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

elongated depressions associated with pockmarks in the western slope of the norwegian trench abstract terraces or elongated depressions have been surveyed in the western slope of the norwegian trench in origin the depressions seem to be closely related to the formation of pockmarks in the norwegian trench a study area d has been selected in the western slope for the description of the elongated depressions and their associated features an area of similar size e has also been selected in the norwegian trench in order to compare the recent geology of the two areas it is found that the elongated depressions probably are caused by a combination of shallow gas seepage the geometry of subsurface bedding and the action of bottom currents furthermore examples of faulting within soft silty unconsolidated clay is documented by use of deep towed boomer data the existence of such faults could explain how gas migrates through the upper layers

classification of sea floor features associated with methane seeps along the gulf of cadiz continental margin abstract based on recently gathered swath bathymetry high to ultra high resolution seismic and underwater camera data along with dredging and coring samples this paper examines the relationship between sea floor features and the nature of hydrocarbon enriched fluid and gas leaks from degassing of deeply buried sediments along the continental margin of the gulf of cadiz eastern central atlantic a classification into three main groups is proposed on the basis of the morphology and nature of deposits 1 mud volcanoes 2 methane derived authigenic carbonates mdac mounds and 3 crater like pockmarks mud volcanoes are topographically cone shaped sea floor edifices built up from catastrophic mud and fluid degassing intercalated with periods of inactivity so far more than 25 mud volcanoes have been discovered in the gulf of cadiz named in memory of deceased colleagues e g ginsburg and baraza or researchers birth places e g faro cibeles almazan san petersburgh yuma rabat bonjardim coruna gades these structures range from 800 to 2500 m in diameter and tower 150 300 m above the seabed the volcanoes consistently feature a well defined outer ring or circular terrace and an inner dome all mud volcanoes are built up of episodes of mud breccia flows intercalated with deep current deposits with evident indications of gas saturation degassing structures a strong h2s smell and chemosynthetic fauna such as pogonophora sp tube worms and calyptogena sp commonly observed carbonate crusts and slabs overlying some mud volcanoes are thought to have been formed by slow diffuse venting during periods of inactivity or slower rates of fluid venting following the ejection of mud a fermentation process the result of microbial mediated oxidation of hydrocarbon enriched fluids seems to play an important role in the growth of large deep water carbonate mounds and chimneys during periods of low methane seep fluid pressure more than 400 crater like pockmarks a common sea floor feature in the tasyo field have been observed their seabed diameter ranges from 500 to 3000 m with steep slopes ranging from 12 to 35 according to their internal structure pockmarks can be grouped into 1 pockmarks with low central mounds and 2 crater like pockmarks apparently collapse structures lacking central mounds with a flat to slightly undulating sea bottom surrounded by steep slopes up to 25

stable isotope anomalies in mediterranean pleistocene records oxygen and carbon isotope records of twelve mediterranean cores raised from west alboran sea to east off israel and of four atlantic cores raised from the western side of gibraltar and located respectively inside below and beyond the modern mediterranean outflow are compared to open ocean records of core 71 139c from the north atlantic and core v19 30 from the east pacific oxygen isotope stratigraphies can be established for deep mediterranean records and the major events of the last 150 ka are easily identifiable for instance the five substages of stage 5 or the 18 ka event stage 2 are identified but the 10 ka event younger dryas is only recorded by globigerina bulloides and benthic foraminifera suggesting that the cooling affected only winter sst 14c dates for the two terminations ia and ib of the last deglaciation which are recorded in sediment cores from areas with high sedimentation rates faro ridge alboran sea sicilian strait are in good agreement with the dates published for the same termination in atlantic cores an intermediate step however has been identified in some cores at 14 ka and divides termination ia into two phases oxygen isotope anomalies are recorded by the shallow water species globigerinoides ruber they correspond to 1 a higher amplitude glacial interglacial change in 18o values which exceeds that of the open ocean by 1 in the western basin and by 1 7 in the eastern basin 2 large isotopic depletions during the deposition of sapropel s1 s3 and s5 they are interpreted as the result of fresh water run off responsible for a strong dilution in the shallower layers of the east mediterranean waters the isotopic effect of this dilution averages 0 75 for sapropel s1 and 1 6 for sapropel s5 during the last glacial maximum the mediterranean still acted as a concentration basin and a gradient of 18o values similar to the modern one 0 75 existed from west to east in the deep waters apparently deep water temperatures were lower during the last glacial but the decrease did not exceed 4 c in the eastern basin major anomalies are also contained in the 13c records of g bulloides and benthic foraminifera the 13c records from surface intermediate and deep waters of the eastern basin and from intermediate and deep waters of the western basin are opposite to that of the open ocean during glacial time 13c values were higher in intermediate and deep waters of the whole mediterranean and also in surface water of the eastern basin at the same time the 13c gradient of surface to deep waters was low this gradient increased during the last deglaciation while 13c values of g bulloides and benthic foraminifera decreased this decrease interrupted at 10 ka may have been related to increased fresh water run off from the bordering contintents due to increased precipitation especially in the tropical belt the 13c depletion is clearly recorded in the levantine and deep water masses and can be traced westward in the mediterranean outflow oxygen and carbon isotope anomalies recorded simultaneously in the east mediterranean by the shallow water species globigerinoides ruber during sapropel deposition indicate that increased precipitation on the bordering continents and strong fresh water run off during the time of the last deglaciation may have caused a density stratification and deep stagnation in the eastern basin uncertainties arise from the fact that in the mixing model deglacial mediterranean waters are mixed with continental fresh waters the isotopic characteristics of which are not entirely understood although increased surface water production of biogenic material can not be excluded as a possible cause of 13c depletion it appears that the river discharge including the nile as well as black sea input are also plausible sources for the fresh water layering

characteristic features of pockmarks on the north sea floor and scotian shelf this paper presents the results of a comparative study of pockmarks and associated features appearing on both sides of the north atlantic on the scotian shelf off nova scotia and in the northern north sea pockmarks are formed in seabed material consisting of soft silty clay the seismic sonar and lithologic characteristics of the sediments on the scotian shelf are remarkably similar to those found in the northern north sea sediment clouds suspended in the water column immediately over the seabed have previously been observed on side scan records associated with gas charged sediments on corresponding shallow seismic records these and similar observations strongly suggest that most pockmarks are caused by gas efflux through the seafloor however the detailed mechanism of formation and the origin of the gas in the sediments is still unknown

potential geologic hazards on the eastern gulf of cadiz slope sw spain abstract geologic hazards resulting from sedimentary oceanographic and tectonic processes affect more than one third of the offshore gulf of cadiz and are identified by interpreting high resolution seismic profiles and sonographs hazards of sedimentary origin include the occurrence of slope instability processes in the form of single or multiple slumps occupying up to 147 km 2 mainly concentrated in the steeper upper slope area besides the presence of steep slopes the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the sediment gassy sediments and associated seafloor pockmarks cover more than 240 km 2 in the upper slope hazards from oceanographic processes result from the complex system of bottom currents created by the interaction of the strong mediterranean undercurrent and the rough seafloor physiography the local intensification of bottom currents is responsible for erosive processes along more than 1900 km 2 in the upper slope and in the canyons eroded in the central area of the slope undermining slopes and causing instability the strong bottom currents also create a mobile seafloor containing bedforms in an area of the gulf that extends more than 2500 km 2 mostly in the continental slope terraces hazards of tectonic origin are important because the gulf of cadiz straddles two major tectonic regions the azores gibraltar fracture zone and the betic range which results in diapir uplift over an area of more than 1000 km 2 and in active seismicity with earthquakes of moderate magnitude also tsunamis produced by strong earthquakes occur in the gulf of cadiz and are related to the tectonic activity along the azores gibraltar fracture zone

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

seabed morphology and hydrocarbon seepage in the gulf of cadiz mud volcano area acoustic imagery multibeam and ultra high resolution seismic data abstract extensive mud volcanism mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are observed throughout the spanish portuguese margin of the gulf of cadiz all the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome accretionary complex units which were emplaced in the late miocene in response to nw directed convergence between the african and eurasian plates the study area was investigated by multibeam echo sounder high and ultra high resolution seismic profiling dredging and coring the structures observed on multibeam bathymetry at water depths between 500 and 1300 m are dominated by elongate mud ridges mud cones mud volcanoes and crater like collapse structures ranging in relief from 50 to 300 m and size from 0 8 to 2 km in diameter the main morphotectonic features named the guadalquivir diapiric ridge gdr and the cadiz diapiric ridge cdr are longitudinally shaped diapirs which trend ne sw and consist of lower middle miocene plastic marly clays the gdr field and the tasyo field which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development are described in this paper the gdr field is characterised by numerous single sub circular mud volcanoes and mud cones the single mud volcanoes are cone shaped features with relatively gentle slopes of 3 6 consisting of several generations of mud breccia deposition with indications of gas saturation degassing structures and the presence of h2s the mud cones have asymmetric profiles with steep slopes of up to 25 and contain large surficial deposits of hydrocarbon derived carbonate chimneys and slabs the tasyo field is characterised by an extensive concentration of small sub circular depressions oval and multi cone mud volcanoes and large sediment slides mud volcanoes in this area are characterised by moderate slopes 8 12 have bathymetric relief ranging from 100 to 190 m and consist of sulphide rich mud breccia calcite chimneys carbonate crusts and chemosynthetic fauna pogonophora tube worms we propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic accretionary wedge this stress results in the uplifting and squeezing plastic marly clay deposits additionally the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon enriched fluids to migrate

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

some sedimentological and geochemical characters of the late triassic noto formation source rock in the ragusa basin sicily in the ragusa basin southeastern sicily the late triassic noto formation is considered as the main oil source rock we provide a detailed description of the sedimentary facies determined from core samples and discuss geochemical results obtained for both kerogens and chloroform extractable hydrocarbons from samples where sedimentary organic matter is immature two main sedimentary sequences were encountered i layers of limestones and of marls or shales alternating at a metric scale and ii laminites having various types and carbonate contents the high petroleum potentials s2 up to 100 kg hc t rock are related to sedimentary lithologies that contain a minimum of clay two types of algal organic matter are probably mixed in the sediments studied these two types can be distinguished by the oxygen content they also do not have the same kinetic behavior in thermal evolution special attention has been paid to the possible use of bitumens and of pyrolysates obtained from kerogens for understanding organic facies bulk properties together with several biomarker distributions were examined through different facies but most of the parameters measured or computed are more clearly related to the maturity of the sequences analyzed rather than to the intrinsic nature of the in situ organic matter a modeling approach is proposed taking into account geological and geochemical data different situations of the noto source rock lead to different timings of oil formation the importance of source rock heterogeneities at different scales in basin evaluation is underlined

chemosynthetic mussels at a brine filled pockmark in the northern gulf of mexico a large 540 square meters bed of bathymodiolus n sp mytilidae bivalvia rings a pool of hypersaline 121 35 practical salinity units brine at a water depth of 650 meters on the continental slope south of louisiana the anoxic brine dissolved oxygen 0 17 milliliters per liter contains high concentrations of methane which nourishes methanotrophic symbionts in the mussels the brine which originates from a salt cored diapir that penetrates to within 500 meters ofthe sea floor fills a depression that was evidently excavated by escaping gas the spatial continuity of the mussel bed indicates that the brine level has remained fairly constant however demographic differences between the inner and outer parts of the bed record small fluctuations

geometry and structure associated to gas charged sediments and recent growth faults in the ebro delta spain abstract in the ebro delta ne spain small to large scale synsedimentary deformation has been observed by means of high and low resolution seismics correlated with a network of wells lithoseismic units involved are middle upper pleistocene clays and gravels and holocene deltaic sequence gassy sediments and large river floor pockmarks are identified from seismic profiles along the ebro delta gas charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles most of the gas is believed to be biogenic in origin resulting from the decay of organic matter contained within rapidly developing shelf margin deltas a group of large symmetric river floor pockmarks also occurs along the inner delta plain both on the river floor surface and buried in the sediment column listric fault geometry is a typical scope shaped plane with a steep upper surface 40 60 that passes downward into a horizontal shear plane the shear plane appears seismically as chaotic and convoluted high amplitude reflectors and is correlated with well data showing layers that include gravel sand and clays associated structural features include reverse drag of reflectors antithetic faults and fluid escapes methane water the growth faulting along the ebro delta includes a break away and thin near surface layer created by overstepping of a sandy holocene delta slope the over steeping lies down dip from growth faults along the shelf break the overall growth faults province appears to represent a deeper seated general movement of the delta mass involving differential loading overpressure of fluids diapiric movements and extensional tectonics that cause listric faults and associated structures at different scales the recent growth faults are regularly spaced with the distance between successive faults controlled by the depth to the detachment layer three main fault systems have been identified a first or primary system spaced 5 2 10 5 km above a detachment layer at 55 95 m deep a second system spaced 3 5 6 km and a third system spaced 1 5 2 5 km above a detachment layer at 25 30 m the slope angle calculated by empirical equations ranges between 0 5 and 1 0 for the three systems

the aquitaine shelf edge bay of biscay a primary outlet for microbial methane release a few thousand 2 612 seeps are releasing microbial methane bubbles from the seafloor at the aquitaine shelf edge bay of biscay at shallow water depths 140 220 m this methane contributes to the formation of meter scale subcircular carbonate structures which are sub outcropping over 375 km 2 based on in situ flow rate measurements and acoustic data and assuming steady and continuous fluxes over time the methane entering the water column is estimated at 144 mg yr microbial methane circulation has been ongoing for at least a few thousand years this discovery highlights the importance of microbial methane generation disconnected from deep thermogenic sources and gas hydrates at continental shelves the shelf edge may be viewed as a focus area for methane circulation and release and related diagenesis all having an impact on the shaping of continental shelves and potentially on the oceanic and atmospheric carbon budget plain language summary at the aquitaine shelf of the bay of biscay northeast atlantic ocean the recent acoustic chemical and visual investigations of microbial methane release at the seafloor have led to the discovery of a vast fluid system this methane escapes as bubbles from the seafloor into the seawater at 2 612 sites all located at shallow water depths 140 220 m along the edge of the continental shelf methane derived authigenic carbonates that are by products of gas seepage cover the sub seafloor over a large area of 375 km 2 these carbonates form subcircular meter scale pavements and mounds less than 2 m in height above the surrounding seafloor based on the growth rate of authigenic carbonates it can be inferred that methane circulation has occurred for at least a few thousand years the amount of methane released from the aquitaine shelf seafloor into the water column estimated at 144 t yr questions the fate of the methane in the ocean and its possible passage to the atmosphere with therefore consequent potential contribution to the oceanic and atmospheric carbon budget over time

geotechnical characteristics and slope stability in the gulf of cadiz abstract sedimentological and geotechnical analyses of thirty seven core samples from the gulf of cadiz continental margin were used to define the regional variability of sediment properties and to assess slope stability considering the sediment property data set as a whole there is an association between grain size plasticity and water content any one of these properties can be mapped regionally to provide an indication of the dominant surface sediment lithology based on static sediment strength a simplified slope stability analysis showed that only steep slopes 16 for even the most vulnerable sediment can fail under static loading conditions accordingly transient loads such as earthquakes or storms are needed to cause failure on more moderate slopes a regional seismic slope stability analysis of the cadiz margin was performed based on detailed geotechnical testing of four gravity core samples the results showed that the stability of these slopes under seismic loading conditions depends upon sediment density the cyclic loading shear strength the slope steepness and the regional seismicity sediment density and cyclic loading shear strength are dependent upon water content which can act as a proxy for plasticity and texture effects specifically sediment in the water content range of 50 56 is most vulnerable to failure under cyclic loading within the cadiz margin as a result for a uniform seismicity over the region susceptibility to failure during seismic loading conditions increases with increasing slope steepness and is higher if the sediment water content is in the 50 56 range than if it is not the only sampled zone of failure on the continental slope contains sediment with water content in this critical range storm wave induced instability was evaluated for the continental shelf the evaluation showed that a storm having hundreds of waves with a height in the range of 16 m might be capable of causing failure on the shelf however no sediment failures were observed on the shelf that might have been caused by this mechanism

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

characteristics of pockmarks in the norwegian trench a 3 km wide corridor across the norwegian trench was surveyed in detail with high resolution side scan sonar and subbottom profiler boomer in order to study pockmarks the pockmarks in the norwegian trench are dish shaped depressions found in the soft silty clays they range from 2 m to 300 m in diameter and appear with a maximum density of 40 50 per km2 lineated depressions and strings of small pockmarks were discovered the strings are generally composed of individual pockmarks 10 15 m in diameter each string can be up to 400 m long some medium sized pockmarks 40 50 m in diameter were found in groups often associated with a large parental pockmark the deep towed subbottom profiler records indicated columnar disturbances fountains in the sediments below some of the pockmarks these fountains are interpreted as vents or fissures through which gas or liquids migrate buried pockmarks and anomalies interpreted as boulders possibly gas bubbles trapped in the substrata were also found the recordings bear evidence of gas or liquid migration originating from layers lying beneath the soft clay sequence a large hydrocarbon gas reservoir has recently been found during exploration drilling in the immediate vicinity of the studied pockmark area pockmarks may be indicators of deep lying gas or oil reservoirs and therefore prove interesting features in relation to hydrocarbon prospecting

tectonics and mud volcano development in the gulf of cadiz many structures associated with fluid escape including mud volcanoes mud carbonate mounds pockmarks and slides have been identified and characterized in the gulf of cadiz most of the mud volcanoes following ne sw and nw se main trends are found at 350 2000 m depth in the eastern domain of the gulf of cadiz which corresponds to the betic rifean margin scattered mud volcanoes have also been recognized on the lower slope at 2300 3900 m depth the major tectonic structures are thrust faults extensional faults strikeslip faults and diapirs all these tectonic structures have provided escape pathways for overpressured material and fluids or have favoured upward fluid movement along the sedimentary column and eventually the build up of mud volcanoes in this work we present images of the mud volcano plumbing systems and the relationship between regional tectonics and mud volcano development seismic profiles acquired during the tasyo 2000 and mvseis ttr 15 cruises are used to image and interpret the link between the mud volcano edifices and the subsurface tectonic structures

spatial variation in fluid flow and geochemical fluxes across the sediment seawater interface at the carlos ribeiro mud volcano gulf of cadiz submarine mud volcanism is an important pathway for transfer of deep sourced fluids enriched in hydrocarbons and other elements into the ocean numerous mud volcanoes mvs have been discovered along oceanic plate margins and integrated elemental fluxes are potentially significant for oceanic chemical budgets here we present the first detailed study of the spatial variation in fluid and chemical fluxes at the carlos ribeiro mv in the gulf of cadiz to this end we combine analyses of the chemical composition of pore fluids with a 1 d transport reaction model to quantify fluid fluxes and fluxes of boron lithium and methane across the sediment seawater interface the pore fluids are significantly depleted in chloride but enriched in lithium boron and hydrocarbons relative to seawater pore water profiles of sulphate hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34 180 cm depth below seafloor clay mineral dehydration and in particular the transformation of smectite to illite produces pore fluids that are depleted in chloride and potassium profiles of boron lithium and potassium are closely related which suggests that lithium and boron are released from the sediments during this transformation pore fluids are expelled into the water column by advection fluid flow velocities are 4 cm yr 1 at the apex of the mv but they rapidly decrease to 0 4 cm yr 1 at the periphery the associated fluxes of boron lithium and methane vary between 7 301 0 5 6 and 0 806 mmol m 2 yr 1 respectively we demonstrate that fluxes of li and b due to mud volcanism may be important on a global scale however release of methane into the overlying water column is suppressed by microbial methanotrophy

geological and oceanographic controls on seabed fluid escape structures in the northern zhongjiannan basin south china sea abstract mega pockmarks which can reach a few kilometers in width and a few hundred meters in depth and associated fluid escape features in the northern zhongjiannan basin south china sea have received much attention in the recent years due to their large variety of morphologies and sizes in this work an integrated interpretation of detailed multibeam bathymetry conventional multi channel seismic reflection and seismic oceanography sections covering the northern zhongjiannan basin south china sea are used to image the seafloor morphology the subsurface geology and the water column in order to better understand the geological and oceanographic controls on the formation and evolution of the widespread fluid escape features in this region regional uplift and volcanism took place in this area near the palaeogene neogene boundary and during the neogene quaternary post rift thermal subsidence period differential subsidence and diapirism deformed the overlying kilometer scale successions mainly after the miocene focused flow of fluids originated at depth occurred along gas pipes polygonal faults tapered reflectors and faults into the shallow sediments resulting in intense fluid blowouts and complex fluid escape structures on the seafloor our results show that elongated pockmarks pockmark gullies and gullies in the northern slope are generally controlled by underlying buried channels and gullies however pockmark gullies and gullies in the southern slope are controlled by gravity sliding slumping along with crescent pockmarks developing parallel to water depth contours to varying degrees intense fluid escape activities associated with mud diapirism and gas escape in the study area resulted in mud extrusion and the formation of mud volcanoes complex seabed morphologies and irregular pockmarks mega pockmarks collapse structures and blind valleys have formed at the apexes of tectonic uplifts related to volcanism and above basement highs which suggests that these processes played a role in creating pathways for fluid escape bottom currents associated with the south china sea western boundary current imaged here for the first time with the seismic oceanographic method mainly flow to the south and erode the seafloor making pockmark walls and slopes of gullies and channels in the north steeper than those in the south some fluid escape structures such as the mega pockmarks and pockmark gullies are buried by sediment transported by the bottom currents turbidity currents flowing downslope along the long pockmark gullies also erode the seafloor and make the nw slopes of the mud volcanoes less steep than those in the se eddies imaged by the seismic sections may also play a role in transporting sediment to the deep sea in this area some of the reported fluid escape structures are still active as indicated by the fluid pathways that reach the seafloor and plumes in the water column imaged by the seismic sections

mediterranean undercurrent sandy contourites gulf of cadiz spain abstract the pliocene quaternary pattern of contourite deposits on the eastern gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late miocene opening of the strait of gibraltar coincident with the northwestward decrease in undercurrent speeds from the strait there is the following northwestward gradation of sediment facies associations 1 upper slope facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins compared to this the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand further northwestward sediment drift grades to biogenous silt near the faro drift at the portuguese border because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean undercurrent the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies the bottom current deposits of valleys and the contourites of the cadiz slope intervalley areas are distinct from turbidite systems the valley sequences are not aggradational like turbidite channel levee complexes but typically exhibit bedrock walls against ridges extensive scour and fill into adjacent contourites transverse bedform fields and bioclastic lag deposits both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse grained layers low deposition rates and a regional pattern of bedform zones textural variation and compositional gradation the surface sandy contourite layer of 0 2 1 2 m thickness that covers the gulf of cadiz slope has formed during the present holocene high sea level because high sea level results in maximum water depth over the gibraltar sill and full development of the mediterranean undercurrent the late pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea level lowstand and apparent weak mediterranean undercurrent development thus the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late pliocene and quaternary sea level changes and mediterranean water circulation patterns since its pliocene origin the contourite sequence has had low deposition rates of

giant irregular pockmark craters in the palaeogene of the outer moray firth basin uk north sea a cluster of irregular pockmark craters of palaeogene age in block 15 18 in the outer moray firth uk north sea have been mapped on 3d seismic data with well control the craters range from 0 5 to 4 km in diameter between 50 and 200 m in depth and have been buried by tertiary subsidence and sedimentation to present day depths of between 1000 and 1400 m sub sea they are circular to elliptical in plan view and are located in a delta front and upper pro deltaic slope position on the dornoch delta the structures are confined to the top of the lower eocene balder formation and regional mapping across the moray firth basin shows they are restricted to an area of ca 15 15 km2 well data show that the craters are sand filled and have excavated an argillaceous and tuffaceous unit the crater filling sands are sealed by overlying mudstone units planar wing like features emanate from the margins of the craters well control indicates that these seismically identified wings consist of sandstone suggesting that the craters have been modified by an episode of sand remobilisation and injection the proposed mechanism for the formation of these structures is large scale fluid eruption probably shallow gas at the early eocene seafloor these craters are significantly larger in size than those typically observed on contemporary sea floors we argue that the gas is more likely to have been sourced from deeper jurassic or carboniferous sediments within the witch ground graben than from shallow biogenic origins these structures rank in scale alongside the largest known pockmarks and demonstrate that natural fluid and hydrocarbon seeps have a long ancestry in this basin

carbonate mounds in a mud volcano province off north west morocco key to processes and controls abstract this paper presents a new cluster of carbonate mounds discovered in 2002 in the gulf of cadiz off morocco r v belgica 2002 in water depths of 500 to 600 m amidst a field of giant mud volcanoes multibeam bathymetry side scan sonar imagery and 2d seismics are analyzed to present four mound provinces 1 the pen duick mound province on the pen duick escarpment 2 the renard mound province on the renard ridge 3 the vernadsky mound province on the vernadsky ridge and the al idrisi mound province on the gas blanked sediments above the buried al idrisi ridge video imagery and surface samples are described to ground truth the different mound areas the paradox is that nearly no live corals are presently being observed at the surface of the mounds while the mound cores display throughout a high number of reef forming cold water coral fragments scleractinians in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework environmental and oceanographic conditions during the recent past glacials stadials were probably more favourable for cold water coral growth pore water analyses in on mound cores at the south eastern edge of pen duick escarpment give evidence of focused higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments a three phase model for carbonate mound evolution in these settings is proposed 1 in a first stage external controls positive oceanographic and environmental conditions the presence of an active planktonic food chain based on a high primary production and a suitable substrate are responsible for the initiation of cold water coral growth 2 once the cold water corals established an initial framework sedimentation becomes an important factor controlling mound growth the cold water corals baffle the sediments 3 throughout mound growth the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation

the enya mounds a lost mound drift competition the genesis and evolution of cold water coral banks along the northeastern atlantic margin is known to be influenced by several factors among which the palaeotopography and nature of the coral settling surface the presence of bottom currents and sediment supply in this paper a case study is presented of the enya mound cluster located in the southernmost tip of the belgica mound province west of ireland below this mound cluster seismic stratigraphy revealed a yet unmapped local unconformity rd1b being part of a composite erosion event rd1 as such from the late miocene to late pliocene at least two erosional events have incised the margin ending with the final rd1a moundbase unconformity acting as a base for the enya mounds during the quaternary the mounds became outgrown and were covered by a mounded contourite drift in addition they are closely associated with a cluster of seabed pockmarks the bottom current regime which became active since the middle pleistocene has certainly influenced the fate of this mound cluster the occurrence of the pockmarks seems to be related to relatively recent fluid migration processes however leaving an open question if any previous seepage phases were involved in the growth or initiation of the coral banks

active pockmarks in a large lake lake constance germany effects on methane distribution and turnover in the sediment in the eastern part of lake constance the second largest prealpine lake in europe about 500 pockmarks morphological depressions on the lake floor were recently discovered the diameters of these pockmarks are as large as 16 m and about 24 of them continuously release methane in the form of visible bubbles the isotopic composition of the escaping gas indicated that the methane was of biogenic origin and was predominantly produced by the co2 reduction pathway in shallow water pockmarks 9 m and 12 m pore water analysis revealed increased methane concentrations of 1100 mol l 1 in the sediment diffusive methane fluxes and oxidation rates within these shallow pockmarks were much higher than in sediments outside pockmarks an estimated methane ebullition into the water column of about 40 l h 1 per pockmark showed that methane ebullition is by far the dominant pathway of methane release from these shallow pockmarks meiofauna abundance and organic carbon contents of the sediment were also higher in the shallow pockmarks but there were no differences in macrofauna in a deep pockmark 80 m even though bubble release was observed the sediment inside the pockmark had almost the same methane concentrations and isotopic compositions as a site outside of the pockmark at both sites the shallow and deep pockmark act mainly as sedimentation traps the emanating gas has only minor influence on the methane inventory of the sediments and therefore gas bubbles enter the water column essentially unaffected

hydrocarbon plumbing systems above the snohvit gas field structural control and implications for thermogenic methane leakage in the hammerfest basin sw barents sea abstract based on the analysis of the high resolution 3d seismic data from the sw barents sea we study the hydrocarbon plumbing system above the snohvit and albatross gas field to investigate the geo morphological manifestation and the dynamics of leakage from the reservoir fluid and gas escape to the seafloor is manifested in this area as mega pockmarks 1 2 km wide large pockmarks

submarine holocene sedimentary disturbances in the olkiluoto area of the gulf of bothnia baltic sea a case of postglacial palaeoseismicity abstract high resolution echo soundings were used to study the fault activity in the olkiluoto area of the gulf of bothnia southwestern finland echo sounding profiles in holocene submarine sediments show normal faults structures related to gas or groundwater seepage submarine slides and slumps and debris flow type structures the faults and other disturbance structures occur along two bedrock lineaments all faults occur at the same stratigraphical level whose age is estimated at 10 700 calendar years bp 9500 radiocarbon years bp it is suggested that their movement has been triggered by palaeoseismic event s when the late weichselian ice sheet was retreating from the site and bedrock stresses were released through lineaments and fracture zones these are the first observations of postglacial fault movements in quaternary sediments from southern finland but the timing of these movements is contemporaneous with the incidence of postglacial bedrock faulting 9000 10 000 radiocarbon years bp in northern fennoscandia the origin of the gas groundwater seepages remains unclear their reflections in the echo sounding profiles imply that part of the gas is derived from the organic bearing litorina and recent gyttja clays however at least some part of the gas is derived from the bedrock deep gas

lateral relations of deltaic palaeosols from the lower jurassic ronne formation on the island of bornholm denmark abstract the ronne formation which crops out on the southern coast of the danish island of bornholm consists of coal bearing siliciclastic strata a major feature in these sediments is successions of different types of stacked palaeosol profiles beneath coal seams of varying thickness the most obvious and diagnostic properties of these palaeosols are the occurrence of fossil roots and the colour of the horizons penetrated by roots three types of palaeosol profiles represent an association of soils that developed on the ancient tidally influenced lower deltaic plains low relief areas were characterized by hydromorphic conditions due to poor drainage this resulted in the development of waterlogged gleysols in backswamp areas and less acid histosols in closed bays of coastal swamps in the latter case peat bogs were formed in topographically higher areas as levee banks and crevasse splays freely drained podzols developed this soil type was dominated by kaolinite in the clay fraction promoted by acid leaching the morphology and geochemistry of all palaeosols were controlled by the position of the ground water table topography climate and rate of subsidence in the podzol profiles occasional flooding resulted in the formation of spodic b horizons accumulated close to the contemporaneous water table of that time the organic rich top horizons of the podzol and histosols profiles acted as a geochemical barrier and trapped various elements litter decomposition by microorganisms in these soils resulted in a concentration of potassium at the surface thus giving an opportunity to form illite the presence of siderite in the peat bogs suggests that reducing conditions prevailed here at the time of their formation the soils were formed in a humid tropical climate this is indicated by the overall leaching pattern on cations in the podzols and histosols the accumulation of peat in the histosols and the fossil flora

molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the gulf of cadiz ne atlantic investigations of molecular and carbon isotopic variability of hydrocarbon gases from methane through butanes pentanes have been performed on six mud volcanoes from two fluid venting provinces located in the gulf of cadiz ne atlantic the main aims were to define the basic gas types to describe their geochemical characteristics in relationship to their sources and to determine the secondary effects due to migration mixing and microbial alteration hydrocarbon gas data reveal two groups of gases despite the different maturation characteristics both gas groups are allochthonous to the erupted mud breccia and represent a complex of redeposited secondary migrated mixed and microbially altered hydrocarbons it may possibly imply the presence of hydrocarbon accumulations in the deep subsurface of the gulf of cadiz

effects of oceanographic changes on controlling the stability of gas hydrates and the formation of authigenic carbonates at mud volcanoes and seepage sites on the iberian margin of the gulf of cadiz abstract the gulf of cadiz is characterized by extensive active and inactive fluid escape structures which manifest as mud volcanoes diapiric ridges pockmarks and cold seeps the high methane content and the presence of gas hydrates in the shallow sediments of the most active fluid escape structures indicate that these are areas of preferential migration and escape of fluids enriched in hydrocarbons and methane extensive fields of methane derived authigenic carbonates are found at fluid escape structures along the upper and mid continental slope where the mediterranean outflow water is in direct contact with the seafloor at water depths generally 1400 m parts of the basin the estimated u th ages of the authigenic carbonates suggest that they were probably formed during discrete episodes of gas hydrate activity that may have occurred as a result of rapid climatic changes such as the termination of the last glacial stage numerical modelling indicates that seafloor temperature variations associated with glacial interglacial cycles and the changes of the mediterranean outflow pathway could have restricted or eliminated the gas hydrate stability zone at the mud volcanoes shallower than 1400 m resulting in intense fluxes of methane rich fluids to the shallow sediments or even the seafloor this fluid flux was recorded by the microbially mediated precipitation of authigenic carbonates

acoustic evidence of shallow gas accumulations and active pockmarks in the izmir gulf aegean sea abstract based on high resolution chirp seismic multibeam bathymetry and side scan sonar data collected in the izmir gulf aegean sea in 2008 and 2010 gas related structures have been identified which can be classified into three categories 1 shallow gas accumulations and gas chimneys 2 mud diapirs and 3 active and inactive pockmarks on the chirp profiles shallow gas accumulations were observed along the northern coastline of the outer izmir gulf at 3 20 m below the seabed they appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic rich highstand fan sediments from the gediz river the diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter clockwise rotation of anatolian microplate furthermore the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs several pockmarks exist close to fault traces to the east of hekim island most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008 another chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active the surveys indicate that the gas seep is an ongoing process in the gulf based on the chirp data we proposed that the pockmark formation in the area can be explained by protracted seep model whereby sediment erosion and re distribution along pockmark walls result from ongoing or long lasting seepage of fluids over long periods of time the existence of inactive pockmarks in the vicinity however implies that gas seepage may eventually cease or that it is periodic most of the active pockmarks are located over the fault planes likely indicating that the gas seepage is controlled by active faulting

gas induced erosion features in the north sea pockmarks are prominent features on the seafloor at two main regions of the north sea the south fladen area and the norwegian trench the pockmarks vary in size from 5 m across to about 150 m and in depth from 1 m to about 20 m some of the features consist of several very small unit pockmarks these are probably gas induced erosion features and found only in soft finegrained marine and glaciomarine sediments they are probably caused by shallow or deep seated gas or fluid release through the seabed whereby the finest particles are thrown into suspension and redistributed by nearbottom currents on the north sea plateau a small pockmarked area has recently been found a close relationship exists here between mobile gas in the upper sediments faulting in the soft sediments and the morphology of the seabed other morphological features in the north sea such as coast parallel depressions and elongated depressions or terraces are also interpreted here as gas induced erosion products this applies yet again to the mottled seabed features which appear as patches of high reflectivity on side scan sonar records from the north sea plateau they correspond to shallow seabed depressions on the deep towed boomer records judging from reported observations from several shallow seas world wide volatile transport through the seabed is probably quite common and several morphological features are undoubtably induced by this process several erosion features of similar origin are probably common also in deep ocean basins

hydrocarbon derived ferromanganese nodules in carbonate mud mounds from the gulf of cadiz mud breccia sediments and clasts as nucleation sites abstract more than 500 fe mn nodules were sampled during the anastasya 01 cruise tasyo project along the continental margin of the gulf of cadiz eastern central atlantic at the confluence of the mediterranean sea with the atlantic ocean where extensive nodule fields were discovered based on wide previous studies that included swath bathymetry multi channel and very high resolution seismic reflection gravimetry magnetism and underwater photography surveys nodules were collected at water depths ranging from 850 to 1000 m associated with hydrocarbon derived ankerite and dolomite chimneys and crusts thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties morphology colour surface texture sphericity weight and size mineralogy xrd optical and electronic microscopy and geochemistry xrf aas icp ms epma gc ms s c and o isotopes nodules show a wide range of sizes densities weights and morphologies nodules are formed of multiple millimetre thick layers of fe and mn oxyhydroxides surrounding the nucleus composed of early middle miocene plastic marl the textures developed by the fe and mn oxyhydroxides layers are massive laminated detrital and mottled to dendritic goethite lepidocrocite mn oxides 7 a manganates and 10 a manganates quartz and phyllosilicates are the main components accessory minerals are calcite dolomite siderite rhodochrosite kutnohorite pyrite chalcopyrite potassium feldspar zircon rutile ilmenite and chlorite fe mn carbonates from the siderite rhodochrosite continuous series are forming part of the marl nuclei framboidal filamentous and globular textures are observed in fe mn oxides and pyrite suggesting biogenic origin nodules show a high mean abundance of fe 38 6 moderate mn 6 0 and low contents of trace metals and rees compared to the average content of deep seabed polymetallic nodules from other oceanic areas the mn fe ratio ranges from 0 07 to 0 25 the studied nodules hold in their oxide layers hydrocarbons n alkanes derived from marine bacterial activity aromatic hydrocarbons in the form of phenanthrene are also present which is characteristic of mature petroleum the structure mineralogy and chemical composition in the studied nodules are similar to those of diagenetic hydrogenetic continental margin nodules rather than deep sea nodules we suggest that the formation of this type of nodule occurred through a combined diagenetic hydrogenetic growth process involving fluid venting from deep seated hydrocarbon reservoirs bio mineralization processes and bottom current erosion

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

tracing organic compounds in aerobically altered methane derived carbonate pipes gulf of cadiz sw iberia abstract the primary geochemical process at methane seeps is anaerobic oxidation of methane aom performed by methanotrophic archaea and sulfate reducing bacteria srb the molecular fingerprints biomarkers of these chemosynthetic microorganisms can be preserved in carbonates formed through aom however thermal maturity and aerobic degradation can change the original preserved compounds making it difficult to establish the relation between aom and carbonate precipitation here we report a study of amino acid and lipid abundances in carbonate matrices of aerobically altered pipes recovered from the seafloor of the gulf of cadiz sw iberian peninsula this area is characterized by a complex tectonic regime that supports numerous cold seeps studies so far have not determined whether the precipitation of carbonate pipes in the gulf of cadiz is a purely chemical process or whether microbial communities are involved samples from this site show signs of exposure to oxygenated waters and of aerobic alteration such as oxidation of authigenic iron sulfides in addition the degradation index calculated from the relative abundance of preserved amino acids indicates aerobic degradation of organic matter although crocetane was the only lipid identified from methanotrophic archaea the organic compounds detected n alkanes regular isoprenoids and alcohols are compatible with an origin from aom coupled with bacterial sulfate reduction bsr and subsequent aerobic degradation we establish a relation among aom bsr and pipe formation in the gulf of cadiz through three types of analysis 1 stable carbon and oxygen isotopic composition of carbonate minerals 2 carbonate microfabrics and 3 mineralogical composition our results suggest that carbonate pipes may form through a process similar to the precipitation of vast amounts of carbonate pavements often found at cold seeps our approach suggests that some organic compound patterns in combination with additional evidence of aom and bsr may help indicate the source of altered methane derived carbonates commonly occurring in ancient and modern deposits

application of lipid biomarkers to detect sources of organic matter in mud volcano deposits and post eruptional methanotrophic processes in the gulf of cadiz ne atlantic abstract the gulf of cadiz is known as an active mud volcano mv and fluid venting region discovered during the training through research ttr expeditions in 1999 ttr 9 here we report a study of lipid biomarkers in the mud breccia matrix from three mvs in this region the lipid biomarker composition revealed strong compositional resemblance as well as similar thermal maturity properties for the studied mvs this indicates that the primary source of the erupted material for these mvs is located in similar litho stratigraphic units in both areas upward migrated fluid went through the sedimentary series of the allochthonous olistrostrome and through the upper cretaceous horizons the relatively immature characteristics of organic matter from both mud breccia rock clasts and matrix together with the more mature properties of hydrocarbon gases from the same sampling locations indicate that source strata for the initial fluid are located deeper in the subsurface than the source strata for the erupted sedimentary material the presence of microbial lipid biomarkers derived from archaea and sulfate reducing bacteria in the mud breccias revealed anaerobic oxidation of methane aom as a dominant microbial process in these habitats our data suggest that the aom derived lipids are at least in part fossil in origin i e they do not reflect active aom communities and thus reflect aom activity in these mvs integrated over time the low concentrations of aom related biomarkers in these mvs suggest relatively low intensities of aom in the studied mv deposits the distribution of biomarkers suggests an abrupt and brief cold seep activity in the past and the absence of a continuous methane influx in the studied locations

landscape evolution and geodynamic controls in the gulf of cadiz huelva coast sw spain during the late quaternary abstract the coastal evolution of the el abalario area huelva southern spain during the late pleistocene and holocene is reinterpreted after a refinement of the available geochronology by means of optically stimulated luminescence osl dating new data come from the analysis of soft sediment deformation palaeosols geomorphological mapping and published seismic surveys on the onshore and offshore gulf of cadiz the present structure of el abalario dome resulted from the complex interaction of littoral catchment processes and sea level changes upon an emergent coastal plain conditioned by the upwarping of the underlying pliocene pleistocene prograding deltaic sequence upwarping is probably related to escape of over pressurized fluids accompanied by dewatering prior to and during ois oxygen isotopic stage 5 continued upwarping produced the large nw se gravitational fault of torre del loro tlf in the southwestern flank of the dome roughly parallel to the present coastline during ois 5 ois 4 the resulting escarpment favoured the accumulation of aeolian sand dunes units u1 u2 and u3 from ois 5 to early ois 1 unit u1 ois 5 ends upwards in a supersurface with a thick weathering profile that suggests moist and temperate climatic conditions unit u2 accumulated mainly during ois 4 and ois 3 with prevailing w e winds the supersurface between u2 and u3 records a part of ois 2 with relative low sea level sedimentation of unit u3 took place during the last deglaciation radiocarbon and osl ages with prevailing w sw winds under a temperate moist climate that became more arid towards the top holocene a major supersurface with an iron crust like layer ssfe developed during the holocene climatic optimum ois 1 under wetter and more temperate conditions than before fossilizing the tlf the supersurface is covered by younger aeolian dunes u4 u5 u6 and u7 transported by w sw winds since the late neolithic chalcolithic cultural period 5 0 ky cal bp

authigenic barite records of methane seepage at the carlos ribeiro mud volcano gulf of cadiz submarine mud volcanoes mvs are dynamic features that episodically expel gas charged fluids and mud onto the seafloor transferring various chemical constituents into the overlying water column the temporal variability in mv activity is however poorly understood so their importance as a source of methane ch4 and higher hydrocarbons for the oceanic carbon budget although thought to be significant cannot be properly constrained in this study the history of fluid and gas seepage at the carlos ribeiro mv gulf of cadiz is assessed via geochemical analyses and transport reaction modelling of pore fluids and barium ba rich layers ba fronts in sediment cores recovered along a transect from the eye to the periphery of the mv x ray fluorescence data reveal that ba fronts are absent at the eye while a single front with up to 1740 ppm ba is present at the margin of the summit three ba fronts occur at 45 85 and 130 cm depth within a mudflow to the southeast of the crater spectrometric analyses indicate that barite is the ba rich mineral in these layers upward advecting pore fluids are enriched in barium but depleted in calcium ca2 relative to seawater modelling of the ba2 and ca2 pore fluid profiles indicates that the positions of the ba fronts reflect both the present day hydrodynamic conditions as well as higher fluxes of methane in the past fluid advection appears to have decreased since 340 cal yr before present but degassing of the mudflow is ongoing and is potentially an important source of ch4

the imprint of methane seepage on the geochemical record and early diagenetic processes in cold water coral mounds on pen duick escarpment gulf of cadiz abstract the diagenetic history and biogeochemical processes in three cold water coral mounds located in close proximity to each other on pen duick escarpment in the gulf of cadiz were examined the influence of ascending methane rich fluids from underlying sediment strata delineated two mound groups alpha and beta mound showed evidence for the presence of a sulfate methane transition zone smtz at shallow depth whereas gamma mound appeared to lack a shallow smtz in the methane influenced alpha and beta mound upward diffusion of hydrogen sulfide from the shallow smtz caused extensive pyritization of reactive iron phases as indicated by values for the degree of pyritization 0 7 this secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction the almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane aom diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in alpha mound this mound has been affected by strong fluctuations in the depth of the smtz as observed by distinct layers with abundant diagenetic high mg calcite with a 13 c depleted carbon isotope composition in the non methane influenced gamma mound low sulfate reduction rates elevated concentrations of dissolved iron and solid phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling the latter process led to 34 s depletion in pyrite of up to 70 relative to pore water sulfate