oceanic crustal thickness and seismic character along a central pacific transect seismic reflection data along a flow line of crustal generation in the central pacific that spans ages of zero to 85 ma and spreading half rates of 30 to 100 km m y 1 shows a nearly constant travel time of 2 s through igneous crust to reflection moho the highest amplitude and most laterally continuous moho reflections were recorded over 20 30 ma crust that was emplaced at superfast 75 95 km m y 1 spreading rates the superfast spread portion also records the lowest scatter about the 2 s average travel time to reflection moho seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest these dipping reflectors are truncated by the reflection moho

dust deposition pulses to the eastern subtropical north atlantic gyre does ocean s biogeochemistry respond 1 enhancement of primary and export production following dust deposition pulses is well established for high nutrient low chlorophyll regions but the effect of atmospheric dust on the biogeochemistry of oligotrophic gyre regions remains unclear here we report atmospheric dust concentrations measured on gran canaria canary islands concomitantly with upper water biogeochemistry at the oligotrophic time series station estoc european station for time series in the ocean canary islands during a 2 year period from winter 1997 to 1999 estoc is located in the eastern subtropical atlantic gyre about 100 km north of gran canaria and 500 km west of the sahara and receives aeolian dust episodically mainly in winter and summer we found a close correlation of the magnitude of atmospheric dust concentration in winter with the magnitude of downward particle flux at estoc mainly due to a relative increase in lithogenic matter and carbonate sedimentation higher aerosol concentration was not accompanied by higher primary or export production however indicating that phytoplankton production remained unaffected by atmospheric nitrogen supply on a seasonal or yearly timescale however by estimating bioavailable iron input and the need of the phytoplankton population we found that the highly episodic dust pulses might exert a feast and famine effect on the phytoplankton despite the high lithogenic matter input carbonate mainly stemming from coccolithophorids exceeded in importance the role of lithogenic matter as ballasting agent of sinking organic matter

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

crustal structure measurements near fram ii in the pole abyssal plain abstract an extensive refraction profiling program was carried out during the fram ii experiment march may 1980 in the eastern arctic ocean two structural areas were covered north of the ice camp 86 n 24 w into the basin of the pole abyssal plain and south onto the flanks of the morris jesup rise digital multichannel data on an 800 by 800 m 24 channel hydrophone array and a single 2 component ocean bottom seismometer obs were recorded for offsets from 2 5 to 100 km arrival times amplitudes and phase velocities of the seismic signals recieved on the hydrophone array were determined using high resolution array processing from these measurements and the obs data preliminary velocity structural models of the crust have been derived for the purposes of this paper 2 refraction lines have been analyzed a 40 km line on a flat region of the pole abyssal plain and an 86 km line on a slightly dipping region taken as the drifting ice camp shoaled on the morris jesup rise these preliminary analyses yield a sedimentary layer with a gradually increasing velocity 1 5 2 km thick this cover overlays a crust with a thin layer 2 and yields a total ocean bottom to mantle thickness of 4 7 km

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

cenozoic and late mesozoic stratigraphy and subsidence history of the norwegian margin the stratigraphy of the passive continental margin off norway is mapped utilizing both deep sea drilling project dsdp data at five sites on the outer margin and multichannel seismic mcs data collected by lamont doherty geological observatory distinct units in the acoustic stratigraphy are tied to biostratigraphic and lithostratigraphic units in the drill data and mapped throughout most of the margin stratigraphy of the pre lower eocene section not encountered in drilling is inferred from depositional patterns and correlation with the continental shelf sequences the stratigraphy of which has been developed independently from detailed surveys the margin can be divided into three physiographic provinces two broad shelf and slope regions separated by the jan mayen fracture zone and the steep narrow shelf and slope off the lofoten islands each experienced a separate cenozoic subsidence history the voring plateau is partly underlain by oceanic crust it subsided nearly 1 000 m less than expected from thermal contraction but it did so following the development of a relatively deep water environment in the paleocene north of the voring plateau the margin may have subsided a few hundred metres more than thermal studies had predicted there is no evidence for a phase of extensional deformation associated with the inception of spreading the youngest faulted level occurs at the late jurassic level preceding sea floor spreading by nearly 100 ma and deformation in the cretaceous section is characterized by large uplift structures that may be of halokinetic origin the physiographic provinces as well as the cenozoic subsidence patterns along the margin appear to be related to the style and extent of mesozoic basin development and tectonics along the margin

equatorial pacific seismic reflectors as indicators of global oceanographic events the origin of a series of regionally correlatable seismic horizons in the neogene sediments of the central equatorial pacific is examined through seismic modeling and the detailed analyses of stratigraphic and physical property relationships in deep sea drilling project cores these regionally traceable reflectors are synchronous the younger reflectors are the direct result of carbonate dissolution events the older ones of stratigraphically selective diagenetic processes the changes in ocean chemistry associated with these events appear to be linked to global reorganizations of surface and bottom water circulation patterns the most dramatic of which are associated with reorganizations of north atlantic bottom waters these deepwater seismic horizons appear to correlate with the major events on the relative sea level curve of vail et al for the neogene

seasonal and interannual variability in deep ocean particle fluxes at the oceanic flux program ofp bermuda atlantic time series bats site in the western sargasso sea near bermuda abstract since 1978 the oceanic flux program ofp time series sediment traps have measured particle fluxes in the deep sargasso sea near bermuda there is currently a 20 yr flux record at 3200 m depth a 12 yr flux at 1500 m depth and a 9 yr record at 500 m depth strong seasonality is observed in mass flux at all depths with a flux maximum in february march and a smaller maximum in december january there is also significant interannual variability in the flux especially with respect to the presence absence of the december january flux maximum and in the duration of the high flux period in the spring the flux records at the three depths are surprisingly coherent with no statistically significant temporal lag between 500 and 3200 m fluxes at our biweekly sample resolution bulk compositional data indicate an extremely rapid decrease in the flux of organic constituents with depth between 500 and 1500 m and a smaller decrease with depth between 1500 and 3200 m depth in contrast carbonate flux is uniform or increases slightly between 500 and 1500 m possibly reflecting deep secondary calcification by foraminifera the lithogenic flux increases by over 50 between 500 and 3200 m depth indicating strong deep water scavenging repackaging of suspended lithogenic material concurrent with the rapid changes in flux composition there is a marked reduction in the heterogeneity of the sinking particle pool with depth especially within the mesopelagic zone by 3200 m depth the bulk composition of the sinking particle pool is strikingly uniform both seasonally and over variations in mass flux of more than an order of magnitude these ofp results provide strong indirect evidence for the intensity of reprocessing of the particle pool by resident zooplankton within mesopelagic and bathypelagic waters the rapid loss of organic components the marked reduction in the heterogeneity of the bulk composition of the flux and the increase in terrigenous fluxes with depth are most consistent with a model of rapid particle turnover and material scavenging from the suspended pool during new particle formation we suggest that much of the deep mass flux is generated in situ by deep dwelling zooplankton and that mass flux as well as scavenging of suspended materials from the deep water column varies in proportion to changes in grazer activity labile very rapidly sinking aggregates e g salp fecal material arriving in the bathypelagic zone within days of their upper ocean production may act to stimulate zooplankton grazing rates and increase large particle production and deep mass flux days to weeks in advance of the arrival of bulk of surface produced material this process could reconcile mean particle sinking rate estimates with the phase coherence observed between upper and deep ocean mass fluxes

crustal thickness and moho character of the fast spreading east pacific rise from 9 42 n to 9 57 n from poststack migrated 3 d mcs data we computed crustal thickness 5740 270 m and mapped moho reflection character using 3 d seismic data covering 658 km2 of the fast spreading east pacific rise epr from 9 42 n to 9 57 n moho reflections are imaged within 87 of the study area average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last 180 ka however individual crustal thickness measurements differ by as much as 1 75 km indicating that the mantle melt delivery has not been uniform third order but not fourth order ridge discontinuities are associated with changes in the moho reflection character and or near axis crustal thickness this suggests that the third order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth order ridge segmentation arises from midcrustal to upper crustal processes in this light we assign fourth order ridge discontinuity status to the debated ridge segment boundary at 9 45 n and third order status at 9 51 5 n to the ridge segment boundary previously interpreted as a fourth order discontinuity our seismic results also suggest that the mechanism of lower crustal accretion varies along the investigated section of the epr but that the volume of melt delivered to the crust is mostly uniform more efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half this south to north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the lamont seamounts

a comparison of velocity and attenuation between the nicobar and bengal deep sea fans the compressional wave velocity and attenuation measurements from a deep source deep receiver seismic refraction experiment from the nicobar fan indian ocean are presented and compared to a similar set of measurements in the bengal fan the two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories travel time inversion of the data into a velocity depth model uses the tau zeta linear inversion scheme we present a revised reparameterization of the travel time data and improved error analysis for the inversion process although the velocity structures with depth are morphologically similar there exist discrete differences between the two stations in the nicobar fan the initial velocity assumed is 1 513 km s with an initial gradient of 2 32 s 1 the velocity increases smoothly with depth while the velocity gradient decreases rapidly to a constant 0 81 s 1 the bengal fan station showed the same trend although the velocity gradient at depth was only 0 67 s 1 the attenuation profiles are similar only in gross structure with the nicobar fan values of q 1 being a factor of 3 to 4 less than the bengal station the differences in velocity and attenuation structure can be attributed to a number of causes including porosity and sedimentation rates which are related to the distance from the sediment source and the presence of a normal fault near the nicobar fan site this fault may act as a conduit for water removal in the presence of overburden pressure it is believed that porosity changes due to whatever cause dominate the lateral and vertical variations of velocity velocity gradients and together with intrabed multiples attenuation in thick sedimentary sections

fluxes and distribution of dissolved iron in the eastern sub tropical north atlantic ocean aeolian dust transport from the saharan sahel desert regions is considered the dominant external input of iron fe to the surface waters of the eastern sub tropical north atlantic ocean to test this hypothesis we investigated the sources of dissolved fe dfe and quantified dfe fluxes to the surface ocean in this region in winter 2008 surface water dfe concentrations varied between 1 5 nm correlated positively with apparent oxygen utilization aou and showed the importance of organic matter remineralization as an dfe source as a consequence vertical diffusive mixing formed an important fe flux to the surface ocean in this region even surpassing that of a major dust event

trapping efficiencies of sediment traps from the deep eastern north atlantic the 230th calibration bottom tethered sediment traps deployed in the deep eastern north atlantic between 54 n 20 w and 33 n 20 w l1 l2 l3 at the european continental margin at 49 n omex and off the canary islands estoc were investigated for the determination of 230th trapping efficiencies the ratios of 230th flux measured in the traps fa to the expected 230th flux from the production rate of 230th in the overlying water column fp ranged between 0 09 and 1 26 for the traps with deployment periods 300 days the interannual variation of fa fp ratios different years but same location and water depth were up to 10 suggesting that the average 230th flux to the sediment traps did not vary significantly the influence of lateral advection on the 230th flux was taken into account either by applying a mass balance of 230th and 231pa or by assuming a constant removal rate of 230th from the water column an assumption based on similar 230th concentration depth profiles observed at most locations investigated 230th trapping efficiencies were between 9 and 143 showing a trend of increasing efficiencies with increasing water depth no relation was found between current velocities and 230th trapping efficiencies our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases the correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern north atlantic

coherent noise in marine seismic data three mechanisms that might produce noise patterns on stacked sections are examined direct and trapped waves that propagate outward from the seismic source cable motion caused by the tugging action of the boat and tail buoy and scattered energy from irregularities in the water bottom and sub bottom different noise patterns can be observed on shot profiles and common midpoint cmp gathers these patterns can be diagnostic of the dominant mechanism in a set of data field data suggest that the dominant noise is from waves scattered within the shallow sub bottom this type of noise is enhanced by cmp stacking moreover it can be as strong as surface wave noise on stacked land seismic data as well moveout filtering is best for suppressing the noise while preserving signal since the scattered noise does not exhibit a linear moveout pattern on cmp sorted gathers moveout filtering must be applied either to traces within shot records and common receiver gathers or to stacked traces our data example demonstrates that although it is more costly moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data dependent processing steps of predictive deconvolution and velocity analysis modified journal abstract

mineral ballast and particle settling rates in the coastal upwelling system off nw africa and the south atlantic the ocean off nw africa is the second most important coastal upwelling system with a total annual primary production of 0 33 gt of carbon per year carr in deep sea res ii 49 59 80 2002 deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes due to a low supply of dissolved silicate from subsurface waters the ocean off nw africa is characterized by predominantly carbonate secreting primary producers i e coccolithophorids these algae which are key primary producers since millions of years are found in organic and chlorophyll rich zooplankton fecal pellets which sink rapidly through the water column within a few days particle flux studies in the mauretanian upwelling area cape blanc confirm the hypothesis of armstrong et al deep sea res ii 49 219 236 2002 who proposed that ballast availability e g of carbonate particles is essential to predict deep ocean organic carbon fluxes the role of dust as ballast mineral for organic carbon however must be also taken into consideration in the coastal settings off nw africa there high settling rates of larger particles approach 400 m day 1 which may be due to a particular composition of mineral ballast an assessment of particle settling rates from opal production systems in the southern ocean of the atlantic sector in contrast provides lower values consistent with the assumptions of francois et al global biogeochem cycles 16 4 1087 2002 satellite chlorophyll distributions particle distributions and fluxes in the water column off nw africa as well as modelling studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean these transport processes have implications for paleo reconstructions from sediment cores retrieved at continental margin settings

nw african climate variations during the last 47 000years evidence from organic walled dinoflagellate cysts nw african climate shows orbital and millennial scale variations which are tightly connected to changes in marine productivity we present an organic walled dinoflagellate cyst dinocyst record from a sediment core off cape yubi at about 27 n in the canary basin covering the time period from 47 to 3 ka before present bp the dinocyst record reflects differences in upwelling intensity and seasonality as well as the influence of fluvial input sea level changes play an important role for the upwelling pattern and productivity signals at the core site within the studied time interval four main phases were distinguished 1 from 45 to 24 ka bp when sea level was mostly about 75 m lower than today high relative abundances of cysts of heterotrophic taxa point to enhanced upwelling activity especially during heinrich events while relatively low dinocyst accumulation rates indicate that filament activity at the core location was strongly reduced 2 at sea level lowstand during the lgm to h1 dinocyst accumulation rates suggest that local filament formation was even more inhibited 3 from the early holocene to about 8 ka bp extraordinary high accumulation rates of most dinocyst species especially of lingulodinium machaerophorum suggest that nutrient supply via fluvial input increased and rising sea level promoted filament formation at the same time the upwelling season prolongated 4 a relative increase in cysts of photoautotrophic taxa from about 8 ka bp onwards indicates more stratified conditions while fluvial input decreased our study shows that productivity records can be very sensitive to regional features from the dinocyst data we infer that marine surface productivity off cape yubi during glacial times was within the scale of modern times but extremely enhanced during deglaciation

seismic stratigraphy of the oceanic moho based on ophiolite models synthetic seismograms that model vertical incidence seismic reflections from the oceanic moho were calculated from detailed laterally varying velocity depth models of two ophiolite complexes the synthetic seismograms reproduce the rapid lateral variability of waveforms amplitudes and two way travel times of reflections from the oceanic moho that are observed in existing mcs data the large variations observed in the synthetic seismograms were caused solely by horizontal changes in the geologic structure along the crust mantle boundary because focusing or defocusing of seismic energy by the upper crustal units was not modeled the petrologic contrast producing the strongest reflector from the oceanic crust mantle transition is highly variable so it is difficult to identify what the particular moho reflections represent geologically although with broad band data velocity gradients can be distinguished from laminated zones more important however is our finding that the seismic refraction oceanic moho and the seismic reflection oceanic moho usually do not coincide and that either or both may cross major geologic contacts

uppermantle anisotropy and the oceanic lithosphere rayleigh wave and love wave dispersion data for oceanic paths cannot be satisfied by an isotropic uppermantle and it is incorrect to invert these two datasets independently for separate isotropic structures available phase and group velocity data are inverted for oceanic structure as a function of age taking into account anelastic dispersion sphericity and anisotropy the resulting models are quite different from previous results which ignore the above effects and the extra parameters involved in proper anisotropic inversion the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age and an anisotropic low velocity zone with an age dependent anisotropy the lid or seismic lithosphere reaches a maximum thickness of 50 km which is about one half the thickness obtained in previous isotropic or pseudo isotropic inversions the seismic lithosphere therefore may be comparable in thickness to the elastic or flexural lithosphere raising the possibility that both are controlled by effects other than temperature stress and time such as mineralogy crystal orientation or partial melting the velocities in the low velocity zone are higher than inferred by previous surface wave studies

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

constraints on melt content of off axis magma lenses at the east pacific rise from analysis of 3 d seismic amplitude variation with angle of incidence we use 3d multichannel seismic data to form partial angle p wave stacks and apply amplitude variation with angle ava crossplotting to assess melt content and melt distribution within two large mid crustal off axis magma lenses oamls found along the east pacific rise from 9 37 5 n to 9 57 n the signal envelope of the partial angle stacks suggests that both oamls are partially molten with higher average melt content and more uniform melt distribution in the southern oaml than in the northern oaml for ava crossplotting the oamls are subdivided into seven 1 km2 analysis windows the ava crossplotting results indicate that the oamls contain a smaller amount of melt than the axial magma lens aml for both oamls a higher melt fraction is detected within analysis windows located close to the ridge axis than within the most distant windows the highest average melt concentration is interpreted for the central sections of the oamls the overall low oaml melt content could be indicative of melt lost due to recent off axis eruptions drainage to the aml or limited mantle melt supply based on the results of this and earlier bathymetric morphological geochemical and geophysical investigations we propose that the melt poor oaml state is largely the result of limited melt supply from the underlying mantle source reservoir with smaller contribution attributed to melt leakage to the aml we hypothesize that the investigated oamls have a longer period of melt replenishment lower eruption recurrence rates and lower eruption volumes than the aml though some could be single intrusion events

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

seasonality of surface water properties and particle flux along a productivity gradient off nw africa abstract three stations along a productivity gradient north of the canary islands were investigated for surface water properties particle flux and composition biogenic and lithogenic components and stable nitrogen isotope composition 15n and export production investigation sites along the east west transect off the nw african upwelling margin included the european station for time series in the ocean canary islands estoc one location contiguous to the nw african upwelling zone in the eastern boundary current ebc and one station north of the island la palma lp the seasonality of surface water properties along the transect was mainly influenced by the winter cooling and simultaneous phytoplankton maximum and in addition at ebc by nearby upwelling accordingly particle flux and composition along the transect were closely linked to the winter bloom sedimentation and upwelling related enhanced plankton biomass stemming from the primary upwelling and the cape yubi filament at ebc during all seasons particle flux was highest at ebc and had the highest contribution of biogenic opal and lithogenic components and the lowest 15n compared to the offshore stations but contrary to what would be expected from the productivity gradient particle flux did not decrease from estoc to lp below the upper several hundred meters particle flux was enhanced by additional particle input along the entire transect manifested by an increase of flux with depth and lower 15n values we offer a scenario in which intermediate nepheloid layers originating from the primary upwelling as well as particle dispersion from upwelling filaments mainly the cape ghir filament impact on the trap stations as far as 700 km into the open ocean this study contributes to our understanding of the poorly resolved biogeochemical transition between the productive shelf and subtropical gyre provinces

the eastern boundary current system between the canary islands and the african coast to study the easternboundarycurrentsystem off northwest africa in detail several ctd adcp sections and long term mooring work were carried out in the channel between lanzarote and africa the observations are compared with a fine resolution model which was developed in the framework of the canigo project the water masses which are observed in this area are characterised and classified in density ranges the current field shows a high spatial and temporal variability with maximum velocities of about 35 cm s seasonal means as well as currents averaged across the channel are only a few cm s in the surface water a steady southward flow in the middle of the channel indicates the canarycurrent in this area during fall a strong northward current is observed close to the african shelf though the canarycurrent strengthens during summer and fall due to an increase of the trade winds the transport in the channel decreases or turns northward during that time due to the enhanced poleward current at the eastern side a northward undercurrent with a mean velocity of 2 3 cm s is observed at the african slope in 950 m depth the poleward transport of aaiw increases during fall and a strong influence of relatively fresh aaiw is observed during that time most of the observations fit well to the results of the canigo model but the occurrence of mw at the bottom of the channel and the corresponding southward flow cannot be resolved by the model

high resolution modeling of sediment erosion and particle transport across the northwest african shelf 1 the region off cape blanc along the northwest african coast is dominated by persistent upwelling and strong activity of small scale eddies filaments and jets vertical particle camera profiles obtained during recent cruises in this region show that there exist two well marked maxima of particle abundance in the water column one at the surface and the other in subsurface layers between 200 m and 400 m depths using a highresolution 2 7 km terrain following coordinate ocean model with built in ecosystem and sediment transport modules we show that the surface particle maximum can be explained by local productivity while the deeper subsurface particle cloud most likely originates from particulate material eroded from the shallow shelf and transported offshore by vigorous filament activity and dynamic features of the flow in the numerical experiments particles are produced either by primary production in the surface layer or from prescribed sediment sources to mimic suspension and erosion along the shelf areas good agreement of modeled particle distributions with the data is achieved with a typical settling velocity of 5 m day 1 time averaged effective transport patterns of particles reveal distinct maxima between 20 5n and 23 5n off cape blanc in the south of cape bojador and off cape timiris on the other hand the effective transport distance patterns suggest energetic offshore activity

seismicity and tectonics of the rivera plate and implications for the 1932 jalisco mexico earthquake seismicity on and near the boundaries of the rivera plate is examined in order to determine the subduction regime of the rivera plate and its influence on the earthquake behavior of the northernmost section of the mid america trench in particular we address the question of a rivera cocos boundary there have been several large historic earthquakes in the coastal areas of the mexican states colima and jalisco but the last large event was in june 1932 the 1932 jalisco earthquake ms 8 1 the present quiescence has lasted longer than the average recurrence interval for the mexican subduction zone and the jalisco area is now categorized as a seismic gap however if subduction in this area is dominated by the motion of the rivera plate which is subducting at half the rate of the cocos plate the deficiency of earthquake activity may not be unusual in an attempt to better delineate the rivera cocos boundary intermediate sized earthquakes along and near the rivera fracture zone and the mid america trench were relocated with the joint epicenter determination method we found that in general the catalog locations are fairly accurate with the average change in epicenter upon relocation being 12 km seismicity along the rivera fracture zone follows the physiographic trend of the fracture zone but east of its intersection with the east pacific rise seismicity is more diffuse two intermediate sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent rivera cocos boundary motion reexamination of arrival time data for the 1932 jalisco earthquake yields an epicenter at 19 57 n 104 42 w close to the boundary zone inferred from these two earthquakes we conclude that the possibility that the 1932 jalisco earthquake broke the northernmost section of the cocos north american plate interface as opposed to the rivera north american plate interface cannot be ruled out

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

siliceous sedimentary record of the last 280 kyr in the canary basin nw africa abstract siliceous microfossils were quantified in four sedimentary sequences off northwest africa in order to disclose the productivity story of the last 280 kyr three sediment cores retrieved along an e w profile at about 29 n geob4240 geob4241 and geob4242 and one core geob4216 from the highly productive area off cape ghir in the canary island region at 29 n along all but the most offshore sequences geob4242 coincidence of high diatom accumulation rates dar and high abundances of chaetoceros spores the upwelling related species indicate glacial stages isotope stages is 4 and 2 as more productive than the interglacial intervals is5 and holocene the dar in the coastal area of the canary basin during is2 is of the same order of magnitude 10 8 valves cm 2 kyr 1 reported for cape blanc and one order of magnitude higher than reported for the iberian margin the deviation observed between the offshore higher dar at is3 and 6 and the coastal diatom records higher dar at is2 and 4 may be a reflection of local productivity generated by wind input of nutrients and advection from coastal sources the occurrence of dar spikes at the timing of some heinrich events seems to support the hypothesis of productivity maxima associated with the presence of meltwater off nw africa as previously proposed by other authors

modern coccolithophore and carbonate sedimentation along a productivity gradient in the canary islands region seasonal export production and surface accumulation rates abstract coccolithophores one of the main primary and carbonate producers in the surface waters of the canary islands region were investigated to quantify coccolith and coccolith carbonate export fluxes and to determine possible trends in species composition related to different productivity conditions present day fluxes were compared to the long term records of coccolith and carbonate accumulation in holocene sediments underneath the mooring locations coccolithophore flux variations in sediment traps at three stations along an e w transect at approximately 29 n from the upwelling influenced north west african shelf region to the open ocean were recorded from january to september 1997 total coccolith fluxes along the studied transect were highly seasonal with pronounced sedimentation events occurring in february upper traps and march deep traps related to the winter phytoplankton bloom in surface waters while total coccolith flux rates and fluxes of different species at the two oceanic sites lp and estoc were comparable mean 6 4 and 5 4 10 8 coccoliths m 2 d 1 fluxes increased significantly towards the shallower site ebc mean 13 7 10 8 coccoliths m 2 d 1 this gradient points to increased productivity in surface waters at the near shore station probably associated with nutrient rich upwelling filaments detaching from the coast and or the easternmost islands additionally a strong influence of laterally advected coccolith rich material was demonstrated for the deeper sediment traps at the two offshore sites resulting in an increase of mean daily coccolith fluxes by a factor 3 lp and by a factor 2 estoc respectively estimated coccolith carbonate fluxes along the transect were 2 8 g m 2 ebc and 1 2 1 3 g m 2 estoc lp in the upper traps representing 31 33 of the total annual carbonate flux the coccolith carbonate supply was higher in the deeper traps with 63 at lp and 44 at estoc respectively average holocene coccolith and coccolith carbonate accumulation rates derived from surface samples at the mooring locations generally confirm the sediment trap results discrepancies mainly result from near bottom and down slope particle transfer processes which especially influence the site closer to the shelf dissolution processes are likely to affect primarily the deep sediments at the oligotrophic site lp the diverse subtropical coccolithophore assemblages were quite similar in all sediment trap samples from the three mooring sites with three species emiliania huxleyi florisphaera profunda gephyrocapsa ericsonii contributing significantly to the assemblage despite the good correspondence between the assemblage compositions along the transect some trends towards the upwelling influenced site ebc could be observed whereas all gephyrocapsa species mainly g ericsonii respond positively to higher productivity conditions at the near shore station calcidiscus leptoporus and the deep photic zone species f profunda increased in mean relative abundances towards the two offshore sites pointing to a preference for the more stable oligotrophic conditions of the subtropical gyre

influence of lateral particle advection and organic matter degradation on sediment accumulation and stable nitrogen isotope ratios along a productivity gradient in the canary islands region abstract we compare total and biogenic particle fluxes and stable nitrogen isotope ratios 15n at three mooring sites along a productivity gradient in the canary islands region with surface sediment accumulation rates and sedimentary 15n higher particle fluxes and sediment accumulation rates and lower 15n were observed in the upwelling influenced eastern boundary region ebc compared to the oligotrophic sites north of gran canaria european station for time series in the ocean canary islands estoc and north of la palma lp the impact of organic matter degradation and lateral particle advection on sediment accumulation was quantified with respect to the multi year flux record at the estoc remineralisation of organic matter in the water column and at the sediment surface resulted in an organic carbon preservation of about 0 8 and total nitrogen preservation of about 0 4 of the estimated export production higher total and carbonate fluxes and accumulation rates in the lower traps and surface sediment compared to the upper traps indicated that at least 50 of the particulate matter at the estoc was derived from allochthonous sources low 15n values in the lower traps of the estoc and lp point to a source region influenced by coastal upwelling we conclude from this study that the reconstruction of export production or nutrient regimes from sedimentary records in regions with strong productivity gradients might be biased due to the mixture of particles originating from autochthonous and allochthonous sources this could result in an imprint of high productivity signatures on sedimentation processes in oligotrophic regions

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

seismic evidence for a magma chamber beneath the slow spreading mid atlantic ridge seismic reflections from magma chambers have been observed along the fast spreading east pacific rise1 2 and the intermediate spreading valu fa ridge3 4 sub axial reflections also exist beneath the intermediate spreading juan de fuca ridge5 but no magma chambers have been identified beneath the slow spreading mid atlantic ridge suggesting that here magma chambers lie unusually deep or are transient features6 11 seismic reflection profiles acquired in 1989 over the snake pit hydrothermal area in the rift valley of the mid atlantic ridge 25 km south of the kane fracture zone showed no evidence of magmatic activity12 although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1a 2 km13 14 by suppressing in these data high amplitude coherent noise generated at the sea floor i have obtained images in an otherwise non reflective crust of seismic reflections beneath and just south of the snake pit hydrothermal area these reflections define a small 4 km wide dome whose apex is 1 200 m beneath the sea floor as bright reflections from the upper flanks of this dome occur in the depth range suggested by the vent fluid geochemistry i interpret the dome to be the seismic expression of a small magma chamber

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

two dimensional seismic reflection modeling of the inferred fossil oceanic crust mantle transition in the bay of islands ophiolite we investigate the origin and character of oceanic moho reflections by computing two dimensional synthetic seismogram profiles of the inferred fossil oceanic crust mantle transition observed in the bay of islands ophiolite to simulate a seismic reflection experiment we calculated near vertical incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length in the synthetic profiles the moho reflection event varies from a single phase to two or more phases of up to 1 s two way travel time total duration individual phases show lateral variation in amplitude and their two way travel times vary by as much as 0 25 s over horizontal distances as short as 10 km lateral discontinuity of phases results in abrupt variations in the travel times of first arriving high amplitude moho phases the geological structures generating the highest amplitude moho reflections vary from high and low velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the moho transition zone reflection amplitudes from the residual upper mantle are insignificant and our modeling suggests that using the first arriving high amplitude moho phase to estimate thickness of magmatic material might result in errors of up to 1 s two way travel time 3 4 km multichannel seismic data from both the western pacific and western north atlantic show moho travel time variations similar to those observed in the synthetic profiles the western north atlantic data also show multiphase moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data suggesting that the structures observed in the inferred fossil crust mantle transition of the ophiolite are characteristic of oceanic lithosphere

the structure of young oceanic crust near a very fast spreading ridge we have modeled the structure of the young oceanic crust near the fastest spreading ridge ever to be profiled using seismic refraction techniques the data were originally collected during the capricorn expedition of the scripps institute of oceanography in 1953 one of the split leg profiles from that expedition c22 was conducted near the east pacific rise at 15 s where the half spreading rate is over 80 mm yr by digitizing the data and modeling the two splits of c22 using synthetic seismograms we have found that the crust is normal in thickness 5 to 7 km and in velocity structure an upper crustal transition zone is present on both legs and would appear to be a regular feature of young oceanic crust it is probably too deep 1 5 to 2 km beneath the seafloor to be correlated with the boundary between pillow basalts and sheeted dikes instead we attribute this transition zone to changes in crustal porosity and or hydrous minerals in the oceanic crust from the appearance of wide angle moho reflections near the rise axis we suggest that the very large crustal magma chamber that might be expected under this fast spreading ridge is not present this presumably results from the cooling of the crust by pervasive hydrothermal circulation

the transition zone of the canary current upwelling region abstract like all the major upwelling regions the canary current is characterised by intense mesoscale structure in the transition zone between the cool nutrient rich waters of the coastal upwelling regime and the warmer oligotrophic waters of the open ocean the canary island archipelago which straddles the transition introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and trade winds the combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability on the basis of historical data and a series of interdisciplinary field studies the principal features of the region are described these include a prominent upwelling filament originating near 28 n off the african coast cyclonic and anti cyclonic eddies downstream of the archipelago and warm wake regions protected from the trade winds by the high volcanic peaks of the islands the filament is shown to be a recurrent feature apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone its role in the transport and exchange of biogenic material including fish larvae is considered strong cyclonic eddies observed throughout the year drift slowly southwestward from gran canaria one sampled in late summer was characterised by large vertical isopycnal displacements apparent surface divergence and strong upwelling producing a fourfold increase in chlorophyll concentrations over background values such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen the lee region of gran canaria is shown to be a location of strong pycnocline deformation resulting from ekman pumping on the wind shear boundaries which may contribute to the eddy formation process

old pacific crust near hawaii a seismic view an expanding spread profile esp over 80 ma pacific ocean crust near hawaii was modeled with a one dimensional solution for travel times relative amplitudes and sometimes frequency content the resulting profiles of compressional and shear wave velocities and poisson s ratio are similar to those determined by other ocean crustal studies except that the shear wave q in the sediments is very low 10 and some unusual high frequency high phase velocity arrivals require a laminated structure within the mid crustal section at about 3 km below the base of the sediment the laminations are probably alternating layers of gabbro and ultramafic cumulates the cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation reflections which may correspond to these laminations have been observed in reflection records both near this esp and in the north atlantic comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity lithology model is probably in error for the pillow and dike basalts resulting in an interpretation of overly thick layers in previous studies of refraction lines the preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1 0 and 1 5 km

microparticle concentration variations linked with climatic change evidence from polar ice cores the microparticle concentrations in three deep ice cores reveal a substantial increase in the concentration of insoluble particles in the global atmosphere during the latter part of the last major glaciation the ratio of the average particle concentration in the late glacial strata to that in the holocene strata is 6 1 for the core from dome c antarctica 3 1 for the core from byrd station antarctica and 12 1 for the core from camp century greenland whether this temporal correlation between increased atmospheric particle load and the lower surface temperatures is directly causal is unknown however the variations in these two parameters must be satisfactorily resolved in any successful hypothesis that addresses the causes of climatic change

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

lava flow layer east pacific rise a multichannel seismic reflection profile across the east pacific rise near 9 n reveals an approximately 400 m thick layer at the top of the oceanic crust with seismic velocity of about 2 8 km s at the rise crest this velocity increases to about 3 8 km s 210 km east of the rise crest these low velocities suggest that this layer is probably a porous lava flow layer layer 2a of seismic refraction studies generated by volcanic activity at the spreading center reflections from the base of the layer imply a relatively sharp transition between the lava flows and the underlying layer presumably a sheeted dike complex the underlying layer is a zone about 1 4 km thick with velocity about 5 7 km s whose base may coincide with the top of a low velocity magma chamber from which the crust develops

multichannel seismic evidence for anomalously thin crust at blake spur fracture zone a two ship wide aperture multichannel seismic reflection profile obtained across a small western north atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones reflections from the crust mantle boundary typically observed at about 2 s below the basement reflection rise to only 0 8 s beneath the fracture zone trough this corresponds to crustal thicknesses of perhaps as little as 2 to 2 4 km in the fracture zone less than half the thickness of normal oceanic crust this is the first time that moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset the existence of thin crust at both large and small offset fracture zones suggests that the magnitude of the thermal edge effect at ridge transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

nitrate isotope anomalies reflect n2 fixation in the azores front region subtropical ne atlantic 1 the subtropical northeast atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production to better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of n2 fixation in providing new nitrogen n we investigated the depth distribution of nitrate 15n and 18o at six stations across the azores front in the ne atlantic in addition we measured the 15n of dissolved organic n don in surface waters and of sinking particulate n collected in sediment traps at 2000 m depth between 2003 and 2005 at station kiel276 the nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the 15n from depth toward low nitrate surface waters concomitant with an increase in 18o given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate 15n and 18o the observed surface water nitrate isotope anomalies 15 18 up to 6 indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed n that is remineralized in surface and subsurface waters both the concentration of don and its 15n in surface water were spatially invariant showing mean values of 4 7 0 5 mol l 1 and 2 6 0 4 n 35 respectively supporting the conjecture of a mostly recalcitrant don pool the weighted biannual mean 15n of sinking particulate n 1 8 0 8 n 33 was low with respect to thermocline nitrate the anomalous dual nitrate isotope signatures together with the low 15n of export production and elevated nitrate to phosphate ratios in surface and subsurface waters strongly suggest that n2 fixation represents a substantive source of n in this part of the subtropical northeast atlantic simple isotope mass balance suggests that locally n2 fixation supplies between 56 and 259 mmol n m 2 a 1 for phytoplankton growth in the photic zone accounting for up to 40 of the estimated export production

the mohorovicic discontinuity in ocean basins some observations from seismic data abstract since the late 1970s studies of the oceanic crust using airgun sources and towed hydrophone arrays have been conducted in a wide range of ocean basin settings including mid ocean ridges and old oceanic crust the very earliest studies were performed at the fast spreading east pacific rise epr and revealed a very distinct almost continuous vertical incidence reflection event at a depth corresponding to the crust mantle transition as inferred from seismic refraction studies this suggested that the transition was quite sharp in comparison to the source wavelength that moho was observed very close to or even exactly beneath the ridge crest implied that it was formed at essentially zero age since then many experiments using progressively improving airgun arrays and streamer systems have expanded these observations here we review the literature presenting studies of normal oceanic crust produced at mid ocean ridges with the objective of assessing the age of formation of moho and the nature of variability of moho signature in multi channel seismic data moho is observed as a consistent feature for all spreading rates but appears quite variable being very distinct in some areas complex in form in others and absent in many regions as much as 40 although fast spread crust is associated with the strongest simplest and most laterally continuous moho images we see significant variability at almost all spreading rates and ages where moho is absent from vertical incidence data this cannot be fully explained by the effect of scattering or attenuation in the crustal section above moho can be observed at zero age in only a small number of areas at or near overlapping spreading centers on the epr after three decades of marine seismic studies many fundamental questions remain concerning the nature of the moho that will require targeted experiments to solve

glacial interglacial variability of particle accumulation in the canary basin a time slice approach abstract particle supply and sediment accumulation in the canary basin are strongly modified by seasonal coastal upwelling and dust input from the sahara along an e w depth profile and fertility gradient north of the canary islands we demonstrate an increase in total mass accumulation rates from a low of 1 2 g cm 2 ka 1 in the outer oceanic domain to 6 12 g cm 2 ka 1 closer to the moroccan shelf in holocene sediments during the last glacial maximum lgm oxygen isotopic event 2 2 a steeper gradient was found with two to four times higher mass accumulation rates at the near shore sites total mass accumulation rates during the last interglacial climax lic isotopic event 5 5 and end of the penultimate glaciation epg isotopic event 6 2 resemble holocene values with lowered near shore maxima of 3 and 4 g cm 2 ka 1 respectively analyses of separate sediment components show that total organic carbon total carbonate lithogenic and diatom accumulation rates were generally enhanced during glacial times especially during the lgm in contrast glacial accumulation rates of biogenic carbonate particles coccoliths and planktic foraminifera shells remained constant or were lowered with respect to interglacial time periods at the site closest to the moroccan shelf despite a doubling of bulk carbonate accumulation this could indicate lowered supply from the productive euphotic zone or a latitudinal offshore shift of the main sedimentary depocentre however enhanced carbonate dissolution could by itself account for this decrease in biogenic carbonate accumulation increased glacial input of detrital carbonate derived from the exposed moroccan shelf appears to be responsible for the overall enhanced carbonate accumulation at the same sites east of lanzarote