the impact of oxic alteration on plume derived transition metals in ridge flank sediments from the east pacific rise we describe transition metal behaviour during early diagenesis in sediments collected along a transect across the eastern flank of the east pacific rise epr at 14 s overlying crust with magnetic ages of 0 36 ma to 4 6 ma the sediments are lithologically simple and consist of biogenic carbonate hydrothermal plume precipitates and minor basaltic debris sedimentation rates range from 1 cm ka in the near axis region 0 36 ma to 0 1 0 2 cm ka across the young ridge flank by 1 9 ma or within 142 km from axis redox indicators porewater mn ii aq and solid phase mo mn ratios indicate fully oxic conditions in near axis sediments changing to mildly suboxic conditions across the ridge flank sequential extraction analysis of sediments from the 0 36 ma site indicates that oxic alteration of fe oxyhydroxides is dominated by transformation of ferrihydrite to goethite implying that alteration occurs via dissolution followed by nucleation and precipitation the extent of alteration amorphous fe total plume derived fe ranges from 0 3 to 0 8 and the time to half conversion is on the order of 100 ka v associated with plume derived ferrihydrite is strongly discriminated against during alteration to goethite and the distribution coefficient dv v fe goethite v fe dissolved is in the range 0 45 0 59 cu zn and ni are delivered to the sediment in association with both fe oxyhydroxides and mn oxides where 70 of cu and zn and 10 of ni are associated with fe oxyhydroxides the fe adsorbed cu and zn are incorporated quantitatively into the lattice structure of goethite during alteration the fe associated ni most likely represents ni co precipitated as ni fe ferrihydrite and incorporated into goethite during alteration co is associated only with mn oxides the oxic alteration of hydrothermal fe oxyhydroxides therefore represents a relatively permanent sink where goethite dissolution is a prerequisite for metal release for a maximum of 70 of plume derived cu and zn 50 of v 10 of ni and 0 of co in oxic sediments the remaining plume derived metals are retained in association with amorphous fe oxyhydroxides and mn oxides although this fraction is highly susceptible to diagenetic remobilization under suboxic conditions

iron rich metalliferous sediments on the east pacific rise prototype of undifferentiated metalliferous sediments on divergent plate boundaries abstract sediments in a zone on the east pacific rise with an especially high spreading rate were studied chemically mineralogically and microscopically they consist of a mixture of metalliferous sediments and plankton tests the metalliferous sediments were formed by an acidic hydrothermal leaching of tholeiitic basalt with seawater and subsequent precipitation in contact with cold near bottom seawater we assume the precipitation from hydrothermal solutions in this part of the east pacific rise to be undifferentiated due to the high spreading rate and the resulting rapid flow on the water through the basalt thus these metalliferous sediments are an initial stage type that have not undergone differentiation mn mo la cu v ni fe zn co and y all of which are leachable in acidic hydrothermal solutions are enriched in the metalliferous sediments in comparison to the tholeiitic basalts zr al and ti on the other hand which under the same conditions are not easily leached are reduced in their concentrations all components of the metalliferous sediments precipitated as hydroxides or as adsorbed ions on the hydroxides of other elements this is due to the oxidizing conditions in the near bottom seawater the sedimentation rate is high the almost 3 m long cores reach only to the late pleistocene the only distinctly observable diagenetic process for this period of time is the formation of goethite from amorphous iron oxides only for na k and rb does it seem possible that a distinct enrichment in the sediments by adsorption from the seawater could have taken place ca sr pb and perhaps sc are primarily bound to the planktonic carbonate part of the sediments

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

the role of prokaryotes in subsurface weathering of hydrothermal sediments a combined geochemical and microbiological investigation a detailed geochemical and microbiological study of a 2 m sediment core from the inactive alvin mounds within the tag hydrothermal field was conducted to examine for the first time the role of prokaryotes in subsurface weathering of hydrothermal sediments results show that there has been substantial post depositional remobilisation of metal species and diagenetic overprinting of the original high temperature hydrothermal minerals and aspects have involved prokaryotic processes prokaryotic enumeration demonstrates the presence of a population smaller than the average for deep sea sediments probably due to the low organic carbon content but not inhibited by and hence adapted to the metal rich environment there was a small but significant increase in population size associated with the active redox boundary in an upper metal sulphide layer 50 70 cm around which active metal remobilisation was concentrated cu au cd ag u zn and zn hence subsurface prokaryotes were potentially obtaining energy from metal metabolism in this near surface zone close association of numbers of culturable mn and fe reducing prokaryotes with subsurface fe2 and mn2 pore water profiles suggested active prokaryotic metal reduction at depth in core cd102 43 to 175 cm in addition a prokaryotic mechanism which is associated with bacterial sulphate reduction is invoked to explain the u enrichment on pyrite surfaces and zn and pb remobilisation in the upper sediment although prokaryotic populations are present throughout this metalliferous sediment thermodynamic calculations indicated that the inferred low ph of pore waters and the suboxic anoxic conditions limits the potential energy available from fe ii oxidation which may restrict prokaryotic chemolithotrophic biomass this suggests that intense prokaryotic fe oxidation and weathering of seafloor massive sulphide deposits may be restricted to the upper portion of the deposit that is influenced by near neutral ph and oxic seawater unless there is significant subsurface fluid flow

reconstruction of past climate conditions over central europe from groundwater data abstract here we present a 30 000 years low resolution climate record reconstructed from groundwater data the investigated site is located in the bohemian cretaceous basin in the corridor between the scandinavian ice sheet and the alpine ice field noble gas temperatures ngt obtained from groundwater data preserved multicentennial temperature variability and indicated a cooling of at least 5 7 c during the last glacial maximum lgm this is further confirmed by the depleted 18 o and 2 h values at the lgm high excess air ne at the end of the pleistocene is possibly related to abrupt changes in recharge dynamics due to progression and retreat of ice covers and permafrost these results agree with the fact that during the lgm permafrost and small glaciers developed in the inner valleys of the giant mountains located in the watershed of the aquifers a temporal decrease of deuterium excess from the pre industrial holocene to present days is linked to an increase of the air temperatures and probably also to an increase of water pressure at the source region of precipitation over the past few hundred years

hydrothermal scavenging at the mid atlantic ridge radionuclide distributions we have carried out a study of u series radionuclide distributions in the neutrally buoyant plume overlying the tag hydrothermal vent field at 26 n mid atlantic ridge limited uptake of u accompanies the formation of fe oxyhydroxide particulate material within the hydrothermal plume the particulate u concentrations are too small for measurable dissolved u anomalies to be produced within the water column at plume height however such uptake followed by deposition in the underlying sediments could account for 10 of the u enrichment that has been reported previously for metalliferous ridge crest sediments particulate 234th 230th and 228th activities in the tag hydrothermal plume represent 65 of total th activities typical of the deep ocean low dissolved th activities coincide with the high particulate th activities indicating that hydrothermal scavenging strongly affects th isotope distributions in the water column close to hydrothermal vents much of the scavenging occurs during the initial formation of fe rich hydrothermal precipitates but some further scavenging continues as the material is dispersed away from its source distributions of particulate 210pb and 231pa activities in the neutrally buoyant plume are also extremely high 7 12 dpm 102 l and 30 160 dpm 106 l respectively like th 210pb and231pa show evidence for scavenging within the hydrothermal plume total th activities at plume height are 50 higher than background this focusing of th into the plume is consistent with the entrainment and re emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume

quantitative assessment of chemical and mineralogical changes due to progressive low temperature alteration of east pacific rise basalts from 0 to 9 ma many studies have focused on low temperature alteration of the oceanic crust nevertheless little is known about the initial processes of low temperature alteration and their influence on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis the lack of such studies is primarily related to the difficulty of recovering samples representative of this initial alteration style such information is nevertheless important not only for crust ocean mass balances but also to characterize an important input to the subduction zones we have studied dredged basalts from the eastern flank of the east pacific rise at 14 15 s concentrating on the products of a single spreading segment in a corridor perpendicular to the spreading axis and covering a range of crustal ages from 0 to 9 ma electron microprobe x ray fluorescence x ray diffraction and icp ms analyses have been carried out to examine the mineralogical and chemical changes in the basalts which make up the surface of the upper crust caused by low temperature alteration fresh rocks were sampled at the ridge axis off axis basalts show features of progressive alteration celadonite is the main alteration component in 0 12 4 6 ma old rocks whereas phillipsite is more abundant in rocks older than 4 6 ma changing compositions of secondary minerals progressive sealing of fractures and the occurrence of more alteration rinds on older rocks show evidence for a slight change in redox conditions from an oxidizing water dominated to a more reducing rock dominated environment with time iron oxyhydroxide and celadonite are the first alteration products partly replaced or covered by saponite under more reducing conditions the fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine phillipsite is present in fractures and veins in rocks older than 1 ma analyses also indicate an illite smectite mixed layered mineral which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite smectite mixed layered mineral all samples are characterized by the lack of minerals formed by hydrothermal processes we conclude that the alteration took place under seawater dominated conditions at low temperature a comparison of trace element analyses from altered whole rock samples and their appendant fresh glass chips provides a record of element mobility during alteration off axis basalts show significant uptakes of rb cs and ba which are supplied by seawater and incorporated in or on secondary minerals an enrichment of u is also apparent and appears to be especially strong when alteration conditions are oxidative k2o is also gained in all altered off axis basalts believed to be linked to the formation of celadonite however the volume of rock being altered is so small relative to the volume of the oceans that this scavenging has no noticeable effect on the composition of the oceans the composition of the oceanic crust on the other hand is affected significantly

latitudinal variations in benthic processes in the abyssal equatorial pacific control by biogenic particle flux abstract the equatorial pacific forms a band of high globally significant primary production this productivity drops off steeply with distance from equatorial upwelling yielding large latitudinal gradients in biogenic particle flux to the abyssal seafloor as part of the us jgofs program we studied the translation of these particle flux gradients into the benthic ecosystem from 12 s to 9 n along 135 140 w to evaluate their control of key benthic processes and to evaluate sediment proxies of export production from overlying waters in october december 1992 the remineralization rates of organic carbon calcium carbonate and biogenic opal roughly matched the rain rates of these materials into deep sediment traps exhibiting peak values within 3 of the equator rates of bioturbation near the equator were about ten fold greater than at 9 n and appeared to exhibit substantial dependence on particulate organic carbon flux tracer time scale i e age dependent mixing and pulsed mixing from burrowing urchins organic carbon degradation within sediments near the equator was dominated by a very labile component reaction rate constant k approximately 15 per year that appeared to be derived from greenish phytodetritus accumulated on the seafloor organic carbon degradation at the highest latitudes was controlled by a less reactive component with a mean k of approximately 0 075 per year where measured megafaunal and macrofaunal abundances were strongly correlated with annual particulate organic carbon flux macrofaunal abundance in particular might potentially serve as a proxy for export production in low energy abyssal habitats sedimentary microbial biomass also was correlated with the rain rate of organic carbon but less strongly than larger biota and on shorter time scales i e approximately 100 days we conclude that the vertical flux of biogenic particlues exerts tight control on the nature and rates of benthic biological and chemical processes in the abyssal equatorial pacific and suggest that global changes in productivity on decadal or greater time scales could yield profound changes in deep sea benthic ecoystems

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

effects of relative plate motion on the deep structure and penetration depth of slabs below the izu bonin and mariana island arcs an increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the earth s lower mantle below some island arcs but are deflected or rather laid down in the transition zone below others recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection with intermittent layering we present a multi disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth central in our discussion are the izu bonin and mariana subduction zones we argue that the differences in the tectonic evolution of these subduction zones in particular the amount and rate of trench migration can explain why the slab of subducted oceanic lithosphere seems to be at least temporarily stagnant in the earth s transition zone below the izu bonin arc but penetrates into the lower mantle below the mariana arc we briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones although further investigation is necessary our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate tectonic history of the subduction zones involved we therefore hope that the ideas outlined here will stimulate and direct new research initiatives

composition oxygen isotope geochemistry and origin of smectite in the metalliferous sediments of the bauer deep southeast pacific the sediments of the bauer deep an open ocean basin situated on the northwest nazca plate in the southeast pacific constitute a regional metalliferous deposit dominated by authigenic smectite two 2 metre long cores from the bauer deep were examined to investigate the nature and origin of the smectite infra red and mossbauer spectroscopy and wet chemical analysis libo2 fusion of isolated smectite indicate the mineral is a mg rich al rich nontronite oxygen isotopic compositions for isolated smectite are uniform and translate to a non hydrothermal temperature of formation of about 3 c sem observations show an abundance of well preserved biogenic opal in surface and near surface sediment but postburial dissolution and transformation of this phase to smectite is evident at depth smectite formation is the result of interaction between iron oxyhydroxide ponded in the bauer deep following a hydrothermal origin at the adjacent east pacific rise and biogenic opal a reaction mechanism is proposed regional factors control smectite formation in particular formation is inhibited in areas of caco3 accumulation topographic elevations but favoured in areas of oxyhydroxide and opal ponding topographic depressions

the effect of hydrothermal processes on midwater phosphorus distributions in the northeast pacific abstract the distributions of dissolved particulate and sedimentary phosphorus were measured in the region of the juan de fuca ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans significant negative dissolved phosphate anomalies ranging from 0 to 60 nmol l were observed in the water column at depths between 1900 and 2300 db the largest anomalies

sediment in the black smoker area of the east pacific rise 18 5 s abstract a sediment profile perpendicular to the east pacific rise epr was investigated in an area in which massive sulfides have been observed along the axis of the epr a holocene to late pleistocene age is concluded for all of the up to 3 m long cores leading to sedimentation rates of 0 3 2 6 cm 1000 a the sediments are composed of hydrothermal precipitate calcareous tests and in traces hyaloclastic material the proportion of carbonate increases with increasing distance from the spreading center the hyaloclastic material is more or less weathered and is concentrated near the spreading center the hydrothermal component is composed mainly of iron and manganese hydroxides the proportion of hydrothermal matter in the sediment is higher near the axis of the epr also near the axis the elements b p cr and v are enriched in the hydrothermal component whereas mn zn cu fe and as are depleted the core profiles show a fluctuation in the proportion of hydrothermal material periods of higher hydrothermal input correlate with periods of the best calcite preservation diagenetic alteration of the sediment increases with increasing distance from the spreading center i e with decreasing sedimentation rate mo zr co ni rb th y ba la and ree are enriched in the sediment by diagenetic processes

plume dispersed hydrothermal particles a time series record of settling flux from the endeavour ridge using moored sensors abstract settling particles collected with moored sediment traps near an area of active hydrothermal venting in the northeast pacific ocean provide information about the deep sea distribution of hydrothermal material results of an 8 day near field single trap experiment and a 340 day multi instrument 5 trap deployment two kilometers from the endeavour ridge vent field show variability in the quantity and composition of the plume to sediment particle flux over time with depth and with distance from the vent field hydrothermal elemental fluxes vary by a factor of two among the four sampling intervals of the longer experiment a factor of three variation in fe s among these periods reflects temporal changes in phase composition changes in flux with depth for example a 200 fold increase in cu and zn between above and below plume traps are evidence of hydrothermal input comparison of fluxes determined in the near and far field experiments reveals the spatial influence of vent effluent near field particle flux exceeds that measured two kilometers away by a factor of 10 to 20 and is relatively enriched in fe cu and organic carbon but depleted in mn calculations of the residence time of the plume with respect to settling indicate that more than 90 percent of the particulate material is transported farther than two kilometers from the vent field interpretation of elemental mineralogical and grain size analyses suggest that chemical precipitation kinetics other physical and biological particle forming processes changes in vent output and lateral transport of hydrothermal material by abyssal currents are tenable explanations for our observations our findings suggest that these rate interdependent processes be included in realistic models of hydrothermal particle flux

hydrothermal phase stabilization of 2 line ferrihydrite by bacteria poorly ordered 2 line ferrihydrite deposits from the caldera of axial volcano a site of hydrothermal vent activity along the juan de fuca ridge are characterized by an abundance of bacterial structures resembling gallionella ferruginea leptothrix ochracea and pv 1 filaments encrusted with nanoparticulate ferrihydrite ferrihydrite formed in the absence of bacteria can be metastable and will subsequently react to form more structurally ordered iron oxide phases such as hematite or goethite inspection of samples from 2 day 80 c aqueous heating experiments by electron microscopy and x ray diffraction revealed that a natural sample of bacteriogenic ferrihydrite from axial volcano did not undergo a phase transition whereas synthetic ferrihydrite absent of bacteria transformed to hematite after 2 days a series of bacterial ferrihydrite composites exhibited decreasing amounts of transformation to hematite with increased amounts of fe iii bound to bacteria complete inhibition was observed when 50 of the total fe was bound as ferric iron and 50 was present as ferrihydrite for the synthetic ferrihydrite samples containing bacteria the inhibition of ferrihydrite transformation is attributed to microbial surface functional groups i e carboxyl phosphoryl amine constraining the brownian motion of surface bound ferrihydrite nanoparticles thereby eliminating the aggregation and rotation of particles required for crystal growth the stabilization of ferrihydrite will prolong the longevity of a highly reactive mineral phase which may have implications for marine geochemistry and environmental remediation preservation of bacteria encrusted with transformation resistant ferrihydrite may enhance the potential usefulness of bacteriogenic ferrihydrite to harbour bacterial microfossils imparting these types of samples with potential use for paleontology and astrobiology

hydrothermal scavenging at the mid atlantic ridge modification of trace element dissolved fluxes abstract we have carried out an icp ms study of the distributions of trace elements fe ca mg al mn cu zn v as cr co pb y mo ag sn cd and be in particulate material collected from the tag hydrothermal plume 26 n mid atlantic ridge to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent fluids are balanced by the precipitation of associated particulate phases comparison of particulate trace element distributions with particulate fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes i co precipitation from vent fluids ii scavenging from seawater and iii preferential settling from the plume and oxidative dissolution within the water column for y and be continuous scavenging from seawater appears to be important whereas for v as and cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume from a comparison of trace element fe ratios in the particles and associated mid atlantic ridge vent fluids it appears that for many elements e g cu co ag pb al cd be and zn the flux of dissolved material from vent fluids is matched closely by the precipitation of hydrothermal particulate phases of the various elements studied here hydrothermal activity only appears to act as a significant source for dissolved ca and dissolved mn

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

manganese scavenging and oxidation at hydrothermal vents and in vent plumes abstract hydrothermal vents provide a major source of dissolved mn ii to the oceans where concentrations range from 5 mm within the 350 c hot smokers to just above ambient seawater concentration in far field vent plumes the mn ii rich environments within warm vents and vent plumes provide a suitable habitat for mn ii oxidizing bacteria in order to compare rates of scavenging and oxidation of mn ii proximally within vent fields

distribution and composition of hydrothermal plume particles from the ashes vent field at axial volcano juan de fuca ridge in 1986 and 1987 buoyant and neutrally buoyant hydrothermal plume particles from the ashes vent field within axial volcano were sampled to study their variations in composition with height above the seafloor individual mineral phases were identified using standard x ray diffraction procedures elemental composition and particle morphologies were determined by x ray fluorescence spectrometry and scanning electron microscopy x ray energy spectrometry techniques the vent particles were primarily composed of sphalerite anhydrite pyrite pyrrhotite chalcopyrite barite hydrous iron oxides and amorphous silica grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume the neutrally buoyant plume consisted of a lower plume which was highly enriched in fe s zn and cu and an upper plume which was highly enriched in fe and mn the upper plume was enriched in fe and mn oxyhydroxide particles and the lower plume was enriched in suspended sulfide particles in addition to the fe and mn oxyhydroxide particles the chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles short term sediment trap experiments indicate that the fallout from the ashes vent field is not as large as some of the other vent fields on the juan de fuca ridge

the scavenging of u 230th and 231pa during pulsed hydrothermal activity at 20 s east pacific rise abstract uranium th and pa isotopes were measured in eight box cores from two transects to the west of the east pacific rise at 10 s and 20 s sediment geochemistry al ca fe mn and water column paniculates mn suggest that highly metalliferous plumes rich in fe and mn are emanating from the rise crest at 19 s measurements of 230thxs and 231paxs indicate decreasing sediment accumulation rates from the rise flank 0 4 cm ky 1 to the tiki basin 0 23 cm ky 1 over this geographical area al is found to have a constant flux of 3 6 10 4 g cm 2 ky 1 application of an al constant flux model to ridge crest metalliferous sediment reveals two 1 000 year pulses of hydrothermal activity during the holoccne since 11 000 y bp these hydrothermal pulses are characterised by hydrothermal fe mn ratios 3 5 1 enhanced 230th and 231pa scavenging and low 230thxs 231paxs ratios 3 4 the similarity of the 230thxs and 231paxs ratio to the dissolved seawaterrauo suggests that in fracuonation of the isotopes occurs during scavenging supplementary supply of dissolved 230th and 231pa in a westwards moving water mass is suggested

history and geochemistry of a metalliferous sediment core from the mid atlantic ridge at 26 n fourteen thousand years of hydrothermal deposition are recorded in a metalliferous sediment core recovered from the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 n an estimated 26 to 96 of the sediment at this core site is hydrothermally derived as determined from the caco3 al and fe data layers of essentially pure vent precipitates contain 3 caco3 and 0 5 al with high concentrations of fe 43 cu 4 1 zn 1 2 mn 1 1 v 480 ppm pb 175 ppm cd 32 ppm and hg 3 8 ppm sediment accumulation rates vary from 1 to 30 g cm2 1000 y throughout the core a function of the intensity of hydrothermal inputs distinct hydrothermal events are recorded at 6000 and 8500 y b p in layers containing 90 vent derived material vertical metal profiles and interelement relationships in the core result from variable deposition of oxides and sulfides oxidation and dissolution of sulfide phases and scavenging of metals from seawater

early organic diagenesis the significance of progressive subsurface oxidation fronts in pelagic sediments abstract porewater and solid phase geochemical data at two contrasting ne atlantic stations are reported station 10552 on the cape verde abyssal plain is a site of slow pelagic accumulation ca 0 4 cm kyr 1 molecular oxygen is present in the sediment column to at least 2 m and probably much deeper labile organic carbon is almost totally consumed in the upper few centimetres of the sediment by contrast at station 10554 on the madeira abyssal plain the pelagic sequence has been interrupted by the occasional deposition of organic rich turbidites porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column this metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time this phenomenon exerts a controlling influence on the present nutrient profile and redox succession in a less extreme form substrate distributions of this latter type are not uncommon in atlantic sediments a model has been developed which is controlled by both oxygen and nitrate data this model permits a vertical profile of metabolic activity to be derived and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations about 30 of the total activity at station 10554 is located within the turbidite at the deepening reaction front this is a non steady state condition in fact it is found that the integrated metabolic activity at the two stations is not dissimilar ca 1 2 10 13 moles cm 2 sec 1 the striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column

field and laboratory studies of metal uptake and release by hydrothermal precipitates data for suspended particles from hydrothermal plumes on the mid atlantic ridge mar support uptake of dissolved v and as during formation of fe oxides and release of cu and cd during alteration of sulfides to complement these field observations we carried out laboratory time series experiments to determine the stability of v as cu and cd in precipitates from hydrothermal plumes on the mar and juan de fuca ridge for v and as the metal fe ratio in plume particles remained uniform after sampling in contrast to v and as particulate cu and cd values decreased relative to fe over time in laboratory studies using both plume particles and mound derived sulfides the combined field and laboratory results show that scavenging is an important removal mechanism for seawater v and as whereas oxidative dissolution of hydrothermal precipitates in seawater adds cu and cd to the ocean

seamount abundances and abyssal hill morphology on the eastern flank of the east pacific rise at 14 s bathymetric data from a hydrosweep multibeam sonar survey of a 720 km long tectonic corridor on the east flank of the southern epr at 14 14 s covered about 25 000 km of zero age to 8 5 m y old crust magnetic anomaly 4a in this corridor we document a strong correlation of robust along flowline changes in abyssal hill morphology and seamount size distribution with spreading rate changes deduced from our magnetic data indeed we find that both rms height of abyssal hills and abundance and height of seamounts increase significantly as spreading rate changes from 75 mm yr to over 85 mm yr half rate moreover we identified 46 seamounts taller than 100 m previous studies on the southern epr reported a larger density of seamounts organized primarily in chains our investigation however revealed seamounts not associated with major chains leading us to the conclusion that different forms of off axis volcanism occur along the spreading center

a multibeam sonar magnetic and geochemical flowline survey at 14 14 s on the southern east pacific rise insights into the fourth dimension of ridge crest segmentation a detailed bathymetric and magnetic survey of the eastern flank of the east pacific rise at 14 14 s covering seafloor ages of 0 10 ma has been carried out and used along with a flowline profile on the conjugate western ridge flank to reveal the spreading history and the temporal ridge crest segmentation additional information from basaltic lavas is included to study the relationship between physical and magmatic segment boundaries the sequence of magnetic reversals indicates a total spreading rate of 150 mm yr since 10 ma symmetric spreading however occurred only since 2 8 ma between 7 and 2 8 ma spreading was asymmetric with a higher spreading rate toward the east migration events of at least five overlapping spreading centres osc left discordant zones on the nazca plate consisting of hummocky basins and motley texture of curved lineations striking a few degrees oblique to the strike of the ridge crest four of the oscs were right stepping and migrated northward and one was left stepping and migrated southward by transferring pacific lithosphere to the nazca plate these migration events may account for most of the asymmetric accretion observed the basaltic samples from the eastern flank have been analysed and back tracked to the position of eruption on the ridge crest in terms of their geochemical signature mg 0 41 0 68 the samples reveal that the magmatic segment boundary between the garrett transform and 14 30 s has remained stationary over the last 10 myr and therefore provide no evidence for a link between magmatic and physical segmentation we therefore propose that migrating non transform ridge axis discontinuities are governed by propagating giant cracks as a crack front advances a melt reservoir is tapped and magma rises passively into the crack and erupts subsequently on the seafloor some of the oscs seem to have originated close to transform faults and therefore argue that far field stresses perhaps caused by the evolution of the bauer microplate rather than mantle upwelling create non transform ridge axis discontinuities