seabed pockmarks associated with deepwater corals off se brazilian continental slope santos basin several factors bear evidence of chemosynthetic ecosystem occurrence in the brazilian coast these motivated our investigation to sample three pockmarks located on the continental slope off se brazil 26js and 46jw at f700 m depth the crater like features were f1 km in diameter and 100 m in depth and were identified with an echo sounder ek 500 they were sampled along two transects using a rectangular dredge our main objective was to identify possible evidence of a seep related community the surrounding sediments of the pockmarks seem to be filled with coral rubble harboring a rich fauna of metazoans including epilithic and chasmolithic organisms a total of 9565 organisms were sampled the most abundant group was the ophiuroidea followed by cirripedia and polychaeta a number of new occurrences and new species were registered the polychaete worm amelinna was the first record for the atlantic ocean which has been found in reducing habitats such as wood islands the echo sounder records also showed the presence of big deepwater coral mounds associated with the edge of the pockmarks these features are f 20 m in height and from 180 to 360 m wide records of demersal fishes were also registered right above coral mounds the presence of deepwater coral mounds at the edge of pockmarks may be related to the flow of the cold oxygen rich antarctic intermediate water over the bottom or else may benefit from the fertilization of the water by the microseepage of hydrocarbons from the sediment we suggest a more thorough survey in order to map and recognize these environments furthermore our findings may shed some light on new research perspectives concerning the ecology biogeography and evolution of reducing habitats off the brazilian coast d 2004 elsevier b v all rights reserved

opening of the fram strait gateway a review of plate tectonic constraints abstract we have revised the regional crustal structure oceanic age distribution and conjugate margin segmentation in and around the lena trough the oceanic part of the fram strait between the norwegian greenland sea and the eurasia basin arctic ocean the lena trough started to open after eurasia greenland relative plate motions changed from right lateral shear to oblique divergence at chron 13 times 33 3 ma earliest oligocene a new bouguer gravity map supported by existing seismic data and aeromagnetic profiles has been applied to interpret the continent ocean transition and the influence of eocene shear structures on the timing of breakup and initial seafloor spreading assuming that the onset of deep water exchange depended on the formation of a narrow oceanic corridor the gateway formed during early miocene times 20 15 ma however if the initial lena trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep water circulation the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around chron 5 times 9 8 ma late miocene paleoceanographic changes at odp site 909 northern hovgard ridge are consistent with both hypotheses of gateway formation we cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the lena trough the gravity seismic and magnetic observations question the prevailing hypotheses on the yermak plateau and the morris jesup rise as eocene oceanic plateaus and the hovgard ridge as a microcontinent

variability of arctic sea ice topography and its impact on the atmospheric surface drag over the polar oceans near surface atmospheric transport of momentum is strongly influenced by sea ice surface topography the latter is analyzed on the basis of laser altimeter data obtained during airborne campaigns between 1995 and 2011 over more than 10 000 km of flight distance in different regions of the arctic ocean spectra of height and spacing between topographic features averaged over 10 km flight sections show that typical values are 0 45 m for the mean height and about 20 m for the mean spacing nevertheless the variability is high and the spatial variability is stronger than the temporal one the total topography spectrum is divided into a range with small obstacles between 0 2 m and 0 8 m height and large obstacles 0 8 m results show that large pressure ridges represent the dominant topographic feature only along the coast of greenland in the central arctic the concentration of large ridges decreased over the years accompanied by an increase of small obstacles concentration and this might be related to decreasing multiyear ice the application of a topography dependent parameterization of neutral atmospheric drag coefficients reflects the large variability in the sea ice topography and reveals characteristic differences between the regions based on the analysis of the two spectral ranges we find that the consideration of only large pressure ridges is not enough to characterize the roughness degree of an ice field and the values of drag coefficients are in most regions strongly influenced by small obstacles

seabed features caused by shallow gas in north sea seepage of shallow gas mainly methane recently has been documented visually in the central section of the north sea at water depths of about 70 80 m the surface expressions caused by these seeps are not dramatic they mainly consist of small funnel shaped craters 20 cm across in the cover sand however in deeper parts of the north sea where the cover sediments consist of silt and clay the surface features are more dramatic here the craters pockmarks caused by shallow gas eruptions or seepages are up to 20 m deep and 100 m wide in some areas the pockmarks are paved with a crust of calcium carbonate cement which is believed to have formed as a result of gas efflux through the seabed furthermore the presence of these calcium carbonate reefs seems to have attracted a wide variety of marine life the presence of shallow gas has caused an enrichment of marine life that seems to be significant beside the semicircular and composite pockmarks gas induced erosion also has caused elongate depressions these occur where the top sediment bedding has caused the gas to migrate along certain axes the mode and speed of formation of the gas induced erosion features are ofmore major concern to oil exploration and development in the northern north sea research into these aspects recently has been stimulated by the discovery of one of the world s largest offshore gas reservoirs which coincidentally lies within a pockmarked area less

ebullition and storm induced methane release from the east siberian arctic shelf vast quantities of carbon are stored in shallow arctic reservoirs such as submarine and terrestrial permafrost submarine permafrost on the east siberian arctic shelf started warming in the early holocene several thousand years ago however the present state of the permafrost in this region is uncertain here we present data on the temperature of submarine permafrost on the east siberian arctic shelf using measurements collected from a sediment core together with sonar derived observations of bubble flux and measurements of seawater methane levels taken from the same region the temperature of the sediment core ranged from 1 8 to 0 degrees c although the surface layer exhibited the lowest temperatures it was entirely unfrozen owing to significant concentrations of salt on the basis of the sonar data we estimate that bubbles escaping the partially thawed permafrost inject 100 630 mg methane m 2 d 1 into the overlying water column we further show that water column methane levels had dropped significantly following the passage of two storms we suggest that significant quantities of methane are escaping the east siberian shelf as a result of the degradation of submarine permafrost over thousands of years we suggest that bubbles and storms facilitate the flux of this methane to the overlying ocean and atmosphere respectively

validation of sea ice topographic heights derived from tandem x interferometric sar data with results from laser profiler and photogrammetry in this paper the retrieval of sea ice surface heights from the interferometric tandem x data is investigated the data were acquired over fast and drifting ice in fram strait located between greenland and svalbard additional measurements of the sea ice surface topography were carried out using a stereo camera and a laser altimeter the comparison of the surface elevation retrieved from tandem x imagery with the results of the stereo camera measurements revealed that sea ice ridges greater than 0 5 m can be estimated with a root mean square error of 0 3 m or less with the error decreasing as a function of ridge height although the helicopter borne laser data are only available as 1 d profiles with a much higher across track spatial resolution than the tandem x data they proved to be useful for the validation the need for multilook averaging to reduce the phase noise is identified as the main challenge in achieving the spatial resolution necessary for retrieving sea ice surface topography using synthetic aperture radar interferometry

decorrelation in interferometric radar echoes a radar interferometric technique for topographic mapping of surfaces implemented utilizing a single synthetic aperture radar sar system in a nearly repeating orbit is discussed the authors characterize the various sources contributing to the echo correlation statistics and isolate the term which most closely describes surficial change they then examine the application of this approach to topographic mapping of vegetated surfaces which may be expected to possess varying backscatter over time it is found that there is decorrelation increasing with time but that digital terrain model generation remains feasible the authors present such a map of a forested area in oregon which also includes some nearly unvegetated lava flows such a technique could provide a global digital terrain map

a pockmark field in the patras gulf greece and its activation during the 14 7 93 seismic event abstract during a recent oceanographical geophysical survey carried out in the southeastern part of the gulf of patras in western greece for the construction of an outfall an active pockmark field was found the pockmark field was formed in soft layered holocene silts the pockmarks are associated with acoustic anomalies attributed to gas charged sediments the pockmarks vary in size and shape from 25 to 250 m in diameter and from 0 5 to 15 m in depth and are among the largest and deepest observed in the world on july 14th 1993 during the survey a major earthquake of magnitude 5 4 on the richter scale occurred in the area during the 24 hour period prior to the earthquake the bottom water temperature anomalously increased on three occasions whilst for a few days after the earthquake it was noted that the majority of the pockmarks were venting gas bublles it is considered that the three abrupt sea water temperature increases were probably the result of upward migrating high temperature gas bubbles in the water column it is further suggested that the earthquake was the triggering mechanism and that the gas expulsion was caused by the reduction in the pore volume in the sediments resulting from changes in the stress regime prior to the earthquake therefore it can be suggested that in seismic areas adjacent to pockmark fields earthquake prediction may be achieved by monitoring the water temperature and or the rate of gas venting in the pockmark field our analysis indicates that the pockmark field in the patras gulf has formed slowly during the holocene by continuous gas venting which is periodically being interrupted by short duration events of enhanced gas seepage triggered by earthquakes

geological and morphological setting of 2778 methane seeps in the dnepr paleo delta northwestern black sea abstract the dnepr paleo delta area in the nw black sea is characterized by an abundant presence of methane seeps during the expeditions of may june 2003 and 2004 within the eu funded crimea project detailed multibeam seismic and hydro acoustic water column investigations were carried out to study the relation between the spatial distribution of methane seeps sea floor morphology and sub surface structures 2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 all seeps are located in the transition zone between the continental shelf and slope in water depths of 66 to 825 m the integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area but are concentrated in specific locations the depth limit for the majority of the detected seeps is 725 m water depth which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature 8 9 c in this part of the black sea this suggests that where gas hydrates are stable they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column higher up on the margin gas seeps are widespread but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub surface features on the shelf the highest concentration of seeps is found in elongated depressions pockmarks above the margins of filled channels on the continental slope where no pockmarks have been observed seepage occurs along crests of sedimentary ridges there seepage is focussed by a parallel stratified sediment cover that thins out towards the ridge crests on the slope seepage also appears in the vicinity of canyons bottom flanks and margins or near the scarps of submarine landslides where mass wasting breaches the fine grained sediment cover that acts as a stratigraphic seal the seismic data show the presence of a distinct gas front which has been used to map the depth of the free gas within the sea floor sediments the depth of this gas front is variable and locally domes up to the sea floor where the gas front approaches the seafloor gas bubbles were detected in the water column a regional map of the sub surface depth of the gas front emphasises this gas front versus seep relationship the integration of all data sets indicates that the spatial distribution of methane seeps in the dnepr paleo delta is mainly controlled by the gas hydrate stability zone as well as by stratigraphic and sedimentary factors

large pockmarks gas charged sediments and possible clay diapirs in the skagerrak abstract extensive patches of shallow gas charged sediments have previously been documented in the eastern skagerrak the strait between norway and denmark recently obtained high resolution side scan sonar and sub bottom profiler data presented here document several surface and sub surface features at the border and inside zones of such gas charged shallow sediments four examples are presented and discussed 1 a low profile 3 m deep pockmark and gas charged sediments 2 a low profile 6 m deep pockmark associated with a rugged sub surface reflector 3 large up to 24 m deep and 500 m wide pockmarks and 4 patches of exposed material from the rugged sub surface reflector from similarities with previously described features and also with features found in the kattegat further south it is concluded that the pockmarks are formed by gas eruption and seepage and that the rugged sub surface reflector could possibly represent the upper surface of a diapirically deformed unit of gas charged plastic clays

an integrated view of the methane system in the pockmarks at vestnesa ridge 79 n accepted manuscript version published version available at https doi org 10 1016 j margeo 2017 06 006 accepted manuscript version licensed cc by nc nd 4 0

extreme dynamic weakening of faults during dehydration by coseismic shear heating the dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release high velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0 6 to 0 15 as the slip velocity reaches 1 1 m s at normal stresses up to 24 5 mpa the extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding the rapid heating at asperities causes serpentine dehydration in impermeable fault zones in nature serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening this dynamic fault weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the san andreas

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

geophysical and geochemical evidence for subsurface hydrocarbon leakage in the bering sea alaska abstract geophysical and geochemical surveys were conducted on the bering sea shelf to investigate whether hydrocarbons found in the surficial sediment particularly in association with certain geophysical events indicate the presence and nature of more deeply reservoired oil and gas deposits surface sediment samples were obtained at 1000 sites where high resolution seismic data indicated the presence of gas seepage and or surface faulting in addition sites for background measurements were occupied away from areas of gas seepage results of the geophysical survey show seismic wipeout zones to be present throughout the bering sea shelf sediment samples obtained within the shallow wipeout zones confirm that these features are caused by gas in addition compositional and isotopic analyses indicate that the gas is of biogenic and thermogenic origins the thermogenic leakage appears to be related to localized faulting and shallow structuring the high resolution seismic data therefore provide an excellent tool for locating leakage points thereby enhancing the opportunity of finding migrated thermogenic hydrocarbons in surficial sediments an examination of vertical variability shows the hydrocarbon concentrations to be strongly related to various processes occurring in the surficial sediments core samples must be taken from below the zone of maximum disturbance to obtain unaltered hydrocarbon samples

gas charged sediments and large pockmark like features on the gulf of cadiz slope sw spain abstract gassy sediments and large seafloor pockmarks are identified from seismic profiles in the gulf of cadiz slope gas charged sediments are represented by zones of acoustic turbidity and other acoustic anomalies on seismic profiles most of the gas is believed to be biogenic in origin resulting from the decay of organic matter contained within rapidly developed regressive shelf margin deltas a group of large asymmetric seafloor pockmarks also occurs downslope of the gassy area both on the seafloor surface and deeply buried in the sediment column sediment sorting by the mediterranean undercurrent seems to act as a controlling factor for the development of each type of gas originated feature the area where the strong mediterranean undercurrent outflow impinges the seafloor is covered by sandy sediments of high porosity which allow the interstitial gas to escape through the sediment pores whereas the areas affected by weaker currents are covered by clays and the gas tends to accumulate in the sediment beneath impermeable horizons

patterns of carbonate authigenesis at the kouilou pockmarks on the congo deep sea fan abstract different types of seep carbonates were recovered from the kouilou pockmarks on the congo deep sea fan in approximately 3100 m water depth the carbonate aggregates are represented by pyritiferous nodules crusts and slabs tubes and filled molds the latter are interpreted to represent casts of former burrows of bivalves and holothurians the nodules consisting of high mg calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic anme 1 group whereas aragonitic crusts resulted from the activity of archaea of the anme 2 cluster evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts probably reflecting higher porosities of the host sediment during carbonate formation this finding is supported by lower 18 o values of crusts agreeing with precipitation from pore waters similar in composition to seawater the aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate rich pore waters similar to seawater moreover the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that anme 1 archaea tend to occur deeper in the sediment than members of the anme 2 group methane represents the predominant carbon source of all carbonates 13 c values as low as 58 9 v pdb and the encrusted archaeal biomarkers 13 c values as low as 140 v pdb oxygen isotope values of some nodular carbonates ranging from 3 9 to 5 1 v pdb are too high for precipitation in equilibrium with seawater probably reflecting the destabilization of gas hydrates which are particularly abundant at the kouilou pockmarks

acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the northern gulf of mexico seeps of free methane gas escaping the seabed can be found throughout the ocean basins to understand the role of methane gas seeps in the global carbon cycle including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume it is important to quantify the amount of methane escaping the seabed few large scale mapping projects of natural methane seeps have been undertaken however and even among these quantitative estimates of flux are rare here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region 6000 km2 of the northern gulf of mexico and outline a general approach for methane seep mapping using a combination of multibeam and split beam echo sounders using additional measurements collected with a remotely operated vehicle rov together with the acoustic mapping results we estimate the total gas flux within the 6000 km2 region to be between 0 0013 and 0 16 tg yr or between 0 003 and 0 3 of the current estimates for global seabed methane seepage rates

characterisation and correlation of active hydrocarbon seepage using geophysical data sets an example from the tropical carbonate yampi shelf northwest australia abstract for the first time in australia active present day hydrocarbon seepage has been imaged on the tropical carbonate yampi shelf in 50 and 90 m water depth seepage is evidenced by gas plumes in the water column that are associated with seabed features such as clusters of reflective blocks hard grounds pockmark fields and mounds seepage activity and intensity seems to vary with changes in pressure related to macro tidal cycles the seabed features coincide with sub surface features such as areas of seismic signal attenuation under high amplitude reflectors seismic discontinuities and bright spots hydrocarbon migration seepage pathways appear to be controlled by the reactivation of pre existing fractures and dykes within the basement the types of seabed features and their preservation on a tropical carbonate shelf are strongly influenced by the coarse bioclastic nature of sediments and the high energy of macro tidal currents and storm reworking

methane seepage along the hikurangi margin new zealand overview of studies in 2006 and 2007 and new evidence from visual bathymetric and hydroacoustic investigations this paper is an introduction to and an overview of papers presented in the special issue of marine geology methane seeps at the hikurangi margin new zealand in 2006 and 2007 three research cruises to the hikurangi margin at the east coast of new zealand s north island were dedicated to studying methane seepage and gas hydrates in an area where early reports suggested they were widespread two cruises were carried out on rv tangaroa and one on rv sonne using the complete spectrum of state of the art equipment for geophysics seismic sidescan controlled source electromagnetics ocean bottom seismometers and hydrophones singlebeam and multibeam seafloor observations towed camera systems rov sediment and biological sampling tv guided multi corer gravity corer grab epibenthic sled deployment of in situ observatories landers as well as water column sampling and oceanographic studies ctd moorings the scientific disciplines involved ranged from geology geophysics petrography geochemistry to oceanography biology and microbiology these cruises confirmed that a significant part of the hikurangi margin has been active with locally intense methane seepage at present and in the past with the widespread occurrence of dead seep faunas and knoll forming carbonate precipitations offshore and on the adjacent land a close link to seismically detected fluid systems and the outcropping of the base of the gas hydrate stability zone can be found at some places pore fluid and free gas release were found to be linked to tides currents as well as density layers modulate the methane distribution in the water column the paper introduces the six working areas on the hikurangi margin and compiles all seep locations based on newly processed multibeam and multibeam backscatter data water column hydroacoustic and visual data that are combined with results presented elsewhere in this special issue in total 32 new seep sites were detected that commonly show chemoherm type carbonates or carbonate cemented sediment with fissures and cracks in which calyptogenid clams and bathymodiolid mussels together with sibloglinid tube worms live white bacterial mats of the genus beggiatoa and dark gray beds of heterotrophic ampharetid polychaetes typically occur at active sites bubble release has frequently been observed visually as well as hydroacoustically flares and geochemical analyses show that biogenic methane is released all seep sites bubbling or not were inside the gas hydrate stability zone gas hydrate itself was recovered at three sites from the seafloor surface or 2 5 m core depth as fist sized chunks or centimeter thick veins the strong carbonate cementation that in some cases forms 50 m high knolls as well as some very large areas being paved with clam shells indicates very strong and long lasting seep activity in the past this activity seems to be less at present but nevertheless makes the hikurangi margin an ideal place for methane related seep studies in the sw pacific

massive vein filling gas hydrate relation to ongoing gas migration from the deep subsurface in the gulf of mexico abstract a gas hydrate mound that contains massive vein filling structure ii gas hydrate occurs on the upper continental slope 540 m water depth of the gulf of mexico southwest of the mississippi delta the mound is located in the green canyon gc block 185 adjacent to jolliet field in gc 184 jolliet field contains oil and gas that filled fault traps caused by salt deformation during late pleistocene holocene time in contrast to reservoir oil in jolliet field which shows bacterial oxidation effects the c 1 c 5 reservoir gas is unaltered by bacterial oxidation disassociated gas is assumed to have recently entered from the subsurface hydrocarbon system vertical migration of gas along faults is ongoing manifested on the sea floor by gas vents gas hydrate complex chemosynthetic communities and by a large gas plume that extends from the vents to the sea surface the isotopic properties of c 1 c 5 hydrocarbons from reservoirs gas vents and gas hydrate correlate closely although outcropping gas hydrate is transiently stable because of variations in seawater temperature the bulk of buried gas hydrate at gc 185 is stable and perhaps increasing in volume because of the copious gas flux the massive accumulation of gas hydrate at the gc 185 site is attributed to the gas that has recently entered the vents largely from jolliet field and to the synchronous activation of fault conduits allowing gas migration to the sea floor synchronous late gas charge and faulting could also explain the wide distribution of gas hydrate across the upper gulf slope

trace element anomaly in fault rock induced by coseismic hydrothermal reactions reproduced in laboratory friction experiments friction experiments at high velocities 0 1 to 0 4 m s at room temperature under high pore fluid pressures 2 or 5 mpa and trace element analyses of the run products were carried out on simulated fault gouge derived from the chelungpu fault taiwan the friction coefficient decreased to 0 1 during sliding and the slip surface temperature reached 300 c by 6 m of slip displacement the slip caused a small but distinct decrease of li in the deformed gouge when the slip surface temperature exceeded 300 c the observed li depletion agreed with calculated results indicating that it resulted from a hydrothermal reaction at a high temperature produced by short duration up to 40 s frictional heating the geochemical data suggest a small fluid rock ratio and low reaction temperature in the simulated fault which is explained by higher normal stress or repeated earthquakes which elevated fluid rock ratios in the natural fault

structurally controlled hydrocarbon seeps on a glaciated continental margin hecate strait offshore british columbia abstract this paper describes structurally controlled gas seeps and their influence on seafloor morphology at sites within queen charlotte basin qcb a sedimentary basin situated offshore british columbia western canada numerous surficial and shallow sediment features related to both ice cover and shallow gas have been identified in high resolution multichannel seismic huntec deep towed seismic multibeam bathymetry and backscatter and piston core data that were acquired over a 10 km by 10 km grid in an eastern part of qcb pockmarks seafloor mounds and iceberg ploughmarks are visible in the multibeam data small scale folds vertical blank zones and acoustic turbidity are observed in the huntec data the multichannel seismic data show a faulted anticline phase reversed bright spots zones of low instantaneous frequency and velocity pull ups one seafloor mound lies directly above a shallow fault that offsets an asymmetric fold the remaining mounds lie either above or in close proximity to asymmetric folds that are faulted bright spots are observed along some fault traces and within a cut and fill sequence at two way travel times between 0 42 and 1 0 s velocity pull ups are observed in the multichannel data below gas expansion voids present in piston cores backscatter data show that seafloor mounds are highly reflective compared to the surrounding seafloor these observations indicate that gas has been migrating upward through the rock column and expelled into the shallow sediments the shallow gas has been biologically mediated to form carbonate sediments within and around seafloor mounds shallow gas has also deformed surficial sediments into positive relief mounds and negative relief pockmarks

surface roughness of baltic sea ice small medium and large scale measurements of sea ice surface roughness were carried out in the baltic sea over a 3 year period a clear relationship was found between the rms height and correlation length of baltic sea ice in most cases the surface correlation function was close to exponential the logarithm of rms height showed a linear dependence on that of measured distance similarly the correlation length was in many cases linearly dependent on the measured distance this dependence on length suggests that ice surface roughness could be characterized as a fractal

thermal stress cracking in granite we have developed a comprehensive experimental capability to study thermally induced microfractures in rock we count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300 c under confining pressures up to 55 mpa after a heating cycle we measure crack porosity and crack compressibility we count the cracks under an sem sorting them by grain boundary and intragranular types and by mineral pair occurrence our observations reveal that 1 significant thermal cracking occurs in granites heated above 200 or 250 c under 28 and 55 mpa and above 100 c in granites heated under 7 mpa 2 most newly created thermal cracks close at pressures below 40 mpa 1 5 km depth 3 grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked 4 the threshold temperature for acoustic emissions is a positive linear function of pressure 5 total acoustic emission counts in westerly granite heated to 300 c are slightly lower under 55 than 28 mpa confining pressure and are substantially lower under 28 than 7 mpa 6 the compressional velocity decreases 22 as westerly granite is heated to 300 c at 7 mpa 15 at 28 mpa and 10 at 55 mpa and 7 greater hysteresis in compressional velocities occurs in samples heated at lower pressures the acoustic emission counts in westerly granite during heating show one or more peaks of activity as a function of temperature with more peaks at lower pressures we interpret this result to mean that additional crack populations are able to open at lower confining pressures the curves of acoustic emission count rates can be fit by summing one to three gaussian density functions

tomographic imaging of fjord ice using a very high resolution ground based sar system this paper presents new experimental results of 3 d imaging using tomographic techniques over a snow covered sea ice medium sensed with an x band radar system the available data are from a ground based synthetic aperture radar data collection campaign carried out over kattfjord tromso norway direct imaging of the vertical structures of the radar reflectivity of the snow and sea ice layers is achieved by focusing the signal from a 2 d synthetic array in the 3 d space the effect of a change in propagation velocity of the wave inside the considered medium is investigated in the focusing process and the tomograms are effectively corrected for this effect the distribution of the scattering contributions in the vertical direction reveals a strong response from the sea ice cover tomograms at two different polarizations are investigated and compared the results and the interpretations are also supported by the simulated data from the same system

the effect of water on strain localization in calcite fault gouge sheared at seismic slip rates abstract strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating the earthquake energy budget and the evolution of fault rock microstructure to assess the nature of strain localization during shearing of calcite fault gouges high velocity v max 1 m s rotary shear experiments at normal stresses of 3 20 mpa were conducted under room dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers when sheared at 1 m s the room dry gouges showed a prolonged strengthening phase prior to dynamic weakening whereas the wet gouges weakened nearly instantaneously microstructural analysis revealed that a thin

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

late quaternary contourites and glaciomarine sedimentation in the fram strait two sites in the eastern fram strait the vestnesa ridge and the yermak plateau have been surveyed and sampled providing a depositional record over the last glacial interglacial cycle the fram strait is the only deep water connection from the arctic ocean to the north atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction on the vestnesa ridge the western svalbard margin a sediment drift was identified in 1226 m of water gravity and multicores from the crest of the drift recovered turbidites and contourites 14c dating indicates an age range of 8287 to 26 900 years bp early holocene to late weichselian the yermak plateau is characterized by slope sediments in 961 m of water gravity and multicores recovered contourites and hemipelagites 14c ages were between 8615 and 46 437 years bp early holocene to mid weichselian downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid weichselian suggesting variable sea ice extent productivity and polynyas present even during the last glacial maximum four layers of ice rafted debris were also identified and correlated within the cores these events occurred ca at 9 24 to 25 26 to 27 and 43 ka asynchronous with heinrich layers in the wider north east atlantic and here interpreted as reflecting instability in the svalbard barents ice sheet and the northward advection of warm atlantic water during the late weichselian the activity of the ancestral west spitsbergen current is interpreted using mean sortable silt records from the cores on the vestnesa ridge drift the modern mass accumulation rate calculated using excess 210pb is 0 076 g cm 2 year 1 on the yermak plateau slope the modern mass accumulation rate is 0 053 g cm 2 year 1

acoustic evidence for shallow gas accumulations in the sediments of the eastern black sea the black sea contains immense gas accumulations exploration of gas accumulations is geologically and economically important because migration of methane in sediments may cause massive slope failures and the methane seeps may indicate deeper hydrocarbon reservoirs human activity both in and on the seafloor oil industry and natural activity earthquakes cyclones trigger mechanisms for seafloor failure and gas release that may have a local and possibly global environmental impact recently sonar and high resolution seismic surveys were carried out to obtain information about the effects of gas and gas filled sediments throughout the turkish margin of the eastern black sea and shallow gas was detected on the subbottom profiler records it continues about 25 65 m below the sea floor and is marked by bright and cloudy spots sometimes pockmarks and acoustic voids the lower section of the turkish shelf is an extensive pockmarked plateau the pockmarks are seen as circular structures with high backscattering on the sonar records

depositional environment and source rock potential of miocene strata from the central fram strait introduction of a new computing tool for simulating organic facies variations abstract organic rich sediments were recognized in early miocene strata from the norwegian greenland sea during odp leg 151 three organic geochemical subunits were distinguished in hole 909c using detailed organic geochemical and microscopic analyses toc up to 3 wt hi up to 200 mg hc g toc and 13corg values 24 26 indicate the predominance of terrestrial type iii organic matter in subunit 3 18 16 2 ma biomarker and vitrinite reflectance values r0 0 5 point to rather fresh immature terrestrial organic matter supplied by river discharge from adjacent vegetated coastal areas although the sediments of subunit 3 have a fair to good source rock potential they are insufficiently mature to generate significant amounts of oil or gas to test if the moderate generation potential and source rock quality of subunit 3 are applicable to the entire area the computer software of mod was applied several modelling runs testing the most probable depositional scenarios result even under conservative assumptions in the formation of good to excellent source rocks towards the basin margins

velocity dependence of shear induced permeability associated with frictional behavior in fault zones of the nankai subduction zone 1 we measured frictional properties and permeability of core materials from the megasplay fault zone site c0004 and the frontal thrust site c0007 in the shallow part of the nankai subduction zone permeability was measured before and after 7 9 m slip displacement at high 1 05 m s and low velocities 0 013 m s under normal stresses of 1 5 mpa using the rotary shear apparatus from which we estimated the shear induced permeability change in an experimental fault gouge prepared from core material gouge permeability 10 18 m2 decreased after sliding for wet gouge and increased for dry gouge the high velocity friction test under wet conditions yielded a smaller reduction in permeability than the low velocity test whereas the opposite trend was observed in dry conditions we attribute the differences in permeability to the effects of thermal mechanical pore pressurization upon shear induced compaction symmetric boudin structures may represent evidence of hydrofracturing induced by pore fluid pressurization the large friction coefficient of the megasplay fault material in the slow and wet friction tests is explained by homogeneous shear deformation and higher permeability that promotes faster shear induced compaction the similarity in post shear permeability for the gouges from the both faults may account for the similar friction coefficients in high velocity friction assuming that the pore fluid pressurization process controls high velocity frictional behavior this velocity dependence on friction suggests that a large dynamic stress drop is expected for the megasplay fault implying that large slip displacement followed by a giant tsunami is plausible when a rupture from depth propagates to the megasplay fault

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

effects of permeability and porosity evolution on simulated earthquakes numerical simulations are a fundamental tool to access the typical conditions attained during earthquake instabilities and to simulate the large number of dissipative processes taking places during faulting in this study we consider a single degree of freedom spring slider system a simplified fault model which can describe the whole seismic cycle and the dynamics of a fault with spatially homogeneous properties we assume a rate and state dependent friction in which we incorporate the effects of pore fluid pressure thermally pressurized as a consequence of the frictional heat produced during sliding we explore in a single framework the role of the time variations of the porosity permeability or both ultimately leading to changes in hydraulic diffusivity which has been recognized as one of the key parameters in thermally pressurized faults our synthetic ruptures show that the changes in the hydraulic diffusivity only due to porosity variations do not markedly affect the earthquake recurrence cycle time the traction evolution and the thermal history of the fault on the contrary when the evolutions of both the porosity and the permeability are accounted for the cycle time is significantly reduced this result has a clear implication in the context of the hazard assessment

combined observations of arctic sea ice with near coincident colocated x band c band and l band sar satellite remote sensing and helicopter borne measurements in this study we compare colocated near coincident x c and l band fully polarimetry sar satellite images with helicopter borne ice thickness measurements acquired during the norwegian young sea ice 2015 n ice2015 expedition in the region of the arctic ocean north of svalbard in april 2015 the air borne surveys provide near coincident snow plus ice thickness surface roughness data and photographs this unique data set allows us to investigate how the different frequencies can complement one another for sea ice studies but also to raise awareness of limitations x band and l band satellite scenes were shown to be a useful complement to the standard sar frequency for sea ice monitoring c band for lead ice and newly formed sea ice identification this may be in part be due to the frequency but also the high spatial resolution of these sensors we found a relatively low correlation between snow plus ice thickness and surface roughness therefore in our dataset ice thickness cannot directly be observed by sar which has important implications for operational ice charting based on automatic segmentation

rupture to the trench frictional properties and fracture energy of incoming sediments at the cascadia subduction zone abstract the mechanical properties of sediment inputs to subduction zones are important for understanding rupture propagation through the accretionary prism during megathrust earthquakes clay minerals strongly influence the frictional behavior of fault gouges and the clay content of subduction input materials varies through a stratigraphic section as well as for subduction margins globally to establish the frictional properties of the shallow cascadia subduction zone and place the results in a global context we conducted high velocity rotary shear experiments on odp core samples retrieved from cascadia input sediments 35 45 clay and a suite of individual clay species we compared our results to a compilation of published high velocity experiments conducted on samples of wet gouge dry gouge and intact rock for each sample type three trends were identified with increasing normal stress 1 the stress drop p s s increases linearly 2 the characteristic thermal weakening distance d t h decreases as a power law function except for wet clay rich gouges and 3 the fracture energy w b shows no dependence however fracture energy does vary with sample type clay rich gouges under wet conditions have the lowest fracture energy and fracture energy for both dry and wet gouges is at least an order of magnitude lower than estimates from intact rocks therefore when clay rich lithologies are present they may minimize spatial variations in frictional behavior allowing earthquakes to propagate to the trench for cascadia input sediments there is little variation in the fracture energy between lithologies but the fracture energy of cascadia sediments is around an order of magnitude higher than input sediments from other subduction margins the high fracture energy of cascadia sediments relative to other subduction margins may inhibit large amounts of shallow earthquake slip and dynamic overshoot

laser altimetry sampling strategies over sea ice with the conclusion of the science phase of the ice cloud and land elevation satellite icesat mission in late 2009 and the planned launch of icesat 2 in late 2015 nasa has recently established the icebridge program to provide continuity between missions a major goal of icebridge is to obtain a sea ice thickness time series via airborne surveys over the arctic and southern oceans typically two laser altimeters the airborne topographic mapper atm and the land vegetation and ice sensor lvis are utilized during icebridge flights using laser altimetry simulations of conventional analogue systems such as icesat lvis and atm with the multi beam system proposed for icesat 2 we investigate differences in measurements gathered at varying spatial resolutions and the impact on sea ice freeboard we assess the ability of each system to reproduce the elevation distributions of two sea ice models and discuss potential biases in lead detection and sea surface elevation arising from variable footprint size and spacing the conventio nal systems accurately reproduce mean freeboard over 25 km length scales while icesat 2 offers considerable improvements over its predecessor icesat in particular its dense along track sampling of the surface will allow flexibility in the algorithmic approaches taken to optimize the signal to noise ratio for accurate and precise freeboard retrieval

sensing the ups and downs of las vegas insar reveals structural control of land subsidence and aquifer system deformation land subsidence in las vegas nevada united states between april 1992 and december 1997 was measured using spaceborne interferometric synthetic aperture radar the detailed deformation maps clearly show that the spatial extent of subsidence is controlled by geologic structures faults and sediment composition clay thickness the maximum detected subsidence during the 5 75 yr period is 19 cm comparison with leveling data indicates that the subsidence rates declined during the past decade as a result of rising ground water levels brought about by a net reduction in ground water extraction temporal analysis also detects seasonal subsidence and uplift patterns which provide information about the elastic and inelastic properties of the aquifer system and their spatial variability

geophysical evidence for gas geohazards off iskenderun bay se turkey abstract high resolution seafloor and sub surface data were acquired as part of a site survey in iskenderun bay se turkey to characterize the geohazards at the location of the proposed drilling site a 3 km 3 km geophysical study reveals a pockmark field which trends ne and nne similar to the trend of major fault systems in the area the pockmarks with an average diameter of 35 m reach their highest density in the northern part of the detailed survey area with 13 features km2 acoustic anomalies in the seismic records acoustic turbidity blanking enhanced reflectors below the proposed drilling site indicated potential shallow gas beneath it the local seismic anomalies amplitude and frequency parallel to stratigraphy were assigned a low gas risk as a result of the active neotectonics in the area the pockmark field presented a potential hazard for drilling at the original location the geohazard study resulted in moving the proposed drilling site eastward to an area of fewer pockmarks less sub surface seismic anomalies and thus a location interpreted as a lower geohazard environment

formation of carbonate chimneys in the mediterranean sea linked to deep water oxygen depletion marine sediments at ocean margins vent substantial amounts of methane1 2 microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3 4 the factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained here we characterize the petrology and geochemistry of rocks sampled from metre size build ups of methane derived carbonate chimneys located at the amon mud volcano on the nile deep sea fan we find that these carbonates comprise porous structures composed of aggregated spherules of aragonite and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the black sea5 using u series dating we show that the amon carbonate build ups formed between 12 and 7 thousand years ago contemporaneous with the deposition of organic rich sediments in the eastern mediterranean the so called sapropel layer s1 we propose that the onset of deep water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor and thus the formation of carbonate chimneys

seabed features caused by shallow gas in north sea abstract seepage of shallow gas mainly methane recently has been documented visually in the central section of the north sea at water depths of about 70 80 m the surface expressions caused by these seeps are not dramatic they mainly consist of small funnel shaped craters 20 cm across in the cover sand however in deeper parts of the north sea where the cover sediments consist of silt and clay the surface features are more dramatic here the craters pockmarks caused by shallow gas eruptions or seepages are up to 20 m deep and 100 m wide in some areas the pockmarks are paved with a crust of calcium carbonate cement which is believed to have formed as a result of gas efflux through the seabed furthermore the presence of these calcium carbonate reefs seems to have attr cted a wide variety of marine life the presence of shallow gas has caused an enrichment of marine life that seems to be significant beside the semicircular and composite pockmarks gas induced erosion also has caused elongate depressions these occur where the top sediment bedding has caused the gas to migrate along certain axes the mode and speed of formation of the gas induced erosion features are of major concern to oil exploration and development in the northern north sea research into these aspects recently has been stimulated by the discovery of one of the world s largest offshore gas reservoirs which coincidentally lies within a pockmarked area end of article last page 267

the contribution of structural geology experimental rock deformation and numerical modelling to an improved understanding of the seismic cycle preface to the special volume physico chemical processes in seismic faults

dynamic weakening of smectite bearing faults at intermediate velocities implications for subduction zone earthquakes the hydrous clay mineral smectite which is pervasive in sediments on subducting oceanic plates is thought to weaken and stabilize subduction thrust faults however these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 m s to 1 3 m s for water saturated smectite quartz mixtures with various smectite contents so as to shed light on the frictional response of smectite bearing faults at intermediate to high slip rates at slip rates of 30 to 150 m s the friction coefficients decreased gradually from 0 5 0 6 to 0 1 with an increase in smectite content from 20 to 50 wt in contrast at slip rates higher than 1 3 mm s friction exhibited marked slip weakening resulting in low friction coefficients of 0 1 0 05 even for low smectite contents roughly 30 wt drastic slip weakening occurred at smectite contents of 10 30 wt at slip rates of 10 mm s which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types the intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear enhanced compaction and microscopic thermal pressurization of pore fluids this process could weaken the fault even below seismic slip rates leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench

positive trend in the mean speed and deformation rate of arctic sea ice 1979 2007 using buoy data from the international arctic buoy program we found that the sea ice mean speed has substantially increased over the last 29 years 17 per decade for winter and 8 5 for summer a strong seasonal dependence of the mean speed is also revealed with a maximum in october and a minimum in april the sea ice mean strain rate also increased significantly over the period 51 per decade for winter and 52 for summer we check that these increases in both sea ice mean speed and deformation rate are unlikely to be consequences of a stronger atmospheric forcing instead they suggest that sea ice kinematics play a fundamental role in the albedo feedback loop and sea ice decline increasing deformation means stronger fracturing hence more lead opening and therefore a decreasing albedo this accelerates sea ice thinning in summer and delays refreezing in early winter therefore decreasing the mechanical strength of the cover and allowing even more fracturing larger drifting speed and deformation and possibly a faster export of sea ice through the fram strait the september minimum sea ice extent of 2007 might be a good illustration of this interplay between sea ice deformation and sea ice shrinking as we found that for both winter 2007 and summer 2007 exceptionally large deformation rates affected the arctic sea ice cover

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

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

studies of multibaseline spaceborne interferometric synthetic aperture radars the authors have utilized a set of seasat synthetic aperture radar sar data that were obtained in nearly repeat ground track orbits to demonstrate the performance of spaceborne interferometric sar insar systems an assessment of the topography measurement capability is presented a phase measurement error model is described and compared with the data obtained at various baseline separations and signal to noise ratios finally the implications of these results on future spaceborne insar design are discussed

experimental investigation of the influence of slip velocity and temperature on permeability during and after high velocity fault slip abstract we used a rotary shear apparatus to investigate changes of fluid transport properties in a fault zone by real time measurement of gas flow rates during and after shearing of hollow sandstone cylinders at various slip rates our apparatus measures permeability parallel to the slip plane in both the slip zone and wall rocks in all cases permeability decreased rapidly with increasing friction but recovered soon after slip reaching a steady state within several tens of minutes the rate of reduction of permeability increased with increasing slip velocity permeability did not recover to pre slip levels after low velocity ca 0 0019 m s tests but recovered to exceed them after high velocity ca 0 29 m s tests frictional heating of gases at the slip surface increased gas viscosity which increased gas flow rate to produce an apparent permeability increase the irreversible permeability changes of the low velocity tests were caused by gouge formation due to wearing and smoothing of the slip surface the increase of permeability after high velocity tests was caused by mesoscale fracturing in response to rapid temperature rise changes of pore fluid viscosity contributed more to changes of flow rate than did permeability changes caused by shear deformation although test results from different rocks and pore fluids might be different

anomalous sea floor backscatter patterns in methane venting areas dnepr paleo delta nw black sea the relation between acoustic sea floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single beam echosounder this study is further supported by side scan sonar recordings high resolution 5 khz seismic data pore water analysis grain size analysis and visual sea floor observations the datasets were acquired during the 2003 and 2004 expeditions of the ec funded crimea project in the dnepr paleo delta area northwestern black sea more than 600 active methane seeps were hydroacoustically detected within a small 3 96 km by 3 72 km area on the continental shelf of the dnepr paleo delta in water depths ranging from 72 m to 156 m multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area seeps generally occur within medium to high backscatter areas which often coincide with pockmarks high resolution seismic data reveals the presence of an undulating gas front i e the top of the free gas in the subsurface which domes up towards and intersects the sea floor at locations where gas seeps and medium to high backscatter values are detected pore water analysis of 4 multi cores taken at different backscatter intensity sites shows a clear correlation between backscatter intensity and dissolved methane fluxes all analyzed chemical species indicate increasing anaerobic oxidation of methane aom from medium to high backscatter locations this is confirmed by visual sea floor observations showing bacterial mats and authigenic carbonates formed by aom grain size analysis of the 4 multi cores only reveals negligible variations between the different backscatter sites integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane derived authigenic carbonates mdacs caused by aom the carbonate formation also appears to lead to a gradual self sealing of the seeps by cementing fluid pathways horizons followed by a relocation of the bubble releasing locations

monitoring temporal variability of bubble release at seeps the hydroacoustic swath system gasquant 1 a lander based hydroacoustic swath system gasquant was deployed in an intensely bubbling seep area at the shelf west of the crimea peninsula black sea with its horizontally oriented swath 21 beams 63 swath angle 180 khz gasquant operates in a sonar like mode and monitors bubbles remotely exploiting their strong backscattering when crossing the swath all active seep spots were monitored simultaneously within the covered area 2075 m2 even applying simple processing and visualization techniques moving average for filtering fft for spectrum analyses swath and trace plots identified 17 seeps of different activity patterns that have been grouped as follows 1 sporadically active with one to a few long bursts up to 18 min or randomly occurring short bursts 350 bursts or 20 active studying the bubble release variability of single seeps and of the entire area allows speculation about the external and internal processes that modulate the bubble release in the study area none of the 17 seeps was found to be permanently active only one was active for 75 and another one for 45 of the time monitored the rest only released bubbles during less than 20 of the time with an overall average of only 12 this would have strong implications for flux extrapolations if these were based on very accurate but few short term measurements both strong overestimates and underestimates are possible high resolution monitoring over at least one tidal cycle as with the gasquant system might help to get an idea of the temporal variability thus flux extrapolations can be corrected to better reflect the real seep activity

estimation of forest structure ground and canopy layer characteristics from multibaseline polarimetric interferometric sar data this paper concerns forest parameter retrieval from polarimetric interferometric synthetic aperture radar polinsar data considering two layers one for the ground under the vegetation and one for the volumetric canopy a model is designed to combine a physical model based polarimetric decomposition with the random volume over ground rvog polinsar parameter inversion approach the combination of a polarimetric scattering media model with a polinsar rvog vertical structure model provides the possibility to separate the ground and the volume coherency matrices based on polarimetric signatures and interferometric coherence diversity the proposed polarimetric decomposition characterizes volumetric media by the degree of polarization orientation randomness and by the particle scattering anisotropy using the full model enhances the estimation of the vertical forest structure parameters by enabling us to estimate the ground to volume ratio the temporal decorrelation and the differential extinction for forest vegetation observed at l band this model accounts for the ground topography forest and canopy layer heights wave attenuation in the canopy tree morphology in the form of the angular distribution and the effective shapes of the branches and the contributions from the ground level consisting of surface scattering and double bounce ground trunk interactions as well as volumetric understory scattering the parameter estimation performance is evaluated on real airborne l band sar data of the traunstein test site acquired by the german aerospace center dlr s e sar sensor in 2003 in both single and multibaseline configurations the retrieved forest height is compared with the ground truth measurements revealing for the given test site an average root mean square error rmse of about 5 m in the repeat pass configuration this implies an improvement in rmse by over 2 m in comparison to the pure coherence based rvog polinsar parameter inversion

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

thermogenic methane injection via bubble transport into the upper arctic ocean from the hydrate charged vestnesa ridge svalbard we use new gas hydrate geochemistry analyses echosounder data and three dimensional p cable seismic data to study a gas hydrate and free gas system in 1200 m water depth at the vestnesa ridge offshore nw svalbard geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source the presence of thermogenic gas and temperatures of similar to 3 3 degrees c result in a shallow top of the hydrate stability zone thsz at similar to 340 m below sea level mbsl therefore hydrate skinned gas bubbles which inhibit gas dissolution processes are thermodynamically stable to this shallow water depth this was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl at the seafloor bubbles are released from acoustically transparent zones in the seismic data which we interpret as regions where free gas is migrating through the hydrate stability zone hsz these intrusions result in vertical variations in the base of the hsz bhsz of up to similar to 150 m possibly making the shallow hydrate reservoir more susceptible to warming such arctic gas hydrate and free gas systems are important because of their potential role in climate change and in fueling marine life but remain largely understudied due to limited data coverage in seasonally ice covered arctic environments

radar interferogram filtering for geophysical applications the use of sar interferometry is often impeded by decorrelation from thermal noise temporal change and baseline geometry power spectra of interferograms are typically the sum of a narrow band component combined with broad band noise we describe a new adaptive filtering algorithm that dramatically lowers phase noise improving both measurement accuracy and phase unwrapping while demonstrating graceful degradation in regions of pure noise the performance of the filter is demonstrated with sar data from the ers satellites over the jakobshavns glacier of greenland

active gas venting through hydrate bearing sediments on the vestnesa ridge offshore w svalbard abstract gas hydrate systems offshore western svalbard are extensive and include the whole vestnesa ridge an elongated sediment drift north of the molloy transform and just east of the molloy ridge one of the shortest segments of the slow spreading north atlantic ridge system the crest of the vestnesa ridge at water depth between 1200 1300 m is pierced with fluid flow features seafloor pockmarks vary in size up to 700 m in diameter pockmarks are generally larger at the eastern start of the vestnesa ridge than at its western termination a recent cruise with r v jan mayen discovered methane flares in the water column above the pockmark field at the onset of the vestnesa ridge over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m it is still unclear what has triggered the increase in gas expulsion from seafloor sediments high resolution 3d seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems at the western end of the vestnesa ridge sub seafloor fluid flow features resemble well described chimney structures however the seismic expression of the active fluid flow features is much broader much more chaotic dome shaped and is not very similar to a typical chimney structure the vestnesa ridge gas hydrate and free gas system occurs within few kilometers of a mid oceanic ridge and transform fault which makes this gas hydrate system unique on earth the close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas hydrate and free gas system the high heat flow together with the high tectonic activity of this region a thick sedimentary cover a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free gas system leading to high variability in gas supply gas migration and gas hydrate build up and dissociation

influence of shear and deviatoric stress on the evolution of permeability in fractured rock 1 the evolution of permeability in fractured rock as a function of effective normal stress shear displacement and damage remains a complex issue in this contribution we report on experiments in which rock surfaces were subject to direct shear under controlled pore pressure and true triaxial stress conditions while permeability was monitored continuously via flow parallel to the shear direction shear tests were performed in a pressure vessel under drained conditions on samples of novaculite arkansas and diorite coso geothermal field california the sample pairs were sheared to 18 mm of total displacement at 5 mm s at room temperature and at effective normal stresses on the shear plane ranging from 5 to 20 mpa permeability evolution was measured throughout shearing via flow of distilled water from an upstream reservoir discharging downstream of the sample at atmospheric pressure for diorite and novaculite initial preshear fracture permeability is 0 5 1 10 14 m 2 and largely independent of the applied effective normal stresses these permeabilities correspond to equivalent hydraulic apertures of 15 20 mm because of the progressive formation of gouge during shear the postshear permeability of the diorite fracture drops to a final steady value of 0 5 10 17 m 2 the behavior is similar in novaculite but the final permeability of 0 5 10 16 m 2 is obtained only at an effective normal stress of 20 mpa

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

numerical modeling of carbonate crust formation at cold vent sites significance for fluid and methane budgets and chemosynthetic biological communities at many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments the range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear here we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate starting with the conditions prevailing at a previously investigated reference site located on hydrate ridge off oregon several parameters are systematically varied in a number of numerical experiments these parameters include coefficients of bioturbation and bioirrigation sedimentation rate fluid flow velocity methane concentration in the ascending vent fluids and ph and saturation state at the sediment water interface the simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane 50 mm and if bioturbation coefficients are low 50 cm ka 1 inhibit crust formation bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust crusts only form over a rather narrow range of upward fluid flow velocities 20 60 cm a 1 which is somewhat enlarged up to 90 cm a 1 if the overlying bottom waters are supersaturated with respect to calcite at higher flow rates methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment the formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years 100 500 a crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport therefore rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation due to the moderate flow rates and the slow diffusive transport only very little methane escapes into the bottom water overlying carbonate encrusted vent areas

laser profiling of the ice surface topography during the winter weddell gyre study 1992 a laser profiler mounted on a helicopter was used to measure the surface topography of sea ice in the weddell sea the measurements were carried out during the cruise antarktis x 4 of r v polarstern as part of the winter weddell gyre study 1992 wwgs 92 the total length of the measured surface profiles amounts to 793 km the observed mean ridge heights and mean ridge spacings are fairly well correlated therefore the ridging intensity r1 which is the ratio of ridge height and spacing was used as a quantitative classifier in order to arrange the laser data into different groups at a cutoff height of 0 8 m the average ridge height and spacing are 1 16 m and 160 m at the lowermost range of ridging intensities r1 0 04 respectively the best fits to the observed ridge height distributions are achieved by negative exponentials at lower ridging intensities as a function of ridge height at the regime of highest ridging intensity as a function of the ridge height squared the distributions of ridge spacing are well described by a lognormal distribution function a simple model is used to estimate the average thickness hd and the areal fraction of ridged ice from the laser data the values of hd range from 3 1m with an areal coverage of 5 at the lowermost range of r1 to 3 7 m with an areal coverage of 31 at the uppermost range if the ridged ice is distributed as a uniform layer over a unit area the corresponding effective thickness values are 0 14 m and 1 16 m respectively mean ridge heights and spacings are determined for different cutoff heights in order to enable comparisons to other measurements the effect of ridges on the surface stress over the ice is estimated using the drag partition theory which is applicable to the regime of lowest ridging intensity the contribution of ice ridge form drag to the total wind drag for an average ridging intensity of 0 007 and a cutoff height of 0 8 m is about 35 to 40 at neutral stability if it is assumed that the aerodynamic roughness length z0 of the intervening ice surface varies between 3 3 10 4 m and 5 5 10 4 m the drag coefficient cdn ranges from 2 08 10 3 to 2 24 10 3 at a reference height of 10 m

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

icesat measurements of sea ice freeboard and estimates of sea ice thickness in the weddell sea 1 sea ice freeboard heights in the weddell sea of antarctica are derived from the ice cloud and land elevation satellite icesat laser altimeter measurements which have a unique range precision to flat surfaces of 2 cm within 70 m footprints spaced at 172 m along track although elevations of flat surfaces can be obtained to an accuracy of 10 cm 1s per footprint direct determination of freeboard heights is precluded by errors in knowledge of the geoid and temporal variability of the ocean surface therefore freeboards are determined relative to an ocean reference level detected over areas of open water and very thin ice within the sea ice pack using an along track filtering method the open water thin ice selections show good agreement in the combined analysis of icesat segments and envisat synthetic aperture radar sar imagery the average residual between the icesat measured ocean level and the egm96 geoid is 1 4 m estimates of snow depth on the sea ice from amsr e passive microwave data along with nominal densities of snow water and sea ice are used to estimate sea ice thickness four periods of icesat data in may june fall and october november late winter of 2004 and 2005 between longitudes 298e and 360e are analyzed in the fall the mean freeboards are 0 28 m in 2004 and 0 29 m in 2005 and the mean thicknesses are 1 33 m in 2004 and 1 52 m in 2005 in late winter the freeboards grew to 0 37 m in 2004 and 0 35 in 2005 and the thicknesses grew to 2 23 m in 2004 and 2 31 m in 2005 the interannual differences in freeboard are small and the larger interannual change in estimated thickness mainly represents differences in the snow depth estimates seasonal changes in the spatial patterns of freeboard and thickness over the 4 months correlate with the expected circulation of sea ice in the weddell sea as indicated by sea ice velocity fields

sea ice surface roughness estimates from airborne laser scanner and laser altimeter observations in fram strait and north of svalbard we present sea ice surface roughness estimates i e the standard deviation of relative surface elevation in the arctic regions of fram strait and the nansen basin north of svalbard acquired by an airborne laser scanner and a single beam laser altimeter in 2010 we compare the scanner to the altimeter and compare the differences between the two survey regions we estimate and correct sensor roll from the scanner data using the hyperbolic response of the scanner over a flat surface measurement surveys had to be longer than 5 km north of svalbard and longer than 15 km in fram strait before the statistical distribution in surface roughness from the scanner and altimeter became similar the shape of the surface roughness probability distributions agrees with those of airborne electromagnetic induction measurements of ice thickness the ice in fram strait had a greater mean surface roughness 0 16m vs 0 09m and a wider distribution in roughness values than the ice in the nansen basin an increase in surface roughness with increasing ice thickness was observed over fast ice found in fram strait near the coast of greenland but not for the drift ice

experimental investigations on dating the last earthquake event using osl signals of quartz from fault gouges abstract obtaining a reliable age of the latest seismic slip event along an active fault is important for seismic hazard assessment here we observe changes in the optically stimulated luminescence osl signal of quartz crystals due to frictional heating in artificial fault gouges comprising a mixture of quartz grains and ca bentonite powder the fault gouge was deformed using a high velocity rotary shear apparatus at room temperature and room humidity at a seismic slip rate of 1 31 m s 1 intense slip localization occurred along a very thin layer 300 m thick within the simulated fault zones 1 mm thick the estimated temperature of the slip localized layer sll increased by 475 c from frictional heating the quartz osl signals of the gouges were fully reset most noticeably for the sll in contrast there was rare slip localization at subseismic slip rates 0 06 0 001 m s 1 for which the estimated temperature rise in the sll was 120 c hence the quartz osl signal was not reset under this condition the results suggest that quartz osl dating can be used to constrain the age of the latest seismic event in natural quartz bearing fault zones where a sll occurs

a new 6 myr stratigraphic framework for the atlantic arctic gateway abstract a comprehensive stratigraphic framework for the atlantic arctic gateway has been established for the last 6 myr on the yermak plateau nw svalbard based on new paleomagnetic and biostratigraphic sampling and correlation of high resolution seismic data between ocean drilling program odp holes 911a 910c and 912a the new results indicate a late miocene age for the base of both holes 911a and 910c providing the first complete late neogene record for the marginal arctic ocean and extending the previously published age model by several million years during the late miocene pleistocene the southern yermak plateau was dominated by contourite deposition from the intensification of the northern hemisphere glaciation at 2 7 ma glacial fans from nw svalbard prograded onto the southernmost yermak plateau this indicates that the ice sheet on nw svalbard reached the shelf break at 2 7 ma much earlier than on the western barents sea svalbard margin 1 5 ma simultaneous with the first shelf break glaciation on the western barents sea svalbard margin at 1 5 ma we observe the first signs of extensive glacial erosion on the yermak plateau indicating a regional glacial intensification at this time for the whole barents sea svalbard region our results agree well with the widening deepening of the fram strait and the opening of the atlantic arctic gateway during the middle miocene