petroleum formation by fischer tropsch synthesis in plate tectonics a somewhat speculative hypothesis of petroleum genesis in the upper lithosphere is proposed based on fischer tropsch synthesis this hypothesis is distinct from both the organic biogenic model and the inorganic model of hydrocarbon degassing from the earth s interior fischer tropsch synthesis is a well known industrial process whereby millions of tons of hydrocarbon oil resembling petroleum are produced annually from carbon monoxide or carbon dioxide and hydrogen gases reacting on a metallic iron or iron oxide catalyst like natural petroleum this synthetic oil consists of gas gasoline diesel oil and wax fractions all rich in saturated aliphatic hydrocarbons and enriched in the light 12c isotope the hypothesis presented in this paper proposes that petroleum liquids form by fischer tropsch synthesis on magnetite and hematite catalysts when carbon dioxide derived by massive metamorphic or igneous decarbonation of subducted sedimentary carbonates reacts with hydrogen generated by the serpentinization in the absence of air of shallow mantle lithosphere and ophiolite thrust sheets oblique plate movements may favor hydrocarbon formation by creating deep faults that aid fluid flow and serpentinization the world s richest oil provinces including those of the middle east may be tentatively interpreted to have formed by this mechanism

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

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

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

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

erosional processes topographic length scales and geomorphic evolution in arid climatic environments the lluta collapse northern chile the lluta collapse of northern chile is one of the oldest recognizable landslides 2 5 ma in a hyperarid continental setting this paper develops a conceptual landscape evolution model of the lluta collapse and analyzes the controls of mass wasting and erosion sediment transport in channels on the topographic development the data presented here imply that high relief along a topographic scarp surface fracturing elevated groundwater table during a more humid climate and an aquitard underlying permeable ignimbrites are preparatory causal factors for landsliding 2 5 ma ago a strong seismic event then possibly resulted in the displacement of ca 26 km3 of mass subsequent modification of the landslide scar occurred by backward erosion resulting in the establishment of a dendritic drainage network and the removal of an additional ca 24 km3 of material it appears that this mass was produced by mass wasting in the headwaters and exported by high concentrated debris flows in channels in addition morphometric information suggest that whereas the geometrical development of the lluta collapse has been controlled by gravitational mass wasting the rates of the development of this geomorphic unit have been limited by the export rates of mass and hence by the transport capacity of the flows

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

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

comparing dust flux records from the subarctic north pacific and greenland implications for atmospheric transport to greenland and for the application of dust as a chronostratigraphic tool we present a new record of eolian dust flux to the western subarctic north pacific snp covering the past 27 000 years based on a core from the detroit seamount comparing the snp dust record to the north greenland ice core project ngrip ice core record shows significant differences in the amplitude of dust changes to the two regions during the last deglaciation while the timing of abrupt changes is synchronous if dust deposition in the snp faithfully records its mobilization in east asian source regions then the difference in the relative amplitude must reflect climate related changes in atmospheric dust transport to greenland based on the synchronicity in the timing of dust changes in the snp and greenland we tie abrupt deglacial transitions in the 230th normalized 4he flux record to corresponding transitions in the well dated ngrip dust flux record to provide a new chronostratigraphic technique for marine sediments from the snp results from this technique are complemented by radiocarbon dating which allows us to independently constrain radiocarbon paleoreservoir ages we find paleoreservoir ages of 745 140 years at 11 653 year b p 680 228 years at 14 630 year b p and 790 498 years at 23 290 year b p our reconstructed paleoreservoir ages are consistent with modern surface water reservoir ages in the western snp good temporal synchronicity between eolian dust records from the subantarctic atlantic and equatorial pacific and the ice core record from antarctica supports the reliability of the proposed dust tuning method to be used more widely in other global ocean regions

landscape response to the intentional use of the birds point new madrid floodway on may 3 2011 during the spring of 2011 massive coalescing storm fronts in the upper mississippi and ohio river basins caused extreme weather conditions that flooded the bottleneck section of the mississippi river in the vicinity of cairo il in order to alleviate the volume of floodwaters acting on downstream levees and to stop flooding near cairo il the us army corps of engineers activated the birds point new madrid floodway by intentionally breaching the birds point levee releasing floodwaters onto active agricultural fields the impacts of floodwater on the landscape were regionally significant concentrated erosion generated deep scour holes rills and gullies in localized areas of the floodway inflow and outflow of waters formed large deposits of sands and gravels sand sheets of a few centimeters to a meter thick blanketed the vicinity of the impacted areas and in some cases sand accumulations exhibited current ripples indicating multiple flow directions this study directly addresses geomorphic changes in the new madrid floodway resulting from activation and the diversion of floodwaters from the mississippi river results from this study can be used along with other field data to conduct a thorough flood damage assessment that includes the costs associated with geomorphic alteration within the floodway

radiocarbon marine reservoir effect in human remains from the kitakogane site hokkaido japan abstract accelerator mass spectrometry ams was used to measure radiocarbon 14 c in bones excavated from the late jomon shell midden at the kitakogane site hokkaido japan comparison between 14 c ages of terrestrial and marine mammals from the same site showed systematic age differences which could be attributed to the 14 c marine reservoir effect in the western north pacific ocean furthermore this effect was clearly observed in human remains from this site indicating a heavy dependence on seafood dietary habit was estimated from 13 c and 15 n measured in the same human remains apparent 14 c ages indicated 79 of protein originated from marine sources

groundwater controlled valley networks and the decline of surface runoff on early mars 1 fluvial erosion on early mars was dominated by valley networks created through a combination of groundwater processes and surface runoff a reduced greenhouse effect due to co 2 loss together with a declining geothermal heat flux promoted the growth of a cryosphere and a hesperian hydrologic regime dominated by outflow channel formation we test the hypothesis that the transition from valley network to outflow channel formation was preceded by a more subtle evolution characterized by a weakening of surface runoff leaving groundwater processes as the dominant final source of valley network erosion our hypothesis supported by a terrestrial analog in the atacama desert of chile is related to the groundwater sapping reactivation hypothesis for densely dissecting highland valley networks on mars suggested by baker and partridge in 1986 and focuses on the age analysis of large sparsely dissecting valley networks such as nanedi valles nirgal vallis valleys in fretted terrain and tributaries of outflow channels and valles marineris chasmata we find that these features are consistently late noachian to hesperian in age younger than noachian densely dissecting dendritic valley networks in the southern highlands in the tharsis region the observation of dense and sparse valley network morphologies on hesperian terrain suggests that while surface runoff gave way to groundwater processes consistent with our hypothesis the transition may have occurred later than elsewhere on the planet the volcanic nature of tharsis suggests that geothermal heat and volatile production led to episodically higher volumes of surface runoff in this region during the hesperian

accretion of moon and earth and the emergence of life the discrepancy between the impact records on the earth and moon in the time period 4 0 3 5 ga calls for a re evaluation of the cause and localization of the late lunar bombardment as one possible explanation we propose that the time coverage in the ancient rock record is sufficiently fragmentary so that the effects of giant sterilizing impacts throughout the inner solar system caused by marauding asteroids could have escaped detection in terrestrial and martian records alternatively the lunar impact record may reflect collisions of the receding moon with a series of small original satellites of the earth and their debris in the time period about 4 0 3 5 ga the effects on earth of such encounters could have been comparatively small the location of these tellurian moonlets has been estimated to have been in the region around 40 earth radii calculations presented here indicate that this is the region that the moon would traverse at 4 0 3 5 ga when the heavy and declining lunar bombardment took place the ultimate time limit for the emergence of life on earth is determined by the effects of planetary accretion existing models offer a variety of scenarios ranging from low average surface temperature at slow accretion of the mantle to complete melting of the planet followed by protracted cooling the choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere further exploration of the sedimentary record on earth and mars and of the chemical composition of impact generated ejecta on the moon may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life

regional groundwater flow and accumulation of a massive evaporite deposit at the margin of the chilean altiplano focused groundwater discharge in closed basins provides opportunities to investigate mechanisms for closing hydrologic and solute budgets in arid regions the salar de atacama sda adjacent to the central andean plateau in the hyperarid atacama desert provides an extreme example of halite 1800 km3 and lithium brine 5 000 ppm accumulation spanning late miocene to present minimum long term water discharge needed to sustain halite accumulation over this timescale at sda is 9 20 times greater than modern recharge and double wet climate paleo recharge within the topographic watershed closing this imbalance requires sourcing water from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed prolonged water discharge at sda requires long residence times deep water tables in recharge zones coupled with persistent near surface water tables in discharge areas and large contributing areas characterized by strong gradients in landscape and climate resulting from plateau uplift

the violent environment of the origin of life progress and uncertainties dating of terrestrial fossils and returned lunar samples reveals that the origin of life on earth occurred not in a quiescent peaceful environment but rather in a violent impact ridden one this realization has important consequences on the one hand sufficiently large and fast impactors can erode planetary atmospheres and the very largest of these may have sterilized the surface of the earth in this regard deep sea hydrothermal vents become especially interesting for the history of early life as they provide an environment protected against all but the greatest impact devastation at the same time impactors would have been delivering key biogenic elements such as carbon and nitrogen to earth s surface and with much greater difficulty intact organic molecules as well estimates of the various sources of prebiotic organics suggest that the heavy bombardment either produced or delivered quantities of organics comparable to those produced by other energy sources however substantial uncertainties exist after reviewing the current understanding of the role of the heavy bombardment in the origins of life a number of remaining key uncertainties are considered and attempts are made to both quantify their magnitude and point to means of resolving them

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

paleoceanographic control on a large marine reservoir effect offshore of tokai south of japan nw pacific during the last glacial maximum deglaciation abstract knowledge of the marine reservoir effect is important in calibrating radiocarbon ages to calendar ages and in correlating geologic and paleoceanographic events among different sites including global events large explosive volcanic eruptions have yielded numerous volcanic ash layers tephras over geological time because tephra layers are deposited both on land and upon the seafloor they represent a unique and important link between the geological records of terrestrial and marine areas comparison of the radiocarbon ages of a tephra layer deposited in marine planktonic foraminifera and terrestrial peat materials reveals a local marine reservoir age 600 700 years or more during the early part of the last deglaciation off tokai south of japan this age is larger than that determined for the present day pacific coast of japan 82 years and similar to that of the present day subarctic nw pacific 350 580 years this difference between modern day and deglaciation marine reservoir values is thought to reflect the contrasting properties of surface water masses between the two periods

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

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

modeling of middle pliocene climate with the ncar genesis general circulation model abstract a climate modeling study was carried out which used boundary conditions created by the usgs prism group the model used for this study is the national center for atmospheric research ncar genesis climate model genesis is an atmospheric general circulation model gcm that is coupled to a land surface model and contains submodels for snow and soil boundary conditions incorporated into the model which were specific to the middle pliocene 3 ma are 1 present day continent configuration modified by a 35 m rise in sea level 2 modified present day elevations 3 reduced continental ice cover on both greenland and antarctica 4 pliocene vegetation and 5 pliocene sea surface temperatures ssts and sea ice distribution atmospheric pco 2 and orbital parameters were set at current values the greatest influence of the specified warm ssts upon the resulting climate was occurrence of warmer and more humid high latitudes than observed in the current climate especially in the winter season the global mean surface temperature was 3 6 c warmer than in a present day control case and global mean precipitation increased by 5 relative to the control case with most excess precipitation occurring over the african continent and the oceans zonal winds weakened slightly in the pliocene case but hadley cell extent and jet stream locations were unchanged relative to the control case model results predict specific areas in which middle pliocene climate should have differed substantially from the present climate which can be tested with paleoclimate data

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

effect of regional slope on drainage networks drainage networks that develop under conditions of no structural control and homogeneous lithology are generally dendritic depending upon the shape and inclination of the surface on which they form an experimental study was designed to investigate the effect of an increase of slope on the evolution and development of dendritic drainage patterns as slope steepens the pattern changes from dendritic at 1 slope to subdendritic at 2 to subparallel at 3 to parallel at 5 and higher the change from a dendritic type pattern to a parallel type pattern occurs at a low slope between 2 and 3 and primary channel junction angles decrease abruptly from about 60 to 43

origin and development of theater headed valleys in the atacama desert northern chile morphological analogs to martian valley networks abstract understanding planetary landforms including the theater headed valleys box canyons of mars usually depends on interpreting geological processes from remote sensing data without ground based corroboration here we investigate the origin and development of two mars analog theater headed valleys in the hyperarid atacama desert of northern chile previous workers attributed these valleys to groundwater sapping based on remote imaging topography and publications on the local geology we evaluate groundwater sapping and alternative hypotheses using field observations of characteristic features strength measurements of strata exposed in headscarps and estimates of ephemeral flood discharges within the valleys the headscarps lack evidence of recent or active seepage weathering such as spring discharge salt weathering alcoves or vegetation their welded tuff caprocks have compressive strengths multiple times those of the underlying epiclastic strata flood discharge estimates of cubic meters to tens of cubic meters per second derived using the manning equation are consistent with the size of transported clasts and show that the ephemeral streams are geomorphically effective even in the modern hyperarid climate we interpret that headscarp retreat in the quebrada de quisma is due to ephemeral flood erosion of weak miocene epiclastic strata beneath a strong welded tuff with erosion of the tuff facilitated by vertical jointing the quebrada de humayani headscarp is interpreted as the scar of a giant landslide maintained against substantial later degradation by similar strong over weak stratigraphy this work suggests that theater headed valleys on earth and mars should not be attributed by default to groundwater sapping as other processes with lithologic and structural influences can form theater headscarps

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

radiocarbon reservoir effects in human bone collagen from northern iceland human bone collagen from a series of icelandic human pagan graves was radiocarbon 14c dated to aid understanding of early settlement landnam chronologies in northern iceland these individuals potentially consumed marine protein the 14c age of samples containing marine carbon requires a correction for the marine 14c reservoir effect the proportion of non terrestrial sample carbon was quantified via measurement of carbon stable isotopes 13c using a simple mixing model based on 13c measurements of archaeofaunal samples non terrestrial carbon was also quantified in six pig bones from the archaeofaunal dataset assuming all non terrestrial carbon in human and pig bone collagen was marine derived calibrated age ranges calculated using a mixed intcal09 marine09 calibration curve were consistent with an early settlement date close to landnam but several samples returned pre landnam age ranges measurements of nitrogen stable isotopes 15n strongly suggest that many of the human bone collagen samples contain freshwater diet derived carbon icelandic freshwater systems frequently display large freshwater 14c reservoir effects of the order of 10 000 14c years and we suggest that the presence of freshwater carbon is responsible for the anomalously early ages within our dataset in pig samples the majority of non terrestrial carbon is freshwater in origin but in human samples the proportion of freshwater carbon is within the error of the marine component 10 this presents a major obstacle to assessing temporal patterns in the ages of human remains from sampled graves although the majority of grave ages are within the same broad calibrated range

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

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

solute and isotope geochemistry of subsurface ice melt seeps in taylor valley antarctica the mcmurdo dry valleys of antarctica are a polar desert region with watersheds dominated by glacial melt recent ground exploration reveals unusual surface fl ow seep features not directly supplied by glacial melt much of this seep water is potentially derived from permafrost snow patches refrozen precipitation accumulated in the subsurface buried glacier ice or even groundwater from the deep subsurface flow features that lack obvious glacier melt sources were identifi ed in archived aerial photographs of taylor valley this valley was surveyed for extant and extinct seeps and the locations of geomorphic features in fi active seeps were documented water samples from seeps were analyzed for major ions and stable isotopes of hydrogen and oxygen solute chemistry and isotopic signatures of seeps are distinct from those of nearby streams and glaciers with the seeps having elevated solute concentrations all but one seep had water isotopically heavier than water from nearby glaciers and streams suggesting that seep waters have been substantially modifi ed if they had been derived originally from the same meteoric sources that supply local glaciers and streams the seeps are important because they compose a previously overlooked component of the desert hydrological cycle seep features in the dry valleys are potential terrestrial analogs for the geologically young gullies observed on mars which are thought to be evidence of groundwater seepage and surface runoff

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

evolution of the west andean escarpment at 18 s n chile during the last 25 ma uplift erosion and collapse through time abstract the geological record of the western andean escarpment warp reveals episodes of uplift erosion volcanism and sedimentation the lithological sequence at 18 s comprises a thick pile of azapa conglomerates 25 19 ma an overlying series of widespread rhyodacitic oxaya ignimbrites up to 900 m thick ca 19 ma which are in turn covered by a series of mafic andesite shield volcanoes between 19 and 12 ma the surface of the oxaya ignimbrites evolved into a large monocline on the western slope of the andes a giant antithetically rotated block oxaya block 80 km 20 km formed on this slope at about 10 12 ma and resulted in an easterly dip and a reversed drainage on the block s surface morphology topography and stratigraphic observations argue for a gravitational cause of this rotation a secondary gravitational collapse 50 km3 extending 25 km to the west occurred on the steep western front of the oxaya block alluvial and fluvial sediments 11 2 7 ma accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall indicating further shortening between 8 and 6 ma flatlying upper miocene sediments

marine reservoir correction for the pacific coast of central japan using 14c ages of marine mollusks uplifted during historical earthquakes in this study we utilize marine shell samples from two levels of historically uplifted sessile mollusk assemblages and raised wave cut benches to evaluate the marine reservoir correction r for the pacific coast of central japan elevation measurements of the uplifted marine shells indicate that the lower assemblage emerged during the 1923 taisho kanto earthquake m7 9 whereas uplift of the upper assemblage is most likely but less confidently ascribed to the 1703 genroku kanto earthquake m8 2 radiocarbon dating of carefully selected samples from the upper and lower assemblages yielded very similar r values of 82 33 and 77 32 yr respectively we regard the former r value as a representative and more reliable value given the uncertainty in correlation of the upper assemblage with the 1703 earthquake this result is consistent with previously reported r values for the pacific coast of south central japan and areas around the sea of japan that are influenced by warm ocean currents radiocarbon dating of coseismically uplifted shells can aid in estimating marine reservoir ages in the tectonically active japan islands

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

metasomatic origin of quartz pyroxene rock akilia greenland and implications for earth s earliest life a quartz pyroxene rock interpreted as a banded iron formation bif from the island of akilia southwest greenland contains 13 c depleted graphite that has been claimed as evidence for the oldest 3850 million years ago life on earth field relationships on akilia document multiple intense deformation events that have resulted in parallel transposition of early archean rocks and significant boudinage the tails of which commonly form the banding in the quartz pyroxene rock geochemical data possess distinct characteristics consistent with an ultramafic igneous not bif protolith for this lithology and the adjacent schists later metasomatic silica and iron introduction have merely resulted in a rock that superficially resembles a bif an ultramafic igneous origin invalidates claims that the carbon isotopic composition of graphite inclusions represents evidence for life at the time of crystallization

origin of life from apatite dating mojzsis et al 1 2 reported the carbon isotope composition of carbonaceous inclusions in grains of apatite from sediment sequences of akilia island southwest greenland that are more than 3 850 million years myr old the 13c values measured by mojzsis et al 1 led them to conclude that these carbonaceous materials are evidence of early life 3 850 myr ago but if other isotopes u pb and pb pb are used the apatites are estimated at just 1 504 336 2 and 1 459 160 2 myr old this value is consistent with measurements of 1 600 1 700 myr for rb sr mineral isochrons on biotites3 and 1 670 myr for k ar muscovite4 from amitsog gneiss in the region we conclude that about 1 500 myr ago these apatites in akilia island experienced a metamorphic event of about 600 c estimated by the closure temperature of the u pb system5 6

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

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

morphometric analysis of martian valley network basins using a circularity function 1 this paper employs a circularity function to quantify the internal morphology of martian watershed basins in margaritifer sinus region and to infer the primary erosional processes that led to their current geomorphologic characteristics and possible climatic conditions under which these processes operated the circularity function describes the elongation of a watershed basin at different elevations we have used the circularity functions of terrestrial basins that were interpreted as having been modified by 1 erosion related to primarily groundwater sapping and 2 erosion related to primarily rainfall and surface run off as well as the circularity functions of cratering basins on the moon in order to formulate discriminant functions that are able to separate the three types of landforms the spatial pattern of the classification of martian basins based on discriminant functions shows that basins that look morphologically similar to terrestrial fluvial basins are mostly clustered near the mainstream at low elevation while those that look morphologically similar to terrestrial basins interpreted as groundwater sapping origin are located near the tributaries and at higher elevation there are more of the latter than the former this spatial distribution is inconsistent with a continuous earth like warm and wet climate for early mars instead it is more aligned with an overall early dry climate punctuated with episodic wet periods alternatively the concentrated erosion in the mainstream could also be caused by a change of water source from rainfall to snowfall or erosion cut through a duricrust layer

massive middle miocene gypsic paleosols in the atacama desert and the formation of the central andean rain shadow abstract the central andes of south america are one of the highest and broadest mountain ranges on earth and have a profound influence on climate in the southern hemisphere yet the timing of andean uplift and its subsequent impact on atmospheric circulation is not well known here we use fossil soils from along an 800 km transect 19 25 s in the atacama desert of northern chile to identify the initiation of extreme aridity along the west coast of south america and infer a minimum age for the development of the central andean rain shadow paleosols record the onset of extreme aridity at the boundary between the middle and late miocene 12 10 ma lower miocene 24 20 ma soils are chemically weathered and contain soil carbonate indicative of a vegetated landscape and semi arid climate in contrast gypsum dominates massive 2 5 m thick upper middle miocene soils that formed on a hyperarid landscape devoid of vegetation the development of extreme hyperaridity is linked to the formation of the central andean rain shadow through the divergence of oxygen isotope values oxygen isotope values of lower miocene soil carbonates in the atacama desert 5 0 1 1 vpdb and central andes 7 8 1 are similar indicating no or a limited rain shadow by 15 mya however 18 o values of soil carbonate differ by 8 with values of soil carbonate in altiplano paleosols getting lower due to uplift while soil carbonate values in the atacama desert get higher due to aridity therefore by 15 mya there is clear evidence for a rain shadow from the central andes by the latest middle miocene ca 12 10 ma paleosols in the altiplano decreased to 11 8 1 0 while soils in the atacama desert increase to 1 5 4 5 this almost 14 divergence in the 18 o values of soil carbonate for soils only 250 km apart is evidence of a strong central andes rain shadow effect in place during the late middle miocene 12 10 ma

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

marine climatic responses to neogene tectonics of the pacific ocean seaways abstract based upon surface marine climatic evolution on both sides of the pacific ocean recently revealed by biochronostratigraphy in japan and south america the tectonics of pacific ocean seaways and associated responses of adjacent seas and ocean currents are discussed the rapid opening of the sea of japan at about 15 ma in early middle miocene time mainly by the rotation of the southwest japan arc can be correlated with the abrupt appearance of a cold water fauna on the pacific coast of northeast japan beginning ca 15 ma and an extensive hiatus along the pacific coast of southwestern japan spanning 15 ma to 12 ma the abrupt and extensive development of biosiliceous lithofacies along the pacific coast of peru beginning about 14 ma might also represent a response to the tectonic anticlockwise rotation of the peruvian block of the andean mountain range estimated to have occurred sometime during the miocene as the tectonic closing of the indonesian seaway would have strongly intensified the warm kuroshio current in the northwest pacific it is most probable that this seaway was effectively closed just prior to 15 5 ma in the earliest middle miocene when tropical marine faunas were developed in most of the japanese islands the closing of the central american seaway is estimated to have been completed at 3 5 3 ma in pliocene time the closure of this seaway would likely have intensified the warm kuroshio current affecting the marine fauna along the pacific coast of southwestern japan it is also possible that a decrease in surface temperatures at about 3 ma along the pacific coast of ecuador was due to this pliocene tectonic event

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

climate change and tectonic uplift triggered the formation of the atacama desert s giant nitrate deposits the giant nitrate deposits of the hyperarid atacama desert chile are one of the most extraordinary yet enigmatic mineral occurrences on earth these deposits are complex assemblages of highly soluble nitrates chlorides sulfates perchlorates iodates and chromates and their preservation is the result of prevalent hyperarid climate conditions in the atacama desert since the late miocene with average rainfall rates of 10 mm yr in the past 3 m y although several hypotheses have been proposed since the mid 1800s the formation of these extensive deposits still remains highly controversial despite the fact that recent studies have argued toward an atmospheric source for the nitrate sulfate and perchlorate components in this report we focus on the often overlooked and poorly studied iodine and chromium components of atacama s nitrates we present the fi rst cosmogenic iodine 129 i and stable chromium 53 52 cr isotope data of nitrates showing that groundwater has played an unforeseen role in the formation of these massive deposits the isotopic signature of i in the nitrates 129 i i 150 600 10 15 share similarities with deep sedimentary marine pore waters and shales deviating signifi cantly from atmospheric iodine 129 i i 1500 10 15 while the positive and highly fractionated 53 52 cr srm979 values 0 7 to 3 are indicative of intense cr redox cycling due to groundwater transport our evidence points toward a multi source genetic model for the atacama desert nitrate deposits where these extensive accumulations were the result of long lived near surface mineral precipitation driven by groundwater i e chromates iodates coupled with dry atmospheric deposition i e nitrates perchlorates and sea spray inputs i e sulfates chlorides triggered by increasing aridity and tectonic uplift

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

ams 14c chronology of the world s southernmost woolly mammoth mammuthus primigenius blum the world s definite southernmost woolly mammoth record is a molar from ji nan around 36 degrees n shandong province china ams c 14 dating of the specimen gave a conventional c 14 age of 33 150 250 bp the period of 40 30 ka bp corresponds to the later phase of the marine oxygen isotope stage 3 mis 3a recognized as the global interstadial of the last glacial period however it is known that the winter monsoon strengthened in asia during the period 35 33 ka bp and the age of the woolly mammoth specimen from ji nan corresponds to that age the specimen suggests that this area became cold and dry in 33 ka bp and grassland or open forest suitable habitat for woolly mammoths developed during this short time span this age is similar to the age of the southernmost woolly mammoth record in europe therefore supporting a hypothesis by porter and an 1995 correlation between climate events in the north atlantic and china during the last glaciation nature 375 305 308 that an important component of chinese palaeoclimates may be linked to changes in north atlantic oceanic conditions c 2007 elsevier ltd all rights reserved

andean growth and the deceleration of south american subduction time evolution of a coupled orogen subduction system article i nfo present day orography at the andean margin is a result of isostasy tectonic accretion and erosional processes the resulting excess mass of the andes gives rise to frictional stresses on the seismogenic plate interface that resist the sinking of the subducting slab into the upper mantle thus subduction rates should be sensitive to the time dependent dynamics of a back arc orogen as well as erosional or accretional processes that affect orogen growth here we develop a two dimensional coupled orogen slab model that allows for the prediction of orogen size and plate motion in response to both tectonic and erosional forcing we find that the frictional force exerted by the orogen on the subducting slab grows quadratically with orogen width and that the frictional resistance typically balances 10 50 of the slab pull force the time evolution of the coupled orogen subduction zone system is largely controlled by the rate of orogen growth which is controlled by the rate of convergence and the erosivity of the climate state in the case of the andean margin our models show that miocene aridification leads to reduced erosion increased orogen growth greater frictional resistance to subduction and ultimately to a 50 reduction in the convergence rate between the nazca and south american plates

timing and nature of alluvial fan development along the chajnantor plateau northern chile abstract alluvial systems in the atacama desert provide a unique opportunity to elucidate the sedimentary response to climate variability particularly changes in precipitation in hyperarid environments alluvial fans along the eastern margin of the salar de atacama adjacent to the chajnantor plateau in the atacama desert of northern chile provide an archive of climate modulated sediment transfer and erosion at an extreme of earth s climate three regional alluvial fan surfaces qf1 oldest to qf3 youngest were mapped along the western flank of the chajnantor plateau the alluvial fans were examined with geomorphic and terrestrial cosmogenic 36 cl surface exposure dating methods to define the timing of alluvial fan formation and to determine the role of climatic processes on fan development in a hyperarid environment alluvial fans in the study area are comprised of hyperconcentrated flow and boulder rich debris flow deposits that reflect deposition transitioning between cohesive and noncohesive regimes alluvial fan surfaces yield exposure ages that range from 49 6 4 4 to 194 12 ka while debris flow boulders yield exposure ages ranging from 12 4 2 1 to 229 53 ka cosmogenic 36 cl exposure ages indicate that abandonment of alluvial fan surfaces qf1 qf2 and qf3 date to 175 22 6 ka mis 6 134 5 9 18 ka mis 6 and 20 07 6 26 ka mis 2 respectively a 36 cl concentration depth profile through alluvial fan qf1 suggests a simple depositional history with minimal nuclide inheritance implying relatively rapid aggradation 6 m in ca 25 kyr followed by surface abandonment ca 180 200 ka our data support a strong climatic control on alluvial fan evolution in the region and we propose that the alluvial fans along the margins of the salar de atacama form according to the humid model of fan formation

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

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

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

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

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

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

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

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

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

vestiges of life in the oldest greenland rocks a review of early archean geology in the godthabsfjord region and reappraisal of field evidence for 3850 ma life on akilia abstract the godthabsfjord region of west greenland contains the most extensive best exposed and most intensely studied early archean rocks on earth a geological record has been described of numerous magmatic events between 3 9 and 3 6 ga and evidence of life at 3 85 ga and 3 8 3 7 ga has been proposed from two widely separated localities some of these claims have recently been questioned and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the isua greenstone belt are being reinvestigated and substantially reinterpreted the first part of this article reviews the evolution of geological research and interpretations outlining the techniques by which the geological history has been determined and the ensuing controversies the second part re examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of akilia

the cretaceous tertiary boundary event in ecuador reduced biotic effects due to eastern boundary current setting a multidisciplinary study of a new cretaceous tertiary k t boundary section near guayaquil ecuador reveals an unusually cool water low diversity planktic foraminiferal fauna and a high diversity radiolarian fauna similar to those found in southern high latitude k t sequences despite the fact that this section was deposited near the cretaceous equator the k t boundary is located by planktic foraminifera within a narrow interval bounded by last appearances of tropical cretaceous species and first appearances of tertiary species including parvularugoglobigerina eugubina as in southern high latitudes there is no major mass extinction of either planktic foraminifera or radiolarians at this level a major radiolarian faunal discontinuity occurs some 6 m higher in the section within foraminiferal zone plc some 300 500 kyr after the k t event 13c values from bulk carbonates show both high and low latitude characteristics similarly to low latitudes there is a 3 negative 13c excursion at the k t boundary which is generally interpreted as a major decrease in primary productivity but unlike the low latitudes recovery occurs within a few thousand years as compared with 300 500 kyr and suggests rapid nutrient influx from the antarctic region via a current similar to the humboldt current today similarly to high latitude k t sequences a negative 13c shift occurs in the early danian zone plb about 300 kyr after the k t boundary sedimentologic and mineralogic data indicate a late maastrichtian with relatively low biogenic quartz and high carbonate followed by increasing biogenic quartz 50 and decreasing carbonate 5 during the early danian this suggests intensified atmospheric and oceanic circulation and upwelling off ecuador during the early danian the k t transition is marked by increased volcanic activity continental erosion and terrigenous influx but this also occurs in the early danian zone at the p1ap1b zonal transition and is thus not unique to the k t boundary we suggest that the catastrophic biotic effects normally observed at the k t boundary in low latitudes are greatly reduced or absent in the eastern equatorial pacific because this region was dominated then as now by upwelling and current transport of nutrient rich waters from the antarctic ocean as a result the biotic patterns are characteristic of southern high latitudes whereas the 13c pattern combines ameliorated low latitude effects with predominantly high latitude trends

a record of past ocean atmosphere radiocarbon differences from the northeast pacific a record of the 14c age differences between atmosphere and the mixed layer of the northeast pacific ocean was obtained from 58 radiocarbon measurements on shells and associated wood from coastal sediments in british columbia this series of mixed layer reservoir ages spans the holocene and extends into the period of the younger dryas cold event in the north atlantic and surrounding regions the data reflect the extent of upwelling in the northeast pacific and thus constrain models of carbon cycling and ocean circulation most of the reservoir ages are only slightly less than than the present preindustrial value of about 800 years and may be identical when probable sampling errors are taken into account either the north pacific reached a state close to its present mode of circulation at or before the end of the younger dryas or a different circulation fortuitously produced a similar reservoir age our results support the hypothesis that slow changes in atmospheric radiocarbon concentrations over the holocene arose from 14c production rate changes data corresponding to a radiocarbon age of around 6400 years b p give a reservoir age of 1200 years if this result is confirmed it indicates a much greater variability for the holocene ocean circulation than was previously believed possible

late pleistocene fans and terraces in the majes valley southern peru and their relation to climatic variations this study investigates the connection between sediment aggradation erosion and climate in a desert environment of the majes valley southern peru luminescence dating of terraces and fans shows that sediment aggradation correlates with wet time intervals on the altiplano suggesting a climatic influence on the aggradation degradation cycles major periods of aggradation occurred between 110 100 60 50 and 12 8 ka more precipitation in the majes catchment resulted in increased erosion and transportation of sediment from the hillslopes into the trunk river as a result the sediment loads exceeded the transport capacity of the majes river and aggradation started in the lower reaches where the river gradient is less depletion of the hillslope sediment reservoirs caused a relative increase in the capacity of the trunk river to entrain and transport sediment resulting in erosion of the previously deposited sediment consequently although climate change may initiate a phase of sediment accumulation degradation can be triggered by an autocyclic negative feedback and does not have to be driven by climatic change

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

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

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

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

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

on the formation of dissolution pipes in quaternary coastal calcareous arenites in mediterranean settings a large number of uniform cone shaped dissolution pipes has been observed and studied in quaternary coastal calcareous arenites in apulia and sardinia italy and tunisia these cylindrical tubes have a mean diameter of 52 8 cm and are up to 970 cm deep mean depth for sediment free pipes is 1 38 m they generally have smooth walls along their length are perfectly vertical and taper out towards their bottoms their development is not influenced by bedding nor fractures sometimes their walls are coated by a calcrete crust their morphology has been studied in detail and their relationships with the surrounding rocks and with the environment have been analysed the perfectly vertical development is a clear evidence of their genesis controlled by gravity the depth of the dissolution pipes can be described by an exponential distribution law the milanovic distribution strongly suggesting they developed by a diffusion mechanism from the surface vertically downward we believe dissolution pipes preferentially form in a covered karst setting local patches of soil and vegetation cause infiltration water to be enriched in carbon dioxide enhancing dissolution of carbonate cement and local small scale subsidence this process causes the formation of a depression cone that guides infiltrating waters towards these spots giving rise to the downward growth of gravity controlled dissolution pipes a change of climate from wetter phases to drier and hotter ones causes the formation of a calcrete lining fossilizing the pipes when the pipes become exposed to surface agents by erosion of the sediment cover or are laterally breached the loose quartz sand filling them may be transported elsewhere copyright 2010 john wiley sons ltd

cenozoic polyphase landscape and tectonic evolution of the cordillera occidental southernmost peru landforms on the pacific slope of the cordillera occidental western cordillera in southernmost peru reflect the episodic epeirogenic uplift of the central andes since the earliest miocene in the geomorphic history three major stages are distinguished initial uplift followed a mid tertiary period of tectonic and magmatic quiescence in which a low lying subdued landscape formed through dissection of an eocene subaerial volcano plutonic terrain uplift and resulting erosional beveling of this landscape coinciding in their later stages with the explosive reactivation of the magmatic arc gave rise to the regionally extensive lower miocene altos de camilaca surface i which was mantled soon after its formation by ignimbrites of the huaylillas formation major relief was achieved in the middle and early late miocene when a succession of uplift episodes generated a sequence of four discrete pediments constituting the multiple pediment stage ii uplift was accompanied by widespread eruption of ignimbrites of the chuntacala formation subsequently in the pliocene pedimentation was succeeded as the major erosional process by canyon incision across the lower slopes of the cordillera during this valley and terrace stage iii the immediate littoral margin experienced the development of two major terraces contemporaneous with a transition from ignimbrite eruption to predominantly andesitic volcanism in the high cordillera in the early pleistocene during this time it is probable that epeirogenic uplift rather than glacio eustatic sea level changes exerted the dominant control on landform development in the absence of evidence for large scale crustal compression in this immediate region since the cretaceous it is inferred that neogene uplift resulted from episodic thickening of the continental crust through underplating by basic and or intermediate magmas the neogene chronology of landform development herein defined is remarkably similar to that outlined in other transects of the central andes implying that the continental margin has on a broad scale responded uniformly to plate subduction since the early miocene however the low relief evident in the mid tertiary in the study area and not for instance in northern chile 27 s suggests that crustal thickening through the neogene was more rapid and more extensive in the vicinity of the arica deflection than in transects to the north and south the estimated neogene uplift rate for the transect was in the range 0 06 to 0 19 mm yr

canyon incision chronology based on ignimbrite stratigraphy and cut and fill sediment sequences in sw peru documents intermittent uplift of the western central andes abstract based on an 40ar 39ar and u pb based chronostratigraphy of ignimbrite sheets and the geomorphological features of watersheds river profiles and slope deposits in the ocona cotahuasi maran ocm and colca valleys of southwest peru we reconstruct the valley incision history of the western central andes over the last c 25 myr we further document the pleistocene and holocene evolution of deep valleys on the basis of 14 10be surface exposure ages obtained on debris avalanche deposits and river straths the data suggest that uplift was gradual over the past 25 myr but accelerated after c 9 ma valley incision started around 11 9 ma and accelerated between 5 and 4 ma incision was followed by several pulses of valley cut and fill after 2 3 ma evidence presented suggest that the post 5 ma sequence of accelerated canyon incision probably resulted from a combination of drainage piracy from the cordilleran drainage divide towards the altiplano accentuated flexural tilting of the western cordillera towards the se and increased rainfall on the altiplano after late miocene uplift of the eastern cordillera the valley deepening and slope steepening driven by tectonic uplift gave rise to large occurrences of rockslope failure the collapsed rock masses periodically obstructed the canyons thus causing abrupt changes in local base levels and interfering with the steadiness of fluvial incision as a result channel aggradation has prevailed in the lower gradient u shaped pacific rim canyons whereas re incision through landslide deposits has occurred more rapidly across the steeper v shaped upper valleys existing canyon knickpoints are currently arrested at the boundary between the plutonic bedrock and widespread outcrops of middle miocene ignimbritic caprock where groundwater sapping favouring rock collapse may be the dominant process driving headward erosion

tectonic record of strain buildup and abrupt coseismic stress release across the northwestern peru coastal plain shelf and continental slope during the past 200 kyr 200 kyr both segments uplifted at high rates of 10 to 20 mm yr 1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e respectively the uplift and related extensive emersion of the coastal plain require high coupling along the subduction zone and or underplating at depth offshore industry acquired reflection seismic lines combined with em12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf major dipping seaward detachments control the long term subsidence of this area these main tectonic features define a tectonic segmentation the talara paita and sechura segments are identified from north to south no clear tectonic correlation in time exists between the onshore and the continental margin segmentations or in space either the longterm subsidence of the offshore indicative of subduction erosion working at depth requires low coupling along the subduction channel at depth the distribution of permanent deformation along the northern peru forearc area includes long term uplift along the coastal plain and long term subsidence along the continental margin the neutral line being located within the 10 km seaward from the present coastline an extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step 20 23 ka to present along the cabo blanco segment is related to a sequence of major earthquakes we infer that eustacy exerts important feedback coupling to the seismogenic behavior of the north peru subduction zone we speculate that during sea level fall pore fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone i e migration of the updip limit reaching a maximum by the end of the eustatic low stand during eustatic sea level rise pore fluid pressure increases along the subduction channel this in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence earth s orbital variations are a potential external cause that may control the physical processes at work along plate interface

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

andean growth and monsoon winds drive landscape evolution at sw margin of south america abstract in the atacama desert the driest place on earth located at the subduction margin of the andes the landscape evolves very slowly and changes in tectonic or erosion processes remain for a long time in the memory of topography at latitude 19 30 s a threshold between exoreic and endoreic drainage regimes is clearly associated with the latitudinal gradient imposed by the modern monsoon carrying humidity from the atlantic and disposed obliquely over catchments draining the andes to the pacific we summarize the geomorphic geological and climatic data in the threshold area we then use these data to constrain numerical experiments of drainage evolution data and experimental results are consistent with the development of a flat low energy morphology close to sea level interrupted at 10 ma by tectonic uplift prevailing to the present suggesting trench ward relief growth by incorporation of the coastal atacama region to the andes mountain belt

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

evidence for the development of the andean rain shadow from a neogene isotopic record in the atacama desert chile abstract varying ages from triassic to pliocene have been proposed for the onset of hyperaridity in the atacama desert the exact timing for the initiation of hyperaridity is critical for determining potential causes which range from regional effects of global cooling to andean uplift above elevations conducive to extreme rain shadows analysis of the stable isotopic composition of lower miocene quaternary 21 0 015 ma palustrine and lacustrine carbonates in the calama basin reveals extreme changes in their oxygen and carbon isotopic composition during the miocene limestone 18 o values increased by 5 from middle to late miocene ranging from 5 5 at 12 ma to 1 at 6 ma carbon isotopic values increase by 9 over the neogene from average values of 3 at 21 ma to 3 at 12 ma and reaching a maximum of 6 at 5 ma the increase in oxygen isotopic values occurred over a time span in which the catchment area of the basin experienced significant uplift causing the 18 o value of precipitation to become more negative we attribute the shift towards higher 18 o values to enhanced evaporative enrichment both of soil water or snow prior to infiltration and within shallow lakes or wetlands prior to carbonate precipitation the large increase in 13 c values was likely caused by a transition from a vegetated landscape influenced primarily by soil respired co 2 to a landscape largely devoid of vegetation and influenced by atmospheric and volcanic co 2 isotopic values of palustrine carbonates therefore indicate that hyperaridity commenced in the calama basin during the middle to late miocene in agreement with other paleoclimatic records from the basin the cause for the onset of this climate change is thought to be due to the development of a strong andean rain shadow associated with the uplift of the andes to mean elevations 2 km

vegetation invasions into absolute desert a 45 th000 yr rodent midden record from the calama salar de atacama basins northern chile lat 22 24 s plant macrofossils percentage abundance of grass taxa fecal pellet d13 c and plant cuticle contents from 49 fossil rodent middens dated by 14 c record changes in local vegetation and precipitation since 45 ka calibrated or measured to thousands of calendar years before present in the central atacama desert lat 228 248s of northern chile the midden sites are along the hyperarid upper margin 2400 3100 m of the absolute desert in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs conditions between 40 and 22 ka may have been at least intermittently dry and possibly cooler as implied by four middens with low species richness we infer a large increase in summer rainfall between 16 2 and 10 5 ka on the basis of the lowering of steppe grasses by as much as 1000 m prominence of c4 grasses and summer annuals high species richness and displacement of northern species at least 50 km south of their modern ranges the precipitation increase was greatest for a cluster of middens between 11 8 and 10 5 ka abrupt drying evident in a dramatic decrease in grass abundance occurred after 10 5 ka at all four midden localities increased percentages of

comparing morphologies of drainage basins on mars and earth using integral geometry and neural maps 1 we compare morphologies of drainage basins on mars and earth in order to confine the formation process of martian valley networks basins on both planets are computationally extracted from digital topography integral geometry methods are used to represent each basin by a circularity function that encapsulates its internal structure the shape of such a function is an indicator of the style of fluvial erosion we use the self organizing map technique to construct a similarity graph for all basins the graph reveals systematic differences between morphologies of basins on the two planets this dichotomy indicates that terrestrial and martian surfaces were eroded differently we argue that morphologies of martian basins are incompatible with runoff from sustained homogeneous rainfall fluvial environments compatible with observed morphologies are discussed we also construct a similarity graph based on the comparison of basins hypsometric curves to demonstrate that hypsometry is incapable of discriminating between terrestrial and martian basins