off axis hydrothermal circulation parametric tests of a refined model of processes at deep sea drilling project ocean drilling program site 504 we have developed a refined model of coupled heat and fluid flow to numerically simulate off axis hydrothermal circulation through the upper oceanic crust the new representation includes mesh elements with curved sides and noncentered nodes these curvilinear elements allow improved simulation of topography at the seafloor and within underlying sediment and basaltic layers curvilinear simulations of circulation at deep sea drilling project ocean drilling program site 504 confirm the broad conclusions derived from the rectilinear simulations of fisher et al 1990 but also suggest that permeability within the upper few hundred meters of crust around hole 504b must be concentrated within several narrow zones this interpretation is consistent with core observations and wireline logs from hole 504b within these highly permeable zones absolute permeability may be several orders of magnitude higher than the bulk permeability measured for the upper crust as a whole a series of detailed parametric simulations was conducted to determine the quantitative importance of basal heat flow seafloor bathymetry basement relief and differential sediment thickness in modifying the geometry and intensity of off axis hydrothermal circulation these studies reveal that basement relief and differential sediment thickness are more important than seafloor bathymetry in enhancing off axis convection within the seafloor observed natural correlations between seafloor bathymetry and heat flow may exist because bathymetry is a proxy for the other two important parameters basement relief and differential sediment thickness off axis convection at geochemically significant velocities is possible even with heat input appropriate for some of the oldest oceanic crust a thick sediment layer over older crust may cause enough conductive refraction at the seafloor to mask the variations in seafloor heat flow often associated with off axis hydrothermal convection

pore fluid advection and reaction in sediments of the eastern flank juan de fuca ridge 48 n abstract pore water data collected during ocean drilling program odp leg 168 provide an opportunity to study advection and reaction in hemipelagic sediments in an off axis hydrothermal setting we have modelled the pore water so42 profiles to obtain estimates of vertical advective darcy velocities strong downwelling at rates of 0 3 0 9 mm a 1 occurs at a site 4 km from the boundary between sediment free and sediment covered ridge downwelling velocities weaken away from the boundary twenty kilometres to the east strong upwelling at 2 mm a 1 is indicated at two sites 1030 and 1031 drilled above a basement high further eastwards where sediment thicknesses are in excess of 200 m vertical advective pore fluid flow through the sediment column is weak confining flow to the basement aquifer extension of the so42 model to pore fluid alkalinity mg2 and ca2 profiles enables the effects of reactions within the sediment column to be examined sulfate reduction leads to the production of alkalinity but the alk so42 ratios are less than the theoretical ratio of 2 at all the sites studied the discrepancy is largely explained by the precipitation of calcium carbonate and by the diagenetic reactivity of magnesium in the pore fluids alkalinity results are consistent with both sources and sinks for mg2 within the sediment column dependent on the direction of pore fluid advection

heat and fluid flux through sediment on the western flank of the mid atlantic ridge a hydrogeological study of north pond a comprehensive survey of heat flow sediment thickness and bathymetry in a large intermontane sediment pond on the western flank of the mid atlantic ridge north pond shows that only 20 to 25 of the heat escaping from the lithosphere flows through the sediments in the pond even though they completely cover a 70 km2 area of the seafloor three in situ pore pressure gradient measurements gave values that were negative relative to hydrostatic indicating that there is drawdown of water through the sediment at rates of 1 to 5 mm yr north pond is a recharge zone for hydrothermal circulation that is probably driven by lateral pressure gradients produced by topographic relief

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

heat flow sediment faulting and porewater advection in the madeira abyssal plain abstract this report describes 21 closely spaced heat flow measurements which were made along two transects in an area of faulted sediments east of great meteor seamount in the madeira abyssal plain the heat flow was found to be correlated with basement topography as mapped by seismic reflection profiling data modelling suggests that this is due both to the thermal conductivity contrast between sediments and basement rocks and to the presence of hydrothermal circulation within basement highs the existence of non linear temperature profiles in sediments covering basement highs suggests that the underlying circulation is causing an upward movement of porewater there is no firm evidence to show that sediment faults act as preferred pathways for porewater advection

hydrothermal flow through the mariana mounds dissolution of amorphous silica and degradation of organic matter on a mid ocean ridge flank abstract we apply one dimensional advection diffusion reaction equations to interpret amorphous silica dissolved silica organic carbon and dissolved nitrate concentrations from the mariana mounds ridgeflank hydrothermal system as a means of elucidating the effects of convectively driven porewater flow on the dissolution of amorphous silica and degradation of organic carbon in sediments we develop these mass transfer equations into models that predict an increase in the dissolution of amorphous silica with increasing porewater upwelling velocity rate of reaction and difference between the concentration of dissolved silica in basement from the steady state dissolved silica concentration in the sediment column one prediction of models for the degradation of organic matter is the depth of oxygen penetration and thus the depth at which dissolved reduced chemical species will oxidize and precipitate the models can also be used to determine the porewater upwelling velocity our dissolution and degradation models are applied to two mid ocean ridge flank hydrothermal systems the mariana mounds and the galapagos mounds which have different basement fluid compositions in the mariana mounds seawater enters basement through faults and outcrops and the resultant basement fluid has a similar composition to that of bottom seawater as this fluid upwells through the sediment it enhances the rate of dissolution and degradation in contrast seawater enters the basement in the southern section of the galapagos mounds by downwelling through the sediment column resulting in a basement fluid that is saturated with amorphous silica and reducing as this fluid upwells through the sediment it limits dissolution and degradation the amounts of amorphous silica dissolution and organic matter degradation in ridge flank hydrothermal systems are greater on an areal basis than those that would occur in nonhydrothermal settings fluid flow through the sediments of these hydrothermal systems may alter indicators of paleoproductivity and impede the formation of cherty deposits thereby yielding a potential indicator of ancient ridge flank hydrothermal systems ridge flank hydrothermal systems also may introduce old dissolved organic carbon and large organic molecules to the deep ocean

the relationship between organoclastic sulfate reduction and carbonate precipitation dissolution in marine sediments abstract pore fluid sulfate calcium and magnesium concentrations in ocean drilling sites 981 984 925 929 and 803 807 are modeled to estimate the rates of sulfate reduction and carbonate precipitation dissolution these sites represent scenarios where the sulfate reduction rate is high moderate and low results show that the rate of carbonate precipitation decreases with decreasing sulfate reduction rate when sulfate reduction rate is sufficiently low carbonate minerals dissolve sulfate reduction produces h and bicarbonate which compete in driving carbonate minerals to dissolve or precipitate the results suggest that the carbonate reactions are mainly driven by the produced h when sulfate reduction rate is low and by the produced bicarbonate when the sulfate reduction rate is high

numerical models for diagenesis and the neogene sr isotopic evolution of seawater from dsdp site 590b abstract a numerical model for the diagenetic exchange of sr between carbonates and their pore fluids during sedimentation and compaction has been developed the model has been applied to data from dsdp site 590b in order to assess the accuracy with which the sr isotope record in the carbonate sediment reflects that of seawater the most important process affecting the sr in the solid carbonate is exchange with the pore fluid due to solution reprecipitation but the concentration or isotopic composition of sr in the solid itself gives little or no information as to the magnitude of this exchange the key to determining the rate of exchange is the pore fluid where the variations of sr2 and87sr 86sr with depth are very sensitive indicators the logical structure of applying the model to data from dsdp 590b is to find by successive iteration an ocean history i e the initial87sr 86sr and sr concentration of each increment of carbonate deposited and a rate of sr exchange between pore water and solid carbonate such that the model matches the present sr concentration and87sr 86sr of both pore water and solid carbonate once all the data are matched the model provides an estimate of the rate of sr exchange due to solution reprecipitation and the evolution of87sr 86sr in seawater over the past 20 million years for dsdp 590b we find that solution reprecipitation decreases rapidly with depth from a near surface value of about 10 per million years to about 1 per million years below 200 m this rate of exchange of sr results in the carbonates of dsdp 590b preserving an accurate record of the sr isotopic evolution of the ocean over the past 5 million years but for ages greater than 5 million years the87sr 86sr ratio of the carbonate is systematically displaced from that of the seawater in which it was deposited the maximum difference is of order 5 10 5

alteration of volcanic matter in deep sea sediments evidence from the chemical composition of interstitial waters from deep sea drilling cores six deep sea drilling project dsdp sites 252 285 315 317 336 386 were examined for the chemical composition of the dissolved salts in interstitial waters the oxygen isotopic composition of the interstitial waters and the major ion composition of the bulk solid sediments an examination of the concentration depth profiles of dissolved calcium magnesium potassium and h218o in conjunction with oxygen isotope mass balance calculations confirms the hypothesis that in dsdp pelagic drill sites concentration gradients in ca mg k and h218o are largely due to alteration reactions occurring in the basalts of layer 2 and to alteration reactions involving volcanic matter dispersed in the sediment column oxygen isotope mass balance calculations require substantial alteration of layer 2 up to 25 of the upper 1000 m but only minor exchange of ca mg and k occurs with the overlying ocean this implies that alteration reactions in layer 2 are almost isochemical

significant contribution of authigenic carbonate to marine carbon burial carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals an analysis of pore water profiles collected from marine sediments around the globe suggests that the precipitation of authigenic calcium carbonate accounts for around 10 of the carbonate that accumulates in marine sediments globally carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals1 2 the formation of calcium carbonate and its burial in marine sediments accounts for around 80 of the total carbon removed from the earth s surface however the fraction of calcium carbonate that precipitates in the oceans versus that which precipitates authigenically in marine sediments is unclear here we compile measurements of the calcium concentration of pore fluids collected at 672 seafloor sites around the globe to calculate the global flux of calcium within marine sediments we use these data combined with alkalinity measurements of pore fluids to quantify authigenic calcium carbonate precipitation we estimate that the net calcium flux into marine sediments that can be ascribed to authigenic carbonate precipitation amounts to around 1 1012 mol yr 1 as such we estimate that authigenic carbonate precipitation accounts for at least 10 of global carbonate accumulation we show that much of the precipitation occurs along the eastern margins of ocean basins where organic matter delivery to the sea floor is likely to be high we suggest that authigenic calcium carbonate precipitation represents a non negligible component of the global carbon cycle

fluid seepage velocities through marine sediments constrained by a global compilation of interstitial water so42 mg2 and ca2 profiles the depth dependence of the so42 mg2 and ca2 contents of interstitial waters extracted from sediments in 140 deep sea drilling project ocean drilling program integrated ocean drilling program drill holes on oceanic crust has been fit using a model of transport advection and diffusion and reaction these drill holes come from a range of crustal ages sediment types sediment thicknesses and sediment accumulation rates the best fitting specific discharge through the sediment at these locations is estimated to be generally 500 m myr 1 0 05 cm yr 1 where sediments are thicker than 100 m although an order of magnitude faster seepage is estimated for some locations with sediment tens of meters thick assuming that the drill holes are globally representative then seepage of fluids through marine sediments at specific discharges of a few hundred m myr 1 is estimated to be only a few percent of the total ridge flank hydrothermal fluid flux this is consistent with the previous suggestion that hydrothermal fluid exchange between the ocean and the crustal aquifer primarily occurs through basement outcrops chemical fluxes of so42 mg2 and ca2 across the sediment water interface globally excluding continental margins are estimated to be less than 10 of the riverine input to the ocean

petrology of oligocene coal makum coalfield assam northeast india abstract there are five workable coal beds in the tikak parbat formation of the barail group in the makum coalfield tinsukia district assam two of these beds 18 and 6 m thick are persistent across the field the coal is high volatile bituminous b c has excellent coking properties and is of great importance as a blending coal to improve the coking properties of the lower quality gondwanan coals these coals are bimacerites as vitrinertite or trimacerites as duroclarite virtrinite predominates with minor amounts of other macerals and minerals the high percentage of vitrinite indicates that the bark and woody tissues were the dominant contributors to the precursor peat these peats were strongly decomposed under anaerobic conditions as indicated by the abundance of the collinite type of vitrinite

pelagic sedimentation on rough seafloor topography 2 inversion results from the north atlantic acoustic reverberation corridor seafloor slope distributions are sensitive to basement topography the surface shape of sediment distributions and the average thickness of sediment within a region by comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography we determine how characteristic values of average sediment thickness l rms basement variability h and apparent diffusivity vary throughout the office of naval research acoustic reverberation corridor located on the western flank of the mid atlantic ridge north of the kane fracture zone 25 5 27 25 n 44 5 49 w values of l from basin centered subregions show thinner sediments near the axis thickening westward with maximum values found on 10 20 ma seafloor lower values of l are found for older late oligocene early miocene seafloor values of l on young seafloor suggest a sediment accumulation rate of 6 5 2 3 m m y since 10 ma values of h computed from hill centered regions range from 217 to 306 m with an average of 267 27 m hills are generally larger to the north of a low running through the study area values of range from 0 12 to 0 77 m2 yr with an average of 0 32 0 19 m2 yr lower values of kbetter describe the surface shapes of individual high standing ponds while higher values are needed to model the surface shapes of thick deposits in regional lows this difference in with geological setting is probably due to variations in sediment transport mechanisms

heat transfer through the sediments of the mounds hydrothermal area galapagos spreading center at 86 w heat transfer processes at the mounds area of the galapagos spreading center at 86 w are revealed by temperatures measured at 10 m intervals in the 30 10m sediment at each of 12 holes at dsdp leg 70 sites 506 509 and by temperatures of up to five thermistors on eleven 8 12 m long piston cores the 325 needle probe values show a significant linear increase of thermal conductivity with depth in each core about half of the temperature thermal resistance profiles are nonlinear and are fit to a steady state vertical pore water advection model results indicate high and variable total heat flow and localized hydrothermal discharge at 10 8 m s associated with individual mounds recharge is indicated at similar rates in the low heat flow belt 5 km south of the mounds and is suggested at slower rates in the intermediate heat flow 0 17 0 42 w m2 belt surrounding the mounds heat flow high possible slow entrained recharge within 100 m of discharging mounds is suggested also suggested is strong local discharge along the major fault bounding the mounds crustal block to the north about 95 km north of the spreading axis at dsdp site 510 temperatures in the 114 m sediment cover on 2 7 m y crust are linear consistent with the suggestion that the hydraulic resistance of this layer is sufficient to seal off free hydrothermal exchange between basement and bottom water the combination of heat flow data and the physical properties data of karato and becker this issue suggests that 50 m of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement where the topography is smooth we suggest that the formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment as its thickness approaches this threshold value

hydrothermal circulation juan de fuca ridge eastern flank factors controlling basement water composition pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the juan de fuca ridge as part of flankflux 90 a study of hydrothermal circulation through mid ocean ridge flanks seismic reflection and heat flow surveys davis et al 1992a indicate that the three areas differ in sediment thickness basement topography abundance of outcrops basement temperature and fraction of heat lost by advection versus conduction area 1 is on 0 6 ma crust with nearly continuous basement outcrop area 2 is on 1 3 ma crust over the first buried ridge parallel to the present ridge axis and area 3 is on 3 5 3 8 ma crust over two axis parallel buried ridges that penetrate the sediment cover in three locations each area includes a hydrothermal system in which seawater flows into basement reacts with crustal basalt and then exits basement either through the sediment or directly into the overlying water column as constrained by concentrations of sulfate and lithium in the pore waters at least some seawater enters basement in all three areas without reacting fully with the overlying sediment even where no outcrops are known nearby speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity which behave conservatively in these areas to numerical time dependent transport models the estimated velocities range from 0 1 to 7 4 cm yr faster flows probably occur but were not sampled upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness the correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment we have documented pore water upwelling through sediment up to 100 m thick we estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick corresponding to a heat flow of 0 44 w m2 in area 2 and 0 3 w m2 in area 3 concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next both species remain at the bottom seawater concentration in area 1 where basement is cooled to 10 c at the base of the sediments mainly by advection the concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2 5 mmol kg at about 90 c in area 3 the transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement as the uppermost basement is heated from 10 to 40 50 c chlorinity of the basement fluid increases above the present day bottom seawater concentration in areas 2 and 3 and in nearly all other mid ocean ridge flanks studied to date as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period while chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems it reaches a maximum in area 2 at only 40 c probably because alteration there occurs at a lower water rock ratio than elsewhere for all mid ocean ridge flanks studied to date the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age

heat transfer in the oceanic crust of the brazil basin a detailed geothermal survey has been made on the western flank of the mid atlantic ridge in the brazil basin where the crust is 20 m y old the area is generally covered by a 150 to 200 m thick layer of foraminiferal ooze but basement outcrops are numerous sedimentary temperature profiles indicate anomalously low temperatures at the base of the sedimentary layer which results from cold bottom water penetrating laterally into the upper igneous crust and absorbing much of the geothermal heat net influx rates of 10 8 m s 1 are implied by a simple aquifer model a negative pressure drop across the sedimentary layer is required by this interpretation refraction studies indicate low seismic velocities 3 3 to 4 5 km s 1 near the top of layer 2 and a large velocity gradient with depth the velocity structure in the upper kilometer of the crust is related to the porosity from which it is inferred that permeabilities there are high many of the 52 temperature profiles show a significant decrease in gradient with depth the curvature can be attributed to one or a combination of the following causes 1 upward advection of pore water at rates of 10 7 s 1 2 bottom water temperature variations 3 variation of conductivity with depth and 4 experimental errors interpretation of curvature in terms of an advective model leads to results hard to reconcile with present knowledge of pressure variations and permeabilities of the oceanic crust

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

impact of seafloor sediment permability and thickness on off axis hydrothermal circulation juan de fuca ridge eastern flank sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust interlayered silt rich and clay rich sediments almost completely bury basement on the eastern flank of the juan de fuca ridge laboratory measurements indicate that silt rich sediment permeability is 1 to 2 orders of magnitude greater than clay rich sediment permeability at similar depths numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor the layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data two distinct patterns of hydrothermal circulation are obtained depending on whether silt rich or clay rich sediments compose the sediment layer above a permeable upper basement aquifer a clay rich sediment column nearly isolates circulation within the basement aquifer resulting in closed convection computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the juan de fuca ridge computed vertical fluid fluxes across the sediment column are small in agreement with fluid fluxes estimated from sediment geochemical profiles when the sediments are silt rich 17 of the flow within a convection cell crosses the sediment column open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow a transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt rich column thus the addition of only a small amount of clay to an otherwise silt rich sediment column may effectively isolate the crustal aquifer from the ocean

diagenesis and sr isotopic evolution of seawater using data from dsdp 590b and 575 new sr isotopic and concentration analyses of bulk carbonate and porewaters from a carbonate rich miocene section sampled at dsdp site 575 are reported and used along with our earlier data from a section of similar age at dsdp site 590b in a numerical model to determine for each site the rate of sr exchange during diagenesis and the degree to which this amount of exchange has shifted the87sr86sr of the bulk carbonate from the ratio each increment of sediment had when first deposited despite the fact that the two sites have very different water depth 4536 m at 575 vs 1299 m at 590b different sedimentation rate 10 m my at 575 vs 40 m my at 590b and significantly different porewater sr2 gradients we find that the rate of sr exchange as a function of sediment age is almost indistinguishable between the two sites the rate of sr exchange determined at the two sites is such that the resulting shift in bulk carbonate87sr86sr due to diagenesis is small compared to the total range of87sr86sr values measured but large compared to the analytical uncertainty of the individual isotopic ratio measurements by taking this shift into account we reconstruct the original87sr86sr of each increment of carbonate sampled which when plotted as a function of age becomes our best estimate of the sr isotopic evolution of seawater because sr is very well mixed in the ocean at any given time there is a single worldwide value of seawater87sr86sr therefore if we are quantitatively accounting for the effect sr exchange we should find the same seawater evolution curve regardless of what dsdp site is used when we compare the observed bulk carbonate87sr86sr vs age at the two sites they are seen to differ by amounts that are sometimes large compared to the analytical uncertainties of the measurements however when these data are corrected for the post depositional sr isotopic shifts predicted by our diagenetic model we find almost perfect agreement this agreement suggests that we have made a realistic determination of the rate of sr exchange and its consequences in terms of shifting the87sr86sr of the bulk carbonate and more importantly that sr isotopes can be used to correlate marine sediments with an accuracy comparable to the very small analytical uncertainties of modern isotopic measurements

liquefaction mitigation using bentonite suspensions abstractottawa sand specimens premixed with 0 3 and 5 bentonite by dry mass of sand were tested under undrained static and cyclic loading to investigate the effects of bentonite on the static and cyclic shear strength of the sand the results show that allowing the bentonite to hydrate within the sand pore space increases the cyclic resistance of the sand for the same skeleton relative density and cyclic stress ratio cyclic tests on specimens with sufficient hydration times showed a significant increase in the number of cycles required for liquefaction compared with clean sand when the specimens were allowed an extended postconsolidation aging period the cyclic resistance increased further resonant column and cyclic triaxial tests showed that this is a result of the delay in the generation of excess pore pressure in the presence of the bentonite suspension in the pore space the improvement in cyclic behavior does not occur at the expense of the static resistance of the soil under working loads be

mapping the fluid flow of the mariana mounds ridge flank hydrothermal system pore water chemical tracers we present a conceptual model of fluid circulation in a ridge flank hydrothermal system the mariana mounds the model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep sea research vessel alvin in small meter sized mounds situated on a local topographic high these mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow we have interpreted the chemical data with time dependent transport reaction models to estimate pore water velocities in the mounds themselves pore water velocities reach several meters per year to kilometers per year within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year a larger area of low heat flow surrounds these heat flow and topographic highs with upwelling pore water velocities less than 2 cm yr in some nearby cores downwelling of bottom seawater is evident but at speeds less than 2 cm yr downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement we conclude that the principal source of seawater recharge to basement is where basement outcrops exist most likely a scarp about 2 4 km to the east and southeast of the study area

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

hydrothermal circulation through mid ocean ridge flanks fluxes of heat and magnesium thermally driven convection of seawater occurs through oceanic crust of all ages at the seafloor spreading axis on mid ocean ridge flanks and in the ocean basins at the ridge axis and on the flanks circulating seawater produces a major chemical exchange between the oceans and the crust based on heat budget constraints and the composition of hot springs between 10 and 40 of the river flux of mg can be taken up during high temperature alteration of the basaltic crust along the ridge axis most of the hydrothermal heat loss however occurs on the mid ocean ridge flanks where the temperatures are lower and the seawater flux correspondingly larger the estimated heat loss on the flanks is so large that upwelling must occur over a large fraction 5 30 of the seafloor less than 65 ma in age if temperatures are 20 c and seepage velocities are on the order of 10 to 100 cm y the circulating seawater needs to lose on average less than 1 2 of its mg content in order to solve the mg mass balance for the oceans chemical fluxes through mid ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date the most critical parameter for characterizing crustal conditions is temperature in basement which is a function of crustal age basement topography and sediment thickness and permeability basement temperature which can usually be inferred from heat flow and seismic reflection surveys largely determines the change in the composition of seawater circulating through basement this change can be inferred in turn from profiles of sediment porewater chemistry all sites studied to date with temperatures at the sediment basement interface 25 c have a large component of advective heat loss and show only a small 80 mg loss both types of sites may be important for chemical fluxes whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust only small changes are needed whether the warmer sites are important depends on how much heat is lost by advection in this type of setting the warmer sites could produce significant chemical fluxes even if they are scarce they could account for the entire river input of mg to the oceans even if they represent only 8 20 of the total advective heat loss on ridge flanks

magnesium in the marine sedimentary environment mgnh4 ion exchange abstract the release of exchangeable mg in marine sediments from displacement by ammonium ions was estimated by way of experimentally determining the parameters that govern this ion exchange equilibrium on solid geochemical phases smectite humic acid illite and opal we showed that a both the conditional selectivity constant as well as the solid concentration are important parameters in determining the relative contribution of ammonium exchangeable mg from smectite organic matter illite and opal and b that except in the cases where opal or organic matter concentrations are very high the clays are the dominant carrier phases for labile mg which is exchangeable by ammonium a model based on the sum of the contributions from the major geochemical phases present in the sediment reliably predicts the amount of mg released by exchange with ammonium in marine sediments

major cation gradients in dsdp interstitial waters the role of diffusive exchange between seawater and upper oceanic crust abstract interstitial water analyses from dsdp legs 1 53 were surveyed to identify sites at which the dominant process supporting gradients of major seawater cations is alteration of the underlying oceanic crust sites with thin sediment cover are generally without gradients sites with appreciable sediment accumulation are generally with gradients the transition is probably the result of cessation of convective circulation within the sediments a model is developed for the diffusive flux through an accumulating unreactive sedimentary layer supported by continued reaction within the crust the model is consistent with the observations provided that convection within the crust extends to depths 1 km and that convective flow within the sediments ends once 100 150 m of sediment accumulates the model suggests a steady state gradient inversely proportional to the sedimentation rate this dependence is apparent in the data however some variation in the reaction rate is also required fluxes of mg and na are systematically related to the flux of ca which because of its mobility serves as a tracer of hydrolysis of the basalt when the ca flux is low the mg flux serves as the principal sink for alkalinity produced during basalt hydrolysis when the ca flux is high the mg flux becomes diffusion limited and nabearing alteration phases must form the fluxes resulting from these alteration reactions exert little influence on either the chemistry of seawater or that of the upper crust

chemical evidence for advection of hydrothermal solutions in the sediments of the galapagos mounds hydrothermal field pore water profiles of ca mg f po4 3 and mn in the galapagos mounds hydrothermal field are believed to reflect in part upwelling of hydrothermal solutions through the sediments concentration depth profiles in a low heat flow area just north of the mounds field display diagenetic changes typical of those found in pore waters underlying highly productive surface waters consistent with the inference of no water flow or very slow downwelling w 5 cm yr of bottom water through these sediments rates of upward advection calculated from mounds field pore water profiles of ca mg and f profiles agree well with each other averaging about 1 cm yr in the pelagic sediments near the mounds and 15 30 cm yr within the hydrothermal mounds themselves the upward advection also modifies the shape of po4 3 and mn profiles advection rates inferred from the pore water data are generally in reasonable agreement with those made from heat flow data the higher ca and lower mg f po4 3 and mn concentrations in mounds field pore waters compared with those of the low heat flow area suggest chemical exchange between the solution and basalt prior to upwelling li k rb sr and so4 concentrations are indistinguishable from bottom water this suggests very high effective water rock ratios during the reactions which produced the upwelling solutions perhaps due to extensive prior alteration of basalt adjacent to the flow path of water through the crust inferred reaction temperatures are between 70 150 c