a sea floor spreading event captured by seismometers two thirds of earth s surface is formed at mid ocean ridges yet sea floor spreading events are poorly understood because they occur far beneath the ocean surface at 9 50 n on the east pacific rise ocean bottom seismometers recently recorded the microearthquake character of a mid ocean ridge eruption including precursory activity a gradual ramp up in activity rates since seismic monitoring began at this site in october 2003 suggests that eruptions may be forecast in the fast spreading environment the pattern culminates in an intense but brief 6 hour inferred diking event on 22 january 2006 followed by rapid tapering to markedly decreased levels of seismicity

segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

the sensitivity of seafloor compliance to two dimensional low velocity anomalies summary seafloor compliance the deformation of the seafloor under pressure forcing by long wavelength ocean surface waves is a passive source geophysical measurement that is particularly sensitive to subsurface s wave velocity existing inversions of seafloor compliance data for s wave structure assume a 1 d velocity model whereas in reality there may be significant lateral velocity variation the effect of 2 d structure on compliance measurements has previously been investigated using finite differences but computational constraints limit the number and type of models that can be tested we present a quasi analytical calculation of seafloor compliance over a cylinder that allows us to efficiently investigate the effect of an important subset of subsurface models including pure fluid inclusions the compliance displays the same sensitivity to regions of low shear modulus as is seen for 1 d situations generating a strong peak over the low velocity zone the spatial width of this peak which is a measure of the horizontal resolution of seafloor compliance is approximately proportional to the depth of the cylinder while its amplitude is controlled by the radius and depth of the cylinder and the s wave speed of the cylinder and the background minimum structure 1 d inversions of the compliance measured above a fluid cylinder recover distinct low velocity zones although the recovered s wave speed is much greater than zero and the low velocity zone is smeared out over a greater depth range than that occupied by the cylinder

fine scale segmentation of the crustal magma reservoir beneath the east pacific rise mid ocean ridges are composed of segmented faults and magma reservoirs seismic images from the east pacific rise show that the magma reservoirs are segmented on the same fine scale as the surface faults and that distinct lava eruptions are sourced from largely isolated magma lenses

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

the first measurements of hydrothermal heat output at 9 50 n east pacific rise abstract despite the importance of the heat output of seafloor hydrothermal systems for the earth s energy budget hydrothermal heat output measurements have been very limited in this paper we report the first measurements of hydrothermal heat output at the ridge 2000 integrated study site on the east pacific rise we focused our work on the bio 9 complex situated at 9 50 n where there has been an extensive measurement and sampling program since 1991 this site is located along the eruptive fissure of the 1991 1992 event and the site of the 1995 earthquake swarm we made direct measurements of advective heat output at several individual vents and at one site of diffuse flow tica although these data do not describe the complete heat flux picture at this vent field the data yield a total hydrothermal heat output of 325 160 mw with 42 21 mw coming from high temperature vents along this 2 km segment of ridge this result assumes a diffuse flux similar to that measured at tica occurs at each high temperature vent site our initial measurements thus suggest that the heat output of the low temperature diffuse venting is approximately 10 times that of the high temperature vents but may also be one or two orders of magnitude greater

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

influence of enhanced melt supply on upper crustal structure at a mid ocean ridge discontinuity a three dimensional seismic tomographic study of 9 n east pacific rise we present a three dimensional upper crustal model of the 9degrees03 n overlapping spreading center osc on the east pacific rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels our p wave velocity model obtained from tomographic inversion of similar to 70 000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs extrusive volcanic rocks above the western melt sill are inferred to be thin similar to 250 m more extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill the extrusive layer inferred above the eastern melt sill thickens from similar to 350 at the neovolcanic axis to 550 m to the west of the melt sill volcanic construction is likely to be significant in the formation of ridge crest morphology at the osc particularly at the tip of the eastern limb on the basis of our interpretation of the velocity model we propose that enhanced magma supply at crustal levels at the osc may provide an effective mechanism for the migration of ridge discontinuities this dynamic magma supply model may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs

sea surface salinity structure of the meandering gulf stream revealed by smos sensor measurements from the soil moisture ocean salinity smos satellite acquired during 2012 in the western north atlantic are used to reveal the evolution of the sea surface salinity sss structure of the meandering gulf stream with an unprecedented space and time resolution combined with in situ surface and profile measurements satellite derived surface currents sea surface height ssh surface temperature sst and chlorophyll chl data smos sss observations are shown to coherently delineate meanders pinching off from the current to form well identified salty warm and fresh cold core gulf stream rings a covariance analysis at two locations along the separated gulf stream path south of nova scotia and in the gulf stream extension reveals a systematically higher correlation between sss and sea level variability than between sst and ssh during the warmer half of the year within 75 w 40 w 30 n 50 n chl concentration is also found to significantly depend on the sss as sst increases above 20 c

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

two dimensional finite difference model of seafloor compliance summary seafloor compliance is the measure of seafloor deformation under a pressure signal our new 2 d finite difference compliance modelling algorithm presents several advantages over the existing compliance models including the ability to handle any gridded subsurface structure with no limitations on the gradients of the material properties as well as significantly improved performance applying this method to some of the problems inaccessible to previously existing methods demonstrates that lateral variations in subsurface structure must be accounted for to adequately interpret compliance data in areas with significant lateral variations the utilization of 1 d modelling and inversion is likely to result in high interpretation errors even when additional subsurface structure information is available we find that flattened pure melt bodies have a significantly higher compliance than cylindrical melt bodies with the same cross sectional area the compliance created by such bodies often has side peaks over their edges which are as strong as or stronger than the central peak requiring a series of measurements to best constrain their size and shear velocity finally we find that the compliance data are far and away most sensitive to the broad thick lower crustal partial melt zone our simple data fitting model for the compliance measurements on the east pacific rise at 9 48 n required shear velocities as low as 700 m s 1 in the centre of this zone far below the values previously estimated using 1 d model based inversions suggesting higher melt percentages than those previously estimated while small melt bodies in the upper part of the crust were found to have little or no effect on the measured compliance

volcanic eruptions on mid ocean ridges new evidence from the superfast spreading east pacific rise 17 19 s uniform sediment cover were recovered from lava that buries older faulted terrain the boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector consistent with magmas from two separate reservoirs being erupted in the same event chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments a comparison of our results to global data on submarine mid ocean ridge eruptions suggests consistent dependencies of erupted volume activated fissure lengths and chemical heterogeneity with spreading rate consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths index terms 3035 marine geology and geophysics midocean ridge processes 8414 volcanology eruption mechanisms 8439 volcanology physics and chemistry of magma bodies 3655 mineralogy and petrology major element composition keywords lava flow chemical heterogeneity erupted volume lava morphology side scan sonar

faulting and hydration of the upper crust of the sw okinawa trough during continental rifting evidence from seafloor compliance inversion the elastic response of seafloor to ocean gravity wave loading or seafloor compliance provides a constraint on the elastic properties of the crust we measured seafloor compliance at three ocean bottom seismometer obs sites around taiwan two in the southwestern sw okinawa trough and one on the ryukyu arc and performed inversion for crustal structures beneath them models best fitting the data demonstrate a decrease in upper crustal shear velocity and an increase in the compressional shear velocity ratio from the arc site to the trough sites with increasing amount of back arc extension this variation suggests that the upper continental crust is highly faulted and hydrated during rifting of the eurasian lithosphere

seasonal evolution of upper ocean horizontal structure and the remnant mixed layer 1 we discuss the seasonal evolution of upper ocean thermohaline structure at small horizontal scales the upper 350 m of a 1000 km long section in the subtropical north pacific was observed in winter spring and summer with 3 14 km horizontal resolution four vertical regions had distinct density and salinity structure the mixed layer remnant mixed layer high stratification layer and permanent thermocline the remnant mixed layer consists of water from the winter mixed layer left over after restratification the remnant mixed layer was most similar to the mixed layer in winter and spring and most similar to the high stratification layer below in summer the high stratification layer had elevated stratification that varied seasonally the permanent thermocline varied little seasonally and was horizontally and vertically uniform in comparison in all seasons density ratios showed that mixed layer s differences tended to compensate in density with the strongest tendency toward compensation in winter density ratios were temperature dominated in the remnant mixed layer consistent with salt fingering salinity anomalies were largest at the surface and decayed with depth in all seasons spectra of isopycnal depth and s anomalies along isopycnals are compared between the three seasons and four vertical layers isopycnal depth variance at 30 46 km wavelengths decreased from winter to spring to summer by a factor of 2 10 in stratified regions by treating salinity anomalies as a tracer the effective isopycnal diffusivity in the remnant mixed layer was estimated to be 1 4 m2 s 1 over 30 46 km wavelengths

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

network of off axis melt bodies at the east pacific rise at faster spreading mid ocean ridges the creation of new oceanic crust through magmatism usually occurs within a narrow zone on the ridge axis three dimensional seismic images of the fast spreading east pacific rise reveal a network of magmatic bodies 4 8 km away from the ridge axis that seem to be connected to the axial magma chamber

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

modes of crustal accretion and their implications for hydrothermal circulation hydrothermal convection at mid ocean ridges links the ocean s long term chemical evolution to solid earth processes forms hydrothermal ore deposits and sustains the unique chemosynthetic vent fauna yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained here based on coupled models of crustal accretion and hydrothermal circulation we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast spreading ridges we integrate numerical models with observations of melt lens depth thermal structure and melt fraction models matching all these observations always require a deep crustal scale hydrothermal flow component and less than 50 of the lower crust crystallizing in situ

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

variations in bone collagen δ13c and δ15n values of fauna from northwest europe over the last 40 000 years we report here a signal in the temporal variation of stable isotopes in protein from surviving animal bone in northwest europe over the past glacial cycle there is a change in the average 13c values of fauna in the holocene and there is also a significant reduction in 15n values of herbivore bone collagen towards the end of the last glaciation with a subsequent recovery soon after the start of the holocene this change is observed for several species and is restricted to those regions most affected by the glacial advance comparison with ice core data shows that there is a strong correlation between the average 13c values of three herbivore species and ice core co2 concentration the data presented here show how readily available faunal bone collagen 13c and 15n measurements provide a record of past climate and environmental change on a regional to continental scale

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

stable carbon and nitrogen isotope analysis on human remains from the early mesolithic site of la vergne charente maritime france we report here the results of stable carbon and nitrogen isotope analysis of human and faunal remains from la vergne charente maritime western france a rare early mesolithic burial site ca 8500e8000 cal bc the results for nine humans average d13c 19 3 d15n 9 4 indicate a strongly terrestrial diet dominated by animal protein with the possibility of at best a slight contribution of marinederived protein given lower sea levels in the early holocene the site would have been some 60e80 km from the sea at the time of its use nevertheless contacts with the coast are shown by the presence of numerous marine shell beads in the graves in the light of the stable isotope results it is suggested here that such contacts most likely took the form of exchange with coastal communities whose remains now lie underwater 2007 elsevier ltd all rights reserved

new insights into mid ocean ridge volcanic processes from the 2005 2006 eruption of the east pacific rise 9 46 n 9 56 n digital seafloor imagery collected on 37 camera tows and alvin dives in which we identify 186 contacts between new and old lava are used to create the most detailed map of a mid ocean ridge mor eruption to date lava flows erupted in 2005 2006 at the east pacific rise epr covered an area of 14 6 km2 along 18 km of the epr crest between 9 46 and 9 56 n the 2005 2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts numerous lava channels 10 50 m wide and 1 5 m deep trending approximately east west served as distributory pathways eruptions were sourced from fissures within the epr axial summit trough as well as fissures located on an off axis fissure mound 600 m east of the epr axis between 9 52 and 9 56 n portions of the lava flow reached as far as 2 km east of the axis near 9 51 2 n using a conservative estimate of 1 5 m for the average flow thickness implies that the 2005 2006 eruptions produced 22 106 m3 of lava 4 5 times larger than estimated volumes of 1991 1992 epr lava flows estimated lava volume for the 2005 2006 eruptions represents 15 of the magma available in the axial magma chamber

mesoscale flow and thermohaline structure around fieberling seamount the circulation and thermohaline structure around fieberling and the three neighboring seamounts to the ese are examined on the basis of high resolution observations in august 1989 in the upper layers the primary interaction is with well defined intrusions of water of subtropical and subarctic origin linked to the outer boundary of a southward meandering california current the effects of flow topography interaction are complex and vary with depth at the level of fieberling s top about 450 m the impinging subtropical flow is deflected anticyclonically and several eddies are generated uplift of isopycnals suggestive of upwelling is observed above the tops of all four seamounts the height of the upwelling cones varies between 120 and 240 m depending upon background stratification and their axes are sometimes tilted apparently by shear of the large scale flow strong subsurface jets 10 20 km wide and 500 1000 m thick with speeds up to 30 cm s have been found in the vicinity of the seamounts in the lower layers the primary interaction is with large scale flow from the southeast the flow is deflected anticyclonically around the seamount group as a whole with individual branches of this flow passing through the larger gaps between seamounts the observations are compared with simple models of subinertial flow past individual and multiple seamounts though suggestive in predicting some of the observed features the models need further development based on less restrictive assumptions on the impinging mean flow and the ambient density stratification

a three dimensional study of a crustal low velocity region beneath the 9 03 n overlapping spreading center overlapping spreading centers oscs play a key role in models of magma distribution at fast spreading ridges to investigate the relationship between ridge axis discontinuities and magma supply we conducted a three dimensional seismic reflection and tomography experiment at the 9 03 n osc along the east pacific rise tomographic analysis imaged a broad mid crustal low velocity zone lvz beneath parts of the overlapper and the associated overlap basin demonstrating that it is magmatically robust the complementary datasets reveal a complex storage and tapping of melt the lvz and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards the subsequent focussing of the lvz and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip the osc western limb is less magmatically robust and may be in the process of dying

anelasticity and viscosity of partially molten rock analogue toward seismic detection of small quantities of melt 1 anelasticity and diffusion creep viscosity of partially molten rock analogue were measured experimentally at various melt fractions 0 0025 0 04 the presence of even a small quantity of melt phase causes a significant increase in attenuation and dispersion and decrease in viscosity melt effect similar changes are additionally caused by the presence of a secondary solid component chemical effect the similitude that was observed in the anelasticity of melt free systems when scaled by the maxwell frequency for temperature and grain size effects was observed too in that of melt bearing systems when scaled by the maxwell frequency for melt and chemical effects the combined melt and chemical effects on anelasticity offer a potential means of identifying small quantities of melt with seismic velocity perturbations

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

spatial distribution and temporal evolution of crustal melt distribution beneath the east pacific rise at 9 10 n inferred from 3 d seafloor compliance modeling determining the melt distribution in oceanic crust at mid ocean ridges is critical to understanding how magma is transported and emplaced in the crust seafloor compliance deformation under ocean wave forcing is primarily sensitive to regions of low shear velocity in the crust making it a useful tool to probe melt distribution analysis of compliance data collected at east pacific rise between 9 and 10 n through 3 d numerical modeling reveals strong along axis variations in the lower crustal shear velocities as well as temporal variation of crustal shear velocity near 9 48 n between measurements spanning 8 years compliance measured across the rise axis at 9 48 n and 9 33 n suggest a deep crustal low velocity zone beneath the ridge axis with a low vs vp ratio consistent with melt in low aspect ratio cracks or sills changes in compliance measured at 9 48 n between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval perhaps following the 2005 2006 seafloor eruption this temporal variability provides direct evidence for short term variations of the magmatic system at a fast spreading ridge

smos salinity in the subtropical north atlantic salinity maximum 2 two dimensional horizontal thermohaline variability the horizontal thermohaline seasonal variability of the surface ocean is investigated in the subtropical north atlantic surface salinity maximum sssmax region satellite sea surface temperature and salinity are used along with high resolution thermosalinograph data and argo interpolated products to study the horizontal two dimensional field of density and thermohaline variability during late winter compensated temperature and salinity gradients at large and mesoscale are observed northeast of the sssmax in the azores front current region in spite of the large and sharp surface thermohaline fronts satellite measurements reveal a rather weak surface horizontal density gradient during summer the front is dominated by salinity gradients south of the sssmax at large scales the density ratio is controlled by the salinity gradient and the horizontal density gradient is enhanced by a constructive contribution of opposite salinity and temperature gradients

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

epr microplates a model for the oman ophiolite structural mapping in the southern half of the oman ophiolite has revealed a palaeoridge organization with similarities to an epr microplate forming in superfast spreading conditions a nw trending propagator was rapidly opening in a lithosphere no more than 1 myr older and itself created in a ne sw ridge system the nw trending propagator underlined by small mantle diapirs was active or dying when detached as part of a future ophiolite local thrusting of the future ophiolite was also initiated very early between 1 and 5 myr after ridge accretion it was accompanied by a 40 rotation within this time lapse such an activity supports the comparison of the oman palaeoridge system with an oceanic microplate and provides evidence to suggest the existence of thrusts at active microplates

on the formation of barrier layers and associated vertical temperature inversions a focus on the northwestern tropical atlantic 1 a unique barrier layer bl system in terms of persistence extension and associated subsurface temperature maximum is present seasonally in the northwestern tropical atlantic based on climatological output of a general circulation ocean model we show here that its development consists of two phases in summer the bl is relatively shallow and thin but subsurface temperature maxima are intense the latter develop as a result of the specific seasonality of the freshwater discharge in this area which limits the mixed layer to a very thin depth while the intense radiative heat flux penetrates significantly below thereby heating the subsurface waters protected from air sea interactions and inducing a barrier layer between the mixed layer and the ocean interior in winter the bl development is due to a surface decrease in salinity associated with the surface freshwater capping which decouples the pycnocline and hence the winter mixed layer from the thermocline the mechanism is ubiquitous in the sense that it is very similar to that of other areas at the same latitude as well as at high latitudes in regions of seasonal surface freshening results are discussed in the light of a simple linear equation linking the bl development to time evolution of sea surface temperature and sea surface salinity stratification

observational evidence of winter spice injection abstract temperature and salinity t s profiles from the global array of argo floats support the existence of spice formation regions in the subtropics of each ocean basin where large destabilizing vertical salinity gradients coincide with weak stratification in winter in these characteristic regions convective boundary layer mixing generates a strongly density compensated sdc layer at the base of the well mixed layer the degree of density compensation of the t s gradients of an upper ocean water column is quantified using a bulk vertical turner angle tub between the surface and upper pycnocline the winter generation of the sdc layer in spice formation zones is clearly seen in argo data as a large amplitude seasonal cycle of tub in regions of the subtropical oceans characterized by high mean tub in formation regions argo floats provide ample evidence of large abrupt spice injection t s increase on subducted isopycnals due to vertical mixing associated with the winter increase in tub a simp

distribution of magma beneath the east pacific rise near the 9 03 n overlapping spreading center from forward modeling of common depth point data we have reprocessed six cross axis and three along axis common depth point cdp profiles near the 9 03 n overlapping spreading center osc to understand the relationship between axial magma chamber amc width and seafloor morphology travel time modeling of the amc reflector reveals an asymmetric distribution of melt across the 9 03 n osc the variation of modeled amc width beneath either osc limb is minimal but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4 15 km near the 9 17 n ridge axis discontinuity additionally melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis neovolcanic zone but is displaced toward the western rise flank depth migration based on a continuum velocity model consistent with postcritical reflections from the base of layer 2a places the skewed amc reflector beneath a nearly constant thickness sheeted dike section which dips 10 away from the rise axis to confirm amc continuity beneath the western rise flank we use the maslov synthetic seismogram method to show that amplitude enhancement of the amc reflector is consistent with a continuous melt body underlying a thickening extrusive layer analysis of along strike cdp profiles indicates an amc which is neither overlapping nor discontinuous when projected onto the along strike plane identifying intracrustal events on along axis cdp lines however requires extreme caution we have modeled out of plane scattering using a kirchhoff formulation and we show that a coherent event identified beneath the overlap basin results from diffraction off the amc which lies nearly 3 km to the west of the profile we attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust in this model melt ascends upward buoyancy forces until deflected by the impermeable sheeted dike complex melt then migrates updip beneath the base of the sheeted dikes toward the neovplcanic zone where fissuring produces a temporary conduit for emplacement discrete jumps in modeled amc width toward the overlap basin represent a further displacement defocusing of melt supply western amc edge relative to the neovolcanic zone eastern amc edge the asymmetric pattern of melt therefore represents a gradual en echelon accommodation of melt supply across the 9 km of ridge axis offset at 9 03 n thus for asymmetric configurations amc width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone here we present a new model for osc development which invokes a significant component of cross axis melt migration moreover abrupt changes in amc width near ridge axis discontinuities e g 9 17 n deviation in axial linearity suggest that any along axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models the transition of melt supply beneath the overlap basin might favor a continuous low velocity zone underlying this feature if true basin development may be related to the subsidence of a mechanically weak crustal lid the proposed model for osc development therefore views ridge axis discontinuities as the surficial response of misalignment and or defocusing of melt supply in the uppermost mantle

seafloor deformation and forecasts of the april 2011 eruption at axial seamount the axial seamount submarine volcano exhibits an inflation deflation cycle comparable to similar volcanoes on land measurements of ocean bottom pressure document the entire inflation deflation cycle between eruptions at axial seamount in 1998 and 2011 and imply that the timing of submarine eruptions could be more predictable than that of their subaerial counterparts

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

shear wave structure from joint analysis of seismic and seafloor compliance data summary determining shear wave structure is the key to identifying the amount and location of fluid within the crust seismic and seafloor compliance methods provide independent estimates of shear wave structure and a joint analysis of the two data sets should provide better constraints on the properties of the uppermost oceanic crust we consider an example from 9 33 n on the east pacific rise seismic data from an on axis expanding spread profile have been reanalysed to determine a shear wave structure for layer 2b ps arrivals require a high shear wave velocity within this layer poisson ratio in the range 0 22 0 25 compliance data from the same location are seemingly inconsistent with this result requiring that layer 2b is a region of low shear wave velocity poisson ratio in the range 0 33 0 44 the quantitative differences between the two results can be explained by anelasticity and anisotropy conversely a knowledge of this discrepancy can be used to constrain the attenuation structure

vertical deformation monitoring at axial seamount since its 1998 eruption using deep sea pressure sensors pressure measurements made on the seafloor at depths between 1500 and 1700 m at axial seamount an active submarine volcano on the juan de fuca ridge in the northeast pacific ocean show evidence that it has been inflating since its 1998 eruption data from continuously recording bottom pressure sensors at the center of axial s caldera suggest that the rate of inflation was highest in the months right after the eruption 20 cm month and has since declined to a steady rate of 15 cm year independent campaign style pressure measurements made each year since 2000 at an array of seafloor benchmarks with a mobile pressure recorder mounted on a remotely operated vehicle also indicate uplift is occurring in the caldera at a rate up to 22f1 3 cm year relative to a point outside the caldera the repeatability of the campaign style pressure measurements progressively improved each year from f15 cm in 2000 to f0 9 cm in 2004 as errors were eliminated and the technique was refined assuming that the uplift has been continuous since the 1998 eruption these observations suggest that the center of the caldera has re inflated about 1 5f0 1 m thus recovering almost 50 of the 3 2 m of subsidence that was measured during the 1998 eruption this rate of inflation can be used to calculate a magma supply rate of 1410 6 m 3 year if this rate of inflation continues it also suggests a recurrence interval of 16 years between eruptions at axial assuming that it will be ready to erupt again when it has re inflated to 1998 levels d 2005 elsevier b v all rights reserved

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

architecture of on and off axis magma bodies at epr 9 37 40 n and implications for oceanic crustal accretion abstract crustal accretion at fast spreading mid ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis however there is increasing evidence for off axis magmatism occurring beyond this narrow zone here we present 3d multichannel seismic mcs images from the east pacific rise 9 37 40 n extending to 11 km on the ridge flanks in the axial region two offset axial magma bodies underlie a small ridge axis discontinuity at 9 37 n displaying an overlapping geometry similar to that of the seafloor structures above on the ridge flanks a series of off axis magma lenses oaml are imaged they are located 2 10 km from the ridge axis at 700 to 1520 ms two way travel time below seafloor bsf 1 6 to 4 5 km bsf with variable areas ranging from 0 5 km2 to 5 2 km2 the largest body is centered 4 km east of the ridge axis and is composed of a large continuous flat topped lens and a series of small discontinuous westward dipping bodies along its western edge the flat crest of the oaml lies at approximately the same depth beneath layer 2a as the axial magma lens and we infer that this oaml has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section a cluster of reflections underlying the oaml at 1260 1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens this largest oaml is associated with moho travel time anomalies of 120 260 ms within a zone that extends up to 2 km from the edge of the oaml suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8 18 and or thicker than normal by up to 1 km local volcanic edifices are found above two of the three oamls imaged in our study area and are inferred to be the eruptive products of the oamls from the volume of these edifices and the moho travel time anomalies we estimate the potential contribution of off axis magmatism to the total volume of the crust to be 0 01 3 the oamls imaged in our study area are present over roughly the same distance range as the zone of formation of near axis seamounts we speculate that oamls and the volcanic edifices found above them are small scale manifestations of the off axis magmatism that gives rise to near axis seamounts

ocean eddy freshwater flux convergence into the north atlantic subtropics for a quasi steady state condition the water vapor flux from the ocean to atmosphere typical of the salty subtropics must be compensated by ocean processes that transfer freshwater into the evaporative regime observations of the north atlantic subtropical sea surface salinity maximum region frequently reveal the presence of eddies with distinct salinity temperature signatures of up to 0 2 psu 1 c with horizontal scales of up to 200 km using the surface layer salinity and meridional velocity from the simple ocean data assimilation soda reanalysis data we find that the eddy flux can accomplish 50 to 75 of the required freshwater convergence into the subtropical regime the rest being delivered by ekman transport convergence and therefore represents a significant component of the marine hydrological cycle interannual fluctuations of the eddy freshwater flux are reflected in sea surface salinity variability

variations in axial magma lens properties along the east pacific rise 9 30 n 10 00 n from swath 3 d seismic imaging and 1 d waveform inversion we use three dimensional multistreamer seismic reflection data to investigate variations in axial magma lens aml physical properties along the east pacific rise between 9 30 n and 10 00 n using partial offset stacks of p and s converted waves reflecting off the top of the aml we image four 2 4 km long melt rich sections spaced 5 10 km from each other one dimensional waveform inversion indicates that the aml in a melt rich section is best modeled with a low vp 2 95 3 23 km s and vs 0 3 1 5 km s indicating 70 melt fraction in contrast the aml in a melt poor section requires higher vp 4 52 4 82 km s and vs 2 0 3 0 km s which indicates 40 melt fraction the thicknesses of the aml are constrained to be 8 32 m and 8 120 m at the melt rich and poor sites respectively based on the aml melt mush segmentation imaged in the area around the 2005 2006 eruption we infer that the main source of this eruption was a 5 km long section of the aml between 9 48 n and 51 n the eruption drained most of the melt in this section of the aml leaving behind a large fraction of connected crystals we estimate that during the 2005 2006 eruption a total magma volume of 9 83 106 m3 was extracted from the aml with a maximum of 71 106 m3 left unerupted in the crust as dikes from this we conclude that an eruption of similar dimensions to the 2005 2006 one would be needed with a frequency of years to decades in order to sustain the long term average seafloor spreading rate at this location

distribution of melt along the east pacific rise from 9 30 to 10 n from an amplitude variation with angle of incidence ava technique author posting the author s 2015 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 203 2015 1 21 doi 10 1093 gji ggv251

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

new cnes cls09 global mean dynamic topography computed from the combination of grace data altimetry and in situ measurements 1 an accurate knowledge of the ocean mean dynamic topography mdt is mandatory for the optimal use of altimetric data including their assimilation into operational ocean forecasting systems a new global 1 4 resolution mdt was computed for the 1993 1999 time period with improved data and methodology compared to the previous rio05 mdt field first a large scale mdt is obtained from the cls01 altimetric mean sea surface and a recent geoid model computed from 4 5 years of grace gravity recovery and climate experiment data altimetric sea level anomalies and in situ measurements are then combined to compute synthetic estimates of the mdt and the corresponding mean currents while the rio05 mdt was based on 10 years of in situ dynamic heights and drifting buoy velocities the new field benefits from an enlarged data set of in situ measurements ranging from 1993 to 2008 and includes all hydrological profiles from the argo array moreover the processing of the in situ data has been updated a new ekman model was developed to extract the geostrophic velocity component from the drifting buoy measurements the handling of hydrologic measurements has also been revisited compared to the previous rio05 solution the new global mdt resolves much stronger gradients in western boundary currents with mean velocities being doubled in some places moreover in comparison to several other recent mdt estimates we find that the new cnes cls09 mdt is in better agreement with independent in situ observations

isotope evidence for the use of marine resources in the eastern iberian mesolithic there are relatively few coastal mesolithic sites in the iberian mediterranean region probably due to a number of factors including sea level changes and the disappearance of sites due to agriculture and urbanisation however recent excavations have uncovered inland sites that have marine faunal remains i e molluscs and fish and lithics from the coastal area which both indicate interactions between the coast and the upland valleys these inland sites are located at a distance of 30 50 km from today s coastline and are at altitudes higher than 1000 m we report on additional information on the links between the coast and these inland sites through the use of dietary isotope analysis carbon and nitrogen stable isotope analysis of collagen extracted from human and faunal remains at the sites of coves de santa maira penya del comptador and cingle del mas nou the results indicate that mesolithic diet in this region was largely based on c3 terrestrial resources but there was measurable evidence of low level consumption of marine resources at both coastal and inland sites

evidence and implications of crustal magmatism on the flanks of the east pacific rise article i nfo article history formation of oceanic crust along the east pacific rise is thought to be complete within a few kilometers of the rise axis here however we present evidence for magmatism 20 km from the spreading center in 300 ka old crust seismic data reveal an intrusive complex 2 km beneath the seafloor which is limited in lateral extent b5 km and comprises a melt lens underlain by low velocity high attenuation crust the crustal magma body overlies a region of off axis delivery of mantle melt that results from a long lived skew between the axes of mantle upwelling and plate spreading magmatism on the flanks of the epr may be commonplace thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off axis hydrothermal activity

mechanisms of differentiation in the skaergaard intrusion thanks to its magnificent exposures and extraordinary petrologic development the skaergaard intrusion provides an excellent test for three basic mechanisms of crystal liquid fractionation diffusive exchange compaction convective exchange 1 diffusive exchange at the solidification front does not seem to have played a major role for no obvious difference is seen in the behavior of components of widely differing diffusivities the effects of diffusion were probably masked by porous flow of the interstitial liquid 2 compaction and 9filter pressing9 of residual liquids can be tested by comparing concentrations of incompatible elements in parts of the intrusion that crystallized in different orientations with respect to gravity the contrast between the large concentrations of these elements in the upper border series and consistently smaller abundances in the layered series long thought to be due to contamination of the upper levels with crustal rocks is more likely a result of compaction that took place at the floor but not under the roof 3 convective exchange driven by differences in the compositional densities of the main magma and interstitial liquids seems to have been important at the steep walls and to a lesser degree under the roof it may also have been responsible for a large compositional anomaly near the centre of the layered series where an iron rich liquid percolated down through parts of lower and middle zones at later stages of crystallization when density relations were reversed a buoyant liquid rich in volatiles and incompatible elements rose from the layered series and permeated the upper part of the intrusion a liquid of this latter kind could have caused the pervasive metasomatic alteration seen in much of the layered series in both compaction and convective exchange re equilibration and reactions between migrating liquids and early formed crystals augmented permeabilities and brought about extensive compositional and textural changes although inhomogeneities in the initial magma could account for some of the compositional variations the main trend of differentiation can be ascribed to a combination of compaction and convective fractionation the convective flow was of two kinds one of descending iron rich liquids and another of more evolved rising liquids and fluids with low densities and large concentrations of incompatible elements in the early stages the proportions of iron rich liquids were much greater than those of the silica rich variety and until the crystallization fronts converged at the sandwich horizon the former made the greatest contribution to evolution of the main body of magma as crystallization advanced however light silica rich liquids became more important they permeated the upper border series and much of the upper part of the layered series this late stage liquid continued to differentiate after the crystallization fronts met a secondary sandwich horizon was formed where incompatible elements reach maximum concentrations in the lower part of the upper border series

bone as a stable isotope archive for local climatic information this brief review outlines the contribution that the study of stable isotope composition in bone can make to palaeoclimatic investigation with the focus almost entirely restricted to the last 50 000 years in nw europe bone can provide a useful archive of the prevailing isotopic condition and represents a quite different and often less specialised sampling of the environment than most other archives on the other hand chronological sequences and dating generally can be a problem and the link between the isotopic value registered in the bone and the environmental conditions which gave rise to it is both complex and not fully understood carbon oxygen and nitrogen isotopes are all available from bone nitrogen only where sufficient protein collagen survives and are all subject in different ways to climatic influences such as temperature rainfall changes in floristic composition and soil chemistry these are all briefly discussed and the datasets that are being published are considered in the context of the environmental information they provide undoubtedly environmental signals are recoverable but their interpretation is still primitive a dataset for carbon and nitrogen isotope composition of herbivores that spans the last 50 000 years is shown together with some of the issues it raises

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n