late quaternary climate and hydrology of tropical south america inferred from an isotopic and chemical model of lake titicaca bolivia and peru abstract a simple mass balance model provides insight into the hydrologic isotopic and chemical responses of lake titicaca to past climatic changes latest pleistocene climate of the altiplano is assumed to have been 20 wetter and 5 c colder than today based on previous modeling our simulation of lacustrine change since 15 000 cal yr b p is forced by these modeled climate changes the latest pleistocene lake titicaca was deep fresh and overflowing the latest pleistocene riverine discharge from the lake was about 8 times greater than the modern average sufficient to allow the expansion of the great paleolake tauca on the central altiplano the lake 18o value averaged about 13 smow the modern value is about 4 2 the early holocene decrease in precipitation caused lake titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake tauca continued evaporation caused the 100 m drop in lake level but only a slight 1 2 increase relative to modern in 18o of early holocene lake waters this holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease relative to modern of about 40 the lake was saline as recently as 2000 cal yr b p the timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the south american summer monsoon

a vegetation and fire history of lake titicaca since the last glacial maximum fine resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding lake titicaca 3810 m peru bolivia since ca 27 500 cal yr bp hereafter bp time control was based on 26 accelerator mass spectrometer ams radiocarbon dates seventeen ams dates and 155 pollen and charcoal samples between ca 17 500 bp and ca 3100 bp allow a centennial scale reconstruction of deglacial and early to mid holocene events local and regional fire signals were based on the separation of two charcoal size fractions 180 m and 179 65 m charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra little or no pollen was detected in the sedimentary record prior to ca 21 000 bp very cold climatic conditions prevailed with temperatures suggested to be at least 5 8 c cooler than present increases in pollen concentration suggest initial warming at ca 21 000 bp with a more significant transition toward deglaciation ca 17 700 bp between 17 700 bp and 13 700 bp puna brava is progressively replaced by puna and sub puna elements the most significant changes between the pleistocene and the holocene floras were largely complete by 13 700 bp providing an effective onset of near modern conditions markedly earlier than in other andean records fire first occurs in the catchment at ca 17 700 bp and becomes progressively more important as fuel loads increase no evidence is found of a rapid cooling and warming coincident with the younger dryas chron a dry event between ca 9000 bp and 3100 bp with a peak between 6000 and 4000 bp is inferred from changes in the composition of aquatics and the marsh community as pollen of cyperaceae is replaced by poaceae apiaceae plantago and the shrub polylepis human disturbance of the landscape is evident in the pollen spectra after ca 3100 bp with the appearance of weed species

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

modern and last local glacial maximum snowlines in the central andes of peru bolivia and northern chile late pleistocene snowlines in the central andes were 500 1200 m lower than at present radiocarbon dates imply that the late pleistocene glacial maximum in the region occurred prior to 20 14c ka but lack of maximum limiting ages adds considerable uncertainty to the exact timing snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central andes respond differently to temperature and precipitation changes in the eastern cordillera the snowline is near the level of the annual 0 c isotherm and melting occurs throughout the year here snowlines are sensitive to temperature changes but relatively insensitive to accumulation changes in the western cordillera the snowline rises 1000 higher owing to increasing aridity and the snowline exhibits a much stronger sensitivity to accumulation changes the consistent 1200 m snowline depression along the eastern cordilleras of the central andes can be modeled by a mean annual cooling of 5 9 c this is inconsistent with the 2 c cooling in tropical sea surface temperatures suggested by climap reconstructions the 800 1000 m snowline depression in the western cordillera cannot be accounted for solely by cooling but also requires an increase in precipitation

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

verifying discontinuous deformation analysis simulations of the jointed rock mass behavior of shallow twin mountain tunnels abstract this study extends discontinuous deformation analysis dda to simulate the interactions of twin tunnels in a jointed rock mass the ground subsidence and the rock stress distribution around the tunnels from the dda simulation are compared to those from the trap door model both the experimental and computational results show that the changing tunnel interactions in the jointed rock mass between the two tunnels are coincidently associated with the tunnel distance in addition the dip angle of the rock strata significantly affects the surface subsidence patterns and the stress distribution around the excavation zone therefore the dda method is a useful tool for describing the mechanical behavior of a jointed rock mass when a twin tunnel is constructed in mountain terrain

four thousand years of environmental change and human activity in the cochabamba basin bolivia the cochabamba basin bolivia is on the ancient road network connecting andean and lowland areas little is known about the longevity of this trade route or how people responded to past environmental changes the eastern end of the cochabamba valley system constricts at the vacas lake district constraining the road network and providing an ideal location in which to examine past human environmental interactions multi proxy analysis of sediment from lake challacaba has allowed a c 4000 year environmental history to be reconstructed fluctuations in drought tolerant pollen taxa and calcium carbonate indicate two periods of reduced moisture availability c 4000 3370 and c 2190 1020 cal yr bp compared to adjacent wetter episodes c 3370 2190 and c 1020 cal yr bp present the moisture fluctuations broadly correlate to el nino southern oscillation variations reported elsewhere high charcoal abundance from c 4000 to 2000 yr ago indicates continuous use of the ancient road network a decline in charcoal and an increase in dung fungus sporormiella c 1340 1210 cal yr bp suggests that cultural changes were a major factor in shaping the modern landscape despite undisputable impacts of human populations on the polylepis woodlands today we see no evidence of woodland clearance in the challacaba record

vegetation and climate changes during the late pliocene and early pleistocene in sw anatolia turkey abstract pollen analysis was done on lacustrine sedimentary sequences dated by micromammals as late pliocene early pleistocene that outcrop in two neogene graben basins from sw turkey this study shows vegetation changes from steppe like to more forested environments very similar to the cyclic oscillations related to late pleistocene glacial interglacial climate changes artemisia was abundant during cold arid periods indicating that this species was already widespread in this area during the latest pliocene and the beginning of the pleistocene a review of pollen records from anatolia agrees with this study suggesting that the spreading of this arid species occurred during a major climatic change the beginning of the first glaciations and probably a change in seasonality towards summer aridity artemisia temporarily disappeared from the region during warm wet periods and thus we suggest that glacial interglacial type oscillations already occurred in the area during the late pliocene early pleistocene

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

post seismic stress evolution for a strike slip fault in the presence of a viscoelastic asthenosphere summary a model for post seismic near field effects on transcurrent vertical faults in the presence of a viscoelastic asthenosphere is presented the lithosphere is modelled as an elastic layer welded to a viscoelastic half space the asthenosphere the earthquake is modelled as a stress drop event crack whereas in previous similar papers it has usually been modelled as a constant slip event unlike crack models constant slip dislocations are not suitable for describing the stress field in the proximity of the fault which is the main concern of the present paper the sliding section is supposed to have a fixed extension and to reach the free surface two cases are considered a fault which becomes welded immediately after an earthquake locked problem and the case in which the slip distribution evolves in the post seismic phase unlocked problem following the stress relaxation of the asthenosphere in both cases the singular integral equation of crack equilibrium is solved in the laplace transform domain the locked problem is assumed to reproduce the post seismic stress recovery following an earthquake on a fault which preserves the initial strength threshold after the earthquake solutions show that major aftershocks in the deep section of the fault can occur if the fault cuts most of the lithosphere owing to relaxation of the asthenosphere the unlocked problem concerns a fault on which the post seismic strength is equal to the residual stress left after the earthquake and post seismic creep occurs owing to relaxation of the asthenosphere the fault keeps sliding after the earthquake and afterslip can be observed at the ground surface we compare the results obtained with the advanced phases of afterslip observed in some californian faults the stress evolution is a slower process in this case with respect to the locked model owing to the two way relaxation i e through fault creep and asthenosphere flow the comparison of the unlocked model with afterslip data seems to require a fault cutting most of the lithosphere if the relaxation time for the asthenosphere is greater than 10 yr

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

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

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

on the existence of a periodic dislocation cycle in horizontally layered viscoelastic models horizontally stratified viscoelastic models are employed to characterize the lithosphere asthenosphere interaction following dislocation events occurring on long transcurrent faults the aim is to investigate the ability of such models to reproduce the periodic behavior often assumed in the geophysical literature for the earthquake source process if the lithosphere is elastic and the asthenosphere has a maxwell rheology the earthquake recurrence time is proved to become zero model i or infinity model ii if boundary conditions of constant strain rate or limited tectonic stress respectively are imposed over the fault region model assumptions that may be responsible for these paradoxical results include the choice of boundary conditions the half space two dimensional geometry and the assumed rheological behavior this third possibility has been investigated in some detail the adoption of the standard linear solid for the asthenosphere model iii yields a stationary nonvanishing recurrence time if constant strain rate is applied another possibility is to assign a viscoelastic rheology to the lithosphere also model iv in this case an earthquake cycle is obtained whose period depends on the assigned boundary conditions in any case the results obtained clearly indicate the importance of past earthquakes in the earthquake cycle

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

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

anelastic processes and stress transfer from shallow to deep fault regions abstract post seismic displacement and strain fields after a major strike slip earthquake are studied in terms of the stress taking place at depth over localized mylonitized shear belts the coseismic stress field produced by the earthquake is represented in terms of crack theory and its post seismic relaxation is studied in terms of a crack model for creep processes recently proposed in the geophysical literature the existence of a sequence of relaxation times which are roughly reciprocal integer numbers and the faster relaxation of stronger stress heterogeneities are shown to be responsible for features of the deformation field observed at the free surface in the post seismic phase deformation becomes progressively slower as to relaxation times and its wavelength becomes accordingly longer similar features are also shown by viscoelastic models of the lithosphere asthenosphere system in which anelastic processes are entirely due to transient post earthquake flow in the asthenosphere the two possibilities are discussed and a major role of intracrustal stress relaxation in the postseismic phase is proposed

investigation into the influence of excavation of twin bored tunnels within weak rock masses adjacent to slopes the focus of this investigation is twin tunnelling projects designed and constructed adjacent to slopes to this end several state of the art 2d and 3d numerical models for multiple scenarios were utilized the novelty of this research further highlighted by two case studies explicitly demonstrates that for 2d and 3d elastic and plastic analyses for weak rock masses the interaction between the two branches for various conditions show how the pillar width minimally influences such interaction past 2 diameter widths of separation as well the 3d sequential longitudinal tunnel excavation sequence of the twin tunnel does not significantly influence the crown displacements of the two tunnels prior to the mid span of the twin construction slope stability analysis was conducted by employing the novel shear strength reduction ssr method these findings confirm that changes in the twin tunnel location with respect to the slope and one another in combination with the natural angle of the slope can improve t

a 94 10 ka pollen record of vegetation change in qaidam basin northeastern tibetan plateau drill core isl1a was obtained from the qarhan salt lake in central eastern qaidam basin northeastern tibetan plateau ne tp fossil pollen and the lithology of the core sediment were analyzed in conjunction with ams c 14 and th 230 dating the results indicated that artemisia and chenopodiaceae dominated the steppe desert steppe vegetation developed around the lake between 94 and 51 2 ka corresponding with the organic rich silty clay deposited in the core sediments pediastrum continuously appeared in the core sediments between 94 and 51 2 ka indicating freshwater to oligohaline conditions of the paleo qarhan lake during the late marine isotope stage mis 5 mis 4 and early mis 3 during the 51 2 to 32 5 ka period ephedra dominated shrub desert vegetation expanded in the basin while pediastrum disappeared in the core sediments the core sediments consisted of interbedded layers of halite silt and clay rich halite between 51 2 and 32 5 ka signifying a shift toward drier hydrologic conditions thus the paleo qarhan lake experienced periods of desiccation and shallow water levels artemisia and chenopodiaceae dominated steppe desert steppe vegetation appeared again from 32 5 to 253 ka with clay rich sediments in the core thereby suggesting an increase of runoff from the lake under relatively wetter climatic conditions since 253 ka however pollen concentrations declined corresponding with the amount of halite deposited in the core sediment and suggesting a cold dry climatic condition during mis2 our findings have important implications for understanding complex regional vegetation and climatic responses to large scale forcings in the qaidam basin c 2015 elsevier b v all rights reserved

empirical estimation of rock mass modulus the deformation modulus of a rock mass is an important input parameter in any analysis of rock mass behaviour that includes deformations field tests to determine this parameter directly are time consuming expensive and the reliability of the results of these tests is sometimes questionable consequently several authors have proposed empirical relationships for estimating the value of rock mass deformation modulus on the basis of classification schemes these relationships are reviewed and their limitations are discussed based on data from a large number of in situ measurements from china and taiwan a new relationship based upon a sigmoid function is proposed the properties of the intact rock as well as the effects of disturbance due to blast damage and or stress relaxation are also included in this new relationship

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

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

albedo changes milankovitch forcing and late quaternary climate changes in the central andes late quaternary humidity changes resulted in substantial modifications of the land surface characteristics in the altiplano of the atacama desert central andes reconstructions of surface albedo top of atmosphere toa albedo and short wave net radiation in the andes of northern chile for 20 14 10 7 and 0 ka suggest that surface and toa albedo increased substantially during periods of relatively humid environmental conditions i e with large palaeolakes glaciers and dense vegetation the decrease of summer shortwave net radiation and seasonality during the late glacial early holocene humid phase 14 to 10 ka due to earth s surface and atmospheric characteristics added to the effect of orbitally driven negative deviations of southern hemisphere austral summer insolation and minimum seasonality at 20 s therefore in situ radiative forcing is in contrast to the northern hemisphere tropics not a suitable explanation for enhanced convective precipitation and ultimately humid climatic conditions our results suggest that late quaternary humidity changes on the altiplano reflect a collective response to 1 environmental changes in the source area of the moisture e g re expansion of the rain forest and increased release of latent heat over amazonia and the chaco warm sea surface temperatures in the e pacific and 2 large scale circulation patterns and wave structures in the upper troposphere strength and position of the bolivian high divergent flow stimulating convection over the altiplano or that they even reflect a response to 3 interhemispherical teleconnections

glacial to holocene changes in sea surface temperature and coastal vegetation in north central chile high versus low latitude forcing abstract north central chile is located at an important climatic transition zone between humid conditions under the influence of the southern hemisphere westerly winds in the south and the extremely dry climate of the atacama desert in the north offshore equatorward flowing cold waters of the peru chile current converge with relatively warmer waters coming from the low latitudes based on terrestrial and marine proxies from the same archive geob 7139 2 30 12 s 71 59 w we show that during the glacial between 33 and 19 kyr cold sea surface temperatures paralleled enhanced humidity with high terrestrial input and abundant vegetation on the adjacent land suggesting a stronger influence of cold waters from the antarctic circumpolar current and the westerly winds while the deglacial sea surface temperature warming started at 19 kyr the humidity decrease occurred around 17 16 kyr bp the early to mid holocene is characterized by extremely warm and dry conditions we suggest that climate changes were driven by fluctuations in antarctic sea ice extent and the circulation of the hadley cell both ultimately linked to insolation changes our records further imply warm and dry conditions in north central chile during the northern hemisphere heinrich events

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

timing of past glaciation at the sierra de aconquija northwestern argentina and throughout the central andes abstract advances in cosmogenic nuclide exposure dating have made moraines valuable terrestrial recorders of palaeoclimate a growing number of moraine chronologies reported from the central andes show that tropical glaciers responded sensitively to past changes in precipitation and temperature over timescales ranging from 103 to 105 years however the causes of past glaciation in the central andes remain uncertain explanations have invoked insolation modulated variability in the strength of the south american summer monsoon teleconnections with the north atlantic ocean and or cooling in the southern hemisphere the driver for these past climate changes is difficult to identify partly due to a lack of dated moraine records especially in climatically sensitive areas of the southern central andes moreover new constraints are needed on precisely where and when glaciers advanced we use cosmogenic 10be produced in situ to determine exposure ages for three generations of moraines at the sierra de aconquija situated at 27 s on the eastern flank of the southern central andes these moraines record glacier advances at approximately 22 ka and 40 ka coincident with summer insolation maxima in the sub tropics of the southern hemisphere as well as at 12 5 ka and 13 5 ka during the younger dryas and the antarctic cold reversal respectively we also identify minor glaciation during bond event 5 also known as the 8 2 ka event these moraines register past climate changes with high fidelity and currently constitute the southernmost dated record of glaciation on the eastern flank of the central andes to contextualise these results we compile 10be data reported from 144 moraines in the eastern central andes that represent past glacier advances we re calculate exposure ages from these data using an updated reference production rate and we re interpret the moraine ages by taking the oldest clustered boulder age after the exclusion of outliers attributed to nuclide inheritance as closest to the timing of glacier advance an approach for which we provide empirical justification this compilation reveals that central andean glaciers have responded to changes in temperature and precipitation we identify cross latitude advances in phase with insolation cycles the last global glacial maximum and episodes of strengthened monsoonal moisture transport including the younger dryas and heinrich stadials 1 and 2 our results from the sierra de aconquija allow us to constrain the southerly limit of enhanced precipitation associated with heinrich stadials at 25 s more broadly our findings demonstrate at both local and regional scales that moraines record past climate variability with a fine spatial and temporal resolution

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

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

early warming of tropical south america at the last glacial interglacial transition glaciation in the humid tropical andes is a sensitive indicator of mean annual temperature here we present sedimentological data from lakes beyond the glacial limit in the tropical andes indicating that deglaciation from the last glacial maximum led substantial warming at high northern latitudes deglaciation from glacial maximum positions at lake titicaca peru bolivia 16 s and lake junin peru 11 s occurred 22 000 to 19 500 calendar years before the present several thousand years before the bolling allerod warming of the northern hemisphere and deglaciation of the sierra nevada united states 36 5 to 38 n the tropical andes deglaciated while climatic conditions remained regionally wet which reflects the dominant control of mean annual temperature on tropical glaciation

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

a 106ka paleoclimate record from drill core of the salar de atacama northern chile abstract a 100 m long salt core sqm 2005 from the salar de atacama northern chile 23 s 68 w a dry lake bed contains a 106 kyr paleoclimate record of hydrologic balances on the western slopes of the central andes of south america six u series disequilibrium dates range sequentially from 106 1 6 4 to 5 4 2 7 ka based on sedimentary structures and petrographic textures of salts and associated siliciclastic sediments interpretations of paleoenvironments and net hydrologic balance suggest that relatively wet periods saline lakes and expanded mudflats existed in the salar de atacama from 75 7 to 60 7 ka and from 53 4 to 15 3 ka with the wettest perennial lake interval from 26 7 to 16 5 ka short relatively wet periods also occurred in the holocene from 11 4 to 10 2 ka and from 6 2 to 3 5 ka these wet periods at salar de atacama correspond well with other late pleistocene climate records from the central andes that are a function of the net hydrologic budget the minchin tauca lake sequence from salar de uyuni bolivia is synchronous with the 53 4 15 3 ka saline lake wet period at salar de atacama the tauca phase coincided with the wettest perennial lake interval at salar de atacama from 26 7 to 16 5 ka the coipasa lake phase from uyuni was probably contemporaneous with the early holocene wet period at salar de atacama from 11 4 to 10 2 ka the early holocene wet interval at salar de atacama was also synchronous with the maximum holocene lake levels of the chilean altiplano lakes to the east and with grass rich rodent middens between 11 8 and 10 5 ka in age from the atacama basin the lake period at salar de atacama between 26 7 and 15 3 ka compares closely with low 18o values low anion concentrations and high accumulation rates from the sajama glacial ice core southwestern bolivia between 25 and 15 5 ka it is concluded that wet and dry phases were synchronous in the central andean plateau of southwestern bolivia and along the western flank of the andes of northern chile over the past 50 kyr this region which today receives its moisture from the amazon basin to the east probably also did so in the past the wet periods at salar de atacama are in phase with january insolation maxima at 15 s at 70 45 and 20 ka which suggests that past changes in summer insolation influenced precipitation patterns in the central andes however there are no major wet periods linked to the january insolation maximum at 93 ka and the present day extremely dry climate in the atacama desert is associated with a peak in summer insolation therefore there must be other important influences over precipitation in the central andes which modify the first order insolation controls

preseismic rupture progression and great earthquake instabilities at plate boundaries we present a procedure for modeling the initially quasi static upward progression of a zone of slip from some depth in the lithosphere toward the earth s surface along a transform plate margin culminating in a great crustal earthquake stress transmission in the lithosphere is analyzed with a generalized elsasser model in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a maxwellian viscoelastic asthenosphere upward progression of rupture over a finite length of plate boundary corresponding to a seismic gap along strike is analyzed by a method based on the line spring concept whereby a two dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness averaged stress and slip used as a boundary condition in the generalized elsasser plate model the formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike a simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip softening slope at the boundary falling below the elastic unloading stiffness of the surroundings the results also indicate a delay of ultimate seismic instability due to the stiffer short versus long time asthenospheric response and predict a final period of self driven creep toward instability the procedures for prediction of rupture progression and instability are illustrated in detail for an elastic brittle crack model of slip zone advance and parameters of the model are chosen consistently with great earthquake slips and stress drops for example an effective crack fracture energy of the order 4 106 j m2 at the peak 7 to 10 km below surface of a gaussian bell shaped distribution of fracture energy with depth with variance of the order 5 km simulating strength build up in a seismogenic layer leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike precursory surface straining in the self driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place

relationship between amazon and high andes rainfall 1 a regional climate model is used to study the connections between amazon and high andes rainfall the modeling effort focuses on understanding the rainfall response during the last glacial maximum lgm 21 000 years ago as paleoproxy evidence generally suggests that the high andes were wet during this period while conditions over the amazon were drier although there is more uncertainty about the latter analysis of the yearlong present day and full lgm integrations reveals that the delayed onset of amazon convection in the lgm during austral spring is associated with an enhancement of rainfall over the high andes in the absence of convection over the amazon in the austral spring zonal geopotential height gradients over the amazon strengthen resulting in anomalous easterly flow between the amazon and the high andes instead of the low level flow paralleling the andes e g northwesterly the flow impinges more directly onto the eastern slopes of the andes during the lgm where it rises and transports moisture from the amazon up into the mountains enhancing convection over the andes this circulation mechanism explains how the moisture and rainfall over the high andes of peru and bolivia can increase such as the case for the lgm despite large scale drying over south america present day observations are then explored for 1984 2002 and 2003 to identify whether mechanisms identified in the paleoclimate modeling are observable in the present day climate comparison of these particular springs reveals a response analogous to the lgm model response suggesting that this mechanism that connects tropical atlantic sea surface temperatures amazon convection and high andes rainfall is operating in the present climate

rheology heterogeneities on fault surfaces inferred from the time history of afterslip events a model of shallow afterslip on transcurrent vertical faults is presented the creeping region is represented with a 2 d quasi static crack open to the earth surface the fault gouge is treated as a viscoplastic layer with vertically heterogeneous bingham rheology and a deep earthquake is assumed to generate a two level stress distribution on the fault plane in the deeper level the stress drops to values less than the plastic threshold of the fault gouge while in the shallower level the earthquake concentrates stress to values greater than the plastic threshold in this region aseismic slip takes place initially the creeping region is assumed to extend from the earth surface to the depth of transition between stress levels but subsequently it penetrates deeper thanks to the stress concentration created by the crack beyond its tip slip at the earth surface is computed for some stress drop distributions and viscosity profiles and the results are compared with afterslip observations on three california faults the whole evolution is sensibly affected by the initial stress profile variations mainly because the crack gradually extends to increasing depths the initial part of the evolution is also affected by the viscosity profile assumed on the fault plane in particular the advanced phase of the evolution is not affected by a shallow region with low viscosity on the fault plane the fast relaxation of this region in fact provides a quick displacement superimposed from the beginning on a slip evolution very similar to that with a uniform viscosity after removal from the data of this initial displacement treated as a free parameter since it is not well constrained from data themselves a suitable choice of the stress parameter values gives a good reproduction of the data in all the cases analysed even if a uniform viscosity on the fault plane is assumed in all the analysed cases these parameter values are consistent with a temporal deepening of the creeping region even if the computed displacement at the earth surface along a direction orthogonal to the fault strike is scarcely sensitive to maximum depth of creep

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

millennial scale record of landslides in the andes consistent with earthquake trigger landslide deposits are often interpreted as the consequence of precipitation a millennial scale record of landslides inferred from river cobbles in the arid andes is instead consistent with earthquake triggering

investigating ground movements caused by the construction of multiple tunnels in soft ground using laboratory model tests the prediction of the ground movements above single tunnels in soft ground is well established and can be estimated using semi empirical methods based on the gaussian curve however the prediction

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

drainage basin response to climate change in the pisco valley peru the quaternary development of the pisco valley in central peru has been characterized by multiple phases of sediment accumulation and erosion that formed distinct levels of cut and fill terraces and alluvial fans luminescence dating shows that they formed in response to at least two different stages of sediment accumulation and erosion during the past 60 ka the main phase of sediment aggradation occurring between ca 54 and 38 ka ago the ages show that sediment accumulation was contemporaneous with the time intervals of the minchin 47 8 36 ka ago with enhanced precipitation beginning ca 54 8 ka ago and tauca 26 14 9 ka ago paleolakes on the altiplano where the headwaters of the pisco river are located we conclude that sediment accumulation was triggered by shifts toward a more humid climate whereas erosion is the response of the fluvial system to the depletion of the hillslope sediment reservoirs

paleo erosion rates and climate shifts recorded by quaternary cut and fill sequences in the pisco valley central peru fluvial cut and fill sequences have frequently been reported from various sites on earth nevertheless the information about the past erosional regime and hydrological conditions have not yet been adequately deciphered from these archives the quaternary terrace sequences in the pisco valley located at ca 13 s offer a manifestation of an orbitally driven cyclicity in terrace construction where phases of sediment accumulation have been related to the minchin 48 36 ka and tauca 26 15 ka lake level highstands on the altiplano here we present a 10be based sediment budget for the cut and fill terrace sequences in this valley to quantify the orbitally forced changes in precipitation and erosion we find that the minchin period was characterized by an erosional pulse along the pacific coast where denudation rates reached values as high as 600 80 mm ka600 80 mm ka for a relatively short time span lasting a few thousands of years this contrasts to the younger pluvial periods and the modern situation when 10be based sediment budgets register nearly zero erosion at the pacific coast we relate these contrasts to different erosional conditions between the modern and the minchin time first the sediment budget infers a precipitation pattern that matches with the modern climate ca 1000 km farther north where highly erratic and extreme el nino related precipitation results in fast erosion and flooding along the coast second the formation of a thick terrace sequence requires sufficient material on catchment hillslopes to be stripped off by erosion this was most likely the case immediately before the start of the minchin period because this erosional epoch was preceded by a 50 ka long time span with poorly erosive climate conditions allowing for sufficient regolith to build up on the hillslopes finally this study suggests a strong control of orbitally and ice sheet forced latitudinal shifts of the itcz on the erosional gradients and sediment production on the western escarpment of the peruvian andes at 13 during the minchin period

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

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

hydrological and ecosystem response to abrupt changes in the indian monsoon during the last glacial as recorded by sediments from xingyun lake yunnan china abstract the indian summer monsoon ism is the major moisture source of precipitation in southwest china and plays an important role in regional environmental change and in economic and social development since abrupt changes of the ism can cause crop failures and flooding that impact the livelihood of a large population however with the exception of speleothem records abrupt changes of the ism during the last glacial are not well documented in terrestrial records here we present a sediment record from xingyun lake in yunnan province southwest china spanning the interval 36 4 ka bp to 13 4 ka bp 1 ka 1000 cal a which documents the occurrence of a series of millennial scale episodes which may reflect the abrupt weakening of the ism seven ams 14c dates from terrestrial wood fragments provide a robust chronology the results demonstrate that increases in the coarse sediment fraction 30 m correspond to increases in the concentration of pediastrum the grain size of magnetic minerals carbonate content and c n ratios which together indicate episodes of low lake level in addition pollen analysis indicates that quercus oak increased and that ferns decreased during intervals of abrupt weakening of the ism indicating a drier environment nine abrupt shifts to a low lake level are documented for the studied interval and three of them are correlative with heinrich events 1 to 3 our study supports the hypothesis that in response to the cold events in the north atlantic associated with massive iceberg influxes and the corresponding slowing down of atlantic meridional overturning circulation the mean latitudinal extent of the itcz shifted southwards as a result of an atmospheric teleconnection there was a corresponding decrease in precipitation associated with ism precipitation which caused reduced fluvial runoff in yunnan

a crack model of creep processes on deep sections of faults a crack model in antiplane shear configuration is shown representing creep processes interpreted in terms of viscous deformation of a narrow plastic layer characterized by inhomogeneous rheological properties embedded within a homogeneous elastic medium the evolution in time of slip and stress over the crack plane is studied through a truncated expansion in chebyshev polynomials and convergence is proved to be fast in the simple examples considered finite stress solutions are found which are compatible with constitutive relations of elasto plastic materials and furthermore these allow us to simulate creep propagation and stress transfer between locked and unlocked fault segments this model provides a simple interpretation of the shallow depth of the seismogenic layer observed in several areas of the world and lends itself to modelling creep processes during either post seismic rebound or pre seismic stress buildup stress transfer is accomplished mostly by the slow extension of the creeping section during a seismic cycle it is envisaged that different regimes dominate over deep intermediate and shallow sections of faults i slow pre seismic stress build up accompanied by creep and stress migration toward intermediate depths ii brittle fracture over shallow and intermediate sections of faults iii post seismic rebound over intermediate and deep sections of faults the present crack model while providing finite stress solutions allows a better understanding of how stress may accommodate at different depths over a fault plane during a seismic cycle

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

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

foraminiferal faunal estimates of paleotemperature circumventing the no analog problem yields cool ice age tropics the sensitivity of the tropics to climate change particularly the amplitude of glacial to interglacial changes in sea surface temperature sst is one of the great controversies in paleoclimatology here we reassess faunal estimates of ice age ssts focusing on the problem of no analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods a new calibration strategy developed here which uses past variability of species to define robust faunal assemblages solves the no analog problem and reveals ice age cooling of 5 o to 6oc in the equatorial current systems of the atlantic and eastern pacific oceans classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core top assemblages misrepresented the ice age faunal assemblages our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by climate long range investigation mapping and prediction climap 1981 and thus may help to resolve a long standing controversy our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems however and supports climap s inference of stability of the subtropical gyre centers

late quaternary lowstands of lake titicaca evidence from high resolution seismic data abstract approximately 600 km of high resolution seismic reflection data were collected to investigate the late quaternary stratigraphic development of lake titicaca the focus of this report is on two seismic sequence boundaries which are interpreted as erosional surfaces formed at times of low lake level the younger erosional surface occurs as much as 90 m below the present lake level and up to 8 m below the present sediment water interface this erosional surface is interpreted to be coeval with a well documented early to mid holocene lowstand dated between 8000 and 3600 cal yr bp an earlier and previously unknown erosional surface occurs at a sub bottom depth of approximately 30 m and as much as 240 m below the present lake level which implies a major late pleistocene lowstand of lake titicaca by extrapolation of sedimentation rates from the upper 14 m of sediment we estimate the age of this older lowstand at 90 000 cal yr bp both lowstands of lake titicaca indicated by the seismic data are likely to have been a response to climatic change in the region

last glacial maximum in an andean cloud forest environment eastern cordillera bolivia comment and reply comment whether the climate of tropical south america during the last glacial maximum lgm was colder and drier or colder and wetter than present day has been widely debated it is accepted however that the lgm in tropical south america was 2 9 c colder than today e g betts and ridgway 1992 1