hydrothermal alteration of basalts beneath the bent hill massive sulfide deposit middle valley juan de fuca ridge burial of midocean ridges by clastic sediments particularly at continental margins profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust middle valley the sediment covered northern extension of the bare rock endeavour segment of the juan de fuca ridge is host to the base metal rich cu zn bent hill massive sulfide deposit at a water depth of 2 400 m the 9 mt bent hill deposit is a steep sided body 200 m across and 100 m thick ocean drilling program odp hole 856h penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized quartz chlorite sediment pile and into the uppermost volcanic basement the basaltic rocks beneath the bent hill deposit include narrow sills intruded into indurated sediments a volcanic flow erupted on top of sediments and pillow lavas below the lowermost sediments recovered similar styles of alteration are present in both the sills and flows and alteration is dominated by the effects of large scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions the basalts are slightly to completely altered to greenschist facies secondary minerals principally quartz chlorite and titanite with subsidiary epidote cu fe sulfides and rare actinolite there are steep mineralogical chemical and isotopic alteration gradients from the highly altered basalt sediment interfaces down to the less altered flow interiors suggesting the channeling of hydrothermal fluids along the basalt sediment boundaries alteration is reflected in intense metasomatic changes in the basalts assuming immobile tio 2 the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite quartz rocks with depletions in silica alumina and alkali alkali earth and base metals chloritized pillow margins with strongly light rare earth element enriched chondrite normalized patterns la sm n 1 5 cf fresh basalts 0 7 that mimic profiles for midocean ridge hydrothermal fluids require fluid rock exchange with large quantities of hydrothermal fluid w r 27 000 oxygen isotope compositions of chlorite quartz rocks 18 o 1 8 2 4 suggest that alteration occurred between 320 and 370 c strontium isotope compositions of the altered basalts and the chlorite quartz rocks are not homogeneous and range from 87 sr 86 sr ratio 0 7037 to 0 7046 there is a strong mode in 87 sr 86 sr ratio at 0 7038 suggesting that much of the alteration occurred by isotopic exchange with a hydrothermal fluid of that composition this ratio is significantly lower than that measured for 265 c fluids venting from the nearby odp mound 87 sr 86 sr 0 7044 the occurrence of epidote and isocubanite in the chloritized glassy pillow margins suggests that these rocks may retain a record of the high temperature 350 c hydrothermal fluid responsible for the formation of the overlying bent hill massive sulfide deposit the strontium isotope composition of the chloritized glassy pillow margins and hence the mineralizing fluid is slightly more radiogenic 87 sr 86 sr 0 7046 this composition could result from the addition of 15 percent of a pelagic or sedimentary component to the 87 sr 86 sr ratio 0 7038 fluid responsible for most of the sr isotope exchange with the upper basement the sediments beneath the bent hill deposit are also strongly recrystallized to quartz and chlorite although their strontium isotope compositions are much lower than those in pelagic or terrigenous sediments in the region 87 sr 86 sr 0 709 0 720 the range of compositions 87 sr 86 sr 0 7046 0 7060 has little overlap with that of the altered basalts and chlorite quartz rocks from the sills and uppermost basement this lack of overlap suggests that the sediments either retain some of their original sedimentary strontium or that there is a range of fluid compositions in the sediment pile beneath the bent hill deposit sediments from the margins of middle valley far from zones of active black smoker venting odp site 855 have 87 sr 86 sr ratios with a significant hydrothermal component 0 7059 0 7086 these ratios indicate that the subsurface hydrology of middle valley is dominated by evolved fluids rather than seawater and that recharge into this system is not through boundary faults or through the sedimentary blanket as suggested by previous models rather exposed basement rocks that form the flanks of middle valley are the most likely zones of regional seawater recharge to the deep high temperature hydrothermal systems

on the resource evaluation of marine gas hydrate deposits using sea floor transient electric dipole dipole methods methane hydrates are solid nonstoichiometric mixtures of water and the gas methane they occur worldwide in sediment beneath the sea floor and estimates of the total mass available there exceed 1016kg since each volume of hydrate can yield up to 164 volumes of gas offshore methane hydrate is recognized as a very important natural energy resource the depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures in sea depths greater than about 300 m the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor the fraction of hydrate in the sediment usually increases with increasing depth the base of the hydrate zone is a phase boundary between solid hydrate and free gas and water its depth is determined principally by the value of the geothermal gradient it stands out on seismic sections as a bright reflection the diffuse upper boundary is not as well mar

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

biogenic grooving on glass shards the occurrence of grooves on the surface of volcanic glass shards in marine and nonmarine environments suggests the existence of microorganisms that can dissolve silica substrates the etchings occur within a 30 cm thick marine turbidite bed in the miocene obispo tuff at shell beach central california and also in a tuff layer deep creek tuff in the miocene john day formation of eastern oregon the etchings occur in three different patterns with two subtypes depending on the formation in which they occur type a is an anastomosing pattern of semicircular grooves covering the entire grain subtypes of a are distinguished by the presence obispo tuff type a 1 or lack john day type a 2 of annulations type b has some sinuous branching grooves covering only part of the grain subtypes are the same as those of type a type c occurring only on obispo tuff grains is a series of straight nonbranching grooves on one side of platy shards the grooves range in size from 4 20 m in the obispo formation to 15 18 m in the john day formation the grooves are strikingly similar to grooves occurring on carbonate grains

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

relationship between the equatorial and meridional modes of climatic variability in the tropical atlantic the tropical atlantic ocean exhibits two primary modes of interannual climate variability an equatorial mode analogous to but weaker than the pacific el nino phenomenon and a meridional mode that does not have a pacific counterpart the equatorial mode is responsible for warm and cold sea surface temperature sst events mainly in the gulf of guinea and is identifiable by abnormal changes in the equatorial thermocline slope resulting from zonal wind anomalies in the western tropical atlantic the meridional mode is characterized by a north south interhemispheric gradient of sst anomalies here it is shown using observed surface and subsurface oceanic temperatures that the meridional mode is linked to the equatorial mode at both decadal and short interannual 1 2 years time scales both modes involve north south displacements of the itcz as in the annual response

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

biological mediation in ocean crust alteration how deep is the deep biosphere abstract biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration and the fraction of biotic alteration may be determined by point counting methods we used this method at dsdp odp sites 417d and 418a 110 ma crust south of bermuda rise and holes 504b and 896a 5 9 ma costa rica rift biotic alteration dominates glass alteration in the upper 250 m of the oceanic crust 60 85 of the total glass alteration and steadily declines in importance down to 10 20 at 500 m the consistency of data between two crustal sections of very different age and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceanic crust however both sites studied are sealed by thick sedimentary layers and thus are typical for ocean crust underlying the oceanic basins rather than crust at mid ocean ridges with possibly deep and rapid hydrothermal circulation down hole temperature measurements at the costa rica rift suggest that glass altering microbes are hyperthermophilic and thrive at least up to temperatures of about 90 c microbial activity does occur at higher temperatures up to about 110 c but with reduced apparent abundance

seasonal response of the sea surface to the wind in the equatorial atlantic from february 1983 through september 1984 18 inverted echo sounder deployments yielded time series from 10 locations as part of the seasonal response of the equatorial atlantic sequal program using historical and in situ data the sounders travel time data are interpreted as dynamic height 0 relative to 500 dbar and then related to representations of the synoptic wind field the seasonal variations of the zonal pressure gradient along the equator and the height of the dynamic ridge along 3 n relative to the equator and 9 n are described the pressure gradient between 10 w and 34 w has a minimum each april and a maximum each july its value ranges between 1 4 not significantly different from zero and 8 3 10 5 dyn g 1 during the observational period the zonal pressure gradient is correlated with zonal wind stress from an in situ wind sensor at midfetch and a 10 day lag time is indicated the north equatorial countercurrent trough attains a maximum in the late boreal summer and fall and a minimum in the spring it ranges from 39 to 8 dyn cm deep at 38 5 w and from 18 to 1 dyn cm at 28 w a two layer model calculation using monthly mean winds derived by servain et al 1986 for the appropriate time period generally reproduces the observed seasonal variation along the two meridians and the large zonal variation between them tracking the individual terms month by month illustrates the rate of the sverdrup adjustment to the local wind stress with surface elevation rising as fast as 10 cm per month

forced oscillations on the equatorial atlantic basin during the seasonal response of the equatorial atlantic program 1983 1984 a subset of data from an array of eight inverted echo sounders and a meteorological station deployed and maintained for 15 months in the equatorial atlantic as part of the seasonal response of the equatorial atlantic program are analyzed to determine the variability of forced oscillations of the basin the analysis is done to study both low and high frequency variability at low frequencies the equatorial basin responds as a whole to the seasonally varying winds the dominant signal is the annual cycle of the equatorial system as a composite of the response to the strong seasonal wind curl that is maximum in the western basin and the part of the zonal transfer to mass associated with the seasonal equatorial upwelling and downwelling system three bands of energy appear as dominant in the combined spectrum of the ocean and the atmosphere at higher frequencies 20 30 days 13 16 days and the inertial gravity wave band each one of the bands is analyzed and discussed instability waves are detected on the echo sounder records the waves have periods centered at 30 2 days and propagate westward with a wavelength of 1000 km and a phase speed of 0 35 m s the most energetic oscillation in the analyzed wind records is at periods centered around 14 days the response of the ocean to this oscillation has a meridional distribution with maximum amplitude at 3 n the most significant oceanic oscillations in the inertial gravity band are centered at 5 2 and 3 5 days from the study of their meridional structure it is concluded that those oscillations correspond to forced inertial gravity waves with meridional structures corresponding to the meridional numbers n 1 and n 3 respectively

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

validation of an equatorial atlantic simulation model using inverted echo sounder data changes in the vertically averaged temperature of the water column observed over a 12 month period at eight locations in the tropical atlantic with inverted echo sounders are described in terms of empirical orthogonal functions these modes are compared with those from a similar analysis for data from a general circulation model of the tropical atlantic the method proves to be useful for the validation of models

elemental abundance distributions in suboceanic basalt glass evidence of biogenic alteration 1 the alteration of subseafloor volcanic glass from three locations is qualitatively attributed to biological biotic or chemical abiotic reactions on the basis of microscopic morphology of the boundary between the unaltered and altered glass eleven element composition of fresh basalt glass sideromelane and its alteration products were determined by electron microprobe principal component analysis pca using these eleven elements as input na11 mg12 al13 si14 p15 cl17 k19 ca20 ti22 mn25 and fe26 extracts three factors accounting for 80 5 of the variance in the data these three factors can serve as inputs to a hierarchical cluster analysis hca algorithm for data driven discovery of three sample classes this autonomous classification agrees with the petrographic microscopic classification in 14 of 15 biotic clay analyses from a set of 34 analyses identified microscopically as abiotic clay the autonomous system identifies 4 with elemental abundance characteristics similar to the biotic clay and 4 similar to unaltered glass pca factors are then used as inputs to train an artificial neural network to produce a bayesian probability of correct classification using the classes discovered by hca mean bayesian probabilities of correct classification for abiotic clays biotic clays and glass were 76 1 8 5 64 9 9 0 and 77 0 7 2 respectively interestingly in the 9 of 74 cases where visual and elemental analysis disagree the bayesian probability estimate of correct classification using only elemental abundance data is low 60 0 11 7 compared to analyses where visual and elemental data agree 75 5 7 8 to our knowledge this is the first demonstration of a quantitative method for discrimination of biotic and abiotic alteration of subocean basalt glass as such the techniques make possible the systematic assessment of the impact of microbial life on subsurface basalts

upper equatorial atlantic variability during 2002 and 2005 associated with equatorial kelvin waves on the basis of moored 35 degrees w 0 degrees and 23 degrees w 0 degrees and satellite observations we contrast the upper equatorial atlantic variability during 2002 with those during 2005 inspection of the interannual boreal summer cold tongue variability revealed a warm cold event in 2002 2005 with relaxed intensified winds in the west and the equatorial undercurrent euc embedded in a shallower deeper thermocline at 23 degrees w during both years 2002 and 2005 equatorial kelvin waves are present in moored observations as 20 degrees c isotherm depth anomalies and dynamic height anomalies basin wide sea surface height anomalies are used to derive an equatorial kelvin wave mode the time evolution of this mode represents the basis for a regression analysis to investigate related oceanic variability compared to the exceptionally strong wave activity in 2002 equatorial kelvin waves were generally weaker during 2005 the main effect of equatorial kelvin waves on zonal velocity anomalies at 23 degrees w 0 degrees is evident well below the euc core their direct influence on cold tongue sea surface temperature is small but they are found to affect the equatorial thermocline slope prior to the cold tongue onset in 2002 2005 the presence of equatorial kelvin waves is associated with a flattened steeper thermocline slope that is crucial for the shallowing deepening of the euc core at 23 degrees w and that might precondition the development of the warm cold event

high resolution magnetics reveal the deep structure of a volcanic arc related basalt hosted hydrothermal site palinuro tyrrhenian sea high resolution magnetic surveys have been acquired over the partially sedimented palinuro massive sulfide deposits in the aeolian volcanic arc tyrrhenian sea surveys flown close to the seafloor using an autonomous underwater vehicle auv show that the volcanic arc related basalt hosted hydrothermal site is associated with zones of lower magnetization this observation reflects the alteration of basalt affected by hydrothermal circulation and or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids to discriminate among these inferences estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area the stockwork we use high resolution vector magnetic data acquired by the auv abyss geomar above a crater shaped depression hosting a weakly active hydrothermal site our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit forming lobe shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

marine dipole dipole controlled source electromagnetic and coincident loop transient electromagnetic experiments to detect seafloor massive sulphides effects of three dimensional bathymetry seafloor massive sulphides smss are regarded as a potential future resource to satisfy the growing global demand of metals including copper zinc and gold aside from mining and retrieving profitable amounts of massive sulphides from the seafloor the present challenge is to detect and delineate significant sms accumulations which are generally located near mid ocean ridges and along submarine volcanic arc and backarc spreading centres currently several geophysical technologies are being developed to detect and quantify sms occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock here we use a short fixed offset controlled source electromagnetic csem system and a coincident loop transient electromagnetic tem system which in theory allow the detection of sms in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings in 2016 csem and tem experiments were carried out at several locations near the trans atlantic geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor measurements were conducted in an area that contains distinct sms sites located several kilometres off axis from the mid atlantic ridge some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased based on the quality of the acquired data both experiments were operationally successful however the data analysis indicates bias caused by three dimensional 3d effects of the rough bathymetry in the study area and thus data interpretation remains challenging therefore we study the influence of 3d bathymetry for marine csem and tem experiments focusing on shallow 3d conductors located beneath mound like structures we analyse synthetic inversion models for attributes associated with 3d distortions of csem and tem data that are not sufficiently accounted for in conventional 1d tem and 2d csem interpretation schemes before an adequate quantification of sms in the region is feasible these 3d effects need to be studied to avoid over underestimation of sms using the acquired em data the sensitivity of csem and tem to bathymetry is investigated by means of 3d forward modelling followed by 1d tem and 2d csem inversion of the synthetic data using realistic error conditions subsequently inversion models of the synthetic 3d data are analysed and compared to models derived from the measured data to illustrate that 3d distortions are evident in the recorded data sets

equatorial kelvin waves in the atlantic long term inverted echo sounder records along and north of the equator in the atlantic are analyzed to show the dominance of equatorially trapped and eastward propagating disturbances with first baroclinic mode kelvin wave characteristics in the frequency band of 10 1 to 40 1 cycles day 1 propagation speed is estimated as 2 35 0 22 m s 1 along the equator from 34 w to 1 w coherence is present to 3 n but absent at 6 n from mooring data and 9 n coherence is shown with the zonal wind stress observed in midbasin and it is suggested that the origin of the disturbance is not far from 40 w

basin wide thermocline displacements along the equator of the atlantic in 1983 observations from five inverted echo sounders deployed between 10 w and 38 30 w along the equator during the spring and summer of 1983 are presented the annual development of the eastward acting zonal pressure gradient is described and related to the observed zonal wind stress the minimum and maximum basin wide gradients are observed in mid april and in the latter part of july respectively the upwelling season in the gulf of guinea is identified at the easternmost sounder during may june and july pulses in thermocline displacement first deepening then shallowing propagate eastward across the basin at speeds of 2 8 and 2 1 m s 1 they arrive at 10 w at the onset and just after the cessation of the seasonal upwelling at that location their initialization is related to relaxation periods in the zonal wind for four of the last five years the time of the pulse preceding the upwelling is tentatively identified

the response of equatorial oceans to a relaxation of the trade winds abstract the trade winds over the central pacific are observed to weaken several months after the appearance of anomalously warm surface waters in the eastern equatorial pacific ocean the following results obtained with a numerical model indicate how this relaxation of the winds affect the later stages of el nino a weakening of the westward trade winds causes a zonal redistribution of heat in the equatorial oceans and a warming of the eastern part of the basin the warming depends on the zonal extent of the region over which the winds relax and on the length of time t for which the winds relax as t increases the warming in the east increases until it asymtotes to a maximum value when t exceeds the adjustment time of the basin which is 400 days in the case of the pacific ocean maximum heating is associated with a permanent weakening of the winds unless the winds reverse direction and become eastward even weak eastward winds for a short period can cause disproportionately large temperature increa

diversity of life in ocean floor basalt abstract electron microscopy and biomolecular methods have been used to describe and identify microbial communities inhabiting the glassy margins of ocean floor basalts the investigated samples were collected from a neovolcanic ridge and from older sediment covered lava flows in the rift valley of the knipovich ridge at a water depth around 3500 m and an ambient seawater temperature of 0 7 c successive stages from incipient microbial colonisation to well developed biofilms occur on fracture surfaces in the glassy margins observed microbial morphologies are various filamentous coccoidal oval rod shaped and stalked forms etch marks in the fresh glass with form and size resembling the attached microbes are common precipitation of alteration products around microbes has developed hollow subspherical and filamentous structures these precipitates are often enriched in fe and mn the presence of branching and twisted stalks that resemble those of the iron oxidising gallionella indicate that reduced iron may be utilised in an energy metabolic process analysis of 16s rrna gene sequences from microbes present in the rock samples show that the bacterial population inhabiting these samples cluster within the and proteobacteria and the cytophaga flexibacter bacteroides subdivision of the bacteria while the archaea all belong to the crenarchaeota kingdom this microbial population appears to be characteristic for the rock and their closest relatives have previously been reported from cold marine waters in the arctic and antarctic deep sea sediments and hydrothermal environments

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

heat flow and mineralogy of tag relict high temperature hydrothermal zones mid atlantic ridge 26 n 45 w submersible investigations employing heat flow measurements 12 stations sampling and imagery of the two relict high temperature hydrothermal zones of the tag field the alvin and mir sulfide zones elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies values of heat flow in the mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers this observation supports the hypothesis that intrusions at volcanic centers adjacent to the high temperature hydrothermal zones supply the heat to drive hydrothermal activity the chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field wide high temperature hydrothermal events recurring at intervals of tens of thousands of years while activity at individual zones may recur at intervals of hundreds to thousands of years a sequence of hydrothermal deposits ranges to at least 140 000 years ago from massive sulfides forming at the active mound to recrystallization of sulfides in the active and relict zones to pyritization of an inactive mound in the alvin zone low temperature mineral phases precipitate before during and after the sulfides

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

alteration of oceanic volcanic glass textural evidence of microbial activity the subsurface biosphere may constitute as much as 50 percent of earth9s biomass direct and indirect evidence suggests that an extensive biosphere exists in the rocks below the sea floor this survey of basalts of the atlantic pacific and indian oceans supports the hypothesis that bacteria have colonized much of the upper oceanic crust which has a volume estimated at 1018 cubic meters although this is the largest habitat on earth its low abundance of bacteria constitutes much less than 1 percent of earth9s biomass

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

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

wind driven meridional eddy heat flux in the gulf of guinea seasonal response of the equatorial atlantic programme francais ocean et climat dans l atlantique equatorial data for 1984 are analyzed to estimate heat fluxes in the interior of the gulf of guinea between 1 n and 4 n in this region during may july the mixed layer roughly 35 m deep cools uniformly by nearly 4 c in 3 months representing a mean rate of heat loss of approximately 70 w m2 climatic heat budget calculations by hastenrath and lamb 1978 show a surface flux heat loss of 30 w m2 during this time this suggests that although surface flux makes a significant contribution to the mixed layer cooling additional divergent heat flux is required to balance the local heat budget all advective and diffusive terms are estimated to be small except for the meridional eddy heat flux v t the cospectrum of v and t has a peak near 15 days at this frequency v is coherent with the meridional wind the thermocline displacements forced by this meridional wind are nearly antisymmetric about the equator the frequency meridional length scale and asymmetry of these fluctuations suggest that a second baroclinic rossby gravity wave is being locally forced thus in the interior of the gulf of guinea the eddy flux driven by the fortnightly wind fluctuations makes a significant contribution to the cooling of the mixed layer during the seasonal upwelling this is in contrast to the central and western equatorial atlantic ocean where a meridional heat flux is produced by 25 to 30 day fluctuations generated by current instabilities

resources of the sea floor in his perspective rona reviews recent advances in knowledge of sea floor resources knowledge is expanding from those minerals eroded from land metals gemstones sand and gravel to those minerals concentrated by plate tectonic processes in ocean basins exploration of the oceans is at an early stage and development of marine mining is subject to economic and environmental considerations

seafloor massive sulphide exploration using deep towed controlled source electromagnetics navigational uncertainties deep towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the sub seafloor we address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data we show that the data error is laterally variable due to irregular motion during deep towing but also due to lateral variations in conductivity including those resulting from topography to address this variability and quantify the data error prior to inversion we propose a two dimensional perturbation study our workflow enables stable and geologically reliable results for multi component and multi frequency inversions an error estimation workflow is presented which comprises the assessment of navigational uncertainties perturbation of navigational parameters and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous sub seafloor conductivity model some navigational uncertainties are estimated from variations of direct measurements other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation some navigational parameters show direct correlation with the measured electric fields for example the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field for the perturbation study each standard deviation is added to the navigational parameters forward models are run for each perturbation amplitude deviations are summed in quadrature with the stacking error for a total laterally varying data error the error estimation is repeated for a heterogeneous sub seafloor model due to the large conductivity range several orders of magnitude which affects the forward model the approach enables us to utilize data from several components multiple electric fields frequencies and receivers in the inversion to constrain the final model and reduce ambiguity the final model is geologically reasonable in this case enabling the identification of conductive metal sulphide deposits on the seafloor

wind generated equatorial kelvin waves observed across the pacific ocean abstract sea level fluctuations during 1978 80 at equatorial pacific islands separated by as much as one quarter of the earth s circumference are coherent at periods of 1 6 weeks with phases implying eastward propagation eastward speeds are 16 7 5 higher than expected for a linear first baroclinic mode kelvin wave based upon hydrography zonal winds in the western pacific exhibit variation on meridional scales comparable to those of equatorial ocean baroclinic motions roughly one quarter of sea level variance in the 1 6 week period range can be explained by local zonal wind alone the observed admittance magnitude o 0 1 cm sea level per m s 1 2 zonal wind pseudo stress and phase lag a few days sea level lagging wind can be accounted for in a linear model of baroclinic equatorial kelvin waves generated by a crudely idealized wind patch of 1000 km zonal scale zonal winds at the equator excite among other things low mode kelvin waves which are recognizable o 10000 km to the east of the for

low temperature alteration of the upper oceanic crust and the alkalinity budget of seawater abstract while the input of river alkalinity into seawater is relatively well known the complementary acidity production is poorly understood using the major element budget of seafloor alteration of the upper 500 m of 120 ma old oceanic crust at dsdp odp sites 417a 417d and 418a in the central western atlantic we estimate the acidity flux associated with the low temperature weathering of the upper oceanic crust the acidity flux is calculated based on major element fluxes and charge balance considerations the relevant chemical fluxes from seawater to the upper crust are 4 1 0 1 1 4 1 4 2 2 0 6 and 12 2 1011 mol yr 1 for k mg na and silicate ca respectively the associated acidity flux is 3 5 3 1011 eq yr 1 relative to continental weathering these fluxes are significant for k and silicate ca but are minor for na mg and acidity thus riverine fluxes of alkalinity are not significantly balanced by acidity fluxes from low temperature upper ocean crust alteration

preservation of biosignatures in metaglassy volcanic rocks from the jormua ophiolite complex finland evidence of microbially mediated alteration of basaltic glass is preserved in originally glassy basalts rims of pillow lavas hyaloclastite breccias and chilled margins of dykes from the well preserved 1 95ga jormua ophiolite complex joc of northeastern finland although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the joc however another robust biosignature is found in the generally depleted d 13 c values of disseminated carbonate extracted from originally glassy basalts relative to crystalline samples the same distribution of d 13 c values is well documented in samples from recent oceanic crust as well as ophiolites of phanerozoic age this characteristic contrastinthe d 13 cvaluesofdisseminatedcarbonateisinterpretedtoresultfrommicrobe inducedfractionationduringoxidation of organic matter x ray mapping of initial alteration zones has identified residual carbon associated with highly concentrated s that is unrelated to carbonate we attribute these biosignatures to microbially mediated alteration of originally glassy material prior to ophiolite emplacement 2004 elsevier b v all rights reserved

on the role of microbes in the alteration of submarine basaltic glass a tem study abstract the alteration of submarine basaltic glass that has been attributed to microbial activity was investigated via transmission electron microscopy in order to document glass alteration and to understand the chemical reactions that bacteria could utilize the features that were examined occur within fresh glass at the alteration front and consist of 0 1 2 3 m patches or tubes of phyllosilicates rimmed by 0 1 0 3 m wide zones of altered glass the altered glass has lost significant mg fe ca and na slight amounts of al and mn and exhibits an order of magnitude increase of k this incongruent dissolution occurred at neutral to acid ph and temperatures 100 fracture volumes which may be important for the supply of nutrients for microbial activity

the boron isotopic composition of altered oceanic crust samples of drilled oceanic crust from dsdp holes 417a 417d and 418a and odp hole 735b and oceanic crust from the oman and cyprus ophiolites were analyzed for b contents and 11b composite samples from dsdp holes 417a 417d and 418a were used to represent the upper 550 m of altered oceanic crustal layer 2a whole rock samples from the troodos ophiolite cyprus and the oman ophiolite wer selected to represent crustal layer 2b dikes composite samples from odp hole 735b were used to represent crustal layer 3 the b content of the dsdp composites ranges from 7 2 ppm to 104 ppm and correlates with both 18o and k showing that it is a good indicator of the extent of low temperature alteration the 11b of the dsdp composites varies between 2 5 and 5 4 the b content of the samples from the troodos ophiolite ranges from 2 4 ppm to 8 1 ppm 11b varies from 0 9 to 7 8 the b content of the oman ophiolite samples ranges from 5 0 ppm to 11 1 ppm 11b varies from 1 6 to 16 9 the b content of the samples from odp hole 735b ranges from 1 1 ppm to 7 1 ppm 11b varies from 4 3 to 24 9 the general pattern displayed by these samples is one of greatest and most variable b enrichment at the top of the crust and least enrichment at the bottom of the section all of these samples are enriched compared to unaltered morb which is believed to have a b content of approximately 0 5 ppm the 11b values of deeper samples from layers 2b and 3 are more variable and generally higher than those from layer 2a boron contents and 11b are not correlated the data from the dsdp site 417418 composites indicate that the 11b of fluid circulating in the upper crust changes only slightly during alteration increasing by an average of 5 1 with an accompanying decrease in b concentration of 7 low temperature alteration appears to be a water dominated process resulting in minor modification of circulating seawater a minimum water rock ratio of 400 is calculated for these samples implying a minimum low temperature seawater flux through the upper oceanic crust of 3 4 1014 l y the average b content of altered oceanic crust as represented by these samples is 5 2 1 7 ppm and the average 11b is 3 4 1 1 this average isotopic composition is measurably different from the apparent average of oceanic sediments supporting the idea that 11b could be useful for identifying the source s of b in island arcs

seasonal adjustments in the equatorial atlantic during 1983 as seen by surface moorings upper ocean seasonal variations in velocity and temperature observed with current meters on surface moorings along the equator in the atlantic during february september 1983 are shown the longitudinal variation in the duration of upwelling and downwelling responses to zonal wind stress changes are discussed these responses exhibit an integral character dependent upon longitudinal position which is consistent with forced linear long wave theory

near seafloor magnetics reveal tectonic rotation and deep structure at the tag trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n we take advantage of geological constraints from ocean drilling program drill holes and high resolution bathymetry to revisit the near seafloor magnetic anomaly at the trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n the dipolar anomaly associated with the site is better reduced to the pole if the magnetization vector is tilted by 34 an observation suggesting that the strongly faulted basalt surrounding the site has been rotated by 53 along an axis parallel to the mid atlantic ridge as a probable consequence of the detachment tectonics inferred in this area the faults and the deeper detachment focus and guide the hot ascending hydrothermal fluid magnetic modeling shows that the nonmagnetic stockwork zone is a significant contributor to the observed negative anomaly the rest being accounted for by a deeper source probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site

dynamics of intraseasonal sea level and thermocline variability in the equatorial atlantic during 2002 03 abstract satellite and in situ observations in the equatorial atlantic ocean during 2002 03 show dominant spectral peaks at 40 60 days and secondary peaks at 10 40 days in sea level and thermocline within the intraseasonal period band 10 80 days a detailed investigation of the dynamics of the intraseasonal variations is carried out using an ocean general circulation model namely the hybrid coordinate ocean model hycom two parallel experiments are performed in the tropical atlantic ocean basin for the period 2000 03 one is forced by daily scatterometer winds from the quick scatterometer quikscat satellite together with other forcing fields and the other is forced by the low passed 80 day version of the above fields to help in understanding the role played by the wind driven equatorial waves a linear continuously stratified ocean model is also used within 3 s 3 n of the equatorial region the strong 40 60 day sea surface height anomaly ssha and thermocline variability result mainly from the

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

biologically mediated dissolution of volcanic glass in seawater we studied the effects of biological mediation on the dissolution of basaltic glass in seawater experiments with typical seawater microbial populations were contrasted with a sterile control and reactions were monitored chemically and isotopically biologically mediated experiments produce twice the mass of authigenic phases than abiotic experiments and the phases are different abiotic alteration of glass dissolves basaltic si and ca and scavenges seawater mg while biotic alteration removes ca from seawater such opposing behavior of ca and mg in biotic and abiotic alteration of basaltic glass may have important implications for the carbon cycle and the exchange processes between ocean crust and seawater 87sr 86sr data of glass and alteration products suggest that biological mediation enhances both the diffusion of seawater sr into glass by a factor of 3 4 and the dissolution of basaltic sr into seawater by a factor of 20 40 the dependence of chemical exchange processes between seawater and glass on biological activity implies that chemical fluxes from water rock interaction at low temperatures may change as life on earth evolves

early life recorded in archean pillow lavas pillow lava rims from the mesoarchean barberton greenstone belt in south africa contain micrometer scale mineralized tubes that provide evidence of submarine microbial activity during the early history of earth the tubes formed during microbial etching of glass along fractures as seen in pillow lavas from recent oceanic crust the margins of the tubes contain organic carbon and many of the pillow rims exhibit isotopically light bulk rock carbonate 13 c values supporting their biogenic origin overlapping metamorphic and magmatic dates from the pillow lavas suggest that microbial life colonized these subaqueous volcanic rocks soon after their eruption almost 3 5 billion years ago

vulcan a deep towed csem receiver we have developed a three axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1000 m of seafloor geology by careful adjustment of buoyancy and the use of real time monitoring of depth and altitude we are able to deep tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor thereby obtaining good coupling to geology the rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing and allows the measurement of the vertical electric field component which modeling shows to be particularly sensitive to near seafloor resistivity variations the positions and orientations of the receivers are continuously measured and realistic estimates of positioning errors can be used to build an error model for the data during a test in the san diego trough offshore california inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1 in amplitude and 1 in phase and vertical fields to about 5 in amplitude and 5 in phase the geological target of the tests was a known cold seep and methane vent in 1000 m water depth which inversions show to be associated with a 1 km wide resistor at a depth between 50 and 150 m below seafloor given the high resistivity 30 m and position within the gas hydrate stability field we interpret this to be massive methane hydrate

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

direct dating of archean microbial ichnofossils well preserved archean pillow lavas from the ca 3 35 ga euro basalt of the pilbara craton western australia contain micron sized tubular structures mineralized by titanite catisio 4 with residual organic carbon preserved along their margins direct u pb dating of titanite in the tubular structures demonstrates an archean age these tubular microstruc tures are identical to microbial ichnofossils in modern basalts ophiolites and greenstone belts and are interpreted as a biogenic signature in these ancient rocks microbial colonization of basaltic glass thus appears to have been part of a deep subsurface biosphere established early in earth s history

tropical atlantic sea level variability from geosat 1985 1989 geosat altimeter data between april 1985 and september 1989 are analyzed in the tropical atlantic ocean first improvements due to the use of new corrections and orbit computations are found to be effective especially in the gulf of guinea where part of the previously missing signal is recovered then the variability of the ocean is examined using empirical orthogonal functions eofs only three eofs are needed to describe 80 of the seasonal variance the first one describes the meridional tilting of the tropical atlantic along the mean location of the intertropical convergence zone with an annual period the second describes a mass redistribution due to the equatorial upwelling peaking in june july the third function presents a clear semiannual signal looking at interannual variability the first eof reveals a mass redistribution between the equatorial region 10 n to 10 s and the northern and southern ones 10 n to 30 n 10 s to 30 s in the equatorial region the upper layer volume increases from about 1 5 to 1 5 x 1014 m3 between 1987 and 1989 occurring 1 year after the pacific el nino this phenomenon recalls the 1984 anomaly observed during the programme francais occan et climat en atlantique tropicavseasonal equatorial atlantic experiments

discrimination of hydrothermal mound structures using transient electromagnetic methods we present numerical modeling results to support the use of seafloor transient electromagnetic em techniques in characterizing the buried component of active hydrothermal mounds we construct 2 d electrical models of a volcanic hosted sulphide mound and compute the em fields created by a transient electric dipole source deployed on the seafloor the time it takes for fields to diffuse between the source and receivers is diagnostic of the intervening electrical conductivity in the case considered the highly conductive outcropping sulphide mound is essentially invisible to the em probe exposing the moderately conducting alteration and upflow zone below transient em responses are measurably different for the model featuring concentrated upflow compared to the model containing a spatially broad fluid supply beneath the mound

waves along the equator in the atlantic abstract spectra of two hundred days of data from five inverted echo sounders deployed along the equator in 1983 84 and a thousand days of 30 crossings of the equator by the topex poseidon altimeter in 1993 95 are both found to have enhanced variance of sea surface displacement in two frequency bands centered nominally about 50 1 and 25 1 cycles day complex empirical orthogonal function analysis of the two bands finds much of the variance being accounted for by eastward and westward propagating waves respectively that are coherent across the basin the eastward propagating wave is identifiable as a first mode kelvin wave with a period of 54 49 days in the sounder altimeter dataset a basinwide wavelength and a speed of 2 1 1 8 m s 1 it is suggested that the westward propagating wave is a surface expression of the meridional oscillation of the equatorial undercurrent about the equator presumably induced by a tropical instability wave with a period of 24 28 days a wavelength of 600 580 km

comparing petrographic signatures of bioalteration in recent to mesoarchean pillow lavas tracing subsurface life in oceanic igneous rocks abstract bioalteration of basaltic glass in pillow lava rims and glassy volcanic breccias hyaloclastites produces several distinctive traces including conspicuous petrographic textures these biologically generated textures include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material such bioalteration textures have been described from upper in situ oceanic crust spanning the youngest to the oldest oceanic basins 0 170 ma the granular type consists of individual and or coalescing spherical bodies with diameters typically around 0 4 m these are by far the most abundant having been traced up to 550 m depths in the oceanic crust the tubular type is defined by distinct straight to irregular tubes with diameters most commonly around 1 2 m and lengths exceeding 100 m the tubes are most abundant between 50 m and 250 m into the volcanic basement we advance a model for the production of these bioalteration textures and propose criteria for testing the biogenicity and antiquity of ancient examples similar bioalteration textures have also been found in hyaloclastites and well preserved pillow lava margins of phanerozoic to proterozoic ophiolites and archean greenstone belts the latter include pillow lavas and hyaloclastites from the mesoarchean barberton greenstone belt of south africa and the east pilbara terrane of the pilbara craton western australia where conspicuous titanite mineralized tubes have been found petrographic relationships and age data confirm that these structures developed in the archean thus these biologically generated textures may provide an important tool for mapping the deep oceanic biosphere and for tracing some of the earliest biological processes on earth and perhaps other planetary surfaces

3 5 billion years of glass bioalteration volcanic rocks as a basis for microbial life abstract alteration textures in volcanic glass from the seafloor fall into two classes one suggestive of abiotic diffusive hydration and chemical exchange and another likely to be caused by microbial cavity forming congruent dissolution glass bioalteration is common in submarine lavas throughout the world s ocean dominant in the upper 300 m of the oceanic crust and found in all well preserved ophiolites and greenstone belts dating back to 3 5 ga it may yield a significant fraction of the global biomass and geochemical fluxes and is relevant to the development of the earliest life on earth we present a critical review concerning these glass bioalteration textures and present new data on their microchemical environment we explore arguments for their biogenicity and further develop the prevalent model for their formation by relating corrosion morphology to the mechanism of microbial dissolution biological alteration produces conspicuous micron scale granular and tubular textures granular glass alteration is well explained by colonizing microbes that selectively dissolve the glass in their contact area forming a sponge like interconnected network of micron sized cavities along glass surfaces tubular alteration meanwhile is more likely to be caused by filamentous cell extensions in a process similar to fungal tunneling of soil feldspars and marine carbonates while we see clear functional similarities to fungal dissolution behavior we do not know whether fungal or prokaryotic organisms are involved however this functional constraint may eventually help to identify potential microbes responsible for these features potentially including eukaryotic or prokaryotic organisms yet we caution that these organisms may be difficult to identify and to study because they are likely to be sparsely distributed slow growing and difficult to cultivate

iron and sulfide oxidation within the basaltic ocean crust implications for chemolithoautotrophic microbial biomass production microbial processes within the ocean crust are of potential importance in controlling rates of chemical reactions and thereby affecting chemical exchange between the oceans and lithosphere we here assess the oxidation state of altered ocean crust and estimate the magnitude of microbial biomass production that might be supported by oxidative and nonoxidative alteration compilations of fe2o3 feo and s concentrations from dsdp odp drill core samples representing upper basaltic ocean crust suggest that fe3 fe increases from 0 15 0 05 to 0 45 0 15 within the first 10 20 myr of crustal evolution within the same time frame 70 25 of primary sulfides in basalt are oxidized with an annual production of 4 0 1 8 1015 g of upper 500 200 m crust and average initial concentrations of 8 0 1 3 wt fe and 0 125 0 020 wt s we estimate annual oxidation rates of 1 7 1 2 1012 mol fe and 1 1 0 7 1011 mol s we estimate that 50 of fe oxidation may be attributed to hydrolysis producing 4 5 3 0 1011 mol h2 yr thermodynamic and bioenergetic calculations were used to estimate the potential chemolithoautotrophic microbial biomass production within ridge flanks combined aerobic and anaerobic fe and s oxidation may support production of up to 48 21 1010 g cellular carbon c hydrogen consuming reactions may support production of a similar or larger microbial biomass if iron reduction nitrate reduction or hydrogen oxidation by o2 aq are the prevailing metabolic reactions if autotrophic sulfate reduction or methanogenesis prevail the potential biomass production is 9 7 1010 g c yr and 3 2 1010 g c yr respectively combined primary biomass production of up to 1 1012 g c yr may be similar to that fueled by anaerobic oxidation of organic matter in deep seated heterotrophic systems these estimates suggest that water rock reactions may support significant microbial life within ridge flank hydrothermal systems these estimates suggest that water rock reactions may support significant microbial life within ridge flank hydrothermal systems

a time domain electromagnetic survey of the tag hydrothermal mound a conductivity survey of the tag mound was made using innovative transient electric dipole dipole instrumentation the receiver was deployed on the sulfide mound 30 m south of the central black smoker while the submersible alvin carried the transmitter on a survey path around the receiver at a radius of approximately 70 m conductivity measurements of the seafloor between the transmitter and receiver were made at 5 minute intervals the survey lasted 4 hours measurements made at 12 sites are initially interpreted in terms of an apparent conductivity of a uniform seafloor a more complex model in which the seafloor has two layers is then used apparent conductivities range from 1 4 to 15 9 s m generally higher than that of seawater the results suggest possible focusing of hydrothermal convection in the northern quadrant of the mound adjacent zones of anhydrite accumulation and fluid convection in the north eastern quadrant heterogeneity and layering in the kremlin zone and unexpected spatial variability in the western quadrant the data from several sites show evidence of distortion attributed to 3 d anomalies

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

microbial alteration of 0 30 ma seafloor and sub seafloor basaltic glasses from the australian antarctic discordance scanning electron microscopic observations of alteration rims in basaltic glasses dredged from 0 2 5 ma seafloor and drilled from 18 28 ma ocean crust ocean drilling program odp leg 187 in the australian antarctic discordance aad document the presence of endolithic microbes in altered basalt glass in very young aad lavas 10 m thick alteration rims are developed along intersecting fractures and cracks and the altered glass contains numerous spherical rod and star shaped partially fossilised microbial cells similar to those from the arctic ridges 1 2 in 2 5 ma basalt glasses altered rims are up to 250 m thick and zeolite phillipsite is present within many fractures spherical cells occur in porous outer zones of alteration rims and on zeolite crystal surfaces within fractures indicating that microbial activity persists in the region for at least 2 5 ma mn rich microbial cell encrustations associated with zeolite suggest that mn is used in an energy yielding metabolic process combined with recent results from the arctic ridges the results from this study demonstrate that endolithic microbial growth is a persistent feature of mid ocean spreading ridges in glasses from odp cores 1 mm thick alteration rims are developed along the widest fractures lined with mn fe oxyhydroxides and or clay and filled by zeolite and calcite most common however are 10 200 m thick rims developed along zeolite filled more narrow fractures and cracks zeolite filled fractures with only minor to no alteration indicate several episodes of fracturing followed by relatively fast sealing there is no age progression in alteration thickness along fractures or other characteristics suggesting that alteration is essentially completed between 2 5 and 18 ma a comparison of alteration in the 2 5 ma glass with that in the odp samples indicates that a significant proportion of the glass alteration in the drilled samples developed prior to burial although one type of diffuse highly irregular front that is only observed in the odp samples most likely developed after burial these diffuse alteration fronts are caused by dissolution and alteration of the glass into minute globules 0 05 0 2 m in diameter with no associated microbial morphologies fossilised mn rich cells do occur within zeolite filled fractures possibly indicating that microbial activity continued in the fractures for as long as circulation continued the apparent non biological origin of diffuse irregular alteration fronts in buried aad glasses indicates that these textural features are not reliable as diagnostic criteria for the existence of a deep biosphere in the volcanic ocean crust

p wave velocity measurements for preliminary assessments of the mineralization in seafloor massive sulfide mini cores during drilling operations highlight it is important to develop systems able to detect and classify mineralized zones from waste materials while drilling deep water seismic p wave velocities vp were measured on 40 sms and unmineralized mini cores the porosity was back calculated from vp the results were compared with electrical resistivity measurements using archie s law it is possible to observe that metallic conduction exists abstract deep sea mining exploration for seafloor massive sulfide sms deposits is currently increasing at present most exploration activities are surficial and use indirect methods to identify potential sites and perform first assessments for a proper resource estimate however drilling is inevitable by using seabed drill rigs exploration costs can be reduced considerably sms deposits are normally found at depths between 1000 and 4000 m and in order for deep sea mining to be implemented reliable technologies are needed additionally the development of geophysical systems that can detect and classify mineralized zones from waste materials while drilling could decrease costs and speed up offshore operations by limiting the amount of drilling of unmineralized materials this paper shows how the physical properties of sms can be used to discriminate between host rocks and mineralization seismic p wave velocities vp were measured on 40 sms and unmineralized mini cores by back calculating the porosity from vp comparing the results with electrical resistivity measurements and using archie s law it is possible to observe that metallic conduction exists for deep sea mineral exploration the combination of seismic tests electrical resistivity and magnetic susceptibility could support the preliminary discrimination of mineralized samples in the cores while drilling at the seafloor