detailed morphology of the tag active hydrothermal mound insights into its formation and growth the detailed structure and morphology of the tag active hydrothermal mound mid atlantic ridge are investigated using near bottom high resolution bathymetry data and digital photographic imagery the mound comprises two distinct circular platforms with one superposed asymmetrically on the other areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales years we propose a model for the growth of the tag active mound in which changes in the near surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed

numerical tests of 3d true amplitude zero offset migration an amplitude preserving migration aims at imaging compressional primary zero or non zero offset reflections into 3d time or depth migrated reflections so that the migrated wavefield amplitudes are a measure of angle dependent reflection coefficients the principal objective is the removal of the geometrical spreading factor of the primary reflections various migration inversion algorithms involving weighted diffraction stacks proposed recently are based on born or kirchhoff approximations here a 3d kirchhoff type zero offset migration approach also known as a diffraction stack migration is implemented in the form of a time migration the primary reflections of the wavefield to be imaged are described a priori by the zero order ray approximation the aim of removing the geometrical spreading loss can in the zero offset case be achieved by not applying weights to the data before stacking them this case alone has been implemented in this work application of the method to 3d synthetic zero offset data proves that an amplitude preserving migration can be performed in this way various numerical aspects of the true amplitude zero offset migration are discussed

hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

a new fossil vent biota in the ballynoe barite deposit silvermines ireland evidence for intracratonic sea floor hydrothermal activity about 352 ma considerable controversy exists as to the timing of the important mississippian carbonate hosted irish type zn pb ba ag deposits the silvermines deposits have been defined as an end member of this style in that they have been interpreted to display textures indicative of sea floor deposition one of the strongest arguments in favor of this interpretation was the report of a hydrothermal vent field including pyritic chimneys in the ballynoe open pit barite deposit this paper adds to that body of evidence by describing a hydrothermal vent fauna from the same vent field consisting of a delicately pyritized worm tube hosted by massive pyrite and hematized filaments of apparent microbial origin the worm tube is remarkably similar to fossil worm tubes from modern and ancient volcanic hosted massive sulfide deposits and the filamentous microfossils have similarities to modern fe oxidizing bacteria we have found no correlation between the worm tube and normal mississippian fossils such as crinoids whose replacement by pyrite in the immediately underlying ballynoe footwall destroys original morphology the sulfur isotope composition of the worm tube and host pyrite is essentially identical to that of the vent field pyrite and the main sulfide ore stage of silvermines sulfides all having a mean value about 20 per mil indicating an open system bacteriogenic sulfide source these discoveries provide additional evidence for the exhalative nature of parts of the silvermines orebodies and imply that mineralization had begun in the irish ore field by the late tournaisian 352 ma

filamentous microfossils in a 3 235 million year old volcanogenic massive sulphide deposit the record of archaean microfossils is sparse1 of the few bona fide fossil assemblages most are from shallow water settings and they are typically associated with laminated stromatolitic sedimentary rocks2 3 4 microfossils from deep sea hydrothermal systems have not been reported in precambrian rocks 544 million years old although thermophilic microbes are ubiquitous in modern sea floor hydrothermal settings5 6 and apparently have the most ancient lineages7 8 here i report the discovery of pyritic filaments the probable fossil remains of thread like microorganisms in a 3 235 million year old deep sea volcanogenic massive sulphide deposit from the pilbara craton of australia from their mode of occurrence the micro organisms were probably thermophilic chemotropic prokaryotes which inhabited sub sea floor hydrothermal environments they represent the first fossil evidence for microbial life in a precambrian submarine thermal spring system and extend the known range of submarine hydrothermal biota by more than 2 700 million years9 such environments may have hosted the first living systems on earth consistent with proposals for a thermophilic origin of life10 11 12 13

environmental controls on biomineralization and fe mound formation in a low temperature hydrothermal system at the jan mayen vent fields abstract diffuse low temperature hydrothermal vents on the seafloor host neutrophilic microaerophilic fe oxidizing bacteria that utilize the fe ii supplied by hydrothermal fluids and produce intricate twisted and branching extracellular stalks the growth behavior of fe oxidizing bacteria in strongly opposing gradients of fe ii and o2 have been thoroughly investigated in laboratory settings to assess whether extracellular stalks and aligned biomineralized fabrics may serve as biosignatures of fe oxidizing bacteria and indications of palaeo redox conditions in the rock record however the processes controlling the growth of biogenic fe oxyhydroxide deposits in natural modern hydrothermal systems are still not well constrained in this study we aimed to establish how variations in the texture of stratified hydrothermal fe oxyhydroxide deposits are linked to the physicochemical conditions of the hydrothermal environment we conducted 16s rrna gene analyses microscopy and geochemical analyses of laminated siliceous fe mounds from the jan mayen vent fields at the arctic mid ocean ridge chemical analyses of low and high temperature hydrothermal fluids were performed to characterize the hydrothermal system in which the fe deposits form our results reveal synchronous inter laminar variations in texture and major and trace element geochemistry the fe deposits are composed of alternating porous laminae of mineralized twisted stalks and branching tubes mn rich horizons with abundant detrital sediment domal internal cavities and thin p and ree enriched lamina characterized by networks of 1 m wide fibers zetaproteobacteria constitute one third of the microbial community in the surface layer of actively forming mounds indicating that microbial fe oxidation is contributing to mound accretion we suggest that mn oxide precipitation and detrital sediment accumulation take place during periodically low hydrothermal fluid discharge conditions the elevated concentrations of p and ree in distinct laminae suggest fe cycling and accumulation of diagenetic species at depth in the deposits during hydrothermal quiescence and co precipitation of these species with fe oxyhydroxides at the mound surface with reinitiated hydrothermal discharge the origin of the low temperature hydrothermal source fluid and the fe deposits are evident by low lree hree ratios and negative eu anomalies which clearly differ from the lree and eu enrichment of nearby high temperature white smoker venting fluids our study demonstrates that hydrothermal fluctuations exert the primary control on the formation of laminae and the activity of fe oxidizing bacteria in marine hydrothermal fe deposits and indicates that ree patterns may be used to distinguish high temperature plume fallout and biomineralized low temperature fe deposits in the rock record

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

relict hydrothermal zones in the tag hydrothermal field mid atlantic ridge 26 n 45 w two relict hydrothermal zones were delineated between water depths of 3400 and 3500 m at the lower part of the east wall of the rift valley of the mid atlantic ridge in the tag hydrothermal field using a deep towed side scan sonar tow and a camera temperature tow along the northern 3 km of the wall and a submersible transect named the north and mir relict zones they are located about 4 km and 2 km northeast respectively of the known active high temperature sulfide mound between water depths of 3625 and 3670 m on the rift valley floor near the base of the east wall the north zone extends about 2 km along the northern end of the lower east wall the zone includes two moundlike features up to 30 m high by 200 m in diameter imaged by side scan sonar within a 2 km long line of discontinuous hydrothermal deposits comprising inactive toppled and standing chimneys layered material and patchy dark stains on sediment photographed by the camera temperature tow several other moundlike features were imaged with the side scan sonar outside of the photographic coverage the mir relict hydrothermal zone 2 km south of the north zone named after the mir submersible used to investigate it consists of three subzones 1 a 200 m wide area of diverse types of hydrothermal materials exposed by normal faulting at its western margin 2 a 400 m wide by 700 m long central area of discrete groups of toppled and standing inactive sulfide chimneys up to 25 m high on a substrate of red metalliferous sediment and carbonate lutite spires sampled on the highest chimneys are composed of coarse grained recrystallized sulfides dominated by pyrite and chalcopyrite which contain the first primary free gold grains 2 3 m diameter found at a hydrothermal site on a mid ocean ridge and 3 a 150 m wide hummocky area of layered hydrothermal material with the appearance of low temperature precipitates and carbonate lutite with patchy dark stains at its eastern margin the active sulfide mound the north zone and the mir zone are each located on the fractured western margins of gentle dome shaped areas of pillow flows typically 500 m in diameter interpreted as summits of volcanic centers that may have supplied heat to drive adjacent hydrothermal activity the distribution and size of the active and inactive hydrothermal zones of the tag field the chronology and the characteristics of relict samples recovered indicate a history of at least 100 103 years of high temperature hydrothermal episodes with multiple overprinting stages of mineralization accompanied by alteration

geological fate of seafloor massive sulphides at the tag hydrothermal field mid atlantic ridge deep sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply increasing demand for these metals many of which are

testate amoebae record from the laptev sea coast and its implication for the reconstruction of late pleistocene and holocene environments in the arctic siberia abstract our study aims to look for testate amobae ta in the surface and fossil sediments from the cape mamontov klyk 73 60 73 63 n 116 88 117 18 e southern coast of the laptev sea ca 150 km west of the lena delta and to discuss the potential of ta analysis for glacial interglacial environmental reconstructions in arctic siberia the radiocarbon age determination suggests that the studied sediments accumulated between ca 45 000 14c yr bp and present a total of 98 ta taxa were identified in the 10 recent surface and 59 fossil samples results of taxonomical identification and ecological analysis of ta in the modern and fossil samples suggest that major changes in the soil moisture conditions took place our results show that soil living taxa dominated the ta assemblages at the study sites during the past 45 000 years the environmental conditions of the study area were most favourable relatively warm and humid during the kargin interstadial ca 45 000 25 000 14c yr bp an opposite situation is reconstructed for the sartan stadial ca 25 000 15 000 14c yr bp during the kargin interstadial optimum conditions occurred between ca 44 000 and 40 000 14c yr bp characterised by highest ta abundances and taxa diversity this initial optimal phase was followed by the interval with drier and colder conditions about 40 000 30 000 14c yr bp the sediments dated between ca 24 000 and 18 000 14c yr bp show low ta abundances and diversity in agreement with the much colder and drier environments during the maximum phase of the last glacial the onset of the holocene is indicated by a broad representation of obligate hydrophilic taxa especially from genus difflugia suggesting wet and relatively warm conditions by comparison with other environmental proxies used in the studied sections as well as from the neighbouring arctic regions our results suggest that ta analysis can provide valuable information contributing to the better understanding of the late quaternary climate and environments in arctic siberia

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

volcanic degassing hydrothermal circulation and the flourishing of early life on earth a review of the evidence from c 3490 3240 ma rocks of the pilbara supergroup pilbara craton western australia abstract new data gathered during mapping of c 3490 3240 ma rocks of the pilbara supergroup in the pilbara craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism for most cherts fossil hydrothermal fluid pathways are preserved as silica barite fe bearing veins that cut through the footwall and up to the level of individual bedded chert units but not above indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events silica barite fe bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units and occurred under alternating high sulphidation and low sulphidation conditions these veins provided pathways to the surface for elements leached from the footwall e g si ba fe and volcanogenic emissions from underlying felsic magma chambers e g co 2 h 2 s hs so 2 stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of warrawoona group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and or degassing of these magmas heat from these intrusions drove episodes of hydrothermal circulation listric normal faulting during caldera collapse produced basins with restricted circulation of seawater eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components thereby providing habitats suitable for early life forms fossil stromatolites from two distinct stratigraphic units in the north pole dome grew in shallow water conditions but in two very different geological settings with different morphologies stratiform and domical stromatolites in the stratigraphically lower c 3490 ma dresser formation of the warrawoona group are intimately associated with barite and chert precipitates from hydrothermal vents suggesting that component microbes may have been chemoautotrophic hyperthermophiles evidence of shallow water to periodically exposed conditions active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water tectonically active caldera and concentrated therein to produce an extreme habitat for early life widespread conical and pseudocolumnar stromatolites in the c 3400 ma strelley pool chert at the base of the unconformably overlying kelly group occur in shallow marine platform carbonates silicification was the result of later hydrothermal circulation driven by heat from the overlying newly erupted euro basalt the markedly different morphology and geological setting of these only slightly younger stromatolites compared with the dresser formation suggests a diversity of microbial life on early earth the biogenicity of putative microfossils from this and younger hydrothermal silica veins in the warrawoona group remains controversial and requires further detailed study

multiproxy approach for holocene paleoenvironmental reconstructions from microorganisms testate amoebae and foraminifera and sediment analyses the infilling of the loire valley in nantes france foraminifera and testate amoebae are jointly used with sediment characteristics sediment size calcium carbonate c h n and s proportions and clay mineralogy to reconstruct the holocene sediment infilling of the loire estuary nantes france the results reveal the infilling of the loire valley during the holocene deposited on the hercynian substratum the first deposits are made of gravels with rare associated fauna this corresponds to the gravel lag usually present as basal deposits in the river course the clay mineralogy indicates an erosion of the substratum then from 8850 to 5850 cal yr bp estuarine fauna settled in a laminated sediment that alternates between clay and sand the smectite associated with kaolinite and illite suggests increased input from the sevre nantaise river the fauna progressively shifts from estuarine to marine indicating a rise in the sea level from 5850 cal yr bp the pace of the sea level rise slowed and channels migrated implying intense erosion after 2100 cal yr bp the fauna was dominated by testate amoebae indicating a continentalization of the environment the top samples reflect recent human activity and urbanization

where are the undiscovered hydrothermal vents on oceanic spreading ridges abstract in nearly four decades since the discovery of deep sea vents one third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits as well as by academic research here we incorporate recent data for back arc spreading centers and ultraslow and slow spreading mid ocean ridges mors to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate we apply this equation globally to predict a total number of vent fields on spreading ridges which suggests that 900 vent fields remain to be discovered almost half of these undiscovered vent fields comparable to the total of all vent fields discovered during 35 years of research are likely to occur at mors with full spreading rates less than 60 mm yr we then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction with the majority expected to occur outside of states exclusive economic zones we hope that these predictions will prove useful to the community in the future in helping to shape continuing ridge crest exploration

a seafloor gravity profile across the tag hydrothermal mound a series of on bottom gravity stations were completed across the tag hydrothermal mound during a dive program with the japanese shinkai 6500 submersible the mound appears as a gravity high 2 mgal with respect to the surrounding seafloor reflecting the high density sulfides of which it is comprised three dimensional modeling of the data shows no evidence of a dense sulfide root extending deep into the seafloor below the mound which would be associated with a broad highly altered stockwork zone sulfide thickness appears to be greatest 50m to the north of the mound the thickness decreases sharply to the south reaching a steady value of about 10m just to the south of the kremlin area the mass of sulfides within the mound is estimated at 3 million tonnes the surrounding basaltic seafloor appears to have a density of 2400 100 kgm 3 indicating a surficial porosity of between 20 30

early jurassic hydrothermal vent community from the franciscan complex san rafael mountains california the figueroa massive sulfide deposit located in franciscan complex rocks in the san rafael mountains of california preserves the only known jurassic hydrothermal vent fossils the figueroa fossil assemblage is specimen rich but of low diversity and comprises in order of decreasing abundance vestimentiferan worm tubes the rhynchonellid brachiopod anarhynchia cf gabbi and a species of nododelphinulid gastropod the figueroa fossil organisms lived at a deep water high temperature vent site located on a mid ocean ridge or seamount at an equatorial latitude the fossil vent site was then translated northwestward by the motion of the farallon plate and was subsequently accreted to its present location an iron silica exhalite bed the probable lateral equivalent of the figueroa deposit contains abundant filamentous microfossils with two distinct morphologies and probably represents a lower temperature diffuse flow environment the figueroa fossil community was subject to the same environmental conditions as modern vent communities but it is unique among modern and other fossil vent communities in having rhynchonellid brachiopods

cyanobacteria and testate amoeba as potential proxies for holocene hydrological changes and climate variability evidence from tropical coastal lowlands of sw india cyanobacteria and thecamoebians are the two major constituents of non pollen palynomorphs npp found in the palynological preparations the former is well represented in the sediment archives while the latter has restricted occurrence in the holocene sequence apart from the conventional organic matter of pollen and spores well preserved morphotypes of gleotrichia and rivularia are the best forms of cyanobacteria found in almost all the studied profiles and their abundance towards early to middle holocene coincides with the heavy rainfall and wet climate conditions during the holocene climate optimum hco contrary to cyanobacterial diversity thecamoebians are restricted to late holocene in most of the studied profiles and their occurrence is correlated with the prevailing dry and stressful environmental conditions the major elements of thecamoebians identified belong to arcellaceans centropyxids and cucurbitella types abundance of centropyxids and arcellaceans in varapuzha west kadungallur koonamavu pachcha and ernakulam indicates low humidity and low precipitation however the relative abundance of centropyxids in mid holocene at ernakulam and koonamavu reflects the early colonization of this type competing with the cyanobacteria whereas arcellaceans are essentially confined to late holocene dominance of cyanobacteria and its proliferation is directly related to freshwater influx of the rainfall as well as the local hydrological regimes associated with the active aquifers controlled by the geomorphological features of the region in which the wetland system sustained considering the signatures of cyanobacteria and thecamoebians observed in the sediment archives their occurrence in the wetlands during the holocene corresponds to the hydrodynamic regimes brought in by the freshwater influx as a result of monsoon variability the well preserved cyanobacterial signatures in sediment archives of kerala coast and their scarcity or even absence elsewhere along the west coast of india indicates that they are exclusively adapted to the wetland habitats where the hydrodynamic regimes are regularly controlled by fresh water influx in the form of extended rainy days of the monsoon system

a near bottom magnetic survey of the mid atlantic ridge axis at 26 n implications for the tectonic evolution of the tag segment 1 an extensive deep tow magnetic survey of the tag ridge segment on the mid atlantic ridge reveals new information about the relationship between the magnetic anomaly field and the tag hydrothermal deposits results show the strongest magnetization is located over the neovolcanic axis and asymmetrically toward the western side of the central brunhes anomaly a well defined linear magnetization low is located over the eastern rift valley wall of the tag segment the near bottom data show no direct correlation between this crustal magnetization low and the hydrothermal deposits the magnetization low is explained by crustal thinning caused by 4 km of horizontal extension along a normal fault previous observations and sampling indicate exposures of gabbros and dikes in the eastern rift valley wall suggesting slip along a normal fault has revealed this crust modeling suggests the fault has been active since 0 35 0 1 ma at a horizontal slip rate of roughly half the spreading rate of 22 km myr the tag hydrothermal system is located on the hanging wall of this fault within 3 km of its termination over the past several hundred thousand years movement on the detachment fault may have episodically increased the permeability of the hanging wall reactivating the overlying hydrothermal systems significant vents like tag may be typically associated with hanging walls of long term detachment faults near seafloor spreading centers this would imply that it is the reactivation of permeability in the hanging wall related to repeated fault movement that controls the longevity of these hydrothermal systems

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

crs stack based seismic imaging considering top surface topography in this case study we consider the seismic processing of a challenging land data set from the arabian peninsula it suffers from rough top surface topography a strongly varying weathering layer and complex near surface geology we aim at establishing a new seismic imaging workflow well suited to these specific problems of land data processing this workflow is based on the common reflection surface stack for topography a generalized high density velocity analysis and stacking process it is applied in a non interactive manner and provides an entire set of physically interpretable stacking parameters that include and complement the conventional stacking velocity the implementation introduced combines two different approaches to topography handling to minimize the computational effort after initial values of the stacking parameters are determined for a smoothly curved floating datum using conventional elevation statics the final stack and also the related residual static correction are applied to the original prestack data considering the true source and receiver elevations without the assumption of nearly vertical rays finally we extrapolate all results to a chosen planar reference level using the stacking parameters this redatuming procedure removes the influence of the rough measurement surface and provides standardized input for interpretation tomographic velocity model determination and post stack depth migration the methodology of the residual static correction employed and the details of its application to this data example are discussed in a separate paper in this issue in view of the complex near surface conditions the imaging workflow that is conducted i e stack residual static correction redatuming tomographic inversion prestack and post stack depth migration leads to a significant improvement in resolution signal to noise ratio and reflector continuity

reduced crustal magnetization beneath relict hydrothermal mounds tag hydrothermal field mid atlantic ridge 26 n submersible magnetic profiles over the mir and alvin relict hydrothermal zones of the tag hydrothermal field on the mid atlantic ridge at 26 n show magnetic anomalies that are consistent with the presence of reduced crustal magnetization these magnetic anomalies are similar to the anomaly found associated with the tag active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems

structural control evolution and accumulation rates of massive sulfides in the tag hydrothermal field the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge is one of the best studied hydrothermal systems to date however high resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle auv reveal new information about the distribution of active and inactive hydrothermal deposits and their relation to structural features the discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow spreading ridges the interpretation of ship based and high resolution auv based data sets allowed for the determination of the main tectonic stress regimes that have a first order control on the location and distribution of past and present hydrothermal activity the data reveal the importance of cross cutting lineament populations and temporal variations in the prevalent stress regime a dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material 29 mt the accumulation rate of 1 500 t yr is comparable to those of other modern seafloor vent fields however our observations suggest that the tag segment is different from many other slow spreading ridge segments in its tectonic complexity which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation the inactive and weakly active mounds contain almost 10 times the amount of material as the active high temperature mound providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

diffraction imaging by multifocusing correct identification of geologic discontinuities such as faults pinch outs and small size scattering objects is a primary challenge of the seismic method seismic response from these objects is encoded in diffractions our method images local heterogeneities of the subsurface using diffracted seismic events the method is based on coherent summation of diffracted waves arising in media that include interface discontinuities and local velocity heterogeneities this is done using a correlation procedure that coherently focuses diffraction energy on a seismic section by flattening diffraction events using a new local time correction formula to parameterize diffraction traveltime curves this time correction which is based on the multifocusing method depends on two parameters the emergent angle and the radius of curvature of the diffracted wavefront these parameters are estimated directly from prestack seismic traces the diffraction multifocusing stack dmfs can separate diffracted and reflected energy on a stacked section by focusing diffractions to the diffraction location and defocusing the reflection energy over a large area

response of testate amoeba assemblages to environmental and climatic changes during the lateglacial holocene transition at lake lautrey jura mountains eastern france we tested the response of lacustrine testate amoebae thecamoebians to climate and environmental changes for the lateglacial holocene transition the palaeoenvironmental history of the study site lake lautrey jura mountains eastern france was previously established based on high resolution multi proxy studies of the same core the present study is characterised by a high taxonomic resolution 54 taxa inclusion of small species clown to 25 mu m and high total counts 500 individuals per sample on average changes in the composition of testate amoeba assemblages dominant species and assemblage structure as well as in the accumulation rate tests cm 2 a 1 corresponded to major climatic phases i e oldest dryas bolling allerod interstadial younger dryas preboreal as well as changes in organic matter inputs furthermore decreases in the accumulation rate characterised minor short lived cooling events such as older dryas event or gerzensee oscillation however the preboreal oscillation which was well registered by other proxies at lake lautrey could not be recognised in the testate amoeba record this work demonstrates that lacustrine testate amoebae can be used for palaeoclimatic and palaeoecological reconstructions nevertheless a better understanding of the relation between climate organic matter and lacustrine testate amoebae requires further high resolution studies based on multi proxy approaches and the development of appropriate modern analogues copyright c 2010 john wiley sons ltd

new age data for mid atlantic ridge hydrothermal sites tag and snakepit chronology revisited the chronologies of tag and snakepit hydrothermal fields have been established using 210pb pb 230th 234u and 14c dating at the tag field a mn oxide record indicative of low temperature events began at least 125 000 years and possibly 140 000 years ago with maximum intensities at 15 000 7000 and 4000 years before present high temperature events giving rise to sulfide deposits began about 100 000 years ago and have been intermittent to the present day a presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20 000 years decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000 7000 and 9000 years ago after a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago the snakepit field is much younger and no sulfides older than 4000 years have been recovered relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at tag reactivation of snakepit took place about 80 years ago and is presently concurrent with that of tag comparison with hydrothermal sites on the east pacific rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits

multi proxy lake sediment record of prehistoric paleoindian archaic archaeological paleoenvironments at rice lake ontario canada abstract the arrival of paleoindian peoples in the lower great lakes ca 11 5 ka cal bp coincided with a phase of lowstand lakes and rapid environmental change in southern ontario at rice lake an archaeologically rich area north of lake ontario rising early holocene water levels inundated wetland areas exploited by paleoindian and archaic peoples in order to better understand the distribution of submerged archaeological sites and the water level history of rice lake a detailed multi proxy study was conducted on two vibracores extracted from the open lake and a shallow the basin stratigraphy consisted of holocene organic rich muds gyttja laminated marls and peats overlying glacial lake iroquois sands and clay deposits two erosional hiatuses indicating lowstand phases eh 1 ca 12 5 11 ka cal bp eh 2 ca 6 3 ka cal bp were identified by rapid shifts in particle size and thecamoebian abundance and assemblages the dominant thecamoebian species in the open lake basin were arcella vulgaris and centropyxis constricta aerophila indicating the trophic status of the lake has remained stable throughout the holocene in the mcintyre lagoon the eh 1 lowstand was associated with a unique bog and wetland species bullinularia and phryganella sp along with the recovery of microdebitage indicating that paleoindian peoples were likely exploiting wetland resources such as plants fish and waterfowl we show that the study of lacustrine paleoenvironments using a multi proxy approach provides additional information for identifying areas of archaeological potential and gaining additional understanding of settlement patterns and resource procurement strategies of paleoindian and archaic peoples in southern ontario

imaging improvement by prestack signal enhancement the quality of recorded seismic data depends on many factors and a low signal to noise ratio leads to a low quality of processing and imaging the zero offset common reflection surface stack and multifocusing methods have been successfully applied to improve the prestack signal to noise ratio by the partial summation of coherent seismic events however in the case of non hyperbolic traveltime behaviour of seismic events these approaches can result in non optimal partial summation we develop a local common offset approximation for the traveltime stacking surface it allows us to adequately approximate the traveltimes of reflection events in the vicinity of an arbitrary offset here the stacking operator is interpreted as a local second order traveltime approximation of the seismic event and it is used for the purpose of partial summation an algorithm and a numerical implementation scheme are discussed the proposed signal enhancement procedure was applied to synthetic and real 3d data imaging results of the enhanced data show a high potential for reliable imaging in complex subsurface environments

a review of current testate rhizopod thecamoebian research in canada thecamoebians are a diverse group of testate rhizopods present in a variety of lacustrine and terrestrial habitats among this group certain agglutinating forms primarily arcellacea have tests that are highly resistant to decay and thus fossilize well although the canadian fossil record of thecamoebians extends back as far as the carboniferous the group is particularly common in quaternary lacustrine and peatland environments from temperate to arctic regions of the country research on thecamoebians during the past few years has resulted in great progress on realizing the potential of this group as an important new class of paleoenvironmental indicator applied research on thecamoebian faunas in canada has emphasized aspects of paleolimnology e g eutrophication ph temperature oxygen levels monitoring of land use changes monitoring effectiveness of remediation efforts in contaminated substrates paleo sea level reconstruction paleo water table assessment and paleoclimatic reconstruction 2002 elsevier science b v all rights reserved

inversion of traveltime data under a statistical model for seismic velocities and layer interfaces we invert large aperture seismic reflection and refraction data from a geologically complex area on the northeast atlantic margin to jointly estimate seismic velocities and depths of major interfaces our approach combines this geophysical data information with prior information on seismic compressional velocities and the structural interpretation of seismic sections we constrain expected seismic velocities in the prior model using information from well logs from a nearby area the layered structure and prior positions of the interfaces follow information from the seismic section obtained by processing the short offsets instead of using a conventional regularization technique to smooth the interface velocity model we describe the spatial correlation of interfaces and velocities with a geostatistical model using a multivariate gaussian probability density function we impose a covariance function on the velocity field in each layer and on each interface in the model to control the smoothness of the solution the inversion is performed by minimizing an objective function with two terms one term measuring traveltime residuals and the other measuring the fit to the statistical model we calculate the posterior uncertainties and evaluate the relative influence of data and the prior model on estimated interface depths and seismic velocities the method results in the estimation of velocity and interface geometry beneath a basaltic sill system down to 7 km depth this method aims to enhance the interpretation process by combining multidisciplinary information in a quantitative model based approach

are undiscovered hydrothermal vents in the southern red sea the main source of silica and helium 3 for the red sea deep water geochemical ocean sections study geosecs indian ocean expedition data show that silica and 3he in the red sea deep water are linearly correlated suggesting a common hydrothermal source silica sections for the red sea from the geosecs and mer rouge merou expeditions show a strong vertical silica maximum centered at 500 to 600 m depth the merou sections which have higher horizontal resolution than the geosecs sections suggest that the primary silica source lies in the south at that depth the interpretation is that the large scale silica and 3he distributions are due to the dispersal in the red sea deep water of warm or hot hydrothermal vent waters associated with the seafloor spreading center of the southern red sea the helium and silica rich hot brines of the central red sea probably cannot provide the large fluxes of silica or 3he required to maintain the inventory of either of these deep water properties in steady state the hypothesis of a common hydrothermal source for silica and 3he can most readily be tested by measurements of the germanium silicon ratio in red sea deep water

structural asymmetry of the tag rift valley evidence from a near bottom survey for episodic spreading comprehensive high resolution sidescan sonar imagery swath bathymetry and photographic data from the tag rift valley mid atlantic ridge 26 n suggest significant asymmetry in crustal accretion discontinuities and asymmetry in structural and volcanic morphology and age distribution about the spreading axis suggest a recent episode of preferential accretion of crust to the east throughout the central 10 km of this spreading segment such episodes may contribute to the long term spreading asymmetry in the area

microbial involvement in the formation of cambrian sea floor silica iron oxide deposits australia the cambrian ordovician mount windsor volcanic belt in northern australia is host to stratiform lenses of massive ferruginous chert that are spatially associated with volcanogenic massive sulfide occurrences in particular the thalanga zinc lead copper silver deposit the rocks are composed principally of fe2o3 and sio2 with very low concentrations of alkalic elements and lithogenous elements such as al zr and ti they are interpreted as nearly pure chemical sediments textural evidence is documented of the integral role of filamentous bacteria and or fungi in depositing iron from hydrothermal fluids and of the inorganic precipitation of silica iron oxyhydroxide gels that subsequently matured to subcrystalline and crystalline silica forms at least three distinct iron accumulating microbial forms are distinguished networks of septate filaments nonseptate filament networks and extremely coarse branching filaments that do not reconnect values for 34s in disseminated pyrite are up to 50 lighter than those of contemporaneous cambrian seawater suggesting postdepositional colonization of some ironstones by sulfur reducing bacteria the site not only preserves the textural interplay of biological and inorganic depositional processes in exhalites but also extends the oldest known instance of microbial mediation in vent proximal hydrothermal iron precipitation to at least 500 ma

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

fresnel zone a pitfall in seismic imaging of mid ocean ridge magma lenses we present estimates of the fresnel zone diameter for the axial magma chamber lens along the southern and northern east pacific rise and compare them to the magma lens widths derived from migrated seismic sections the mean values of that diameter are 996m and 1052m and the mean values of the magma lens width are 680m and 1056m respectively most individual estimates of the magma lens width fall below the fresnel zone diameter similar results are presented for three other mid ocean ridges the registered signal is therefore a mix of responses from the magma sill and the encasing rock and lateral resolution is not achieved we conclude that data analysis procedures that assume a laterally homogeneous earth cannot provide reliable estimates of the magma lens physical properties including whether it is liquid or not

chemosynthetic microbialites in the devonian carbonate mounds of hamar laghdad anti atlas morocco abstract veins cutting across the devonian conical mounds of the hamar laghdad ridge eastern anti atlas morocco were the carriers of geofluids although not completely determined the composition of these fluids seems related to hydrothermal and seepage fluxes that were the primary contributors to the establishment of sub seafloor cryptic environments linking deeper fluid sources to the seafloor the optical and scanning electron microscope investigation of the laminated carbonate infill of these veins has revealed that they consist of stromatolite and oncolite like fabrics in the early and middle devonian mounds the laminated carbonates contain morphologies that are attributable to microbially mediated processes that in turn suggest a direct biological contribution to the deposition of the veins infill microbial evidence include rod shaped and cocci clusters embedded in amorphous membranes iron rich filaments organized as mat like and biodictyon filamentous aggregates or isolated filaments microbial clotted textures and peloids moreover the oncoids from the veins of the middle devonian hollard mound have been interpreted as the mineral replacement of crypto microorganisms as frutexites other microbial evidence are biominerals and include carbonate with spheroids and pyrite framboids in which typifying hydrocarbon seep environments the hollard mound which is the only one with a well documented origin by hydrocarbon seepage shares a number of geomicrobiological evidence with other early devonian mounds these evidence which are supplemented by 13 c depleted data suggest that the infill of the veins system in the hamar laghdad mounds would be the product of chemotrophic chemosynthetic microbial communities that are adapted to crypto habitats sustained by hydrocarbon and other components fluid fluxes

what is a seismic reflector like the spatial region that is in the vicinity of an interface and actually affects the interface response and hence the reflected wavefield is of particular interest for the characterization of reflectors from a seismic viewpoint this region is represented by a volume of integration of medium properties above and below the interface whose maximum lateral extent corresponds to the lateral extent of the interface fresnel zone and whose maximum vertical extent is equal to a thickness we evaluate approximately for subcritical incidence angles for a plane interface as well as for curved interfaces of anticline and syncline type the maximum vertical extent might be larger than the seismic wavelengths for subcritical incidence angles close to the critical angle and for a strong impedance contrast at the interface although the part of the reflector volume lying below the interface and affecting traveltime measurements actually is smaller than described in previous studies the whole part of the reflector volume that affects the amplitude of the reflected wavefield is larger than estimates in previous studies which considered only the spatial region below the interface for a syncline respectively an anticline it is larger respectively smaller than described for a plane interface in addition to providing more physical insights into the wave reflection process this study might have significant implications for seismic interpretation using amplitude variation with angle methodologies

marine mineral exploration with controlled source electromagnetics at the tag hydrothermal field 26 n mid atlantic ridge seafloor massive sulphide sms deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand the trans atlantic geotraverse tag hydrothermal field at 26 n on the mid atlantic ridge hosts several such deposits this study presents new controlled source electromagnetic csem bathymetric and magnetic results from the tag field potential sms targets were selected based on their surface expressions in high resolution bathymetric data high resolution reduced to the pole magnetic data show negative anomalies beneath and surrounding the sms deposits revealing large areas of hydrothermal alteration csem data sensitive to the electrical conductivity of sms mineralization further reveal a maximum thickness of up to 80 m and conductivities of up to 5 s m sms samples have conductivities of up to a few thousand s m suggesting that remotely inferred conductivities represent an average of metal sulphide ores combined with silicified and altered host basalt that likely dominates at greater depths

the generation at hot springs of sedimentary ore deposits microbialites and life abstract notwithstanding the current fashion which favours an epigenetic origin for what used to be termed sedex deposits there are several lines of evidence to indicate that phanerozoic base metal orebodies of this type have at least some exhalative aspects the fossil polychaete worms which occur in lower carboniferous pyrite mounds at tynagh and silvermines in ireland have affinities to paralvinella an organism that lives attached to hydrothermal chimneys at the juan de fuca hot spring site in the northeast pacific in addition fossil tube worms and their moulds occur both in silica masses underlying the carboniferous giant red dog sulphide orebody in alaska and in devonian barite and base metal deposits in north america and in russia respectively the development of sulphide and carbonate fossil microbialites over exhalative centres further supports generation of some mineral deposits on sea or lake floors carbonate microbialite mounds are also developing today over warm springs and seepages the existence of an environment in which sulphide mineralisation developed at the sea floor has implications also in a different sphere life itself may have emerged in a similar milieu at 4 2 ga from iron monosulphide bubbles a primitive metabolism could have been driven by the high long lived and constant redox potential of 300 mv made available across an iron monosulphide membrane which would have been spontaneously generated where sulphide bearing submarine alkaline springs issued into the acidic iron bearing hadean ocean the alkaline spring provided bisulphide to the iron rich carbonic acid ocean for the precipitation of iron monosulphide bubbles probotryoids as well as acetate shock 1992 the feeder to the biochemical krebs cycle driven in reverse by the high partial pressure of co2 in addition to its scientific significance an understanding of these beginnings may well benefit research into many aspects of economic geology even more extreme redox contrasts are revealed by the presence of sedimentary jasper or iron formation in three of the major carboniferous sulphide orebodies in ireland pyritic sulphide microbialites also grew over some of the associated fossil hot spring sites and may be recognised by their bacteriogenic 34s values 20 to 40 recognition of such fossil hot spring sites could lead to further discoveries of sedex deposits

lacustrine thecamoebians mainly arcellaceans as potential tools for palaeolimnological interpretations abstract a number of thecamoebian genera of potential or demonstrated importance for palaeolimnological research mainly arcellacea are discussed what is known about their biology ecology spatial distribution mechanisms of dispersal and characteristic associations is briefly reviewed in the introductory part after a short discussion on the concept of species and the somewhat peculiar way in which it applies to this group of organisms a chapter on systematic taxonomy including an illustrated guide to the identification of the genera is presented the scarce fossil record of the group is then briefly discussed practical instructions on collection preparation and observation of thecamoebians for palaeolimnological research are supplied the paper closes with the presentation of a few studies performed by the authors as examples of how arcellacea can become tools for the solution of palaeolimnological problems

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

determination of fresnel zones from traveltime measurements for a horizontally stratified isotropic earth the rms velocity of a primary reflection is a key parameter for common midpoint cmp stacking interval velocity computation by the dix formula and true amplitude processing geometrical spreading compensation as shown here it is also a very desirable parameter to determine the fresnel zone on the reflector from which the primary zero offset reflection results hence the rms velocity can contribute to evaluating the resolution of the primary reflection the situation that applies to a horizontally stratified earth model can be generalized to three dimensional 3 d layered laterally inhomogeneous media the theory by which fresnel zones for zero offset primary reflections can then be determined purely from a traveltime analysis without knowing the overburden above the considered reflector is presented the concept of a projected fresnel zone is introduced and a simple method of its construction for zero offset primary reflections is described the pr

extreme 18o enriched volcanics and 18o evolved marine water aljustrel iberian pyrite belt transition from high to low rayleigh number convective regimes abstract quartz eye keratophyric tuffs at aljustrel s portugal iberian pyrite belt possess unusually high uniform whole rock values up to 18 1 with a mean of 16 7 0 7 because the quartz eye cores have 18o 12 0 to 13 5 the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11 the deduced enrichments of up to 8 in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to 100 c during thermally driven convective cooling the isotopic uniformity requires elevated permeability of 10 8 cm2 such that the second critical rayleigh number was exceeded and drifting non steady state convection cells dominated with relatively smooth thermal structures in mineralised counterparts of the tuffs beneath the feitais estacao zn pb cu massive sulphide orebodies whole rock values are 12 and quartz 13 3 15 4 chlorite 3 2 6 3 fractionations correspond to temperatures of 220 270 c and a calculated fluid 18o of 1 4 to 5 7 this local isotopic overprinting in vent domains of the regionally high 18o was induced by a stable second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts and or progressive spilitisation of the tuffs at lower water rock coupled with higher temperatures the recirculating marine water underwent variable 18o enrichment up to 5 7 by exchange with the high 18o tuffs an upwards increase in 18o quartz and quartz reflects progressive cooling of the geothermal discharge from 15 4 270 c in the stockwork through 18 3 220 240 c in the orebody to 20 1 110 130 c in overlying hydrothermal cherts probably induced by entrainment of ambient marine bottom water in sub seafloor aquifers

giant pyritic base metal deposits the example of feitais aljustrel portugal detailed study of hanging wall rocks and alteration above the giant feitais orebody of aljustrel iberian pyrite belt portugal shows that cherts that constitute the immediate hanging wall to the sulfide deposit result from hydrothermal alteration of laterally equivalent fe and or mn rich jaspers textures geochemistry and lack of fossil radiolarians indicate that these jaspers are hydrothermal precipitates hydrothermal alteration of the jaspers took place concomitantly with sulfide precipitation through reaction with hot 150 c reduced fluids with replacement of hematite by magnetite pyrite and fe2 phyllosilicates on the basis of the above and considering also the results of previous studies on hydrothermal activity in the aljustrel volcanic rocks and sulfide ores we propose that sulfide precipitation may have taken place under a thin veneer of siliceous sediments present day massive sulfide formation in the guaymas basin and in the salton sea under hundreds of meters of sediments may be approximate analogues in the sense that the presence of a caprock reduces dispersion of black smoke and causes more efficient precipitation

relationship of foraminifera and thecamoebian distributions to sediments contaminated by pulp mill effluent saguenay fiord quebec canada gravity cores and grab samples collected in the saguenay fiord between 1976 and 1988 contain the record of a 20th century benthic marine environment contaminated primarily by organic matter discharges from several local pulp and paper mills spiroplectammina biformis is the dominant arenaceous species its living percent abundance decreases between 1982 and 1988 as a consequence of the apparent recolonization of the upper reaches of the fiord by several arenaceous taxa the most important beingtextularia earlandi cassidulina reniforme the dominant living calcareous species shows about a three fold increase in relative abundance over the six year interval grab sample observations also suggest a recolonization of some formerly barren benthic environments near the head of the fiord by foraminiferal species between 1982 and 1988 this recolonization may reflect the combined effect of government regulations imposed on local industrial polluters in the early 1970 s and the capping of a large area of contaminated sediment by a layer of clay that was transported to the basin at the head of the fiord as the result of a catastrophic landslide in 1971 x radiographs of core sections from a basin at the head of the fiord show sharp laminations that reflect both the absence of bioturbators and the seasonal variation of fluvial depositional processes organic waste deposition from pulp and paper mill outfalls first increased around 1910 and shows a first order inverse relationship to benthonic foraminiferal assemblage diversity the presence of only allochthonous thecamoebian and reworked planktonic foraminifera tests in many benthonic foraminifera barren core intervals supports the idea that common indigenous estuarine species such asspiroplectammina biformis were excluded from these areas at certain times as a consequence of pollution stress rather than having been removed from the sediment record as a result of post depositional diagenetic effects however future studies of estuarine pollution history based on proxy foraminiferal data from cores must give greater emphasis to distinguishing ecological stress responses from diagenetic process i e test dissolution effects since both mechanisms can be reflected in the proxy pollution signal in a similar way

microbial remains and other carbonaceous forms from the 3 24 ga sulphur springs black smoker deposit western australia well preserved carbonaceous materials including filamentous microbial remains have been isolated from an archaean sedimentary package overlying the sulphur springs volcanic hosted massive sulphide vhms deposit in the pilbara of western australia one of the oldest best preserved vhms deposits on earth the isotopically light carbonaceous material 13 c values of 26 8 to 34 0 v pdb occurs as finely striated lenticular to banded organic matter om within fine grained epiclastic sediments and silicified materials that overlie the deposit the predominant banded lenticular carbonaceous material occurs parallel to original sedimentary bedding planes and is distinctly visible in light microscopy transmission electron microscopy tem reveals this om as bundles of filamentous and tubular structure closely resembling both modern day and more ancient microbial forms documented from sea floor hydrothermal environments total organic carbon toc has a range of 1 0 to 2 3 while the thermal maturity of the filamentous microbial bundles indicated by reflectance ro points to maximum temperatures since deposition ca 3 24 ga of around 90 100 c a factor that has enabled the preservation of their morphology these results and the intimate association of sulphides with the om are suggestive of a well developed archaean sediment hosted microbial community situated within a basinal environment associated with an active centre of seafloor hydrothermal activity 2007 elsevier b v all rights reserved

petrology and geochemistry of chert on the marginal zone of yangtze platform western hunan south china during the ediacaran cambrian transition the ediacaran cambrian transition was one of the most critical intervals in earth history during this interval widespread chert was precipitated commonly as a stratal wedge in carbonates along the southern margin of the yangtze platform south china the chert wedge passes into a full chert succession further basinward to the south east four lithotypes of chert are identified across the marginal zone in western hunan mounded vein brecciated and bedded chert the mounded chert is characterized by irregular to digitiform internal fabrics generally with abundant original vesicles and or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals the host chert or matrix of these mounds is dominated by amorphous cryptocrystalline silica commonly disseminated with pyrite the vein chert with minor quartz locally generally cross cuts the overlying dolostone and chert horizons and terminates under the mounded and or bedded chert bodies the brecciated chert commonly occurs as splayed intrusions or funnel shaped wedges and cross cuts the topmost dolostones the bedded chert the most common type generally is thin to medium bedded and laminated locally it is composed of amorphous silica with minor amounts of black lumps microthermometry of fluid inclusions from vein and void lining minerals mainly quartz and barite revealed homogenization temperatures from 120 to 180 c for the trapped primary fluids compositionally these chert deposits generally are pure with sio2 92 wt and only minor fe2o3 and al2o3 contents most of which show positive europium anomalies in rare earth element patterns especially for the mounded chert all these data suggest that the marginal zone chert deposits resulted from a low temperature silica rich hydrothermal system in which the mounded chert was precipitated around the releasing vents i e as silica chimneys the vein and splayed brecciated chert however was formed along the syndepositional fault fracture conduits that linked downward while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor these petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation rapid tectonodepositional and oceanic changes during the ediacaran cambrian transition in south china

the relationship between the first fresnel zone and the normalized geometrical spreading factor1 conventionally the fresnel zone and the geometrical spreading factor are investigated separately because they belong to different theories of wave propagation however if the paraxial ray method is used for establishing the fresnel kirchhoff diffraction formula for a laterally inhomogeneous multilayered medium it can be shown that the normalized geometrical spreading factor is inversely proportional to the area of the first fresnel zone associated with the reflection point therefore if no diffracting edge cuts the first fresnel zone the geometrical optics approximation represents the principal part of the wavefield obtained by fresnel kirchhoff diffraction theory otherwise the geometrical optics approximation has to be corrected by adding edge diffractions it is also shown that kirchhoff type migration and geometrical spreading factor correction both reduce the first fresnel zone to a zone with unit area

postglacial stratigraphy and sea level change in southwestern new brunswick cores from small basins in southwestern new brunswick above present sea level but below the former marine limit were examined for microfaunal changes indicating a transition from marine to freshwater conditions carbon 14 dates of those transition points add to and strengthen a previous sea level curve from the area the oldest date places sea level at about 75 m above present at 16 000 years bp dates from the lower lakes 10 m appear to be anomalously old and it is suggested that after 12 000 years bp erosion became a factor in marine areas this erosion may be related to the overall breakup of ice in the north atlantic and may explain why few dates younger than 12 000 years bp are found in raised marine deposits of new brunswick and maine

hydrothermal oxide and nontronite deposits on seamounts in the eastern pacific deposits of fe oxide mud nontronite and fe mn crusts were sampled from the summits of two seamounts in the eastern pacific where low temperature 0 15 c hydrothermal fluids are issuing the deposits consist of x ray amorphous fe oxyhydroxide and are rich in fe 43 wt contain minor si and p 4 and 3 5 respectively and have very low mn 0 01 and other trace element contents other deposits where no current hydrothermal activity was observed consist of mud and crusts composed of amorphous material and poorly crystalline hematite goethite and smectite these deposits have slightly higher mn up to 2 and other trace element contents the mineralogical and chemical differences between the active and inactive deposits and crusts are interpreted to be due to recrystallization and contamination with detrital biogenic hydrogenous and possibly hydrothermal sulfide material nontronite deposits capped by fe mn oxides were sampled from one of the seamounts these deposits have compositions similar to other seafloor nontronite deposits and formed at low temperatures 30 c during mixing of low temperature hydrothermal fluids with seawater the oxide deposits consist almost entirely of long delicate filaments remarkably similar in morphology to genera of fe oxidizing bacteria such bacteria were probably important in catalyzing the oxidation and precipitation of iron from hydrothermal fluids on the seamounts the occurrence of nearly identical filament rich oxide deposits on two widely separated seamounts suggests that this type of deposit may be common on seamounts but previously unrecognized