el nino la nina and sahel precipitation during the middle holocene simulations with a synchronously coupled atmo sphere land ocean sea ice model indicate el nino la nina like events in the eastern tropical pacific ocean during the middle holocene 6 ka with similar intensities and frequencies as at present 0 ka july august september sahel precipitation shifts northward as expected from proxy data for present day interannual decadal variability of western sahel precipi tation is correlated with both pacific el nino la nina sea sur face temperature sst anomalies and the atlantic dipole teleconnections at 6 ka between sahel precipitation and el nino la nina sst anomalies are absent with tropical atlantic ssts asserting a dominant influence these results illustrate potential problems with using present day teleconnection patterns in interpreting past climate variability from proxy data

correlation of changes in gravity elevation and strain in southern california measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity elevation and strain in several southern california networks precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long laser ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern california during 1978 and 1979 although the precision of these measuring systems has been questioned the rather good agreement among them suggests that the observed changes reflect true crustal deformation

at what depth is the eemian layer expected to be found at neem no continuous record from greenland of the eemian interglacial period 130 115 ka bp currently exists however a new ice core drill site has been suggested at 77 449 n 51 056 w in north west greenland north eemian or neem radio echo sounding images and flow model investigations indicate that an undisturbed eemian record may be obtained at neem in this work a two dimensional ice flow model with time dependent accumulation rate and ice thickness is used to estimate the location of the eemian layer at the new drill site the model is used to simulate the ice flow along the ice ridge leading to the drill site unknown flow parameters are found through a monte carlo analysis of the flow model constrained by observed isochrones in the ice the results indicate that the eemian layer is approximately 60m thick and that its base is located approximately 100m above bedrock

review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

identification of cyclone track regimes in the north atlantic a langrangian type climatology of north atlantic cyclones is established based on the high resolution european centre for medium range weather forecasts data set of the 1000 hpa height field first an algorithm is introduced to identify mid latitude cyclones and cyclone paths with as few constraints as possible cluster analysis of relative cyclone displacements yields three types of cyclone tracks characterizing stationary north eastward and zonally travelling storms the internal lagrangian statistics of these cyclone track types reveal representative life cycles for central pressure and geopotential height gradients and a power law scaling behaviour of cyclone displacements finally a basic climatology of north atlantic cyclone track regimes is deduced in terms of a time series and circulation statistics

land surface feedbacks and palaeomonsoons in northern africa we ran a sequence of climate model experiments for 6000 years ago with land surface conditions based on a realistic map of palaeovegetation lakes and wetlands to quantify the effects of land surface feedbacks in the saharan region vegetation induced albedo and moisture flux changes produced year round warming forced the monsoon to 17 25 n two months earlier and shifted the precipitation belt 300 km northwards compared to the effects of orbital forcing alone the addition of lakes and wetlands produced localised changes in evaporation and precipitation but caused no further extension of the monsoon belt diagnostic analyses with biome and continental hydrology models showed that the combined land surface feedbacks although substantial could neither maintain grassland as far north as observed 26 n nor maintain lake megachad 330 000 km

early holocene climate oscillations recorded in three greenland ice cores a new ice core chronology for the greenland dye 3 grip and ngrip ice cores has been constructed making it possible to compare the 18 o and accumulation signals recorded in the three cores on an almost annual scale throughout the holocene we here introduce the new time scale and investigate 18 o and accumulation anoma lies that are common to the three cores in the early holocene 7 9 11 7 ka before present three time periods with significant and synchronous anomalies in the 18 o and accumulation signals stand out the well known 8 2 ka event an event of shorter duration but of almost similar amplitude around 9 3 ka before present and the preboreal oscillation during the first centuries of the holocene for each of these sections we present a 18 o anomaly curve and a common accumulation signal that represents regional changes in the accumulation rate over the greenland ice cap

southeast greenland high accumulation rates derived from firn cores and ground penetrating radar despite containing only 14 of the greenland ice sheet by area the southeastern sector has the highest accumulation rates and hence receives 30 of the total snow accumulation we present accumulation rates obtained during our 2010 arctic circle traverse derived from three 50 m firn cores dated using geochemical analysis we tracked continuous internal reflection horizons between the firn cores using a 400 mhz ground penetrating radar gpr gpr data combined with depth age scales from the firn cores provide accumulation rates along a 70 km transect we followed an elevation gradient from 2350 to 1830m to understand how progressive surface melt may affect the ability to chemically date the firn cores and trace the internal layers with gpr from the firn cores we find a 52 0 43 m w e a 1 increase in average snow accumulation and greater interannual variability at the lower site than the upper site the gpr profiling reveals that accumulation rates are influenced by topographic undulations on the surface with up to 23 variability over 7 km these measurements confirm the presence of high accumulation rates in the southeast as predicted by the calibrated regional climate model polar mm5

tectonomagnetism and tectonoelectricity the last four years has seen a considerable waning of efforts in the u s to use the inherent stress sensitivity of the magnetization of rocks in tectonically active regions as a tool for understanding and perhaps predicting earthquake activity and volcanic eruptions the primary reasons for this decreased interest derive from accumulating evidence that the mean deviatoric stress release during earthquakes and the ambient shear stress near active faults is lower than than had been previously assumed and secondly the observational limits of magnetometers separated by about 10 km after application of a variety of noise reduction techniques appear to be no better than about 0 1 nt lower estimates of the average change in deviatoric stress associated with earthquake rupture correspondingly reduce the fields calculated from tectonomagnetic models at the same time some unexpected and exciting results have been obtained which indicate many aspects of magnetic field changes near active faults and volcanoes are still not completely understood the more important of these results are 1 the observation of correlated magnetic strain uplift and gravity changes during the latter stages of a remarkable deformation episode near the san andreas fault in southern california 2 indications of anomalous change in magnetic fields along the san andreas fault to the southeast of palm springs 3 an unambiguous observation of a volcanomagnetic effect at mt st helens prior to an extrusive eruption and 4 the first observations of coseismic seismomagnetic effects on two independent magnetometers during the july 8 1986 ml 5 9 north palm springs earthquake a resurgence of interest has occurred during the quadrennium in a related area concerning electrical phenomena and electromagnetic effects related to earthquakes and volcanoes mechanisms have been proposed to explain the occurrence of earthquakes lights and the possibility of electromagnetic emission prior to the 1960 chilean earthquake has been proposed self potential anomalies have been observed before and during several eruptions on kilauea volcano in hawaii

accumulation over the greenland ice sheet from historical and recent records water accumulation defined as precipitation minus evaporation was estimated over all of greenland as part of a program to understand changes in ice sheet mass and elevation over 360 historical and recent point accumulation estimates on the greenland ice sheet were evaluated and 276 estimates that were judged to be high quality were used to develop the accumulation map the data set includes 99 points developed as part of four investigations of the past 5 15 years these are judged to have the greatest accuracy using kriging the average accumulation over greenland is estimated to be 30 g cm 2 yr 1 for the interior part of the ice sheet above 1800 m elevation where most of the data were acquired the average accumulation is also estimated to be 30 g cm 2 yr 1 there are still many areas on the ice sheet including northwest southeast and southern greenland where accumulation is highly uncertain exceeding the mean ice sheet uncertainty at a point of 20 25 in these regions further sampling will be required to reduce uncertainty in both regional and ice sheet wide accumulation

greenland ice sheet mass balance reconstruction part i net snow accumulation 1600 2009 ice core data are combined with regional atmospheric climate model version 2 racmo2 output 1958 2010 to develop a reconstruction of greenland ice sheet net snow accumulation rate at g spanning the years 1600 2009 regression parameters from regional climate model rcm output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values each core site s residual variance is used to inversely weight the cores respective contributions the interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the rcm data uncertainty and significance of changes is measured using statistical models a 12 or 86 gt yr 21 increase in ice sheet accumulation rate is found from the end of the little ice age in 1840 to the last decade of the reconstruction this 1840 1996 trend is 30 higher than that of 1600 2009 suggesting an accelerating accumulation rate the correlation of at g with the average surface air temperature in the northern hemisphere satnht remains positive through time while the correlation of at g with local near surface air temperatures or north atlantic sea surface temperatures is inconsistent suggestinga hemispheric scale climateconnection an annualsensitivityof at g to satnhtof 6 8 k 21 or 51 gt k 21 is found the reconstuction at g correlates consistently highly with the north atlantic oscillation index however at the 11 yr time scale the sign of this correlation flips four times in the 1870 2005 period

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

the 1979 palmdale california strain event in retrospect although the long term 1971 1984 strain accumulation across the san andreas fault near palmdale california is simply right lateral shear at the rate of 0 20 0 01 strain a tensor shear annual measurements indicate that an anomalous contraction perpendicular to the fault accumulated in the interval 1973 1978 and was abruptly released in mid 1979 this anomaly is referred to as the 1979 palmdale strain event more frequent measurements of strain since 1979 suggest a more irregular accumulation of strain than prior to 1979 this irregularity may be due to measurement errors larger than previously estimated as a consequence of this post 1979 irregularity the 1979 strain event no longer appears as anomalous as it had in the shorter 1971 1980 record to provide independent support for existence of the 1979 palmdale strain event we review gravity measurements at palmdale by jachens 1983 and water level measurements in wells near palmdale by merifield and lamar 1981 that have previously been shown to provide a marginal correlation with the 1979 strain recovery these data taken together with the original strain observations suggest a real strain anomaly did occur near palmdale in 1979 but the evidence is not overwhelming

role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges california evidence for intracontinental subduction bouguer gravity anomalies in the transverse ranges of southern california show a pronounced asymmetry in north south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike while individual gravity topography profile pairs could be consistent with a number of isostatic compensation mechanisms the unusual relationship between gravity and topography in the san bernardino mountains severely limits the class of acceptable models if the bouguer gravity field is caused by flexure of the crust mantle boundary the simplest solution consistent with the data in the western transverse ranges the san gabriel mountains and the san bernardino mountains models the great valley and mojave provinces to the north as a 50 km thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south the best fit to the gravity data in the san bernardino mountains requires a terminal bending moment on the southern plate beneath the mountains the magnitude of the terminal bending moment decreases to the west in the san gabriel and western transverse ranges we equate the load responsible for this terminal moment with the high velocity slablike feature mapped in the upper mantle beneath the transverse ranges using seismic tomography the coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small scale convective feature

three dimensional adjustment of trilateration data the three dimensional locations of the monuments in the usgs hollister trilateration network were adjusted to fit line length observations observed in 1977 using a bayesian approach and incorporating prior elevation estimates as data in the adjustment procedure no significant discrepancies in the measured line lengths were found but significant elevation adjustments up to 1 85 m were needed to fit the length data

reconstructing climate variability from greenland ice sheet accumulation an era40 study 1 re analysis data covering the period 1958 2001 are used to investigate the relationship between regional inter annual snow accumulation variability over the greenland ice sheet gris and large scale circulation patterns cyclone frequency and strength four regions of the gris have been identified that are highly independent with respect to accumulation variability accumulation indices of three of these regions are associated with distinct large scale circulation patterns central western gris reveals an inverse relationship with a nao like pattern the south west a positive correlation with a high pressure bridge from central north atlantic to scandinavia and the south eastern gris a positive correlation with a high pressure anomaly over the greenland sea these large scale patterns also impact european climate in different ways accumulation variability in north eastern gris however is dominated by cyclones originating from the greenland sea thus greenland ice core accumulation records offer the potential to reconstruct various large scale circulation patterns and regional storm activity

interseismic deformation along the san andreas fault in southern california eight trilateration networks located along the san andreas fault in southern california have been surveyed 8 to 19 times within a 14 to 17 year interval between 1971 and 1992 the data measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models the corrected data a total of 2027 measurements are displayed in the form of plots of measured distance versus time for each of the 167 lines measured the hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error a significant deviation from steady deformation is found only for the network located neax palmdale california in that network many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys the deviations are not of the form fixed proportional error one would generally expect from systematic survey error if the survey of early 1982 is excluded the remaining data at palmdale are consistent with steady deformation thus the apparent strain event observed near palmdale in 1982 was transient

recent accumulation variability in northwest greenland from ground penetrating radar and shallow cores along the greenland inland traverse accumulation is a key parameter governing the mass balance of the greenland ice sheet several studies have documented the spatial variability of accumulation over wide spatial scales primarily using point data remote sensing or modeling direct measurements of spatially extensive detailed profiles of accumulation in greenland however are rare we used 400mhz ground penetrating radar along the 1009 km route of the greenland inland traverse from thule to summit during april and may of 2011 to image continuous internal reflecting horizons we dated these horizons using ice core chemistry at each end of the traverse using density profiles measured along the traverse we determined the depth to the horizons and the corresponding water equivalent accumulation rates the measured accumulation rates vary from 0 1mw e a in the interior to 0 7mw e a near the coast and correspond broadly with existing published model results though there are some excursions comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10 increase in accumulation rates over the past 52 years along most of the traverse route this implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of greenland

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

north atlantic oscillation dynamics recorded in greenland ice cores carefully selected ice core data from greenland can be used to reconstruct an annual proxy north atlantic oscillation nao index this index for the past 350 years indicates that the nao is an intermittent climate oscillation with temporally active coherent and passive incoherent phases no indication for a single persistent multiannual nao frequency is found in active phases most of the energy is located in the frequency band with periods less than about 15 years in addition variability on time scales of 80 to 90 years has been observed since the mid 19th century

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

contribution to the glaciology of northern greenland from satellite radar interferometry interferometric synthetic aperture radar insar data from the ers 1 and ers 2 satellites are used to measure the surface velocity topography and grounding line position of the major outlet glaciers in the northern sector of the greenland ice sheet the mass output of the glaciers at and above the grounding line is determined and compared with the mass input we find that the grounding line output is approximately in balance with the input except for the three largest glaciers for which the mass loss is 4 3 km3 ice year 1 or 11 8 of the mass input along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with insar which implies that the outlet glaciers are thinning the inferred coastal thinning is too large to be explained by a few warm summers glacier thinning must be of dynamic origin that is caused by spatial and temporal changes in ice velocity iceberg production from the glaciers is uncharacteristically low it accounts for only 8 of the ice discharge to the ocean about 55 of the ice is lost through basal melting 5 8 m ice year 1 on average from the underside of the floating glacier tongues that are in contact with warm ocean waters mass losses are highest in the first 10 km of floating ice where ice reaches the greatest depths and basal melting is 3 times larger than on average only a small increase in basal melting would suffice to disintegrate the floating glacier tongues copyright 2001 by the american geophysical union

a late holocene palaeoenvironmental snapshot of the angamma delta lake megachad at the end of the african humid period abstract during the african humid period ahp there was a large increase in the area of lakes and wetlands lake megachad one of several huge lakes underwent dramatic fluctuations during the ahp prior to regression in the mid holocene however the timing and nature of ahp termination has been disputed we present evidence from sediments of the angamma delta from the northern end of the palaeolake for lake megachad lake level fluctuations at the end of the ahp delta slope deposits were deposited around 7000 cal bp at the height of the ahp overlying bioclastic sediments from 4300 to 4800 cal bp at an elevation of 285 290 m lie below the palaeolake highstand 339 m but close to the elevation of the bahr el ghazal sill which divided the lake s two sub basins ostracod 18o values indicate that the waters of the northern sub basin were evaporated to levels similar to modern lake chad palaeoecological evidence suggests that the lake was perennial and evaporative enrichment is attributed to restricted circulation of lake waters as the sill emerged the age and elevation of the bioclastic sediment coupled with published lake level reconstructions suggest a complex lake level history with a major regression at the end of the ahp followed by a short lived lake level rise and then a further regression this new evidence for changes in lake level provides support for other geological records and some modelling experiments that suggest rapid fluctuations in hydroclimate at the end of the ahp

micromorphological approach to polycyclic pedogenesis on the messak settafet plateau central sahara formative processes and palaeoenvironmental significance the messak settafet is a wide plateau located in the libyan central sahara sw fezzan it is cut into the nubian sandstone and interpreted as relic of a tertiary peneplain heavily affected by aeolian deflation notwithstanding degradation phenomena discontinuous thin soil bodies are locally preserved below the hamada several profiles located in correspondence of different geomorphological units the hamada the endorheic depressions and the southern escarpment were described and sampled for micropedological analyses micromorphology outlined that the main characteristics of the residual b horizons from each geomorphological units are i high concentration of ferruginous nodules ii multiple generations of clay illuviation iii fragmentation and deformation of textural pedofeatures and iv calcite mobilization and redeposition according to horizon macro and micromorphological characteristics these pedologic bodies do not seem to be in equilibrium with the present day climatic conditions and should be regarded as paleosols generated by processes regulated by large water availability and warm temperature such as rubification clay translocation and calcite redistribution the development of horizons acted mainly since the early middle pleistocene during subsequent pluvial periods and was interrupted by several erosional and aeolian input phases arid periods pedological processes ended at the onset of the severe dry environmental conditions at the mid late holocene transition the development of this kind of paleosols in the central sahara is also discussed in comparison with pedogenetic processes occurring in temperate regions during interglacial and interstadial intervals

strain effects near palmdale associated with the san fernando earthquake 1971 abstract field data from the southeast palmdale network in california are the basis for a practical strain analysis with triangulation data available for the year 1938 and 1958 and trilateration data at 1 year intervals from 1971 to date a uniform although small strain accumulation over the last forty years appears to be evident from this data set a possible soceismic strain jump of 2 strain in both shearing strain components related to the 1971 san fernando earthquake may be postulated in the observations however it was already pointed out previously that this coseismic strain jump needs further qualifications due to the corrupting influence of uncertainties in the network height differences this particular situation arises because triangular data have to be compared with trilateration data for the computation of the coseismic strain changes to clarify this situation the height differences of the southeast palmdale network were determined by trigonometric levelling in spring 1981 the effect of this new height data on the interpretation of the coseismic strain change is discussed in detail a comparison of this result with strain model calculations is presented

tectonic motions and systematic errors in leveling and trilateration data for california abstract we examined leveling and trilateration data for southern california we used mathematical models that allow for both tectonic motion and systematic errors for both data types we find evidence of systematic errors the leveling data reveal no significant tectonic motion except the direct effects of earthquakes trilateration data show steadily accumulating shear strain but previously reported episodic dilatations probably result from systematic errors

annual accumulation over recent centuries at four sites in central greenland 1 accurate accumulation records are fundamental to understanding mass balance of the greenland ice sheet ice cores can provide multicentury high resolution ground based point estimates of water accumulation in 2003 and 2004 four new ice cores were collected in the central western d4 and d5 and summit sandy and katie regions of greenland annual layer counting based on multiple glaciochemical parameters primarily hydrogen peroxide sea salt and dust proxies was used to date the ice cores resulting in annual dating the bottom depth ages for d4 d5 sandy and katie were estimated at 1738 1673 1844 and 1934 a d with mean annual accumulation rates of 41 4 35 2 22 4 and 22 4 cmweq yr 1 respectively these new records were in good agreement with previous accumulation maps and four previously published records from nearby multicentury ice cores spatial variability was quantified through an analysis of variance thereby allowing for more meaningful comparisons to atmospheric processes more regionally representative accumulation records were constructed for the central western and summit regions spanning recent centuries using a principle component analysis of both the new and previously reported ice core measurements these regional records exhibited 6 7 interannual variability 1 standard deviation of the mean and had lower spatial variability uncertainties than individual accumulation records correlations indicate that 20 of the variance in the central western region was explained by the north atlantic oscillation nao and suggests that an array of more northern ice cores may correlate more closely with nao

reconstruction of sea level pressure fields over the eastern north atlantic and europe back to 1500 spatially and temporally high resolution estimates of past natural climate variability are important to assess recent significant climate trends the mid latitude atmospheric circulation is the dominant factor for regional changes in temperature rainfall and other climatic variables here we present reconstructions of gridded monthly sea level pressure slp fields back to 1659 and seasonal reconstructions from 1500 1658 for the eastern north atlantic european region 30 w to 40 e 30 n to 70 n these were developed using principal component regression analysis based on the combination of early instrumental station series pressure temperature and precipitation and documentary proxy data from eurasian sites the relationships were derived over the 1901 1960 calibration period and verified over 1961 1990 under the assumption of stationarity in the statistical relationships a transfer function derived over the 1901 1990 period was used to reconstruct the 500 year large scale slp fields systematic quality testing indicated reliable winter reconstructions throughout the entire period lower skill was obtained for the other seasons although meaningful monthly reconstructions were available from around 1700 onwards when station pressure series became available the quality and the reconstructed slp fields for two exceptionally cold years 1573 1740 are discussed and climatologically interpreted an eof analysis of the 1500 1999 winter slp revealed firstly a zonal flow pattern with pronounced decadal to centenial time scale variations secondly a monopole pattern over northwest europe and thirdly a pattern modulating the meridional flow component over europe these 500 year slp reconstructions should be useful for modelling studies particulary for analyses of low frequency atmospheric variability and for circulation dynamics

paleosols on pleistocene dunes as indicators of paleo monsoon events in the sahara of east niger abstract in a sw ne traverse across the tenere desert and the southern tchigai mountainous region only one generation of ancient dunes was found overlain by recent active eolian sand sheets and dunes paleosols on these dunes display red brown to yellow brown bw horizons up to 100 cm thick and are classified as chromi cambic arenosols and cambic arenosols respectively the structure of the soil horizons is stabilized by pedogenic cementation and often shows effects of bioturbation near the shores of previously more extensive paleolakes these paleosols change into gleyic arenosols the former shore lines are frequently marked by seams of goethite rhizoconcretions bog iron ores within the paleolake depressions ancient dune sediments bleached by gleying are covered by silt rich lacustrine sediments neolithic artifacts on the lacustrine sediments indicate that during an arid climatic period the paleolakes contracted in size and did not reach their fullest original extent during the late neolithic humid period this suggests the existence of earlier periods with enhanced humidity because of a decrease in humidity from sw to ne the degree of rubefication and several physical chemical and mineralogical properties of the paleosols are related to their position along the traverse a gradient of decreasing weathering intensity from sw to ne is paralleled by local variations in mineralogical properties of the parent materials and in rates of dust deposition consequently the two effects on soil properties are difficult to separate

palaeosols in saharan and sahelian dunes of chad archives of holocene north african climate changes limnic sediments and palaeosols are frequently present in dunes of the sahara and sahel deserts of chad this study focused on genesis and degree of development of the palaeosols and the timing of soil formation the soils are cambic arenosols formed by precipitation induced silicate weathering and leaching the upper parts of the cambic arenosols are indurated due to accumulation of salt and amorphous silica during a drier climate following soil formation the optical age data reveal the occurrence of one or two distinct short lived humid climate intervals shortly after 5 ka and before 2 5 ka and the formation of local desert lakes shortly after 5 ka the study further suggests that rapid climate changes occurred mainly in the mid holocene whereas the early holocene humid period lasted longer

statistical tests of aries data statistical tests are performed on project aries preliminary baseline measurements in the southern california triangle formed by the jet propulsion laboratory the owens valley radio observatory and the goldstone tracking complex during 1976 1980 in addition to conventional one dimensional tests a two dimensional test which allows for an arbitrary correlation between errors in individual components is formulated using the hotelling statistic on two out of three baselines the mean rate of change in baseline vector is statistically significant apparent motions on all three baselines are consistent with a pure shear with north south compression and east west expansion of 1 x 10 to the 7th year the aries measurements are consistent with the usgs geodolite networks in southern california and the safe laser satellite ranging experiment all three experiments are consistent with a 6 cm year motion between the pacific and north american plates and a band of diffuse shear 300 km wide except that corresponding rotation of the entire triangle is not found

holocene vegetation zonation in the eastern sahara recent discoveries of fossil bearing holocene lake sediments from the eastern sahara have brought further confirmation and detail to earlier indirect evidence that a major pluvial episode occurred between 9 500 and 4 500 yr bp1 3 the botanical results mainly palynological show that savanna and desert grassland occupied regions that today are plantless hyperarid deserts we present here the first evidence from the entire saharan region of a latitudinal zonation of holocene vegetation extending across a 500 km wide belt in north west sudan we have completed detailed pollen analyses of three of the several sites discovered so far and the salient features from two the northernmost and the southernmost are presented here to demonstrate a steep gradient of past vegetative cover

probability of earthquake recurrence with nonuniform stress rates and time dependent failure current methods for calculation of long term probabilities for the recurrence of large earthquakes on specific fault segments are based upon models of the faulting process that implicitly assume constant stress rates during the interval separating earthquakes and instantaneous failure at a critical stress threshold however observations indicate that the process of stress recovery following an earthquake involves rate variations at all time scales in addition to stress steps caused by nearby earthquakes additionally the existence of foreshocks aftershocks and possible precursory processes suggest that there may be significant time dependence of the earthquake nucleation process a method for determining the conditional probabilities for earthquake occurrence under conditions of irregular stressing is developed that could be useful at all time scales including those pertinent to short and intermediate term prediction used with models for earthquake occurrence at a stress threshold the addition of variable stressing introduces a simple scaling of the conditional probabilities by stress level and stress rate a model for the time dependent nucleation of earthquake slip has been proposed recently that is based upon laboratory observations of fault strength this failure criterion results in large but relatively short duration changes in the probability of earthquake recurrence particularly following stress steps applied to populations of earthquakes the models predicts a 1 t decay of seismicity following stress steps as observed for aftershocks and for frequency of foreshock mainshock pairs the model suggests that variations of seismicity rates of small earthquakes in the nucleation zone of the expected earthquake directly indicate variations in probability of recurrence of the large earthquake

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

recurrent climate winter regimes in reconstructed and modelled 500 hpa geopotential height fields over the north atlantic european sector 1659 1990 recurrent climate winter regimes are examined from statistically reconstructed and modelled 500 hpa geopotential height fields over the north atlantic european sector for the period 1659 1990 we investigate the probability density function of the state space spanned by the first two empirical orthogonal functions of combined winter data regimes are detected as patterns that correspond to areas of the state space with an unexpected high recurrence probability using a monte carlo approach the reconstruction and the model reveal four recurrent climate regimes they correspond to the two phases of the north atlantic oscillation and two opposite blocking patterns complemented by the investigation of the temporal evolution of the climate regimes this leads to the conclusion that the reconstructed and the modelled data for this geographic sector reproduce low frequency atmospheric variability in the form of regime like behaviour the overall evidence for recurrent climate regimes is higher for the model than for the reconstruction however comparisons with independent data sources for the period 1659 1990 revealed a more realistic temporal evolution of the regimes for the reconstructed data

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

the north atlantic oscillation and its imprint on precipitation and ice accumulation in greenland interannual to decadal fluctuations in net precipitation and ice accumulation are examined over greenland it is shown that in western greenland these fluctuations are correlated with the north atlantic oscillation the analysis is based on two complementary data sources a highly resolved net precipitation and accumulation history over 15 years derived form the reanalysis data of the european centre for medium range weather forecasts and a composite ice accumulation record representative for the western part of central greenland it is suggested that western greenland snow accumulation is a good proxy for the nao index with the potential for the reconstruction of a long time series

retrieving a common accumulation record from greenland ice cores for the past 1800 years 1 in the accumulation zone of the greenland ice sheet the annual accumulation rate may be determined through identification of the annual cycle in the isotopic climate signal and other parameters that exhibit seasonal variations on an annual basis the accumulation rate in different greenland ice cores is highly variable and the degree of correlation between accumulation series from different ice cores is low however when using multiyear averages of the different accumulation records the correlation increases significantly a statistical model has been developed to estimate the common climate signal in the different accumulation records through optimization of the ratio between the variance of the common signal and of the residual using this model a common greenland accumulation record for the past 1800 years has been extracted the record shows significant 11 9 years periodicity a sharp transition to very dry conditions is found just before a d 1200 and very dry conditions during the 13th century together with dry and cold spells during the 14th century may have put extra strain on the norse population in greenland and may have contributed to their extinction accumulation rates gradually decrease from a distinct maximum in a d 1394 to very dry conditions in the late 17th century and thus reflect the little ice age

delineation of carbonate dust aluminous dust and sea salt deposition in a greenland glaciochemical array using positive matrix factorization 1 high resolution trace element records from polar ice cores are fundamental to identifying spatial and temporal variability in dust and sea salt aerosols here we developed high temporal resolution glaciochemical records over recent centuries from an array of seven recently collected greenland ice cores positive matrix factorization pmf was used to assess apportionment of trace elements between three unique sources which were identified as proxies of carbonate dust aluminous dust and sea salt the three pmf factors agree well with literature values and represent the first time carbonate and aluminous dust sources have been quantifiably differentiated using high resolution glaciochemistry the concentrations of all three sources varied with ice core site however the large array of ice cores provided a regional scale understanding of the sources and magnitude of dust and sea salt impurities deposited on the greenland ice sheet at the regional level all three sources varied on seasonal annual and multiyear timescales the seasonal deposition of sea salt impurities reached a maximum in the winter followed by spring peaks of carbonate and aluminous dust over the 331 year composite record only aluminous dust exhibited a long term increasing trend that peaked in the 1930s and was followed by a 20 decline in recent decades

charophyte remains from wadi howar as evidence for deep mid holocene freshwater lakes in the eastern sahara of northwest sudan abstract field research in the eastern sahara northwest sudan revealed extensive early to mid holocene lake marl deposits bearing gyrogonites of charophytes green algae stoneworts along the lower course of the wadi howar an extinct tributary to the nile the charophyte flora of a paleolake southwest of jebel rahib 17 31 n 26 52 e dated prior to 4720 110 yr b p is mainly composed of the ecologically sensitive species nitellopsis obtusa which indicates permanent relatively deep 4 12 m and cool oligotrophic freshwater the study of nitellopsis sites provides a new and promising approach to the reconstruction of quaternary paleoenvironments of deserts

interferometric estimation of three dimensional ice flow using ascending and descending passes satellite radar interferometry sri provides an important new tool for determining ice flow velocity interferometric measurements made from a single track direction are sensitive only to a single component of the three component velocity vector observations from along three different track directions would allow the full velocity vector to be determined a north south looking synthetic aperture radar sar could provide these observations over large portions of the globe but not over large areas of the polar ice sheets the authors develop and demonstrate a technique that allows the three component velocity vector to be estimated from data acquired along two track directions ascending and descending under a surface parallel flow assumption this technique requires that there are accurate estimates of the surface slope which are also determined interferometrically to demonstrate the technique the authors estimate the three component velocity field for the ryder glacier greenland their results are promising although they do not have yet ground truth data with which to determine the accuracy of their estimates

forecasting southern california earthquakes since 1978 and 1979 california has had a significantly higher frequency of moderate to large earthquakes than in the preceding 25 years in the past such periods have also been associated with major destructive earthquakes of magnitude 7 or greater and the annual probability of occurrence of such an event is now 13 percent in california the increase in seismicity is associated with a marked deviation in the pattern of strain accumulation a correlation that is physically plausible although great earthquakes magnitude greater than 7 5 are too infrequent to have clear associations with any pattern of seismicity that is now observed the san andreas fault in southern california has accumulated sufficient potential displacement since the last rupture in 1857 to generate a great earthquake along part or all of its length

holocene lake deposits and palaeoenvironments in central sahara northeastern mali abstract extensive and fossiliferous lacustrine deposits have been found in a now hyperarid area south of the tanezrouft 20 30 21 n 1 w between the erg ine sakane and the djebel takabart the assemblage of fish molluscs ostracods and charophytes among which lychnothamnus is observed in africa for the first time testify to fresh water conditions however stable carbon and oxygen isotopic values provide evidence for phases of strong evaporation the presence of limicolaria sp a terrestrial pulmonate first observed in the sahara indicate permanent soil humidity around the palaeolake s the remains of large steppe type mammals elephant antelopes buffalo as well as neolithic sites and burials in brown soils also contribute to evidence a northward shift of the sahel belt 3 4 in latitude this period is radiocarbon dated from 6130 180 b p to 7520 70 b p on molluscan shells the human material could be younger 3750 100 b p on charcoal

interannual variations of snow accumulation on the greenland ice sheet 1985 1996 new observations versus model predictions newly measured time series of net water equivalent accumulation from ice cores are reported for 11 sites located near the 2000 m contour of the greenland ice sheet many of these sites are located in regions where accumulation has not been previously measured we compared these new time series of annual accumulation with previously reported modeled precipitation covering the same time range because ice core accumulation records include both regional and local accumulation components we used adjacent ice core records at six multiple core sites to estimate the local component and extrapolated these results to the five single core sites we investigated the impact of this short scale spatial variability in accumulation on statistical comparisons between the observed accumulation and model precipitation although sealers were required to match the 12 year mean modeled precipitation to the mean observed accumulation a high degree of correspondence between observed interannual accumulation and scaled modeled precipitation was found at 7 of 11 locations the need for scaling and the low correspondence between model simulations and observations at some sites clearly indicate that model improvements are needed although very encouraging a more spatially distributed array of ice core accumulation measurements spanning the same time period as the precipitation modeling is necessary for continued model improvement and validation in agreement with earlier studies the large temporal variability in snow accumulation predicted by modeling and confirmed by ice cores indicates that the ice sheet elevation varies by tens of centimeters from year to year simply because of changing accumulation

seismicity near palmdale california and its relation to strain changes we evaluate the relationships between the spatio temporal patterns and faulting mechanisms of small earthquakes and the recent temporal changes in horizontal strain observed along the big bend portion of the san andreas fault near palmdale california microearthquake activity along the entire big bend of the san andreas fault increased in november 1976 concurrent with the initiation of an earthquake swarm at juniper hills this activity then decreased abruptly to the northwest and southeast of juniper hills during the beginning of 1979 this drop in seismic activity occurred around the time that crustal dilatation was observed on the u s geological survey palmdale trilateration network focal mechanisms from the study region are predominantly thrust there are two time periods when the mechanisms are closer to strike slip than to thrust the first period december 1976 to february 1977 corresponds to the beginning of the juniper hills swarm the second period november 1978 to april 1979 approximately coincides with a change in trend of the strain data from uniaxial n s compression to dilatation

modelling palaeosol preservation in aeolian dunes in some of the world s desert and desert marginal areas e g simpson strzelecki australia dunefields preserve well developed palaeosols whereas in other regions with broadly similar climatic regimes and topography e g southwest kalahari the dunes are characterized by very poorly developed internal stratigraphy it has been postulated that dunes such as those in the kalahari may never have had conditions conducive to soil formation or that soils once formed but any evidence of palaeosols has been lost due to reworking this study develops and applies a one dimensional numerical model to simulate dune development soil formation and soil preservation variables in the model allowed experimentation on the influence of sediment supply the time taken for soil to form and the additional resistance to erosion offered by the soil reduced sediment supply plays a vital role in landscape development during periods of initial pedogenesis although re exhumed palaeosols influence sediment supply the effect is minimal although under almost all parameterized conditions more than half and up to 80 90 of those soils initially formed are lost due to reworking evidence of their past formation remains in the large majority of profiles and the dominant factor in controlling the preservation of palaeosols is the frequency of their formation the implication is that where dunes are found without palaeosols the most likely albeit not certain inference is that they have never formed counter intuitively the limited sediment supply means their additional resistance to erosion becomes almost inconsequential to their preservation at least until the unit approaches complete invulnerability short chronostratigraphic hiatuses around palaeosols are normal and although long gaps can occur they are extremely infrequent where such gaps are observed in field studies external forcing factors e g climatic or environmental changes are implied as they are highly unlikely to result from stochastic net preservation copyright 2014 john wiley sons ltd

observed and modeled greenland ice sheet snow accumulation 1958 2003 and links with regional climate forcing abstract annual and monthly snow accumulation for the greenland ice sheet was derived from ecmwf forecasts mainly 40 yr ecmwr re analysis era 40 and further meteorological modeling modeled accumulation was validated using 58 ice core accumulation datasets across the ice sheet and was found to be 95 of the observed accumulation on average with a mean correlation of 0 53 between modeled and observed many of the ice core datasets are new and are presented here for the first time central and northern interior parts of the ice sheet were found to be 10 30 too dry in era 40 in line with earlier ecmwf analysis although too much 50 locally snow accumulation was modeled for interior southern parts of greenland nevertheless 47 of 58 sites show significant correlation in temporal variability of modeled with observed accumulation the model also captures the absolute amount of snow accumulation at several sites most notably das1 and das2 in southeast greenland mean modeled accumulation over the

a mini surge on the ryder glacier greenland observed by satellite radar interferometry satellite radar interferometry reveals that the speed of the ryder glacier increased roughly threefold and then returned to normal 100 to 500 meters year over a 7 week period near the end of the 1995 melt season the accelerated flow represents a substantial though short lived change in ice discharge during the period of rapid motion meltwater filled supraglacial lakes may have drained which could have increased basal water pressure and caused the mini surge there are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in greenland but because most other outlet glaciers are at lower latitudes they should experience more extensive melting making them more susceptible to meltwater induced surges

aseismic crustal deformation in the transverse ranges of southern california abstract geodetic data at a plate boundary can reveal the pattern of subsurface displacements that accompany plate motion we model these displacements as the sum of rigid block motion and the elastic effects of frictional drag between blocks the frictional interactions are represented by uniform dislocation on each of several rectangular fault patches we then estimate the block velocities and fault parameters from geodetic data our bayesian inversion procedure employs prior estimates based on geological and seismological data we apply the method to the transverse ranges using prior data from bird and rosenstock 1984 and geodetic data from the usgs trilateration networks and nasa vlbi measurements our model consists of 12 blocks and 27 rectangular fault patches the block motion inferred from the geodetic data has the same order of magnitude as the geologic estimates and for many faults the agreement is excellent however the geodetic data imply a displacement rate of about 20 mm yr 1 across the san andreas fault while the geologic estimates exceed 30 mm yr 1 the final model implies about 6 mm yr 1 of crustal shortening normal to the trend of the san andreas fault this is about half the rate implied by the prior model most of this shortening occurs on the sierra madre cucamonga and the white wolf fault systems aseismic fault motion is a major contributor to plate motion and the thickness of the frictional surface varies considerably from one fault to another the geodetic data can help to identify faults that are suffering rapid stress accumulation in the transverse ranges those faults are the san andreas san gabriel sierra madre pleito and white wolf faults

northern hemisphere midlatitude cyclone variability in gcm simulations with different ocean representations the impact of different ocean models or sea surface temperature sst and sea ice concentrations on cyclone tracks in the northern hemisphere midlatitudes is determined within a hierarchy of model simulations a reference simulation with the coupled atmosphere ocean circulation model echam hope is compared with simulations using echam and three simplified ocean and sea ice representations 1 a variable depth mixed layer ml ocean 2 forcing by varying sst and sea ice and 3 with climatological sst and sea ice the latter two are from the coupled echam hope integration the reference simulation reproduces the observed cyclone tracks the cyclones are tracked automatically by a standard routine and the variability of individual cyclone trajectories within the storm tracks is determined by a cluster approach in the forced simulation with varying sst the geographical distribution and the statistics of the cyclones are not altered compared to the coupled reference simulation in the ml and the climatological simulation deviations of the mean cyclone distribution are found which occur mainly in the north pacific and can partially be traced back to missing el nino southern oscillation enso variability the climatological experiment is superior to the ml experiment the variability of the individual cyclone trajectories as determined by the cluster analysis reveals the same types and frequencies of propagation directions for all four representations of the lower boundary the largest discrepancies for the cluster occupations are found for the climatological and the ml simulation

anomalous water level changes and possible relation with earthquakes since late 1976 abandoned water wells have been monitored along the san andreas and san jacinto faults to detect water level changes which may be premonitory to earthquakes an m5 5 earthquake occurred 25 february 1980 within the san jacinto fault zone near anza the water level in a continuously recorded well in borrego valley 35 km from the epicenter rose 0 5 m and returned to its prior level during a four hour period beginning 88 hours before the earthquake the water level in this well had been remarkably steady compared to other wells being monitored another well in borrego valley showed a much smaller rise and fall in water level at the same time because these changes are unique for the long term records of these wells they may represent precursors to the earthquake several wells along the palmdale valyermo segment of the san andreas fault showed possibly anomalous long term water level changes which are not clearly related to rainfall between early 1979 and early 1980 water levels in six wells were 6 to 4 5 m higher and levels in two wells were about 3 m lower than would have been predicted based on the previous records the water level increases could have been the result of delayed response to seasonal rainfall following a long drought however it is interesting that the possibly anomalous water level changes first occurred at the same time as the reported changes in strain pattern from compression to expansion along the san andreas fault in the same area

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

palaeoenvironmental reconstructions from linear dunefields recent progress current challenges and future directions abstract this paper reviews recent progress in the use of linear dunes as geoproxies of late quaternary environmental change summarises the challenges facing their use and explores some potential solutions to these challenges large areas of the swathes of linear dunes which occupy the continental interior of southern africa australia and parts of central asia and southern america currently have limited or negligible aeolian activity they have been recognised as offering potential information about past environments for more than a century but only with the widespread application of luminescence dating during the 1990s did they realistically start to offer the prospect of being an extensive dateable proxy of late quaternary palaeoenvironments and possibly palaeoclimates dating aeolian dune sands with luminescence methods is generally although not always relatively straightforward over the past twenty years a large number 1000 of luminescence ages have been added to the global dataset yet there has also been significant criticism of some of the rationale underpinning much of the interpretation of the records derived at the landscape scale developments of arguably equal importance have come from improved geomorphological understanding based on the wider availability of remotely sensed data and the paradigm of dunefield evolution as a self organising complex system current challenges are identified in three key regions incomplete understanding of how the process geomorphology of linear dunes affect the accumulation and preservation of sediment a lack of clarity regarding the temporal and spatial scale of the response in a dynamic environmental setting and uncertainty surrounding the drivers of changing rates of net accumulation solutions to these challenges lie within diverse research methodologies certainly further field study is required with improvement required in understanding system responses to changing environmental stimuli at scales from sedimentological to landscape in parallel the full implications of complex systems approaches to aeolian geomorphology for linear dunes lag behind the adoption of the concept to some other dune forms e g barchans the relationship between observed dune complexity and age from field data complicates previous suggestions that dune patterning may be purely a function of development time parallel to this lies the need to improve interpretation of the results of dune geochronological studies a suggestion is offered which aims to test the statistical significance of dune accumulation time series by comparison with a modelled system with unchanging external forcing despite the initial promise of linear dune records as a revolutionary source of palaeoenvironmental information in drylands the outcomes to date have been coarse although still valuable in resolution perhaps the most valuable realization has been that geomorphological understanding of these widespread landforms is incomplete

palaeosols and their cover sediments of a glacial landscape in northern central europe spatial distribution pedostratigraphy and evidence on landscape evolution abstract knowledge of the distribution types and properties of buried soils i e palaeosols is essential in understanding how lowlands in northern central europe have changed over past millennia this is an indispensable requirement for evaluating long term human impact including soil erosion and land cover dynamics in the serrahn area 62 km2 a young glacial landscape representative for northeastern germany and part of the muritz national park 26 pedosedimentary sections were documented and analysed to this end a multiproxy approach was applied using pedology micromorphology geochronology and palaeoecology statistical and spatial analyses of c 5200 soil profiles of which 10 contain palaeosols show that buried soils cover an area of 5 7 km2 i e 9 of the area studied most palaeosols are cambisols arenosols and gleysols palaeosols are mainly covered by aeolian and colluvial sands as well as by lacustrine sands and peat radiocarbon and luminescence dating together with palynological and anthracological data reveal that former land surfaces were dominantly buried through erosion triggered by human activity in the late holocene in addition but to a clearly smaller extent lateglacial early holocene palaeosols and cover sediments occur following medieval clear cutting and intensive land use the study area is today again widely forested the high share of buried land surfaces detected here is expected to be representative for the hilly glacial landscapes even in the wider region i e in northern central europe and should be considered in soil mapping soil carbon budgeting and assessments of past human impact

dune ages in the sand deserts of the southern sahara and sahel in this paper we aim to document the history of aeolian processes within the southern sahara as part of the inqua dune atlas we review available luminescence ages for sand dunes across the southern sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation this was achieved by analysing dune age by country as well as by latitude and longitude the results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas despite these gaps some related patterns in dune morphology and stratigraphy appear to be consistent between northern nigeria and southern mali where older linear dunes are distinct from younger late holocene transverse and barchanoid dunes elsewhere in mauretania linear dunes with different orientations appear to have accumulated at different times most likely in response to changes in atmospheric circulation regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins we identify four locations where dune accumulation is terminated by lacustrine transgressions two of which in lake chad and the bodele depression occur shortly after the last glacial maximum lgm the third example at gobiero in niger occurred later in the early holocene around 8 4 ka and a fourth marks a later transgression of palaeolake megachad after 4 7 ka larger scale latitudinal and longitudinal distributions in dune ages across the southern sahara do not show any consistent patterns though this may be due to the small sample size relative to the study area in addition local variations in external controls such as wind regime rainfall vegetation and sand supply need to be considered sometimes on a site by site basis limiting the analysis to dune ages determined using the single aliquot regenerative dose sar protocol indicates a lack of dune preservation during the lgm and the younger dryas times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity the sar dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record the most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions

variability of accumulation in northwest greenland over the past 250 years during summer 1996 a 120 m firn and ice core was drilled to determine annual accumulation rates at a northwest greenland site gits 77 1392 n 61 0422 w 1910 m elevation annual layers were identified in the core using multiple parameters 18o and concentrations of dust hydrogen peroxide ammonium calcium and nitrate using all parameters together to define annual layers resulted in a 251 year record with a dating uncertainty of one year within that period annual accumulation over the period of record averaged about 0 37 m water equivalent comparing this record with four other multi century long records from the west central and northwest portion of the ice sheet shows many periods when decadal scale fluctuations in accumulation at the different sites are in phase overall variations in accumulation in this portion of the ice sheet were 8 9 per decade versus 25 for individual cores annual accumulation at gits showed a significant correlation with a 12 month north atlantic oscillation index pearson s r 0 32 with a significance level of 99 though the correlation was slightly lower than for two cores roughly 350 and 700 km south

migration of historical earthquakes in california most large earthquakes of magnitude 6 0 in california during 1852 1987 appear to show a southeast to northwest tendency of epicenter migration this finding is consistent with earlier findings ofsavage 1971 for a relatively few large earthquakes along the west coast of north america and ofwood andallen 1973 for smaller events along the san andreas fault in central california the average speed of migration is approximately 130 km yr which is within the range of speeds observed for other major seismic zones in the world the epicenter migration in california may be the result of some small but broad scaled episodic strain changes associated with creep waves induced by magma injections at the east pacific rise and propagating northwestwardly along a broad transform boundary between the pacific and north american plates at subseismogenic depths as proposed bysavage 1971

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

precision of geodolite surveys a reply to jackson and cheng jackson and cheng have suggested that the changes in areal dilatation measured in geodolite surveys by the u s geological survey may be simply an artifact of the measuring system although systematic error could conceivably account for the observed excursions in dilatation we maintain that the specific criticisms by jackson and cheng are incorrect the excursions in dilatation cannot be attributed to the offset correction nor to proportional error associated with temperature the absence of both errors is demonstrated by using data that are particularly sensitive to the two effects