an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

polarimetric signatures of sea ice 2 experimental observations experimental observations of polarimetric signatures are presented for sea ice in the beaufort sea under cold winter conditions and interpreted with the composite model developed in part 1 polarimetric data were acquired in march 1988 with the jet propulsion laboratory multifrequency airborne synthetic aperture radar sar during the beaufort sea flight campaign the experimental area was located near 75 n latitude and spanned 140 145 w longitude selected sea ice scenes contain various ice types including multiyear thick first year and thin lead ice additionally the c band sar on the first european remote sensing satellite provides supplementary backscattering data of winter beaufort sea ice for small incident angles 20 26 at vertical polarization sea ice characterization and environmental data used in the model were collected at the applied physics laboratory drifting ice station to the northeast of prudhoe bay additional data from field and laboratory experiments are also utilized in this analysis the model relates sea ice polarimetric backscattering signatures to physical structural and electromagnetic properties of sea ice scattering mechanisms contributing to sea ice signatures are explained and sensitivities of polarimetric signatures to sea ice characterization parameters are studied

tropical climates at the last glacial maximum a new synthesis of terrestrial palaeoclimate data i vegetation lake levels and geochemistry palaeodata in synthesis form are needed as benchmarks for the palaeoclimate modelling intercomparison project pmip advances since the last synthesis of terrestrial palaeodata from the last glacial maximum lgm call for a new evaluation especially of data from the tropics here pollen plant macrofossil lake level noble gas from groundwater and delta 18 o from speleothems data are compiled for 18 2 ka c 14 32 degrees n 33 degrees s the reliability of the data was evaluated using explicit criteria and some types of data were re analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month mtco mean annual temperature mat plant available moisture pam and runoff p e cold month temperature mat anomalies from plant data range from 1 to 2 k near sea level in indonesia and the s pacific through 6 to 8 k at many high elevation sites to 8 to 15k in s china and the se usa mat anomalies from groundwater or speleothems seem more uniform 4 to 6 k but the data are as yet sparse a clear divergence between mat and cold month estimates from the same region is seen only in the se usa where cold air advection is expected to have enhanced cooling in winter regression of all cold month anomalies against site elevation yielded an estimated average cooling of 2 5 to 3 k at modern sea level increasing to approximate to 6 k by 3000m how ever neotropical sites showed larger than the average sea level cooling 5 to 6 k and a non significant elevation effect whereas w and s pacific sites showed much less sea level cooling 1 k and a stronger elevation effect these findings support the inference that tropical sea surface temperatures ssts were lower than the climap estimates but they limit the plausible average tropical sea surface cooling and they support the existence of climap like geographic patterns in sst anomalies trends of pam and lake levels indicate wet lgm conditions in the w usa and at the highest elevations with generally dry conditions elsewhere these results suggest a colder than present ocean surface producing a weaker hydrological cycle more arid continents and arguably steeper than present terrestrial lapse rates such linkages are supported by recent observations on freezing level height and tropical ssts moreover simulations of greenhouse and lgm climates point to several possible feedback processes by which low level temperature anomalies might be amplified aloft

holocene history of the el nino phenomenon as recorded in flood sediments of northern coastal peru significant precipitation along the north central coast of peru lat 5 10 s occurs exclusively during el nino incursions of warm water into the peruvian littoral flood deposits from this region therefore provide a proxy record of extreme el nino events i present a 3500 yr chronology of the extreme events based on radiocarbon dating of overbank flood sediments from the rio casma lat 9 2 s the flood plain stratigraphy suggests that the el nino phenomenon has occurred throughout the holocene and that flood events much larger than that which occurred during 1982 1983 occur here at least once every 1000 yr

independence of geostatic stress from overconsolidation in some beaufort sea clays reply since 1982 extensive use has been made of the self bored pressuremeter in arctic offshore site investigations one finding of this work is that the in situ geostatic stress ratio k0 of beaufort sea clays is apparently unrelated to the overconsolidation ratio ocr the procedures and data to support this finding are documented and the finding is discussed in light of the present geological understanding for the area it is concluded that the independence of k0 from ocr may be quite widespread a finding that contradicts the widely held tenet regarding the existence of a single valued function relating k0 and ocr if k0 is regarded as important then it should be measured in situ key words self bored pressuremeter geostatic stress ratio k0 beaufort shelf geologic history overconsolidation ratio

application of hyperbolic stress strain models for sandstone and shale to fold wavelength in the mexican ridges foldbelt abstract the mexican ridges foldbelt on the continental slope east of mexico has major folds with wavelengths averaging 7 km locally there are also small amplitude near surface folds with wavelengths of 2 km initial dominant wavelength is modelled by the finite element method using the hyperbolic stress strain relationship originally derived for soils hyperbolic stress strain models for sandstone and shale are calibrated using field porosity depth relationships for shale and laboratory one dimensional consolidation data for sand and clay effective tangent young s modulus calculated from hyperbolic stress strain models is generally higher for sandstones than for shales and for both rock types modulus increases substantially with depth the finite element models of dominant wavelength duplicate the two orders of folds observed in the mexican ridges and match their wavelengths in the models fold wavelength is nearly constant for large variations in both sandstone fraction in the folded section and pore pressure magnitude in the overpressured basal shale below the folded section but fold amplification is increased by high pore pressure in the basal shale and by high sandstone fraction in the folded section fold wavelength is affected more by depth distribution of high pore pressure than by pressure magnitude and on the lower continental slope where grain size may be smaller a thicker zone of high pressure would partly explain the modest downslope increase in fold wavelength

el nino southern oscillation events recorded in the stratigraphy of the tropical quelccaya ice cap peru snow accumulation measured during 1982 1983 on the quelccaya ice cap peru was 70 percent of the average from 1975 through 1983 inspection of 19 years 1964 through 1983 of accumulation measured near the summit of quelccaya reveals a substantial decrease 30 percent in association with the last five el nino southern oscillation enso occurrences in the equatorial pacific the enso phenomenon is now recognized as a global event arising from large scale interactions between the ocean and the atmosphere understanding this extreme event with the goal of prediction requires a record of past occurrences the quelccaya ice cap which contains 1500 years of annually accumulated ice layers may provide a long and detailed record of the most extreme enso events

physical properties of fine grained sediments with segregated hydrate lenses abstract the physical properties of natural gas hydrate bearing sediments are critical for the analysis of natural systems and for the design of gas production strategies this work explores the properties of fine grained sediments containing segregated hydrate lenses our analyses show that hydrate formation is grain displacive when the product of the effective stress and the grain radius is r

towards identification of optimum radar parameters for sea ice monitoring modele theorique permettant de prevoir la diffusion par la glace multi annuelle et par la glace de premiere annee en conditions froides les resultats experimentaux montrent que les bandes de frequence ku et x ainsi que les angles d incidence superieure a 30 sont les meilleurs pour distinguer les types de glace de mer

late holocene phytolith and carbon isotope record from a latosol at salitre south central brazil abstract the reliability of paleovegetation records inferred from soil phytolith assemblages and som soil organic matter carbon isotope analysis was examined in light of previous pollen and charcoal reconstructions the sampled latosol is located in south central brazil salitre at a boundary between forest and cerrado the derived mean age of phytoliths and som at each level is the result of a balance between continuous translocation and selective dissolution it increases with depth in a regular quantifiable fashion that allows paleoenvironmental interpretation phytoliths and som tracers first record a savanna phase associated with the last holocene long dry period occurring between ca 5500 and 4500 yr b p two periods of tree community development followed between ca 4000 and 3000 and after ca 970 yr b p leading to the present cerrado forest association the dry spell interrupted this trend about 970 60 yr b p the second development of woody elements was contemporaneous with an increase in anthropogenic fires therefore climate was more important than fires and human activities in constraining the growth of vegetation during the last nine centuries at salitre more generally despite pedogenic processes soil phytoliths and 13c values of the som may be accurate tracers of vegetation changes

sources and transport pathways of marine aerosol species into west antarctica sixteen high resolution marine aerosol na so4 records from spatially distributed international trans antarctic scientific expedition itase ice cores spanning the last 200 years from the pine island thwaites and ross drainage systems and the south pole are used to examine sources sea spray and frost flowers and transport pathways of marine aerosols into west antarctica factors contributing to the amount of marine aerosols transported inland include sea ice extent the presence of open water features polynyas leads wind strength and direction and the strength and positioning of low pressure systems analysis of so4 na ratios indicates that frost flowers can contribute significantly 40 to the na budget of antarctic ice cores higher na concentrations in the ross drainage system may result from greater production of marine aerosols related to frost flowers in the ross sea region in association with greater sea ice extent and larger open water areas significant positive correlations of sea ice extent and the na time series exist in some regions of west antarctica higher wind speeds in winter and higher na concentrations when slp is

late quaternary vegetational change in the central peruvian andes abstract new fossil pollen records from the central peruvian andes at elevations above 4000 m including pollen concentration data and additional radiocarbon dates provide information on late quaternary vegetation change for the region a pollen assemblage typical of superpuna plus low pollen concentrations and organics in the late glacial section of laguna tuctua indicate a cold dry climate but strong evidence for a late glacial climate reversal i e the younger dryas is not presently confirmed an abrupt increase in urticales pollen as compositae and polylepis acaena type pollen decline signals the beginning of the holocene subsequent increases in urticales and plantago rigida type pollen point to increased moisture as well as higher temperatures from about 11 000 to 7000 yr b p a possible correlation exists between the rapid expansion of puna and closed continuous forest and the rise in importance of camelids in the early holocene in the mid holocene plantago rigida cushion bogs and continuous montane forest elements decline suggesting a drier climate grasses and cheno ams then increase reaching maxima about 4000 yr b p pastoralism and agriculture are thought to have developed at this time according to ethnobotanical studies from this area and pollen evidence seems to support these findings between 3000 and 2000 yr b p pollen percentages of poaceae plantago rigida type and other local taxa decline as glacial activity was renewed in the higher mountains zea mays is present by 2000 yr b p and pollen of weeds and secondary successional plants associated with increasing agricultural disturbance are evident

variations in brightness temperature over cold first year sea ice near tuktoyaktuk northwest territories microwave radiometric temperature tb profiles of first year sea ice were obtained along 70 to 100 m traverses with sled mounted radiometers at 10 18 7 33 6 and 37 ghz and an effective spot size of 30 cm measurements of tb as a function of nadir angle were obtained at selected sites along the traverses snow and ice properties were recorded and correlated with the tb measurements to infer the effect of snow cover and ice conditions on the radiometric temperature tb correlated positively with the brine volume profile in the ice at several sites suggesting that brine volume has a strong effect on tb under these conditions an overall statistical comparison of snow thickness with tb when compared with previously published models suggests that the effect of snow cover on the microwave transmission coefficient of the snow ice interface may be an important contribution to the radiometric temperature at these frequencies a model is proposed to explain the data

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

late quaternary vegetation and climate change in the amazon basin based on a 50 000 year pollen record from the amazon fan odp site 932 hemipelagic sediments from the amazon deep sea fan odp site 932 5 12 7 n 47 1 8 w and continental shelf provide a 50 000 yr long pollen record of amazon basin vegetation the age model for hole 932a is constrained by eight magnetic remanence intensity features one paleomagnetic excursion and three ams14c dates alchornea melastomataceae myrtaceae and moraceae urticaceae are dominant taxa in the pollen record between 40 200 and 19 800 cal yr b p andean taxa such aspodocarpusandhedyosmum increase in abundance between 19 800 and 11 000 cal yr b p and prior to 40 200 cal yr b p the holocene pollen assemblage derived from amazon river and continental shelf sediments is dominated by secondary growth taxa such ascecropia climatic factors influencing the development of glacial and interglacial tropical vegetation are considered by comparing marine with terrestrial records of vegetation change this comparison shows that the amazon basin forests were not extensively replaced by savanna vegetation during the glacial period contradicting the refugia hypothesis

equivalent number of uniform stress cycles for soil liquefaction analysis the seismic demand on potentially liquefiable soils can be approximated by a series of uniform shear stress cycles procedures are reviewed for converting an arbitrary acceleration time history to a series of uniform cycles with amplitude 0 65 of the peak the number of cycles n at this amplitude is evaluated so as to represent a seismic demand for liquefaction triggering equivalent to that of the accelerogram an assumed relationship between n and magnitude m underlies so called magnitude scaling factors used to adjust the liquefaction resistance of soil for the effects of duration magnitude scaling factors can alternatively be related directly to n which enables the effects of factors other than m for example site source distance r to be quantified we develop empirical models for n that are applicable to active tectonic regions and find a strong dependence on m and r and a weaker dependence on site condition and near fault rupture directivity effects the model for n is used to develop new s

simulations of a centrifuge test with lateral spreading and void redistribution effects abstractnumerical simulations of a centrifuge test in which dissipation patterns lateral spreading and shear strain localization were measured and recorded are performed for validation of the numerical modeling approach and insight on the deformation mechanisms the constitutive model calibration process is performed based on available laboratory data and is described in detail the model container and construction sequence are simulated to provide good approximations of the boundary conditions and initial stress distributions two approaches for simulating successive shaking events are presented and compared the key observations and mechanisms from the centrifuge test namely 1 the dynamic response and onset of liquefaction 2 the amount and pattern of surface deformation 3 the patterns of pore pressure dissipation and void ratio redistribution and 4 the difference in response between a sand profile treated and not treated with liquefaction drains are all reasonably captured and bounded b

snow depth on arctic sea ice derived from radar in situ comparisons and time series analysis the snow radar being flown on nasa s operation icebridge ongoing aircraft campaigns to the arctic and the antarctic are providing unique observations of the depth of snow on the sea ice cover in this paper we focus on the radar derived snow depth results from the 2009 2012 arctic campaigns we develop and evaluate the use of a distinct snow layer tracker to measure snow depth based on a support vector machine svm supervised learning algorithm the snow radar is designed to detect both the air snow and snow ice interfaces using ultrawideband frequencies from 2 to 8 ghz the quality errors and repeatability of the snow radar snow depth estimates are examined based on comparisons with in situ data obtained during two separate sea ice field campaigns the greenarc 2009 and the cryovex 2011 campaigns off greenland in the lincoln sea finally we analyze 4 years 2009 2012 of three annually repeated sea ice flight lines obtained in early spring located off greenland and the canadian arctic we examine the annual variations of snow depth differences between perennial and seasonal ice when available overall the snow layer tracker produced consistent accurate results for snow depths between 0 10 and 0 60 m this was confirmed with comparisons with the two data sets from the in situ measurement campaigns as well as with the time series analysis and is consistent with other published results

evaluation of the lateral earth pressure coefficients at rest in granular soil deposits using the anisotropic components of s wave velocity abstract determination of the lateral earth pressure coefficients at rest k0 in natural soil deposits is an important issue in geotechnical and geological engineering in this study evaluation of this parameter in a granular soil deposit was examined by using two anisotropic components of the s wave velocity sv and sh wave components to this end thirteen pairs of p and s wave seismic refraction profiles were acquired at two different granular sites in iran the collected shot gathers were processed from which the sv and sh wave velocities were derived the sv wave velocities were obtained by using the multichannel analysis of surface waves masw method while the sh wave velocities were extracted by utilizing the multichannel analysis of love waves malw technique the reference k0 values in the soil deposits were calculated by the mayne and kulhawy s formula based on the friction angles and overconsolidation ratios ocr determined from laboratory tests the curves of the reference k0 values versus the sh sv velocity ratios at both sites strongly confirmed the feasibility of k0 evaluation from the velocity components to develop a global equation for k0 evaluation in granular soil deposits a general regression analysis was conducted on the obtained data from this study combined with those reported in the literature the results showed that reliable values of k0 can be estimated using the sh sv velocity ratios the findings of this study may appropriately serve as references for economic and non invasive in situ k0 identification in granular soil deposits using the surface wave methods

letter to the editor climate of east africa 6000 14c yr b p as inferred from pollen data by odile peyron dominique jolly raymonde bonnefille annie vincens and joel guiot

an optical luminescence chronology for late pleistocene aeolian activity in the colombian and venezuelan llanos the lowland savannas llanos of colombia and venezuela are covered by extensive aeolian landforms for which little chronological information exists we present the first optically stimulated luminescence osl age constraints for dunes in the llanos orientales of lowland colombia and new ages for dunes in the venezuelan llanos the sampled dunes are fully vegetated and show evidence of post depositional erosion ages range from 4 5 0 4 to 66 4 ka with the majority dating to 27 10 ka marine isotope stage 2 some dunes accumulated quickly during the last glacial maximum although most were active 16 10 ka accretion largely ceased after 10 ka all dunes are elongated downwind from rivers parallel with dry season winds and are interpreted as source bordering features as they are presently isolated from fluvial sediments by gallery forest it is proposed that activity was associated with a more prolonged dry season which restricted gallery forest leading to greater sediment availability on river shorelines such variability in dry season duration was potentially mediated by the mean latitude of the itcz the cessation of most dune accretion after ca 10 ka suggests reduced seasonality and a more northerly itcz position consistent with evidence from the cariaco basin

postshaking shear strain localization in a centrifuge model of a saturated sand slope this paper presents analyses of a test conducted on a 9 m radius centrifuge to study the redistribution of pore water during diffusion of earthquake induced excess pore pressures in a sand slope with embedded silt layers the centrifuge model developed large postshaking deformations associated with shear strain localization at the interface between the sand and silt layers dense arrays of pore pressure transducers provided detailed measurements of pore pressure variations in time and space within the slope a new data analysis approach is presented in which measured pore pressures are used to compute flow rates and volumetric strains as a function of time and position throughout the slope hydraulic gradients were calculated by numerical differentiation of measured pore pressure distributions with respect to position flow rates that were based on darcy s law were then integrated with respect to time to obtain flow quantities from which volumetric strains were computed a second data analysis approach that computes volumetric strains on the basis of soil compressibility and changes in pore pressure provided an independent computation of strains in consolidating zones results using these data analysis procedures confirm that a dilating loosening zone of significant thickness developed in the sand immediately beneath an embedded silt layer that had impeded the drainage of high pore pressures these results support the hypothesis that the dilating zone corresponds to regions where the mobilized friction angle exceeds the critical state friction angle and that the dilating zone can be initially relatively thick before its size diminishes to the thickness of a thin shear band after the peak friction angle is mobilized quantification of the evolution of the size of the dilating zone is a key to understanding the magnitude of deformations associated with void redistribution

polarimetric signatures of sea ice 1 theoretical model physical structural and electromagnetic properties and interrelating processes in sea ice are used to develop a composite model for polarimetric backscattering signatures of sea ice physical properties of sea ice constituents such as ice brine air and salt are presented in terms of their effects on electromagnetic wave interactions sea ice structure and geometry of scatterers are related to wave propagation attenuation and scattering temperature and salinity which are determining factors for the thermodynamic phase distribution in sea ice are consistently used to derive both effective permittivities and polarimetric scattering coefficients polarimetric signatures of sea ice depend on crystal sizes and brine volumes which are affected by ice growth rates desalination by brine expulsion drainage or other mechanisms modifies wave penetration and scattering sea ice signatures are further complicated by surface conditions such as rough interfaces hummocks snow cover brine skim or slush layer based on the same set of geophysical parameters characterizing sea ice a composite model is developed to calculate effective permittivities and backscattering covariance matrices at microwave frequencies for interpretation of sea ice polarimetric signatures

natural fractures in a united kingdom shale reservoir analog cleveland basin northeast england faults and fractures within the well exposed lower jurassic cleveland ironstone and whitby mudstone formations may provide insights into the tectonic history of gas prospective mississippian shale in northern england subvertical opening mode fractures occur throughout the cleveland basin bed parallel fractures some of which contain blocky calcite fills occur preferentially within well bedded clay rich mudstones of the cleveland ironstone and whitby mudstone formations at jet wyke and port mulgrave subvertical fractures display abutting or curving parallel relationships with under and overlying bed parallel fractures together these observations suggest that bed parallel fractures at times acted as free surfaces some bed parallel fractures curve toward and branch from calcite filled fault slip surfaces indicating that bed parallel fracturing and normal faulting were synchronous occurring within a regional stress field with vertical maximum principal stress this apparent paradox can be explained by normal compaction followed by cementation and coupling between pore pressure and minimum horizontal stress driven by poroelastic deformation or incipient slip along critically stressed normal faults causing elevation of horizontal stress in excess of the vertical stress within clay rich units propagation of bed parallel fractures was enhanced by dilatational strains adjacent to normal fault planes bed parallel fractures have not been observed within more rich units at the top of the whitby mudstone formation at whitby east cliff or within well bedded clay rich shale at saltwick nab this observation is consistent with the lack of normal faulting at saltwick nab and the whitby mudstone formation having been drained by structural and or stratigraphical juxtaposition against permeable middle jurassic sandstones at both these localities

modeling and measurement of c band radar backscatter from snow covered first year sea ice in this paper we present model and measurement results for c band hh and vv normalized radar cross sections nrcs from winter snow covered first year sea ice with average snow thicknesses of 16 4 and 3 cm the brine content in snow pack was low in all three case studies which is typical for cold winter conditions we used the first order approximation of the small perturbation theory accounting for surface scattering from the air snow and snow ice rough interfaces and continuously layered snow and sea ice the experimental data were collected during the circumpolar flaw lead system study in the winter of 2008 in the southern beaufort sea from the research icebreaker amundsen good agreement between the model and experimental data were observed for all three case studies the model results revealed that the scattering at the snow ice rough interface is usually stronger than that at the air snow interface furthermore both model and experimental nrcs values at vv and hh polarizations were considerably higher for thin snow cover compared with the thick snow cover case we associate this effect with the lower attenuation of the propagated wave within the thin snow pack in comparison to the thick snow pack we also demonstrated that different brine volume contents in snow with close thicknesses of 4 and 3 cm did not affect the backscattering coefficients at certain incidence angles and polarization our findings provide the physical basis for winter snow thickness retrieval and suggest that such retrievals may be possible from radar observations under particular scattering conditions

k o ocr relationships in soil the relationships between k o and ocr are investigated for primary loading unloading reloading conditions the study reviews laboratory data from over 170 different soils and presents an approach common to clays silts and sands simple empirical methods for predicting k o for normally consolidated and overconsolidated soils are evaluated the validity of the methods is supported by statistical analyses on the basis of the findings only the effective stress friction angle o and prior stress history ocr and ocrmax are needed to predict approximate values of k o

co2 and temperature controlled altitudinal shifts of c4 and c3 dominated grasslands allow reconstruction of palaeoatmospheric pco2 abstract during the pleistocene the vegetation changes in the high colombian andes included changes from c3 to c4 plants this is inferred from 13c values of the c31 n alkane from the funza 2 sedimentary record taken from the high plain of bogota at 2550 m elevation the environmental factors thought to be responsible for these changes were investigated using a single point simulation of the biome3 vegetation model including changes in precipitation temperature and atmospheric co2 concentrations the model shows that changes are for a major part caused by these latter two factors the isotopic signature of the n alkanes of several extant c3 and c4 grasses from the area were determined to calibrate the interpretation of the isotopic record from the geochemical record we estimated the altitudinal distribution of c3 and c4 plants using present grass distribution patterns based on floristic data as a template this information in combination with palaeotemperature estimates enabled the reconstruction of atmospheric co2 concentrations the reconstructed co2 concentrations follow the trends of the vostok antarctic ice core through three glacial and two interglacial stages the lowest calculated co2 concentration is ca 210 ppmv for the glacial maxima and within the range of lowest values from vostok our highest value 310 ppmv is for interglacial mis 7 this represents a new method to reconstruct palaeoatmospheric pco2 it is less accurate than measurements from ice cores but has potential to be used for sediments that are much older than the ice cores

evaluation of simulation models of lateral spread sites treated with prefabricated vertical drains abstractfinite element analyses are performed of centrifuge tests consisting of untreated and drain treated lateral spread sites with the goal of evaluating the ability of numerical simulations to predict the response of sites improved with prefabricated vertical drains finite element models of varying complexity are analyzed a two dimensional model of the full centrifuge test a three dimensional unit cell model around a single drain and a two dimensional unit cell model all the finite element models predict pore pressures consistent with the centrifuge tests the deformation responses of the finite element models show that unit cell analyses produce significantly larger deformations than those observed in the centrifuge this result is caused by the fact that the centrifuge test geometry is not well represented by the infinite slope condition assumed in a unit cell the full two dimensional model predicts deformations that are more consistent with those of the centrifuge because it can capture the

yield stress history evaluated from paired in situ shear moduli of different modes abstract a special compiled database of shear moduli from different directional and polarization modes hh hv and vh waves at well documented geologic sites shows that stiffness ratio g 0 hh g 0 vh can be used to assess the yield stress ratio ysr in soils in general the reference profiles of ysr for these sites were determined using series of laboratory consolidation tests on undisturbed samples at varied elevations coupled with a good understanding of the engineering geology background of the formations for stress history assessment in soils various empirical correlations were derived from multiple regression analyses using the stiffness ratio and additional variables

inverse electromagnetic scattering models for sea ice inverse scattering algorithms for reconstructing the physical properties of sea ice from scattered electromagnetic field data are presented the development of these algorithms has advanced the theory of remote sensing particularly in the microwave region and has the potential to form the basis for a new generation of techniques for recovering sea ice properties such as ice thickness a parameter of geophysical and climatological importance moreover the analysis underlying the algorithms has led to significant advances in the mathematical theory of inverse problems in particular the principal results include the following 1 inverse algorithms for reconstructing the complex permittivity in the helmholtz equation in one and higher dimensions based on layer stripping and nonlinear optimization have been obtained and successfully applied to a lossless laboratory system in one dimension causality has been imposed to obtain stability of the solution and layer thicknesses can be obtained from the recovered dielectric profile or directly from the reflection data through a nonlinear generalization of the paley wiener theorem in fourier analysis 2 when the wavelength is much larger than the microstructural scale the above algorithms reconstruct a profile of the effective complex permittivity of the sea ice a composite of pure ice with random brine and air inclusions a theory of inverse homogenization has been developed which in this quasistatic regime further inverts the reconstructed permittivities for microstructural information beyond the resolution of the wave rigorous bounds on brine volume and inclusion separation for a given value of the effective complex permittivity have been obtained as well as an accurate algorithm for reconstructing the brine volume from a set of values 3 inverse algorithms designed to recover sea ice thickness have been developed a coupled radiative transfer thermodynamic sea ice inverse model has accurately reconstructed the growth of a thin artificial sea ice sheet from time series electromagnetic scattering data

surface based passive microwave observations of sea ice in the bering and greenland seas measurements of brightness temperatures of sea ice were carried out during both the mizex field experiments in 1983 the first in februrary in the bering sea and the second in june july in the northern greenland sea in the bering sea thin growing sea ice types from black ice to 400 mm thick snow covered floes were investigated brightness temperatures increased with ice thickness up to 100 mm from values of 100 k for open water to as high as 250 k e 0 97 for thick ice and a moderate dependence on snow thickness was found in the greenland sea thick first year and multiyear ice types were studied brightness temperatures were quite variable depending on the daily melt freeze cycle superimposed on the seasonal warming ranging from near blackbody values for melting conditions to multiyear like spectra when the surface layers refroze the melt season was sufficiently advanced however that first year and multiyear ice could not be differentiated radiometrically

late pleistocene temperature depression and vegetation change in ecuadorian amazonia abstract paleoecological pollen phytolith and wood analyses of sediments radiocarbon dated 33 000 to 26 000 yr b p from two sites in ecuadorian amazonia provide data that suggest a cooling of ca 7 5 c below present in equatorial lowlands from 33 000 to 30 000 yr b p a period of warning followed in which novel species assemblages a blend of montane and lowland floral components persisted for at least 4000 years these data of forest community change from sites lying within the postulated glacial rain forest napo refugium provide the strongest paleoecological refutation of the refugial hypothesis yet obtained the large temperature depression at ca 30 000 yr b p allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum lgm of the northern hemisphere freezing temperatures would have been experienced in parts of lowland amazonia between 25 000 and 18 000 b p

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

late holocene environmental history of southern choco region pacific colombia sediment diatom and pollen analysis of core el caimito abstract we present a multi proxy study of pollen diatoms sediment characteristics and major elements of a 610 cm sediment core from lake el caimito located in the humid rain forest of southern choco pacific colombia we propose an integrated reconstruction of the local basin development and of the regional vegetation which is possibly related to the tectonic activity of an unstable coastal area time control is based on 7 ams 14 c dates that show that the record represents the last 3850 cal yr bp from 3850 to 2700 cal yr bp sandy deposits low carbon content absence of diatoms and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system erosive event s removed part of the sediment record and we observed a hiatus representing 700 years after that the basin became more isolated from the river drainage system from 2010 to 1430 cal yr bp mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record benthic and littoral benthic diatom species indicate a shallow water body and a stable water chemistry mangrove forest was close to the lake apparently growing along the close by inlets regionally the main vegetation elements were palms and taxa in the families of moraceae urticaceae melastomataceae fabaceae and from a number of other families and genera characteristic of tropical lowland rain forest from 1430 to 810 cal yr bp the river impact gradually diminished each fluvial event that affected the local forest is shown in the pollen record by an expansion of cecropia dominated pioneer forest decreasing intensity of forest disturbance coincides with an increase in the diversity of fossil pollen taxa possibly reflecting an increasing plant diversity of the forest mangrove pollen declined indicating that the coastline moved seaward and suggesting tectonic uplift of the coastal area between 810 and 580 cal yr bp mangrove forest was closer to the lake again reflecting an inland migration of the coastline suggesting tectonic subsidence from 580 to 300 cal yr bp the last fluvial events were recorded diatom associations indicate oligotrophic and acidic water the mangrove belt moved seaward again suggesting tectonic uplift palms and cecropia became more abundant suggesting increased human impact in the near shore lowland forest during the last 300 years stable lacustrine conditions and lowland rain forest with the highest floral diversity is registered

early holocene atmospheric co2 concentrations wagner et al 1 1 present a high resolution record of leaf stomatal index si from which they reconstruct the concentration of atmospheric co2 in the early holocene 10 070 to 9 38014c yr bp 11 300 to 10 600 cal yr bp 1 1 2 2 the resulting record shows a co2 concentration of

structure and dynamics of the sea of okhotsk marginal ice zone from ocean satellite radar sensing data radar images of the sea of okhotsk ice cover are analyzed the images were obtained by side looking radars slr installed on the ussr ocean series of satellites since september 1983 slr operates at wavelength 3 15 cm at vertical polarization a swath width is 460 km at a spatial resolution of 1 3 km satellite optical images air reconnaissance and meteorological information are used to interpret sea ice radar contrasts the radar images permit monitoring of the formation and transformation of the ice cover and its displacement under the influence of winds and currents radar contrast is enhanced in the marginal ice zone on account of the great difference in the scattering properties of pancake ice and grease ice in the marginal ice zone different regular motions are registered for ice edge waves ice bands and jets eddies and mushroomlike structures with dimensions ranging between 10 20 and 100 120 km

estimating liquefaction induced lateral displacements using the standard penetration test or cone penetration test a semiempirical approach to estimate liquefaction induced lateral displacements using standard penetration test spt or cone penetration test cpt data is presented the approach combines available spt and cpt based methods to evaluate liquefaction potential with laboratory test results for clean sands to estimate the potential maximum cyclic shear strains for saturated sandy soils under seismic loading a lateral displacement index is then introduced which is obtained by integrating the maximum cyclic shear strains with depth empirical correlations from case history data are proposed between actual lateral displacement the lateral displacement index and geometric parameters characterizing ground geometry for gently sloping ground without a free face level ground with a free face and gently sloping ground with a free face the proposed approach can be applied to obtain preliminary estimates of the magnitude of lateral displacements associated with a liquefaction induced lateral spread

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

paleoclimate reconstruction along the pole equator pole transect of the americas pep 1 examples are presented of inter hemispheric comparison of instrumental climate and paleoclimate proxy records from the americas for di erent temporal scales despite a certain symmetry of seasonal precipitation patterns along the pep 1 transect decadal variability of winter precipitation shows di erent characteristics in terms of amplitude and frequency in both the last 100 and last 1000 years such di erences in variability are also seen in a comparison of time series of di erent el nin8 o southern oscillation proxy records from north and south america however these di erences do not appear to a ect the spatial correlation with paci c sea surface temperature patterns local and regional di erences in response to climate change are even more pronounced for records with lower temporal resolution and inter hemispheric synchroneity may or may not be indicative of the same forcing this aspect is illustrated in an inter hemispheric comparison of the last 1000 years of glacier variability and of the full and lateglacial lake level history 1999 elsevier science ltd all rights reserved

late quaternary palaeoecology and palaeoclimatology from pollen records of the savannas of the llanos orientales in colombia abstract pollen analysis of sediment cores from laguna angel 4 28 n 70 34 w 200 m elevation and laguna sardinas 4 58 n 69 28 w 80 m a s l in the savannas of the llanos orientales of colombia are presented the pollen record of laguna angel includes 7 radiocarbon dates and shows the holocene vegetation and climatic history since 10 030 yr b p the record of laguna sardinas includes 7 radiocarbon dates and shows the late glacial and holocene history since 11 570 yr b p both pollen records show that the present day savanna vegetation dominated by grassland poaceae with only few woody savanna taxa e g curatella bryrsonima was present since the late glacial indicating dry climatic conditions with a marked dry season shrubs and trees of forest and or gallery forest e g mauritia mauritiella moraceae urticaceae melastomataceae alchornea and myrtaceae are well represented during the recorded period the proportions between forest and savanna were subject to change in the past but also the composition of the forest and savanna ecosystems changed laguna sardinas shows a greater abundance during the late glacial holocene transition than today for alchornea especially from 10 680 to 10 070 yr b p for this interval synchronous to the el abra stadial younger dryas equivalent in colombia more humid climatic conditions have been inferred which are primarily the result of higher moisture levels under colder conditions during the early and middle holocene from 9730 to 5260 yr b p in laguna angel and from 9390 to 6390 yr b p in laguna sardinas the maximum expansion of grassland occurred indicating very dry climatic conditions the following period from 5260 to 3890 yr b p in laguna angel and from 6390 to 3680 yr b p in laguna sardinas shows a decrease of savanna herbs and an increase of forest and gallery forests reflecting a wetter period probably with a shorter annual dry season in both lake records the period after ca 3500 yr b p is incomplete but is characterized by a high representation of the palm genera mauritiella in laguna angel and mauritia in laguna sardinas this marked change has been interpreted as evidence of strong a anthropogenic influence on the vegetation but also wetter climatic conditions than during the late glacial and early and middle holocene periods

palaeoenvironmental changes during the last ca 8590 calibrated yr 7800 radiocarbon yr in the dry forest ecosystem of the patia valley southern colombian andes a multiproxy approach two sediment cores were recovered from two swamps in the dry forest lowland ecosystem of the patia valley colombia the 500 cm long core el patia 1 was analysed using diatoms pollen lithostratigraphy geochemistry charcoal and radiocarbon dates the 270 cm long core patia 2 was analysed using pollen lithostratigraphy and radiocarbon dates the environmental reconstruction includes the local development and history of the patia 1 swamp and the regional vegetation history of the two swamps the environmental reconstruction shows a fluvial system in the area before 7830 14c yr bp ca 8590 cal yr bp after this date the river gradually moved away leading to the formation of the swamp patia 1 open vegetation with patches of dry forest surrounded the swamp between 7670 and 6860 14c yr bp 8350 7690 cal yr bp the swamp become partially isolated from the river and was a permanent water body which was alkaline and mesotrophic during this period the surroundings were covered by dry forest from 6860 to 5870 14c yr bp 7690 6690 cal yr bp the swamp was totally isolated from the river and suffered from frequent periods of desiccation and erosion from 5870 to 3570 14c yr bp 6690 3890 cal yr bp the swamp turned into a marsh terrestrial environment and in the surrounding area the dry forest alternated with open vegetation it is possible that there were human impacts on the environment during this period between 3570 and 1990 14c yr bp 3890 2000 cal yr bp unstable conditions prevailed after 1990 14c yr bp 2000 cal yr bp swamp conditions of permanent water were re established open vegetation surrounded the swamp agriculture and human activities were present and have continued since then the climatic reconstruction suggests an increase in humidity at ca 7670 14c yr bp 8350 cal yr bp and a dry climate between 6860 and 5870 14c yr bp 7690 6690 cal yr bp

development of polygonal fault systems a test of hypotheses polygonal networks of normal faults in layer bound sequences of fine grained mudstones have formed without regional tectonic extension the two leading hypotheses concerning the generic mechanism responsible for their development are 1 horizontal stresses are reduced by syneresis 2 coefficients of residual friction are very low to discriminate between these hypotheses the ratios between the minimum horizontal and the vertical effective stresses have been estimated in four north sea wells penetrating oligocene and miocene sequences that contain polygonal fault networks the effective stress ratios are c 0 8 consistent with very low coefficients of friction but not with syneresis

contraction driven shear failure in compacting uncemented sediments shear failure in sediments is universally linked with active boundary conditions such as those imposed by tectonic stresses under conditions of no lateral strain and in the absence of tectonic stress soil mechanics theories predict a simple one dimensional compaction in which sediment particles displace vertically without shear failure during pressure diffusion conflicting with this theory shear failure planes are often found in sediments that formed under near horizontal burial conditions we investigated the effect of particle scale volume contraction as a potential cause of shear failure in uncemented particulate materials and found that loss of particle volume under confined conditions no external loading resulted in pronounced lateral stress reduction often reaching coulomb frictional failure conditions shear strain localization was analytically predicted and modeled numerically due entirely to volume loss at the grain scale we define this mode of internally driven shear failure as contractile to distinguish it from that caused by external loading and show that it can explain many natural fracture systems without invoking regional tectonics

neoglacial fluctuations and sedimentation of an iceberg calving glacier resolved with tree rings kenai fjords national park alaska abstract the mccarty fjord iceberg calving glacier and its land terminating tributaries have fluctuated with some asynchroneity during the past two millennia during advance mccarty glacier shed outwash along the fjord and into ice dammed tributaries inundating forests a radioacarbon framework has revealed at least two major late holocene glacial advances that occurred following a poorly documented expansion about 3600 bp these two later advances are resolved within this fjord with tree ring dating of in situ and transported tree trunks the first event was an early medieval expansion of the mccarty trunk glacier beyond midfjord about 596 a d this followed an interval of tree growth of at least 206 years tributary glaciers probably also advanced at this time however continuous tree growth occurred in the distal southern tributary valleys during this advance while northern tributaries were being dammed by the advancing trunk glacier the down valley extent of this expansion is thus constrained to a position within 12 km of the present contracted ice margin the tree ring refined chronology shows that a second mccarty ice expansion began in the 9th century and reached midfjord by 900 a d and continued to advance through the little ice age in contrast expansions of land terminating glaciers here began after 1300 a d in concert with mountain glaciers worldwide a log from a diamict cross dated with a living tree ring chronology shows that mccarty glacier was advancing within a kilometer of its little ice age maximum at 1790 a d since about 1905 a d dramatic ice retreat has uncovered more than 20 km of mccarty fjord

effective stress vane shear strength correction factor correlations a recent effective stress model of vane shear strength testing in soils can relate measured torques to vane shear strengths using theoretical analysis in terms of effective stress parameters the strength estimates are based on known in situ stresses and soil parameters derived from laboratory testing the model may be applied for example in obtaining theoretical estimates of conventional undrained vane shear strengths for comparison with field data and for use in stability analyses however the model incorporates a correction factor v analogous to bjerrum s field vane shear strength correction factor intended to compensate for pore pressure and shearing rate effects this correction factor must be evaluated before reliable torque or shear strength estimates can be made in any given case to facilitate this the paper presents correlations of v with both liquid limit and plasticity index based on world wide data from clays and silts the correlations are compared with independent data from norwegian clays

absence of last glacial maximum records in lowland tropical forests abstract environmental conditions of the lowland tropical forests during the last glacial maximum lgm between ca 20 000 and 18 00014c yr b p are reevaluated in terms of dating control and lithology analyzed in seven pollen records from south america the reevaluation shows that probably in none of the published records are lgm sediments present or abundant this conclusion is based on the occurrence of abrupt lithologic changes coupled with changes in sedimentation rate interpolated from radiocarbon dates these findings suggest that the lgm was represented probably by a hiatus of several thousand years indicative of drier climates than before or after

comparison of the measured and finite element predicted ground deformations of a stiff lodgement till the use of the finite element method to model excavations and tunnels in dublin black boulder clay has in the past had limited success owing to the failure of the available constitutive models in commercially available software programs to adequately represent the essential features of a stiff soil a stiff soil lodgement till underlies much of the city of dublin ireland consequently the successful prediction of deformations in this soil arising from structural and infrastructural projects is of considerable importance research has shown that the stress strain response of a stiff soil is complex and depends on many factors including stress history stress level and strain direction these important features are included in the hardening plasticity small strain stiffness hss soil model that is incorporated in the plaxis v8 4 finite element code this paper describes the field and laboratory methods that were used to determine the parameters for incorporation in this soil model and the validati

bistatic scattering of gps signals off arctic sea ice this paper evaluates the potential of a global positioning system gps bistatic radar for the retrieval of information concerning the presence and condition of sea ice cover for this purpose the permittivity and roughness of a ground scattering target at l band are extracted from reflected gps waveforms collected from an airborne platform using the kirchhoff approximation for the surface cross section the retrieved gps estimates are then evaluated against collocated measurements of surface roughness obtained from a lidar profiler and a reference classification of sea ice types inferred from a multisensor data set that includes polarimetric microwave emissions radarsat backscatter and moderate resolution imaging spectroradiometer infrared visible imagery

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

influence of stratigraphic setting and simple shear on layer bound compaction faults offshore mauritania abstract we have used three dimensional seismic data to examine the geometry of layer bound compaction faults on the mauritanian continental margin the faults deform a fine grained neogene unit which mantles buried canyons above the canyons most layer bound faults strike perpendicular to the canyon axes whereas adjacent to the canyons the faults are omni directional and intersect bedding planes in polygonal patterns on the flanks of a large structural high faults also strike perpendicular to the slope direction displaying a concentric pattern faults that show preferential alignment are located in areas with increased bedding dip and most aligned faults are antithetic to the bedding dip where the bedding dip is greater than 1 the synthetic faults dip more steeply than the antithetic faults this discovery was unexpected because the stress tensor due to the effect of gravity on a dipping margin would cause the antithetic faults to form with steeper dips which we show theoretically we hypothesise that the layer hosting the faults has been subjected to simple shear of 20 or more with the top of the layer displaced downdip by 100 m relative to its base a potential application of these and similar observations is to assess slope stability on continental margins

holocene history of the choco rain forest from laguna piusbi southern pacific lowlands of colombia abstract a high resolution pollen record from a 5 m long sediment core from the closed lake basin laguna piusbi in the southern colombian pacific lowlands of choco dated by 11 ams14c dates that range from ca 7670 to 22014c yr b p represents the first holocene record from the choco rain forest area the interval between 7600 and 610014c yr b p 500 265 cm composed of sandy clays that accumulated during the initial phase of lake formation is almost barren of pollen fungal spores and the presence of herbs and disturbance taxa suggest the basin was at least temporarily inundated and the vegetation was open the closed lake basin might have formed during an earthquake probably about 440014c yr b p from the interval of about 600014c yr b p onwards 200 different pollen and spore types were identified in the core illustrating a diverse floristic composition of the local rain forest main taxa are moraceae urticaceae cecropia melastomataceae combretaceae acalypha alchornea fabaceae mimosa piper protium sloanea euterpe geonoma socratea andwettinia little change took place during that time interval compared to the pollen records from the rain forests of the colombian amazon basin and adjacent savannas the choco rain forest ecosystem has been very stable during the late holocene paleoindians probably lived there at least since 346014c yr b p evidence of agricultural activity shown by cultivation ofzea maissurrounding the lake spans the last 1710 yr past and present very moist climate and little human influence are important factors in maintaining the stable ecosystem and high biodiversity of the choco rain forest

stress release undrained storage and reconsolidation in simulated underwater clay triaxial testing has examined the influence of storage time and reconsolidation procedures on normally consolidated and overconsolidated samples of reconstituted clay prepared with high pore water pressures to simulate underwater deposits samples of illite were consolidated one dimensionally off loaded without drainage stored undrained reconsolidated and sheared undrained the storage times were 15 min 1 day and 7 days reconsolidation pressures were 1 isotropic to 2 isotropic to and 3 anisotropic to the results were compared with specimens that had not been off loaded different undrained storage times had no effect on the strengths after reconsolidation but the strengths were affected by the reconsolidation pressures anisotropic reconsolidation produced the best agreement between the control specimens and samples in terms of their strengths and af values the duration of storage time did affect stress strain parameters such as the undrained compression modulus e50 and the pore

holocene climatic variations in the western cordillera of colombia a multiproxy high resolution record unravels the dual influence of enso and itcz abstract the paramo de frontino 3460 m elevation in colombia is located approximately halfway between the pacific and atlantic oceans it contains a 17 kyr long stratigraphically continuous sedimentary sequence dated by 30 ams 14c ages our study covers the last 11 500 cal yr and focuses on the biotic pollen and abiotic microfluorescence x or xrf components of this high mountain ecosystem the pollen record provides a proxy for temperature and humidity with a resolution of 20 35 yr and xrf of ti and fe is a proxy for rainfall with a sub annual ca 6 month resolution temperature and humidity display rapid and significant changes over the holocene the rapid transition from a cold mean annual temperature mat 3 5 c lower than today and wet younger dryas to a warm and dry early holocene is dated at 11 410 cal yr bp during the holocene mat varied from ca 2 5 c below to 3 5 above present day temperature warm periods 11 410 10 700 9700 6900 4000 2400 cal yr bp were separated by colder intervals the last 2 4 kyr of the record is affected by human impact the holocene remained dry until 7500 cal yr bp then precipitations increased to reach a maximum between 5000 and 4500 cal yr bp a rapid decrease occurred until 3500 cal yr bp and the late holocene was dry spectral analysis of xrf data show rainfall cyclicity at millennial scale throughout the holocene and at centennial down to enso scale in more specific time intervals the highest rainfall intervals correlate with the highest activity of enso variability in solar output is possibly the main cause for this millennial to decadal cyclicity we interpret enso and itcz as the main climate change driving mechanisms in frontino comparison with high resolution xrf data from the caribbean cariaco basin a proxy for rainfall in the coastal venezuelian cordilleras demonstrates that climate in frontino was pacific driven enso dominated during the yd and early holocene whereas it was atlantic driven in cariaco itcz dominated from ca 8000 cal yr bp climate in both areas was under the dual influence of enso and itcz thereby showing existing teleconnections between the tropical pacific and atlantic oceans the frontino record is to date the highest resolution holocene study in nw colombia an implication of these results is that new records should be analyzed with multiproxy tools in particular those providing high resolution time series such as xrf

preceramic occupations in the orinoco river valley two sites in the orinoco valley containing preceramic from excavated contexts are described radiocarbon dating and stylistic comparisons indicate that the northern tropical lowlands were inhabited at the onset of the holocene suggesting a time depth of 9000 years before the present for tropical forest savanna adaptations in northern south america

seismic liquefaction lateral spreading and flow slides a numerical investigation into void redistribution experience from past earthquakes indicates that seismically induced large lateral spreads and flow slides in alluvial sand deposits have taken place in coastal and river areas in many parts of the world the ground slope in these slides was often not very steep gentler than a few percent recent research indicates that the presence of low permeability silt or clay sublayers within the sand deposits is responsible for this behaviour such layers form a barrier to upward flow of water associated with earthquake generated pore pressures this causes an accumulation of pore water at the base of the layers resulting in greatly reduced strength and possible slope instability this paper uses an effective stress coupled stress flow dynamic analyses procedure to demonstrate the effects of a low permeability barrier layer on ground deformations from an earthquake event the analyses show that an expansion zone develops at the base of barrier layers in stratified soil deposits under seismic loading which can grea

effects of multipropped excavation on an adjacent floating pile abstractdeep excavations inevitably induce stress relief and soil movement which may affect the safety and serviceability of nearby existing pile foundations although the effects of excavation on

simple shear testing of sensitive very soft offshore clay for wide strain range stiffness and damping properties of sensitive very soft clay sediments of the italian adriatic continental shelf are determined by means of two series of cyclic simple shear tests one with 12 stages and one with two stages the apparatus used in this research is capable of investigating the stress strain behaviour of the soil in a wide range of shear strains from about 0 0004 to 1 test results were expressed in terms of small strain shear modulus g0 normalized equivalent shear modulus geq g0 and damping ratio d versus cyclic shear strain amplitude c these parameters were analyzed in the framework of existing literature by comparison with empirical correlations developed for onshore materials of different plasticity and limited to g0 also for soft soils the dependence of g0 geq g0 c and d c on factors such as void ratio stress history and loading cycles is analyzed and discussed

winter snow cover on sea ice in the weddell sea measurements of snow thickness temperature salinity density and stratigraphy acquired during the 1992 winter weddell gyre study are presented results indicate that the winter snow cover on sea ice in the weddell sea is extremely variable extreme fluctuations in antarctic synoptic conditions air temperature precipitation humidity and wind speed occur during the austral winter they result in unique modifications and additions to the snow layer during the aging process and act to stabilize an otherwise easily wind redistributed shallow snow cover and develop well packed drift features the latter occur even over relatively undeformed areas of sea ice and have a significant localized effect on the snow thickness distribution significant variability in snow grain size mean 2 73 3 12 mm and density 0 32 0 09 g cm 3 is observed as a result of cyclical switches between high and low temperature gradient metamorphism multiple icy layers indicate multiple thaw freeze events one such event occurred during a 3 day station during which the air temperature rose by 22 c in 12 hours to approximately 0 c this paper also examines mechanisms for flooding of the snow ice interface including snow loading even where the latter is not a factor the layer of snow immediately above the snow ice interface is commonly damp and saline 10 limitations in the data set are discussed and comparisons are drawn with other experiments

late quaternary pollen records from the middle caqueta river basin in central colombian amazon abstract three pollen records are presented from the pantano de monica region 0 42 s 72 04 w 160 m elevation on the lower terrace of rio caqueta of the central colombian amazonas ten radiocarbon dates from three cores indicate that the deposits are of holocene age but the pollen data suggest that the record may also contain the late glacial the core pantano de monica 1 covers the time interval from 11 150 bp extrapolated to 4730 bp during the late glacial and early holocene this swamp was smaller in size and waters were more shallow than today with abundant mauritia palm trees this indicates that the lower terrace of the caqueta river was better drained than today which might be related to changes in the drainage system and or drier conditions during that time late glacial and early holocene vegetational changes in the rain forests surrounding the swamp pantano de monica indicate successional stages probably related to changes in the drainage system and or climate changes presence of podocarpus pollen grains up to 2 6 of the total sum and influx of 78 grains cm 2 yr 1 point to the regional presence of podocarpus at the beginning of the holocene evidence of podocarpus during glacial times in other pollen records from the amazon basin has been taken as indicative of cooling core pantano de monica 2 was taken in a small swamp 1000 m distance to pantano de monica 1 and starts at 4000 bp when this location was well drained several changes in the composition of the rain forest are documented such as increase in disturbance increase of cecropia followed by a quick recovery increase of psychotria protium and caryocar tree taxa were frequent suggesting that the lower terrace area was well drained until 3080 bp after which small swamps developed core pantano de monica 3 is from the wet forest in between the two swamps and represents the period since 3260 bp pollen data show a continuously increasing presence of palms probably euterpe suggesting either a transition from well drained to less drained conditions in the lower terrace during the late holocene and or a more intensive human impact of the rain forest on the lower terraces of rio caqueta the three records from the lower terrace of rio caqueta show different forest compositions in the past and indicate that the rain forest environments were not stable during late glacial and holocene times comparison with other data from lowland records of northwestern south america suggest that climate change is a major factor for environmental change in central colombian amazon

pollen based biome reconstructions for the past 450 000 yr from the funza 2 core colombia comparisons with model based vegetation reconstructions abstract the 586 m funza 2 sequence from the high plain of bogota colombia has provided one of the longest pollen based vegetation reconstructions in the world affinity scores to seven biomes are compared to the record of co2 fluctuation from the vostok ice core for the period spanning from approximately 25 000 to 450 000 yr before present bp results are compared to output from the biome 3 vegetation model run under different environmental scenarios at regional and local scales the model is run with temperature and precipitation reductions relative to the present values of up to 10 c and 700 mm yr 1 respectively for this particular investigation we are interested in the role of co2 as a forcer of vegetation change we run the model with concentrations of atmospheric co2 co2 atm within the range of 170 to 340 ppmv during glacial periods the cool grass shrubland biome is highly dominant less so during interglacial periods when the cool evergreen and cool mixed forest biomes become co dominant in addition to this climate driven altitudinal oscillation of the vegetation there is also a signal that indicates longer term evolution of the vegetation the cool grass shrubland and cool evergreen and cool mixed forest biomes become co dominant for the first time over the 450 000 yr record at approximately 170 000 yr bp become completely anti phase during a period of extreme low co2 atm and then become co dominant from approximately 120 000 yr bp until the core top just prior to the last glacial maximum the model results for the funza area indicate that changes in co2 atm temperature and precipitation are inter linked by the vegetation response a shift of 5 5 c is required to lower the cool grass shrubland biome to altitudes about the funza catchment where the andean forest biome dominant at low co2 atm concentrations particularly below 180 ppmv the composition of the high latitudinal tropical vegetation about the funza catchment changed it is suggested that this low co2 atm combined with a period when the climate was characterised by extreme cooling and drying caused a readjustment of the tropical high altitudinal vegetation zonation and the formation of plant communities that are presently recorded

ice core palaeoclimate records in tropical south america since the last glacial maximum ice core records spanning the last 25 000 yr from the tropical andes of south america are reviewed these records from quelccaya huascaran and sajama present a high temporal resolution picture of both the late glacial stage lgs and the holocene climatic and environmental conditions in the south american andes late glacial stage conditions at high elevations appear to have been cooler than today although the magnitude of the inferred cooling differs with the particular proxy used e g snowline depression pollen ice cores insoluble dust and anion concentrations in the ice cores reveal that lgs hydrological conditions in the tropics 9 s were much drier than today whereas in the subtropics 18 s lgs conditions were much wetter this probably reflects the migration of the tropical hadley cell in response to a different meridional temperature gradient low nitrate concentrations in the lgs ice from both huascaran and sajama suggest that the amazon basin forest cover may have been much less extensive discussed is the conundrum surrounding the use of d 18 o as a palaeothermometer in the tropics where temperatures exhibit little seasonal variation yet the ice core records suggest that d 18 o records temperature variations on decadal to millennial time scales finally evidence is presented for a strong twentieth century warming copyright 2000 john wiley sons ltd

late quaternary palaeoclimate in the savannas of south america the paper reviews late quaternary palynological evidence from eight sites in the savannas of northern south america the sites reviewed are lake valencia 10 n carajas 6 s aguas emendadas 15 s crominia 17 s salitre 19 s lagoa de serra negra 18 s lagoa santa 20 s and lagoa dos olhos 20 s four of the sites show evidence for a climate more humid than at present from ca 36 ka bp to ca 22 18 ka bp maximum dryness was reached at 14 ka to 10 5 ka bp at lake valencia there was an increase in moisture from 8 8 ka bp at lake valencia and from 7 ka bp in central brazil the presence of charcoal indicates human impact from ca 8 6 ka bp 1997 john wiley sons ltd

tomographic imaging of fjord ice using a very high resolution ground based sar system this paper presents new experimental results of 3 d imaging using tomographic techniques over a snow covered sea ice medium sensed with an x band radar system the available data are from a ground based synthetic aperture radar data collection campaign carried out over kattfjord tromso norway direct imaging of the vertical structures of the radar reflectivity of the snow and sea ice layers is achieved by focusing the signal from a 2 d synthetic array in the 3 d space the effect of a change in propagation velocity of the wave inside the considered medium is investigated in the focusing process and the tomograms are effectively corrected for this effect the distribution of the scattering contributions in the vertical direction reveals a strong response from the sea ice cover tomograms at two different polarizations are investigated and compared the results and the interpretations are also supported by the simulated data from the same system

a holocene palynological record from the northeastern laptev sea and its implications for palaeoenvironmental research a 844 cm long core ps51 118 3 77 53 54 n 132 11 92 e recovered from the upper slope 122 m water depth of the laptev sea russian arctic has been studied for pollen spores and aquatic palynomorphs including freshwater green algae and cysts of marine dinoflagellates the age model was established on the basis of radiocarbon dates obtained on marine bivalve mollusk shells the available dates suggest that the analyzed sediment was accumulated during the last ca 10 8 cal ka and reveal two intervals with markedly different sedimentation rates reflecting the sedimentary regime changes of the laptev sea shelf during postglacial sea level rise very high sedimentation rates ca 4 7 mm per year in the lower part of the core 120 866 cm between ca 9 2 and 10 8 cal ka bp reflect lower than present sea levels high erosion activity and much closer position of the palaeo shoreline with the lena and yana river mouths to the core site dramatic decrease in sedimentation rates ca 0 1 mm per year during the middle and late holocene interval reflects high sea level and decreased amount of suspended material transported to the outer shelf by rivers despite the location of the core site at the continental slope and far away from the modern coastline pollen spores and fresh water algae constitute a major part of the microfossils throughout the whole record indicating great impact of the lena and yana rivers and possibly prevalent wind regime on the pollen and non pollen palynomorph npp assemblages although a number of short term decadal to multi century oscillations deviate from the mean holocene values pollen taxa percentages and pollen based numerical biome reconstructions do not show very clear trends the latter is likely a result of the mixed environmental signal and complex pollen contribution of several large environmental regions and vegetation zones of siberia drained by the lena and yana rivers the greater pollen contribution of the forested regions to the ps51 118 3 record reflects higher pollen production of the boreal trees and shrubs over the low productive arctic vegetation the intervals of the relative increase in the tundra biome scores in the ps51 118 3 record reflect decreased arboreal pollen production or and increased landscape openness within the pollen source area and can be correlated within the uncertainty of the age models with the cold episodes observed in the greenland ice and north atlantic sediment records

forest changes in the amazon basin during the last glacial maximum we report here the discovery of forest beds in the amazon basin radiocarbon dated to the wisconsin wurm glaciation these are the first dates from the last glaciation determined on amazonian samples the locality is in ecuador at 1 100 m elevation near the upper limit of the present amazonian rainforest both pollen spectra and podocarpus wood in the deposits suggest that vegetation comparable to the present andean forest grew at least 700 m lower than it does now suggesting a temperature depression of at least 4 5 c for the amazon lowlands

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

an archaeological perspective of the pleistocene holocene boundary in northern south america abstract in this paper i examine late pleistocene early holocene adaptations of hunter gatherers to different ecosystems in northern south america the role played by the climatic events associated with the pleistocene holocene boundary in the evolution of culture in that part of the americas is assessed in the light of those adaptations the conclusion is that the pleistocene holocene boundary is not very useful in understanding the human process of early colonization and adaptation to the various ecosystems of northern south america because a mechanism of cultural change cannot be linked to that boundary

colombian dry moist forest transitions in the llanos orientales a comparison of model and pollen based biome reconstructions colombian vegetation at the ecological level of the biome is reconstructed at six sites using pollen data assigned a priori to plant functional types and biomes the chosen sites incorporate four savanna sites laguna sardinas laguna angel el pinal and laguna carimagua a site on the transition between savanna and amazon rainforest loma linda and a site within the amazon rainforest pantano de monica the areal extent of tropical moist forest tropical dry forest and steppe have been subject to significant change differential responses of the vegetation to climatic shifts are related to changes in plant available moisture duration of dry season and edaphic controls on the vegetation the record from el pinal shows that the present day savanna vegetation dominated by steppe poaceae with little occurrence of woody savanna taxa e g curatella byrsonima was present since the last glacial period of the northern hemisphere unfortunately el pinal is located on an edaphic savanna and is not particularly responsive to registering change most records cover the early holocene one site records the el abra stadial younger dryas equivalent when forest expansion reflects more humid climatic conditions and higher plant available moisture during the early and middle holocene the maximum expansion of steppe and tropical dry forest occurred indicating that dry climatic conditions continued to around 4000 14 c bp the following period from shortly before 4000 14 c bp is characterised by an increase in forest and gallery forests reflecting a wetter period probably with a shorter annual dry season anthropogenic influence on the vegetation is recorded by all the records over the last millennial particularly characterised by a reduction in forest cover and high amplitude changes in vegetation biome transitions from one type to another and the environmental controls on this shift are investigated by applying a vegetation model biome 3 the model uses climatic data from six meteorological stations that encompass a range of environments within lowland colombia which are similar to the pollen data the signals of vegetation change can be translated to the main environmental controls of temperature and moisture to indicate the degree of change needed in these parameters to record the vegetation change depicted by the pollen data moisture balance is the dominant control on driving vegetation change whether under seasonal or annual control the combined reconstruction from pollen data and model output of biome scale vegetation dynamics for lowland colombia allows an understanding of the environmental controls to be developed d 2005 elsevier b v all rights reserved

an assessment of the earth pressure coefficient in overconsolidated clays the determination of the earth pressure coefficient k0 in a natural clay deposit is a problem of considerable significance in geotechnical engineering while the methods for evaluation of k0 are reliable for normally consolidated soils significant difficulties still exist in evaluating k0 in overconsolidated clays given that it is influenced by the stress history of the material together with the age structure mineralogical composition and depositional environment indeed some of these factors are responsible for the soil becoming anisotropic the existing framework for prediction of k0 in overconsolidated soils does not account for any influences caused by anisotropy the work reported in this paper evaluates the validity of a revised relationship between k0oc and ocr overconsolidation ratio using data obtained from laboratory investigations the tests were performed on reconstituted and undisturbed samples of belfast upper boulder clay london clay and gault clay tests were also performed on re

the holocene history of the ne black sea and surrounding areas an integrated record of marine and terrestrial palaeoenvironmental change here we present an almost complete and integrated holocene record of marine and terrestrial palaeoenvironmental change from the ne shelf of the black sea a dinoflagellate cyst record used to recon

physical dielectric and c band microwave scattering properties of first year sea ice during advanced melt 1 this paper investigates the influence of solar heating and intermittent cloud cover on the physical and dielectric properties of naturally snow free warm 2 first year sea ice fyi in the southeastern margin of the beaufort sea during advanced melt a simple three layer physical model describing the surface is introduced and copolarized c band microwave signatures are simulated using a multilayer scattering model forced with four sets of measured surface parameters modeled backscatter signatures are compared to coincident surface based c band scatterometer signatures in order to elucidate the signature controlling properties of the ice results show that 50 mhz impedance probe dielectric measurements of desalinated upper ice layers exhibit statistically significant diurnal variations due to the link between solar forcing and the availability of free water in brine free upper ice layers enhanced downwelling longwave radiation to the surface from low level stratus clouds is positively linearly associated r 0 709 with volumetric moisture mv detected in upper ice layers model results show that desalinated upper ice layers contribute volume scattering from smooth snow free fyi under the observed surface mv range sustained cloud free periods result in the formation of a 0 5 2 5 cm granular surface layer composed of 5 2 mm ice grains which enhances backscatter under relatively dry conditions sensitivity analyses show that layer thickness plays a significant role in scattering due to the increased number density of inclusions which act as discrete scatterers and sufficient energy may penetrate to and scatter from the saline columnar ice layer under relatively dry conditions only mv 2

significance of localized pore pressures to the genesis of septarian concretions the burial stress and hydrologic conditions existing during concretion formation in mudrocks are evaluated and integrated into a model for the genesis of septarian cracks initial concretion cement formation will lower concretion permeability through the filling of pre existing pore space during progressive burial this may lead to increased excess pore pressure localized within the concretion body causing a reduction of the effective stress analysis of the stress conditions and crack morphology suggests that cracks in septarian concretions result from tensional failure sub critical crack growth as a consequence of this localized excess pore pressure conditions suitable for crack formation will depend upon the magnitude of the excess pore pressure and the stress corrosion limit of the concretion body a review of the likely strength of such concretions indicates that cracking could be initiated at depths less than 10 m a variety of observed crack morphologies can be explained with this model depending upon the spatial distribution of strength and effective stress in the concretion crack orientations mostly reflect stress anisotropy but are also influenced by directional anisotropy in the crack growth rates locally increased pore pressure also likely occurs in non septarian concretions but is not sufficient to cause cracking this enhanced local pressure may assist the crystal surface growth reactions of the carbonate cement through this enhancement process the shape of concretions may be a response to the local anisotropic pore pressure contours which reflect the permeability anisotropy of the concretion and surrounding mudrock

late quaternary environmental and climatic changes in central brazil paleoenvironmental changes dating back to 30 000 yr b p documented in a pollen record from central brazil lat 19 s permit the reconstruction of climatic changes related to shifts of the antarctic polar fronts the paleoclimatic inferences were obtained by a study of modern vegetation and pollen distribution taking into account present day climatic parameters at 30 000 yr b p the climate must have been warmer and moister than today judging from the high amount of tree pollen taxa characteristic of floodplain forest from 17 000 to 14 000 yr b p the climate was drier although tree pollen percentages were relatively high after 12 000 yr b p araucaria forest elements increased suggesting a moister and cooler climate the araucaria forest disappeared during a short interval between 11 000 and 10 000 yr b p this could be related to the younger dryas event at the beginning of the holocene the climate became cool and moist again as indicated by the reexpansion of the araucaria forest the latter was progressively replaced by a mesophytic semideciduous forest indicating warmer and drier climate after 8500 yr b p at 5000 yr b p an arid interval was followed by the expansion of mesophytic semideciduous forest elements

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

in situ lateral stress measurement in glaciolacustrine seattle clay using the pressuremeter pressuremeter testing was conducted for the state route sr 99 bored tunnel project in seattle washington to estimate in situ soil stress deformation parameters along the tunnel alignment many of the tests were conducted in a very stiff to hard glaciolacustrine clay known as seattle clay this unit is historically known for deep seated slope failures and many of these failures have been attributed to the release of high locked in lateral stresses estimation in situ lateral stresses along the tunnel alignment was a primary focus of the exploration program due to the hard consistency of this unit and the potential for cobbles neither self boring pressuremeter nor dilatometer testing was feasible therefore prebored pressuremeter testing was used using several lateral stress estimation techniques including a novel in situ creep testing approach the in situ lateral stresses in the seattle clay were estimated to be significantly higher than what would be expected by assuming a simple laterally constrained vertical loading and unloading stress path due to glaciation deformational features commonly encountered in seattle clay indicate its stress history also has included significant lateral shearing the memory of this shearing within the fabric of the clay may influence the in situ stress state and response to lateral unloading

analysis of pv drains for mitigation of seismically induced ground deformations in sand slopes abstractprefabricated vertical pv drain arrays have been proposed as a minimally intrusive technique for mitigating seismically induced ground deformations in sandy liquefiable slopes this paper

evaluating the in situ lateral stress coefficient k0 of soils via paired shear wave velocity modes abstractthe utilization of shear wave velocities toward the evaluation of the in situ geostatic horizontal stress state in soils is validated specifically the lateral stress coefficient k0 field shear wave velocities from paired sets of different directional and polarization modes are compiled from 16 well documented test sites involving a variety of geomaterials focus is particularly placed on shear wave velocities measured by downhole tests vsvh crosshole tests vshv and special rotary type crosshole tests vshh at these sites field k0 stress states have been quantified using one or more direct assessment techniques including self boring pressuremeter total stress cells and hydrofracture in field testing as well as suction measurements special consolidometers and or triaxial arrangements on undisturbed samples in the laboratory although the specific delineation of stress induced versus inherent or fabric anisotropy may be difficult it is shown that the ratio of horizontally polarized