mid holocene climate and culture change in the atacama desert northern chile abstract twenty archaeological campsites intercalated between more than 30 debris flows caused by heavy rainfall events between 6200 and 310014c yr b p have recently been discovered at quebrada puripica in the atacama desert of northern chile this record provides detailed information about extreme short lived climatic events during the hyperarid mid holocene period for the first time we found evidence of continuous human occupation in this area filling the regional hiatus in the atacama basin silencio arqueologico between 8000 and 480014c yr b p the transformation of early archaic hunters into the complex late archaic cultural tradition was an adaptive process during this time the site was a local ecological refuge with abundant resources in a generally hostile environment

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

how are large western hemisphere warm pools formed during the boreal summer the western hemisphere warm pool whwp stretches from the eastern north pacific to the tropical north atlantic and is a key feature of the climate of the americas and africa in the summers following nine el nino events during 1950 2000 there have been five instances of extraordinarily large warm pools averaging about twice the climatological annual size these large warm pools have induced a strengthened divergent circulation aloft and have been associated with rainfall anomalies throughout the western hemisphere tropics and subtropics and with more frequent hurricanes however following four other el nino events large warm pools did not develop such that the mere existence of el nino during the boreal winter does not provide the basis for predicting an anomalously large warm pool the follow ing summer in this paper we find consistency with the hypothesis that large warm pools result from an anomalous divergent cir culation forced by sea surface temperature sst anomalies in the pacific the so called atmospheric bridge we also find significant explanations for why large warm pools do not always develop if the el nino event ends early in the eastern pacific the pacific warm anomaly lacks the persistence needed to force the atmospheric bridge and the atlantic portion of the warm pool remains normal if sst anomalies in the eastern pacific do not last much beyond february of the fol lowing year then the eastern north pacific portion of the warm pool remains normal the overall strength of the pacific el nino does not appear to be a critical factor we also find that when conditions favor a developing atmospheric bridge and the winter atmosphere over the north atlantic conforms to a negative north atlantic oscillation nao pattern as in 1957 58 and 1968 69 the forcing is reinforced and the warm pool is stronger on the other hand if a positive nao pat tern develops the warm pool may remain normal even if other circumstances favor the atmospheric bridge as in 1991 92 finally we could find little evidence that interactions internal to the tropical atlantic are likely to mitigate for or against the formation of the largest warm pools although they may affect smaller warm pool fluctuations or the warm pool persistence

mid and late holocene limnogeology of laguna del negro francisco northern chile and its palaeoclimatic implications lacustrine sediments from the high elevation endorheic saline lake negro francisco 27 28 s 69 14 w 4125 m provide a detailed mid and late holocene environmental history of the southern chilean altiplano mineralogy chemical composition and sedimentary facies analyses of the deposits confirm the regional significance of mid holocene aridity between 6000 and 3800 bp precipitation rates were signifi cantly lower than the c 250 mm yr 1 of today fully arid conditions were interrupted by short term moist spells with an average return period of about 200 years effective moisture increased after 3800 bp and peaked between 3000 and 2600 bp and between 2200 and 1800 bp in two phases with conditions more humid than today comparison with regional palaeodata suggest that these two late holocene humid spells were most likely related to pacific moisture sources salinity levels increased again in laguna del negro francisco after 1800 bp the onset of modern climatic conditions was a stepwise nonlinear proce

holocene palaeoenvironmental reconstruction in norteastern brazi inferred from polen charcoal and carboon isotope records soils in the barreirinhas region maranhao state were sampled for 613 c analysis and buried charcoal fragments in the soils were radiocarbon dated three soil profiles collected in forested areas around the lagoa do caqo and one in a woody savanna mixture of non arboreal and arboreal species located approximately 10 km southeast of the lagoa were studied a high resolution pollen record was obtained from lake sediments showing that forest vegetation was predominant in the area in the early holocene from approximately 10 000 14c yr bp the pollen spectrum gradually changed suggesting the dominance of open savanna communities these were transformed to a more forested landscape woody savanna from approximately 7500 yr bp the lake sediments also record evidence of fire indicated by buried charcoal particles at several soil depths during the holocene the 613 c analysis of soil organic matter som indicates that from between approximately io0000 yr bp and 9000 yr bp to 4000 yr bp a woody savanna

a large amplitude gravity wave in the lower stratosphere detected by radiosonde temperature profiles in the lower stratosphere commonly show fluctuations typical peak to peak amplitude of a few degrees on a vertical scale of a few kilometres the routine operational radiosonde launched from shanwell 56 43 n 2 87 w at 2318 gmt on 12 december 1986 recorded very large amplitude excursions in temperature 15 9 k change over a vertical distance of 2 km in the lower stratosphere which were identified as being exceptional by the staff at the radiosonde station the temperature fluctuations were accompanied by strong shear in the horizontal wind and large variations in the rate of ascent of the balloon the disturbance is argued to be due to a quasi stationary gravity wave with vertical momentum flux about 3 or 4 n m 2 averaged over a horizontal scale of at least 20km these observations their meteorological context and their interpretation are described in detail

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

gravity waves above the andes detected from gps radio occultation temperature profiles jet mechanism 1 a significant wave activity wa in the upper troposphere and lower stratosphere mainly during winter was detected at midlatitudes in the southern hemisphere 30 40s above the andes range from an analysis of global positioning system radio occultation gps ro temperature profiles retrieved by champ challenging minisatellite payload and sac c satelite de aplicaciones cientificas c low earth orbit leo satellites between may 2001 and february 2006 the possible main gravity wave sources in this region are i orographic forcing ii geostrophic adjustment and iii deep convection the available vertical resolution of gps ro soundings does not rule out any of these alternatives based on satellite imaginary the wa enhancements cannot be attributed to deep convection events inertia gravity waves igws could be generated after a geostrophic adjustment process following a perturbation of the zonal jet situated above the andes mountains by mountain waves mws the monthly wa intensity follows the zonal wind velocity strength according to its seasonal variability at jet altitudes as the gps leo lines of sight are roughly meridionally aligned and the morphology of the andes at middle latitudes is predominantly north south it was possible to detect mws as well as igws from gps ro temperature profiles this characteristic does not apply for other mountain range alignments from the analysis of a numerical simulation at the time and location of a single ro event with very strong wa two main modes of oscillation with horizontal wavelength around 40 and 200 km were identified the first one is attributed to a mw and the second one to an igw

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

evidence of gravity wave saturation and local turbulence production in the summer mesosphere and lower thermosphere during the state experiment this study utilizes the unique data set obtained during the structure and atmospheric turbulence environment state experiment conducted during june 1983 at poker flat alaska to examine the structure and characteristics of the wave field near the summer mesopause it is shown that the rocket and radar data together permit a much more detailed specification of wave parameters than would be possible using either data set alone the results of this analysis suggest that the wave field near the summer mesopause is composed in general of a superposition of wave motions which act collectively to produce regions in which the wave field is convectively or dynamically unstable these regions are found to correlate well with zones of enhanced turbulence and small scale wave activity suggesting the processes and effects of wave field saturation

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

high resolution temperature profiles measured with stratospheric balloons near the andes mountains a spectral analysis of two balloon soundings of temperature performed in the troposphere and lower stratosphere over the andes mountains in argentina with a vertical resolution of 10 m has been performed the first and second soundings launched at the same location were performed during unusually calm and disturbed atmospheric conditions respectively considerable deviations from the 3 value predicted for the spectral slopes in the power law range by current saturation theories are found these departures are reduced if the data resolution at disposal is diminished thereby eliminating the highest wavelength harmonics slopes do not clearly steepen with increasing height as suggested by other authors and topography seems to be responsible for this fact at low altitudes the spectral amplitude increase with height detected from recent lidar data is corroborated only in the sounding with calm atmospheric conditions spectra are found to scale with the buoyancy frequency not as simply as suggested by the saturation models

on the relationship between stress and elastic strain for porous and fractured rock modeling the mechanical deformations of porous and fractured rocks requires a stress strain relationship experience with inherently heterogeneous earth materials suggests that different varieties of hooke s law should be applied within regions of the rock having significantly different stress strain behavior we apply this idea by dividing a rock body conceptually into two distinct parts the natural strain volume change divided by rock volume at the current stress state rather than the engineering strain volume change divided by the unstressed rock volume should be used in hooke s law for accurate modeling of the elastic deformation of that part of the pore volume subject to a relatively large degree of relative deformation i e cracks or fractures this approach permits the derivation of constitutive relations between stress and a variety of mechanical and or hydraulic rock properties we show that the theoretical predictions of this method are generally consistent with empirical expressions from field data and also laboratory rock experimental data

characterizing production induced anisotropy of fractured reservoirs having multiple fracture sets since natural fractures in petroleum reservoirs play an important role in determining fluid flow during production knowledge of the orientation and density of fractures is required to optimize production this paper outlines the underlying theory and implementation of a fast and efficient algorithm for upscaling a discrete fracture network dfn to predict the fluid flow elastic and seismic properties of fractured rocks potential applications for this approach are numerous and include the prediction of fluid flow elastic and seismic properties for fractured reservoirs model based inversion of seismic amplitude versus offset and azimuth avoa data and the optimal placement and orientation of infill wells to maximize production given that a single fracture network may comprise hundreds of thousands of individual fractures the sheer size of typical dfns has tended to limit their practical applications this paper demonstrates that with efficient algorithms the utility of discrete fracture networks can be extended far beyond mere visualization

magnesium content of non marine ostracod shells a new palaeosalinometer and palaeothermometer abstract ostracods are microcrustaceans with calcitic carapaces each consisting of two valves or shells that grow by moulting and regeneration of sequentially larger valves up to nine times before reaching maturity the mg content of these valves is species dependent and is a function of their growth stage and the temperature salinity mg content and mg ca ratio of their host water newly formed partly calcified valves of the non marine nektic ostracod mytilocypris henricae have high mg contents 100 000 ppm mg in caco3 at individual shell weights of 2 g this relative mg content decreases as calcification proceeds until shell weights exceed 120 g whereupon the mg content is constant and useful as an indicator of the ostracod s environment mg ca ratios in individual ostracod valves from several species of the genera mytilocypris and australocypris from culture experiments and a series of australian lakes indicate that the valve s mg content increases as a function of increasing temperature and salinity within the range 11 25 c and for waters containing 24 2325 ppm mg the molar mg ca ratio in these ostracods fully calcified valves increases by 0 0015 or 370 ppm mg in caco3 per 1 c and per change of 125 ppm mg in solution for most mainland australian lakes a change in water composition of 125 ppm mg corresponds to a change of 3 5 in salinity for a given mg content of a lake the mg ca ratio of the water may vary within the range of common lacustrine values without significantly affecting mg uptake in the ostracod valves these relationships can provide palaeosalinities and or palaeotemperatures for ostracods recovered from continental sediments for mg partitioning an increase in temperature and salinity produce increased mg content in ostracods whereas for 18o fractionation in biogenic calcite an increase in temperature produces a decrease in 18o of the ostracod and an increase in salinity leads to an increase in 18o of the ostracod thus a combination of 18o and mg measurements on non marine ostracods should enable unique resolution of palaeotemperature and palaeosalinity variations

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

southern oscillation signal in south american palaeoclimatic data of the last 7000 years abstract during strong el nino events rainfall anomalies and changes in wind patterns are observed in different regions of south america along the central brazilian coast during the 1983 el nino year the frontal systems were blocked to the south provoking a reversal of the longshore sand transport long duration reversals of longshore transport were also recorded in holocene beach ridge terraces from the rio doce coastal plain this led to the formulation of a model relating these reversals of longshore transport to el nino like conditions el nino like conditions are past average climate situations that generate the same perturbations as the strong el nino events observed during the last decade they are likely to correspond to the long duration low phase of the southern oscillation to confirm this hypothesis we compared the holocene beach ridge record with other palaeoenvironmental records from regions where strong el nino events would have a substancial signal as well 1 water level fluctuations of lake titicaca 2 a pollen and sediment record in an eastern amazonian lake 3 changes of the rio xingu discharge in eastern amazonia and 4 variations of sand supply at the rio piura and rio chira outlets in the sechura desert the occurrences of el nino like conditions were numerous before 3900 3600 yr b p absent between 39003600 and 2800 2500 yr b p and infrequent after 2800 2500 yr b p

abrupt early to mid holocene climatic transition registered at the equator and the poles paleoclimatic records from equatorial east africa antarctica and greenland reveal that atmospheric circulation changed abruptly at the early to mid holocene transition to full postglacial conditions a climatic reorganization occurred at all three sites between 8200 and 7800 years ago that lasted 200 years or less and appears to have been related to abrupt transitions in both marine and terrestrial records around the world

the little ice age as recorded in the stratigraphy of the tropical quelccaya ice cap the analyses of two ice cores from a southern tropical ice cap provide a record of climatic conditions over 1000 years for a region where other proxy records are nearly absent annual variations in visible dust layers oxygen isotopes microparticle concentrations conductivity and identification of the historical a d 1600 huaynaputina ash permit accurate dating and time scale verification the fact that the little ice age about a d 1500 to 1900 stands out as a significant climatic event in the oxygen isotope and electrical conductivity records confirms the worldwide character of this event

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

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

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

multiple landslide clusters record quaternary climate changes in the northwestern argentine andes the chronology of multiple landslide deposits and related lake sediments in the semi arid eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary i e between 40 and 25 and after 5 14 c kyr bp these clusters may correspond to the minchin maximum at around 28 27 14 c kyr bp and titicaca wet periods after 3 9 14 c kyr bp the more humid conditions apparently caused enhanced landsliding in this environment in contrast no landslide related damming and associated lake sediments occurred during the coipasa 11 5 10 14 c yr bp and tauca wet periods 14 5 11 14 c yr bp the two clusters at 40 25 and after 5 14 c kyr bp may correspond to periods where the el nin o southern oscillation enso and tropical atlantic sea surface temperature variability tav were active this however was not the case during the coipasa and tauca wet periods lake balance modelling of a landslide dammed lake suggests a 10 15 increase in precipitation and a 3 4 c decrease in temperature at v30 14 c kyr bp as compared to the present in addition time series analysis reveals a strong enso and tav during that time the landslide clusters in northwestern argentina are therefore best explained by periods of more humid and more variable climates 2003 elsevier science b v all rights reserved

acoustic emissions monitoring during inelastic deformation of porous sandstone comparison of three modes of deformation in some reservoirs large deformations can occur during oil or gas production because of the effective stress change for very porous rocks these production operations can be sufficient to cause inelastic deformation and irreversible damage rock formations can undergo deformation by different mechanisms including dilatancy or pore collapse in the laboratory it has been shown that the inelastic deformation and failure mode of porous rocks are pressure sensitive indeed when subjected to an overall compressive loading a porous rock may fail by shear localization compaction localization or by cataclastic compaction acoustic emission ae records provide important information to understand the failure mode of rocks the spatial evolution of damage as well as the source mechanisms can be followed using this technique in this paper we present three different laboratory axisymmetric compression experiments performed on bleurswiller sandstone which enable us to compare the acoustic emission signature of these three modes of deformation our data show that compaction localization and cataclastic compaction are characterized by similar acoustic signatures in terms of ae sources characteristics and evolution of ae number in comparison to the acoustic signature from shear localization this implies similar micromechanisms involved during compaction bands formation and cataclastic compaction

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

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

paleoecology of limnic ostracodes a review of some major topics abstract the paleoecology of limnic ostracodes is reviewd in the context of the evolution of their environment in order to understand why there is a rich ostracode fossil record we examine data such as the quantitative and qualitative distribution of living ostracode species and ostracode assemblages ways of describing and limitations importance of biological and physico chemical factors for the local accumulation of high numbers of ostracodes and diversified assemblages is emphasized we discuss the limits of extrapolations of information from ecology to paleocology we stress how paleontological knowledge can be applied to neontological studies the possibility of using holocene ostracodes as indicators of morphometric changes of lakes temperature salinity eutrophication and as a criterion of meromictic conditions is demonstrated relationships between ostracodes and the chemical environment and particularly the morphological response of the carapace are considered e g diversity and qualitative composition of assemblages and chemical stability types of carapace mineralization and ornamentation and chemistry of bottom waters these ideas developed from studies on recent material are tested for limnic cenozoic faunas in the mesozoic the main questions arising from the study of nonmarine ostracodes deal with the possible evaluation of paleosalinities in marginal lagoonal environments purbeck wealden and with evaluation of depth and hydrochemical and paleoclimatological regime in truly lacustrine intracratonic basins some peculiarities of paleozoic nonmarine ostracode assemblages are described the paleoecological cause of changes in diversity of several ostracode groups in the carboniferous and permian is emphasized

saturated gravity wave spectra measured with balloons in mendoza argentina spectral results from a sounding of temperature and wind velocity between 11 9 and 23 1 km performed with an open type stratospheric balloon near the andes mountains are reported the use of this kind of balloons and high performance instruments allowed to attain better spatial resolution around 2 5 meters during the descent than previous results and other sounding techniques this high resolution permitted to study the complete interval of altitude data in three consecutive subintervals below around and above the tropopause each of these subintervals shows different saturation characteristics some differences have been found with the general pattern of power spectral densities reported by different theories a large amplitude wave of around 1 8 km and possibly originated in the mountain relief is clearly observed in the temperature vertical and shear wind velocity components

holocene paleohydrology of the tropical andes from lake records two century scale time series in northern bolivia constrain the ages of abrupt changes in the physical geochemical and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late pleistocene glacial maximum the watersheds of laguna viscachani 16 12 s 68 07 w 3780 m and lago taypi chaka kkota 16 13 s 68 21 w 4300 m located on the eastern and western slopes of the cordillera real respectively contain small cirque glaciers a high resolution chronology of the lake sediments is provided by 23 ams14c dates of discrete macrofossils late pleistocene glaciers retreated rapidly exposing the lake basins between 10 700 and 970014c yr b p the sedimentary facies suggest that after 890014c yr b p glaciers were absent from the watersheds and remained so during the middle holocene an increase in the precipitation evaporation balance is indicated above unconformities dated to 230014c yr b p in both lago taypi chaka kkota and laguna viscachani an abrupt increase in sediment accumulation rates after 140014c yr b p signals the onset of neoglaciation a possible link exists between the observed millennial scale shifts in the regional precipitation evaporation balance and seasonal shifts in tropical insolation

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

permeability tensor for discontinuous rock masses rock masses which commonly contain a large number of discontinuities are treated as homogeneous anisotropic porous media to formulate the corresponding permeability tensor this has been successfully achieved by introducing a symmetric tensor crack tensor which depends only on the geometry of the related cracks aperture size and orientation the principal directions associated with the symmetric crack tensor are coaxial with those of the permeability tensor the first invariant of the crack tensor is proportional to the mean permeability while the deviatoric part is related to the anisotropic permeability these results are well supported by numerical experiments on the permeability of cracked media by long remer wilson and witherspoon an actual rock mass moderately jointed granite was studied to see whether the crack tensor can be determined in situ stereology based on geometrical statistics provides a sound basis for determining the crack tensor in situ the crack tensor is obtained by

quantification of paleotemperature changes during isotopic stage 2 in the la draga continental sequence ne spain based on the mg ca ratio of freshwater ostracods past summer temperature changes are quantified from the la draga sequence banyoles ne spain during stage 2 24 10 ka bp using trace element content in lacustrine ostracod valves the high resolution results from this new method are compared to summer paleotemperature reconstructions from sequences from the north atlantic area providing a discussion about the magnitudes of some temperature changes in ne spain and offshore portugal the coldest part of stage 2 is not documented at the classical date of similar to 18 c 14 ka bp similar to 21 5 calendar ka bp but rather occurred later date of ca 17 15 ka cal bp in the north atlantic area the onset of the bolling allerod interstadials was relatively synchroneous at similar to 14 6 ka cal bp although the magnitude of the bolling allerod improvement was more important in the southern area than in the northern area the duration of the younger dryas is discussed even if different durations are reported from calendar ages it seems that the summer temperature cooling of the younger dryas was similar around the north atlantic area c 1996 elsevier science ltd

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

changes in geophysical properties caused by fluid injection into porous rocks analytical models author s pride sr berryman jg commer m nakagawa s newman ga vasco dw abstract 2016 european association of geoscientists a engineers analytical models are provided that describe how the elastic compliance electrical conductivity and fluid flow permeability of rocks depend on stress and fluid pressure in order to explain published laboratory data on how seismic velocities and electrical conductivity vary in sandstones and granites the models require a population of cracks to be present in a possibly porous host phase the central objective is to obtain a consistent mean field analytical model that shows how each modeled rock property depends on the nature of the crack population the crack populations are described by a crack density a probability distribution for the crack apertures and radii and the averaged orientation of the cracks the possibly anisotropic nature of the elasticity conductivity and permeability tensors is allowed for however only the isotropic limit is used when comparing to laboratory data for the transport properties of conductivity and permeability the percolation effect of the crack population linking up to form a connected path across a sample is modeled however this effect is important only in crystalline rock where the host phase has very small conductivity and permeability in general the importance of the crack population to the transport properties increases as the host phase becomes less conductive and less permeable

limb sounders tracking topographic gravity wave activity from the stratosphere to the ionosphere around midlatitude andes several studies have shown that the surroundings of the highest andes mountains at mid latitudes in the southern hemisphere exhibit gravity waves gw generated by diverse sources which may traverse the troposphere and then penetrate the upper layers if conditions are favorable there is a specific latitude band where that mountain range is nearly perfectly aligned with the north south direction which favors the generation of wavefronts parallel to this orientation this fact may allow an optimization of procedures to identify topographic gw in some of the observations we analyze data per season to the east and west of these andes latitudes to find possible significant differences in gw activity between both sectors gw effects generated by topography and convection are expected essentially on the eastern side we use satellite data from two different limb sounding methods the global positioning system gps radio occultation ro technique and the sounding of the atmosphere using broadband emission radiometry saber instrument which are complementary with respect to the height intervals in order to study the effects of gw from the stratosphere to the ionosphere activity becomes quantified by the gw average potential energy in the stratosphere and mesosphere and by the electron density variance content in the ionosphere consistent larger gw activity on the eastern sector is observed from the stratosphere to the ionosphere night values however this fact remains statistically significant at the 90 significance level only during winter when gw generated by topography dominate the eastern sector on the contrary it is usually assumed that orographic gw have nearly zero horizontal phase speed and will therefore probably be filtered at some height in the neutral atmosphere however this scheme relies on the assumption that the wind is uniform and constant our results also suggest that it is advisable to separate night and day cases to study gw in the ionosphere as it is more difficult to find significant statistical differences during daytime this may happen because perturbations induced by gw during daytime are more likely to occur in a disturbed environment that may hinder the identification of the waves

imaging of a fluid injection process using geophysical data a didactic example author s commer m pride sr vasco dw finsterle s kowalsky mb abstract the authors in many subsurface industrial applications fluids are injected into or withdrawn from a geologic formation it is of practical interest to quantify precisely where when and by how much the injected fluid alters the state of the subsurface routine geophysical monitoring of such processes attempts to image the way that geophysical properties such as seismic velocities or electrical conductivity change through time and space and to then make qualitative inferences as to where the injected fluid has migrated the more rigorous formulation of the time lapse geophysical inverse problem forecasts how the subsurface evolves during the course of a fluid injection application using time lapse geophysical signals as the data to be matched the model unknowns to be estimated are the multiphysics forward modeling parameters controlling the fluid injection process properly reproducing the geophysical signature of the flow process subsequent simulations can predict the fluid migration and alteration in the subsurface the dynamic nature of fluid injection processes renders imaging problems more complex than conventional geophysical imaging for static targets this work intents to clarify the related hydrogeophysical parameter estimation concepts

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

fractured reservoirs an analysis of coupled elastodynamic and permeability changes from pore pressure variation equivalent medium theories can describe the elastic compliance and fluid permeability tensors of a layer containing closely spaced parallel fractures embedded in an isotropic background we propose a relationship between effective stress background or lithostatic stress minus pore pressure and both permeability and elastic constants this relationship uses an exponential decay function that captures the expected asymptotic behavior i e low effective stress gives high elastic compliance and high fluid permeability while high effective stress gives low elastic compliance and low fluid permeability the exponential decay constants are estimated for physically realistic conditions with relationships coupling pore pressure to permeability and elastic constants we are able to couple hydromechanical and elastodynamic modeling codes a specific coupled simulation is demonstrated where fluid injection in a fractured reservoir causes spatially and temporally varying changes in pore pressure permeability and elastic constants these elastic constants are used in a 3d finite difference code to demonstrate time lapse seismic monitoring with different acquisition geometries changes in amplitude and traveltime are seen in surface seismic p to s reflections as a function of offset and azimuth as well as in vertical seismic profile p to s reflections and in crosswell converted s waves these observed changes in the seismic response demonstrate seismic monitoring of fluid injection in the fractured reservoir

11 years of rayleigh lidar observations of gravity wave activity above the southern tip of south america gravity wave gw activity is analyzed using temperature t data retrieved from a rayleigh lidar at rio gallegos argentina 51 6 s 69 3 w gw characteristics are derived from 302 nights of observations providing more than 1018 h of high resolution lidar data between 20 km and 56 km height from august 2005 to december 2015 t measurements are performed by a differential absorption lidar dial instrument this lidar was the southernmost outside antarctica until the end of 2017 rio gallegos is an exceptional place to observe large amplitude gw every lidar measurement is classified according to its relative position to the polar vortex the lidar measurements are compared with collocated sounding of the atmosphere using broadband emission radiometry saber and global positioning system radio occultation gps ro data the different instruments show different windows of the gw spectrum providing complementary observations in general the geometric mean of the specific gw potential energy pe is larger during winter and spring than during summer and autumn the largest geometric mean of pe is found inside the vortex and decreases monotonically at its edge outside it and when there is no vortex the same behavior is observed with satellite data on average it can be seen that lidar observations provide larger pe values than limb sounding measurements from a morlet continuous wavelet transform analysis 3 distinct modes are captured from saber and from gps ro data at the upper and lower stratosphere respectively in particular a systematic 3 5 4 years oscillation possibly related to el nino southern oscillation is observed

the use of effective medium theories for seismic wave propagation and fluid flow in fractured reservoirs under applied stress summary methods for predicting the permeability and elastic properties of hydrocarbon reservoirs containing natural fractures are important for the production optimization and seismic characterization of fractured reservoirs effective medium theories may be used for this purpose but need to be validated using numerical calculations of the fluid flow and seismic properties for realistic fracture networks the prediction of effective permeability and elastic properties using effective medium theories of fractured media are compared with numerical simulations using a discrete fracture network dfn containing two sets of non orthogonal vertical fractures discrepancies between the permeability obtained from the simulations and using effective medium theory are attributed to an oversimplified treatment of fracture interconnectivity in the effective medium theory used the simulations indicate that the effect of interconnectivity on fluid flow varies with stress and flow direction by contrast the effective elastic compliances obtained from numerical simulation and effective medium theory are in good agreement even for relatively complicated fracture networks although permeability and seismic anisotropy both vary with stress the relation between them is not simple the variation in reflection amplitude with offset and azimuth is found to be sensitive to the ratio of the normal to shear compliance of the fractures whereas permeability is less sensitive to this ratio for the dfn studied the permeability rapidly increases at high level of stress due to dilation of the fractures when the in situ stress field is strongly anisotropic

effective elasticity of rocks with closely spaced and intersecting cracks the noninteraction approximation nia is the simplest effective media theory that describes the overall elasticity of fractured rocks if the nia is used for fracture characterization its accuracy and the range of applicability must be estimated we do it by performing a series of 3d finite element simulations of effective elasticity for models that contain several sets of fractures embedded in otherwise isotropic host rock we intentionally place the cracks close to each other to create strong interactions in their local stress fields in addition we allow the cracks to intersect in such a way that they do not break a rock specimen apart perhaps surprisingly we find that fracture interactions and intersections have little influence on the effective elasticity and the nia performs well in all cases while it has a tendency to slightly underestimate the effective stiffnesses the incurred errors are small their typical magnitudes are just a few percent in the entire range of the crack densities expe