marine ice sheet stability we examine the stability of two dimensional marine ice sheets in steady state the dynamics of marine ice sheets is described by a viscous thin film model with two stefan type boundary conditions at the moving boundary or grounding line that marks the transition from grounded to floating ice one of these boundary conditions constrains ice thickness to be at a local critical value for flotation which depends on depth to bedrock at the grounding line the other condition sets ice flux as a function of ice thickness at the grounding line depending on the shape of the bedrock multiple equilibria may be possible using a linear stability analysis we confirm a long standing heuristic argument that asserts that the stability of these equilibria is determined by a simple mass balance consideration if an advance in the grounding line away from its steady state position leads to a net mass gain the steady state is unstable and stable otherwise this also confirms that grounding lines can only be stable in positions where bedrock slopes downwards sufficiently steeply

the mera site revisited ice age amazon in the light of new evidence abstract the mera site is the only fossil pollen and forest bed radiocarbon dated 33 26 ka bp in tropical lowland amazonia here i conclude from the study of the geology geomorphology and paleopedology of the mera site area as well as from new 14c and u th age determination of the organic layers that the pollen sections are not of last glacial age but of middle pleistocene age at least no evidence of last glacial age has been found for the mera fossil forest beds the data from the mera sites therefore should neither be used for climate reconstructions of the last glacial maximum nor for disputing the refugial theories

continued retreat of thwaites glacier west antarctica controlled by bed topography and ocean circulation the amundsen sea sector is experiencing the largest mass loss glacier acceleration and grounding line retreat in antarctica enhanced intrusion of circumpolar deep water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat here we investigate the dynamics and evolution of thwaites glacier with a novel fully coupled ice ocean numerical model we obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model coupled simulations over the coming decades indicate a continued mass loss at a sustained rate uncoupled simulations using a depth dependent parameterization of sub ice shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat bed topography controls the pattern of grounding line retreat while oceanic thermal forcing impacts the rate of grounding line retreat the importance of oceanic forcing increases with time as thwaites grounding line retreats farther inland

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

asynchronous behavior of outlet glaciers feeding godthabsfjord nuup kangerlua and the triggering of narsap sermia s retreat in sw greenland we thank w dryer and d podrasky for assistance with fieldwork and l kenefic for assisting with digitizing glacier front positions ch2 m hill polar services and air greenland provided logistics support the spot 5 images used for the 2008 dem were provided by the spirit program centre national d etudes spatiales france the digitalglobe worldview images used for the 2014 dem were obtained from p morin terminus positions were derived from landsat images courtesy of the u s geological survey funding was provided by the us national science foundation nsf office of polar programs opp grants nsf plr 0909552 and nsf plr 0909333 cassotto is supported by nasa under the earth and space science fellowship program grant nnx14al29h k k kjeldsen acknowledges support from the danish council research for independent research grant no dff 409000151 k kjaer is thanked for his support during the earlier phases of this study on ice weather stations are operated by geus denmark within the programme for monitoring of the greenland ice sheet promice j mortensen acknowledges support from iiknn greenland defrost project of the nordic centre of excellence program interaction between climate change and the cryosphere and the greenland ecosystem monitoring programme www g e m dk s rysgaard was funded by the canada excellence research chair programme additional funding was provided by the geophysical institute university of alaska fairbanks and greenland climate research centre scientific editor h fricker and reviewers h jiskoot and g cogley provided very constructive feedback that helped improve the paper

historical ecology in beringia the south land bridge coast at st paul island a 14 m core of lake sediments from st paul island yields a long environmental history of the south coast of the bering land bridge tritium assay demonstrates that sands in the bottom 8 m of deposit are injected with modern water suggesting that a radiocarbon anomaly is the result of modern carbon introduced in groundwater the remaining radiocarbon chronology coupled with a time stratigraphic pollen zone boundary suggests that the record penetrates to the mid wisconsin interstadial pollen percentage data picea pollen influx and pollen species lists allow reconstruction of the land bridge vegetation which was tundra without shrubs or trees with bare ground and comparable to bering land bridge tundras found further north there was no coastal mild or wet strip plant associations comparable to those of the modern aleutians or pribilofs probably did not exist along the land bridge coast and the region was probably not suitable for breeding by fur seals and other marine mammals a cold dry continental air mass system reached to the coast itself the south land bridge coast did not offer an environment to aboriginal human populations that was significantly milder than that of the land bridge plains to the north at about 11 000 yr b p the wisconsin dry climate was replaced by a regimen comparable to that at the modern tree line of the interior and this climate in turn was replaced with the modern system at about 9500 yr b p climatic change was independent of fluctuating sea level

a physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics we present results from numerical ice flow models that include calving criteria based on penetration of surface and basal crevasses which in turn is a function of longitudinal strain rates near the glacier front the position of the calving front is defined as the point where either 1 surface crevasses reach the waterline model cdw or 2 surface and basal crevasses penetrate the full thickness of the glacier model cd for comparison with previous studies results are also presented for a height above buoyancy calving model qualitatively both models cdw and cd produce similar behaviour unlike previous models for calving the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues the numerical ice flow model is applied to an idealized geometry characteristic of marine outlet glaciers results indicate that grounding line dynamics are less sensitive to basal topography than previously suggested stable grounding line positions can be obtained even on a reverse bed slope with or without floating termini the proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate in contrast to previous studies in which calving rate or position of the terminus is linked to local water depth the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for greenland marine outlet glaciers the contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change

a late glacial holocene tephrochronology for glacial lakes in southern ecuador despite the presence of numerous active volcanoes in the northern half of ecuador few if any distal tephras have been previously recognized in the southern one third of the country in this article we document the presence of thin 0 1 1 0 cm thick distal tephras comprising glass and or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in las cajas national park 2 40 3 00 s 79 00 79 25 w the lake cores contain from 5 to 7 tephras and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of 710 glass shards and 440 phenocrysts of feldspar and hornblende the loss of sodium with exposure to the electron microbeam causes a 10 7 wt 1 reduction in na content which we empirically determined and corrected for before correlating tephras among the sediment cores we use a similarity coefficient to correlate among the sediment cores pair wise comparison of all tephras generally yields an unambiguous correlation among the cores six tephras can be traced among all or most of the cores and several tephras are present in only one or two of the cores twenty six accelerator mass spectrometry 14c dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology the widespread tephras were deposited 9900 8800 7300 5300 2500 and 2200 cal yr b p the oldest tephras were deposited 15 500 and 15 100 cal yr b p but these are not found in all cores two of the tephras appear correlative with volcaniclastic strata on the flanks of volcan cotopaxi and one tephra may correlate with strata on the flanks of volcan ninahuilca both volcanoes are in central ecuador the absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions which did deposit tephras in the lakes must have been either especially voluminous or southerly winds must have prevailed at the time of the eruption or both

greenland ice sheet surface melt extent and trends 1960 2010 observed meteorological data and a high resolution 5 km model were used to simulate greenland ice sheet surface melt extent and trends before the satellite era 1960 79 and during the satellite era through 2010 the model output was compared with passive microwave satellite observations of melt extent for 1960 2010 the average simulated melt extent was 15 5 for the period 1960 72 simulated melt extent decreased by an average of 6 whereas 1973 2010 had an average increase of 13 with record melt extent in 2010 the trend in simulated melt extent since 1972 indicated that the melt extent in 2010 averaged twice that in the early 1970s the maximum and mean melt extents for 2010 were 52 9 5 10 5 km 2 and 28 5 2 10 5 km 2 respectively due to higher than average winter and summer temperatures and lower than average winter precipitation for 2010 the southwest greenland melt duration was 41 60 days longer than the 1960 2010 average while the northeast greenland melt duration was up to 20 days shorter from 1960 to 1972 the melting period with a 10 melt extent decreased by an average of 3 days a 1 after 1972 the period increased by an average of 2 days a 1 indicating an extended melting period for the ice sheet of about 70 days 40 and 30 days in spring and autumn respectively

a variational approach to ice stream flow ice sheets are susceptible to the formation of ice streams or narrow bands of fast flowing ice whose high velocities are caused by rapid sliding at the contact between ice and the underlying bed based on recent geophysical work which has shown that the sliding motion of ice streams may be described by a coulomb friction law we investigate how the location of ice streams depends on the geometry of an ice sheet and on the mechanical properties of the underlying bed more generally this problem is relevant to the flow of thin films with coulomb or solid friction laws applied at their base by analogy with friction problems in elasticity we construct a variational formulation for the free boundary between ice streams where bed failure occurs and the surrounding ice ridges where there is little or no sliding this variational problem takes the form of a non coercive variational inequality and we show that solutions exist provided a force and moment balance condition is satisfied in that case solutions are also unique except under certain specialized circumstances which are unlikely to arise for a real ice sheet further we show how the variational formulation of the ice flow problem can be exploited to calculate numerical solutions and to simulate the effect of changing ice geometry and bed friction on the location and velocities of streaming flow lastly we study the effect of ice shelf buttressing on the flow of ice streams whose spatial extent is determined by the yield stress distribution of the bed in line with previous studies of ice shelf buttressing we find that the removal of an ice shelf can cause an ice stream feeding the ice shelf to speed up considerably which underlines the important role ice shelves may play in controlling the dynamics of marine ice sheets

stress transfer during three sequential moderate earthquakes along the central calaveras fault california seismic and geodetic information on three sequential moderate earthquakes the 1979 m5 9 coyote lake the 1984 m6 2 morgan hill and the 1988 m5 1 alum rock along the central calaveras fault is used to examine how tractions on fault segments are transferred from one fault segment to the other geodetic data obtained before and after each earthquake are inverted for slip at depth the perturbed stress field induced by each earthquake is calculated using 3 d elastic dislocation theory although only long wavelength features of the slip distributions are well resolved the model suggests that the distribution of slip is nonuniform the largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter according to the model the coyote lake earthquake induced 0 1 1 bar 0 01 0 1 mpa shear stress across the northern part of the morgan hill segment and only about 0 1 bar at the hypocenter of the morgan hill mainshock a shear stress of 0 5 bar was induced by the morgan hill earthquake at the hypocenter of the alum rock mainshock the average stress drops for the coyote lake morgan hill and alum rock earthquakes are estimated to be 30 25 and 15 bars respectively thus the earthquake induced shear stress of 0 1 0 5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes however the induced shear stress can trigger the adjacent earthquakes in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer it is inferred that fault geometry may influence the local stress field thereby initiation and termination of the rupture but not the regional stress accumulation

stress heterogeneity and its impact on seismicity pattern along the equatorial bifurcation zone of the great sumatran fault indonesia abstract a stress transfer modeling revealed the stress heterogeneity and its correlation with the seismicity pattern in the bifurcation zone of the great sumatran fault gsf where the gsf is divided into two separate sub parallel segments up to 35 km apart i e angkola and barumun forming a strike slip duplex at around equator the analysis was conducted using a seismic catalog compiled from five institutions i e the indonesian agency for meteorology climatology and geophysics bmkg united states geological survey usgs harvard and global centroid moment tensor gcmt international seismological center isc and geoforschungzentrum gfz germany in the period of 1970 2017 our frequency magnitude distribution analysis indicates that this zone has a high stress regime especially at the tip of the bifurcation zone in which 20 clockwise rotation of principal stress and 20 increase in stress ratio are observed from the stress inversion consequently events with magnitude larger than 6 0 are concentrated in this area in addition our stress transfer modeling reveals the mechanism of the stress heterogeneity at the bifurcation zone and a possible interaction between segment ruptures two main internal structures perpendicular to the strike slip duplex are also observed from the displacement vector due to its high stress concentration and its close distance to populous cities of padang sidempuan bukittinggi solok and padang panjang in western sumatra the tip of gsf bifurcation poses a big potential of seismic hazard the information of the principal stress rotation and stress ratio obtained in this study can be used further to assess the potential of great ruptures and estimate the possible damage in the study area

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

transient response of idealized glaciers to climate variations the transient response of glaciers to climate variations is investigated with a novel low order model of an idealized glacier resting on uniformly inclined bedrock the model consists of two volumes representing the accumulation and ablation areas which are joined by a flux gate controlling mass flow according to the shallow ice approximation under the assumption of a constant vertical mass balance gradient a volume length scaling relation is derived which depends explicitly on mass balance gradient and bedrock slope analytic expressions for the volume and area timescales are given which are inversely proportional to the mass balance gradient and a geometric factor which is the ratio between the vertical extent of the ablation area and the ice thickness at the equilibrium line from the low order model a dynamical system in length and volume is obtained results from this system are in good agreement with solutions obtained from a transient finite element model solving the full force balance and mass conservation equations under periodic forcing there are significant deviations of the length response which show that the usual relaxation type parameterization of length change is not well suited for short term reactions switching on and off periodic climate forcing the model glaciers show surprisingly large initial and final transient responses that have not been investigated before these results are of significance for the interpretation of length and thickness changes observed on glaciers

reconstruction of the mid holocene paleoclimate of the ecuadorian andean paramo at tres lagunas ecuador previous paleoclimate studies have suggested a warm dry event during the mid holocene in paramo vegetation of the tropical andes of south america however the timing of the mid holocene warm dry event in tres lagunas ecuador remains uncertain since a previously reported bog core record characterized the warm dry event during a hiatus in sediment deposition in order to understand the timing of the warm dry event in tres lagunas in relation to regional records a lake sediment core was collected subsamples from the core were analyzed for radiocarbon dates pollen magnetic susceptibility and charcoal to reconstruct the depositional vegetational and fire history of the area a near constant sedimentation rate in the lake core indicated that the lake did not dry in contrast to the adjacent previously reported bog core increases in lower elevation pollen types suggest

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

time scale for adjustment of glaciers to changes in mass balance the length of time tm over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models we show the following tm may in general be estimated by a volume time scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states the factor f in the classical estimate of m l u where i is glacier length and u is terminus velocity has a simple geometrical interpretation ft is the ratio of thickness change averaged over the full length i to the change at the terminus although both u and f relate to dynamic processes local to the terminus zone the ratio f u and therefore tm are insensitive to details of the terminus dynamics in contrast to conclusions derived from some simplified kinematic wave models a more robust estimate of tm independent of terminus dynamics is given by tm h b where h is a thickness scale for the glacier and b is the mass balance rate negative at the terminus we suggest that tm for mountain glaciers can be substantially less than the 1o2 103 years commonly considered to be theoretically expected

interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry the relative timings of the last deglacial warming in the southern and northern hemispheres are not well constrained but are a crucial component in understanding the mechanisms of deglaciation1 a clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high resolution temperature records that can be tied to the absolute calendar timescale moreover the quantification of tropical temperatures during the last glacial cycle is controversial2 8 here we apply the alkenone method of sea surface temperature reconstruction9 10 to several high resolution sediment cores recovered from the tropical indian ocean between 20 n and 20 s the inferred initial sea surface temperature warming 15 000 calendar years ago at 20 s is in phase with northern hemisphere sea this study and air11 temperature changes but lags antarctic warming12 14 by several millennia this finding along with the results of recent modelling studies15 16 provides strong support for the idea that changes in the ocean s global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation

sustained retreat of the pine island glacier we use satellite observations to show that between 1992 and 2011 the pine island glacier hinge line retreated at a rate of 0 95 0 09 km yr 1 despite a progressive steepening and shoaling of the glacier surface and bedrock slopes respectively which ought to impede retreat the retreat has remained constant because the glacier terminus has thinned at an accelerating rate of 0 53 0 15 m yr 2 with comparable changes upstream this acceleration is consistent with an intensification of ocean driven melting in the cavity beneath the floating section of the glacier the pattern of hinge line retreat meanders and is concentrated in isolated regions until ice becomes locally buoyant because the glacier ocean system does not appear to have reached a position of relative stability the lower limit of sea level projections may be too conservative

leveraging the rapid retreat of the amundsen gulf ice stream 13 000 years ago to reveal insight into north american deglaciation we examine the influence of different deglacial histories of the north american ice saddle which connected the cordilleran ice sheet and western laurentide ice sheet on the stability of the fast flowing amundsen gulf ice stream located in the northwestern laurentide ice sheet we use a simplified marine terminating ice stream model to simulate grounding line retreat and compare our predictions to geologic evidence for the rapid collapse of the amundsen gulf ice stream at 13 ka we show that observations of ice stream retreat can be used to distinguish between past ice unloading histories and that the rapid retreat of the amundsen gulf ice stream near 13 ka is most dynamically consistent with substantial deglaciation of the north american ice saddle from 13 to 11 5 ka during the younger dryas cooling episode these results suggest that short lived changes in ice stream behavior may be used to reveal larger scale and longer duration ice sheet dynamics

response of marine terminating glaciers to forcing time scales sensitivities instabilities and stochastic dynamics recent observations indicate that many marine terminating glaciers in greenland and antarctica are currently retreating and thinning potentially due to long term trends in climate forcing in this study we describe a simple two stage model that accurately emulates the response to external forcing of marine terminating glaciers simulated in a spatially extended model the simplicity of the model permits derivation of analytical expressions describing the marine terminating glacier response to forcing we find that there are two time scales that characterize the stable glacier response to external forcing a fast time scale of decades to centuries and a slow time scale of millennia these two time scales become unstable at different thresholds of bed slope indicating that there are distinct slow and fast forms of the marine ice sheet instability we derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes the slow response rate of marine terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing finally we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes such as ice shelf calving changes the equilibrium glacier state

the response of glaciers to intrinsic climate variability observations and models of late holocene variations in the pacific northwest discriminating between glacier variations due to natural climate variability and those due to true climate change is crucial for the interpretation and attribution of past glacier changes and for the expectations of future changes we explore this issue for the well documented glaciers of mount baker in the cascades mountains of washington state usa using glacier histories glacier modeling weather data and numerical weather model output we find that natural variability alone is capable of producing kilometer scale excursions in glacier length on multi decadal and centennial timescales such changes are similar in magnitude to those attributed to a global little ice age the null hypothesis that no climate change is required to explain the glacier fluctuations in this setting cannot be rejected these results for mount baker glaciers are also consistent with an earlier study analyzing individual glaciers in scandinavia and the alps the principle that long timescale fluctuations of glacier length can be driven by short timescale fluctuations in climate reflects a robust and fundamental property of stochastically forced physical systems with memory it is very likely that this principle also applies to other alpine glaciers and that it therefore complicates interpretations of the relationship between glacier and climate history however the amplitude and timescale of the length fluctuations depends on the details of the particular glacier geometry and climatic setting and this remains largely unevaluated for most glaciers

how similar were the 1934 and 1966 parkfield earthquakes bakun and mcevilly 1984 showed that surface waves from the 1934 and 1966 parkfield earthquakes are remarkably similar this observation together with other seismic and geologic data led to the forecast of a repeat of a characteristic parkfield earthquake by 1993 in this paper we model near field geodetic measurements collected before and after the 1934 and 1966 parkfield earthquakes the slip in 1934 on the northwestern half of the rupture is poorly constrained due to the limited station coverage in this part of the rupture near the hypocenter the geodetic measurements permit the two earthquakes to have the same slip distribution on the other hand the data clearly show that the 1966 earthquake ruptured through the right step in the surface trace of the fault while the 1934 earthquake stopped northwest of the step if we assume only that the slip distribution is to some degree smooth we find that the centroid of the 1934 slip distribution could be as much as 10 km northwest of the centroid of the 1966 slip the data are consistent with but do not demand the largest slip in 1934 occurring in a region of low slip in 1966 the geodetically determined moments of the two earthquakes from smooth inversions are indistinguishable m0 4 4 0 4 1018 n m we also examine inversions in which we assume that the two earthquakes start identically and estimate how far down the fault the two earthquakes could have remained the same we find that the 1934 event could have had the same slip as in 1966 up to the fault step the 1934 earthquake must have stopped at the step whereas the 1966 event slipped southeast of the step in this quasi characteristic model m0 of the 1934 quake is only 60 of that in 1966 the difference presumably occurring as afterslip in 1966 if the 1934 geodetic moment was indeed less than in 1966 and the loading rate was constant this observation could explain the early occurrence of the 1934 earthquake

seasonal to multiyear variability of glacier surface velocity terminus position and sea ice ice melange in northwest greenland glacier ice discharge which depends on ice velocity and terminus fluctuations is a primary component of greenland ice sheet mass loss some research suggests that ice melange influences terminus calving in turn affecting glacier velocity the details and broad spatiotemporal consistency of these relationships however is undetermined focusing on 16 northwestern greenland glaciers during 2009 through summer 2014 we examined seasonal surface velocity changes glacier terminus position and sea ice and ice melange conditions for a longer term analysis we also produced extended records of four glaciers from 1999 to 2014 there is a strong correspondence between seasonal near terminus sea ice melange conditions and terminus change with rigid ice melange conditions associated with advance and open water associated with retreat extended sea ice free periods and reduced rigid melange are also linked with anomalously large terminus retreat in all but one case sustained multiyear retreat of greater than 1 km during both the 15 year and 6 year records was accompanied by interannual velocity increases seasonal velocity patterns however correspond more strongly with runoff changes than terminus behavior projections of continued warming and longer sea ice free periods around greenland indicate that notable retreat over wide areas may continue this sustained retreat likely will contribute to multiyear speedup longer melt seasons and earlier breakup of melange may also alter the timing of seasonal ice flow variability

grounding line migration from 1992 to 2011 on petermann glacier north west greenland we use satellite radar interferometry to investigate changes in the location of the petermann glacier grounding line between 1992 and 2011 the grounding line location was identified in 17 quadruple difference interferograms produced from european remote sensing ers 1 2 data the most extensive time series assembled at any ice stream to date there is close agreement 20 6 cm between vertical displacement of the floating ice shelf and relative tide amplitudes simulated by the arctic ocean dynamics based tide model 5 aodtm 5 arctic tide model over the 19 a period the groundling line position varied by 470 m on average with a maximum range of 7 0 km observed on the north east margin of the ice stream although the mean range 2 8 km and variability 320 m of the grounding line position is considerably lower if the unusually variable north east sector is not considered our observations demonstrate that large isolated movements cannot be precluded thus sparse temporal records should be analysed with care the grounding line migration observed on petermann glacier is not significantly correlated with time r 2 0 22 despite reported ice shelf thinning and episodes of large iceberg calving which suggests that unlike other ice streams on the south west margin of the greenland ice sheet petermann glacier is dynamically stable

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

increasing rates of ice mass loss from the greenland and antarctic ice sheets revealed by grace 1 we use monthly measurements of time variable gravity from the grace gravity recovery and climate experiment satellite gravity mission to determine the ice mass loss for the greenland and antarctic ice sheets during the period between april 2002 and february 2009 we find that during this time period the mass loss of the ice sheets is not a constant but accelerating with time i e that the grace observations are better represented by a quadratic trend than by a linear one implying that the ice sheets contribution to sea level becomes larger with time in greenland the mass loss increased from 137 gt yr in 2002 2003 to 286 gt yr in 2007 2009 i e an acceleration of 30 11 gt yr2 in 2002 2009 in antarctica the mass loss increased from 104 gt yr in 2002 2006 to 246 gt yr in 2006 2009 i e an acceleration of 26 14 gt yr2 in 2002 2009 the observed acceleration in ice sheet mass loss helps reconcile grace ice mass estimates obtained for different time periods

an observationally validated theory of viscous flow dynamics at the ice shelf calving front an analytical theory is developed for ice flow velocity in a boundary layer couplet at the calving front the theory has simple quantitative characteristics that relate ice front velocity to thickness strain rate and shelf width matching one set of empirically derived relationships alley and others 2008 and implying that these relationships predict ice velocity rather than calving rate the two boundary layers are where longitudinal and transverse flow fields change from the interior flow to patterns consistent with the calving front stress condition numerical simulations confirm the analytical theory the quantitative predictions of the theory have low sensitivity to unmeasured parameters and to shelf plan aspect ratio while its robustness arises from its dependence on the scale invariance of the governing equations the theory provides insights into calving the stability of ice shelf calving fronts the stability of the grounding line of laterally resisted ice streams and also suggests that the calving front is an instructive dynamical analogue to the grounding line

rapid grounding line migration induced by internal ice stream variability observations indicate that the grounding line position of west antarctica is sensitive to both forced and unforced ice stream variabilities this study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration we use a flowline ice stream model with an undrained plastic bed lateral shear stresses and a stretched grid refined in the grounding zone this model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases as in previous spatially resolved ice flow models thermal activation propagates as an activation wave this model s fine resolution of the grounding zone allows for accurate simulations of transient unforced grounding line migration upstream of the grounding zone horizontal grid spacing of 1 km is required to accurately resolve activation waves activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km yr this is followed during the active phase by retreat which then continues for the duration of the stagnant phase grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability the novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes

abrupt changes in early holocene tropical sea surface temperature derived from coral records for many high latitude regions of the globe it is now clear that the transition to modern climate following the last glacial maximum was punctuated by a number of rapid and substantial climate oscillations1 2 in contrast relatively little is known about how the tropics responded to the deglaciation because few high resolution records are available from lower latitudes corals have recently been shown to provide an important source of tropical climate records because they can be easily and accurately dated using either 14c or 230th and because past sea surface temperatures can be recovered from the sr ca ratios in coral skeletons here we use this technique to derive several early holocene sea surface temperature records from a coral drill core recovered from espiritu santo vanuatu in the tropical southwest pacific ocean these records indicate that sea surface temperatures in this region were depressed by as much as 6 5 c below modern values at 10 350 calendar years bp but rose very abruptly during the following 1 500 years this temperature increase lags the post younger dryas increase observed in a coral record from the tropical atlantic ocean3by about 3 000 years an unexpected phase shift that may ultimately shed light on the mechanisms of deglacial climate change

grounding line retreat of pope smith and kohler glaciers west antarctica measured with sentinel 1a radar interferometry data we employ sentinel 1a c band satellite radar interferometry data in terrain observation with progressive scans mode to map the grounding line and ice velocity of pope smith and kohler glaciers in west antarctica for the years 2014 2016 and compare the results with those obtained using earth remote sensing satellites ers 1 2 in 1992 1996 and 2011 we observe an ongoing rapid grounding line retreat of smith at 2 km yr 40 km since 1996 an 11 km retreat of pope 0 5 km yr and a 2 km readvance of kohler since 2011 the variability in glacier retreat is consistent with the distribution of basal slopes i e fast along retrograde beds and slow along prograde beds we find that several pinning points holding dotson and crosson ice shelves disappeared since 1996 due to ice shelf thinning which signal the ongoing weakening of these ice shelves overall the results indicate that ice shelf and glacier retreat in this sector remain unabated

the response of glaciers to climatic persistence 1 attribution of past glacier fluctuations to changes in the climate requires character2 izing glacier mass balance variability the available observational records which are 3 relatively short are consistent with random fluctuations uncorrelated in time plus 4 an anthropogenic trend however longer records of other climate variables suggest 5 that in fact there is a degree of temporal persistence associated with internal that 6 is unforced climate variability and that it varies with location and climate variable 7 therefore it is likely that climatic persistence does exist for mass balance but that 8 records are too short to confirm its presence or establish its magnitude with conven9 tional statistical tests extending previous work we explore the impact of potential 10 climatic persistence on glacier length fluctuations we use a numerical model and a 11 newly developed analytical model to establish that even a degree of persistence so 12

stochastic marine ice sheet variability it is well known that deterministic two dimensional marine ice sheets can only be stable if the grounding line is positioned at a sufficiently steep downward sloping bedrock when bedrock conditions favour instabilities multiple stable ice sheet profiles may occur here we employ continuation techniques to examine the sensitivity of a two dimensional marine ice sheet to stochastic noise representing short time scale variability either in the accumulation rate or in the sea level height we find that in unique regimes the position of the grounding line is most sensitive to noise in the accumulation rate and can explain excursions observed in field measurements in the multiple equilibrium regime there is a strong asymmetry in transition probabilities between the different ice sheet states with a strong preference to switch to the branch with a steeper bedrock slope

marine ice sheet instability amplifies and skews uncertainty in projections of future sea level rise sea level rise may accelerate significantly if marine ice sheets become unstable if such instability occurs there would be considerable uncertainty in future sea level rise projections due to imperfectly modeled ice sheet processes and unpredictable climate variability in this study we use mathematical and computational approaches to identify the ice sheet processes that drive uncertainty in sea level projections using stochastic perturbation theory from statistical physics as a tool we show mathematically that the marine ice sheet instability greatly amplifies and skews uncertainty in sea level projections with worst case scenarios of rapid sea level rise being more likely than best case scenarios of slower sea level rise we also perform large ensemble simulations with a state of the art ice sheet model of thwaites glacier a marine terminating glacier in west antarctica that is thought to be unstable these ensemble simulations indicate that the uncertainty solely related to internal climate variability can be a large fraction of the total ice loss expected from thwaites glacier we conclude that internal climate variability alone can be responsible for significant uncertainty in projections of sea level rise and that large ensembles are a necessary tool for quantifying the upper bounds of this uncertainty

marine ice sheet profiles and stability under coulomb basal conditions the behavior of marine terminating ice sheets such as the west antarctic ice sheet is of interest due to the possibility of rapid grounding line retreat and consequent catastrophic loss of ice critical to modeling this behavior is a choice of basal rheology where the most popular approach is to relate the ice sheet velocity to a power law function of basal stress recent experiments however suggest that near grounding line tills exhibit coulomb friction behavior here we address how coulomb conditions modify ice sheet profiles and stability criteria the basal rheology necessarily transitions to coulomb friction near the grounding line due to low effective stresses leading to changes in ice sheet properties within a narrow boundary layer ice sheet profiles taper off towards a flatter upper surface compared with the power law case and basal stresses vanish at the grounding line consistent with observations in the coulomb case the grounding line ice flux also depends more strongly on flotation ice thickness which implies that ice sheets are more sensitive to climate perturbations furthermore with coulomb friction the ice sheet grounds stably in shallower water than with a power law rheology this implies that smaller perturbations are required to push the grounding line into regions of negative bed slope where it would become unstable these results have important implications for ice sheet stability in a warming climate

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

greenland ice sheet response to external forcing kinematic wave modeling is used to evaluate possible responses of the greenland ice sheet to changes in its surface mass balance in the approach followed here the reference state is defined based on measured velocity and discharge flux along the central flow line of petermann glacier in the northwest and perturbations on this state are considered the results indicate that significant rates of thickness change can occur immediately after the prescribed change in surface mass balance but adjustments in flow rapidly diminish these rates to a few centimeters per year at most full adjustment of the ice sheet requires times of the order of 1000 years the instability mechanism known as the jakobshavn effect is discussed and based on observational evidence as well as results from prior modeling studies it is concluded that this is an unlikely mechanism for destabilizing major drainage basins of the greenland ice sheet

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

a macroscopic approach to glacier dynamics a simple approach to glacier dynamics is explored in which there is postulated to be a relationship between area and volume with three parameters the time for area to respond to changes in volume a thickness scale and an area characterizing the condition of the initial stale this approach gives a good fit to the measurements of cumulative balance and area on south cascade glacier from 1970 97 the area time scale is roughly 8 years the thickness scale about 123 m and the 1970 area roughly 4 larger than required for adjustment with volume combining this relationship with a version of mass continuity expressed in terms of area and volume produces a theory of glacier area and volume response to climate in which another time constant the volume time scale appears area and volume both respond like a damped spring and mass system the damping of the south cascade response is approximately critical and the volume time scale is roughly 48 vexes six times the area time scale the critically damped spring and mass analogy reproduces the time dependence predicted by the more complicated traditional theory of nyc

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

ice stream stability on a reverse bed slope marine ice streams whose beds deepen inland are thought to be inherently unstable numerical modelling of the maguerite bay ice stream retreat in west antarctica since the last glacial maximum suggests that an ice stream can stabilize on an inland sloping bed owing to increased lateral drag where the ice stream narrows marine based ice streams whose beds deepen inland are thought to be inherently unstable1 2 3 this instability is of particular concern because significant portions of the marine based west antarctic and greenland ice sheets are losing mass and their retreat could contribute significantly to future sea level rise4 5 6 7 however the present understanding of ice stream stability is limited by observational records that are too short to resolve multi decadal to millennial scale behaviour or to validate numerical models8 here we present a dynamic numerical simulation of antarctic ice stream retreat since the last glacial maximum lgm constrained by geophysical data whose behaviour is consistent with the geomorphological record we find that retreat of marguerite bay ice stream following the lgm was highly nonlinear and was interrupted by stabilizations on a reverse sloping bed where theory predicts rapid unstable retreat we demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed we conclude that as well as bed topography ice stream width and long term retreat history are crucial for understanding decadal to centennial scale ice stream behaviour and marine ice sheet vulnerability

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

local stress perturbations associated with the 2008 wenchuan m 8 0 earthquake near the longmenshan fault zone in the eastern margin of the tibetan plateau abstract the 2008 wenchuan m 8 0 earthquake induced local stress perturbations near the longmenshan lms fault zone using in situ measurements and stress tensor inversions from focal mechanisms we investigated the post earthquake perturbed stress field at shallow depths 1 2 km in the upper crust above a basal attachment 5 18 km as well as in the middle crust beneath a basal attachment 19 32 km at shallow depths in the beichuan jiangyou section the maximum principal stress 1 orientations changed from a pre earthquake orientation of nww to a post earthquake orientation of nee however near the baoxing an xian and qingchuan mianxian sections the pre earthquake 1 orientation of nw nww was largely unaltered before the mainshock in the upper crust above the basal detachment the entire lms fault zone exhibited a consistent 1 orientation of nw nww however after the earthquake some sections such as the beichuan nanba section achieved 1 orientations of nee that were distinctly different from the pre seismic stress field in the middle crust beneath the basal detachment the stress field was mostly unchanged the crust beneath the basal detachment along the eastern margin of the tibetan plateau is dominated by nww oriented maximum principle stress and strain directions and a thrust faulting stress regime along the entire lms fault zone these observations are representative of the southeastward motion of the bayan har block and its ongoing collision with the south china block along the lms thrust belt the spatially heterogeneous stress field that occurred in the upper crust 18 km after the mainshock was most likely caused by the co seismic static stress changes associated with the wenchuan earthquake prior to the onset of seismic activity the deviatoric stress magnitudes were estimated at less than 22 mpa and 4 mpa at depths of 10 km and 20 km respectively suggesting that the mainshock fault zone has a relatively low fault strength to overcome in order to initiate significant co seismic rupture and aftershocks especially in the crust beneath the basal detachment these smaller deviatoric magnitudes are indicative of the weaker fault materials that resides in the lms region

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

dynamic thinning of glaciers on the southern antarctic peninsula growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice especially if it is resting on bedrock below sea level much of the southern antarctic peninsula satisfies this condition and also possesses a bed slope that deepens inland such ice sheet geometry is potentially unstable we use satellite altimetry and gravity observations to show that a major portion of the region has since 2009 destabilized ice mass loss of the marine terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of 56 8 gigatons per year constituting a major fraction of antarctica s contribution to rising sea level the widespread simultaneous nature of the acceleration in the absence of a persistent atmospheric forcing points to an oceanic driving mechanism

effects of syn pandemic fire reduction and reforestation in the tropical americas on atmospheric co2 during european conquest abstract a new reconstruction of late holocene biomass burning in the tropical americas is consistent with the expansion of fire use by mesoamerican and amazonian agriculturalists and a subsequent period of fire reduction beginning 500 years bp the marked reduction of biomass burning after 500 years bp a unique feature of the fire history of the tropical americas relative to other regions of the globe is synchronous with the collapse of the american indigenous population during pandemics accompanying european conquest we predict that fire reduction contemporaneous with pandemics in the tropical americas was associated with massive forest regeneration on 5 105 km2 of land and sequestration of 5 10 gt c into the terrestrial biosphere which contributed to the 2 global reduction in atmospheric co2 levels and the 0 1 increase in 13c of atmospheric co2 from 1500 to 1750 a d this study 1 builds upon prior fire history reconstructions by synthesizing a substantially greater number of stratigraphic charcoal accumulation records and soil charcoal 14c dates to resolve features of the late holocene biomass burning record in the tropical americas and 2 corroborates the hypothesis advanced by ruddiman ruddiman w f 2003 the anthropogenic era began thousands of years ago climatic change 61 261 293 ruddiman w f 2005 plows plagues and petroleum princeton university press princeton new jersey that biospheric carbon sequestration via reforestation of cropland abandoned during pandemics contributed to changes in atmospheric co2 concentration during the past millennium

inland thinning on the greenland ice sheet controlled by outlet glacier geometry greenland s contribution to future sea level rise remains uncertain and a wide range of upper and lower bounds has been proposed these predictions depend strongly on how mass loss which is focused at the termini of marine terminating outlet glaciers can penetrate inland to the ice sheet interior previous studies have shown that at regional scales greenland ice sheet mass loss is correlated with atmospheric and oceanic warming however mass loss within individual outlet glacier catchments exhibits unexplained heterogeneity hindering our ability to project ice sheet response to future environmental forcing using digital elevation model differencing we spatially resolve the dynamic portion of surface elevation change from 1985 to present within 16 outlet glacier catchments in west greenland where significant heterogeneity in ice loss exists we show that the up glacier extent of thinning and thus mass loss is limited by glacier geometry we find that 94 of the total dynamic loss occurs between the terminus and the location where the down glacier advective speed of a kinematic wave of thinning is at least three times larger than its diffusive speed this empirical threshold enables the identification of glaciers that are not currently thinning but are most susceptible to future thinning in the coming decades greenland s ice loss depends on propagation of mass loss from the marine glacier termini to the interior an analysis of surface elevation change in 16 glacier catchments shows that the up glacier extent of thinning is limited by glacier geometry

sensitivity of grounding line dynamics to the choice of the friction law basal slip accounts for a large part of the flow of ice streams draining ice from antarctica and greenland into the ocean therefore an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections various friction laws have been proposed to describe basal slip in models here we compare the influence on grounding line gl dynamics of four friction laws the traditional weertman law and three effective pressure dependent laws namely the schoof tsai and budd laws it turns out that even when they are tuned to a common initial reference state the weertman budd and schoof laws lead to thoroughly different steady state positions although the schoof and tsai laws lead to much the same result in particular under certain circumstances it is possible to obtain a steady gl located on a reverse slope area using the weertman law furthermore the predicted transient evolution of the gl as well as the projected contributions to sea level rise over a 100 year time horizon vary significantly depending on the friction law we conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models

the july 1986 oceanside ml 5 3 earthquake sequence in the continental borderland southern california an earthquake of m l 5 3 occurred at 32 58 7 n 117 51 5 w southwest of oceanside in san diego county at 13 47 13 july 1986 ut this main shock was followed by an extensive aftershock sequence with 55 events of m l 3 0 during july 1986 the epicenters of the main shock and aftershocks are located at the northern end of the san diego trough bahia soledad fault zone sdt bs where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault the northwest striking sdt bs is one of three strike slip fault systems that constitute the offshore agua blanca fault system the spatial distribution of the aftershocks indicates a unilateral 7 to 9 km long rupture to the east southeast away from the epicenter of the main shock the focal mechanism of the main shock also has an east southeast striking and south dipping plane with mostly reverse movement on it focal mechanisms of the m l 3 0 aftershocks show both reverse and strike slip movement the reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the san diego trough fault as movement is transferred to the west along the santa cruz catalina island escarpment some of the aftershocks that are located to the southeast of the main shock and have strike slip focal mechanisms suggest activation of the northwest trending san diego trough fault a stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of s30 w plunging 18 the maximum principal stress determined from 30 aftershocks that occurred from 15 july to 2 october 1986 has an azimuth of s20 w plunging 18 the values the measure of the relative sizes of the principal stresses are approximately 0 07 and 0 1 respectively indicating that the intermediate and minimum principal stress are of similar magnitude the results of the stress inversion and the focal mechanisms that showed reverse faulting suggest that the inner continental borderland offshore from oceanside is not currently a pure strike slip tectonic regime but rather a strike slip mixed with reverse faulting regime when the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated the change in stress can be described as a progressive counter clockwise rotation of 14 of the orientation of the maximum principal stress to a more southerly direction with the greatest change in stress orientation observed shortly after the main shock the abundance of aftershocks may be related to the large temporal variation in stress orientation that in turn may have resulted from the small stress drop of the main shock

the statistical physics of iceberg calving and the emergence of universal calving laws determining a calving law valid for all glaciological and environmental regimes has proven to be a difficult problem in glaciology for this reason most models of the calving process are semi empirical with little connection to the underlying fracture processes in this study i introduce methods rooted in statistical physics to show how calving laws valid for any glaciological domain can emerge naturally as a large spatial scale long temporal scale limit of an underlying continuous or discrete fracture process an important element of the method developed here is that iceberg calving is treated as a stochastic process and that the probability an iceberg will detach in a given interval of time can be described by a probability distribution function using limiting assumptions about the underlying probability distribution the theory is shown to be able to simulate a range of calving styles including the sporadic detachment of large tabular icebergs from ice tongues and ice shelves and the more steady detachment of smaller sized bergs from tidewater outlet glaciers the method developed has the potential to provide a physical basis to include iceberg calving into numerical ice sheet models that can be used to produce more realistic estimates of the glaciological contribution to sea level rise

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

increased mass loss and asynchronous behavior of marine terminating outlet glaciers at upernavik isstrom nw greenland in order to model and predict future behavior of marine terminating glaciers it is essential to understand the different factors that control a glaciers response to climate change here we present a detailed study of the asynchronous changes in dynamic behavior of four adjacent marine terminating glaciers at upernavik isstrom ui northwest greenland between 1992 and 2013 velocities were stable for all outlets at ui between 1992 and 2005 the northernmost glacier started to accelerate and thin in 2006 and continued to do so into 2011 after which time the velocities stabilized the second most northerly glacier started to accelerate and thin in 2009 and continued to do so until the last observations in 2013 dramatically increasing the area affected by dynamically induced thinning the southern glaciers show little change with the most southerly glacier undergoing slight retreat and deceleration between 1992 and 2013 these observations point out the fact that the ui glaciers are reacting to climate change on different timescales the asynchronous behavior of the four neighboring glaciers is explained in terms of the individual glaciers geometry and terminus position the northernmost glacier is believed to have had a floating tongue between 1985 and 2007 which disintegrated in 2007 2008 this release of back stress destabilized the glacier causing it to accelerate and thin rapidly we suggest that the ice tongue broke up due to ocean warming induced thinning in the late 1990s recent response on ui glaciers is found to be related to increased surface melt our investigations suggest that three out of the four main glaciers in the ui are likely to be in unstable positions and may have the potential to rapidly thin and accelerate and increase their contribution to sea level in the future

ice sheet model sensitivities to environmental forcing and their use in projecting future sea level the searise project ten ice sheet models are used to study sensitivity of the greenland and antarctic ice sheets to prescribed changes of surface mass balance sub ice shelf melting and basal sliding results exhibit a large range in projected contributions to sea level change in most cases the ice volume above flotation lost is linearly dependent on the strength of the forcing combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that nonlinear feedbacks are modest our models indicate that greenland is more sensitive than antarctica to likely atmospheric changes in temperature and precipitation while antarctica is more sensitive to increased ice shelf basal melting an experiment approximating the intergovernmental panel on climate change s rcp8 5 scenario produces additional first century contributions to sea level of 22 3 and 8 1 cm from greenland and antarctica respectively with a range among models of 62 and 14 cm respectively by 200 years projections increase to 53 2 and 26 7 cm respectively with ranges of 79 and 43 cm linear interpolation of the sensitivity results closely approximates these projections revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice sheet response to complicated forcings over 200 years can be linearized

a mass flux perspective of the tidewater glacier cycle i explore the tidewater glacier cycle with a 1 d depth and width integrated flow model that includes a mass flux calving parameterization the parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity the model demonstrates variable sensitivity to climate from an advanced stable configuration a small warming of the climate triggers a rapid retreat that causes large scale drawdown and is enhanced by positive glacier dynamic feedbacks eventually the terminus retreats out of deep water and the terminus velocity decreases resulting in reduced drawdown and the potential for restabilization terminus readvance can be initiated by cooling the climate terminus advance into deep water is difficult to sustain however due to negative feedbacks between glacier dynamics and surface mass balance despite uncertainty in the precise form of the parameterization the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability it also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models

cyclic ductile and brittle deformation related to coseismic thrust fault propagation structural record at the base of a basement nappe preveli crete 1 the structural record at the base of a basement nappe preveli nappe crete greece thrust upon sedimentary rocks is investigated aimed on understanding mechanisms which result in decoupling of the thrust sheet from its original substratum we identify several superimposed deformation stages each with characteristic structural style and indications of episodic deformation at initially high differential stress the final stage involves formation of a matrix supported breccia transected by pseudotachylytes comprising the lowermost 30 m of the nappe brecciation and pseudotachylyte formation occurred in a single event and structures were not modified afterward complete induration of breccia and composition of phengite crystallized during devitrification of pseudotachylytes place the sequence of events into the middle crust we propose a model relating episodic deformation and cyclic stress history to propagation of a thrust fault in a limited number of seismic events terminal brecciation and frictional fusion record passage of the fault front beneath the site of observation and decoupling of the thrust sheet absence of discernible further deformation is consistent with negligible basal friction during transport as a nappe brecciation and pseudotachylyte formation mark the switch from a history of repeated coseismic loading and postseismic stress relaxation in the plastosphere driven by seismic events on the approaching thrust fault to passive transport with deformation localized in a weak thrust plane for a sequence of superimposed ductile to brittle structures our model provides an alternative to progressive cooling and exhumation concomitant with deformation over millions of years

a numerical model of interactions between a polar ice stream and the ocean application to ice stream e west antarctica a time dependent numerical model has been developed to study the dynamics of a polar ice stream grounded below sea level the model was applied to ice stream e one of the two most active ice streams draining the ross sea sector of the west antarctic ice sheet the retarding force or back stress from the floating ross ice shelf needed to hold the model grounding line in dynamic equilibrium was found to compare closely with retarding force mapped from field data by earlier investigators this implies that the grounding line of ice stream e is in fact close to dynamic equilibrium neither advancing nor retreating rapidly if the ice stream and its catchment area are approximately in mass balance sensitivity tests showed that the model ice stream is more sensitive to changes in back stress from the ice shelf than to changes in the accumulation rate changes in back stress can be caused by changes in the average thickness of the ice shelf the model was used to simulate retreat of the ice stream from the edge of the continental shelf in the ross sea during the holocene period of rising sea level the sea floor was assumed to be isostatically depressed but to remain rigid during retreat the effect of the assumed retreat history of the ross ice shelf on the computed timing of grounding line retreat was investigated by considering several alternative ice shelf histories the results support the conclusion of earlier investigators that the ross ice shelf caused the grounding line to stop retreating near its present position after extensive retreat during the holocene in addition the timing of retreat of the calving front of the ross ice shelf exerted major influence on the timing of retreat of the ice stream grounding line

glacial and postglacial pollen records from the ecuadorian andes and amazon abstract a long pollen record is derived from sediments of a lake dammed behind a low moraine of the last glaciation at 3 s latitude in the ecuadorian andes and is compared with a glacial age pollen record from the amazon rainforest immediately below lake surucucho llaviucu lies at 3180 m on the amazonian flank of the andes and above the glacial age pollen record from san juan bosco at 970 m the surucucho pollen record is interpreted as showing treeless vegetation in glacial times advance of treeline in late glacial time and holocene development of modern andean forests combining the surucucho and san juan bosco records shows that andean vegetation was affected by glacial cooling at all elevations vegetation did not move up and down slope as belts rather plant associations were reformed as temperature sensitive species found different centers of distribution with changing temperature

rotation of the principal stress directions due to earthquake faulting and its seismological implications earthquake faulting results in stress drop over the rupture area because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault the principal stress directions must rotate to adapt such a change of the state of stress using two constraints i e the normal stress on the fault and the vertical stress the overburden pressure which do not change before and after the earthquake we derive simple expressions for the rotation angle in the 1 axis for a dip slip earthquake the rotation angle is only a function of the stress drop ratio defined as the ratio of the stress drop to the initial shear stress and the angle between the 1 axis and the fault plane but for a strike slip earthquake the rotation angle is also a function of the stress ratio depending on the faulting regimes the 1 axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal the rotation of the stress field has several important seismological implications it may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks the rotation angle can be used to estimate the stress drop ratio which has been a long lasting topic of debate in seismology

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

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

pollen studies in the junin area central peruvian andes laguna junin is a large lake 675 km long but only 4 m deep located in the grass covered altiplano at an altitude of 4 100 m northeast of lima pollen analysis of a 30 m core indicates that from about 12 000 yr ago until some time after 3 000 yr ago about 30 of the pollen was carried in by easterly winds from the east andean forests which today are at least 600 m lower in elevation contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the junin plain was dominated by local grassland types and had little contribution from this easterly source at some time after 3 000 yr ago perhaps about 1 200 yr ago the pollen influx to laguna junin from this distant forest source greatly diminished perhaps because of deforestation by increased human populations between about 12 000 and 24 000 yr ago in the junin core the sparse pollen concentration was dominated by grassland types supplemented by polylepis acaena alnus and compositae blown in from the sub puna shrublands that may have occupied much of the east andean slopes in place of the humid forest that occurs there today the silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain especially from the cordillera to the west these glaciers left a distinctive pattern of moraines and outwash fans which served to dam tine lake basin an unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago when the lake was probably dry before this time the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation recorded on the landscape by an older set of moraines and outwash fans which initially dammed the lake basin

seasonal variability of submarine melt rate and circulation in an east greenland fjord 1 the circulation in a glacial fjord driven by a large tidewater glacier is investigated using a nonhydrostatic ocean general circulation model with a melt rate parameterization at the vertical glacier front the model configuration and water properties are based on data collected in sermilik fjord near helheim glacier a major greenland outlet glacier the approximately two layer stratification of the fjord s ambient waters causes the meltwater plume at the glacier front to drive a double cell circulation with two distinct outflows one at the free surface and one at the layers interface in summer the discharge of surface runoff at the base of the glacier subglacial discharge causes the circulation to be much more vigorous and associated with a larger melt rate than in winter the simulated double cell circulation is consistent in both seasons with observations from sermilik fjord seasonal differences are also present in the vertical structure of the melt rate which is maximum at the base of the glacier in summer and at the layers interface in winter simulated submarine melt rates are strongly sensitive to the amount of subglacial discharge to changes in water temperature and to the height of the layers they are also consistent with those inferred from simplified one dimensional models based on the theory of buoyant plumes our results also indicate that to correctly represent the dynamics of the meltwater plume care must be taken in the choice of viscosity and diffusivity values in the model

intermittent thinning of jakobshavn isbrae west greenland since the little ice age rapid thinning and velocity increase on major greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the jakobshavn effect hughes 1986 with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat to assess whether recent trends deviate from longer term behavior we measured glacier surface elevations and terminus positions for jakobshavn isbrae west greenland using historical photographs acquired in 1944 1953 1959 1964 and 1985 these results were combined with data from historical records aerial photographs ground surveys airborne laser altimetry and field mapping of lateral moraines and trimlines to reconstruct the history of changes since the little ice age lia we identified three periods of rapid thinning since the lia 1902 13 1930 59 and 1999 present during the first half of the 20th century the calving front appears to have been grounded and it started to float during the late 1940s the south and north tributaries exhibit different behavior for example the north tributary was thinning between 1959 and 1985 during a period when the calving front was stationary and the south tributary was in balance the record of intermittent thinning combined with changes in ice marginal extent and position of the calving front together with changes in velocity imply that the behavior of the lower parts of this glacier represents a complex ice dynamical response to local climate forcings and interactions with drainage from the interior

localization patterns in sandbox scale numerical experiments above a normal fault in basement abstract the finite element program elfen is used to study the effect of basement fault dip on the evolution of shear band patterns in unconsolidated sand the material properties and boundary conditions of the model were chosen to correspond to generic sandbox experiments model results reproduce the range of structural styles found in corresponding sandbox experiments with a basement fault dip of 60 and lower a graben structure is formed composed of a synthetic shear band followed by one or more antithetic shear bands with a basement fault dip of 70 and steeper a reverse precursor shear band forms first followed by a synthetic normal shear band that accommodates all further displacement the dip of the synthetic shear band is close to the basement fault dip for basement fault dips between 60 and 70 we observe a transition in localization patterns an analysis of the stress fields and velocity vectors in the model explains the first order aspects of the relationships observed we consider the observed precursor dominated and graben dominated structural domains to be important components of normal fault systems in which the first order structural style and deformation patterns are only weakly dependent on the details of the rheology of the model materials and explore the interesting problem of the change in structural style from precursor dominated to graben dominated structural domains above a normal fault in basement we find similar structural domains in sandbox experiments for the same set of boundary conditions but with slightly different material properties suggesting that the modeled patterns are robust within these two structural domains i e will occur over a range of similar material properties and boundary conditions the results of this study contribute to our ability to validate numerical models against experiments in order to finally better simulate natural systems

particle motion and the theory of charcoal analysis source area transport deposition and sampling principles from particle motion physics were applied to recurring problems of the interpretation of stratigraphic charcoal data 1 fires within catchments of lakes often produce no record in fossil charcoal curves and 2 periods characterized by no local fire e g 20th century fire suppression often display as much charcoal as times when local fire was frequent quantitative theory on source area transport deposition and sampling of charcoal shows the relationship between particle sizes counted by alternative methods of charcoal analysis pollen slides for particles 5 80 m in diameter petrographic thin sections for particles 50 10 000 m diameter and charcoal diagrams the relationship between diameter and critical and deposition velocities results in fundamental aerodynamic differences between the sizes of particles quantified by the two methods charcoal recorded on pollen slides is of a size that is difficult to lift but once entrained it remains in suspension thin section charcoal is lifted at relatively low wind velocities but it is not suspended at normal surface wind speeds thermal buoyancy during fire lofts charcoal above the forest canopy depending on particle size and wind speed pollen slide charcoal sizes are underrepresented near the fire because they remain in suspension and are preferentially exported from the burn area thin section charcoal is convected to lower heights on average and is deposited nearby following fire thin section charcoal is redistributed locally by wind and thus may enter lakes because of cohesive forces and aerodynamics more pollen slide charcoal remains on the ground and less enters lakes source areas for pollen slide charcoal are subcontinental to global and diagrams of pollen slide charcoal are biased toward nonlocal charcoal they can be used to interpret importance of fire for broad spatial and temporal scales thin section charcoal represents the catchment fire regime simulation models that generate charcoal during fire mix sediments and then sample at specified intervals indicate that 1 in the absence of sediment mixing the pollen slide method should consistently resolve individual fires that occur with an expectation of 30 50 yr 2 unless samples are continuous neither method will produce useful estimates of fire frequency and 3 even a modest amount of sediment mixing will obscure the signal

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

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line