geochronology stratigraphy and geochemistry of cindery tuff in pliocene hominid bearing sediments of the middle awash ethiopia cindery tuff is a subalkaline rhyolitic air fall deposit that was probably produced by a mixed magma eruption it is a distinctive datable regional isochronous marker bed within the pliocene sediments of the middle awash district and is stratigraphically situated between two new fossil hominid discoveries based on 40ar 39ar analyses of plagioclase rhyolitic glass and basaltic glass as well as fission track analyses of zircons we estimate its age to be 3 8 4 0 myr this implies that associated hominid skull fragments are at least 3 9 myr old

extensive carbonate algal bioherms in upper pleistocene saline lakes of the central altiplano of bolivia during the upper pleistocene the central altiplano of bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands discontinuously covering the 300 km long and 100 km wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of uyuni distribution size and shape of the bioherms are diverse due to various factors e g the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth on larger palaeoterraces the build ups coalesced to form platform like carbonate accumulations although the morphologies closely resemble those induced by cyanobacteria they were predominantly constructed by other plant communities probably dominated by filamentous green algae cyanobacterial communities flourished in association with these plants but they did not contribute significantly to the architecture of the bioherms they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build ups a prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects the build ups located between 3660 and 3680 m elevation display up to three major parts 1 a massive inner core formed during an early minchin highstand before 40 ka 2 a large peripheral envelope deposited at about 40 ka late minchin and 3 a thinner outermost crust formed during a late glacial event lake level dropped during interlacustrine stages sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system i e the present day salar of uyuni

viscosity structure of the crust and upper mantle in western nevada from isostatic rebound patterns of the late pleistocene lake lahontan high shoreline received 13 july 2005 revised 26 october 2006 accepted 31 january 2007 published 8 june 2007 1 large lakes can both produce and record significant crustal deformation we present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake lahontan in western nevada using a layered maxwell viscoelastic model the inferred viscosity structure depends on loading history we use three variants of a well documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure a simple two layer model with an elastic plate over an inviscid half space fits the observed elevation pattern quite well with a residual variance of 32 of the data variance using multilayered finite viscosity models the residual variance is reduced to 20 of the data variance which is very near to the noise level in the higher resolution models the viscosity is below 10 18 pa s over the depth range from 80 to 160 km the minimum viscosity is very similar to the value that has been seen in the eastern great basin from similar analyses of lake bonneville shorelines but the lowviscosity zone is thinner beneath bonneville making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations additionally we find that small variations in proposed loading models can result in presumably spurious density inversions and suggest that this modeling approach provides a test for loading histories

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

light element geochemistry and spallogenesis in lunar rocks abstract the abundances and isotopic compositions of carbon nitrogen and sulfur were measured in eleven lunar rocks samples were combusted sequentially at three temperatures to resolve terrestrial contamination from indigenous volatiles sulfur abundances in apollo 16 highland rocks range from 73 to 1165 g g whereas sulfur contents in apollo 15 and 17 basalts range from 719 to 1455 g g and correlate with tio 2 content lunar rocks as a group have a remarkably uniform sulfur isotopic composition which may reflect the low oxygen fugacity of the basaltic magmas much of the range of reported 34 s cd values 2 to 2 5 permil is caused by systematic analytical discrepancies between laboratories lunar rocks very likely contain less than 0 1 g g of nitrogen the measured spallogenic production rate 4 1 10 6 g 15 n g sample m y agrees remarkably closely with previous estimates an estimate which includes all available data is 3 7 10 6 g 15 n g sample m y lunar basalts may contain no indigenous lunar carbon in excess of procedural blank levels 0 7 g g highlands rocks consistently release about 1 to 5 g g of carbon in excess of blank levels but this carbon might either derive from ancient meteoritic debris or be a mineralogie product of terrestrial weathering the average measured spallogenic 13 c production rate is 4 1 10 6 g 13 c g sample m y the 13 c spallation exposure ages of rocks 15058 and 15499 are 184 and 135 m y respectively

deglaciation and holocene climate change in the western peruvian andes pollen charcoal magnetic susceptibility and bulk density data provide the first paleoecological record spanning the last 33 000 years from the western cordillera of the peruvian andes sparse super puna vegetation existed before 30 000 cal yr b p around lake compuerta 3950 m elevation prior to a sedimentary hiatus that lasted until c 16 200 cal yr b p when sedimentation resumed a glacial foreland or super puna flora is represented in which polylepis was a significant element glacial outwash marked by high sedimentary magnetic susceptibility increased from c 16 200 cal yr b p and reached a peak at c 13 200 cal yr b p between c 12 500 cal yr b p and 10 000 cal yr b p magnetic susceptibility was reduced vegetation shifts suggest a cool dry time consistent with regional descriptions of the younger dryas event deglaciation resumes by 10 000 cal yr b p and the last ice is lost from the catchment at 7500 cal yr b p during the early holocene warm and dry period between 10 000 and 5500 cal yr b p alnus expanded in downslope forests alnus declined in abundance at 5500 cal yr b p when wetter and cooler conditions returned and human activity intensified maize zea mays pollen first occurred in the core at 2600 cal yr b p indicating a minimum age for local agriculture an increase in alnus pollen abundance at 1000 cal yr b p could be due to human activity or perhaps due to a regional climate change associated with cultural turnover elsewhere in the andes at this time

a 130 ka reconstruction of rainfall on the bolivian altiplano new efforts to link climate reconstructions from shoreline deposits and sediment cores yield an improved and more detailed lake history from the bolivian altiplano on the southern altiplano 10 lake oscillations have been identified from this new unified chronology each coincident with north atlantic cold events such as heinrich events h5 h2 h1 and the younger dryas by coupling this new lake history to a hydrologic budget model we are able to evaluate precipitation variability on the southern bolivian altiplano over the last 130 ka these modeling efforts underscore the relative aridity of the altiplano during the rare and small lake cycles occurring between 80 and 20 ka when colder temperatures combined with little or no change in rainfall produced smaller paleolakes relative aridity between 80 and 20 ka contrasts with the immense tauca lake cycle 18 1 14 1 ka which was six times larger than modern lake titicaca and coincided with heinrich event 1 this improved paleolake record from the southern altiplano reveals a strong link between central andean climate and atlantic sea surface temperature gradients during the late pleistocene even though today rainfall variability is driven mostly by pacific sea surface temperature anomalies associated with el nino southern oscillation however not all heinrich events appear to result in lake expansions most conspicuously during the global cold interval between 80 and 20 ka when the altiplano and amazon basin were relatively arid

avulsion processes at the terminus of low gradient semi arid fluvial systems lessons from the rio colorado altiplano endorheic basin bolivia abstract the rio colorado dryland river system in the southeast of the endorheic altiplano basin bolivia terminates on a very flat coastal plain at the edge of the salar de uyuni the world s largest salt pan with an area of ca 12 500 km 2 since the pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods respectively at present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the rio colorado onto the former lake bottom the present field study of the terminus of the rio colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth this bankfull decrease is caused by the combined effects of 1 extremely low gradient of the lake bottom and hence loss of flow energy and 2 downstream transmission losses due to high evaporation potential and river water percolation through the channel floor peak water discharge in seasonal short duration rain periods causes massive overbank flooding and floodplain inundation on satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels however field observations show that only one channel is active at low flow stage and at high flow stage an abandoned partially infilled channel may be active as well the active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain the rest of the channels which form the divergent network are older sediment filled abandoned sinuous river courses with multiple random avulsion points these channel deposits together with extensive amalgamated crevasse splay deposits form an intricate network of fluvial sand deposits successive stages of progressively deeper crevasse channel incision into the floodplain are the result of waning stage return flow of floodwaters towards the main channel this results in increased crevasse channel width depth and length and redeposition of eroded lacustrine coastal plain sediment at the junction of the crevasse channel with the present day river a sedimentary model is proposed in which deep river bank incision by return flow in crevasse channels creates the preferential locus for river avulsion in a lowstand systems tract with little vertical accommodation increase the recurring avulsed river paths result in a thin but laterally extensive network of amalgamated channel fill point bar crevasse channel and crevasse splay deposits the resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

a 350014c yr high resolution record of water level changes in lake titicaca bolivia peru abstract sediment cores collected from the southern basin of lake titicaca bolivia peru on a transect from 4 6 m above overflow level to 15 1 m below overflow level are used to identify a new century scale chronology of holocene lake level variations the results indicate that lithologic and geochemical analyses on a transect of cores can be used to identify and date century scale lake level changes detailed sedimentary analyses of subfacies and radiocarbon dating were conducted on four representative cores a chronology based on 60 accelerator mass spectrometer radiocarbon measurements constrains the timing of water level fluctuations two methods were used to estimate the14c reservoir age both indicate that it has remained nearly constant at 25014c yr during the late holocene core studies based on lithology and geochemistry establish the timing and magnitude of five periods of low lake level implying negative moisture balance for the northern andean altiplano over the last 3500 cal yr between 3500 and 3350 cal yr b p a transition from massive inorganic clay facies to laminated organic matter rich silts in each of the four cores signals a water level rise after a prolonged mid holocene dry phase evidence of other significant low lake levels occurs 2900 2800 2400 2200 2000 1700 and 900 500 cal yr b p several of the low lake levels coincided with cultural changes in the region including the collapse of the tiwanaku civilization

isotopic tracers of paleohydrologic change in large lakes of the bolivian altiplano we have developed an 87sr 86sr 234u 238u and 18o data set from carbonates associated with late quaternary paleolake cycles on the southern bolivian altiplano as a tool for tracking and understanding the causes of lake level fluctuations distinctive groupings of 87sr 86sr ratios are observed ratios are highest for the ouki lake cycle 120 95 ka at 0 70932 lowest for coipasa lake cycle 12 8 11 4 ka at 0 70853 and intermediate at 0 70881 to 0 70884 for the salinas 95 80 ka inca huasi 45 ka sajsi 24 20 5 ka and tauca 18 1 14 1 ka lake cycles these sr ratios reflect variable contributions from the eastern and western cordilleras the laca hydrologic divide exerts a primary influence on modern and paleolake 87sr 86sr ratios waters show higher 87sr 86sr ratios north of this divide most lake cycles were sustained by slightly more rainfall north of this divide but with minimal input from lake titicaca the coipasa lake cycle appears to have been sustained mainly by rainfall south of this divide in contrast the ouki lake cycle was an expansive lake deepest in the northern poopo basin and spilling southward these results indicate that regional variability in central andean wet events can be reconstructed using geochemical patterns from this lake system

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

non conservative behavior of bromide in surface waters and brines of central andes a release into the atmosphere abstract the transfer of reactive bromine into the atmosphere was recently observed by honninger et al honninger g bobrowski n palenque e r torrez r platt u 2004 reactive bromine and sulfur emission at salar de uyuni bolivia geophys res lett 31 doi 10 1029 2003gl018818 in a large salt pan of the bolivian altiplano the salar de uyuni however bromide is considered to be an excellent conservative tracer which leads to the questioning of its actual conservation in surficial geochemical processes the relation between bromide and lithium thought to be a conservative component in waters and brines of the central andes points to a depletion of br relative to li in uyuni brines of about 50 300 kg day a flux close to that measured by honninger et al 2004 200 kg day such values are very low in regard to the size of the salar 5 30 g km 2 day salt efflorescences have a much higher surface area than a flat salt crust which should enhance the release of bromine leach solutions of salt efflorescences in closed basins of northern chile are compared to their parent waters conservative components should have the same concentration ratios in both solutions actually a strong depletion in br is observed in the leach solutions which could suggest a significant release of br from the salt into the atmosphere during the rainy season efflorescences are leached and their dissolved components brought in saline lakes and salars evaporative profiles show a slight but noticeable fractionation between li and br which could be due to the contribution of br depleted leach solutions of salt efflorescences therefore bromide does not behave conservatively in surface waters and brines of the central andes if the loss of br is really due to its transfer into the atmosphere then the flux would be much higher than that estimated for the salar de uyuni alone numerous salt pans saline lakes and widespread efflorescences covering large land surfaces would contribute significantly more reactive bromine into the atmosphere

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

martian gases in an antarctic meteorite significant abundances of trapped argon krypton and xenon have been measured in shock altered phases of the achondritic meteorite elephant moraine 79001 from antarctica the relative elemental abundances the high ratios of argon 40 to argon 36 2000 and the high ratios of xenon 129 to xenon 132 2 0 of the trapped gas more closely resemble viking data for the martian atmosphere than data for noble gas components typically found in meteorites these findings support earlier suggestions made on the basis of geochemical evidence that shergottites and related rare meteorites may have originated from the planet mars

bromine geochemistry of salar de uyuni and deeper salt crusts central altiplano bolivia abstract the salar of uyuni in the central bolivian altiplano is probably the largest salt pan in the world 10 000 km 2 a 121 m deep well drilled in the central salar disclosed a complex evaporitic sequence of 12 salt crusts separated by 11 mud layers in the lower half of the profile thick halite beds alternate with thin mud layers the mud layers thicken upwards and show clear lacustrine features the thickness of the salt beds decreases markedly from the base upward the bromine content of the halite ranges from 1 3 to 10 4 mg kg the halite does not originate from the evaporation of the dilute inflow waters of the altiplano which would lead to br content of tens of mg kg the presence of halite of very low br content 2 mg kg in a gypsum diapir strongly suggests that most of the halite deposited at uyuni originated from the leaching of ancient salt formations associated with the numerous gypsum diapirs of the altiplano the deep and thick halite beds were probably deposited in a playa lake as suggested by their very low and fairly constant br content 1 6 2 3 mg kg and by the abundance of detrital minerals thereafter perennial salt lakes of increasing duration flooded the central altiplano increasing amounts of dilute inflow waters of low cl br ratio and the exhaustion of diapiric halite lead to an increase of both the thickness of the lacustrine layers and of the bromine content of the salt crusts 5 10 mg kg deposited as the lakes dried up paleolake levels in the central altiplano rose as those in the northern altiplano the titicaca basin were simultaneously falling the progressive fluvial erosion of the threshold between the northern and the central altiplano lowered the levels of the northern lakes and allowed more dilute waters of low cl br ratio to flood the central altiplano therefore both the thickness of lacustrine sediments and the bromine content of the salt crust increased to the present the three upper crusts show a decrease in bromine content which may reflect a recent modification of the inflow regime to the last lakes the salt crust deposited by the last salt lake 10 000 years ago shows a bromine distribution that unexpectedly increases with depth as the lake dried the salt crust was probably initialy distributed over a wider area than the present flat crust covering the topographic surface up to the level of the lake where halite reached saturation the dissolution of the initial salt of low br content from the margins of the lake to the center is probably responsible for the observed br distribution in the surface salt as in a playa during the holocene a severe drought hit the altiplano and drastically lowered the intracrustal brine table within the salt crust leaving a thin layer of high concentrated residual brine at the bottom of the crust bromine rich halite replaced primary halite by a temperature activated mechanism in the lower crust further increasing the br gradient with depth

topography of the salar de uyuni bolivia from kinematic gps the salar de uyuni in the bolivian andes is the largest salt flat on earth exhibiting less than 1 m of vertical relief over an area of 9000 km 2 we report on a kinematic global positioning system gps survey of a 45 by 54 km area in the eastern salar conducted in september 2002 to provide ground truth for the ice cloud and land elevation satellite icesat mission gps post processing included corrections for long period gps noise that significantly improved survey accuracy we fit corrected gps trajectories with 2 d fourier basis functions from which we created a digital elevation model dem of the surface whose absolute accuracy we estimate to be at least 2 2 cm rmse with over two magnitudes better vertical resolution than the shuttle radar topography mission data this dem reveals decimetre level topography that is completely absent in other topographic data sets longer wavelengths in the dem correlate well with mapped gravity suggesting a connection between broad scale salar topography and the geoid similar to that seen over the oceans

primary igneous carbon in ureilites petrological implications the ureilite meteorites are carbonaceous olivine pyroxene achondrites they typically contain up to 4 wt carbon carbonaceous matrix as graphite diamond and lonsdaleite shock degradation has effectively obliterated primary textures in the carbonaceous matrix of previously described ureilites a factor that has hampered efforts to explain the origin of this material in contrast the antarctic ureilite alha78019 displays perfectly preserved primary textures in the carbonaceous matrix characterized by euhedral graphite blades intergrown with fe ni metal and sulfide diamonds are absent this petrographic feature suggests that most graphite in ureilites originated by crystallization from a c rich metallic phase assuming that fo2 is controlled by c co co2 reactions the compositions of silicates and metals in ureilites imply a two stage redox history the noble gases and rare earths of ureilites are discussed in light of this model

revisiting the chronology and environmental conditions for the accretion of late pleistocene early holocene pampean loess argentina abstract the present research work reviews the age models of three previously studied loess sections across 700 km in the loess belt of the pampean region in central argentina the loess sections were investigated for the first time through high resolution luminescence dating in order to determine the chronology of the loess deposits and its temporal correlation with important palaeoclimatic records of the southern hemisphere reliable estimations of mass accumulation rates mars for loess were used to infer the temporal dust flux variation during the last glacial interglacial transition in southern south america the new age models have significant differences compared to the previously published ones of likely last glacial maximum lgm ages the three studied sections showed general bimodal grain size distributions consistent with multiple sources contributing dust to the pampas it is observed that higher mars values are associated with an increased presence of finer loess increases in loess accumulation are coeval with climatic transitions from wetter to drier periods in the puna altiplano plateau and high mars are related to synchronous climatic shifts to humid conditions in the pampean plain contrasting with dust fluxes observed in the more distal palaeoarchives i e the south atlantic ocean and the antarctic ice sheet the new data suggest increased dust accumulation in the pampas during the antarctic cold reversal acr and the holocene this supports the idea that changes related to atmospheric transport efficiency can better explain dust flux variations observed over glacial interglacial periods in distant palaeoarchives

a 14kyr record of the tropical andes the lago chungara sequence 18 s northern chilean altiplano abstract high resolution geochemical analyses obtained using an x ray fluorescence xrf core scanner as well as mineralogical data from the lago chungara sedimentary sequence in the northern andean chilean altiplano 18 s provided a detailed reconstruction of the lacustrine sedimentary evolution during the last 14 000 cal yr bp the high resolution analyses attained in this study allowed to distinguish abrupt periods identify the complex structures of the early and mid holocene arid intervals and to compare their timing with titicaca lake and sajama ice records three main components in the lake sediments have been identified a biogenic component mainly from diatoms b volcanics ash layers from the nearby parinacota volcano and c endogenic carbonates the correlation between volcanic input in lago chungara and the total particles deposited in the nevado sajama ice core suggests the parinacota volcano as the common source the geochemical record of lago chungara indicates an increase in siliceous productivity during the early holocene lagging behind the rise in temperatures inferred from the nevado sajama ice core the regional mid holocene aridity crisis can be characterized as a number of short events with calcite and aragonite precipitation in the offshore lake zones

misr based passive optical bathymetry from orbit with few cm level of accuracy on the salar de uyuni bolivia abstract we demonstrate that under ideal circumstances passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters our target area is the salar de uyuni in bolivia it is a large active salt flat or playa which is maintained as an almost perfectly level and highly reflective surface by annual flooding to a mean depth of 30 50 cm we use misr data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle we use a single icesat laser altimetry profile to calibrate our water depth model though the salt surface is probably the smoothest surface of its size on earth with less that 30 cm rms height variations over an area of nearly 104 km2 it is not completely featureless topography there includes a peripheral depression or moat around the edge of the salt and several sets of prominent parallel ridges with 5 km wavelength and 30 cm amplitude the process by which these features form is still not well characterized

quaternary geochemical evolution of the salars of uyuni and coipasa central altiplano bolivia risacher f and fritz b i99 i quaternary geochemical evolution ofthe salars of uyuni and coipasa central altiplano bolivia chem geol 90 21 1 231 the central trough of the bolivian altiplano is occupied by two wide salt crusts the salar of uyuni which is probably the largest salt pan in the world 10 000 km and the salar of coipasa 2 500 km both crusts are essentially made of porous halite filled with an interstitial brine very rich in li k mg b up to 4 7 g l li 4 3 g l b 30 g l k and 75 g l mg lithium reserves are the highest known in the world around 9x io6 tons potassium magnesium and boron reserves in brines are also important around 194x io6 tons k 8x 10 tons b and 21 1 x io6 tons mg the cnists are the remnant of saline lake tauca 13 000 10 000 yr bp its salinity was estimated approximately at 80 g l its paleochemistry was derived in two ways 1 by dissolving the present amounts of all chemical components in the former lake volume and 2 by simulating the evaporation of the major inflows to the basin the resulting chemical compositions are quite different the dissolution derived one is 5 to 50 times less concentrated in li k mg b than the evaporation simulated ones however all compositions present the same na and ci contents this suggests either a removal of bittern salts or an enrichment of the former lake water in na and ci the most probable interpretation is that lake tauca redissolved a salt crust akin to that existing today several older lakes have been detected on the altiplano nevertheless such an explanation only pushes the problem back it is likely that the anomaly was transferred from one lake to an other three hypotheses may be put forward 1 bittern seepage through bottom sediments 2 uptake of the missing components by minerals and 3 leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin the excess halite could have been recycled from lake to lake this latter process seems to be the most effective to explain the large excess of na and ci over the bittern solutes li k mg and b the occurrence of almost pure na ci saline springs flowing out from a gypsum diapir in the northern altiplano gives substantial support to this hypothesis

vegetation and climate change on the bolivian altiplano between 108 000 and 18 000 yr ago abstract a 90 000 yr record of environmental change before 18 000 cal yr b p has been constructed using pollen analyses from a sediment core obtained from salar de uyuni 3653 m above sea level on the bolivian altiplano the sequence consists of alternating mud and salt which reflect shifts between wet and dry periods low abundances of aquatic species between 108 000 and 50 000 yr ago such as myriophyllum and isoetes and marked fluctuations in pediastrum suggest generally dry conditions dominated by saltpans between 50 000 yr ago and 36 000 cal yr b p lacustrine sediments become increasingly dominant the transition to the formation of paleolake minchin begins with marked rises in isoetes and myriophyllum suggesting a lake of moderate depth similarly between 36 000 and 26 000 cal yr b p the transition to paleolake tauca is also initiated by rises in isoetes and myriophyllum the sustained presence of isoetes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level polylepis tarapacana dominated communities were probably an important component of the altiplano terrestrial vegetation during much of the last glacial maximum lgm and previous wet phases

last glacial maximum in an andean cloud forest environment eastern cordillera bolivia climatically sensitive tropical ecosystems provide important information that may help us to fill the gaps in our knowledge concerning the evolution of rainforests during periods of full glaciation small changes in precipitation in the amazon basin have immediate consequences for the survival of the andean cloud forest because its dominant source of moisture today is the atlantic ocean a 40 k y lacustrine record from the eastern cordillera in bolivia in an endemic species rich and ecologically threatened region shows a dry last glacial maximum indicating a drastic decrease of the amazonian moisture source to explain this aridity we infer steep temperature gradients between the pole and equator in both hemispheres that would have reduced considerably the size and displacement of the intertropical convergence zone and the austral summer precipitation this major change in water supply induced a dramatic reduction in species diversity and suggests that the andean cloud forest did not provide refugia for tropical lowland taxa during full glacial times

nature and causes of quaternary climate variation of tropical south america abstract this selective review of the quaternary paleoclimate of the south american summer monsoon sasm domain presents viewpoints regarding a range of key issues in the field many of which are unresolved and some of which are controversial 1 el nino southern oscillation variability while the most important global scale mode of interannual climate variation is insufficient to explain most of the variation of tropical south american climate observed in both the instrumental and the paleoclimate records 2 significant climate variation in tropical south america occurs on seasonal to orbital i e multi millennial time scales as a result of sea surface temperature sst variation and ocean atmosphere interactions of the tropical atlantic 3 decadal scale climate variability linked with this tropical atlantic variability has been a persistent characteristic of climate in tropical south america for at least the past half millennium and likely far beyond 4 centennial to millennial climate events in tropical south america were of longer duration and perhaps larger amplitude than any observed in the instrumental period which is little more than a century long in tropical south america these were superimposed upon both precession paced insolation changes that caused significant variation in sasm precipitation and eccentricity paced global glacial boundary conditions that caused significant changes in the tropical south american moisture balance as a result river sediment and water discharge increased and decreased across tropical south america lake levels rose and fell paleolakes arose and disappeared on the altiplano glaciers waxed and waned in the tropical andes and the tropical rainforest underwent significant changes in composition and extent to further evaluate climate forcing over the last glacial cycle 125 ka we developed a climate forcing model that combines summer insolation forcing and a proxy for north atlantic sst forcing to reconstruct long term precipitation variation in the sasm domain the success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation in addition we propose a critical correction for speleothem stable oxygen isotopic ratios which are among the most significant of paleoclimate proxies in tropical south america for reconstruction of variation of paleo precipitation or sasm intensity however it is already well known that any particular 18o value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount the influence of temperature on carbonate water isotopic fractionation in the cave and the influence of changing 18o of seawater quantitatively accounting for both artifacts can significantly alter the interpretations of speleothem records in tropical south america both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo hydrologic signal but by different amounts in glacial and inter glacial stages these corrections have even graver implications for the interpretation of tropical northern hemisphere speleothem records e g chinese speleothems where the combined adjustments tend to decrease or even eliminate the true signal amplitude

the discovery and initial characterization of allan hills 81005 the first lunar meteorite antarctic meteorite alha81005 discovered in the allan hills region of victoria land is a polymict anorthositic breccia which differs from other meteorites in mineralogical and chemical composition but is strikingly similar to lunar highlands soil breccias the petrologic character and several independent lines of evidence identify alha81005 as a meteorite from the moon two small clasts of probable mare basalt occur among the highlands lithologies in thin section 81005 22 this lunar specimen which shows relatively minor shock effects has generated new ideas on the types of planetary samples we may find on the earth and has added fresh excitement to the antarctic meteorite program

late pleistocene climate and water budget of the south american altiplano abstract geomorphic evidence from the peruvian bolivian altiplano indicates that during episodes prior to 28 000 and around 12 500 11 000 yr b p lakes covered an area about six and four times as large as at present respectively within the constraints of the heat and water budget model calculations are used to estimate the precipitation rate that would allow hydrologic equilibrium on this basis it is suggested that rainfall on the altiplano during the episodes of enlarged lakes was respectively some 300 and 200 mm annum 1 larger than at present representing increases of about 75 and 50 respectively field evidence suggests that the episodes of enlarged lakes on the altiplano may have preceded or coincided with periods of maximum glaciation in the neigh boring andes in this region with high elevation of the ice equilibrium line increased precipitation is particularly conducive to glaciation

high resolution seismic reflection profiles from lake titicaca peru bolivia evidence for holocene aridity in the tropical andes high resolution seismic reflection profiles of the sediments of lake titicaca peru bolivia suggest that lake levels in the recent past were considerably lower than today incised channels on the major deltas extend to depths of 85 m below modern lake level erosional truncation of onlapping seismic reflectors is found at similar depths this interpretation of the seismic data is supported by analyses of sediment cores from the lake which indicate that there was a significant drop in lake level during the early to mid holocene

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

paleo eolian dynamics in the andean plateau inferences from sedimentology of fossil and modern dunes fil lopez steinmetz romina lucrecia universidad nacional de jujuy instituto de ecorregiones andinas consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet salta instituto de ecorregiones andinas argentina universidad nacional de jujuy argentina consejo nacional de investigaciones cientificas y tecnicas argentina

carbon abundance and isotopic studies of shergotty and other shergottite meteorites consortium samples of the shergotty meteorite have been measured for c abundance and isotopic composition by stepped combustion and the results compared to different samples of the same meteorite and the other known shergottites clearly the shergottite meteorites have a complex c chemistry and contain components of both low and high thermal stability two components labile at low temperature can be tentatively identified one which is enriched in 13c and may be related to the carbonate thought to be produced by pre terrestrial weathering in nakhla the other which is isotopically light is presently of unknown origin but we speculate that it may be related to shock effects at high temperatures two of the samples examined show evidence for a component of co2 trapped from the martian atmosphere possibly indicating that shock produced glass is heterogeneously distributed throughout the shergottite group this interpretation is corroborated by n isotope measurements made on one of the specimens all samples appear to contain evidence of a high temperature magmatic component of c

holocene palaeohydrology of lake titicaca estimated from an ostracod based transfer function lacustrine deposits of holocene age 8000 14c yr bp present were studied in lake titicaca bolivia ostracod assemblages were identified in six cores taken in the three basins which today form lake titicaca factorial correspondence and multiple linear regression analysis of the ostracods have permitted the reconstruction of 1 lake level 2 water salinity and 3 water mg ca ratio variations palaeolake levels indicate a complex evolution of the three basins with some major phases after a flooding at about 8000 yr 14c bp the lake level fluctuated around a mean position until 3900 14c yr bp when a second rise took place it seems however that the present day lake level was not reached until after 680 yr 14c bp some major dry events are also recorded at about 5300 14c yr bp and 2300 14c yr bp the lake titicaca data are consistent with other records from south america and also seem to reflect some world wide events

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

sources and behavior of arsenic and trace elements in groundwater and surface water in the poopo lake basin bolivian altiplano water management in semiarid and arid catchments such as the poopo lake basin requires improved understanding of the complex behavior of the various contaminants which affect the drinking water quality and considered as crucial for sustainable development of the region mechanisms of arsenic as release in the surface and groundwater were studied hydrochemical data for surface water 4 samples and groundwater 28 samples were collected in a small watershed in the poopo catchment at the highland of the bolivian andes altiplano all of them show high electrical conductivity values and moderately oxidizing conditions the surface water contains high concentration of sulfate and the trace elements as zn and pb in the zone affected by acid mine drainage there is a large variability of the concentration of as and of the trace elements in the groundwater in the five different regions within the poopo catchment the metal concentrations sensitive to changes of redox state and results of speciation modeling suggest that as v is a predominant aqueous species which conforms to the prevailing oxidizing conditions in the shallow groundwater environment two generalized trends for as distribution were identified in groundwater a high concentrations are found in the arid zone 100 250 g l in the southern region iii and in the northwestern region v regions and b low concentrations 50 g l are found in the remaining part of the basin region i ii and iv however the spatial distribution within these regions needs to be investigated further a conclusion from the present study is that there are multiple sources of as as well as other trace elements such as cd mn and zn in the poopo lake basin among the sources and the processes which led to the mobility of as and other trace metals in the region are a weathering of sulfide minerals b oxidation of pyrite and or arsenopyrite in mineralized areas and c desorption from hydrous ferric oxide hfo surfaces in non mining areas volcanic ash is suggested to be a significant source of as

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

glacial interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical andes bolivia peru abstract a palynological investigation of the last glacial interglacial cycle in the southern hemisphere tropical andes reveals changes in the moisture balance as the main driver in vegetation change thirty accelerator mass spectrometry radiocarbon dates biostratigraphy and tephra correlation reveal that a 119 m sediment core recovered from the huinaimarca sub basin of lake titicaca 16 0 to 17 5 s 68 5 to 70 w 3810 masl contains sediments covering 151 000 years correlation of aridity indicators with precessional variations in insolation is used to fine tune the structure of the age depth curve within this period variations in isoetes concentration above below 10 000 grains cm3 identify the extent of shallow water environments examination of another palaeolimnological indicator pediastrum and consideration of the bathymetry of the huinaimarca sub basin allow the reconstruction of lake level fluctuations these data indicate five wet dry cycles between c 151 000 and 14 200 cal yr bp high stands are suggested during the transition into c 134 000 cal yr bp and out of c 114 000 and 92 000 cal yr bp the last interglacial and during full glacial conditions c 70 000 and 45 000 cal yr bp these cycles are superimposed on a general trend of deepening lake levels through the glacial period this interpretation is supported by correlation with sediments from salar de uyuni 20 s 68 w 3653 masl the youngest wet episode is concurrent with palaeolake minchin c 45 000 cal yr bp with further evidence for an additional wet period commencing c 28 000 cal yr bp concomitant with palaeolake tauca the timing of lake level fluctuations is also supported by palaeoshoreline reconstructions from the uyuni poopo region however our data do not suggest a major peak in lake level in huinaimarca during the ouki lake cycle c 120 000 98 000 cal yr bp as inferred from u th ages obtained from palaeoshorelines around lago poopo the most extreme dry event occurs during the last interglacial period and resulted in a sedimentary hiatus tentatively dated to c 121 000 129 000 cal yr bp the observed wet dry cycles are shown to have a marked and rapid impact on the vegetation the aridity of the last interglacial promoted a community dominated by chenopodiaceae amaranthacae with no modern andean analogue polylepis acaena pollen is also shown to fluctuate markedly 0 20 particularly during the transitions into and out of the last interglacial it is probable that this pollen taxon is primarily representative of the high altitude arboreal genus polylepis which is a key component of highly biodiverse andean woodlands today rapid fluctuations indicate the sensitivity of this ecosystem to natural environmental pressure and potential vulnerability to future human impact and climate change the 100 000 year eccentricity solar cycle is shown to be the major controlling factor in moisture balance and vegetation over the last glacial interglacial cycle however significant fluctuations in moisture balance are also evident on timescales considerably shorter than the full glacial interglacial cycle we have linked these to precessional 21 000 year forcing nevertheless precise independent dating during the full glacial cycle is required to confirm the importance of this forcing mechanism

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

associated chemical and carbon isotopic composition variations in diamonds from finsch and premier kimberlite south africa abstract the carbon isotopic composition of 66 inclusion containing diamonds from the premier kimberlite south africa 93 inclusion containing diamonds and four diamonds of two diamond bearing peridotite xenoliths from the finsch kimberlite south africa was measured the data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates for both kimberlites similar trends are noted for diamonds containing peridotite suite inclusions p type and for diamonds containing eclogite suite inclusions e type higher 13c p type diamonds tend to have inclusions lower in sio2 ol al2o3 opx gt cr2o3 mgo mg mg fe ol opx gt and higher in feo ol opx gt and cao gt higher 13c e type diamonds tend to have inclusions lower in sio2 al2o3 gt cpx mgo mg mg fe gt na2o k2o tio2 cpx and higher in cao ca ca mg gt cpx consideration of a number of different models that have been proposed for the genesis of kimberlites their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle regions exist that are characterized by different mean carbon isotopic compositions

long range high resolution stratigraphic correlation of rotliegend fluvial fan deposits in the central dutch offshore abstract the rotliegend feather edge area in the central part of the endorheic southern permian basin in the dutch offshore is characterized by a predominance of mud prone evaporite bearing playa and lake deposits with a subordinate amount of interbedded thin fluvial sheet sandstones the distribution and lateral facies changes of the sandstone bodies have been analyzed by generating a long range high resolution chronostratigraphic correlation framework the correlation technique of pattern matching of gr logs was applied supported by calculating spectral trend curves flooding events are the primary near synchronous correlation surfaces which can be traced up to and over 100 km the basin setting of the southern permian basin the studied sandstone depositional architecture logs and sedimentary characteristics core are analogous to the depositional setting of laterally amalgamated terminal lobes of dryland river systems in an endorheic basin such as the holocene altiplano basin in bolivia present day lake eyre australia and the miocene ebro basin spain the integrated approach has yielded a stratigraphic reservoir architecture framework in which the reservoir sandstones with net sand up to 10 m have been identified as amalgamated terminal splay sandstone sheets formed at the end of dryland river pathways alternating with lacustrine mudstone layers deposited during short duration high magnitude flooding in intermittent wet climate periods

the origin of brines and salts in chilean salars a hydrochemical review abstract northern chile is characterized by a succession of north south trending ranges and basins occupied by numerous saline lakes and salt crusts collectively called salars fossil salt crusts are found to the west in the extremely arid central valley while active salars receiving permanent inflows fill many intravolcanic basins to the east in the semiarid cordillera sea salts and desert dust are blown eastward over the cordillera where they constitute an appreciable fraction of the solute load of very dilute waters salt content evaporative concentration of inflow waters leads to sulfate rich or calcium rich near neutral brines alkaline brines are almost completely lacking the alkalinity calcium ratio of inflow waters is lowered by the oxidation of native sulfur reducing alkalinity and the deposition of eolian gypsum increasing ca concentration theoretically so4 rich inflow waters and their derived so4 rich brines should be found in the intravolcanic basins of the cordillera because of the ubiquity of native sulfur while ca rich brines should prevail in sedimentary basins where ca rich minerals are abundant this relation is perfectly observed in the salar de atacama the largest in chile however several salars located within the volcanic cordillera belong to the ca rich group inflows and brines may have acquired their ca rich composition in pleistocene time when their drainage basins were mainly sedimentary later on recent lava flows and ignimbrites covered the sedimentary formations underground waters may have kept their early sedimentary signature by continuous recycling however the weathering of volcanic rocks tend to slowly shift the water compositions from the ca rich to the so4 rich type

carbon isotopes in three snc meteorites samples of three snc meteorites shergotty chassigny and elephant moraine eeta 79001 have been analysed for their contents and stable isotopic compositions of carbon using stepped combustion in an attempt to resolve indigenous components from terrestrial contaminants the rarity of most of the snc meteorites precludes the use of large samples for analysis hence carbon isotopic compositions have been determined by static mass spectrometry in all three cases ca 90 of the carbon was liberated below 500 c the isotopic composition of this material being strongly suggestive of terrestrial contamination at higher extraction temperatures a range in values of 11 to 41 was found which although not uncommon for carbon in terrestrial igneous rocks is clearly distinct from the values postulated for the bulk earth 13c ca 5 the presence of several carbonaceous components was inferred from the data although only tentative suggestions can be made on the nature of most of them certain similarities in the carbon isotope versus temperature profiles are observed that may indicate a common origin for the three meteorites but the lack of isotopic data from analogous terrestrial rocks and the absence of petrographic evidence of carbonaceous material in the snc s remains an obstacle to the complete interpretation of the present results

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

near infrared spectroscopy of lacustrine sediments in the great salt lake desert an analog study for martian paleolake basins the identification and characterization of aqueous minerals within ancient lacustrine environments on mars are a high priority for determining the past habitability of the red planet terrestrial analog studies are useful both for understanding the mineralogy of lacustrine sediments how the mineralogy varies with location in a lacustrine environment and for validating the use of certain techniques such as visible near infrared vnir spectroscopy in this study sediments from the pilot valley paleolake basin of the great salt lake desert were characterized using vnir as an analog for martian paleolake basins the spectra and subsequent interpretations were then compared to mineralogical characterization by ground truth methods including x ray diffraction automated scanning electron microscopy and several geochemical analysis techniques in general there is good agreement between vnir and ground truth methods on the major classes of minerals present in the lake sediments and vnir spectra can also easily discriminate between clay dominated and salt dominated lacustrine terrains within the paleolake basin however detection of more detailed mineralogy is difficult with vnir spectra alone as some minerals can dominate the spectra even at very low abundances at this site the vnir spectra are dominated by absorption bands that are most consistent with gypsum and smectites though the ground truth methods reveal more diverse mineral assemblages that include a variety of sulfates primary and secondary phyllosilicates carbonates and chlorides this study provides insight into the limitations regarding the use of vnir in characterizing complex mineral assemblages inherent in lacustrine settings

dating of paleolakes in the central altiplano of bolivia abstract the deposits of the extensive paleolakes of the bolivian altiplano correspond to an alternation of lacustrine episodes and dry periods a phenomenon commonly explained by climatic changes the lack of precise dating of the deposits has prevented detailed reconstruction of the evolution of paleolakes that would allow a determination of the relative influence of interconnections between neighboring basins and local climatic conditions among the five paleolakes of the northern altiplano titicaca basin and the three of the central altiplano poopo coipasa and uyuni basins only the two most recent paleolakes have been dated a 121 m long sediment core recovered from the salar de uyuni contained 11 lacustrine layers l1 l11 separated by 12 salt crusts radiocarbon dating shows that the younger layers l1 and l2 are related to the tauca lacustrine phase 12 000 16 000 cal yr bp and layers l3 and l4 to the minchin lacustrine phase 30 000 73 000 cal yr bp layer l5 located at 46 m below the surface contains a volcanic ash with well preserved biotites is here dated by the 40 ar 39 ar method the measured age of 191 000 5000 y is the first date ever obtained from a pre minchin paleolake lake escara this result combined with 14 c and u th data obtained on lakes tauca and minchin from various authors suggests that the duration of the lacustrine events increased regularly during the pleistocene in contrast paleolake levels in the northern altiplano decreased from the oldest known lake mataro 3950 m to the present titicaca level 3806 m a progressive erosion of the threshold between the northern and central altiplano may have allowed important volumes of water to overflow from the northern basin into the central altiplano the existence of large paleolakes in the central altiplano was thus not only dependent on local climatic conditions but also on additional inputs from the north where climatic conditions could have been quite different from those prevailing in the south

quantitative seismic stratigraphic interpretation of the evaporite sequence in the santos basin abstract on the account of the absence of modern analogues the interpretation of evaporite bearing basins relies extensively on seismic and well data recent studies describe the intrasalt kinematic geometries and compositions relating the change of reflection patterns in seismic data to variations in evaporite sequences however these interpretations are only qualitative this study describes a step further in proceeding from qualitative to quantitative interpretation of an evaporite dominated interval it integrates geophysical and geological foundations by combining rock physics seismic inversion and statistical techniques to deliver seismic driven facies volume of the evaporite sequence in the santos basin offshore brazil synthetic seismogram demonstrate the ambiguities and uncertainties in relating the seismic amplitude to salt types rock physics analysis indicates that acoustic impedance is an effective differentiator of salt types in comparison to seismic amplitude therefore we perform seismic inversion to transform interface property into layer property the acoustic impedance volume mitigated significantly the risk of seismic interpretation in the salt sequence the bayesian classification integrates acoustic impedance and rock physics analysis to deliver the seismic driven facies volume which categorizes bittern salts halite and anhydrite this enables the refinement of the seismic stratigraphic interpretation of intrasalt sequences of the ariri formation based on the sedimentary cycles of evaporites additionally we calculate proportion maps to evaluate the spatial variability of salt deposition we observe an increase in bittern salts and anhydrite proportions from the bottom to the top of the sequence the comparison between well based and seismic based proportions testifies to the quantitative potential of seismic data the approach provides quantitative analysis of the evaporite sequence and can be applied to other salt bearing basins

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

cosmogenic 3he production rate in the high tropical andes 3800 m 20 s implications for the local last glacial maximum abstract to improve both precision and accuracy of cosmic ray exposure dating methods there is a crucial need of calibration sites constraining the production rate of cosmogenic isotopes this is particularly true in the high tropical area where existing scaling models present significant discrepancies this study presents a new calibration site for cosmogenic 3he located at 3800 m in the tropical altiplano 19 9 s 67 6 w on the southern flank of the tunupa volcano in the vicinity of the salar de uyuni it consists in a fluvio glacial outwash that is stratigraphically bracketed by two successive lacustrine shorelines these shorelines are well dated by 14c and u series dating n 57 allowing determination of the age of the boulder field at the surface of the delta at 15 3 0 5 ka 1 eleven andesitic boulders were sampled on this fluvio glacial surface and the cosmogenic 3he contents of their pyroxene phenocrysts and amphiboles was analyzed the nucleogenic contribution from 6li capture is well constrained by determining the u th sm 4he age of these rocks this correction is minimal combined with the absolute dating of the delta these 3hec data yield a local production rate of 999 38 1 at g 1 yr 1 at 3800 m and 19 89 s after scaling with the time dependent scaling scheme of stone 2000 this result yields a rate of 121 5 1 at g 1 yr 1 at high latitude and sea level this new calibration is the highest among the existing global dataset it will thus permit the establishment of new robust glacial chronologies in the high tropics with an uncertainty lower than 5 at 1 such precision may have important implications in paleoclimatology notably because it will allow comparison with other well dated paleoclimatic archives including this new data the updated global 3hec production rate is 122 15 at g 1 yr 1 using the time dependent scaling of stone 2000 the new site specific 3hec production rate is used here to refine glacier fluctuations on cerro tunupa confirming that the local last glacial maximum was synchronous with the lake tauca highstand 15 5 ka data also suggest that a dramatic glacial retreat occurred at about 15 ka few hundred years before the lake tauca regression synchronously with the onset of the bolling allerod

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

sources and a proposal for comprehensive exploitation of lithium brine deposits in the qaidam basin on the northern tibetan plateau china evidence from li isotopes abstract a thorough understanding of the origin and distribution of lithium brine deposits is the foundation for lithium resource exploration development and utilization the lithium isotope system a sensitive geochemical tracer can be used to track a wide range of geological processes and sources this study investigates major cations li and 7li of samples brine lakes intercrystalline brines river waters rain hot spring waters spring waters and rocks from the qaidam basin on the northern tibetan plateau china li values vary from less than 0 01 mg l 1 to 408 83 mg l 1 there are significant differences in li for different sample types spring water rain

low holocene level 7700 to 3650 years ago of lake titicaca bolivia abstract preliminary results of sedimentological and paleontological analysis of two cores taken in lake titicaca bolivia indicate that between 7700 and 3650 yr b p the lake level was up to 50 m lower than at present evaporitic deposits occurred at the maximum low lake level

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

hydrologic variation during the last 170 000 years in the southern hemisphere tropics of south america despite the hypothesized importance of the tropics in the global climate system few tropical paleoclimatic records extend to periods earlier than the last glacial maximum lgm about 20 000 years before present we present a well dated 170 000 year time series of hydrologic variation from the southern hemisphere tropics of south america that extends from modern times through most of the penultimate glacial period alternating mud and salt units in a core from salar de uyuni bolivia reflect alternations between wet and dry periods the most striking feature of the sequence is that the duration of paleolakes increased in the late quaternary this change may reflect increased precipitation geomorphic or tectonic processes that affected basin hydrology or some combination of both the dominance of salt between 170 000 and 140 000 yr ago indicates that much of the penultimate glacial period was dry in contrast to wet conditions in the lgm our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50 000 years than in earlier times d 2003 university of washington all rights reserved

hydro isostatic deflection and tectonic tilting in the central andes initial results of a gps survey of lake minchin shorelines sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle lake minchin was the largest of the late pleistocene pluvial lakes in the central andes prominent shorelines which formed during temporary still stands in the climatically driven lake level history preserve records of lateral variations in subsequent net vertical motions at its maximum extent the lake was 140 m deep and spanned 400 km n s and 200 km e w the load of surficial water contained in lake minchin was sufficient to depress the crust and underlying mantle by 20 40 m depending on the subjacent rheology any other differential vertical motions will also be recorded as departures from horizontality of the shorelines we recently conducted a survey of shoreline elevations of lake minchin with the express intent of monitoring the hydro isostatic deflection and tectonic tilting using real time differential gps we measured topographic profiles across suites of shorelines at 15 widely separated locations throughout the basin horizontal and vertical accuracies attained are roughly 30 and 70 cm respectively geomorphic evidence suggests that the highest shoreline was occupied only briefly probably less than 200 years and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr the basin wide pattern of elevations of the highest shoreline is composed of two distinct signals 27 1 m of hydro isostatic deflection due to the lake load and a planar tilt with east and north components of 6 8 0 4 10 5 and 5 3 0 3 10 5 this rate of tilting is too high to be plausibly attributed to steady tectonism and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate the history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology however it is already clear that the effective elastic plate thickness is closer to 40 km than the 60 70 km crustal thickness in the central andes and the effective viscosity is less than 5 1020 pa s

late quaternary palaeolakes rivers and wetlands on the bolivian altiplano and their palaeoclimatic implications drill cores of sediments from the rio desaguadero valley bolivia provide new information about the climate of tropical south america over the past 50 000 years the modern rio desaguadero is fed by lake titicaca overflow and by local tributaries in the wetter northern altiplano and discharges into lake poopo in the more arid central altiplano during the late quaternary the rio desaguadero valley was the site of several generations of palaeolakes and wetlands that formed during periods of increased precipitation and local runoff augmented by increased overflow from lake titicaca sediments recovered by drilling in eight localities along the 390 km long valley of the rio desaguadero yield a regional history of lacustrine sedimentation and effective precipitation lacustrine strata in the drill cores record 12 distinct wet periods in the past 50 000 years four of these wet periods resulted in the formation of major palaeolakes in the rio desaguadero valley during the last glacial maximum from before 20 000 to 16 000 cal yr bp during the late glacial from about 14 000 to 12 000 cal yr bp in the early holocene from about 10 000 to 7900 cal yr bp and in the late holocene from 4500 cal yr bp to present the period that appears to have been most arid was between 7900 and 4500 cal yr bp the altiplano wet periods were generally synchronous with north atlantic cold events respectively the last glacial maximum the younger dryas the 8200 cal yr bp event and the neoglacial implying a relationship between past precipitation variability in tropical south america and north atlantic sea surface temperature copyright 2005 john wiley sons ltd

lake level variability in salar de coipasa bolivia during the past 40 000 yr various paleoclimatic records have been used to reconstruct the hydrologic history of the altiplano relating this history to past variability of the south american summer monsoon prior studies of the southern altiplano the location of the world s largest salt flat the salar de uyuni and its neighbor the salar de coipasa generally agree in their reconstructions of the climate history of the past 24 ka some studies however have highly divergent climatic records and interpretations of earlier periods in this study lake level variation was reconstructed from a 14 m long sediment core from the salar de coipasa these sediments span the last 40 ka lacustrine sediment accumulation was apparently continuous in the basin from 40 to 6 ka with dry or very shallow conditions afterward the fossil diatom stratigraphy and geochemical data 13 c 15 n ca c n indicate fluctuations in lake level from shallow to moderately deep with the deepest conditions correlative with the heinrich 1 and younger dryas events the stratigraphy shows a continuous lake of variable depth and salinity during the last glacial maximum and latter stages of marine oxygen isotope stage 3 and is consistent with environmental inferences and the original chronology of a drill core from salar de uyuni

morphological and speleothemic development in brujas cave southern andean range argentine palaeoenvironmental significance abstract brujas cave in the southern andean range is a well known endokarstic site in argentina however the origin and evolution of this cave system are poorly known based on morphological cave features as well as characteristics of cave deposits we propose a meteogene drawdown cave genesis including a change from phreatic to vadose conditions related to the high rate of fluvial downcutting in the area during the vadose period various cave related deposits including authogenic calcite and gypsum speleothems allogenic volcanic ash and external tufas were deposited gypsum crusts are the oldest cave deposits identified 90 2 64 3 ky bp their origin deduced from isotopic characteristics 34 s 9 6 is related to the oxidation of pyrite contained in the jurassic limestone bedrock as well as the dissolution of overlying jurassic triassic evaporite formations gypsum crust deposition is associated with evaporation of water flowing and seeping into the cave during arid environmental conditions calcite deposits precipitated from flowing water under equilibrium conditions represent the main speleothem growth period 67 6 34 ky bp in age their stable isotope values 13 c 3 to 5 and 18 o 9 to 11 may indicate slightly humid and warm conditions related to the regional minchin lacustrine phase and global oxygen isotope stage 3 following this stage a seismic event is evidenced by accumulations of broken stalactites seepage calcite speleothems covering cave walls were deposited under disequilibrium conditions by evaporation probably during holocene time finally another more recent gypsum deposition period represented by gypsum balls has been differentiated micromorphological as well as isotopic 34 s 5 6 data indicate that these gypsum forms are related to cyclic processes solution deposition from water seeping into the cave under arid conditions in addition intense volcanic activity in the area during holocene time is deduced from allogenic volcanic ash and lapilli located mainly at the entrance cave at present limited hydrological activity is observed in the cave and small tufa deposits at karstic discharge points are evidenced we conclude that the geomorphological evolution of brujas cave was controlled by climatic changes wet and dry stages under semiarid environmental conditions in a very active tectonic and volcanic setting during late quaternary time

the u th pb systematics of zircons from the type nuk gneisses godthabsfjord west greenland abstract u pb analyses were made on zircons separated from different lithologies within the nuk gneisses taken from godthab nuuk townsite and from the south west shore of bjorneoen size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the nuk gneisses of 2890 3065 myr this spread of ages is not a result of multiple intrusion but more likely results from different degrees of diffusion and metamorphic episodic lead loss by the zircons since u pb and th pb discordance are similar loss of radiogenic lead and not u or th loss gain by radiation damage diffusion and through metamorphism is the main factor causing discordance of the u pb dates the results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed this interpretation is supported by rb sr whole rock analyses of the same rocks in which the nuk townsite rocks scatter about a reference isochron of 2840 myr while the bjorneoen samples give an isochron mswd 0 55 of 3076 27 myr and the initial ratio at 0 70205 0 00009 the zircon results do not rule out the possibility that more than one metamorphic episode affected the original nuk gneiss parents prior to 2520 myr qorqut granite intrusion a date of 2660 myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south western bjorneoen and godthab town

climate variation and the rise and fall of an andean civilization paleolimnological and archaeological records that span 3500 years from lake titicaca and the surrounding bolivian peruvian altiplano demonstrate that the emergence of agriculture ca 1500 b c and the collapse of the tiwanaku civilization ca a d 1100 coincided with periods of abrupt profound climate change the timing and magnitude of climate changes are inferred from stratigraphic evidence of lake level variation recorded in14c dated lake sediment cores paleo lake levels provide estimates of drainage basin water balance archaeological evidence establishes spatial and temporal patterns of agricultural field use and abandonment prior to 1500 b c aridity in the altiplano precluded intensive agriculture during a wetter period from 1500 b c to a d 1100 the tiwanaku civilization and its immediate predecessors developed specialized agricultural methods that stimulated population growth and sustained large human settlements a prolonged drier period ca a d 1100 1400 caused declining agricultural production field abandonment and cultural collapse