mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge east pacific rise 17 to 19 s naudur cruise 1993 phase separation processes controlled by volcanic and tectonic activity during the naudur cruise in december 1993 23 dives using the french submersible nautile were conducted on the axis of the east pacific rise between 17 and 19 s where the spreading rate is among the fastest in the ocean 14 to 16 cm yr twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered respectively at 17 25 s 18 15 s and 18 26 s the fluids exhibit a very wide range of temperature chemical and gas compositions on the 17 25 s and 18 26 s segments fluids have quite uniform compositions low chlorinities are gas enriched and are low in dissolved metals relative to fluids from the 18 15 s segment which show high chlorinities are less gas enriched and show high metal concentrations chloride and metal depletion associated with gas enrichment is consistent with phase separation whereas ch 4 end members show large variations between sites the 13 ch 4 data are very similar with 13 c values in a narrow range 22 0 to 23 9 versus pee dee belemnite pdb 13 co 2 measured in fluids within the 18 15 s and 18 26 s segments are respectively 7 9 and 5 8 versus pdb similar to 13 c of co 2 trapped in mid oceanic ridge basalts suggesting a magmatic origin the variability in fluid composition is linked to the variability of the accretion process observed on the three segments the uniform venting of low chlorinity fluids in the 17 25 s and 18 26 s segments is connected with volcanic activity which causes boiling with preferential venting of vapor enriched fluids high salinity fluids are emitted on the 18 15 s segment where the ridge is tectonics dominated and subseafloor circulation controlled by faults phase separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area as previously observed on the juan de fuca ridge or in the north fiji basin ridge

circulation and transport of water along the western weddell sea margin ocean current temperature and salinity data obtained from the western weddell sea during the austral winter 1992 u s russian drifting ice station experiment ice station weddell 1 isw 1 are used to describe water circulation and transport surface to bottom baroclinic currents were computed by applying the geostrophic approximation to derived density data these were corrected using current measurements obtained from drifting current meter arrays and the resulting total currents were vertically integrated to obtain volume transports transport was found to be northward in the region which encompassed the western boundary current of the cyclonic weddell sea gyre this northward transport increased from south to north by more than a factor of 2 from about 12 106m3 s 1 in the southwestern weddell to about 28 106m3s 1 farther north the increase in northward transport was compensated for by westward flow from the interior of the gyre into the western boundary region about 5 6 106m3s 1 of the northward transport was contained in a 300 500 m thick bottom layer of cold water this layer whose transport increased by about 1 106 m3 s 1 from south to north was identifiable by its water mass characteristics as weddell sea bottom water originating on the southwestern and western shelf regions its north flowing volume was consistent with past estimates of a 1 5 2 106 m3 s 1 production rate coupled with a 300 400 transport increase due to entrainment during downslope flow from the shelves to the deep basin the maximum northernmost northward transport less the bottom water transport is consistent with previous estimates for wind driven transport in the weddell gyre provided that bottom friction and the sea ice influence on wind forcing are taken into consideration

chemistry of hydrothermal fluids from the 17 s active site on the north fiji basin ridge sw pacific abstract in june 1989 a diving cruise of the french submersible nautile was made along the north fiji basin spreading ridge the objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the kaiyo cruise in 1988 active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge the transparent venting fluid showed significant salinity depletion the extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the cl depleted hydrothermal fluid from the ashes vent site in the juan de fuca ridge these features suggest that the present hydrothermal end member resulted from subcritical phase separation which generated a low salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents

fluctuation of chemical compositions of the phase separated hydrothermal fluid from the north fiji basin ridge ishibashi j grimaud d nojiri y auzende j m and urabe t 1994 fluctuation of chemical compositions of the phase separated hydrothermal fluid from the north fiji basin ridge in j m auzende and t urabe editors north fiji basin starmer french japanese program mar geol 116 215 226 on the central ridge of the north fiji basin southwest pacific hydrothermal activities were located in 1989 it was the second discovery of the hydrothermal fluid undergo phase separation during a dive mission of slrinkai 6500 in 1991 we revisited the hydrothermal area and collected fluid samples from the same anhydrite chimney fluid composition of the end member shows no significant changes during this two years other fluid ventings 150 m apart show similar chemistry suggesting that the phase separation and segregation processes occur throughout the hydrothermal area small fluctuation in chemical composition of the fluids from two sites is attributed to subseafloor fluid rock interaction after the phase segregation

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

geochemistry of some gases in hydrothermal fluids from the southern juan de fuca ridge five samples of hydrothermal fluids from two vent areas on the southern juan de fuca ridge were analyzed for dissolved gases concentrations in the end member hydrothermal fluid of h2 270 527 mol kg ch4 82 118 mol kg and co2 3920 4460 mol kg are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems the carbon isotopic ratios of the ch4 13c 17 8 to 20 8 and co2 13c 3 6 to 4 7 suggest that at least some of the ch4 and co2 in the fluids is basalt derived the range of 13c values for the basalt derived co2 is 6 8 to 9 7 calculated by assuming conservation of recharge co2 during hydrothermal circulation apparent temperatures of equilibration between the ch4 and the basalt derived co2 range from 640 c to 750 c small amounts of ethane c2h6 ch4 0 9 10 3 2 2 10 3 propane and butane detected in the samples may also have formed in the basalt one sample of almost pure 95 5 hydrothermal fluid contained a significant fraction up to 63 and 74 respectively of the recharge ar and n2 this suggests that the fluid has not undergone extensive vapor liquid phase separation

consequences of phase separation on the distribution of hydrothermal fluids at ashes vent field axial volcano juan de fuca ridge recent studies of hydrothermal fluid chemistry at the axial seamount hydrothermal emissions study ashes vent field of axial volcano indicate separation of liquid and vapor phases during the migration of fluids to the seafloor the brine phase liquid emanates from discrete sulfide chimneys while fluid which has been through vapor phase separation flows diffusely from fissures in the surrounding basalt one possible mechanism to explain this spatial segregation of the fluid phases is based on the relative permeability terms in the equations of two phase flow a qualitative model for the distribution of fluids observed at ashes is postulated in which brine phase fluids are confined within flow conduits by a surrounding relative permeability barrier while vapor phase fluids flow diffusely into the surrounding host rock

factors affecting the rare earth element compositions in massive sulfides from deep sea hydrothermal systems to reconstruct the evolution of ore forming fluids and determine the physicochemical conditions of deposition associated with seafloor massive sulfides we must better understand the sources of rare earth elements rees the factors that affect the ree abundance in the sulfides and the ree flux from hydrothermal fluids to the sulfides here we examine the ree profiles of 46 massive sulfide samples collected from seven seafloor hydrothermal systems these profiles feature variable total ree concentrations 37 2 4092 ppb and ree distribution patterns la cn lu cn ratios 2 00 73 8 eu eu cn ratios 0 34 7 60 the majority of the ree distribution patterns in the sulfides are similar to those of vent fluids with the sulfides also exhibiting light ree enrichment we demonstrate that the variable ree concentrations eu anomalies and fractionation between light rees and heavy rees in the sulfides exhibit a relationship with the ree properties of the sulfide forming fluids and the massive sulfide chemistry based on the sulfide ree data we estimate that modern seafloor sulfide deposits contain approximately 280 t of rees according to the flux of hydrothermal fluids at mid ocean ridges mors and an average ree concentration of 3 ng g in these fluids hydrothermal vents at mors alone transport more rees 360 t to the oceans over the course of just 2 years than the total quantity of rees in seafloor sulfides the excess rees i e the quantity not captured by massive sulfides may be transported away from the systems and become bound in sulfate deposits and metalliferous sediments

helium and carbon geochemistry of hydrothermal fluids from the north fiji basin spreading ridge southwest pacific the north fiji basin is one of the active marginal basins in the southwestern pacific hydrothermal fluid emanations were located at two sites on the central ridge of the basin high temperature fluids 230 290 c venting from anhydrite chimneys at a 17 s site have end member compositions of 11 0 14 5 mmol kg of co2 30 4 43 5 mol kg of ch4 and 2 3 4 5 10 5 cm3stp g of he taking phase separation processes into account the gas abundances are not enriched compared with the mid oceanic ridge hydrothermal systems isotopic compositions of co2 13c 6 2 to 5 7 pdb ch4 13c 20 to 18 pdb and helium rra 9 0 10 0 are comparable to the mid oceanic ridge signature together with basalt helium data the helium isotopic signature may be attributed to the incorporation of a hotspot like primitive component low temperature shimmering fluids the highest measured temperature was 5 2 c associated with biological communities at a site at 18 50 s show slight chemical anomalies in some species in sio2 mn li ph and ch4 and helium isotope ratios distinct from ambient seawater evaluated helium isotopic compositions prior to dilution fall between rra 8 4 and 8 9 supporting the suggestion of high temperature hydrothermal activity at this site although this was not observed by dive expeditions the gas geochemistry of these two different types of fluids show several similar characteristics to the mid oceanic ridge hydrothermal systems this result is in accordance with previous petrological studies which demonstrated a dominant n morb source signature and a co existing oib source influence of the north fiji basin magmatism

measurements of total carbon dioxide and alkalinity by potentiometric titration in the geosecs program abstract approximately 6000 determinations of the alkalinity and total carbon dioxide content of seawater have now been made in the atlantic pacific and indian oceans as part of the geosecs program by a computer controlled potentiometric titration technique the equations used to locate the equivalence points of the carbonic acid system on this titration curve were developed in 1971 but have not previously been published these functions may be represented by f 1 v 2 v v 0 n h k 1 c v 0 v v 0 h h s o 4 h f b o h 4 1 h k 1 c f 2 v 0 v v 0 h h s o 4 h f h c o 3 upon inspection these functions are analogous to the modified gran functions of hansson and jagner 25 with the omission of the contributions of oh and co 3 2 and with the contribution of b oh 4 being assessed at a chlorinity of 19 for all samples reprocessing the original titration e m f volume data with appropriate corrections and modified gran functions reveals an error of about 12 mol kg in the geosecs total carbon dioxide data in addition the protonation of dissolved phosphate species during the titration results in a contribution to measured total carbon dioxide equal to the total phosphate concentration differences in the application of the geosecs functions between the atlantic and the pacific indian oceans expeditions are also to be found so that the error deriving from this source for the atlantic expedition was only 5 mol kg the application of the correct functions increases precision enabling smaller differences such as those attributable to fossil fuel carbon dioxide potentially to be observed and increases accuracy so that the error in titrator total carbon dioxide previously diagnosed by takahashi 14 can be logically accounted for

regions where lightning flashes began regions where 773 flashes began during 13 thunderstorms were located by calculating centroids of the sources of the first six or 10 vhf pulses that were emitted by each flash sources were located by measuring differences in the times at which their pulses arrived at five widely spaced receivers stationed on the ground we found that the distribution of origin heights was bimodal with peaks at 5 3 and 9 2 km above mean sea level amsl standard errors in the coordinates of flash origins were estimated to be 20 80 m in x and y and 160 240 m in the height coordinate z there were 431 flashes in the lower group and 342 in the higher flashes in the lower group were more numerous in 10 storms in three storms high flashes were in the majority there was evidence to suggest that this condition depended partly on the phase of the host thunderstorm recorded e field changes produced by 165 flashes whose paths we had mapped convinced us that the vast majority of the 773 flashes including the 342 high flashes had also been negative flash origins tended to cluster in regions that were a few kilometers or less in horizontal diameter densities of flash origins in these regions ranged from 1 25 to 25 km 3 the origins of 658 flashes were mapped onto radar precipitation patterns of their host storms we found that 66 of the flashes began within one picture element approximately 270 m of the contours for 20 dbz 27 began inside these contours and most of these began at edges of high reflectivity cores the remaining 7 began outside the 20 dbz contours similar results 73 19 8 were obtained for 276 high flashes that began at heights above 7 4 km amsl but 195 ground flashes scored 54 36 9 and showed a greater tendency to begin inside the 20 dbz contours the distribution of distances between origins and their nearest 20 dbz contours showed a marked peak near zero we concluded that charge density in thunderclouds was affected by the presence of heavy precipitation and that 20 dbz surfaces enclosed regions that carried excess negative charge

lightning and precipitation in a small multicellular thunderstorm radio images of 26 consecutive lightning flashes are described in relation to radar pictures of the storm these flashes all started in two vertical turrets where reflectivity factors exceeded 25 dbz five large cloud flashes pervaded an almost horizontal plume or low level anvil and ground flashes occurred on the opposite side in the propagating flank of this storm observers on the ground reported a light hailfall and copious rain most of the lightning occurred in a region that was 3 5 km above ground agl flashes originated at heights that ranged from 3 1 km where the environmental air temperature was 5 c to 5 1 km agl 16 c and they began at a mean height of 3 94 km agl 10 c the standard deviation of 26 heights at which flashes began was 440 m recorded electric field changes showed that the channels were charged negatively and we inferred that negative charge had accumulated at these heights in the vertical turrets and that the paths followed by cloud flashes were influenced by positive charge which had accumulated in a plume that was nearly horizontal and which had formed on the flank opposite to the propagating flank of the storm the estimated total length of 18 consecutive lightning flashes that had occurred during a 3 min interval was 486 km measurements of electric field change magnitudes were used to estimate totals of 440 c and 106 c that had been discharged in 180 s from two cells whose volumes enclosed by 30 dbz surfaces were 260 and 80 km3 respectively this paper also describes the geometries of stepped leaders that cause electric field changes of the bil form it discusses a continuing current which was carried by a cloud flash

on the cooling of a buoyant boundary current abstract through a steady state reduced gravity model we examine the downstream evolution of a buoyant boundary current as it is subjected to surface cooling it is found that the adverse pressure gradient associated with the diminishing buoyancy is countered by falling pressure head so the overall strength of the current as measured by the transport weighted mean square velocity remains unchanged this constancy also applies to the cross stream difference of the square velocity because of the vorticity constraint which leads to the general deduction that the net current shear is enhanced regardless of its upstream sign as a consequence if the upstream flow contains near shore and offshore branches that are comparable in strength this parity would persist downstream but if the near shore branch is weaker to begin with it may be stagnated by cooling with the ensuing generation of anti cyclonic eddies on account of the geostrophic balance the buoyant layer narrows as the square root of the buoyancy the same rate as the falling pressure head but more rapid than that of the local deformation radius some of the model predictions are compared with observations from the tsushima current in the japan east sea

paleoecology reconstruction from trapped gases in a fulgurite from the late pleistocene of the libyan desert when lightning strikes the ground it heats melts and fuses the sand in soils to form glass tubes known as fulgurites we report here the composition of co 2 co and no contained within the glassy bubbles of a fulgurite from the libyan desert the results show that the fulgurite formed when the ground contained 0 1 wt organic carbon with a c n ratio of 10 15 and a 13 c of 13 96 compositions similar to those found in the present day semiarid region of the sahel where the vegetation is dominated by c 4 plants thermoluminescence dating indicates that this fulgurite formed 15 k y ago these results imply that the semiarid sahel at 17 n reached at least to 24 n at this time and demonstrate that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology

molecular evidence for microorganisms participating in fe mn and s biogeochemical cycling in two low temperature hydrothermal fields at the southwest indian ridge we examined two low temperature hydrothermal deposits rich in fe si mn collected from the recently discovered hydrothermal fields at the southwest indian ridge using mineralogical geochemical and molecular biological techniques the mineralogical and geochemical analyses indicated that the low temperature hydrothermal fields would provide a warm and chemical species rich habitat for chemosynthetic based hydrothermal ecosystems analyses of 16s rrna sequences showed that proteobacteria pseudoalteromonas leptothrix and pseudomonas were potential fe and mn oxidizers in the low temperature hydrothermal environments but they were not present in equal abundance among the subniches some potential fe and mn reducers were also recovered they were more commonly found in the exterior black fe mn oxides the difference between the exterior black fe mn oxides and the interior opal a could be related to differences in in situ physicochemical conditions we also identified microbial players that may participate in sulfur s geochemical cycling in these low temperature hydrothermal environments via analyses of 16s rrna sequences and the apra functional gene the results indicated that members of proteobacteria and proteobacteria were involved in the s oxidation process while members of proteobacteria nitrospirae firmicutes and archaea might participate in the s reduction process fe mn and s oxidizers and reducers might actively participate in hydrothermal biogeochemical processes which could influence the transfer of chemical species and the formation of biogenic minerals

chemistry of ore forming fluids and mineral formation rates in an active hydrothermal sulfide deposit on the mid atlantic ridge the chemical composition of diffuse effluent from the tag hydrothermal mound 26 n mid atlantic ridge directly demonstrates that mixing of entrained seawater and high temperature black smoker fluids leads to the precipitation of sulfides silica and anhydrite and to the zone refining of metals including cu zn u and the rare earth elements in an actively forming sulfide ore body rates of mineral formation deduced from these data are extremely rapid and are up to an order of magnitude greater for anhydrite 5 15 107 kg yr than for sulfides 106 107 kg yr comparison with mineral inventories for tag suggests that virtually all of the anhydrite in the tag mound is contemporary and confirms that the sulfide precipitated episodically during the history of mound growth and that much of it has been lost by oxidation and reworking

surface vitrification caused by natural fires in late pleistocene wetlands of the atacama desert we describe extended occurrences of unusual silicate glass surface layers from the atacama desert chile these glasses found near the town of pica at four localities separated by up to 70 km are neither fulgurites nor volcanic glasses nor metallurgical slags related to anthropic activity but show close similarities to other glasses that have been previously attributed to large airbursts created by meteoroids entering the earth s atmosphere the glasses are restricted to specific late pleistocene terrains paleo wetlands and soils rich in organic matter with sio2 rich plant remains salts and carbonates 14c dating and paleomagnetic data indicate that the glasses were formed during at least two distinct periods this rules out the hypothesis of a single large airburst as the cause of surface melting instead burning of organic rich soils in dried out grassy wetlands during climate oscillations between wet and dry periods can account for the formation of the pica glasses large oases did indeed form in the hyperarid atacama desert due to elevated groundwater tables and increased surface discharge during the central andean pluvial event roughly coeval with the mystery interval and younger dryas finally we discuss the implications of our results for the other surface glasses previously attributed to extraterrestrial events

eddy formation by dense flows on slopes in a rotating fluid properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated with a variety of laboratory experiments the dense fluid may initially flow down the slope but it turns under the influence of rotation to flow along the slope and initial geostrophic adjustment gives it an anticyclonic velocity profile some of the dense fluid drains downslope in a viscous ekman layer which may become unstable to growing waves provided that the viscous draining is not too strong cyclonic vortices form periodically in the upper layer and the dense flow breaks up into a series of domes three processes may contribute to the formation of these eddies first initial downslope flow of the dense current may stretch columns of ambient fluid by the taylor column process which we term capture secondly the initial geostrophic adjustment implies lower layer collapse which may stretch the fluid column and thirdly viscous drainage will progressively stretch and spin up a captured water column overall this last process may be the most significant but viscous drainage has contradictory effects in that it progressively removes dense lower layer fluid which terminates the process when the layer thickness approaches that of the ekman layer the eddies produced propagate along the slope owing to the combined effects of buoyancy coriolis balance and beta gyres this removes fluid from the vicinity of the source and causes the cycle to repeat the vorticity of the upper layer cyclones increases linearly with l d where l is the rossby deformation radius the bottom slope and d the total depth reaching approximately 2 f in the experiments presented here the frequency at which the eddy dome structures are produced also increases with while the speed at which the structures propagate along the slope is reduced by viscous effects the flow of dense fluid on slopes is a very important part of the global ocean circulation system and the implications of the laboratory experiments for oceanographic flows are discussed

gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss hydrothermal fluid samples collected in 1984 1987 and 1988 from a large vent field near 47 57 n on the endeavour segment of the juan de fuca ridge jfr have been analyzed for major and minor elements and gases there are of the order of 100 individual smoker vents on 10 large sulfide structures which are localized along faults and fault intersections across the vent field each sulfide structure has a characteristic fluid composition which varies very little from one vent orifice to the next or from year to year on a given structure however there are large gradients in fluid composition across the vent field with endmember chlorinity increasing from 255 mmol kg in the sw to 505 mmol kg in the ne end member concentrations of major elements are well correlated with chlorinity and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids the gradients in composition across the vent field and measured vent fluid temperatures 400 c are consistent with supercritical phase separation and loss of brine phase below the seafloor the factor of 2 variation in co2 and h2s is larger than expected for loss of a very high chlorinity brine concentrations of iron and manganese are not positively correlated with chlorinity suggesting that temperature and ph are more important in controlling metal solubility elevated ammonia and bromide chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter and high boron concentrations point to a sedimentary source

geochemistry of north cleft segment vent fluids temporal changes in chlorinity and their possible relation to recent volcanism hydrothermal vent fluids from the north cleft segment of the juan de fuca ridge between 44 54 and 45 00 n were sampled in 1988 1990 1991 and 1992 in 1988 chloride depleted relative to seawater diffuse fluids with low dissolved metal concentrations relative to other mid ocean ridge mor fluids were sampled over a distance of 10 km along axis in 1990 and 1991 both high temperature 200 c and diffuse vents were consistently metal and chloride enriched the end member compositions of high temperature vents and nearby diffuse vents were very similar there were small but significant correlated changes in diffuse and high temperature composition from 1990 to 1991 the large scale but temporary venting of low chlorinity fluids over the entire north cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride enriched fluids it is possible that a volcanic event along the north cleft segment prior to 1987 for which there is firm geologic evidence initially caused a boiling event which resulted in the preferential venting of vapor enriched fluids through 1988 followed by a transition to brine enriched fluids by 1990 high iron low sodium and low sr ca ratios in the high chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event the absence of chloride depleted fluids from 1990 onward and the systematics of lithium boron and manganese with chloride in the high temperature fluids from north cleft suggest that the evolution toward lower chlorinity at monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater

manganese and methane in hydrothermal plumes along the east pacific rise 8 40 to 11 50 n abstract in november 1991 we surveyed the water column for hydrothermal plumes along 350 km of the east pacific rise axis from 8 40 to 11 50 n using a combination of physical and chemical measurements our survey included the two major ridge segments north and south of the clipperton transform fault at about 10 10 n both limbs of the overlapping spreading centers osc s at 9 03 n and 11 45 n and a 30 km section of the next ridge segment to the south we found vigorous plumes along most of this ridge axis in keeping with its magmatically robust cross section axial summit caldera and shallow magma related seismic reflector these plumes were detectable by both physical temperature and light attenuation and chemical dissolved mn and ch4 measurements although the chemical measurements were more sensitive the least active sections were the southern third of the northern segment from 10 20 to 52 n and the oscs especially the osc at 11 45 n plumes there had weak mn and ch4 signals and were barely detectable by physical methods these axial sections were the only ones surveyed that lie deeper than 2600 m and appear to be magma starved the most active sections on the northern segment gave stronger signals for mn and temperature than for ch4 and light attenuation whereas the opposite was true on the southern segment which was the site of a volcanic eruption at 9 45 52 n only seven months prior to our cruise on the northern segment the four physical and chemical plume tracers correlated positively and linearly with one another suggesting that the segment was fed by relatively uniform end member fluids with a mean ch 4 mn molar ratio of 0 075 the southernmost section surveyed from 8 42 to 9 08 n closely resembled the northern segment the rest of the southern segment fell into three sections with different ch 4 mn ratios 9 39 to 53 n with ch 4 mn as high as 10 9 08 to 39 n with ch 4 mn of 0 51 and 9 53 to 10 07 n with ch 4 mn of 0 85 the section with the highest ch 4 mn was the site of the volcanic eruption which produced high temperature low salinity gas rich vent fluids carrying abundant bacterial particles the high ch4 concentrations are clearly associated with the volcanic eruption but the origin of the ch4 is unclear

acidic and sulfate rich hydrothermal fluids from the manus back arc basin papua new guinea hot 88 120 c and acidic ph 2 1 hydrothermal fluids rich in sulfate were discovered venting in the desmos caldera depth 1926 m eastern manus back arc basin bismarck sea surrounded by papua new guinea the abundant sulfate 32 8 m m higher than the seawater value of 28 m m with elemental sulfur deposition around the vents and remarkably low d h 2 o and 34 s h 2 s values 8 1 and 5 6 respectively are suggestive of the incorporation of a magmatic fluid and the disproportionation of the exsolved so 2 from a magma body the desmos fluid may be similar in origin to the acidic sulfate chloride hot springs associated with subaerial volcanic activity in contrast to the typical hydrothermal end member mg concentration of 0 the desmos fluids are rich in mg 46 52 m m probably because of mg dissolution by acid attack upon magnesium silicate minerals

geochemical and thermal fluxes high temperature venting and diffuse flow from mid ocean ridge hydrothermal systems the tag hydrothermal field mid atlantic ridge 26 n

a numerical study of three dimensional dense bottom plumes on a southern ocean continental slope three dimensional features of dense bottom plumes flowing over continental slopes with or without along slope topographic variations are investigated by simulating the evolution of a density front at a southern ocean continental shelf break using a three dimensional primitive equation numerical model with a second order turbulence closure scheme embedded the focus of our investigation is the role of topography in determining mixing and the offshore transport of dense shelf water during the transient adjustment process of a density front over a continental slope we compare and discuss the numerical simulations for two cases a uniform shelf and slope case and a case with a canyon that leads from the coast to the deep ocean crossing the shelf and the slope the numerical simulations indicate that baroclinic instability planetary rotation bottom friction and topography play major roles in determining the surface and bottom plume formation the growth and penetration depth of the bottom plumes and the characteristics of water mass on the slope mesoscale eddies play a fundamental role in transporting mass heat and salt from shelf to deep ocean the effects of the depth and width of the canyon are examined with two more experiments the presence of a wide and deep canyon in the continental shelf and slope enhances considerably the drainage of coastal shelf water into the deep ocean

hydrothermal fluid emanations from the submarine kick em jenny volcano lesser antilles island arc abstract little is known about the role of island arcs as hydrothermal sources to the ocean when compared to the extensive research that has been carried out on hydrothermal systems at spreading ridges although increasingly more work is being done on intraoceanic arcs here we present fluid geochemistry data from the kick em jenny kej submarine volcano of the lesser antilles island arc discharge of diffuse hydrothermal fluid was discovered on the southwestern flanks of the volcano hydrothermal input into the water column was recognized from ch4 and trace metal anomalies in water column profiles in a depth range of 500 700 m samples collected directly on the seafloor also showed evidence for hydrothermal emissions where mg and cl were depleted in the fluids compared to ambient seawater while ch4 and several trace metals were enriched the chemical composition of these samples including o and h isotope data suggests that the fluids are a mixture between a condensed vapor phase derived from a phase separated hydrothermal fluid which was conductively cooled in the sub surface and seawater increased dissolved organic carbon doc concentrations of the fluids also indicate interaction with the sediment layer through which the fluids percolated although the volcanic crater could not be sampled directly due to safety concerns 3he and some trace metal values in seawater samples indicate fluid input containing a mantle component probably from a hot focused flow source in the crater region around 200 300 m depth

evolution of the deep western boundary current of antarctic bottom water in the brazil basin abstract a synthesis of woce and other hydrographic data shows that the deep western boundary current of antarctic bottom water has a double core structure and that it is differentially modified during its northward transit through the brazil basin at 25 s it descends about 200 m while continuing equatorward as a paired double core current over the relatively broad sloping topography of the western region the downslope core is faster 4 6 cm s 1 versus 3 cm s 1 and is coincident with the high density core equatorward of 19 s the cores begin to diverge and by 15 s the velocity cores have separated and the high speed pattern has inverted between 13 and 11 s the colder slower flowing branch around 3 cm s 1 deepens by 350 m with a noticeable decrease in the vertical stratification of overlying water characteristics subsequently it continues to the equator toward the romanche chain fracture zones the warmer faster flowing branch around 8 cm s 1 proceeds to the equator along the continent

an eddy resolving numerical model of the ventilated thermocline abstract a three dimensional primitive equation numerical model is used to study the effects of mesoscale eddies within the subtropical thermocline solutions are obtained for an ocean bounded by idealized topography are driven by simple wind and buoyancy forcing at the surface results using an eddy resolving fine grid are compared to those using a noneddy resolving coarser grid relatively little difference is realized in the basic mean flow patterns of the two solutions however more significant differences are seen in the distributions of a passive tracer and the potential vorticity mixing by eddies in the westward flowing sector of the subtropical gyre is quite effective in homogenizing these quantities on isopycnals whereas previous theory has predicted homogenization of potential vorticity on long time scales within recirculating gyres the present model demonstrates homogenization on a much shorter time scale across recirculating ventilating flow boundaries anomalous potential vorticity th

systematics of and postulated controls on submarine hydrothermal solution chemistry the chemical compositions of seafloor hydrothermal solutions from the galapagos spreading center 21 n and 13 n on the east pacific rise the southern juan de fuca ridge and the guaymas basin gulf of california are examined for systematic variations several elemental ratios elemental products and empirical geothermometers show much consistency among the various areas in spite of large absolute differences in elemental concentrations the results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites examination of the solution chemistry in conjunction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry as potassium appears to be solubility controlled its use to calculate water rock ratios is inappropriate lithium appears to be a better element to use in these calculations a model to explain the observed solution chemistries as a result of three component mixing is discussed all of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing a gas rich vapor phase and a gas poor brine phase or any one of these individual components alone the vapor and brine phases are the result of phase separation which may have occurred at some time in the past essentially all of the reported data fall within the field defined by these three components the solutions apparently have had sufficient time after the mixing event s to reequilibrate with the rock this is true for quartz as well as other chemical components the best test of this model is chloride together with gas and isotropic data as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18 o and low chlorinity solutions will have elevated gas contents and lower 18 o

formation of hydrothermal vents in the north fiji basin sulfur and lead isotope constraints abstract hydrothermal sulfides were recovered from the 16 50 s triple junction area in the north fiji basin at a water depth of ca 1900 m the chimney samples can be divided into three groups based on their major metal contents 1 type 1 fe cu rich 2 type 2 fe cu rich with minor zn and 3 type 3 zn rich chimneys type 1 chimneys are mainly composed of chalcopyrite and pyrite and are enriched in elements commonly precipitated under high temperature conditions 300 c such as cu co mo and se type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite and are enriched in elements commonly associated with relatively low temperatures 150 to 250 c such as zn cd pb as and ga type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples type 2 chimneys appear to be the products of low temperature replacement which might be related to formation of type 3 chimneys of type 1 chimneys the trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources n morb and e morb sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys the type 1 sulfides 34 s 2 8 1 2 206 pb 204 pb 18 082 to 18 132 207 pb 204 pb 15 440 to 15 481 208 pb 204 pb 37 764 to 37 916 are depleted in 34 s and have lower pb isotope ratios compared to types 2 and 3 chimneys 34 s 4 1 0 9 206 pb 204 pb 18 122 to 18 193 207 pb 204 pb 15 475 to 15 554 208 pb 204 pb 37 882 to 38 150 the higher 34 s values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater the more radiogenic pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to e morb volcanism that shows higher pb isotope composition than n morb rocks however ferromanganese crust samples have radiogenic pb isotope compositions that are identical to those of e morb therefore input of hydrogenous pb is also a possible source for radiogenic pb isotopes in the types 2 and 3 chimneys

distribution patterns of the elements in the ocean a synthesis the observed partition patterns of elements between a marine solid phase e g pelagic clays deep sea manganese nodules seamount manganese crusts and plankton and seawater can be to the first approximation explained by the james and healy adsorption model except for ce co mn pb and tl which are concentrated in the solid phase through oxidative uptake processes in other words the composition of seawater is mainly controlled by the compositions of source materials i e the crustal igneous rocks and primary magmatic volatiles and the physicochemical principles regulating the adsorption processes of various chemical species in seawater onto oceanic pelagic clays

submarine venting of phase separated hydrothermal fluids at axial volcano juan de fuca ridge since the discovery of high temperature venting on the east pacific rise in 19791 it has been expected2 3 because of the physical properties of sea water at pressures and temperatures encountered during submarine hydrothermal circulation4 5 that phase separated fluids would discharge from ridgecrest vents although this notion is supported by the reported large deviations in vent fluid chlorinity relative to that of sea water 40 200 6 7 by observations of venting at p t conditions clearly within the two phase region 220 bar and 420 c 8 and by fluid inclusion data9 10 unequivocal identification11 of phase separated venting fluids has remained elusive here we report observations of chloride and metal depleted gas enriched fluids from a shallow vent field on the juan de fuca ridge which confirm the expectation that phase separated effluents are delivered to the deep ocean from some sea floor venting systems

export of weddell sea deep and bottom water an extensive set of conductivity temperature depth ctd lowered acoustic doppler current profiler ladcp data obtained within the northwestern weddell sea in august 1997 characterizes the dense water outflow from the weddell sea and overflow into the scotia sea along the outer rim of the weddell gyre there is a stream of relatively low salinity high oxygen weddell sea deep water defined as water between 0 and 0 7 c constituting a more ventilated form of this water mass than that found farther within the gyre its enhanced ventilation is due to injection of relatively low salinity shelf water found near the northern extreme of antarctic peninsula s weddell sea shelf shelf water too buoyant to descend to the deep sea floor the more ventilated form of weddell sea deep water flows northward along the eastern side of the south orkney plateau passing into the scotia sea rather than continuing along an eastward path in the northern weddell sea weddell sea bottom water also exhibits two forms a low salinity better oxygenated component confined to the outer rim of the weddell gyre and a more saline less oxygenated component observed farther into the gyre the more saline weddell sea bottom water is derived from the southwestern weddell sea where high salinity shelf water is abundant the less saline weddell sea bottom water like the more ventilated weddell sea deep water is derived from lower salinity shelf water at a point farther north along the antarctic peninsula transports of weddell sea deep and bottom water masses crossing 44 w estimated from one ladcp survey are 25 106 and 5 106 m3 s 1 respectively the low salinity better ventilated forms of weddell sea deep and bottom water flowing along the outer rim of the weddell gyre have the position and depth range that would lead to overflow of the topographic confines of the weddell basin whereas the more saline forms may be forced to recirculate within the weddell gyre

a three component mixing model for ridge crest hydrothermal fluids the hydrothermal fluid chemistry data set for sediment starved ridge axes is used to estimate endmember compositions and to constrain a three component mixing model based on that of von damm using numerical methods to estimate the endmember compositions the data set includes fe cl h2s na k ca and sr at 11 13 n and 21 n east pacific rise north and south cleft segment southern juan de fuca ridge tag and mark q mode factor analysis shows that the data can be explained 99 9 by 3 factors a constrained optimization of a weighted least squares algorithm assuming conservative mixing and with the constraints that all endmember concentrations and contributions are non negative and that fractional contributions sum to one is used to partition the data set into endmembers mixing of the calculated endmembers consistently found to be phase separated brine vapor and hydrothermally altered seawater reproduces the composition of the vent fluids the model is consistent with the presence of a convecting brine layer in the primary reaction zone at depth as suggested by bischoff and rosenbauer that transfers properties to the overlying circulating seawater by release of a vapor and by diffusion we suggest that future experimental work examine solubility controls in conjunction with and subsequent to phase separation and work with a wider range of fluid salinities

dynamics of dense water descending a continental slope abstract a reduced gravity model is used to examine the dynamics of dense water descending a continental slope the model solves for the geostrophically adjusted state before it is subjected to significant frictional decay for such bottom mounted flow it is argued that frictional torque would dominate the net vorticity balance to equalize the edge flows resulting in double velocity cores constrained by the geostrophic balance the dense water thus may settle only over a concave bottom and is sheetlike covering typically the whole slope rise as such the adjustment is characterized by a spreading rather than sinking of the layer with little descent of the upper edge but a swift downslope current propelling the lower edge through the mechanical energy balance it is found in addition that a greater density anomaly would increase the total entrainment flux to more strongly dilute the original anomaly yielding a product water that is less varied in the water mass properties model predictions compare

a model of buoyant throughflow with application to branching of the tsushima current abstract a reduced gravity model is used here to investigate the dynamics of a buoyant flow through a strait driven by pressure difference of the adjoining basins assuming the flow to be hydraulically controlled so that the transport is maximized flow structures in the upstream basin during its transit through the strait and along the downstream coast are determined it is found in particular that the combined effect of friction exerted by the sill and stretching of the buoyant layer as it exits the strait may cause the downstream flow to exhibit two velocity maxima along the layer outcrop and the coastal boundary when applied to the tsushima current of the japan east sea the required conditions for branching are amply satisfied the model thus provides a plausible explanation of this observed feature in addition a favorable comparison between predicted and observed transports supports the hydraulic control of the flow

momentum and energy balance of the mediterranean outflow field data taken in the gulf of cadiz have been analyzed to describe some aspects of the momentum and energy balance of the mediterranean outflow a crucial component of the momentum balance is the total stress entrainment stress and bottom drag which has been estimated from a form of the bernoulli function evaluated from density and current observations for the first 60 km west of the camarinal sill the outflow was confined within a narrow channel on the continental shelf at about 70 km downstream the outflow crossed over the shelf slope break and began to descend the continental slope the buoyancy force increased substantially and the outflow underwent a geostrophic adjustment albeit one heavily influenced by mixing and dissipation the current direction turned 90 degrees to the right at a near inertial rate in this region the estimated geostrophic velocity greatly underestimated the actual current and the estimated curvature rossby number was about 0 5 current speeds were in excess of 1ms 2 1 and the total stress was as large as 5 pa the entrainment stress estimated independently from property fluxes reached a maximum of about 1 pa or considerably smaller than the inferred bottom stress by about 130 km downstream the current was aligned approximately along the local topography the current amplitude and the estimated stress were then much less about 0 3 m s21 and 0 3 pa the entrainment stress was also very small in this region well downstream of the strait this slightly damped geostrophic flow continued on to cape st vincent where the outflow began to separate from the bottom bottom stress thus appears to be a crucial element in the dynamics of the mediterranean outflow allowing or causing the outflow to descend more than a kilometer into the north atlantic in the regions of strongest bottom stress the inferred drag coefficient was about 2 2 12 3 10 23 depending upon which outflow speed is used in the usual quadratic form entrainment stress was small by comparison to the bottom stress but the entrainment effect upon the density anomaly was crucial in eroding the density anomaly of the outflow the observed entrainment rate appears to follow roughly a critical internal froude number function

helium and methane measurements in hydrothermal fluids from the mid atlantic ridge the snake pit site at 23 n methane and helium isotopes concentrations have been measured in hydrothermal fluids of black smoker vents at the snake pit area mar 23 n as part of the hydrosnake cruise with the submersible nautile the samples contain large quantities of helium and methane with endmember values of 4 6 10 5 cm3 stp g for helium and 1 38 10 3 cm3 stp g for methane the helium isotopic ratio 3he 4he is 8 4 times the atmospheric ratio these values are strikingly similar to those already obtained on the east pacific rise the ch4 3he ratio is also quite similar ch4 3he 2 6 106 suggesting a non biogenic origin of this gas in the fluids assuming that the range of variation of the 3he concentration data obtained so far from various tectonic settings is close to the genuine range of values encountered on mid ocean ridges the global mass flow involved in the hydrothermal circulation at the ridge s axis is estimated between 1000 and 3000 m3 of water sec leading to a residence time of the global ocean with respect to the hydrothermal circulation between 15 and 45 million years this estimate implies that in addition to the individual vents smokers there is an extensive diffuse circulation of hydrothermal fluids along the ridges in agreement with recent observations concerning heat budgets at hydrothermal sites in the eastern pacific

source analysis of siliceous minerals and uranium in early cambrian shales south china significance for shale gas exploration abstract efficient shale gas exploration in the early cambrian shales of southern china requires the identification of favorable target areas this study used logs for u si al and total organic carbon toc from three typical wells to assess a new framework for gas exploration in this area the excess siliceous mineral content from a non terrigenous clastic source ranged from 20 to 30 in most layers reaching 50 analysis of these excess siliceous minerals using an al fe mn ternary plot showed that most were probably derived from hydrothermal fluids within the boundary of the yangtze and cathaysian plates excess siliceous mineral content and uranium content had a good positive correlation both tending to decrease away from the plate boundary indicating that both were derived from hydrothermal fluids radioactive uranium in hydrothermal fluids can enhance biological productivity which is conducive to the increased accumulation of sedimentary organic matter therefore shale gas exploration in the early cambrian layers of south china should focus on target areas near the plate boundary that have increased uranium levels moderate maturity and good preservation conditions

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

lightning activity and severe storm structure space time mapping of vhf sources from four severe storms on 19 june 1980 reveals that lightning processes for cloud to ground cg and large intracloud ic flashes are confined to an altitude below about 10 km and closely associated with the central regions of high reflectivity another class of ic flashes produces a splattering of sources within the storms main electrically active volumes and also within the large divergent wind canopy aloft there is no apparent temporal association between the small high altitude ic flashes that occur almost continuously and the large ic and cg flashes that occur sporadically in the lower portions of storms

fulgurites in the southern central sahara republic of niger and their palaeoenvironmental significance the mineralogical and chemical characteristics of fulgurites natural glasses formed by lightning strikes to the ground from the southern central sahara niger are presented the fulgurites are indicators of thunderstorms the northernmost important fulgurite formation in the study area reached up to about 18 n with decreasing fulgurite concentration from south to north their distribution pattern and the relative dating of their formation in relation to landscape history from the late pleistocene onwards e g palaeolakes palaeosols and to neolithic settlement reveals their value as palaeoenvironmental indicators they indicate 1 local palaeoenvironmental conditions depending on the topographical situation in a complex dune relief 2 climatic change during the mid holocene from northerly rains to southerly rains and 3 the northernmost limit of important thunderstorms and rainfall activity since this time in the southern central sahara

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

lagrangian observations of meddy formation during a mediterranean undercurrent seeding experiment mediterranean eddies meddies play an important role in maintaining the temperature and salinity distributions in the north atlantic but relatively little is known about their early life histories including where how often and by what mechanism they form a major field program called a mediterranean undercurrent seeding experiment has been carried out to directly observe meddy formation and the spreading pathways of mediterranean water into the north atlantic between may 1993 and march 1994 49 rafos floats were deployed sequentially in the mediterranean undercurrent south of portugal and tracked acoustically for up to 11 months the float deployments were accompanied by high resolution xbt sections across the undercurrent nine meddy formation events were observed in the float trajectories six near cape st vincent at the southwestern corner of the iberian peninsula and three near the estremadura promontory along the western portuguese continental slope meddy formation thus occurs where the continental slope turns sharply to the right when facing in the downstream direction of the undercurrent after conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent the authors have estimated a meddy formation rate of 15 20 meddies per year the timescale for meddy formation at cape st vincent was found to be 3 7 days shorter than previous estimates based on the volume of larger meddies meddies were observed to form most frequently when the speed of the mediterranean undercurrent was relatively fast the meddy formation process at cape st vincent resembles the conceptual model of e a d asaro whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer with negative relative vorticity at a sharp corner comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent this confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean may not be the principle mechanism at work in the formation of meddies at cape st vincent

watermass properties of the antarctic slope front a simple model abstract in the antarctic dense shelf water is formed in coastal polynyas and is differentiated from the fresher surface water by the wind induced ice motion that displaces offshore the ice melt from production zones where the shelf water discharges into the deep ocean the antarctic slope front asf is v shaped separating the shelf and surface waters referred to as frontal waters from the circumpolar deep water cdw to elucidate basic constraints on frontal properties a minimal model of homogeneous water masses forced by offshore wind and freshwater input in a perpetual winter is considered with the surface water stirred by and hence aligned with the ice cover there is little leakage of ice or meltwater from the frontal system so ice production and melt merely redistribute heat and salt between frontal waters as such the heat loss to the atmosphere needs to be supplied by entraining cdw which then necessitates the shelf water discharge on account of the mass balance because of the fresh

the mediterranean outflow splitting a comparison between theoretical models and canigo data abstract the mediterranean outflow in the atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima hydrological data collected west of the strait of gibraltar and off the south and south west coasts of portugal in september 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the mediterranean outflow these models are i a steady one dimensional streamtube model and ii a local time dependent model that includes cross stream variations the streamtube model applied at the northern respectively the southern part of the outflow appears capable of reproducing the observed density difference between the two cores the vertical separation however was underestimated the results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

geochemical and physical structure of the hydrothermal plume at the ultramafic hosted logatchev hydrothermal field at 14 45 n on the mid atlantic ridge abstract the hydrothermal plume generated in deep waters above the logatchev hydrothermal field lhf about 15 n on the mid atlantic ridge was investigated and mapped for its 3d distributions using a combination of in situ optical light scattering data temperature and salinity data as well as concentrations of hydrogen methane total dissolvable fe and total dissolvable mn based on the results obtained for these meaningful parameters we report the geochemical and physical characteristics of the fluids expelled from the ultramafic lhf and the chemical structure of its hydrothermal plume in the water column the hydrothermal plume is sourced by at least seven distinct vent sites and possibly additional diffusive fluid and gas discharge it comprises a water body characterized by strong nephelometric anomalies expressed as ntu nephelometric turbidity units and high concentrations of fe and mn 5 times seawater concentration and the gas plume with several times the h 2 and ch 4 concentrations of normal seawater up to three plume levels with a total vertical extension of about 350 m from the seafloor were classified in the hydrothermal plume the ntu plume could be followed to approximately 2 5 km to the north and to the south from the vent site while the gas plume spread several km farther from the hydrothermal source high concentrations of h 2 up to 1598 nmol l 1 and ch 4 up to 323 nmol l 1 accompanied by relatively low dissolvable fe concentrations up to 270 nmol l 1 as well as low concentrations of dissolvable mn 112 nmol l 1 compared to basaltic hydrothermal systems are the characteristics of the plume the low metal gas ratios showed a decrease with increasing distance from the vent site our data demonstrate that ultramafic systems such as the lhf serve both as sources and sinks for elements with respect to metal and gas inputs into the oceanic water column the relevance of such systems is underlined by the discovery and postulated frequency of further ultramafic hosted hydrothermal systems on slow spreading ridges

thunderstorm electrification a case study the paper presents interpretation of triple doppler radar observations of a thunderstorm that occurred at the kennedy space center florida precipitation particle motions inside the storm as evaluated from the three radial velocity radar data sets are presented in a selected vertical plane throughout the storm development these data are interpreted and compared with observations of vhf radio emission sources and lightning charge centers derived from an earlier analysis krehbiel 1981 the main feature of the particle motion field is the persistence at nearly the same altitude throughout the entire period of analysis of a balance level characterized by a zero value of the precipitation particle mean vertical velocity further analysis based on considerations of radar reflectivity and air velocity suggests the presence at this altitude of relatively large precipitation particles suspended in an updraft the lightning negative charge centers appear to be less than 1 km above the particle balance level

discrete and diffuse heat transfer atashes vent field axial volcano juan de fuca ridge abstract heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ashes vent field located in the caldera of axial volcano on the central juan de fuca ridge the discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor the investigation centered on the high temperature portion of the field a 100 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge 326 c from seven individual vents and diffuse flow from intervening areas a total of 4 4 2 10 6 w was estimated for the discrete heat flux based on measurements of effluent temperature flow rate and orifice diameter at all the known individual vents by d s v alvin in september 1987 a total of 15 75 10 6 w was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of alvin s instrument platform the study indicates that 1 the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component and that 2 the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high temperature portion of the ashes vent field

hydrothermal fe cycling and deep ocean organic carbon scavenging model based evidence for significant poc supply to seafloor sediments abstract submarine hydrothermal venting has recently been identified to have the potential to impact ocean biogeochemistry at the global scale this is the case because processes active in hydrothermal plumes are so vigorous that the residence time of the ocean with respect to cycling through hydrothermal plumes is comparable to that of deep ocean mixing caused by thermohaline circulation recently it has been argued that seafloor venting may provide a significant source of bio essential fe to the oceans as the result of a close coupling between fe and organic carbon in hydrothermal plumes but a complementary question remains to be addressed does this same intimate fe corg association in hydrothermal plumes cause any related impact to the global c cycle to address this scor interridge working group 135 developed a modeling approach to synthesize site specific field data from the east pacific rise 9 50 n hydrothermal field where the range of requisite data sets is most complete and combine those inputs with global estimates for dissolved fe inputs from venting to the oceans to establish a coherent model with which to investigate hydrothermal corg cycling the results place new constraints on submarine fe vent fluxes worldwide including an indication that the majority of fe supplied to hydrothermal plumes should come from entrainment of diffuse flow while this same entrainment is not predicted to enhance the supply of dissolved organic carbon to hydrothermal plumes by more than 10 over background values what the model does indicate is that scavenging of carbon in association with fe rich hydrothermal plume particles should play a significant role in the delivery of particulate organic carbon to deep ocean sediments worldwide

direct observation of the evolution of a seafloor black smoker from vapor to brine a single hydrothermal vent f vent occurring on very young crust at 9 16 8 n east pacific rise was sampled in 1991 and 1994 in 1991 at the measured temperature of 388 c and seafloor pressure of 258 bar the fluids from this vent were on the two phase curve for seawater these fluids were very low in chlorinity and other dissolved species and high in gases compared to seawater and most sampled seafloor hydrothermal vent fluids in 1994 when this vent was next sampled it had cooled to 351 c and was venting fluids 1 5 times seawater chlorinity this is the first reported example of a single seafloor hydrothermal vent evolving from vapor to brine the 1991 and 1994 fluids sampled from this vent are compositionally conjugate pairs to one another these results support the hypothesis that vapor phase fluids vent in the early period following a volcanic eruption and that the liquid phase brines are stored within the oceanic crust and vent at a later time in this case 3 years these results demonstrate that the venting of brines can occur in the same location in fact from the same sulfide edifice where the vapor phase fluids vented previously

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

energetic plumes over the western ross sea continental slope rapid descent of dense drygalski trough western ross sea antarctica shelf water over the continental slope within 100 to 250 m thick benthic plumes is described speeds of up to 1 0 m s are recorded flowing at an average angle of 35 to the isobaths entraining ambient lower circumpolar deep water en route this process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor antarctic bottom water nonetheless a 4 hour duration pulse of undiluted shelf water was observed at depth 1407 m directly north of the drygalski trough moving at around 90 degrees to isobaths and at a speed of 1 4 m s thus the export of ross sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths punctuated by rapid downhill cascades

mixing and spreading of the mediterranean outflow abstract hydrographic and current profiler data taken during the 1988 gulf of cadiz expedition have been analyzed to diagnose the mixing spreading and descent of the mediterranean outflow the s properties and the thickness and width of the outflow were similar to that seen in earlier surveys the transport of pure mediterranean water i e water with s 38 4 psu was estimated to be about 0 4 106 m3 s 1 which is lower than historical estimates most of which were indirect but comparable to other recent estimates made from direct velocity observations the outflow transport estimated at the west end of the strait of gibraltar was about 0 7 106 m3 s 1 of mixed water and the transport increased to about 1 9 106 m3 s 1 within the eastern gulf of cadiz this increase in transport occurred by entrainment of fresher north atlantic central water and the salinity anomaly of the outflow was consequently reduced the velocity weighted salinity decreased to 36 7 psu within 60 km of the strait and decre

alteration of the oceanic crust implications for geochemical cycles of lithium and boron abstract fresh tholeiitic basalt glass has been reacted with seawater at 150 c water rock mass ratio of 10 and fresh diabase has been reacted with a na k ca cl fluid at 375 c water rock mass ratios of 1 2 and 5 to understand better the role of temperature basalt composition and water rock mass ratio on the direction and magnitude of b and li exchange during basalt alteration at 150 c slight but nevertheless significant amounts of b and li were removed from seawater and incorporated into a dominantly smectite alteration phase at 375 c however b and li were leached from basalt b behaved as a soluble element and attained concentrations in solution limited only by the b concentration in basalt and the water rock mass ratio li however was less mobile for example at water rock mass ratios of 1 2 and 5 the percent of li leached from basalt was 58 70 and 92 respectively this suggests some mineralogic control on li mobility during hydrothermal alteration of basalt especially at low water rock mass ratios in general these results as well as those for b are consistent with the temperature dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids based on the distribution and chemistry of products of seafloor weathering low 150 c and high temperature hydrothermal alteration of basalt and the chemistry of ridge crest hydrothermal fluids it was estimated that alteration of the oceanic crust is a li source for seawater this is not true for b however since the hot spring flux estimated for b is balanced by low temperature basalt alteration these data coupled with b and li flux estimates for other processes e g continental weathering clay mineral adsorption authigenic silicate formation and formation of siliceous skeletal material yield new insight into the b and li geochemical cycles calculations performed here indicate relatively good agreement between the magnitude of b and li sources and sinks the geochemical cycle of b however may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones serpentinites are conspicuously enriched in b and if the b source for these rocks is seawater then an additional b sink exists which must be integrated into the b geochemical cycle however until more data are available in terms of areal extent of serpentinization serpentite chemistry and isotopic composition the importance of b in these rocks with respect to the b geochemical cycle remains speculative at best

trace metals in hydrothermal solutions from cleft segment on the southern juan de fuca ridge concentrations of trace metals in fe and cl rich hydrothermal solutions from the southern juan de fuca ridge sjfr have been determined and corrected for residual precipitates formed in the sampler precipitate corrections for ag cd cu mo pb sb and zn were variable contributing an average of 20 to total solution concentrations whereas corrections for co averaged less than 5 and essentially no corrections were required for fe and mn values for cu co and mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320 c in addition and unlike most other metals studied all vent fluids from the sjfr were almost completely depleted in mo relative to seawater values of about 110 nmol kg 1 in contrast to the cu co mo group concentrations of ag cd sb and pb correlated well with those for zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332 c range encountered in this study and more by the combined chemical processes that control zn levels along the transport pathway from the deep reaction zone to the vent orifice

lightning location relative to storm structure in a supercell storm and a multicell storm relationships between lightning location and storm structure are examined for one radar volume scan in each of two mature severe storms one of these storms had characteristics of a supercell storm and the other was a multicell storm data were analyzed from dual doppler radar and dual vhf lightning mapping systems the distributions of vhf impulse sources were compared with radar reflectivity vertical air velocity and their respective gradients in the supercell storm lightning tended to occur along streamlines above and downshear of the updraft and reflectivity cores vhf impulse sources were most concentrated in reflectivities between 30 and 40 dbz and were distributed uniformly with respect to updraft speed in the multicell storm on the other hand lightning tended to coincide with the vertical reflectivity and updraft core and with the diverging streamlines near the top of the storm the results suggest to us that the location of lightning in these severe storms was most directly associated with the wind field structure relative to updraft and reflectivity cores since the magnitude and vertical shear of the environmental wind are fundamental in determining the reflectivity and wind field structure of a storm we suggest that these environmental parameters are also fundamental in determining lightning location

rhenium osmium abundance and isotopic compositions of massive sulfides from modern deep sea hydrothermal systems implications for vent associated ore forming processes studies of rhenium re and osmium os concentrations and isotopic compositions in seafloor hydrothermal sulfides are an important tool for understanding the evolution of hydrothermal systems allowing the determination of both metal sources and reconstructing the physicochemical conditions of their deposition the re os concentrations and isotopic compositions of 38 massive sulfide samples have been studied in different hydrothermal fields from the east pacific rise epr mid atlantic ridge mar central indian ridge cir southwest indian ridge swir and back arc basin bab the majority of the sulfides possess os 187 os 188 that span a narrow range 1 004 to 1 209 which is most easily explained as a seawater derived component this may suggest that those initial os 187 os 188 isotope compositions of ancient seafloor hydrothermal sulfide deposits which were formed by the mixing process between seawater and hydrothermal fluid are possible for analysing ancient seawater os components only two of samples mar05 tvg1 10 2 and mar05 tvg1 21 from the logatchev hydrothermal field lhf mar possess moderately less radiogenic os 187 os 188 0 645 to 0 730 which may reflect the lesser extent of hydrothermal fluid seawater mixing during hydrothermal ore forming process

response of two phase fluids to fracture configurations within submarine hydrothermal systems submarine hydrothermal systems at mid ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation the interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow especially under conditions leading to aqueous two phase separation within the brittle region fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor in the ductile region fluids may be trapped under near lithostatic pressure while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure in a narrow transition zone between these two extremes a steep pore fluid pressure gradient must exist in the brittle region itself fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits a hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth as a result of the large volume increase that accompanies two phase separation a two phase fluid must either accelerate upward or exert an increased pressure on its surroundings possibly leading to enhanced fracturing the fracture configuration will also affect the behavior of the mixed fluid after phase separation a two phase fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well mixed suspension of vapor and liquid whereas a two phase mixture in a constricted branching series of channels moving more slowly in a laminar flow regime is more likely to undergo physical isolation of the phases leading to venting of a vapor phase and retention of a saline brine phase at depth we propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets progressively increasing in diameter and decreasing in abundance feeding into a main upflow stockwork zone in which flow directly to the surface takes place in relatively large unconstricted conduits fluid movement within a feeder zone may be modified by capillary flow tortuous channelways disconnected fractures blocking of cracks by mineral deposition and opening of new fractures

mineralogical and compositional features of rock fulgurites a record of lightning effects on granite fulgurites are a naturally occurring glass formed when sand rock or soil is struck by atmospheric electrical discharges lightning the aim of this paper is to provide insights into the conditions occurring in rocks during the lightning strike rock fulgurites collected from mt mottarone baveno piedmont italy have been investigated to assess the mineralogical and compositional changes occurring in granite due to a lightning strike x ray powder diffraction showed that the samples represent the dominant granitic rock type of the baveno massif the so called pink baveno fulgurite coats the surface of the granite as a brown black glassy to very fine grained porous layer powder diffraction data for the fulgurite reveal the presence of cristobalite and quartz crystals in a glass matrix suggesting that temperature exceeded 1700 c at near atmospheric conditions assuming thermodynamic equilibrium electron probe microanalysis of the glass revealed that it is mainly composed of sio 2 and al 2 o 3 and that it has a porosity of 5 7 area in the studied zones the presence of the amorphous phase indicates that the abrupt electrical joule heating of the rock surface yielded high temperatures producing a thin melt layer on the surface which then cooled adiabatically idealized physical model was developed to simulate the effects of joule heating and subsequent thermal conduction close to the rock surface during and after a lightning strike the quantity of organic matter in the glass obtained via elemental analyzer suggests that rapid quenching of the melt trapped no x and co x gases produced during heating raman spectroscopy revealed the presence of polyaromatic hydrocarbon molecules which combined with the elemental analyzer data suggest that organic matter was pyrolyzed at around 300 350 c and then trapped in the glass matrix of the studied rock fulgurites

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

western ross sea continental slope gravity currents antarctic bottom water of the world ocean is derived from dense shelf water that is carried downslope by gravity currents at specific sites along the antarctic margins data gathered by the anslope and clima programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the ross sea when high salinity shelf water exits the shelf through drygalski trough joides trough immediately to the east offers an additional escape route for less saline shelf water while the glomar challenger trough still farther east is a major pathway for export of the once supercooled low salinity ice shelf water that forms under the ross ice shelf the drygalski trough gravity currents increase in thickness from 100 to 400 m on proceeding downslope from 600 m the shelf break to 1200 m upper slope sea floor depth while turning sharply to the west in response to the coriolis force during their descent the mean current pathway trends 35 downslope from isobaths benthic layer current and thickness are correlated with the bottom water salinity which exerts the primary control over the benthic layer density a 1 year time series of bottom water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of shelf water export through drygalski trough these cold 34 75 pulses correlate with strong downslope bottom flow extreme examples occurred during austral summer fall 2003 comprising concentrated high salinity shelf water 1 9 c 34 79 and approaching 1 5 m s 1 at descent angles as large as 60 relative to the isobaths such events were most common during november may consistent with a northward shift in position of the dense shelf water during austral summer the coldest saltiest bottom water was measured from mid april to mid may 2003 the summer fall export of high salinity shelf water observed in 2004 was less than that seen in 2003 this difference if real may reflect the influence of the large iceberg c 19 over drygalski trough until its departure in mid may 2003 when there was a marked decrease in the coldest saltiest gravity current adjacent to drygalski trough northward transport of cold saline recently ventilated antarctic bottom water observed in march 2004 off cape adare was 1 7 sv including 0 4 sv of high salinity shelf water

geochemistry of diffuse low temperature hydrothermal fluids in the north fiji basin low temperature 13 c diffuse hydrothermal fluids were sampled directly at the seafloor with a specially designed hydro bottom station in the north fiji basin and analyzed for gases major and minor elements and a large number of trace metals the fluids were significantly enriched in co2 si li rb cs ba mn and several trace metals compared to ambient seawater had high ch4 and h2s concentrations and had a slightly decreased salinity calculated end member concentrations of the low temperature fluids show a strong similarity to the neighboring hot vents implying that the diffuse fluids are hot vent waters diluted by seawater according to the chemical composition the sampled fluids derive from vapor phase fluids produced by subseafloor boiling and phase separation compared to hot vents from other regions mn fe and other trace metal concentrations are low subsurface metal sulfide precipitation during cooling and dilution with seawater has further decreased the dissolved metal contents of the diffuse fluids thus creating a very favorable environment for the hydrothermal fauna as indicated by a very low fe h2s ratio therefore the fluids support high bioproductivity but no hydrothermal mineral precipitation the emanation of the condensed vapor phase appears to have been stable during the past 10 years however the occurrence of metal sulfide particles in some fluid and sediment samples and small areas of dead fauna indicate that the hydrothermal system may be evolving toward the emanation of the stored brine phase

compared geochemical signatures and the evolution of menez gwen 37 50 n and lucky strike 37 17 n hydrothermal fluids south of the azores triple junction on the mid atlantic ridge during the diva 1 cruise in may 1994 a series of 19 dives was conducted using the french submersible nautile at the topographic highs of three volcanic segments centered at 37 17 n 37 50 n and 38 20 n respectively south of the azores triple junction atj on the mid atlantic ridge mar hydrothermal fluids were collected at the lucky strike ls hydrothermal field discovered at 37 17 n in 1993 and at a new hydrothermal site called menez gwen mgw discovered at 37 50 n during this cruise both systems are relatively shallow compared to other mar systems with seafloor depths of 1700 and 850 m respectively characteristics which make them unique among the already known sites on the mar the characteristics of the ls fluids are the same as in 1993 and include temperatures ranging from 170 c to 324 c variable chlorinities lower than seawater low hydrogen sulfide 3 0 mmol kg high ba concentrations up to 80 mol kg low metal concentrations and high gas contents and distinct chemical end members indicative of significant geographic control of the venting system in contrast the very clear mgw fluids at 37 50 n show a rather uniform exit temperature 285 c and chemical compositions with chlorinities 360 380 mmol kg lower than at ls and corresponding lower concentrations of cations h2s 1 8 mmol kg metals and silica 8 11 5 mmol kg due to the lower temperature t and pressure p of the system in the two systems fluid chemistry is strongly affected by phase separation at the low pressure conditions of these sites phase relations in the nacl h2o system dictate production of an extremely low salinity the higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single phase hydrothermal seawater is the dominant process controlling cl concentrations depletions and enrichments of elements in solutions are also explained by the involvement of fluid rock reactions the enrichment of ba k cs and rb in the fluids is linked to the enriched character of these elements in basalts relatively high ph and ca low li sr and metals are related to the highly altered basaltic substrate in the reaction zone low sr ca and high ca na ratios are explained by albitization within the crust the increase of degassing when approaching the azores may be related to the carbon enriched basaltic crust near the azores hot spot isotopic ratios show that co2 ch4 and helium gases have a magmatic signature compared to other deeper sites on the mar all fluids collected at ls and mgw are gas enriched except for h2s and helium whose both concentration and isotopic ratio compare well with other hydrothermal fluids the high ch4 concentration associated with unsaturated hydrocarbons and the high ch4 3he ratios also suggest a contribution of ch4 generated by serpentinization of ultramafic rocks by fischer tropsch catalysis of co2 reduction