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

folding within seconds hypervelocity impacts of cosmic projectiles larger than 200 m diameter are capable of forming complex craters on earth at these craters shock loading shock damage and excavation flow are followed by a gravity driven collapse of the deep transient cavity such impact structures are characterized by a central uplift a flat crater floor and a terraced crater rim collapse induced deformation features like folds and brittle fault zones have many similarities to tectonic structures typical deformation patterns of complex terrestrial impact craters of 5 15 km diameter are compiled and analyzed with respect to their kinematic development unlike their tectonic counterparts deformation structures are always the result of non plane strain deformation and are formed in a single event that takes place in seconds to minutes to understand the high strain rate processes the microstructure of an impact induced fold of the crooked creek impact crater 7 km diameter missouri united states is investigated in detail a period of 20 30 s at the most is determined for the collapse phase of this crater the gross plastic deformation behavior of the fold is achieved by localized brittle deformation along millimeter to centimeter spaced fault zones forming a network of veins shock damage has fractured 40 of grain boundaries the onset of collapse and associated deformation started in rocks with a reduced cohesion and is friction controlled

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

diachronous retreat of the greenland ice sheet during the last deglaciation abstract the last deglaciation is the most recent interval of large scale climate change that drove the greenland ice sheet from continental shelf to within its present extent here we use a database of 645 published 10be ages from greenland to document the spatial and temporal patterns of retreat of the greenland ice sheet during the last deglaciation following initial retreat of its marine margins most land based deglaciation occurred in greenland following the end of the younger dryas cold period 12 9 11 7 ka however deglaciation in east greenland peaked significantly earlier 13 0 11 5 ka than that in south greenland 11 0 10 ka or west greenland 10 5 7 0 ka the terrestrial deglaciation of east and south greenland coincide with adjacent ocean warming 14c ages and a recent ice sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west greenland showing deglaciation occurring later than observed in 10be ages this model data misfit likely reflects the absence of realistic ice ocean interactions we suggest that oceanic changes may have played an important role in driving the spatial temporal ice retreat pattern evident in the 10be data

an early holocene greenland whale from melville bugt greenland abstract radiocarbon age determination of a greenland whale balaena mysticetus vertebra from melville bugt in northwestern greenland yields an age of 9259 8989 cal yr bp the margin of the greenland ice sheet in melville bugt was situated behind its ad 1950 2000 position in the early holocene at a similar position to that being reached following rapid retreat in recent years such an early deglaciation of areas close to the greenland ice sheet is unusual this probably reflects the unique glaciological setting resulting from the narrow fringe of ice free islands and peninsulas and offshore waters with deep areas that characterize this part of greenland the timing of greenland ice sheet retreat to its present margin varies significantly around greenland

sea surface conditions in northernmost baffin bay during the holocene palynological evidence the analysis of cores collected in northernmost baffin bay from within the area of the north water polynya permits definition of a composite sedimentary sequence ca 12 m thick spanning the last 10 000 14 c yr with only a few discontinuities palynological analyses were performed in order to reconstruct changes in surface water conditions and biogenic production transfer functions using dinocyst assemblages were applied to estimate sea surface temperature sst and salinity as well as the seasonal duration of sea ice cover at the base of the record prior to 9300 14 c yr bp dinocysts and organic linings of benthic foraminifers are sparse indicating harsh conditions and low productivity after ca 9300 14 c yr bp the increased concentration of benthic foraminifers up to 10 3 linings cm 3 and dinocyst fluxes 10 2 10 3 cysts cm 2 yr 1 reveals high biological productivity related to open water conditions the early to middle holocene from ca 9000 to ca 3600 14 c yr bp is marked by relatively high species diversity in dinocyst assemblages and the significant occurrence of autotrophic taxa such as spiniferites elongatus pentapharsodinium dalei and impagidinium pallidum this assemblage suggests conditions at least as warm as at present from ca 6400 to ca 3600 14 c yr bp transfer functions indicate warmer conditions than at present with sst in august fluctuating up to 5 5 c after 3600 14 c yr bp the dinocyst record suggests a trend of decreasing temperature toward modern values marked by recurrent cooling events copyright 2001 john wiley sons ltd

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

aminochronology and time averaging of quaternary land snail assemblages from colluvial deposits in the madeira archipelago portugal understanding the properties of time averaging age mixing in a stratigraphic layer is essential for properly interpreting the paleofauna preserved in the geologic record this work assesses the age and quantifies the scale and structure of time averaging of land snail rich colluvial sediments from the madeira archipelago portugal by dating individual shells using amino acid racemization calibrated with graphite target and carbonate target accelerator mass spectrometry radiocarbon methods gastropod shells of actinella nitidiuscula were collected from seven sites on the volcanic islands of bugio and deserta grande desertas islands where snail shells are abundant and well preserved in quaternary colluvial deposits results show that the shells ranged in age from modern to 48 cal ka bp calibrated radiocarbon age covering the last glacial and present interglacial periods snail shells retrieved from two of the colluvial sites exhibit multimillennial age mixing 6 ka which significantly exceeds the analytical error from dating methods and calibration the observed multimillennial mixing of these assemblages should be taking into consideration in upcoming paleoenvironmental and paleoecological studies in the region the extent of age mixing may also inform about the time span of colluvial deposition which can be useful in future geomorphological studies in addition this study presents the first carbonate target radiocarbon results for land snail shells and suggests that this novel rapid and more affordable dating method offers reliable age estimates for small land snail shells younger than 20 cal ka bp

impact generated hydrothermal systems on earth and mars it has long been suggested that hydrothermal systems might have provided habitats for the origin and evolution of early life on earth and possibly other planets such as mars in this contribution we show that most impact events that result in the formation of complex impact craters i e 2 4 and 5 10 km diameter on earth and mars respectively are potentially capable of generating a hydrothermal system consideration of the impact cratering record on earth suggests that the presence of an impact crater lake is critical for determining the longevity and size of the hydrothermal system we show that there are six main locations within and around impact craters on earth where impact generated hydrothermal deposits can form 1 crater fill impact melt rocks and melt bearing breccias 2 interior of central uplifts 3 outer margin of central uplifts 4 impact ejecta deposits 5 crater rim region and 6 post impact crater lake sediments we suggest that these six locations are applicable to mars as well evidence for impact generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration depending on the setting and nature of the system a variety of hydrothermal minerals have been documented in terrestrial impact structures and these can be grouped into three broad categories 1 hydrothermally altered target rock assemblages 2 primary hydrothermal minerals precipitated from solutions and 3 secondary assemblages formed by the alteration of primary hydrothermal minerals target lithology and the origin of the hydrothermal fluids strongly influences the hydrothermal mineral assemblages formed in these post impact hydrothermal systems there is a growing body of evidence for impact generated hydrothermal activity on mars although further detailed studies using high resolution imagery and multispectral information are required such studies have only been done in detail for a handful of martian craters the best example so far is from toro crater marzo g a davila a f tornabene l l dohm j m fairen a g gross c kneissl t bishop j l roush t l mckay c p 2010 icarus 208 667 683 we also present new evidence for impact generated hydrothermal deposits within an unnamed 32 km diameter crater 350 km away from toro and within the larger holden crater synthesizing observations of impact craters on earth and mars we suggest that if there was life on mars early in its history then hydrothermal deposits associated with impact craters may provide the best and most numerous opportunities for finding preserved evidence for life on mars moreover hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long after hydrothermal activity has ceased

dating deep lake sediments by using amino acid racemization in fossil ostracodes the long term rate of racemization for amino acids preserved in fossil ostracode shells was determined by using independently dated sediment cores from five deep lakes the racemization rates for aspartic and glutamic acids in the common ostracode genus candona were calibrated for the past 100 k y providing the basis for an age equation with a realistic age uncertainty of 20 the new age equation can be applied to other hypolimnic settings because the temperature below the thermocline of deep lakes from middle and high latitude regions can reasonably be assumed to remain close to 4 c the sample size required for analysis is an order of magnitude less than for 14c dating and presents new opportunities to date deposits that are organic matter poor or those with large 14c reservoir effects

glacial history of the greenland ice sheet and a local ice cap in qaanaaq northwest greenland in this study we present new information on the glacial history of the greenland ice sheet gris and a local ice cap in qaanaaq northwest greenland we use geomorphological mapping 10be exposure dating of boulders analysis of lake cores and 14c dating of reworked marine molluscs and subfossil plants to constrain the glacial history our 14c ages of reworked marine molluscs reveal that the ice extent in the area was at or behind its present day position from 42 2 0 4 to 30 6 0 3k cal a bp after which the gris expanded to its maximum position during the last glacial maximum we find evidence of early ice retreat in the deep fjord inglefield bredning at 11 9 0 6 ka whereas the taserssuit valley was deglaciated 4 ka later at 7 8 0 1k cal a bp a proglacial lake record suggests that the local ice cap survived the holocene thermal maximum but moss kill dates reveal that it was smaller than present for a period of time before 3 3 0 1k until 0 9 0 1k cal a bp following which the ice in the area expanded towards its little ice age extent

the central pit and dome at cerealia facula bright deposit and floor deposits in occator crater ceres morphology comparisons and formation abstract the prominent bright deposit cerealia facula ceres coincides with the central depression or central pit of the recently formed 92 km wide complex crater occator the central pit is 9 10 km wide and up to 1 km deep and is partially filled with a 700 m high 2 km wide dome the upper surface of the central dome is densely fractured but the flanks are not indicating that uplift of the dome surface occurred after the bright deposit was emplaced and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion the pit is rimless except for two prominent massifs that flank it to the east and west this pit dome morphology bears a strong resemblance in morphology topography and dimensions to central pit craters observed on ganymede and callisto but is essentially absent on the similar sized ice rich moons of uranus saturn and pluto the lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of ceres suggest that temperature is a significant controlling factor in central pit formation beyond the canonical inverse gravity scaling of complex crater transitions while renewed modeling will benefit from these new constraints the mapping of central pits on ceres including occator suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on ceres lack of similar high resolution imaging on the galilean icy satellites precludes definitive evaluation of models there large volumes of lobate floor fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of occator and small outcrops of this unit are found perched high on closed topographic depressions on the occator terrace zone and other higher areas of the crater floor the swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as tycho these observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates salts and or carbonates and large quantities of fragmented debris melt of ice phases within ceres crust is also supported by independent numerical models of impact at occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright na carbonates on the surface of the central pit at occator if the deposit formed within a million years or so of impact the central dome finds analogs in salt domes on earth and pingos frost mounds on earth and mars which form by water and solute migration and freeze expansion later freezing of a central reservoir of volatiles beneath the central pit at occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome finally no changes were detected in the cerealia and vinalia facula bright deposits during the course of the prime dawn mapping mission placing additional constraints on their formation time and limits on ongoing activity

late holocene relative sea level rise and the neoglacial history of the greenland ice sheet in west greenland early and mid holocene relative sea level rsl fall was replaced by late holocene rsl rise during the neoglacial after 4 3 cal ka bp thousand calibrated years before present here we present the results of an isolation basin rsl study completed near to the coastal town of sisimiut in central west greenland rsl fell from 14 m above sea level at 5 7 cal ka bp to reach a lowstand of 4 0 m at 2 3 1 2 cal ka bp before rising by an equivalent amount to present differences in the timing and magnitude of the rsl lowstand between this and other sites in west and south greenland record the varied interplay of local and non greenland rsl processes notably the reloading of the earth s crust caused by a neoglacial expansion of the greenland ice sheet gis and the subsidence associated with the collapse of the laurentide ice sheet forebulge this means that the timing of the sea level lowstand cannot be used to infer directly when the gis advanced during the neoglacial the rise in late holocene rsl is contrary to recently reported bedrock uplift in the sisimiut area based on repeat gps surveys this indicates that a belt of peripheral subsidence around the current ice sheet margin was more extensive in the late holocene and that there has been a switch from subsidence to uplift at some point in the last thousand years or so

holocene climate change in arctic canada and greenland abstract this synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the holocene in arctic canada and greenland our synthesis includes 47 records from a recently published database of highly resolved holocene paleoclimate time series from the arctic sundqvist et al 2014 we analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records mostly from datasets that did not fit the selection criteria for the arctic holocene database combined we review evidence from a variety of proxy archives including glaciers ice cores and glacial geomorphology lake sediments peat sequences and coastal and deep marine sediments the temperature sensitive records indicate more consistent and earlier holocene warmth in the north and east and a more diffuse and later holocene thermal maximum in the south and west principal components analysis reveals two dominant holocene trends one with early holocene warmth followed by cooling in the middle holocene the other with a broader period of warmth in the middle holocene followed by cooling in the late holocene the temperature decrease from the warmest to the coolest portions of the holocene is 3 0 1 0 c on average n 11 sites the greenland ice sheet retracted to its minimum extent between 5 and 3 ka consistent with many sites from around greenland depicting a switch from warm to cool conditions around that time the spatial pattern of temperature change through the holocene was likely driven by the decrease in northern latitude summer insolation through the holocene the varied influence of waning ice sheets in the early holocene and the variable influx of atlantic water into the study region

a proposed origin for palimpsests and anomalous pit craters on ganymede and callisto palimpsests and anomalous pit craters are two classes of craters found on the icy satellites ganymede and callisto that have no obvious counterparts among the crater populations found on rocky planets the distinct characteristics of palimpsests and anomalous pit craters in morphology morphometry and population statistics which differentiate them from the more numerous normal craters and basins on the icy satellites also preclude their origin as normal craters that have nearly vanished through viscous relaxation an alternate origin is proposed palimpsests are formed when the flow of material during the modification stage of crater formation is dominated by wet fluid flow as opposed to dry granular flow which forms normal craters conditions of wet modification flow occur when the volume of melt remaining in the crater attains a volume comparable to the volume of the transient crater by use of an impact model that includes 1 plausible impact velocity distributions 2 crater scaling in ice 3 shock melting in ice 4 thermal profiles as a function of time in ganymede and 5 cratering flow field constraints conditions for wet modification flow are found to occur for sufficiently large impacts or high velocity impactors the range of normal crater palimpsest transition diameters and the morphologic characteristics inferred from the impact model are consistent with the observed characteristics of palimpsests thus normal craters are inferred to result from the majority of impactors which have impact velocities near the average of 15 km s anomalous pit craters inferred to be transitional forms from intermediate velocity impactors at about 30 km s and palimpsests by impactors travelling in excess of about 40 km s the thermal state of the crust at the time of impact is a second order effect but sufficient to account for the difference in palimpsest population between ganymede and callisto if ganymede once had a liquid mantle while callisto remained unmelted and undifferentiated or solidified prior to the formation of the presently observed crater population

a global inventory of central pit craters on the moon distribution morphology and geometry abstract the origin of central pit craters on the moon has long been an enigma and a primary reason is that their geographic distribution and morphometric characteristics were unknown we investigated a global inventory of lunar central pit craters using high resolution image and topography data obtained from the lunar reconnaissance orbiter 56 certain and 35 probable central pit craters are found on both the lunar maria and highlands the certain pit craters are 9 57 km in diameter the average diameter ratio between the central pits and their parent craters is 0 12 and the average depth diameter ratio for the central pits is 0 072 with irregular shaped rims the central pits have conical profiles and some have flat floors the central pits occur on both crater floors and central peaks the floor pits are generally larger deeper and with more irregular shape compared with summit pits both the summit and floor pit craters have formed in every lunar stratigraphic epoch from nectarian to copernican target properties of background terrains affect the morphology and size of central pits but they do not determine whether or not a central pit forms during a cratering event the lunar central pits may have formed by deformation of central peaks caused by some mechanical processes during or soon after the cratering process of their parent craters

differential melt scaling for oblique impacts on terrestrial planets abstract analytical estimates of melt volumes produced by a given projectile and contained in a given impact crater are derived as a function of impact velocity impact angle planetary gravity target and projectile densities and specific internal energy of melting applications to impact events and impact craters on the earth moon and mars are demonstrated and discussed the most probable oblique impact 45 produces 1 6 times less melt volume than a vertical impact and 1 6 and 3 7 times more melt volume than impacts with 30 and 15 trajectories respectively the melt volume for a particular crater diameter increases with planetary gravity so a crater on earth should have more melt than similar size craters on mars and the moon the melt volume for a particular projectile diameter does not depend on gravity but has a strong dependence on impact velocity so the melt generated by a given projectile on the moon is significantly larger than on mars higher surface temperatures and geothermal gradients increase melt production as do lower energies of melting collectively the results imply thinner central melt sheets and a smaller proportion of melt particles in impact breccias on the moon and mars than on earth these effects are illustrated in a comparison of the chicxulub crater on earth linked to the cretaceous tertiary mass extinction gusev crater on mars where the mars exploration rover spirit landed and tsiolkovsky crater on the moon the results are comparable to those obtained from field and spacecraft observations other analytical expressions and hydrocode simulations

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

the spatial and temporal complexity of the holocene thermal maximum the holocene thermal maximum a period of relatively warm climate between 11 000 and 5 000 years ago1 2 is most clearly recorded in the middle and high latitudes2 3 of the northern hemisphere where it is generally associated with the local orbitally forced summer insolation maximum however proxy based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions2 3 4 suggesting the involvement of extra feedbacks and forcings here we simulate the holocene thermal maximum in a coupled global ocean atmosphere vegetation model we find that before 7 000 years ago summers were substantially cooler over regions directly influenced by the presence of the laurentide ice sheet whereas other regions of the northern hemisphere were dominated by orbital forcing our simulations suggest that the cool conditions arose from a combination of the inhibition of labrador sea deep convection by the flux of meltwater from the ice sheet which weakened northward heat transport by the ocean and the high surface albedo of the ice sheet we thus conclude that interglacial climate is highly sensitive to relatively small changes in ice sheet configuration

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

balancing and restoration of piercement structures geologic insights from 3d kinematic models abstract restoration of cross sections and map surfaces of volume balanced 3d flexural slip models are presented to provide insight into the structural interpretation and validation of highly 3d structures like faulted salt domes 2d restoration results in apparently unbalanced cross sections most sections show apparent thickening on dome flanks because the section line is oblique to the dip except the radial cross section that bisects a flank horst or graben along the slip direction a section is corrected by adjusting the thickness according to the true dip as determined in map view a cross section from the flank of a 3d piercement structure is best restored by flexural slip unfolding between pin lines placed at each edge of the section parallel flexural slip unfolding is a reasonably good and practical restoration method for 3d piercement structures small gaps nearly always exist along every fault trace restored in the model this systemically distributed small inconsistency does not however indicate a problem in the interpretation these small gaps are due to the use of the same unfolding direction for every restored block insights from model dome restoration are applied to test the interpretation of natural salt dome examples from eastern texas and northern germany

a 100 yr record of ocean temperature control on the stability of jakobshavn isbrae west greenland an understanding of the interaction between ice sheet dynamics and forcing mechanisms such as oceanic and atmospheric circulation is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea level change here we report new benthic foraminiferal data from disko bugt west greenland showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier jakobshavn isbrae over the past 100 yr in particular our faunal data show that warm ocean currents entered a bay disko bugt during the retreat phases of jakobshavn isbrae from a d 1920 to 1950 and since 1998 we also show a link between west greenland ocean temperature and the atlantic multidecadal oscillation a key climate indicator in the north atlantic ocean the close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea level change

surface and sub surface multi proxy reconstruction of middle to late holocene palaeoceanographic changes in disko bugt west greenland we present new surface water proxy records of meltwater production alkenone derived relative sea surface temperature diatom alkenones and sea ice diatoms changes from the disko bugt area off central west greenland we combine these new surface water reconstructions with published proxy records benthic foraminifera bottom water proxy dinocyst assemblages surface water proxy along with atmospheric temperature from greenland ice core and greenland lake records this multi proxy approach allows us to reconstruct centennial scale middle to late holocene palaeoenvironmental evolution of disko bugt and the western greenland coastal region with more detail than previously available combining surface and bottom water proxies identifies the coupling between ocean circulation west greenland current conditions the atmosphere and the greenland ice sheet centennial to millennial scale changes in the wider north atlantic region were accompanied by variations in the west greenland current wgc during periods of relatively warm wgc increased surface air temperature over western greenland led to ice sheet retreat and significant meltwater flux in contrast during periods of cold wgc atmospheric cooling resulted in glacier advances we also identify potential linkages between the palaeoceanography of the disko bugt region and key changes in the history of human occupation cooler oceanographic conditions at 3 5 ka bp support the view that the saqqaq culture left disko bugt due to deteriorating climatic conditions the cause of the disappearance of the dorset culture is unclear but the new data presented here indicate that it may be linked to a significant increase in meltwater flux which caused cold and unstable coastal conditions at ca 2 ka bp the subsequent settlement of the norse occurred at the same time as climatic amelioration during the medieval climate anomaly and their disappearance may be related to harsher conditions at the beginning of the little ice age

varve and radiocarbon dating support the rapid advance of jakobshavn isbrae during the little ice age abstract large outlet glaciers draining the greenland ice sheet significantly influence overall ice sheet mass balance considerable short term years to decades retreat and fluctuations in velocity of jakobshavn isbrae western greenland illustrate the complex nature by which large outlet glaciers respond to climate change making predictions of future ice sheet change challenging to provide a longer term view centuries we investigate the geological record of jakobshavn isbrae change we use continuous sediment records from lakes that were influenced by the recent advance of jakobshavn isbrae which took place during the little ice age in particular we explore the use of annually laminated lake sediments varves to precisely constrain the advance of the ice margin as it approached its late holocene maximum extent we find that the ice margin advanced recently at least after 1650 to 1700 ad and more likely 1800 ad we suggest that during this period jakobshavn isbrae advanced at a rate that was similar to its historically documented average retreat since 1850 ad our results indicate that jakobshavn isbrae and presumably other large marine calving glaciers have the ability to advance quickly in response to climate forcing

holocene temperature history at the western greenland ice sheet margin reconstructed from lake sediments abstract predicting the response of the greenland ice sheet to future climate change presents a major challenge to climate science paleoclimate data from greenland can provide empirical constraints on past cryospheric responses to climate change complementing insights from contemporary observations and from modeling here we examine sedimentary records from five lakes near jakobshavn isbrae in central west greenland to investigate the timing and magnitude of major holocene climate changes for comparison with glacial geologic reconstructions from the region a primary objective of this study is to constrain the timing and magnitude of maximum warmth during the early to middle holocene positive anomaly in summer insolation temperature reconstructions from subfossil insect chironomid assemblages suggest that summer temperatures were warmer than present by at least 7 1 ka the beginning of the north lake record ka thousands of years before present and that the warmest millennia of the holocene occurred in the study area between 6 and 4 ka previous studies in the jakobshavn region have found that the local greenland ice sheet margin was most retracted behind its present position between 6 and 5 ka and here we use chironomids to estimate that local summer temperatures were 2 3 c warmer than present during that time of minimum ice sheet extent as summer insolation declined through the late holocene summer temperatures cooled and the local ice sheet margin expanded gradual insolation driven millennial scale temperature trends in the study area were punctuated by several abrupt climate changes including a major transient event recorded in all five lakes between 4 3 and 3 2 ka which overlaps in timing with abrupt climate changes previously documented around the north atlantic region and farther afield at 4 2 ka

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

impact basin relaxation on rhea and iapetus and relation to past heat flow abstract evidence for relaxation of impact crater topography has been observed on many icy satellites including those of saturn and the magnitude of relaxation can be related to past heat flow e g moore j m schenk p m bruesch l s asphaug e mckinnon w b 2004 icarus 171 421 443 dombard a j mckinnon w b 2006 j geophys res 111 e01001 http dx doi org 10 1029 2005je002445 we use new global digital elevation models of the surfaces of rhea and iapetus generated from cassini data to obtain crater depth diameter data for both satellites and topographic profiles of large basins on each in addition to the factor of three lower amplitude of global topography on rhea compared to iapetus we show that basins on iapetus 100 km in diameter show little relaxation compared to similar sized basins on rhea because of the similar gravities of rhea and iapetus we show that iapetus basin morphologies can be used to represent the initial unrelaxed morphologies of the rhea basins and we use topographic profiles taken across selected basins to model heat flow on both satellites we find that iapetus has only experienced radiogenic heat flow since formation whereas rhea must have experienced heat flow reaching a few tens of mw m 2 although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes rhea experienced a different thermal history from iapetus which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system a recent model for the formation of saturn s mid sized icy satellites interior to and including rhea charnoz s et al 2011 icarus 216 535 550 describes how rhea s orbit would have expanded outwards after its accretion from a giant primordial ring which would have instigated early heating through rapid despinning and tidal interaction with saturn and other satellites rhea s basins would therefore be required to have formed within the first few tens of myr of rhea s formation in order to relax due of this heating and if so may provide an important anchor point for saturn system chronology none of these heating mechanisms are viable for iapetus in its isolated position far from saturn and as such it has remained dynamically inert since formation confirming conclusions based on thermal modeling of iapetus interior rapid and complete relaxation and subsequent erosion by bombardment of a first generation of large basins on rhea is regarded as an explanation for the lower counts of large basins on rhea relative to iapetus and the overall lower amplitude of topography on rhea compared to iapetus

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

age of the fjord stade moraines in the disko bugt region western greenland and the 9 3 and 8 2 ka cooling events abstract retreat of the western greenland ice sheet during the early holocene was interrupted by deposition of the fjord stade moraine system the fjord stade moraine system spans several hundred kilometers of western greenland s ice free fringe and represents an important period in the western greenland ice sheet s deglaciation history but the origin and timing of moraine deposition remain uncertain here we combine new and previously published 10 be and 14 c ages from disko bugt western greenland to constrain the timing of fjord stade moraine deposition at two locations 60 km apart at jakobshavn isfjord the northern of two study sites we show that jakobshavn isbrae advanced to deposit moraines ca 9 2 and 8 2 8 0 ka in southeastern disko bugt the ice sheet deposited moraines ca 9 4 9 0 and 8 5 8 1 ka our ice margin chronology indicates that the greenland ice sheet in two distant regions responded in unison to early holocene abrupt cooling 9 3 and 8 2 ka as recorded in central greenland ice cores although the timing of fjord stade moraine deposition was synchronous in jakobshavn isfjord and southeastern disko bugt within uncertainties we suggest that jakobshavn isbrae advanced while the southeastern disko bugt ice margin experienced stillstands during the 9 3 and 8 2 ka events based on regional geomorphology and the distribution of 10 be ages at each location the contrasting style of ice margin response was likely regulated by site specific ice flow characteristics jakobshavn isbrae s high ice flux results in an amplified ice margin response to a climate perturbation both warming and cooling whereas the comparatively low flux sector of the ice sheet in southeastern disko bugt experiences a more subdued response to climate perturbations our chronology indicates that the western greenland ice sheet advanced and retreated in concert with early holocene temperature variations and the 9 3 and 8 2 ka events although brief were of sufficient duration to elicit a significant response of the western greenland ice sheet

the deglaciation and neoglaciation of upernavik isstrom greenland abstract we constrain the history of the greenland ice sheet margin during the holocene at upernavik isstrom a major ice stream in northwestern greenland radiocarbon dated sediment sequences from proglacial threshold lakes adjacent to the present ice margin constrain deglaciation of the sites to older than 9 6 0 1 ka this age of deglaciation is confirmed with 10 be ages of 9 9 0 1 ka from an island adjacent to the historical ice position the lake sediment sequences also constrain the ice margin to have been less extensive than it is today for the remainder of the holocene until 1100 to 700 yr ago when it advanced into two lake catchments the ice margin retreated back out of these lake catchments in the last decade the early holocene deglaciation in melville bugt one of few locations around greenland where a vast stretch of the current ice margin is marine based preceded deglaciation in most other parts of greenland earlier deglaciation in this ice sheet sector may have been caused by additional ablation mechanisms that apply to marine based ice margins furthermore despite ice sheet models depicting this sector of greenland as relatively stable throughout the holocene our data indicate a 20 km advance retreat cycle within the last millennium

holocene palaeoceanographic evolution off west greenland benthic foraminiferal assemblages from a core southwest of disko bugt provide a holocene perspective last 7 ka bp on ice sheet ocean interactions between the west greenland current wgc and the west greenland ice sheet changes in the fauna reveal significant variations in the water mass properties temperature and salinity of the wgc through time from 7 3 to 6 2 ka bp a relatively warm strong wgc influences ice sheet melt in disko bugt and causes enhanced meltwater production resulting in low surface water productivity the most favourable oceanographic conditions occur from 5 5 to 3 5 ka bp associated with thermal optimum like conditions encompassing minimum ice sheet extent in the disko bugt area these conditions are attributed to 1 reduced meltwater influence as the ice sheet is land based and 2 enhanced contribution of warm saline water masses from the irminger current to the wgc the transition into the late holocene last 3 5 ka bp is characterized by a cooling of oceanographic conditions caused by increased advection of cold low salinity water masses from the east greenland current a longer term late holocene cooling trend within the wgc is attributed to the onset of neoglacial cooling within the north atlantic region superimposed on this cooling trend multicentennial scale variability within the wgc matches reconstructions from a nearby coring site in disko bugt as follows 1 cooling at 2 5 ka bp linked to the 2 7 ka bp cooling event 2 a warm phase centred at 1 8 ka bp associated with the roman warm period 3 slight warming between 1 4 and 0 9 ka bp linked to the medieval climate anomaly 4 severe cooling of the wgc after 0 9 ka bp culminating at 0 3 ka bp during the little ice age we show that multicentennial scale palaeoceanography variability along the west greenland margin is driven by ocean forcing i e variations in the relative contribution of atlantic irminger current and polar east greenland current water masses to the wgc during the last 7 ka bp influencing ice sheet dynamics

10be dating reveals early middle holocene age of the drygalski moraines in central west greenland abstract we reconstruct the history of the greenland ice sheet margin on the nuussuaq peninsula in central west greenland through the holocene using lake sediment analysis and cosmogenic 10 be exposure dating of the prominent drygalski moraines erratics perched on bedrock outboard of the drygalski moraines constrain local deglaciation to 9 9 0 6 ka n 2 three drygalski moraine crests yield mean 10 be ages of 8 6 0 4 ka n 2 8 5 0 2 ka n 3 and 7 6 0 1 ka n 2 from outer to inner perched erratics between the inner two moraines average 7 8 0 1 ka n 2 and are consistent with the moraine ages sediments from a proglacial lake with a catchment area extending an estimated 2 km beneath inland of the present ice sheet terminus constrain an ice sheet minimum extent from 5 4 ka to 0 6 ka the moraine chronology paired with the lake sediment stratigraphy reveals that the ice margin likely remained within 2 km of its present position from 9 9 to 5 4 ka this unexpected early holocene stability preceded by rapid ice retreat and followed by minimum ice extent between 5 4 and 0 6 ka contrasts with many records of early holocene warmth and the northern hemisphere summer insolation maximum we suggest ice margin stability may instead be tied to adjacent ocean temperatures which reached an optimum in the middle holocene

thermal stability of ice on ceres with rough topography the dwarf planet ceres may have an ice rich crust and subsurface ice exposed by impacts or endogenic activity would be subject to sublimation we model surface and subsurface temperatures on ceres to assess lifetimes of water ice and other volatiles topographic shadowing allows a small but non negligible fraction 0 4 of ceres surface to be perennially below the 110 k criterion for 1 gyr of stability these areas are found above 60 latitude other molecules ch3oh nh3 so2 co2 may be cold trapped in smaller abundances a model for the transport gravitational escape and photoionization of h2o molecules suggests net accumulation in the cold traps buried ice is stable within a meter for 1 gyr at latitudes higher than 50 an illuminated polar cap of water ice would be stable within a few degrees of the poles only if it maintained a high albedo 0 5 at present obliquity if the obliquity exceeded 5 in the geologically recent past then a putative polar cap would have been erased at latitudes 0 30 ice is stable under solar illumination only briefly 10 100 yr unless it has high albedo and thermal inertia in which case lifetimes of 104 yr are possible finally a small hemispheric asymmetry exists due to the timing of ceres perihelion passage which would lead to a detectable enhancement of ice in the northern hemisphere if the orbital elements vary slowly relative to the ice accumulation rate our model results are potentially testable during the dawn science mission

central pit and dome craters exposing the interiors of ganymede and callisto central pit craters on the large icy satellites ganymede and callisto are an unusual crater class perhaps related to the unusual properties of water ice the detailed morphology and dimensions of pit crater features differ from those of smaller normal complex craters on these two satellites pit crater depths appear to be constant regardless of diameter rim collapse is also restricted relative to that in lunar craters or smaller ganymede craters the floors of pits in craters larger than 60 km are occupied by smooth bright domes pit to crater and dome to crater diameter ratios increase linearly with crater diameter and are indistinguishable on ganymede and callisto and with terrain type or surface longitude the floors and central structures of young bright rayed pit craters such as osiris are covered by a thin uniformly bright impact melt or frost deposit thus domes and pits form rapidly on the time scale of the impact itself rather than by long term postimpact intrusion or extrusion the bright domes in pit craters are most simply explained as the uplift and exposure of relatively ice rich material from depths of 3 5 to 5 km during impact this process is almost directly analogous to the uplift and exposure of anamolous deep seated material observed in terrestrial and lunar craters the unusual pit morphology on icy satellites may be the result of impact into crust that is mechanically much weaker at shallow depth than on rocky bodies such as the moon because crater morphology is strongly dependent on ice rock composition the similarity of pit and dome dimensions on ganymede and callisto indicates that the structure and rheology of the crusts of these bodies down to depths of 10 km are very similar and have been for several b y pit crater morphology indicates that the crusts of both satellites are probably ice rich and differentiated

widespread crater related pitted materials on mars further evidence for the role of target volatiles during the impact process abstract recently acquired high resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process a densely pitted crater related unit has been identified in images of 204 craters from the mars reconnaissance orbiter this sample of craters are nearly equally distributed between the two hemispheres spanning from 53 s to 62 n latitude they range in diameter from 1 to 150 km and are found at elevations between 5 5 to 5 2 km relative to the martian datum the pits are polygonal to quasi circular depressions that often occur in dense clusters and range in size from 10 m to as large as 3 km pit sizes scale with both the host crater s diameter and the host deposit size these pits have subtle raised rims and unlike primary and secondary impact craters they lack well defined ejecta deposits and overlapping stratigraphic relationships they also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow like deposits on crater floors behind terraces and infilling the lowest local topographic depressions atop the ejecta blanket similar to the distribution of impact melt bearing bodies on the moon based on the observations and comparisons to terrestrial and lunar analogs we conclude that the pit bearing materials are impactite deposits the presence of these deposits in older craters where preserved suggests that they have formed on mars throughout most of its geologic history thus understanding their origin may help to constrain the hydrological and climate history of mars

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

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

localized sources of water vapour on the dwarf planet 1 ceres the largest asteroid of the solar system 1 ceres has been thought to have an icy surface here it is observed to be emitting water vapour the presence of hydrated minerals on the surface of ceres the largest body in the solar system s main asteroid belt suggested that there may be water there too now infrared spectra obtained by esa s herschel space observatory provide unambiguous evidence that there is water ice at or near the surface of ceres water vapour is issuing at a rate of at least 1026 molecules per second from sources on ceres localized to mid latitude regions the water evaporation could be due to comet like sublimation or to cryo volcanism in which volcanoes erupt volatiles such as water instead of molten rocks this finding supports models that propose that the icy bodies such as comets may have migrated into the asteroid belt from beyond the notional snowline dividing the early solar system into a dry inner and icy outer regions the snowline conventionally divides solar system objects into dry bodies ranging out to the main asteroid belt and icy bodies beyond the belt models suggest that some of the icy bodies may have migrated into the asteroid belt1 recent observations indicate the presence of water ice on the surface of some asteroids2 3 4 with sublimation5 a potential reason for the dust activity observed on others hydrated minerals have been found6 7 8 on the surface of the largest object in the asteroid belt the dwarf planet 1 ceres which is thought to be differentiated into a silicate core with an icy mantle9 10 11 the presence of water vapour around ceres was suggested by a marginal detection of the photodissociation product of water hydroxyl ref 12 but could not be confirmed by later more sensitive observations13 here we report the detection of water vapour around ceres with at least 1026 molecules being produced per second originating from localized sources that seem to be linked to mid latitude regions on the surface14 15 the water evaporation could be due to comet like sublimation or to cryo volcanism in which volcanoes erupt volatiles such as water instead of molten rocks

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

amino acid ratios in reworked marine bivalve shells constrain greenland ice sheet history during the holocene reconstructions of ice sheet fluctuations during the holocene which encompassed cooler and warmer conditions than those that are captured in the historic record help to elucidate ice margin sensitivity to climate change we used amino acid geochronology to constrain the history of the western greenland ice sheet margin during intervals of relative warmth in the middle holocene we measured the extent of amino acid racemization in 251 ice sheet reworked marine bivalve shells from three locations spanning western greenland a significant relationship between shell age and the ratio of aspartic acid asp isomers asp d l was revealed using bayesian model fitting on 20 radiocarbon dated shell fragments the range of asp inferred bivalve ages at each site corresponds well with independent records of early holocene ice retreat and late holocene ice advance furthermore the frequency of asp inferred bivalve ages from the three widely separated locations is nearly identical with most ages between 5 and 3 ka coinciding with optimum oceanic conditions because ice margin changes in western greenland are tightly linked with oceanographic conditions the distribution of reworked bivalve ages provides important information about relative ice margin position during smaller than present ice sheet configurations this approach adds a new chronometer to our toolkit for constraining smaller than present ice sheet configurations and may have wide applicability around greenland

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

meltwater chemistry and solute export from a greenland ice sheet catchment watson river west greenland summary solute export from the greenland ice sheet gris to coastal regions around greenland is likely to increase in the future as a consequence of increasing icemelt production here we present hydrochemical characteristics solute and major ion exports and chemical denudation rates for 2007 2010 for the watson river sector of the gris that drains into the fjord kangerlussuaq the hydrochemistry is dominated by ca2 and hco3 with a relatively high molar k na ratio of 0 6 0 1 typical for meltwaters draining a gneissic lithology low molar ca2 na and mg2 na ratios indicate that weathering of disseminated carbonates contributes less than silicate weathering to the chemical composition the solute export varied between 33 103 2009 and 61 103 tons 2010 showing that increasing discharge leads to increasing solute export at the catchment scale deviations between ion yield estimates derived from use of discharge weighted and mean daily concentrations methods were generally less than 5 indicating that the choice of method is of less importance the chemical denudation rates ranged between 36 and 56 meq m 2 per year which are lower than previous records from glacierized catchments however when normalized by discharge the denudation rates are comparable to other arctic sites when extrapolating the results from the watson river catchment to the entire greenland for 2007 2010 the solute export from greenland meltwater varied between 7 1 106 and 7 8 106 tons whilst the major ion export was between 6 4 106 and 7 3 106 tons dissolved fe a potential biolimiting nutrient for primary productivity in the north atlantic had annual export rates from greenland between 15 103 and 52 103 tons

central pit craters on ganymede central pit craters are common on mars ganymede and callisto and thus are generally believed to require target volatiles in their formation the purpose of this study is to identify the environmental conditions under which central pit craters form on ganymede we have conducted a study of 471 central pit craters with diameters between 5 and 150 km on ganymede and compared the results to 1604 central pit craters on mars diameter range 5 160 km both floor and summit pits occur on mars whereas floor pits dominate on ganymede central peak craters are found in similar locations and diameter ranges as central pit craters on mars and overlap in location and at diameters 60 km on ganymede central pit craters show no regional variations on either ganymede or mars and are not concentrated on specific geologic units central pit craters show a range of preservation states indicating that conditions favoring central pit formation have existed since crater retaining surfaces have existed on ganymede and mars central pit craters on ganymede are generally about three times larger than those on mars probably due to gravity scaling although target characteristics and resolution also may play a role central pits tend to be larger relative to their parent crater on ganymede than on mars probably because of ganymede s purer ice crust a transition to different characteristics occurs in ganymede s icy crust at depths of 4 7 km based on the larger pit to crater diameter relationship for craters in the 70 130 km diameter range and lack of central peaks in craters larger than 60 km diameter we use our results to constrain the proposed formation models for central pits on these two bodies our results are most consistent with the melt drainage model for central pit formation

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

an assessment of evidence for pingos on mars using hirise pingos are small hills with cores of ice formed by injection and freezing of pressurized water the possibility of pingos on mars is of particular interest because of the associated implications for liquid water we have systematically searched for candidate pingos using images from the high resolution imaging science experiment hirise camera since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core we have searched for fractured features and identified a variety of mounds these features are confined to the martian mid latitudes in the bands where gullies are also most common the observed fractured mounds have a variety of morphologies and are likely of multiple origins isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes so these features should still be interpreted with caution other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures flat topped mounds in utopia have some characteristics fracture pattern and latitudinal distribution consistent with pingos but differ in other aspects such as shape and setting while we do not rule out a pingo origin we prefer an erosional model for these enigmatic features

calibrating a glaciological model of the greenland ice sheet from the last glacial maximum to present day using field observations of relative sea level and ice extent we constrain a three dimensional thermomechanical model of greenland ice sheet gris evolution from the last glacial maximum lgm 21 ka bp to the present day using primarily observations of relative sea level rsl as well as field data on past ice extent our new model huy2 fits a majority of the observations and is characterised by a number of key features i the ice sheet had an excess volume relative to present of 4 1 m ice equivalent sea level at the lgm which increased to reach a maximum value of 4 6 m at 16 5 ka bp ii retreat from the continental shelf was not continuous around the entire margin as there was a younger dryas readvance in some areas the final episode of marine retreat was rapid and relatively late c 12 ka bp leaving the ice sheet land based by 10 ka bp iii in response to the holocene thermal maximum htm the ice margin retreated behind its present day position by up to 80 km in the southwest 20 km in the south and 80 km in a small area of the northeast as a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka bp which corresponds to a deficit volume relative to present of 0 17 m ice equivalent sea level our results suggest that remaining discrepancies between the model and the observations are likely associated with non greenland ice load differences between modelled and observed present day ice elevation around the margin lateral variations in earth structure and or the pattern of ice margin retreat

modeling the morphological diversity of impact craters on icy satellites abstract impact craters on icy satellites display a wide range of morphologies some of which have no counterpart on rocky bodies numerical simulation studies have struggled to reproduce the diversity of features such as central pits and transitions in crater depth with increasing diameter observed on the icy galilean satellites the transitions in crater depth at diameters of about 26 and 150 km on ganymede and callisto have been interpreted as reflecting subsurface structure using the cth shock physics code we model the formation of craters with diameters between 400 m and about 200 km on ganymede using different subsurface temperature profiles our calculations include recent improvements in the model equation of state for h 2 o and quasi static strength parameters for ice we find that the shock induced formation of dense high pressure polymorphs ices vi and vii creates a gap in the crater excavation flow which we call discontinuous excavation for craters larger than about 20 km discontinuous excavation concentrates a hot plug of material 270 k and mostly on the melting curve in the center of the crater floor the size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters and we propose that a genetic link exists between them we also derive depth versus diameter curves for different internal temperature profiles in a 120 k isothermal crust calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior hence the depth to diameter transition at about 26 km reflects thermal weakening of ice finally simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean

assessing ice margin fluctuations on differing timescales chronological constraints from sermeq kujatdleq and nordenskiold gletscher central west greenland the observational record of ice margin position reveals asynchrony in both the timing and magnitude of greenland ice sheet gris margin fluctuations and illustrates the complex reactions of ice sheets to climatic perturbations in this study we reconstruct the timing and pattern of middle and late holocene gris margin fluctuations at two locations 190 km apart in central west greenland using radiocarbon dated sediment cores from proglacial threshold lakes our results demonstrate that deglaciation occurs at both sites during the early holocene with the ice sheet remaining in a smaller than present ice margin configuration until 500 years ago when it readvanced into lake catchments at both sites at our northern site sermeq kujatdleq the late holocene advance of the gris approached maximum position during the past 280 years with the culmination of the advance occurring at ad 1992 1994 and modern retreat was underway by ad 1998 2001 in contrast field and observational evidence suggest that the

ceres evolution and current state 1 we modeled several thermal evolution scenarios for ceres to explore the nature of large wet protoplanets and to predict current day evidence that might be found by close inspection such as by the dawn mission the density for ceres is near 2 1 suggesting a water content between 17 and 27 by mass short and long lived radioactive nuclide heating is considered even if only long lived radionuclide heating is assumed the water ice in ceres melts quickly and a water mantle forms but an approximately 10 km crust does not melt the circulating warm water would alter the silicates as heat is lost by conduction through the frozen crust water begins to freeze out at the base of the crust solid state convection begins and transports more heat as well as perhaps material dissolved or entrained in the water to or near the surface ceres water layer eventually but perhaps not entirely freezes forming a layered density structure with perhaps some liquid water remaining today our differentiated models are in agreement with the recently measured difference between the equatorial and polar radii we find that ceres existence and evolution depend critically on it containing water at formation and this depends strongly on the combination of when it accreted and the amount of 26al present in the pre ceres 1 km sized objects slightly more 26al or earlier accretion produces a dry vesta like object melting and freezing plus mineralization would lead to several dimensional changes over time creating topographic features zones of weakness and perhaps disruptions in the crust

early holocene collapse of marine based ice in northwest greenland triggered by atmospheric warming abstract knowledge about the future response of the greenland ice sheet to global climate change including ice sheet contributions to sea level rise is important for understanding the impact of climate change on society such studies rely in ice sheet model predictions and improved chronological constraints of past ice sheet extents and paleoclimatic trends many regions in greenland are well studied but northwest greenland and especially melville bay being one of the most important regions in terms of dynamical ice mass loss lack a firm chronology of holocene ice marginal fluctuations in this study we present the first comprehensive chronology for melville bay spanning 73 1 75 7 n based on 36 new 10be exposure ages of boulders and 39 new radiocarbon ages of marine molluscs in little ice age moraines from weighted mean 10be exposure ages excluding 6 outliers we find that the outer coast in melville bay was deglaciated 11 6 0 3 ka n 15 and the ice margin reached its present day position 40 km farther inland 11 5 0 3 ka n 15 our results suggest an interval of rapid ice marginal retreat i e collapse of the northwest gris in melville bay most likely triggered by rapidly rising atmospheric temperatures in early holocene additionally combining the comprehensive dataset of new radiocarbon ages with 26 radiocarbon ages from previous studies shows a restricted ice sheet extent from 9 1 0 2 to 0 4 0 1 cal ka bp which coincides with increased sea surface temperatures our results highlight past ice sheet sensitivity towards climate changes in one of the least explored and most vulnerable regions of greenland furthermore comparing our new results to already existing ice sheet models huy3 and huy3b emphasize the proximal relevance of the agassiz ice core temperature reconstruction for melville bay which indicates the possible sensitivity of the ice sheet to a warming climate and place improved constraints on ice sheet simulations

enhanced age resolution of the marine quaternary record in the arctic using aspartic acid racemization dating of bivalve shells abstract aspartic acid asp racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better temporal resolution of arctic marine deposits from alaska spitsbergen and baffin island heating experiments at 100 c on the bivalvesmyaandhiatellashow the asp racemization rate decreases with increasing d l values as is typical for biogenic carbonates based on these experimental racemization rates and rates determined from racemization of samples radiocarbon dated to ca 10 000 12 000 yr b p activation energies formyaandhiatellaare estimated to be 30 6 and 30 0 kcal mol respectively for asp racemization and 29 0 and 29 5 for isoleucine epimerization analysis of a time series of plio pleistocenehiatellafrom the north coast of alaska shows that last interglacial mollusks can be readily distinguished from modern samples by asp but not by isoleucine d l asp values indicate a younger age for the fishcreekian transgression than does isoleucine epimerization for spitsbergen d l asp shows a slight age difference ca 12 000 yr between two units of the episode b interstadial and suggests that the age of these units may be closer to 65 000 than to 80 000 yr b p two possible ages suggested by other evidence the age of the loks land interstadial on baffin island is likely to be greater than that indicated by radiocarbon ages within deposits from each region d l asp values are less variable among individual shells than isoleucine epimerization values this may indicate better reliability of asp for geochronology

the theoretical plausibility of central pit crater formation via melt drainage central pit craters are seen in large craters on some icy satellites and on mars we investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater for this process to occur the volume of melt generated during the impact the volume of void space in fractures beneath the impact crater and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits we estimate the volume of melt generated using results from previous numerical modeling studies the fracture volume is estimated using gravity anomalies over terrestrial craters to estimate the amount of melt able to drain before freezing we consider flow through plane parallel fractures these calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on ganymede to form a central pit on earth and the moon silicate impact melt will freeze before a large volume is able to drain so we do not expect to see central pits in impact craters in targets with no ice in summary we find our calculations are consistent with observed central pits in craters on ganymede and the lack of central pits in craters on earth and the moon

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

evidence for an explosive origin of central pit craters on mars abstract kilometer scale pits are nested in the centers of many impact craters on mars as well as on icy satellites they have been inferred to form in the presence of a water ice rich substrate however the process es responsible for their formation is still debated previous models invoke origins by either explosive excavation of potentially water bearing crustal material or by subsurface drainage of meltwater and or collapse if explosive excavation forms central pits pit derived ejecta should be draped around the pits whereas internal collapse should not deposit significant material outside pit rims using visible wavelength images from the mars reconnaissance orbiter mro context camera ctx and high resolution imaging science experiment hirise instruments and thermal infrared images from the odyssey thermal emission imaging system themis instrument we conducted a survey to characterize in detail the global population of central pits in impact craters 10 km in diameter we specifically examined the morphology and thermophysical characteristics of the pits for evidence of pit ejecta our analysis of thermal images suggests that coarse grained materials are distributed proximally around many central pits on the floors of their host craters the decrease in average grain size with distance from pit rims is consistent with pit derived ejecta these observations and interpretations better support an explosive origin for central pits on mars than they do an origin of subsurface meltwater drainage and collapse of the overlying substrate a major weakness to previous explosive central pit formation models is the inability for them to form pits late enough in the impact process to be preserved to address this we present an alternative melt contact model where a central uplift brings ice bearing substrate into contact with impact melt to generate steam explosions and excavate central pits during the impact modification stage theoretical calculations show that more than enough thermal energy is available via impact melt from the host crater to form central pits by steam explosions and such explosions would require only a modest amount 2 6 by volume of uplifted water ice we therefore propose that central pits on mars could have formed explosively by the interaction of impact melt and subsurface water ice

sr nd pb isotope evidence for ice sheet presence on southern greenland during the last interglacial to ascertain the response of the southern greenland ice sheet gis to a boreal summer climate warmer than at present we explored whether southern greenland was deglaciated during the last interglacial lig using the sr nd pb isotope ratios of silt sized sediment discharged from southern greenland our isotope data indicate that no single southern greenland geologic terrane was completely deglaciated during the lig similar to the holocene differences in sediment sources during the lig relative to the early holocene denote however greater southern gis retreat during the lig these results allow the evaluation of a suite of gis models and are consistent with a gis contribution of 1 6 to 2 2 meters to the 4 meter lig sea level highstand requiring a significant sea level contribution from the antarctic ice sheet