painting a picture of gas hydrate distribution with thermal images 1 a study of cold thermal anomalies identified in continuous down core records of thermal structure acquired by an infrared ir camera during ocean drilling program odp leg 204 to southern hydrate ridge is combined with core based observations of structure and lithology to present new insights into the influence of lithology on hydrate distribution in an accretionary margin hydrate distribution is spatially associated with 0 5 cm thick laterally discontinuous sand horizons that compose 1 of the sedimentary section at all sites proximity to the fractured ridge crest determines the extent to which individual sand horizons host hydrate with 70 of the horizons hosting hydrate at the crest and only 29 in the basin this trend is consistent with focused up dip migration of fluids along permeable lithologic conduits and shows that lateral flow may be as important as vertical diffusion and fracture flow in determining gas hydrate distribution at hydrate ridge

post subduction margin structures along boyd strait antarctic peninsula abstract the pacific margin of the antarctic peninsula to the southwest of the hero fracture zone hfz is a former subducting margin which became inactive following the arrival of ridge crest segments of the antarctic phoenix ridge at the margin during the tertiary in contrast the part of the margin to the northeast of the hfz remains active tertiary convergence was approximately perpendicular to the margin and ongoing motion is thought to have the same orientation a new seismic reflection profile running along boyd strait just northeast of the landward projection of the hfz shows major structural components similar to those typically observed along the margin to the southwest of the hfz in order of increasing proximity to the margin these components are the inner shelf the shelf basin the mid shelf basement high msbh and the outer shelf the continuation of the post subduction margin structures to the active margin suggests that the boundary between crust with passive and active margins characteristics is not sharply defined our postulated scenario for tectonic evolution along boyd strait is that 1 before the arrival of the last ridge crest segment to the southwest of the hfz the inner shelf and the shelf basin were part of a cretaceous tertiary arc and forearc area 2 after the arrival thermal effects resulting from interaction of the ridge crest with the margin just southwest of the hfz lead to the formation of the msbh to the northeast but msbh uplift in boyd strait did not prevent concurrent cross shelf sediment transport contributing to development of an extensive outer shelf on the seaward flank of the msbh 3 recent extension in bransfield strait a marginal basin to the northeast of the landward projection of the hfz has caused about 10 km of seaward deflection in the strike of the part of the msbh to the northeast of the projection of the hfz

ice growth and oceanic heat flux models and measurements heat fluxes at the ice ocean interface and ice thickness are investigated by comparing field data from the coordinated eastern arctic experiment cearex drift phase with model calculations the calculations are based on two types of models the first one is a one dimensional ice ocean model with high vertical resolution this model is based on the conservation equations for heat salt and momentum and uses turbulence models to achieve closure a discrete element approach is also introduced to explicitly parameterize the ice roughness the second model is a simple one dimensional bulk heat transfer model in this version the interfacial salinity is modelled on the basis of salt conservation at the ice ocean interface the bulk heat transfer model is then calibrated using the former model the two models predict ocean heat fluxes that are quite variable in time owing to short term variations in the ice drift both models calculate realistic ice thicknesses it is demonstrated that the observed time variation in ice thickness from eight different experimental sites with varying initial thicknesses and bottom topographies can be reproduced by applying bulk heat transfer coefficients in the range 2 8 1 10 4 horizontal variation of the thermal state within a single pack ice floe results in simultaneous freezing and melting over relatively small spatial scales when modeling or averaging ice data in space these aspects need to be considered

migration and venting of deep gases into the ocean through hydrate choked chimneys offshore korea it has recently been recognized that in addition to low concentrations of widespread natural gas hydrate associated with bottomsimulating seismic refl ectors highly concentrated hydrate can occur in local seafl oor fl uid venting structures such structures extend upward into the regional gas hydrate stability fi eld and sometimes allow gases to escape into the overlying ocean these hydrate choked chimneys are especially prospective as an energy resource because they contain high hydrate concentrations furthermore they may be one of the most important conduits into the ocean atmosphere system for deep methane we present an analysis of two dimensional seismic refl ection data from offshore korea that give a complete picture of gas migrating from a deep source zone to feed hydrate choked vent structures at the seafl oor the gases migrate upward through networks of fractures imaged as steep amplitude striations in both diffuse and concentrated distributions we present an example of a high fl ux gas vent fed through fracture swarms emanating from a 10 15 km wide catchment zone of source gases residing 3 4 km below the seabed the geological context and the inferred distribution of hydrate within this feature are consistent with recent models in which seabed gas venting is a consequence of elevated pore fl uid salinities that are produced by a high fl ux of gases migrating independently of the pore water in contrast a nearby vent that is not plumbed into the same high fl ux system appears to be dominated more by gas rich liquids we present these two vent structures as type cases for high and lowfl ux fl uid escape systems in the seabed furthermore we suggest that since the amount of gas trapped as hydrate within the vents is small compared with the amount in the underlying reservoir the greatest risk for increased methane input to the atmosphere associated with climate driven oceanic warming is not the melting of hydrate but an increase in the number of deep reservoirs able to vent gases through the seabed that occurs as the regional gas hydrate stability zone thins

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

permeability evolution during the formation of gas hydrates in marine sediments 1 fluxes of gas energy and fluid water with dissolved gas are known to control the distribution of gas hydrate and free gas in gas hydrate reservoirs but theoretical studies have so far not focused on the effect of sediment properties e g permeability on these fluxes during the formation of gas hydrate deposits using a transient one dimensional two phase gas hydrate and fluid numerical model based on a coupled conservation approach we examine permeability clogging during gas hydrate formation in end member porous media and fractures initially lacking hydrate and determine the resulting changes in fluid and advective energy flux gas hydrate accumulates most rapidly at depth in both systems a result consistent with borehole logging and seismic studies on the blake ridge consideration of the ratio of surface area to fluid volume explains model results that imply more rapid permeability reduction in the porous media system and more uniform hydrate formation as a function of depth in the fracture system depending on fluid flux fracture aperture or pore sizes and other factors the timescale for permeability clogging due to the accumulation of gas hydrate ranges from thousands to millions of years regenerating gas hydrate to a concentration of 10 15 of pore space in marine settings with fluid fluxes characteristic of hydrate ridge or blake ridge requires 103 105 years meaning that hydrate deposits in these settings cannot be viewed as a naturally renewable resource these results also imply that significant time is required to reestablish gas hydrate deposits after large scale dissociation events associated with global scale climate change

in situ measurements of the evolution of young sea ice 1 we present data from in situ measurements of the salinity evolution of young sea ice in the arctic the measurements were carried out with very high vertical and temporal resolution over the course of a few days until the ice had reached a thickness of around 20 cm the measured bulk salinity profiles show that during ice growth sea ice salinity is continuous across the ice ocean interface and that there is no instantaneous loss of salt at the advancing front measured salt fluxes emanating from the ice are as high as 90 g m 2 h 1 during the first few hours of new ice formation and are roughly half as large during later stages of the experiments the bulk salinity within the ice decreases continuously with time from the ocean water salinity to a near steady state value of around 4 parts per thousand ppt these results are interpreted with ideas from mushy layer theory using a rayleigh number to analyze gravity drainage as the driving mechanism for the observed salt loss in our experiments gravity drainage occurs for a critical rayleigh number of around 10 in close agreement with earlier theoretical and experimental studies

polynya dynamics a review of observations and modeling 1 polynyas are nonlinear shaped openings within the ice cover ranging in size from 10 to 105 km2 polynyas play an important climatic role first winter polynyas tend to warm the atmosphere thus affecting atmospheric mesoscale motions second ocean surface cooling and brine rejection during sea ice growth in polynyas lead to vertical mixing and convection contributing to the transformation of intermediate and deep waters in the global ocean and the maintenance of the oceanic overturning circulation since 1990 there has been an upsurge in polynya observations and theoretical models for polynya formation and their impact on the biogeochemistry of the polar seas this article reviews polynya research carried out in the last 2 decades focusing on presenting a state of the art picture of the physical interactions between polynyas and the atmosphere sea ice ocean system observational and modeling studies the surface heat budget and water mass transformation within these features are addressed

depositional models for moraine formation in east antarctic coastal oases this paper examines the origin of moraine ridges in east antarctic coastal oases and derives depositional models appropriate for the reconstruction of quaternary history on the basis of morphology structure and sedimentology four principal types of ridge may be identified 1 type a moraines form whenthe basal debris zone crops out near an ice margin 2 type b moraines form when large recumbent folds develop in the basal debris zone 3 type c moraines are ice contact screes and fans which form when debris accumulates at streep or cliffed ice margins and 4 type d moraines are thrust block moraines that form when unconsolidated sediment is entrained by freezing shearing and thrusting of sediment blocks at the base of the glacier simple calcumations of the rate of debris accumulation at ice margins suggest that type a b and c moraines take thousands of years to form and record stable ice margins type d moraines are structural features that may form relatively quickly when ice margins override unconsolidated sediment constructing models to explain the origin of the moraines is an important part of reconstructing the quaternary history of antarctic coastal oases because the models provide a basis for reconstructing the position and behaviour of the ice sheet during advance and retreat

formation of ice shelf moraines by accretion of sea water and marine sediment at the southern margin of the mcmurdo ice shelf antarctica a combination of ground penetrating radar surveys physical sedimentology and ice composition measurements has been used to characterize ice and sediment accreted to the southern margin of the mcmurdo ice shelf antarctica the radar data and surface observations show that the iceshelf margin consists of strongly layered debris rich ice that contains marine sediment and fossils a modified rayleigh based distillation system has been used to model the isotopic fractionation from sea water to ice in a closed system the model of ice formation is consistent with formation during almost complete freezing of a sea water reservoir by contrast ice on the upstream side of the grounding line has formed during the early stages of freezing in which a small fraction of the sea water reservoir has frozen the model results and the presence of delicate well preserved marine fossils are interpreted as evidence of anchor ice formation on the seabed and rafting of glaciomarine sediment into the bottom of the ice shelf we argue that repeated accretion of sea water and marine sediment has produced a stacked sequence of ice and glaciomarine debris that forms shore parallel ice cored moraines

characterization of excess pore pressures at the toe of the nankai accretionary complex ocean drilling program sites 1173 1174 and 808 results of one dimensional modeling 1 elevated fluid pore pressures play a critical role in the development of accretionary complexes including the development of the decollement zone in this study we used measured permeabilities of core samples from ocean drilling program odp leg 190 to develop a permeability porosity relationship for hemipelagic sediments at the toe of the nankai accretionary complex this permeability porosity relationship was used in a one dimensional loading and fluid flow model to simulate excess pore pressures and porosities simulated excess pore pressure ratios as a fraction of lithostatic pressure hydrostatic pressure using the best fit permeability porosity relationship were lower than predicted from previous studies we then tested sensitivity of excess pore pressure ratios in the underthrust sediments to bulk permeability lateral stress in the prism and a hypothetical low permeability barrier at the decollement our results demonstrated significant increase in pore pressures below the decollement with lower bulk permeability such as obtained by using the lower boundary of permeability porosity data or when a low permeability barrier is added at the decollement in contrast pore pressures in the underthrust sediments demonstrated less sensitivity to added lateral stresses in the prism although the profile of the excess pore pressure ratio is affected both simulations with lateral stress and a low permeability barrier at the decollement resulted in sharp increases in porosity at the decollement similar to that observed in measured porosities furthermore in both scenarios maximum excess pore pressure ratios were found at the decollement suggesting that either of these factors would contribute to stable sliding along the decollement

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

mapping of large scale diapir structures at the paleo ice tongue bed in western latvia from geophysical investigations and borehole data abstract this study comprises the investigation of the complex and deformed late pleistocene sedimentary sequence in western latvia composed of sandy sediments that are protruded by large scale clayey silt diapirs and covered by glaciotectonically disturbed discontinuous patches of the late weichselian till we use detailed measurements by ground penetrating radar gpr and electrical resistivity tomography ert combined with borehole data and sedimentological investigations at the baltic sea cliffs to map and characterize the large scale deformation structures and discontinuous till patches we distinguish the intriguing and previously not detected characteristics of the spatial distribution pattern of discontinuous till layer represented as elongated up to 150 m long and up to 50 m wide patches parallel to the seacoast and perpendicular to the latest ice flow direction the upper surface of till patches is flat and not inclined in any direction that is partially explainable by the postglacial erosion of the baltic ice lake the possible locations of diapirs inland are drawn from gpr data we relate the formation of observed large scale deformations to subglacial diapirism induced by the ice loading of the advancing apri i ice tongue clayey silt sediments become loaded and overpressurized and flowed upwards intruding into the overlying sandy sediments the initial deposition and deformation of till layer could have been affected by the diapirism process and preferred accretion of till in inter diapir spaces we demonstrate that the geophysical methods can greatly supplement traditional geological investigations in the field revealing characteristics of deformed sediment sequences that are not recognizable from the outcrop data only and thus can have significant implications on the interpretation of deformation processes

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

an approach to estimate gas hydrate saturation from 3 d heterogeneous resistivity model a study from krishna godavari basin eastern indian offshore abstract a method is proposed to estimate gas hydrate saturation from three dimensional 3 d heterogeneous model of resistivity simulated using resistivity log pure gas hydrates are highly resistive compared to the host sediments and their presence in the pore space of sediments increase the resistivity of the formation the anomalous increase of resistivity is used as a proxy for the delineation of gas hydrates using the resistivity log a 3 d heterogeneous resistivity model has been constructed from one dimensional resistivity log in krishna godavari kg basin eastern indian offshore the simulated model contains all small scale variation in resistivity of the reservoir and maintains all properties associated with covariance like root mean square fluctuation characteristic scales and fractal dimension of the observed log we have estimated volumetric hydrate saturation using the three dimensional simulated model the porosity used for estimating hydrate saturation is calculated from the simulated density field generated using the observed density log estimated average gas hydrate saturation is about 9 84 of the pore volume over a 1000 m 1000 m x 131 m cubic meters area

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

solidification of leads theory experiment and field observations thin sea ice plays a central role in the surface heat and mass balance of the arctic ocean in order to develop understanding of these balances we describe and analyze highly resolved temperature data taken through the air sea ice interface during the transition from an ice free to an ice covered arctic ocean surface the data were taken to observe the thermodynamic evolution of a lead a process that has previously only been accessible to measurement techniques confined to the lead edge our detailed analysis of the field data is guided by recent theoretical and experimental advances in understanding the phase dynamics of directionally solidified alloys because of the dearth of direct observations we also present time series of the relevant heat fluxes inferred from our data and demonstrate the controlling influence that the internal phase evolution has on these quantities we have previously examined the stability of the brine trapped in a growing sea ice matrix both theoretically and experimentally and now find that haline convection driven from within the growing layer is consistent with this previous work and with the nature of direct turbulence measurements the importance of this process is that although ice growth is continuous the local brine flux commences abruptly only after some time in contrast to what had previously been supposed hence the ice growth process itself is a source of intermittency in oceanic boundary layer turbulence furthermore we find that in this particular situation the sea ice growth is not simply a square root function of time in contrast to the model typically used in numerical simulations by far the most practical methods of studying lead convection are numerical simulations and laboratory models and a strong conclusion of this study is the importance of the proper treatment of the boundary conditions describing the buoyancy flux

entrainment of glaciomarine sediments and formation of thrust block moraines at the margin of sorsdale glacier east antarctica the morphology sedimentology and structure of moraines at the margin of an outlet glacier in east antarctica are described and contemporary depositional processes in a marine inlet adjacent to the ice margin are examined results indicate that the principal moraines are thrust block moraines produced by basal freezing and deformation of glaciomarine sediment as the outlet glacier expands into a marine inlet preservation of detailed glaciomarine sedimentary structures and beds of marine shells suggests that the sediment was frozen during entrainment transportation and deposition the presence of low angle faults in the moraines show that the moraines consist of an en echelon arrangement of thrust plates the sedimentology structure thickness of the thrust plates and inferred entrainment processes are consistent with weertman s ice debris accretion hypothesis for debris entrainment at the edge of cold ice sheets a model of thrust block moraine development produced by this study provides a framework for the interpretation of radiocarbon dates from marine macrofossils in the moraines the model may also be useful in the interpretation of similar moraines in coastal east antarctic oases and other polar marginal marine environments 1997 by john wiley sons ltd

pebble fabric in an ice rafted diamicton pebble fabric studies on ice rafted diamictons have been limited to general observations with authors noting preferences toward vertical random or horizontal orientations to clarify such observations pebble fabric data were collected from a fossiliferous diamicton of late pleistocene age located on whidbey island washington the ice rafted origin of this unit is supported by several independent characteristics including in situ macrofauna and microfauna conformity with subaqueous lithofacies containing dropstones lower bulk densities and higher void ratios than associated tills soft sediment deformation structures suggestive of iceberg dumping textural gradations and facies relationships analysis using the eigenvalue method indicates that ice rafted fabrics are nearly random with little consistency of vector orientations between sites and without any relationship to the probable direction of glacial flow the weak fabric is mainly the product of settling through the water column and impact wit

gas hydrate versus geological features the south shetland case study abstract a gas hydrate reservoir is hosted in marine sediments of an accretionary prism located offshore the south shetland islands antarctic peninsula and affected by widespread deformations to analyse gas hydrate distribution and fluid circulation inside sediments available velocity models were used seismic velocities are translated in terms of hydrate porosity which is the difference between the reference porosity i e the porosity without gas hydrate and the effective porosity i e the porosity reduced by the gas hydrate presence the pre stack depth migration sections underlined the presence of several geological features such as gentle and open folds fractures and faults in this paper we observed a relationship between syncline anticline structures and hydrate presence in particular a relationship is underlined between the hydrate porosity values and the distance from the hinge of the anticline the hydrate porosity increases toward the limbs of anticline the micro fracturing model supports the idea that the syncline favours the hydrate formation while the anticline favours the free gas accumulation below the bottom simulating reflector

investigations on physical and textural properties of arctic first year sea ice in the amundsen gulf canada november 2007 june 2008 ipy cfl system study we report sea ice temperature and bulk salinity measurements as well as textural analysis from 33 first year drift and fast ice stations sampled between november 2007 and june 2008 in the southern beaufort sea amundsen gulf canadian arctic during the international polar year circumpolar flaw lead ipy cfl system study we use this significant dataset to investigate the halo thermodynamic evolution of sea ice from growth to melt a strong desalination phase is observed over a small time window in the spring using calculated proxies of sea ice permeability brine volume fraction and of the intensity of brine convection rayleigh number we demonstrate that this phase corresponds to full depth gravity drainage initiated by a restored connectivity of the brine network with warming in the spring most stations had a textural sequence typical of arctic first year ice with granular ice overlying columnar ice unusual textural features were observed sporadically sandwiched granular ice platelet ice and draped platelet ice we suggest that turbulence in leads and double diffusion in strong brine plumes following the refreeze of cracks are plausible mechanisms for the formation of these textures

numerical simulations of the profile properties of undeformed first year sea ice during the growth season a simulation scheme is developed that estimates salinity profiles for first year sea ice during the growth season as a function of the growth history of the ice the model considers the dependence of the initial ice salinity on ice growth velocity and sea water salinity and also the subsequent drainage of brine from the ice the equation for ice growth assumes a linear temperature profile within the ice and is driven by surface heat balance equations that are based on smoothed climatic data for the central arctic basin the estimated salinity profiles are in good agreement with natural profiles although temperature and salinity profiles depend upon the time of the year when ice growth is initiated the brine volume profiles which they specify are essentially a unique function of ice thickness a conclusion that holds even when the insulative effects of snow are considered the temperature and brine volume profiles are then utilized to calculate the ice strength and elastic modulus profiles which in turn specify the composite mechanical properties of the ice sheets through the application of a theory developed by assur 1967 significant differences which are largest for thin ice sheets are observed between ice sheet properties as calculated using composite plate theory and properties calculated from uniform plate theory and average ice properties these results provide a justification for the practice common within the ice modeling community of parameterizing the mechanical behavior of pack ice on the basis of the ice thickness distribution

the oceanography of winter leads leads in pack ice have long been considered important to the thermodynamics of the polar regions a winter lead affects the ocean around it because it is a density source as the surface freezes salt is rejected and forms more dense water which sinks under the lead this sets up a circulation with freshwater flowing in from the sides near the surface and dense water flowing away from the lead at the base of the mixed layer if the mixed layer is fully turbulent this pattern may not occur rather the salt rejected at the surface may simply mix into the surface boundary layer in either event the instability produced at the surface of leads is the primary source of unstable buoyancy flux and as such exerts a strong influence on the mixed layer here as many as possible of the disparate and almost anecdotal observations of lead oceanography are assembled and combined with theoretical arguments to predict the form and scale of oceanographic disturbances caused by winter leads the experimental data suggest the velocity disturbances associated with lead convection are about 1 5 cm s 1 these appear as jets near the surface and the base of the mixed layer when ice velocities across the lead are less than about 5 cm s 1 the salinity disturbances are about 0 01 to 0 05 psu scaling arguments suggest that the geostrophic currents set up by the lead density disturbances are also of the order of 1 5 cm s 1 the disturbances are most obvious when freezing is rapid and ice velocity is low because the salinity and velocity disturbances in the upper ocean are not smeared out by turbulence in this vein lead convection may be characterized at one extreme as free convection in which the density disturbance forces the circulation at the other extreme lead convection may be characterized as forced convection in which the density disturbance is mixed rapidly by boundary layer turbulence the lead number lo which is the ratio of the pressure term to the turbulence term in the momentum equation and the turbulent lead number lot which is the ratio of buoyant production to shear production in the turbulent kinetic energy equation define the boundary between the free and forced regimes for lo and lot less than one both the large scale circulation and the turbulence are forced by surface stress for lo and lot greater than one both the large scale circulation and the turbulence are forced by the buoyancy flux the magnitudes of velocity and salinity disturbances from a model developed elsewhere suitable to free convection agree with what few observations we have the results of a forced convection model developed here suggest salinity disturbances of the order of 0 01 0 02 practical salinity units with the maximum occurring at the surface of the lead and decreasing substantially below 5 10 m this unstable gradient is a unique characteristic of lead convection though the salinity disturbances may be small when ice velocities are large the buoyancy flux in leads has a major effect on the boundary layer turbulence

unglaciated areas in east antarctica during the last glacial marine isotope stage 3 new evidence from rauer group abstract limited information on the east antarctic ice sheet eais geometry during marine isotope stage 3 mis 3 60 25 ka restricts our understanding of its behaviour during periods of climate and sea level change ice sheet models forced by global parameters suggest an expanded eais compared to the holocene during mis 3 but field evidence from east antarctic coastal areas contradicts such modelling and suggests that the ice sheet margins were no more advanced than at present here we present a new lake sediment record and cosmogenic exposure results from bedrock which confirm that rauer group eastern prydz bay was ice free for much of mis 3 we also refine the likely duration of the last glacial maximum lgm glaciation in the region lacustrine and marine sediments from rauer group indicate the penultimate period of ice retreat predates 50 ka the lacustrine record indicates a change from warmer wetter conditions to cooler drier conditions after ca 35 ka substantive ice sheet re advance however may not have occurred until much closer to 20 ka contemporary coastal areas were still connected to the sea during mis 3 restricting the possible extent of grounded ice in prydz bay on the continental shelf in contrast relative sea levels rsl deduced from field evidence indicate an extra ice load averaging several hundred metres thicker ice across the bay between 45 and 32 ka thus ice must either have been thicker immediately inland with a steeper ice profile or there were additional ice domes on the shallow banks of the outer continental shelf further work is required to reconcile the differences between empirical evidence of past ice sheet histories and the history predicted by ice sheet models from far field temperature and sea level records

geomorphological characterisation and interpretation of a mid latitude glacier like form hellas planitia mars we describe and interpret the surface terrain types associated with a widely reported 4 km long midlatitude martian viscous flow feature vff the feature is located in the southern hemisphere on the poleward facing rim of a 60 km diameter crater in eastern hellas planitia high resolution imaging science experiment hirise images analysed in both 2d and 3d reveal that the upper margin of the feature is bounded by steep 30 headwalls typically some tens of metres high that are formed from unconsolidated material and characterised by a series of slope parallel linear incisions below these incised headwalls the feature flows at a general angle of 10 from a broad upper basin to a confined lower tongue that is bounded by a nested sequence of elongate raised ridges these characteristics are typical of several vffs in the region and are strikingly similar to moraine bounded valley glaciers on earth and we sub classify this feature as a glacier like form glf 1 the glf comprises five distinctive surface terrain types that contrast sharply with surface characteristics outside its bounding moraines four of these terrains scaly terrain polygonized terrain linear terrain and mound and tail terrain are located within the glf s innermost bounding moraine while the fifth rectilinear ridge terrain is located between its frontal moraines these terrains are mapped characterised and associated with possible mechanisms of formation to draw inferences about the glf s glaciology and glacial history this analysis suggests that the glf reached its maximal extent in the geologically recent past and that it may have been partially wetbased at that time subsequent to this phase the glf experienced an extended period of general recession that has been punctuated by several episodes of still stand or advance currently the glf s basin appears to be composed of a lower zone that is dominated by an exposed former glacier bed and an upper zone that may still contain a now degraded and dust mantled viscous mass similar to many partially glacierized basins on earth

the phase evolution of young sea ice sea ice forms as a porous matrix consisting of pure ice crystals in equilibrium with brine we present an experimental and theoretical study that reveals that the brine expelled by growing sea ice initially remains trapped within the crystalline interstices but that drainage into the underlying water is ultimately triggered abruptly due to the onset of buoyancy driven convection the onset is quantified by a critical porous medium rayleigh number and we present an empirical marginal stability diagram that defines the transition

development of sediment landform associations at cold glacier margins dry valleys antarctica hambrey m j fitzsimons s j 2010 development of sediment landform associations at cold glacier margins dry valleys antarctica sedimentology 57 3 857 882 michael hambrey is corresponding author sponsorship marsden fund of new zealand university of otago research committee nerc grant no ner a s 2000 01256

seismic character of bottom simulating reflectors examples from the mid norwegian margin three classes of bottom simulating reflectors bsr cross cut the post breakup sediments of the mid norwegian margin the first class is caused by free gas at the base of the pressure and temperature dependent gas hydrate stability zone the second class of bsr is caused by the diagenetic transition from opal a to opal ct the third class of bsr is always observed underneath the opal a opal ct transition but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector odp site 643 drilling results from the voring plateau suggest two possible processes as the reason for this third bsr a smectite illite conversion or b a sudden increase in the abundance of authigenic carbonates the genesis of both is pressure and temperature dependent and could potentially result in a cross cutting seismic reflector the data are not conclusive as to which process is causing the third class of observed bsr

late quaternary environmental and climate history of rauer group east antarctica the rauer group is an archipelago in prydz bay east antarctica the ice free islands and the surrounding shallow marine areas provide valuable archives for the reconstruction of the late pleistocene and holocene environmental and climatic history of the region two sediment records from two marine inlets of rauer group have been studied for their sedimentological geochemical and biological characteristics radiocarbon ages from one of the inlets indicate ice free conditions within the last glacial cycle probably during the second half of marine isotope stage 3 subsequent ice sheet coverage of rauer group during the last glacial maxiumum lgm can be inferred from a till layer recovered in one of the basins the inlets became ice free prior to 11 200 cal yr bp when biogenic sedimentation started deglacial processes in the catchments however influenced the inlets until 9200 cal yr bp as evidenced by the input of minerogenic material marine productivity under relatively open water conditions indicates an early holocene climate optimum until 8200 cal yr bp which is followed by a cooler period with increased sea ice warmer conditions are inferred for the mid holocene when both basins experienced an input of freshwater between 5700 3500 cal yr bp probably due to ice sheet melting and increased precipitation on the islands neoglacial cooling in the late holocene since c 3500 cal yr bp is reflected by an increase in sea ice in both inlets 2010 elsevier b v

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

marine ice recycling at the southern mcmurdo ice shelf antarctica marine ice accretes at the base of ice shelves often infilling open structural weaknesses and increasing ice shelf stability however the timing and location of marine ice formation remain poorly understood this study determines marine ice source water composition and origin by examining marine ice crystal morphology water isotope and solute chemistry in ice samples collected from the southern mcmurdo ice shelf smis antarctica the measured co isotopic record together with the output of a freezing model for frazil crystals indicate a spatio temporally varying water source of sea water and relatively fresher water such as melted meteoric or marine ice this is in agreement with the occurrence of primarily banded and granular ice crystal facies typical for frazil ice crystals that nucleate in a supercooled mixture of water masses we propose that marine ice exposed at the surface of smis which experiences summer melt is routed to the ice shelf base via the tide crack here frazil crystals nucleate in a double diffusion mechanism of heat and salt between two water masses at their salinity dependent freezing point recycling of previously formed marine ice facilitates ice shelf selfsustenance in a warming climate

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

a non destructive method for measuring the salinity and solid fraction of growing sea ice in situ we describe an instrument developed to make in situ measurements of salinity and solid fraction profiles in growing sea ice the vertical resolution of the measurements is up to a few millimeters with a temporal resolution of up to fractions of a second the technique is based on impedance measurements between platinum wires around which sea ice grows data obtained using this instrument in laboratory experiments are in good agreement with theoretical predictions in a field test in the arctic the bulk salinity of growing sea ice has been measured in situ throughout the whole depth of the ice layer the data are compared with bulk salinities obtained from ice cores and confirm the general understanding that the bulk salinity in ice core studies is significantly underestimated in the lower parts of the cores the approach can also be used in other glaciological applications and for general studies of two phase two component porous media

motion of the scotia sea plates earthquake data from the scotia arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the scotia sea region events with magnitude greater than or equal to5 which occurred on or near the boundaries of the scotia and sandwich plates and for which centroid moment tensor cmt solutions are available are examined the newer data fill some of the previous sampling gaps along the boundaries of the scotia and sandwich plates and provide tighter constraints on relative motions variations in the width of the brunhes anomaly on evenly spaced marine magnetic profiles over the east scotia ridge provide new estimates of scotia sandwich plate spreading rates since there are no stable fracture zones in the east scotia sea the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading 18 newrate estimates and four azimuths from the east scotia ridge are combined with 68 selected earthquake slip vectors from the boundaries of the scotia sea in a least squares inversion for the best fitting set of euler poles and angular rotation rates describing the present day motions of the scotia and sandwich plates relative to south america and antarctica our preferred model tlp2003 gives poles that are similar to previous estimates except for scotia plate motion with respect to south america which is significantly different from earlier estimates predicted rates of motion also differ slightly our results are much more robust than earlier work we examine the implications of the model for motion and deformation along the various plate boundaries with particular reference to the north and south scotia ridges where rates are obtained by closure

the anatomy of a freezing lead winter leads are regions of intense ice growth with resultant large fluxes of heat to the atmosphere and salt to the ocean they constitute a major source of new ice in the arctic basin during the 1988 drift phase of the coordinated eastern arctic experiment we were afforded a unique opportunity to conduct a detailed long term study of a freezing lead measurements were made from september 17 to november 18 during which time the ice grew from open water to a thickness of 0 56 m cores were removed from the lead ice on a routine basis and analyzed for ice temperature salinity density and structure from these measurements the derived quantities of brine volume porosity heat flux to the atmosphere and salt flux to the ocean were computed in addition to this 2 month time series study of ice cores the spatial variation in lead ice properties was investigated on september 30 thin section studies of ice structure indicated that the upper 0 05 0 15 m of the ice sheet was granular and that the lower portion was columnar typically a portion of the granular layer was snow ice once the transition from granular to columnar ice had occurred granular ice did not reappear as the ice grew thicker the c axes of the ice crystals became aligned within the horizontal plane this alignment direction corresponded closely with the inferred direction of the current at the ice water interface vertical temperature profiles in the ice were approximately linear salinity profiles were usually c shaped with bulk salinities ranging from 9 to 6 before stabilizing at 6 for ice thicker than 0 35 m core data were used to compute the flux of heat to the atmosphere and the flux of salt to the ocean for seven time intervals during the experiment heat fluxes ranged from 89 to 29 w m2 with an average of 50 w m 2 roughly 3 times the corresponding value from multiyear ice the flux of salt from the lead ice to the ocean varied from 0 51 to 0 06 kg m 2 d 1 averaging 0 21 kg m 2 d 1

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

seasonal variations in the upper arctic ocean as observed at t 3 hydrographic data from t 3 are analyzed to illustrate the behavior of the arctic mixed layer the mixed layer depth fluctuates 11 m annually and mixed layer salinity fluctuates 0 32 the fluctuations in total salt content are consistent with theoretical work by maykut and are in phase with mixed layer depth indicating changes in the mixed layer are controlled by salt flux deepening of the mixed layer brings 0 3 kcal cm 2 yr 1 of heat to the surface from the ocean below

formation of glaciolacustrine late pleistocene end moraines in the tasman valley new zealand abstract detailed examination of the tekapo formation in the tasman valley new zealand has identified 20 facies and five facies associations these associations are delta foresets and bottomsets sediment density flows ice contact lake sediments with ice rafted debris and resedimentation deposits and outwash gravels interpretation of the sediment landform associations informed by observations at modern glacier termini suggests that the late pleistocene tekapo formation moraines have been formed by downwasting of a more expanded tasman glacier during the early stages of glacier retreat ponds on the glacier surface develop into thermokarst lakes which enlarge and coalesce to form a large supraglacial lake continued downwasting causes the lake outlet river to entrench into the impounding latero frontal ice cored moraine lowering the lake level this exposes lake bottom sediments and forms shorelines on the proximal slopes of the ice cored moraine as the ice cored moraine melts these lake sediments are deformed and deposited against the mt john moraine the observations and interpretations reported here suggest the late pleistocene end moraine is a constructional feature not a structural glaciotectonic feature as suggested by previous studies

discriminating glacier thermal and dynamic regimes in the sedimentary record abstract this paper provides a description and evaluation of the sedimentary facies and environments associated with a range of glacier thermal and dynamic regimes with additional consideration given to the tectonic context new and previously published data are evaluated together and are presented from modern terrestrial and marine glacial sedimentary environments in order to identify a set of criteria that can be used to discriminate between different glacier thermal regimes and dynamic styles in the sedimentary record sedimentological data are presented from a total of 28 glaciers in 11 geographical areas that represent a wide range of contemporary thermal dynamic and topographic regimes in the context of landsystems representatives from terrestrial environments include temperate glaciers in the european alps patagonia new zealand the cordillera blanca peru cold glaciers in the dry valleys of antarctica and the antarctic peninsula region and polythermal valley glaciers in svalbard northern sweden the yukon and the khumbu himal nepal the glaciomarine environment is illustrated by data from cold and polythermal glacier margins on the east antarctic continental shelf and from a polythermal tidewater glacier in svalbard along with general observations from temperate glaciers in alaska these data show that temperate glacial systems particularly in high relief areas are dominated by rockfall and avalanche processes although sediments are largely reworked by glaciofluvial processes debris in polythermal glaciers is both thermally and topographically influenced in areas of moderate relief debris is mainly of basal glacial origin and the resulting facies association is dominated by diamicton in high relief areas such as the himalaya the debris load in polythermal glaciers is dominated by rockfall and avalanche inputs resulting in extensive accumulations of sandy boulder gravel cold glaciers are dominated by basal debris entrainment but sediments are little modified from the source materials which are typically sandy boulder gravel from older till and sand from glaciofluvial glaciolacustrine and aeolian sources similar facies associations but with different facies geometry and thickness occur in equivalent glaciomarine settings application of these concepts can aid the interpretation of glacier thermal regime and hence palaeoclimate in quaternary and ancient glacial systems

seafloor reflectivity an important seismic property for interpreting fluid gas expulsion geology and the presence of gas hydrate a bottom simulating reflection bsr is a seismic reflectivity phenomenon that is widely accepted as indicating the base of the gas hydrate stability zone the acoustic impedance difference between sediments invaded with gas hydrate above the bsr and sediments without gas hydrate but commonly with free gas below are accepted as the conditions that create this reflection the relationship between bsrs and marine gas hydrate has become so well known since the 1970s that investigators when asked to define the most important seismic attribute of marine gas hydrate systems usually reply a bsr event research conducted over the last decade has focused on calibrating seafloor seismic reflectivity across the geology of the northern gulf of mexico gom continental slope surface to the seafloor this research indicates that the presence and character of seafloor bright spots sbs can be indicators of gas hydrates in surface and near surface sediments figure 1 it has become apparent that sbss on the cont

fracture networks and hydrate distribution at hydrate ridge oregon abstract analysis of resistivity at the bit rab images acquired during ocean drilling program odp leg 204 to southern hydrate ridge oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled fractures on the ridge crest at sites 1247 1248 and 1250 are typically high angle 60 with isotropically distributed strikes while those on the ridge flanks at sites 1244 1246 have lower dips 20 60 and strike parallel to the trend of the ridge the eastern slope basin site 1251 lacks significant fracturing these differences indicate that the minimum principal stress 3 direction rotates to horizontal at the ridge crest as a result of topographically driven extension and gravitational collapse of the ridge chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration which is confined to high permeability sediments and fracture zones by capillary pressure constraints in contrast on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping 3 and the predominance of aqueous rather than gaseous flow

sedimentary facies and landform genesis at a temperate outlet glacier soler glacier north patagonian icefield glasser n f hambrey m j 2002 sedimentary facies and landform genesis at a temperate outlet glacier soler glacier north patagonian icefield 49 1 43 64

geochemical constraints for the formation and dissociation of gas hydrate in an area of high methane flux eastern margin of the japan sea abstract the umitaka spur in the joetsu basin on the eastern margin of the japan sea is characterized by a number of features indicative of high methane flux such as a bsr and gas plumes rising through the water column these are likely linked to the presence of hydrocarbon source rocks underneath the spur the interstitial water chemistry at the spur was measured in order to delineate the behavior and evolution of the gas hydrate system across the region the sulfate depth profiles indicate that the smi depths become shallower toward the crest of the spur and are shallowest less than 200 cmbsf at two plume sites suggesting that the methane fluxes are highest around the plumes depth profiles of dissolved cl all fall into one of four distinguishable trends type i where concentrations linearly increase with sediment depth type ii where concentrations linearly decrease with depth type iii where concentrations remain constant and type iv where concentrations exhibit negative spikes caused by dissociation of gas hydrate during core recovery and handling type i cores commonly recovered at central plume site are characterized not only by steep cl gradients 13 2 to 51 0 mm m but also extreme depletions in d and 18o 0 97 to 0 54 vsmow m for dh2o and 0 16 to 0 10 vsmow m for 18oh2o suggesting gas hydrate formation at shallow depth the cl gradients of type ii cores recovered from the southern part of the spur have little variation 14 8 to 8 3 mm m gas hydrate dissociation along base of the gas hydrate stability zone best explains both the similar pattern of freshening with depth and the widespread distributions of type ii cores although gas hydrate dissociation is known to release isotopically heavy water type ii cores show progressive depletions of both d and 18o with downward depth the apparent contradiction between cl concentrations and the isotopic compositions of type ii cores is likely due to re equilibration by strong burial diagenesis observed all over the japan sea sediments regional dissociation of gas hydrate is likely to have been triggered by sea level drop which facilitated dissociation of subsurface gas hydrate

lead convection measured with an autonomous underwater vehicle the winter lead experiment leadex was conducted during 1992 in the beaufort sea there the autonomous conductivity temperature vehicle actv was operated under and around leads these are the first measurements of this kind measurements of temperature salinity and turbulence parameters were also made at lead edges with sensors fixed to rigid masts by combining information from the fixed measurements and the actv an inertial dissipation method is used to derive horizontal profiles of salt flux from actv salinity measurements comparisons also show that turbulent vertical velocity perturbations can be estimated from the vertical motion of the actv with these velocity estimates horizontal profiles of the turbulent fluxes of heat w t and salt w s are also computed directly data from a wide 1000 m wide lead with rapid ice motion indicate stronger turbulence under the lead than downstream but the character of the flow is that of a convective boundary layer modified by a change in surface buoyancy flux the convection is characterized by multiple intermittent plumes that scale in size with the mixed layer depth at a medium sized 100 m wide lead with slower ice motion the convective circulation set up by the lead is more apparent convective plumes are stronger and more prevalent at the lead edges particularly the downstream edge the comparison between the horizontal profiles from the actv and time series from fixed sensors suggests that lead convection is a combination of a near surface convective boundary layer and a deeper quasi stationary convection pattern

estimating the amount of gas hydrate and free gas from marine seismic data marine seismic data and well log measurements at the blake ridge offshore south carolina show that prominent seismic bottom simulating reflectors bsrs are caused by sediment layers with gas hydrate overlying sediments with free gas we apply a theoretical rock physics model to 2 d blake ridge marine seismic data to determine gas hydrate and free gas saturation high porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity effective pressure mineralogy elastic properties of the pore filling material and water gas and gas hydrate saturation of the pore space to apply this model to seismic data we first obtain interval velocity using stacking velocity analysis next all input parameters to the rock physics model except porosity and water gas and gas hydrate saturation are estimated from geologic information to estimate porosity and saturation from interval velocity we first assume that the entire sediment does not contain gas hydrate o

evidence for fluid circulation overpressure and tectonic style along the southern chilean margin the southern chilean convergent margin between 50 and 57 s is shaped by the interaction of the three main plates antarctic south america and scotia north of 53 s the convergence between antarctic and south america plates is close to orthogonal to the continental margin strike here the deformational style of the accretionary prism is mainly characterized by seaward verging thrusts and locally by normal faults and fractures a very limited lateral extension of prism a very shallow dip 6 decollement and subduction of a thick and relatively undeformed trench sedimentary sequence south of 53 s convergence is oblique to the margin locally the trench sediments are proto deformed by double vergence thrusting and the front of the prism grows through landward verging thrusting the decollement is sub horizontal and deep involving most of the sediment over the oceanic crust in the accretionary process building a comparatively wide and thicker prism a bottom simulating reflector is present across the whole prism to the abyssal plan suggesting the presence of gas in the sediments the analysis of p and s wave velocity reflectivity sections derived by amplitude versus offset technique avo detailed velocity information and the velocity derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre stack depth migrated seismic lines aiming to explain the relation between fluid circulation and tectonics accretion along double vergence thrust faults may be associated with the presence of overpressured fluid which decreases the effective shear stress coefficient along the main decollement and within the sediments and modify the rheolgical properties of rocks the presence of an adequate drainage network represented by interconnected faults and fractures affecting the entire sedimentary sequence can favour the escape of pore fluid toward the sea bottom while less permeable and not faulted sediments can favour fluid accumulations gravitational and tectonic dewatering and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench the results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation

laminated sediments in glacimarine environments diagnostic criteria for their interpretation abstract fine grained laminated deposits are often reported from contemporary and ancient glacimarine sedimentary sequences laminated glacimarine sediments form by a variety of processes including suspension settling from turbid overflow plumes turbidity current deposition and contour current activity subglacial deformation of pre existing sediments may also produce laminated deposits in the form of glacitectonite and deformation till the development of diagnostic criteria for genetic discrimination of laminated glacimarine sediments is important both for their correct interpretation in sedimentary sequences and also for the accurate reconstruction of former glacier extent and geometry as laminated sediments may form in subglacial ice proximal and ice distal environments this paper reviews the processes by which laminated glacimarine sediments form and their resulting sedimentology the aim is to provide a synthesis of diagnostic sedimentological criteria for laminated glacimarine deposits that will assist in the interpretation of such sediments in the geological record laminated sediments are genetically differentiated on the basis of sedimentary structures bedding contacts stratigraphic relationships and macrofaunal content bioturbation these attributes also facilitate discrimination between glacimarine and glacilacustrine rhythmites the paper concludes by discussing the glaciological significance of laminated glacimarine sediments in terms of glacier hydrology reconstructions of former ice dynamics and basal thermal regime