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

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

bubble growth and rise in soft sediments the mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance we offer definitive evidence on the mode of bubble formation in the form of x ray computed tomographic images and comparison with theory natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids disks that grow either by fracturing the sediment or by reopening preexisting fractures in contrast bubbles in soft sandy sediment tend to be spherical suggesting that sand acts fluidly or plastically in response to growth stresses we also present bubble rise results from gelatin a mechanically similar but transparent medium that suggest that initial rise is also accomplished by fracture given that muddy sediments are elastic and yield by fracture it becomes much easier to explain physically related phenomena such as seafloor pockmark formation animal burrowing and gas buildup during methane hydrate melting

a climatic trigger for the giant troll pockmark field in the northern north sea abstract pockmarks are seafloor craters usually formed during methane release on continental margins however the mechanisms behind their formation and dynamics remain elusive here we report detailed investigations on one of the world s largest pockmark fields located in the troll region in the northern north sea seafloor investigations show that 7000 pockmarks are present in a 600 km2 area a similar density of pockmarks is likely present over a 15 000 km2 region outside our study area based on extensive monitoring coring geophysical and geochemical analyses no indications of active gas seepage were found still geochemical data from carbonate blocks collected from these pockmarks indicate a methanogenic origin linked to gas hydrate dissociation and past fluid venting at the seafloor we have dated the carbonates using the u th method in order to constrain the pockmark formation the carbonates gave an isochron age of 9 59 1 38 ka i e belonging to the initial holocene moreover radiocarbon dating of microfossils in the sediments inside the pockmarks is consistent with the ages derived from the carbonates based on pressure and temperature modelling we show that the last deglaciation could have triggered dissociation of gas hydrates present in the region of the northern part of the norwegian channel causing degassing of 0 26 mtch4 km2 at the seafloor our results stress the importance of external climatic forcing of the dynamics of the seafloor and the role of the rapid warming following the younger dryas in pacing the marine gas hydrate reservoir

new style of honeycomb structures revealed on 3d seismic data indicate widespread diagenesis offshore great south basin new zealand abstract in the great south basin within the eocene section at time depths around 700 900 ms two way time below the seafloor unusual features are observed on 3d seismic data closely associated with polygonal faults the features referred to as honeycomb structures hs cover an area of 600 km 2 are packed circular oval to polygonal depressions 150 400 m across in plan view and several to 10 m in amplitude polygonal faults rapidly die out at the marshall paraconformity which is overlain by the oligocene penrod formation hence the polygonal faults are inferred to have formed prior to the marshall paraconformity and they cross cut hs features consequently the top of the hs probably formed at burial depths of around 375 500 m which is their decompacted depth below the paraconformity the interval containing hs is about 125 m vertical thick there are several possible origins for the hs the most probable is related to bulk contraction of the sediment volume accompanied by fluid expulsion which suggests a diagenetic origin in particular the opal a ct transition there are actually two polygonal fault systems pfs present in the area the southern tier 1 pfs lies laterally to the hs and overlaps with it the northern pfs tier 2 lies above the hs appears to be independent of the hs and formed in the upper 200 300 m of the sediment column the tier 1 pfs probably formed by shear failure related to the same diagenetic effects that caused the hs

spatial and temporal patterns of benthic macrofaunal communities on the deep continental margin in the gulf of guinea abstract density taxonomic composition at higher taxon level and vertical distribution of benthic macrofaunal communities and sediment characteristics pore water nitrogen organic carbon sulfur c n ratio n alcohol biomarkers were examined at three deep sites on the congo gabon continental margin this study was part of the multidisciplinary biozaire project that aimed at studying the deep benthic ecosystems in the gulf of guinea sampling of macrofaunal communities and of sediment was conducted during three cruises january 2001 december 2001 and december 2003 at two downslope sites 4000 m depth one located near the congo submarine channel 15 km in the south and the other one far from the channel 150 km in the south the third area located 8 km north of the congo channel in the surroundings of a giant pockmark at 3160 m depth was sampled during one cruise in december 2003 at these three locations the macrofaunal communities presented relatively high densities 327 987 ind 0 25 m 2 compared with macrofaunal communities at similar depths that is due to high levels of food input related to the congo river and submarine system activities that affect the whole study area the communities were different from each other in terms of taxonomic composition at higher taxon level phylum class order for all the groups except for the polychaetes classified into families the polychaetes dominated the communities and were responsible for the increase in densities observed at both deep sites 4000 m between january 2001 and december 2003 whereas the tanaidaceans the isopods and the bivalves were the other most abundant taxa responsible for the spatial differences between these sites the community at 3150 m differed from the two deep communities by higher abundances in bivalves nemerteans and holothuroids the composition of the polychaete community also differed among sites in the vicinity of the congo channel the expected positive effect of the additional organic matter transported through the turbiditic currents on to the surrounding benthic communities was not observed as the increase in densities during the study period was higher at the site located away from the congo channel than near the channel 80 vs 30 that may be due to the low food value of the organic matter of terrestrial origin carried through the turbidites and or to the disturbance caused by these turbidites conversely far from the channel the macrofaunal communities benefit from organic matter of higher energetic value originating mainly from marine sources but also from continental sources carried by the congo plume or by near bed currents across or along the continental slope spatial and temporal variability in trophic and physical characteristics of the sediment habitat at both deep sites also affected the vertical distribution of the macrofaunal communities the activities of the very active congo system structure the deep macrofaunal communities on a large area in terms of densities composition and vertical distribution the food input is enhanced at regional scale as well as the heterogeneity of the sediment characteristics mainly in terms of organic matter quality marine vs terrigenous in turn the densities are enhanced as well as the regional diversity of the macrofaunal communities in terms of taxonomic composition and distribution

natural gas formation in the western nile delta eastern mediterranean thermogenic versus microbial abstract the offshore part of the nile delta is an emerging gas and condensate province and one of the most promising areas for future petroleum exploration and supply in northern africa while the nile delta s hydrocarbon potential and its geological and stratigraphic pattern has been the focus of numerous studies the origin of its gas has been poorly studied in order to characterize the natural gas and to determine whether microbial or thermogenic processes are the dominant generation pathways of methane a regional geochemical study was performed on 38 test gases collected from 9 neogene western nile delta discoveries main constituents of the gases are methane 89 4 99 4 up to pentane with traces of carbon dioxide and nitrogen methane carbon isotopic composition ranges from 66 0 to 39 6 displaying a mixed microbial and thermogenic character for most of the gases microbial gas contributions range from 15 to 95 the mixed and the more thermogenic gases are frequently accompanied by light oils and condensates 41 59 api calculated maturities of the thermogenic gas proportions suggest source rock maturities between 0 8 and 1 4 ro which are actually present in the pre miocene section evidence for microbial alteration of petroleum accumulations in the nile delta is largely manifested in anomalously heavy isotopic compositions of both wet gas components propane and n butane and preferential removal of higher n alkanes from condensates the hydrocarbon accumulations affected by biodegradation are trapped within shallow pliocene reservoirs

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

geo and hydro acoustic manifestations of shallow gas and gas seeps in the dnepr paleodelta northwestern black sea shallow gas occurs between 0 and 1000 m below the sea floor it consists mainly of microbial formed or thermogenic methane or a combination of both sometimes with a limited admixture of higher hydrocarbons propane butane etc

the spatial and temporal distribution of pipe formation offshore namibia a group of nearly 400 pipe structures from the continental slope of northern namibia are analysed for their spatial and temporal distribution the pipes most likely formed as a result of highly focused fluid venting and understanding the factors controlling their distribution in space and time is key to their genesis we analysed their spatio temporal distribution using an arbitrary chronostratigraphic timescale from which it is concluded that the pipes did not form at the same time pipe formation is shown to be intermittent and persistent with 2 29 pipes forming in each of the 20 arbitrary time intervals that are considered to span the neogene period the spatial distribution of these pipes is clustered to dispersed spatial statistics conducted on the distribution of pipe formation timings have shown that two statistically significant groups of pipes exist within the population 1 in the north and west and 2 in the south with the former occurring prior to the latter locally pipe formation is sporadic with clusters and outliers occurring during the same time period a conceptual model is proposed whereby pipe formation in specific locations is the result of localised breaching of the seals for isolated pressure cells which are locally independent yet broadly controlled an inferred basinal fluid source is thought to determine the broader patterns of pipe formation and the focus of this fluid source shifts from north to south with time at a local scale multiple local factors interact producing a sporadic pipe formation distribution through a prolonged period of highly focused fluid migration once formed the pipes continued to focus fluids intermittently leading in some cases to later pockmark formation

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

melt inclusion record of the conditions of ascent degassing and extrusion of volatile rich alkali basalt during the powerful 2002 flank eruption of mount etna italy 1 two unusual highly explosive flank eruptions succeeded on mount etna in july august 2001 and in october 2002 to january 2003 raising the possibility of changing magmatic conditions here we decipher the origin and mechanisms of the second eruption from the composition and volatile h2o co2 s cl content of olivine hosted melt inclusions in explosive products from its south flank vents our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive volatile rich 4 wt basaltic magma that rose from 10 km depth below sea level bsl and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading this magma the most primitive for 240 years probably represents the alkali rich parental end member responsible for etna lavas evolution since the early 1970s few of it was directly extruded at the eruption onset but its input likely pressurized the shallow plumbing system several weeks before the eruption this latter was subsequently fed by the extrusion and degassing of larger amounts of the same but slightly more evolved magma that were ponding at 6 4 km bsl in agreement with seismic data and with the lack of preeruptive so2 accumulation above the initial depth of sulphur exsolution 3 km bsl we find that while ponding this magma was flushed and dehydrated by a co2 rich gas phase of deeper derivation a process that may commonly affect the plumbing system of etna and other alkali basaltic volcanoes

multiproxy late quaternary stratigraphy of the nile deep sea turbidite system towards a chronology of deep sea terrigeneous systems abstract understanding the recent formation of a sedimentary system such as a deep sea turbidite system dsts requires an accurate stratigraphic control on deposits due to the important terrigeneous input which disrupts the sedimentary record dsts is an environment where stratigraphic control is difficult to assess most of the time traditional stratigraphic tools are not accurate enough this has led to a rather limited number of studies concerning stratigraphy in dsts in this study we examine several hemipelagic long piston cores collected from the nile dsts eastern mediterranean in order to understand the recent evolution of the complex sedimentary system in this area the first aim of this study is to show how to obtain a reliable timeframe in dsts indeed we provided a detailed ecostratigraphical scheme based on planktonic foraminiferal distribution oxygen isotope records and lithostratigraphy sapropels and tephra of three cores where the sedimentation is least disturbed we have identified 29 foraminiferal ecozones during the last 250 000 years bp with an approximately 2000 year time resolution the time span of each ecozone was constrained by the oxygen isotope record 14 c ams radiometric data tephrochronology and the sapropel chronology these high resolution ecostratigraphical time subdivisions have been applied in discontinuous mixed hemipelagic turbiditic sequences of a levee record this example shows how to date gravity events formation and time periods of sedimentary accumulations

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

he ar o sr and nd isotope constraints on the origin and evolution of mount etna magmatism abstract the 0 5 ma history of the origin and evolution of mount etna sicily has been investigated by analysing the isotopic composition of he ar o sr and nd in 21 selected lava samples the near constancy of the isotopic compositions of oxygen 18 o 5 4 0 3 and of helium trapped in olivine phenocrysts 6 7 0 4 ra is interpreted as evidence of a single mantle source despite drastic petrological changes during the volcano s history he analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower 3 he 4 he ratios in pyroxenes this is best explained by crystallization of pyroxenes at a depth shallower than that of olivines and or by exchange of helium trapped in pyroxenes with atmospheric or radiogenic he before eruption 87 sr 86 sr ratios of recent lavas tend to increase with time and to correlate with rb th ratios and for historical lavas these variations are tentatively attributed to shallow selective contamination from underlying sediments based on the similarity of the 3 he 4 he ratios at etna to those of european mantle xenoliths 1 we propose that the baseline geochemical signature of isotopic tracers at etna reflects the composition of the subcontinental mantle comparison to other southern italian active volcanoes etna vulcano ischia campi flegrei vesuvius shows gradual dilution of the predominantly mantle etnean end member by more radiogenic sr and he and material with higher 18o and c 3 he which is reasonably explained by the progressively important influence of subducted continental crust

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

shallow water pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay abstract a systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western gulf of maine has identified more than 70 biogenic natural gas deposits occupying 311 km2 in nearshore muddy embayments many of these embayments also contain pockmark fields with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment pockmarks hemispherically shaped depressions of various size and depths formed through fluid escape of gas and or pore water are sometimes found within or outside gas fields although many gas fields lack pockmarks altogether although the origin of the natural gas remains unclear if coastal environments at times of lower sea level were similar to the present numerous lake wetland valley fill and estuarine sources of organic rich material may have formed on the inner shelf if these deposits survived transgression and remain buried they are potential gas sources intensive mapping of the belfast bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a gulf of maine pockmark field with a corresponding 3 dimensional geological model generated from seismic data statistical analysis of pockmark geometry gas deposit loci and subsurface evidence for gas enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas curtain seismic reflections the coarse grained transgressive ravinement unconformity between pleistocene glacial marine mud and holocene mud may act as a conduit for distributing methane to the field s margins

pockmarks in the witch ground basin central north sea marine sediments host large amounts of methane ch4 which is a potent greenhouse gas quantitative estimates for methane release from marine sediments are scarce and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios here we use 2d and 3d seismic reflection multibeam bathymetric geochemical and sedimentological data to i map and describe pockmarks in the witch ground basin central north sea ii characterize associated sedimentological and fluid migration structures and iii analyze the related methane release more than 1500 pockmarks of two distinct morphological classes spread over an area of 225 km2 the two classes form independently from another and are corresponding to at least two different sources of fluids class 1 pockmarks are large in size 6 m deep 250 m long and 75 m wide show active venting and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources class 2 pockmarks which comprise 99 5 of all pockmarks are smaller 0 9 3 1 m deep 26 140 m long and 14 57 m wide and are limited to the soft fine grained sediments of the witch ground formation and possibly sourced by compaction related dewatering buried pockmarks within the witch ground formation document distinct phases of pockmark formation likely triggered by external forces related to environmental changes after deglaciation thus greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present day fluxes but require an analysis of the pockmark forming processes through geological time

seismic morphologic and scale variabilities of subsurface pipes and vent complexes in a magma rich margin subsurface pipes related to fluid flow are observed in seismic reflection data as vertical to sub vertical features of low amplitude reflections linking chaotic reflections at the base root zones of volcanic constructs to their summits which can comprise craters mounds or eye shaped vents to date uncertainties remain regarding the linear correlation and scaling relationship between pipes and their overlying vent structures using 3d seismic reflection data from a magma rich basin the vigrid syncline in the voring basin offshore norway this study provides a seismic morphometric and statistical description of forty 40 pipes and their associated structures these pipes include conical bifurcated fault controlled and columnar types which are a consequence of the intrusion of two mappable magmatic sills of early eocene age the heights av 1098 m widths av 1740 m and slenderness ratios i e the heights widths of the pipes av 0 6 show low to moderate correlation coefficients with vent parameters such as height av 258 m area av 2 3 km2 and ellipticity av 1 5 the morpho statistical analyses provided here show that pipe formation mechanisms are mutually exclusive and that vent geometries are not particularly systematic in nature

2001 flank eruption of the alkali and volatile rich primitive basalt responsible for mount etna s evolution in the last three decades abstract since the early 1970s enhanced eruptive activity of mount etna has been accompanied by selective geochemical changes in erupted lavas among which a gradual increase of alkalis whose origin is still debated here we provide further insight into the origin of this recent evolution based on a detailed study of the chemistry and dissolved volatile content of melt inclusions trapped in olivine crystals of unusual plagioclase poor primitive basalt that was extruded during a highly explosive flank eruption in july august 2001 two types of lava were erupted simultaneously along a n s fracture system trachybasalts from the upper vents 2950 2700 m were simply drained out by fracturing of the central volcanic conduit they are identical to summit crater lavas and contain mg poor olivines fo 70 72 with evolved and volatile poor melt inclusions that represent late stage crystallisation during shallow open conduit degassing in contrast plagioclase poor basalt 80 of total extruded through the lower vents 2550 2100 m derived from lateral dyke intrusion of a more primitive and volatile rich magma across the sedimentary basement this primitive melt is best preserved in rare fo 82 4 80 5 skeletal olivines present in lapilli deposits from the most powerful activities at the 2550 m vent its high dissolved contents of h 2 o 3 4 wt co 2 0 11 to 0 41 wt s 0 32 wt cl 0 16 wt and f 0 094 wt point to its closed system ascent from 400 to 250 mpa 12 to 6 5 km depth b s l however the predominance of euhedral olivine phenocrysts with common reverse zoning cores fo 76 78 and rims fo 78 80 and decrepited inclusions shows that most of the erupted basalt derived from a slightly more evolved crystallizing body of the same magma that was invaded by the uprising primitive melt prior to erupting the few preserved inclusions in these olivines indicate pre eruptive storage of that magma body at about 5 km depth b s l in coherence with seismic data we propose that the 2001 flank eruption resulted from gradual overpressuring of etna s shallow plumbing system due to the influx of volatile rich primitive basalt that may have begun several months in advance we find that this basalt is much richer in alkalis 2 0 wt k 2 o and has higher s cl 2 0 but lower cl k and cl f ratios than all pre 1970s etnean lavas 1 4 wt k 2 o s cl 1 5 as further exemplified by melt inclusions in entrained olivine xenocrysts combining these new observations with previously published data we argue that the 2001 basalt represents a new alkali rich basic end member feeding mt etna only few amount of which had previously been extruded during a 1974 peripheral eruption and more recently during brief paroxysmal summit events over the last three decades this new magma has progressively mixed with and replaced the former k poorer trachybasalts filling the plumbing system leading to extrusion of gradually more primitive and alkali richer lavas its geochemical singularities cannot result from shallow crustal contaminations instead they suggest the involvement of an alkali richer but cl poorer arc type component during recent magma genesis beneath etna

chemical heterogeneity of mt etna magmas in the last 15 ka inferences on their mantle sources abstract primitive basaltic magmas are crucial in the study of the geochemical heterogeneity documented in etna magmas and their inferred mantle sources we undertook a systematic sampling of the less evolved basalts mg 50 erupted over the last 15 ka a time period which corresponds to the activity of the youngest volcanic edifice of mt etna complex i e mongibello volcano we focused on lava flows and pyroclastites emplaced during deep dyke fed ddf eruptions which were driven by the rapid ascent of deeply rooted magma intrusions that bypassed the shallow plumbing system of the volcano all the samples were analyzed by the same laboratory to avoid analytical bias to build a comprehensive dataset on their major and trace element compositions and to propose a coherent framework for interpreting the geochemical fingerprints of present day etna basalts trace element modeling together with literature data for sr isotopes gave insight into long term magmatic processes related to different melting degrees of the heterogeneous mantle beneath mt etna ddf magma batches provide good snapshots of their mantle source heterogeneities that point to the variable involvement of clinopyroxenitic lithology rb 87 sr cl rich fluid component s possibly controlled by their source mineralogy and slab derived fluids selectively enriched in alkalis rb k the ongoing alkali rb k enrichment of the present day magmas well manifest since the 1970s is decoupled from that of sr and cl we propose that this process is linked to mantle source composition and is concomitant with changes in both volcanological and seismotectonic patterns of the volcano there is no time evolution of ddf magma chemistry

the spatial pattern and drainage cell characteristics of a pockmark field nile deep sea fan over 25 300 seabed pockmarks were mapped from the rosetta channel region of the western niledeepseafan ndsf using concurrent high resolution 2d chirp profiler and multibeam bathymetry data which spans the holocene pleistocene period within the region apockmarkfield containing 13 800 pockmarks was analysed using spatial statistics to determine the distribution of pockmarks within the field pockmarks within the field are small 16 m diameter shallow 0 5 m deep circular depressions which formed within the last 6500 years the fluid source for the field is identified as an accumulation generation of gas beneath a hemipelagic seal c 20 40 ms beneath the seabed the position of the pockmarks is shown to be unrelated to the depth to the fluid source and an irregular high amplitude acoustic anomaly which is tentatively interpreted as a possible carbonate precipitate of biogenic microbial activity statistical spatial analysis of the field confirms the distribution of pockmarks is not random an exclusion zone surrounding each individual pockmark is identified the exclusion zone is a unique minimum radius around each pockmark which is not penetrated by any other pockmark the exclusion zone works in unison with self organised criticality soc in determining the spatial distribution of pockmarks within the field the exclusion zone is interpreted as apockmark drainagecell a conceptual model for apockmarkdrainagecell is proposed whereby pockmark formation dissipates a radius area of fluid and overpressure thereby preventing the formation of another pockmark within that cell consequently pockmarks are observed to separate or produce anti clustering tendencies within the field

thermohaline circulation in the central indian basin cib during austral summer and winter periods of 1997 abstract as a part of indian deep sea environment experiment index aimed at assessing the environmental impact of manganese nodule mining in the central indian basin cib a study on baseline physical conditions of water column viz potential temperature salinity and potential density together with geostrophic circulation regime in the deeper depths of the basin was conducted the hydrography data used in the present analysis were collected over a wide area of the western part of cib 71 79 e 9 14 s during austral summer january 1997 from the indian research vessel orv sagar kanya while during the austral winter season june july 1997 hydrographical stations were occupied by russian research vessel rv yuzhmorgeologia in the central part of cib 75 77 e 9 11 s where a benthic disturbance on experimental scale was carried out the spatial variations in the physical parameters decreased below 3500 m inferring a restricted basin scale deep circulation the dynamic topography field at 5000 m relative to 2000 db surface in the central part of cib representing the abyssal circulation was generally characterized by a southwestward weak flow around 10 s flanked by cyclonic and anti cyclonic eddies on its right and left sides respectively this flow regime agreed with the earlier one inferred by warren j mar res 40 1982 823 linking the source of deep water in cib to a saddle overflow across ninetyeast ridge from west australian basin around 10 s

south malvinas falkland basin hydrocarbon migration and petroleum system abstract we report a detailed seismic interpretational study of direct hydrocarbon indicators dhis using 2d multichannel seismic reflection data the study includes analyses of four distinctive types of dhi 1 a bottom simulating reflection bsr 2 enhanced reflections ers 3 pipes and vertical anomaly clusters which indicate focused migration of hydrocarbon fluids and 4 flat spots which indicate a deeper source for the shallower hydrocarbon occurrences the interpretation of these dhis included analysis of spatial distribution amplitude variation with offset avo and instantaneous spectral decomposition these analyses are set in a tectono stratigraphic context in order to evaluate the relationship between intra basinal structures hydrocarbon occurrences and migration pathways the recent discovery of a gas condensate accumulation in the study area darwin well ca 190mmbbl confirms the presence of a mature source rock in the basin and supports a model whereby the observed dhis are connected via an active hydrocarbon plumbing system

dynamic plumbing system beneath volcanoes revealed by kinetic modeling and the connection to monitoring data an example from mt etna abstract our ability to monitor volcanoes using seismic signals ground deformation gas fluxes or other ground and satellite based observations as well as our understanding of melt reservoirs that feed eruptions have evolved tremendously in recent years the complex plumbing systems that are thought to feed eruptions are however difficult to relate to the monitoring signals here we show that the record preserved in compositional zoning of erupted minerals may be used to reconstruct sections of the plumbing system kinetic modeling of such zoning can yield information on the residence time of magma in different segments of the plumbing systems this allows a more nuanced evaluation of the link between observed monitoring signals or eruption styles and the magmatic processes and movement of batches of melts at depth the approach is illustrated through a study of the compositional zoning recorded in olivine crystals from the 1991 1993 se flank eruption products of mt etna sicily the zoning patterns in crystals reveal that the plumbing system of the volcano consisted of at least three different magmatic environments between which magma was transported and mixed in the year or two preceding the start of eruption quantification of this history indicates that two main pathways of melt migration and three timescales dominated the dynamics of the system combination of this information with the timing of observation of various monitoring signals allows a reconstruction of the dynamic evolution of this section of the plumbing system during the early stages of the 1991 1993 eruption it is seen for example how the migration of melt through the same sections of the plumbing system can cause pre eruptive triggering enhance strombolian activity and through the ensuing eruption cleanse and flush the plumbing system different kinds of mixing occur simultaneously at different sections of the plumbing system on different timescales a few days up to two years

shallow gas and fluid migration in the northern arafura sea offshore northern australia abstract a variety of shallow gas indicators and fluid migration pathways have been interpreted from new sub bottom profiler multibeam bathymetry side scan sonar and echo sounder data together with geochemical analyses of sampled sediments from the northern arafura sea offshore northern australia this study provides new geological data and a seismic stratigraphy of the youngest units in the money shoal basin in an area that has not been closely studied in over 30 years the shallow gas indicators include pockmarks low frequency enhanced reflectors and acoustic blanking these indicators are supported by gas within shallow cores geochemistry indicates that this gas has a microbial origin but deeper fluid movement is also suggested by the presence of interpreted hydrocarbon slicks based on synthetic aperture radar data a region of likely hydrocarbon seepage is indicated by the clear coincidence of shallow gas indicators with an area of increased faulting and a zone of poor quality seismic data recognised on conventional seismic data together with anomalies interpreted on remote sensing data aligned with mapped deep structures

the holocene gas system of the ria de vigo nw spain factors controlling the location of gas accumulations seeps and pockmarks abstract high resolution seismic profiles cored sediment gravity cores and vibro cores and radiocarbon data from suitable shell material were used to establish the stratigraphic context of gassy sediments in the ria de vigo mapping of seeps and pockmarks were compared with the stratigraphy of gassy sediment to ascertain the relationships in the ria de vigo three holocene seismic units were identified the first two uh 1 and uh 2 correspond to the transgressive systems tract tst and the third uh 3 corresponds to the highstand systems tract hst the presence of acoustic turbidity within the three seismic units indicates that generation of gas took place during the relative sea level rise that occurred after the younger dryas event i e during the tst and mainly during the hst the relative sea level variations are shown to be a major factor controlling the type fine vs coarse grained and distribution of facies lateral and vertical that generate and or allow gas accumulation or escape acoustic turbidity due to presence of free gas was recognized in the three holocene units with a total of 12 acoustic turbidity fields mapped in the ria de vigo no significant variations in the spatial distribution or extent of the gas fields or in their vertical location below the present sea bed have been observed during the seismic surveys carried out in the ria during the last decade furthermore gas escape from the sediment and active pockmark has been observed therefore we conclude that gas escape seeps pockmark generation and maintenance and methane formation are in equilibrium the combined analysis of high resolution seismic profiles sediment temperature pore water salinity and hydrostatic pressure revealed 1 a direct relationship between depth of free gas and hydrostatic pressure and 2 a direct link between sediment temperature and number of acoustic plumes from seabed to water column more acoustic plumes were detected in summer when the temperature of the sediment was higher than in winter indicating a seasonal variation of seeps within the ria salinity showed no significant differences between sea bottom water and sediment pore water and was therefore not a major factor controlling gas front variations

indian deep sea environment experiment index an appraisal abstract indian deep sea environment experiment index is a multi disciplinary study to establish baseline conditions and evaluate the possible impact of deep seabed mining in central indian basin a disturbance was simulated to study the effects of sediment re suspension and re settlement in the benthic areas monitoring the process of restoration and recolonisation of benthic environment and development of predictive models for environmental impact of deep seabed mining are underway significant information on physical chemical biological and geological characteristics of water column and benthic baseline conditions has been generated in the programme evaluation of impact of simulated disturbance on the seafloor shows vertical mixing of sediment on the seafloor lateral migration of sediment plume changes in geochemical and biochemical conditions as well as reduction in biomass in the benthic environment the results obtained are useful in determining the indicator parameters and standardising the methods for assessment of effects of large scale mining

occurrence of pockmarks on the ortegal spur continental margin northwestern iberian peninsula a total of 445 pockmarks were observed on the upper continental slope of the northwest corner of the iberian peninsula the ortegal spur area by swath bathymetric and ultrahigh resolution seismic data the pockmarks are u v and w shaped and have terraces or indentations in cross section and are dish shaped circular to oval in plan view they occur on the surface of the seabed and buried within the plio quaternary and neogene sediments four types of pockmarks were identified and mapped on the basis of their plan view and cross section morphology regular irregular asymmetric and composite the concentration of pockmarks is attributed to seepage of fluids migrating up dip from deeper parts of the sedimentary basin a linear high density concentration with a nnw to n ne and ese trend of pockmarks is observed above inferred basement faults that do not affect the quaternary succession these pockmarks are thus caused by seepage of thermogenic gas and or other pore fluids from deeper late cretaceous units and their distribution may help to improve our understanding of the fluid system and migration regime in this part of the galicia continental margin

recent depositional patterns of the nile deep sea fan from echo character mapping the nile deep sea fan was surveyed in 1998 using swath multibeam bathymetry backscatter imagery and 3 5 khz and seismic profiling on the basis of this new data set the fan has been divided into three main provinces western central and eastern recent sedimentary patterns as deduced from echo character mapping show that gravity induced sedimentary deposits are predominant and are expressed through either slides debris flows or turbidites turbidity processes are particularly active in the westernmost province through a well developed network of deep sea channels this network controls turbiditic flows that subsequently overflow levees mass wasting processes are extremely efficient all over the nile deep sea fan in the upper central province a broad allochthonous tongue of mass flow deposits generated by slope destabilizations may have led to channel avulsions and migrations in the eastern province a northwest southeast deformed belt resulting from combined deep seated and salt related tectonics contains large proportions of destabilized sediments i e originating from destabilization processes such as mass movements probably settled by recent tectonic activity this article highlights 1 the influence of both thin skinned and thick skinned tectonics on sedimentary distribution and 2 the predominance of gravity induced sediments turbidites slides and mass flows in such submarine fans the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential

thirty years of satellite derived lava discharge rates at etna implications for steady volumetric output 1 we present a 30 year long data set of satellite derived time averaged lava discharge rates tadr for mount etna volcano sicily italy spanning 1980 2010 and comprising 1792 measurements during 23 eruptions we use this to classify eruptions on the basis of magnitude and intensity as well as the shape of the tadr time series which characterizes each effusive event we find that while 1983 1993 was characterized by less frequent but longer duration effusive eruptions at lower tadrs 2000 2010 was characterized by more frequent eruptions of shorter duration and higher tadrs however roughly the same lava volume was erupted during both of these 11 year long periods so that the volumetric output was linear over the entire 30 year period increasing at a rate of 0 8 m3 s 1 between 1980 and 2010 the cumulative volume record can be extended back in time using data available in the literature this allows us to assess etna s output history over 5 centuries and to place the current trend in historical context we find that output has been stable at this rate since 1971 at this time the output rate changed from a low discharge rate phase which had characterized the period 1759 to 1970 to a high discharge rate phase this new phase had the same output rate as the high discharge rate phase that characterized the period 1610 1669 the 1610 1669 phase ended with the most voluminous eruption of historic times

types of fluid related features controlled by sedimentary cycles and fault network in deepwater nigeria abstract the seismic characteristics of focused fluid related features such as pockmarks and carbonate build ups on the nigerian continental slope have been investigated using complementary seismic data sets reprocessed 3d exploration data and 2d very high resolution hull mounted and near bottom seismic data coupled with the results of previous studies sedimentological geotechnical and geochemical analyses the results show different types of fluid related features within the hemipelagic phase of long duration 1 myr turbidite hemipelagic cycles they are the product of the disturbance of the hosting sediments following two main distinct processes 1 no or low sediment deposition caused by fluid escape associated with seafloor settlement benthic fauna carbonate precipitation gas hydrate formation and 2 post depositional erosion caused either by fluid release dewatering from underlying features such as channels and upward migration from a deep source or by collapse destabilisation of gas hydrates the temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation the spatial distribution of fluid related features seems to be controlled by the type of fault small fault or structural lineament

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

lava fountains during the episodic eruption of south east crater mt etna 2000 insights into magma gas dynamics within the shallow volcano plumbing system mt etna in sicily italy is well known for frequent effusive and explosive eruptions from both its summit and flanks south east crater se crater one of the four summit craters has been the most active in the last 20 years and often produces episodic lava fountains over periods lasting from a few weeks to months the most striking of such eruptive phases was in 2000 sixty four lava fountains separated by quiescent intervals and sometimes associated with lava overflows occurred that year between january and june a time period during which we consider the volcano to have been in episodic eruption this paper presents mainly results of petrochemical investigations carried out on both tephra and lavas collected during a number of the lava fountain episodes in 2000 the new data have been integrated with volcanological and seismic information in order to correlate the features of the eruptive activity with magma gas dynamics in the plumbing system of se crater the main findings allow us to characterise the 2000 episodic eruption in the framework of the recent se crater activity in particular we infer that the onset of the 2000 eruption was triggered by the ascent of new more primitive and volatile rich magma that progressively intruded into the se crater reservoir where it mixed with the resident more evolved magma furthermore we argue that the 2000 se crater lava fountains largely resulted from the instability of a foam layer accumulated at the top of the underlying reservoir and rebuilt prior to each episode in agreement with the collapsing foam model for lava fountains

deep currents in the arabian sea in 1987 abstract new features of the deep circulation in the arabian sea were revealed by four sections of deep ctd o 2 stations occupied by the r r s charles darwin late in 1986 and early in 1987 during the northeast monsoon and repeated during the succeeding southwest monsoon the most prominent elements in the bottom water layer were 1 a northward flowing western boundary current of volume transport of about 4 10 6 m 3 s 1 in the southern somali basin which appeared to turn eastward at the equator and supply water to the northern somali basin from along the equator and 2 a southeastward flowing boundary current along the northeastern flank of the carlsberg ridge in the arabian basin which demonstrated that the bottom water of the arabian basin enters mainly from the somali basin through the owen fracture zone rather than from the central indian basin the strength of the boundary current in the somali basin implies an unusually large upward velocity at the top of the bottom water about 12 10 5 cm s 1 mapping of water properties in the deep water layer above the bottom water during successive monsoons was undertaken because three earlier surveys had shown differing horizontal distributions there and had thus raised the possibility of a monsoonal reversal of the circulation paralleling that in the near surface water however the patterns of variation in the salinity and oxygen fields were essentially the same on the two darwin cruises suggesting a broad southwestward flow in the deep water layer of the somali basin during both monsoons we propose that this pattern was not forced by the monsoons but reflected mainly the mean circulation of the deep water as driven by the upwelling of the bottom water from below that that pattern might be enhanced during the northeast monsoon if the wind forcing penetrates into the deep water and that a moderately strong southwest monsoon in contrast to the extraordinarily weak monsoon of 1987 would be required to disrupt this distribution of water properties

explosive eruption of a picrite the 3930 bp subplinian eruption of etna volcano italy 1 we report on a subplinian eruption fed by a picritic magma that occurred 3930 60 bp at etna volcano the eruption produced a fallout deposit of 0 183 km3 0 055 km3 dre widely dispersed eastward which represents a marker bed for the holocene stratigraphy of the volcano tephra have a composition of nearly primary magma owing to their high mg 72 79 glass geothermometry indicates eruptive temperatures 1159 1171 c 100 higher than those measured in recent eruptions high volatile content and unusual enrichment of lil elements in feeding magma both suggest addition of fluids in the source zone accounting for a large production of highly buoyant fast rising melt vigorous degassing of volatile rich picritic magma is responsible for the explosive nature of this eruption

3d kinematics of a thick salt layer during gravity driven deformation abstract we present the results of an interpretation of 2d and 3d seismic from offshore lebanon in which we identify a suite of 5 linearly distributed trails of fluid escape pipes with pockmarks at their upper terminus these features transect the thick messinian evaporites and root within prominent ne sw oriented pre salt folds the pipe trails are oriented orthogonal to the strike of the pre salt folds with a synchronous initial expulsion episode in each trail dated at 1 7 ma 0 3 ma approximately coeval with the onset of salt detached growth faulting along the basin margin each expulsion episode has been systematically offset to the nw away from the pre salt fold by the flow of the salt resulting in deformation of the fluid escape pipes in the salt the orientation of the pipe trails thus provides a direct kinematic indicator for the flow direction of the salt layer during early stages of gravity driven deformation of the salt and overburden concomitant with basin margin uplift and tilting the unidirectional nw oriented flow is recorded over a region of some 50 km width within the translational domain of the salt tectonic deformation synchronicity in the onset of fluid expulsion from overpressured reservoirs within the pre salt succession evidenced by these pipe trails and growth fault development at the basin margins implies that the pipe trails record the kinematics of the deforming salt layer throughout its post messinian phase of deformation the deformed pipe trails demonstrate a couette flow regime for the salt layer and document subtle changes in cumulative strain and velocity 2 4 mm yr 0 3 mm yr over distances of a few km it is proposed that this novel method of using fluid flow features as natural markers for the kinematics of deforming salt layers could be utilized in other parts of eastern mediterranean as well as other salt basins on earth

natural gas hydrates in the rio grande cone brazil a new province in the western south atlantic the rio grande cone is a large scale fanlike feature in the continental slope of the pelotas basin southern brazil where ubiquitous world class bottom simulating reflectors bsrs are readily observed in seismic records with the purpose of searching for natural gas hydrate deposits in the cone area four oceanographic cruises were carried out between may 2011 and july 2013 leading to the discovery of two pockmark fields active faults and gas hydrates in shallow sediments multichannel seismic multibeam echo sounder side scan sonar and sub bottom profiler records were used to map the shallow section and select sites for piston core sampling gas hydrates were recovered in several piston cores within muddy sediments collected inside pockmarks displaying high backscatter in the multibeam and side scan sonar data we present two representative piston cores where numerous levels of gas hydrates occur along with degassing features authigenic carbonate and soupy sediments gas dissociated from gas hydrate samples is dominantly methane 99 78 with minor quantities of ethane the chemical and isotopic compositions of the gas strongly suggest a biogenic origin for the analyzed samples these new findings are regarded as strong enough evidence to consider the rio grande cone as a new gas hydrate province

seasonal variability in the vertical current structure and kinetic energy in the central indian ocean basin abstract seasonal variability in the currents at the subsurface 450 670 m intermediate 1150 1370 m deeper 3450 3670 m and near bottom 4270 5100 m depths at three mooring stations in the central indian ocean basin ciob were documented in 1995 1997 the current meter moorings were deployed thrice at station 1 ms 1 ms 1a and ms 1b in the northern basin 10 s and twice at stations 2 ms 2 and ms 2a and 3 ms 3 and ms 3a in the southern basin 15 s in the northern basin seasonal variability in the currents at 500 m and 1200 m depth is due to the seasonal north south shift in the westward flowing south equatorial current sec low frequency 30 60 day oscillation superimposed on high frequency inertial semi diurnal and diurnal fluctuations is noticed at all the depths including near bottom depth 5100 m where the u component of current was dominant the variability of the currents at the intermediate depth range 1150 1300 m encompassing the three stations of 500 km apart indicates the existence of anticyclonic gyral circulation the depth variation of kinetic energy ke emphasises the bottom intensification of currents with minimum ke at deeper depths followed by relatively higher ke at abyssal depths in the northern basin the total ke exhibits seasonal variation it is high during southern spring summer and low during southern fall winter the eddy ke constitutes a larger part of the total ke at synoptic scale 12 15 day fluctuations in the northern basin and meso scale inertial semi diurnal and diurnal oscillations in the southern basin

the role of supercritical fluids in the potassium enrichment of magmas at mount etna volcano italy abstract the enrichment in potassium shown by the basic lavas erupted at mount etna volcano after 1971 k2o max 2 2 wt has been considered by previous researchers to be too high to be related to simple crystal fractionation and instead linked this high k2o content to either crustal assimilation or changes in the magma source unfortunately all existing models for the post 1971 k2o enrichment fail to explain the phenomenon satisfactorily leaving the question still open we present a critical re examination of published data for major elements 633 whole rock analyses trace elements 376 whole rock analyses and isotopic ratios 136 87sr 86sr analyses for historical and pre historical lavas potassium enrichment is not limited to the products of the last 35 years a comparable increase in potassium is noticed in lavas erupted during the pre historic phase of the recent mongibello k2o max 2 5 wt and in lavas related to the early phase of the ancient mongibello k2o max 3 wt moreover data from melt inclusions in olivines from the 2001 and 2002 eruptions reveal that potassium contents remain constant for melts with entrapment pressure between 490 to 100 mpa and increase significantly in melts entrapped at pressures below 100 mpa we propose that supercritical fluids coming from deeper magmas and carrying alkali cl complexes migrate through basic to intermediate magmas residing in the shallow feeding system as chlorine exsolves and leaves the system alkalis are released contributing to the observed potassium enrichment of the shallow magma fluctuations of the volatiles influx throughout time are likely related to the magma supply rate considering that the amount of magma entering a plumbing system is determined by the rate of regional extension the flux of alkali cl complexes entering the melt might be related to an extensional regime acting in the etnean area

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