first results of biostratigraphical investigations of lago d averno near naples relating to the period 800 bc 800 ad abstract remains of diatoms molluscs ostracods foraminifera and pollen exines preserved in the sediments of lago d averno a volcanic lake in the phlegrean fields west of naples allowed us to reconstruct the changes in the ecological conditions of the lake and of the vegetation around it for the period from 800 bc to 800 ad lago d averno was at first a freshwater lake temporarily influenced by volcanic springs salinity increased slowly during greek times as a result of subsidence of the surrounding land saline conditions developed only after the lake was connected with the sea by a canal when portus julius was built in 37 bc the first post roman period of uplift ended with a short freshwater phase during the 7th century after christ deciduous oakwoods around the lake was transformed into a forest of evergreen oaks in greek times and thrived there apparently almost uninfluenced by man until it was felled when the avernus was incorporated into the new roman harbour in 37 bc to construct a shipyard and other military buildings there land use was never more intense than during roman times and weakest in greek and early roman times when the avernus was considered a holy place the entrance to the underworld

lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

icesat s new perspective on ice shelf rifts the vertical dimension 1 the small footprint 70 m and 172 m along track spacing of the geoscience laser altimeter system glas on the ice cloud and land elevation satellite icesat provides unprecedented horizontal resolution for a satellite altimeter this enables icesat to map many previously unresolved features on ice shelves such as crevasses rifts grounding zones and ice fronts we present examples of icesat derived elevation data showing topography over rifts on the amery and ross ice shelves widths of rifts and as estimates of the thickness of melange a collection of ice and snow trapped inside the rifts we show that melange thickness remains constant over the icesat data period and tends to be thicker in older rifts we validate the icesat derived melange depth estimate with an in situ measurement on the ross ice shelf

transoceanic wave propagation links iceberg calving margins of antarctica with storms in tropics and northern hemisphere 1 we deployed seismometers on the ross ice shelf and on various icebergs adrift in the ross sea including b15a a large 100 km by 30 km fragment of b15 which calved from the ross ice shelf in march 2000 the data reveal that the dominant energy of these floating ice masses is in the 0 01 to 0 1 hz band and is associated with sea swell generated in the tropical and extra tropical pacific ocean in one example a strong storm in the gulf of alaska on 21 october 2005 approximately 13 500 km from the ross sea generated swell that arrived at b15a immediately prior to and during its break up off cape adare on 27 october 2005 if sea swell influences iceberg calving and break up a teleconnection exists between the antarctic ice sheet mass balance and weather systems worldwide

a model of marine ice formation within antarctic ice shelf rifts large rifts that open in antarctic ice shelves are known to be filled by snow accumulations ice shelf fragments and sea ice this work demonstrates how these rifts may also be filled from below through their interaction with ocean water by marine ice the model presented here quantifies both the rate of marine ice accumulation at the top of the rift which results from melt driven convection at its sides and the impact of this process on waters occupying the confined environment of the rift the results show that such a system could fill the rift with tens of meters of marine ice in the process the temperature and salinity of the ambient water in the rift evolve through two distinct phases the first is characterized by rapid change that takes the properties of the ambient water to near equilibrium values followed by a second phase of much slower change the melting rate at the lower part of the walls of the rift emerges as the principal factor influencing many aspects of model behavior testing the model against field observations from an antarctic rift showed it to be robust and successful in reproducing the main observed features including the presence of a thick layer of supercooled ocean water inside the rift

geochemical mineralogical and isotopic data relating to the origin and tectonic setting of the rossland volcanic rocks southern british columbia bulk rock and mineral chemical and isotopic analyses of rossland volcanic rocks are used to infer the nature of the magma extruded in the nelson rossland area of southern british columbia during th

nd sr isotopic constraints on the interactions of the intermontane superterrane with the western edge of north america in the southern canadian cordillera sr and nd isotopic compositions of the late paleozoic metavolcanics and late triassic to early tertiary granitoids from four magmatic episodes in the southern canadian cordillera from the kootenay arc to the fraser fault have been used to i identify the sources of these rocks ii constrain the compressive tectonic history from middle jurassic to paleocene and iii constrain the western boundary of the basement in this region the 215 190 ma old primitive granitoids end 3 1 to 8 7 87sr 86sr 0 7028 0 7043 of the late triassic and early jurassic magmatic episode were emplaced in the paleozoic oceanic crust of quesnellia end 2 9 to 9 3 prior to its obduction over the basement in contrast during the younger magmatic episodes middle late jurassic cretaceous and early tertiary the granitoids from western quesnellia show primitive isotopic compositions and those from eastern quesnellia show eastward increasing crust contaminated compositions the contaminated characters of the middle

episodic propagation of a rift on the amery ice shelf east antarctica 1 we investigate ice shelf rift propagation using a combination of gps and seismic measurements near the tip of an active rift in the amery ice shelf these measurements reveal that propagation occurs in episodic bursts which were identified based on swarms of seismicity accompanied by rapid rift widening the bursts last approximately 4 hours and are separated by 10 24 days in between bursts the rift widens at a rate comparable to that of ice shelf spreading comparison of automatic weather station data and tidal amplitudes show that the propagation bursts are not directly triggered by winds or tides suggesting that rift propagation is driven by the background glaciological stress in the ice shelf we show that the ice debris that partly fills the rift may play a role in controlling the rate of propagation

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

basal mass budget of ross and filchner ronne ice shelves antarctica derived from lagrangian analysis of icesat altimetry traditional methods of deriving temporal variability of antarctic ice shelf elevation from satellite altimetry use a fixed eulerian reference frame where the measured changes include advection of ice thickness gradients between measurement epochs we present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving lagrangian reference frame applying the technique to icesat laser altimetry for the period 2003 2009 over the two largest antarctic ice shelves ross and filchner ronne we show that the lagrangian approach reduces the variability of derived elevation changes by about 50 compared to the eulerian approach and reveals clearer spatial patterns of elevation change the method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice shelf elevation changes we use field data and icesat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content and remove the effect of ice flow divergence using surface velocity and ice thickness data the results show highest basal melt rates 5 m a 1 near the deep grounding lines of major ice streams but smaller melt rates 5 m a 1 near the ice shelf fronts are equally important to total meltwater production since they occur over larger areas integrating over the entire ice shelf areas we obtain basal mass budgets of 50 64 gt a 1 for ross and 124 66 gt a 1 for filchner ronne with changes in firn air content as the largest error source

basal crevasses on the larsen c ice shelf antarctica implications for meltwater ponding and hydrofracture a key mechanism for the rapid collapse of both the larsen a and b ice shelves was meltwater driven crevasse propagation basal crevasses large scale structural features within ice shelves may have contributed to this mechanism in three important ways i the shelf surface deforms due to modified buoyancy and gravitational forces above the basal crevasse creating 10 m deep compressional surface depressions where meltwater can collect ii bending stresses from the modified shape drive surface crevassing with crevasses reaching 40 m in width on the flanks of the basal crevasse induced trough and iii the ice thickness is substantially reduced thereby minimizing the propagation distance before a full thickness rift is created we examine a basal crevasse 4 5 km in length 230 m in height and the corresponding surface features in the cabinet inlet sector of the larsen c ice shelf using a combination of high resolution 0 5 m satellite imagery kinematic gps and in situ ground penetrating radar we discuss how basal crevasses may have contributed to the breakup of the larsen b ice shelf by directly controlling the location of meltwater ponding and highlight the presence of similar features on the amery and getz ice shelves with high resolution imagery 2012 american geophysical union all rights reserved

three dimensional crustal structure of the eastern cordillera southwestern canada and northwestern united states lithoprobe and cocorp crustal seismic reflection profiles across the southwestern foreland and omineca belts of southern british columbia and the adjacent united states provide regional three dimensional coverage of orogen parallel structural variations in this area north american cratonic basement and cover rocks are deformed into east verging folds and thrusts which increase in scale and depth to the west this simple pattern may be complicated by out of sequence faulting in the upper and middle crust of the kootenay arc and purcell anticlinorium in southern british columbia the southern omineca belt is the structural and metamorphic hinterland of the foreland belt and contains the monashee priest river crustal scale tectonic ramp which records a protracted history of east directed contraction it is overlain by the eagle river vernon colville crustal scale antiform of probable late cretaceous to paleocene age which locally may have formed above blind thrusts an embayment in the autochthonous craton between the monashee and priest river ramps corresponds spatially with the kootenay arc structural salient and eastward deflection of the lower paleozoic shelf to basin facies transition the south flank of the monashee and valhalla complexes and the north flank of the purcell anticlinorium near the international border correspond with the lateral margins of this east trending embayment on an even larger scale the increasing northward tectonic overlap of accreted terranes onto thick cratonic crust from the northern united states into british columbia and associated basement thickening coincide spatially with the northward increased width and depth of the early tertiary foreland basin in this part of the cordillera eocene crustal extension did not completely mask older contractional fabrics and appears to have been accomplished by two mechanisms 1 in the east by crust penetrating shear zones that separate the omineca and foreland belts and 2 in the west by crustal scale boudinage within and west of the omineca belt

crevasse propagation on brittle ice application to cycloids on europa existing lineaments on the surface of the jovian moon europa are thought to be the result of ongoing brittle crack formation in the elastic regime arcuate features are called cycloids and can be modeled using linear elastic fracture mechanics here we build on existing terrestrial models of rift propagation and extend them to cycloids on the moon we propose that these cracks tend to grow as a series of nearly instantaneous events spaced by periods of inactivity the behavior is similar to what is observed on antarctic ice shelves where rifts can remain dormant for years we argue that dormant periods between growth events could explain the presence of cycloids on europa even without invoking secular motion of the crust furthermore being able to model propagation events and their timing should help future missions exploring the moon

seismicity within a propagating ice shelf rift the relationship between icequake locations and ice shelf structure iceberg calving is a dominant mass loss mechanism for antarctic ice shelves second only to basal melting an important process involved in calving is the initiation and propagation of through penetrating fractures called rifts however the mechanisms controlling rift propagation remain poorly understood to investigate the mechanics of ice shelf rifting we analyzed seismicity associated with a propagating rift tip on the amery ice shelf using data collected during the austral summers of 2004 2007 we apply a suite of passive seismological techniques including icequake locations back projection and moment tensor inversion we confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h even during periods of quiescence we find significant deformation around the rift tip moment tensors calculated for a subset of the largest icequakes mw 2 0 located near the rift tip show steeply dipping fault planes horizontal or shallowly plunging stress orientations and often have a significant volumetric component they also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf this suggests a complex system of deformation that involves the propagating rift the region behind the rift tip and a system of rift transverse crevasses small scale variations in the mechanical structure of the ice shelf especially rift transverse crevasses and accreted marine ice play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts

the nicola group late triassic and early jurassic subduction related volcanism in british columbia mafic lavas of the nicola group are divided into three distinct petrographic and geochemical types type 1 lavas are strongly augite porphyritic picrites basalts and andesites that belong to a high potassium to shoshonitic rock series type 2 lavas are augite and plagioclase porphyritic basalts and andesites that belong to a low potassium calc alkaline series and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites low concentrations of ti zr y and nb and moderate to high concentrations of k rb ba and sr in type 1 and 2 lavas clearly indicate a subduction related tectonic setting of eruption type 3 lavas show chemical affinities intermediate between modern day island arc and intraplate volcanics type 1 shoshonitic lavas generally lie east of and are younger than type 2 calc alkaline lavas a relationship that implies an east dipping early mesozoic subduction zone beneath the nicola arc these interpretations resolve previous uncertainties regarding th

how iceberg calving and grounding change the circulation and hydrography in the filchner ice shelf ocean system the formation of bottom water in the southern weddell sea is strongly influenced by the flow of ice shelf water isw out of the filchner ronne ice shelf cavity the breakout of three giant icebergs in 1986 and their grounding on the shallow berkner bank modified the circulation and water mass formation in the filchner trough and the adjacent sea areas hydrographic measurements along the filchner ice shelf front carried out with rv polarstern in 1995 show significant changes in the water mass characteristics and flow patterns in the filchner trough in comparison to measurements from the early 1980s changes in the trough will affect the flow over the sill to the deep weddell abyssal plain we combine a three dimensional ocean circulation model with conductivity temperature depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the filchner ice shelf we assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties results indicate variations of the flow pattern in the filchner trough and on berkner bank where high salinity shelf water the feedstock for isw is produced the calving and grounding impacts illustrate the sensitivity of the ice shelf ocean system to perturbations in local bathymetric settings

calving and rifting on the mcmurdo ice shelf antarctica on 2 march 2016 several small en echelon tabular icebergs calved from the seaward front of the mcmurdo ice shelf and a previously inactive rift widened and propagated by 3 km 25 of its previous length setting the stage for the future calving of a 14 km2 iceberg within 24 h of these events all remaining land fast sea ice that had been stabilizing the ice shelf broke up the events were witnessed by time lapse cameras at nearby scott base and put into context using nearby seismic and automatic weather station data satellite imagery and subsequent ground observation although the exact trigger of calving and rifting cannot be identified definitively seismic records reveal superimposed sets of both long period 10 s sea swell propagating into mcmurdo sound from storm sources beyond antarctica and high energy locally sourced short period 10 s sea swell in the 4 days before the fast ice break up and associated ice shelf calving and rifting this suggests that sea swell should be studied further as a proximal cause of ice shelf calving and rifting if proven it suggests that ice shelf stability is tele connected with far field storm conditions at lower latitudes adding a global dimension to the physics of ice shelf break up

interactions of wind transported snow with a rift in the ross ice shelf antarctica 1 rifts in ice shelves accumulate a melange of snow and firn from above and marine ice from below material that has been postulated to negatively influence iceberg calving from measurements and modeling we show that a 100 m wide rift near the front of the ross ice shelf captures all wind transported snow traveling in saltation and a substantial fraction of the snow blowing in suspension across the rift moderate winds and snow transport appear sufficient to fill the rift in several years whereas the melange surface profile remains relatively constant within the 30 m deep rift observed subsidence and ocean temperatures in the rift suggest that net basal melting dissolves the melange from below limiting its effectiveness as an anti calving agent near ice fronts

experimental and theoretical fracture mechanics applied to antarctic ice fracture and surface crevassing recent disintegration of ice shelves on the antarctic peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally in this paper we present a fracture criterion incorporating new experimental fracture data coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the filchner ronne ice shelf we have developed experiments that have enabled us to quantify for the first time quasi stable crack growth in antarctic ice core specimens using a fracture initiation toughness kinit for which crack growth commences the tests cover a full range of near surface densities 560 871 kg m 3 10 9 75 7 m depth results indicate an apparently linear dependence of fracture toughness on porosity such that kinit 0 257 80 7 predicting a zero porosity toughness of ko 155 kpa m1 2 we have used this data to test the applicability to crevassing of a two dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field the growth of an initial flaw into a larger crevasse which involves a purely tensile crack opening depends on the size of the flaw the magnitude of kinit and the nature of the applied stress field by incorporating the criterion into a stress map of the filchner ronne ice shelf derived from a depth integrated finite element model of the strain rate field we have been able to predict regions of potential crevassing these agree well with satellite imagery provided an initial flaw size is assumed in the range 5 50 cm

calving of large tabular icebergs from ice shelf rift systems 1 we used interferometric synthetic aperture radar to study the detachment process that allowed two large icebergs to calve from the ross ice shelf antarctica time series of rift geometries indicate that rift widths increased steadily whereas rift lengths increased episodically through several discrete rift tip propagation events we also conducted modeling experiments constrained by the observed rift geometry both the observations and model suggest that rift opening and thus tabular iceberg calving are largely driven by glaciological stresses stress introduced by the effect of gravity on the ice shelf rather than by stress introduced by the ocean and atmosphere e g tides and storms this style of rift propagation is expected to determine the steady background calving rate of ice shelves and thus differs significantly from styles that led to the recent disintegration of ice shelves in response to climate warming e g the larsen b ice shelf on the antarctic peninsula

lithospheric flexure on venus summary topographic flexural signatures on venus are generally associated with the outer edges of coronae with some chasmata and with rift zones using magellan altimetry profiles and grids of venusian topography we identified 17 potential flexure sites both 2 d cartesian and 2 d axisymmetric thin elastic plate models were used to establish the flexural parameter and applied load bending moment these parameters can be used to infer the thickness strength and possibly the dynamics of the venusian lithosphere numerical simulations show that the 2 d model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter however an axisymmetric model must be used to obtain a reliable estimate of load bending moment 12 of the 17 areas were modelled with a 2 d thin elastic plate model yielding best fit effective elastic thicknesses in the range 12 to 34 km we find no convincing evidence for flexure around smaller coronae though five possible candidates have been identified these five features show circumferential topographic signatures which if interpreted as flexure yield mean elastic thicknesses ranging from 6 to 22 km we adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on venus are similar to or lower than strain rates on earth many of the flexural signatures correspond to relatively high plate bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow respectively for areas where the curvatures are not too extreme the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere the large amplitude flexures in aphrodite terra predict complete failure of the plate rendering mechanical thickness estimates from these features unreliable one smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data reliable mechanical thicknesses found by forward modelling in this study are 21 km 37 km significantly greater than the 13 km 20 km predictions based on heat flow scaling arguments and chondritic thermal models if the modelled topography is the result of lithospheric flexure then our results for mechanical thickness combined with the lack of evidence for flexure around smaller features are consistent with a venusian lithosphere somewhat thicker than predicted dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere also consistent with low temperature gradients recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed corresponding time scales for gravitational relaxation are 108 109 yr making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features if dry olivine is also found to be stronger than previously believed the mechanical thickness estimates for venus will be reduced and will be more consistent with the predictions of global heat scaling models

does size matter statistical limits of paleomagnetic field reconstruction from small rock specimens as samples of ever decreasing sizes are being studied paleomagnetically care has to be taken that the underlying assumptions of statistical thermodynamics maxwell boltzmann statistics are being met here we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field it is found that for samples with a thermoremanent magnetic moment larger than 10 11am2 the assumption of a sufficiently large number of grains is usually given standard 25 mm diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible but single silicate crystal works on for example zircon plagioclase and olivine crystals are approaching the limits of what is physically possible leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error the reliability of nanopaleomagnetic imaging techniques capable of resolving individual grains used for example to study the cloudy zone in meteorites however is questionable due to the limited area of the material covered

the effects of antarctic iceberg calving size distribution in a global climate model icebergs calved from the antarctic continent act as moving sources of freshwater while drifting in the southern ocean the lifespan of these icebergs strongly depends on their original size during calving in order to investigate the effects if any of the calving size of icebergs on the southern ocean we use a coupled general circulation model with an iceberg component iceberg calving length is varied from 62 m up to 2 3 km which is the typical range used in climate models results show that increasing the size of calving icebergs leads to an increase in the westward iceberg freshwater transport around antarctica in simulations using larger icebergs the reduced availability of meltwater in the amundsen and bellingshausen seas suppresses the sea ice growth in the region in contrast the increased iceberg freshwater transport leads to increased sea ice growth around much of the east antarctic coastline these results suggest that the absence of large tabular icebergs with horizontal extent of tens of kilometers in climate models may introduces systematic biases in sea ice formation ocean temperatures and salinities around antarctica

evolution of ice shelf rifts implications for formation mechanics and morphological controls abstract ice shelf rifts are predecessors to iceberg calving events observations of their morphology over time and interaction with their surroundings are presented newly available landsat derived annual velocity data and laser altimetry from nasa s icesat icesat 2 and operation icebridge missions were used to characterize several ice shelf rift systems actively propagating ice shelf rifts exhibit uplift on both sides with an offset in height between the two rift walls suggesting an asymmetric buoyancy force resulting from the rifting process itself it is observed that the direction of the asymmetry is the same for rifts within a single ice shelf but not between ice shelves this finding suggests a rheological or basal properties influence on rift formation and buoyancy asymmetry it is shown that this asymmetry can be accounted for by modeling ice shelf rifts as subvertical faults with time and inactivity rift walls eventually relax and become rounded off measurements of widening suggest that rift infilling is active on many active rifts active rifts experience higher spreading rates than do dormant rifts lastly two candidate processes are identified for rift nucleation subvertical basal crevassing and localized melt it is found that more than any other ice shelf in this study rifting in pine island glacier is driven by ocean heat variations and is showing signs of increasing destabilization

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

nature of the basement to quesnel terrane near christina lake southeastern british columbia the character of the paleozoic basement of quesnel terrane and the position of the terrane accretion surface that separates quesnel and kootenay terranes from rocks of the ancient north american margin are subjects of debate to address these problems detailed mapping and u pb geochronologic studies were carried out in the christina lake area to define the relationship of the mollie creek assemblage josh creek diorite and fife diorite to similar lithologies in the greenwood grand forks and rossland regions and to place limits on the ages of regional deformation and local position of the terrane accretion surface deformed metasedimentary rocks of the mollie creek assemblage may correlate with sedimentary rocks of the pennsylvanian to early triassic mount roberts formation in the rossland area the mollie creek assemblage is intruded by the foliated late triassic josh creek diorite the josh creek diorite and mollie creek assemblage have been deformed together as a result of phase two deformation fo

icesat profiles of tabular iceberg margins and iceberg breakup at low latitudes 1 icesat elevation profiles of tabular iceberg margins and the ronne ice shelf edge reveal shapes indicative of two types of bending forces icebergs and shelf fronts in sea ice covered areas have broad 1000 m wide rounded 0 6 m high berms and outer edges that slope down several meters toward the water bergs in warmer water have 2 to 5m ramparts with 1500 m wide edge parallel moats inboard of the edge this latter pattern was first revealed in images from international space station iss showing edge parallel melt ponds on one iceberg just prior to its disintegration model results indicate the patterns are caused by hydrostatic and lithostatic forces acting on the ice face berm profiles arise from differences between ice and water pressure along the face rampart moat profiles result from waterline erosion creating a submerged bench of ice that lifts the ice edge we use the results to discuss iceberg breakup at low latitudes

structural and environmental controls on antarctic ice shelf rift propagation inferred from satellite monitoring 1 iceberg calving from ice shelves accounts for nearly half of the mass loss from the antarctic ice sheet yet our understanding of this process is limited the precursor to iceberg calving is large through cutting fractures called rifts that can propagate for decades after they have initiated until they become iceberg detachment boundaries to improve our knowledge of rift propagation we monitored the lengths of 78 rifts in 13 antarctic ice shelves using satellite imagery from the moderate resolution imaging spectroradiometer and multiangle imaging spectroradiometer between 2002 and 2012 this data set allowed us to monitor trends in rift propagation over the past decade and test if variation in trends is controlled by variable environmental forcings we found that 43 of the 78 rifts were dormant i e propagated less than 500 m over the observational interval we found only seven rifts propagated continuously throughout the decade an additional eight rifts propagated for at least 2 years prior to arresting and remaining dormant for the rest of the decade and 13 rifts exhibited isolated sudden bursts of propagation after 2 or more years of dormancy twelve of the fifteen active rifts were initiated at the ice shelf fronts suggesting that front initiated rifts are more active than across flow rifts although we did not find a link between the observed variability in rift propagation rate and changes in atmospheric temperature or sea ice concentration correlated with we did find a statistically significant correlation between the arrival of tsunamis and propagation of front initiated rifts in eight ice shelves this suggests a connection between ice shelf rift propagation and mechanical ocean interaction that needs to be better understood

the ice cloud and land elevation satellite 2 icesat 2 science requirements concept and implementation abstract the ice cloud and land elevation satellite icesat mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets as well as sea ice thickness distribution these measurements have provided important information on the response of the cryopshere earth s frozen surfaces to changes in atmosphere and ocean condition icesat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean terrestrial and atmospheric scientific communities the conclusive assessment of significant ongoing rapid changes in the earth s ice cover in part supported by icesat observations has strengthened the need for sustained high accuracy repeat observations similar to what was provided by the icesat mission following recommendations from the national research council for an icesat follow on mission the icesat 2 mission is now under development for planned launch in 2018 the primary scientific aims of the icesat 2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet and from 10s of meters to the entire polar oceans for sea ice freeboard icesat carried a single beam profiling laser altimeter that produced 70 m diameter footprints on the surface of the earth at 150 m along track intervals in contrast icesat 2 will operate with three pairs of beams each pair separated by about 3 km cross track with a pair spacing of 90 m each of the beams will have a nominal 17 m diameter footprint with an along track sampling interval of 0 7 m the differences in the icesat 2 measurement concept are a result of overcoming some limitations associated with the approach used in the icesat mission the beam pair configuration of icesat 2 allows for the determination of local cross track slope a significant factor in measuring elevation change for the outlet glaciers surrounding the greenland and antarctica coasts the multiple beam pairs also provide improved spatial coverage the dense spatial sampling eliminates along track measurement gaps and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals the icesat 2 instrumentation concept uses a low energy 532 nm green laser in conjunction with single photon sensitive detectors to measure range combining icesat 2 data with altimetry data collected since the start of the icesat mission in 2003 such as operation icebridge and esa s cryosat 2 will yield a 15 year record of changes in ice sheet elevation and sea ice thickness icesat 2 will also provide information of mountain glacier and ice cap elevations changes land and vegetation heights inland water elevations sea surface heights and cloud layering and optical thickness

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

accelerated ice shelf rifting and retreat at pine island glacier west antarctica pine island glacier has undergone several major iceberg calving events over the past decades these typically occurred when a rift at the heavily fractured shear margin propagated across the width of the ice shelf this type of calving is common on polar ice shelves with no clear connection to ocean ice dynamic forcing in contrast we report on the recent development of multiple rifts initiating from basal crevasses in the center of the ice shelf resulted in calving further upglacier than previously observed coincident with rift formation was the sudden disintegration of the ice melange that filled the northern shear margin resulting in ice sheet detachment from this margin examination of ice velocity suggests that this internal rifting resulted from the combination of a change in ice shelf stress regime caused by disintegration of the melange and intensified melting within basal crevasses both of which may be linked to ocean forcing

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

distribution of marine ice beneath the amery ice shelf we present a map of the marine ice accreted to the base of the amery ice shelf ais east antarctica this map is obtained by converting a digital elevation model dem of the ais generated from satellite radar altimeter data to an ice thickness map assuming hydrostatic equilibrium and subtracting from that a second ice thickness map derived from airborne radio echo sounding res measurements the res signal does not penetrate the marine ice so the measurement is only to the meteoric marine ice boundary and therefore the difference between the two maps is the marine ice thickness the marine ice is up to 190 m thick and accounts for about 9 of the shelf volume it is concentrated in the northwest of the shelf a result of the clockwise ocean circulation in the cavity below

u pb zircon monazite and sphene ages for granitic orthogneiss of the barkerville terrane east central british columbia granitic orthogneiss forms an important component of the barkerville terrane of southeastern british columbia rb sr whole rock ages for the orthogneisses are ambiguous and range from late proterozoic to mid paleozoic with large associated errors u pb dating of zircon monazite and sphene has been employed in an attempt to establish precise crystallization ages for two of the orthogneiss bodies u pb systematics for zircons from both bodies show the combined effects of inheritance of zircon cores and postcrystallization pb loss this complexity precludes a precise estimate of the age of emplacement of the granitic protoliths of the gneiss the data do however constrain possible emplacement ages for the bodies to between 335 and 375 ma late devonian mid mississippian a u pb age of 174 4 ma for metamorphic sphene from one of the orthogneiss bodies is interpreted as dating the end of the second phase of deformation in the area two nearly concordant u pb ages of 114 and 117 ma for monazite from th

an investigation into the forces that drive ice shelf rift propagation on the amery ice shelf east antarctica for three field seasons 2002 03 2004 05 2005 06 we have deployed a network of gps receivers and seismometers around the tip of a propagating rift on the amery ice shelf east antarctica during these campaigns we detected seven bursts of episodic rift propagation to determine whether these rift propagation events were triggered by short term environmental forcings we analyzed simultaneous ancillary data such as wind speeds tidal amplitudes and sea ice fraction a proxy variable for ocean swell we find that none of these environmental forcings separately or together correlated with rift propagation this apparent insensitivity of ice shelf rift propagation to short term environ mental forcings leads us to suggest that the rifting process is primarily driven by the internal glaciological stress our hypothesis is supported by order of magnitude calculations that the glaciological stress is the dominant term in the force balance however our calculations also indicate that as the ice shelf thins or the rift system matures and iceberg detachment becomes imminent short term stresses due to winds and ocean swell may become more important

modelling of rift propagation on ronne ice shelf antarctica and sensitivity to climate change 1 the calving of icebergs from large antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding using insar we observe the evolution of rifts along hemmen ice rise on ronne ice shelf antarctica prior to the large calving event of october 1998 we couple these observations with a computer model combining the viscous flow of an ice shelf with a linear elastic fracture mechanics description of the propagation of rifts the model reveals that the ice melange trapped in between the rifts exerts a major control on the propagation of rifts and in turn on ice shelf stability melting of the ice melange from oceanic or atmospheric warming would significantly increase the propagation rate of rifts and threaten the ice shelf stability

u pb and k ar dates related to the timing of magmatism and deformation in the cache creek terrane and quesnellia southern british columbia u pb dating of zircon from the guichon creek batholith indicates an emplacement age of 210 3 ma comparison with previously published k ar 211 188 ma and rb sr 205 and 196 ma dates reveals that intrusion mineralization cooling and uplift of the batholith took some 20 ma spanning the triassic jurassic boundary on the decade of north american geology dnag time scale the mount martley pluton and tiffin creek stock yield late jurassic dates of 155 2 ma u pb zircon and 152 5 ma k ar hornblende respectively and provide a reliable minimum age kimmeridgian for penetrative deformation in the cache creek terrane k ar whole rock dates from cache creek terrane and ashcroft formation argillites range from early permian 266 8 ma and early jurassic 194 6 ma to late jurassic kimmeridgian 154 5 ma we interpret the younger dates as recording middle late jurassic tectonism and the older ones as possible relics from earlier deformation episodes an early cretaceous k ar date 129 5 ma for a lamprophyre dike that cuts the nicola group suggests that the early cretaceous magmatic arc of the coast plutonic complex had an eastern alkalic fringe in the intermontane belt

continental influence on canadian cordilleran terranes from nd isotopic study and significance for crustal growth processes nd isotopic data are presented for rocks of the terrane assembly that lies inboard of the stikine terrane in the canadian cordillera of british columbia and yukon these are from most inboard outward cassiar kootenay slide mountain quesnel and cache creek terranes they are regarded as documenting a transition from terranes whose evolution was closely tied to that of the north american continental margin out to far traveled oceanic terranes the results emphasize sedimentary rocks as indicators of tectonic position of the crustal fragments sedimentary rocks of the cassiar and kootenay terranes show a strong connection to miogeoclinal sediment sources argillites of pennsylvanian permian age from a paleontologically controlled section in the slide mountain terrane are also consistent with sediment sources in the north american miogeocline igneous rocks of the slide mountain quesnel and cache creek terranes show juvenile oceanic or arc origins based on nd values between 3 and 10 and are essent

high altitude radar measurements of ice thickness over the antarctic and greenland ice sheets as a part of operation icebridge the national aeronautics and space administration nasa initiated a program called operation icebridge for monitoring critical parts of greenland and antarctica with airborne lidars until icesat ii is launched in 2016 we have been operating radar instrumentation on the nasa dc 8 and p 3 aircraft used for lidar measurements over antarctica and greenland respectively the radar package on both aircraft includes a radar depth sounder imager operating at the center frequency of 195 mhz during high altitude missions flown to perform surface elevation measurements we also collected radar depth sounder data we obtained good ice thickness information and mapped internal layers for both thicker and thinner ice we successfully sounded 3 2 km thick low loss ice with a smooth surface and also sounded about 1 km or less thick shallow ice with a moderately rough surface the successful sounding required processing of data with an algorithm to obtain 56 db or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross track surface clutter in this paper we provide a brief description of the radar system discuss range sidelobe reduction and array processing algorithms and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the antarctic and greenland ice sheets

rock debris in an antarctic ice shelf we have discovered a band of stones and coarse sand in the ronne ice shelf antarctica some 60m above the ice shelf s base 40km from its seaward edge and 420km from the point where the ice originally went afloat a study of ice sounding radar data from across the ronne ice shelf has revealed other areas likely to contain debris in significant quantities it appears that basal debris at the margins of ice streams feeding the ice shelf can be buried in the ice shelf by sea water freezing on at the ice shelf base these findings are evidence for a mechanism active in a present day ice sheet shelf system which enables icebergs to transport large volumes of ice rafted debris and which also provides a potential mechanism for the formation of ice rises near ice fronts we anticipate that a seismics study of debris melted from the ice shelf and deposited beneath will provide a valuable control on the history of ice shelf ocean interactions