the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

the distribution of geothermal fields on the juan de fuca ridge near bottom water temperatures were mapped along 400 km of the strike of the juan de fuca ridge as part of a combined sea marc seabeam experiment to image the variability of morphology and structure along a spreading center segment the water temperature data collected by a continuously towed thermistor chain in addition to salinity data indicate that there are four geothermal areas spaced at distances of 100 km from each other south of the cobb propagator and one field just to the north of the propagator on the endeavor ridge segment each thermal region is located above a morphological dome on the spreading center these domes are an average of 100 200 m shallower than the rest of the axis the structure of bottom water suggests that the geothermal regions are on average 20 km long and that the heat from these fields raises the temperature in the water column by a minimum of 0 06 sup 0 c up to 300 m above the bottom two simple models are used to estimate the heat flux associated with these features

electrical image of passive mantle upwelling beneath the northern east pacific rise sea floor magnetotelluric soundings along a portion of the east pacific rise reveal a symmetric high conductivity triangular region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle the east pacific rise is a fast spreading mid ocean ridge where sea floor volcanoes and hydrothermal vents result from a robust mantle melt supply to explore how this magma is supplied to the surface kerry key and colleagues use sea floor magnetotelluric soundings to image mantle electrical structure to a depth of about 160 km along the northern east pacific rise the images reveal a symmetrical high conductivity anomaly beneath the ridge consistent with partial melting of passively upwelling mantle they explain a deeper conductor observed east of the ridge by asymmetric upwelling due to viscous coupling across two nearby transform faults these findings suggest that plate driven passive upwelling dominates this ridge segment while dynamic forces are negligible melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid ocean ridges yet the forces controlling this process are debated1 2 passive flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates but have been challenged by geophysical observations of asymmetric upwelling3 4 5 that suggest anomalous mantle pressure and temperature gradients2 6 7 and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2 here we use sea floor magnetotelluric soundings at the fast spreading northern east pacific rise to image mantle electrical structure to a depth of about 160 kilometres our data reveal a symmetric high conductivity region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle9 10 11 the triangular region of conductive partial melt matches passive flow predictions suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere a deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis where high vertical conductivity at depths of 10 20 kilometres indicates localized porous conduits this suggests that a coincident seismic velocity anomaly12 is evidence of shallow magma transport channels13 14 rather than deeper off axis upwelling we interpret the mantle electrical structure as evidence that plate driven passive upwelling dominates this ridge segment with dynamic forces being negligible

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

structural and volcanic expression of a fast slipping ridge transform ridge plate boundary sea marc i and photographic surveys at the clipperton transform fault a high resolution near bottom survey has been conducted of the clipperton transform fault and adjoining segments of the east pacific rise epr using the sea marc i side looking sonar system and the lamont doherty geological observatory olympus based camera system the transform fault zone tfz is a narrow well defined belt of transform parallel lineaments which varies along strike from a single sharp edged notch to a complex band of subparallel lineaments up to 1 km wide the tfz is set within a 5 km wide band of unusually fine grained side scan texture which could indicate nonbasaltic seafloor and or pervasively sheared and mass wasted basaltic crust the fine grained swath is surrounded by constructional volcanic terrain with no hint of strike slip motion this observation puts an upper limit of 5 km on the extent of lateral migration of the tfz in the last 1 5 m y both ridge transform intersections rtis are dominated by bathymetric highs located on the old plate opposite the spreading center a mantling of fresh looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism thermal expansion may also play a part the epr south of clipperton has recently experienced extrusion of high effusion rate basalts burial of faults and fissures by lava flows and development of vigorous hydrothermal circulation on the epr north of clipperton the axial zone of faults and fissures tapers toward the transform fault this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault

hydrothermal venting in magma deserts the ultraslow spreading gakkel and southwest indian ridges 1 detailed hydrothermal surveys over ridges with spreading rates of 50 150 mm yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting but the validity of this relation at slow and ultraslow ridges is unproved here we compare hydrothermal plume surveys along three sections of the gakkel ridge arctic ocean and the southwest indian ridge swir to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges along the gakkel ridge most apparent vent sites occur on volcanic highs and the extraordinarily weak vertical density gradient of the deep arctic permits plumes to rise above the axial bathymetry individual plumes can thus be extensively dispersed along axis to distances 200 km and 75 of the total axial length surveyed is overlain by plumes detailed mapping of these plumes points to only 9 10 active sites in 850 km however yielding a site frequency fs sites 100 km of ridge length of 1 1 1 2 plumes detected along the swir are considerably less extensive for two reasons an apparent paucity of active vent fields on volcanic highs and a normal deep ocean density gradient that prevents extended plume rise along a western swir section 10 23 e we identify 3 8 sites so fs 0 3 0 8 along a previously surveyed 440 km section of the eastern swir 58 66 e 6 sites yield fs 1 3 plotting spreading rate us versus fs the ultraslow ridges and eight other ridge sections spanning the global range of spreading rate establish a robust linear trend fs 0 98 0 015us implying that the long term heat supply is the first order control on the global distribution of hydrothermal activity normalizing fs to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster spreading ridges in supporting active vent fields this increased efficiency could derive from some combination of three dimensional magma focusing at volcanic centers deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle and nonmagmatic heat supplied by exothermic serpentinization

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

chemical and physical diversity of hydrothermal plumes along the east pacific rise 8 45 n to 11 50 n we conducted a survey of water column hydrothermal plumes along a 350 km long section of the east pacific rise epr axis stretching from 8 45 n to 11 50 n including the region at 9 45 54 n where 1991 alvin dives found evidence for recent seafloor volcanic eruptions our survey included measurements of temperature anomaly light attenuation suspended particles methane hydrogen iron manganese helium and aluminum we detected strong light attenuation plumes in two main regions south of the clipperton transform fault ctf from 9 27 n to 9 57 n and north of the ctf from 11 05 n to 11 35 n however the plumes at these two regions had very different physical and chemical characteristics south of the ctf where the ridge is thought to be magmatically robust the plumes had very high ratios of he heat methane mn and s fe plumes north of the ctf had much lower ratios of he heat methane mn and s fe these striking differences in volatile heat and volatile metal ratios suggest that the plumes in the vicinity of 9 50 n were derived from a young evolving hydrothermal system while the activity at 11 10 n is from an older stable system this survey demonstrated that water column plume measurements can be used not only to accurately locate regions of hydrothermal activity but also to determine the first order chemical and physical characteristics of the hydrothermal fluids exiting from the seafloor

the axial summit graben and cross sectional shape of the east pacific rise as indicators of axial magma chambers and recent volcanic eruptions abstract the axis of the east pacific rise epr undulates up and down hundreds of meters over distances of 30 200 km along strike the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers oscs we have suggested that systematic variations in depth and cross sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment 1 a comparison of recently collected multichannel seismic mcs data 2 with our sea beam and seamarc ii data have allowed us to test and advance this hypothesis along the epr from 9 to 13 n there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment the cross sectional shape of the rise the presence or absence of an axial summit graben and the presence or absence of a shallow axial magma chamber as interpreted from mcs data where the axial magma chamber is present the cross sectional shape of the ridge is broad and an axial summit graben is recognized along the axis in contrast where the cross sectional shape of the rise is narrow and triangular an axial magma chamber is not detected and an axial summit graben is absent these ridge axis characteristics tend to occur along deeper portions of a given ridge segment often near ridge axis discontinuities we suggest that these systematic variations in ridge axis morphology cross sectional shape and structure presence or absence of an axial graben reflect spatial and temporal variations in the magmatic budget of the ridge axis where the magmatic budget is waxing shallow level magma reservoirs in the crust and underlying upper mantle swell creating a broad axial bulge with a summit graben where the magmatic budget is diminished the crustal magma chamber is small this proposed correlation of shape structure and magmatic parameters fails along only two short portions of the ridge in these areas there is evidence for an axial magma chamber and the rise has a broad cross sectional shape but there is no summit graben bottom photographs and submersible results however show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting suggesting that the summit graben has been recently filled in by lava flows and the development of a summit graben or a linear caldera by volcano tectonic collapse has not yet occurred

hydrothermal plumes along the east pacific rise 8 40 to 11 50 n plume distribution and relationship to the apparent magmatic budget abstract the interactions between hydrothermal circulation and large scale geological and geophysical characteristics of the mid ocean ridge cannot be ascertained without large scale views of the pattern of hydrothermal venting such multi ridge segment surveys of venting are accomplished most efficiently by mapping the distribution and intensity of hydrothermal plumes in november 1991 we mapped hydrothermal temperature and light attenuation c anomalies above the east pacific rise epr continuously from 8 40 to 11 50 n a fast spreading ridge crest portion bisected by the clipperton transform fault plume distributions show a precise correlation with the distribution of active vents where video coverage of the axial caldera is exhaustive 9 09 54 n elsewhere in the study area the sketchy knowledge of vent locations gleaned from scattered camera tows predicts only poorly the large scale hydrothermal pattern revealed by our plume studies plumes were most intense between 9 42 and 9 54 n directly over a march april 1991 seafloor eruption these plumes had exceptionally high c ratios compared to the rest of the study area we suggest that the phase separated gas rich vent fluids discharging here fertilize an abundant population of bacteria hydrothermal plume distributions define three categories intense and continuous 8 48 8 58 n 9 29 10 01 n and 11 05 11 27 n weak and discontinuous 8 58 9 29 n and negligible the location of each category is virtually congruent with areas that are respectively magmatically robust magmatically weak and magmatically starved as inferred from previous measurements of axial bathymetric undulations cross axis inflation and magma chamber depth and continuity this congruency implies a fine scale spatial and temporal connection between magmatic fluctuations and hydrothermal venting we thus speculate that at least along this fast spreading section of the epr cyclic replenishment eruption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mush and hot rock we found intense and continuous plumes along 33 of the surveyed ridge crest an observation implying that any point on the ridge is on average hydrothermally active one third of the time combining this result with the 20 plume coverage found along the medium rate juan de fuca ridge suggests that superfast 150 mm yr spreading ridges should support vigorous venting along 50 of their length if spreading rate and along axis plume coverage are linearly related

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

hydrothermal venting and the apparent magmatic budget of the juan de fuca ridge the fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large scale geological controls on the distribution of hydrothermal venting along the mid ocean ridge we undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the juan de fuca ridge a medium rate spreading center with segments of markedly differing morphology hydrothermal anomalies were detected by towing a high precision conductivity temperature depth transmissometer ctdt package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment these surveys conducted between 1985 and 1989 have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid ocean ridge hydrothermal discharge is strongest on those segments or portions of segments where the apparent magmatic budget is highest as indicated by the degree of along axis inflation and other morphological characteristics this correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber we propose that the probability of discharge occurring at any point on the mid ocean ridge is proportional to the spreading rate this probability scaled by the observation that significant plumes presently emanate from 20 of the juan de fuca ridge is expected to range from 5 on the slowest spreading ridge to 50 on the fastest spreading ridge

deep tow studies of the overlapping spreading centers at 9 03 n on the east pacific rise the deeply towed instrument package of the scripps institution of oceanography was used to study for the first time the fine scale structure of an overlapping spreading center osc system the 9 03 n osc on the east pacific rise epr the eastern and western limbs of this osc system which are 8 km apart and overlap by 27 km show marked differences away from the tips the spreading centers are highly tectonized and differ greatly from the ridge tips the western ridge tip exhibits a few fissures but no sign of recent volcanic activity whereas the eastern ridge tip is highly fissured and has experienced more recent volcanic episodes which are confined to a narrow zone within this highly fissured area in both cases the ridge tip fissures are 10 15 oblique to the strike of the faults that occur on the flanks of the adjacent portion of the opposite spreading axis indicating a recent change in the direction of deviatoric tension and suggesting that the eastern and western spreading centers have been propagating southward and northward respectively the eastern ridge tip fissures are transecting the flank of the western spreading axis suggesting that the eastern spreading center is about to become the through going trace of the east pacific rise in contrast the western spreading center disappears into abyssal hill terrain that was presumably created at the eastern ridge axis some of the very few tectonic features observed in the overlap basin may reflect the existence of a short lived shear couple due to the interaction of the two overlapping spreading centers sediment cover over the whole area and especially in the overlap basin is relatively thick supporting the idea of a time averaged deficit in the magmatic budget at this osc we suggest that the 9 03 n osc developed where two magmatic pulses independent in space and time and propagating along the strike of the east pacific rise away from the loci of melt emplacement failed to meet misalignment of the magmatic conduits resulted in the propagation of the two spreading centers past each other and the development of the ensuing osc geometry independent lines of evidence derived from the three dimensional inversion of the magnetic field at the 9 03 n osc sempere et al 1984 from numerical modelling of the growth of two en echelon elastic cracks in a tensile stress field sempere and macdonald 1986 and from geochemical data langmuir et al 1986 support our interpretation of the deep tow data we suggest that deviations from axial linearity of the spreading centers devals langmuir et al 1986 and small nonoverlapping offsets snoos batiza and margolis 1986 are simply local lows or saddle points along the axial depth profile that arise when two magmatic pulses propagating toward one another meet head on oscs and some saddle points i e devals snoos are all small rapidly evolving ridge axis discontinuities which may represent the surficial expression of the distal ends of small scale longitudinal convection cells beneath the east pacific rise

bulk properties of hot smoker plumes the hot buoyant fluid from a submarine vent rises as a plume entraining fluid from the stratified ocean until a height is reached where the driving buoyant force vanishes and shortly thereafter the diluted plume fluid intrudes into the ocean interior at its equilibrium depth while a large positive heat flux issues from the vent the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height depends on the oceanic stratification of both salinity and potential temperature and on the salinity of the source fluid at the vent for example in the atlantic ocean the oceanic stratification causes the spreading plume fluid to be cooler and fresher than the surrounding seawater of the same density while in the pacific the spreading fluid is relatively warm here the well known solution for a turbulent plume rising in a linearly stratified environment is used to obtain simple formulae for 1 the potential temperature anomaly of the horizontally spreading fluid 2 the heat flux associated with this injection of diluted vent water at this density level and 3 the nondimensional solution for the evolution of tracer properties e g salinity and potential temperature in the rising plume

identification of karst sinkholes in a forested karst landscape using airborne laser scanning data and water flow analysis abstract karst sinkholes dolines play an important role in a karst landscape by controlling infiltration of surficial water air flow or spatial distribution of solar energy these landforms also present a limiting factor for human activities in agriculture or construction therefore mapping such geomorphological forms is vital for appropriate landscape management and planning there are several mapping techniques available however their applicability can be reduced in densely forested areas with poor accessibility and visibility of the landforms in such conditions airborne laser scanning als provides means for efficient and accurate mapping of both land and landscape canopy surfaces taking the benefits of als into account we present an innovative method for identification and evaluation of karst sinkholes based on numerical water flow modelling the suggested method was compared to traditional techniques for sinkhole mapping which use topographic maps and digital terrain modelling the approach based on simulation of a rainfall event very closely matched the reference datasets derived by manual inspection of the als digital elevation model and field surveys however our process based approach provides advantage of assessing the magnitude how sinkholes influence concentration of overland water flow during extreme rainfall events this was performed by calculating the volume of water accumulated in sinkholes during the simulated rainfall in this way the influence of particular sinkholes on underground geomorphological systems can be assessed the method was demonstrated in a case study of slovak karst in the west carpathians where extreme rainfalls or snow thaw events occur annually we identified three spatially contiguous groups of sinkholes with a different effect on overland flow concentration these results are discussed in relation to the known underground hydrological systems

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

evidence for volcanic eruption on the southern juan de fuca ridge between 1981 and 1987 the formation of new ocean crust at mid ocean ridges is known to be a discontinuous process in both space and time but little is known about the frequency and duration of eruptions along an active ridge segment here we present evidence from sea beam surveys and underwater photography for the eruption of lavas along a segment of the juan de fuca ridge between 1981 and 1987 although previous studies have inferred volcanic activity on ridges in areas where recent seismicity or young lava flows have been observed1 4 none has yet had direct evidence to date such a recent submarine eruption the temporal coincidence between this eruptive episode and the megaplumes huge sudden emissions of hot mineral laden water observed over this part of the ridge5 6 in 1986 and 1987 supports previous suggestions5 10 that megaplumes are caused by sea floor spreading events

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

geologic form and setting of a hydrothermal vent field at lat 10 56 n east pacific rise a detailed study using angus and alvin a hydrothermal vent field here called the feather duster site occurs on the eastern marginal high near the edge of a narrow 95 m and shallow 15 20 m axial graben within an area dominated by sheet flows and collapse features the sheet flows are intermediate in relative age between younger fluid flow lavas on the floor of the axial graben and older pillow constructional lavas on the marginal highs hydrothermal activity occurs in two zones within a 65 by 45 m area the main zone is located where a fissure system and sulfide sulfate chimneys vent warm 9 47 sup 0 c and hot 347 sup 0 c hydrothermal fluids here two mounds of massive sulfide totaling about 200 t are forming one occurs at the base of a 3 m high scarp which is the wall of a drained lava lake the other is perched on top of the scarp 19 references 4 figures

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

famous and amar segments on the mid atlantic ridge ubiquitous hydrothermal mn ch4 δ3he signals along the rift valley walls and rift offsets dynamic hydrocast experiments enabled mn tdm ch4 concentrations and 3he ratio to be recorded through vertical cross sections of hydrothermal plumes along the famous segment and the southern part of the amar segment on the mid atlantic ridge between 36 n and 37 n mn ch4 and 3he figures all along both segments are well above the seawater background in the open ocean they are interpreted to be the result of time integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends hydrothermal activity along the famous and amar segments appears to be similar a comparison of the residence times of the three tracers from the dispersed time integrated signals is proposed although the background values in these closed basin are high some proximal and or large hydrothermal inputs overprinted on the general time integrated plume can be detected i e the rainbow site south of amar based on the depth and the location of plumes hydrothermal activity is not by far limited to the neo volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley considering the mn and ch4 concentrations in these plumes two types of ocean mantle interaction may be represented hot focused discharges on ultramafic exposures rainbow site and low temperature diffuse serpentinisation

chemical controls on the composition of vent fluids at 13 11 n and 21 n east pacific rise six vent fields sampled at 13 11 n east pacific rise epr in may 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21 n measured ph at 25 c are acidic ranging from 3 1 to 3 7 sodium and chloride vary from 40 lower to 30 higher than seawater iron concentrations range from 2 to 10 mmol kg compared with 0 7 2 5 mmol kg at 21 n other sulfide forming metals cu zn cd and pb are generally lower at 11 13 n than at 21 n reliable temperature measurements were obtained at only two of the six vents and both were 350 5 c the vent fields at 21 n epr were resampled in august 1985 thus extending to almost 6 years the time period over which they have been monitored previous expeditions were made in november 1979 and november 1981 campbell et al this issue have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation equilibrium calculations for the fluids from the fields at 13 n and 21 n using a greatly improved thermodynamic data base are described in this report they indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist type mineral assemblage at depth calculated high temperature ph of the fluids range from 4 1 to 4 7 with those from 13 11 n at the more acidic end of the range calculated affinities show that the fluids are close to or at saturation with respect to quartz albite muscovite smectite chlorite epidote and pyrrhotite the computations imply that lower temperature vents such as national geographic smoker ngs 273 c may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz a comparison of fluid chemistry between ngs and vent 5 at 11 n suggests that the latter vent may also have a temperature 300 c while the fluid chemistry of a given field is compositionally stable the compositions themselves are unique for a particular field and cover a wide range between fields the processes controlling this variability probably include the depth of reaction i e pressure which is difficult to treat thermodynamically at present in addition increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water rock ratio it may be difficult to separate these two effects if the age differences between the fields at present unknown are of the order of decades then their temporal histories may have an important influence on their particular chemistries

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

physical characteristics of the endeavour ridge hydrothermal plume during july 1988 abstract we conducted ctd transmissometer tows from 8 to 26 july 1988 within 15 km of the central hydrothermal vent site 47 57 n 129 06 w on the endeavour segment of juan de fuca ridge anomalies of temperature salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site as a result of widespread plume dispersion background values of potential temperature salinity and light attenuation below the 1900 m depth exceeded those for pristine ambient waters by 0 05 c 0 05 psu and 0 03 m 1 respectively maximum plume anomalies relative to the background waters were of the order of 0 10 c 0 010 psu and 0 10 m 1 at core depths of 2000 2050 m heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation particle anomalies were confined to within 2 5 km of the vent site based on the background water property anomalies and moored current meter records the mean time averaged heat fluxes for the survey region were 2 3 1 5 10 8 w in the along ridge direction 20 t and 7 7 4 7 10 8 w in the cross ridge direction 110 t mean along and cross ridge salt fluxes were 7 5 and 25 15 kg s 1 mean particle fluxes were 0 09 0 06 and 0 29 0 18 kg s 1 estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1 2 0 6 10 10 w and corresponding particulate fluxes as high as6 3 kg s 1

detection and response to volcanic hydrothermal events on the mid ocean ridge the mid ocean ridge is by far the most continuous volcanic system on the planet but since it lies beneath more than 2 km of seawater a thorough study of its thermal and chemical interaction with the ocean remains very difficult until 1990 the end of the period covered by the last iugg report literally all of our understanding of the dynamics of magmatic accretion on the mid ocean ridge was based on inferences from 1 geochemical geophysical and morphologic studies of long sections of the ridge using a variety of instruments mounted on ships hulls or towed on long cables near the seafloor and 2 submersible based mapping and sampling of small sections of the ridge particularly hydrothermally active ones these types of studies have continued to flourish parmentier 1995 humprhis 1995 but the past 4 years have also seen the first near real time observations of accretion events on two pacific spreading centers and the development of technology and techniques that can monitor the occurrence and extent of such events over a selected portion of the ridge system these monitoring capabilities allow interdisciplinary studies of seafloor hydrothermal systems so that the thermal and chemical effects of hydrothermal venting on the oceans can be assessed

effusive and explosive volcanism on the ultraslow spreading gakkel ridge 85 e 1 we use high definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 arctic gakkel vents expedition agave to evaluate the volcanic characteristics of the 85 e segment of the ultraslow spreading gakkel ridge 9 mm yr 1full rate our seafloor imagery reveals that the axial valley is covered by numerous small volume order 1000 m3 lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm the valley floor contains two prominent volcanic lineaments made up of axis parallel ridges and small cratered volcanic cones the lava flows appear to have erupted from a number of distinct source vents within the 12 15 km wide axial valley only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site and these are associated with the oden and loke volcanic cones we model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of co2 that accumulated in possibly deep crustal melt reservoirs the energy released during explosive discharge combined with the buoyant rise of hot fluid lofted fragmented clasts of rapidly cooling magma into the water column and they subsequently settled onto the seafloor as fall deposits surrounding the source vent

deep ocean circulation and transport where the east pacific rise at 9 10 n meets the lamont seamount chain 1 we report the first 3 d numerical model study of abyssal ocean circulation and transport over the steep topography of the east pacific rise epr and adjoining lamont seamount chain in the eastern tropical pacific we begin by comparing results of hydrodynamical model calculations with observations of currents hydrography and sf6 tracer dispersion taken during larval dispersal on the deep east pacific rise ladder field expeditions in 2006 2007 model results are then used to extend observations in time and space regional patterns are pronounced in their temporal variability at m2 tidal and subinertial periods mean velocities show ridge trapped current jets flowing poleward west and equatorward east of the ridge with time varying magnitudes weekly average maximum of 10 cm s 1 that make the jets important features with regard to ridge originating particle larval transport isotherms bow upward over the ridge and plunge downward into seamount flanks below ridge crest depth the passage p1 between the epr and the first lamont seamount to the west is a choke point for northward flux at ridge crest depths and below weekly averaged velocities show times of anticyclonic flow around the lamont seamount chain as a whole and anticyclonic flow around individual seamounts results show that during the ladder tracer experiment sf6 reached p1 from the south in the western flank jet a short lived change in regional flow direction just at the time of sf6 arrival at p1 started the transport of sf6 to the west on a course south of the seamounts as field observations suggest approximately 20 days later a longer lasting shift in regional flow from west to sse returned a small fraction of the tracer to the epr ridge crest

geological chemical and biological evidence for recent volcanism at 17 5 s east pacific rise the superfast spreading portion of the east pacific rise at 17 5 s is a magmatically robust section of mid ocean ridge plumes characterized by high volatile metal ratios which has been hypothesized to be indicative of recent magmatism were found between 17 22 s and 17 35 s in november december 1993 dives with the french submersible nautile in december 1993 observed young sheet flows and widespread venting centered on the segment s shoalest point at 17 26 s eight submersible dives made one year later with the japanese submersible shinkai 6500 in september november 1994 found glassy unsedimented lavas a range of diffuse and high temperature vents and several types of biologic communities on the ridge crest beneath the volatile rich 1993 plumes the diffuse vents range from small areas in the youngest lavas characterized by very low temperature flow 10 c and a relatively low macrofaunal diversity to larger areas where the apparent older surface was covered with a dense and diverse biotope including anemones tubeworms mussels and serpulids several inactive vents marked by dead mussel clam beds are present in the southernmost portion of the site in the older lavas submersible mapping indicates that the youngest lava l0 unit erupted for at least 4 km along the strike of the ridge at the southern site and for 5 10 km at the northern site the flow is up to 400 m in width at the southern site and may be as much as 1 2 km in width at the northern site comparison of the observed 1993 fe s h2s heat and 3he heat plume values with time series measurements at sites on the northern east pacific rise and the juan de fuca ridge where the onset of the magmatic event is known implies that the last magmatic event at 17 5 s occurred within several years prior to 1993 continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives the type and diversity of the vent biota found on the l0 vents also suggests a young system the spatial and temporal scales of the accretion event at the 17 5 s epr site are similar in scale to eruptions on the intermediate rate spreading north cleft segment juan de fuca ridge but the 17 5 s event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

geological indexes of hydrothermal venting hydrothermal venting occurs on mid ocean ridge axes with a diverse array of morphological structural and petrological characteristics consequently it is not clear if certain geological environments are more conducive than others to hydrothermal activity this paper uses complementary data sets recently collected on multiple tectonic segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue the spatial density of venting is gauged by plume incidence the fraction of linear ridge crest overlain by hydrothermal plumes five geological indexes are compared with plume incidence ridge axis elevation ridge axis cross sectional area the percentage of ridge axis underlain by an axial magma chamber amc reflector mgo wt in basaltic glass and spreading rate a mean value for plume incidence and each index where available is calculated for each of 14 second to fourth order tectonic segments from the juan de fuca ridge 44 30 to 48 45 n and the northern 9 to 11 50 n and southern 13 50 to 18 40 s east pacific rise plumes are most common on segments with a cross sectional area 3 5 km2 a net elevation 0 35 km amc coverage 60 and mgo 7 wt spreading rate is not deterministic on the segment scale the data suggest two classes of ridge segments segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget those with high index values have a plume incidence ranging from low to complete coverage apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity the best individual predictors of hydrothermal activity are cross sectional area and net elevation applying these indexes to the east pacific rise between 18 n and 33 s suggests that the unexplored segments with the highest probability of activity are those at 15 30 16 n 6 9 n 5 6 s 27 28 s and 30 30 33 s

hydrothermal methane and manganese variation in the plume over the superfast spreading southern east pacific rise abstract onboard analyses of dissolved ch4 and mn were conducted on the water column along the southern east pacific rise between 13 50 and 18 40 s during the 1993 ridge flux cruise many hydrothermal plumes were identified by chemical anomalies which overlie more than half of the surveyed ridge crest the ch 4 mn ratio in the plumes shows wide variability ranges from about 0 05 to 3 9 due to wide range of ch 4 concentration the ch 4 rich plumes were detected in the region of 16 00 18 40 s where vigorous magmatic budget was confirmed by previous studies moreover the ch 4 mn ratio of the plumes is highly correlated with other volatile metal parameters such as the s fe ratio in plume particulate these results strongly suggest that volatile input caused by magmatic perturbation to hydrothermal systems is responsible for the ch 4 mn variability in the southern epr plumes based on analogy of the case at the epr 9 50 n site the detection of several volatile rich plumes and the extensive hydrothermal flux of volatile species are attributed to frequent magmatic events in the superfast spreading southern epr

methane manganese and helium in hydrothermal plumes following volcanic eruptions on the east pacific rise near 9 50 n 1 as part of a rapid response cruise in may 2006 we surveyed water column hydrothermal plumes and bottom conditions on the east pacific rise between 9 46 0 n and 9 57 6 n where recent seafloor volcanic activity was suspected real time measurements included temperature light transmission and salinity samples of the plume waters were analyzed for methane manganese helium concentrations and the 13c of methane these data allow us to examine the effects of the 2005 2006 volcanic eruption s on plume chemistry methane and manganese are sensitive tracers of hydrothermal plumes and both were present in high concentrations methane reached 347 nm in upper plume samples 250 m above seafloor and exceeded 1085 nm in a near bottom sample mn reached 54 nm in the upper plume and 98 nm in near bottom samples the concentrations of methane and mn were higher than measurements made after a volcanic eruption in the same area in 1991 but the ratio of ch4 mn at 6 7 is slightly lower though still well above the ratios measured in chronic plumes high concentrations of methane in near bottom samples were associated with areas of microbial mats and diffuse venting documented in seafloor imagery the isotopic composition of the methane carbon shows evidence of active microbial oxidation however neither the fractionation factor nor the source of the eruption associated methane can be determined with any certainty considerable scatter in the isotopic data is due to diverse sources for the methane as well as fractionation as methane is consumed one sample at 21 versus peedee belemnite standard is among the most enriched methane carbon values reported in a hydrothermal plume to date

hydrothermal activity and sulphide formation in axial valleys of the east pacific rise crest between 18 and 22 s abstract deep grabens some 90 m in depth occur on the crest of the east pacific rise between 17 9 and 18 6 s and between 21 1 and 21 7 s and are found within the topographic highs in these regions along the rise axis these axial valleys mark areas of recent faulting and intensive hydrothermal activity which is associated with the formation of metalliferous sediments and of massive sulphides of iron copper and zinc most iron and manganese released from the ore fluids continues to be deposited outside the grabens preferentially on the western flank of the rise a tendency which can be recognised also in older sediments of this area

hydrothermal methane venting between 12 n and 26 n along the mid atlantic ridge hydrographic surveys along the mid atlantic ridge mar between 12 n and 26 n carried out from 1984 to 1990 show a variable pattern of ch4 rich water column plumes the vertical distribution of ch4 at stations located every 20 40 km is presented along this 1200 km long section of the mar ch4 venting is clearly demonstrated ch4 enriched fluids rise from vents as plumes spreading is confined to the axial valley due to the topography of the mar ch4 contents from 45 nmol to 675 nmol kg are measured in the buoyant plumes above the two active hydrothermal sites mark 23 n tag 26 n known at present whereas ch4 anomalies up to 3 6 nmol kg are typically observed in plumes emitted either on the inner floor on the walls and or at the top of the rift mountains along the studied ridge section ch4 concentrations 45 mol to 144 mol kg in mark and tag vent fluids are of the same order of magnitude as those found in the east pacific rise fluids even though ch4 is known to be unstable with respect to oxidation by dissolved oxygen and in spite of its microbial oxidation in plumes these results confirm ch4 to be a good indicator to track hydrothermal plumes and to map the variation of hydrothermal activity along mid oceanic ridges moreover between 12 and 26 n along the mar ch4 results show that while hydrothermal activity is present everywhere along the ridge it is predominant near fracture zones fz kane fz 15 20 n fz comparison of ch4 tracer with total dissolvable manganese tdm tracer in plumes allows us to differentiate subseafloor hydrothermal processes the high tdm ch4 found above tag and mark areas is indicative of basalt seawater interaction while at 15 n the low tdm ch4 provides evidence of fluid circulation in ultrabasic rocks ch4 data confirm the association between mantle degassing hydrothermal activity and serpentinization along this 12 26 n section of the mid atlantic ridge

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise