geotectonic significance of gneissic amphibolites from the vema fracture zone equatorial mid atlantic ridge a large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the vema fracture zone serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites in the shallowest of the dredge hauls petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies on the other hand the associated meta gabbros display sequences of secondary minerals indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium we tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the vema fracture zone hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions k ar dating suggests within relatively large analytical uncertainties that the amphibolite metamorphism took place 10 m y ago i e at a time when the dredging sites were located in the vicinity of the spreading center

bed form genesis from bed defects under unidirectional oscillatory and combined flows new experimental data on bed form initiation under unidirectional oscillatory and combined flows are presented to gain quantitative insight into bed form genesis from artificially generated defects on a flat sediment laden bed planform changes revealed in time lapse photography allowed study of the evolution of the downstream and upstream edges of defects from their initial geometric center based on this temporal data set and flow velocity profiles it was observed that combined flow bed forms share the same bed form initiation processes as unidirectional and oscillatory flows which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions the development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle under current dominated combined flow conditions no defect propagation occurred in the upstream direction despite the presence of flow reversal in addition spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that i periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows ii wave dominated combined flows possess a period driven peak with respect to defect growth in experiments with relatively short wave periods but this peak is absent in experiments possessing relatively long wave periods t 15 s and iii current dominated combined flows and pure unidirectional flows do not display a period driven peak in defect growth these results suggest that the occurrence of long period combined flow bed forms may be underrepresented in the sedimentary record

oxygen isotope evidence for submarine hydrothermal alteration of the del puerto ophiolite california abstract the oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the del puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the great valley sequence indicate that their alteration occurred in a submarine hydrothermal system whole rock 18o compositions decrease progressively down section with increasing metamorphic grade 22 4 smow to 13 8 for zeolite bearing volcaniclastic sedimentary rocks overlying the ophiolite 19 6 to 11 6 for pumpellyite bearing metavolcanic rocks in the upper part of the ophiolite s volcanic member 12 3 to 8 1 for epidote bearing metavolcanic rocks in the lower part of the volcanic member 8 5 to 5 7 for greenschist facies rocks from the ophiolite s plutonic member 7 6 to 5 8 for amphibolite facies or unmetamorphosed rocks from the plutonic member modelling of fluid rock interaction in the del puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock 18o can be best explained by isotopic exchange with discharging18o shifted seawater at fluid rock mass ratios near 2 and temperatures below 500 c 18o depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism submarine weathering and later burial metamorphism at the base of the great valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions the pervasive zeolite and prehnite pumpellyite facies mineral assemblages found in the del puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center

transport mixing and stirring processes in a louisiana estuary a model study transport and mixing processes in a broad and shallow estuary in louisiana terrebonne timbalier basin are examined using a depth integrated two dimensional numerical model using current meter records previously obtained the model calibration yielded correlation coefficients between simulated and observed current components of 0 89 0 95 it appears that the bottom friction in the bay is relatively large with a manning s coefficient of 0 07 producing the best results the large bottom friction appears to be due to a combined effect of currents and the surface wave field which is not explicitly accounted for in the numerical model despite the small tidal range tidal forcing dominates circulation in the bay during equatorial tides tidal currents on the order of 20 cm s 1could develop in a broad area of the bay while in tidal passes currents could reach 50 60 cm s 1 during tropic tides strengths of the currents in the bay could easily be double those during equatorial tides local wind forcing is also important in controlling general flow direction inside the bay in particular during equatorial tides flushing time estimated by a particle tracking technique was 27 days that appears to be in agreement with observations horizontal diffusivities computed using tracer particles are comparable to the previous estimates of horizontal diffusion coefficients compiled by okubo 1974 the larger values appear to be due to coastal trapping mixing of water masses based on particle tracking is found to consist of continuous stretching folding and break up of material lines due to interaction of wind driven and tidal currents with bottom and coastal topography time evolution of the boundary between the two water masses depends on the initial tidal phase however this dependence lasts only until coastal trapping becomes dominant in controlling the time evolution of the boundary coastal trapping appears to be an important process by which stirring and mixing processes are enhanced thus making stirring and mixing more efficient and chaotic those observations point to the need to resolve small scale shear flow patterns in both space and time and detailed bottom and coastal topography in order to understand transport stirring and mixing processes in the broad shallow estuaries typical of the northern gulf of mexico

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

isotopic exchange in mineral fluid systems ii oxygen and hydrogen isotopic investigation of the experimental basalt seawater system abstract oxygen and hydrogen isotopic exchange reactions between basalt and seawater at t 300 to 500 c were investigated using oceanic tholeiitic basalt 18 o 5 7 d 70 natural seawater 18 o 1 2 d 5 and artificial seawater 18 o 5 3 as starting materials the starting basalts varied in the crystallinity from holocrystalline to glass but were ground to approximately the same grain size 100 mesh the water rock mass ratios ranged from 1 to 3 and the duration of the experiments ranged from 167 to 576 days in general depletion of 18o and enrichment of d in basalts occur at all temperatures with the magnitudes of change being greater as temperature time and to a lesser degree glass content increases the trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the basalt e g smectite at 300 c talc actinoiite at 400 500 c the changes in the 18 o values of basalts and seawater in the experimental systems were observed to follow closely with those expected from a first order rate law rate constants for the oxygen isotopic exchange between rock and water range from 10 9 5 to 10 8 0 moles oxygen m2 of solid surface sec for temperatures of 300 to 500 c the activation energy for the isotopic exchange reaction was calculated to be 11 5 kcal mol an application of our experimental rate data to natural systems suggests that the oxygen isotope equilibrium between basalt and seawater in the mid oceanic ridge may take place within approximately 1000 years at 350 c our experimental data also suggest the equilibrium oxygen isotopic fractionation factors between the altered basalt and seawater to be 3 5 0 5 at 300 c 2 0 0 4 at 400 c and 0 5 0 25 at 500 c the observed hydrogen isotopic fractionation factors between the altered basalts and seawater in our experimental systems were about 74 at 300 c about 62 at 400 c and about 48 at 500 c these fractionation factors are probably attributable to the equilibrium fractionation between fe rich secondary phases and seawater

high temperature alteration of oceanic gabbros by seawater hess deep ocean drilling program leg 147 evidence from oxygen isotopes and elemental fluxes two major stages of high temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the hess deep rift valley at the east pacific rise 2 n 101 w the 154 m of plutonic rocks cored at site 894g show mineralogies and textures that suggest that they represent the roof of an approximate 1 myr magma chamber a late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite upper greenschist facies conditions the hydration of the gabbros occurred locally through the development of well crystallized green amphiboles significant chemical fluxes were associated with the fluid flow and precipitation dissolution mechanisms oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the hess deep gabbros at temperatures above 500 c hydration was limited with only minute amphibole lamellae in pyroxenes that trapped cl na and minor k from seawater ca plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns we propose that this high temperature event lowered the 18o of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion although it is unclear whether the 18o depletion of hole 894g gabbros is representative of the entire lower oceanic crust it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans during this stage water rock ratios calculated using sr isotopes are underestimated 0 1 0 5 because of insignificant diffusion of this cation in plagioclases better estimates of water rock ratios are provided by oxygen isotopes 0 2 1 which are important in unraveling the cooling history of the oceanic crust

dynamics diffusion and geomorphological significance of tidal residual eddies many shallow tidal areas show the occurrence of ebb and flood surplusses organized in cell like structures or tidal residual eddies they seem to arise from a nonlinear transfer of vorticity from the oscillating tidal field to the mean field the irregular structure of the bottom topography or the coastline acting as a catalyst to produce the necessary vorticity gradients the dynamical characteristics of the eddies assist our understanding of the geomorphology of the sea bed

chaotic stirring in a tidal system an analysis of a two dimensional tidal model of the wadden sea reveals lagrangian chaos in the trajectories of water parcels the associated chaotic stirring results from the transverse intersection of the stable and unstable curves of hyperbolic fixed points in the lagrangian residual displacement field the tidal poincare map this tidal dispersion mechanism produces rapid water exchange along the channel axis and could be representative of many shallow tidal seas

an investigation of dispersion characteristics in shallow coastal waters abstract hydrodynamic dispersion has a significant impact on the mass transport of sediments and contaminants within coastal waters in this study apparent horizontal dispersion in a tidally dominated shallow estuary was investigated using field observations and a numerical model a cluster of four lagrangian drifters was released in two shallow regions inside moreton bay australia between two small islands and in an open water area during a 16 h tracking period the drifters generally showed similar behaviour initially moving with the dominant current and remaining together before spreading apart at the change of tide two dispersion regimes were identified a slow dispersion during the earlier stage and a rapid dispersion during the latter stage of deployment such change in regime typically occurred during the succeeding ebb or flow tides which may be attributable to residual eddies breaking down during reversal of tidal direction in addition a power function of the squared separation distance over the apparent dispersion coefficient produced an r2 exceeding 0 7 indicating a significant relationship between them by applying a three dimensional hydrodynamic model the trajectories of artificial particles spreading in the bay were simulated which allowed the calculation of dispersion coefficients throughout the entire bay the study results demonstrate that the tidal effects on dispersion were dependent on the effect of tidal excursion and residual current the tide was found to be the most dominant driver of dispersion in the bay when unobstructed by land however bathymetric and shoreline characteristics were also significant localised drivers of dispersion between the two islands as a result of island wake

the 18o 16o ratio of 2 billion year old seawater inferred from ancient oceanic crust an oxygen isotope profile of the 2 billion year old purtuniq ophiolite overlaps with similar profiles of younger ophiolites and the modern oceanic crust this overlap implies i that there was a similar style of seawater ocean crust interaction during the past 2 billion years ii that the oxygen isotope composition of early proterozoic seawater was similar to the modern value iii that early proterozoic sea floor spreading rates were similar to or greater than average modern rates and iv that early proterozoic carbonate rocks and cherts with low 18o 16o ratios do not reflect global scale 18o depletion of early proterozoic oceans

energy partitioning and horizontal dispersion in a stratified rotating lake abstract the response of a stratified rotating basin to the release of a linearly tilted interface is derived this case is compared with a uniformly forced basin in the two limits when the duration of the forcing is much greater than the period of the dominant internal waves and when it is much smaller energy partitioning is studied as a function of the burger number s relative importance of stratification versus rotation showing the dominance of a geostrophic component over the wave field for low s trajectories are integrated numerically revealing the stokes drift of the waves to be always cyclonic transport properties are classified in terms of s and the wedderburn number w relative importance of the disturbance versus stratification the geostrophic flow is the main source of advection but only the waves allow particles to break the barrier to transport between the two geostrophic gyres ultimately leading to stretching and folding for low values of w advection can become chaotic conserva

oxygen isotope analyses of co existing carbonate and phosphate in biogenic apatite a way to monitor diagenetic alteration of bone phosphate abstract the oxygen isotopic composition of phosphate 18 op and structural carbonate 18 oc of hydroxylapatite was determined in 31 bone and tooth samples of modern mammals from different countries these two variables are highly correlate r 2 0 98 and the calculated best fit of linear regression is very similar to the equation calculated from the phosphate and carbonate palaeotemperature equations 1 2 according to previous measurements 3 6 on fossils of different ages from different areas it seems quite improbable to find isotopically altered skeletal remains showing a good correlation between 18 op and 18 oc as is the case with modern samples it therefore seems possible at least in some cases to use these measurements for monitoring fossil bone and tooth diagenetic alteration when a set of points lie on the equilibrium line or close to it the 18 o values could be considered close to the original values in contrast when the points lie to the left or to the right of this line this probably means that the values are diagenetically modified due to interaction with meteoric water or 18 o enriched water respectively

characterization of three dimensional lagrangian circulation associated with tidal rectification over a submarine bank abstract two approaches to the quantitative characterization of lagrangian circulation in three dimensional 3d time periodic current fields are used to describe major features of a model derived flow field for barotropic tidal rectification on georges bank the first approach is an extension of the 2d saddlepoint method used by ridderinkhof and loder to describe the vertically averaged lagrangian circulation associated with various forcings over outer gulf of maine banks application of the method to representative sigma terrain following surfaces and comparison with particle trajectories in the 3d current field indicate that the circulation saddlepoints and associated separation lines between recirculation and throughflow regions in the vertically averaged flow over georges bank are qualitatively representative of the entire water column but that there is significant vertical structure in the lagrangian circulation in particular the horizontal extent of the tidally rectified clockwise gyre on

modelling of the oxygen isotope evolution of seawater implications for the climate interpretation of the δ18o of marine sediments it is often argued that the 18o value of oceanic water was maintained close to 0 for hundreds of millions of years as a consequence of oxygen isotope exchange between oceanic crust and seawater however for several decades the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because with increasing age a general trend of decreasing 18o values about 6 is observed in most carbonates cherts and phosphates especially for the paleozoic and early mesozoic we developed a dynamical model of seawater crust interaction that computes the 18o value in these two reservoirs as function of time this model takes into account the continuous production of crust at oceanic ridges its expansion rate the permeability profile with space and time the mineralogical mode of the crust and the kinetics of oxygen isotope exchange between rock forming minerals and seawater the model indicates that the 18o value of seawater may vary by 2 with a time response ranging from 5 to 50 ma for expansion rates of 1 to 10 cm a 1 the variation of 2 is fixed by both integrated water rock ratio and closure time of the seawater crust system by sediments variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low 18o values of pre jurassic marine sediments according to our model marine paleotemperatures could be up to 10 c lower than those expected when applying the classical hypothesis of an ice free ocean with a 18o value of 1

circulation and dispersion in a cancellate coast the rivers bays and estuaries of central maine the glacially carved central coast of maine is incised by river systems with interconnecting channels offshore trending submarine ridges and narrow passages between nearshore islands and headlands the tidal range exceeds 3 m leading to complex and vigorous circulation patterns with strong flows in narrow channels near river mouths and between islands the spongiform coastal morphology allows enhanced exchange between offshore waters estuaries and internecine bays resulting in rapid dispersal of nutrients larvae and contaminants throughout the region a fine grid numerical circulation model has been used to examine the influences of the tides river flows and winds on the dispersion of lobster larvae and pollutants in the nearshore and riverine environment this paper describes the model application presents a few salient features of the circulation patterns and examines some implications for the coastal environment for example under realistic tides and variable southwest summer winds about 80 of neutral near surface particles introduced near the offshore islands a proxy for stage iv lobster larvae from offshore sources remain within a few km of the islands over a two week period on the other hand a persistent periodic sea breeze can remove more than two thirds of the particles from the domain over the same period tidal mixing disperses pollutants entering the upper kennebec river to the offshore and through internecine passages in about one week

a model of tidal rectification by potential vorticity mixing part i homogeneous ocean abstract in previous studies of tidal generation of mean flow over varying topography the rectification mechanism has generally invoked bottom friction as a source of tidal flux of momentum and vorticity hence referred as friction mechanism the author proposes a different mechanism based on horizontal mixing of potential vorticity drawing analogy from tidal dispersion of passive tracers this mixing is parameterized through a diffusivity hence called diffusivity mechanism that is quadratic in the tidal amplitude in this part 1 the mean along isobath flow near a shelf break is determined for a homogeneous ocean and contrasted with that induced by friction mechanism in part 2 the effect of a front will be considered it is found that although the mean flow is pointing in the same direction as that induced by friction mechanism i e to the right when facing deep water in the northern hemisphere it varies more slowly with the tidal amplitude in the typical situation when the mean shear is

mapping exchange and residence time in a model of willapa bay washington a branching macrotidal estuary 1 the numerical model getm is used to examine transport pathways and residence time in willapa bay washington a macrotidal estuary with a complex channel geometry when the model is run with realistic forcing it reproduces both tidal velocities and the decrease of the salt intrusion length with increasing river flow with errors of 5 20 furthermore a more stringent test when the model is run with tidal forcing only it reproduces the along channel profile of the effective horizontal diffusivity k a direct measure of the strength of subtidal dispersion which is known from previous empirical estimates a lagrangian particle tracking method is used to map subtidal transport pathways at the resolution of the model grid this analysis reveals an interweaving of coherent lateral exchange flows with discontinuous small scale dispersion as well as tidal residual currents that in some locations sharpen rather than smooth gradients between water masses comparison between these lagrangian results and an eulerian salt flux decomposition suggests that along channel complexity channel junctions and channel curvature is at least as important as cross sectional depth variation in shaping the subtidal circulation finally a nonconservative tracer method is used to produce high resolution three dimensional maps of residence time this analysis shows that consistent with previous observational work in willapa at all except the highest winter storm level river flows river and ocean density driven exchanges are discernable but secondary to tidal stirring in all seasons despite the fact that half the volume of the bay enters and leaves with every tide average retention times in the upper third of the estuary are 3 5 weeks

application of the tidal random walk model to calculate water dispersion coefficients in the ems estuary abstract longitudinal and lateral dispersion coefficients are computed for the ems estuary using a modified version of the tidal random walk model developed by zimmerman this model relates the mixing properties of the flow to the velocity and length scales of the tidal current and the residual eddies it is found that values of the longitudinal dispersion coefficient are 200 1200 m 2 s 1 depending on the position in the estuary and lateral dispersion coefficients are a factor of c 50 smaller these results are three to four times larger than earlier reported values which were determined from field observations this difference is attributed to the fact that in the tidal random walk model the dependence of the dispersion mechanism on the detailed kinematics of the flow is neglected since a statistical distribution of residual eddies is assumed however the model predicts the spatial distribution of the dispersion coefficients in the estuary quite well particular attention is paid to the presence of three peaks in the dependence of these coefficients on the distance to the river mouth using the concepts underlying the tidal random walk model these peaks are related to the presence of tidally induced residual circulations around sand banks it is also concluded that the effect of shear dispersion on the horizontal mixing processes in the ems estuary can be neglected

episodicity within a mid cretaceous magmatic flare up in west antarctica u pb ages of the lassiter coast intrusive suite antarctic peninsula and correlations along the gondwana margin long lived continental margin arcs are characterized by episodes of large volume magmatism or flare ups that can persist for 30 m y before steady state arc conditions resume flare up events ar

oxygen isotopes in phosphatic fish remains from israel paleothermometry of tropical cretaceous and tertiary shelf waters the isotopic composition of oxygen in phosphate was measured on 24 fossil fishes from israel and jordan ranging in age from cretaceous to eocene from these the first paleotemperatures were calculated for shallow water cretaceous paleotropics the results were compared to those of fish from other mediterranean localities and to late cretaceous marine reptiles from israel 18o in the fish average 18 0 in the early cretaceous 17 8 in the cenomanian 18 3 in the coniacian 18 2 in the santonian 19 4 in the campanian 20 1 in the maastrichtian and 18 4 in the early eocene such variations suggest a warm cenomanian turonian cooling towards the end of the cretaceous and warming up again in the eocene the deduced temperatures for the cretaceous range between 20 c and 34 c which is about 10 higher than cretaceous water surface temperatures in northern europe apatite from marine reptile skeletons yields similar 18o to coexisting fish the good agreement between these and previously known paleotemperature data give further credence to the validity of paleotemperatures obtained by isotopic analysis of oxygen in phosphates the relatively large thermal gradient between middle and low paleolatitudes favors barron s 1983 coolest cretaceous model

a review of oceanographic and meteorological controls on the north sea circulation and hydrodynamics with a view to the fate of north sea methane from well site 22 4b and other seabed sources abstract the north sea hydrodynamics are key to the redistribution of methane released at the 22 4b site located at 57 55 n 1 38 e in the uk central north sea 200 km east of the scottish mainland this review summarizes the current state of knowledge on the north sea circulation stratification and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22 4b or other seabed sources astronomical tidal waves follow a counter clockwise path and tide topography interaction generates a residual circulation in the same direction wind stress forcing can enhance reduce or even reverse this circulation variations in the strength of the fair isle current fic are important the fic enters the north sea between the orkneys and shetland follows approximately the 100 m isobath passes along site 22 4b and ends up in the norwegian trench the north atlantic oscillation nao also causes variability a positive negative nao index is associated with stronger weaker than normal westerly winds nao situations strengthen the circulation in the north sea whereas it weakens during nao conditions and is directed northeastward high positive correlations exist between the sst at site 22 4b and the nao index climate change can have a long term effect on the hydrodynamics of the north sea seasonal stratification has potentially the most important imprint on methane derived from the well 22 4b site summertime heating stratifies the northern part of the north sea in autumn loss of heat to the atmosphere causes the stratification to breakdown until tides and storms mix the entire water column during the period of stratification the bulk of dissolved methane released from site 22 4b gets trapped below the thermocline the loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation oxidation in the water column

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

hydrothermal vent waters at 13 n on the east pacific rise isotopic composition and gas concentration abstract gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13 n on the east pacific rise in the most mg depleted samples 5 10 3 moles kg the gas concentrations are 3 4 5 10 5 cm 3 stp kg helium 0 62 1 24 cm 3 stp kg ch 4 10 80 16 71 10 3 moles kg co 2 the samples contain large quantities 95 126 cm 3 kg of h 2 and some carbon monoxide 0 26 0 36 cm 3 kg which result from reaction with the titanium sampling bottles 13 c in methane and co 2 16 6 to 19 5 and 4 1 to 5 5 respectively indicate temperatures between 475 and 550 c whereas 13 c co is compatible with formation by reduction of co 2 on ti at 350 c close to the sampling temperature 3 he 4 he are very homogeneous at 7 5 0 1 r a 3 he 4 he 1 0 10 5 and very similar to already published data as well as ch 4 3 he ratios between 1 4 and 2 1 10 6 18 o and d in water show enrichments from 0 39 to 0 69 and from 0 62 to 1 49 respectively these values correspond to w r ratios of 0 4 7 the distinct 18 o enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations the 3 he enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3 he mantle flux

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

gabbroic rocks from the mathematician ridge failed rift plutonic rocks from the ocean crust provide direct evidence on the petrological processes leading to its formation and evolution the nature of such rocks is important for interpreting the origin of on land ophiolites so far oceanic plutonic rocks have been mainly recovered from slow spreading ridges and their associated fractures1 13 and from trenches14 15 here we report the results of petrological studies of a suite of gabbros and related plutonic and volcanic rocks dredged from the mathematician ridge mr which was a fast spreading ridge 43 mm yr 1 until it was abandoned 3 5 myr ago16 textures and mineral chemistry of the rocks together with structural considerations indicate that before abandonment the mr produced fractionated mid ocean ridge basalt morb and fractionated gabbros the gradual decrease of spreading rate during abandonment resulted in morphological change eruption of more primitive morb and possibly tectonic emplacement of the older ferrogabbros alternatively the latter could have been tectonically emplaced after abandonment

dynamics of willapa bay washington a highly unsteady partially mixed estuary results from 3 yr of hydrographic time series are shown for willapa bay washington a macrotidal partially mixed estuary whose river and ocean end members are both highly variable fluctuating ocean conditions alternations between wind driven upwelling and downwelling and intrusions of the buoyant columbia river plume are shown to force order of magnitude changes in salinity gradients on the event 2 10 day scale an effective horizontal diffusivity parameterizing all up estuary salt flux is calculated as a function of riverflow results show that willapa s volume integrated salt balance is almost always far from equilibrium at very high riverflows the top 15 of observations the estuary loses salt on average while at all other riverflow levels it gains salt under summer low riverflow conditions in fact the effective diffusivity k is large enough to drive a net increase in salinity that is 3 6 times the seaward river driven salt flux this diffusion process is amplified not damped by increased tidal forcing contrary to the expectation for baroclinic exchange furthermore k varies along the length of the estuary as 5 of the rms tidal velocity times channel width a scaling consistent with density independent stirring by tidal residuals to summarize willapa s event and seasonal scale variability a simple diagnostic parameter space for unsteady estuarine salt balances is presented a generalization from the hansen and rattray steady state scheme

oxygen isotope variations in phosphate of biogenic apatites iii conodonts abstract the method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite down to 10 mg this extension enables one to analyze hand picked conodont samples with an analytical reproducibility better than 0 5 1 46 samples of conodonts ranging from the ordovician to the pennsylvanian of north america were analysed some insoluble phosphatic residues ichthyoliths and inarticulate brachiopods of the same time range were also measured the range of the 18o values of the analysed conodonts is between 15 and 19 it shows a general trend of decreasing d18o with increasing age from an average value of about 19 in the pennsylvanian to 17 in the ordovician this trend parallels that previously detected in marine phosphorites for the time range between the devonian and pennsylvanian our data are in agreement with independent paleoclimatic information specifically we detect maximum18o enrichment at the end of the pennsylvanian and minimum enrichment at the end of the devonian the difference between these two extremes is equivalent to about 10 c from about 40 to 30 c assuming constant isotopic composition of ocean water the success of oxygen isotopic analysis of conodonts raises the possibility of their use in paleozoic paleo oceanography in a similar way to foraminifera in the mesozoic cenozoic

determination of oxygen isotope fractionation between water and phosphate from living lingulids potential application to palaeoenvironmental studies abstract lingulids are inarticulate brachiopods with phosphatic shells which live exclusively in marine waters their distribution is generally restricted to the continental shelf within the 40 belt from temperate to equatorial areas they show a range of morphological physiological and behavioral features that have remained remarkably constant since the cambrian the oxygen isotope fractionation between water and phosphate from living lingulids was determined as a tool to investigate the temperature and or salinity of past coastal seawater oxygen isotope measurements were performed according to the silver phosphate method on a collection of lingulids coming from well known and varied marine environments a global 18o variation of 3 5 was found for these lingulids which proves their good sensitivity to environmental factors namely the oxygen isotope composition and the temperature of seawater oceanographic data provided average temperature and salinity values in the living sites of the studied specimens least squares treatment of the data gave the following fractionation equation t c 112 2 15 3 4 20 0 71 18o po4 18o h2o which is similar to the equations determined by longinelli and nuti 1973a and kolodny et al 1983 for other phosphate bearing invertebrates and fishes all these results suggest that a unique equation describes the phosphate water fractionation of many marine organisms except for mammals

near surface horizontal convergence and dispersion near the tidal mixing front on northeastern georges bank abstract horizontal dispersion and convergence are evaluated from near surface drifter data collected during the summer and autumn of 1988 and summer of 1989 in the vicinity of the tidal mixing front on northeastern georges bank the effective horizontal diffusivity coefficient k e estimated from the relative motion of the drifters over the duration of the cluster tracking increases as the cluster length scale with a range from approximately 100 to 450 m 2 s 1 for length scales of approximately 5 to 58 km at length scales k e is near zero with many negative estimates indicative of convergence at length scales 20 km k e increases in magnitude and scatter above 30 km length scales k e is typically between 200 400 m 2 s 1 the dispersion as a function of length scale is compared to published estimates using apparent diffusivities which are adjusted for the finite size of the drifter cluster at release georges bank shows lower apparent diffusivities at length scales 20 km the latter are believed to be principally due to the large horizontal gradients in the seasonal flow field although other processes perhaps associated with the large tidal currents may also contribute at the shorter length scales the low dispersion is due to convergent processes examples of convergence at different locations and under different wind and stratification conditions are presented convergence near the tidal mixing front tends to occur under light wind conditions and was observed in both seasons and in both years during two intense southwestward windstorms drifters also converged in the bank s vertically well mixed zone apparently due to a combination of spatially varying ekman depth and topographic effects

review of recent developments in tidal hydrodynamic modeling i spectral models in this paper we present a brief overview with a comprehensive set of references to recently published papers for detail of a range of techniques used in tidal hydrodynamic modeling and recent progress in the field although the paper aims to introduce the topic to someone new to the area the main emphasis is on new developments in near coastal shallow sea tidal modeling and consequently we concentrate on three dimensional modeling which has progressed significantly in recent years the paper gives a brief account of methods used to discretize the hydrodynamic equations in the horizontal namely finite difference finite element and boundary fitted coordinates and through time the representation of tidal current structure in a three dimensional model using a spectral method in the vertical is also described in this method the subgrid scale vertical diffusion of momentum which has a major influence on tidal current profiles is parameterized using a flow dependent eddy viscosity recent work on

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

hydrothermal activity in a peculiar oceanic ridge oxygen and hydrogen isotope evidence in the xigaze ophiolite tibet china the xigaze ophiolite locally displays a 7 km thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge it consists from top to bottom of 1 1 pillow lavas with 18o between 9 and 16 and d around 90 vs smow 2 2 rare trondjhemites 9 7 18o 13 4 and d around 75 3 3 dolerite sills and dikes with 18o between 7 0 and 9 5 and d between 77 and 40 4 4 isotropic gabbros occurring as screens of the sills and dikes and rare layered gabbros these gabbros have 18o between 4 8 and 11 0 and d around 65 and bear high 18o albitized plagioclases 18o 10 which coexist with clinopyroxenes of mantle values 18o 5 5 or having values being slightly depleted down to 3 0 5 5 ultramafic rocks are either fresh or serpentinized they have 18o mainly around mantle values and d s are between 250 and 100 for the mafic part of the ophiolite the isotopic compositions correspond to hydrothermal interaction with seawater derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in d in particular the gabbros reacted with seawater derived fluids under amphibolite facies conditions as seen from their secondary mineralogy and their 18o depleted pyroxenes and subsequently were pervasively reacted under greenschist zeolite facies conditions during which the high 18o albitized plagioclase formed this occurrence of amphibolite facies conditions followed by greenschist zeolite facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower temperature hydrothermal interaction if compared with other ophiolites in contrast the d s of the serpentine 100 from the ultramafics are attributed to fluids of meteoric origin at high elevation nevertheless the particular location of the serpentinized harzburgites just beneath the gabbros and the high d dolerite sills and dikes 60 d 40 enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction the xigaze ophiolite sequence displays very little 18o depleted rocks relative to mantle oxygen composition they cannot balance the amount of 18o enriched rocks consequently the result of the hydrothermal interaction between the xigaze ophiolite sequence and the seawater is an 18o depletion of the seawater however the contribution of xigaze type ridges to the buffering of 18o in cretaceous seawater was probably small