oxygen isotopes in titanite and apatite and their potential for crustal evolution research abstract oxygen isotope analysis of zircon often combined with geochronology and hf isotope analysis has been pivotal in understanding the evolution of continental crust in this contribution we expand the use of underexplored accessory phases titanite and apatite by demonstrating that their oxygen isotope systems can be robust and by developing geochemical indicators involving o isotopes and trace element concentrations to better constrain magma petrogenesis these minerals have the advantage over zircon of being present in less evolved magmas and being more responsive to igneous processes and crustal metamorphism we present new data on titanite apatite and zircon from carefully selected granitoids through geological time the phanerozoic high ba sr granites caledonian province scotland archean sanukitoids karelia province finland and a neoproterozoic basalt andesite dacite rhyolite suite badr guernsey channel island we demonstrate i that 18o values of the studied accessory minerals are not affected by crystal fractionation ii a strong correlation between 18o in all three accessory minerals showing that apatite and titanite can faithfully record the magmatic 18o iii that these accessory minerals can also record metamorphic and or fluid circulation events during the syn to post magmatic history of granitoids

seismic evidence for shallow gas escape features associated with a retreating gas hydrate zone offshore west svalbard active gas venting occurs on the uppermost continental slope off west svalbard close to and upslope from the present day intersection of the base of methane hydrate stability bmhs with the seabed in about 400 m water depth in the inter fan region between the kongsfjorden and isfjorden cross shelf troughs from an integrated analysis of high resolution two dimensional pre stack migrated seismic reflection profiles and multibeam bathymetric data we map out a bottom simulating reflector bsr in the inter fan region and analyze the subsurface gas migration and accumulation gas seeps mostly occur in the zone from which the bmhs at the seabed has retreated over the recent past 1975 2008 as a consequence of a bottom water temperature rise of 1 c the overall margin parallel alignment of the gas seeps is not related to fault controlled gas migration as seismic evidence of faults is absent there is no evidence for a bsr close to the gas flare region in the upper slope but numerous gas pockets exist directly below the predicted bmhs while the contour following trend of the gas seeps could be a consequence of retreat of the landward limit of the bmhs and gas hydrate dissociation the scattered distribution of seeps within the probable hydrate dissociation corridor and the occurrence of a cluster of seeps outside the predicted bmhs limit and near the shelf break indicate the role of lithological heterogeneity in focusing gas migration

gravity and magnetic studies in the channel islands the gravity and total magnetic field over jersey guernsey herm and jethou and the gravity field over alderney and sark have been surveyed in detail at 470 gravity stations and 847 magnetic locations as the basis of bouguer and magnetic anomaly maps which can be integrated with the marine gravity and aeromagnetic maps of the region there is a striking correlation in location and relative amplitude of gravity and magnetic anomalies on the islands the st peter port gabbro of guernsey can be modelled as a laccolith no more than 0 8 km thick and extending several km ne subsurface beyond its present outcrop limit the aeromagnetic and gravity high over se jersey can be modelled as a circular laccolithic lens c 12 km in diameter and c 2 km thick with about 1 km of cover thus the dioritic and gabbroic outcrop in that area does not reflect a great volume of such material at

gas hydrate reservoir and active methane venting province in sediments on 20 ma young oceanic crust in the fram strait offshore nw svalbard seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas hydrate stability zone ghsz the present day flux through these degassing systems is presumably at a low level on most rifted continental margins a pockmark field on the nw svalbard passive margin is located on young ocean crust 20 ma and shows evidence of ongoing episodic degassing new geophysical data from the vestnesa ridge 79 n a mounded and elongated sediment drift in the eastern fram strait reveal a gas hydrate free gas and venting system that is exceptionally more dynamic than documented elsewhere along the northeastern north atlantic margin the prominent bottom simulating reflection bsr about 200 mbsf separates anomalously high p wave velocities in the ghsz from a remarkable underlying low velocity zone indicating the presence of gas hydrate and gas in the pore space inversion of p wave velocity data using the differential effective medium theory yields a two dimensional concentration model of methane hydrate and free gas the model predicts saturations of up to 11 in the hydrate reservoir which due to the seafloor topography forms a large anticlinal permeability barrier below in the low velocity zone i e 1350 1500 m s up to 3 of free gas is predicted across the apex of the vestnesa ridge and in the immediate vicinity of extensional faults a conservative estimate indicates that 225 kg m2 of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column an elongated pockmark field consisting of 100 individual pockmarks up to 600 m wide systematically aligns the apex of the vestnesa ridge active vigorous degassing from the topography controlled pressure valve system was evident from a 750 m high and 150 m wide gas flare observed in the water column during a cruise with r v jan mayen in october 2008 the gas flare documents dynamic degassing through the corresponding chimney which penetrates the entire ghsz and into the underlying free gas zone cruises in 2006 and 2007 did not detect active gas venting above the pockmark field accordingly vigorous degassing may operate in an episodic mode where hydrothermal circulation systems through young ocean crust may play a significant role

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

a fast robust method for detecting and characterizing azimuthal anisotropy with marine ps converted waves and its application to the west svalbard continental slope summary a method for using ps converted waves to detect and characterize azimuthal seismic anisotropy is developed and applied to four component ocean bottom seismometer data acquired near the base of the continental slope west of svalbard the method determines the distribution of coherent signal vibrating perpendicular to the sagittal plane the transverse direction and employs an optimized rotation technique to separate the effects of anisotropy from other contributions to the seismic amplitude the magnitude of the ps wave transverse amplitude when normalized by its radial counterpart is shown to correspond to the degree of ellipticity of particle motions and is used to constrain the anisotropic symmetry class in the near surface sediments results indicate the presence of orthorhombic or horizontal transversely isotropic symmetry that is consistent with the presence of near vertical aligned cracks from its orientation the observed anisotropy is associated with 1 gravitational downslope stresses in the sediment sequence which dips by approximately two degrees to the sw and 2 tectonic stresses associated with the segmented spreading centres of the northern knipovich ridge which spread in the nw se direction aligned cracks are likely to influence the local distribution of gas hydrate and free gas in these sediments an investigation of the effect of seismic noise on optimized rotation shows that the vectorial combination of independent geophone records has variable sensitivity to source related noise this sensitivity peaks where the sagittal azimuth bisects the deployment orientation of the geophones on the seabed a method to reduce the sensitivity to directional noise is developed and shown to improve the transverse amplitude plots this approach to noise reduction may be used to improve the quality of seismic images derived from multicomponent data

magma mingling and melt modification between granitic pipes and host diorite guernsey channel islands within the northern igneous complex of guernsey cylindrical pipe intrusions 20 40 cm in diameter developed by buoyant emplacement of granite magma into overlying denser diorite magma the high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing and mesoscopically sharp well defined interfaces developed the granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re equilibration these formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and or melt interdiffusion across the pipe host interface such processes between partially crystallized magmas may in part provide an explanation for textural mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites

waterflood response of reservoirs in an estuarine valley fill upper mannville g u and w pools little bow field alberta canada lower cretaceous upper mannville g u and w pools in little bow field are hosted by separate parallel elongate estuarine sandstone bodies within an incised valley fill each sandstone body is 3 4 km long 300 500 m wide and up to 22 m thick g pool was discovered in 1972 and placed on primary production oil production declined gradually and was accompanied by modestly increasing gas to oil ratios gor and water to oil ratios wor u and w pools were discovered in 1982 and 1983 respectively and were produced by primary methods until initiation of waterflooding in 1985 response to waterflooding these two pools has been a rise then decline in the gor followed by rapidly rising wor which is currently up to 10 1 in wells adjacent to water injectors production re ponse indicates control by mesoscale and microscale reservoir heterogeneities mesoscale heterogeneities include permeable sandstone beds several meters thick that are continuous between adjacent wells and stochastic shale beds up to 80 cm thick which are not yet correlatable between wells rapid breakthrough of water occurred in producing wells adjacent to injectors due to channeling in thick permeable sandstone beds between shale beds microscale heterogeneities are principally mineralogically segregated laminae of cross stratified sandstones permeability values from cores indicate variations of one order of magnitude among laminae it appears that water passes preferentially through low permeability sandstone laminae due to the higher mobility of water compared to oil

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

methane generated pockmarks on young thickly sedimented oceanic crust in the arctic vestnesa ridge fram strait acoustic backscatter imagery in the fram strait between greenland and spitzbergen reveals a 1 3 km wide 50 km long belt of 50 pointlike backscatter objects decorating the 1300 m deep crest of vestnesa ridge a 1 2 km thick sediment drift possibly underlain by a transform parallel oceanic basement ridge crustal ages 3 14 ma a 3 5 khz seismic reflection profile indicates that at least some objects are pockmarks 100 200 m in diameter and 10 20 m deep the pockmarks possibly also mud diapirs may have been formed by evolution of methane generated by the decomposition of marine organic matter in the vestnesa ridge sediment drift the ridge may be underlain by an anticlinal carapace of methane hydrate calculated to be 200 300 m thick comparable to the hydrate thickness measured just to the south the rising methane would collect in the ridge crest trap intermittently escaping to the sea floor this hypothesis is supported by multichannel evidence for bright spots and bottom simulating reflectors in the area the pockmark belt may also be located above a transcurrent fault sediment slumps on the flanks of vestnesa ridge and northeast of molloy ridge may have been triggered by plate boundary earthquakes and facilitated by methane hydrates

characterization of a stratigraphically constrained gas hydrate system along the western continental margin of svalbard from ocean bottom seismometer data the ongoing warming of bottom water in the arctic region is anticipated to destabilize some of the gas hydrate present in shallow seafloor sediment potentially causing the release of methane from dissociating hydrate into the ocean and the atmosphere ocean bottom seismometer obs experiments were conducted along the continental margin of western svalbard to quantify the amount of methane present as hydrate or gas beneath the seabed p and s wave velocities were modeled for five sites along the continental margin using ray trace forward modeling two southern sites were located in the vicinity of a 30 km long zone where methane gas bubbles escaping from the seafloor were observed during the cruise the three remaining sites were located along an e w orientated line in the north of the margin at the deepest northern site vp anomalies indicate the presence of hydrate in the sediment immediately overlying a zone containing free gas up to 100 m thick the acoustic impedance contrast between the two zones forms a bottom simulating reflector bsr at approximately 195 m below the seabed the two other sites within the gas hydrate stability zone ghsz do not show the clear presence of a bsr or of gas hydrate however anomalously low vp indicating the presence of free gas was modeled for both sites the hydrate content was estimated from vp and vs using effective medium theory at the deepest northern site modeling suggests a pore space hydrate concentration of 7 12 if hydrate forms as part of a connected framework and about 22 if it is pore filling at the two other northern sites located between the deepest site and the landward limit of the ghsz we suggest that hydrate is present in the sediment as inclusions hydrate may be present in small quantities at these two sites 4 5 of the pore space the variation in lithology for the three sites indicated by high resolution seismic profiles may control the distribution concentration and formation of hydrate and free gas

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

microseismicity linked to gas migration and leakage on the western svalbard shelf the continental margin off prins karls forland western svalbard is characterized by widespread natural gas seepage into the water column at and upslope of the gas hydrate stability zone we deployed an ocean bottom seismometer integrated into the masox monitoring arctic seafloor ocean exchange automated seabed observatory at the pinch out of this zone at 389 m water depth to investigate passive seismicity over a continuous 297 day period from 13 october 2010 an automated triggering algorithm was applied to detect over 220 000 short duration events sdes defined as having a duration of less than 1 s the analysis reveals two different types of sdes each with a distinctive characteristic seismic signature we infer that the first type consists of vocal signals generated by moving mammals likely finback whales the second type corresponds to signals with a source within a few hundred meters of the seismometer either due east or west that vary on short tens of days and seasonal time scales based on evidence of prevalent seafloor seepage and subseafloor gas accumulations we hypothesize that the second type of sdes is related to subseafloor fluid migration and gas seepage furthermore we postulate that the observed temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally forced seasonal gas hydrate decomposition our analysis presents a novel technique for monitoring the duration intensity and periodicity of fluid migration and seepage at the seabed and can help elucidate the environmental controls on gas hydrate decomposition and release

layer stripping of shear wave splitting in marine ps waves summary the properties of split s waves can be used to infer 1 the state of stress and strain in the earth 2 the directional dependence of hydraulic conductivity and 3 small changes in pore fluid pressure in the rock mass that occur in response to dynamic processes such as the earthquake cycle measurements of split s waves are particularly useful in shallow 1000 m subseabed marine sediments where s wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near vertical aligned cracks here we present a method of measuring small amounts of s wave splitting in marine p to s mode converted waves and illustrate the technique with data from an ocean bottom seismometer obs deployed on the west svalbard continental slope the analysis applies a modified version of the alford rotation and layer stripping technique developed for zero offset s wave sources and treats ps waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth when the seismic record contains coherent signal on both the in plane and out of plane components the layer stripping technique is capable of decoupling the swave splitting from the effects of p wave velocity anisotropy and reflector dip that influence the downgoing p wave part of the ray path the amount of s wave splitting in the data is small however and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer stripping theory such as use of a finite offset source for the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal to noise levels these include using a wide range of shot receiver azimuths to generate data redundancy methods of identifying and rejecting poor measurements and a predictive layer stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer by layer results with the ps waves of the example data which have a dominant period about 30 ms we find the technique is capable of measuring s wave splitting to a precision of about 0 5 ms for 4 or 5 layers the number of layers successfully treated would increase if the amount of s wave splitting were larger than in these data for which the total cumulative s wave splitting was about 10 ms over a 450 m depth interval the orientation of the fast split s wave was measured with a precision of about 15 our results give an s wave velocity anisotropy of 1 2 per cent in the shallowest 25 30 m subseabed that implies the presence of a differential horizontal stress at or very close to the seabed the s wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel fluid filled cracks with crack density about 0 015 the fast s wave was found to be oriented at 75 15 for the uppermost 150 m before drifting clockwise to an azimuth of 190 210 the clockwise drift in the fast s wave polarization direction implies a transfer of dominance between one set of cracks in the near surface probably related to the slope of the seabed and a different set in deeper sediments of tectonic origin a zone of azimuthal isotropy where the mechanisms that produce the elastic anisotropy may cancel each other out occupies the interval between the two almost orthogonal sets of cracks from interpretation

co oscillating tide controls long term sedimentation in the gironde estuary france abstract study of the sedimentation of the gironde estuary in france over the last three centuries has revealed that there are areas where the volume of the river under the lowest water level has varied greatly and others where it has remained remarkably stable the processes of erosion and sedimentation thus do not take place uniformly throughout the estuary the distribution of stable and unstable areas along the estuary moreover is in no way due to change the unstable areas are 25 26 km and 18 20 km long and regularly alternate with

gas hydrates at the storegga slide constraints from an analysis of multicomponent wide angle seismic data geophysical evidence for gas hydrates is widespread along the northern flank of the storegga slide on the mid norwegian margin bottom simulating reflectors bsr at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 outside but also inside the storegga slide scar area traveltime inversion and forward modeling of multicomponent wide angle seismic data result in detailed p and s wave velocities of hydrate and gas bearing sediment layers the relationship between the velocities constrains the background velocity model for a hydrate free gas free case the seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3 and 6 in a zone that is as much as 50 m thick overlying the bsr hydrates are most likely disseminated neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably average free gas concentrations beneath the hydrate stability zone are approximately 0 4 to 0 8 of the pore volume assuming a homogeneous gas distribution the free gas zone underneath the bsr is about 80 m thick amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone bghs this gives rise to enhanced reflections that terminate at the bghs the stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations gas bearing layers can be as thin as 2 m

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

geochemistry and u pb and 40ar 39ar geochronology of the man of war gneiss lizard complex sw england pre hercynian arc type crust with a sudeten iberian connection the man of war gneiss is a variably deformed meta igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the lizard ophiolite complex sw england u pb dating of zircon demonstrates that the parental magmas crystallized in the earliest ordovician 499t s 3 ma but 40 ar 39 ar step heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite grade metamorphic event at c 374 ma probably associated with thrust emplacement of the ophiolite the gneiss exhibits major trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase probably with minor garnet and zircon the elemental abundances and variations are characteristic of granitoids formed in supra subduction zone environments a granitoid sill within the adjacent old lizard head series schists previously inferred to be related to the man of war gneiss is trondhjemitic and exhibits higher sio 2 tio 2 and mgo but lower zr v ba and sr contents and therefore is probably not genetically related to the latter the sill may record local melting during ophiolite emplacement the man of war gneiss probably represents a structural inlier of pre hercynian basement incorporated in the hercynian nappes of sw england during nnw directed thrusting we emphasize the geological and geochronological similarities between variably deformed granitoid units of the lizard complex the sudeten massif of southern poland and the allochthonous complexes of northern portugal and nw spain these granitoid rocks may represent dispersed remnants of tremadoc arcs that formed oceanward from the northern margin of gondwana this is further convincing evidence that at this time eastern avalonia was separated from northern gondwana by an ocean basin that may have been connected with the tornquist sea

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california

petrology of a magma chamber the plutonic complex of guernsey channel islands uk the northern plutonic complex of guernsey is a calc alkaline intrusion emplaced within a metamorphic complex composed mainly of the icart gneiss and the perelle gneiss the northern plutonic complex is made up of four distinct bodies the st peter port gabbro the bordeaux diorite group the l ancresse granodiorite and the cobo granite which are roughly contemporaneous emplaced at the end of the cadomian orogeny at around 550 560 ma the st peter port gabbro and the bordeaux diorite group are relicts of a magma chamber characterized by layered rocks along the margins and heterogeneous rock associations in the core of the chamber resulting from stirring by convection oxygen isotopic data are consistent with melting of the country rock possibly the perelle gneiss in response to the intrusion of the gabbro diorite magma chamber the resulting acidic melts mixed with crystallizing gabbro diorite magma to yield the cobo granite and the l ancresse granodiorite this mechanism of acid magma genesis can probably be extended to other late cadomian magmatic events in the northern armorican massif suggesting that basic plutonism in this area may be more important than previously considered

an effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate bearing sediments the presence of gas hydrate in marine sediments alters their physical properties in some circumstances gas hydrate may cement sediment grains together and dramatically increase the seismic p and s wave velocities of the composite medium hydrate may also form a load bearing structure within the sediment microstructure but with different seismic wave attenuation characteristics changing the attenuation behaviour of the composite here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate bearing sediments the velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure the attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures we relate velocity and attenuation increases in hydrate bearing sediments to their hydrate content using an effective medium inversion algorithm based on the self consistent approximation sca differential effective medium dem theory and biot and squirt flow mechanisms of fluid flow the inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space we applied our algorithm to a data set from the mallik 2l 38 well mackenzie delta canada and to data from laboratory measurements on gas rich and water saturated sand samples predictions using our algorithm match the borehole data and water saturated laboratory data if the proportion of hydrate contributing to the load bearing structure increases with hydrate saturation the predictions match the gas rich laboratory data if that proportion decreases with hydrate saturation we attribute this difference to differences in hydrate formation mechanisms between the two environments

evidence from three dimensional seismic tomography for a substantial accumulation of gas hydrate in a fluid escape chimney in the nyegga pockmark field offshore norway in recent years it has become evident that features commonly called gas chimneys provide major routes for methane to pass through the methane hydrate stability zone in continental margins and escape to the ocean one of many such chimneys lying beneath pockmarks in the southeastern voring plateau off norway was investigated with a high resolution seismic experiment employing a 2 d array of sixteen 4 component ocean bottom seismic recorders at approximately 100 m separation and a dense network of shots to define the 3 d variation of the chimney s structure and seismic properties the tomographic model derived from p wave travel times shows that p wave velocity inside the chimney is up to 300 m s higher than in the surrounding strata within the methane hydrate stability zone the zone of anomalously high velocity about 500 m wide near its base narrowing to about 200 m near the seabed extends to a depth of 250 m below the seafloor the depth extent of this zone and absence of high velocity beneath the base of the methane hydrate stability field make it more likely that it contains hydrate rather than carbonate if a predominantly fracture filling model is appropriate for the formation of hydrate in low permeability sediment the maximum hydrate concentration in the chimney is estimated to be 14 27 by total volume depending on how host sediment properties are affected by hydrate formation doming of the strata penetrated by the chimney appears to be associated with the emplacement of hydrate accompanying the invasion of the gas hydrate stability zone by free gas

paleo methane emissions recorded in foraminifera near the landward limit of the gas hydrate stability zone offshore western svalbard we present stable isotope and geochemical data from four sediment cores from west of prins karls forland ca 340 m water depth offshore western svalbard recovered from close to sites of active methane seepage as well as from shallower water depths where methane seepage is not presently observed our analyses provide insight into the record of methane seepage in an area where ongoing ocean warming may be fueling the destabilization of shallow methane hydrate the 13c values of benthic and planktonic foraminifera at the methane seep sites show distinct intervals with negative values as low as 27 8 that do not coincide with the present day depth of the sulfate methane transition zone smtz these intervals are interpreted to record long term fluctuations in methane release at the present day landward limit of the gas hydrate stability zone ghsz shifts in the radiocarbon ages obtained from planktonic foraminifera toward older values are related to methane derived authigenic carbonate overgrowths of the foraminiferal tests and prevent us from establishing the chronology of seepage events at shallower water depths where seepage is not presently observed no record of past methane seepage is recorded in foraminifera from sediments spanning the last 14 ka cal bp 14c ams dating 13c values of foraminiferal carbonate tests appear to be much more sensitive to methane seepage than other sediment parameters by providing nucleation sites for authigenic carbonate precipitation foraminifera thus record the position of even a transiently stable smtz which is likely to be a characteristic of temporally variable methane fluxes

in situ temperature measurements at the svalbard continental margin implications for gas hydrate dynamics during expedition maria s merian msm57 2 to the svalbard margin offshore prins karls forland the seafloor drill rig marum mebo70 was used to assess the landward termination of the gas hydrate system in water depths between 340 and 446 m the study region shows abundant seafloor gas vents clustered at a water depth of 400 m the sedimentary environment within the upper 100 meters below seafloor mbsf is dominated by ice berg scours and glacial unconformities sediments cored included glacial diamictons and sheet sands interbedded with mud seismic data show a bottom simulating reflector terminating 30 km seaward in 760 m water depth before it reaches the theoretical limit of the gas hydrate stability zone ghsz at the drilling transect we present results of the first in situ temperature measurements conducted with mebo70 down to 28 mbsf the data yield temperature gradients between 38 c km 1 at the deepest site 446 m and 41 c km 1 at a shallower drill site 390 m these data constrain combined with in situ pore fluid data sediment porosities and thermal conductivities the dynamic evolution of the ghsz during the past 70 years for which bottom water temperature records exist gas hydrate is not stable in the sediments at sites shallower than 390 m water depth at the time of acquisition august 2016 only at the drill site in 446 m water depth favorable gas hydrate stability conditions are met maximum vertical extent of 60 mbsf however coring did not encounter any gas hydrates

trough mouth fans palaeoclimate and ice sheet monitors abstract trough mouth fans are fans at the mouth of transverse troughs channels on glaciated continental shelves on the northwest european glaciated continental margin eight trough mouth fans varying in size between 2700 and 215 000 km 2 have been identified the trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest european ice sheets the debris flow units are separated by hemmipelagic interglacial interstadial sediments it is inferred that the number of debris flow units record the number of shelfbreak positions of the ice sheet margins the number found varies between three in the south and eight in the north typical trough mouth fans and related debris flow units seem to have been formed later than the early mid pleistocene thus the north european ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid pleistocene besides being loci for sediment deposition the trough mouth fans were also the main sites of fresh water supply to the ocean in the form of icebergs during the mid late pleistocene ice ages

acoustic imaging of gas hydrate and free gas at the storegga slide 1 analysis of p wave velocities of an ocean bottom cable obc data set demonstrates the existence of gas hydrates and free gas on the northern flank of the storegga slide the distribution and concentration of gas hydrates and free gas show significant variation over the length of the obc average gas hydrate saturation of pore space is 5 when modeled by effective medium theory with hydrates as a sediment frame component average gas saturation is 0 45 assuming homogeneous distribution the bottom simulating reflector bsr is generally identified as termination of enhanced reflection but in some places appears as a reflection proper amplitude anomalies related to the bsr are primarily caused by the gas and not the hydrates the gas hydrates at the storegga slide develop from gas rich fluids migrating into the gas hydrate stability zone ghsz from the sediments below development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area the physical and geological properties of the sediments control the distribution and concentration of gas which migrates predominantly along strata rather than along the base of the ghsz this migration mechanism controls the flux of gas rich fluids into the ghsz and hence the distribution of gas hydrates and the variation in gas hydrate concentration

the mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

timing of post tectonic cadomian magmatism on guernsey channel islands evidence from 40ar 39ar mineral ages the channel island of guernsey exposes pre and post tectonic calc alkaline plutonic rocks attributed to late precambrian cadomian subduction activity 40 ar 39 ar mineral cooling ages presented here clarify the age and time span of post tectonic intrusions amphibole from the bon repos meladiorite yields an isotope correlation age of 605 3 4 6 ma indicating emplacement during late stages of the tectonothermal event recorded within its host rocks igneous hornblendes from the northern igneous complex record consistent isotope correlation ages st peter port gabbro 570 2 1 1 ma bordeaux diorite complex 569 0 1 1 and 565 3 2 2 ma and a comagmatic enclave within cobo granite 570 2 1 5 ma hornblende and biotite from contact metamorphosed host rocks adjacent to the cobo granite yield isotope correlation ages of 569 6 1 1 and 563 8 3 3 ma respectively these ages together with field data demonstrate that the various members of the northern igneous complex were emplaced contemporaneously at c 570 ma significantly earlier than a c 500 ma rb sr whole rock isochron age previously reported for the cobo granite the 40 ar 39 ar ages indicate that post tectonic magmatism on guernsey was broadly coeval with post metamorphic cooling around the baie de st brieuc on the adjacent french mainland and occurred c 30 ma prior to peak cadomian deformation and metamorphism in the st malo area

fate of rising methane bubbles in stratified waters how much methane reaches the atmosphere there is growing concern about the transfer of methane originating from water bodies to the atmosphere methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport this work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere the bubble model predicts the evolving bubble size gas composition and rise distance and is suitable for almost all aquatic environments the model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep oxic saline water with excellent results methane bubbles from within the hydrate stability zone typically below 500 m water depth in the ocean are believed to form an outer hydrate rim to explain the subsequent slow dissolution a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone the calibrated model explains the impressively tall flares 1300 m observed in the hydrate stability zone of the black sea this study suggests that only a small amount of methane reaches the surface at active seep sites in the black sea and this only from very shallow water areas 100 m clearly the black sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere although substantial amounts of methane may reach the surface in shallow lakes and reservoirs

the coastal transition at the mouth of a small mountainous river in taiwan the tseng wen river is a small mountainous river in southern taiwan that has distinct dry and flood seasons several lines of evidence have revealed that the depositional system at the mouth of the river has transformed from a deltaic system to an estuarine system the evidence includes long to short term shoreline changes geomorphology of the river mouth mixed sediment sources inside the river mouth mixing and suspended sediment characteristics inside the river mouth and the grain size distribution patterns on the river bed unlike the long term evolution of many other coastal systems the transformation at the tseng wen river mouth is largely caused by the building of a reservoir in the middle reaches of the river the present study provides an example that coastal environments can be influenced by human activities farther inland on a time scale much faster than what would occur naturally

ocean warming and gas hydrate stability on the mid norwegian margin at the storegga slide abstract the sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment in the storegga slide area on the mid norwegian margin evidence for the existence of gas hydrates both inside and outside the storegga slide complex comes from reflection seismic profiles where a bottom simulating reflection marks today s base of the gas hydrate stability zone paleo bottom water temperatures show a relatively fast increase at approximately 12 5 10 ka calendar years following the younger dryas while stable warm water conditions have prevailed since then despite sea level rise this warm water inflow caused a major reduction in the thickness of the gas hydrate stability zone along the upper slope of the mid norwegian margin modelling results indicate that critical hydrate stability conditions and consequently the maximal potential pore pressure build up occur at the location of the storegga slide headwall although the major phase of hydrate melting predates the storegga slide event dated at 8 2 ka calendar years reduced hydrate stability conditions could have facilitated or contributed to sub marine slope failure additionally the bottom simulating reflections within the slide complex seem to have nearly adjusted to new equilibrium conditions highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes climate change geohazards

the blast in the past the rapidity and amount of carbon being released through human agency have been thought to be unprecedented not so it emerges from the evidence of anomalies in the ratios of carbon isotopes in an ocean core it seems that around 55 million years ago there was an equivalently swift and massive blast of carbon into the ocean and atmosphere the source was probably methane hydrates that are usually locked up in huge deposits on continental shelves

dynamic response of oceanic hydrate deposits to ocean temperature change the dynamic response of oceanic hydrate deposits to ocean temperature change matthew t reagan and george j moridis earth sciences division lawrence berkeley national laboratory 1 cyclotron rd berkeley ca 94720 usa

time dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing 1 buffett and archer 2004 showed a model of the steady state global ocean clathrate reservoir that reproduces drill data and predicts a global methane inventory of about 5000 gton c the model inventory is very sensitive to ocean temperature with factor of two changes in methane inventory in response to temperature changes of 1 5 c here we explore the potential time dependent behavior of the clathrate reservoir assuming simple first order rate constants for methane accumulation and meltdown in response to changes in the predicted steady state reservoir for melting we divide the reservoir into two parts one melting catastrophically and the other slowly evolution of clathrates through geologic time is very sensitive to the choice of these rate constants a positive feedback exists during a transient meltdown event stronger if the catastrophic melting fraction is high and its melting timescale is fast a clathrate reservoir governed by such kinetics would melt down periodically on a multimillion year timescale determined by the rate constant for clathrate accumulation ideally we would like a model that predicts runaway melting in the paleocene and stability in the pleistocene however the model is most unstable when climate is cold rather than warm opting for stability in the pleistocene constrains the melting kinetic parameters leaving one degree of freedom the grow in timescale must be 5 myr or longer to get the deglacial 13c right but shorter than 10 myr in order for clathrates to have built up through geologically recent cooling the model thus constrained predicts methane fluxes of 200 gton c or less on deglaciations but eventual releases of 2000 4000 gton c in response to a 2000 gton c anthropogenic carbon release anthropogenic climate change differs from a deglaciation in that it warms the ocean to temperatures not seen in millions of years

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

the pore scale distribution of sediment hosted hydrates evidence from effective medium modelling of laboratory and borehole seismic data much of our knowledge of hydrate distribution in the subsurface comes from interpretations of remote seismic measurements a key step in such interpretations is an effective medium theory that relates the seismic properties of a given sediment to its hydrate content a variety of such theories have been developed these theories generally give similar results if the same assumptions are made about the extent to which hydrate contributes to the load bearing sediment frame we have further developed and modified one such theory the self consistent approximation differential effective medium approach to incorporate additional empirical parameters describing the extent to which both the sediment matrix material clay or quartz and the hydrate are load bearing we find that a single choice of these parameters allows us to match well both p and s wave velocity measurements from both laboratory and in situ datasets and that the inferred proportion of hydrate that is load bearing varies approximately linearly with hydrate saturation this proportion appears to decrease with increasing hydrate saturation for gas rich laboratory environments but increase with hydrate saturation when hydrate is formed from solution and for an in situ example

contourites in the fram strait abstract multichannel seismic reflection profiles from the fram strait show many features that can be attributed to bottom current influence on the sediments by comparison with well known contourite sequences elsewhere a greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of svalbard the currents here have probably been controlled by local topography we interpret sequences with abundant diffractions in the broader basins to be current related probably deposited during periods of intensified circulation the continental slopes on the svalbard side have fairly continuous sequences whereas the younger sequences on the greenland lower slope onlaps the lowermost one this can be attributed to differences in current regime sediment supply or margin subsidence the contourites of the lower slope in the northeastern part of the strait about 1000 m water depth can be explained by long term influence of the west spitsbergen current but current influenced patterns down to 3000 m water depth are not compatible with the weak deep water circulation observed at present the oldest contourites west of svalbard are estimated to be of late miocene to pliocene age between 10 and 3 ma based on correlation to the dsdp site 344 and inferred basement ages near the present plate boundary hiatuses may be present beneath the contourites the onset of currents may be related to opening of the gateway between greenland and svalbard to climatic cooling and to palaeogeographic changes outside the fram strait contourite deposition has continued through the late neogene and quaternary but vertical changes in the pattern suggest that current intensities have decreased through time

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

mixing between granitic and dioritic crystal mushes guernsey channel islands uk abstract the contact zone between the cobo granite and bordeaux diorite complex of guernsey channel islands uk displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas initial interaction resulted in the formation of chilled mafic enclaves in granite magma as thermal equilibrium was approached some physical mixing took place to produce a heterogeneous marginal facies continued interaction resulted in incorporation of previously mixed magma into intruding magma the early mixed material is locally preserved as enclaves but more commonly underwent disaggregation promoted by its incompletely crystallised nature the mineral components becoming distributed as xenocrysts and often as microenclaves or glomeroxenocrysts into surrounding magma further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres these processes produced geochemical profiles which do not exhibit perfect mixing trends the petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition but also provide criteria by which such interaction may be recognised elsewhere many features in particular mineral scale disequilibria and small scale modal heterogeneity bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent as such it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events

pleistocene iceberg dynamics on the west svalbard margin evidence from bathymetric and sub bottom profiler data large icebergs leave evidence of their drift via ploughing of the seabed thereby providing a geological record of episodes of calving from thick ice sheets we interpret large scale curvilinear depressions on the western svalbard margin as ploughmarks produced by the keels of icebergs that grounded on the seafloor as they drifted through this area iceberg ploughmarks were identified at modern water depths between 300 m and 1000 m and in two distinct stratigraphic units combining data from sediment cores with seismic stratigraphy from sub bottom profiler data suggests that the ploughmarks developed in two phases 1 during marine isotope stage mis 6 and 2 during mis 2 indicating the presence of large drifting icebergs on the western svalbard margin during both the late saalian and late weichselian glaciations sediment core data along the western svalbard margin indicate a sharp increase in mass transported sediments dated at 23 7 0 2 ka consistent with the mis 2 age of the younger iceberg ploughed surface the ploughmarks are oriented in two main directions sw ne and s n s n oriented ploughmarks which shallow to the north indicate iceberg drift from the south with a sw ne component marking the zone of splitting of the west spitsbergen current wsc into the yermak slope current ysc and north spitsbergen current nsc large mis 6 and mis 2 icebergs most likely had an arctic ocean source we suggest that these icebergs probably left the arctic ocean southward through fram strait and circulated within the norwegian greenland sea before being transported northwards along the svalbard margin by the wsc an additional likely source of icebergs to the western svalbard margin during mis 2 was the ice sheet terminating in the western barents sea from which icebergs drifted northward

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

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the gironde estuary southwestern france abstract this study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment the sandbar is elongated and attached to a headland that is located in the gironde estuary southwestern france bathymetric surveys side scan sonar sonograms grain size trends of surficial sediment and 2dh numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar the sandbar is oriented anticlockwise with respect to the main flow the channel oriented bar face is ebb dominated whereas the opposite face is flood dominated resulting in a clockwise residual transport the line of sediment flux convergence corresponds to the crest of the bar the residual flow over the bar appears to originate from the regional estuarine flow the dominant process responsible for the maintenance of the sandbar seems to be the depth averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed to generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents pingree and maddock 1979 zimmerman 1981 huthnance 1982 and others the seabed stability approach developed by huthnance 1982 seems to be the most reliable theory to explain the saint georges bank dynamics even though many natural factors are not taken into account

sea bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents in many engineering and sedimentological applications it is usual to relate sediment movement to the time averaged bed shear stress however the shear stress exerted on the sea bed by tidal currents is intermittent in nature due to the turbulent bursting phenomenon1 2 sediment movement at the sea bed also occurs in bursts3 4 however attempts to link these two processes have failed because of the poor temporal resolution of existing sediment transport measurement techniques recent experimental advances now make this possible and we describe here the first detailed observations of sediment movement and fluid turbulence in tidal currents flowing over sandy gravels these results show that high stress values are a necessary but not sufficient condition for bed load sediment movement and that form drag on individual particles may have greater dynamical significance than the stress this work also suggests that it may now be possible to describe sediment movement at a level which is deterministic in terms of the turbulence and at very much shorter timescales than have been used previously

seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the beaufort sea the upper continental margin of the beaufort sea north of alaska is underlain by a strong bottom simulating reflection bsr that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above bsrs of similar origin are common worldwide where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment with or without underlying free gas in the sediment surprisingly little is known about the origin of these strong reflections in this paper we analyze the contrasting physical properties which produce bsrs by comparing synthetic bsr amplitudes and waveforms for varying source receiver offsets with multichannel seismic reflection data across the well developed bsr of the beaufort sea in order to discriminate whether free gas is present under the bsr or not it was necessary to supplement near vertical incidence data with prestack offset data the amplitude versus offset avo analysis indicates that the bsr is produced mainly by the existence of free gas in the clastic sediments beneath the bsr the zone of free gas is based on vertical incidence synthetics estimated to be thinner than 11 16 m it is possibly thicker than 16 m if the gas concentration decreases with depth saturation of gas hydrate in the sediment above the bsr is tentatively estimated from the avo modeling to be less than 10 of the pore volume

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data

switching of a paleo ice stream in northwest svalbard ice streams are the fast flowing zones of ice sheets that can discharge a large flux of ice the glaciated western svalbard margin consists of several cross shelf troughs which are the former ice stream drainage pathways during the pliocene pleistocene glaciations from an integrated analysis of high resolution multibeam swath bathymetric data and several high resolution two dimensional reflection seismic profiles across the western and northwestern svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin our results show that the northwestern margin of svalbard experienced a switching of a major ice stream based on correlation with the regional seismic stratigraphy as well as the results from odp 911 on yermak plateau and odp 986 farther south on the western margin of spitsbergen off van mijenfjord we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling starting 2 8 2 6 ma a switch in ice stream flow direction to the present day kongsfjorden cross shelf trough took place during a glaciation at 1 5 ma or probably later during an intensive major glaciation phase known as the mid pleistocene revolution starting at 1 0 ma the seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier highlights reflection seismic data reveal two glacial fans at northwest svalbard margin the fans are result of ice stream activities during pliocene pleistocene glaciations based on seismic and bathymetric data we find the flow directions of the ice streams we find a switch in ice stream flow direction the switch resulted as the ice sheet became thick and overcame a topographic barrier