climatic control on the late permian main dolomite ca2 deposition in northern margin of the southern permian basin and implications to its internal cyclicity

distribution and origin of protodolomite from the late miocene pliocene red clay formation chinese loess plateau the pliocene epoch is considered the most recent analog of modern warming because co2 levels were similar to the present to explore the carbonate minerals formed in the warmer pliocene epoch we studied two continuous sections of the red clay formation on the chinese loess plateau clp by x ray diffraction xrd fourier transform infrared spectroscopy ftir scanning electron microscopy sem and stable isotope mass spectrometry the red clay formation on the clp exhibits diagnostic ftir absorption features of calcite and protodolomite this allowed quantification of the two carbonate minerals by the ftir method using the ftir method we estimate the average concentration of protodolomite in bajiazui is 3 6 whereas the duanjiapo section is 6 0 protodolomite occurrence is more consistent and the concentration is higher from similar to 6 5 4 2 ma b p and decreases markedly from 4 2 2 6 ma b p red clay protodolomite is depleted in both delta c 13 pdb and delta o 18 pdb ranging from 4 1 to 10 4 and from 6 7 to 11 6 respectively and has a slightly higher delta o 18 value than the calcites sem observations show that red clay protodolomite is composed of euhedral rhombic crystals that range from 1 20 mu m in diameter grow into the soil voids and coexist with authigenic calcite and palygorskite these observations imply that the protodolomite grew in situ and is authigenic from pedogenesis dolomitization in the red clay sequence appears to be the result of overcoming kinetic barriers we propose that in the red clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to mg2 and significantly increasing the mg ca ratio bringing about the formation of protodolomite

observed and modelled ice temperature and velocity along the main flowline of east rongbuk glacier qomolangma mount everest himalaya knowledge of present day ice temperature and velocity is important in order to determine how fast a glacier will respond to present and future climate change we apply a two dimensional higher order thermomechanical flowband model to simulate present day ice temperature and velocity along the main flowline of east rongbuk glacier qomolangma mount everest himalaya we use recent 2005 11 observational data to validate the numerical model modelled and observed ice surface velocities exhibit good agreement modelled ice temperatures agree well with observed values in two shallow boreholes that are similar to 18 m deep the model suggests that the ablation zone of east rongbuk glacier km 4 to km 10 from the glacier head is underlain by temperate ice

modeling the dynamic response of a crater glacier to lava dome emplacement mount st helens washington usa the debris rich glacier that grew in the crater of mount st helens after the volcano s cataclysmic 1980 eruption was split in two by a new lava dome in 2004 for nearly six months the eastern part of the glacier was squeezed against the crater wall as the lava dome expanded glacier thickness nearly doubled locally and surface speed increased substantially as squeezing slowed and then stopped surface speed fell and ice was redistributed downglacier this sequence of events which amounts to a field scale experiment on the deformation of debris rich ice at high strain rates was interpreted using a two dimensional flowband model the best match between modeled and observed glacier surface motion both vertical and horizontal requires ice that is about 5 times stiffer and 1 2 times denser than normal temperate ice results also indicate that lateral squeezing and by inference lava dome growth adjacent to the glacier likely slowed over a period of about 30 days rather than stopping abruptly this finding is supported by geodetic data documenting dome growth

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

the origin and microstructures of metamorphic felsic dykes emplaced during brittle ductile extension southeast sinai structural petrographic and chemical analyses of a set of northeast trending quartzofeld spathic dykes in wadi kid area southeastern sinai show that they are the product of anatexis of pelitic rocks metamorphic p t data indicate that anatexis could have occurred only 2 km below the present erosion level the partial melts intruded into overlying rocks during regional metamorphism along a set of northeast oriented extensional fractures thereby recrystallizing and in some cases obtaining tectonite fabrics the fractures were formed perpendicular to the regional mineral lineation and show that the area was within the deeper parts of the brittle ductile transition zone the original depth of the presently exposed crustal level corresponds well with the theoretical and experimentally derived depths of the brittle ductile transition zone in high heat flow regimes the emplacement of a granitoid pluton or gneiss dome possibly of dioritic composition could be responsible for both melting and tensile fracturing of the overlying rocks

investigating the stability of subglacial lakes with a full stokes ice sheet model despite the large amount of subglacial lakes present underneath the east antarctic ice sheet and the melt processes involved the hydrology beneath the ice sheet is poorly understood changes in subglacial potential gradients may lead to subglacial lake outbursts discharging excess water through a subglacial drainage system underneath the ice sheet such processes can eventually lead to an increase in ice flow in this paper a full stokes numerical ice sheet model was employed which takes into account the ice flow over subglacial water bodies in hydrostatic equilibrium with the overlying ice sensitivity experiments were carried out for small perturbations in ice flow and basal melt rate as a function of ice thickness general surface slope ice viscosity and lake size in order to investigate their influence on the subglacial potential gradient and the impact on subglacial lake drainage experiments clearly demonstrate that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events lake drainage can therefore be regarded as a common feature of the subglacial hydrological system and may influence to a large extent the present and future behavior of large ice sheets

a hydrologically coupled higher order flow band model of ice dynamics with a coulomb friction sliding law 1 the influence of hydrologic transience and heterogeneity on basal motion is an often neglected aspect of numerical ice flow models we present a flow band model of glacier dynamics with a coulomb friction sliding law that is coupled to a model of the basal drainage system by means of subglacial water pressure the ice flow model contains higher order stress gradients from the stokes flow approximation originally conceived by blatter 1995 the resulting system of nonlinear equations is solved using a modified picard iteration that is shown to improve the rate of convergence a parameterization of lateral shearing is included to account for the effects of three dimensional geometry we find that lateral drag has a discernible effect on glacier speed even when glacier width exceeds glacier length variations in flow band width are shown to have a greater influence on flow line speed than either different valley cross sectional shapes or the presence or absence of glacier sliding along valley walls modeled profiles of subglacial water pressure depart significantly from pressures prescribed as a uniform fraction of overburden thus producing profiles of glacier sliding that are distinctly different from those that would be described by a sliding law controlled by overburden pressures simulations of hydraulically driven glacier acceleration highlight the value of including a representation of basal hydrology in models aiming for improved predictive capability of glacier dynamics

climate controlled fabric destructive reflux dolomitization and stabilization via marine and synorogenic mixed fluids an example from a large mesozoic calcite sea platform croatia the jurassic cretaceous dolomites adriatic platform croatia resulted from climate influenced post depositional reflux dolomitization as opposed to synsedimentary peritidal and deeper burial dolomitization and subsequent stabilization within a mesozoic calcite sea isolated platform the dolomites are stratiform 10 to 200 m thick fabric destructive 20 to over 500 m crystals nonluminescent and zoned with respect to ca bulk dolomites have low mn 10 to 30 ppm moderate sr 60 to over 200 ppm positive 13c and 18o values only moderate ordering 0 25 to 0 6 and single phase fluid inclusions temperatures 3 vpdb and sr 100 ppm composes most jurassic and 40 of cretaceous dolomites making up turbid dolomite cores and initial clear dolomite rims a first generation of low ca dolomite lcd 1 50 to 53 mol ca 18o 1 to 3 vpdb 100 180 ppm sr forms cement and variably replaces stabilizes earlier hcd cores hcd and lcd 1 formed in refluxing marine dominated pore waters under semi arid climate 55 mol phases a second generation of much younger low ca dolomite lcd 2 fracture associated more negative 18o from 1 4 to 1 vpdb sr 100 ppm enriched in radiogenic 87sr overgrows and variably replaces earlier cretaceous dolomites it formed during eocene oligocene deformation and uplift to 1 km burial depths 50 c from synorogenic mixed marine meteoric pore fluids circulating via faults fractures and local permeable zones porosity loss by dolomite cementation coupled with localized rapid cenozoic uplift variably arrested stabilization to lcd

transient glacier response with a higher order numerical ice flow model in this paper a higher order numerical flowline model is presented which is numerically stable and fast and can cope with very small horizontal grid sizes 10 m the model is compared with the results from blatter and others 1998 on haut glacier d arolla switzerland and with the european ice sheet modelling initiative benchmarks huybrechts and others 1996 results demonstrate that the significant difference between calculated basal drag and driving stress profiles in a fixed geometry disappears when the glacier profile is allowed to react to the surface mass balance conditions and reaches a steady state dynamic experiments show that the mass transfer in higher order models occurs at a different speed in the accumulation and ablation areas and that the front position is more sensitive to migration compared to the shallow ice approximation

permeability loss with depth in the cenozoic carbonate platform of west central florida relationships between matrix permeability and depth in shallow buried 500 m cenozoic limestones of florida were investigated using more than 12 000 minipermeameter measurements on 1210 m of slim hole core these data are considered representative of the matrix permeability reduction that accompanies early burial of shallow water subtidal to peritidal nonreefal carbonates of sta ble cratonic margins platforms shelves and ramps measured matrix permeabilities range from less than 4 0 to 6450 md for both limestones and dolostones although the median permeability of limestones 88 md exceeds that for dolostones 15 md sucrosic dolostones which have high intercrystalline po rosities have median matrix permeabilities of 1000 md non sucrosic dolostones which are much less porous have median matrix permeabilities of only 13 md neither population of dolostones exhibits a permeability with depth trend low permeability limestones consist of mudstones wackestones tidal laminites pedogenetically altered limestone lime mud rich breccia and well cemented grain supported rocks these low permeability limestones all exhibit median matrix permeabilities less than 35 md and none exhibit permeability with depth trends in contrast high mud packstones low mud packstones very low mud packstones and grainstones exhibit larger median matrix permeabilities 69 129 181 and 418 md respectively and a systematic reduction with depth in their mean and maximum permeabilities least squares exponential regressions indicate that burial depth explains 22 31 of the permeability variance in these grain supported rocks the balance of the variability within each textural class is attributed to variation in facies grain types grain sizes sorting etc and the amount of early preburial cementation in the absence of permeability enhancement by deep subsurface diagenesis or fracturing extrapolation of the permeability with depth trends suggests that the limestones that have the best reservoir potential in the deeper subsurface are grain supported limestones that have higher than begin page 1254 average matrix permeabilities in the near surface realm petrographic analyses and consideration of diagenetic histories suggest that matrix permeability of the dolostones and low permeability limestones was determined by early near surface dolomitization and cementation respectively and has yet to be affected by burial processes petrographic observations also suggest that the depth signal in grain supported limestones results from mechanical compaction and the onset of grain to grain pressure solution extrapolation of the matrix permeability data indicates that permeability loss with burial occurs faster than the documented burial related porosity loss for the same group of rocks this indicates that limestone permeability is more sensitive to burial compaction than limestone porosity thus preservation of reservoir quality rock during burial is more dependent on permeability preservation than on porosity preservation

meltwater production due to strain heating in storglaciaren sweden 1 storglaciaren northern sweden is temperate in most parts except for a cold surface layer in the ablation zone one of four possible sources for liquid water in temperate ice is melting due to strain heating velocity fields are calculated with an ice flow model so that calculated and observed surface velocities agree meltwater accumulation is computed by integrating strain heating along trajectories starting at the surface in the accumulation area and ending at the cold temperate transition surface in the ablation zone the distribution of moisture content due to strain heating alone is mapped in a longitudinal section of storglaciaren values reach more than 10 g of water per kilogram ice water mixture in the lowest parts of the temperate domain for this moisture content the rate factor is more than 3 times higher than for water free ice and therefore water production by strain heating is important for the modeling of temperate and polythermal glaciers

an example of mixing zone dolomite middle eocene avon park formation floridan aquifer system abstract a late formed dolomite cement in a core of the middle eocene avon park formation peninsular florida provides an example of dolomite cement from a mixing zone and illustrates how dolomite textural alteration and stabilization can occur at earth surface conditions the avon park formation is a pervasively dolomitized peritidal platform carbonate 400 m thick in the floridan aquifer system typical avon park dolomite is inclusion rich fine grained 40 mm noncathodoluminescent highly porous average 20 and formed during the eocene by normal to hypersaline seawater 18o 3 7 pdb 13c 2 0 87sr 86sr 0 70778 sr 167 ppm in a 20 m interval in a core from southwest florida inclusion free cathodoluminescent dolomite overgrows the early formed noncathodoluminescent marine dolomite the cathodoluminescent dolomite cement profoundly alters the texture of avon park dolomite from typical cenozoic like porous poorly crystalline dolomite to hard dense low porosity highly crystalline paleozoic like dolomite the dolomite cement is not a replacement of limestone but an overgrowth of early formed marine dolomite and pore occluding cement from analyses of samples containing both the early marine dolomite and the late dolomite cement quantitative estimates of pure cathodoluminescent dolomite cement are 13c 0 5 and 18o 1 7 87sr 86sr 0 7085 sr 225 ppm the stable isotope compositions are intermediate between avon park marine dolomite and dolomite predicted to be in equilibrium with floridan aquifer freshwater estimated 18o 0 to 0 5 p b indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater due to the rise in seawater 87sr 86sr since the middle eocene the 87sr 86sr composition of the cathodoluminescent dolomite cement demonstrates that it must have formed after the middle miocene incorporating radiogenic sr from seawater or rocks of middle miocene age or younger a water sample was taken from this same core from an interval in the avon park formation in the dilute part of the modern mixing zone approximately 60 m above the first occurrence of altered dolomite fluids from this interval have 3400 ppm tds and are a mixture of 94 floridan aquifer freshwater and 6 normal seawater these pore fluids are sr rich 24 ppm and so4 rich 755 ppm due to extensive avon park gypsum dissolution and have a 87sr 86sr composition 0 7078 in equilibrium with avon park host rock the salinity of the fluid that precipitated the cathodoluminescent dolomite cement is quantitatively calculated with a fluid fluid mixing model using 1 the sr concentration 24 ppm and 87sr 86sr composition 0 7078 of the von park pore fluid sample from the core 2 the sr concentration 8 ppm and 87sr 86sr compositions of modern 0 7092 and late miocene 0 7089 seawater and 3 the 87sr 86sr composition 0 7085 of the cathodoluminescent dolomite cement these calculations indicate that the dolomite cement precipitated from a mixed marine meteoric fluid with salinity approximately 75 seawater this same fluid would be both calcite supersaturated and contain about 12 ppm sr such a fluid could precipitate dolomite cement with 225 ppm sr using a reasonable kd 0 05 this accounts for the higher sr concentrations in the mixing zone dolomite cement 225 ppm relative to avon park marne dolomite 167 ppm this study demonstrates that 1 dolomite precipitated from a 75 seawater mixing zone fluid that was both calcite saturated and sulfate rich and 2 dramatic textural maturation and stabilization in dolomite can occur in the near surface environment without elevated temperature and burial conditions

ice flow sensitivity to geothermal heat flux of pine island glacier antarctica model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation bedrock position or surface temperatures among others of all these inputs geothermal heat flux is the one for which uncertainty is greatest in the area of pine island glacier antarctica available data sets differ by up to a factor of 2 5 here we evaluate the impact of such uncertainty on ice flow using sampling analyses based on the latin hypercube method first we quantify the impact of geothermal heat flux errors on ice hardness a thermal parameter that critically controls the magnitude of ice flow second we quantify the impact of the same errors on mass balance specifically on the mass flux advecting through thirteen fluxgates distributed across pine island glacier we contrast our results with similar uncertainties generated by errors in the specification of ice thickness model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5 7 with maximum uncertainty reaching 15 resulting uncertainties in mass balance remain however below 1 we discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction viscous and geothermal heating we also show that comparatively errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux especially for fast flowing ice streams 2012 american geophysical union all rights reserved

petrography and significance of zoned dolomite cements from the cayman formation miocene of cayman brac british west indies abstract cavities in dolostones from the cayman formation miocene from cayman brac commonly contain complex cement successions that record three main phases of diagenesis phase i cements formed of low ca calcian dolomite lcd and high ca calcian dolomite hcd are divided into monomineralic lcd crystals oscillatory zoned lcd hcd crystals and step zoned hcd lcd crystals dissolution during phase ii produced dolomite crystals with hollow cores hollow growth zones and hollow crosscutting zones calcite precipitated during phase iii filled many of the cavities that had not been filled by the dolomite cements etching shows that the lcd is formed of blocky subcrystals that have their long axes perpendicular to the growth surfaces whereas hcd is characterized by a modulated structure that is evident as growth parallel slightly sinuous strands 10 20 nm wide the contrast in crystallographic microstructures supports the notion that lcd and hcd are distinct types of dolomite two levels of variability that operated on different time scales controlled diagenesis in the cayman formation of cayman brac long term variability controlled the evolution from dolomite precipitation to dolomite dissolution to calcite precipitation short term variability controlled variations in the physiochemical environments that characterized each phase of diagenesis thus during phase i diagenesis short term fluctuations in water chemistry and or crystal growth patterns controlled the mol caco3 in the dolomite cement crystals similarly short term variations in pore water compositions controlled the style of dolomite dissolution during the second phase of diagenesis

insights into the dolomitization process and porosity modification in sucrosic dolostones avon park formation middle eocene east central florida u s a abstract the avon park formation middle eocene in central florida u s a contains shallow water carbonates that have been replaced by dolomite to varying degrees ranging from partially replaced limestones to highly porous sucrosic dolostones to less commonly low porosity dense dolostones the relationships between dolomitization and porosity and permeability were studied focusing on three 305 m long cores taken in the city of daytona beach stable isotope data from pure dolostones mean 18o 3 91 v pdb indicate dolomite precipitation in eocene penesaline pore waters which would be expected to have been at or above saturation with respect to calcite nuclear magnetic log derived porosity and permeability data indicate that dolomitization did not materially change total porosity values at the bed and formation scale but did result in a general increase in pore size and an associated substantial increase in permeability compared to limestone precursors dolomitization differentially affects the porosity and permeability of carbonate strata on the scale of individual crystals beds and formations at the crystal scale dolomitization occurs in a volume for volume manner in which the space occupied by the former porous calcium carbonate is replaced by a solid dolomite crystal with an associated reduction in porosity dolomite crystal precipitation was principally responsible for calcite dissolution both at the actual site of dolomite crystal growth and in the adjoining rock mass carbonate is passively scavenged from the formation which results in no significant porosity change at the formation scale moldic pores after allochems formed mainly in beds that experienced high degrees of dolomitization which demonstrates the intimate association of the dolomitization process with carbonate dissolution the model of force of crystallization controlled replacement provides a plausible explanation for key observations concerning the dolomitization process in the avon park formation and elsewhere 1 volume for volume replacement at a crystal scale 2 coupled growth of dolomite crystals and dissolution of host calcium carbonate matrix and 3 automorphic replacement by euhedral dolomite crystals the force of crystallization model also does not require an influx of externally derived water that is undersaturated with respect to calcite to dissolve calcite a fact that could simplify diagenetic models of porosity generation in dolostones the later addition of external carbonate can result in a substantial reduction in porosity by the precipitation of dolomite cement which could convert a high porosity sucrosic dolostone into a dense paleozoic type dolostone

finite element modeling of subglacial cavities and related friction law 1 sliding velocity and basal drag are strongly influenced by changes in subglacial water pressure or subglacial water storage associated with opening and closing of water cavities in the lee of bedrock obstacles to better understand this influence finite element simulations of ice flowing past bedrock obstacles with cavity formation are carried out for different synthetic periodic bedrock shapes in the numerical model the cavity roof is treated as an unknown free surface and is part of the solution as an improvement over earlier studies the cases of nonlinear ice rheology and infinite bedrock slopes are treated our results show that the relationship between basal drag and sliding velocity the friction law can be easily extended from linear to nonlinear ice rheology and is bounded even for bedrocks with locally infinite slopes combining our results with earlier works by others a phenomenological friction law is proposed that includes three independent parameters that depend only on the bedrock geometry this formulation yields an upper bound of the basal drag for finite sliding velocity and a decrease in the basal drag at low effective pressure or high velocity this law should dramatically alter results of models of temperate glaciers and should also have important repercussions on models of glacier surges and fast glacier flows

a full stokes flow thermo mechanical model for firn and ice applied to the gorshkov crater glacier kamchatka the gorshkov crater glacier at ushkovsky volcano kamchatka is characterized by a large aspect ratio and special thermodynamic conditions at the bedrock caused by a locally enhanced and spatially varying geothermal heat flux furthermore large parts of this glacier consist of firn rather than pure ice which alters the rheological properties such as viscosity and compressibility of the glacier we present a newly developed thermo mechanically coupled three dimensional flow model based on the finite element fe modeling software elmer and apply it to the gorshkov crater glacier by assuming steady state conditions the present day velocity field temperature field basal melting rate and age distribution are simulated we find that flow velocities are generally small 10 s of centimeters per year horizontal and vertical velocities are of comparable magnitude which shows that the shallow ice approximation is not applicable owing to the spatially variable volcanic heat flux the thermal regime at the ice base is cold in the deeper parts of the glacier and temperate in the shallower parts the measured temperature profile and age horizons at the k2 borehole are reproduced quite well and remaining discrepancies may be attributed to transient non steady state conditions firn compressibility is identified as a crucial element for the modeling approach

a parallel high order accurate finite element nonlinear stokes ice sheet model and benchmark experiments 1 the numerical modeling of glacier and ice sheet evolution is a subject of growing interest in part because of the potential for models to inform estimates of global sea level change this paper focuses on the development of a numerical model that determines the velocity and pressure fields within an ice sheet our numerical model features a high fidelity mathematical model involving the nonlinear stokes system and combinations of no sliding and sliding basal boundary conditions high order accurate finite element discretizations based on variable resolution grids and highly scalable parallel solution strategies all of which contribute to a numerical model that can achieve accurate velocity and pressure approximations in a highly efficient manner we demonstrate the accuracy and efficiency of our model by analytical solution tests established ice sheet benchmark experiments and comparisons with other well established ice sheet models

numerical modelling of historical front variations and the 21st century evolution of glacier ax010 nepal himalaya due to the lack of measurements of ice velocity mass balance glacier geometry and other baseline data model based studies of glacial systems in the nepal himalaya are very limited here a numerical ice flow model has been developed for glacier ax010 in order to study its relation to local climate and investigate the possible causes of its general retreat since the end of the little ice age first an attempt is made to simulate the historical front variations considering each climatic parameter separately good agreement between the observations and model projections can be obtained under the assumption that variations in glacier front position are a response to changes in temperature alone the same assumption is made about future changes to explore the 21st century evolution of the glacier under a no change scenario the glacier will retreat by another 600 m by ad 2100 whereas it is projected to vanish completely during this century for all trends with a temperature rise larger than 2 58 cb yad 2100 with respect to the 1980 99 mean with constant precipitation at the 1980 99 mean the model predicts that the glacier will cease to exist at ad 2083 2056 or 2049 if the temperature rises linearly by 38c 4 58 co r 68c respectively by the end of this century with an additional range of precipitation changes between 30 and 30 the life expectancy of glacier ax010 varies by 18 6 and 2 years for the respective temperature rises thus the role of precipitation becomes minimal for the higher trends of temperature rise

the origin of channels on lower taylor glacier mcmurdo dry valleys antarctica and their implication for water runoff well developed surface channels on taylor glacier mcmurdo dry valleys antarctica begin as medial moraines incised as shallow narrow surface depressions and retain this geometry for tens of km over a distance of 1100m the channel geometry dramatically changes reaching depths 20m and widths 100m after rapidly enlarging the channels appear to evolve toward a new equilibrium geometry compared to the glacier surface the air temperature in the channels is warmer by 1 78c wind speed is reduced by 2 4m s and net shortwave radiation is greater by 14wm the microclimate in the channel shifts the energy balance towards enhanced melt field evidence and energy balance modeling indicate ablation in the deep channels is 4 5 times greater than the local horizontal glacier surface and that melt accounts for 99 of the summer ablation compared to 75 on the adjacent horizontal glacier surface melt in these channels supplies 65 of the unaccounted water discharge into the neighboring lake in large part the channels generate the water they carry rather than merely route water generated elsewhere

carbonate diagenesis in a high transmissivity coastal aquifer biscayne aquifer southeastern florida usa abstract cores collected through the biscayne aquifer plio pleistocene in hollywood southeastern florida as part of the hollywood coastal salinity barrier project provided an opportunity to examine the diagenesis of limestones and sandstones at the meteoric to marine water transition of one of the most transmissive aquifers in the world the saline water front and thus coastal mixing zone has migrated landward approximately 1 km in the hollywood area as the result of wellfield withdrawals no changes in mineralogy such as dolomitization cement types and abundances paragenetic sequence or porosity are evident that can be correlated with the current or likely pre development wellfield withdrawals location of the mixing zone an approximately 2 downhole increase in calcite 18 o values is present in a core hmw 6d that penetrates the pre development mixing zone which may be related to either a down hole increase in salinity or to the interaction of meteoric waters with marine carbonate sediments during calcite cementation the biscayne aquifer in hollywood is currently a relatively quiescent diagenetic environment the limited current diagenesis appears to consist of the dissolution of trace skeletal aragonite remaining in the aquifer as suggested by a meteoric water sr ca ratio similar to that of molluscan aragonite it is proposed that a punctuated equilibria model may be applicable to diagenesis in the biscayne and other aquifers in which limestones and sandstones entered a long period of diagenetic stasis after a period of relatively rapid textural and mineralogical stabilization

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

localized basal motion of a polythermal arctic glacier mccall glacier alaska usa we analyzed the ice flow of mccall glacier alaska usa by numerical glacier modeling and radio echo sounding res model experiments were carried out with a higher order numerical ice flow model and results were validated with measurements of annual ice velocities and compared with previous estimates of ice flow dynamics during the 2003 summer campaign detailed res measure ments were carried out along the central flowline of the ablation area with a 5 mhz central frequency ice penetrating radar where 10 m ice temperatures are approximately 7 5 c the bed reflection power brp beneath this central flowline abruptly increases at one location area followed by a slow decrease down glacier the model experiments show that basal sliding 50 is necessary to match the observed annual mean surface velocities in the area that is characterized by high brp values however when thermomechanical effects are taken into account a temperate basal ice layer is apparent in the ablation area which locally softens the ice and can explain to a certain extent the anomalous flow field the model results confirm that the present temperature field is a remnant of a larger glacier geometry that was near steady state before the onset of enhanced surface thinning in the 1970s

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

parameterization of lateral drag in flowline models of glacier dynamics given the cross sectional geometry of a valley glacier effects of lateral drag can be parameterized in flowline models through the introduction of nye shape factors lateral drag also arises due to lateral variability in bed topography and basal flow which induce horizontal shear stress and differential ice motion for glaciers with various geometric and basal conditions we compare three dimensional stokes solutions to flowline model solutions to examine both sources of lateral drag we calculate associated correction factors that help flowline models to capture the effects of lateraldrag such parameterizations provide improved simulations of the dynamics of narrow channelized fast flowing glacial systems we present an example application for athabasca glacier in the canadian rocky mountains

the eismint benchmarks for testing ice sheet models we present a series of benchmark experiments designed for testing and comparing numerical ice sheet models following the outcome of two eismintworkshops organized to intercompare large scale ice sheet models currently in operation model benchmark experiments are described for ice sheets underfixed and moving margin conditions these address both steady state and time dependent behavior under schematic boundary conditions and with prescribedphysics a comparison was made of each model s prediction of basic geophysical variables such as ice thickness velocity and temperature consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice sheet modeling codes can be assessed

simulations of the greenland ice sheet 100 years into the future with the full stokes model elmer ice it is likely that climate change will have a significant impact on the mass balance of the greenland ice sheet contributing to future sea level rise here we present the implementation of the full stokes model elmer ice for the greenland ice sheet which includes a mesh refinement technique in order to resolve fast flowing ice streams and outlet glaciers we discuss simulations 100 years into the future forced by scenarios defined by the searise sea level response to ice sheet evolution community effort for comparison the same experiments are also run with the shallow ice model sicopolis simulation code for polythermal ice sheets we find that elmer ice is 43 more sensitive exhibits a larger loss of ice sheet volume relative to the control run than sicopolis for the ice dynamic scenario doubled basal sliding but 61 less sensitive for the direct global warming scenario based on the a1 b moderate emission scenario for greenhouse gases the scenario with combined a1b global warming and doubled basal sliding forcing produces a greenland contribution to sea level rise of 15cm for elmer ice and 12cm for sicopolis over the next 100 years