toward the west in the surface convergent pattern north of point conception the surface flow is equatorward while the flow at 45 m depth is poleward east of point conception along the mainland coast the flow is westward at all depths and there results a convergence at the surface between point conception and point arguello with offshore transport over a distance on the order of 100 km beneath the surface layer the direction of the flow is consistently poleward the relaxation pattern is almost the reverse of the upwelling pattern with the exception that in the sbc the cyclonic circulation is such that the flow north of the channel islands remains eastward although weak the upwelling pattern is more likely to occur in march and april after the spring transition when the winds first become upwelling favorable and while the surface pressure is uniform the surface convergent pattern tends to occur in summer when the wind is still strong and persistently upwelling favorable and the alongshore variable upwelling has build up alongshore surface pressure gradients the relaxation pattern occurs in late fall and early winter after the end of the period of persistent upwelling favorable winds

paleomagnetism of the north patagonian batholith southern chile an exercise in shape analysis abstract over 100 sites in granitic country rock mafic inclusions within granitic rock and mafic dikes were sampled in the northernmost 550 km of the patagonian batholith after magnetic cleaning some sites were rejected as magnetically unstable and others were combined because they were contiguous and carried similar directions hence probably representing duplicate samplings of the same direction of the geomagnetic field a total of 42 well determined directions 29 from granitic country rock and inclusions and 13 from dikes survived to be included in the interpretation most of these had a normal magnetic polarity probably indicating that a disproportionate fraction of the north patagonan batholith formed during the mid cretaceous long normal interval tectonic interpretation of paleomagnetic directions from intrusive rocks often is difficult because paleohorizontal indicators in such rocks are usually lacking rather than struggle against this limitation we yielded to it and elected to concentrate on the shapes of our various data sets the method used is simple and straightforward involving bingham statistics and small circle fitting from this analysis it appears that the main structural feature within the north patagonian batholith npb the liquine ofqui fault zone is a site of crustal detachment a sliver of south america west of the fault is detached from the rest of the continent and has moved relatively northward because the main strands of the liquine ofqui fault are curved and concave to the west northward movement was accompanied by counterclockwise rotation east of the fault the npb appears to have been the locus of distributed shear mainly dextral the principal period of deformation of the batholith appears to have been mid or late cenozoic and may have been initiated by the arrival of the south america nazca farallon phoenix aluk triple junction

a forcing mechanism for the poleward flow off the southern california coast it is shown that when the wind distribution along a coast is anisotropic such that its cross shore scale is smaller than its alongshore scale the coastal sea level or the upper layer anomoly a of an ocean forced by both wind and wind curl is governed by a modified nondimensionalized kelvin wave equation a t a y k0 0 y t k1 y dtl where k0 and k1 are wind stress and wind stress curl at the coast respectively y is the alongshore distance and t is the time numerical experiments from a simple reduced gravity type with idealized forcing and coastline to a three dimensional primitive equation model with a realistic coastline bottom topography of the southern california bight and the santa barbara channel and observed wind stresses were carried out to show that the observed near coast near surface poleward flow in the region is primarily forced by the equatorward weakening of the wind curl k1 y 0 in the bight beta provides natural damping by weakening and widening the current through westward propagating rossby waves and causes the current to lead the coastal pressure field by 1 2 months which improves the agreement with observations of the phasing of the modeled currents but is otherwise not required in forcing the poleward flow

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

oxygen isotope compositions of phosphate from middle miocene early pliocene marine vertebrates of peru phosphatic remains of marine vertebrates recovered from five fossil sites of the pisco formation ranging from the latest middle earliest late miocene ca 11 13 ma to the early pliocene ca 3 5 ma have been analysed for their oxygen isotope compositions 18op coexisting seals dolphins whales penguins and sharks from each locality have distinct 18op values reflecting ecology and physiology differences ranging from 18 2 to 21 4 for marine mammals from 19 5 to 21 5 for marine birds and from 20 9 to 23 1 for sharks systematic offsets observed between dolphin teeth and bones as well as between dolphin and whale bones indicate that the fractionation equation established by using data from extant cetaceans may not be directly applicable to miocene cetaceans in order to estimate water 18ow values assuming that polar ice caps were not totally developed during this time interval marine palaeotemperatures ranging from 13 0 1 3 c to 17 2 1 3 c were estimated comparison of our results with those obtained in other world s areas suggests that the oxygen isotope ratios of pisco vertebrates reflect the influence of both global and local events such as the setting of the atacama desert the cold humboldt current or the global phases of ice cap growth and decay

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

seasonal cycles of currents temperatures winds and sea level over the northeast pacific continental shelf 35 n to 48 n seasonal cycles of coastal wind stress adjusted sea level asl shelf currents and water temperatures off the west coast of north america 35 n to 48 n are estimated by fitting annual and semiannual harmonics to data from 1981 1983 longer records 9 34 years of monthly asl indicate that these two harmonics adequately represent the long term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles we characterize the differences between fall winter and spring summer as follows for fall winter monthly mean winds north of 35 n are northward for 3 6 months longer in the north than in the south south of 35 n the mean winds are near zero or weakly southward monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures for spring summer monthly mean wind stresses are southward at all latitudes for 3 6 months longer in the south than in the north sea levels are low and water temperatures are relatively cool monthly mean currents at 35 m depth over the shelf are southward for 1 6 months longer at the shelf break than over midshelf and longer in the north than in the south while the deeper currents are less southward or northward the magnitudes of the seasonal cycles of all variables are maximum between approximately 38 n and 43 n generally decreasing slightly to the north and greatly to the south at each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1 2 months the seasonal cycles of all variables show a south to north progression south leads north by 1 2 months at 48 n annual mean currents at 50 m depth over the shelf break oppose the annual mean wind northward wind and southward current similarly at 35 n annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind southward wind and northward current from 35 n to 43 n both summer and winter regimes are dominated by strongly fluctuating currents

k ar and 40ar 39ar geochronology of supergene processes in the atacama desert northern chile tectonic and climatic relations new k ar and 40 ar 39 ar data obtained for 29 samples of supergene alunite jarosite and hollandite group minerals from the central depression of northern chile document almost continuous supergene oxidation from 33 to 9 ma the downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m ma 1 these calculated rates are sensitive to pre existing topography and show that the present relief observed in the central depression represents a modified miocene landscape the results presented here are in good agreement with previously published geochronological data for the atacama desert the combination of all published data including the results presented here defines a continuous skewed distribution with a maximum between 21 and 14 ma the relative paucity of ages greater than 30 ma reflects the removal of supergene profiles by erosion during and immediately after the incaic compressive deformation from 29 to 9 ma supergene oxidation profiles were generated under semi arid conditions throughout the atacama region ages between 9 and 5 ma on the northern and southern margins of the desert mark the end of supergene oxidation processes consistent with a pliocene onset of present day hyperaridity

warm upwelling regions in the pliocene warm period 1 given the importance of upwelling processes to coastal productivity and regional climate it is critical to study the role of upwelling regions within the context of global climate change we generated sea surface temperature sst records for the last 5 million years in three important upwelling regions the eastern equatorial pacific the california margin and the peru margin prior to 3 0 ma ssts at all sites were significantly warmer than today by 3 9 c indicating that cold upwelling regions that characterize the modern pacific ocean did not exist in the early pliocene warm period 4 6 to 3 1 ma earth s most recent period of sustained global warmth alkenone phosphorus and organic carbon mass accumulation rate records indicate that changes in productivity and sst were decoupled and that upwelling of nutrient enriched water occurred even when ssts were warm during the early pliocene thus the long term trends in sst are likely explained by changes in the temperature of upwelled water rather than in the strength of upwelling favorable winds alone the fact that gradual cooling of upwelling regions began before the onset of significant northern hemisphere glaciation provides further evidence that the growth of ice sheets and their influence on atmospheric winds alone can not explain the cooling of upwelling regions our results suggest that the long term average ssts of upwelling regions are influenced by global changes in the depth and or temperature of the ventilated thermocline

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

scale of relief growth in the forearc of the andes of northern chile arica latitude 18 s the stream profiles of rivers of northern chile reveal two graded segments separated by 20 km long knickzones their formation was initiated in the late miocene in response to surface uplift of the western flank of the altiplano this phase of uplift that was coeval with the shift of deformation from the altiplano to the sub andean zone caused relief to grow at the scale of the whole drainage basin above and beneath these knickzones the presence of braided channels and the absence of erosion on adjacent pediplains suggest no substantial modification in the local relief the knickzones however show bedrock channels and fluvial dissection rates have exceeded erosion rates on adjacent pediplains by two orders of magnitudes hence the data imply that the only geomorphic recorders of relief growth are the knickzones that currently transpose the effects of the late miocene phase of surface uplift from the coast to the altiplano

invited review paper neogene to quaternary tectonics of the coastal cordillera northern chile abstract the extreme aridity of the northern chilean coastal cordillera enables the complete preservation of permanent deformation related to the coupling between nazca and south america the region between antofagasta and arica is characterized by four types of late cenozoic structures ew striking reverse faults ns striking normal faults sparse wnw and nnw striking right lateral strike slip faults and unimodal and bimodal populations of surface cracks the ew reverse faults occur only between 19 and 21 40 s latitude and have been active since at least 6 ma to present a march 2007 earthquake demonstrates that this deformation can occur during the interseismic phase of the plate boundary seismic cycle the ns normal faults are most abundant near antofagasta and mejillones and diminish in significance northward several of these faults in both the antofagasta and the salar grande area have been reactivated as reverse faults causing minor topographic inversion we suggest that normal faults move during both interseismic and coseismic deformation and are fundamentally very weak surface cracks are ubiquitous throughout the region and form by both non tectonic and tectonic mechanisms a significant proportion forms during coseismic deformation and thus may be used to identify long lived rupture segments on the plate boundary overall forearc deformation is very slow with permanent strain rates of 1 5 nstrain year and fault slip rates less than 0 5 mm year

quantifying the role of paleoclimate and andean plateau uplift on river incision 1 river incision over geologic timescales can be a valuable indicator of regional surface uplift however extracting the timing of surface uplift relative to the onset of incision is complicated by changes in precipitation commensurate with topographic development evidence of large scale river incision on the flanks of the andean plateau has been cited in support of a rapid and recent surface uplift event recent climate modeling studies demonstrate large magnitudes of regional climate change associated with surface uplift of the andes which may have influenced river incision processes here we present an analysis of mid miocene 16 ma to present river incision of the southwest peruvian ocona river a monte carlo approach with 1 6 105 different simulations is used to explore the range of surface uplift and paleoclimate histories that are compatible with the modern river profile and geological constraints on the incision timing and magnitude a range of channel properties including the erodibility coefficient and erosion threshold are considered results indicate that deep canyon incision on the plateau flanks may not be as diagnostic of rapid surface uplift as previously thought more specifically the evolution of the ocona river is consistent with local plateau elevations of 1 3 km at 16 ma and either a steady or punctuated uplift of 1 5 3 5 km since then the range of acceptable uplift histories is sensitive to the long term magnitude and temporal evolution of precipitation similar paleoprecipitation changes are expected to have modified river profile evolution elsewhere in the andes

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

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

non biological fractionation of stable ca isotopes in soils of the atacama desert chile abstract we measured ca stable isotope ratios 44 40 ca in an ancient 2 my hyperarid soil where the primary source of mobile ca is atmospheric deposition most of the ca in the upper meter of this soil 3 5 kmol m 2 is present as sulfates 2 5 kmol m 2 and to a lesser extent carbonates 0 4 kmol m 2 in aqueous extracts of variably hydrated calcium sulfate minerals 44 40 ca e values vs bulk earth increase with depth 1 4 m from a minimum of 1 91 to a maximum of 0 59 the trend in carbonate 44 40 ca in the top six horizons resembles that of sulfate 44 40 ca but with values 0 1 0 6 higher the range of observed ca isotope values in this soil is about half that of 44 40 ca values observed on earth linear correlation among 44 40 ca 34 s and 18 o values indicates either a a simultaneous change in atmospheric input values for all three elements over time or b isotopic fractionation of all three elements during downward transport we present evidence that the latter is the primary cause of the isotopic variation that we observe sulfate 34 s values are positively correlated with sulfate 18 o values r 2 0 78 and negatively correlated with sulfate 44 40 ca e values r 2 0 70 if constant fractionation and conservation of mass with downward transport are assumed these relationships indicate a 44 40 ca fractionation factor of 0 4 in caso 4 the overall depth trend in ca isotopes is reproduced by a model of isotopic fractionation during downward ca transport that considers small and infrequent but regularly recurring rainfall events near surface low ca isotope values are reproduced by a rayleigh model derived from measured ca concentrations and the ca fractionation factor predicted by the relationship with s isotopes this indicates that the primary mechanism of stable isotope fractionation in caso 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events

beryllium 10 concentrations in the hyper arid soils in the atacama desert chile implications for arid soil formation rates and el nino driven changes in pliocene precipitation abstract meteoric 10be concentrations in soil were measured to understand the mechanism timescale and climatic dependence of soil formation in the hyper arid atacama desert the observed systematic decline of soil 10be concentrations with depth has been reproduced using a simple model that assumes soil matrix including 10be builds up as layers over time while 10be decays in situ this suggests a mechanism of soil accumulation via atmospheric deposition which is in agreement with stable isotopic evidence the model estimates an age of 6 6 0 4 ma for the total soil profile small discrepancies between the model and observations are likely mainly due to changes in precipitation rates that can impact 10be delivery rates and 10be movement within the profile interpreted in this way the 10be data suggest drying in the atacama after 4 7 ma and returning to an insignificant wet period at 1 ma which was possibly connected to el nino or la nina like climate change

slow long term exhumation of the west central andean plate boundary chile we present a regional analysis of new low temperature thermochronometer ages from the central andean fore arc to provide insights into the exhumation history of the western andean margin to derive exhumation rates over 10 million year timescales 38 new apatite and zircon u th he ages were analyzed along six 500 km long near equal elevation coast parallel transects in the coastal cordillera cc and higher elevation precordillera pc of the northern chilean andes between latitudes 18 5 s and 22 5 s these transects were augmented with age elevation profiles where possible results are synthesized with previously published thermochronometric data corroborating a previously observed trenchward increase in cooling ages in peru and northern chile one dimensional thermal kinematic modeling of all available multichronometer equal elevation samples reveals mean exhumation rates of 0 2 km myr since 50 ma in the pc and 100 ma in the cc regression of pseudovertical age elevation transects in the cc yields comparable rates of 0 05 to 0 12 km myr between 40 and 80 ma differences between the long term mean 1 d rates and shorter term age elevation derived rates indicate low variability in the exhumation history modeling results suggest similar background exhumation rates in the cc and pc younger ages in the pc are largely a function of increased heat flow and consequently an elevated geothermal gradient near the arc slow exhumation rates are suggestive of semiarid conditions across the region since at least the eocene and deformation and development of the andean fore arc around this time

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

late pliocene age for the atacama desert implications for the desertification of western south america the atacama desert forms one of the major hyperarid deserts of the world previous studies suggest that desertification commenced at 14 ma during global climate desiccation sedimentologic data from middle miocene to upper pliocene successions in the modern atacama desert indicate that a semiarid climate persisted from 8 to 3 ma punctuated by a phase of increased aridity at ca 6 ma as such hyperaridity did not commence until the late pliocene implications are 1 that the rain shadow generated by the andean cordillera has had a minor influence on climate change and 2 that the upwelling north flowing cold humboldt current although important in establishing the generally arid climate of western south america did not control the shift to hyperaridity the formation of the hyperarid atacama desert in the late pliocene accompanied the development of the current phase of aridification in the sahara and namib deserts and is attributed to a phase of global climate cooling

rapid regional surface uplift of the northern altiplano plateau revealed by multiproxy paleoclimate reconstruction the central altiplano is inferred to have experienced 2 5 1km surface uplift between 10 and 6 ma while the southern altiplano experienced a similar magnitude of surface uplift that began earlier between 16 and 9 ma to properly constrain the along strike timing of the altiplano plateau surface uplift it is necessary to know how and when the northernmost part of the altiplano plateau evolved we reconstruct the paleoclimate and infer the corresponding paleoelevation from the miocene pliocene deposits of the descanso yauri basin 14 15 s in the northernmost part of the altiplano plateau using 4 different proxies including carbonate clumped isotope composition i e 47 values carbonate 18 o c leaf wax d wax and pollen assemblages from paleosol lacustrine and palustrine carbonates and organic rich sediments the isotopic signatures reflect past climate conditions of mean annual air temperature 47 and meteoric water isotope values 18 o c d wax our results show that the northernmost plateau remained at low elevation 0 9 0 8 to 2 1 0 9km until late miocene time 9 ma characterized by 15 c warmer than modern temperature mean annual air temperature of 23 4 c 2 low elevation vegetation and precipitation signature with reconstructed 18 o mw vsmow of 8 3 2 0 2 from carbonate 18 o c and 8 6 1 8 2 from leaf wax d wax modern elevations of 4 km were not reached until 5 4 1 0ma as indicated by a negative shift in d wax vsmow from 143 4 12 8 2 to 209 2 21 1 2 between 9 1 0 7 and 5 4 1 0ma the timing of surface uplift of the northernmost altiplano is consistent with the evidence for late miocene surface uplift of the central altiplano 16 19 s between 10 and 6 ma and indicates that regional scale uplift in the northern central plateau significantly postdates the onset of surface uplift in the southern altiplano 19 22 s between 16 and 9 ma these results are consistent with piecemeal removal of the lower dense lithosphere combined with possible lower middle crustal flow from south to north in the plateau acting as the main mechanisms for the formation of the altiplano plateau

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

temperature and carbonate ion effects on mg ca and sr ca ratios in benthic foraminifera aragonitic species hoeglundina elegans received 16 march 2005 revised 20 june 2005 accepted 3 november 2005 published 21 february 2006 1 core top samples from atlantic little bahama banks lbb and pacific hawaii and indonesia depth transects have been analyzed in order to assess the influence of bottom water temperature bwt and aragonite saturation levels on mg ca and sr ca ratios in the aragonitic benthic foraminifer hoeglundina elegans both the mg ca and sr ca ratios in h elegans tests show a general decrease with increasing water depth although at each site the decreasing trends are consistent with the in situ temperature profile mg ca and sr ca ratios in lbb are substantially higher than in indonesia and hawaii at comparable water depths with a greater difference observed with increasing water depth because we find no significant difference between results obtained on live and dead specimens we propose that these differences are due to primary effects on the metal uptake during test formation evaluation of the water column properties at each site suggests that in situ co3 ion concentrations play an important role in determining the h elegans mg ca and sr ca ratios the co3 ion effect is limited however only to aragonite saturation levels dco3 aragonite below 15 mmol kg 1 above this level temperature exerts a dominant effect accordingly we propose that mg ca and sr ca in h elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships mg ca 0 034 0 002 bwt 0 96 0 03 and sr ca 0 060 0 002 bwt 1 53 0 03 for dco3 aragonite 1 5mmol kg 1 the standard error associated with these equations is about 1 1 c reconstruction of deeper water temperatures is complicated because in undersaturated waters changes in mg ca and sr ca ratios reflect a combination of changes in co3 and bwt overall we find that sr ca rather than mg ca in h elegans may be a more accurate proxy for reconstructing paleotemperatures

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable

late cenozoic structural and tectonic development of the western margin of the central andean plateau in southwest peru 1 structural and thermochronologic studies of the western margin of the central andean plateau show changing styles of deformation through time that give insights into tectonic evolution in southwest peru uplift of the plateau proceeded in several distinct phases first nw striking ne dipping reverse faults accommodated uplift prior to 14 16 ma subsequent uplift of the plateau relative to the piedmont between the plateau and the pacific ocean occurred between 14 and 2 2 ma and was accommodated by nw striking sw dipping normal faults and subparallel monoclinal folds the youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block although the uplift magnitude associated with phase 1 is unknown phases 2 and 3 resulted in at least 2 4 3 0 km of uplift up to 1 km of this may have occurred during phase 3 geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift

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

wind relaxations and poleward flow events in a coastal upwelling system on the central california coast 1 when upwelling favorable winds weaken or relax in coastal upwelling systems prevailing equatorward currents often weaken and then reverse to form propagating poleward currents here the statistics of wind relaxations and poleward flow events along the south central california coast are derived using meteorological and oceanographic times series during 2000 2006 over the 7 year record 169 wind relaxations were observed about 1 every 2 weeks and 127 of these were followed by poleward flow events thermistor moorings and current profilers along the 15 m isobath at alegria point arguello point purisima and point sal recorded the poleward flows the poleward flows propagate northward at 10 30 km d 1 and appear as sequential temperature increases at the moorings wind relaxations occur throughout the year but are most frequent in september and least frequent in april when upwelling winds are strong and persistent poleward flows follow wind relaxations frequently during may through november and rarely in december and january sea level differences between santa monica and port san luis california decreased as winds relaxed consistent with forcing by alongshore pressure gradients temperature distributions at point arguello point purisima and point sal were skewed toward higher values because of the poleward flows the alongshore distance traveled by the poleward flows increased with duration of the wind relaxations and magnitudes of alongshore temperature and sea level differences prior to the relaxations

a comment on rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere by c n garzione et al earth planet sci lett 241 2006 543 556

87sr 86sr of calcium sulfate in ancient soils of hyperarid settings as a paleoaltitude proxy pliocene to quaternary constraints for northern chile 19 5 21 7 s

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system