chemistry of uranium thorium and radium isotopes in the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal abstract the most comprehensive data set on uranium thorium and radium isotopes in the ganga brahmaputra one of the major river systems of the world is reported here the dissolved 238u concentration in these river waters ranges between 0 44 and 8 32 1 and it exhibits a positive correlation with major cations na k mg ca the 238 u cations ratio in waters is very similar to that measured in the suspended sediments indicating congruent weathering of uranium and major cations the regional variations observed in the 234 u 238 u activity ratio are consistent with the lithology of the drainage basins the lowland tributaries chambal betwa ken and son draining through the igneous and metamorphic rocks of the deccan traps and the vindhyan bundelkhand plateau have 234 u 238 u ratio in the range 1 16 to 1 84 this range is significantly higher than the near equilibrium ratio 1 05 observed in the highland rivers which drain through sedimentary terrains the dissolved 226ra concentration ranges between 0 03 and 0 22 dpm 1 the striking feature of the radium isotopes data is the distinct difference in the 228ra and 226ra abundances between the highland and lowland rivers the lowland waters are enriched in 228ra while the highland waters contain more 226ra this difference mainly results from the differences in their weathering regimes the discharge weighted mean concentration of dissolved 238u in the ganga at patna and in the brahmaputra at goalpara are 1 81 and 0 63 1 respectively the ganga brahmaputra river system constitutes the major source of dissolved uranium to the bay of bengal these rivers transport annually about 1000 tons of uranium to their estuaries about 10 of the estimated global supply of dissolved uranium to the oceans via rivers the transport of uranium by these rivers far exceeds that of the amazon although their water discharge is only about 20 of that of the amazon the high intensity of weathering of uranium in the ganga brahmapura river system can also be deduced from the 232 th 238 u and 230 th 238 u activity ratios measured in the suspended sediments 230th is enriched by about 19 in the suspended sediments relative to its parent 238u the flux of excess 230th supplied to the bay of bengal via these river sediments is 980 1012 dpm a about six times more than its in situ production from seawater in the entire bay of bengal

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

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

near vent chemical processes in a hydrothermal plume insights from an integrated study of the endeavour segment support from onc for sample collection is gratefully acknowledge as is the analytical expertise of jody spence bruno keiffer and maureen soon who helped generate the data set reported here funding for sample analysis came from an nserc discovery grant to l a c ocean networks canada is supported through the canadian foundation for innovation all data are provided in the supporting information or through onc at http dmas uvic ca datasearch

low temperature fluid flow through sulfidic sediments from tag modification of fluid chemistry and alteration of mineral deposits low temperature fluid flow has been identified as an important component of the heat mass and chemical flux at sites of active venting one way to study the movement of water through hydrothermal deposits is to analyse the pore fluids extracted from sulfidic sediments the composition of diffuse flow fluids in one sediment core from the south of the active tag mound reflects the dilution of high temperature black smoker fluids with modification inside the mound during ongoing circulation fluid modification includes conductive cooling removal of si from solution and entrainment and removal of u from seawater which can represent significant sinks of these elements precipitation of clay minerals and fe oxides is currently occurring within this sediment core generating ochreous type material subsurface extensive low temperature alteration of basaltic material at the core top is apparent in conjunction with supergene enrichment of cu and mn

238u 226ra and 210pb in some vent waters of the galapagos spreading center the concentrations of sup 226 ra sup 238 u and sup 210 pb have been measured in waters from the mussel bed and the rose garden thermal fields at the galapagos spreading center over a temperature range of approx 2 5 16 sup 0 c the sup 226 ra t plots yield slopes of 0 112 and 0 036 dpm kg sup 0 c for the mussel bed and the rose garden respectively yielding a global hydrothermal sup 226 ra flux less than 5 of that required to sustain the oceanic inventory sup 238 u concentration in waters 9 sup 0 c is the same as that in ambient sea water whereas water hotter than approx 9 sup 0 c shows a decreasing trend with temperature to zero sup 238 u at approx 29 sup 0 c sup 210 pb concentration in mussel bed increases with temperature and extrapolated to approx 350 sup 0 c yields a sup 210 pb concentration considerably less than that expected from sup 222 rn decay and basalt alteration

topographical and meteorological control on snow avalanching in the longyearbyen area central svalbard 2006 2009 abstract both topography and meteorology control snow avalanching around svalbard s main settlement longyearbyen in the high arctic extensive plateau mountains in combination with wind activity in this barren landscape lead to significant cornice accretion preferably on west facing slopes these cornices deform detach and eventually breaking down as cornice fall avalanches the most observed avalanche type with 45 2 of the total in the 2006 2009 observation period slab avalanches as the second most dominant type 32 6 accounted for the largest observed avalanches up to 250 000 m 3 debris releasing equally in all aspects the main triggering mechanism for slab avalanches seems to be additional snow loading as well as distinctive cooling or warming of the upper snow layers both mechanisms were caused by daily or weekly air temperature fluctuations mainly due to winter storm activity when frequent low pressure systems passed the islands these snowstorms resulted in 19 major avalanche cycles in which 80 of all observed avalanches released these avalanche cycles had a minimum of five releases per day lasting between 1 and 12 days in total we observed 423 avalanches with an avalanche debris volume of 100 m 3 or larger the majority released between march to may due to the slow onset of the snow cover in the early snow season and a lack of direct insolation during the polar night cold and stable high pressure periods often occurring during the spring months however reduced the avalanche activity we also use our results from this first systematic avalanche quantification to assess how future climate variability will affect the avalanche regime around longyearbyen as almost 50 of all avalanches were cornice fall avalanches which are topographically controlled we expect the same avalanche type distribution in the future as the rest of the avalanches are controlled mainly by the frequency of low pressure systems which is projected to decrease in a warming climate these avalanche occurrences will most likely decrease in amount thus cornice fall avalanches will become a more pronounced natural hazard to the infrastructure in longyearbyen this first comprehensive analysis of the recent avalanche regime in svalbard provides an important first step towards the development of an avalanche forecasting service

redox zonation of elements at an oxic post oxic boundary in deep sea sediments abstract deep sea sediments generally provide limited evidence of redox control on the post depositional redistributions of elements this is because only a fraction of any element is reactive and observed elemental distributions are the result of a number of competing processes which are simultaneously active so that redox effects are not dominant a detailed investigation is reported of a core which is unusual in showing unequivocal evidence of direct redox control operating in a homogeneous environment that is relatively free from competing processes a progressive oxidation front has migrated slowly downwards into a uniform turbidite over a long period ca 250 ky the reactive fractions of several redox sensitive elements now show a remarkable degree of organisation around the redox boundary of this front with the different elements exhibiting individual responses solid phase enrichments are not coincident but are arranged in a clear succession of concentration peaks along the redox gradient from oxic to postoxic conditions the succession is mn i fe se and pt cu and pb v zn and sb and u the movement directions of most elements can be inferred from the peak shapes against the homogeneous turbidite background composition the system seems to have been essentially isolated within the sediment for most elements so that no loss or gain can be ascribed to redox processes exceptions are u and v which have been augmented by uptake from ocean bottom waters into the sediments by an efficient redox trap mechanism

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

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

tube fossils from gossanites of the urals vhms deposits russia authigenic mineral assemblages and trace element distributions abstract the occurrence types morphology and mineralogical characteristics of tube microfossils were studied in gossanites from twelve vhms deposits of the urals several types of tube microfossils were recognized including siboglinids polychaetes and calcerous serpulids replaced by a variety of minerals e g hematite quartz hematite chlorite carbonate hematite depending on the nature of the substrate prior to the formation of the gossanites colonial hematite tube microfossils 150 m across 1 2 mm long are composed of hematitic outer and inner walls and may exhibit a cellular structure within their cavities spherical forms are saturated with fe oxidizing bacteria inside the tubes probably analogues of trophosomes colloform stromatolitic outer wall surfaces are characterized by the presence of numerous interlaced filaments of hematite 2 3 m diameter up to 1 2 mm long between tube microfossils the hematitized cement contains bundles of hematitized filaments with structures similar to the hyphae of fungi hematite chlorite tube microfossils are scattered in gossanites mostly as biological debris they are typically 30 to 300 m in diameter and 1 to 5 mm long the layered structure of their tube walls is characterized by hematite quartz and chlorite layers abundant filamentous bacteria coated by glycocalix and chlorite stromatolite are associated with hematite chlorite tubes the carbonate hematite tube microfossils up to 300 m across 2 3 mm long occur in carbonate rich gossanites the tubes are characterized by fine 10 m thick walls of hematite and cavities dominated by relatively dark carbonate or hematite carbonates may be present both in walls and cavities stromatolite like leucoxene or hematite carbonate aggregates were also found in association with tubes randomly oriented filaments are composed of ankerite single filaments are composed of individual cells typically smaller than 100 nm across similar to that of magnetotactic bacteria three dimensional tomographic images of all types of tube microfossils demonstrate a clear wavy microlayering from outer and inner walls which may reflect segmentation of the tube worms the traces of burrowing or fragments of glycocalix with relict spheres are typical of tube microfossils from gossanites the carbon isotopic composition of carbonates associated with tube microfossils from hematite quartz hematite carbonate and hematite chlorite gossanites average 7 2 6 8 22 8 pdb respectively these values are indicative of a biogenic origin for the carbonates the oxygen isotopic composition of these carbonates is similar in all three gossanite types averaging 13 5 14 2 13 0 relative to smow and indicative of active sulfate reduction during the diagenetic and anadiagenetic stages of the sediments evolution the trace element characteristics of hematite from tube microfossils are characterized by high contents of following trace elements average ppm mn 1529 as 714 v 540 w 537 mo 35 and u 5 such high contents are most likely the result of metal and metalloid sorption by fine particles of precursor iron hydroxides during the oxidation of sulfides and decomposition of hyaloclasts via microbially mediated reactions

magmatic processes under mid ocean ridges a detailed mineralogic study of lavas from east pacific rise 9 30 n 10 30 n and 11 20 n 1 detailed petrologic study has been made for lavas from the northern east pacific rise epr 9 30 n 11 20 n and 10 30 n 9 30 n and 11 20 n have robust magma supply and a shallow melt lens while 10 30 n has nonrobust magma supply and no shallow melt lens lavas from all three localities are sparsely phyric and glassy containing plagioclase olivine pyroxene typically the lavas contain several to many up to seven distinct chemical groups of plagioclase that are not always distinct texturally the lavas may also contain up to three chemically distinct groups of olivine and two groups of pyroxene the lavas contain both individual crystals and groups comprising reticulate and dendritic clots that we interpret to represent bits of crystal networks forming in mushy zones of an axial magma chamber 21 of the 23 samples studied in detail have diverse crystal compositions and require mixing which most likely occurs when mostly liquid magma passes through mushy zones we find no significant systematic differences between robust and nonrobust segments in terms of their crystal content proportion of texturally distinct xenocrysts crystal size aspect ratio roundness modal abundance magma residence time number of diverse mineral chemical groups and characteristics of mixing like the total range of composition of disequilibrium minerals present and the magnitude of the chemical gaps between disequilibrium and equilibrium compositions this unexpected and remarkable similarity suggests that the presence or absence of a seismically imaged shallow melt lens has essentially no effect on the mineralogy of erupted lavas we conclude that the fundamentally important magmatic process under mid ocean ridges from slow to fast is the formation of crystal networks and their subsequent compaction the seismically detected shallow melt lenses likely contain highly evolved magma formed by expelled interstitial melt during crystal network compaction and play very little role in crustal accretion our conclusion implies that the thick 3 km low velocity zone near the base of the crust produces the thin 30 m shallow melt lenses not the other way around

a dual origin for the hydrothermal component in a metalliferous sediment core from the mid atlantic ridge a highly metalliferous sediment core collected from tag 26 n mid atlantic ridge has been analyzed for uranium series isotopes rare earth elements ree and radiogenic isotopes two types of deposition of hydrothermal material have been identified and characterized the first is sulfide material which has been rapidly deposited and is sometimes clearly turbiditic this material exhibits high u fe and eu sm ratios and low 230th fe and ree fe ratios it is proposed that u is incorporated into the sulfides by diffusion through the oxidized coating with reductive fixation near the sulfide oxide boundary the second hydrothermal component is particulate fallout from the neutrally buoyant plume which is diluted by the background pelagic sedimentation this component exhibits u fe ratios around 2 10 5 an order of magnitude greater than reported plume particulate levels but the 230th fe ree fe and eu sm ratios are similar to plume values nd and pb isotopes have been used to differentiate the hydrothermal and pelagic end members within this sediment core

enzymatic iron and uranium reduction by sulfate reducing bacteria abstract the potential for sulfate reducing bacteria srb to enzymatically reduce fe iii and u vi was investigated five species of desulfovibrio as well as desulfobacterium autotrophicum and desulfobulbus propionicus reduced fe iii chelated with nitrilotriacetic acid as well as insoluble fe iii oxide fe iii oxide reduction resulted in the accumulation of magnetite and siderite desulfobacter postgatei reduced the chelated fe iii but not fe iii oxide desulfobacter curvatus desulfomonile tiedjei and desulfotomaculum acetoxidans did not reduce fe iii only desulfovibrio species reduced u vi u vi reduction resulted in the precipitation of uraninite none of the srb that reduced fe iii or u vi appeared to conserve enough energy to support growth from this reaction however desulfovibrio desulfuricans metabolized h2 down to lower concentrations with fe iii or u vi as the electron acceptor than with sulfate suggesting that these metals may be preferred electron acceptors at the low h2 concentrations present in most marine sediments molybdate did not inhibit fe iii reduction by d desulfuricans this indicates that the inability of molybdate to inhibit fe iii reduction in marine sediments does not rule out the possibility that srb are important catalysts for fe iii reduction the results demonstrate that although srb were previously considered to reduce fe iii and u vi indirectly through the production of sulfide they may also directly reduce fe iii and u vi through enzymatic mechanisms these findings as well as our recent discovery that the so reducing microorganism desulfuromonas acetoxidans can reduce fe iii demonstrate that there are close links between the microbial sulfur iron and uranium cycles in anaerobic marine sediments

geochemistry and morphology of metalliferous sediments and oxyhydroxides from the endeavour segment juan de fuca ridge abstract we present first data on the geochemistry mineralogy and morphology of near vent sediments 35 and 200 m from active vent and ridge flank sediments approximately 3 km from the vent field as well as oxyhydroxide deposits from the endeavour segment juan de fuca ridge the purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains hydrothermal components in sediments are fe si s rich and mn fe si rich phases sulfides and barite which were exclusively derived from plume fallout sulfides are only a minor constituent of near vent sediments 2 4 wt and were not detected in ridge flank sediments the study suggests that the distribution of hydrothermal phases and associated elements in near vent and ridge flank sediments is affected mainly by processes of agglomeration dissolution absorption and settling that take place within a plume and to a lesser extent post depositional processes rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the cu concentrations in sediments with increasing distance from the vents besides sulfides important carriers of pb cu zn and co in near vent sediments are fe mn oxyhydroxides that occur together with silica as aggregates of gel like material and flaky particles and as coatings on filaments away from the vents trace metals are mostly in fe mn oxyhydroxides and authigenic fe rich montmorillonite oxyhydroxides at the main endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary fe oxyhydroxide silica from low temperature fluids at the mothra field seafloor deposits and chimney crusts composed of fe oxyhydroxide mn silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities

uranium removal in oceanic sediments and the oceanic u balance pore water uranium data from the nw atlantic ocean cariaco trench black sea and san clemente basin are used to calculate fluxes across the sediment water interface all samples were collected in situ and show depletions in pore water u relative to the overlying water concentration the uptake rate of u derived from pore water gradients reflects the redox environment with lowest fluxes 10 g cm 2 ky in oxic su ic sediments of the nw atlantic dominated by no 3 reduction and higher fluxes 25 70 g cm 2 ky in the anoxic sulfate reducing sediments of the cariaco trench black sea and san clemente basin the anoxic fluxes compare well with previous estimates based on solid phase data calculations indicate that the areally integrated removal of u into su ic hemipelagic sediments is of greater magnitude than that into anoxic sulfate reducing sediments these fluxes are discussed in the context of the oceanic u balance the removal of u in all oceanic sediments is estimated to account for about 65 of the riverine input and this sink plus removal of u in altered oceanic crust balances the most probable value for riverine input

geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field east pacific rise 18 s an inactive vent field comprised of dead chimneys was discovered on the ultrafast east pacific rise epr at 18 degrees s during the research campaign naudur with the r v le nadir in december 1993 one of these chimneys was sampled studied and found to be largely composed of silica mineralized bacterial like filaments the filaments are inferred to be the result of microbial activity leading to silica fe oxyhydroxide precipitation the chimney grew from the most external layer precipitated 226 4 yr b p towards the central chimney conduit hydrothermal activity ceased 154 13 yr b p and the chimney conduit was completely sealed mixing between an end member hydrothermal fluid and seawater explains the sr nd isotopic composition of the chimney seawater was the major source of sr to the chimney whereas the dominant nd source was the local mid ocean ridge basalt morb leached by the hydrothermal fluids the mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions the proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85 pb isotope data indicates that all of the pb in the chimney was derived from the underlying morb the precipitation temperatures of the chimney layers varied between 55 and 71 degrees c and were a function of the seawater end member hydrothermal fluid mixing ratio delta si 30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the si isotope composition of the chimney concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end member hydrothermal fluid the inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater hydrothermal fluid mixing ratio which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids the silica chimney is significantly enriched in u likely a result of bacterial concentration of u from the seawater dominated vent fluid the chimney is poor in rare earth elements ree it inherited its ree distribution patterns from the parent end member hydrothermal fluids the dilution of the hydrothermal fluid with over 85 seawater could not obliterate the particular ree features positive eu anomaly of the hydrothermal fluids

geochemistry of sediments from the tag hydrothermal field m a r at latitude 26 n abstract sediments from the tag hydrothermal field show evidence of hydrothermal inputs in the form of fe cu zn and mn enrichments the fe cu and zn are thought to have been introduced as finely divided sulphides precipitated from suspension after dispersion away from now inactive hydrothermal vents and have since been oxidised by contrast the manganese is believed to be in the form of primary oxide precipitate the processes of metal enrichment have been variably active during discrete intervals in the past geochemical partition analysis has demonstrated that the elements determined can be distributed differently between various phases in the different types of sediments studied and that diagenetic transfer of elements from one phase to another occurs on burial the general lack of metal enrichments in the sediments of the intensity sometimes found on the east pacific rise is thought to be largely the result of major metal precipitation subsurface

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

uranium enrichment in metalliferous sediments from the mid atlantic ridge abstract previous studies of metal rich sediments at mid ocean ridge spreading centres have suggested that u fe ratios are relatively constant from site to site a suite of cores from the mid atlantic ridge has been analysed for u and fe content and most samples exhibit constant u fe ratios sulphide bearing parts of one core however contain excess u compared with that observed in other metalliferous sediments but apparently only during oxidative alteration of pyrite phases fission track registration has demonstrated that this excess u is intimately associated with sulphide grains in the sediment scanning electron microscope examination indicates that this u enrichment is limited to very small areas within the oxidation layer and is associated with an enhanced p concentration ion probe analyses allow semi quantitative analyses of various elements through a u rich grain the observed distribution of u coupled with its specific enrichment suggests that microbial mediation might be controlling this enrichment u fe ratios in fully oxidised tag sediments are constant and approximately an order of magnitude higher than in young plume particulates this discrepancy is attributed to a second type of post depositional u enrichment producing an homogeneous distribution of u throughout the sediment

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

pb sr and nd isotopes in basalts and sulfides from the juan de fuca ridge pb sr and nd isotopes of seven basalt glasses collected by the submersible alvin from the southern juan de fuca ridge sjfr are almost identical 206pb 204pb 18 45 207pb 204pb 15 47 208pb 204pb 37 81 87sr 86sr 0 70249 143nd 144nd 0 51315 whereas all basalts appear cogenetic four of the samples have uniform abundances of u th rb nd sm pb and sr indicating that they are also comagmatic two basalt glasses dredged previously at the sjfr have similiar isotopic compositions but higher concentrations of u th and pb the 206pb 204pb ratios are intermediate between generally less radiogenic ridge basalts from south of the juan de fuca ridge jfr and often more radiogenic basalts from the northern jfr and ne pacific seamounts sr and nd isotopic compositions closely resemble data of other ridge basalts from the northernmost east pacific rise and are intermediate between isotopically more diverse seamount basalts produced nearby eight sulfides collected from three hydrothermal vent sites at sjfr have very similar pb isotopic compositions to that of the associated basalts sr and nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater

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

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

tracing low temperature fluid flow on ridge flanks with sedimentary uranium distribution the uranium content of sediments and pore fluids along a ridge axis to ridge flank transect perpendicular to the southern east pacific rise is used to assess the impact of low temperature fluid flow on u distribution two distinct regions are identified 1 a young crustal site 0 36 ma crust where significant solid phase u enrichments occur and 2 the ridge flank 1 9 4 6 ma crust with uniform u association with hydrothermal fe oxide phases in the solid phases upward advection and diffusion of cool u depleted basement fluids occurs at many coring sites at the 0 36 ma site the oxic basal fluids strip the plume derived sediment of the excess u effectively migrating the u upwards through the sediment into the overlying water column at the ridge flank sites the pore fluid advection rates are highest at bathymetric basement highs and advection velocities of up to 7 5 mm yr 1 are inferred from the pore fluid profiles these estimates are consistent with previous calculations based on fluoride pore fluid distributions the basal fluid u depletions are in the range 10 70 relative to seawater depending on temperature and redox state low temperature uptake of u during basalt alteration is a significant sink from seawater in the global seawater u budget 6 7 29 mmol yr 1 pore fluid u content is a sensitive tracer of extremely low temperature advection through ridge flank systems and the basal sediment u fe ratio is potentially a useful proxy for basement alteration history where low ratios indicate extensive oxic alteration of basal sediments during fluid flow

seawater metasomatism in hydrothermal sediments escanaba trough northeast pacific abstract hydrothermal sediments from the nesca vent field escanaba trough are enriched in a number of typical vent fluid metals for which fluid data for the same sediment hosted site exhibit anomalous depletion these results are consistent with cooling of rising fluids and precipitation of various metal sulphide sulphate and oxide phases in situ within the sediment column prior to expulsion of the fluids from the seabed significant enrichments of mg and u elements which are quantitatively removed from high temperature vent fluids are also observed in the escanaba trough sediments however enrichments of these elements cannot derive from simple conductive cooling of the rising vent fluids but instead are believed to result from extensive seawater metasomatism as relatively fresh unreached seawater is heated from below by hot sediments and or percolating vent fluids in the upper levels of the circulation system calculated seawater sediment interaction ratios range from 5 1 to 2500 l of seawater per gram of sediment pb sr and nd isotope distributions for the escanaba sediments are consistent with combined sediment fluid and seawater sediment interactions pb isotope ratios for hydrothermal and background sediments coincide with local sulphide deposits but are much more radiogenic than local morb this indicates not only that complete isotopic re equilibration must occur between the hot rising fluids and the host sediments but in addition that sediment composition appears to dominate the pb isotopic composition of these fluids sr and nd isotope ratios in the hydrothermally altered sediments which are distinct from background ratios coincide closely with modern seawater values providing confirmatory evidence that extensive interactions must also have occurred between these hydrothermal sediments and relatively fresh unreacted seawater

age and paragenesis of mineralisation at coronation hill uranium deposit northern territory australia coronation hill is a u au platinum group elements deposit in the south alligator valley sav field in northern australia south of the better known unconformity style u east alligator rivers ear field the sav field differs from the ear by having a more complex basin basement architecture a volcanically active fault trough jawoyn sub basin developed on older basement and then was disrupted by renewed faulting before being buried beneath regional mcarthur basin sandstones that are also the main hanging wall to the ear deposits primary mineralisation at coronation hill formed at 1607 26 ma rather than 600 900 ma as previously thought and so it is likely that the sav was part of a single west mcarthur basin dilational event most ore is hosted in sub vertical faults and breccias in the competent volcanic cover sequence this favoured fluid mixing acid buffering forming illite and oxidation of fe2 and reduced c rich assemblages as important uranium depositional mechanisms however reduction of u in fractured older pyrite pb model age of 1833 67 ma is an important trap in diorite some primary ore was remobilised at 675 21 ma to form coarse uraninite ni co pyrite networks containing radiogenic pb coronation hill is polymetallic and in this respect resembles the egress style u deposits in the athabascan basin canada however these are all cover hosted a hypothesis for further testing is that coronation hill is also egress style with ores formed by fluids rising through basement hosted fault networks u reduction by diorite pyrite and carbonaceous shale and into veins and breccias in the overlying jawoyn sub basin volcano sedimentary succession

hydrothermal sediments record changes in deep water oxygen content in the se pacific the distribution of redox sensitive metals in sediments is potentially a proxy for past ocean ventilation and productivity but deconvolving these two major controls has proved difficult to date here we present a 740 kyr long record of trace element concentrations from an archived sediment core collected at 15 s on the western flank of the east pacific rise epr on 1 1 myr old crust and underlying the largest known hydrothermal plume in the world ocean the downcore trace element distribution is controlled by a variable diagenetic overprint of the inferred primary hydrothermal plume input two main diagenetic processes are operating at this site redox cycling of transition metals and ferrihydrite to goethite transition during aging the depth of oxidation in these sediments is controlled by fluctuations in the relative balance of bottom water oxygen and electron donor input organic matter and hydrothermal sulfides these fluctuations induce apparent variations in the accumulation of redox sensitive species with time subsurface u and p peaks in glacial age sediments in this and other published data sets along the southern epr indicate that basin wide changes in deep ocean ventilation in particular at glacial interglacial terminations ii iii iv and v alter the depth of the oxidation front in the sediments these basin wide changes in the deep pacific have significant implications for carbon partitioning in the ocean atmosphere system and the distribution of redox sensitive metals in ridge crest sediment can be used to reconstruct past ocean conditions at abyssal depths in the absence of alternative proxy records

fe mn micronodules born in the metalliferous sediments of two spreading centres the east pacific rise and mid atlantic ridge abstract a geochemical study of fe mn micronodules associated with the metalliferous sediments at two spreading centres has shown that their composition depends on the site of micronodule formation close to the hydrothermal mounds they exhibit significant variation in elemental content related to the type of hydrothermal discharge low or high temperature the nature of primary hydrothermal matter plume fall out oxidised sulfides and the extent of diagenesis in this environment three types of micronodules can be distinguished although not observed as pure end members 1 diagenetic micronodules 2 micronodules formed generally from the plume fall out of oxyhydroxide matter and 3 micronodules grown on the oxidised sulfide grains supplied to the sediments by slumping or fall out of nearby buoyant plume away from the active spreading centre the hydrothermal signatures of primary precipitates are gradually masked and hydrogenous diagenetic processes lead the micronodule formation composition of micronodules becomes less variable well pronounced deep rift valleys confine the hydrothermal plume which brings the hydrothermal suspension into contact with restricted volumes of seawater and consequently weakens the hydrogenous influence on the primary hydrothermal matter shallow rift valleys do not confine hydrothermal plumes which are scattered over hundreds of kilometres by bottom currents this brings the hydrothermal suspended matter into contact with large volumes of seawater extensive scavenging occurs which masks the hydrothermal signal away from the spreading axis and enhances the hydrogenous one thus the ridge crest morphology defined by the spreading rate is supposed to play a certain role though indirect in the chemical composition of the primary precipitates and consequently in the composition of the micronodules formed

evolution of an active sea floor massive sulphide deposit hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to 400 c before venting at spectacular black smokers these hydrothermal systems exert a strong influence on ocean chemistry1 yet their structure longevity and magnitude remain largely unresolved2 the active transatlantic geotraverse tag deposit at 26 n on the mid atlantic ridge is one of the largest oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black smoker fields here we report ages of sulphides and anhydrites from the recently drilled3 tag substrate structures determined from 234u 230th systematics analysed by thermal ionization mass spectrometry the new precise ages combined with existing data4 5 show that the oldest material 11 000 to 37 000 years old forms a layer across the centre of the deposit with younger material 2 300 7 800 years old both above and below this stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6 the age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2 000 years