upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

distribution of anthropogenic co2 in the pacific ocean 1 this work presents an estimate of anthropogenic co2 in the pacific ocean based on measurements from the woce jgofs oaces global co2 survey these estimates used a modified version of the c technique modifications include a revised preformed alkalinity term a correction for denitrification and an evaluation of the disequilibrium terms using an optimum multiparameter analysis the total anthropogenic co2 inventory over an area from 120 e to 70 w and 70 s to 65 n excluding the south china sea the yellow sea the japan east sea and the sea of okhotsk was 44 5 5 pg c in 1994 approximately 28 pg c was located in the southern hemisphere and 16 5 pg c was located north of the equator the deepest penetration of anthropogenic co2 is found at about 50 s the shallowest penetration is found just north of the equator very shallow anthropogenic co2 penetration is also generally observed in the high latitude southern ocean one exception to this is found in the far southwestern pacific where there is evidence of anthropogenic co2 in the northward moving bottom waters in the north pacific a strong zonal gradient is observed in the anthropogenic co2 penetration depth with the deepest penetration in the western pacific the pacific has the largest total inventory in all of the southern latitudes despite the fact that it generally has the lowest average inventory when normalized to a unit area the lack of deep and bottom water formation in the north pacific means that the north pacific inventories are smaller than the north atlantic

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

robust empirical relationships for estimating the carbonate system in the southern california current system and application to calcofi hydrographic cruise data 2005 2011 1 the california current system ccs is expected to experience the ecological impacts of ocean acidification oa earlier than most other ocean regions because coastal upwelling brings old co2 rich water relatively close to the surface ocean historical inorganic carbon measurements are scarce so the progression of oa in the ccs is unknown we used a multiple linear regression approach to generate empirical models using oxygen o2 temperature t salinity s and sigma theta as proxy variables to reconstruct ph carbonate saturation states carbonate ion concentration co32 dissolved inorganic carbon dic concentration and total alkalinity ta in the southern ccs the calibration data included high quality measurements of carbon oxygen and other hydrographic variables collected during a cruise from british columbia to baja california in may june 2007 all resulting empirical relationships were robust withr2values 0 92 and low root mean square errors estimated and measured carbon chemistry matched very well for independent data sets from the calcofi and imecocal programs reconstructed ccs ph and saturation states for 2005 2011 reveal a pronounced seasonal cycle and inter annual variability in the upper water column deeper in the water column conditions are stable throughout the annual cycle with perennially low ph and saturation states over sub decadal time scales these empirical models provide a valuable tool for reconstructing carbonate chemistry related to ocean acidification where direct observations are limited however progressive increases in anthropogenic co2 content of southern ccs water masses must be carefully addressed to apply the models over longer time scales

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

shear and compressional wave structure of the east pacific rise 9 10 n we use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9 10 n region of the east pacific rise modeling indicates that shear conversion occurs at the base of layer 2a producing ps energy that travels through oceanic crust as p and converts to s at the base of layer 2a on upgoing path and ps energy that travels as p through layer 2a and converts to s at the base of layer 2a on downgoing path arrivals the travel times of these arrivals require a shear wave velocity within layer 2a of 0 4 0 8 km s poisson s ratio of 0 46 0 49 waveform inversion was used to model the poisson s ratio structure of layers 2b and 3 for nine instruments with good shear arrivals poisson s ratio within layer 2b was highly variable with values as low as 0 24 and a mean value of 0 263 some of this variability might be due to lateral variability in layer 2a structure the mean poisson s ratio of layer 3 was 0 271 according to the cracking model of shaw 1994 the low poisson s ratios within the upper portion of layer 2b indicate that thick cracks aspect ratio 0 1 extend to depths of 1 5 1 7 km in this region two dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities with layer 2b and layer 3 velocities reduced by up to 11 from the regional values cracking theory suggests that these low velocities could be caused by porosities of 0 3 5 5 depending on the crack aspect ratio a maximum porosity of 1 5 is predicted from our one poisson s ratio measurement in this area the anomalous velocities are located near the western discordant zone left by the 9 03 n overlapping spreading center we suggest that shearing associated with on axis rotation of the overlap basins is responsible for the low seismic velocities the pattern of anomalies indicates that faulting extends to distances of 10 15 km from the basins

poleward reach of the california undercurrent extension 1 the california undercurrent is known to transport relatively warm high salinity nutrient rich water from the equatorial pacific to vancouver island along the western continental slope of north america this transport helps maintain the high productivity of the eastern boundary california current system in this study we use several decades of water property survey data for the coasts of oregon washington british columbia and alaska to show that equatorial pacific water carried poleward by the undercurrent can eventually reach the aleutian islands roughly 11 000 km from the source region long term current meter records confirm the undercurrent as far north as vancouver island where the current is found to be weakest in spring but then to strengthen through the summer and fall before merging with the wind forced poleward flowing davidson current in winter the core depth of the equatorial water increases from 150 m 25 m off northwest washington near the northern end of the western north america coastal upwelling domain to 225 m 25 m off southeast alaska near the southern end of the gulf of alaska coastal downwelling domain

seasonal variations in the aragonite saturation state in the upper open ocean waters of the north pacific ocean seasonal variability of the aragonite saturation state ar in the upper 50 m and 100 m depths north pacific ocean npo was investigated using multiple linear regression mlr the mlr algorithm derived from a high quality carbon data set accurately predicted the ar of evaluation data sets three time series stations and p02 section with acceptable uncertainty 0 1 ar the algorithm was combined with seasonal climatology data and the estimated ar varied in the range of 0 4 0 6 in the midlatitude western npo with the largest variation found for the tropical eastern npo these marked variations were largely controlled by seasonal changes in vertical mixing and thermocline depth both of which determine the degree of entrainment of co2 rich corrosive waters from deeper depths our mlr based subsurface ar climatology is complementary to surface climatology based on pco2 measurements

velocity structure in upper ocean crust at hole 504b from vertical seismic profiles hole 504b provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust providing a baseline for comparison with seismic measurements elsewhere we determine p and s velocities from vertical seismic profiles vsps obtained on ocean drilling program odp legs 111 and 148 four issues are considered the location of the seismic layer 2 3 boundary p to s wave conversion by scattering transverse isotropy and poisson s ratio as an indicator of lithology porosity and structure 1 in the p velocity profile the change in slope marking the layer 2 3 boundary coincides with the top of the sheeted dike unit seismic layer 2 is composed of the extrusives and the lithologic transition zone the layer in which flows and dikes interfinger 2 even in these normal incident vsps several second arrivals with velocities indicative of vertically polarized shear energy are observed p to s wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis 3 vertical velocities determined from the vsp differ by 10 from horizontal velocities obtained from the oblique seismic experiment ose on deep sea drilling project dsdp leg 92 the p wave velocity structure is determined by small and intermediate 1 cm pore structure with no measurable anisotropy the large scale well oriented vertical fractures which are formed tectonically do not have a detectable effect on compressional wave velocities 4 high poisson s ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows low poisson s ratio at 850 1150 m below seafloor mbsf coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes relatively large aspect ratio cracks are required to produce such low values of poisson s ratio the cracks were likely created by hydraulic fracturing when hot dikes encountered low temperature seawater

tomographic image of the magma chamber at 12 50 n on the east pacific rise the east pacific rise at 12 50 n fig 1 is a fast spreading ridge with intense hydrothermal activity1 and ophiolite studies2and thermal modelling3 indicate that this is a likely setting for a magma chamber a recent seismic reflection experiment4 imaged the top of the magma chamber at this site at a depth of 1 4 km below the sea floor and found that it is continuous for tens of kilometres along the rise axis here we examine a large set of accurate p wave travel times from a detailed seismic refraction experiment at the same site5 the patterns observed in the travel times demonstrate that a zone of low seismic velocities exists beneath the rise axis throughout the region studied the best fitting two dimensional structure obtained from linear inversion of the travel times includes an axial low velocity zone magma chamber only 6 km wide in which velocities are depressed by more than 0 5 km s 1

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

isolation of source and receiver statics and scale factors in waveform inversion of marine refraction data in this paper i present a method that uses multiple ocean bottom instruments to separate variations in seismic amplitudes and travel times caused by near source topography and variable source amplitudes from similar variations related to the overall one dimensional seismic structure the results indicate that the standard correction for topography sediment thickness and source size can be improved with this method reducing uncertainties in the velocity solution

mesoscale biogeochemical responses to iron fertilization in the upper layers of the southern ocean iron experiment areas 1 during the southern ocean iron experiment sofex january february 2002 two iron fertilization experiments were conducted at the south 66 45 s 171 8 w and north 56 23 s 172 w patches the south patch was replete with all macronutrients whereas the north patch was nearly depleted of silicate using a towed water sampling measurement system high resolution three dimensional observations of temperature salinity co2 partial pressure pco2 and the concentrations of total co2 tco2 nitrate phosphate and silicate in the upper 100 m were obtained 10 and 28 days after initial iron additions in the south and north patches respectively tco2 and nutrient drawdowns observed in the south patch were small due to wind events that limited biological response in the early postfertilization period at the north patch drawdown signals were stronger resulting from the longer time since fertilization the north patch was dispersed along a frontal zone between two distinct waters with the denser subducted under the lighter subduction limited co2 uptake from the air but provided alternative mechanisms for vertical carbon export based on the biogeochemical drawdowns and published estimates of the rate of the entrainment of nonfertilized waters into the patch the mean net community productivity over 28 days was estimated to be 94 12 mmol carbon m 2 day 1 to which the air to sea flux contributed only 2 despite low silicate concentrations and potential limitation of growth of silicifying plankton in the north patch our estimates of the enhanced carbon uptake are similar to estimates from higher silicate environments citation hales b and t takahashi 2012 mesoscale biogeochemical responses to iron fertilization in the upper layers of the southern ocean iron experiment areas j geophys res 117 c01018 doi 10 1029 2011jc006956

new observations of the shallow seismic structure of young oceanic crust the results are presented of three new experiments carried out on the mid atlantic ridge mar near latitude 23 n using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust the data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography these 1 to 2 km long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200 300 m of the young igneous crust one experiment was carried out over the site of hole 648b of the ocean drilling program on a small volcano within the median valley of the mar the seafloor velocity was observed to be 2 1 km s 1 underlain by an approximately linear velocity gradient of 4 s 1 given that we know the crust at this location consists of fresh basalt lavas with laboratory measured velocities in excess of 5 8 km s 1 porosities at the seafloor of as high as 30 50 are inferred the two other experiments were located over 7 m y old crust near deep sea drilling project site 395 west of the mar separated by only 14 km laterally but by over 1400 m in water depth the experiment positioned in the 100 m thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable the velocity of the uppermost basaltic basement was 4 1 km s 1 and the velocity gradient was less than 0 5 s 1 this 2 km s 1 difference in velocity between the two sites is assumed to be a consequence of age related modifications to the physical properties of the crust if the primary change is in only the total porosity then a 15 20 reduction is required to explain these observations this seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data e g off axis volcanism secondary mineral deposition tectonic compression a combination of many processes acting in unison would be necessary to produce such a decrease in porosity if the geometry of the cracks and voids were modified in just the right way then no change in total porosity is required to explain the velocity difference these on bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor s geologic features they produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements

breaking new ground estimates of crack depth along the axial zone of the east pacific rise 9 12 54 n using simple fracture mechanics models the depths of fissures that were observed along the axial zone of the east pacific rise epr from 9 12 to 54 n with the deep towed argo z vehicle are estimated the widest cracks 4 8 m along this fast spreading portion of the epr are the deepest and are spatially correlated with the broadest youngest and most hydrothermally active portions of the ridge crest if the widest deepest cracks are not primarily eruptive they should be most abundant in areas of older lava flows having increased in width with time this is not observed some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at 9 45 52 n and may be deep enough to reach the sheeted dikes of layer 2b these cracks may have tapped melt during the eruption and facilitated the flux of vapor rich hydrothermal fluids through overlying lava flows the narrowest presumably shallowest cracks correspond to the narrowest oldest and least hydrothermally active portions of the ridge crest we interpret the wide deep cracks as primarily eruptive in origin and suggest that they may be associated with the inflation of an axial magma chamber whereas the narrow shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far field plate stresses

observations from a yakutat eddy in the northern gulf of alaska 1 the impingement of deep basin eddies on the shelf in the gulf of alaska has been implicated as an important mechanism for cross shelf exchange the influence of eddies on biological processes has been confirmed with data from the sea viewing wide field of view sensor showing elevated chlorophyll associated with eddies altimetry data suggest that an eddy formed in the winter of 2003 near yakutat alaska and propagated along the shelf break reaching the head of the gulf by spring this eddy was sampled during may and september of 2003 the eddy core water was warm salty and high in nitrate relative to basin water of the same density but was similar to historical water properties from the shelf near yakutat this suggests a shelf origin for the eddy core water

earthquake evidence for along arc extension in the mariana islands 1 analysis of data from a deployment of ocean bottom and land seismographs in 2003 2004 detected four swarms of earthquakes in the overriding plate of the mariana subduction system between the fore arc and the back arc spreading center two additional shallow swarms were identified by analyzing the teleseismic earthquake catalogs from 1967 to 2003 focal mechanism solutions for these swarms determined from regional waveform inversion for the 2003 2004 events or retrieved from the centroid moment tensor catalog for previous years suggest a complex system of deformation throughout the arc we observe arc parallel extension near volcanic cross chains arc perpendicular extension along the frontal arc and arc parallel compression farther into the back arc near the mariana trough a swarm beneath the middle and eastern summits of the diamante cross chain may have recorded magmatic activity volcanic cross chains showing evidence of adiabatic decompression melting from extensional upwelling are localized at regions of enhanced along strike extension the earthquake data are consistent with recent gps results indicating 12 mm a of extension between guam and agrihan the along arc extension may result from either increasing curvature of the mariana system with time or from deformation induced by oblique subduction in the northernmost and southernmost regions of the arc

contrast in crustal structure across the clipperton transform fault from travel time tomography a three dimensional 3 d seismic refraction study of the clipperton transform fault northern east pacific rise reveals anomalously low compressional velocities from the seafloor to the moho we attribute this low velocity anomaly to intensive brittle deformation caused by transpression across this active strike slip plate boundary the seismic velocity structure south of the clipperton transform appears unaffected by these tectonic forces but to the north seismic velocities are reduced over 10 km outside the zone of sheared seafloor this contrast in seismic velocity structure corresponds well with the differences in mid ocean ridge morphology across the clipperton transform we conclude that the amount of fracturing of the upper crust which largely controls seismic velocity variations is strongly dependent on the shallow temperature structure at the ridge axis intermittent supply of magma to the shallow crust north of the clipperton transform allows seawater to penetrate deeper and the cooler crust is brittle to a greater depth than south of the transform where a steady state magma lens is known to exist the crustal thickness averages 5 7 km only slightly thinner than normal for oceanic crust and variations in moho depth in excess of 0 3 km are not required by our data the absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal east pacific rise type crust perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies

some characteristics of yakutat eddies propagating along the continental slope of the northern gulf of alaska abstract data from array for real time geostrophic oceanography argo profiling floats oil tanker thermosalinographs shipboard adcp and towed ctd surveys and satellite altimetry are used to examine properties of two 200 km diameter anticyclonic yakutat eddies that propagated westward at 1 5 km day 1 along the continental slope of the northern gulf of alaska goa in 2001 and 2003 the eddies had lifetimes of up to 5 years remained close to the shelfbreak and had relatively constant size and strength until they encountered the alaskan stream where they appeared to spawn smaller shorter lived anticyclones the azimuthal velocity field was vigorous 25 40 cm s 1 and in gradient wind balance with rossby numbers of 0 05 conservation of salt and azimuthal mass transports between 20 m and the depth of the 32 2 isohaline from shipboard surveys in may and august 2003 suggest little mass exchange occurred between the surface layers of the eddy and ambient waters chlorophyll concentrations were greater in the eddy than in ambient waters in both may and august in may the chlorophyll was patchily distributed while in august dense chlorophyll concentrations occurred in and beneath the seasonal thermocline within 50 km of the eddy center the high august chlorophyll concentrations might have been fostered by a broad and shallow it appears likely that as yakutat eddies approach the shelfbreak non linear processes will modify the slope flow field and the stability and structure of the shelfbreak front leading to cross slope flows and flow reversals this interaction may induce 30 km wide streamers of shelf water to flow around the trailing edge of the anticyclone the role of streamers in the freshwater and nutrient budgets of the goa shelf and basin is unknown but their contribution to these budgets will depend on the trajectory of a yakutat eddy especially its proximity to the shelfbreak as the eddy propagates along the goa continental slope

the importance of freshwater to spatial variability of aragonite saturation state in the gulf of alaska high latitude and subpolar regions like the gulf of alaska goa are more vulnerable than equatorial regions to rising carbon dioxide co2 levels in part due to local processes that amplify the global signal recent field observations have shown that the shelf of the goa is currently experiencing seasonal corrosive events carbonate mineral saturation states 1 including suppressed in response to ocean acidification as well as local processes like increased low alkalinity glacial melt water discharge while the glacial discharge mainly influences the inner shelf on the outer shelf upwelling brings corrosive waters from the deep goa in this work we develop a high resolution model for carbon dynamics in the goa identify regions of high variability of and test the sensitivity of those regions to changes in the chemistry of glacial melt water discharge results indicate the importance of this climatically sensitive and relatively unconstrained regional freshwater forcing for variability in the nearshore the increase was nearly linear at 0 002 per 100 mol kg increase in alkalinity in the freshwater runoff we find that the local winds biological processes and freshwater forcing all contribute to the spatial distribution of and identify which of these three is highly correlated to the variability in given that the timing and magnitude of these processes will likely change during the next few decades it is critical to elucidate the effect of local processes on the background ocean acidification signal using robust models such as the one described here

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

prediction of sea of japan east sea acidification over the past 40 years using a multiparameter regression model 1 a multiparameter linear regression model mlr of aragonite saturation state arg as a function of temperature pressure and o2 concentration in the upper 1 000 m of the sea of japan east sea was derived with an uncertainty of 0 020 1 the arg data n 1 482 used to derive the basin wide arg prediction model were collected during a field survey in 1999 and were corrected for anthropogenic co2 some biases were resolved by addition of a pressure and o2 concentration interaction term to the proposed model correlation between the two predictor terms caused by addition of this term was minimized by centering the data for the three variables thus subtracting the mean from each individual data point validation of the model against data sets obtained in 1992 and 2007 yielded correlation coefficients of 0 995 0 013 for 1992 n 64 p 0 001 and 0 995 0 009 for 2007 n 137 p 0 001 and root mean square errors of 0 064 for 1992 and 0 050 for 2007 the strong correlation between measurements and predictions suggests that the model can be used to estimate the distribution of arg in the sea of japan east sea including dynamic coastal waters on varying time scales when basic hydrographic data on temperature pressure and o2 concentration are available application of the model to past measurements for the sea of japan east sea indicated that interdecadal variability 2 from the mean in arg corrected for anthropogenic co2 was generally high 0 1 0 7 in the upper water layer 200 m depth and decreased 0 05 0 2 with depth for waters deeper than 500 m the interdecadal variability is largely controlled by variations in the degree of water column ventilation superimposed on this natural variability the input of co2 derived from fossil fuels has markedly acidified the upper water layers during the anthropocene and thereby moved the aragonite saturation horizon upward by 50 250 m the impact of co2 derived from fossil fuels on upper ocean acidification will increase in the future the present study indicates that in combination with other easily measurable parameters a multifunctional model can be a powerful tool for predicting the temporal evolution of arg in the ocean including coastal waters that are highly likely to be susceptible to ocean acidification in the future

cracked media poisson s ratio and the structure of the upper oceanic crust summary theoretical models of water filled cracks within a solid are in general agreement and indicate that the poisson s ratio of the composite material will typically increase for thin cracks with aspect ratios less than a but will decrease for thick cracks with aspect ratios between and i near spherical pores cause little change in poisson s ratio this is true both for randomly oriented isotropic crack models and at certain directions for aligned crack models which predict seismic anisotropy observed anomalous values of poisson s ratio in oceanic layer 2 can often be explained using these models as an effect of cracks and do not necessarily require the presence of rocks of unusual composition in particular the low poisson s ratios reported by spudich orcutt 1980 and au clowes 1984 can approximately but not always exactly be fitted with a model consisting of thick cracks within higher poisson s ratio rock more typical of the upper oceanic crust however this interpretation is near the limit of existing theories of cracked material so other explanations for the anomalous observations must also be considered

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

effect of trench outer rise bending related faulting on seismic poisson s ratio and mantle anisotropy a case study offshore of southern central chile summary several trench outer rise settings in subduction zones worldwide are characterized by a high degree of alteration fracturing and hydration these processes are induced by bending related faulting in the upper part of the oceanic plate prior to its subduction mapping of p and s wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere and serves as a baseline for comparison with seismic measurements elsewhere active source seismic investigations at the outer rise off southern central chile 43 s were carried out in order to study the seismic structure of the oceanic nazca plate seismic wide angle data were used to derive 2 d velocity models of two seismic profiles located seaward of the trench axis on 14 5 ma old crust p01a approximately parallel to the direction of spreading and p03 approximately parallel to the spreading ridge and trench axes we determined p and s velocity models using 2 d traveltime tomography we found that the poisson s ratio in the upper crust layer 2 ranges between 0 33 at the top of the crust to 0 28 at the layer 2 3 interface while in the lowermost crust and uppermost mantle it reaches values of 0 26 and 0 29 respectively these features can be explained by an oceanic crust significantly weathered altered and fractured relative high poisson s ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization thus the seismic structure of the oceanic lithosphere at the southern central chile outer rise exhibits notable differences from the classic ophiolite seismic model normal oceanic crust these differences are primarily attributed to fracturing and hydration of the entire ocean crust which are direct consequences of strong bending related faulting at the outer rise on the other hand the comparison of the uppermost mantle p wave velocities at the crossing point between the perpendicular profiles 90 km oceanward from the trench axis reveals a low degree of pn anisotropy 2 per cent

lateral heterogeneity in the upper oceanic crust at deep sea drilling project site 504 the lateral variability of the upper oceanic crust can be studied by mapping the amplitude or power content of arrivals from a dense array of identical sources to a single receiver in this paper we present the results from a borehole seismic experiment at deep sea drilling project site 504 in the eastern equatorial pacific seismic amplitude anomalies of 20 db and more are present with a horizontal correlation length of 1 3 km finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry basement relief and lateral variations within basement a three dimensional model of the velocity structure around the borehole can be constructed the resultant model has horizontal gradients greater than 2 0 s 1 in the upper crust these gradients are comparable to the vertical gradients normally associated with oceanic crust assuming that velocity anomalies in the upper crust reflect changes in porosity the resultant structure provides constraints on the hydrothermal circulation at the site in addition the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver

physical processes that enhance nutrient transport and primary productivity in the coastal and open ocean of the subarctic ne pacific abstract in comparison to the open ocean several additional processes including coastal upwelling river discharge tidal mixing estuarine circulation and benthic remineralization enhance nutrient supply to the surface waters of the continental shelf in general coastal waters become nitrate limited during the phytoplankton growing season whereas iron and dissolved silicate limit phytoplankton growth in the less productive oceanic waters of the gulf of alaska if coastal processes supply ample amounts of macro and micro nutrients during the growing season diatom communities dominated by species such as skeletomema costatum chaetoceros spp and thalassiosira spp will bloom growth rates of these bloom populations typically range from 0 5 to 1 5 doublings per day which place a high demand on nutrients any transport of coastal waters away from the shelf will enhance productivity in oceanic waters the general circulation of the eastern subarctic pacific does not allow for offshore transport except in special circumstances these include anticyclonic mesoscale eddy formation which can export as much as 5000 km 3 of nutrient rich waters from the shelf in a single eddy and recirculation of waters away from southwestern alaska due to the cyclonic circulation around the western edge of the alaskan gyre recirculation can carry nutrient rich water from the coast to the vicinity of ocean station papa 50 n 145 w within a few months for both eddies and gyre recirculation much of the water being carried into the open ocean lies below the euphotic zone iron enrichment occurs to a depth of at least 1000 m in the gulf of alaska as a result periods of enhanced eddy formation or recirculation may supply iron that enriches the open ocean for several years damming of the columbia river and human uses of its waters have resulted in more winter and less summer discharge of fresh water and dissolved si coastal currents in this area flow north in winter and south in summer as a consequence there has been a net temporal change in si inputs to the ocean since the mid 1970s with about 4 10 9 mol more si flowing north in winter and 9 10 9 mol less si flowing south in summer compared with the early 1900s since silicate limits diatom growth and diatoms are responsible for most carbon export in phytoplankton implications for ocean productivity are significant

travel time analysis of borehole seismic data a method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity depth structure the technique which treats simultaneously data from any number of depths in the borehole in addition to surface data is based on the linear inversion scheme of dorman and jacobson 1981 given the parameters p t p px p and p t p px p the method solves for depth to specified slownesses assuming linear gradients in velocity between slowness values a practical limitation on the use of surface travel time measurements to resolve velocity depth structure is the necessity of assuming a surface velocity this is not necessary for the case of borehole data for a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity depth function which would be obtained from surface data alone the technique is applied to data from three borehole seismic experiments in oceanic crust the experiments were carried out in the western atlantic crustal age 110 m y the gulf of california crustal age 1 m y and the costa rica rift area crustal age 6 m y all three experiments show relatively high upper crustal velocities 4 0 km s suggesting that layer 2a is not present even in the very young crust all sites had over 100 m of sediment thickness and it is postulated that sediment thickness and sediment permeability not merely age govern the velocity of the upper crust by accelerating the cementation of fractures and cracks

crustal structure beneath the galapagos archipelago from ambient noise tomography and its implications for plume lithosphere interactions 1 to constrain the seismic velocity structure of the crust beneath the galapagos archipelago we conducted a tomographic study using high frequency rayleigh waves obtained from cross correlations of ambient noise we analyzed waves with periods between 5 and 8 5 s sensitive to shear wave velocity vs structure between about 3 and 10 km depth after accounting for the effect of water depth crustal velocities are up to 25 lower than those of very young crust at the east pacific rise and are comparable to those of hawaii we attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust on average vs between 3 and 10 km depth beneath the western archipelago is up to 15 lower than beneath the eastern archipelago we attribute the west to east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the galapagos plume the results of this study in combination with previous work indicate that many of the unusual aspects of the galapagos archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres our findings indicate that observed variations in the flexural response to loading observed in the galapagos cannot be explained by the current thermal state of the lithosphere instead the flexural response likely represents varying elastic strength at the time of loading we also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago the darwinian lineations may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume lithosphere interactions

seismic layer 2a variations in the lucky strike segment at the mid atlantic ridge from reflection measurements 1 the sismomar experiment carried out seismic measurements of the slow spreading lucky strike segment of the mid atlantic ridge located approximately 300 km south of the azores platform we present results from a segment scale reflection study of seismic layer 2a that is centered on the lucky strike volcano and covers the central 54 km of the 70 km long segment and extends 30 km on both sides of the ridge axis our study allows new conclusions about the role of tectonic magmatic and hydrothermal processes in shaping the upper crustal structure of the lucky strike segment the seismic reflection measurements show an almost constant layer 2a two way time within the median valley and an abrupt layer 2a two way time decrease off axis this two way time decrease is caused by a layer 2a velocity increase on the order of 1 km s the uniform two way time in the median valley results from a constant thickness layer 2a which could be caused by the existence of a porosity threshold linked to the lithologic lava dike boundary the regularity of the layer 2a 2b interface within the median valley indicates that layer 2a is built entirely inside the median valley the off axis velocity increase is consistent with hydrothermal alteration and the aging of the crust

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

tidal triggering of earthquakes in the northeast pacific ocean summary there have been many searches for evidence of tidal triggering in earthquake catalogues with the exception of volcanically active regions the more rigorous studies in continental settings tend to find no correlation or only a very weak correlation in the oceans the effect of loading by the ocean tides can increase tidal stresses by about an order of magnitude over continental settings in recent years several studies have reported evidence of tidal triggering in oceanic regions and such observations can represent a useful constraint on models of earthquake rupture in this paper i systematically search for a link between ocean tide height and the incidence of earthquakes in the northeast pacific ocean a region of high amplitude open ocean tides the focal mechanisms of most of the earthquakes in these catalogues are unknown but it can be shown that tidal stresses will in most instances promote failure at low tides i investigate three declustered data sets comprising 1 earthquakes from 1980 to 2007 on the juan de fuca plate and in the queen charlotte fault region from land based catalogues 2 earthquakes from 1992 to 2001 on the juan de fuca plate located with the us navy s sound surveillance system sosus hydrophone array and 3 earthquakes from 1980 to 2001 south of alaska and the aleutians located with land based networks i look at the distributions of earthquakes with ocean tide phase height and tidal range and apply schuster and binomial tests and monte carlo simulations to determine if they deviate significantly from random the results show no evidence of triggering during intervals of increased tidal range but all three data sets show a significant increase in earthquake incidence at low tides the signal is particularly strong in the land based catalogue for the juan de fuca plate and queen charlotte fault regions where there is a 15 per cent increase in the rate of seismicity within 15 of the lowest tides the signal is weakest in the sosus data set which may reflect the lower average tidal range at epicentres in this data set or an analysis that is influenced by gaps in the catalogue the triggering signal in the alaska aleutian may be partially obscured by earthquakes in the aleutians where the total tidal stresses can be significantly out of phase with the ocean tide height the increase in the rates of seismicity i observe at low tides is less than observed by local networks on mid ocean ridges similar to the prediction from an analysis of global thrust earthquakes and greater than inferred by extrapolating laboratory simulations of fault failure under tidal loading

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

a regression modeling approach for studying carbonate system variability in the northern gulf of alaska 1 the northern gulf of alaska goa shelf experiences carbonate system variability on seasonal and annual time scales but little information exists to resolve higher frequency variability in this region to resolve this variability using platforms of opportunity we present multiple linear regression mlr models constructed from hydrographic data collected along the northeast pacific global ocean ecosystems dynamics globec seward line the empirical algorithms predict dissolved inorganic carbon dic and total alkalinity ta using observations of nitrate no3 temperature salinity and pressure from the surface to 500 m with r2s 0 97 and rmse values of 11 mol kg 1 for dic and 9 mol kg 1 for ta we applied these relationships to high resolution no3 data sets collected during a novel 20 h glider flight and a globec mesoscale seasoar survey results from the glider flight demonstrated time space along isopycnal variability of aragonite saturations arag associated with a dicothermal layer a cold near surface layer found in high latitude oceans that rivaled changes seen vertically through the thermocline the seasoar survey captured the uplift to 100 m of dense high pco2 waters at the shelf break that had been forced by the passage of a yakutat eddy during this event the aragonite saturation horizon depth where arag 1 shoaled to a previously unseen depth in the northern goa this work is similar to recent studies aimed at predicting the carbonate system in continental margin settings albeit demonstrates that a no3 based approach can be applied to high latitude data collected from platforms capable of high frequency measurements

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

crustal fissuring on the crest of the southern east pacific rise at 17 15 40 s s with the near bottom dsl 120 andargo ii imaging systems we observe that the youngest lava flows on a relative age scale aresparsely fissured and that there is a cumulative increase in fissure abundance with time thatproduces a strong positive correlation between fissure density and relative age of lava flows average fissure widths were used to estimate fissure depths in the 17 15

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

physical properties of upper oceanic crust ocean drilling program hole 801c and the waning of hydrothermal circulation 1 the hydrologic evolution of oceanic crust from vigorous hydrothermal circulation in young permeable volcanic crust to reduced circulation in old cooler crust causes a corresponding evolution of geophysical properties ocean drilling program odp hole 801c which obtained the world s oldest section of in situ normal oceanic crust provides the opportunity to examine relationships among hydrologic properties porosity permeability fluid flow crustal alteration and geophysical properties at both core plug and downhole log scales within these upper crustal basalts fluid flux in zones with high porosity and associated high permeability fosters alteration particularly hydration consequently porosity is correlated with both permeability and a variety of hydration indicators porosity dependent alteration is also seen at the log scale potassium enrichment is strongly proportional to porosity we extend the crustal alteration patterns observed at hole 801c to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of deep sea drilling project and odp core physical properties and downhole logs increasing crustal age entails macroporosity reduction and large scale velocity increase despite intergranular velocity decrease with little microporosity change the changes in macroporosity and velocity are significant for pillows but minor for flows matrix densities provide the strongest demonstration of systematic age dependent alteration on the basis of observed decreases in matrix density that are proportional to the logarithm of age approximately half of all intergranular scale crustal alteration occurs after the first 10 15 myr apparently crustal alteration continues at a decreasing rate throughout the lifetime of oceanic crust

velocity porosity relationships in the upper oceanic crust theoretical considerations we consider here the application of rock physics theories to investigate relationships between seismic velocities and porosities in the shallow oceanic crust classical hashin shtrikman limits ignore void shapes and are too broad to provide useful constraints on velocities and porosities making some assumptions about the distribution of void shapes improves the constraints theories which ignore crack crack interactions underestimate the effects of porosities on velocities thus providing upper bounds on velocities and porosities self consistent theories overestimate crackcrack interactions and so provide lower bounds at the high porosities required to reduce basalt from a p velocity of 7km s in massive form to the 2 2km s observed in zero age oceanic crust however the bounds are too far apart to be useful the theories are strictly valid only for very small porosities using an algorithm proposed by cheng for iteratively building up porosity to create a highly porous medium analogous to differential computation methods traditionally used to improve upon the self consistent approach we have devised two hybrid theories which we term extended walsh and extended kuster toksoz these two theories remain approximately valid at the high porosities of oceanic crustal layer 2a to provide useful upper and lower bounds on velocity for a given porosity and pore aspect ratio distribution we attempt the inverse problem determining porosity from a given velocity using on bottom refraction data collected on the flank of the east pacific rise for 120ka material with a p velocity of 2 5km s if our assumptions regarding the aspect ratio distribution are correct porosity lies somewhere between 24 and 34 resolution on slower zero age crust 2 2km s is poorer there we predict a porosity between 26 and 43 use of shear wave information would tighten these bounds

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

upper crustal velocity structure beneath the central lucky strike segment from seismic refraction measurements we present a three dimensional velocity model of the upper crust around the central volcano of the lucky strike segment mid atlantic ridge the model constructed from a 3 d array of air gun shots 37 5 m spacing along line and 100 m between lines to ocean bottom seismometers fired during a 3 d seismic reflection survey shows an off axis velocity increase 1 km s a low velocity region within the median valley and a low velocity anomaly underneath the lucky strike volcano our observations indicate a porosity decrease of 1 9 corresponding to a velocity increase of 0 5 1 km s over a distance of 8 km from the ridge axis 0 7 ma and a porosity decrease of 4 11 corresponding to a velocity increase of 2 km s between a depth of 0 5 and 1 75 km below seafloor a sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis we favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion lava sheeted dike boundary and active extension faults fractures and fissures the porosity variation with depth probably depends on pore space collapse hydrothermal alteration and a change of stress accommodation the off axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust

seismic structure of the upper crust from 0 75 ma in the equatorial atlantic ocean on the african plate using ultralong offset seismic data

automatic 1 d waveform inversion of marine seismic refraction data summary full waveform synthetic seismograms are now used as standard in the interpretation of marine refraction data but only with a laborious trial and error fitting procedure we show how the matching of observed waveforms with wkbj synthetic seismograms can be efficiently and automatically performed for 1 d earth models automation of the inversion is difficult because of the multi modal irregular form of the misfit of data and synthetics a sequence of three steps is required for a complete inversion location of the deepest valley of the misfit function descent to the global minimum and description of the neighbourhood of the minimum for error analysis the first step is accomplished with a monte carlo search through a very large model space defined by poor prior knowledge of velocities and gradients of the solution weak traveltime constraints are used to eliminate poorly fitting models from waveform calculations a comparison of the traveltime and waveform misfits of the monte carlo models clearly illustrates that waveforms are providing more information than traveltimes alone bayesian statistics are used to construct marginal probability distributions and the covariance matrix which give a rough preliminary error analysis in the second step damped least squares linearized inversion makes small adjustments to the best fitting monte carlo model as it descends to the global minimum finally the immediate neighbourhood of the global minimum is explored with constrained least squares inversion realistic error bounds on each parameter are defined from the resulting slices through the misfit function these bounds are much narrower than traveltime inversion provides correlations between parameters are obtained from the covariance matrix constructed from models examined during this error analysis the inversion methods are illustrated on the ff2 refraction data set of the scripps institution of oceanography the monte carlo search successfully locates the valley of the global minimum as well as a nearby secondary minimum the error analysis puts realistic error bounds on the detailed inversion of this data set by spudich orcutt

marine geodynamics depth age revisited more than 20 years have passed since the subsidence of the seafloor away from midocean ridges was explained by the cooling of the lithosphere figure 1 a thermal boundary layer at the earth s surface turcotte and oxburgh 1967 although the general concept has proven quite sound and found much practical application the details of the model have never been completely agreed upon for example if no process limits the growth of the thermal boundary layer the seafloor should continue to deepen as the square root of lithospheric age in contrast the observations indicate that little deepening occurs for lithosphere aged greater than about 100 ma ma million years before present early on this fact was explained by supposing that there was a limit to the depth of cooling of the boundary layer such that the lithosphere cools as a plate of finite thickness rather than as a semi infinite half space mckenzie 1967 according to the plate model the depth of old lithosphere approaches a maximum as the geothermal gradient in the plate reaches thermal equilibrium however this theoretical model merely provided a better fit to the observations without addressing the fundamental question of what process might limit the growth of the earth s uppermost thermal boundary layer

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

mesoscale eddies dominate surface phytoplankton in northern gulf of alaska abstract the hnlc waters of the gulf of alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters even in spring and summer our observations of chlorophyll sensed by seawifs north of 54 n in pelagic waters 500 m depth of the gulf found that on average more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies the ratio approaches 80 in spring months yet these contours enclosed only 10 of the total surface area of pelagic waters in the gulf therefore eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters we outline several eddy processes that enhance primary productivity eddies near the continental margin entrain nutrient and fe rich and chlorophyll rich coastal waters into their outer rings advecting these waters into the basin interior to directly increase phytoplankton populations there in addition eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin as these anticyclonic eddies decay their depressed isopycnals relax upward injecting nutrients up toward the surface layer we propose that this transport brings iron and macro nutrients toward the surface mixed layer where they are available for wind forced mixing to bring them to surface these mesoscale eddies decay slowly but steadily perhaps providing a relatively regular upward supply of macro nutrients and iron toward euphotic layers they might behave as isolated oases of enhanced marine productivity in an otherwise iron poor basin we note that much of this productivity might be near or just below the base of the surface mixed layer and therefore poorly sampled by colour sensing satellites it is possible then that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only

quantitative comparison of seismic data sets with waveform inversion testing the age dependent evolution of crustal structure studies of seismic variability play a more important role in the understanding of processes that form oceanic crust than individual velocity depth solutions found in isolation for such variability studies solutions must be contrasted with one another this is normally not straightforward owing to the lack of rigorous uncertainty estimates available for solutions here i present a method for the direct comparison of such solutions by hypothesis testing a null hypothesis is proposed that a single velocity solution is consistent with two sets of data the hypothesis is tested by seeking such a model using waveform inversion and comparing its misfit against the misfit from fully independent solutions the degree of similarity of the velocity structures can be further quantified by solving for the pair of solutions that are as similar as possible while consistent with each set of data the f test is used to evaluate the significance of each additional effective model parameter as it lends independence to the two solutions the method possesses advantages of a hypothetical monte carlo search of all models because the complete forward problem is carried out at each step yet it is tractable because the search direction is well defined the method is applied to two data sets collected near the mid atlantic ridge on 0 and 7 m y crust in order to quantify the differences between the solutions the comparison finds quantifiable differences in both layers 2 and 3 between these two regions suggesting that significant evolution of the entire crustal section has taken place in the first 7 m y

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

variability of the southern hemisphere winter split flow a case of two way reinforcement between mean flow and eddy anomalies abstract a new split jet index is defined in this study and composites based on this index show that the split flow regime is characterized by a cold warm cold tripolar temperature anomaly in the south pacific that extends equatorward from the southern hemisphere sh high latitudes while nonsplit flow occurs when the phase of the tripolar temperature anomaly is reversed analyses of the heat budget reveal that the temperature anomalies associated with the split nonsplit flow are mainly forced by mean flow advection instead of local diabatic heating or convergence of eddy heat fluxes localized eliassen palm e p flux diagnostics suggest that the zonal wind anomalies are maintained by the eddy vorticity flux anomalies these diagnostic results are confirmed by numerical experiments conducted using a stationary wave model forced by observed eddy forcings and diabatic heating anomalies the model results show that the effects of the vorticity flux dominates over those of the heat flux which tend to damp

poisson s ratio structure of young oceanic crust we have applied full waveform inversion to wide aperture seismic reflection data from the southern east pacific rise near 14 s the data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the p and s wave velocities vp vs and hence poisson s ratio and seismic attenuation qp qs structure of seismic layer 2 layer 2a is highly attenuating qp 18 30 and qs 8 15 and layer 2b is moderately attenuating qp 30 50 and qs 20 25 our results show high at the seafloor and in layer 2a 0 48 across the top of the 2a b transition the rapid increase in vp is accompanied by a sharp drop in to 0 25 within just 200 m of the seafloor we perform simple calculations to gain an insight into the porosity and crack distribution with depth these calculations suggest that porosity is in excess of 30 in layer 2a but reduces to 6 7 at the top of the 2a b transition and to about 5 at a depth of 600 m below seafloor within layer 2b our results suggest that there is an increase in the average aspect ratio with depth across the 2a b transition the most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth our results show there to be an abrupt change in the pore structure across the 2a b transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition mechanical or hydrothermal within the extrusive pile

compressional and shear wave structure of the east pacific rise at 11 20 n constraints from three component ocean bottom seismometer data we report the analysis of seismograms from two orthogonal refraction lines in the rose area of the east pacific rise at 11 20 n recorded by a three component ocean bottom seismometer obs converted shear waves are clearly observed on the horizontal seismometers for both lines including paths crossing the rise axis neither a travel time delay nor measurable attenuation is observed for shear or compressional waves which have passed beneath the rise crest at depths down to 4 km on the basis of these observations we conclude that no large crustal magma chamber is present beneath the rise at this latitude we invert the travel times of p and converted s waves to infer the crustal structure in the vicinity of the rise and we use p wave amplitudes to constrain further the range of solutions the p wave velocity distribution does not differ markedly from that of normal young oceanic crust but the shear velocity distribution results in a poisson s ratio 0 32 that may be more typical of highly fractured crust in rise axis regions a thin 80 m layer of sediment is inferred to be present beneath the obs on the basis of the measured particle motions the observation of an sp phase and the reverberatory character of the seismograms a combined consideration of the results of this experiment and other observations of rise crossing crustal phases elsewhere in the rose area leads us to conclude that any magma within the crust beneath the rise axis of this region must be confined to narrow dikes or pipes or to small isolated bodies with a vertical thickness of less than about 1 km

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

satellite and hydrographic observations of eddy induced shelf slope exchange in the northwestern gulf of alaska 1 satellite and hydrographic observations of oceanographic conditions in the northwestern gulf of alaska indicate that anticyclonic eddies propagating adjacent to the continental shelf alter the structure of the shelf break front and in doing so influence the shelf slope exchange of biota and water mass properties eddies typically form in the northern gulf of alaska during fall and winter and propagate southwestward within an 200 km wide corridor along and above the continental slope eddy activity within this corridor diminishes in the downstream direction the trajectories of faster propagating 4 km day eddies tend to lie closer to the shelf break than do the trajectories of slower propagating 1 5 km day eddies the interaction between azimuthal eddy currents and the shelf break frontal jet 1 establishes an upwelling zone associated with the leading flank of the eddy that strengthens cross slope gradients 2 weakens cross slope gradients and promotes shelf slope exchange where the eddy is adjacent to the shelf and 3 reestablishes a weak upwelling zone associated with the trailing flank of the eddy

distribution of porosity in a section of upper oceanic crust exposed in the troodos ophiolite the porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the troodos ophiolite the distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages eight sites with one dominant lithology and contrasting alteration histories were studied two types of porosity were quantified macroscopic macrofractures vesicles vugs interpillow zones and total macroscopic and laboratory measured microscopic porosity crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction mean initial macroscopic porosity ranges from 14 to 17 in uppermost pillows and from 6 to 10 in the underlying pillows and massive flows macroscopic porosity was reduced by 10 11 where alteration was most pervasive i e 250 m of the paleoseafloor beneath this zone pillow and flow outcrops showed decreases of 1 3 and 2 5 respectively the combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity 4 8 the greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone which comprises 10 of the volcanic pile thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography where crests of abyssal hills remain unsedimented for long periods of time porosity data derived from deep sea drilling project ocean drilling program cores and borehole logs show similar relationships which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data

crustal structure in the rose area of the east pacific rise one dimensional travel time inversion of sonobuoys and expanded spread profiles we study the variability of the young oceanic crust as a function of age by analyzing 11 digitally recorded sonobuoys and three expanded spread profiles over 0 5 2 5 and 4 5 m y old oceanic crust the data were collected during the rivera ocean seismic experiment at the east pacific rise between 11 and 13 n we analyze the data within a one dimensional travel time scheme using the tau p method and travel time forward modeling we perform the tau p mapping by using a polynomial least squares technique that is fast and can be applied even to poor quality data as in this case we do not see large changes in the velocity structure between 0 5 and 4 5 m y old oceanic crust the overall structure consists of a rapid increase in velocity from about 2 5 6 km s within the uppermost 1 km of crust layer 2a a zone 1 km in thickness with little or no velocity gradient layer 2b a thin transition layer with a relatively rapid increase in velocity from about 6 2 to 7 km s layer 2c and a thick nearly homogeneous basal layer with velocities around 7 2 km s layer 3 we find that the low velocities at the seafloor and large velocity gradients within the shallow crust can be explained by the hypothesis of large scale cracks and their behavior under confining pressure for the variation of velocity v versus depth below seafloor z this hypothesis yields v v02 bz 1 2 a relationship that fits our solutions

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

crustal velocity structure of the lucky strike segment of the mid atlantic ridge at 37 n from seismic refraction measurements we estimate the seismic structure of the slow spreading lucky strike segment of the mid atlantic ridge located approximately 300 km south of the azores platform using seismic reflection and seismic refraction data acquired in june 2005 as a part of the seismic study for monitoring of the mid atlantic ridge sismomar survey the three dimensional velocity model shows an upper crustal low velocity anomaly running parallel to the ridge axis which is limited by the median valley bounding faults the velocity models also show a low velocity anomaly underlying the axial melt lens reflector located at the segment center below the lucky strike volcano this lower crustal low velocity region can be explained by elevated temperatures and possibly small amounts of melt the lower crustal low velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the lucky strike volcano the presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center

shear wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies summary a comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity depth models for the oceanic crust and upper mantle the data were obtained as part of an investigation of lithospheric structure carried out in the nootka fault zone area off the west coast of canada one dimensional s wave velocity depth models are derived from the data by trial and error comparisons with wkbj synthetic seismograms the results of this study confirm crustal characteristics interpreted previously from the compressional wave data layers 3a and 2b in the upper crust are characterized by rapid increases in seismic velocities shear wave velocity increases from 2 2 to 3 6 km s l over an interval of 3 5 km although the exact nature of the shearwave velocity gradient is uncertain in contrast the shear velocity in layer 3a remains fairly constant with depth at values near 3 6km s no velocity discontinuity is observed at the moho instead the transition from crust to upper mantle is marked by the presence of a velocity gradient zone layer 3b the shear wave velocities in the upper mantle are 4 5 and 4 6 km s l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively this low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the pn velocities combining the p wave interpretation with this s wave study enables evaluation of poisson s ratio as a function of depth except for low values of poisson s ratio found in the upper crust the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples

waveform inversion of time corrected refraction data summary seismic refraction data collected in land or marine surveys are often complicated by small near random time shifts of the individual seismograms relative to one another for the case of fixed receiver geometry these shifts arise primarily from basement topography often masked by sediments beneath each source although a topographic timing correction is normally applied to the seismograms the effects of poorly known topography produce residual shifts in the seismograms the application of additional time corrections statics can bias solutions computed using iterative waveform inversion because a suitable set of statics can make a poor starting velocity model appear to match the data without any correction however seismograms computed from even the best fitting laterally homogeneous model may not agree with the data to within one half period in this paper i present a formalism for iterative waveform inversion of refraction data containing residual time shifts in this method the statics are directly modelled in the synthetic seismograms and new values estimated in the inversion together with new velocity model parameters the method is similar to studies of statics in seismic reflection data but in this case values for the statics are estimated simultaneously with the velocity model with the use of suitable norms the new velocity model is required to be smooth and the statics required to be small convergence is achieved when the velocity profile s roughness the rms size of the statics and the waveform misfit are in equilibrium because the starting synthetics need not match the data to within one half period a much larger set of velocity models used for an initial guess will converge on the family of solutions than for waveform inversions in which time shifts are not used the original arbitrary starting model loses its importance after the first iteration and solutions meeting the convergence criterion appear to be independent of the particular choice of starting model numerical tests show the method to be robust an inaccurate starting model that fits the data only through a biased set of statics is accurately corrected the method is illustrated on a set of refraction data collected in the vicinity of the east pacific rise in this application two quite different starting velocity models converge to a common family of solutions

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

a note on cross shelf exchange in the northern gulf of alaska the continental shelf of the gulf of alaska goa is a complex system characterized by large freshwater runoff and strong winds the goa supports one of the world s richest ecosystems including numerous species of fishes marine mammals and sea birds the mechanisms that provide nutrients to support this ecosystem are not well understood the rivers and streams that provide freshwater to the shelf are low in nitrate and the regional winds favor downwelling high concentrations of nitrate are available in the deep basin of the goa but these must be introduced to the shelf in order to support the high productivity we present evidence for cross shelf exchange due to three different mechanisms episodes of downwelling relaxation result in a flux of saline nutrient rich water onto the shelf at depth eddies formed in the northeastern goa propagate along the shelf break influencing cross shelf exchange by carrying shelf origin water from the formation region into the basin and by interacting with the shelf break circulation bathymetric steering in the many canyons that incise the goa shelf results in flow into the canyons where strong tidal mixing results in cross isobath movement of water properties

evidence of prolonged aragonite undersaturations in the bottom waters of the southern bering sea shelf from autonomous sensors abstract the southeastern shelf of the bering sea is a dynamic area that experiences seasonal variability in primary production and remineralization of organic matter both of which control the carbon biogeochemistry of the water column surface water partial pressure of carbon dioxide p co 2 is greatly reduced in summer by biological production which increases carbonate mineral saturation states in contrast the export of large quantities of organic matter from surface blooms drives an active remineralization loop that sharply increases p co 2 near the bottom lowering ph and suppressing new observations from moored biogeochemical sensors in 2011 showed that seasonal net community production lowers surface water p co 2 causing large gradients between the ocean and atmosphere that are sustained throughout the summer confirming that these waters likely remain supersaturated with respect to aragonite throughout the open water season on the other hand moored sensors deployed near the bottom showed that p co 2 levels exceed 500 atm by early june and remain at these high levels well into the autumn months indicating that the bottom waters are likely continuously undersaturated in aragonite for at least several months during each year only a small fraction of the increased p co 2 can currently be attributed to the intrusion of anthropogenic co 2 from the atmosphere while the majority is due to natural respiration processes the biological impacts along with the timing and duration of these undersaturation events could play a role in the development of larval and juvenile calcifiers in the region and will change as anthropogenic co 2 concentrations continue to rise