axial topographic relief associated with ridge transform intersections the axial depths of mid ocean ridges are consistently observed to increase along strike from the central part of a ridge segment to its intersection with a transform fault variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment constrained by observations from 41 ridge transform intersections we investigate the relative effect on axial depths of transform offset segment length regional depth and spreading rate we first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length both scaled by spreading rate through a series of regression analyses covering a range of spreading rate scalings we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation we find that half segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault a second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis we can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common but not ubiquitous existence of a median valley there these results in combination with the segment relief analysis clearly indicate that the axial depth at a ridge transform intersection is a function of both long wavelength processes such as may control hot spot distribution and local tectonic factors including but not limited to transform offset

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

abyssal hill topography as an indicator of episodicity in crustal accretion and deformation abstract recent studies of seafloor morphology suggest that abyssal hills are created at the ridge axis by the interplay of magmatic and tectonic processes therefore abyssal hills should be ideal indicators of episodicity in this study we use abyssal hill topography created near the ridge axis and subsequently transported onto the ridge flanks as a time series reflecting the episodicity in magmatism tectonism at the ridge axis if the creation of an abyssal hill is viewed as a discrete event i e a pulse of magmatism or faulting we can parameterize episodicity by considering a sequence of events of magnitude m proportional to the relief or cross sectional area of a hill separated by time intervals t proportional to the spacing between hills this analysis will allow us to decide if abyssal hills are periodic or if they are randomly distributed in time as a poisson process ten profiles 200 1000 km long of unaveraged centerbeam sea beam located along flowlines and covering a range of spreading rates 15 60 km ma half rate were used to measure locations and characteristics of the relief cross sectional area and height of abyssal hills the principal results of the analysis are that 1 abyssal hills are clearly not periodic and their distribution in time corresponds to a poisson process with a characteristic frequency the characteristic frequency is roughly proportional to the square of the spreading rate and varies from 12 8 events ma at fast rates to 1 4 events ma at slow rates and 2 while the magnitudes m of abyssal hills cross sectional areas and reliefs are positively correlated with the time elapsed since the formation of the previous hill there is no correlation between m and the amount of time to the next event these results agree with a scenario in which magma or stress accumulate at a steady rate at the ridge axis and are episodically released in discrete events of volcanism or faulting whose magnitude is proportional to the time elapsed since the last event

why some mediterranean sapropels survived burn down and others did not abstract we have evaluated using a numerical model the burn down depth depth of post depositional oxygenation of organic matter rich sapropels under aerobic conditions at the seafloor the model suggests that sapropels may be erased by burn down to depths of at least 5 cm after 10 kyr of bottom water reoxygenation a test of the sensitivity shows that the parameters exerting a major control over the burn down process are the flux of freshly sedimenting organic carbon the oxygen concentration of the bottom water the amount of organic matter in the sapropel and the sedimentation rate in comparison bioturbation is of minor importance the result implies that burial of elevated concentrations of non refractory organic carbon is generally improbable when the sedimentation rate is lower than 1 2 cm kyr 1 the record of sapropel deposition in the mediterranean sea and elsewhere may thus be biased towards depositional environments where the sedimentation rate is large enough to shield the organic rich layer from oxygenation

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

a metrological approach to measuring 40ar concentrations in k ar and 40ar 39ar mineral standards in geochronology isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals for dating of geological material using the k ar system the simultaneous determination of 40ar and 40k concentrations on the same aliquot is not possible therefore a widely used variant the 40ar 39ar technique involves the production of 39ar from 39k by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux referred to as mineral standards many mineral standards still in use rely on decades old determinations of 40ar concentrations resulting uncertainties both systematic and analytical impede the determination of higher accuracy ages using the k ar decay system we discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of 40ar and will allow for the sensitivity calibration of noble gas mass spectrometers the design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system a number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system

axial depth anomalies from 10 to 50 north along the mid atlantic ridge correlation with other mantle properties abstract we present an analysis of intermediate to long wavelength a few hundred to a few thousand kilometers axial depth anomalies along the mid atlantic ridge between 10 and 50 n the maximum depth of the rift valley is chosen as the elevation datum for oceanic crust of zero age the large depth anomalies are correlated for short wavelengths less than a few hundred kilometers with some fracture zones irrespective of their offset and for intermediate to long wavelengths with mantle properties expressed in 1 excess elevation near triple junctions 2 trace element and isotope geochemistry of the basaltic liquids emplaced at the ridge axis and 3 anomalies in the earth s gravity field we suggest that the correlations may be explained in two ways the depth anomalies of intermediate to long wavelength may represent the sites of upwelling and downwelling of the deep mantle alternatively the depth anomalies could be due to a regionalization in temperature or mineralogy in the asthenosphere

a review of nitrogen isotopic alteration in marine sediments key points use of sedimentary nitrogen isotopes is examined on average sediment 15n 14n increases approx 2 per mil during early burial isotopic alteration scales with water depth abstract nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record however bulk sedimentary nitrogen isotope ratios which can be determined routinely and at minimal cost may be altered during burial and early sedimentary diagenesis particularly outside of continental margin settings the causes and detailed mechanisms of isotopic alteration are still under investigation case studies of the mediterranean and south china seas underscore the complexities of investigating isotopic alteration in an effort to evaluate the evidence for alteration of the sedimentary n isotopic signal and try to quantify the net effect we have compiled and compared data demonstrating alteration from the published literature a 100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that at sites located off of the continental margins an increase in sediment 15n 14n occurs during early burial likely at the seafloor the extent of isotopic alteration appears to be a function of water depth depth related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of n isotopic alteration moreover the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global n cycle

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

morphology and crustal structure of the kane fracture zone transverse ridge the kane fracture zone kfz transverse ridge is an anomalously shallow ridge which parallels the kfz for over 200 km east of its intersection with the mid atlantic ridge rift valley sea beam bathymetry and gravity data have been used to determine the morphology and density structure of the ridge and travel time data from two seismic refraction experiments have been modeled to constrain its seismic velocity structure the transverse ridge first appears on older lithosphere opposite the eastern ridge transform intersection it rises to a maximum height of about 2000 m above the bordering kfz trough 40 km east of the intersection area although bathymetry data along the western limb of the kfz are relatively sparse a ridge of similar dimensions is not present the kfz transverse ridge has a decidedly flexural shape in cross section with a high steep south facing wall and a gently sloping northern side the grain of the ridge parallel topography north of the kfz continues undisrupted across the transverse ridge suggesting that the ridge has formed by vertical uplift of this crust seismic ray tracing and gravity analyses indicate that the kfz transverse ridge is not locally compensated by an overthickened crust but is underlain by crustal thicknesses and velocities similar to that of normal oceanic crust on either side of the kfz anomalously thin crust is only present immediately beneath the kfz trough and along the south facing wall of the transverse ridge these constraints and the formation of the kfz transverse ridge on the older plate preclude several previously proposed models for its origin including flexural bending due to differential subsidence across the fracture zone constructional volcanism and serpentine diapirism although no single mechanism provides a completely satisfactory explanation for the origin of the kfz transverse ridge the most likely mechanism appears to be a combination of thermal and viscodynamic forces operating near the ridge transform intersection

influence of diagenesis on sedimentary delta n 15 in the arabian sea over the last 130 kyr abstract sedimentary 15n records are valuable archives of ocean history but they are often modified during early diagenesis here we quantify the effect of early diagenetic enrichment on sedimentary n isotope composition in order to obtain the pristine signal of reactive n assimilated in the euphotic zone this is possible by using paired data of 15n and amino acid composition of sediment samples which can be applied to estimate the degree of organic matter degradation we determined 15n and amino acid composition in coeval sediments from ocean drilling program odp hole 772 b in the central arabian sea and from hole 724 c situated on the oman margin in the western arabian sea coastal upwelling area the records span the last 130 kyr and include two glacial interglacial cycles these new data are used in conjunction with data available for surface sediments that cover a wide range of organic matter degradation states and with other cores from the northern and eastern arabian sea to explore spatial variations in the isotopic signal in order to reconstruct pristine n values we apply the relationship between organic matter degradation and 15n enrichment in surface sediments to correct the core records for early diagenetic enrichment reconstructed 15n values suggest a significant role of n2 fixation during glacial stages an evaluation of two preservation indices based on amino acid composition reactivity index ri jennerjahn and ittekkot 1997 and the degradation index di dauwe et al 1999 in both recent sediments and core samples suggests that the ri is more suitable than the di in correcting arabian sea 15n records for early diagenetic enrichment

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

fracture zones in the north atlantic morphology and a model a morphology is presented for the typical cross section of an inactive fracture zone in the atlantic inactive fracture zones typically consist of an asymmetric valley to the young side of the fracture zone plane and a high wall or scarp to the old side large offset fracture zones may be accompanied by a marginal valley on the other side of the high wall this topography is superimposed on the depth age step due to lithospheric cooling a model is developed which accounts for this morphology the model relates the inactive fracture zone morphology to the topography found at present day intersections of the spreading axis with transform faults the asymmetry of the median valley near fracture zones plays an important role in explaining the typical fracture zone morphology the existence of a median valley is related to the viscous delay of the upwelling mantle material at the spreading axis its asymmetry near fracture zones can be accounted for by modelling the viscous drag exerted by the lithosphere on the asthenosphere if the viscosity is low as under reykjanes ridge and the east pacific rise no median valley develops and a different morphology may be expected the occurrence of marginal valleys is interpreted as the result of lithospheric warping when the graben walls in the transform domain which are caused by tension due to horizontal thermal contraction are excessively high

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

analysis of the cooling of a variable viscosity fluid with applications to the earth summary analysis shows that the convective cooling of a fluid with a temperature dependent viscosity should exhibit simple behaviour which is consistent with geophysical data on the cooling of the oceanic lithosphere and asthenosphere the cooling rate of convecting fluid is predicted to vary approximately linearly with time 12 this relationship has also been demonstrated for numerical solutions of the full governing equations of convection it is found that a similarity solution can describe the evolution of temperature within the stagnant high viscosity lid which forms over an actively convecting region the rate of thickening of the lid or lithosphere is directly proportional to a single parameter a and the overall cooling rate is also a function of a expressions are derived which relate changes in the parameter a to the initial average viscosity or temperature of the fluid and to the temperature and pressure dependence of viscosity with assumptions about how the cooling of variable viscosity fluids can be used to describe the cooling of the oceanic lithosphere and asthenosphere values of sea floor depth and the isostatic geoid height can be predicted as a function of time sea floor depth is predicted to increase linearly with and geoid height to vary nearly linearly with time in agreement with observations the rate of change of the depth or geoid height is a strong function of the initial temperature of the convecting region the mantle is taken to have physical properties appropriate for olivine a change in the mantle temperature under the ridge of 150k causes only about a 20 per cent change in the rate of subsidence but causes nearly a 100 per cent change in the rate of change of the geoid height with time the observed variations in these rates over different sections of oceanic ridges are of about this magnitude simple conductive cooling would require more than l000k variation in mantle temperature to account for the geoid height observations this analysis can also be used to estimate the cooling rate of magma sills

stochastic modeling of seafloor morphology one of the basic requirements for modeling ocean bottom scattering and acoustic reverberation is a realistic quantitative description of the seafloor because of the small scales usually required say 1 m or smaller for such modeling deterministic descriptions of the seafloor are often prohibitive fortunately the gap between what is needed by acoustic modelers and what can be delivered by seafloor mappers can be narrowed or eliminated through the application of stochastic modeling in the methodology presented here the statistical characteristics of abyssal hill morphology the predominant ocean bottom morphology are modeled using parametrized forms of the topographic moments to order four the first moment or mean is considered known and removable the model for the second moment or covariance function contains parameters describing the rms height characteristic length and width azimuthal orientation and fractal dimension parametrization of the third and fourth moments provides information

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

a new analysis of gravity and topography data over the mid atlantic ridge non compensation of the axial valley abstract topographic and associated gravity signals often result from the interference of several phenomena in the present paper an admittance function for the superposition of several physical mechanisms acting in the same waveband is derived if the processes are not phase related the resulting admittance function is the average of the individual admittance functions weighted by the squared amplitude of the topography for each process we use this concept of superposition of different processes to understand the compensation of the atlantic ridge valley classical transfer function studies applied to slow spreading ridges have concluded that the topography was emplaced on an elastic plate about 10 km thick these analyses of gravity and topography seem to contradict the physical models explaining the median rift valley which is thought to be due to dynamical effects and thus to be uncompensated we obtain an average axial valley gravity and topography profile by stacking several profiles perpendicular to the mid atlantic ridge and subtracting a long wavelength thermal effect we find that the gravity over topography spectral ratio of the average axial valley signal is consistent with an uncompensated process our study thus confirms that the mode of formation of the axial valley of a slow spreading ridge involves an uncompensated mechanism the presence of an additional process characterized by low admittance values uncorrelated from one profile to the other is also suggested in order to explain the observed admittance function the study of the long wavelength 300 km gravity and topography signal leads us to invoke the cooling of the upper portion of the crust by water circulation and to exclude the presence of a large amount of partial melt at depth more than 5 over a 20 km thick layer at a mean depth of 60 km

isotope fractionation during nitrogen remineralization ammonification implications for nitrogen isotope biogeochemistry abstract the 15 n 14 n isotopic ratio of marine particulate organic matter usually increases as a consequence of preferential 14 n remineralization ammonification during decay in the oxic water column and at the oxic seafloor however a fractionation factor 15 e of sediment ammonification in natural environments has not been determined yet here the 15 e of sedimentary ammonification is calculated in a set of seven sapropel containing sediment cores two subrecent sediment cores and four sediment trap deployments from the eastern mediterranean sea data clearly reveal rayleigh type isotope fractionation and a fractionation factor 15 e between 1 43 and 2 3 hence sediments and probably sinking particulate organic matter as well can be important sources of isotopically depleted reactive nitrogen the knowledge of 15 e further enables to reconstruct paleo export fluxes from sediment cores if original and actual 15 n as well as n accumulation rates are known

dynamic topography in rift zones implications for lithospheric heating extended abstract extension of the lithosphere in rift zones results in heating of the crust and mantle beneath the rift based on simple stretching models mckenzie 1978 the subsidence history of rifts has been attributed to crustal thinning during extension and subsequent conductive cooling of the lithosphere beneath the rift the amount of subsidence caused by cooling should provide an estimate of the amount of heat added to a column of lithosphere as it extends models of this type which assume local isostatic compensation and uniform extension over the whole thickness of the lithosphere underpredict the amount of post rift thermal subsidence royden keen 1980 and do not explain the flanking uplifts associated with rifts predictions of central subsidence from models that do not explain flanking uplifts should be suspect since the same mechanisms producing the uplift may also affect the central subsidence the origin of flanking uplifts may have important implications for the amount and horizontal distribution of lithospheric heating beneath a rift in a study of the gulf of suez rift steckler 1985 proposes that flanking uplifts are isostatically supported by thermal expansion due to heating because horizontal heat conduction occurs too slowly to explain the amplitude and width of flanking uplifts convection due to stretching induced horizontal temperature variations beneath the rift buck 1986 has been suggested as a heat transfer mechanism

a quantitative study of the axial topography of the mid atlantic ridge some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults for example the presence of an axial depression coincides in most cases with slow spreading rates and the overall depth of the ridge axis increases toward ridge transform intersections rtis on the other hand it is also well known that the relief and width of the axial valley on say the mid atlantic ridge mar vary along strike in an unpredictable manner the purpose of the present study is to quantify how much of the observed variation in the first order topography at the axis is related to changes in other parameters such as spreading rate and distance from rtis to carry out this test the zero age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the mar between the equator and 50 n full spreading rates 22 36 km ma zero age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a gaussian bell the measured axial valley relief varies between 600 and 2100 m average 1300 m while the valley width varies between 16 and 62 km average 35 km the correlation between zero age depth axial valley relief and width latitude of axial crossing spreading rate distance from nearest rtt and offset on the nearby transforms has been investigated using linear regression techniques the main results of the present study are that 1 zero age depth significantly correlates with latitude of crossing distance from nearest rti and offset on the nearby transforms and 2 the variation in axial valley relief and width is essentially uncorrelated with spreading rate zero age depth distance from nearest rtt and offset on nearby transforms the preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation in fact if the rough abyssal hill topography typical of the mar flanks is created within the axial valley the shape of the axial valley cannot be steady state although the existence of an axial valley may be a steady state phenomenon this hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography measured as the residual on the thermal subsidence trend

a global marine fixed nitrogen isotopic budget implications for holocene nitrogen cycling 1 a nitrogen stable isotopic model was constructed in order to constrain the holocene marine fixed nitrogen budget the primary sources and sinks considered were riverine and atmospheric sources nitrogen fixation sedimentary and water column denitrification and sediment burial the source budget was found to be insensitive to changes in nitrogen fixation rates and thus could not be used to constrain this term however the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification if the isotopic value of marine fixed nitrogen has not changed during the holocene as supported by sedimentary records then in order to balance the isotopic value of sinks and sources approximately 280 tg n yr 1 of sedimentary denitrification is required if such a high rate of denitrification has been sustained throughout the holocene it implies that present day estimates of marine nitrogen fixation are grossly underestimated it also implies that the marine nitrogen budget has a residence time of less than 2000 years

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km