chirp 2 7 khz echo characters of the south korea plateau east sea styles of mass movement and sediment gravity flow abstract the south korea plateau is a complex of ridges seamount chains troughs and a basin detailed analysis of 12 019 line km chirp 2 7 khz subbottom profiles from the south korea plateau and the eastern continental slope of korea reveals a distinctive zonal distribution of echo types depending systematically on seafloor morphology pelagites hemipelagites type i 2 and basement highs type iii 1 prevail on the ridge summits seamount chains and the upper to middle part of the eastern continental slope of korea and are commonly bounded downslope by creeps type iii 3 and slides slumps type iv 1 that occur extensively over the entire slope areas of the ridges seamount chains and eastern continental slope the mass movement deposits change downslope to debrites and turbidites types ii iii 2 iv 2 and iv 3 in the troughs and onnuri basin suggesting successive downslope evolution from slide and slump to debris flow and turbidity current the voluminous creeps slides and slumps over the entire slope areas of the plateau and eastern continental slope deeper than 300 m in water depth were most likely generated by frequent seismic shakings

southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

three dimensional seismic imaging of paleogene dike fed submarine volcanoes from the northeast atlantic margin we use three dimensional seismic data to describe the architecture of shallow intrusive igneous sill and dike complexes intruded into the upper crust as well as the internal and external geometry of extrusive conical igneous mounds that were fed by the magma conduits the conical mounds accreted on the seabed directly above the tips of the basaltic dikes between 54 9 and 54 6 ma in plan view the mounds are broadly circular 1 1 7 km in diameter and have a relief of 50 300 m they occur above a complex network of linked cuspate shaped sills and dikes intruded into paleocene and cretaceous sediments of the faeroe shetland basin northeast atlantic margin the highly organized internal structure consisting of bulbous layers arranged concentrically around a central axis along with the clear vertical link to the underlying sills and dikes indicates that they most likely represent pillowed basaltic lava and hyaloclastite mounds that were fed by the dikes and extruded onto the seafloor mounds of similar external geometry have been described from sidescan sonar images above active spreading centers in oceanic settings however such structures are rarely recognized in passive margin settings and prior to this investigation our knowledge of their three dimensional internal geometry has been largely speculative

carbonate mounds from paradox to world heritage the recent marine carbonate world comprises two major compartments 1 the surface domain of the photozoan carbonates confined in space by water depth and by the penetration of light and 2 a deep domain where heterozoan mound builder guilds directly forage on fluxes of nutrients which primarily percolate from the photic zone and or are generated by in situ benthic processes locally giant cold water coral mounds tower up to heights of 150 to 250 m above the sea floor in general between 500 and 1300 m water depth and within sharply delineated provinces some 15 years of research on these giant mound provinces conveys a picture of their distribution in space and possibly sheds light on controls acting in concert globally there is no counterpart for the prolific north atlantic mound basin namb a chemical control is seen by an overlay of the mound provinces on a map of the aragonite saturation horizon ash an external physical control is inferred fromthe position of themound provinces girdling a vigorous north atlantic subtropical gyre systemand clustering close to the roof of the intermediate to deep water masses of a dynamically stratified ocean on the eastern boundary of the namb nutrient fluxes are enhanced by mixing processes driven either by internal waves between galicia and the shetlands or by the vast and heterogeneous eastern boundary upwelling system along the iberian african margins down to 10 n early diagenesis by carbonate dissolution and re precipitation driven by convecting or advecting internal fluids can contribute to stabilize such constructions facilitating an exuberant vertical accretion it is speculated that in thenorth atlantic ocean the deep water carbonate factory outclasses in size the shallow water coral reefs giant mound formation is a recurrent play of life since the dawn of the metazoans nama group upper neoproterozoic however with actors and plots varying from act to act remarkably literature reports only three occurrences of deep water mounds in the phanerozoic the modern ocean possibly the danian and the carboniferous some striking parallelisms in the development of the atlantic and the paleo tethys oceans combined with the developing insights in the controls on deep watermounds in the present ocean invite for a comparative study this has the potential to eventually shed light on the full circulation pattern of the paleo tethys ocean surface and deep comparative studies will build upon 1 modeling of ocean circulation constrained by the record of deep water carbonate systems and supported by advances in tracer and proxy tools and 2 field studies on representative and accessible continental locations the mound route that develops in morocco under the auspices of ioc unesco will provide tomulti disciplinary teams with marine and continental experience opportunities for confronting observations from the modern ocean and on key records of past oceanic basins it has the potential to eventually qualify for a unesco recognition as world heritage

anomalous seafloor mounds in the northern natal valley southwest indian ocean implications for the east african rift system abstract the natal valley southwest indian ocean has a complicated and protracted opening history as has the surrounding southwest indian ocean recently collected multibeam swath bathymetry and 3 5 khz seismic data from the natal valley reveal anomalous seafloor mounds in the northern natal valley the significance of these domes as recorders of the geological history of the natal valley and se african margin has been overlooked with little attempt made to identify their origin evolution or tectonic significance this paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin the seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the natal valley between 25 km and 31 km long and 16 km and 18 km wide these features rise some 400 m above the sedimentary deposits that have filled in the natal valley such macro scale features have not previously been described from the natal valley or from other passive margins globally they are not the result of bottom water circulation salt tectonics rather igneous activity is favoured as the origin for these anomalous seafloor features we propose a hypothesis that the anomalous seafloor mounds observed in the natal valley are related to igneous activity associated with the ears the complicated opening history and antecedent geology coupled with the southward propagation of the east african rift system creates a unique setting where continental rift associated features have been developed in a marine setting

growth and erosion of a cold water coral covered carbonate mound in the northeast atlantic during the late pleistocene and holocene the first detailed stratigraphic record from a deep water carbonate mound in the northeast atlantic based on absolute datings u th and ams 14c and stable oxygen isotope records reveals that its top sediment sequences are condensed by numerous hiatuses according to stable isotope data mainly sediments with an intermediate signal are preserved on the mound while almost all fully glacial and interglacial sediments have either not been deposited or have been eroded later the resulting hiatuses reduce the late pleistocene sediment accumulation at propeller mound to amounts smaller than the background sedimentation the hiatuses most likely result due to the sweeping of the mound in turn with the re establishment of vigour interglacial circulation patterns after sluggish current regimes during glacials thus within the discussion if internal fluid driven or external environmentally driven processes control the evolution of such carbonate mounds our findings for propeller mound clearly point to environmental forcing as the dominant mechanism shaping deep water carbonate mounds in the ne atlantic during the late pleistocene and holocene

structure of mud volcano systems and pockmarks in the region of the ceuta contourite depositional system western alboran sea abstract new high resolution seismic and swath bathymetry data reveal the detailed structure of nine mud volcanoes and two large fields of pockmarks at water depths of 400 1100 m in the western alboran sea these features are related to episodes of focused fluid flow through pliocene quaternary contourite deposits of the ceuta drift we describe nine mud volcano systems composed of two key structural elements i extrusive edifices comprising cones bicones and low dip mudflows interfingering with adjacent drift sediments and ii intrusive feeder complexes comprising pipes and unstratified downward tapering cones underlying the extrusive edifices the mud volcano systems are grouped into four types i complex edifices ii stacked single edifices iii single edifices fed by buried bicones and iv single edifices all except the last record episodic extrusive activity since the pliocene the timing of which is constrained by correlation between the main mud extrusion episodes and dated unconformities within the ceuta drift complex mud volcanoes composed of twin to multiple seabed cones are the longest lived systems with at least six extrusive episodes since the mid pliocene most of the mud volcanoes appear to have been reactivated in the mid pleistocene 0 9 ma mpr discontinuity and all at mis12 0 4 ma while the onset of focused fluid flow was triggered by mid pliocene tectonically driven diapirism of lower miocene shales that form the rooting zone of the mud volcanoes we suggest that some of the pleistocene phases of mud extrusion might be related to erosion of the contourite drift during major sea level falls in addition to vertical focused fluid flow we propose that leakages from the mud volcano feeder complexes into high permeability contourite sediments may cause lateral fluid migration and form gas accumulations lateral fluid flow in the near surface sediments could be responsible of forming subsurface reservoirs and consequently random pockmarks by fluidification and or liquefaction of the non consolidated shallow sediments stacked buried paleo pockmarks indicate that this mechanism has been persistent along recent times at least after the mid pleistocene

evidence of episodic fluid gas and sediment venting on the northern gulf of mexico continental slope surficial sediments and sea floor features of the northern gulf of mexico continental slope record dramatic episodic venting and slower seepage of formation fluids brines crude oil gases and fine grained sediments faults activated by massive sediment input during periods of plio pleistocene falling to low sea levels accompanied by compensating salt movement provide avenues of vertical transport to the continental slope surface many of these faults cut thick sedimentary sequences that frequently contain geopressured zones which provide the driving force for fluid and gas expulsion flux rate and fluid characteristics are interpreted as important determinants of modern sea floor geology and biology qualitatively under conditions of rapid flux of sediment rich fluids mud volcanoes up to 1 km wide and 50 m high and extensive sheets of extruded mud result these sediments contain old and displaced microfaunas muds are frequently extruded with large volumes of crude oil and gas both biogenic and thermogenic in water depths greater than approximately 500 m rapid to moderate vertical flux of hydrocarbon gases and fluids results in the construction of relief features composed of gas hydrates ejected mud and authigenic carbonates areas with near surface hydrates frequently support complex chemosynthetic communities with associated hardgrounds containing calcareous remains of mussels clams and associated gastropods slow flux seeps promotes the formation of 13 c depleted carbonate hardgrounds stacked carbonate slabs and moundlike carbonate buildups frequently 20 m relief barite along with carbonate has been found in areas where rapid flux of sediment carrying fluids has waned in the recent past and mineral rich brines are now being ejected crests chimneys and cones of these minerals are found in such settings both recent numerical simulations of fluid release from geopressured zones and direct observations at the sea floor support a pattern of episodic venting short term episodes of venting are probably regulated by fault movement perhaps controlled by local salt adjustment destabilization of gas hydrates by oceanographic processes also causes short term episodic gas expulsion these events occur with interannual to intra annual frequencies longer gas hydrate stabilization destabilization cycles in continental margin settings are forced by hydrostatic loading and unloading at the frequency of sea level change radiometric dating suggests that other long term episodes of venting and major venting expulsion events are also probably modulated by sea level change time scales of thousands of years in response to low stand sedimentary loading results of this summary of vent seep related phenomena suggest that sediment input from the shelf margin associated with cyclic plio pleistocene falling to low sea level periods followed by major listic fault and salt adjustments to a new sediment load have activated venting seepage throughout this period and perhaps longer

seismic interpretation of pelagic sedimentation regimes in the 18 53 ma eastern equatorial pacific basin scale sedimentation and infilling of abyssal valleys understanding how pelagic sediment has been eroded transported and deposited is critical to evaluating pelagic sediment records for paleoceanography we use digital seismic reflection data from an integrated ocean drilling program site survey amat03 to investigate pelagic sedimentation across the eastern central equatorial pacific which represents the first comprehensive record published covering the 18 53 ma eastern equatorial pacific our goals are to quantify 1 basin hill scale primary deposition regimes and 2 the extent to which seafloor topography has been subdued by abyssal valley filling sediments the eastern pacific seafloor consists of a series of abyssal hills and basins with minor late stage faulting in the basement ocean crust rarely outcrops at the seafloor away from the rise crest both hills and basins are sediment covered the carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data overall we recognized three different sedimentation regimes depositional high sedimentation rate transitional and minimal sedimentation low sedimentation rate regimes in all areas the sedimented seafloor mimics the underlying basement topography although the degree to which topography becomes subdued varies depositional regimes result in symmetric sedimentation within basins and subdued topography whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief regardless of sedimentation regime enhanced sediment deposition occurs within basins however we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills if this variation is due to sediment focusing the focusing factor in the basins as measured by 230th is no more than a factor of 1 3 of the total vertical particulate rain

multistage collapse of eight western canary island landslides in the last 1 5 ma sedimentological and geochemical evidence from subunits in submarine flow deposits volcaniclastic turbidites in the madeira abyssal plain provide a record of major landslides from the western canary islands in the last 1 5 ma these volcaniclastic turbidites are composed of multiple fining upward turbidite sands known as subunits the subunits indicate that the landslides responsible for the sediment gravity flows occurred in multiple stages the subunits cannot result from flow reflection or splitting because the compositions of volcanic glasses from each individual subunit in an event bed are subtly different this indicates that each subunit represents a discrete failure as part of a multistage landslide this has significant implications for geohazard assessments as multistage failures reduce the magnitude of the associated tsunami the multistage failure mechanism reduces individual landslide volumes from up to 350 km3 to less than 100 km3 thus although multistage failure ultimately reduce the potential landslide and tsunami threat the landslide events may still generate significant tsunamis close to source

geology sulfide geochemistry and supercritical venting at the beebe hydrothermal vent field cayman trough the beebe vent field bvf is the world s deepest known hydrothermal system at 4960m below sea level located on the mid cayman spreading centre caribbean the bvf hosts high temperature 401 c black smoker vents that build cu zn and au rich sulphide mounds and chimneys the bvf is highly gold rich with au values up to 93 ppm and an average au ag ratio of 0 15 gold precipitation is directly associated with diffuse flow through beehive chimneys significant mass wasting of sulphide material at the bvf accompanied by changes in metal content results in metaliferous talus and sediment deposits situated on very thin 2 3km thick oceanic crust at an ultraslow spreading centre the hydrothermal system circulates fluids to a depth of 1 8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies we suggest hydrothermal interaction with chalcophile bearing sulphides in the mantle rocks together with precipitation of au in beehive chimney structures has resulted in the formation of a au rich volcanogenic massive sulphide vms deposit with its spatial distribution of deposit materials and metal contents the bvf represents a modern day analogue for basalt hosted au rich vms systems this article is protected by copyright all rights reserved

slope failures on the flanks of the western canary islands abstract landslides have been a key process in the evolution of the western canary islands the younger and more volcanically active canary islands el hierro la palma and tenerife show the clearest evidence of recent landslide activity the evidence includes landslide scars on the island flanks debris deposits on the lower island slopes and volcaniclastic turbidites on the floor of the adjacent ocean basins at least 14 large landslides have occurred on the flanks of the el hierro la palma and tenerife the majority of these in the last 1 million years with the youngest on the northwest flank of el hierro as recent as 15 thousand years in age older landslides undoubtedly occurred but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments landslides on the canary island flanks can be categorised as debris avalanches slumps or debris flows debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence up to a maximum thickness of 1 to 2 km they are the commonest type of landslide mapped in contrast slumps move short distances and are deep rooted landslides which may affect the entire thickness of the volcanic edifice debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks some landslides are complex events involving more than one of the above end member processes individual debris avalanches have volumes in the range of 50 500 km3 cover several thousand km2 of seafloor and have runout distances of up to 130 km from source overall debris avalanche deposits account for about 10 of the total volcanic edifices of the small relatively young islands of el hierro and la palma some parameters such as deposit volumes and landslide ages are difficult to quantify the key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks broadening into a depositional lobe below 3000 m debris avalanche deposits have a typically blocky morphology with individual blocks up to a kilometre or more in diameter however considerable variation exists between different avalanche deposits at one extreme the el golfo debris avalanche on el hierro has few large blocks scattered randomly across the avalanche surface at the other icod on the north flank of tenerife has much more numerous but smaller blocks over most of its surface with a few very large blocks confined to the margins of the deposit icod also exhibits flow structures longitudinal shears and pressure ridges that are absent in el golfo the primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes observations in experimental debris flows show that the differences between the el golfo and icod landslide deposits are probably controlled by the greater proportion of fine grained material in the icod landslide this in turn relates to the nature of the failed volcanic rocks which are almost entirely basalt on el hierro but include a much greater proportion of pyroclastic deposits on tenerife landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands with landslides propagating perpendicular to the rift orientation however this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a weakness that can be carried forward during island development this may occur because certain island flanks are steeper extend to greater water depths or are less buttressed by the surrounding topography and because volcanic production following a landslide my be concentrated in the landslide scar thus focussing subsequent landslide potential in this area landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight although the actual triggering factors are poorly understood they may include or be influenced by dyke intrusion pore pressure changes related to intrusion seismicity or sealevel climate changes a possible relationship between caldera collapse and landsliding on tenerife is not in our interpretation supported by the available evidence

geomorphology of the oceans abstract we present the first digital seafloor geomorphic features map gsfm of the global ocean the gsfm includes 131 192 separate polygons in 29 geomorphic feature categories used here to assess differences between passive and active continental margins as well as between 8 major ocean regions the arctic indian north atlantic north pacific south atlantic south pacific and the southern oceans and the mediterranean and black seas the gsfm provides quantitative assessments of differences between passive and active margins continental shelf width of passive margins 88 km is nearly three times that of active margins 31 km the average width of active slopes 36 km is less than the average width of passive margin slopes 46 km active margin slopes contain an area of 3 4 million km2 where the gradient exceeds 5 compared with 1 3 million km2 on passive margin slopes the continental rise covers 27 million km2 adjacent to passive margins and less than 2 3 million km2 adjacent to active margins examples of specific applications of the gsfm are presented to show that 1 larger rift valley segments are generally associated with slow spreading rates and smaller rift valley segments are associated with fast spreading 2 polar submarine canyons are twice the average size of non polar canyons and abyssal polar regions exhibit lower seafloor roughness than non polar regions expressed as spatially extensive fan rise and abyssal plain sediment deposits all of which are attributed here to the effects of continental glaciations and 3 recognition of seamounts as a separate category of feature from ridges results in a lower estimate of seamount number compared with estimates of previous workers

condor seamount azores ne atlantic a morpho tectonic interpretation abstract high resolution datasets collected by multibeam and acoustic backscatter surveys were used to produce fine scale seafloor nature and morpho tectonic interpretations of the condor seamount condor constitutes an elongated volcanic ridge that extends for 39 km and rises more than 1800 m from the surrounding seafloor constructive morphologies include i linear eruptive centres ii volcanic cones with or without summit depressions iii lava flows and iv hummocky sectors eruptive type is interpreted to vary with depth on the deeper seamount extremities the predominance of highly acoustically backscattering volcanic cones and hummocky terrain is interpreted to result from effusive eruptions not yet covered by sediment deposits in contrast the smoother relief of the central seamount flanks is interpreted as draping and infilling of the underlying effusive relief by i primary volcaniclastic deposits produced by explosive eruptions on the shallowest parts of the ridge together with ii secondary volcanigenic sediments resulting from truncation of the seamount top by swell erosion and iii sediments resulting from biogenic production a set of wnw ese to nw se trending volcano tectonic structures are shown to control most of the fissural volcanism that formed the ridge a network of nnw sse trending faults is identified on the sea floor around condor but they show little relation with the distribution of volcanic edifices or with post emplacement dismantling of the seamount these fault sets are related to the transtensional regime acting on the azorean segment of the eurasia nubia plate boundary erosional features include i palaeo wave cut platforms on the seamount summit ii landslide scars produced by lateral collapses of the ne and sw facing flanks iii gullies and turbidity current channels and iv mass wasting deposits iceberg drag and bump marks are also identified on the seamount upper flanks representing the first reference to such features in the azores and an additional low latitude record given the lack of major erosional and tectonic dismantling condor is suggested to be a relatively young seamount a revised factoring of eustatic erosional and isostatic processes does not exclude that the summit may have been eroded as late as the last glacial maximum

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

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick and are common where gas is present in near seafloor sediments this paper investigates how gas might lead to pockmark formation the process is envisioned as follows a capillary seal traps gas beneath a fine grained sediment layer or layers perhaps layers whose pores have been reduced in size by hydrate crystallization gas accumulates until its pressure is sufficient for gas to invade the seal the seal then fails completely a unique aspect of capillary seals releasing a large fraction of the accumulated gas into an upward propagating gas chimney which displaces water like a piston as it rises near the seafloor the water flow causes the sediments to become quick i e liquefied in the sense that grain to grain contact is lost and the grains are suspended dynamically by the upward flow the quickened sediment is removed by ocean bottom currents and a pockmark is formed equations that approximately describe this gas piston water drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor for uniform near surface sediment permeability this is a buoyancy control not a permeability control the rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure plausible estimates of these parameters suggest gas chimneys at blake ridge could reach the seafloor in less than a decade or more than a century depending mainly on the permeability of the deforming near surface sediments since these become quick before gas is expelled gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation however detecting gas chimney growth might be a useful risk predictor any area underlain by a gas chimney that extends half way or more to the surface should be avoided c 2009 elsevier ltd all rights reserved

a new type of intra plate volcanism young alkali basalts discovered from the subducting pacific plate northern japan trench alkali pillow basalts were collected from the toe of the oceanward slope of the northern japan trench these alkali basalts formed as a result of a low degree of partial melting of pacific ocean mantle and rapid rise of the magma no fractionation in shallow magma chambers reconstructing pacific plate motion based on 40ar 39ar age dates of 5 95 0 31 ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the japan trench in the nw pacific we suggest that these alkali basalts represent a new form of intra plate volcanism whereby magmatic activity occurs off the forebulge of the downgoing pacific slab perhaps using conduits related to fracturing of the slab during bending prior to subduction

submarine volcanic mounds in the pearl river mouth basin northern south china sea a number of enigmatic km scale mound structures have been discovered in the baiyun sag of the pearl river mouth basin using high resolution 2d and 3d seismic data combined with multi beam bathymetry based on detailed seismic characterization we interpret these as sill fed volcanic mounds associated structures include igneous sills trans tensional faults lava flows and compactional drape folds the igneous intrusions and associated mounds were emplaced in the late early miocene c 18 5 ma or shortly thereafter constrained by the onlap relationship between the mounds and their overburden the volcanic mounds are preferentially developed above or adjacent to basement highs indicating a structural control on the igneous plumbing system we propose that the fractures and tectonic faults above the raised basement follow pre existing zones of weakness and may have acted as feeding conduits for the magmatic material the discovery of the early miocene igneous province has important implications for the understanding of the evolution of the pearl river mouth basin and for regional hydrocarbon prospectivity c 2014 elsevier b v all rights reserved

earlier history of the 70 ma old canary hotspot based on the temporal and geochemical evolution of the selvagen archipelago and neighboring seamounts in the eastern north atlantic major element trace element and sr nd pb isotope data combined with 40ar 39ar age determinations of volcanic rocks from the selvagen islands and neighboring seamounts in the eastern north atlantic reveal the earlier history of the 70 ma old canary hotspot a basanitic to phonolitic late shield stage intrusive complex 29 ma is exposed on selvagem pequena the evolution of selvagem grande can be divided into three magmatic phases a tephritic to phonolitic late shield stage intrusive complex 24 26 ma and two rejuvenated or post erosional stages 8 12 and 3 4 ma consisting of alkali basalt basanite and rare phonolite during the early to mid miocene volcanic hiatus 12 24 ma the top of the volcano was beneath sea level as evidenced by marine carbonate sediments 13 24 ma dated through correlation of 87sr 86sr with the seawater sr isotope curve the geochemistry of the shield stage lavas indicates that they derive from plume sources whereas the post erosional lavas are derived from metasomatized lithospheric sources five sampled seamounts to the east and northeast of the islands range in composition from alkali basalt and basanite to phonolite samples from dacia conception bank and lars were dated at 9 17 and 68 ma respectively geochemical data suggest that the dredged samples come from the post erosional stage of volcanism and therefore the dates represent minimum ages for the seamount volcanoes the elevation of erosional platforms formed at wave base decrease from selvagen grande 100 m above sea level to lars seamount 900 m below sea level suggesting a southwest to northwest age progression and that all of these seamounts are older than the selvagen islands trace element and sr nd pb isotopic composition of the selvagen islands and neighboring seamounts are consistent with their origin from the canary plume interaction of the weak canary mantle plume with a slow moving plate appears to be responsible for generating a 450 km wide irregular hotspot track extending 800 km from the youngest canary island of hierro in the southwest to lars seamount in the northeast

susceptibility of mid ocean ridge volcanic islands and seamounts to large scale landsliding 1 with a view to assessing the incidence of large scale landsliding a morphologic database was created for volcanic islands and seamounts on young oceanic lithosphere the database included 44 mid ocean ridge seamounts jasper seamount and the islands of ascension bouvet guadalupe and several of the galapagos and azores islands supplemented with published reports from a further five volcanic edifices the data reveal that major landslides are common on edifices taller than 2500 m but are rare in shorter edifices implying a threshold of instability at around 2500 m a number of causes of this threshold are discussed for example many structures taller than 2500 m are or were originally volcanic islands and therefore their flanks probably include extensive weak hyaloclastite built up from lava sea interactions around coasts compaction of hyaloclastites in larger edifices lead to regions of low permeability which may help to explain the more deeply seated slope failures it is intriguing that the threshold also coincides with the edifice height at which volcanic ridges become observable a stage at which dike intrusion is probably common because slope oversteepening or excess pore pressure associated with dike intrusions have been proposed elsewhere as landslide triggers the current indications of landsliding reveal no observable relation to rainfall as landslides occur equally in wet and dry climates and no relation to tectonic setting as there are relatively few major landslides in the seismically active azores group

volcanic mound fields on the east pacific rise 16 19 s low effusion rate eruptions at overlapping spreading centers for the past 1 myr 1 volcanic mound fields identified on seamarc ii and hmr1 12 khz side scan data from the southern east pacific rise sepr occur near overlapping spreading centers oscs and migration traces of oscs the volcanic mound fields appear as a distinctive hummocky seafloor fabric due to side scan backscatter reflections from clusters of mound shaped reflectors the lack of growth of the mound fields away from the ridge axis and their occurrence in association with osc traces suggests that mound fields form along the ridge crest near oscs volcanic mound fields are found where 120 khz side scan and visual observations find fields of pillow mounds since pillow mounds are constructed by low effusion rate eruptions the volcanic mound fields found near the oscs and in their migration traces indicate that volcanic effusion rates tend to be lower near ridge discontinuities than midsegment regions this tendency for low effusion rate eruptions at oscs is documented for the past 1 myr three independent measurements of ridge segmentation 1 volcanic segment boundaries marked by the low effusion rate eruptions 2 tectonic segments defined by oscs and 3 magmatic segment boundaries based on continuity of parental magma composition all coincide in the study area high backscatter off axis lava fields not associated with seamounts are found on seafloor younger than 0 2 ma the 0 2 ma corridor corroborates previous results from the distribution of small isolated volcanoes that indicates randomly distributed off axis eruptions mainly occur on crust younger than 0 2 ma

saucer shaped sill with lobate morphology revealed by 3d seismic data implications for resolving a shallow level sill emplacement mechanism we use 3d seismic data to describe the 3d geometry of a large igneous intrusion the solsikke compound sill and address a number of issues related to sill emplacement the solsikke compound sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer shaped depressions and linear discontinuities one of the saucer shaped sub elements of the compound sill the solsikke lobate sill has a previously unrecognized morphology it has a basal feeder and consists of a bifurcating network of interlinked lobe shaped sill segments we propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments the lobate pattern indicates that the solsikke lobate sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation the feeder location is coincident with a fault intersection suggesting that magma transport from the underlying source exploited the zone surrounding the intersection

1000 m long gas blow out pipes abstract this study presents seismic observation of pipe anomalies from offshore nigeria outcrops of blow out pipes from rhodes greece and geophysical modelling of an acoustic pipe the studies give insight into how pipes form their internal structure the seismic image and geophysical artefacts related to the pipes over one hundred seafloor craters 100 m 700 m wide and up to 30 m deep have been observed on the seafloor offshore nigeria they are underlain by interpreted cones and seismic pipe anomalies that can be traced down to reservoir zones at 1000 m 1300 m below the seafloor the seismic pipe anomalies are 50 m 150 m wide and almost vertical they are interpreted as up scaled pipes found in outcrops on rhodes greece the outcrops show pipe related structures at three levels lowest the reservoir rock contains metre sized cavities which are filled with a mixture of clay derived from the overlying cap rock in the middle several circular to oval structures in plane view of pipes are observed in the lowest part of the cap rock highest 15 m into the clay cap rock strongly sheared country rock forms circular structures with a core of structureless clay based on outcrop observation on rhodes we constructed an acoustic model of a 50 m wide and 1000 m long pipe seismic modelling proves that such pipes would be expressed in seismic data that they are similar to the seismic pipe anomalies offshore nigeria but this study also revealed that prominent intra pipe reflections are artefacts a formation model for the pipes is suggested high fluid overpressure in the reservoir generated hydro fractures from the reservoir to seafloor where a mixture of gas and fluid flowed at high speed to form pipes cones and seafloor craters after hours to weeks of gas and fluid flow through the pipe the pore pressure in the reservoir dropped and the blow out terminated muddy slurry fell back and plugged the cavity in the reservoir and the pipe

geologic setting of pacmanus hydrothermal area high resolution mapping and in situ observations abstract this study presents a systematic analysis and interpretation of autonomous underwater vehicle based microbathymetry combined with remotely operated vehicle rov video recordings rock analyses and temperature measurements within the pacmanus hydrothermal area located on pual ridge in the bismarck sea of eastern manus basin the data obtained during research cruise magellan 06 and so 216 provides a framework for understanding the relationship between the volcanism tectonism and hydrothermal activity pacmanus is a submarine felsic vocanically hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries vent temperatures and morphologies the total area of hydrothermal activity is estimated to be 20 279 m2 the microbathymetry maps combined with the rov video observations allow for precise high resolution mapping estimates of the areal extents of hydrothermal activity we find the distribution of hydrothermal fields in the pacmanus area is primarily controlled by volcanic features that include lava domes thick and massive blocky lava flows breccias and feeder dykes spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field we define a three stage chronological sequence for the volcanic evolution of the pacmanus based on lava flow morphology sediment cover and lava sio2 concentration in stage 1 sparsely to moderately porphyritic dacite lavas 68 69 8 wt sio2 erupted to form domes or cryptodomes in stage 2 aphyric lava with slightly lower sio2 concentrations 67 2 67 9 wt sio2 formed jumbled and pillowed lava flows in the most recent phase stage 3 massive blocky lavas with 69 to 72 5 wt sio2 were erupted through multiple vents constructing a volcanic ridge identified as the pacmanus neovolcanic zone the transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot mantle derived melts

transition from circular to stellate forms of submarine volcanoes large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones multiple volcanic centers and embayments due to giant flank failures small mid ocean ridge volcanoes in contrast are commonly subcircular in plan view and show only embryonic rift zones in order to characterize the transition between these two end members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice irregularity was characterized by measuring perimeter distance elongation and moment of inertia of the contours assuming an ideal edifice is circular the analysis reveals a general transition over 2 4 km edifice height best transition estimate 3 km while some large edifices 4 5 km high show no major embayments or ridges suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones the various origins of the transition are discussed and the upper limit of magma chambers many of which lie above the basement of the larger edifices is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth the origins of the truncated cone shape of mid ocean ridge volcanoes are also discussed of the eruption mechanisms that have been proposed to explain their flat summits the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low density upper oceanic crust the discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes supporting table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on searching and submitting electronic supplements is found at http www agu org pubs csupp about html

seismic reflection imaging and controls on the preservation of ancient sill fed magmatic vents two dimensional seismic reflection data from offshore southern australia image a series of 2 13 km wide by 50 800 m high sub circular middle eocene age mounds the majority of the mounds occur above tips of igneous bodies intruded into the underlying upper cretaceous succession the mounds are interpreted as the seismic expression of submarine volcanic vents which are amongst the largest and oldest examples ever documented these vents are pristine and have been preserved because of their submarine environment of emplacement and burial in a post eruption sediment drape supplementary material uninterpreted version of the seismic sections presented in figure 3 is available at www geolsoc org uk sup18545

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

the northwest african slope apron a modern analogue for deep water systems with complex seafloor topography the northwest african slope apron is an interesting modern analogue for deep water systems with complex seafloor topography a sediment process map of the northwest african continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin fine grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin and is considered to result from background sedimentary processes alongslope bottom currents smooth and mould the seafloor sediments and produce bedforms such as erosional furrows sediment waves and contourite drifts downslope gravity flows debris avalanches debris flows and turbidity currents are infrequent but important events on the margin and are the dominant processes shaping the morphology of the slope and rise the overall distribution of sedimentary facies and morphological elements on the northwest african margin is characteristic of a fine grained clastic slope apron however the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models in particular the distribution and thickness of turbidite sands are controlled by the location of the break of slope which is itself controlled by the pre existing submarine topography

morphological characteristics and emplacement mechanism of the seamounts in the central indian ocean basin abstract the morphotectonic features of the central indian ocean basin ciob provide information regarding the development of the basin multibeam mapping of the ciob reveals presence of abundant isolated seamounts and seamount chains sub parallel to each other and major fracture zones along 73 e 79 e and 75 45 e morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub parallel chains the identified eight parallel seamount chains that trend almost north south and reflecting the absolute motion of the indian plate probably originated from the ancient propagative fractures inspite of the differences in their height the seamounts of these eight chains are morphologically correlatable in the study area the seamounts are clustered north and south of 12 s latitude interestingly in the area north of 12 s area ii 9 12 s the seamounts are distinctly smaller 400 m height whereas the area south of 12 s area i 12 15 s has a mixed population of seamounts the normalized abundance of the ciob seamount is 976 seamounts 10 6 km 2 but on a finer scale this value varies from 500 to 1600 seamounts 10 6 km 2 which is less than the seamount concentrations of the pacific and atlantic oceans 9000 to 16 000 seamounts 10 6 km 2 three categories of seamounts are present in the ciob e g 1 single peaked 2 multi peaked and 3 composite the study indicate that single peaked seamounts are dominant 89 while multi peaked is less 8 and composite ones are rare 3 in the ciob the progressive northward movement of the indian continent caused collision between india and asia at around 62 ma ago a majority of the near axis originated seamounts in the ciob seemed to have formed as a consequence of the temporally widespread cretaceous 65 ma to late eocene

structure temporal evolution and heat flux estimates from the lucky strike deep sea hydrothermal field derived from seafloor image mosaics 1 here we demonstrate with a study of the lucky strike hydrothermal field that image mosaicing over large seafloor areas is feasible with new image processing techniques and that repeated surveys allow temporal studies of active processes lucky strike mosaics generated from 56 000 images acquired in 1996 2006 2008 and 2009 reveal the distribution and types of diffuse outflow throughout the field and their association with high temperature vents in detail the zones of outflow are largely controlled by faults and we suggest that the spatial clustering of active zones likely reflects the geometry of the underlying plumbing system imagery also provides constraints on temporal variability at two time scales first based upon changes in individual outflow features identified in mosaics acquired in different years we document a general decline of diffuse outflow throughout the vent field over time scales up to 13 years second the image mosaics reveal broad patches of seafloor that we interpret as fossil outflow zones owing to their association with extinct chimneys and hydrothermal deposits these areas encompass the entire region of present day hydrothermal activity suggesting that the plumbing system has persisted over long periods of time loosely constrained to hundreds to thousands of years the coupling of mosaic interpretation and available field measurements allow us to independently estimate the heat flux of the lucky strike system at 200 to 1000 mw with 75 to 90 of this flux taken up by diffuse hydrothermal outflow based on these heat flux estimates we propose that the temporal decline of the system at short and long time scales may be explained by the progressive cooling of the amc without replenishment the results at lucky strike demonstrate that repeated image surveys can be routinely performed to characterize and study the temporal variability of a broad range of vent sites hosting active processes e g cold seeps hydrothermal fields gas outflows etc allowing a better understanding of fluid flow dynamics from the sub seafloor and a quantification of fluxes

submarine volcanic morphology of the western galapagos based on em300 bathymetry and mr1 side scan sonar 1 a compilation of high resolution em300 multibeam bathymetric and existing mr1 side scan sonar data was used to investigate the volcanic morphology of the flanks of the western galapagos islands the data portray an assortment of constructional volcanic features on the shallow to deep submarine flanks of fernandina isabela and santiago islands including rift zones and groups of cones that are considered to be the primary elements in constructing the archipelagic apron ten submarine rift zones were mapped ranging in length from 5 to 20 km comparable in length to western canary island rift zones but significantly shorter than hawaiian submarine rift zones a detailed analysis of the northwestern fernandina submarine rift including calculated magnetization from a surface towed magnetic study suggests that the most recent volcanism has focused at the shallow end of the rift small submarine volcanic cones with various morphologies e g pointed cratered and occasionally breached are common in the submarine western galapagos both on rift zones and on the island flanks where no rifts are present at depths greater than 3000 m large lava flow fields in regions of low bathymetric relief have been previously identified as a common seafloor feature in the western galapagos by geist et al 2006 however their source s remained enigmatic the new em300 data show that a number of the deep lava flows originate from small cones along the mid lower portion of the nw submarine rift of fernandina suggesting that the deep flows owe their origin at least in part to submarine rift zone volcanism

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

growth structure instability and collapse of canarian volcanoes and comparisons with hawaiian volcanoes recent onshore and offshore investigations in the canarian archipelago especially in the western islands of la palma and el hierro have greatly improved the understanding of the genesis and evolution of these islands and allow interesting comparisons with other hotspot induced oceanic island volcanoes especially the hawaiian archipelago genesis induced by hotspot activity and consequently shield and post erosional stages of growth allow the definition of similar main stratigraphic units the canarian and hawaiian volcanoes show common constructional and structural features such as rift zones progressive volcano instability and multiple gravitational collapses however the canaries present some important geological differences from the prototypical hotspot volcanoes of the hawaiian islands particularly the absence of significant subsidence the canarian volcanoes remain emergent until completely mass wasted by gravitational collapses and erosion volcanic formations over 20 million years are observable in outcrop in the canaries including the seamount stages of growth in several of the islands

zoned facies of mass flow deposits in the ulleung tsushima basin east sea sea of japan abstract high resolution 3 5 khz reflection profiles in the ulleung tsushima basin made it possible to classify the uppermost ca 50 m thick sedimentary deposits into rock fall slide slump debris flow deposit debrite and turbidite in terms of their acoustic properties these deposits generally occur in a distinctive zonal facies slides and slumps on the slope debrites on the base of slope and turbidites on the basin floor the rock fall deposits occur along the bases of the korea plateau and ulleung and dok islands the zonal distribution of mass flow deposits suggests that they most likely are derived from a line source at the shelfbreak shelf sediments prograde toward the shelfbreak on the slope sediment masses move downslope mainly accompanied by progressive slumping upslope individual slides and slumps on the slope probably develop into debris flows on the base of slope and successively into turbidity currents on the basin floor mass flow processes on the ulleung basin slope played an important role in transporting a large volume of terrigenous sediments into the deep basin

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

seamount morphology in the bowie and cobb hot spot trails gulf of alaska full coverage multibeam bathymetric mapping of twelve seamounts in the gulf of alaska reveals that they are characterized by flat topped summits rarely with summit craters and by terraced or step bench flanks these summit plateaus contain relict volcanic features e g flow levees late stage cones and collapse craters and as such must have been constructed by volcanic processes such as lava ponding above a central vent rather than by erosion above sea level the terraced flanks are composed of a sequence of stacked lava deltas and cones probably tube fed from a central lava pond a morphology which is suggestive of long lived stable central lava sources and low to moderate eruption rates indicative of significant time spent above a hot spot outlet most of these seamounts have summit plateaus surrounded and cut into by amphitheater headwall scarps and flanks that are scarred by debris chutes but lack visible debris accumulations at their base we interpret the lack of blocky debris fields as evidence that the slope failures are mainly small scale debris flows rather than large scale flank collapses however we cannot rule out the possibility that large flank collapse blocks from early in the histories of these seamounts are now hidden beneath the thick glacio fluvial fan deposits that cover the gulf of alaska seafloor these slope failure features become smoother and longer and increase in size and abundance with increasing age of a seamount suggesting that slope failure processes continue long after volcanic activity ceases

carbonate mounds in shallow and deep time over a decade of research on recent cold water coral mounds in various oceans has set the stage for comparative studies between recent and ancient carbonate mound systems with the aim to unravel generic processes and reveal the red thread in a fundamental strategy of life building geology a strategy nearly as ancient as life itself natural laboratories have been identified in the present ocean which provide new insights in oceanographic controls on species migration and settlement in the interaction of currents and carbonate build up in the earliest diagenesis which overprints environmental signals and shapes the template of compartmentalization of carbonate build ups and in so many other processes and factors ultimately shaping carbonate bodies comparable in size and properties to the large scale carbonate hydrocarbon reservoirs in the geological record ocean drilling and coring is an essential component of this research ideally this process is a two way avenue between shallow and deep time where fundamental and industrial knowledge about fossil carbonate mounds can drive further investigations and even experimentation in the present seas while the discoveries and process studies on live systems can yield new insights in the architecture and evolution of ancient reservoir systems this bridging exercise is the quintessence of cocarde cold water carbonate reservoir systems in deep environments an international network under the auspices of ioc unesco http www cocarde eu cocarde has organised two workshops in 2009 with a significant support of european programmes e g esf magellan workshop series and partner projects e g esf eurodiversity project microsystems this special issue groups 12 papers all addressing observations which by their nature have the potential to provide keys to generic processes of relevance for past carbonate systems as cocarde proceeds it is the objective to reciprocate in near future with an equivalent grouping of contributions from the study of fossil studies to guide further research in the present ocean it is the purpose of cocarde to strengthen such a reflux from the studies of fossil systems by stimulating relevant continental drilling exercises with comparable sampling protocols resolution and analytical procedures where relevant to allow direct comparisons with records from ocean drilling the papers in this special issue have been structured in four themes i keys to palaeoenvironmental control ii mound initiation growth and demise iii of microbes and mounds and iv tracking organominerals recent and ancient

factors affecting arsenic content of unconsolidated sediments and its mobilization in the ishikari plain hokkaido japan arsenic as contamination of groundwater is often caused by as leaching from geological formations this study focused on factors affecting as content in unconsolidated sediments by using three sediment cores in the ishikari plain hokkaido japan the geochemical properties of the sediments were characterized by the chemical and mineral compositions as well as leaching experiments and sequential extractions of as the results showed that higher as content of 4 9 mg kg was observed in peat layers with organic matter content 10 wt and that higher as contents deeper than 8 m below the surface were also observed the deeper sediments were likely to be formed in brackish water conditions due to lower weight ratios of organic carbon content to sulfur content c s 10 the results of the leaching experiments showed that as was mobilized at slightly alkaline region these indicate that the distribution of as depended on the organic content in the sediments and sedimentation condition and that the mobility of as in the sediment was affected by ph

subsurface gas offshore of northern california and its link to submarine geomorphology abstract the northern california continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology analyses of seismic reflection sidescan sonar and high resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope subsurface gas inferred from enhanced reflectors and other geophysical indicators is spatially variable and related more to total depth and stratigraphy than to underlying structure with the exception of one band of gas that follows the regional structural trend shallow depressions on the seafloor pockmarks are used to infer expulsion sites the largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m where expulsion features are rare the upper slope water depths 400 600 m has a high concentration of pockmarks diameter 10 20 m in contrast to a near absence of pockmarks at water depths shallower than 400 m of nearly 4000 pockmarks observed on sidescan sonar records more than 95 are located in water depths deeper than 400 m bottom simulating reflectors bsrs on some seismic reflection profiles indicate the possible presence of gas hydrate we find that gas and pore fluid migration in the offshore eel river basin is 1 correlated to surface morphology 2 a contributor to seabed roughness 3 a significant mode of sediment redistribution on the upper slope and 4 potentially a factor in large slope failures

morphology and shallow structure of seafloor mounds in the canary basin eastern central atlantic ocean abstract the increasing volume of high resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments in this paper seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub bottom profilers in the deep central area of the canary basin 350 550 km west off el hierro island between 4800 and 5200 mbsl these features have circular to elongated shapes with heights of 10 to 250 m diameters of 2 24 km and with flank slopes of 2 50 based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size the first second subgroup a and third mound types show heights lower than 80 m and maximum slopes of 35 with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet the second subgroup b fourth and fifth mound types show higher heights up to 250 m high maximum slopes of 47 and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and or volcanics from extrusive deep seated systems based on the constraints on their morphological and structural analyses we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds we propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution these structures are also affected by tectonics bulge like structures clearly deformed by faulting and mass movements slide scars and mass transport deposits in this work we report how intrusive and extrusive processes may affect the seafloor morphology identifying a new type of geomorphological feature as intrusive domes that have to date only been reported in fossil environments but might extend to other oceanic areas

volcano growth mechanisms and the role of sub volcanic intrusions insights from 2d seismic reflection data abstract temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location however the relationship between the external and internal characteristics of volcanoes and sub volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data we use high quality 2d seismic reflection data from the ceduna sub basin offshore southern australia to quantitatively analyse 56 pristinely preserved eocene age volcanogenic mounds and a genetically related network of sub volcanic sills and laccoliths detailed seismic mapping has allowed the 3d geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised forty six continental basaltic shield volcanoes have been identified that have average flank dips of 3 parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material suggesting that a series of temporally separate eruptions emanated from a central vent the shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill complex eight volcanogenic hydrothermal vents which are also associated with the lateral tips of sills were also recognised and these appear to have formed from the seepage of intrusion related hydrothermal fluids onto the seafloor via emplacement induced fractures this work highlights that deformation patterns preceding volcanic eruptions may i be offset from the eruption site ii attributed to intrusions with complex morphologies and or iii reflect magma movement along pre existing fracture systems these complexities should therefore be considered in eruption forecasting models that link pre eruption ground deformation to subterranean magma emplacement depth and volume more generally our study highlights the key role that seismic reflection data can play in understanding the geometry distribution and evolution of ancient and modern volcanic systems

release of methane from a volcanic basin as a mechanism for initial eocene global warming a 200 000 yr interval of extreme global warming marked the start of the eocene epoch about 55 million years ago negative carbon and oxygen isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid approximately 10 000 yr input of isotopically depleted carbon it has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change large scale hydrate melting however requires a hitherto unknown triggering mechanism here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the voring and more basins in the norwegian sea we propose that intrusion of voluminous mantle derived melts in carbon rich sedimentary strata in the northeast atlantic may have caused an explosive release of methane transported to the ocean or atmosphere through the vent complexes close to the palaeocene eocene boundary similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the siberian traps approximately 250 million years ago and the karoo igneous province approximately 183 million years ago