hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

hypsometric control on surface and subsurface runoff 1 a fundamental problem in hydrology is relating the basin hydrological response to the geomorphologic properties of a catchment in this technical note we show that the hypsometric distribution exerts control on surface and subsurface runoff partitioning by isolating its effect with respect to other basin characteristics we conduct simulations using a distributed watershed model for hypsometric realizations developed by modifying the contour line values of a real basin results indicate that the runoff components are a function of the basin hypsometric form in general a relatively less eroded convex basin exhibits higher total runoff that is more dominated by subsurface processes while a relatively more eroded concave basin shows less total runoff with a higher fraction of surface response hypsometric differences are also observed in the relations between base flow discharge and the mean groundwater depth and the variable source area furthermore the hypsometric form reveals clear signatures on the spatial distribution of soil moisture and runoff response mechanisms

pore pressures and permeabilities measured in marine sediments with a tethered probe a new probe for measuring pore fluid pressures in marine sediments has been constructed and tested in two hydrologic environments on the juan de fuca ridge the instrument utilizes a highly incompliant high sensitivity differential pressure transducer and a small 4mm diameter sensing probe that telescopes within a larger diameter 1 5 m long currently 2 5 m strength member the small probe diameter and the low compliancy serve to minimize the time required for pressures to approach equilibrium after penetration observed decay times are short enough to permit the instrument to be used in a tethered mode the mechanical configuration enables the small diameter probe to penetrate the sediments without buckling and to be pulled out without bending this allows multiple penetrations to be completed during a single instrument lowering ten measurements were made during the development and testing of the instrument in fine grained turbidite sediments on the juan de fuca ridge four penetrations were uninterrupted by mechanical disturbances or instrumentational problems and although equilibrium conditions were approached during only one penetration 2 7 hours maximum undisturbed decay period all provided useful constraints on equilibrium pore pressures values at two sites where sediment cover is thick and heat flow relatively low were unresolvably different from hydrostatic values estimated from two penetrations located within a few hundred meters of a hydrothermal vent field within the sedimented axial valley of the northern juan de fuca ridge middle valley exceeded hydrostatic and yielded pressure gradients of roughly 0 5 kpa m 1 permeabilities were estimated from the rates of decay of the penetration transients values ranged from 5 10 16 to 1 10 14 m2 and agreed well with values measured on core samples collected near the probe penetrations which ranged from 8 10 16 to 1 10 14 m2 using the values determined from the probe measurements from middle valley the implied rate of fluid flow upward through the sediments in the vicinity of the vent field is roughly 5 10 10 m s 1 or 15 mm yr 1 with the current practical limit of resolution of roughly 10 10 m s 1 3 mm yr 1 this direct measurement technique provides a means of determining pore fluid flow rates that is roughly 1 order of magnitude more sensitive than the method of estimating the rate of flow from the perturbation of the conductive thermal regime measured by typical marine heat flow instruments

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

heat flow variations correlated with buried basement topography on the juan de fuca ridge flank systematic heat flow variations are commonly observed on sedimented flanks of mid ocean ridges these variations have been attributed to the convective circulation of pore fluids within the oceanic crust and have been used to estimate the scale and pattern of this circulation here we report similar systematic variations observed in a detailed heat flow and seismic reflection survey over sediment covered 3 myr old sea floor on the eastern flank of the juan de fuca ridge we find that heat flow peaks are located above buried basement ridges and adjacent to two minor basement outcrops heat flow lows are found where thick sediments fill basement valleys the simple inverse relationship between heat flow and sediment thickness defined by the data indicates that hydrothermal circulation sealed within the permeable portion of the oceanic crust is vigorous enough to maintain approximately isothermal 10 k conditions at the sediment crust interface as no systematic variations in estimated basement temperatures are observed it seems that at this location the heat flow variations result only from sediment thickness variations and do not reflect a particular convective pattern

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

heat flow over reunion hot spot track additional evidence for thermal rejuvenation of oceanic lithosphere we present 99 new heat flow determinations obtained with a regular sampling 2 3 km along two seismic profiles across the mascarene ridge indian ocean this aseismic ridge is volcanic relief due to the activity of a hot spot now responsible of the active volcanism of reunion island our sampling technique allows us to identify short wavelength processes such as slope driven water flows that affect heat flow on the ridge with the exception of these perturbed sites heat flow appears to be very uniform across the ridge and significantly higher 6 8 mw m 2 than the reference heat flow predicted for the seafloor age although this anomaly is low it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry the set of information now available for the mascarene ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell

estimation of lateral hydrothermal flow distance from spatial variations in oceanic heat flow measured heat flow in young oceanic crust is generally less than predicted by lithospheric cooling models even where thick sediments had been expected to isolate the crust from the ocean and hence eliminate hydrothermal heat transfer at isolated sites with basement outcrops or topographic highs however heat flow sometimes exceeds the model predictions it thus appears that water migrates and upwells at distant sites hence transferring heat laterally simple estimates of the lateral flow distance can be made when heat flow data have adequate areal sampling the data can be parametrized by distance from the presumed upwelling site and integrated to estimate the area from which heat must be transferred along the juan de fuca ridge the minimum effective lateral flow distances are 8 km for 3 5 ma crust and 2 km for younger 1 ma crust

heat flow in the balearic and tyrrhenian basins western mediterranean we present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the balearic and tyrrhenian basins in the western mediterranean analysis of 12 measurements within a 10 km radius from 40 01 4 55 e in the balearic abyssal plain shows a mean heat flow of 92 10 mw m 2 after correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late oligocene age proposed for the basin a similar survey with 18 measurements around 40 16 n 11 19 e in the western tyrrhenian gives a flux of 134 8 mw m 2 while the third survey of 26 measurements in the southern tyrrhenian abyssal plain at 37 17 n 12 58 e yields a slightly higher heat flow of 151 10 mw m 2 these values are within the range predicted by simple plate cooling models for the late miocene ages of the deep tyrrhenian basin thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins the thermal signature is similar also the trend of increasing heat flow from west to east through the balearic and tyrrhenian basins is in agreement with models of the formation of the western mediterranean behind an eastwardly migrating trench system in all three areas the measured flux shows significant local variability in the two westernmost surveys this can be attributed to the presence of buried high conductivity salt structures but in the third area se tyrrhenian the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone instead evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area

the double silica maximum in the north pacific the north pacific has two vertical silica maxima the well known intermediate maximum occurs between 2000 and 2500 m with a potential density relative to 2000 dbar of 36 90 in the northeastern pacific the deep maximum which has not been observed extensively before is found at or near the ocean bottom in the northern north pacific in a narrow latitude range maps of silica on isopycnals which intersect the intermediate and bottom maxima show that the lowest silica is found in the western tropical north pacific suggesting a route for the spread of south pacific water into the deep north pacific low silica water is found along the western boundary of the north pacific with a separate broad tongue south of hawaii the highest silica on both isopycnals is in the northeast pacific a bottom maximum in the cascadia basin in the northeastern pacific can be differentiated from both open ocean maxima four sources for the vertical maxima are considered in situ dissolution of sinking particles bottom sediment dissolution hydrothermal venting and upslope advection in the northeastern pacific because not enough is known about any of these sources only rough estimates of their contributions can be made the bottom maximum is most likely to result from bottom sediment dissolution but requires a flux larger than some current direct estimates the cascadia basin bottom maximum may result from both bottom sediment dissolution and hydrothermal venting the intermediate maximum is likely to result primarily from dissolution of sinking particles there is no quantitative estimate of the effect of possible upslope advection or enhancement of bottom fluxes due to the columbia river outflow

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

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

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

heat flow on the western flank of the east pacific rise at 21 n we report 48 new heat flow measurements on the western flank of the east pacific rise at 21 n the stations were taken in 1 to 30 m of sediment on 0 4 to 1 4 m y old crust the low average measured value of 173 mw 2 including 12 tilted minimum values is about 1 3 the mean heat flow predicted for crust of this age range by cooling plate theory this may be partly due to a bias of measurement locations in sediment ponds nevertheless 4 km running averages of measured heat flows projected normally to the axis display an oscillatory trend suggestive of cellular hydrothermal circulation in the basement we sampled no more than one cycle of the apparent heat flow modulation which is of 15 km wavelength and 200 mw m2 amplitude this variation appears to be uncorrelated with the topographic variation of 10 km wavelength assuming that the porosity sensitive electromagnetic results of young and cox 1981 reflect a permeability distribution that restricts the depth of circulation to 1 4 km average temperatures of circulation may be 50 60 c deeper higher temperature circulation is allowable if a more uniform permeability depth distribution holds

the galapagos spreading center at 86 w a detailed geothermal field study appendix is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20009 document j80 013 01 00 payment mustaccompany order we report here measurements of the heat flow field of the galapagos spreading center on crust of age less than 1 0 m y the 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near axis conductive heat flux the intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern with circulation apparently continuing beyond the limits of our survey the areal average of the conductive heat flux is 7 1 0 8 hfu 295 33 mw m2 about one third the heat flux predicted by plate models the remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis the pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography sharp lateral gradients in the heat flow greater than 10 hfu km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys may indicate a local concentration of the circulation by surface fault systems and or variable sediment thickness

an introduction to the special section on oceanic crustal evolution the concept that the rock formations which make up the ocean floor can change with time is relatively recent and is an idea that has itself undergone considerable evolution prior to the hypothesis of plate tectonics the seafloor was thought of as unchanging and essentially ageless the identification of a specific age for a particular portion of oceanic basement developed naturally from the concept of seafloor spreading early in the 1960s field studies during this period began to show that in fact the upper oceanic crust was far from uniform and that some of this variability could be associated with systematic changes in crustal age

hydrothermal models for the generation of massive sulfide ore deposits the discovery of massive sulfide ore deposits at certain sites at active seafloor spreading centers provides a basis for considering the energetics of processes that have concentrated similar large ore deposits that have been preserved in volcanic rocks on land in this paper we construct transport models based on hydrothermal convection systems in the oceanic crust we investigate models in which the circulation is driven by heat extracted from the permeable crustal rocks themselves as well as by heat conducted through the roof of a replenished vigorously convecting crustal magma chamber if the hydrothermal fluids carry 100 ppm dissolved iron as found in black smokers on the east pacific rise and exit at 350 c or greater calculations show that the heat content of the oceanic crust is insufficient to account for ore deposits of 3 mt or greater heat extraction through the roof of a magma chamber in conjunction with the formation of a thin 1 km layer of plated gabbro may account for ore bodies of the order of 3 mt provided the permeability of the oceanic crust in the magma hydrothermal fluid contact zone is 10 15 m2 or less if the conducting boundary layer were only 1 m thick locally for 103years the permeability could be as high as 10 13 m2 the permeability in the discharge zone could conceivably be two orders of magnitude greater than the permeability at depth more work is needed on the details of the effects of magma chamber replenishment and off axis dike injection and on the role of magmatic fluids very large ore deposits i e 100 mt are probably formed in multiple hydrothermal cycles

influence of sedimentation local and regional hydrothermal circulation and thermal rebound on measurements of seafloor heat flux 1 we quantify the influence of environmental processes on measurements of seafloor heat flux with a new one dimensional thermal model that includes time varying sedimentation and boundary conditions represents several scales of hydrothermal and transient conductive processes within basement and allows fluid seepage through accumulating sediments variations in basement thermal conductivity the extent of hydrothermal mixing in upper basement and fluid seepage through sediments each influence seafloor heat flux by 2 8 conductive thermal rebound following the cessation of advective heat loss from the crust may lower seafloor heat flux values by 5 10 even after several to several tens of million years the new models indicate that thermal rebound takes much longer than suggested by earlier analytical calculations mainly because earlier models did not account for the heat capacitance of the conductive lithosphere application of the new model to 3 5 3 6 ma seafloor on the eastern flank of the juan de fuca ridge suggests that anomalously low heat flux in this area is best explained by incomplete conductive thermal rebound in the last 100 200 ka following the burial of numerous basement outcrops application of the new model to the eastern flank of the east pacific rise and extrapolation out to the age of some of the oldest remaining seafloor indicate that sedimentation corrections may be important even where accumulation rates are typical of global values model results also suggest that conductive thermal rebound through thick sediments may bias measurements made on moderate to old seafloor even where there is little evidence at present for ridge flank hydrothermal circulation

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

geomorphological origin of recession curves 1 we identify a previously undetected link between the river network morphology and key recession curves properties through a conceptual physical model of the drainage process of the riparian unconfined aquifer we show that the power law exponent of dq dt vs q curves is related to the power law exponent of n l vs g l curves which we show to be connected to hack s law where l is the downstream distance from the channel heads n l is the number of channel reaches exactly located at a distance l from their channel head and g l is the total length of the network located at a distance greater or equal to l from channel heads using digital terrain models and daily discharge observations from 67 us basins we find that geomorphologic estimates match well the values obtained from recession curves analyses finally we argue that the link between recession flows and network morphology points to an important role of low flow discharges in shaping the channel network

near axis heat flow measurements on the northern juan de fuca ridge implications for fluid circulation in oceanic crust present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process in axial regions abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid in older crust where the insulating sediment cover is thick previous studies have shown that basement topography is the dominant forcing factor for within crust fluid circulation in the intermediate region where young crust is lightly sedimented heat flow data are difficult to obtain with traditional techniques to determine whether topography or permeability is the dominant process controlling fluid circulation in the near axis region we conducted a profile of heat flow measurements using the submersible alvin on the endeavour segment of the juan de fuca ridge our data indicate that topographic forcing is responsible for the long wavelength variations with high heat flow at the ridge summits and low values in the inter ridge valleys the locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge valley topographic pair comprises a single circulation cell this model predicts that the source area for the high temperature axial vents may be in the flanking inter ridge valleys

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

hydrothermal venting and geothermal heating in cascadia basin observations in cascadia basin on the eastern flank of the northern juan de fuca ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small 1 km2 isolated igneous basement outcrops in the northern sector of the plateaulike basin the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250 to 350 m thick geothermal boundary layer characterized by anomalously high temperature reduced vertical stability and high dissolved silicate concentration using a basinwide average lithospheric heat flux of 0 3 w m 2 and the observed thermal anomaly structure of the water column we obtain a mean residence time of 1 to 2 years for the deep water over cascadia basin detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north central sector of the basin indicate that local bottom water heating arises from low temperature venting through the summit and flanks of the outcrops near the smallest edifice especially well defined layers of anomalously warm particle laden water were found within 20 m of the outcrop summit depth of 2610 m maximum anomalies of temperature light attenuation coefficient and dissolved silicate concentration in the layers were 0 040 c 0 015 m 1 and 5 mol l 1 respectively we estimate the local heat flux fo from the smallest outcrop to be 2 4 0 8 u 109 w where u m s 1 is the mean horizontal current at the venting depth for reasonable mean currents in the range 10 3 to 10 2 m s 1 we find fo 0 2 to 2 4 107 w assuming that the depressed conductive heat flow of 0 05 w m 2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

porosity and hydraulic properties of sediments from the galapagos spreading center and their relation to hydrothermal circulation in the oceanic crust density and porosity of sediments were measured on dsdp legs 69 and 70 samples from the galapagos spreading center permeability and the hydraulic impedance of each sediment layer were estimated from measured values of porosity the gradients of porosity and density with depth where sediment layers are thin or approx 50 m thick are anomalously high compared with those of other areas and with the upper part of thicker sediment layers in this area a good correlation was found between the anomalous porosity and density gradients and the present day heat flow we interpret these observations to suggest that these high gradients may be due to active hydrothermal circulation through a thin sediment cover which is inhibited by a thicker sediment layer and that the pattern of hydrothermal circulation may be essentially fixed with the moving plate hydraulic impedance of the sediment layer was estimated from the observed depth variation of porosity and was shown to increase rapidly with its thickness our interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields an average permeability of the underlying basement layer of about 3 6 x 10 supmore 14 m sup 2 30 60 mdarcies less

strontium oxygen and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in dsdp hole 504b costa rica rift dsdp hole 504b was drilled into 6 ma crust about 200 km south of the costa rica rift galapagos spreading center penetrating 1 35 km into a section that can be divided into four zones zone i oxic submarine weathering zone ii anoxic alteration zones iii and iv hydrothermal alteration to greenschist facies in zone iii there is intense veining of pillow basalts zone iv consists of altered sheeted dikes isotopic geochemical signatures in relation to the alteration zones are recorded in hole 504b as follows zone depth m average87sr 86sr average 18o o average d o i 275 550 0 7032 7 3 63 ii 550 890 0 7029 6 5 45 iii 890 1050 0 7035 5 6 31 iv 1050 1350 0 7032 5 5 36 full size table table options view in workspace download as csv alteration temperatures are as low as 10 c in zones i and ii based on oxygen isotope fractionation strontium isotopic data indicate that a circulation of seawater is much more restricted in zone ii than in zone i fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 20 c in zones iii and iv which is consistent with secondary mineral assemblages the strontium oxygen and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in zones iii and iv are estimated to be 0 7037 2 and 3 respectively strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water rock ratios in weight of about 1 6 this value is close to that of the end member hydrothermal fluids issuing at 21 n epr basement rocks are not completely hydrothermally altered about 32 of the greenstones in zones iii and iv have escaped alteration thus 1 g of fresh basalt including the 32 unaltered portion are required in order to make 1 g of end member solution from fresh seawater in water rock reactions

heat flow and flexure at subduction zones 1 it is often suggested that heat flow in the incoming plate at subduction zones is lower than expected for crust of that age due to increased hydrothermal circulation from flexure extending about 300 km seaward of the trench and faulting observed starting about 50 75 seaward of the trench testing this suggestion using global heat flow data shows no significant difference between heat flow near the trench and the global means for the same age crust however on average heat flow in the overriding plate is about 60 of that in the incoming plate

geomorphological theory of the hydrological response the geomorphological theory of the hydrological response originated by rodriguez iturbe and valdes is reviewed with reference to the general framework of the formulation of transport processes by travel time distributions this review by no means exhaustive of the publications on this subject collects in a reasonably organic manner some relevant theoretical developments and a few significant applications

impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust one of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust layer 2a appears to thin and eventually disappear as its seismic velocity doubles from approximately 2 2 to 4 5 km s 1 in the first 40 m y the most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals a by product of off axis hydrothermal circulation and alteration of basalts new insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases a qualitative model of layer 2a crustal evolution is outlined that satisfies many geophysical and geochemical observations some seemingly unrelated somewhat surprisingly the crustal age at which layer 2a seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment which in turn controls the hydrologic regime necessary for secondary mineral precipitation speculative predictions can be used to verify refine or invalidate this dynamic model of crustal evolution major scientific and technical obstacles must be overcome before a definitive model can be quantified with such a rigorous model of crustal upper crustal evolution in hand remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state composition and past history of oceanic crust

the relationship between flow and permeability field in seafloor hydrothermal systems the permeability of oceanic crust is spatially variable and probably anisotropic as well using realistic permeability fields for young oceanic crust we have performed numerical simulations of finite amplitude steady and unsteady convective fluid flow in layered and or anisotropic porous media heated from below to investigate particular patterns of fluid flow and temperature in mid ocean ridge hydrothermal systems on the flanks of mid ocean ridges permeability measurements in deep sea boreholes suggest that only the top few hundred meters of oceanic crust is permeable given this permeability structure and assuming some minimum permeability and layer thickness our models predict that convection occurs in the form of numerous cells with aspect ratios of order unity within this permeable layer such convection results in fluid flux and diagenetic reactions within the permeable layer with a negligible effect on heat flow at the seafloor in agreement with field observations estimates of oceanic crust permeability based on hydrothermal veins in ophiolites and measurements in deep sea boreholes suggest that pillow basalts lava flows are much more permeable than underlying sheeted dikes this is particularly true at the ridge crest where voids have not yet collapsed and filled given this permeability structure our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting the system at high temperatures in very focused discharge zones in our models these discharge zones narrow considerably at the interface between the sheeted dikes and the pillow basalts lava flows much of the fluid entering the system however never circulates below the pillow basalts lava flows and exits the seafloor at low temperatures in general discharge zones are more focused than recharge zones these results are consistent with observations of narrow and focused discharge zones in ophiolites and localized high temperature venting amid widespread low temperature flow on the ridge crest the spacing of upflow zones at the surface is strongly controlled by convective flow in the bottom permeable layer in our models near field effects dominate over far field effects in systems with lateral variations in permeability and no large scale flow develops between widely spaced areas of contrasting permeability the time to steady state and evolution of convective flow in porous media vary considerably with initial conditions our simulations suggest that the time to steady state is relatively long and that it is possible that hydrothermal convection at the ridge axis never reaches a steady state flow pattern in the sense that the variations in the system boundary conditions such as basal heat flux may occur on a time scale less than the response time of the hydrothermal system it is possible however that these systems may be quasi steady for significant periods of time

geomorphic controls on regional base flow the subsurface hydrological response plays an important role in the hydrology of humid regions in particular the physical relationship between base flow dynamics and the fluctuations in spatially averaged water table depth as described by the groundwater rating curve determine to a significant extent the nature of statistical persistence of hydrological anomalies in the unconfined aquifers level and river flow in this paper we propose that the scale and shape of the groundwater rating curve reflect some of the geomorphological characteristics of the region such as relief drainage density and the hypsometric distribution of the elevation field these connections between geomorphology and hydrology of river basins are investigated using a simple model of unconfined groundwater flow applied to synthetic basins as well as observed basins from illinois

heat flow sediment and pore fluid chemistry and hydrothermal circulation on the east flank of alarcon ridge gulf of california new seismic heat flow sediment and pore fluid geochemistry data from the east flank of alarcon ridge at the mouth of the gulf of california provide evidence for vigorous hydrothermal circulation within young oceanic crust formed at a moderate rate spreading center data and samples were collected 9 20 km from the ridge axis to assess the hydrologic state of 0 30 0 65 ma seafloor conductive heat flow values are 15 55 of that input at the base of the lithosphere heat flow is highest near the center of a sediment covered trough and lowest along the trough margins suggesting that trough bounding faults and basement exposures may help to focus hydrothermal recharge sediment and pore fluid geochemistry data in combination with reactive transport modeling indicate that conditions within the shallow sediments are dominantly diffusive and reactive in two locations but that bottom seawater recharges through the thin sediment layer with velocities on the order of 2 10 mm yr at other sites seafloor heat flow appears to be entirely conductive which is consistent with the slow rate of seepage inferred from pore fluid observations and modeling fluid recharge through sediments requires that basement is underpressured relative to hydrostatic conditions we interpret these observations and inferences to indicate vigorous fluid flow in basement and secondary seepage through overlying sediments the heat flow deficit along the 11 km alarcon basin transect averages 440 mw m 2 equivalent to heat output of 5 mw per kilometer of spreading axis this heat output is similar to the combined focused and diffusive heat output of a single basement outcrop on the east flank of juan de fuca ridge and suggests that sites of concentrated heat and fluid outflow may exist on the east flank of alarcon ridge fl 2001 published by elsevier science b v

heat flow evidence for hydrothermal convection in cretaceous crust of the madeira abyssal plain 36 measurements of heat flow have been made along two transects in an area to the east of great meteor seamount madeira abyssal plain values ranging from 51 to 74 mw m 2 correlate partly with the basement topography these data together with results from a previous heat flow survey have been compared to a numerical heat flux model which incorporates a depth related conductivity structure for both sediment and basement the model suggests that the remaining heat flux variability probably relates to basement hydrothermal circulation which is being driven by topographic relief no new evidence in terms of non linear temperature profiles was found for sediment porewater advection

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

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 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

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

measurements of plume flow from a hydrothermal vent field simultaneous velocity temperature and conductivity data were taken in the convective flow emanating from a hydrothermal vent field located at 10 56 n 103 41 w on the east pacific rise the horizontal profiles obtained indicate that the flow field approaches an ideal plume in the temperature and velocity distribution such parameters as total heat flow and maximum plume height can be estimated using either the velocity or the temperature information the results of these independent calculations are in close agreement yielding a total heat flow from this vent site of 3 7 0 8 mw and a maximum height of 150 10 m the nonlinear effects of large temperature variations on heat capacity and volume changes slightly alter the calculations applied to obtain these values

geomorphic signatures on brutsaert base flow recession analysis this paper addresses the signatures of catchment geomorphology on base ow recessioncurves its relevance relates to the implied predictability of base ow features which arecentral to catchment scale transport processes and to ecohydrological function movingfrom the classical recession curve analysis method originally applied in the finger lakesregion of new york a large set of recession curves has been analyzed from swissstream ow data for these catchments digital elevation models have been preciselyanalyzed and a method aimed at the geomorphic origins of recession curves has beenapplied to the swiss data set the method links river network morphology epitomized bytime varying distribution of contributing channel sites with the classic parameterization ofrecession events this is done by assimilating two scaling exponents and b

geothermal heat flux from hydrothermal plumes on the juan de fuca ridge thesis m s massachusetts institute of technology dept of ocean engineering and woods hole oceanographic institution 1991

on the terrestrial heat flow and physical limnology of crater lake oregon the heat flow into 7000 year old crater lake caldera is dominated by hydrothermal processes the average of the conducted heat flow value is 138 121 mw m sup 2 3 3 2 9 hfu the total heat flow is estimated to be between 669 and 1381 mw m sup 2 16 and 33 hfu hydrothermal fluids penetrate into the lake floor to a depth of at least 1 5 to 2 0 km and possibly much deeper two thermal spring areas were discovered on the deep lake floor these springs and probably other as yet undiscovered ones have a dramatic effect on the water temperature circulation and stratification of the lake dense thermal waters pond in the southwest basin the deep waters of the east and northwest basin are warmed to the point that they convect causing the lake to be vertically mixed a hyperadiabatic temperature gradient results from this convection and possibly the stagnating effects of small quantities of suspended solids thermal fluids carrying dissolved solids cause anomalously high concentrations of chloride and sulfate in the relatively pure lake water

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