late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

timing and structure of mega sacz events during heinrich stadial 1 a substantial strengthening of the south american monsoon system sams during heinrich stadials hs points toward decreased cross equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales in order to better constrain the exact timing and internal structure of hs1 over tropical south america we assessed two precisely dated speleothem records from central eastern and northeastern brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial atlantic during hs1 we recognize at least two events of widespread intensification of the sams across the entire region influenced by the south atlantic convergence zone sacz at 16 11 14 69 kyr b p and 18 1 16 66 kyr b p labeled as hs1a and hs1c respectively separated by a dry excursion from 16 66 to 16 11 kyr b p hs1b in view of the spatial structure of precipitation anomalies the widespread increase of monsoon precipitation over the sacz domain was termed mega sacz

broad synchrony of a late glacial glacier advance and the highstand of palaeolake tauca in the bolivian altiplano the morphology and sedimentology of glacially influenced fan deltas on massifs at the margin of the southern altiplano bolivia suggest a broadly synchronous expansion of glaciers and palaeolake tauca during the late glacial interval this is shown by sedimentary successions of glacigenic glacifluvial and glacideltaic facies linking palaeoglaciers with palaeolake tauca on the flanks of cerro azanaques and cerro tunupa at altitudes of 3770 3720 m radiocarbon dates from peat overlain by glacial diamict and glacifluvial outwash indicate that glaciers in this area reached their last glacial maximum extent after ca 13 300 14 c yr bp glacifluvial fan deltas contiguous with the moraines confirm that the advance coincided with a highstand of palaeolake tauca radiocarbon dated to the interval ca 13 500 11 500 yr bp modeling of climatic controls on this glacier advance suggests the primary forcing was increased summer wet season moisture possibly amounting to 600 mm above the modern values of 200 400 mm greater cloud cover probably depressed local temperatures and reduced the evaporation rate the consequent rise in effective annual moisture p e comfortably accommodates a palaeolake 48 50 10 3 km 2 in area and up to 100 m deep in the southern altiplano because the palaeoglacier equilibrium line altitudes rose toward the south and west like the gradient of modern precipitation totals we conclude that the increased late glacial moisture was brought by weather systems similar to those of the present but that atmospheric conditions were cloudier and cooler 1997 john wiley sons ltd

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

coupled thermo hydro mechanical processes in the near field of a high level radioactive waste repository in clay formations the present paper provides an overview of key coupled thermo hydro mechanical thm processes in clay formations that would result from the development of a high level radioactive waste repository here in this paper clay formations include plastic clay such as the boom clay of belgium as well as more indurated clay such as the callovo oxfordian and upper toarcian of france and opalinus clay of switzerland first we briefly introduce and describe four major underground research laboratories urls that have been devoted to clay repository research over the last few decades much of the research results in this area have been gained through investigations in these urls and their supporting laboratory and modeling research activities then the basic elements in the development of a waste repository in clays are presented in terms of four distinct stages in repository development for each of these four stages key processes and outstanding issues are discussed a summary of the important areas of research needs and some general remarks then conclude this paper

the morphology structural evolution and significance of push moraines abstract push moraines glaciotectonic ice marginal moraines have a restricted distribution at modern glacier margins and consequently are of potential value in reconstructing pleistocene ice sheets providing data both on former glaciodynamics and on the palaeoenvironment of the glacial foreland to the wider earth science community push moraines are of interest as analogues for thin skin tectonics within orogenic belts this paper reviews the morphology structural geology formation and significance of push moraines the morphological and structural characteristics of small seasonal push moraines through to large multi crested examples produced by sustained glacier advances are reported before the primary controls on push moraine formation are examined these controls include the nature of the applied glacial stress field the presence and properties of decollement horizons and the shear strength and rheology of the glacial foreland a conceptual model of push moraine formation is introduced in which the range of observed morphological and structural forms are viewed within a matrix defined by the main variables which control their formation the absence of consensus over which of these variables is of greatest importance currently limits the significance of push moraines in palaeoglaciological research as a consequence this review emphasises the need for future research in order to realise the true potential of push moraine in the reconstruction of pleistocene environments

high latitude forcing of the south american summer monsoon during the last glacial the climate of the last glacial period 10 000 to 110 000 years ago was characterized by rapid millennial scale climate fluctuations termed dansgaard oeschger d o and heinrich events we present results from a speleothem derived proxy of the south american summer monsoon sasm from 16 000 to 50 000 years ago that demonstrate the occurrence of d o cycles and heinrich events this tropical southern hemisphere monsoon reconstruction illustrates an antiphase relationship to northern hemisphere monsoon intensity at the millennial scale our results also show an influence of antarctic millennial scale climate fluctuations on the sasm this high resolution precisely dated tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the northern and southern hemispheres

a one dimensional model for the interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented a significant topographic obstacle to the westerly winds in northern midlatitudes this work explores how consequent changes in the atmospheric stationary wave pattern might have affected the shape and growth of the ice sheets themselves a one dimensional 1 d model is developed which permits an examination of the types and magnitudes of the feedbacks that might be expected when plausible temperature perturbations are introduced at the ice sheet margin which are proportional to the stationary wave amplitude the equilibrium shape of the ice sheet is significantly altered and depends on the sign of the perturbation the proposed feedback also affects the response of the ice sheet to time varying climate forcing the results suggest that the evolution of a continental scale ice sheet with a land based margin may be significantly determined by the changes it induces in the atmospheric circulation

quaternary glaciation and hydrologic variation in the south american tropics as reconstructed from the lake titicaca drilling project a 136 m long drill core of sediments was recovered from tropical high altitude lake titicaca bolivia peru enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370 000 yr within the errors of the age model the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages periods of ice advance in the southern tropical andes generally were periods of positive water balance as evidenced by deeper and fresher conditions in lake titicaca conversely reduced glaciation occurred during periods of negative water balance and shallow closed basin conditions in the lake the apparent coincidence of positive water balance of lake titicaca and glacial growth in the adjacent andes with northern hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global scale temperature changes as well as by precessional forcing of the south american summer monsoon

snowline depression in the tropics during the last glaciation five primary methods have been used to reconstruct pleistocene snowlines or equilibrium line altitudes elas in the tropics 23 53n 23 53s during the last glaciation but each has inherent errors that limit the accuracy of the results additional potential errorsin determiningela depression involveestimates of modernsnowline altitude datingresolution topographicreconstructionof former glaciers orographic e ects the presence of rockfall debris on glaciers and calculation of regional ela gradients eustatic sea level lowering during the last glaciation is an additional factor in uencing estimates of ela depression ela in cases where modern snowline lies above a mountain summit only a minimum value for ela can be obtained at 12 tropical sites in africa the americas to 103s latitude and paci c islands estimates of average ela range from 440 to 1400m but most fall in the range of 800 1000m mean 1 900 135m in a regional study of ela depression in the southern tropical andes 8 223s an average ela of ca 920 250m has been reported based on the assumption that glacier mass balance was controlled solely by ablation season temperature and assuming a full glacial temperature lapse rate of 63c km depression of mean annual temperature in glaciated alpine areas was ca 5 4 0 83c if adjusted for a sea level fall of 120m at the glacial maximum this value is reduced to 4 7 0 83c the gure is based on the unlikely assumption that accumulation on alpine glaciers has been invariant nevertheless it is similar to values of temperaturedepresson 5 6 43c for the last glaciationobtainedfrom variousterrestrial sites but contrasts with tropical sea surface temperature estimates that are only 1 33c cooler than present 2001 elsevier science ltd all rights reserved

800 000 years of abrupt climate variability we constructed an 800 000 year synthetic record of greenland climate variability based on the thermal bipolar seesaw model our greenland analog reproduces much of the variability seen in the greenland ice cores over the past 100 000 years the synthetic record shows strong similarity with the absolutely dated speleothem record from china allowing us to place ice core records within an absolute timeframe for the past 400 000 years hence it provides both a stratigraphic reference and a conceptual basis for assessing the long term evolution of millennial scale variability and its potential role in climate change at longer time scales indeed we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the middle to late pleistocene

evidence of an lgm cooling in nw argentina 22 s derived from a glacier climate model abstract a climate glacier model is used to interpret late pleistocene maximum glaciations in the nw argentinan andes 22 s according to the model massive cooling combined with a moderate precipitation increase is necessary to explain maximum glacier extension even given a factor of 2 4 precipitation increase the model indicates that temperatures must have been lowered by 4 5 8 c suggesting that maximum glaciation probably occurred during full glacial times of mis 2 25 18 ka this conclusion is supported by dated proglacial lake sediments and evidence of past glacial geometry the extremely long tongues and low accumulation area ratios indicate that ablation was reduced during cold periods in agreement with the model a strong relationship exists between glacier geometry and prevailing climate contrary to the precipitation controlled late glacial 12 10 ka maximum glaciation of the arid central andes in southern peru and northern chile the glaciations on the much more humid eastern andean slope are mainly controlled by temperature and therefore reached their maximum extent during the mis 2 cold conditions

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

climatic interpretation of alpine snowline variations on millennial time scales abstract the depression of snowlines or equilibrium line altitudes of alpine glaciers is often used by glacial geologists to infer variations in mass balance the climatic interpretation of snowline depression however is complicated by the number of factors that control glacier mass balance the simple lapse rate method of temperature interpretation ignores the effects of changes in radiation and snow accumulation the statistical approach to temperature interpretation which regresses precipitation and temperature against snowline altitude neglects the effect of radiation the most comprehensive approach for the climatic interpretation of snowline depression couples the heat and mass balances of a glacier surface a sensitivity analysis that utilizes the coupled heat and mass balance approach indicates that the 1000 m variation in snowline of alpine glaciers on glacial to interglacial time scales could be a result of significant changes in temperature and to a lesser extent changes in insolation snowline variations are sensitive only to relatively large changes in annual accumulation which should also be evident in other proxy records of moisture change the approaches outlined here provide glacial geologists with a summary of how various climatic forcings associated with glaciation may be quantified from snowline data

abrupt glacial climate shifts controlled by ice sheet changes the volume of the northern hemisphere ice sheet controlled abrupt millennial scale climate changes during the last glacial dansgaard oeschger events are millennial scale warm periods interrupting cold glacial climates although they are perhaps the most prominent example of abrupt change observed in the ice core record of the past 800 000 years their underlying physical controls remain debated xu zhang and colleagues now use a fully coupled climate model to show that dansgaard oeschger events tend to occur when ice sheets are at an intermediate height a condition in which small perturbations to the coupled atmosphere ocean sea ice system can lead to rapid nonlinear feedbacks and abrupt changes in climate during glacial periods of the late pleistocene an abundance of proxy data demonstrates the existence of large and repeated millennial scale warming episodes known as dansgaard oeschger do events1 this ubiquitous feature of rapid glacial climate change can be extended back as far as 800 000 years before present bp in the ice core record2 and has drawn broad attention within the science and policy making communities alike3 many studies have been dedicated to investigating the underlying causes of these changes but no coherent mechanism has yet been identified3 4 5 6 7 8 9 10 11 12 13 14 15 here we show by using a comprehensive fully coupled model16 that gradual changes in the height of the northern hemisphere ice sheets nhiss can alter the coupled atmosphere ocean system and cause rapid glacial climate shifts closely resembling do events the simulated global climate responses including abrupt warming in the north atlantic a northward shift of the tropical rainbelts and southern hemisphere cooling related to the bipolar seesaw are generally consistent with empirical evidence1 3 17 as a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets minor changes in the height of the nhiss and the amount of atmospheric co2 can trigger the rapid climate transitions via a local positive atmosphere ocean sea ice feedback in the north atlantic our results although based on a single model thus provide a coherent concept for understanding the recorded millennial scale variability and abrupt climate changes in the coupled atmosphere ocean system as well as their linkages to the volume of the intermediate ice sheets during glacials

late pleistocene climate conditions in the north chilean andes drawn from a climate glacier model a climate glacier model was used to reconstruct late glacial climate conditions from two case study glaciers at 18 and 22 s in the arid sub tropical western andes of northern chile the model uses i the geometry of the late glacial maximum glaciation ii modern diurnal and annual cycles amplitudes and lapse rates of the climate iii empirical statistical sublimation melt and accumulation models developed for this area and iv dynamic ice flow through two known cross sections for steady state conditions the model is validated with modern conditions and compares favorably with the glaciological features of today the mass balance model calculates the modern equilibrium line altitude at 18 s as high as 5850 m field data 5800 m whereas no glaciers exist in the fully arid southern area at 22 s despite altitudes above 6000 m and continuous permafrost for late glacial times the model results suggest a substantial increase in tropical summer precipitation ap 840 50 10 mm a 1 for the northern test area 1000 10 30 mm a 1 for the southern test area and a moderate temperature depression t 4 4 0 1 0 2 c at 18 s 3 2 0 1 c at 22 s extratropical frontal winter precipitation june september was 15 of the total annual precipitation a scenario with higher winter precipitation from the westerlies circulation belt does not yield a numerical solution which matches the observed geometry of the glaciers therefore we conclude that an equatorward displacement of the westerlies must be discarded as a possible explanation for the late pleistocene glaciation in the andes of northern chile

hydrologic issues associated with nuclear waste repositories significant progress in hydrology especially in subsurface flow and solute transport has been made over the last 35 years because of sustained interest in underground nuclear waste repositories the present paper provides an overview of the key hydrologic issues involved and to highlight advances in their understanding and treatment because of these efforts the focus is not on the development of radioactive waste repositories and their safety assessment but instead on the advances in hydrologic science that have emerged from such studies work and results associated with three rock types which are being considered to host the repositories are reviewed with a different emphasis for each rock type the first rock type is fractured crystalline rock for which the discussion will be mainly on flow and transport in saturated fractured rock the second rock type is unsaturated tuff for which the emphasis will be on flow from the shallow subsurface through the unsaturated zone to the repository the third rock type is clay rich formations whose permeability is very low in an undisturbed state in this case the emphasis will be on hydrologic issues that arise from mechanical and thermal disturbances i e on the relevant coupled thermo hydro mechanical processes the extensive research results especially those from multiyear large scale underground research laboratory investigations represent a rich body of information and data that can form the basis for further development in the related areas of hydrologic research

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

a new andean deep ice core from nevado illimani 6350 m bolivia abstract a new ice core record from the nevado illimani 16 s bolivia covers approximately the last 18 000 years bp a comparison with two published ice records from sajama 18 s bolivia thompson et al science 282 1998 1858 1864 and huascaran 9 s peru thompson et al science 269 1996 46 50 documents a regionally coherent transition from glacial to modern climate conditions in south america north of 20 s the strong resemblance between the illimani and huascaran water isotope records and their differences from the sajama record in particular during the period from 9000 years bp to 14 000 years bp suggest that local water recycling or local circulation changes played a major role for sajama we interpret the common illimani huascaran water isotope history in terms of a common change from wetter cooler conditions during glacial times to drier warmer conditions in the early holocene

impact of bolivian paleolake evaporation on the δ18o of the andean glaciers during the last deglaciation 18 5 11 7 ka diatom inferred δ18o values and hydro isotopic modeling abstract during the last deglaciation the bolivian altiplano 15 23 s 66 70 w was occupied by paleolake tauca covering at least 51 000 km 2 at its maximum highstand between 16 5 and 15 ka twenty five hundred years later after a massive regression a new transgressive phase produced paleolake coipasa smaller than tauca and restricted to the southern part of the basin these paleolakes were overlooked at the west by the sajama ice cap the latter provides a continuous record of the oxygen isotopic composition of paleo precipitation for the last 25 ka contemporaneously to the end of paleolake tauca around 14 3 ka the sajama ice cap recorded a significant increase in ice oxygen isotopic composition 18 o ice this paper examines to what extent the disappearance of lake tauca contributed to precipitation on the sajama summit and this specific isotopic variation the water 18 o values of paleolakes tauca and coipasa 18 o lake were quantitatively reconstructed from 18 5 to 11 7 ka based on diatom isotopic composition 18 o diatoms and ostracod isotopic composition 18 o carbonates retrieved in lacustrine sediments at a centennial time scale a strong trend appears abrupt decreases of 18 o lake during lake fillings are immediately followed by abrupt increases of 18 o lake during lake level stable phases the highest variation occurred at 15 8 ka with a 18 o lake decrease of about 10 concomitant with the lake tauca highstand followed 400 years later by a 7 increase in 18 o lake a simple hydro isotopic modeling approach reproduces consistently this rapid decrease increase feature moreover it suggests that this unexpected re increase in 18 o lake after filling phases can be partly explained by an equilibration of isotopic fluxes during the lake steady state based on isotopic calculations during lake evaporation and a simple water stable isotopes balance between potential moisture sources at sajama advection versus lake evaporation we show that total or partial evaporation from 5 to 60 of paleolake tauca during its major regression phase at 14 3 ka could explain the pronounced isotopic excursion at sajama ice cap these results suggest that perturbations of the local hydrological cycle in lacustrine areas may substantially affect the paleoclimatic interpretation of the near by isotopic signals e g ice core or speleothems

on the thm behaviour of a sheared boom clay sample application to the behaviour and sealing properties of the edz abstract in this study the effect of the temperature increase on the hydro mechanical properties of sheared boom clay samples is investigated two samples of boom clay are resaturated in a new hollow cylinder triaxial cell with a short drainage path and then sheared by performing an axisymmetric triaxial loading the effect of the undrained heating under deviatoric stress is studied on the first sample it is shown that undrained temperature increase leads to an increase of pore water pressure and consequently to a decrease of the effective mean stress which brings the sample to failure for an initially sheared sample the failure occurs along the existing shear band which behaves as a weakness plane in the sample the responses of the local strain measurement transducers clearly show the sliding of rigid blocks when failure occurs in the sample the effect of the presence of a shear band on the permeability of the other boom clay sample is investigated at ambient temperature and at 80 c it is shown that the presence of a shear band does not affect significantly the permeability these results confirm the good self sealing properties of boom clay at ambient and at high temperature

timing of the last deglaciation in the sierra nevada of the merida andes venezuela abstract in the tropical merida andes northwestern venezuela glacial landforms were found at altitudes between 2600 and 5000 m corresponding to 600 km2 of ice cover during the maximum glacial extension however the lack of sufficient absolute age data prevents detailed reconstruction of the timing of the last deglaciation on the northwestern flank of the mucunuque massif successive moraines and striated eroded basement surfaces were sampled for cosmogenic 10be investigation their compilation with published data allows the establishment of a detailed chronology of the post lgm glacier history the oldest moraines 18 1 and 16 8 ka correspond to the oldest dryas successive moraine ridges indicate stops in the overall retreat between the lgm and the younger dryas the cold and short older dryas stadial has been identified results indicate that most of the ice withdrew during the pleistocene the dataset supports an intensification of the vertical retreat rate from 25 m ka during the late pleistocene to 310 m ka during the pleistocene holocene afterwards the glacier was confined and located in the higher altitude zones the altitude difference of the younger dryas moraines in the mucubaji la victoria and los zerpa valleys indicates a strong effect of valley orientation on the altitude of moraine development

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

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

cosmogenic dating of late pleistocene glaciation southern tropical andes peru we present 30 10be dates on three moraines formed by small glaciers in the southern tropical andes of peru to determine the timing of the local last glacial maximum lgm and subsequent deglaciation two terminal moraines indicate that the local lgm ended at 23 5 0 5 and 21 2 0 8 ka or well before the end of the global lgm 20 19 ka a recessional moraine perched at the base of a steep valley wall has a 10be age of 19 8 0 3 ka suggesting that glaciers had largely retreated from the area during the earliest stages of the last termination and before the onset of the deglacial co2 rise and increase in tropical sea surface temperatures these ages generally agree with other tropical glacial chronologies and reflect a broad synchroneity between low and high latitude deglaciation at the lgm but the trigger for early deglaciation of the tropics remains unclear

how did the hydrologic cycle respond to the two phase mystery interval abstract lake estancia s transition from a big dry episode during the first half of the mystery interval to a big wet episode during the second half has equivalents in records from across the planet at the time of this transition chinese monsoons experienced pronounced weakening closed basin lakes in both the great basin of the western united states and in the southern altiplano of south america underwent a major expansion mountain glaciers in southern hemisphere middle latitudes had retreated and the rates of increase of co2 and of 18o in antarctic ice underwent a decrease finally the precipitous drop in dust rain over antarctica and the southern ocean terminated as did a similar drop in the 13c to 12c ratio in atmospheric co2 these changes are consistent with a southward shift of the thermal equator the cause of such a shift is thought to be an expansion of sea ice caused by a shutdown in deep water production in the northern atlantic this creates a dilemma because a similar southward shift is an expected consequence of the heinrich event 1 which initiated the mystery interval

a cosmogenic 10be chronology for the local last glacial maximum and termination in the cordillera oriental southern peruvian andes implications for the tropical role in global climate abstract resolving patterns of tropical climate variability during and since the last glacial maximum lgm is fundamental to assessing the role of the tropics in global change both on ice age and sub millennial timescales here we present a10be moraine chronology from the cordillera carabaya 14 3 s a sub range of the cordillera oriental in southern peru covering the lgm and the first half of the last glacial termination additionally we recalculate existing 10be ages using a new tropical high altitude production rate in order to put our record into broader spatial context our results indicate that glaciers deposited a series of moraines during marine isotope stage 2 broadly synchronous with global glacier maxima but that maximum glacier extent may have occurred prior to stage 2 thereafter atmospheric warming drove widespread deglaciation of the cordillera carabaya a subsequent glacier resurgence culminated at 16 100 yrs followed by a second period of glacier recession together the observed deglaciation corresponds to heinrich stadial 1 hs1 18 000 14 600 yrs during which pluvial lakes on the adjacent peruvian bolivian altiplano rose to their highest levels of the late pleistocene as a consequence of southward displacement of the inter tropical convergence zone and intensification of the south american summer monsoon deglaciation in the cordillera carabaya also coincided with the retreat of higher latitude mountain glaciers in the southern hemisphere our findings suggest that hs1 was characterised by atmospheric warming and indicate that deglaciation of the southern peruvian andes was driven by rising temperatures despite increased precipitation recalculated 10be data from other tropical andean sites support this model finally we suggest that the broadly uniform response during the lgm and termination of the glaciers examined here involved equatorial pacific sea surface temperature anomalies and propose a framework for testing the viability of this conceptual model

interhemispheric atlantic seesaw response during the last deglaciation the asynchronous relationship between millennial scale temperature changes over greenland and antarctica during the last glacial period has led to the notion of a bipolar seesaw which acts to redistribute heat depending on the state of meridional overturning circulation within the atlantic ocean here we present new records from the south atlantic that show rapid changes during the last deglaciation that were instantaneous within dating uncertainty and of opposite sign to those observed in the north atlantic our results demonstrate a direct link between the abrupt changes associated with variations in the atlantic meridional overturning circulation and the more gradual adjustments characteristic of the southern ocean these results emphasize the importance of the southern ocean for the development and transmission of millennial scale climate variability and highlight its role in deglacial climate change and the associated rise in atmospheric carbon dioxide

constraints on southern hemisphere tropical climate change during the little ice age and younger dryas based on glacier modeling of the quelccaya ice cap peru 2015 the authors improving the late quaternary paleoclimate record through climate interpretations of low latitude glacier length changes advances our understanding of past climate change events and the mechanisms for past present and future climate change paleotemperature reconstructions at low latitude glaciers are uniquely fruitful because they can provide both site specific information and enhanced understanding of regional scale variations due to the structure of the tropical atmosphere we produce little ice age lia and younger dryas yd paleoclimate reconstructions for the huancane outlet glacier of the quelccaya ice cap qic and low latitude southern hemisphere regional sea surface temperatures ssts using a coupled ice flow and energy balance model we also model the effects of long term changes in the summit temperature and precipitiation rate and the effects of interannual climate variability on the huancane glacier length we find temperature to be the dominant climate driver of glacier length change also we find that interannual climate variability cannot adequately explain glacier advances inferred from the geomorphic record necessitating that these features were formed during past colder climates to constrain our lia reconstruction we incorporate the qic ice core record finding a lia air temperature cooling at the ice cap of between 0 7 c and 1 1 c and 0 4 c and regional ssts cooling of 0 6 c for the yd paleoclimate reconstructions we propose two limits on the precipitation rate since the ice core record does not extend into the pleistocene 1 the precipitation rate scales with the clausius clapeyron relationship upper limit on cooling and 2 the precipitation rate increases by 40 lower limit on cooling which is an increase about twice as great as the regional increases realized in gcm simulations for the period the first limit requires 1 6 c cooling in ice cap air temperatures and 0 9 c cooling in ssts and the second limit requires 1 0 c cooling in ice cap air temperatures and 0 5 c cooling in ssts our temperature reconstructions are in good agreement with the magnitude and trend of gcm simulations that incorporate the forcing mechanisms hypothesized to have caused these climate change events

long term low latitude and high elevation cosmogenic 3he production rate inferred from a 107 ka old lava flow in northern chile 22 s 3400 m a s l abstract available geological calibration sites used to estimate the rate at which cosmogenic 3he is produced at the earth s surface are mostly clustered in medium to high latitudes moreover most of them have exposure histories shorter than tens of thousands of years this lack of sites prevents a qualitative assessment of available production models used to convert cosmogenic 3he concentrations into exposure ages and or denudation rates it thus limits our ability to take into account the atmospheric geomagnetic and solar modulation conditions that might have affected the production of cosmogenic nuclides in the past for longer exposure histories and in low latitude regions we present the cosmogenic 3he production rate inferred from a new geological calibration site located in northern chile five samples were collected on the surface of the largest and best preserved lava flow of the san pedro volcano 21 934 s 68 510 w 3390 m a s l which displays pristine crease structure features 40ar 39ar dating yields a reliable plateau age of 107 12 ka for the eruption of this lava flow eight pyroxene aliquots separated from the surface samples yield a weighted average cosmogenic 3he concentration of 99 3 1 2 mat g 1 from which a local cosmogenic 3he production rate of 928 101 at g 1 yr 1 is calculated the local production rate is then scaled to a sea level high latitude slhl reference position using different combinations of geographic spatialization schemes atmosphere models and geomagnetic field reconstructions yielding slhl production rates between 103 11 and 130 14 at g 1 yr 1 consistent with the most recent estimates available from the literature finally we use the same scaling frameworks to re evaluate the mean global scale cosmogenic 3he production rate in olivine and pyroxene minerals at 120 16 at g 1 yr 1 from the compilation of previously published calibration datasets

drying induced shrinkage of boom clay an experimental investigation drying induced shrinkage of geomaterials may have a strong effect on geostructure stability and deformation the settlement of foundations and fracture openings in slopes roads and tunnel walls may be due to drying shrinkage however there is still a lack of knowledge concerning shrinkage evolution with time and shrinkage propagation within the material in this study the shrinkage of a specific clayey rock boom clay under drying conditions is investigated experimentally this rock is a deep geological formation which is under study for high level and long term radioactive waste storage in belgium two experimental campaigns are presented the first based on the vapour equilibrium drying technique and coupled with sample size manual measurement aims to characterize the material shrinkage in balanced states the second based on the convective drying technique and coupled with shape monitoring using x ray tomography aims to analyse how shrinkage develops before reaching a steady state both app

late quaternary glacier response to humidity changes in the arid andes of chile 18 29 s abstract today no glaciers exist between 18 30 s and 27 s even on mountains much higher than 6000 m these dry high mountain environments are very sensitive to changes in climatic boundary conditions particularly humidity since temperatures are far below freezing here we present a reconstruction of climatic changes since the last glacial maximum for the chilean dry andes of the southern hemisphere we reconstructed regional equilibrium line altitudes ela for three different moraine stages representing extensive past glaciations in this currently unglaciated region comparison of the regional pattern of the ela with modern climate conditions allows us to draw implications about the paleoclimatic conditions during the best preserved moraine stage ii glaciations in the northern as well as in the southern part of the research area our results suggest humid conditions in the northern part 18 24 s during late glacial times with strongly increased convective precipitation during austral summer the temporal coincidence of glaciers in the mountains and high lake levels on the altiplano tauca phase is evident to the south no simple shift of the westerlies is implied by our glacier reconstructions the northern limit of effective moisture for glacier formation did not shift significantly but stayed in the area of the present day northernmost glaciers of the westerlies at about 27 s but further south precipitation increased significantly accompanied by at least 2 3 c colder conditions the age of this glaciation is uncertain but probably of late glacial or last glacial maximum lgm times overall glaciers in the dry andes of south america clearly responded to large changes in the humidity not primarily the temperature regime with all its consequences on the environment hydrology vegetation etc illustrating the different regional responses to global climatic change

antarctic like temperature variations in the tropical andes recorded by glaciers and lakes during the last deglaciation abstract the respective impacts of northern and southern hemispheric climatic changes on the tropics during the last deglaciation remain poorly understood in the high tropical andes the antarctic cold reversal acr 14 3 12 9 ka bp is better represented among morainic records than the younger dryas 12 9 11 7 ka bp however in the altiplano basin bolivia two cold periods of the northern hemisphere heinrich stadial 1a 16 5 14 5 ka bp and the younger dryas are synchronous with i major advances or standstills of paleoglaciers and ii the highstands of giant paleolakes tauca and coipasa here we present new cosmic ray exposure cre ages from glacial landforms of the bolivian andes that formed during the last deglaciation termination 1 we reconstruct the equilibrium line altitudes ela associated with each moraine and use them in an inverse algorithm combining paleoglaciers and paleolake budgets to derive temperature and precipitation during the last deglaciation our temperature reconstruction t relative to present day yields a consistent regional trend of progressive warming from t 5 to 2 5 c during 17 14 5 ka bp followed by a return to colder conditions around 4 c during the acr 14 5 12 9 ka bp the coipasa highstand 12 9 11 8 ka bp is coeval with another warming trend followed by t stabilization at the onset of the holocene ca 10 ka bp around 3 c our results suggest that during the last deglaciation 20 10 ka bp atmospheric temperatures in the tropical andes mimicked antarctic variability whereas precipitation over the altiplano was driven by changes in the northern hemisphere

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

too young or too old evaluating cosmogenic exposure dating based on an analysis of compiled boulder exposure ages cosmogenic exposure dating has greatly enhanced our ability to define glacial chronologies spanning several global cold periods and glacial boulder exposure ages are now routinely used to constrai

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

lake highstands on the altiplano tropical andes contemporaneous with heinrich 1 and the younger dryas new insights from 14 c u th dating and δ 18 o of carbonates this study provides new geochronological and stable isotope constraints on late pleistocene fluctuations in lake level that occurred in the closed watershed of the central altiplano between 25 and 12 ka u series isochrons and 14 c ages from carbonates are used to confirm and refine the previous chronology published placzek et al 2006b our new data support three successive lake highstands during the late pleistocene i the lake sajsi cycle from 25 to 19 ka that culminated at 3670 m at about 22 ka almost synchronously with the global last glacial maximum ii the lake tauca cycle that lasted from 18 to 14 5 ka and was characterized by the highest water level reached at least 3770 m from 16 5 to 15 ka iii the lake coipasa cycle from 12 5 to 11 9 ka that reached an elevation of 3700 m 42 m above the elevation of the salar de uyuni 3658 m these high amplitude lake level fluctuations are in phase with the cold warm oscillations that occurred in the north atlantic and greenland during the late pleistocene heinrich 1 bolling allerod younger dryas such temporal coincidence supports the hypothesis that wet events recorded in the central altiplano are controlled by the northesouth displacement of the inter tropical convergence zone resulting from changes in the meridional temperature gradient finally the oxygen isotope ratios measured in these lacustrine carbonates allows for calculation of the 18 o value of paleolake waters estimates of water 18 o v smow are 2 8 0 7 for lake tauca and 1 6 0 9 for lake coipasa these data are used to constrain changes in lake hydrology and can be interpreted to indicate that the proportion of precipitation arising from local water recycling was less than 50

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

late pleistocene glaciations of the arid subtropical andes and new results from the chajnantor plateau northern chile abstract the spatiotemporal pattern of glaciation along the andes mountains is an important proxy record reflecting the varying influence of global and regional circulation features on south american climate however the timing and extent of glaciation in key parts of the orogen particularly the deglaciated arid andes are poorly constrained we present new cosmogenic 10 be and 36 cl exposure ages for glacial features on and near the chajnantor plateau 23 s the new dates although scattered due to cosmogenic inheritance imply that the most recent extensive glacial occupation ended before or during the global last glacial maximum lgm we discuss this new record in the context of published glacial chronologies from glacial features in peru bolivia and northern chile rescaled using the latest cosmogenic 10 be production rate calibration for the tropical andes the results imply regionally synchronous moraine stabilization ca 25 40 ka 15 17 ka and 12 14 ka with the youngest of these moraines absent in records south of 20 s including in our new chajnantor area chronology this spatial pattern implicates easterly moisture in generating sufficient snowfall to glaciate the driest parts of the andes while allowing a role for westerly moisture possibly modulated by the migration of the southern westerly wind belt in the regions near and south of the atacama desert

cosmogenic exposure dating of late pleistocene moraine stabilization in alaska seventy three new 1 0 be 2 6 al ages from 57 moraine boulders and 2 tors together with 43 previously published cosmogenic exposure ages from 41 moraine boulders allow us to critique the use of cosmogenic exposure ce dating of moraine boulders in alaska boulder exhumation during moraine degradation likely gives rise to the largest uncertainty in constraining the timing of initial moraine stabilization following ice retreat isotopic inheritance appears to be most important for moraines deposited close to their cirque headwalls boulder surface bedrock erosion rate can be roughly constrained and leads to a range in moraine stabilization ages snow cover history is difficult to constrain but its effect is thought to be minor for the tall boulders sampled despite these complications the ce ages provide important new information regarding the timing of the last and penultimate glaciations in alaska three penultimate moraines yielded ce ages that overlap with marine isotope stage mis 4 early mis 3 45 65 ka rather than mis 6 ca 140 ka based on a combination of our new ce chronologies and existing 1 4 c ages from six study areas glaciers retreated from their local late wisconsin maxima ca 24 27 ka kokrines hills west interior alaska ca 24 26 ka northeastern brooks range ne alaska ca 21 23 ka yukon tanana upland east interior alaska ca 22 ka ahklun mountains sw alaska ca 20 ka western alaska range central alaska ca 16 18 ka chuilnuk mountains sw alaska overall glacier retreat was concurrent with the peak of the last global glacial maximum probably in response to limited moisture availability

cosmogenic 10be production rate calibrated against 3he in the high tropical andes 3800 4900 m 20 22 s abstract many geomorphologic applications notably glacier chronologies require improvements in both the precision and the accuracy of the cosmogenic dating tool of particular importance is the need to better constrain the spatial variability of the cosmogenic nuclides production rates at high elevation and low latitudes one strategy that can be adopted for this is to couple absolute calibrations from independently dated surfaces with cross calibration studies performed by measuring several cosmogenic nuclides in the same rock in the present study we report the highest elevation 4800 m cross calibration published to date comprising measurements of cosmogenic 3 he and 10 be in cogenetic pyroxene and quartz the samples were collected from six dacitic moraine boulders exposed from 32 to 65 ka at 4820 m on the flanks of the uturuncu volcano 22 s 67 w southern lipez bolivia the samples yield a remarkably tight cluster of 3 he 10 be production ratios with a weighted mean of 33 3 0 9 1 this production ratio is undistinguishable within uncertainties from the 3 he 10 be production ratio of 32 3 0 9 determined in the same mineral pair at low elevation 1333 m by amidon et al 2009 these results agree at the 1 level and suggest that any hypothetical increase of the 3 he 10 be production ratio in pyroxene and quartz is likely to be lower than 5 over this elevation range 1000 5000 m moreover the production ratio is almost insensitive to the li content of the pyroxene 20 to 50 ppm li suggesting that the cosmogenic thermal neutron production of 3 he is very low in this setting the high elevation 3 he 10 be production ratio is used in combination with a local determination of the 3 he production rate in the high central altiplano 3800 m blard et al 2013 to establish a local 10 be production rate of 30 0 1 4 at g 1 yr 1 at 3800 m and 20 s after scaling to sea level high latitude with the time dependent lal stone model this yields a 10 be production rate in quartz of 3 63 0 17 at g 1 yr 1 importantly this rate can be used for high precision geomorphologic dating for example for determining glacial chronologies 1 4 through 10 be dating of moraines in the high tropical andes any inaccuracy attached to the scaling model is canceled out when the calibration site is located close to the dated object the same experiment was also undertaken in pyroxene and plagioclase from two andesitic boulders from the tunupa volcano moraines exposed at 3800 m and 4200 m a 3 he 10 be production ratio of 35 9 1 3 1 is obtained for this mineral pair indicating that the 10 be production rate in plagioclase is about 8 lower than in quartz

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

quaternary glaciations in the verkhoyansk mountains northeast siberia geomorphological mapping revealed five terminal moraines in the central verkhoyansk mountains the youngest terminal moraine i was formed at least 50 ka ago according to new irsl infrared optically stimulated luminescence dates older terminal moraines in the western foreland of the mountains are much more extensive in size although the smallest of these older moraines moraine ii has not been dated moraine iii is 80 to 90 ka moraine iv is 100 to 120 ka and the outermost moraine v was deposited around 135 ka this glaciation history is comparable to that of the barents and kara ice sheet and partly to that of the polar ural mountains regarding the timing of the glaciations however no glaciation occurred during the global last glacial maximum mis 2 based on cirque orientation and different glacier extent on the eastern and western flanks of the verkhoyansk mountains local glaciations are mainly controlled by moisture transport from the west across the eurasian continent thus glaciations in the verkhoyansk mountains not only express local climate changes but also are strongly influenced by the extent of the eurasian ice sheets

a 25 000 year tropical climate history from bolivian ice cores ice cores that were recovered from the summit of sajama mountain in bolivia provide carbon 14 dated tropical records and extend to the late glacial stage lgs oxygen isotopic ratios of the ice decreased 5 4 per mil between the early holocene and the last glacial maximum which is consistent with values from other ice cores the abrupt onset and termination of a younger dryas type event suggest atmospheric processes as the probable drivers regional accumulation increased during the lgs during deglaciation and over the past 3000 years which is concurrent with higher water levels in regional paleolakes unlike polar cores sajama glacial ice contains eight times less dust than the holocene ice which reflects wetter conditions and extensive snow cover

late local glacial maximum in the central altiplano triggered by cold and locally wet conditions during the paleolake tauca episode 17 15 ka heinrich 1 the timing and causes of the last deglaciation in the southern tropical andes is poorly known in the central altiplano recent studies have focused on whether this tropical highland was deglaciated before synchronously or after the global last glacial maximum 21 ka bp in this study we present a new chronology based on cosmogenic 3he 3he c dating of moraines on cerro tunupa a volcano that is located in the centre of the now vanished lake tauca 19 9 s 67 6 w these new 3he c ages suggest that the tunupa glaciers remained close to their maximum extent until 15 ka bp synchronous with the lake tauca highstand 17 15 ka bp glacial retreat and the demise of lake tauca seem to have occurred rapidly and synchronously within dating uncertainties at 15 ka bp we took advantage of the synchronism of these events to combine a glacier model with a lake model in order to reconstruct precipitation and temperature during the lake tauca highstand this new approach indicates that during the tauca highstand 17 15 ka bp the centre of the altiplano was characterized by temperature 6 5 c cooler and average precipitation higher by a factor ranging between 1 6 and 3 compared to the present cold and wet conditions thus persisted in a significant part of the southern tropical andes during the heinrich 1 event 17 15 ka bp this study also demonstrates the extent to which the snowline of glaciers can be affected by local climatic conditions and emphasizes that efforts to draw global climate inferences from glacial extents must also consider local moisture conditions

late pleistocene glacial equilibrium line altitudes in the colorado front range a comparison of methods abstract six methods for approximating late pleistocene pinedale equilibrium line altitudes elas are compared for rapidity of data collection and error rmse from first order trend surfaces using the colorado front range trend surfaces computed from rapidly applied techniques such as glaciation threshold median altitude of small reconstructed glaciers and altitude of lowest cirque floors have relatively high rmses 97 186 m because they are subjectively derived and are based on small glaciers sensitive to microclimatic variability surfaces computed for accumulation area ratios aars and toe to headwall altitude ratios thars of large reconstructed glaciers show that an aar of 0 65 and a thar of 0 40 have the lowest rmses about 80 m and provide the same mean ela estimate about 3160 m as that of the more subjectively derived maximum altitudes of pinedale lateral moraines rmse 149 m second order trend surfaces demonstrate low elas in the latitudinal center of the front range perhaps due to higher winter accumulation there the mountains do not presently reach the ela for large glaciers and small front range cirque glaciers are not comparable to small glaciers existing during pinedale time therefore pleistocene ela depression and consequent temperature depression cannot reliably be ascertained from the calculated ela surfaces

geomagnetic field intensity during the last 60 000 years based on 10be and 36cl from the summit ice cores and 14c abstract based on 10 be 36 cl and 14 c records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic the geomagnetic field derived from the 10 be flux to summit in central greenland agrees well with independent geomagnetic field intensity reconstructions however compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20 000 to 30 000 years bp 36 cl from the grip ice core agrees well with the 10 be record for the last ice age and hence also the reconstructed geomagnetic field based on the tree ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the holocene a comparison of radiocarbon based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10 000 years bp

glacier and climate reconstruction at tres lagunas nw argentina based on 10be surface exposure dating and lake sediment analyses abstract we present 35 10 be surface exposure ages from quartzite moraine boulders at tres lagunas in the sierra de santa victoria nw argentina additionally an 8 m sediment core from a glacial lake was investigated to provide independent radiocarbon age control the oldest and most extensive glaciation occurred 116 3 ka and likely coincides with the lake transgression phase ouki on the altiplano maximum glaciation in phase with the global last glacial maximum lgm is dated to 22 25 ka and documents a glacial mass balance response to substantially lower temperatures and minor precipitation increase in phase with the sajsi lake transgression phase after initial deglaciation and formation of a small paleolake at tres lagunas a glacial readvance is documented by an outwash diamict in the sediment core and radiocarbon dated to 19 17 ka as well as by a moraine surface exposure dated to 17 ka the most prominent moraine was deposited at 14 5 ka coinciding with the lake transgression phase tauca on the altiplano this late glacial advance reflects the massive intensification of the south american summer monsoon sasm in response to northern hemispheric cooling during heinrich i and the resultant southward shift of the intertropical convergence zone itcz and la nina like conditions in the eastern equatorial pacific rapid deglaciation started after 14 5 ka interrupted only by minor glacial still stands at 13 8 1 5 ka and 12 0 1 5 ka the latter one correlating with the coipasa transgression phase on the altiplano and the younger dryas

reconstruction of equilibrium line altitudes for tropical and sub tropical glaciers past fluctuations of tropical and sub tropical glaciers provide important palaeoclimate proxies for regions where other forms of evidence are rare however published equilibrium line altitude ela estimates for tropical and sub tropical glaciers at the lgm vary widely reflecting the diversity of methods and approaches employed by different research groups this complicates regional and global comparisons of ela estimates and emphasises the need for standardised methods the distinctive character of tropical and sub tropical glaciers however means that standard methods for reconstructing former glacier limits elas and palaeoclimate need to be adapted for local conditions many methods of ela reconstruction explicitly or implicitly make assumptions about glacier mass balance gradients and care needs to be taken that the choice of accumulation area ratios aars balance ratios brs and terminus to head ratios thars is appropriate as such indices are influenced by climatic regime debris cover and other factors ela reconstructions should employ multiple methods and should be cross checked and fully reported to allow assessment of the accuracy of ela estimates reliable glacier chronologies are equally important dating should be based on multiple radiometric techniques wherever possible and method of dating the type of material dated and the context of the date must all be reported

twenty five years geotechnical observation and testing in the tertiary boom clay formation the research and development programme on geological disposal for high level and long lived waste hlw in belgium was initiated in 1974 a deep tertiary clay formation the boom clay present under the mol dessel nuclear site was selected as a reference host formation for experimental purposes the construction of the underground laboratory hades at a depth of 223 m initiated in 1980 and extended in 2002 allowed the building of a valuable geotechnical database and led to the development of improved excavation techniques that significantly reduce the excavation damaged zone edz since the operational start of hades about 25 years ago many geotechnical measurements have been performed around excavations comparison between in situ measurements and modelling results allowed a continuous improvement of our knowledge on the boom clay behaviour important issues for interpreting the measurements correctly are good control of the excavation parameters and the boundary conditions an extensive characteri

evaluating the sonic layer depth relative to the mixed layer depth 1 using a global set of in situ temperature and salinity profile observations the sonic layer depth sld and the mixed layer depth mld are analyzed and compared over the annual cycle the sld characterizes the potential of the upper ocean to trap acoustic energy in a surface duct while mld characterizes upper ocean mixing the sld is computed from temperature and salinity profile pairs using a new tunable method while mld is computed using recently developed methods and either temperature only profiles or temperature and salinity profile pairs both sld and mld estimates provide information on different and important aspects of the upper ocean the sld and mld often coincide because sound speed increases with depth down to the mld where typically a decrease in temperature occurs resulting in a local maximum sound speed the depth of this maximum sound speed is the sld the sld and mld are not always the same because sound speed is substantially more sensitive to temperature than to salinity compared to density since mld is a commonly known and studied parameter mld is often used as a proxy for sld in scientific and operational applications in the boreal spring when fresh restratification events occur the sld is 10 m deeper shallower than the mld in 39 7 of the observed profiles a parabolic equation acoustic transmission model is used to evaluate the relative skill of the sld and mld estimates to predict surface acoustic trapping as measured by a simple metric

a review of statistical chronology models for high resolution proxy based holocene palaeoenvironmental reconstruction abstract in this paper we explain the background workings and results obtained from three recently developed statistical age depth models implemented in freely available general purpose software packages bpeat oxcal and bchron these models aim to reconstruct the sedimentation rate in a single core typically lake peat or ocean given a limited number of scientific date estimates usually radiocarbon and fixed depths most importantly they provide a suitably qualified estimate of the uncertainty in the age depth chronology and thus can be used in a variety of applications we perform a large data driven study of the three models and discuss their general utility in chronology construction for palaeoenvironmental research

the age and extent of tropical alpine glaciation in the cordillera blanca peru based on new 10 be data for moraines in the cordillera blanca of central peru we have calculated model ages of three sets of pleistocene glacial moraines in order of decreasing age these are the cojup rurec and laguna baja moraines the oldest moraines occur at the lowest elevations and yield dates of 400 ka pre dating the last interglacial and demonstrating that maximum ice volumes occurred during previous glacial periods and not at the end of the last glacial our new data from the younger moraines indicate that the end of the last glacial cycle comprised two separate advances at ca 29 ka and ca 16 5 ka each reflecting significant 4 c tropical of cooling at 10 s these data combined with published records from the laurentide ice sheet indicate that climate instabilities associated with the close of the last glacial were likely global and synchronous in nature copyright 2005 john wiley sons ltd

climate at the equilibrium line of glaciers the relationships between temperature precipitation and radiation on glacier equilibrium lines are investigated using 70 glaciers for which the mass balance and meteorological observations have been carried out for sufficiently long periods it is found that the characteristic climate at glacier equilibrium lines can be described using the summer 3 months temperature in a free atmosphere annual total precipitation and the sum of global and long wave net radiation all of these are measured at or very near the equilibrium line altitudes then it is shown how the shift of the equilibriumline will occur as a result of a climatic change finally the effect of the shift of the equilibrium line on the annualmean specific mass balance is analytically derived and compared with observations the present results make it possible to identify the altitudes in climate models where glacierization should begin and to evaluate the mass balance changes as a result of possible future changes in the climate

late quaternary climate history of the bolivian altiplano abstract sediment cores and outcrops from the bolivian altiplano are used to interpret late pleistocene holocene paleoclimates and lake evolution based on a multi proxy approach ostracod content palynology sedimentology and radiocarbon dating despite the different sensitivity of the lacustrine basins to environmental changes interpreting the records in terms of paleohydrology climate and more especially timing of events is difficult notwithstanding these problems lake titicaca in the north and lake pocoyu lake poopo salars of coipasa and uyuni in the south reveal a similar evolution in general trends through the course of time during the minchin phase until 30 26 kyr bp the altiplano was wetter than present the last glacial maximum ca 26 14 kyr bp marked by cooler drier conditions was followed by a return to a wetter climate interrupted by short arid events between ca 14 and 10 5 kyr bp another cycle of aridity between 10 5 and 8 kyr bp took place abruptly just after the humid tauca phase the mid holocene is interpreted as climatically unstable with an alternation of humid and dry episodes since 3 9 kyr bp wetter conditions have persisted and intensified to the present the data are interpreted in terms of changes in inter tropical convergence zone extension summer precipitation and in polar air mass intensity winter precipitation

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

mass balance and equilibrium line altitudes of glaciers in high mountain environments abstract the mass balance characteristics of glaciers in high mountain environments complicate the relationship between glacier equilibrium line altitudes elas and climatic variables such as precipitation and air temperature therefore methods of ela reconstruction employed in low relief environments are commonly not applicable in high mountains or require some modification we review the concept of the ela with reference to the mass balance of a range of glacier types found in high mountain regions we examine the applicability of several commonly used methods of ela reconstruction for different glacier types and propose some general principles to guide the choice of appropriate methods

summertime precipitation variability over south america role of the large scale circulation abstract the observed large scale circulation mechanisms associated with summertime precipitation variability over south america are investigated particular attention is paid to the altiplano where a close relationship has been observed between rainfall and the position and intensity of the bolivian high empirical orthogonal function eof correlation and composite analyses suggest that on intraseasonal timescales typically 5 20 days rainy periods on the altiplano are associated with at least three types of circulation anomalies involving either extratropical cyclones cold core lows or the westward enhancement of the south atlantic high in each instance the primary support for high rainfall rates is a moist poleward flow at low levels along the eastern flank of the central andes in association with the south atlantic convergence zone sacz the warm low level flow along the sacz also inflates the overlying atmospheric column resulting in an intensification and southward shift of the bolivi

lateglacial and holocene cosmogenic surface exposure age glacial chronology and geomorphological evidence for the presence of cold based glaciers at nevado sajama bolivia comparisons of palaeo equilibrium line altitudes between the western and eastern cordilleras in the central andes are commonly based on the assumption that the tall outermost moraines visible in remotely sensed images of the western cordillera date to the last glacial maximum lgm however field investigation and geomorphic mapping at nevado sajama bolivia indicates the tall moraines are relic features with shorter moraines overlying and in some cases extending beyond them 36cl exposure ages from the shorter moraines suggest that they date to lateglacial times ca 16 9 10 2 ka although lateglacial deposits have been found throughout the central andes the extent of these deposits relative to lgm deposits varies both between the western and eastern cordilleras and north to south along the western cordillera in the western cordillera in the zone of easterly winds the lateglacial appears to be the most extensive glacial advance of the last glacial cycle geomorphic evidence also suggests that some lateglacial moraines were deposited by cold based ice a previously unreported finding in the tropical andes retreat from other glacial features occurred at about 7 0 4 4 ka and 4 7 3 3 ka these are the first directly dated holocene glacial deposits in the western cordillera of bolivia and their presence suggests that the mid holocene may not have been as warm and dry as previously thought copyright 2008 john wiley sons ltd

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

mass balance of glaciar zongo bolivia between 1956 and 2006 using glaciological hydrological and geodetic methods the longest continuous glaciological mass balance time series in the intertropical zone of south america goes back to 1991 on glaciar zongo bolivia photogrammetric and hydrological data have been used to i check the specific net balance over long periods and ii extend the mass balance time series over the last 50 years these data reveal a bias in the glaciological mass balance which can be explained by the field measurement sampling network our study shows a large temporal variability of the surface mass balances in the ablation area and reveals strong relationships between independent surface mass balance data coming from selected ablation areas with numerous data it demonstrates the very large contribution 80 of low elevation ranges one third of the surface to the specific mass balance and consequently the importance of the reduction of the area of the tongue with these new results glaciar zongo offers the longest and most accurate mass balance series in any andean country the dataset shows that glaciar zongo experienced a relatively steady state over the period 1956 75 with even a slight mass gain over 1963 75 and a rapid and continuous decrease since then

early local last glacial maximum in the tropical andes the local last glacial maximum in the tropical andes was earlier and less extensive than previously thought based on 106 cosmogenic ages from beryllium 10 dating from moraines in peru and bolivia glaciers reached their greatest extent in the last glacial cycle 34 000 years before the present and were retreating by 21 000 years before the present implying that tropical controls on ice volumes were asynchronous with those in the northern hemisphere our estimates of snowline depression reflect about half the temperature change indicated by previous widely cited figures which helps resolve the discrepancy between estimates of terrestrial and marine temperature depression during the last glacial cycle

a major advance of tropical andean glaciers during the antarctic cold reversal a moraine chronology determined by surface exposure dating shows that glaciers in the northern tropical andes expanded to a larger extent during the antarctic cold reversal 14 500 to 12 900 years ago than during the younger dryas stadial 12 800 to 11 500 years ago contrary to previous studies as a result previous chronologies and climate interpretations from tropical glaciers may need to be revisited the younger dryas was a cold period immediately prior to the present warm interglacial period there is clear evidence that it occurred throughout much of the northern hemisphere but its global extent remains debatable prior work in the tropical andes suggested extensive glacial advances during the younger dryas vincent favier and colleagues present new cosmogenic dating of glacial deposits and a reassessment of existing datasets from other glaciated andean landscapes to show that instead maximal glacial advances took place in the preceding antarctic cold reversal rather than a major advance the younger dryas now appears to have been associated with modest glacial retreat the younger dryas stadial a cold event spanning 12 800 to 11 500 years ago during the last deglaciation is thought to coincide with the last major glacial re advance in the tropical andes1 this interpretation relies mainly on cosmic ray exposure dating of glacial deposits recent studies however have established new production rates2 3 4 for cosmogenic 10be and 3he which make it necessary to update all chronologies in this region1 5 6 7 8 9 10 11 12 13 14 15 and revise our understanding of cryospheric responses to climate variability here we present a new 10be moraine chronology in colombia showing that glaciers in the northern tropical andes expanded to a larger extent during the antarctic cold reversal 14 500 to 12 900 years ago than during the younger dryas on the basis of a homogenized chronology of all 10be and 3he moraine ages across the tropical andes we show that this behaviour was common to the northern and southern tropical andes transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of atlantic meridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration during the antarctic cold reversal glaciers advanced primarily in response to cold sea surface temperatures over much of the southern hemisphere during the younger dryas however northern tropical andes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land surface feedbacks triggered by a weakened atlantic meridional overturning circulation conversely glacier retreat during the younger dryas in the southern tropical andes occurred as a result of progressive warming probably influenced by an increase in atmospheric carbon dioxide considered with evidence from mid latitude andean glaciers16 our results argue for a common glacier response to cold conditions in the antarctic cold reversal exceeding that of the younger dryas

progressive glacial retreat in the southern altiplano uturuncu volcano 22 s between 65 and 14 ka constrained by cosmogenic 3he dating this work presents the first reconstruction of late pleistocene glacier fluctuations on uturuncu volcano in the southern tropical andes cosmogenic 3he dating of glacial landforms provides constraints on ancient glacier position between 65 and 14 ka despite important scatter in the exposure ages on the oldest moraines probably resulting from pre exposure these 3he data constrain the timing of the moraine deposits and subsequent glacier recessions the uturuncu glacier may have reached its maximum extent much before the global lgm maybe as early as 65 ka with an equilibrium line altitude ela at 5280 m then the glacier remained close to its maximum position with a main stillstand identified around 40 ka and another one between 35 and 17 ka followed by a limited recession at 17 ka then another glacial stillstand is identified upstream during the late glacial period probably between 16 and 14 ka with an ela standing at 5350 m this stillstand is synchronous with the paleolake tauca highstand this result indicates that this regionally wet and cold episode during the heinrich 1 event also impacted the southern altiplano the ela rose above 5450 m after 14 ka synchronously with the bolling allerod

reconstructing glacier retreat since the little ice age in se tibet by glacier mapping and equilibrium line altitude calculation abstract temperate glaciers in the eastern nyainqentanglha range southeastern tibet are highly sensitive to climate change and therefore of particular high interest for research on late holocene changes of the monsoonal climate in high asia however because of the remoteness of the area the scarcity of empirical data and the challenges to remote sensing work posed by cloud and snow cover knowledge about the glacier dynamics and changes in this region is still very limited in this study we applied a remote sensing approach in which 1964 glaciers were mapped from a landsat etm scene and subsequently parameterized by dem supported measurements geomorphological evidence i e trimlines and latero frontal moraines were used to obtain quantitative data on the glaciers morphological characteristics and the changes since the little ice age lia maximum glacier advance statistical analysis of glacier length change revealed an average retreat of 27 and a trend toward stronger retreat for smaller glaciers an evaluation of different methods to calculate equilibrium line altitudes elas indicates that an optimized toe to ridge altitude method tram is more suitable than other methods in settings with complex topography and a lack of mass balance measurements a large number of glacier measurements are crucial for high quality of tram results and special attention has to be paid to different glacier characteristics in order to determine the best fitting tram ratio value and to test the quality of the calculated elas a remote sensing approach was applied for each investigated glacier the altitudes of transient snowlines visible in the late summer landsat scene were measured from the dem and compared to tram results the interpolated ela results show a se nw gradient ranging from 4400 to 5600 m asl and an average ela rise of 136 m since the lia because of the high spatial resolution of measurements the ela distribution reveals topographic effects down to the catchment scale specifically orographic rainfall and leeward shielding the interpretation of these patterns reveals that the eastern nyainqentanglha range is influenced by both the indian ism and east asian summer monsoon easm however the easm does not reach the western part of the study area the results indicate that the monsoonal temperate glaciers high sensitivity to climate change is driven by two double forcings owing to the coincidence of accumulation and ablation phases