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

glacier fluctuations in the southern peruvian andes during the late glacial period constrained with cosmogenic 3he the occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes accurate constraint of both the timing and magnitude of events such as the younger dryas and antarctic cold reversal is vital in order to test different hypotheses for the causes and propagation of abrupt climate change however in contrast to higher latitude regions well dated records from the tropics are rare and the structure of late glacial tropical climate remains uncertain as a step toward addressing this problem we present an in situ cosmogenic 3 he surface exposure chronology from nevado coropuna southern peru documenting a significant fluctuation of the ice margin during the late glacial period ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the last glacial maximum terminus range from 11 9 to 13 9ka and give an arithmetic mean age of 12 8 0 7ka 1s these data constitute direct evidence for a readvance or prolonged stillstand of glaciers in the arid andes of southwestern peru copyright 2011 john wiley sons ltd

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 tropical mechanism for northern hemisphere deglaciation 1 we investigate the role of the tropics in the melting and reforming of the laurentide ice sheet on glacial timescales using an atmospheric general circulation model it is found that warming of tropical sea surface temperatures ssts from glacial boundary conditions as observed at the end of glacial periods causes a large increase in summer temperatures centered over the ice sheet forming regions of canada this highlatitude response to tropical change is due to relatively small changes in the circulation of the extratropical atmosphere which lead to changes in the vertical profiles of temperature and moisture in the extratropical atmosphere the maximum perturbation in the summer radiative balance over the laurentide ice sheet 25 w m 2 due to the changes in the local atmospheric water vapor inventory is much larger than that induced by glacial to interglacial changes in atmospheric co2 these changes via an atmospheric bridge between the tropics and extratropics represent a mechanism for deglaciations which is consistent with timing constraints in contrast a cold perturbation to tropical sst for interglacial boundary conditions results in only very small changes in the delivery of water vapor to the laurentide region and therefore almost no cooling over the canadian region this implies that tropical ssts could play a more important role in melting ice sheets in the northern hemisphere than in reforming them possibly providing a mechanism which could help to explain the rapidity of deglaciation relative to glacial inception

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 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

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

tropical climate at the last glacial maximum inferred from glacier mass balance modeling model derived equilibrium line altitudes elas of former tropical glaciers support arguments based on other paleoclimate data for both the magnitude and spatial pattern of terrestrial cooling in the tropics at the last glacial maximum lgm relative to the present lgm elas were maintained by air temperatures that were 3 5 to 6 6 c lower and precipitation that ranged from 63 wetter in hawaii to 25 drier on mt kenya africa our results imply the need for a 3 c cooling of lgm sea surface temperatures in the western pacific warm pool sensitivity tests suggest that lgm elas could have persisted until 16 000 years before the present in the peruvian andes and on papua new guinea

marine magnetotellurics for petroleum exploration part ii numerical analysis of subsalt resolution in areas where seismic imaging of the base of salt structures is difficult seaborne electromagnetic techniques offer complementary as well as independent structural information numerical models of 2 d and 3 d salt structures demonstrate the capability of the marine magnetotelluric mt technique to map the base of the salt structures with an average depth accuracy of better than 10 the mapping of the base of the salt with marine mt is virtually unaffected by internal variation within the salt three dimensional anticlinal structures with a horizontal aspect ratio greater than two can be interpreted adequately via two dimensional inversions marine mt can distinguish between salt structures which possess deep vertical roots and those which do not one measure of the relative accuracy of mt and seismic methods can be made by considering the vertical and lateral position errors in the locations of interfaces caused by neglecting velocity anisotropy in migration for the shallow part of the section where two way travel times are on the order of 1 s the vertical and lateral position errors in the locations of salt sediment interfaces from 2 d mt inversion is more than twice the expected migration error in reflectors in transversely isotropic sediments such as those in the gulf of mexico deeper in the section where two way times are on the order of 4 s lateral position errors in migration become comparable to those of the mt inverse whereas seismic vertical position errors remain more than a factor of two smaller than mt errors this analysis shows that structural mapping accuracy would be improved using mt and seismic together

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

1d inversion and resolution analysis of marine csem data we present results from an investigation into 1d inversion of controlled source electromagnetic csem data based on inspection of a data set we formulate a simple empirical noise model described by a few pragmatically determined parameters we also investigate the effects of transmitter height above the seafloor and include the data uncertainty resulting from varying transmitter height in our noise model based on the noise model and assumptions about the available data we analyze model parameter uncertainty estimates derived from the a posteriori model covariance matrix for a resistive layer buried at depth we find that the layer parameters uncertainty primarily depends on the depth of burial and the thickness of the layer we then formulate quantitative bounds for these parameters within which we have a small uncertainty of the parameters of the resistive layer the depth of burial and the transverse resistance of the layer become better determined the higher its resistance we invert a field data

wisconsin glaciation of the sierra nevada 79 000 15 000 yr b p as recorded by rock flour in sediments of owens lake california chemical analyses of the clay sized fractions of 564 continuous sediment samples 200 yr resolution from composite core ol90 92 allow quantification of an abundance of glacial rock flour rock flour produced during glacier advances is represented by clay sized plagioclase k feldspar and biotite in homogeneous internal composition the abundance of rock flour is deemed proportional to the intensity of glacies advances age control for the composite section is provided by combining previously published radiocarbon dates on organics u th dates on ostracode shells and u th dates on saline minerals from nearby searles lake correlated to ol92 by pollen the rock flour record displays three levels of variability 1 a dominant one of about 20 000 yr related to summer insolation and precipitation 2 an intermediate one of 3000 5000 yr perhaps related to north atlantic heinrich events and 3 a minor one of 1000 2000 yr perhaps related to north atlantic thermohaline driven air temperature variation

role of erosion and isostasy in the cordillera blanca uplift insights from landscape evolution modeling northern peru andes abstract the processes driving uplift and exhumation of the highest peruvian peaks the cordillera blanca are not well understood uplift and exhumation seem closely linked to the formation and movement on the cordillera blanca normal fault cbnf that delimits and shapes the western flank of the cordillera blanca several models have been proposed to explain the presence of this major normal fault in a compressional setting but the cbnf and the cordillera blanca recent rapid uplift remain enigmatic whereas the cordillera blanca morphology demonstrates important erosion and thus a significant mass of rocks removal the impact of erosion and isostasy on the evolution of the cordillera blanca uplift rates has never been explored we address the role of erosion and associated flexural rebound in the uplift and exhumation of the cordillera blanca with numerical modeling of landscape evolution we perform inversions of the broad features of the present day topography total exhumation and thermochronological data using a landscape evolution model fastscape to provide constraints on the erosion efficiency factor the uplift rate and the temperature gradient our results evidence the not negligible contribution of erosion and associated flexural rebound to the uplift of the cordillera blanca and allow us to question the models previously proposed for the formation of the cbnf

the effects of 3d topography on controlled source audio frequency magnetotelluric responses we are investigating the 3d topographic effects on controlled source audio frequency magnetotelluric data two 3d topographic models are considered a trapezoidal hill model and a trapezoidal valley model different responses are generated including the amplitude of electric field amplitude of magnetic field apparent resistivity and phase data the responses distorted by the 3d topography are simulated for the source located both next to and on the hill valley our study indicates that all the electric field magnetic field apparent resistivity and phase data are influenced by 3d topography but to different extents these topographic effects depend on the transmission receiver topography geometry the transmission frequency earth resistivity and the roughness of the surface the effects in the near field generated by topography in the survey area are quite different from those in the far field because of the existence of the source compared to those in the far field zone the magnetic field and pha

late quaternary glaciation in the tropics future research directions abstract glacier snowline depressions in the tropics of 800 1000 m indicate an atmospheric cooling of 5 7 c during the late pleistocene such a temperature depression and the presence of large glaciers in the tropics in the past suggests that on glacial interglacial time scales the tropics have undergone substantial climatic and hydrologic change however uncertainties in the chronology of glaciation the response of glacial mass balance in the tropics to a variety of climate forcings and the relation of proxy records in alpine regions to synoptic tropical paleoclimate limit the full utility of the snowline proxy future research in these three areas will provide information on the phasing of climate change in the tropics in comparison to higher latitude regions and the correspondence of tropical glaciation to large scale changes in precipitation temperature and solar radiation

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity 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

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

paleolimnological evidence for the onset and termination of glacial aridity from lake tanganyika tropical east africa geochemical and sedimentological data in a continuous 60 000 year sediment core record from the kalya horst region of central lake tanganyika provide a detailed history of paleoclimate mediated weathering and overflow events from upstream lake kivu univariate elemental profiles bivariate elemental ratios and multivariate analyses of chemical trends show variations between the dry late pleistocene 32 18 ka cal yr bp and the wetter conditions that both preceded and post date that interval this record places important new constraints on the timing of aridity in east africa during the high latitude last glacial maximum lgm based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica elemental indicatorsintheoldestportionofthesedimentaryrecord 60 50kacalyrbp characterizethisintervalasacomparatively wetperiod similar to modern conditions our record demonstrates that the ensuing transition toward arid conditions in tropical africa during high latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50 32 ka cal yr bp followed by intense aridity from 32 18 ka cal yr bp the initiation of inflow from upstream lake kivu into lake tanganyika is evidenced at 10 6 ka cal yr bp through its influence on both elemental profiles mg ca and through its effect on 87 sr 86 sr increases in elemental mg ca sr concentrations coincide with the timing of the lake kivu overflow metal geochemistry suggests that the overflow from lake kivu into lake tanganyika may have ceased between 8 and 6 ka cal yr bp suggesting a period of middle holocene aridity in east africa 2007 elsevier b v all rights reserved

a stalagmite record of changes in atmospheric circulation and soil processes in the brazilian subtropics during the late pleistocene we present a high resolution 116 000 year carbon stable isotope record from a stalagmite from southern brazil which has been precisely dated using the u series method evaluation of carbon and oxygen isotope ratios together with the speleothem growth history suggest that the carbon isotopic composition of the speleothem is primarily controlled by biogenic co2 supply from the soil which is in turn affected by temperature and secondarily rainfall amount thus the speleothem provides evidence of paleoenvironmental change in southern brazil during the last glacial period predominantly high d 13 c values and low stalagmite growth rates reflect persistent cool conditions during most of the glacial period in subtropical brazil this cooling is probably related to an intensified extratropical circulation with more frequent and intense cold surges reaching a maximum at approximately 19 ky b p this cooling tendency is interrupted during periods of high obliquity values within the full glacial period at 93 85 and 47 40 ky b p and after 19 ky b p when a dramatic decrease in d 13 c marks the deglaciation time in the continent unlike d 18 o the d 13 c record does not exhibit a strong response to precessional forcing instead it shows a strong 40 ky obliquity signal here we propose that local temperature and thus the biological processes in the soil are primarily steered by the gradients of temperature between low and mid high latitudes which influence the meridional heat transport these gradient changes in turn are paced by obliquity r 2006 published by elsevier ltd

bathymetric effects and corrections in marine csem data the effect of seafloor topographic variations on controlled source electromagnetic csem responses is investigated using a finite difference modeling approach for both petroleum and gas hydrate exploration scenarios in a deepwater reservoir model distortions in the inline electric field responses are mainly due to galvanic effects particularly at long transmitter receiver tx rx offsets and are characterized by persistent short wavelength anomalies at all tx rx offsets in the amplitude profiles in a shallow water reservoir model the contribution of inductive effects to bathymetric distortions is large at short tx rx offsets compared to the deepwater case whereas at long offsets the distortion pattern is mainly determined by the source receiver geometry relative to the sea surface because of the airwave effect rather than by the resistivity variations associated with the seafloor topography a simple correction technique which is an extension of the topographic correction procedure for magnetotelluric data is effective in removing the bathymetric distortions for the deep marine case but cannot be used for the shallow water case because of the airwave effect for a gas hydrate exploration model it is shown that the bathymetric response can be large enough to mask the target response because higher frequencies and shorter offsets need to be used to detect the shallow targets however the modeling study suggests that the shallow target response can be separated from the bathymetric distortions using the correction method

marine magnetotellurics for base of salt mapping gulf of mexico field test at the gemini structure a sea floor magnetotelluric mt survey was conducted over the gemini subsalt petroleum prospect in the gulf of mexico gom to demonstrate that the base of salt can be mapped using marine magnetotelluric mmt methods the high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for mmt the gemini salt body located at 28 46 n 88 36 w is a relatively complex shape buried 2 5 km below the sea floor in 1 km deep water its geometry has been previously determined using 3 d seismic prestack depth migration with well log control in order not to confuse limitations in interpretation technique with limitations in data acquisition numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2 d inversion even though the body was clearly 3 d the seismic imaging of the base of salt along the chosen profile is considered good thus providing a good control for testing the mt method in many other ar

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

the taylor dome antarctic 18o record and globally synchronous changes in climate abstract the 18 o 16 o profile of a 554 m long ice core through taylor dome antarctica shows the climate variability of the last glacial interglacial cycle in detail and extends at least another full cycle taylor dome shares the main features of the vostok record including the early climatic optimum with later cool phase of the last interglacial period in antarctica taylor dome 18 o fluctuations are more abrupt and larger than those at vostok and byrd station although still less pronounced than those of the greenland gisp2 and grip records the influence of the atlantic thermohaline circulation on regional ocean heat transport explains the partly north atlantic character of this antarctic record under full glacial climate marine isotope stage 4 late stage 3 and stage 2 this marine influence diminished and taylor dome became more like vostok varying degrees of marine influence produce climate heterogeneity within antarctica which may account for conflicting evidence regarding the relative phasing of northern and southern hemisphere climate change

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

useful characteristics of shallow and deep marine csem responses inferred from 3d finite difference modeling hydrocarbon reservoirs can be mapped if sufficient resistivity contrastsexistbetweenthemandtheirconfininglayers butpractical problems remain in target discrimination in deep and shallowwaters especiallyinthepresenceofheterogeneousoverburden wehavedevelopedanefficient3dstaggered gridfinite difference controlled source electromagnetic csem modeling code that enables study of the physics underlying some practical problems we undertook a comparative analysis of reservoir detection in 2000 m and 300 m deep waters using the simulated electric and magnetic field responses of a simple 3d reservoir we examined the effect of two types of near surface heterogeneitymimickingdisconnectedgascloudsand orpatchygeochemicalalterationhalosonthe3dreservoirresponse wefoundthat small scale shallowheterogeneitiescausedistortionsthatarealmost independent of the source frequency these persist at all source receiver offsets in the electric amplitude response in deep and shallow waters and phase response in shallow water they decrease in magnitude with increasing offset in deepwater phase response large scale near surface heterogeneities distort the horizontal electric field response more significantly than the small scale ones but the near surface response gets smaller in amplitude as the offset increases the distortions in shallow water are much smaller in magnitude than those for the deepwater case so that the reservoir signatures still are visible on the responseprofiles thismightbeconsideredasapositivefeaturefor shallow water inline electric field exploration the magnetic field responses for the orthogonal direction provide diagnostic target signatures that are similar to the inline electric field responsesindeepwaterbutthataredifferentinshallowwater the magnetic responses are affected by the airwave in a different manner from the electric field suggesting that combined 3d electric and magnetic field analysis might be vital for handling theairwaveproblem

last glacial maximum equilibrium line altitude and paleo temperature reconstructions for the cordillera de merida venezuelan andes abstract the pattern and magnitude of glacier equilibrium line altitude ela lowerings in the tropics during the last glacial maximum lgm are topics of current debate in the northern tropics paleo ela data are particularly limited inhibiting the ability to make regional and large scale paleoclimatic inferences to improve these records nine paleo glaciers in the venezuelan andes were reconstructed based on field observations aerial photographs satellite imagery and high resolution digital topographic data paleo glacier equilibrium line altitudes elas were estimated using the accumulation area ratio aar and the area altitude balance ratio aabr methods during the local lgm in venezuela 22 750 to 19 960 cal yr bp elas were 850 to 1420 m lower than present local lgm temperatures were are at least 8 8 2 c cooler than today based on a combined energy and mass balance equation to account for an ela lowering this is greater than estimates using an atmospheric lapse rate calculation which yields a value of 6 4 1 c cooler the paleo glacial data from the venezuelan andes support other published records that indicate the northern tropics experienced a greater ela lowering and possibly a greater cooling than the southern hemisphere tropics during the lgm

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

a preliminary investigation of the timing of the local last glacial maximum and deglaciation on hualcahualca volcano patapampa altiplano arid central andes peru abstract we studied the timing and area of the late pleistocene maximum glacier extent the readvance or stillstand phases and the deglaciation processes in the hualcahualca volcano and patapampa altiplano located in the southern arid peruvian andes we used geomorphological mapping and 9 36 cl cosmogenic surface exposure dating of moraine boulders as well as polished and striated bedrock surfaces the cosmogenic exposure ages indicates that the local last glacial maximum occurred at 17 16 ka on the hualcahualca volcano moreover the patapampa altiplano ice cap had already disappeared by 12 ka while glaciers on the hualcahualca also experienced a marked retreat at that time however the low number of samples in the study area and the different chronologies derived from the available production rates do not allow us to provide a detailed glacial reconstruction we recommend a more intensive dating campaign and the development of a local production rate for 36 cl in the central andes

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

climate impact of late quaternary equatorial pacific sea surface temperature variations magnesium calcium data from planktonic foraminifera in equatorial pacific sediment cores demonstrate that tropical pacific sea surface temperatures ssts were 2 8 0 7 c colder than the present at the last glacial maximum glacial interglacial temperature differences as great as 5 c are observed over the last 450 thousand years changes in sst coincide with changes in antarctic air temperature and precede changes in continental ice volume by about 3 thousand years suggesting that tropical cooling played a major role in driving ice age climate comparison of sst estimates from eastern and western sites indicates that the equatorial pacific zonal sst gradient was similar or somewhat larger during glacial episodes extraction of a salinity proxy from the magnesium calcium and oxygen isotope data indicates that transport of water vapor into the western pacific was enhanced during glacial episodes

finite element simulation of 3 d marine controlled source electromagnetic fields in anisotropic media with unstructured tetrahedral grids marine controlled source electromagnetic csem method has proved to be a powerful tool and found its increasing applications for offshore resource exploration and tectonic studies however bathymetric variations and anisotropic structures encountered in practical measurements have posed challenges for reliable interpretation of marine csem data collected in increasingly complex geologic settings in this study we present a finite element fe based forward algorithm for simulating 3 d marine csem responses in geologic settings characterized with rough bathymetry and electrical anisotropy unstructured tetrahedral meshes are employed to permit precise descriptions of arbitrary seafloor topography after validating the accuracy of the algorithm we investigate the effects of bathymetric variations and complicated electric anisotropy separately and jointly on marine csem responses numerical results have demonstrated that both the bathymetry and electrical anisotropy can cause significant distortions on marine csem responses but to different extent the effects of full electrical anisotropy and bathymetry have to be taken into consideration for reliable interpretations of marine csem data in practice

marine magnetotellurics for petroleum exploration part i a sea floor equipment system induction in electrically conductive seawater attenuates the magnetotelluric mt fields and coupled with a minimum around 1 hz in the natural magnetic field spectrum leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s for this reason the marine mt method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure rarely does a sea floor mt response extend to a 100 s period to be useful for mapping continental shelf structure at depths relevant to petroleum exploration however mt measurements need to be made at periods between 1 and 1000 s this can be accomplished using ac coupled sensors induction coils for the magnetic field and an electric field amplifier developed for marine controlled source applications the electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording in deep 1 km water motional noise in magnetic field sensors appears not to be a proble

subdivision of glacial deposits in southeastern peru based on pedogenic development and radiometric ages the cordillera vilcanota and quelccaya ice cap region of southern peru 13 30 14 00 s 70 40 71 25 w contains a detailed record of late quaternary glaciation in the tropical andes quantification of soil development on 19 moraine crests and radiocarbon ages are used to reconstruct the glacial history secondary iron and clay increase linearly in quelccaya soils and clay accumulates at a linear rate in vilcanota soils which may reflect the semicontinuous addition of eolian dust enriched in secondary iron to all soils in contrast logarithmic rates of iron buildup in soils in the cordillera vilcanota reflect chemical weathering high concentrations of secondary iron in vilcanota tills may mask the role of eolian input to these soils soil age estimates from extrapolation of field and laboratory data suggest that the most extensive late quaternary glaciation occurred 70 000 yr b p this provides one of the first semiquantitative age estimates for maximum ice extent in southern peru and is supported by a minimum limiting age of 41 520 14c yr b p a late glacial readvance culminated 16 650 cal yr b p in the cordillera vilcanota following rapid deglaciation of unknown extent an advance of the quelccaya ice cap occurred between 13 090 and 12 800 cal yr b p which coincides approximately with the onset of the younger dryas cooling in the north atlantic region moraines deposited 394 cal yr b p in the cordillera vilcanota and 300 cal yr b p on the west side of the quelccaya ice cap correlate with little ice age moraines of other regions

palaeoenvironmental dynamics and underlying climatic changes in southeast amazonia serra sul dos carajas brazil during the late pleistocene and holocene abstract pollen fern spore micro charcoal sediment and mineral analyses have been carried out on a 466 cm long sediment core from the serra sul dos carajas swamp in southeastern amazonia ten ams radiocarbon dates suggest that the sediment covers at least the last 73 000 years shifts between tropical forests and savanna changes in species composition as well as changes in fire frequency indicate several wet and dry periods during this time interval during the assumed early pleniglacial 73 50 ka the dominance of savanna vegetation reflects drier conditions whereas vegetation changes in the mid pleniglacial 50 40 ka suggest alternating wet and dry phases under generally cool conditions the recorded next period before the last glacial maximum lgm shows a transition to wetter and cooler conditions followed by a cold and dry lgm and late glacial at the beginning of the holocene between 11 4 and 10 2 cal ka tropical forest expanded reflecting stable and very humid conditions during the early to mid holocene an increasingly seasonal generally warm and dry climate established dry seasons were longer than at present during mid holocene times with a pronounced dry phase from 9 to 3 4 cal ka only after 3 4 cal ka did modern tropical rainforests became established under present day climatic conditions comparison with other palaeoecological studies in the amazon basin and eastern andean regions suggests that environmental changes in serra sul were strongly coupled with large scale climatic changes and sea surface temperature variability in the tropical atlantic

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

3d modeling of a deepwater em exploration survey analysis of a current offshore prospect employed 3d numerical modeling of a controlled source electromagnetic csem exploration system the analysis considers the sensitivity of data presentations to assumptions about the background model the numerical simulations show that false anomalies and significant distortion to anomaly magnitude can be caused by normalization of the observed electric fields by reference fields calculated from an incorrect or oversimplified background model bathymetry effects on the measured electric fields if not accounted for can produce anomalies as large as those of target sands the maximum sensitivity to oil water contacts or other strong lateral variations within the modeled channel sands is achieved by profiling along the length of the channel profiles run offset from a simulated oil water contact by as little as 2 km show a response below the expected noise levels good background models can be constructed by taking advantage of the magnetotelluric data recorded by m

late quaternary glaciation and equilibrium line altitudes of the mayan ice cap guatemala central america abstract the sierra los cuchumatanes 3837 m guatemala supported a plateau ice cap and valley glaciers around montana san juan 3784 m that totaled 43 km 2 in area during the last local glacial maximum former ice limits are defined by sharp crested lateral and terminal moraines that extend to elevations of 3450 m along the ice cap margin and to ca 3000 3300 m for the valley glaciers equilibrium line altitudes elas estimated using the area altitude balance ratio method for the maximum late quaternary glaciation reached as low as 3470 m for the valley glaciers and 3670 m for the mayan ice cap relative to the modern altitude of the 0 c isotherm of 4840 m we determined ela depressions of 1110 1436 m if interpreted in terms of a depression of the freezing level during maximal glaciation along the modern lapse rate of 5 3 c km 1 this ela indicates tropical highland cooling of 5 9 to 7 6 1 2 c our data support greater glacial highland cooling than at sea level implying a high tropical sensitivity to global climate changes the large magnitude of ela depression in guatemala may have been partially forced by enhanced wetness associated with southward excursions of the boreal winter polar air mass

rapid ice margin fluctuations during the younger dryas in the tropical andes abstract radiocarbon dated lacustrine sequences in peru show that the chronology of glaciation during the late glacial in the tropical andes was significantly out of phase with the record of climate change in the north atlantic region fluvial incision of glacial lake deposits in the cordillera blanca central peru has exposed a glacial outwash gravel radiocarbon dates from peat stratigraphically bounding the gravel imply that a glacier advance culminated between 11 280 and 10 990 14c yr b p rapid ice recession followed similarly in southern peru ice readvanced between 11 500 and 10 900 14c yr b p as shown by a basal radiocarbon date of 10 870 14c yr b p from a lake within 1 km of the quelccaya ice cap by 10 900 14c yr b p the ice front had retreated to nearly within its modern limits thus glaciers in central and southern peru advanced and retreated in near lockstep with one another the younger dryas in the peruvian andes was apparently marked by retreating ice fronts in spite of the cool conditions that are inferred from the 18o record of sajama ice this retreat was apparently driven by reduced precipitation which is consistent with interpretations of other paleoclimatic indicators from the region and which may have been a nonlinear response to steadily decreasing summer insolation

climatic and hydrologic oscillations in the owens lake basin and adjacent sierra nevada california oxygen isotope and total inorganic carbon values of cored sediments from the owens lake basin california indicate that owens lake overflowed most of the time between 52 500 and 12 500 carbon 14 14 c years before present b p owens lake desiccated during or after heinrich event h1 and was hydrologically closed during heinrich event h2 the magnetic susceptibility and organic carbon content of cored sediments indicate that about 19 sierra nevada glaciations occurred between 52 500 and 23 500 14 c years b p most of the glacial advances were accompanied by decreases in the amount of discharge reaching owens lake comparison of the timing of glaciation with the lithic record of north atlantic core v23 81 indicates that the number of mountain glacial cycles and the number of north atlantic lithic events were about equal between 39 000 and 23 500 14 c years b p

evidence for a prolonged retroflection of the north brazil current during glacial stages the importance of the role played by the tropics in driving and propagating climate change between hemispheres has long been the focus of attention in a bid to evaluate ocean atmosphere interactions on glacial interglacial timescales the amazon fan and ceara rise in the western equatorial atlantic ocean lie directly in the flowpath of the north brazil current nbc which as a conduit for the cross equatorial transport of heat and salinity is a key component for the heat budget of the north atlantic mg ca palaeothermometry and stable oxygen isotope analysis of planktonic foraminifera sampled from 15 sites across the amazon fan and ceara rise reveal variations in oceanic surface currents and of the dispersal of freshwater from the river amazon over five timeslices modern early holocene younger dryas late glacial and marine isotope stage 3 sea surface temperature reconstructions reveal progressive climatic amelioration over the last similar to 30 ka indicating a temperature increase of similar to 3 2 1 1 degrees c since the late glacial in conjunction with this warming values of delta delta o 18 a proxy for water column stratification indicates increased vertical mixing of the glacial ocean the spatial distribution of values of delta 18 w the isotopic composition of ambient seawater is used to infer surface current variations and demonstrates an oceanward shift in the river outflow plume in cold climates representing a prolongation or possibly permanent continuation in the duration of the seasonal retroflection of the nbc causing the curtailment of cross equatorial transport a prolongation of this retroflection could have resulted from a mean southward migration of the southern boundary of the itcz c 2011 elsevier b v all rights reserved

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

sensitivity study of marine csem data for reservoir production monitoring using numerical and analytical modeling we assess the feasibility of marine controlled source electromagnetic csem data for monitoring the flooding front during water flooding of an oil reservoir we discuss the ability of time lapse csem data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes measurement and modeling errors are discussed briefly and analytical calculations comparing time lapse signals with a certain type of time lapse modeling errors are performed numerical calculations performed with a volume integral equation method are used to study the effect on the electromagnetic em fields of relevant conductivity changes the numerical investigation includes robustness with respect to survey parameter values discrimination between different front shapes and ability to overcome time lapse error simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters it is found analytically and numerically that certain modeling errors experience a high degree of time lapse cancellation

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

isotopic evidence for late quaternary climatic change in tropical south america the tropical hydrologic cycle affects atmospheric trace gases and global climate change and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology isotopic analysis of calcite from lake junin peru provides a record of hydrologic variability that spans the last glacial interglacial transition in the southern tropics the record reveals a 6 enrichment in 18 o calcite during the late glacial followed by a gradual depletion during the holocene which can be interpreted as a decrease followed by a long term increase in effective moisture close agreement between 18 o calcite and rainy season insolation indicates that long term changes in tropical hydrology were linked to orbital variations furthermore hydrologic change was out of phase in the northern and southern tropics over this time period

modelling electrical conductivity for earth media with macroscopic fluid filled fractures modeling electrical conductivity for earth media with macroscopic fluid filled fractures james g berryman 1 and g michael hoversten 2 lawrence berkeley national laboratory one cyclotron road ms 90r1116 berkeley ca 94740 usa e mail jgberryman lbl gov chevron energy technology co 6001 bollinger canyon road san ramon ca 94583 e mail hovg chevron com

clastic sediment flux to tropical andean lakes records of glaciation and soil erosion we developed records of clastic sediment flux to 13 alpine lakes in peru ecuador and bolivia and compared these with independently dated records of regional glaciation our objectives are to determine whether a strong relationship exists between the extent of ice cover in the region and the rate of clastic sediment delivery to alpine lakes and thus whether clastic sediment records serve as reliable proxies for glaciation during the late pleistocene we isolated the clastic component in lake sediment cores by removing the majority of the biogenic and authigenic components from the bulk sediment record and we dated cores by a combination of radiocarbon and tephrochronology in order to partially account for intra basin differences in sediment focusing bedrock erosivity and sediment availability we normalized each record to the weighted mean value of clastic sediment flux for each respective core this enabled the stacking of all 13 lake records to produce a composite record that is generally representative of the tropical andes there is a striking similarity between the composite record of clastic sediment flux and the distribution of 100 cosmogenic radionuclide crn exposure ages for erratics on moraine crests in the central peruvian and northern bolivian andes the extent of ice cover thus appears to be the primary variable controlling the delivery of clastic sediment to alpine lakes in the region which bolsters the increasing use of clastic sediment flux as a proxy for the extent of ice cover in the region the crn moraine record and the stacked lake core composite record together indicate that the expansion of ice cover and concomitant increase in clastic sediment flux began at least 40 ka and the local last glacial maximum llgm culminated between 30 and 20 ka a decline in clastic sediment flux that began 20 ka appears to mark the onset of deglaciation from the llgm at least one millennium prior to significant warming in high latitude regions the interval between 20 and 18 ka was marked by near holocene levels of clastic sediment flux and appears to have been an interval of much reduced ice extent an abrupt increase in clastic sediment flux 18 ka heralded the onset of an interval of expanded ice cover that lasted until 14 ka clastic sediment flux declined thereafter to reach the lowest levels of the entire length of record during the early middle holocene a middle holocene climatic transition is apparent in nearly all records and likely reflects the onset of neoglaciation and or enhanced soil erosion in the tropical andes

late quaternary glaciation of the cordillera real bolivia glacial geological studies in tropical mountain areas of the southern hemisphere can be used to address two issues of late pleistocene climate change the global synchroneity of deglaciation and the magnitude of temperature reduction in the tropics radiocarbon dates from the cordillera real and from other areas in peru and bolivia suggest that late pleistocene glaciation culminated between 14 000 and 12 000 yr bp followed by rapid deglaciation because deglaciation was apparently synchronous with that in northern hemisphere regions insolation change at high latitudes may not have been the only factor that produced global deglaciation at this time late pleistocene glaciation in the cordillera real culminated when precipitation was 200 mm yr 1 higher and temperatures were 3 5 1 6 c lower than today this produced an equilibrium line altitude depression of about 300 100 m on the western side of the cordillera prior to this conditions were drier and probably at least as cold however the lack of moraines in the cordillera real dated to the last glacial maximum ca 18000 yr bp precludes using the equilibrium line altitude method to quantitatively evaluate the discrepancy between warm sea surface temperatures and cold terrestrial conditions reconstructed with other proxies for this time period

biogeochemical indicators of environmental changes from 50 ka to 10 ka in a humid region of the brazilian amazon abstract we present a geochemical record of a 113 6 cm sediment core lpt v from lagoa da pata which is located in the forested upper rio negro basin the record reveals significant changes in the environmental history of amazonia during the late quaternary the results of biogeochemical analyses revealed three hydrological and climatic regimes from 50 000 to 10 000 cal yr bp the first phase between 50 000 and 26 300 cal yr bp was characterized by a relatively wet climate as suggested by relatively high total organic carbon toc and chlorophyll derivate concentrations indicating high productivity linked to a high lake level a decrease of the toc and chlorophyll derivate accumulation rates between 43 100 cal yr bp and 26 300 cal yr bp marks a decrease in the productivity linked to a reduced lake level indicating a decrease in moisture at the end of this phase the second phase between 26 300 and 15 300 cal yr bp was characterized by a decrease in productivity reaching a minimum at 21 950 cal yr bp as indicated by a minima in sedimentary chlorophyll and toc accumulation rates values of 13 c increased by 5 in relation to the preceding phase indicating an influence of the c 4 organic matter high iron concentrations and accumulation rates related to intense erosion of the lateritic crust in the watershed were observed all of the observations indicate a dry phase during this period a third phase from approximately 15 300 to 10 000 cal yr bp was characterized by an increase in lacustrine productivity as shown by an increased in toc and chlorophyll derivate concentrations and accumulation rates these increases likely correspond to a lake level rise due to a wetter climate

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

anisotropy of elastic wave velocities in deformed shales part 1 experimental results elastic wave velocity measurements in the laboratory are used to assess the evolution of the microstructure of shales under triaxial stresses which are representative of in situ conditions microstructural parameters such as crack aperture are of primary importance when permeability is a concern the purpose of these experiments is to understand the micromechanical behavior of the callovo oxfordian shale in response to external perturbations the available experimental setup allows for the continuous simultaneous measurement of five independent elastic wave velocities and two directions of strain axial and circumferential performed on the same cylindrical rock sample during deformation in an axisymmetric triaxial cell the main results are 1 identification of the complete tensor of elastic moduli of the transversely isotropic shales using elastic wave velocity measurements 2 assessment of the evolution of these moduli under triaxial loading and 3 assessment of the evolution of the elastic ani

moraine preservation and boulder erosion in the tropical andes interpreting old surface exposure ages in glaciated valleys cosmogenic dating provides a long awaited means of directly dating glacial deposits that pre date the last glacial cycle although the potential benefits of longer chronologies are obvious the greater uncertainty associated with older cosmogenic ages may be less readily apparent we illus trate the challenges of developing and interpreting a long chronology using our data from the peruvian andes we used surface exposure dating with cosmogenic radionuclides crns 10 be and 26 al to date 140 boulders on moraines in valleys bordering the junin plain 11 s 76 w in central peru our chronology spans multiple glacial cycles and includes exposure ages greater than 1 million years which indicate that long term rates of boulder erosion have been very low interpreting the chronology of moraines for glaciations that predate the last glacial cycle is complicated by the need to consider boulder erosion and exhumation surface uplift and inheritance of crns from pre vious exposure intervals as an example we recalculate exposure ages using our boulder erosion rates 0 3 0 5 metres per million years and estimated surface uplift rates to emphasise both the chal lenges involved in interpreting old surface exposure ages and the value of chronological data even with large uncertainties when reconstructing the palaeoclimate of a region copyright 2005 john wiley sons ltd