seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at odp site 801 the subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34s relative to igneous sulfides although these 34s depleted sulfur isotope ratios may point to signatures of biological fractionation previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration here we report results for multiple s isotope 32s 33s 34s compositions of altered basalts at odp site 801 in the western pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust in situ ion microprobe analyses of secondary pyrite in basement rocks show a large range of 34s values between 45 and 1 whereas bulk rock 34s analyses yield a more restricted range of 15 8 to 0 9 these low and variable 34s values together with bulk rock s concentrations ranging from 0 02 up to 1 28 are consistent with loss of magmatic primary mono sulfide and addition of secondary sulfide via microbial sulfate reduction high precision multiple sulfur isotope 32s 33s 34s analyses suggest that secondary sulfides exhibit mass dependent equilibrium fractionation relative to seawater sulfate in both 33s and 34s values these relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates the determination of the s isotope composition of bulk altered oceanic crust demonstrates that s based metabolic activity of subsurface life in oceanic basalt is widespread and can affect the global s budget at the crust seawater interface

a reconsideration of the mass balance of a portion of the ross ice shelf antarctica des estimations raisonnables du degre de diminution de la vitesse juste a l amont de la zone de glace reposant sur le sol indiquent qu il n est plus possible de conclure a un epaississement a l aide de la bande d ecoulement originale de thomas et bentley 1978 c est pourquoi une nouvelle bande d ecoulement situee plus a l est que celle de thomas et bentley et non influencee par quelque zone de glace reposant sur le sol a ete choisie le bilan de masse de cette bande d ecoulement n est pas significativement different de zero une difference de vitesse entre l epaississement et l accretion a la base depassant environ 0 2 m a 1 n est pas vraisemblable

ophiolite obduction pulses as a proxy indicator of superplume events abstract a major new synthesis of ophiolite geochronology and map of global phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume related tectonism a marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction related minor intrusions this episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field formation of massive carbon rich deposits sea level highstands and formation of large flood basalt provinces all generally considered to be superplume proxy indicators a key to interpreting ophiolite obduction as a further proxy is the mid cretaceous superplume associated ocean margin compressional deformation during this event thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc terrane collision marginal basin shoaling back arc basin closure and ophiolite obduction convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

survival of reactive carbon through meteorite impact melting melt fragments in melt breccias from the gardnos impact crater norway contain abundant carbon a high proportion of the carbon present in the original melt was preserved the stripping of hydrogen from carbon during melting prevents later hydrocarbon formation hence the carbon is fixed in place rather than volatilized underlying lithic breccias that were not melted record hydrocarbon generation as a response to less extreme heating despite the high temperature history of the melt the carbon from the gardnos impact crater is highly disordered rather than ordered crystalline graphite and in this respect it is comparable with carbon in chondrite chondrules disordered carbon bears functional groups upon weathering and therefore carbon preserved through impact or other melting may be available for reworking into biologically relevant organic molecules

the deep dark energy biosphere intraterrestrial life on earth most ecosystems on earth exist in permanent darkness one or more steps removed from the light driven surface world this collection of dark habitats is the most poorly understood on earth in particular the size function and activity of these ecosystems and what influence they have on global biogeochemical processes the vastest of these ecosystems constitute the deep biosphere habitats physically located below the surface of continents and the bottom of the ocean the deep biosphere has been the subject of considerable and increasing study and scrutiny in recent years new deep biosphere realms are being explored from deep in mines in south africa to sediments in the middle of oceanic gyres and beyond new technologies are emerging permitting researchers to do active manipulable experimentation in situ within the subsurface this review highlights recent history of the research and the exciting new directions this field of research is going in and discusses some of the most active and interestin

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

a glaciological map of filchner ronne ice shelf antarctica a set of landsat 5 multispectral scanner mss images of filchner ronne ice shelf was commissioned by the institut fur angewandte geodasie and recorded over a short period in early 1986 a mosaic of digitally enhanced images was constructed using all available ground control in due course it is planned to use the data for the production of a series of digital image maps at a scale of 1 1 000 000 meanwhile a line map has been prepared at a scale of 1 2 000 000 to show the principal glaciological features the new mosaic shows much more detail than earlier landsat images and extends coverage to higher latitudes more of the poorly known southern boundary of the ice shelf can be identified but floating ice still extends beyond the limit of coverage extensive ice rumples are revealed many of them undetected in earlier imagery flow bands can be tracked without a break from their ice stream source to the ice front over distances of up to 800 km even minor tributaries and sheet flow from the inland ice sheet between the ice streams yield narrow but identifiable flow bands ice thickness velocity and mass flux data are reviewed in order to underline their dependence on good mapping

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

fracture size estimation using data from multiple boreholes abstract the size of a fracture within a rock mass can be obtained using data sampled from boreholes or exposed surfaces in this study fracture sizes are estimated using data from multiple boreholes the fractures are assumed to be elliptical all having the same aspect ratio but not necessarily occurring parallel to each other formulae for estimating the mean and standard deviation of the characteristic fracture size are developed by considering the number of intersections between the fractures and the boreholes a method is proposed to infer the probability density function of the fracture size from the number of fractures which is obtained by random sampling of fractures in adjacent boreholes the fracture distribution is assumed to follow the form of a fractal exponential or lognormal distribution the performance of the method is validated using monte carlo simulations with different inputs of size distribution fracture density orientation and mean fracture size

extent of the microbial biosphere in the oceanic crust we estimate the depth of the 120 c isotherm by constructing crustal thermal gradients based on theoretical and observed conductive heat flux as a function of lithospheric age we chose the 120 c isotherm because it is close to the upper limit for prokaryotic life and therefore the isotherm approximates the maximum depth at which life can persist in the ocean crust the depth of the potential microbial biosphere increases with lithospheric age from approximately 0 5 to 1 km for 1 ma lithosphere to as much as 5 km at a subduction age of 180 ma we use global models of oceanic plate creation to estimate the volume of crust occupied by the biosphere today and throughout geologic time presently the volume of the ocean crust that is capable of containing life is similar to the volume of the oceans 1018 m3 depending on the model used for the growth of continental crust the volume of rock available to house the subsurface biosphere may have remained constant or doubled since the archean although the thermal models presented here provide estimates for the potential depth and volume of rock in which microbes may live the biomass in this volume is not well constrained using a previously published model the prokaryotic biomass in the igneous ocean crust is estimated to exceed that in all aquatic and soil environments and is similar to that in the continental subsurface and in marine sediment most of the crustal biomass beneath the sediments is likely contained within the extrusive layer and this has probably been the case since microbes first colonized the oceanic crust in the archean

an investigation of low stress ice rheology on the ward hunt ice shelf deformation of the ward hunt ice shelf northwest territories canada can be used to study the rheology of ice in the low stress low strain rate regime 10 12s 1 finite element simulations are separately displayed to demonstrate what the effects of ice salinity ice temperature and sea ice back pressure have on the spreading rates of this ice shelf we conclude that in general these effects tend to obscure any differences between the commonly used non newtonian flow law and the newtonian flow law recently reinterpreted from field measurements the numerical simulations indicate that there are specific locations where useful field measurements could be carried out on the ice shelf

basal mass balance along a flow line on ronne ice shelf antarctica abstract england abstract recent glaciological work on ronne ice shelf has focused on an assumed flow line which extends from rutford ice stream grounding line to the ice front results from doppler satellite surveying and radio echo sounding are used in kinematic calculations to determine the basal mass balance assuming the flow line to be in a steady state models suggest that basal melting dominates over most of the flow line and is most pronounced at the extremities in the region within

the eurasian ice sheet formation and collapse resulting from natural atmospheric co2 concentration variations finite element model simulations of ice sheet dynamics are conducted to investigate the role of natural atmospheric co2 variations obtained from ice core bubbles in creating climatic conditions necessary to glaciate the eurasian arctic effects of co2 variations are sufficient to support extensive marine ice sheet and ice shelf development at the last glacial maximum and to deglaciate the region during the holocene from approximately 14 ka to 9 ka the ice sheet coverage simulated for the last glacial maximum agrees with evidence for extensive ice sheet grounding over the shallow continental shelves of the barents kara laptev and east siberian seas an ice shelf also forms over the arctic ocean and greenland and norwegian seas

green icebergs formed by freezing of organic rich seawater to the base of antarctic ice shelves although most icebergs are blue green icebergs are seen occasionally in the antarctic ocean chemical and isotopic analysis of samples from green icebergs indicate that the ice consists of desalinated frozen seawater as does the basal ice from the amery ice shelf spectral reflectance of a green iceberg measured near 67 s 62 e confirms that the color is inherent to the ice not an artifact of the illumination pure ice appears blue owing to its absorption of red photons addition of a constituent that absorbs blue photons can shift the peak reflectance from blue to green such a constituent was identified by spectrophotometric analysis of core samples from this iceberg and from the amery basal ice and of seawater samples from prydz bay off the amery ice shelf analysis of the samples by fluorescence spectroscopy indicates that the blue absorption and hence the inherent green color is due to the presence of marine derived organic matter in the green iceberg basal ice and seawater thick accumulations of green ice in icebergs and at the base of ice shelves indicate that high concentrations of organic matter exist in seawater for centuries at the depth of basal freezing

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

carbon release by off axis magmatism in a young sedimented spreading centre continental rifting creates narrow ocean basins where coastal ocean upwelling and enhanced silicate weathering remove carbon dioxide from the atmosphere evidence from seismic data sonar backscatter and seafloor images and geochemical water analyses suggest that in young sedimented rifts active magmatism occurs in a broader region than appreciated and releases carbon from the sediments

regime of the filchner ronne ice shelves antarctica seismic radio echo twerle balloon and oceanographic data of the filchner ronne ice shelves are analysed a large thin area in the central ronne ice shelf is found to differ from the morphology of the ross and filchner ice shelves this area is partly filled by basal saline ice which can be attributed to regional regelation caused by oceanic circulation

glaciological studies on rutford ice stream antarctica rutford ice stream drains part of the west antarctic ice sheet into ronne ice shelf much of the drainage basin has a bed well below sea level and is therefore an area which could undergo substantial change if a climatic warming were to cause sustained thinning of ronne ice shelf snow accumulation data suggest that an accumulation rate of 0 51 0 12 mg m 2 yr 1 is required for balance an extensive ice movement survey network was established across the grounding line in 1978 1980 and was extended for more than 100 km upstream in 1984 1986 velocities at five sites have been calculated using doppler satellite position measurements when the velocities are combined with cross sectional areas of the ice stream measured by radio echo sounding mass flux figures show that drag at the sidewalls and on the base must decrease downstream toward the grounding line the increasing influence of buoyancy forces on ice stream motion is also shown by the steady rise in the ratio of bedrock depth to ice thickness going downstream along the network below the grounding line that was found to cross the middle part of the earlier network is a zone stretching for a further 100 km where it can now be shown that the ice stream is intermittently grounded in these grounded areas the glacier bed is raised with respect to the surrounding seabed and forms eskerlike features surface elevations over the grounded areas are less than 30 m above those needed for hydrostatic equilibrium

sulfur isotope evidence for a paleoarchean subseafloor biosphere barberton south africa the archean sub seafloor has been proposed as an environment for the emergence of life with septate clusters of titanite microtextures in pillow lava rims argued to be the earliest traces of microbial microboring here we use nanoscale secondary ion mass spectrometry nanosims to test possible geochemical traces of life in ca 3 45 ga pillow lavas of the barberton greenstone belt south africa sulfide inclusions in the titanite microtextures record strongly negative sulfur isotope fractionations of 34 s vcdt 39 8 to 3 2 vcdt vienna canyon diablo troilite these represent the largest range and most negative 34 s values so far reported from the archean and are consistent with an early biogenic origin for the sulfides extensive in situ elemental mapping did not find any organic linings associated with the microtextures despite the high spatial resolution and sensitivity of the nanosims the absence of organic linings thus excludes a key line of evidence previously used to support the biogenicity of the microtextures in contrast in situ sulfur isotope analysis of basalt hosted sulfides provides an alternative approach to investigating the existence and nature of an archean subseafloor biosphere

combining structural and hydrogeological data conceptualization of a fracture system abstract in this paper a fracture system is studied based on hydrogeological observations and structural measurements at different scales from the bataapati site which has been chosen to store hungary s low and medium level nuclear waste it is argued that the combination of these two approaches may improve our understanding of the structure of the fracture system at different scales this paper presents the orientations sizes intensity spatial locations and transmissivity of fractures at different scales in two hydraulic domains 1 more transmissive zones and 2 less transmissive blocks based on the evaluation of these properties a conceptual model has been developed for the fracture system the hydrostructural concept suggests that the complexity of fracture orientation and intensity increases with distance from more transmissive zones to less transmissive blocks but fracture size and transmissivity decrease it also suggests that these parameters vary continuously between different hydraulic domains and that different parameters are strongly intercorrelated the complex interpretation of the fracture system studied can provide direct inputs for hydrogeological models but can also provide conceptual information for the development of the geosphere module in safety calculations

derivation of parent fracture population statistics from trace length measurements of fractal fracture populations this note describes how to estimate the three dimensional fracture size distribution from trace length statistics

an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k metcalfe r banks d and bottrell s h 1992 an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k chem geol 102 1 21 the builth volcanic group of the builth wells ordovician inlier wales is a sequence of basic intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised domainal calc silicate alteration the pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins the pyrobitumens were emplaced by ordovician hydrothermal fluids which were also responsible for the domainal alteration combined petrographic gas chromatography and stable isotopic studies show that light organic gases c2 c5 were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens these light organic gases mixed with methane characterized by 13c 13 o which was already present in the fluids this methane was most likely mantle derived or produced from inorganic synthesis the organic content of the fluids exerted an important control on the inorganic secondary assemblages a similar control of inorganic assemblages by organic matter might possibly occur deep within modern petroleum bearing hydrothermal systems such as the guaymas basin and escanaba trough hydrothermal systems

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

using the migration of the induced seismicity as a constraint for fractured hot dry rock reservoir modelling the challenge for hot dry rock technology is to develop a heat exchanger in deep hot rock masses and to circulate a fluid in order to extract its energy to be used at the ground surface the present day strategy is to take advantage of natural fractures that pre exist at these depths and to improve their hydraulic properties the extension of the area with enhanced properties must then be evaluated so that the best locations for further boreholes can be proposed to date this development procedure is based on fluid injection at high rates forcing hydro mechanical interactions to take place along pressurised fractures these pore pressure driven mechanisms are accompanied by seismic activity assuming the validity of poro elastic theory in the fractured host rock some authors have derived the virgin hydraulic diffusivity of the fractured reservoir from the analysis of the spatio temporal growth of the induced seismicity the present work is aimed at verifying this approach using a numerical code to solve directly for hydro mechanical interactions in random fracture networks our approach assumes that the seismic activity is controlled by a coulomb shear criterion and we show how the interpretation of spreading rate of the modelled shear activity in a given network coincides with the upscaled virgin hydraulic diffusivity of the same fracture network calculated from an independent numerical procedure at the reservoir scale therefore it is shown that the direct analysis of the seismicity migration is appropriate to give reliable estimates of virgin hydraulic and mechanic parameters these parameters can then be used for performing any further quantitative analysis of a reservoir open to the far field this is of importance as fluid mass balance in multi well exploitation systems will include the contribution from areas surrounding the stimulated zone

pyrite and organic matter in phanerozoic normal marine shales data have been obtained from our own chemical analyses and from the literature for the concentrations of organic carbon c and pyrite sulfur s in over 600 samples of cambrian to recent normal marine shales normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and or indicators of bioturbation all samples were selected to minimize 1 1 loss of c and or s due to weathering at the outcrop by emphasizing the use of drill core material 2 2 analytical errors and non diagenetic effects by avoiding sediments low in c or s content such as sandstones 3 3 fe limitation of pyrite formation by avoiding limestones cherts and euxinic shales 4 4 metamorphic loss of c relative to s by avoiding rocks obviously subjected to even low grade metamorphism our results indicate that 1 there is generally a good positive linear correlation between organic c and pyrite s for normal marine shales of all ages and 2 the mean cs ratio for normal marine shales has varied over time devonian to tertiary shales exhibit mean cs weight ratios 1 8 0 5 somewhat lower than quaternary sediments meancs 2 8 this we believe is due to preferential c loss relative to s during biogenic methanogenesis plus diagenetic heating distinctly lower mean c s values for the cambrian and ordovician 0 5 0 1 cannot be explained solely in terms of diagenetic c loss and instead must represent low original ratios the low original ratios we suggest were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time because the major source of such material vascular land plants had not yet evolved this along with a possibly lower degree of bioturbation contributed to enhanced pyrite formation and preservation and thus lower early paleozoic cs ratios

enhanced microbial activity in carbon rich pillow lavas ordovician great britain and ireland there is extensive evidence for the microbial colonization of seafloor basalts in the modern ocean and in the geological record the sulfur isotope composition of pyrite in the basalts commonly indicates marked isotopic fractionation due to microbial sulfate reduction sections through the nemagraptus gracilis zone ordovician in great britain and ireland are characterized by both widespread pillow lavas and organic rich seafloor sediment allowing an exceptional opportunity to assess whether the availability of organic carbon influenced the extent of microbial activity in the basalts in deep geological time whole rock data from basalts at 10 localities show that there is a relationship between sulfur isotopic composition and the carbon content of the basalt at two localities where organic carbon was entrained in the basalt isotopic compositions are heavy compared to compositions in carbon poor basalt implying that microbial activity exhausted the supply of seawater sulfate in most basalt microbial activity was limited by the supply of carbon but where the basalt incorporated carbon during emplacement on the seafloor microbial activity became sulfate limited

an ordovician lost city venting serpentinite and life oases on iapetus seafloor highly brecciated carbonate rich serpentinites or ophicalcites of early ordovician age in the dunnage zone of quebec are host to fracture fill high temperature 80 230 c carbonate cements away from or crosscut by the fractures centimetre to decimetre thick crusts made up of massive to laminated micrite peloidal layers and threads are associated with low temperature botryoidal calcite cements the peloidal masses are characterized by a clotted texture that is reminiscent of interpreted fossilized ordovician microbial communities the ophicalcite contains carbonate botryoids morphology commonly found at shallow reefal margins but also at modern cold ch4 seeps and recently documented at hot ch4 vents from serpentinite such as the modern lost city hydrothermal field in the atlantic ocean the 18ovpdb ratios of the calcite botryoids and peloidal layers indicate formation and or precipitation out of cold ordovician deep marine waters to the contrary of botryoids at cold methane seeps the botryo

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

platelet ice attachment to instrument strings beneath the amery ice shelf east antarctica oceanographic instruments suspended beneath the amery ice shelf east antarctica have recorded sporadic pressure decreases of 10 20 dbar over a few days at three sites where basal marine ice growth is expected we attribute these events to flotation due to platelet ice accretion on the instrument moorings some events were transient rapidly returning to pre event pressures probably through dislodgement of loosely attached crystals driven by these pressure changes temperatures recorded by the shallowest instruments within 20 m of the shelf base tracked in situ freezing temperatures during the events these observations provide indirect evidence for the presence of frazil ice in the sub ice shelf mixed layer and for active marine ice accretion at one site we infer that a dense layer of platelet ice 1 5 m thick was accreted to the ice shelf over a 50 day period following some permanent abrupt pressure decreases which we interpret as due to the lodgement of the uppermost instrument at the ice shelf base altered background trends in pressure suggest compaction rates of 3 4 m a 1 for the accreted basal platelet layer attachment of platelet ice and resulting displacement of moorings has ramifications for project design and instrument deployment and implications for interpretation of oceanographic data from sub ice shelf environments

tidal flexure of jakobshavns glacier west greenland jakobshavns glacier a floating outlet glacier on the west greenland coast was surveyed during july 1976 the vertical displacements of targets along two profiles perpendicular to the fjord wall bounding the north margin of the glacier were analyzed to determine the effect of flexure caused by tidal oscillations within the fjord an analysis based on the assumption that vertical displacements of the glacier reflected pure elastic bending yielded the conclusion that the effective thickness of the ice i e the thickness which remained unaffected by surface and basal cracking and which behaved as a continuum was 160 m 2 6 km upglacier from the calving front and 110 m 0 6 km from the calving front an analysis based on the more realistic assumption that observed bending reflected elastic and viscoplastic deformation yielded the conclusion that the average effective thickness of the ice was 316 74 m 40 of the estimated 800 m total thickness 2 6 km from the calving front and 160 48 m 21 of the estimated 750 m total 0 6 km from the calving front a constitutive relationship appropriate for hard glide during flexure was used

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

numerical models of the filchner ronne ice shelf an assessment of reinterpreted ice thickness distributions recent radio echo soundings of the filchner ronne ice shelf by the german antarctic expedition 1983 1984 suggest that previous ice thickness measurements may have misinterpreted an internal radio echo reflecting horizon as the true ice shelf bottom this and the analysis of total ice thickness from surface altimetry suggests that a previously defined thin ice region comprising approximately 1 5 of the total ice shelf area may be underlain by a thick layer of possibly saline ice one possible way to verify the existence of such a layer is by measurement of its influence on the ice shelf flow regime here we evaluate this influence by conducting finite element simulations of two alternative ice thickness configurations we conclude that flow differences are sufficiently large to allow verification of the possible saline basal ice layer provided that surface strain rate measurements are conducted in certain key areas

structural controls of fracture orientations intensity and connectivity teton anticline sawtooth range montana the teton anticline is a multiple hinge anticline containing fractured mississippian devonian carbonates in the frontal part of the sawtooth range in montana the structure serves as a good surface analog for fracture patterns and connectivities within subsurface folded carbonate reservoirs the primary fracture sets are longitudinal and transverse relative to the axis of the fold although two additional oblique sets are also present the length and density of the longitudinal fracture sets are strongly controlled by position relative to multiple hinges the transverse fractures are related to changes in fold plunge and exhibit less variation in fracture density fracture connectivity is dependent on the number of fracture sets their orientations and dispersions and the densities of the fracture sets the connectivity is measured using two parameters the fractional connected area fca which represents the fraction of the total sample area that is connected by fractures and the distribution of clusters of different sizes in any given area because the longitudinal fractures represent the dominant fracture set and also show the most variation with structural position the fracture connectivity as measured by both the fcas and the distribution of cluster sizes is greater in the vicinity of the fold hinges the results and approaches used in the study have some important implications for subsurface folded fractured carbonate reservoirs the analysis of sparsely distributed fracture data from wells must be integrated with an understanding of the controls of the macroscopic structure on fracture parameters to effectively simulate fracture patterns and connectivities around subsurface structures

hydrothermally derived petroleum examples from guaymas basin gulf of california and escanaba trough northeast pacific ocean petroleum is associated with sulfide rich sediment at two active oceanic spreading axes in the northeast pacific ocean in the guaymas basin a spreading axis in the gulf of california petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites in escanaba trough at the southern end of the gorda ridge spreading axis offshore northern california petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in quaternary turbiditic river derived sediment comparisons of the distributions of hydrocarbons n alkanes isoprenoids terpanes steranes and aromatics show that chemical differences among four petroleum samp es are such that two samples from guaymas basin can be distinguished from two samples from escanaba trough distinguishing characteristics resulting from differences in sources include n alkane distributions and certain sterane ratios distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons these oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes and their generation expulsion and migration occurred simultaneously over an instantaneous geological time period the potential economic significance of hydrothermally derived petroleum is uncertain but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation expulsion and migration

biogenic alteration of volcanic glass from the troodos ophiolite cyprus alteration of basaltic glass through the entire volcanic sequence of the troodos ophiolite is partly bio mediated the following observations support this conclusion alteration textures resembling microbes in form and size altered glass locally shows high carbon concentrations at the alteration front and organic remains the absence of dna suggests that the bio alteration is a fossil process temperatures calculated from 18 o of carbonate assumed to have been in equilibrium with seawater yield 27 65 c and 18 o silicate versus h 2 o relationships indicate seawater alteration these data are consistent with biogenic alteration during an early stage of ocean floor alteration of the troodos oceanic crust some 70 90 ma ago

snowball earth ice thickness on the tropical ocean 1 on the tropical oceans of a neo proterozoic snowball earth snow free ice would have existed in regions of net sublimation photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear the steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates the heat flux from the ocean to the ice base the latent heat of freezing to the ice base and the solar energy absorbed within the ice a preliminary study using a broadband model for solar radiation and assuming a large freezing rate had indicated that tropical ice might be only a few meters thick here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight we use a spectral model for solar absorption computing radiative transfer at 60 wavelengths considering absorption by the ice and scattering by bubbles with the spectral model the computed ice thickness is much greater for a solar flux of 320 w m 2 at the equatorial surface and expected albedo of 0 5 for bare sea ice we find that surface temperatures below 12 c generate ice layers too thick for photosynthesis 100 m if the albedo were as low as 0 4 thick ice would occur only for surface temperatures below 25 c but such low temperatures would be difficult to maintain with such low albedo for surface temperatures warmer than these limits the ice becomes thin 1 m and is unlikely to represent a coherent ice layer however glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice free patches

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

accumulation and ice core studies on filchner ronne ice shelf antarctica accumulation rates in the eastern part of ronne ice shelf were determined by isotopic stratigraphy ebo the samples were taken from snow pits dug during the filchner i and ii operations in 1984 and 1986 in general the accumulation rate decreases towards the south the greatest decrease from 21 3 to 13 3 g cm 2 a 1 was observed between filchner station and measuring point 341 sited 270 km up stream of the ice edge the slbo values of the near surface layers vary between 25 and 29 0 the 1bo content in the more southerly part is progressively depleted in the direction of mollereisstrom paralleling a decrease in the accumulation rate near the ice edge the 1bo content decreases to the west a 100 m ice core drilled in 1984 at point 340 220 km from the ice edge probably goes back to a d 1460 it has been dated by isotopic stratigraphy the accumulation rate up stream of the drilling site was deduced from the sequence of annual layers using a simple ice flow model the accumulation shows strong variations over the last 200 years which may be caused in part by local variations in the accumulation on ronne ice shelf

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

inorganic chemistry gas compositions and dissolved organic carbon in fluids from sedimented young basaltic crust on the juan de fuca ridge flanks abstract the permeable upper oceanic basement serves as a plausible habitat for a variety of microbial communities there is growing evidence suggesting a substantial subseafloor biosphere here new time series data are presented on key inorganic species methane hydrogen and dissolved organic carbon doc in ridge flank fluids obtained from subseafloor observatory corks circulation obviation retrofit kits at integrated ocean drilling program iodp boreholes 1301a and 1026b these data show that the new sampling methods cowen et al 2012 employed at 1301a result in lower contamination than earlier studies furthermore sample collection methods permitted most chemical analyses to be performed from aliquots of single large volume samples thereby allowing more direct comparison of the data the low phosphate concentrations 0 06 0 2 m suggest that relative to carbon and nitrogen phosphorus could be a limiting nutrient in the basement biosphere coexisting sulfate 17 18 mm hydrogen sulfide 0 1 m hydrogen 0 3 0 7 m and methane 1 5 2 m indicates that the basement aquifer at 1301a either draws fluids from multiple flow paths with different redox histories or is a complex environment that is not thermodynamically controlled and may allow co occurring metabolic pathways including sulfate reduction and methanogenesis the low doc concentrations 11 18 m confirm that ridge flank basement is a net doc sink and ultimately a net carbon sink based on the net amounts of doc oxygen nitrate and sulfate removed 30 m 80 m 40 m and 10 mm respectively from entrained bottom seawater organic carbon may be aerobically or anaerobically oxidized in biotic and or abiotic processes

response of a marine ice sheet to changes at the grounding line abstract a numerical model was designed to study the stability of a marine ice sheet and used to do some basic experiments the ice shelf ice sheet interaction enters through the flow law in which the longitudinal stress is also taken into account instead of applying the model to some measured profile and showing that this is unstable as is common practice in other studies an attempt is made to simulate a whole cycle of growth and retreat of a marine ice sheet although none of the model sheets is particularly sensitive to changes in environmental conditions the question as to what might happen to the west antarctic ice sheet in the near future when a climatic warming can be expecied as a result of the co 2 effect seems to be open for discussion again from the results presented in this paper one can infer that a collapse caused by increased melting on the ice shelves is not very likely

evaluation of sampling methods for fracture network characterization using outcrops outcrops provide valuable information for the characterization of fracture networks sampling methods such as scanline sampling window sampling and circular scanline and window methods are available to measure fracture network characteristics in outcrops and from well cores these methods vary in their application the parameters they provide and therefore have advantages and limitations we provide a critical review on the application of these sampling methods and apply them to evaluate two typical natural examples 1 a large scale satellite image from the oman mountains oman 120 000 m2 1 291 669 ft2 and 2 a small scale outcrop at craghouse park united kingdom 19 m2 205 ft2 the differences in the results emphasize the importance to 1 systematically investigate the required minimum number of measurements for each sampling method and 2 quantify the influence of censored fractures on the estimation of fracture network parameters hence a program was developed to analyze 1300 sampling areas from 9 artificial fracture networks with power law length distributions for the given settings the lowest minimum number of measurements to adequately capture the statistical properties of fracture networks was found to be approximately 110 for the window sampling method followed by the scanline sampling method with approximately 225 these numbers may serve as a guideline for the analyses of fracture populations with similar distributions furthermore the window sampling method proved to be the method that is least sensitive to censoring bias reevaluating our natural examples with the window sampling method showed that the existing percentage of censored fractures significantly influences the accuracy of inferred fracture network parameters

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

numerical flow simulation at local parts of filchner ronne ice shelf antarctica the munster radio echo sounding res work in the central part of filchner ronne ice shelf has revealed new information different signatures are discernible in the airborne res data local models in the area between henry ice rise and berkner island are used to interpret these signatures in terms of thermal and dynamic parameters from our airborne measurements the shapes of the meteoric and marine ice layers were prepared as input data for dynamic model calculations a general increase in the temperature dependent flow parameter is compared with a spacially variable flow parameter that is derived from model calculations concerning freezing and melting processes velocity and strain fields were calculated by a finite difference method on a local data base between henry ice rise and berkner island

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock