maximum late holocene extent of the western greenland ice sheet during the late 20th century 10 be exposure dating abstract the pattern of greenland ice sheet margin change during the 20th century is variable large scale retreat of marine outlet glaciers contrasts with the often negligible retreat observed along land terminating margins of the ice sheet we reconstruct a chronology of ice margin change for two land terminating ice margins in western greenland using radiocarbon and 10 be exposure dating our results indicate that two land terminating lobes attained their maximum late holocene position in the late 20th century this contrasts with the nearby marine terminating jakobshavn isbrae which achieved a maximum late holocene position during the little ice age and has since retreated ca 40 km in addition we survey ice margin change across western greenland utilizing satellite imagery we find that many land terminating sectors of the ice sheet in addition to our study area may have attained their maximum late holocene extent during the 20th century this suggests a lagged ice margin response to prior cooling such as the little ice age which would imply significant retreat of land terminating sections of the greenland ice sheet in response to 20th and 21st century warming may be yet to come

residual shear strength of unsaturated soils via suction controlled ring shear testing abstract results from a comprehensive series of suction controlled ring shear tests conducted on statically compacted specimens of silty clayey sand and silty sand are presented the experiments were accomplished in a newly developed servo suction controlled ring shear apparatus suitable for testing unsaturated soils under large deformations and suction controlled conditions via the axis translation technique the present work focuses primarily on two crucial aspects of compacted unsaturated soil behavior namely the behavior of silty clayey sand under suction controlled ring shear testing and the effects of pre shearing and suction histories on unsaturated residual shear strength of compacted silty sand test results corroborate the important role played by matric suction on residual shear strength properties of unsaturated soils for the range of net normal stresses and suction states investigated the increase in residual shear strength with increasing suction was confirmed to be a linear trend for silty sand but significantly nonlinear for silty clayey sand results from multi stage ring shear tests confirmed that the residual shear strength of unsaturated soils is virtually independent of the pre shearing and suction histories experienced by the soil

multiple stage deglacial retreat of the southern greenland ice sheet linked with irminger current warm water transport 1 there is limited knowledge pertaining to the history of the greenland ice sheet gis during the last glacial interglacial transition as it retreated from the continental margins to an inland position here we use multiproxy data including ice rafted debris ird planktonic isotopes alkenone temperatures and tephra geochemistry from the northern labrador sea off southwest greenland to investigate the deglacial response of the gis and evaluate its implications for the north atlantic deglacial development the results imply that the southern gis retreated in three successive stages 1 early deglaciation of the east greenland margins by tephra rich ird that embrace heinrich event 1 2 progressive retreat during allerod culminating in major meltwater releases 18o depletion of 1 2 at the allerod younger dryas transition 12 8 13 0 kyr b p and 3 a final stage of glacial recession during the early holocene 9 11 kyr b p rather than indicating local temperatures of ambient surface water the alkenones likely were transported to the core site by the irminger current we attribute the timing of gis retreat to the incursion of warm intermediate waters along the base of grounded glaciers and below floating ice shelves on the continental margin the results lend support to the view that gis meltwater presented a forcing factor for the younger dryas cooling

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

the response of the southern greenland ice sheet to the holocene thermal maximum to determine the long term sensitivity of the greenland ice sheet to a warmer climate we explored how it responded to the holocene thermal maximum 8 5 cal kyr b p calibrated to calendar years before present i e a d 1950 when lake records show that local atmospheric temperatures in greenland were 2 4 c warmer than the present records from five new threshold lakes complemented with existing geological data from south of 70 n show that the ice margin was retracted behind its present day extent in all sectors for a limited period between ca 7 and 4 cal kyr b p and in most sectors from ca 1 5 to 1 cal kyr b p in response to higher atmospheric and ocean temperatures ice sheet simulations constrained by observations show good correlation with the timing of minimum ice volume indicated by the threshold lake observations the simulated volume reduction suggests a minimum contribution of 0 16 m sea level equivalent from the entire greenland ice sheet with a centennial ice loss rate of as much as 100 gt yr for several millennia during the holocene thermal maximum our results provide an estimate of the long term rates of volume loss that can be expected in the future as regional air and ocean temperatures approach those reconstructed for the holocene thermal maximum

shear strength characteristics of an unsaturated expansive clay the shear strength characteristics of an expansive clay from china were studied by performing suction controlled direct shear tests on both natural and compacted specimens the tested soil was a silty clay with intermedi ate plasticity and medium expansion potential a modified direct shear apparatus with a newly developed water volume indicator was used for this laboratory study the experimental results clearly show that the dilatancy of the expansive clay increases with an increase in the applied suction for both the natural and compacted specimens matric suction contributes to the shear strength of the expansive clay via two different mechanisms the contribution of capillary force to interparticle normal stress and the effect of suction on soil dilatancy as a result of the second mechanism the con tribution of suction to peak shear strength for the clay is more significant than that to post failure shear strength par ticularly at a high suction range the contribution of suction to post failure shear strength for the natural specimen is basically consistent with that for the compacted specimen the higher peak shear strength and dilatancy for the natural specimen are related to the cementation effect of the iron and manganese oxides the contribution of suction to shear strength for the compacted expansive clay is more significant than that for a compacted kaolin at suctions less than 100 kpa

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

strength and dilatancy of a silt stabilized by a cement and fly ash mixture the mechanical behaviour of a weakly cemented silt was studied experimentally the cementing agent was a cement and fly ash slurry and the samples so formed were slightly cemented in triaxial tes

holocene environmental variability in southern greenland inferred from lake sediments abstract sediments from qipisarqo lake provide a continuous holocene paleoenvironmental record from southern greenland following deglaciation and glacio isostatic emergence of the basin from the sea 9100 cal yr b p proxies of lake paleoproductivity including biogenic silica and organic matter increased markedly until 6000 cal yr b p and thereafter remained stable over the ensuing warm three millennia subsequent decreases in these proxies most dramatically between 3000 and 2000 cal yr b p show the lake s responses to initial neoglacial cooling intervals of ameliorated limnological conditions occurred between 1300 and 900 and between 500 and 280 cal yr b p briefly interrupting the decreasing trend in productivity that culminated in the little ice age increased lake productivity during the latter half of the 20th century which reflects the limnological response to circum arctic warming still has not reached peak holocene values the neoglacial climate of the last 2000 yr includes the most rapid high amplitude environmental changes of the past nine millennia the norse thus migrated around the north atlantic ocean region in the most environmentally unstable period since deglaciation lacustrine sediment records provide a context with which to consider future environmental changes in the labrador sea region in particular any future warming will be superposed on a regional climate system that is currently exhibiting highly unstable behavior at submillennial timescales

pattern and forcing of northern hemisphere glacier variations during the last millennium time series depicting mountain glacier fluctuations in the alps display generally similar patterns over the last two centuries as do chronologies of glacier variations for the same interval from elsewhere in the northern hemisphere episodes of glacier advance consistently are associated with intervals of high average volcanic aerosol production as inferred from acidity variations in a greenland ice core advances occur whenever acidity levels rise sharply from background values to reach concentrations 1 2 equiv h kg above background a phase lag of about 10 15 yr equivalent to reported response lags of alpine glacier termini separates the beginning of acidity increases from the beginning of subsequent ice advances a similar relationship but based on limited and less reliable historical data and on lichenometric ages is found for the preceding 2 centuries calibrated radiocarbon dates related to advances of non calving and non surging glaciers during the earlier part of the little ice age display a comparable consistent pattern an interval of reduced acidity values between about 1090 and 1230 a d correlates with a time of inferred glacier contraction during the medieval optimum the observed close relation between noothern hemisphere glacier fluctuations and variations in greenland ice core acidity suggests that sulfur rich aerosols generated by volcanic eruptions are a primary forcing mechanism of glacier fluctuations and therefore of climate on a decadal scale the amount of surface cooling attributable to individual large eruptions or to episodes of eruptions is simlar to the probable average temperature reduction during culminations of little ice age alacier advances ca 0 5 1 2 c as inferred from depression of equilibrium line altitudes

planetary science occam s origin of the moon following almost three decades of some certainty over how the moon was formed new geochemical measurements have thrown the planetary science community back into doubt we are either modelling the wrong process or modelling the process wrong

abrupt holocene climate change as an important factor for human migration in west greenland west greenland has had multiple episodes of human colonization and cultural transitions over the past 4 500 y however the explanations for these large scale human migrations are varied including climatic factors resistance to adaptation economic marginalization mercantile exploration and hostile neighborhood interactions evaluating the potential role of climate change is complicated by the lack of quantitative paleoclimate reconstructions near settlement areas and by the relative stability of holocene temperature derived from ice cores atop the greenland ice sheet here we present high resolution records of temperature over the past 5 600 y based on alkenone unsaturation in sediments of two lakes in west greenland we find that major temperature changes in the past 4 500 y occurred abruptly within decades and were coeval in timing with the archaeological records of settlement and abandonment of the saqqaq dorset and norse cultures which suggests that abrupt temperature changes profoundly impacted human civilization in the region temperature variations in west greenland display an antiphased relationship to temperature changes in ireland over centennial to millennial timescales resembling the interannual to multidecadal temperature seesaw associated with the north atlantic oscillation

contrasting evidence of holocene ice margin retreat south western greenland constraining the greenland ice sheet s gris response to holocene climate change provides calibrations for ice sheet models that hindcast past ice margin fluctuations ice sheet models predict enhanced ice retreat in south western greenland during the middle holocene however few geological observations corroborating the extensive retreat are available we present new data from lake sediment cores from the isua region south western greenland which provide constraints on holocene fluctuations of the gris margins our data indicate that the main gris margin was 30 km west of its present day extent by 5 7k cal a bp and had retreated to within 3 km of its present day margin by 4 6k cal a bp with subsequent re advances by 1 8 and at 1 7 1 3 and 0 5k cal a bp times of ice retreat are coeval with increasing temperatures in western greenland suggesting that temperature was a dominant factor in ice retreat the late retreat at isua is in contrast to the early retreat observed in the godthabsfjord area and is probably related to the lack of fjords extending to the present isua ice margin our data are not consistent with current ice sheet models that overestimate the middle holocene ice retreat in the isua region of south western greenland

rapid oceanic and atmospheric changes during the younger dryas cold period the younger dryas event which began approximately 12 900 years ago was a period of rapid cooling in the northern hemisphere driven by large scale reorganizations of patterns of atmospheric and oceanic circulation 1 3 environmental changes during this period have been documented by both proxy based reconstructions 3 and model simulations 4 but there is currently no consensus on the exact mechanisms of onset stabilization or termination of the younger dryas 5 8 here we present high resolution records from two sediment cores obtained from lake krakenes in western norway and the nordic seas multiple proxies from lake krakenes are indicative of rapid alternations between glacial growth and melting during the later younger dryas meanwhile reconstructed sea surface temperature and salinity from the nordic seas show an alternation between sea ice cover and the influx of warm salty north atlantic waters we suggest that the influx of warm water enabled the westerly wind systems to drift northward closer to their present day positions the winds thus brought relatively warm maritime air to northern europe resulting in rising temperatures and the melting of glaciers subsequent input of this fresh meltwater into the ocean spurred the formation of sea ice which forced the westerly winds back to the south cooling northern europe we conclude that rapid alternations between these two states immediately preceded the termination of the younger dryas and the permanent transition to an interglacial state the impact of freshwater pulses released from northern hemisphere ice sheets on the north atlantic meridional overturning circulation has been suggested as a likely mechanism for the younger dryas cooling 7 11 in addition a responsive sea ice cover has been proposed as a mechanism that could rapidly alter the heat exchange between the ocean and atmosphere during the last glacial period 12 13 the roles of the ocean and the atmosphere in triggering the climate shifts around the younger dryas are much discussed 9 these hypotheses are difficult to resolve partly because the temporal resolution of most palaeoclimate archives is too coarse to enable the anatomy of the younger dryas stadial to be adequately described 14 to address these questions we here present a lake sediment record at sub annual resolution together with new data on the sea surface conditions of the norwegian atlantic current nwac throughouttheyoungerdryasstadial fig 1

the influence of ice marginal setting on early holocene retreat rates in central west greenland ice sheet reconstructions from diverse ice margin settings spanning multiple millennia are needed to assess the reaction of the greenland ice sheet gris to millennial scale climatic forcing and to place historical records in a longer term context here we present 18 new cosmogenic 10 be exposure ages and five new radiocarbon ages that constrain the early holocene retreat of the gris in the disko bugt region in both a marine and a land based setting results indicate similar rates of early holocene retreat of 40 50m a 1 from transects in torsukattak fjord marine setting and the naternaq area land based setting we compile seven previously published chronologies of deglaciation from west greenland which yield early holocene retreat rates ranging from 10 to 65m a 1 similar to those determined for our two study areas this work demonstrates that when averaged on millennial timescales retreat rates were remarkably similar along the western gris margin furthermore the retreat rates calculated here demonstrate that terrestrial sectors of ice sheets can retreat at net rates comparable to their marine counterparts copyright 2015 john wiley sons ltd

10be dating of the narsarsuaq moraine in southernmost greenland evidence for a late holocene ice advance exceeding the little ice age maximum in southernmost greenland near narsarsuaq the terminal narsarsuaq moraine was deposited well outside of a historical little ice age lia moraine adjacent to the modern ice margin using 10be surface exposure dating we determine narsarsuaq moraine abandonment at 1 51 0 11 ka a second set of 10be ages from a more ice proximal position shows that ice has been within or at its historical i e lia extent since 1 34 0 15 ka notably narsarsuaq moraine abandonment was coincident with climate amelioration in southern greenland southern greenland warming at 1 5 ka was also concurrent with the end of the roman warm period as climate along the northern north atlantic sector of europe cooled into the dark ages the warming of southern greenland and retreat of ice from the narsarsuaq moraine is consistent with studies suggesting possible anti phase centennial scale climate variability between northwestern europe and southern greenland other southernmost greenland ice margin records do not preclude a pre lia ice margin maximum potentially concurrent with a narsarsuaq advance prior to 1 51 ka but also lack sufficient ice margin control to confirm such a correlation we conclude that there is a clear need to further determine whether a late holocene pre lia maximum was a local phenomenon or a regional southern greenland ice maximum and if this advance and retreat reflects a regional fluctuation in climate

holocene evolution of the western greenland ice sheet assessing geophysical ice sheet models with geological reconstructions of ice margin change abstract geophysical ice sheet models are used to predict future ice sheet dimensions and in turn these projections help estimate the magnitude of eustatic sea level rise before models can confidently predict ice sheet behavior they must be validated by being able to duplicate the geological record of ice sheet change here we review geological records of greenland ice sheet gris change with emphasis on the warmer than present middle holocene and compare these records to published studies that numerically simulate gris behavior through the holocene geological records are concentrated in west and southwest greenland which are also the regions where the gris margin likely experienced the greatest distance of inland retreat during the middle holocene several records spanning from melville bugt to jakobshavn isfjord in western greenland indicate the gris achieved its minimum extent between 5 and 3 ka and farther south in the kangerlussuaq region new data presented here indicate the ice margin reached its minimum extent between 4 2 and 1 8 ka in the narsarsuaq region in southern greenland the gris likely achieved its minimum configuration between 7 and 4 ka we highlight key similarities and discrepancies between these reconstructions and model results and finally we suggest that despite some degree of inland retreat the west and southwest gris margin remained relatively stable and close to its current position through the holocene thermal maximum

dating of the narssarssuaq stade in southern greenland following the holocene thermal maximum the greenland ice sheet began to expand and many local glaciers formed the maximum extent of most outlet glaciers from the inland ice was reached around 100 to 200 years ago however some exceptions have been recorded in southern greenland the neoglacial narssarssuaq stade marks an older re advance that exceeded the historical advance the dating of the narssarssuaq stade has been much debated here we present new radiocarbon dates from a sediment core collected from a small lake within the moraines formed during this stade a minimum age for the narssarssuaq stade of around 1200 yr bp is suggested from the dates

performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration a major infrastructure project the south to north water transfer project has been proposed to carry potable water from the yangtze river region in the south to many arid and semi arid areas in the northern regions of china including beijing the proposed 1200 km middle route of the project is likely to be an open channel with a trapezoidal cross section at least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils an 11 m high cut slope in a typical expansive clay with medium plasticity in zaoyang close to the middle route in hubei was selected for a comprehensive and well instrumented field study of the effects of rainfall infiltration two artificial rainfall events were created during a month of field investigation and monitoring prior to the rainfall events high soil suction was measured within the top 1 m of soil this high initial soil suction created an upward flux of water and moisture the abundant cracks and fissures near the ground surface p

rapid early holocene ice retreat in west greenland abstract the possible demise of the greenland ice sheet and its effect on global sea level rank among the most serious climate threats to society to improve our knowledge about the future behaviour of the ice margin we studied the ice sheet s response to early holocene warming in west greenland using 47 cosmogenic 10 be exposure ages 26 optically stimulated luminescence ages as well as 15 new and 28 previously published radiocarbon ages paired bedrock and boulder ages show that the entire area was covered by warm based ice during the last glacial maximum lgm although glacial erosion was insufficient to completely remove the upper rock surface containing 10 be inherited from a previous period of exposure in bedrock samples above an elevation of 800 m our compilation of 10 be and 14 c ages demonstrates that the ice sheet retreated from the outer coast to the present ice margin between c 11 4 and 10 4 cal ka bp in the godthabsfjord system and between 10 7 0 6 and 10 1 0 4 ka ago in buksefjord whereas the coast at sermilik became ice free at c 10 5 cal ka bp we find no significant changes in the retreat rates between the deep godthabsfjord system and the buksefjord sermilik region which is characterized by only a few narrow and shallow fjords however deglaciation was initiated c 700 900 years earlier in the godthabsfjord system indicating that the deep fjords probably triggered land based deglaciation by dynamic ice loss leading to an overall rapid early holocene ice retreat and drawdown of the ice sheet in west greenland these results demonstrate that even if there was a topographic control on the onset of deglaciation fast ice retreat is not restricted to deep fjord systems but may occur independently of the topographic setting

model for the prediction of shear strength with respect to soil suction experimental studies on unsaturated soils are generally costly time consuming and difficult to conduct shear strength data from the research literature suggests that there is a nonlinear increas

effects of soil structure on the shear behaviour of an unsaturated loess at different suctions and temperatures previous studies of shear behaviour at different suctions and temperatures focused mainly on recompacted soils limited attention has been paid to intact soil in this paper the shear behaviour of

behavior of a compacted completely decomposed granite soil from suction controlled direct shear tests a series of single staged consolidated drained direct shear tests are carried out on recompacted completely decomposed granite cdg soil a typical residual soil in hong kong under different matric suctions and net normal stresses matric suction is controlled by applying air pressure in the pressure chamber and water pressure at the bottom of the high air entry ceramic disk the experimental results show that the contribution of suction to shear strength is significant shear strength of cdg soil increases with the increase of matric suction net normal stress has a remarkable influence on the shear strength of unsaturated cdg soil the increase in shear strength due to an increase in matric suction suction envelope is observed as nonlinear i e b value varies with matric suction no soil dilatancy is observed for zero matric suction saturated case but as the suction value is increased higher soil dilatancy is obvious in lower net normal stresses the rate of increase of soil dilatancy is greater

shear strength and dilative characteristics of an unsaturated compacted completely decomposed granite soil shear strength and dilative characteristics of a re compacted completely decomposed granite cdg soil are studied by performing a series of single stage consolidated drained direct shear tests under different matric suctions and net normal stresses the axis translation technique is applied to control the pore water and pore air pressures a soil water retention curve swrc is obtained for the cdg soil from the equilibrium water content corresponding to each applied matric suction value for zero net normal stress using a modified direct shear apparatus shear strength increases with matric suction and net normal stress and the failure envelope is observed to be linear the apparent angle of internal friction and cohesion intercept increase with matric suction a greater dilation angle is found at higher suctions with lower net normal stresses while lower or zero dilation angles are observed under higher net normal stresses with lower suctions also at a saturated condition experimental shear strength

rapid last deglacial thinning and retreat of the marine terminating southwestern greenland ice sheet abstract marine terminating outlet glaciers are a major source of modern ice loss from the greenland ice sheet gris but their role in gris retreat during the last deglaciation is not well constrained here we develop deglacial outlet glacier retreat chronologies for four regions in southwest and south greenland to improve understanding of spatial variations in centennial to millennial scale ice loss under a warming climate we calculate 10be surface exposure ages of boulders located in fjords near the towns of qaqortoq paamiut nuuk and sisimiut our northernmost study site sisimiut deglaciated earliest at 18 ka to 15 ka with an average thinning rate of 0 1 0 3 m yr 1 inland retreat from sisimiut to the modern ice margin took 7 ka at an average retreat rate of 15 20 m yr 1 a 10be dated moraine 25 km from the modern gris margin deposited at 8 ka suggests a possible ice margin still stand but this does not change overall retreat rates after retreat from the small coastal sisimiut fjords the gris margin was mainly land terminating in this region in contrast earliest exposure occurred at 12 ka near qaqortoq and 11 10 ka near nuuk and paamiut with ice thinning at rates of 0 2 0 3 m yr 1 to instantaneous within measurement uncertainty ice retreat inland through the extensive nuuk paamiut and qaqortoq fjord systems to near modern ice margins occurred in

hydrography and climate of the last 4400 years in a sw greenland fjord implications for labrador sea palaeoceanography a multiproxy record including benthic foraminifera diatoms and xrf data of a marine sedi ment core from a sw greenland fjord provides a detailed reconstruction of the oceanographic and cli matic variations of the region during the last 4400 cal years the lower part of our record represents the final termination of the holocene thermal maximum after the onset of the neoglaciation at approxi mately 3 2 ka cal bp the fjord system was subject to a number of marked hydrographical changes that were closely linked to the general climatic and oceanographic development of the labrador sea and the north atlantic region our data show that increased advection of atlantic water irminger sea water from the west greenland current into the labrador sea was a typical feature of northeast atlantic cool ing episodes such as the little ice age and the european dark ages while the advection of irminger sea water decreased significantly during warm episodes such as the mediaeval warm period and the roman warm period whereas the mediaeval warm period was characterized by relatively cool climate as suggested by low meltwater production the preceding dark ages display higher meltwater runoff and consequently warmer climate when compared with european climate these regional climate anomalies indicate persisting patterns of advection of colder respectively warmer air masses in the study region dur ing these periods and thus a long term seesaw climate pattern between west greenland and europe

shear strength characteristics of statically compacted unsaturated kaolin the shear strength characteristics of statically compacted unsaturated soil specimens of commercial kaolin ip 15 were determined using conventional triaxial shear testing apparatus extending total stress analysis the testing program consists of two different types of confined compression shear strength tests on unsaturated soil specimens in the first series of tests the specimens were sheared immediately after the application of the confining pressure under undrained loading conditions in the second series of tests the specimens were first allowed to come to equilibrium with respect to volume change and suction value under the applied confining pressure and sheared at two different rates of strain a psychrometer was used for measuring suction values in addition conventional consolidated undrained tests were also conducted on specimens that were saturated using the back pressure technique before shearing comparisons with respect to the shear strength behavior are provided for both saturated a

critical state soil constitutive model for methane hydrate soil 1 this paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate bearing soil based on the concept of critical state soil mechanics referred to as the methane hydrate critical state mhcs model methane hydrate bearing soil is under certain geological conditions known to exhibit greater stiffness strength and dilatancy which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing the proposed model represents these features by introducing five extra model parameters to the conventional critical state model it is found that for an accurate prediction of ground settlement volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation which are expected during depressurization for methane gas recovery

holocene century scale temperature variability from west greenland lake records synchronous changes in high resolution loss on ignition profiles from widely separated shallow arctic lakes in west greenland suggest that sedimentary records of lake productivity provide a sensitive century scale time series of variability in surface air temperatures throughout the holocene ice cover greatly reduces biological activity in arctic lakes and the common productivity response among lakes is attributed to air temperature controlled variations in the length of the ice cover period comparison with the greenland oxygen isotope records from summit ice cores demonstrates that the inferred temperature fluctuations in the lakes closely follow the century scale fluctuations in the greenland ice core project 18 o record but correlate less well with the greenland ice sheet project 2 record low altitude continental lake records thus not only confirm features of holocene atmospheric temperature variability otherwise seen only in high altitude ice cores from central greenland but also highlight discrepancies between the ice core time series the combined lake and ice core data provide corroborative evidence for an intrinsically unstable holocene climate in greenland including the 8200 yr b p event that is currently recognized as the most pronounced holocene climatic cooling with possible global significance our finding of a link between paleoproductivity and surface air temperature has important implications for reconstructing natural climate variability in areas where such information is still rare the results illustrate the large potential of paleolimnological studies in the arctic

rapid changes in ice discharge from greenland outlet glaciers using satellite derived surface elevation and velocity data we found major short term variations in recent ice discharge and mass loss at two of greenland s largest outlet glaciers their combined rate of mass loss doubled in less than a year in 2004 and then decreased in 2006 to near the previous rates likely as a result of fast re equilibration of calving front geometry after retreat total mass loss is a fraction of concurrent gravity derived estimates pointing to an alternative source of loss and the need for high resolution observations of outlet dynamics and glacier geometry for sea level rise predictions

an extensive and dynamic ice sheet on the west greenland shelf during the last glacial cycle considerable uncertainty surrounds the extent and timing of the advance and retreat of the greenland ice sheet gis on the continental shelf bordering baffin bay during the last glacial cycle here we use marine geophysical and geological data to show that fast flowing ice sheet outlets including the ancestral jakobshavn isbrae expanded several hundred kilometers to the shelf edge during the last glaciation ca 20 ka retreat of these outlets was asynchronous initial retreat from the shelf edge was underway by 14 880 calibrated cal yr b p in uummannaq trough radiocarbon dates from the adjacent disko trough and adjoining trough mouth fan imply later deglaciation of jakobshavn isbrae and significantly an extensive readvance and rapid retreat of this outlet during the younger dryas stadial yd this is notable because it is the first evidence of a major advance of the gis during the yd on the west greenland shelf although the short duration suggests that it may have been out of phase with yd temperatures

using proglacial threshold lakes to constrain fluctuations of the jakobshavn isbrae ice margin western greenland during the holocene abstract the future response of the greenland ice sheet gis and its potential contribution to sea level rise are uncertain rapid changes of greenland s outlet glaciers over the past decade have made it difficult to extrapolate ice sheet change into the future this significant short term variability highlights the need for longer term geologic e g holocene records of ice margin fluctuations however a major challenge with reconstructing the gis during the holocene stems from it having been smaller than it is at present thus traditional glacial geologic approaches are not suitable we use radiocarbon dated sediment sequences from seven proglacial threshold lakes spanning 50 km of the western gis margin near jakobshavn isbrae to constrain the timing of early holocene deglaciation the duration that this sector of the western gis was smaller than its present configuration and the timing of its advance during neoglaciation our reconstructions suggest deglaciation 7300 cal yr bp minimum ice extent 6000 5000 cal yr bp and smaller than present ice configuration until at least 2300 cal yr bp for the ice margin south of jakobshavn isbrae and until 400 cal yr bp for the ice margin north of jakobshavn isbrae one relatively large proglacial lake that became briefly ice free during the middle holocene lies in a catchment that likely extends 10s of km inland beneath the gis suggesting significant middle holocene retreat of this portion of the gis the overall pattern of ice sheet change is inconsistent with existing ice sheet model reconstructions for this region but is consistent with numerous paleoclimate proxy and relative sea level data these continuous lacustrine records corroborate but provide closer age control than existing non continuous records of radiocarbon dated reworked bivalves from historical moraines in the region reconstructing ice margin change from proglacial threshold lakes is one of few approaches with the potential to constrain smaller than present ice sheet extent

determination of the shear strength parameters of an unsaturated soil using the direct shear test multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till a conventional direct shear apparatus was modified in order to use the axis tra

age of the orkendalen moraines kangerlussuaq greenland constraints on the extent of the southwestern margin of the greenland ice sheet during the holocene abstract although greenland ice core records register relatively stable holocene climate conditions the lower elevation margins of the greenland ice sheet gris experienced significant holocene fluctuations these fluctuations include ice sheet recession during the holocene thermal maximum 9 5 ka and advance during the little ice age lia a d 1350 1880 determining the extent and timing of these fluctuations is important for understanding the response of the gris to interglacial climate conditions both warmer and colder than at present and for developing accurate ice sheet models sets of moraines marking past extents of the southwestern gris margin occur in the kangerlussuaq region we focus on the orkendalen moraines a prominent moraine set located within 2 km of the modern ice margin and just outboard of the lia moraines we present the first 10be ages of the orkendalen moraines indicating they were deposited at 6 8 0 3 ka the geomorphic relationship between the orkendalen and lia moraines indicates that the ice sheet margin was inboard of its orkendalen extent between 6 8 ka and the culmination of the lia the age of the orkendalen moraines provides an important constraint on the extent of the southwestern gris during the middle holocene

earliest holocene south greenland ice sheet retreat within its late holocene extent early holocene summer warmth drove dramatic greenland ice sheet gis retreat subsequent insolation driven cooling caused gis margin readvance to late holocene maxima from which ice margins are now retreating we use 10be surface exposure ages from four locations between 69 4 n and 61 2 n to date when in the early holocene south to west gis margins retreated to within these late holocene maximum extents we find that this occurred at 11 1 0 2 ka to 10 6 0 5 ka in south greenland significantly earlier than previous estimates and 6 8 0 1 ka to 7 9 0 1 ka in southwest to west greenland consistent with existing 10be ages at least in south greenland these 10be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late holocene temperatures in the early holocene current south greenland ice margin retreat suggests that south greenland may have now warmed to or above earliest holocene summer temperatures

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

reconstruction of former glacier equilibrium line altitudes based on proglacial sites an evaluation of approaches and selection of sites abstract various approaches are used to record variations in glacier activity and equilibrium line altitudes elas based on proglacial sites lacustrine and terrestrial these approaches are based on a conceptual model of glacier meltwater induced sedimentation in which the minerogenic nonorganic component of the sediments is related to the occurrence of a glacier in the catchment the principal coupling to former glacier activity and elas is common for these approaches however different methods and techniques may complement each other and both possibilities and limitations are demonstrated site selection for reconstructing variations in glacier activity elas is evaluated and critical factors are discussed rerouting of glacier meltwater streams across local watersheds in combination with proglacial sites gives a distinct on off signal for former glacier activity elas together with representative lateral moraines of known age local watersheds are important for calibrating reconstructed glacier activity elas based on a chain of proglacial lakes based on the modern analogue principle various proxies can record whenever glaciers existed in a catchment in a chain of proglacial lakes with different sensitivity to record variations in glacier activity elas these proxies can be calibrated against independent records for one site approaches however variations in glacier activity elas depend on the interpretation and sensitivity of the proxies used

restricted impact of holocene climate variations on the southern greenland ice sheet abstract we use an ice marginal threshold lake lower nordboso in southern greenland to record episodes when the southern greenland ice sheet sgis was close to the present extent represented by beds of clastic sediments whereas gyttja deposition reflects periods when the glacier was behind the threshold and no glacial meltwater input the results show that the sedimentation was initiated at c 10 6 cal ka bp when the ice margin had reached the present extent following the deglaciation after this the ice margin remained close to the present extent until c 6 9 cal ka bp when it retreated behind the threshold and first reappeared with a clear glacial signal at c 3 0 2 8 cal ka bp and from c 0 5 cal ka bp until the present an overall good correlation between clastic input into lower nordboso and ird events from narsaq sound suggests that they represent a general glacial proximity signal of the sgis our data demonstrates that the sgis behaved less vigorously to holocene climate changes compared to previous reconstructions the ice sheet only retreated behind the present ice extent from c 6 9 to 3 0 cal ka bp and c 2 8 to 0 5 cal ka bp in the mountainous southern greenland whereas the more sensitive low lying qassimiut lobe was behind its present margin from c 9 ka until the little ice age accordingly the ice marginal behavior in southern greenland is not only governed by climate changes but also strongly influenced by the local topographic setting

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

work input analysis for soils with double porosity and application to the hydromechanical modeling of unsaturated expansive clays a mechanical approach for unsaturated expansive soils considering double porosity has been developed based on the porous media theory in this approach the adsorbed and the capillary water as well as the micropores and macropores are two distinct phases an interaggregate stress considered as the work conjugate of the macrostructural strain increment has been defined both physicochemical and capillary effects of the pore water have been introduced at the macroscopic level other work conjugate variables relevant for the constitutive modeling of double porosity unsaturated media have also been identified consisting of the modified suction as conjugate of the increment of the macrostructural degree of saturation and the microstructural effective stress as conjugate of the microstructural volumetric strain increment a hydromechanical model for unsaturated expansive clays taking into account the interaction between the micro and the macrostructures in expansive clays can thus be built based on the bou

a microstructure based elastoplastic model to describe the behaviour of a compacted clayey silt in isotropic and triaxial compression the paper focuses on the hydromechanical behaviour of an unsaturated compacted clayey silt accounting for fabric changes induced by drying wetting cycles occurring at low stress levels the respon

a model of greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent an ice sheet model was constrained to reconstruct the evolution of the greenland ice sheet gris from the last glacial maximum lgm to present to improve our understanding of its response to climate change the study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea level rsl model the model reconstruction builds upon the work of simpson et al 2009 through four main extensions 1 a larger constraint database consisting of rsl and ice extent data model improvements to the 2 climate and 3 sea level forcing components 4 accounting for uncertainties in non greenland ice the research was conducted primarily to address data model misfits and to quantify inherent model uncertainties with the earth structure and non greenland ice our new model termed huy3 fits the majority of observations and is characterised by a number of defining features during the lgm the ice sheet had an excess of 4 7 m ice equivalent sea level iesl which reached a maximum volume of 5 1 m iesl at 16 5 cal ka bp modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors southwest and southeast greenland began to retreat from the continental shelf by 16 to 14 cal ka bp thus responding in part to the bolling allerod warm event c 14 5 cal ka bp subsequently ice at the southern tip of greenland readvanced during the younger dryas cold event in northern greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the younger dryas to present day conditions upon entering the holocene 11 7 cal ka bp the ice sheet soon became land based during the holocene thermal maximum htm 9 5 cal ka bp air temperatures across greenland were marginally higher than those at present and the gris margin retreated inland of its present day southwest position by 40 60 km at 4 cal ka bp which produced a deficit volume of 0 16 m iesl relative to present in response to the htm warmth our optimal model reconstruction lost mass at a maximum centennial rate of c 103 4 gt yr our results suggest that remaining data model discrepancies are affiliated with missing physics and sub grid processes of the glaciological model uncertainties in the climate forcing lateral earth structure and non greenland ice particularly the north american component finally applying the huy3 greenland reconstruction with our optimal earth model we generate present day uplift rates across greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the earth structure present day uplift rates due to past changes are spatially variable and range from 3 5 to 7 mm a including earth model uncertainty

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

preliminary 10be chronology for the last deglaciation of the western margin of the greenland ice sheet the now acknowledged thinning of the greenland ice sheet raises concerns about its potential contribution to future sea level rise in order to appreciate the full extent of its contribution to sea level rise reconstruction of the ice sheet s most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment we measured 10be concentrations in 12 samples collected along longitudinal and altitudinal transects from sisimiut to within 10 km of the isunguata sermia glacier ice margin on the western coast of greenland along the longitudinal transect we collected three perched boulders and two bedrocks in addition we sampled seven perched boulders along a vertical transect in a valley within 10 km of the isunguata sermia glacier ice margin our pilot dataset constrains the height of the ice sheet during the last glacial maximum lgm between 500 m and 840 m including the 120 m relative sea level depression at the time of the lgm 21 ka bp from the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12 3 1 5 10be ka n 2 and 8 3 1 2 10be ka n 3 to be 6 cm a 1 over this time period direct dating of the retreat of the western margin of the greenland ice sheet has the potential to better constrain the retreat rate of the ice margin the thickness of the former ice sheet as well as its response to climate change copyright 2008 john wiley sons ltd

shear strength characteristics of two saprolitic soils the saturated and unsaturated shear strength behavior of an undisturbed completely decomposed fine ash tuff and an undisturbed completely decomposed granite from hong kong were studied using direct shear and triaxial tests the completely decomposed fine ash tuff is a fine to medium grained saprolite the completely decomposed granite is a coarse grained saprolite results show that matric suction increases the shear strength of both soils the extent of the increase is the shear strength with matric suction is related to the soil water characteristic curve for the soil and to the amount of dilation during shear the effect of matric suction on the shear strength was more pronounced for the fine to medium grained completely decomposed fine ash tuff than for the coarse grained completely decomposed granite these studies on the saprolitic soils provide insight into the understanding of the shear strength of unsaturated coarse grained soils key words saprolites shear strength matric suction triaxi

ocean regulation hypothesis for glacier dynamics in southeast greenland and implications for ice sheet mass changes synchronous acceleration and thinning of southeast se greenland glaciers during the early 2000s was the main contributor that resulted in the doubling of annual discharge from the ice sheet we show that this acceleration was followed by a synchronized and widespread slowdown of the same glaciers in many cases associated with a decrease in thinning rates and we propose that ice sheet ocean interactions are the first order regional control on these recent mass changes sea surface temperature and mooring data show that the preceding dynamic thinning coincides with a brief decline in the cold east greenland coastal current egcc and east greenland current we suggest this decline was partly induced by a reduction in ice sheet runoff which allowed warm water from the irminger current to reach the se greenland coast a restrengthening of the cold waters coincides with the glaciers subsequent slowdown we argue that this warming and subsequent cooling of the coastal waters was the cause of the glaciers dynamic changes we further suggest that the restrengthening of the egcc resulted in part from cold water input by increased glacier calving during the speedup and increased ice sheet runoff we hypothesize that the main mechanism for ice sheet mass loss in se greenland is highly sensitive to ocean conditions and is likely subject to negative feedback mechanisms copyright 2010 by the american geophysical union

slow retreat of a land based sector of the west greenland ice sheet during the holocene thermal maximum evidence from threshold lakes at paakitsoq abstract records from two connected proglacial threshold lakes at paakitsoq west greenland have been analyzed in order to investigate the response of a land based ice sheet margin to holocene climate change the results are used to test whether or not the land terminating margin at paakitsoq behaved synchronously with the nearby marine terminating jakobshavn isbrae during the holocene the radiocarbon dated lake sediment cores indicate that the ice margin retreated to its present position 6 5 ka ago and thereafter maintained a relatively stable configuration similar to the present for 1000 years despite summer temperatures 2 c higher than today the lakes became non glacial after 5 4 cal ka bp when the ice margin retreated behind a drainage divide situated 1 5 km inland of the present margin by this time jakobshavn isbrae had already reached its minimum configuration the paakitsoq ice margin remained 1 5 km inland of its present margin until after 240 20 cal yr bp after this time both paakitsoq ice margin and jakobshavn isbrae reached their holocene maxima during the later stage of the little ice age our results suggest that the present ice margin position at paakitsoq is relatively stable in a warming climate but after a total retreat of 1 5 km behind the present margin position it may become marine based and more unstable due to marine melting and calving processes

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

an aerial view of 80 years of climate related glacier fluctuations in southeast greenland both marine and land terminating glaciers in southeast greenland have experienced dramatic recent retreat an 80 year record of historical aerial photographs and satellite imagery shows that many land terminating glaciers in this region retreated more rapidly in the 1930s than today whereas marine terminating glaciers have retreated faster in the 2000s