secondary circulation in a region of flow curvature relationship with tidal forcing and river discharge 1 on the basis of a 301 day acoustic doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge during low flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow with bottom flow toward the inside of the bend and surface flow toward the outside of the bend the strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed during times of high river discharge the strength of secondary flow is significantly reduced and its vertical structure undergoes a fundamental change over the spring neap cycle during spring tides the classic helical flow pattern is evident albeit weaker than during low flow conditions however during neap tides a more complex two cell structure is evident the change between these two states occurs with a spring neap transition in the subtidal flow indicating that it is also accompanied by changing stratification simple scaling analysis suggests that during weakly stratified conditions secondary circulation will influence stream wise dynamics and dispersion for channels with widths on order or less than 0 1 h cd where h is the water column depth and cd is a quadratic bottom drag coefficient in contrast during highly stratified conditions lateral excursions due to secondary flows are limited to approximately one tenth of the channel s width and are an ineffective lateral mixing agent

new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

interaction of lateral baroclinic forcing and turbulence in an estuary 1 observations of density and velocity in a channel in northern san francisco bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross channel baroclinic pressure gradient and vertical mixing due to turbulence profiles of velocity salinity temperature and suspended sediment concentration were measured in transects across suisun cutoff in northern san francisco bay on two days over the 12 5 hour tidal cycle during flood tides an axial density front developed between fresher water flowing from the shallows of grizzly bay into the northern side of suisun cutoff and saltier water flowing up the channel north of the front transverse currents were driven by the lateral salinity gradient with a top to bottom velocity difference greater than 30 cm s south of the front the secondary circulation was weak and along channel velocities were greater than to the north the gradient richardson number shows that stratification was stable north of the front while the water column was turbulently mixed south of the front time series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle in estuaries longitudinal dynamics predict less stratification during flood than ebb tides these data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

sediment pumping by tidal asymmetry in a partially mixed estuary 1 observations collected at two laterally adjacent locations are used to examine the processes driving sediment transport in the partially mixed york river estuary estimates of sediment flux are decomposed into advective and pumping components to evaluate the importance of tidal asymmetries in turbulent mixing at the instrumented location in the estuarine channel a strong asymmetry in internal mixing due to tidal straining is documented with higher values of eddy viscosity occurring during the less stratified flood tide as a result of this asymmetry more sediment is resuspended during the flood phase of the tide resulting in up estuary pumping of sediment despite a net down estuary advective flux at the instrumented location on the adjacent shoal where no pronounced tidal asymmetry in internal mixing was found both the pumping flux and advective flux were directed down estuary the down estuary pumping of sediment on the shoal appears to be driven by asymmetries in bed stress the impact of tidal asymmetries in bed stress at the channel location was negated because the amount of sediment available for resuspension was limited as a result the pumping flux was dominated by the overlying asymmetries in internal mixing the asymmetries in stratification appear to exert an important control on the vertical distribution of sediment by both impacting the eddy diffusivity as well as the fall velocity during the more turbulent flood tide the fall velocities are smaller suggesting the kolmogorov microscale is setting the upper bound on floc diameter

lateral circulation and sediment transport driven by axial winds in an idealized partially mixed estuary 1 a 3d hydrodynamic model roms is used to investigate lateral circulation in a partially mixed estuary driven by axial wind events and to explore the associated transport of sediments the channel is straight with a triangular cross section the model results suggest that driving mechanisms for lateral circulation during axial wind events are different between stratified and unstratified conditions when the water column is largely unstratified rotational effects do not drive significant lateral circulation instead differential advection of the axial salinity gradient by wind driven axial flow is responsible for regulating the lateral salinity gradients that in turn drive bottom divergent convergent lateral circulation during down up estuary winds from the subtidal lateral salt balance it is found that the development of lateral salinity gradient by wind induced differential advection is largely counterbalanced by the advection of salt by lateral circulation itself when the water column is stratified the lateral flow and salinity structures below the halocline closely resemble those driven by boundary mixing and rotational effects are important lateral sediment flux and the event integrated sediment transport are from channel to shoals during down estuary winds but reversed for up estuary winds potential impacts of wind generated waves on lateral sediment transport are evaluated with two cases representing event conditions typical of upper chesapeake bay accounting for wind wave effects results in an order of magnitude increase in lateral sediment fluxes because of greater bottom stresses and sediment resuspension

particle trapping in estuarine tidal flows particle trapping in estuarine turbidity maxima etm is caused primarily by convergent mean and or tidal fluxes of sediment the result is an approximately bell shaped along channel distribution of vertically integrated tidal cycle mean suspended sediment concentration observations from the columbia river estuary suggest that 1 strong two layer or internal along channel residual and overtide flows are generated by time varying stratification and 2 correlations between the near bed velocity and the suspended sediment fields at these frequencies are important in landward transport of sediment a new spatially and temporally integrated form of the sediment conservation equation has been derived to analyze this trapping process time changes in tidally averaged sediment concentration between two estuarine cross sections can be shown to be related to the divergence of the seaward river flow transport the divergence of velocity shear sediment stratification correlations for the mean flow and each tidal constituent and net erosion or deposition at the bed vertically integrated variables other than seaward river transport are absent from this integrated balance analysis of sediment fluxes using this balance supports the idea that internal residual and overtide circulations are primarily responsible for the landward sediment transport on the seaward side of etm found near the upstream limits of salinity intrusion the balance also shows that attempts to represent fluxes causing trapping of sediment in an etm as a product of a time mean vertically integrated along channel gradient and a diffusivity inevitably lead to the appearance of countergradient transport and thus a negative diffusivity on the seaward side of the etm this result occurs because the trapping process is inherently nonlinear and at least two dimensional and because a one dimensional representation is physically unrealistic

implications of updated magnetic anomalies for the late cretaceous tectonic evolution of walvis ridge in the south atlantic a reorganization of the mid atlantic ridge began before anomaly c34n 83 6 ma and ended before anomaly c30n 66 4 ma complicating tectonics of rio grande rise and older walvis ridge wr which formed together at the mid atlantic ridge this reorganization is poorly understood because magnetic anomalies c30n c34n are poorly defined near wr we interpreted these anomalies along westernwrto improve knowledge of rio grande rise wrtectonic development anomaly trends indicate that valdivia bank has an e w age progression perpendicular to that predicted by hot spot models anomaly spacing and width is irregular and anomalous near wr implying a series of ridge jumps and possibly a microplate between anomalies c34n and c32n eastward ridge jumps transferred microplate lithosphere to the south american plate this study shows that late cretaceous tectonic evolution of the rio grande rise wrlarge igneous provinces was more complex than previously understood

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

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

locating the spreading axis along 80 km of the mid atlantic ridge south of the atlantis transform the zone of active eruptive fissuring the spreading axis is investigated at an accretionary segment of the mid atlantic ridge high resolution side scan images are used to produce a detailed geologic map of the median valley floor of segment 18 located immediately south of the atlantis transform 30 n the spreading axis is defined by distinguishing between primary eruptive vents seamounts and hummocky ridges fed by underlying dikes and secondary vents seamounts and terraces fed from lava tubes or channels and is found to be about 2 km wide or less along the entire segment it is impossible to narrow the width further with the data we have in hand for comparison at faster spreading ridges such as the east pacific rise the spreading axis is generally equated to the width of the axial summit trough typically 50 100 m wide within the brunhes anomaly the high resolution structure of the central anomaly magnetic high camh obtained from several cross axis deep tow tracks is used to define regions of recent major volcanism the locations of the camh peaks generally conform to our pick of the spreading axis in some places the peaks are located several kilometers away from the spreading axis possibly indicating that large volumes of lava have been deposited there our interpretation of many volcanic structures as secondary vents at segment 18 leads to the conclusion that lava channels and tubes commonly develop to transport lava to these off axis deposition sites

the rhine outflow plume studied by the analysis of synthetic aperture radar data and numerical simulations the dynamics of the rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations the remote sensing data consist of 41 synthetic aperture radar sar images acquired by the first and second european remote sensing satellites ers 1 and ers 2 over the outflow region of the river rhine most of them show sea surface signatures of oceanic phenomena for example surface current and wind variations ship wakes and oil slicks in particular in 36 of these images pronounced frontal features are visible as narrow zones of mainly enhanced sometimes enhanced reduced radar backscatter that can be associated with the rhine surface front within the area enclosed by the frontal line large zones characterized by a lower radar backscatter than in the outer area are often visible the analysis of the ers sar images suggests that the form and the location of the frontal features are mainly linked to the semidiurnal tidal phase in the outflow region although their variability suggests also that they weakly depend on river discharge residual currents and neap spring tidal cycle in order to test this observational hypothesis the results obtained from the analysis of the ers sar images are compared with the results obtained from the numerical simulation of the hydrodynamics of the rhine outflow region carried out using a two layer frontal model which is based on the nonlinear hydrostatic shallow water equations on an f plane the model is forced by prescribing tidal and residual currents and river discharge at the open boundaries several simulations are performed by varying the values of these forcing parameters the numerical results corroborate the observational conjecture it is found that the form and the location of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the outflow region and that river discharge residual currents and neap spring tidal cycle contribute only secondarily to their determination inserting the simulated surface velocity field into a simple radar imaging model that relates the modulation of the backscattered radar power to the surface velocity convergence in radar look direction narrow elongated bands of enhanced radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated rhine plume area the consistency of the model results with the results obtained from the analysis of the sar images enables one to infer a mean spatial and temporal evolution of the rhine outflow plume over a semidiurnal tidal cycle from the analysis of spaceborne sar images acquired during different tidal cycles over the rhine outflow area and suggests the possibility of using numerical modeling in conjunction with the analysis of spaceborne measurements for monitoring the oceanic variability in the rhine outflow area

a new high resolution seafloor age grid for the south atlantic digital grids of basement age of the world s oceans are essential for modern geodynamic and paleoceanographic studies any such grid is built using a plate kinematic model whose accuracy and reli ability directly influence the accuracy and reliability of the grid we present a seafloor age grid for the south atlantic based on a recent high resolution plate kinematic model the grid is built from a data set of points whose ages are defined in or for the plate kinematic model incorporating breaks at tectonic boundaries like fracture zones where the age function is discontinuous we compare predictions of the new grid and of a previously published one which is based on an older plate kinematic model to magnetic isochron pick data sets the comparison shows the new grid to provide a more reliable depiction of seafloor age in the south atlantic numerical estimates of the new grid s uncertainty are determined by interpolation between 1 misfits at grid cells coinciding with magnetic isochron ages 2 misfits implied by locational uncertainties in predicted isochrons propagated from uncertainties in the plate kinematic model and 3 by the proxim ities of cells to fracture zone traces or ridge jump scars estimated total uncertainty is 10 my for 94 of the grid and 5 my for 72 but much larger in areas where magnetic anomaly data are scarce such as the cretaceous normal superchron and in the vicinity of long offset fracture zones

semiautomatic fracture zone tracking oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics satellite altimetry missions have resulted in high resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified and numerous scientists have digitized such lineaments we have initiated a community effort to maintain low cost infrastructure that allows seafloor fabric lineaments to be stored accessed and updated a key improvement over past efforts is our processing software released as a gmt5 supplement that allows for semiautomatic corrections to previously digitized fracture zone traces given improved gridded data sets here we report on our seafloor fabric processing tools which complement our database of seafloor fabric lineations magnetic anomaly identifications and plate kinematic models

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

thermal correction for moho depth estimations on west philippine basin a python code to calculate the gravitational effects of lithospheric cooling under oceanic crust the aim of this work is to present an easy to use python code to calculate the gravitational effect due to lateral variations in the thermal structure of oceanic lithospheric plates necessary to appropriately isolate the contribution of the variations in crustal thickness the model is applied to calculate the moho depth in the west philippine basin and the results are compared with seismic data the estimations of the moho depth taking into account the thermal correction presented a better fit with the seismic data and smoother geographical variations than the model without thermal correction

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

intraplate deformation of oceanic crust near the rio grande rise in the south atlantic abstract globally oceanic crust has relatively uniform thickness and structure compared to continental crust with typical ocean crust being 5 8 km thick however a substantial proportion of oceanic crust is thickened outside the normal range in the form of oceanic plateaus and crust affected by hotspots the rio grande rise rgr is the most prominent bathymetric feature in the western south atlantic ocean and was thickened by activity of the tristan hotspot we use multichannel seismic data collected along the eastern margin of the rgr to investigate the relationship history between the thickened crust of the rgr and the adjacent normal crust we find numerous normal faults spatially coincident with the region where oceanic crust begins to thicken from normal to rgr crust using crosscutting relationships and superposition our interpretation supports protracted extensional deformation at the eastern margin of the rgr including recent activity that appears to deform the modern seafloor we attribute the faulting to differential subsidence linked to crustal thickness variations between the thickened rgr and normal oceanic crust given the fundamental processes we attribute to the cause of faulting and the proportion of thickened oceanic crust globally it is possible that regions of many oceanic plates experience intraplate deformation after formation

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

axial convergence in a well mixed estuary abstract transverse secondary circulations involving surface convergence observed in a well mixed estuary in north wales are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary the circulation and axial convergence however are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross section produced by non uniform advection of the longitudinal gradient through the channel a diagnostic model using measured mean distributions of cross sectional density indicates surface transverse velocities 0 1 ms 1 similar to those observed in the estuary the model further predicts appreciable transverse divergent currents at a fractional depth of 0 75 a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators

lateral circulation and suspended sediment transport in a curved estuarine channel winyah bay sc usa 1 shipborne vertical profiles of flow and suspended sediment concentration collected on a transect across a curved nonsymmetrical estuarine channel are presented analysis of the transient cross channel momentum balance equation shows that the lateral circulation pattern is controlled by the interaction between centrifugal and lateral baroclinic forcings although those two might not be necessarily in balance as suggested earlier by seim and gregg 1997 instead differential along channel advection and local acceleration appear to influence greatly lateral circulation dynamics during ebb when the water column is highly stratified the interaction between centrifugal acceleration and opposite directed lateral baroclinic forcing results in weak lateral flows during flood lateral flows are dominated by centrifugal acceleration which is directed toward the outside of the curvature at the mid depth because of the nonlogarithmic current profile and reinforced by lateral baroclinic forcing this results in strong two layer clockwise circulation looking up estuary during flood the eastward directed bottom currents during ebb deliver only a small amount of suspended sediment from the relatively narrow western shoal to the channel bed during flood the west directed near bed currents deliver a significant amount of sediments from the gentle broad eastern shoal which in conjunction with the locally resuspended sediment load promotes the development of the estuarine turbidity maximum increased lateral advection of sediments during flood reinforces a tidal asymmetry in the development of turbidity maximum decomposition analysis of lateral sediment fluxes averaged over a tidal cycle suggests convergence of sediment toward the center of the channel is driven mainly by the oscillatory tidal component

effects of winds on stratification and circulation in a partially mixed estuary 1 numerical experiments are conducted to investigate how axial winds affect stratification and circulation in the partially mixed estuary of chesapeake bay in the absence of rotational effects stratification in the estuary decreases following both down estuary and up estuary winds but stratification experiences larger reduction and takes longer to recover under up estuary winds in the presence of rotational effects wind driven lateral circulations cause the lateral straining of density field and weaken the shear in the along channel flows under the down estuary winds a counterclockwise lateral circulation steepens isopycnals in the cross channel sections while the coriolis force acting on it decelerates the downwind current in the surface layer and the upwind directed current in the bottom layer under the up estuary winds a clockwise lateral circulation flattens isopycnals in the cross channel sections and reduces the shear between the surface and bottom currents hence in the presence of rotational effects the lateral straining offsets the effects of longitudinal straining such that the asymmetry in stratification reduction is significantly reduced between the down estuary and up estuary winds regime diagrams based on wedderburn w and kelvin ke numbers are constructed to summarize the net effects of winds on estuarine stratification during both wind perturbation and postwind adjustment periods

the influence of lateral advection on the residual estuarine circulation a numerical modeling study of the hudson river estuary abstract in most estuarine systems it is assumed that the dominant along channel momentum balance is between the integrated pressure gradient and bed stress scaling the amplitude of the estuarine circulation based on this balance has been shown to have predictive skill however a number of authors recently highlighted important nonlinear processes that contribute to the subtidal dynamics at leading order in this study a previously validated numerical model of the hudson river estuary is used to examine the forces driving the residual estuarine circulation and to test the predictive skill of two linear scaling relationships results demonstrate that the nonlinear advective acceleration terms contribute to the subtidal along channel momentum balance at leading order the contribution of these nonlinear terms is driven largely by secondary lateral flows under a range of forcing conditions in the model runs the advective acceleration terms nearly always act in concert with the baroclinic pressure gradie

bathymetric controls on sediment transport in the hudson river estuary lateral asymmetry and frontal trapping 1 analyses of field observations and numerical model results have identified that sediment transport in the hudson river estuary is laterally segregated between channel and shoals features frontal trapping at multiple locations along the estuary and varies significantly over the spring neap tidal cycle lateral gradients in depth and therefore baroclinic pressure gradient and stratification control the lateral distribution of sediment transport within the saline estuary sediment fluxes are strongly landward in the channel and seaward on the shoals at multiple locations bottom salinity fronts form at bathymetric transitions in width or depth sediment convergences near the fronts create local maxima in suspended sediment concentration and deposition providing a general mechanism for creation of secondary estuarine turbidity maxima at bathymetric transitions the lateral bathymetry also affects the spring neap cycle of sediment suspension and deposition in regions with broad shallow shoals the shoals are erosional and the channel is depositional during neap tides with the opposite pattern during spring tides narrower deeper shoals are depositional during neaps and erosional during springs in each case the lateral transfer is from regions of higher to lower bed stress and depends on the elevation of the pycnocline relative to the bed collectively the results indicate that lateral and along channel gradients in bathymetry and thus stratification bed stress and sediment flux lead to an unsteady heterogeneous distribution of sediment transport and trapping along the estuary rather than trapping solely at a turbidity maximum at the limit of the salinity intrusion

runoff and tidal influences on the estuarine turbidity maximum of a highly turbid system the upper humber and ouse estuary uk abstract results are presented from longitudinal and vertical measurements of salinity and turbidity made at spring and neap tides throughout the length of the upper humber and ouse estuary uk during a 1 year period that experienced strong freshwater runoff during winter and a long period of very low runoff during summer these data were complemented by tidal cycle profiling measurements at a site in the lower ouse despite the strong suspended particulate matter spm stratification salinity was fairly well mixed vertically except in the presence of high concentration near bed muddy layers the data consistently exhibited a strong estuarine turbidity maximum etm in the low salinity reaches of the estuary the geographical positions of the freshwater saltwater interface fsi x s and the up estuary intrusion of the etm the etm nose x p measured from the tidal limit exhibited a pronounced seasonal cycle a multiple regression for x p in terms of the monthly averaged runoff q 31 averaged over 31 days preceding the survey and the tidal range for the surveyed tide tr explained 97 of the variance in x p the etm nose was located in the lower estuary during high runoff winter conditions more than 50 km from the tidal limit and in the upper estuary during very low runoff summer conditions within 2 km of the tidal limit according to the regression the etm nose was shifted up estuary by 15 km between mean neap tides and mean spring tides the location of the fsi also depended strongly on runoff the regression for x s in terms of tr and q 31 explained 88 of the variance the fsi was located in the lower estuary during winter conditions and in the upper estuary during summer conditions and according to the regression there was an up estuary displacement of 6 km between mean neap tides and mean spring tides the separation between the etm nose and fsi x p x s tended to be positive etm nose down estuary of the fsi when the nose was located more than about 55 km down estuary from the tidal limit but was otherwise negative etm nose up estuary of the fsi when the nose was located within about 30 km of the tidal limit it tended also to be about 10 30 km up estuary of the fsi a multiple regression of x p x s in terms of x p and tr explained 78 of the variance the magnitude of the maximum depth averaged spm concentration within the etm p was most closely related to the location of the etm nose the logarithm of p decreased approximately linearly with x p and p was reduced from 35 g l 1 during summer when x p was less than 2 km to 3 5 g l 1 when x p was 58 km a regression explained 94 of the variance tidal cycle profiling illustrated the seasonal etm movements via the presence of very high turbidity waters in the lower estuary at high water during winter and spring at low water during summer and early autumn and at mid ebb through to mid flood in the late autumn this seasonal movement of the etm also was accompanied by a seasonal variability in sand sized subtidal and intertidal bed sediment distributions within the upper humber and ouse estuary in which very fine sand was transported down estuary over the high runoff winter period

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

apparent polar wander paths for the major continents 200 ma to the present day a palaeomagnetic reference frame for global plate tectonic reconstructions summary synthetic apparent polar wander apw paths for north america south america eurasia india central africa australia and antarctica for the last 200 myr are proposed computation of these apw paths is based upon the latest version 4 5a of the global paleomagnetic database gpmdb a revised global plate tectonic model since the early jurassic and a new technique for generating smoothed apw paths the smoothing technique includes the following steps 1 pre selection of palaeopoles including pre filtering parameters number of sites number of samples per site 95 per cent confidence circle about mean direction cleaning procedure and time uncertainty 2 generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit 3 independent spline regression analyses of the palaeolatitude and declination plots 4 removal of palaeolatitude or declination data that deviate by more than 10 from the regression curves post filtering process 5 generation of synthetic apw paths from the resulting palaeolatitude and declination plots these synthetic apw paths are then rotated into african coordinates to determine the best fit apw path and a global palaeomagnetic reference frame four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

lateral variability of sediment transport in the delaware estuary lateral processes contribute significantly to circulation and material transport in estuaries the mechanisms controlling transport may vary spatially such that shallow and deep regions of an estuary contribute differently to the total transport an observational study was conducted to explore the importance of lateral variability in sediment transport mechanisms in the delaware estuary seven moorings were deployed across the channel in the region of the estuarine turbidity maximum etm zone from april to august 2011 time series of along channel sediment transport reveal a consistent pattern of sediment export across the entire estuary during periods of high river discharge followed by a transition to import within the channel and export on the flanks during low river flow there is a persistent divergence of across channel sediment fluxes on the delaware side where sediment from the flank is transported toward both the channel and wetland coast decomposition of the fluxes highlight that across channel sediment transport is driven by mean lateral circulation whereas along channel transport is driven primarily by mean advection with tidal pumping contributing to about 30 of total transport the spatial and temporal variability of mean advection and tidal pumping were generally complementary with both contributing to the observed sediment transport pathways tidal pumping linked to tidal asymmetries in stratification and sediment resuspension was shown to drive both ebb driven export and flood driven import depending on the tidal variability of stratification the spatiotemporal patterns of sediment transport highlight the three dimensional structure of the etm and shed light on the variability of sediment transport mechanisms

southern ocean hotspot tracks and the cenozoic absolute motion of the african antarctic and south american plates abstract a detailed analysis based on an antarctica africa finite reconstruction at chron c29 64 ma an assumption of no relative wander between the marion prince edward and tristan hotspots and on recently revised bathymetric maps of the southern ocean region shows that the fixed hotspot reference frame is tenable for absolute plate motions bouvet hotspot and probably trinidade as well also shows little or no cenozoic relative motion contrary to previous models bouvet hotspot is unrelated to the meteor rise cape rise seamount chain instead the bathymetric data when compared with the predicted hotspot tracks indicate another hotspot exists near the southernmost south atlantic spreading ridge segment new geochemical evidence from the latter region supports this hypothesis in showing the effects of plume enrichment from a source that is compositionally distinct from bouvet the peculiar zig zag shape of the cape rise meteor rise lineament is the result of this hotspot crossing the active transform segment of the falkland agulhas fracture zone in late mesozoic times followed by an early cenozoic ridge jump to the pre weakened trace on the then south american plate from the averaged cenozoic absolute motions of the african antarctic and south american plates it is evident that antarctica has been most nearly stationary in an absolute motion sense

convergence of lateral flow along a coastal plain estuary a set of velocity profiles obtained in the james river estuary with an acoustic doppler current profiler was used in combination with the results of an analytic tidal model to depict the appearance of surface lateral flow convergences v y during both flood and ebb stages of the tidal cycle the bathymetry of the estuary was characterized by a main channel and a secondary channel separated by relatively narrow shoals lateral surface flow convergences appeared over the edges of the channels and were produced by the phase lag of the flow in the channel relative to the shoals flood convergences developed in the late tidal stages and ebb convergences appeared soon after maximum currents most of these convergences caused fronts in the density field and flotsam lines that also appeared over the edges of the channel and that lasted 2 hours the transverse flows associated with the convergences were mostly in the same direction throughout the water column in fact the vertically averaged flow produced the same convergence patterns as those near the surface the analytic tidal model reproduced well the timing and location of the convergences as observed in the james river model results with different bathymetry emulated the results in other estuaries e g axial convergence in an estuary with a channel in the middle this work showed that the strength of lateral convergences along the estuary was proportional to the tidal amplitude and the channel steepness it also suggested that the convergences were produced mainly by the tidal flow interacting with the channel shoal bathymetry i e that they did not require the presence of density gradients however the analytic model underestimated the magnitude of the convergences and did not account for vertical circulations associated with fronts the formation of fronts resulted from the interaction of the tidal flow with the bathymetry and the density field

influence of lateral advection on residual currents in microtidal estuaries abstract the influence of nonlinear lateral advection on estuarine exchange flow is examined with a scaling analysis and eight groups of idealized numerical experiments nonlinear lateral advection is related to the linkage between lateral circulation and the lateral shear of the along estuary flow the relative contribution of lateral advection to the overall dynamics of a microtidal estuary is found to be a function of width and depth and of vertical mixing lateral advection is dynamically important in narrow and deep estuaries particularly under relatively weak vertical mixing the relative importance of lateral advection and the earth s rotation on estuarine dynamics can be evaluated in terms of the nondimensional rossby and ekman numbers ro and ek lateral advection is most effective at large ro and small ek and is negligible at small ro and large ek as expected the earth s rotation is most significant at small ro and ek and is negligible at large ro and ek under the influence of lateral adv

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

frictional estuarine flow in juan de fuca strait with implications for secondary circulation estuarine flow in juan de fuca strait is highly seasonal in nature and an appreciable along channel surface pressure gradient is coincident with the peak fraser river discharge an upper layer momentum balance requires the vertical eddy viscosity av to be of the order of 0 02 m2 s 1 slightly larger than implied by conventional empirical formulae and used in current models mixing in an estuarine channel and a large vertical eddy viscosity should lead to significant cross channel flows this secondary circulation which may resemble internal ekman layers and intrusions of water mixed at the sloping sides of the strait is much less understood than the basic along channel estuarine exchange unfortunately historical current meter data in juan de fuca strait are of insufficient spatial resolution to show the predicted patterns

modeling of two layer eddies and coastal flows with a particle method an existing particle in cell pic numerical method developed for the study of two layer mesoscale motions with outcropping pycnocline is applied to lens like anticyclonic vortices and buoyant coastal currents from a first series of experiments investigating the evolution of an initially elongated lens like anticyclone it is found that motions induced in the lower layer act only to increase the rotation of the vortex structure and do not appear to affect the process of eccentricity reduction partial axisymmetrization eccentricity reduction if any produces a final vortex of aspect ratio between 1 8 and 1 9 a value that is very close to the stability threshold of large reduced gravity lenses a second series of experiments devoted to vortex mergers determines how the maximal separation distance for which two circular anticyclonic lenses merge spontaneously varies with vortex size ratio of lens radius to deformation radius and stratification ratio of lens central thickness to ocean depth a third series of experiments considers the interaction of a lens vortex in the upper layer with a potential vorticity anomaly in the lower layer second hand relative vorticity generated in the lower layer under the action of vertical stretching induced by the movement of the upper layer vortex interacts with first hand relative vorticity created by the existing potential vorticity to create effects similar to those predicted by studies of two layer point vortices hetons finally the pic method is generalized to simulate the finite amplitude instability of a buoyant geostrophic hydrostatic intrusion flowing along a vertical coastal wall those results however are reported here more as a demonstration on how the pic method can be extended to include coastal boundaries than as a thorough investigation of coastal current instabilities

layer outcropping in numerical models of stratified flows abstract layered models of stratified flow are favored by theoreticians because of their conceptual simplicity but have only seen limited use in numerical modeling due to difficulties related to layer outcropping vanishing layer depth two mass transport schemes capable of handling outcropping are tested against analytic solutions describing wind and buoyancy forced flow in a two and three layer ocean in both cases the model is found to closely reproduce the available analytic solutions this result helps alleviate longstanding reservations about the severity of the outcrop problem in layer models

the role of advection straining and mixing on the tidal variability of estuarine stratification abstractdata from the hudson river estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along channel tidal straining these observations result from three additional processes not accounted for in the traditional tidal straining model 1 along channel and 2 lateral advection of horizontal gradients in the vertical salinity gradient and 3 tidal asymmetries in the strength of vertical mixing as a result cross sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood these observations highlight the three dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters even when direct turbu

new angles on south atlantic opening existing models of relative motion between the south america and africa plates significantly misrepresent the azimuth of lower cretaceous seafloor spreading in the south atlantic ocean an improved model is derived from fits of fracture zones magnetic reversal anomalies and the edges of a rifted large igneous province at the northeast georgia rise and agulhas plateau an absolute date for this rifting event is not known but by assuming least change in spreading rates during magnetic chron c34 rifting can be estimated to have occurred at 100 ma this modelling demands a plate divergence history that involves diachronous opening of the south atlantic consistent with published estimates of the ages of breakup from sedimentary basins on the south american and african passive margins the diachronous opening lasts approximately 40 million years during which time it must be accommodated by significant intracontinental deformation a reconstruction using the intra c34 rotation also illustrates the earliest possibility for direct deep water connection between the central and south atlantic oceans one further consequence of this model is that a total reconstruction derived from it and closure of the central atlantic between north america and africa suggests that the venezuelan and gulf of mexico basins may be conjugates formed during the earliest opening of the central atlantic and not separate marginal basins

lateral entrapment of sediment in tidal estuaries an idealized model study two physical mechanisms leading to lateral accumulation of sediment in tidally dominated estuaries are investigated involving coriolis forcing and lateral density gradients an idealized model is used that consists of the three dimensional shallow water equations and sediment mass balance conditions are assumed to be uniform in the along estuary direction a semidiurnal tidal discharge and tidally averaged density gradients are prescribed the erosional sediment flux at the bed depends both on the bed shear stress and on the amount of sediment available in mud reaches for resuspension the distribution of mud reaches over the bed is selected such that sediment transport is in morphodynamic equilibrium that is tidally averaged erosion and deposition of sediment at the bed balance analytical solutions are obtained by using perturbation analysis results suggest that in most estuaries lateral density gradients induce more sediment transport than coriolis forcing when frictional forces are small ekman number e 0 02 the lateral density gradient mechanism dominates and entraps sediment in areas with fresher water results also show that the lateral sediment transport induced by the semidiurnal tidal flow is significant when frictional forces are small e 0 02 model predictions are in good agreement with observations from the james river estuary

new insights of the tectonic and sedimentary evolution of the rio grande rise south atlantic ocean abstract oceanic plateaus such as the rio grande rise rgr comprise crucial pieces of information about the opening of the south atlantic ocean and the evolution of associated lithospheric plate margins a detailed seismostratigraphic assessment of the volcano sedimentary package of the rgr was conducted in its western portion based on correlations between seismic data and dsdp site 516f the approximately 330 km long seismic transect is characterized by a complex array of normal faults related to extensional tectonics from late cretaceous to the miocene and probably into the present day along the cruzeiro do sul rift the bathymetric and seismic data reveal a the presence of deep faults that rise from the basement to the sea floor b the occurrence of several seamounts with tops at a water depth below 1000 m and c prominent graben structures that represent unidentified volcanic intrusions indicative of the reactivation of lower miocene faults by younger extensional events based on integration of older data with the results presented here we propose a five stage tectono sedimentary model for the western portion of the rgr 1 the first basaltic flow began to form the rio grande rise in the coniacian santonian 2 extensional movements caused rifting during the uplift of the large volcanic structures 3 volcanic islands emerged above sea level increasing the deposition of volcanic breccia and ash layers 4 after the volcanism ceased thermal subsidence took place over the entire rise with intense erosion and sedimentation and 5 the uppermost sedimentary layers were deposited in pelagic conditions and offset by sub vertical normal faults

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

observations of ridge hotspot interactions in the southern ocean the evolution of ridge hotspot systems is not well understood in this investigation satellite derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge hotspot interaction at four sparsely explored systems in the southern ocean these systems illustrate three different stages of ridge hotspot interaction in which a migrating spreading center approaches a hotspot pacific antarctic louisville passes over or is captured by the hotspot mid atlantic shona discovery and ultimately migrates away from the hotspot southeast indian kerguelen all of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading interestingly these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge hotspot systems a simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump the presence of previously uncharted obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered the azimuths of the aseismic ridges cannot be explained by plate kinematics alone they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure the features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets

modeling the lateral circulation in straight stratified estuaries the dynamics of lateral circulation in an idealized straight estuary under varying stratification conditions is investigated using a three dimensional hydrostatic primitive equation model in order to determine the importance of lateral circulation to the momentum budget within the estuary for all model runs lateral circulation is about 4 times as strong during flood tides as during ebbs this flood ebb asymmetry is due to a feedback between the lateral circulation and the along channel tidal currents as well as to time varying stratification over a tidal cycle as the stratification is increased the lateral circulation is significantly reduced because of the adverse pressure gradient set up by isopycnals being tilted by the lateral flow itself when rotation is included a timedependent cross channel ekman circulation is driven and the tidally averaged bottom lateral circulation is enhanced toward the right bank when looking toward the ocean in the northern hemisphere this asymmetry in the tidally averaged bottom lateral circulation may lead to asymmetric sediment transport leading to asymmetric channel profiles in straight estuaries for the weakly stratified model run advection due to lateral currents is a dominant term in both the along channel and cross channel momentum equations over a tidal cycle and for the tidally averaged momentum equations in the tidally averaged along channel momentum equation lateral advection acts as a driving term for the estuarine exchange flow and can be larger than the along channel pressure gradient therefore it should not be ignored when estimating momentum budgets in estuaries

zircon xenocryst evidence for crustal recycling at the mid atlantic ridge abstract the presence of zircon xenocrysts in mid atlantic ridge mar rocks poses essential questions concerning the path they have followed from their sources how they have survived transport through the mantle and why they are much more abundant than in any other ocean sixteen out of twenty five seafloor samples collected between s 21 to n 42 contained xenocrysts here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the atlantic seafloor age from which they were collected the xenocrysts are of two types young and old hf and o isotopes and trace elements indicate that the younger crystals 3 1 ma to 25 6 ma are oceanic derived from previous mar rocks whereas the older crystals 190 ma to 3 2 ga are continental derived from crust slivers located at the mar or immediate vicinity the high 18o 7 to 11 of many older xenocrysts indicates ultrafast recycling through the mantle otherwise the fast diffusivity of oxygen would have driven the xenocrysts 18o to that of their mantle hosts 18o 5 3 the incorporation of xenocrysts into mar magmas mainly occurs after ridge jumps when hot mafic magma pervades a new crustal section scavenging xenoliths or locally melts felsic wall rocks and mixes with the resulting zircon laden low t magmas in either case the fusible component of the xenocrysts source contaminates the upwelling mafic magma when the source was oceanic the resulting chemical and isotopic signatures of the contaminated magma would remain morb like albeit enriched in incompatible elements when the source was continental the resulting magma would reflect a perceptible continental influence crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the mar magmas compared with other morbs these features are ultimately related to the atlantic slow spreading because it promotes ridge jumping and hence active crustal recycling