spatial variability of antarctic peninsula net surface mass balance 1 measurements from ice cores and snow pits collected over the last 50 years are used to examine how net surface mass balance varies across the antarctic peninsula to give the first detailed map of mass balance for the region a total of 211 reliable mass balance measurements were available for the preparation of the map but some areas were found to be very data sparse the analysis suggests that the largest values of mass balance are found along the spine of the northern part of the peninsula where over 2 5 m yr 1 water equivalent we has been measured a secondary peak of more than 2 0 m yr 1 we is determined along the mountains of eastern alexander island precipitation minus evaporation p e fields from the european centre for medium range weather forecasts reanalysis project are compared with our analysis of in situ data the model fields are found to have peak values of p e of only half the amounts found from the measurements the greatest model values are located on the western side of the peninsula areas where a high density of in situ data is available including king george vi sound and the high south central plateau part of the peninsula show a high spatial variability of net surface mass balance suggesting that local orographic features play a major part in dictating the mass balance

the development of antarctic glaciation and the neogene paleoenvironment of the maurice ewing bank abstract a micropaleontologic magnetostratigraphic and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late miocene to holocene depositional and erosional history of the intermediate depth maurice ewing bank meb located at the eastern extremity of the falkland malvinas plateau southwest atlantic ocean sedimentation on the meb is controlled presently by the position of the polar front and strong bottom currents beneath the axis of the antarctic circumpolar current acc fluctuations in the position of the polar front and in the intensity of the acc are inferred to have largely controlled deposition on the meb since the initiation of acc flow over the bank in the miocene the sedimentary record of the meb is used to infer the late miocene to holocene oceanographic and climatic conditions in the south atlantic sector of the southern ocean the meb suffered major erosion by intense acc flow during late miocene time which exposed cretaceous miocene sediment near the surface and shaped the present configuration of the bank this erosional event is inferred to have occurred during late miocene to early pliocene time 7 2 4 7 m y b p with the major phase of erosion occurring between middle magnetostratigraphic chron 7 and late chron 6 7 2 6 2 m y b p we suggest that the miocene sedimentary record of the meb and other paleoenvironmental evidence from the circum antarctic region precludes the existence of a west antarctic ice sheet or extensive ice shelves along the antarctic margin prior to the late miocene during the late miocene a west antarctic ice shelf formed in the former west antarctic sea and rapidly thickened until it grounded below sea level to form the west antarctic ice sheet formation of this ice sheet with floating and partially grounded extensions ice shelves in the ross and weddell embayments led to the first major production of true antarctic bottom water formed in a manner similar to today which permanently altered global abyssal circulation late miocene changes in oceanic ccd levels and a permanent shift in the oceanic 13 c 12 c composition may be attributed to this major change in abyssal circulation a decrease in acc velocity during an early pliocene amelioration of climate led to resumption of deposition on the bank from 4 7 to 3 9 m y b p intensification of the acc during the late gilbert and early gauss chrons again resulted in limited deposition and widespread erosion and or non deposition over most of the meb from 4 0 to 3 2 m y b p acc velocity apparently began to wane during the middle of the gauss chron and allowed widespread deposition on the bank during late gauss time 2 9 2 48 m y b p and over more limited areas during earliest matuyama time 2 48 to about 2 2 m y b p during middle to late matuyama time bottom current intensity reached its greatest post miocene level forming a regional disconformity between sediments of about 2 0 and 1 0 m y in age this increased circumpolar circulation and associated erosion is inferred to have peaked during the late matuyama chron 1 2 1 0 m y b p and is approximately the same age as the tentatively dated greatest patagonian glaciation in nearby southern argentina between 1 0 and 0 7 m y b p the bank was blanketed with a coarse erosion resistant layer of ice rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion sedimentation on the meb during the bruhnes chron 720 000 yr b p present was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest pleistocene time

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

biostratigraphic characterization and quaternary microfossil palaeoecology in sediment drifts west of the antarctic peninsula implications for cyclic glacial interglacial deposition abstract the sedano project recovered 19 gravity cores on sediment drifts from the pacific continental margin of the antarctic peninsula camerlenghi et al 1997a high resolution terrigenous sedimentary record of the sediment drifts on the antarctic peninsula pacific margin in ricci c a ed the antarctic region museo nazionale dell antartide siena pp 705 710 fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils diatoms planktonic and benthic foraminifera framed in a depositional process reconstruction barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles respectively analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles the unit boundaries were drawn more accurately by means of the microfossil distribution on the basis of micropalaeontological and sedimentological evidence interglacial unit c is correlated to oxygen isotope stage 5 thus dating the unit boundaries at 127 and 70 ka peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment a calcareous nannofossil cold taxa association is present in most cores examined and its consistent distribution within interglacial unit c indicates key environmental relationships the occurrence of calcareous nannofossils has been related to temperature tolerance sea ice cover reduction nutrient availability and factors limiting primary productivity our results confirm that coccolithophorids occurred at southern high latitudes in the western marginal basins of the antarctic peninsula during short periods of the late quaternary a foraminiferal assemblage made up of sinistral neogloboquadrina pachyderma and few benthics occurs only in interglacial units correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes

analysis of velocity dispersion using full waveform multichannel sonic logging data a case study seismic attenuation and velocity dispersion are potentially able to reveal the rock physical properties of the subsurface conventionally a frequency independent quality factor q is measured this q is equivalent to the total velocity dispersion in a seismic record and is inadequate for analysing the attenuation mechanism or rock physical properties here a new method is proposed to extract the velocity dispersion curves so that more attributes can be obtained from full waveform multichannel sonic logging data especially the critical frequency fc if it is within the bandwidth of the data this method first decomposes the seismic data into a series of frequency components computes the semblance of each frequency component for different velocity values cross correlates the semblance matrices of adjacent frequency components to get the velocity gradients and finally integrates to obtain a velocity dispersion curve results of this method are of satisfactory accuracy and robustness this method is applied to the data acquired in mallik 5l 38 gas hydrate research well in mackenzie delta northwest territories canada the observed p wave velocity dispersion compares well with the geological setting in the gas hydrate zone about 900 m 1100 m high concentration of gas hydrate causes very strong velocity dispersion and a distinct fc at about 15 khz likely due to strong scattering of centimetre scale inclusions of gas hydrate concurrently water flow in connected cracks in some ranges of this zone adds a large part of velocity dispersion and a dimmer fc at about 9 5 khz immediate underneath the gas hydrate zone abundant free water in weakly laminated sediments causes quite strong velocity dispersion and an fc at about 6 5 khz velocity dispersion is mild and without an obvious fc in sediments above the gas hydrate zone

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

leaky mode a mechanism of horizontal seismic attenuation in a gas hydrate bearing sediment the leaky mode is a possible attenuation mechanism of seismic waves propagating along lamination in gas hydrate bearing sediment layers this horizontal propagation attenuation mechanism occurs when a high velocity layer is embedded in a low velocity zone this is a typical situation for gas hydrate occurrences to quantify this attenuation mechanism a 2d digital rock model based on the crosswell data of the mallik 2002 gas hydrate production research well program is used for simplicity our elastic simulations exclude attenuation mechanisms like scattering loss or intrinsic absorption we demonstrate that the leaky mode is a significant horizontal attenuation mechanism that cannot be neglected the effective attenuation of gas hydrate bearing sediments is a combination of intrinsic and scattering attenuation by small scale heterogeneties and the leaky mode

a comparative sem study on the micromorphology of glacial and nonglacial clasts with varying age and lithology preliminary research on striated clasts from a variety of depositional environments suggests that scanning electron microscopy sem of striated clasts of varying lithology in diamictons may prove useful in interpreting a glacial origin evaluating whether sem analysis of clasts from diamictons is an applicable technique to define a glacial origin requires a better understanding of the microfeatures occurring on glacial and nonglacial clasts we describe microtextures and surface characteristics for samples of quartzite granite limestone basalt chert pillow basalt and quartz pebbles collected from a variety of depositional environments our study suggests that it is possible to differentiate between glacial and nonglacial deposits based on frequency and morphology of diagnostic surface microfeatures observed on entrained clasts these microfeatures are best preserved and identified on competent and monomineralic samples e g quartzite chert limestone and poorly preserved on less resistant and p

ocean drilling program leg 178 antarctic peninsula sedimentology of glacially influenced continental margin topsets and foresets abstract ocean drilling program leg 178 february april 1998 drilled two sites sites 1097 and 1103 on the outer antarctic peninsula pacific continental shelf recovered strata are no older than late miocene or early pliocene site 1097 lies on the outer shelf within marguerite trough which is a major outlet for ice expanding seaward from the antarctic peninsula and reached a maximum depth drilled of 436 6 m below the sea floor mbsf seismic stratigraphic data show flat lying upper strata resting on strata that dip gently seaward uppermost strata to a depth of 150 mbsf were poorly recovered but data suggest they consist of diamictites containing reworked and abraded marine microfauna this interval is interpreted as having been deposited largely as till produced by subglacial cannibalization of marine sediments deformation till recording ice sheet expansion across the shelf underlying gently dipping strata show massive stratified and graded diamictite facies with common bioturbation and slump stuctures that are interbedded with laminated and massive mudstones with dropstones the succession contains a well preserved in situ marine microfauna typical of open marine and proglacial marine environments the lower gently dipping succession at site 1097 is interpreted as a complex of sediment gravity flows formed of poorly sorted glacial debris site 1103 was drilled in that part of the continental margin that shows uppermost flat lying continental shelf topsets overlying steeper dipping slope foresets seaward of a structural mid shelf high drilling reached a depth of 363 mbsf with good recovery in steeply dipping continental slope foreset strata foreset strata are dominated by massive and chaotically stratified diamictites interbedded with massive and graded sandstones and mudstones the sedimentary record and seismic stratigraphy is consistent with deposition on a continental slope from debris flows and turbidity currents released from a glacial source data from sites 1097 and 1103 suggest the importance of aggradation of the antarctic peninsula continental shelf by till deposition and progradation of the slope by mass flow this may provide a model for the interpretation of palaeozoic and proterozoic glacial successions that accumulated on glacially influenced continental margins

microtextures on quartz grains in tills from antarctica quartz grains of the sand 63 2000 m fractions from tills of oligocene to quaternary age in antarctica were analyzed by scanning electron microscope sem to test the hypothesis that relative ice thickness and distance of transport influence the type and range of microtextures observed on individual grains in this study replicated subsamples were analyzed from tills emplaced by thick glaciers with both short and long transport distances till samples with grains transported over long distances up to several hundred km under thick ice 1000 m were compared with quartz grains from relatively thinner e g 500 m thick outlet glaciers with shorter transport distances 100 km a range of microtextures including subparallel linear fractures conchoidal fractures arc shaped steps grain relief edge sharpness grain angularity and roundness straight and curved grooves crescentic gouges dissolution features precipitation features and presence of adhering particles was used to determine the environmental history of these grains microtextures on sand size clasts including microfractures abrasion etching and coatings can provide important information on paleoenvironmental history including glacier activity and severity of weathering inferred from etching and dissolution features

plio pleistocene ice volume antarctic climate and the global δ18o record we propose that from 3 to 1 million years ago ice volume changes occurred in both the northern and southern hemispheres each controlled by local summer insolation because earth9s orbital precession is out of phase between hemispheres 23 000 year changes in ice volume in each hemisphere cancel out in globally integrated proxies such as ocean 18 o or sea level leaving the in phase obliquity 41 000 years component of insolation to dominate those records only a modest ice mass change in antarctica is required to effectively cancel out a much larger northern ice volume signal at the mid pleistocene transition we propose that marine based ice sheet margins replaced terrestrial ice margins around the perimeter of east antarctica resulting in a shift to in phase behavior of northern and southern ice sheets as well as the strengthening of 23 000 year cyclicity in the marine 18 o record

minimal antarctic sea ice during the pliocene antarctic sea ice concentration at ocean drilling program sites 1165 64 380 s 67 219 e and 1166 67 696 s 74 787 e was lower than today through much of the pliocene the low sea ice concentration is evident from the proportion of the diatom eucampia antarctica with intercalary valves eucampia index this sea ice proxy was calibrated by using modern diatom data obtained from core top samples and winter sea ice concentration data september average through 1979 1987 the modern relationship is expressed as a binomial generalized linear model modern sea ice model this model was applied to the pliocene eucampia index within a 95 tolerance interval obtained from bootstrap estimates the results indicate that reduced winter sea ice concentrations persisted through much of the pliocene and at times were 78 and 61 relatively less concentrated than today at sites 1165 and 1166 respectively 2005 geological society of america

direct constraints on antarctic peninsula ice sheet grounding events between 5 12 and 7 94 ma 1 how has the antarctic ice sheet responded to or influenced global climate change this simple question has been difficult to address because the long term records of the ice sheet s fluctuations are poorly constrained with geologic data from antarctica thus studies to date have not convincingly established how specific antarctic ice sheet events correlate with climatic eustatic or other phenomena known from low latitude and deep sea records this study focused on documenting the direct record of ice sheet advance and retreat to the antarctic peninsula s shelf edge on the peninsula s outer shelf seismic reflectors interpreted to be subglacial unconformities were correlated with published results from ocean drilling program leg 178 lithologic and chronologic control at two drill sites provided ground truth for the seismic interpretation and the timing of the antarctic peninsula ice sheet grounding events this synthesis showed that grounded ice advanced to the shelf edge on at least 12 occasions between 5 12 and 7 94 ma

past hydrographic and climatic changes in the subantarctic zone of the south atlantic the pleistocene record from odp site 1090 abstract pleistocene summer sea surface temperatures ssst have been reconstructed on a composite core section recovered in the subantarctic zone of the southern ocean from planktonic foraminifers applying the modern analog technique the composite consists of core ps2489 2 and the sections recovered at odp site 1090 and documents the last 1 83 ma three distinct climatic periods can be identified that mirror the pleistocene development of the southern ocean hydrography cold climatic conditions prevailed at 43 s during glacial as well as during interglacial periods during the early pleistocene 1 83 0 87 ma indicating a northward shift of isotherms that characterize the present day polar front zone by about 7 of latitude evidence shows a strong linkage between southern ocean and low latitude climate during that interval time between the mid pleistocene revolution ca 0 9 ma and the mid brunhes event ca 0 4 ma we observe higher amplitude fluctuations in the ssst between glacial and interglacial periods corresponding to the temperature range between the present polar front and subantarctic front these climatic variations have been related to changes in the northern hemisphere ice sheets the past 0 4 ma are characterized by strong ssst variations of up to 8 c between glacials and interglacials only during the climatic optima stages 11 3 9 3 7 5 7 1 5 5 and the early holocene ssst exceeded present ssst at the core locality 10 2 c although the carbonate dissolution record exhibits high variability during the pleistocene it can be shown that ssst estimates were not significantly biased the mid brunhes dissolution cycle as well as the mid pleistocene enhanced carbonate preservation appear to belong to a global long term variability in carbonate preservation

the depositional pattern and distribution of glacial interglacial sequences on the antarctic peninsula pacific margin the outer shelf on the antarctic peninsula pacific margin south of 63 30 s is underlain by pliocene pleistocene prograding sequences which have been produced mainly by the action of ice sheets grounded out to the shelf edge at times of glacial maximum most sediment in these sequences has probably been transported to the margin in a deforming basal till which implies deposition on a broad front a line source a representative prograding sequence mapped across an extensive network of multichannel seismic reflection lines has an elongate depocentre on the upper palaeoslope which is consistent with the grounded ice sheet model however it is likely that each sequence recognized on existing multichannel seismic data represents several ice advances depth to surface maps reveal a broad variation along the margin in the amount of progradation reflecting differences in sediment supply the pattern of progradation and the bathymetry of the outer shelf suggest that the main depocentre in the area studied was fed by an ice stream at times of glacial maximum seismic lines across the margin farther to the southwest indicate the existence of other depocentres several broad depositional lobes have probably coalesced to form the extensive outer shelf the present continental slope is smooth and steep and is not cut by major canyons a downslope change in seismic facies and scouring on the uppermost rise probably reflect downslope transitions from slumps to debris flows to turbidity currents these processes are likely to be most active at times of glacial maximum deep drilling data indicate that the rise sediments consist mainly of terrigenous turbidite and ice rafted detritus a marked upward change in seismic facies on the continental rise indicates a change to a higher energy sedimentary regime and appears to correlate with the start of glacial progradation on the shelf

seismic modeling of multidimensional heterogeneity scales of mallik gas hydrate reservoirs northwest territories of canada 1 in hydrate bearing sediments the velocity and attenuation of compressional and shear waves depend primarily on the spatial distribution of hydrates in the pore space of the subsurface lithologies recent characterizations of gas hydrate accumulations based on seismic velocity and attenuation generally assume homogeneous sedimentary layers and neglect effects from large and small scale heterogeneities of hydrate bearing sediments we present an algorithm based on stochastic medium theory to construct heterogeneous multivariable models that mimic heterogeneities of hydrate bearing sediments at the level of detail provided by borehole logging data using this algorithm we model some key petrophysical properties of gas hydrates within heterogeneous sediments near the mallik well site northwest territories canada the modeled density and p and s wave velocities used in combination with a modified biot gassmann theory provide a first order estimate of the in situ volume of gas hydrate near the mallik 5l 38 borehole our results suggest a range of 528 to 768 106 m3 km2 of natural gas trapped within hydrates nearly an order of magnitude lower than earlier estimates which did not include effects of small scale heterogeneities further the petrophysical models are combined with a 3 d finite difference modeling algorithm to study seismic attenuation due to scattering and leaky mode propagation simulations of a near offset vertical seismic profile and cross borehole numerical surveys demonstrate that attenuation of seismic energy may not be directly related to the intrinsic attenuation of hydrate bearing sediments but instead may be largely attributed to scattering from small scale heterogeneities and highly attenuate leaky mode propagation of seismic waves through larger scale heterogeneities in sediments

coarse grained terrigenous sediment deposition on continental rise drifts a record of plio pleistocene glaciation on the antarctic peninsula sediment drifts on the continental rise are located proximal to the western side of the antarctic peninsula and recorded changes in glacial volume and thermal regime over the last ca 15 m y at ocean drilling program odp site 1101 leg 178 which recovered sediments back to 3 1 ma glacial interglacial cyclicity was identified based on the biogenic component and sedimentary structures observed in x radiographs magnetic susceptibility and lithofacies descriptions glacial intervals are dominated by fine grained laminated mud and interglacial units consist of bioturbated muds enriched in biogenic components from 2 2 to 0 76 ma planktonic foraminifera and calcareous nannofossils dominate in the interglacials suggesting a shift of the antarctic polar front apf to the south near the drifts prior to 2 2 ma cyclicity cannot be identified and diatoms dominate the biogenic component and high percent opal suggests warmer conditions south of the apf and reduced sea ice over the drifts analyses of the coarse grained terrigenous fraction pebbles and coarse sand from sites 1096 and 1101 record glaciers at sea level releasing iceberg rafted debris ird throughout the last 3 1 m y analyses of quartz sand grains in ird with the scanning electron microscope sem show an abrupt change in the frequency of occurrence of microtextures at similar to 1 35 ma during the late pliocene to early pleistocene the population of quartz grains included completely weathered grains and a low frequency of crushing and abrasion suggesting that glaciers were small and did not inundate the topography debris shed from mountain peaks was transported supraglacially or englacially allowing weathered grains to pass through the glacier unmodified during glacial periods from 1 35 0 76 ma glaciers expanded in size the hid flux was very high and dropstones have diverse lithologies conditions resembling those at the last glacial maximum lgm have been episodically present on the antarctic peninsula since similar to 0 76 ma quartz sand grains show high relief fracture and abrasion common under thick ice and the ird flux is low with a more restricted range of dropstone lithologies

margin architecture reveals the transition to the modern antarctic ice sheet ca 3 ma comment and reply comment rebesco et al 2006 1 present multichannel seismic mcs reflection profiles showing a regional change in the style of margin accretion on the pacific margin of the antarctic peninsula pmap and estimate an age of ca 3 ma for this change by correlation with ocean drilling program odp site

detection and estimation of gas hydrates using rock physics and seismic inversion examples from the northern deepwater gulf of mexico natural gas hydrates composed primarily of water and methane are solid crystalline ice like substances found in permafrost areas and deepwater basins around the world as the search for oil and gas extends into ever deeper waters particularly within the northern gulf of mexico gas hydrates are becoming more of a focus in terms of both safety and as a potential energy resource

sedimentation on the continental rise west of the antarctic peninsula over the last three glacial cycles abstract the continental rise west of the antarctic peninsula includes a number of large sediment mounds interpreted as contourite drifts cores from six sediment drifts spanning some 650 km of the margin and 4 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy interglacial sedimentation rates range from 1 1 to 4 3 cm ka glacial sedimentation rates range from 1 8 to 13 5 cm ka and decrease from proximal to distal sites on each drift late quaternary sedimentation was cyclic with brown biogenic burrowed mud containing ice rafted debris ird in interglacials and grey barren laminated mud in glacials foraminiferal intervals occur in interglacial stages 5 and 7 but not in the holocene processes of terrigenous sediment supply during glacial stages differed meltwater plumes were more important in stages 2 4 turbidity currents and ice rafting in stage 6 the terrigenous component shows compositional changes along the margin more marked in glacials the major oxides al2o3 and k2o are higher in the southwest and cao and tio2 higher in the northeast there is more smectite among the clay minerals in the northeast magnetic susceptibility varies along and between drifts these changes reflect source variations along the margin interglacial sediments show less clear trends and their ird was derived from a wider area downslope processes were dominant in glacials but alongslope processes may have attained equal importance in interglacials the area contrasts with the east antarctic continental slope in the se weddell sea where ice rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial the differences in glacial setting and margin physiography can account for these contrasts

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

acoustic properties of sediments saturated with gas hydrate free gas and water we obtain the wave velocities and quality factors of gas hydrate bearing sediments as a function of pore pressure temperature frequency and partial saturation the model is based on a biot type three phase theory that considers the existence of two solids grains and gas hydrate and a fluid mixture attenuation is described with the constant q model and viscodynamic functions to model the high frequency behaviour we apply a uniform gas water mixing law that satisfies wood s and voigt s averages at low and high frequencies respectively the acoustic model is calibrated to agree with the patchy saturation theory at high frequencies white s model pressure effects are accounted by using an effective stress law for the dry rock moduli and permeabilities the dry rock moduli of the sediment are calibrated with data from the cascadia margin moreover we calculate the depth of the bottom simulating reflector bsr below the sea floor as a function of sea floor depth geothermal gradient below the sea floor and temperature at the sea floor