massive sulfides of mount jourdanne along the super slow spreading southwest indian ridge and their genesis abstract modern massive sulfide deposits are known to occur in diverse tectonic settings and it is generally expected that hydrothermal deposits of similar geological settings shall have more or less similar mineralogical and geochemical signatures however the mount jourdanne sulfide deposits along the super slow spreading southwest indian ridge deviate from this common concept these sulfide precipitates are zn rich up to 35 wt and are characterized by high concentrations of pb 3 5 wt as 1 1 wt ag 0 12 wt au 11 ppm sb 967 ppm and cd 0 2 wt which are unusual for a modern sediment free mid oceanic ridge system therefore we have reinvestigated the sulfide samples collected during the indoyo cruise in 1998 in order to explain their unusual mineralogy and geochemical composition the sulfide samples are polymetallic and are classified as a chimneys b mounds and c hydrothermal breccias the chimneys are small tube like symmetrical bodies 30 40 cm high 10 cm diameter and consist mainly of sphalerite and less chalcopyrite set in a matrix of late amorphous silica the inner wall shows a late stage colloform sphalerite containing co precipitates of galena and or pb as sulfosalts in contrast the mound samples are dominated either by fe sulfides pyrite or by a mixture of pyrite and chalcopyrite with less sphalerite pyrrhotite amorphous silica and barite both the chimney and mound samples are characterized by layering and mineral zonation the hydrothermal breccias are highly altered and mineralogically heterogeneous they consist of silicified basaltic material that are impregnated with sulfides and contain cm sized chimney fragments within a matrix of low temperature minerals such as sphalerite and pyrite the latter fragments mainly consist of chalcopyrite with isocubanite lamellae in addition these breccias contain late stage realgar boulangerite galena pb as sulfosalts and barite that are mostly confined to vugs or fractures at least five mineralogical associations are distinguished that indicate different thermal episodes ranging from black smoker mineralization conditions to cessation of the hydrothermal activity based on the mineralogical associations and established literature in this regard it is inferred that the mineralization at mt jourdanne occurred mainly in three temperature domains above 300 c the chalcopyrite with isocubanite pyrrhotite association formed whereas the sphalerite dominated assemblage with much less chalcopyrite and pyrite formed around and below 300 c the late stage mineralization below 200 c contains colloform sphalerite galena pb as sulfosalts realgar and barite the unusual mineralogy and trace element chemistry for this modern vhms deposit could be explained assuming hydrothermal leaching of some felsic differentiates underneath the basaltic cover and subsequent zone refining processes

formation of large sulfide mineral deposits along fast spreading ridges example from off axial deposits at 12 43 n on the east pacific rise abstract submersible investigations with nautile near the 13 n hydrothermal field on the epr were made in 1992 we selected diving areas based on deep towed side scan sonar imaging close to a pit crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest 70 m high and 200 m in diameter sulfide mounds formed at a fast spreading ridge 12 cm yr despite an overlap in composition between axial and off axial deposits the general trend reflects real differences in compositions off axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment sulfur isotope variation is minimal for the off axis seamounts and suggests that sulfides precipitated from unmodifed end member hydrothermal solutions axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different pb sources and separate convective systems for the seamount and the axial deposits our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount this configuration is by far more efficient than unstable axial processes to create during several hydrothermal episodes major sulfide deposits near a fast spreading ridge off axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis thus we suggest that off axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast to medium spreading ridges

factors controlling riffle scale hyporheic exchange flows and their seasonal changes in a gaining stream a three dimensional groundwater flow model 1 subsurface flow within a single riffle of a low gradient gravel bed stream was modeled in three dimensions using modflow a finite difference groundwater flow model model simulations showed that exchange flows can only occur in this low gradient gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment k 10 4 m s 1 compared with k 10 6 to 10 8 m s 1 the key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium the hydraulic gradient between upstream and downstream ends of the riffle and the flux of groundwater entering the alluvium from the sides and beneath in the study riffle each of these factors changes with season causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0 2 0 5 m 3 d per meter stream length in summer to about 0 008 0 04 m 3 d 1 per meter stream length during fall to spring the model also revealed that exchange flows are up to twice as strong but more variable at the sides of the stream than near the center and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

distribution of bedrock and alluvial channels in forested mountain drainage basins mountain river networks often consist of both bedrock and alluvial channels1 5 the spatial distribution of which controls several fundamental geomorphological and ecological processes6 7 the nature of river channels can influence the rates of river incision and landscape evolution1 2 as well as the stream habitat characteristics affecting species abundance and aquatic ecosystem structure8 11 studies of the factors controlling the distribution of bedrock and alluvial channels have hitherto been limited to anthropogenic badlands12 here we investigate the distribution of channel types in forested mountain drainage basins and show that the occurrence of bedrock and alluvial channels can be described by a threshold model relating local sediment transport capacity to sediment supply in addition we find that valley spanning log jams create alluvial channels hospitable to aquatic life in what would otherwise be bedrock reaches the formation of such jams depends critically on the stabilizing presence of logs derived from the largest trees in the riverside forests suggesting that management strategies that allow harvesting of such trees can have a devastating influence on alluvial habitats in mountain drainage basins

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at odp site 801 the subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34s relative to igneous sulfides although these 34s depleted sulfur isotope ratios may point to signatures of biological fractionation previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration here we report results for multiple s isotope 32s 33s 34s compositions of altered basalts at odp site 801 in the western pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust in situ ion microprobe analyses of secondary pyrite in basement rocks show a large range of 34s values between 45 and 1 whereas bulk rock 34s analyses yield a more restricted range of 15 8 to 0 9 these low and variable 34s values together with bulk rock s concentrations ranging from 0 02 up to 1 28 are consistent with loss of magmatic primary mono sulfide and addition of secondary sulfide via microbial sulfate reduction high precision multiple sulfur isotope 32s 33s 34s analyses suggest that secondary sulfides exhibit mass dependent equilibrium fractionation relative to seawater sulfate in both 33s and 34s values these relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates the determination of the s isotope composition of bulk altered oceanic crust demonstrates that s based metabolic activity of subsurface life in oceanic basalt is widespread and can affect the global s budget at the crust seawater interface

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

trace element and sulfur isotope compositions for pyrite across the mineralization zones of a sulfide chimney from the east pacific rise 1 2 s abstract active black smokers represent important products of seafloor hydrothermal systems and can reveal the fluid evolution and ore genesis in sulfide deposits the key for understanding the details of fluid circulation and mineralization within sulfide zonation of chimneys lies in the understanding of variation in micro scale trace elements and sulfur isotopes in sulfide deposits an active pyrite dominated chimney collected from the east pacific rise 1 2 s hydrothermal field supplies a unique opportunity to exhibit on going ore forming processes especially the detailed fluid processes among the sulfide zonation of chimney as well as to constrain the mineralization environment that is crucial for metal transport and deposition across the chimney pyrites as the ubiquitous sulfide present crystal morphologies varying from anhedral though euhedral to massive textures at different mineralization stages the characterization of trace elements in different types of pyrite from rim to core evolves from zn pb cd tl ag mn and ba to cu se sn bi co and ni in response to increasing mineralization temperature the dominant control on co and ni contents in pyrite from the exterior and interior of the chimney changes from temperature to sulfur fugacity as the hydrothermal circulation becomes active and subsequently wanes in addition to the physicochemical environment the chemical variations like as sb and mo in pyrite from different mineral zonation are mostly like related to the temporal evolution of fluid compositions seawater fluid mixing not only influences seawater derived species of trace elements like v and mg and the spatial distributions of temperature sensitive elements like co cu se and sn but also increases the s isotope values of pyrite in the mineral sequence of the chimney however the incorporation of approximately 8 24 bacteria derived s leads to anhedral pyrite with abnormally light s isotopes in the outer mineral zonation

hyporheic exchange due to channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive a large zone of river groundwater mixing or hyporheic exchange flume experiments and numerical simulations show that river water downwells into the riverbed upstream of a channel spanning log and upwells downstream exchange rates are greatest near the log and decay exponentially with distance upstream and downstream we developed equations for bed pressure profiles and hyporheic exchange rates in the vicinity of a channel spanning log that can be used to evaluate the impact of lwd removal or reintroduction on hyporheic mixing the magnitude of pressure disturbance along the bed and thus hyporheic exchange increases with the fraction of channel depth blocked by the log and channel froude number exchange rates are relatively insensitive to relative depth of the log gap ratio at natural densities lwd in lowland streams drives reach averaged hyporheic exchange rates similar to a ripple covered bed however the length scales and residence times of hyporheic exchange due to lwd are greater by removing lwd from streams humans have altered patterns and rates of hyporheic exchange which influence habitat distribution and quality for invertebrates and fish

s sr and pb isotopic systematics of hydrothermal chimney precipitates from the eastern manus basin western pacific evaluation of magmatic contribution to hydrothermal system 1 sulfur strontium and lead isotopic analyses were performed on chimneys from the hydrothermal vent fields the pacmanus and susu knolls in the eastern manus back arc basin lead isotope composition varies in a narrow range 206pb 204pb 18 75 18 78 207pb 204pb 15 51 15 54 and 208pb 204pb 38 31 38 40 strontium isotopic ratios 87sr 86sr of sulfate minerals vary from 0 7051 to 0 7077 falling between those of seawater 0 7090 and the hydrothermal fluid end member 0 7050 the 34s values of the chimneys are 8 0 to 3 9 for the susu knolls sulfide 1 0 to 4 3 for the pacmanus sulfides and 7 4 to 20 6 for sulfate the susu knolls sulfides have the lowest values so far reported for volcanic hosted massive sulfides from modern seafloor the low 34s values of sulfates and sulfides cannot be explained by a simple seawater circulation model without an additional source of light sulfur this is also supported by the presence of sulfate in hydrothermal end member fluid indicated by sulfur and strontium isotopic ratios in chimney sulfates biogenic sulfur and or oxidation of h2s after boiling are excluded from the possible sources of light sulfur because of the lack of sediment cover and nonequilibrium in isotopic fractionation by boiling at seafloor disproportionation of magmatic so2 introduced into hydrothermal system through preeruptive degassing is the most plausible mechanism for the observed low 34s values the lighter 34s values and more radiogenic lead isotope compositions of the susu knolls samples indicate that magmatic degassing was more vigorous at the susu knolls than pacmanus

sulfur and carbon geochemistry of the santa elena peridotites comparing oceanic and continental processes during peridotite alteration abstract ultramafic rocks exposed on the continent serve as a window into oceanic and continental processes of water peridotite interaction so called serpentinization in both environments there are active carbon and sulfur cycles that contain abiogenic and biogenic processes which are eventually imprinted in the geochemical signatures of the basement rocks and the calcite and magnesite deposits associated with fluids that issue from these systems here we present the carbon and sulfur geochemistry of ultramafic rocks and carbonate deposits from the santa elena ophiolite in costa rica the aim of this study is to leverage the geochemistry of the ultramafic sequence and associated deposits to distinguish between processes that were dominant during ocean floor alteration and those dominant during low temperature continental water peridotite interaction the peridotites are variably serpentinized with total sulfur concentrations up to 877 ppm that is typically dominated by sulfide over sulfate with the exception of one sample the ultramafic rocks are characterized by positive 34 s sulfide up to 23 1 and 34 s sulfate values up to 35 0 carbon contents in the peridotites are low and are isotopically distinct from typical oceanic serpentinites in particular 13 c of the inorganic carbon suggests that the carbon is not derived from seawater but rather the product of the interaction of meteoric water with the ultramafic rocks in contrast the sulfur isotope data from sulfide minerals in the peridotites preserve evidence for interaction with a hydrothermal fluid specifically they indicate closed system abiogenic sulfate reduction suggesting that oceanic serpentinization occurred with limited input of seawater overall the geochemical signatures preserve evidence for both oceanic and continental water rock interaction with the majority of carbon and possibly sulfate being incorporated during continental water rock interaction furthermore there is evidence for microbial activity that was possibly stimulated by carbon sourced from water rock interaction with adjacent sediments or fluid inclusions this study provides detailed insight into the complex hydrothermal history of continental serpentinization systems and adds to our understanding of the carbon and sulfur cycling within peridotite hosted hydrothermal systems

seafloor massive sulfides from mid ocean ridges exploring the causes of their geochemical variability with multivariate analysis abstract the neovolcanic zones of mid ocean ridges are host to seawater derived hydrothermal systems forming seafloor massive sulfide sms deposits these deposits have high concentrations of base metals and potentially economic enrichment of a wide range of trace elements the factors controlling this enrichment are currently poorly understood we have investigated the main factors controlling sms compositional variability through robust principal component analysis and robust factor analysis of published and newly obtained bulk geochemical data for samples collected from sms deposits worldwide we found that a large part of the observed variability is produced by a combination of three independent factors which are interpreted to reflect in order of importance 1 the temperature of deposition 2 the ridge spreading rate and 3 zone refining the first and the third factors are mostly related to processes operating near the seafloor such as conductive cooling mixing of the hydrothermal fluids with seawater and metal remobilization and determine the relative proportions of the main minerals and thus of cu and zn co se sb pb the ridge spreading rate influences the structure of the oceanic lithosphere which exerts a major control on the length and depth of the hydrothermal convection cell and on the rock to water ratios in the reaction zone which in turn control the behavior of the precious metals au and ag and elements including ni mo se despite the obvious role of substrate rocks as metal sources their composition specifically mafic vs ultramafic does not emerge as a statistically significant independent factor

submarine gold mineralization near lihir island new ireland fore arc papua new guinea gold rich siliceous veins with disseminated polymetallic sulfides and pyritic stockwork mineralization have been recovered from the top of conical seamount a shallow 1 050 m water depth submarine volcano located about 10 km south of lihir island papua new guinea grab samples from the summit of conical seamount contain the highest concentration of gold yet reported from the modern sea floor max 230 ppm au avg 26 ppm n 40 the gold occurs in sulfide rich veins of black amorphous silica hosted by intensely altered high k calc alkaline basalts sulfides in the veins consist of sphalerite galena pyrite chalcopyrite marcasite and a variety of cu pb as sb sulfosalts the gold occurs as native gold and electrum in the amorphous silica and as inclusions in the sulfides the highest gold concentrations are associated with high ag as sb and hg zoned alteration adjacent to the veins consists of illite smectite amorphous silica k feldspar secondary plagioclase minor chlorite and trace carbonate the association of gold with illite smectite amorphous silica and k feldspar indicates deposition from near neutral ph hydrothermal fluids however the auriferous polymetallic sulfide veins and the associated alteration are overprinted on stockwork pyrite mineralization that is associated with earlier acid alteration containing alunite aluminum phosphate sulfates kaolinite and other clay minerals the platy habit of the alunite in this assemblage the presence of alumium posphate sulfate minerals and the sulfur isotope ratios of the crystalline pyrite 8 6 to 0 2 34 s n 28 and alunite 7 5 and 6 4 34 s are consistent with a contribution of magmatic volatiles in the earliest stages of the hydrothermal system framboidal pyrite within and at the margins of the mineralized zone has 34 s values suggesting involvement of biogenic activity 11 6 to 13 9 34 s the gold rich veins at conical seamount are distinct from sea floor massive sulfide deposits and represent a new style of mineralization on the modern sea floor the mineralogy alteration geochemistry and texture of the veins resemble those of some subaerial epithermal gold deposits and indicate that features long considered to define a subaerial setting can also form in a submarine environment the proximity of conical seamount to the giant ladolam epithermal gold deposit on nearby lihir island also raises the possibility that both subaerial and submarine gold mineralization in the region may be related to the same district scale magmatic events

where are the undiscovered hydrothermal vents on oceanic spreading ridges abstract in nearly four decades since the discovery of deep sea vents one third of the length of global oceanic spreading ridges has been surveyed for hydrothermal activity active submarine vent fields are now known along the boundaries of 46 out of 52 recognized tectonic plates hydrothermal survey efforts over the most recent decade were sparked by national and commercial interests in the mineral resource potential of seafloor hydrothermal deposits as well as by academic research here we incorporate recent data for back arc spreading centers and ultraslow and slow spreading mid ocean ridges mors to revise a linear equation relating the frequency of vent fields along oceanic spreading ridges to spreading rate we apply this equation globally to predict a total number of vent fields on spreading ridges which suggests that 900 vent fields remain to be discovered almost half of these undiscovered vent fields comparable to the total of all vent fields discovered during 35 years of research are likely to occur at mors with full spreading rates less than 60 mm yr we then apply the equation regionally to predict where these hydrothermal vents may be discovered with respect to plate boundaries and national jurisdiction with the majority expected to occur outside of states exclusive economic zones we hope that these predictions will prove useful to the community in the future in helping to shape continuing ridge crest exploration

the southwest indian ridge between 49 15 e and 57 e focused accretion and magma redistribution abstract bathymetric gravity magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow spreading southwest indian ridge swir full rate 14 mm yr between 49 15 e and 57 e this analysis reveals that the segmentation defined by along axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along axis variations of backscattering strength mantle bouguer anomaly mba and amplitude of the central magnetic anomalies cma at axial discontinuities with offsets larger than 15 km thin crust and reduced volcanic production are suggested by the occurrence of mba highs almost non existent cma and 50 lower backscattering strength relative to the segment centres by contrast smaller non transform discontinuities with offsets smaller than 15 km correspond to very weak variations or to no variation of the mba the cma or the reflectivity of the seafloor suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities these small axial discontinuities bound low relief bathymetric segments 500 700 m corresponding to weak or no mba lows amplitude 1000 m corresponding to large mba lows amplitude 30 mgal we suggest that the magma supply to these low relief segments is controlled by near surface processes such as melt migration and or crustal plumbing from adjacent high relief segments pronounced mba lows at high relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle these spreading cells are spaced by about 100 km along the swir axis we suggest that the spacing of spreading cells on slow spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow spreading ridges than on slow spreading ridges

sulfur isotopic compositions of sulfides along the southwest indian ridge implications for mineralization in ultramafic rocks the recently explored tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow spreading southwest indian ridge displays high temperature sulfide mineralization isocubanite sphalerite and minor pyrrhotite and low temperature pyrite and covellite phases pyrite can be subdivided into pyrite i and ii with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite sphalerite and pyrite i or occurring as individual grains in quartz veinlets the sulfide minerals have the greatest range of 34s values 23 8 to 14 1 found so far among modern sediment starved ridges with distinct 34s values for low and high temperature mineral phases the high 34s values of isocubanite 9 6 to 12 2 and sphalerite 9 1 to 14 1 suggest that sulfate which precipitated from seawater during an early low temperature phase of hydrothermal circulation was the main sulfur source for these sulfides pyrite ii has the lowest and most variable 34s values 23 8 to 3 6 suggesting microbial sulfate reduction pyrite i has variable and generally positive 34s values 0 1 to 12 0 with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate mixed with volcanogenic sulfur intermittent magmatism represented by gabbroic intrusions and high permeability caused by well developed fractures associated with detachment faults contributed to the formation of sulfides in the tianzuo hydrothermal field these factors possibly control sulfide mineralization in amagmatic segments of ultraslow spreading ridges

effect of experimental wood addition on hyporheic exchange and thermal dynamics in a losing meadow stream 1 stream restoration structures such as large wood can enhance shallow river groundwater exchange or hyporheic exchange and alter temperature dynamics in restored reaches we added and then removed channel spanning logs in a second order mountain meadow stream to test short term impacts on hyporheic exchange streambed temperatures and surface water temperatures based on vertical seepage measurements and numerical simulations of hyporheic fluid and heat flow large wood addition increased hyporheic exchange and altered streambed temperatures in this losing stream meter scale hyporheic exchange cells formed beneath large wood upwelling pore water downstream of logs stabilized diel temperature cycles across 8 of the streambed creating localized but potentially valuable thermal refuge exchange rates were 0 1 of channel discharge too small to impact the range of diel temperature signals in surface water however the lag between downstream and upstream diel temperature signals was slightly greater with large wood which may indicate that surface storage zones rather than hyporheic storage zones increased thermal retardation losing conditions limited the spatial extent and rates of hyporheic exchange near large wood impacts of large wood reintroduction on hyporheic exchange depend on ambient groundwater discharge or recharge streambed permeability channel froude number large wood blockage ratio and large wood spacing in many streams large wood reintroduction may increase hyporheic habitat volume and complexity but may not increase exchange rates enough to alter surface water temperature or chemistry surface storage zones such as eddies and pools can still influence heat and solute retention in the channel

reactivation of precambrian faults on the southwestern continental margin of india evidence from gravity anomalies abstract gravimetric and bathymetric studies on the southwestern continental margin of india confirm the extension of onshore nw se nnw sse n s ne sw ene wsw and e w trending lineaments of precambrian age over a considerable distance into the offshore region the bight in the bathymetry off coondapoor the offsets of bessas de pedro bank and the cora divh island of the laccadive group the prathap ridge and the inferred mid shelf basement ridge suggest block movements on the southwestern continental margin the physiographic expression on the prathap ridge around 14 20 n and 72 50 e is unaffected by some of the ene wsw lineaments which probably indicates that these lineaments predate the evolution of the topographic expression as seafloor spreading advanced with respect to the carlsberg ridge some of the ene wsw and ne sw lineaments on the western continental margin appear to have been reactivated and block movements took place the presence of a basement ridge in the mid shelf and the shelf margin basement high prathap ridge west of the slope resembles the structural style of a passive continental margin

the effects of discharge and slope on hyporheic flow in step pool morphologies this paper focuses on surface subsurface water exchange in a steep coarse bedded stream with a step pool morphology we use both flume experiments and numerical modelling to investigate the influence of stream discharge channel slope and sediment hydraulic conductivity on hyporheic exchange the model step pool reach whose topography is scaled from a natural river consists of three step pool units with 0 1 m step heights discharges ranging between base and over bankfull flows scaled values of 0 3 4 5 l s and slopes of 4 and 8 results indicate that the deepest hyporheic flow occurs with the steeper slope and at moderate discharges and that downwelling fluxes at the base of steps are highest at the largest stream discharges in contrast to findings in a pool riffle morphology those in this study show that steep slopes cause deeper surface subsurface exchanges than gentle slopes numerical simulation results show that the portion of the hyporheic zone influenced by surface water temperature increases with sediment hydraulic conductivity these experiments and numerical simulations emphasize the importance of topography sediment permeability and roughness elements along the channel surface in governing the locations and magnitude of downwelling fluxes and hyporheic exchange our results show that hyporheic zones in these steep streams are thicker than previously expected by extending the results from streams with pool riffle bed forms copyright 2014 john wiley sons ltd

geology sulfide geochemistry and supercritical venting at the beebe hydrothermal vent field cayman trough the beebe vent field bvf is the world s deepest known hydrothermal system at 4960m below sea level located on the mid cayman spreading centre caribbean the bvf hosts high temperature 401 c black smoker vents that build cu zn and au rich sulphide mounds and chimneys the bvf is highly gold rich with au values up to 93 ppm and an average au ag ratio of 0 15 gold precipitation is directly associated with diffuse flow through beehive chimneys significant mass wasting of sulphide material at the bvf accompanied by changes in metal content results in metaliferous talus and sediment deposits situated on very thin 2 3km thick oceanic crust at an ultraslow spreading centre the hydrothermal system circulates fluids to a depth of 1 8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies we suggest hydrothermal interaction with chalcophile bearing sulphides in the mantle rocks together with precipitation of au in beehive chimney structures has resulted in the formation of a au rich volcanogenic massive sulphide vms deposit with its spatial distribution of deposit materials and metal contents the bvf represents a modern day analogue for basalt hosted au rich vms systems this article is protected by copyright all rights reserved

bridging the gaps an overview of wood across time and space in diverse rivers abstract nearly 50 years of research focused on large wood lw in rivers provide a basis for understanding how wood enters rivers how wood decays breaks and is transported downstream and how at least temporarily stable wood influences channel geometry fluxes of water sediment and organic matter and the abundance and diversity of aquatic and riparian organisms field based studies have led to qualitative conceptual models and to numerical stimulations of river processes involving wood numerous important gaps remain however in our understanding of wood dynamics the majority of research on wood in rivers focuses on small to medium sized rivers defined using the ratio of wood piece size to channel width as channels narrower than the locally typical wood piece length small and slightly narrower than the longer wood pieces present medium although diverse geographic regions and biomes are represented by one or a few studies in each region the majority of research comes from perennial rivers draining temperate conifer forests regional syntheses most commonly focus on the pacific northwest region of north america where most of these studies originate consequently significant gaps in our understanding include lack of knowledge of wood related processes in large rivers dryland rivers and rivers of the high and low latitudes using a wood budget as an organizing framework this paper identifies other gaps related to wood recruitment transport storage and how beavers influence lw dynamics with respect to wood recruitment we lack information on the relative importance of mass tree mortality and transport of buried or surficial downed wood from the floodplain into the channel in diverse settings knowledge gaps related to wood transport include transport distances of lw and thresholds for lw mobility in small to medium rivers with respect to wood storage we have limited data on longitudinal trends in lw loads within unaltered large and great rivers and on fluctuations in lw load over time intervals greater than a few years other knowledge gaps relate to physical and ecological effects of wood including the magnitude of flow resistance caused by lw patterns of wood related sediment storage for diverse river sizes and channel geometry quantification of channel floodplain lw interactions and potential threshold effects of lw in relation to physical processes and biotic communities finally knowledge gaps are related to management of large wood and river corridors including understanding the consequences of enormous historical reductions in lw load in rivers through the forested portions of the temperate zone and how to effectively reintroduce and manage existing lw in river corridors which includes enhancing public understanding of the importance of lw addressing these knowledge gaps requires more case studies from diverse rivers as well as more syntheses and metadata analyses

sulfide mineralization in an ultramafic rock hosted seafloor hydrothermal system from serpentinization to the formation of cu zn co rich massive sulfides abstract the rainbow vent field is an ultramafic rock hosted seafloor hydrothermal system located on the mid atlantic ridge issuing high temperature acidic metal rich fluids hydrothermal products include cu zn co rich massive sulfides with characteristics comparable to those found in mafic volcanic hosted massive sulfide deposits petrography mineralogy and geochemistry of nonmineralized and mineralized rocks sampled in the rainbow vent field indicate that serpentinized peridotites host the hydrothermal vent system but serpentinization reactions occurred prior to and independently of the sulfide mineralization event the onset of sulfide mineralization is reflected by extensive textural and chemical transformations in the serpentine group minerals that show clear signs of hydrothermal corrosion element remobilization is a recurrent process in the rainbow vent field rocks and during simple peridotite serpentinization ni and cr present in olivine and pyroxene are incorporated in the pseudomorphic serpentine mesh and bastite respectively ni is later remobilized from pseudomorphic serpentine into the newly formed sulfides as a result of extensive hydrothermal alteration bulk rock geochemistry and correlation coefficients discriminate the different processes serpentinization sulfide mineralization and superficial seafloor low temperature processes related to the circulation of seawater e g carbonatization sulfide oxidation and b and u uptake

effects of coarse woody debris and its removal on a channel affected by the 1980 eruption of mount st helens washington during the may 18 1980 eruption of mount st helens washington a pyroclastic surge introduced large volumes of coarse woody debris cwd and fine grained sediment to clearwater creek approximately 15 km northeast of the summit effects of controlled cwd removal on sediment storage substrate and pool frequency and volume were measured in four reaches each with three 200 m segments from 1982 to 1990 in each reach the upstream segment served as a control with no debris removal and cwd was totally and selectively removed from the other two segments unique among similar experiments are the large size and volume of cwd and the large inputs of fine grained sediment except for segments of two reaches that received debris torrents the clearwater channel thalweg scoured until 1985 in three reaches total debris removal caused additional scour and coarsening of the bed surface compared to segments with no or partial debris removal pools contracted from 1982 to 1985 and expanded afterward especially in control segments total debris removal apparently caused pools to become shallower and in segments of low sinuosity decreased the frequency of major pools habitat complexity decreased after total debris removal as indicated by a decrease in the standard deviation of residual depth and an increase in the size of substrate patches

mixing of surface and groundwater induced by riverbed dunes implications for hyporheic zone definitions and pollutant reactions 1 the hyporheic zone is often defined as where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers this mixing is credited with creating unique biogeochemical conditions that can attenuate contaminants from either upstream surface water or groundwater under gaining conditions however reactions of contaminants upwelling from groundwater may be more dependent on such mixing than contaminants from surface water we numerically modeled mixing between hyporheic flow paths induced by riverbed dunes and flow paths of adjacent upwelling of deeper groundwater results show that only 12 7 or less tracer mass upwelling from deeper groundwater dispersed across into hyporheic flow paths originating in surface water the spatial extent of a mixing defined hyporheic zone was smaller than a hyporheic zone defined as hydrologic flow paths leaving and returning to surface water mixing dependent reactions will therefore be localized within a thin mixing zone yet vary considerably with site conditions for example mixing in homogeneous sediments was controlled most by variation in hydraulic conductivity and upwelling flow rate which primarily affected mixing zone length by contrast introduction of heterogeneity increased mixing primarily by increasing mixing zone thickness consistent with studies of flow focusing in aquifers finally dispersivity is a critical parameter for which data are needed for shallow sediments our results help clarify hyporheic zone definitions and potential for mixing dependent reactions in particular the biogeochemically reactive portion of riverbed sediments from the perspective of upwelling contaminants does not necessarily spatially coincide with more traditional hydrologic conceptions of the hyporheic zone

sustained volcanically hosted venting at ultraslow ridges piccard hydrothermal field mid cayman rise at slow spreading mid ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically controlled hydrothermal circulation unrelated to young volcanic activity here by contrast we show that the piccard hydrothermal field phf on the ultraslow spreading mid cayman rise represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting the lateral extent and apparent longevity associated with the phf are comparable to some of the largest tectonically hosted vent sites known along the slow spreading mid atlantic ridge if such systems recur along all ultraslow ridges which comprise 20 of the 55 000 km global ridge crest potential implications would include i a higher probability of locating large economically valuable mineral deposits along ultraslow ridges together with ii larger fluxes than previously anticipated of chemicals released from high temperature venting entering the oceans along the atlantic indian ocean sectors of the deep ocean thermohaline conveyor

thermal regime of dune covered sediments under gaining and losing water bodies 1 we investigate the effects of current bed form induced flow and heat transport through permeable bottom sediments overlain by a marine or terrestrial water column that is gaining or losing deep groundwater heat transport is forced by the diel variation of temperature in the water column the investigation utilizes sequentially coupled simulations of turbulent flow in the water column and darcy flow and heat transport in the sediments the simulations address the question when where and by how much are diel water column temperature variations transmitted into sediments subjected to ambient groundwater discharge this is crucial information for detecting observing and predicting temperature sensitive biogeochemical and ecological processes in the bottom sediments when the groundwater gain or loss is small it has no appreciable effect on temperatures in the sediments which are controlled by heat conduction and current bed form induced heat advection as losing discharge increases the temperature signal from the water column penetrates deeper into the sediments with the largest temperature variations found under a downwelling zone along the stoss side of the bed form and damped temperature variations found near a narrow upwelling zone below the crest similar patterns are observed under gaining conditions but with temperature variations penetrating to shallower depths the interfacial exchange zone is diminished by upward movement of deep groundwater large gains or losses of deep groundwater prevent the formation of an interfacial exchange zone making heat transport almost vertically one dimensional the sensitivity of the sediment thermal regime to hydrodynamic conditions increases with increasing water column current reynolds number and with sediment permeability

magnetic studies of basement off the coast of bombay west of india abstract bathymetric and total magnetic intensity data along 45 closely spaced profiles in 20 100 m of water totalling 2400 line km were collected on the continental shelf off bombay west of india magnetic intensity anomaly maps were prepared the anomalies show prominent trends in e w nnw sse and ene wsw directions correlations of the anomaly signatures with the geology known from the drill well logs and seismic results infer 1 1 the mostly basaltic nature of the basement overlain by the tertiary sediments except in the murud depression and isolated patches of the bombay high and inner shelf where granites are interpreted 2 2 the lateral extent of the two nnw sse trending regional faults between longitudes 70 50 e and 71 40 e 3 3 the outlines of the magnetic signatures associated with the structural highs and depressions two dimensional model and spectral studies of the anomalies under numerical constraints have revealed the structure of the basement overlain by 1 5 5 0 km thick sediments a crustal section along a transect across the inner shelf the bassein high structure and the offshore basin in the middle shelf the bombay high structure in the middle outer shelf and further west is interpreted the section depicts the structure of the tertiary sediments the late cretaceous early tertiary basalts and the archaean precambrian granites thick basalt constructs occur in the paleoslopes of the archaean precambrian basement down faulting of parts of the western indian peninsula into the arabian sea along the regional faults is suggested due to the lithologically similar sequences of the tertiary sediments the basalts deccan traps and granitic gneisses of the study and onshore areas therefore the area is considered to be divided into three structural blocks consisting of the inner shelf the offshore basin and the bombay high separated by nnw sse trending deep seated faults which result from rifting of the crust the basement faults mostly extending to the overlying young sediments and the tectogenes indicate a continued stress regime the continued subsidence both mechanical and thermal of the crust in the study area under increased sediments the drift of the indian continent from the south to the present latitudes and the dynamic interaction of the oceanic continental lithospheres appear to deform the crust by thinning and shearing

sulfur in peridotites and gabbros at lost city 30 n mar implications for hydrothermal alteration and microbial activity during serpentinization variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the lost city hydrothermal field at the southern face of the atlantis massif mid atlantic ridge 30n were examined to better understand serpentinization and alteration processes and to study fluid fluxes redox conditions and the influence of microbial activity in this active peridotite hosted hydrothermal system the serpentinized peridotites are characterized by low total sulfur contents and high bulk d 34 s values close to seawater composition low concentrations of 34 s enriched sulfide phases and the predominance of sulfate with seawater like d 34 s values indicate oxidation loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites the predominance of pyrite in both serpentinites and gabbros indicates relatively high fo2 conditions during progressive serpentinization and alteration which likely result from high fluid fluxes during hydrothermal circulation and evolution of the lost city system from temperatures of 250 to 150 c sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at lost city show d 34 s values that reflect the influence of microbial activity our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long lived moderate temperature peridotite hosted hydrothermal system and high seawater fluxes on the global sulfur cycle

hydraulic and thermal effects of in stream structure induced hyporheic exchange across a range of hydraulic conductivities in stream structure induced hyporheic exchange and associated thermal dynamics affect stream ecosystems their importance is controlled by spatial variability of sediment hydraulic conductivity k we calibrated a computational fluid dynamics cfd model of surface and groundwater hydraulics near a channel spanning weir represents log dams boulder weirs to field data and varied k from 10 7 to 10 2 m s silt to gravel surface water stopped cresting the weir for k 10 3 m s non darcy hyporheic flow was also prevalent for k 10 3 m s and velocity errors using non cfd models ranged up to 32 2 we also modeled weir induced heat transport during summer as k increased from 10 7 to 10 3 m s weir induced hyporheic heat advection steadily increased cooling and buffering along hyporheic flow paths decreased with increasing k particularly above k 10 5 and 10 4 m s respectively vertical heat conduction between surface water and groundwater near the weir decreased with increasing k particularly for k 10 5 m s conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow downstream surface water cooling by hyporheic advection increased steadily with k as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths yet such effects were small 0 016 c even at k 10 3 m s the largest thermal effect of weir induced exchange was therefore spatial expansion of subsurface diel variability particularly for k 10 5 m s which affects benthic habitat and chemical reactions the specific values of k where such trend shifts occur is likely variable among streams based on flow conditions but we expect the presence of such trend shifts to be widespread

subsurface processes at the lucky strike hydrothermal field mid atlantic ridge evidence from sulfur selenium and iron isotopes at lucky strike near the azores triple junction the seafloor setting of the hydrothermal field in a caldera system with abundant low permeability layers of cemented breccia provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution coupled analyses of s isotopes performed in conjunction with se and fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems these data provide a tool for resolving the different abiotic and potential biotic near surface hydrothermal reactions the 34s between 1 5 and 4 6 and se values between 213 and 1640 ppm of chalcopyrite suggest a high temperature end member hydrothermal fluid with a dual source of sulfur sulfur that was leached from basaltic rocks and sulfur derived from the reduction of seawater sulfate in contrast pyrite and marcasite generally have lower 34s within the range of magmatic values 0 1 and are characterized by low concentrations of se 50 ppm for 82se 76se ratios the 82se values range from basaltic values of near 1 5 to 7 the large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between se leached from igneous rock and se derived from seawater we interpret the se isotope signature to be a result of leaching and mixing of a fractionated se source located beneath hydrothermal chimneys in the hydrothermal fluid at lucky strike we consider two sources for s and se 1 the end member hydrothermal fluid with basaltic se isotopic values 1 5 and typical s isotope hydrothermal values of 1 5 2 a fractionated source hosted in subsurface environment with negative 34s values probably from bacterial reduction of seawater sulfate and negative 82se values possibly derived from inorganic reduction of se oxyanions fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by s isotopes fe isotope systematic reveals that se rich high temperature samples have 57fe values close to basaltic values 0 whereas se depleted samples precipitated at medium to low temperature are systematically lighter 57fe values between 1 to 3 an important implication of our finding is that light fe isotope composition down to 3 2 may be explained entirely by abiotic fractionation in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions

hydrothermal activity along the southwest indian ridge twenty years after the discovery of sea floor hot springs vast stretches of the global mid ocean ridge system remain unexplored for hydrothermal venting the southwest indian ridge is a particularly intriguing region as it is both the slowest spreading of the main ridges1 and the sole modern migration pathway between the diverse vent fauna of the atlantic and pacific oceans2 a recent model postulates that a linear relation exists between vent frequency and spreading rate3 and predicts vent fields to be scarcest along the slowest spreading ridge sections thus impeding migration and enhancing faunal diversity2 here however we report evidence of hydrothermal plumes at six locations within two 200 km long sections of the southwest indian ridge indicating a higher frequency of venting than expected these results suggest that fluxes of heat and chemicals from slow spreading ridges may be greater than previously thought and that faunal migration along the southwest indian ridge may serve as an important corridor for gene flow between pacific and atlantic hydrothermal fields

evidence from gravity anomalies for interactions of the marion and bouvet hotspots with the southwest indian ridge effects of transform offsets abstract the ultra slow spreading southwest indian ridge swir presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry using recently available satellite free air gravity and shipboard bathymetry data we obtain mantle bouguer mba and residual mantle bouguer anomalies rmba by removing from free air gravity the attractions of seafloor topography sediment thickness variations a reference crust and theoretically predicted effects of lithospheric cooling the bouvet hotspot previously observed to be associated with anomalous bathymetry and geochemistry near the bouvet triple junction has an mba axial gravity low of 100 mgal implying pronounced localized crustal thickening off axis the rmba lows along previously calculated bouvet hotspot tracks are variable in amplitude suggesting the possibilities that bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center along axis geophysical anomalies suggest that the marion hotspot has a significant effect on accretionary processes in the central portion of the swir importantly the marion axial anomaly appears to be compartmentalized between the andrew bain and discovery ii fracture zones implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented ultra slow spreading system

extension of structural and tectonic trends from the indian subcontinent into the eastern arabian sea abstract trends of basement highs faults and structural patterns on the india pakistan indus continental shelf and slope and in the deep waters extending to the lakshmi laxmi chagos laccadive ridge complex in the eastern arabian sea are parallel to the ancient precambrian structural and tectonic grain of the indian subcontinent these trends the occurrence of horst graben style features of the basement of the lakshmi ridge and of the region north and east of it both in deep waters and shelf areas and the oceanic nature of the basement west of the ridge suggest that the rifted transitional crust extends as far as the lakshmi chagos laccadive ridge complex the extension of rifted crust with horst graben style tectonic features into deep offshore areas of the arabian sea has implications for future hydrocarbon exploration in the region

contrasting evolution of hydrothermal fluids in the pacmanus system manus basin the sr and s isotope evidence ocean drilling program odp leg 193 investigated two sites of hydrothermal activity along the crest of the pual ridge in the eastern manus basin a site of low temperature diffuse venting snowcap site 1188 and a high temperature black smoker site roman ruins site 1189 were drilled to depths of 386 and 206 m below seafloor mbsf respectively although the two sites are 1000 m apart the 87sr 86sr and 34s signatures of anhydrite recovered at both sites are very different the data suggest a complex interplay among hydrothermal fluid magmatic fluid and seawater during alteration and mineralization of the pacmanus papua new guinea australia canada manus system these new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems

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

heat transport in hyporheic zones due to bedforms an experimental study heat transport in hyporheic zones is critical for many temperature sensitive physical chemical and biological processes it also plays a role in the energy balance of rivers to complement previous theoretical studies we investigated the effects of hyporheic flow due to bed topography on hyporheic thermal dynamics through flume experiments diel temperature cycles were imposed in the channel and propagation of temperature signals into the sediment was examined for three different bed morphologies flat bed pool riffle pool and rippled bed three channel flow rates and two sediment grain sizes results show that the downwelling upstream and downstream of the bedforms and upwelling focused near the crest caused substantial advective heat transport and distinct thermal patterns in the sediment with downwelling zones reflecting most of the surface water diel thermal range whereas upwelling zones were more buffered increasing permeability and channel flow rates increased advective transport the results corroborate previous theoretical models further showing that hyporheic flow distinctly affects thermal patchiness in sediment the results also have implications for heat tracing using analytical models which work well for flow fields without pronounced divergence and convergence and minimal dispersion features atypical of hyporheic flow

buffered lagged or cooled disentangling hyporheic influences on temperature cycles in stream channels 1 we monitored summertime base flow water temperatures of hyporheic discharge to surface water in main side and spring channels located within the bank full scour zone of the gravel and cobble bedded umatilla river oregon usa diel temperature cycles in hyporheic discharge were common but spatially variable relative to the main channel s diel cycle hyporheic discharge locations typically had similar daily mean temperatures but smaller diel ranges compressed by 2 to 6 c and desynchronized phases offset by 0 to 6 h in spring channels which received only hyporheic discharge surface water diel cycles were also compressed by 2 to 6 c and desynchronized by 4 to 6 h relative to the main channel creating diverse daytime and nighttime mosaics of surface water temperatures across main side and spring channels despite only minor differences 1 c in daily mean temperatures among the channels the river s hyporheic zone received and stored heat from the channel yet hyporheic return flows carried heat back to the channel minutes to months after removal associated surface water temperature dynamics were therefore complex hyporheic discharge was not simply cooler or warmer than main channel water instead instantaneous temperature differences between channel water and hyporheic discharge typically arose from diel temperature cycles in hyporheic discharge that were buffered and lagged relative to diel cycles in the main channel

how can subsurface modifications to hydraulic conductivity be designed as stream restoration structures analysis of vaux s conceptual models to enhance hyporheic exchange 1 despite the growing interest in hyporheic exchange and the associated stream ecosystem processes few studies consider restoration of hyporheic exchange as a design goal here we study the design of three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago vaux s designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream in this preliminary analysis of subsurface structure design we use two dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities structure performance is evaluated on the basis of total streambed flux physical extent of hyporheic flow paths created and residence time distributions along flow paths modified by the structures high hydraulic conductivity structures bend flow paths toward and through the structures themselves performance is controlled by the structure hydraulic conductivity results show low hydraulic conductivity structure performance is insensitive to the structure material hyporheic exchange is created by deflecting flow paths away from the structure itself time scales of simulated exchange are great enough to promote nitrification denitrification respiration and thermal buffering in the subsurface though these processes will also be controlled by site specific chemical and biological factors general design recommendations for specific restoration objectives are presented results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities e g clay and gravel deposits and bedrock knickpoints and flow fields in creating hyporheic flow paths

crustal structure from deep seismic soundings along the koyna ii kelsi loni profile in the deccan trap area india abstract the crustal depth section obtained from deep seismic soundings along the koyna ii kelsi loni profile which lies near latitude 18 n roughly in the east west direction in that part of the deccan trap maharashtra state india shows a number of reflection segments below the deccan traps down to the moho discontinuity a deep fault below the deccan traps 13 km east of mahad divides the entire cross section including the moho boundary into two crustal blocks the reflection segments show updip towards the west coast in the western block the moho discontinuity which is at a depth of 39 km near the deep fault starts rising towards the coast reaching a depth of 31 5 km at the west coast the eastern block is thrown up by 1 5 km with respect to the western block along the deep fault a structural contour map of the moho discontinuity for the koyna reservoir area has been prepared from the present results and the crustal information obtained along the koyna i profile kaila et al 1979a shows that the deep fault in the koyna area is aligned in the nnw sse direction refraction seismic data analysis by the wave front method reveals that the thickness of the deccan trap increases towards the west coast the deccan trap is 600 700 m thick in the eastern region between nira sp 130 and loni sp 200 and attains a thickness of 1500 m at 10 km east of the west coast the longitudinal wave velocity in the deccan traps along the profile varies from 4 8 to 5 0 km sec and in the crystalline basement from 6 0 to 6 15 km sec a tentative isopach contour map of the deccan traps and a tentative structural contour map of the pre deccan trap contact have been prepared for the koyna reservoir area from the results along the koyna ii and koyna i profiles a flexure aligned in a nnw sse direction in the pre deccan trap contact which is an expression of the deep fault into the basement has been clearly brought out the flexure coincides in general with the orientation of the deccan volcanic scarp in this area

effects of stream morphodynamics on hyporheic zone thermal regime 1 we develop a heat transport model to study the impact of streambed morphology on temperature distribution within the hyporheic zone of gravel bed rivers the heat transport model which includes conduction diffusion and advection is solved by a lagrangian approach neglecting transverse dispersion and considering stream water temperature as boundary condition at the streambed first we show that the model accurately reproduces the temperature distribution measured within the hyporheic zone of a reach of the bear valley creek idaho usa our model reveals spatially complex patterns of hyporheic water temperatures that vary with time within the hyporheic zone and at the streambed surface the analysis shows that temperature distributions are primarily related to the hyporheic residence time and consequently to the bed morphology and in stream flow discharge results show that the hyporheic temperature amplitudes are smaller than the surface water temperature and they decrease with stream size leading hyporheic zones of large streams to be independent from in stream daily temperature fluctuations

seismic observation of an extremely magmatic accretion at the ultraslow spreading southwest indian ridge the oceanic crust is formed by a combination of magmatic and tectonic processes at mid ocean spreading centers under ultraslow spreading environment however observations of thin crust and mantle derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes with little or absence of magmatism using three dimensional seismic tomography at an ultraslow spreading southwest indian ridge segment containing a central volcano at 50 28 e here we report the presence of an extremely magmatic accretion of the oceanic crust our results reveal a low velocity anomaly 0 6 km s in the lower crust beneath the central volcano suggesting the presence of partial melt which is accompanied by an unusually thick crust 9 5 km we also observe a strong along axis variation in crustal thickness from 9 5 to 4 km within 30 50 km distance requiring a highly focused melt delivery from the mantle we conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano which was enhanced by the significant along axis variation in lithosphere thickness at the ultraslow spreading southwest indian ridge

sulfur isotope signatures in gypsiferous sediments of the estancia and tularosa basins as indicators of sulfate sources hydrological processes and microbial activity abstract in order to reconstruct paleo environmental conditions for the saline playa lakes of the rio grande rift we investigated sediment sulfate sources using sulfur isotope compositions of dissolved so 4 2 ions in modern surface water groundwater and so 4 2 precipitated in the form of gypsum sediments deposited during the pleistocene and holocene in the tularosa and estancia basins the major sulfate sources are lower and middle permian marine evaporites 34s of 10 9 14 4 but the diverse physiography of the tularosa basin led to a complex drainage system which contributed sulfates from various sources depending on the climate at the time of sedimentation as inferred from sulfur isotope mass balance constraints weathering of sulfides of magmatic hydrothermal and sedimentary origin associated with climate oscillations during last glacial maximum contributed about 35 50 of the sulfates and led to deposition of gypsum with 34s values of 1 2 to 2 2 which are substantially lower than permian evaporates in the estancia basin microbial sulfate reduction appears to overprint sulfur isotopic signatures that might elucidate past groundwater flows a rayleigh distillation model indicates that about 3 18 of sulfates from an inorganic groundwater pool 34s of 12 6 13 8 have been metabolized by bacteria and preserved as partially to fully reduced sulfur bearing minerals species elemental sulfur monosulfides disulfides with distinctly negative 34s values 42 3 to 20 3 compared to co existing gypsum 3 8 to 22 4 for the tularosa basin microbial sulfate reduction had negligible effect on 34s value of the gypsiferous sediments most likely because of higher annual temperatures 15 33 c and lower organic carbon content median 0 09 in those sediments leading to more efficient oxidation of h2s and or smaller rates of sulfate reduction compared to the saline playas of the estancia basin 5 28 c median 0 46 of organic carbon the white sands region of the tularosa basin is frequently posited as a hydrothermal analogue for mars high temperatures of groundwater 33 3 c and high 18o h2o values 1 1 in white sands however are controlled predominantly by seasonal evaporation rather than the modern influx of hydrothermal fluids nevertheless it is possible that some of the geochemical processes in white sands such as sulfide weathering during climate oscillations and upwelling of highly mineralized waters might be considered as valid terrestrial analogues for the sulfate cycle in places such as meridiani planum on mars

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

cu ni co au bearing massive sulfide deposits associated with mafic ultramafic rocks of the main urals fault south urals geological structures ore textural and mineralogical features comparison with modern analogs abstract cu rich massive sulfide deposits associated with mafic ultramafic rocks in the southern portion of the main urals fault muf are characterized by variable enrichments in ni up to 0 45 wt co up to 10 wt and au up to 16 ppm in individual hand specimens the cu ni co rich composition of muf deposits as opposed to the cu zn rich composition of more eastward massive sulfide deposits of broadly similar age along the western flank of the magnitogorsk arc reflects the abundance of seafloor exposed ni co rich ultramafic rocks in the most external portion of the early devonian magnitogorsk forearc morphological textural and compositional differences between individual deposits are interpreted to be the result of the sulfide deposition style and in part of the original subseafloor lithology one deposit produced by dominantly on seafloor hydrothermal processes is characterized by pyrite marcasite pyrrhotite not so low zn grades occasionally up to 2 wt abundant clastic facies and periodical superficial oxidation deposits produced by dominantly subseafloor hydrothermal processes are characterized by pyrrhotite pyrite very low zn generally

spatial periodicity in bed form scale solute and thermal transport models of the hyporheic zone spatially periodic solute boundaries force symmetry across a model domain by ensuring that concentrations and concentration gradients are identical at the same location on opposite boundaries they have been used in multiple publications on a hyporheic zone model of a single ripple or dune style bed form including variable density flow and reactive transport variants we evaluate simulations of multibed form models without imposing spatially periodic transport to demonstrate that nonphysical solute distributions arise from the periodic solute transport assumption that is the flow field within the single bed form model leads to a transport scenario that violates the forced symmetry of periodic solute boundary conditions culminating in a physically unrealistic solute distribution our results show that lack of symmetry between boundaries is a function of the vertical concentration gradient and two dimensionless parameters characterizing the hyporheic and underflow flow regimes and the solute exchange between them we assess the error associated with the spatially periodic assumption based on an analysis of solute fluxes across the lateral bed form model boundaries while the focus is on steady state concentration distributions the implications for transient solute transport models are also discussed we conclude that periodic solute transport boundary conditions should be applied only to bed form models that have minimal vertical dispersive and diffusive solute transfer this includes gaining systems and tracers such as temperature for which a temporally periodic flux reversal occurs across the top boundary

effect of instream wood on vertical water flux in low energy sand bed flume experiments 1 in streams interaction of current with bed forms and solid objects can produce vertical water flux across the streambed instream wood is a major obstacle to flow and alters the bed topography of natural sand bed streams in the present experimental study in large circulating flumes simulating sand bed streams with low constant slope we compared systems with and without natural quantities of wood the introduction of wood resulted in the production of irregular bed forms as a result flow resistance tripled and vertical water flux across the streambed increased by a factor of 1 8 to 2 5 the mixing depth was spatially variable and seemed unrelated to the local bed form after the addition of wood surface water mixed deeper into the sediments and the total sediment pore water volume involved in mixing increased by factors of 1 2 to 1 5 the practice of increasing instream wood should be a valuable tool for hyporheic rehabilitation of degraded sand bed streams

tectonic and magmatic controls on hydrothermal activity in the woodlark basin the woodlark basin is one of the rare places on earth where the transition from continental breakup to seafloor spreading can be observed the potential juxtaposition of continental rocks a large magmatic heat source crustal scale faulting and hydrothermal circulation has made the woodlark basin a prime target for seafloor mineral exploration however over the past 20 years only two locations of active hydrothermalism had been found in 2009 we surveyed 435 km of the spreading axis for the presence of hydrothermal plumes only one additional plume was found bringing the total number of plumes known over 520 km of ridge axis to only 3 much less than at ridges with similar spreading rates globally particularly the western half of the basin 280 km of axis is apparently devoid of high temperature plumes despite having thick crust and a presumably high magmatic budget this paucity of hydrothermal activity may be related to the peculiar tectonic setting at woodlark where repeated ridge jumps and a re location of the rotation pole both lead to axial magmatism being more widely distributed than at many other more mature and stable mid ocean ridges these factors could inhibit the development of both a stable magmatic heat source and the deeply penetrating faults needed to create long lived hydrothermal systems we conclude that large seafloor massive sulfide deposits potential targets for seafloor mineral exploration will probably not be present along the spreading axis of the woodlark basin especially in its younger western portion

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

two episodes of sulfide mineralization at the yuhuang 1 hydrothermal field on the southwest indian ridge insight from zn isotopes abstract the newly discovered yuhuang 1 hydrothermal field yhf on the southwest indian ridge primarily contains two different sulfide distribution areas the southwest sulfide sws and the northeast sulfide nes distribution areas in this study zn isotope compositions and element ratios of zn fe cu and cd in the surface sulfides were analyzed to characterize zn isotopic fractionation ore forming conditions and mineralization processes of the yhf the results indicate that sulfide rich samples in the yhf exhibit intensive zn isotopic fractionation with 66zn values ranging from 0 43 to 1 24 these values are not only larger than those currently reported for seafloor sulfides but are also larger than the values reported for the deposits on land remarkably the obtained 66zn value 0 43 is the lowest one observed in seafloor sulfides 0 26 1 17 currently a study of zn isotope systematics revealed that zn isotope variations of sulfide rich samples in the yhf were controlled by the sphalerite and pyrite content the study also revealed that early minerals are rich in light zn isotopes but later minerals are rich in heavy zn isotopes a comparative study of previous data on zn isotopes shows that zn isotope fractionations in the yhf are likely attributed to rayleigh fractionation caused by mineral precipitation and multiple generations of pyrite further zn cd ratios of sulfide rich samples indicate that the ore forming temperature of the silicified sulfide rich samples in the sws probably formed under lower temperatures as compared with the silicified sulfide rich chimney in the nes furthermore sulfur and zn isotope compositions in the sws are significantly different from those in sulfide rich samples of the nes which suggests pervasively different metallogenic conditions further indicating that they are probably products of separate mineralization episodes

marine magnetic anomalies off ratnagiri western continental shelf of india abstract total magnetic intensity and bathymetric surveys were carried out in the offshore area of ratnagiri on the western continental shelf of india and an isomagnetic anomalies map at a contour interval of 50 nt was prepared which reveals n s trends of magnetic anomalies two dimensional model and spectral studies of these anomalies were carried out and subsurface models of the geology in the area have been derived from anomalies at a number of places the results suggest that the anomalies occur over a magnetic crystalline basement at a depth of 1 1 2 km which is similar in magnetization to onshore basalts of northwest india these anomalies are believed to be an expression of a considerable thickness around 1 7 km of basalt underlain by sediments identification of these basalts in offshore areas along the northwestern continental shelf of india would support 1 the idea that the onshore deccan basalts of western india and the rhyolitic tuffs at the laccadive ridge system dsdp site 210 are related to the same volcanic events and 2 subsequent downfaulting of onshore deccan basalts into the arabian sea and submergence below the tertiary sediments

effects of current bed form induced fluid flow on the thermal regime of sediments 1 heat transport associated with current bed form fluid exchange affects biogeochemical and ecological process in a variety of aquatic settings we simulate turbulent flow over dunes induced darcy flow in sediments and sediment heat transport forced by a diel variation of water column temperature heat conduction dominates when the water column current cannot induce rapid fluid exchange heat advection becomes important with increasing sediment permeability and or current velocity resulting in a complicated space time temperature pattern weak temperature variations are found beneath dune crests where water upwells out of the sediments strong variations are located below dune stoss faces where water downwells the relative dominance of heat conduction versus advection is captured by a global peclet number that depends on dune geometry and hydrodynamic and thermal properties including the period of thermal forcing thermally induced fluid viscosity variations cause periodic but nonsymmetric changes in fluid flux through the sediments but do not significantly influence temperature patterns

fluid composition of the sediment influenced loki s castle vent field at the ultra slow spreading arctic mid ocean ridge the hydrothermal vent field loki s castle is located in the mohns knipovich bend 73 n of the ultraslow spreading arctic mid ocean ridge amor close to the bear island sediment fan the hydrothermal field is venting up to 320 c hot black smoker fluids near the summit of an axial volcanic ridge even though the active chimneys have grown on a basaltic ridge geochemical fluid data show a strong sedimentary influence into the hydrothermal circulation at loki s castle compelling evidence for a sediment input is given by high alkalinity high concentrations of nh4 h2 ch4 c2 hydrocarbons as well as low mn and fe contents the low 13c values of co2 and ch4 and the thermogenic isotopic pattern of the c2 hydrocarbons in the high temperature vent fluids clearly point to thermal degradation of sedimentary organic matter and illustrate diminution of the natural carbon sequestration in sediments by hydrothermal circulation thus carbon release to the hydrosphere in arctic regions is especially relevant in areas where the active arctic mid ocean ridge system is in contact with the organic matter rich detrital sediment fans

first active hydrothermal vents on an ultraslow spreading center southwest indian ridge the ultraslow spreading southwest indian ridge is a major tectonic province representing one of the important end member mid ocean ridge types for its very slow and oblique spreading and providing the only known route for migration of chemosynthetic deep sea vent fauna between the atlantic and indian oceans we report the investigation of the first active high temperature hydrothermal field found on any ultraslow mid ocean ridge worldwide located on southwest indian ridge at 37 47 s 49 39 e it consists of three zones extending 1000 m laterally and it is one of four recently discovered active and inactive vent sites within a 250 km long magmatically robust section our results provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow spreading ridges at least along magmatically robust segments this implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities it is surprising that the special vent fauna appear to indicate some complex affinity to those on the central indian ridge southern mid atlantic ridge and the southwest pacific ocean

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

hyporheic zone hydrologic science a historical account of its emergence and a prospectus the hyporheic zone defined by shallow subsurface pathways through river beds and banks beginning and ending at the river is an integral and unique component of fluvial systems it hosts myriad hydrologically controlled processes that are potentially coupled in complex ways understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale thus the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of water resources research this manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in water resources research from the birth of the concept of the hyporheic zone as a hydrologic black box sometimes referred to as transient storage zone to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view to its mature emergence as an interdisciplinary research field that employs the wide array of state of the art tools available to the modern hydrologist the field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth there are exciting opportunities for scientists that are able to tightly link the allied fields of geology geomorphology hydrology geochemistry and ecology to tackle the many open problems in hyporheic zone science

high resolution surveys along the hot spot affected galapagos spreading center 1 distribution of hydrothermal activity the spatial density of hydrothermal activity along most mid ocean ridges is a robust linear function of spreading rate or magmatic budget but extreme crustal properties may alter this relationship in 2005 2006 we tested the effect of thickened crust on hydrothermal activity using high resolution mapping of plumes overlying the hot spot affected galapagos spreading center from 95 to 89 42 w 560 km of ridge crest plume mapping discovered only two active high temperature vent fields subsequently confirmed by camera tows though strong plume evidence indicated minor venting from at least six other locations total plume incidence ph the fraction of ridge crest overlain by significant plumes was 0 11 0 014 about half that expected for a non hot spot mid ocean ridge with a similar magmatic budget plume distributions on the galapagos spreading center were uncorrelated with abrupt variations in the depth of the along axis melt lens so these variations are apparently not controlled by hydrothermal cooling differences we also found no statistical difference for a significance level of 0 05 in plume incidence between where the seismically imaged melt lens is shallow 2 0 56 km ph 0 108 0 045 and where it is deep 3 4 0 7 km ph 0 121 0 015 the galapagos spreading center thus joins mid ocean ridges near the iceland reykjanes ridge st paul amsterdam south east indian ridge and ascension mid atlantic ridge hot spots as locations of anomalously scarce high temperature venting this scarcity implies that convective cooling along hot spot affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid ocean ridges

hydrothermal plumes over the carlsberg ridge indian ocean 1 indian ocean ridges north of the rodriguez triple junction remain poorly explored for seafloor hydrothermal activity with only two active sites confirmed north of 25 s we conducted water column surveys and sampling in 2007 and 2009 to search for hydrothermal plumes over a segment of the carlsberg ridge here we report evidence for two separate vent fields one near 3 42 n 63 40 e and another near 3 41 5 n 63 50 e on a segment that is apparently sparsely magmatic both sites appear to be located on off axis highs at the top of the southern axial valley wall at depths of 3600 m or shallower 1000 m above the valley floor at the 63 40 e site plume sampling found local maxima in light scattering temperature anomaly oxidation reduction potential orp dissolved mn and3he no water samples are available from the 63 50 e site but it showed robust light scattering and orp anomalies at multiple depths implying multiple sources orp anomalies are very short lived so the strong signals at both sites suggest that fluid sources lie within a few kilometers or less from the plume sampling locations although ultramafic rocks have been recovered near these sites the light scattering and dissolved mn anomalies imply that the plumes do not arise from a system driven solely by exothermic serpentinization e g lost city instead the source fluids may be a product of both ultramafic and basaltic gabbroic fluid rock interaction similar to the rainbow and logatchev fields on the mid atlantic ridge

hydrothermal cooling along the eastern lau spreading center no evidence for discharge beyond the neovolcanic zone heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades recent observations and models are now challenging this assumption of exclusively axis centric high temperature venting one example is the proposal that intense cooling along the vertical boundaries of a broad low velocity volume lvv of hot crust could generate high temperature fluids several kilometers off axis to test the hypothesis that substantial hydrothermal discharge might occur beyond the lvv we conducted a dense survey grid of the ridge and surrounding seafloor up to 5 km along 175 km of the eastern lau spreading center and valu fa ridge 1800 km of track line our sampling array extended from 50 to 400 m above bottom and included light scattering oxidation reduction potential and hydrographic sensors attached to the tow line and beneath the imi120 sonar mapping system the surveys successfully mapped plumes from several vent fields in the neovolcanic zone 1 5 km about the axis but did not detect evidence of significant discharge anywhere farther off axis at a few locations on the valu fa ridge however we did record oxidation reduction potential anomalies with hydrographic density signatures that imply low temperature hydrothermal sources on the axial flank although these sites are hundreds of meters deeper than the adjacent crest they are above not beyond the previously mapped lvv our results thus do not support a simple picture of high temperature fluids ascending undiluted through the crust to the seafloor several kilometers off axis however we cannot exclude the possibilities that the largely unmapped lvv is narrower here than seen on other ridges that hydrothermal fluids formed beyond the lvv are channeled to the axis or that discharge beyond the neovolcanic zone occurs only as dispersed very low temperature fluids our observations do demonstrate that high temperature discharge predominantly exits the seafloor within a narrow 1 5 km axial ribbon regardless of the presence or absence of an axial magma chamber

hyporheic flow and dissolved oxygen distribution in fish nests the effects of open channel velocity permeability patterns and groundwater upwelling many fish lay their eggs in nests or redds which they construct in sediment the viability of eggs depends on many factors particularly their oxygenation because dissolved oxygen is typically saturated within the stream channel the dissolved oxygen distribution within the redd depends on whether or not hyporheic flow and transport occur within the sediment we conducted a series of flume and numerical flow and age transport modeling experiments with the aim of understanding the effects of salmonid redds on the hyporheic transport of young oxygenated water hyporheic flow was visualized directly through dye injections dissolved oxygen throughout the fish nest was measured using a planar optode experiments were conducted at various open channel flow velocities in order to understand their effect on dissolved oxygen and computational simulations considered various sediment textures and ambient groundwater upwelling rates to add process level insight we found that as also shown by previous studies the redd topography induces multiscale hyporheic flow that effectively flushes the egg pocket location with younger presumably oxygenated water older water upwells and forms anoxic zones this pattern persists even at the lowest channel flow rates and at small upwelling velocities of older ambient groundwater which splits the multiscale hyporheic flow cells into isolated pockets large groundwater upwelling rates can shut down all the hyporheic flushing the relatively coarse texture of the redd further promotes hyporheic flushing of the redd sediment with oxygenated water thus redd morphology and sediment texture optimally combine to induce hyporheic exchange flow that delivers young oxygenated water to the egg pocket

propagation of a melting anomaly along the ultraslow southwest indian ridge between 46 e and 52 20 e interaction with the crozet hotspot summary regional axial depths mantle bouguer anomaly values geochemical proxies for the extent of partial melting and tomographic models along the southwest indian ridge swir all concur in indicating the presence of thicker crust and hotter mantle between the indomed and gallieni transform faults tfs 46 e and 52 20 e relative to the neighbouring ridge sections accreted seafloor between these tfs over the past 10 myr is also locally much shallower 1000 m and corresponds to thicker crust 1 7 km than previously accreted seafloor along the same ridge region two large outward facing topographic gradients mark the outer edges of two anomalously shallow off axis domains on the african and antarctic plates their vertical relief 2000 m locally and their geometry parallel to the present day axis along a 210 kmlong ridge section suggest an extremely sudden and large event dated between 8 magnetic anomaly c4n and 11 ma magnetic anomaly c5n asymmetric spreading and small ridge jumps occur at the onset of the formation of the anomalously shallow off axis domains leading to a re organization of the ridge segmentation we interpret these anomalously shallow off axis domains as the relicts of a volcanic plateau due to a sudden increase of the magma supply this event of enhanced magmatism started in the central part of the ridge section and then propagated along axis to the east and probably also to the west however it did not cross the gallieni and indomed tfs suggesting that large offsets can curtail or even block along axis melt flow we propose that this melting anomaly may be ascribed to a regionally higher mantle temperature provided by mantle outpouring from the crozet hotspot towards the swir

subsurface precambrian ridge on the continental shelf of western india between coondapoor and kasaragod abstract bathymetric and magnetic data along seven ene wsw and seventeen nnw sse and n s profiles collected on the southwestern continental shelf of india between coondapoor and kasaragod within water depths of 20 2000 m are studied model studies of magnetic data suggest that the depth to the top of the basement in the study area varies from 1 0 to 4 0 km and provide evidence for the occurrence of a graben below the inner shelf and basement ridge below the mid shelf it is also suggested that the inferred basement ridge below the mid shelf is of continental type and precambrian in age

bulk geochemistry sulfur isotope characteristics of the yuhuang 1 hydrothermal field on the ultraslow spreading southwest indian ridge abstract the yuhuang 1 hydrothermal field hf was discovered recently on the ultraslow spreading southwest indian ridge segment 29 this field comprises two main sulfide deposition areas including the southwest sulfide area sws and the northeast sulfide area nes all of which are about 500 m in diameter sulfide rich samples collected on the seafloor consist of zinc rich iron rich and silicified samples the zn rich samples contain up to 60 sphalerite and 5 chalcopyrite while iron rich samples primarily contain pyrite and marcasite and silicified sulfide rich samples primarily consist of amorphous silicon 70 pyrite 15 20 and minor sphalerite sulfide rich samples generally show mound features with similar mineral assemblages except that the nes contains pyrrhotite indicating precipitation under relatively reduced conditions bulk geochemistry of the sws and nes massive sulfide samples display similar au co ni and si contents and ni co ratios similar to mafic related hfs while the sn content and the cd zn ratio are comparable to ultramafic related hfs which probably indicates a multi stage origin sulfides rich samples from the sws have 34s values ranging from 1 37 to 6 02 with a median value of 0 83 currently the 34s value of 1 37 is the lowest sulfur isotope composition observed in hfs on ultraslow spreading ridges bacteria derived sulfur which is calculated to be 10 25 of the total sulfur component is likely the cause of the negative sulfur isotope composition conversely the 34s values of the seafloor sulfides rich samples in the nes ranges from 3 75 to 8 73 with a median value of 4 74 which is distinctly heavier than that of the sws this study suggests that the sws and nes of the yuhuang 1 hf probably formed during different mineralization stages

enhanced hydrothermal activity on an ultraslow spreading supersegment with a seismically detected melting anomaly abstract seafloor hydrothermal venting fields occur on all ocean spreading ridges osrs regardless of spreading rates however the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow spreading osrs are poorly known chinese dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system with an array of turbidity sensors and a near bottom camera along the ultraslow spreading southwest indian ridge here we report the discovery of multiple hydrothermal plumes overlying segments 28 29 and 30 between the indomed and gallieni fracture zones from these data and earlier explorations in segments 25 27 we identify nine active venting sites the spatial density fs sites 100 km of active sites along the 394 km of ridge axis in our study area is thus 2 8 nearly 3 higher than predicted by the global trend of fs for ultraslow osrs in the interridge database previous studies concluded that an enhanced magma supply to the central indomed gallieni supersegment 11 8 ma is now limited to segment 27 our results indicate that although hydrothermal activity may be most concentrated in segment 27 the discoveries of active venting in segments 25 30 implies the presence of additional magma bodies across a broad extent of the indomed gallieni supersegment

hydrothermal exploration of mid ocean ridges where might the largest sulfide deposits be forming highlights black smokers on slow spreading ridges can host larger seafloor massive sulfide sms deposits than on fast ridges the largest slow ridge sms deposits are typically associated with long lived tectonic fractures sustained fluid flow ultramafic tectonic hosted sites exhibit high average cu 10wt and au 3ppm contents in their surface sms samples hydrothermal plume surveys are consistent with equal abundances of magmatic vs tectonic vent control along ultraslow ridges the first black smokers on any ultraslow ridges exhibit high cu au sms contents at both magmatic and tectonic vent sites abstract here we review the relationship between the distribution of modern day seafloor hydrothermal activity along the global mid ocean ridge crest and the nature of the mineral deposits being formed at those sites since the first discovery of seafloor venting a sustained body of exploration has now prospected for one form of hydrothermal activity in particular high temperature black smoker venting along 30 of the global mid ocean ridge crest while that still leaves most of that 60 000 km continuous network to be explored some important trends have already emerged first it is now known that submarine venting can occur along all mid ocean ridges regardless of spreading rate and in all ocean basins further to a first approximation the abundance of currently active venting as deduced from water column plume signals can be scaled linearly with seafloor spreading rate a simple proxy for magmatic heat flux what can also be recognized however is that there is an excess of high temperature venting along slow and ultra slow spreading ridges when compared to what was originally predicted from seafloor spreading magmatic heat budget models an examination of hydrothermal systems tracked to source on the slow spreading mid atlantic ridge reveals that no more than half of the sites responsible for the black smoker plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast spreading ridges the other half of all currently known active high temperature submarine systems on the mid atlantic ridge are hosted under tectonic control these systems appear both to be longer lived than and to give rise to much larger sulfide deposits than their magmatic counterparts presumably as a result of sustained fluid flow a majority of these tectonic hosted systems also involve water rock interaction with ultramafic sources importantly from a mineral resource perspective this subset of tectonic hosted vent sites also represents the only actively forming seafloor massive sulfide deposits on mid ocean ridges that exhibit high concentrations of cu and au in their surface samples 10 wt average cu content and 3 ppm average au along ultraslow spreading ridges first detailed examinations of hydrothermally active sites suggest that sulfide deposit formation at those sites may depart even further from the spreading rate model than slow spreading ridges do hydrothermal plume distributions along ultraslow ridges follow the same 50 50 distribution of black smoker plume signals between magmatic and tectonic settings as the slow spreading mar however the first three black smoker sites tracked to source on any ultra slow ridges have all revealed high temperature vent sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings further deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of cu and au respectively an important implication is that ultra slow ridges may represent the strongest mineral resource potential for the global ridge crest despite being host to the lowest magmatic heat budget

hyporheic temperature dynamics and heat exchange near channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive river groundwater mixing or hyporheic exchange and heat transport within the hyporheic zone we quantify hyporheic fluid and heat exchange induced by current interaction with channel spanning logs using two approaches laboratory flume experiments and numerical simulations that link turbulent open channel fluid flow porous fluid flow and heat transport flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio a fraction of the channel depth blocked by the log channel froude number and sediment permeability upstream of a log downwelling water transports the river s diel thermal signal deep into the sediment downstream upwelling water from shallow flow paths has a thermal signal similar to the surface water while upwelling water from deep flow paths forms a wedge of buffered low amplitude temperatures since most hyporheic water emerges from shallow flow paths upwelling water has limited potential to buffer surface water temperature because of historic channel clearing practices modern rivers have unnaturally low densities of lwd a key implication is that lwd removal has contributed to thermal homogenization and potential degradation of hyporheic habitats lwd reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone however hyporheic exchange near lwd may not impact diel surface water temperatures at the reach scale

mineralogy and geochemistry of hydrothermal precipitates from kairei hydrothermal field central indian ridge the kairei hydrothermal field was the first confirmed active submarine hydrothermal system on the central indian ridge it has been suggested to be related to mafic as well as ultramafic host rocks based on vent fluid composition and the presence of ultramafic rocks in its vicinity in this study detailed geochemical and mineralogical analyses have been carried out on the hydrothermal precipitates from the kairei vent field in order to investigate the possible presence of indications for an ultramafic substrate at this vent site the studied samples included fragments of sulfide chimneys massive sulfides and talc bearing and silicified breccias three mineralization stages were identified 1 a high temperature stage consisting largely of chalcopyrite isocubanite and pyrite 2 a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite and 3 a weathering stage characterized by secondary cu sulfides bornite digenite covellite and idaite fe oxihydroxides opal a and cu chloride paratacamite and atacamite the sulfide geochemistry is characterized by high concentrations of cu and zn cu zn up to 29 3 wt n 17 and au mean 5 28 ppm n 17 which is comparable to results from seafloor massive sulfides collected from ultramafic hosted sites in the atlantic ocean but differs from those of typical mafic hosted deposits the high concentrations of cu and au at the kairei hydrothermal field could be an indication for the involvement of ultramafic rocks in the subseafloor ultramafic hosted au rich sulfide deposits may not be restricted to the atlantic ocean and may be common along all slow and intermediate spreading ridges

a model of basement structure from magnetic anomalies of the bombay high oil field west of india abstract bathymetric and magnetic data along 25 profiles covering the bombay high and adjacent area on the mid shelf and inner shelf off the bombay coast are studied except for the nw se and ene wsw trending anomaly closures in the east of the high the magnetic anomalies mostly trend e w correlation of the anomalies with well logs reveals that the positive magnetic anomalies arise from the basalts deccan traps underlain by granites two dimensional model studies of the anomalies under numerical constraints reveal 1 the structure of the magnetic basement and 2 sediments of 2 0 to 3 5 km thickness resting on the basalts the latter reaching 0 8 km in thickness maps of the surface model of the basalts and granites the basalt isopachs and the nature of the basement are prepared the basement structure models with steep slopes in the south and discontinuous zones in the east which are interpreted in terms of pre to post basement faults the faults which are of regional extent divide the crustal blocks underlying the mid shelf from those of the inner shelf the significance of the present study is discussed in the context of hotspot and plate tectonic concepts relating to mascarene plateau and western india