layer formation in sedimentary fingering convection author s reali jf garaud p alsinan a meiburg e abstract when particles settle through a stable temperature or salinity gradient they can drive an instability known as sedimentary fingering convection this phenomenon is thought to occur beneath sediment rich river plumes in lakes and oceans in the context of marine snow where decaying organic materials serve as the suspended particles or in the atmosphere in the presence of aerosols or volcanic ash laboratory experiments of houk a green deep sea res vol 20 1973 pp 757 761 and green sedimentology vol 34 2 1987 pp 319 331 have shown sedimentary fingering convection to be similar to the more commonly known thermohaline fingering convection in many ways here we study the phenomenon using three dimensional direct numerical simulations we find evidence for layer formation in sedimentary fingering convection in regions of parameter space where it does not occur for non sedimentary systems this is due to two complementary effects sedimentation affects the turbulent fluxes and broadens the region of parameter space unstable to the unicode stix x1d6fe instability radko j fluid mech vol 497 2003 pp 365 380 to include systems at larger density ratios it also gives rise to a new layering instability that exists in unicode stix x1d6fe stable regimes the former is likely quite ubiquitous in geophysical systems for sufficiently large settling velocities while the latter probably grows too slowly to be relevant at least in the context of sediments in water

sedimentation from buoyant muddy plumes in the presence of interface mixing an experimental study a series of flume experiments were conducted to study the sediment removal rates characterized by an effective settling velocity from buoyant muddy plumes overriding clear saltwater at steady state conditions under different richardson numbers and initial suspended sediment concentrations the experiments were all carried out in the subcritical regime leading to cusps style instabilities at the interface of the two fluids flocculation in the experiments was not suppressed yet flocs remained small while in the plume layer data from the experiments allowed for calculation of the individual floc settling velocity ws f and the overall effective settling velocity ws eff i e that velocity needed to predict the downward flux with cws eff results showed that ws eff was greater than the floc settling velocity in all runs ws eff was found to increase with plume velocity and interface mixing but not with plume concentration the difference between the effective and floc settling velocities was therefore attributed to turbulent diffusion brought on by mixing at the interface and not to convective sedimentation or leaking the turbulence enhanced settling velocity ws t was found to be a strong function of the richardson number and conceptual and empirical equations are presented to predict ws t as a function of the plume surface velocity ups and richardson number ri this analysis showed that ws t upsri 2 in the runs with low richardson number ws t was approximately equal to the floc settling velocity leading to an overall doubling of the effective settling velocity relative to that of the individual particles this article is protected by copyright all rights reserved

the importance of double diffusion to the settling of suspended material the vertical fingering motions which can occur when warm salty water overlies colder fresher water are important to the vertical flux of salt in the ocean an analogous and not uncommon situation is that where turbid water overlies clearer and denser water for the case of an originally sharp horizontal interface between the two water layers associated as an example with an interflow due to a warm muddy river entering a stratified lake the resulting downward sediment flux is compared with that occurring by way of gravitational settling rules are proposed to ascertain when the fingering effect could be important to sedimentation for example it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 m and the upper layer is about 1 c warmer than the clear lower layer when the upper layer concentration is 1 ppm the effect of fingering on the residence time of a gravitationally stable turbid layer is calculated such layers may be more short lived than commonly thought

debris transport in a temperate valley glacier haut glacier d arolla valais switzerland hambrey m j goodsell becky glasser n f 2005 debris transport in a temperate valley glacier haut glacier d arolla valais switzerland journal of glaciology 51 172 pp 139 146

tectonic processes in a surge type glacier the 1982 1983 surge of variegated glacier involved the development growth and downglacier propagation of a velocity peak associated with rapid basal sliding facilitated by high subglacial water pressures passage of the velocity peak through the glacier was preceded by an episode of longitudinal shortening and followed by an episode of elongation the deformation history of the glacier ice was dependent upon location relative to the surge nucleus and the final position reached by the propagating velocity peak ice above the surge nucleus experienced continuous and cumulative elongation ice below the final position of the velocity peak experienced continuous and cumulative shortening ice between these two points experienced shortening followed by elongation and low cumulative strain the large scale pattern of ice structure development reflects these deformation histories surging is equivalent to thrust sheet emplacement by a combination of gravity gliding over a weakened basal layer and push from behind with the gravity driven motion of the surging part of the glacier providing the push which allows the surge front to propagate the relationships established between the deformation history of surging glaciers and the development of ice structures may facilitate the interpretation of structures in thrust sheets

the influence of diffusive convection on sedimentation from buoyant plumes abstract diffusive convection driven by the differential diffusion of density altering fluid properties may enhance the scavenging of particles from natural buoyant plumes for single phase fluid solute heat systems this phenomenon has been extensively studied because salt fingering generated at the oceanic thermocline is a major mechanism of salt transport in the oceans however the influence of this process on particle laden plumes for example fluvial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown in this paper we present direct experimental measurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes particle flux is measured using a light attenuation technique employing a chain of photodiodes which average concentration over a large number of fingers the results are in good general agreement with earlier studies based on finger velocity flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude for very small particles double diffusive salt finger theory is applicable to two component particulate systems as suggested by earlier studies green t 1987 the importance of double diffusion to the settling of suspended material sedimentology 34 319 331 two component diffusive convection theory is extended to three components in order to predict particle scavenging from marine fluvial plumes which involve the diffusion of sediment salt and heat for larger particles which settle significantly the flux can be approximated by adding the double diffusive and settling fluxes a theory to predict particle transport through the lower layer and sedimentation at the bed is developed based on the observation of strong convection below the plume application of our theory and experimental results indicates that while double diffusion may significantly influence the longitudinal distribution and vertical sorting of deposits from lacustrine plumes the diffusive convection process is generally insignificant in marine plumes observational evidence for lacustrine double diffusion based on water column measurements is presented

the duration of the active phase on surge type glaciers contrasts between svalbard and other regions many glaciers in svalbard and in other glacierized areas of the world are known to surge however the time series of observations required to assess the duration of fast motion is very restricted data on active phase duration in svalbard come from aerial photographs satellite imagery field surveys and airborne reconnaissance evidence on surge duration is available for eight svalbard ice masses varying from 3 to 1250 km 2 worldwide active phase duration is recorded for less than 50 glaciers few observations are available on high polar ice masses the duration of the active phase is significantly longer for svalbard glaciers than for surge type glaciers in other areas from which data are available in svalbard the active phase may last from 3 to 10 years by contrast a surge duration of 1 2 years is more typical of ice masses in northwest north america iceland and the pamirs ice velocities during the protracted active phase on svalbard glaciers are considerably lower than those for many surge type glaciers in these other regions mass is transferred down glacier more slowly but over a considerably longer period svalbard surge type glaciers do not exhibit the very abrupt termination of the active phase over periods of a few days observed for several alaskan glaciers the duration of the active phase in svalbard is not dependent on parameters related to glacier size the quiescent phase is also relatively long 50 500 years for svalbard ice masses detailed field monitoring of changing basal conditions through the surge cycle is required from surge type glaciers in svalbard in order to explain the significantly longer length of the active phase for glaciers in the archipelago which may also typify other high polar ice masses the finding that surge behaviour in the form of active phase duration shows systematic differences between different regions and their environments has important implications for understanding the processes responsible for glacier surges

settling driven convection a mechanism of sedimentation from stratified fluids convection driven by sediment particles may play an important role in sedimentation from the base of buoyant hypopycnal plumes for example fluvial plumes in stratified estuaries and lakes black smokers on the ocean floor volcanic clouds and coastal currents in addition to the well known double diffusive convection mechanism another mode of convective instability development is by settling across the density interface we performed laboratory experiments to investigate this fingering convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed a simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal plume formation at a flat heated plate this model which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid is in good agreement with measurements of finger size and instability wavelength from visualization experiments since fingering at the density interface drives larger scale convection in the fluid below a mass balance model of the lower layer assuming strong mixing i e uniform sediment concentration is successfully applied to predict sediment concentration in the water column and deposition at the bed strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities as convection proceeds the sediment concentrations in the two layers approach each other and convection will die out using the model equations we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

sedimentation from flocculated suspensions in the presence of settling driven gravitational interface instabilities we experimentally examine sedimentation from a freshwater suspension of clay flocs overlying saltwater in the presence of gravitational instabilities the study seeks to determine 1 if flocculation hampers or alters interface instability formation 2 how the removal rates of sediment from the buoyant layer compare to those predicted by individual floc settling and 3 whether or not it is possible to develop a model for effective settling velocity the experiments were conducted in a tank at isothermal conditions all experiments were initially stably stratified but later developed instabilities near the interface that grew into downward convecting plumes of fluid and sediment throughout we measured sediment concentration in the upper and lower layers floc size and plume descent rates the data showed that flocculation modifies the mixture settling velocity and therefore shifts the mode of interface instability from double diffusive what one would expect from unflocculated clay to settling driven leaking and rayleigh taylor instability formation removal rates of sediment from the upper layer in the presence of these instabilities were on the same order of magnitude as those predicted by individual floc settling however removal rates were found to better correlate with the speed of the interface plumes a simple force balance model was found to be capable of reasonably describing plume velocity based on concentration in the buoyant layer this relation coupled with a critical grashof number and geometry relations allowed us to develop a model for the effective settling velocity of the mixture based solely on integral values of the upper layer

glacier surge propagation by thermal evolution at the bed bakaninbreen southern svalbard began a prolonged surge during 1985 in 1986 an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm unfrozen and cold frozen bed ground penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection interpreted as the top of a thick layer of sediment rich basal ice down glacier of the present surge front features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer the base of a permafrost layer and local ice lenses this indicates that this region of the bed is cold up glacier of the surge front a scattering zone above the basal reflection is interpreted as warm ice there is no evidence for this warm zone down glacier of the surge front nor do we see basal permafrost up glacier of it thus as in early surge phase the location of the surge front is now at the transition between warm and cold ice at the glacier bed we suggest that the propagation of the front is associated with this basal thermal transition throughout the surge because propagation of the front occurs rapidly and generates only limited heat basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice bed interface

application of borehole optical televiewing to investigating the 3 d structure of glaciers implications for the formation of longitudinal debris ridges midre lovenbreen svalbard digital optical televiewing optv of hot water drilled boreholes is evaluated as a technique for the investigation of englacial ice and debris structures on the basis of six boreholes drilled in the terminus region of midre lovenbreen svalbard the resulting optv logs successfully reveal several visually distinctive englacial ice properties and deformation structures e g oblique englacial fractures imaged here for the first time combining these optv logs with surface mapping has resulted in the identification of eight separate structural elements several of which can be interpolated onto 3 d grids at a node spacing of 1 m vertically and 10 m horizontally basally derived englacial sediment layers are also found to be intercalated with primary stratification elevated into near vertical planes around a central fold axis by large scale lateral folding the analysis also allows supraglacial longitudinal debris ridges to be subclassified into two types a previously described type i form which are the exposed fold axes of large scale lateral folds and a new type ii form experiencing secondary deformation by small scale horizontal folding in association with vertical displacements across arcuate shear planes in response to longitudinally compressive stresses near the glacier terminus although using boreholes to investigate glacier structure is limited e g by parallelism with vertical planes applying optv to multiple boreholes at midre lovenbreen has successfully revealed a range of 3 d structural elements at high spatial resolution as such interpolating between multiple optv logs overcomes many of the problems associated with interpretations made solely on the basis of surface based structural mapping and combining the two techniques represents a powerful glaciological tool

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

classification of poorly sorted sedimentary rocks abstract the classification and nomenclature of sandstones has evolved over the years into a system which is now generally accepted as standard however no equivalent system exists for the poorly sorted sedimentary rocks which include grain sizes from clay to gravel a descriptive classification is proposed based on an earlier system but using updated nomenclature and lithological fields such a scheme is needed to clearly define lithological terms currently used in the description of sedimentary sequences

enhanced sediment scavenging due to double diffusive convection abstract river water intruding into a lake or ocean typically results in a surface layer of warm sediment laden water overriding relatively dense cold fluid though this system is stable with respect to density instability may arise as a result of the different diffusivities of sediment and heat a series of experiments were conducted which indicated that double diffusive convection ddc was responsible for rapid sedimentation of slowly settling particles a simple theory based upon relevant physical processes is developed that describes this behavior it also incorporates the previously neglected effects of ambient stratification on the strength of the ddc application of the theory and our results to field data taken from oceanic river plumes demonstrates that the flux due to double diffusive sedimentation dds can be at least as strong as flocculation enhanced gravitational settling

the role of saltwater and waves in continental shelf formation with seaward migrating clinoform continental shelves have generally been interpreted as drowned coastal plains associated with the allogenic effect of sea level variation here without disputing this mechanism we describe an alternative autogenic mechanism for subaqueous shelf formation driven by the presence of dissolved salt in seawater and surface waves we use a numerical model describing flow hydrodynamics sediment transport and morphodynamics in order to do this more specifically we focus on two major aspects 1 the role of saltwater in the subaqueous construction of continental shelves and 2 the transformation of these shelves into seaward migrating clinoforms under the condition of repeated pulses of water and sediment input and steady wave effects but no allogenic forcing such as sea level change in the case for which the receiving basin contains fresh water of the same density as the sediment laden river water the hyperpycnal river water plunges to form a turbidity current that can run out to deep water in the case for which the receiving basin contains sea water but the river contains sediment laden fresh water the hypopycnal river water forms a surface plume that deposits sediment proximally this proximate proto shelf can then grow to wave base after which wave supported turbidity currents can extend it seaward the feature we refer to is synonymous with near shore mud belts

convective instability in sedimentation 3 d numerical study to provide a probable explanation on the field observed rapid sedimentation process near river mouths we investigate the convective sedimentation in stably stratified saltwater using 3 d numerical simulations guided by the linear stability analysis this study focuses on the nonlinear interactions of several mechanisms which lead to various sediment finger patterns and the effective settling velocity for sediment ranging from clay single particle settling velocity v0 0 0036 and 0 0144 mm s or particle diameter d 2 and 4 m to silt v0 0 36 mm s or d 20 m for very fine sediment with v0 0 0036 mm s the convective instability is dominated by double diffusion characterized by millimeter scale fingers gravitational settling slightly increases the growth rate however it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface for sediment with v0 0 0144 mm s rayleigh taylor instabilities become dominant before double diffusive modes grow sufficiently large centimeter scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes for sediment with v0 0 36 mm s rayleigh taylor mechanism dominates and the resulting centimeter scale sediment fingers show a plume like structure the flow pattern is similar to that without ambient salt stratification rapid sedimentation with effective settling velocity on the order of 1 cm s is likely driven by convective sedimentation for sediment with v0 greater than 0 1 mm s at concentration greater than 10 20 g l

dynamics of fingering convection part 1 small scale fluxes and large scale instabilities received and in revised form double diffusive instabilities are often invoked to explain enhanced transport in stablystratified fluids the most studied natural manifestation of this process fingering convection commonly occurs in the ocean s thermocline and typically increases diapycnal mixing by two orders of magnitude over molecular diffusion fingering convection is also often associated with structures on much larger scales such as thermohaline intrusions gravity waves and thermohaline staircases in this paper we present an exhaustive study of the phenomenon from small to large scales we perform the first three dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates of the induced turbulent transport our results are consistent with oceanic field measurements of diapycnal mixing in fingering regions we then develop a generalized mean field theory to study the stability of fingering systems to large scale perturbations using our calculated turbulent fluxes to parameterize small scale transport the theory recovers the intrusive instability the collective instability and the instability as limiting cases we find that the fastest growing large scale mode depends sensitively on the ratio of the background gradients of temperature and salinity the density ratio while only intrusive modes exist at high density ratios the collective and instabilities dominate the system at the low density ratios where staircases are typically observed we conclude by discussing our findings in the context of staircase formation theory

influence of filling drawdown cycles of the vajont reservoir on mt toc slope stability abstract in the present work the 1963 vajont landslide has been back analyzed in detail to examine the influence of reservoir operations filling and drawdown on mt toc slope stability the combined seepage slope stability analyses carried out show that the main destabilizing factor that favored the 1963 vajont landslide was the reservoir induced water table that formed as a consequence of rapid seepage inflow within the submerged toe of the slope decrease in the factor of safety fos up to 12 compared to the initial slope stability condition i e in the absence of the vajont reservoir rainfall would only have been a decisive factor if the initial stability condition of the mt toc slope had already been very close to failure decrease in fos caused by heavy or prolonged rainfall is about 3 4 for the worst case scenario analyzed the permeability of the shear zone material occurring at the base of the prehistoric vajont rockslide has been evaluated at 5 10 4 m s and back calculated values of the friction angles range from 17 5 to 27 5 when considering mountain reservoirs slope failures can occur during both filling and drawdown phases in the vajont case owing to the highly permeable materials of the shear zone slope stability decreased during filling and increased during drawdown another displacement dependent phenomenon of a mechanical nature progressive failure of the ne landslide constraint has to be considered to understand the slope collapse that occurred during the last drawdown 26 september 9 october 1963 the results of the combined seepage slope stability models indicate that permeability of bank forming material and filling drawdown rates of reservoirs can strongly influence slope stability slow lowering of the reservoir level is a necessary measure to reduce the occurrence of very dangerous transient negative peaks of fos

controls on glacier surging in svalbard the geographical distribution of surge type glaciers worldwide displays a remarkably non random pattern surge type glaciers tend to be concentrated in certain glacierized areas and to be completely absent in others this observation suggests that special conditions are required for surges to occur however the factors controlling the spatial occurrence of surge type behaviour are not known to investigate this problem we performed probability statistical analysis on a sample population of 615 glaciers in svalbard the probability that a glacier in the sample population is surge type is 36 4 within the sampled area there is a spatial variation in the concentration of surge type glaciers several geometric and environmental factors associated with glaciers in the sample population were measured and tested to determine if they are related to the probability of surging of the geometric factors tested length slope elevation orientation and presence or absence of tributaries only glacier length is related to surging with surge probability increasing with increasing length elevated probabilities of surging were also found for glaciers associated with sedimentary subglacial rocks and sub polar thermal regimes the distributions of related factors were used to predict the spatial distribution of surge type glaciers however in each case the individual factors were unable to reproduce the observed pattern of surge type glacier distribution

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

the formation of a geometrical ridge network by the surge type glacier kongsvegen svalbard traditionally geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge crevasse fill ridges they are widely regarded as a characteristic landform of glacier surges understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record the geometrical ridge network at the surge type glacier kongsvegen in svalbard does not form by crevasse filling the networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin the transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice the thrusts were apparently formed during a glacier surge in 1948 longitudinal ridges form through the meltout of elongated pods of debris which on the glacier surface are subparallel to the ice foliation and pre date the surge this work adds to the range of landforms associated with glacier surges

enhanced sedimentation beneath particle laden flows in lakes and the ocean due to double diffusive convection the transport rate of particles beneath sediment laden overflows and interflows in lakes and in the ocean can be enhanced by double diffusive and settling driven convection in previous experiments with sediment laden fluid overlaying a saline layer visual measurements could only be made in the optically clear lower layer hence there was difficulty distinguishing the two processes hindering predictions of when enhanced sedimentation occurs we used an acoustic doppler velocimeter adv to measure velocities and turbulence above and below the initial sediment salt interface the velocity of the sediment fingers in the lower layer were always larger than the stokes settling velocity of the particles leading to an asymmetry in the flow field of the two convective layers sediment fingers only dominated when there were marginal density differences between the two layers we conclude that double diffusive sediment fingers control sedimentation beneath interflows in most lakes whereas settling driven convection is dominant in most oceanic overflows

the carstairs kames lanarkshire scotland morphology sedimentology and formation the carstairs kames have been the subject of debate for over 150 yr a synthesis of previous work along with new sedimentological data is used to reappraise this classic british esker locality three morphological elements are recognised i large sinuous and anastomosing ridges ii large irregular linear mounds and iii small low and irregular mounds the main ridge system is composed of a core of boulder gravel with sand pebble gravel above and adjacent to the core the large linear mounds contain a varied facies range which include glaciolacustrine sediments deltaic sequences laminites and dropstones debris flows and glaciofluvial channel systems all the deposits both in the main ridges and in the adjacent linear mounds contain syn and post depositional subsidence structures including normal faults synclinal folds and growth faults a range of potential origins for these landforms is reviewed and it is suggested that glaciofluvial sedimentation in a supraglacial environment controlled by a topography of ice cored ridges is the most appropriate depositional model 1997 john wiley sons ltd

the structural geometric and volumetric changes of a polythermal arctic glacier during a surge cycle comfortlessbreen svalbard various parameters of the most recent surge of the polythermal glacier comfortlessbreen in northwest svalbard have been assessed through a combination of remote sensing and ground observations analysis of a digital elevation model time series shows a marked change in the geometry of the glacier from quiescence 1990 and earlier into the late surge phase 2009 the transfer of 0 74 km3 of ice caused up to 80 m of surface drawdown in the reservoir area above the equilibrium line whilst ice built up in a spatially concentrated manner in the receiving zone below the equilibrium line a ramp of ice c 100 m above quiescent level developed in the lower reaches of the glacier late in the surge also in the lower reaches of the glacier structures attributable to the passage of a kinematic wave are identified and the migration of a surge front on the glacier is thus inferred in a conceptual model we consider that a bend in the valley in which the glacier resides and convergence with tributary glaciers to be significant factors in the style of surge evolution their flow restrictive interference results in slow initial mass transfer and the growth of a surge front within 3 4 km of the terminus copyright 2015 john wiley sons ltd

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

sediment laden fresh water above salt water nonlinear simulations when a layer of particle laden fresh water is placed above clear saline water both double diffusive and rayleigh taylor instabilities may arise the present investigation extends the linear stability analysis of burns meiburg j fluid mech vol 691 2012 pp 279 314 into the nonlinear regime by means of two and three dimensional direct numerical simulations dns the initial instability growth in the dns is seen to be consistent with the dominant modes predicted by the linear stability analysis the subsequent vigorous growth of individual fingers gives rise to a secondary instability and eventually to the formation of intense plumes that become detached from the interfacial region the simulations show that the presence of particles with a stokes settling velocity modifies the traditional double diffusive fingering by creating an unstable nose region in the horizontally averaged profiles located between the upward moving salinity and the downward moving sediment interface the effective thickness l s l c of the salinity sediment interface grows diffusively as does the height h of the nose region the ratio h l s initially grows and then plateaus at a value that is determined by the balance between the flux of sediment into the rose region from above the double diffusive rayleigh taylor flux out of the nose region below and the rate of sediment accumulation within the nose region for small values of h l s leqslant o 0 1 double diffusive fingering dominates while for larger values h l s geqslant o 0 1 the sediment and salinity interfaces become increasingly separated in space and the dominant instability mode becomes rayleigh taylor like a scaling analysis based on the results of a parametric study indicates that h l s is a linear function of a single dimensionless grouping that can be interpreted as the ratio of inflow and outflow of sediment into the nose region the simulation results furthermore indicate that double diffusive and rayleigh taylor instability mechanisms cause the effective settling velocity of the sediment to scale with the overall buoyancy velocity of the system which can be orders of magnitude larger than the stokes settling velocity while the power spectra of double diffusive and rayleigh taylor dominated flows are qualitatively similar the difference between flows dominated by fingering and leaking is clearly seen when analysing the spectral phase shift for leaking dominated flows a phase locking mechanism is observed which intensifies with time hence the leaking mode can be interpreted as a fingering mode which has become phase locked due to large scale overturning events in the nose region as a result of a rayleigh taylor instability

the structural geology of a surge type glacier abstract the range of deformation structures found in surge type variegated glacier alaska is similar to that found in many thin skinned thrust belts and includes foliations folds dip slip and strike slip faults thrust faults and fractures the development of structural relationships and the overall structural assemblage can be related to the deformation histories of quiescent and surge phases of motion the surge phase dominates the formation of brittle structures because of the large stresses and strain rates associated with it the relationship between downglacier verging overturned folds with hinge lines transverse to flow and a family of transverse arcuate thrust faults is characteristic of structural development in the lower part of the glacier during the surge longitudinal foliation develops mainly from sedimentary stratification in the upper part of the glacier in a marginal transpressive deformation regime during quiescent phase flow from broad accumulation basins the structural relationships in this surge type glacier may aid in the interpretation of structures in other gravity driven tectonic settings