drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

tidally generated turbulence over the knight inlet sill very high turbulent dissipation rates above 5 1024 wk g 21 were observed in the nonlinear internal lee waves that form each tide over a sill in knight inlet british columbia this turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions away from the sill turbulent dissipation was significantly lower 5 1027 to 5 1028 wk g 21 energy removed from the barotropic tide was estimated using a pair of tide gauges a peak of 20 mw occurred during spring tide approximately twothirds of the barotropic energy loss radiated away as internal waves while the remaining one third was lost to processes near the sill observed dissipation in the water column does not account for the near sill losses but energy lost to vortex shedding and near bottom turbulence though not measured could be large enough to close the energy budget

tidal mixing events on the deep flanks of kaena ridge hawaii abstract a 3 month mooring deployment august november 2002 was made in 2425 m depth on the south flank of kaena ridge hawaii to examine tidal variations within 200 m of the steeply sloping bottom horizontal currents and vertical displacements inferred from temperature fluctuations are dominated by the semidiurnal internal tide with amplitudes of 0 1 m s 1 and 100 m respectively a series of temperature sensors detected tidally driven overturns with vertical scales of 100 m a thorpe scale analysis of the overturns yields a time averaged dissipation near the bottom of 1 2 10 8 w kg 1 10 100 times that at similar depths in the ocean interior 50 km from the ridge dissipation events much larger than the overall mean up to 10 6 w kg 1 occur predominantly during two phases of the semidiurnal tide 1 at peak downslope flows when the tidal stratification is minimum n 5 10 4 s 1 and 2 at the flow reversal from downslope to upslope flow when the tidal stratification is ordinarily increasi

late eocene and maastrichtian paleomagnetic poles for the pacific plate implications for the validity of seamount paleomagnetic data abstract mean paleomagnetic poles for the pacific plate have been calculated for the late eocene 39 ma and the maastrichtian 69 ma the former is located at 77 6 n 7 6 e the latter at 69 9 n 0 9 e although these pole positions are little changed from previous calculations they are better constrained with additional data slightly less than half of the data are derived from the inversion of seamount magnetic fields providing an excellent opportunity to compare such data with other paleomagnetic data of the same age as no significant systematic difference between the two types of data is evident it is inferred that most seamount paleomagnetic data are probably useful indicators of the paleomagnetic field direction

breaking internal lee waves at kaena ridge hawaii shallow water oscillatory flows and deep ocean steady flows have both been observed to give rise to breaking internal lee waves downstream of steep seafloor obstacles a recent theory also predicts the existence of high mode oscillatory internal lee waves in deep water but they have not previously been directly observed here we present repeated spatial transects of velocity isopycnal displacement and dissipation rate measured with towed instruments on the south flank of a supercritical ridge in hawaii known as kaena ridge and compare them with predictions from a 3 d numerical model with realistic tidal forcing bathymetry and stratification the measured and modeled flow and turbulence agree well in their spatial structure time dependence and magnitude confirming the existence and predicted nature of high mode internal lee waves turbulence estimated from thorpe scales increases 2 orders of magnitude following downslope tidal flow when the internal lee wave begins to propagate upslope and breaks

the characterization of viscous remanent magnetization in large and small magnetite particles previous work on the properties of viscous remanent magnetization vrm have shown that it can play an important role in some paleomagnetic investigations the results of this study on the properties of vrm of very fine single domain sd and coarse multidomain md magnetite particles ranging in size from 0 02 to 60 m indicate that the md samples are very sensitive to the previous magnetic history of the samples the af demagnetization causes a dramatic decrease in the ability of md samples to acquire vrm compared to the same sample with an undemagnetized thermal remanent magnetization trm thermal demagnetization of the same md samples causes an increase in the ability to acquire vrm components submicron sd samples show vrm acquisition and decay curves that are independent of previous magnetic treatment both md and sd samples are extremely sensitive to the length of time of zero and weak field storage that occurs prior to the application of the external vrm field a model using the total energy of the system is used to describe the above results

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

contribution of induced magnetization to magnetization of seamounts a fundamental assumption in modelling seamount magnetic anomalies is that the contribution of induced magnetization is negligible the general consistency of seamount and non seamount palaeopoles scarcity of poles skewed toward the present field direction and the high ratio of remanent to induced magnetization koenigsberger ratio of many oceanic basalts have been cited as evidence supporting this assumption1 2 recent discussions concerning the dominance of normally magnetized seamounts have focused attention on the possible role of viscous and induced magnetization in seamount anomalies3 6 here we determine natural remanent magnetization initial volume susceptibility and the koenigsberger ratio for more than 2 000 samples from a subaerially exposed seamount section on la palma canary islands table 1 by contrast to results from the oceanic crust and ophiolites these data indicate that a variety of rock types are potential magnetic sources the significant induced component of intrusives underscores the importance of the lithological distribution in determining the character of seamount magnetic anomalies the la palma data together with a plausible lithological distribution indicate that induced magnetization may account for one sixth of seamount magnetization

energetics of m2 barotropic to baroclinic tidal conversion at the hawaiian islands abstract a high resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide m2 over a region of the hawaiian ridge from niihau to maui this region includes the kaena ridge one of the three main internal tide generation sites along the hawaiian ridge and the main study site of the hawaii ocean mixing experiment the 0 01 horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations moored adcp currents and notably reasonable agreement with microstructure data barotropic and baroclinic energy equations are derived from the model s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget the m2 barotropic tide loses 2 7 gw of energy over the study region of this 163 mw 6 is dissipated by bottom friction and 2 3 gw 85 is converted into internal tides internal tide generation primarily occurs along the flanks of the kaena ridge and

thermal fronts generated by internal waves propagating obliquely along the continental slope abstract rapid temperature falls occurring at semidiurnal periods are observed close to the bottom above the continental slope in the bay of biscay simultaneous current measurements reveal that the abrupt temperature decrease o 0 5 k within one minute is associated with a brief downslope current contrary to previous observations it is suggested that the flow field associated with internal waves propagating obliquely downslope is responsible for advecting denser water higher on the slope than lighter fluid resulting in a gravitationally unstable stratification the collapse of this stratification is observed as a thermal front passing the moored instruments

velocity structure of internal tide beams emanating from kaena ridge hawaii abstractobservations are reported of the semidiurnal m2 internal tide across kaena ridge hawaii horizontal velocity in the upper 1000 1500 m was measured during eleven 240 km long shipboard acoustic doppler current profiler adcp transects across the ridge made over the course of several months the m2 motions are isolated by means of harmonic analysis and compared to numerical simulations using the princeton ocean model pom the depth coverage of the measurements is about 3 times greater than similar past studies offering a substantially richer view of the internal tide beams sloping features are seen extending upward north and south from the ridge and then downward from the surface reflection about 40 km from the ridge crest closely matching theoretical m2 ray paths and the model predictions

large eddy simulation of flow and turbulence at the steep topography of luzon strait steep generation sites for topographic internal gravity waves can also be sites of turbulence the present work uses turbulence resolving large eddy simulation les for tidal flow over multiscale steep bathymetry patterned after a portion of the west ridge of luzon strait at 20 6 n oceanic values of the slope criticality froude number excursion number and aspect ratio are matched the amplitude and phasing of velocity overturns and dissipation are similar to observations made at a deep mooring however a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal downslope jets critical slope boundary layer high mode internal wave beams off slope lee wave breaking and wave breaking in a valley between subridges the model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23 5 is large

gravity inversion using seminorm minimization density modeling of jasper seamount a gravity inversion algorithm for modeling discrete bodies with nonuniform density distributions is presented the algorithm selects the maximally uniform model from the family of models which fit the data ensuring a conservative and unprejudiced estimate of the density variation within the body the only inputs required by the inversion are the gravity anomaly field and the body shape tests using gravity anomalies generated from synthetic bodies confirm that seminorm minimizing inversions successfully represent mass distribution trends but do not reconstruct sharp discontinuities we apply the algorithm to model the density structure of seamounts inversion of the seasurface gravity field observed over jasper seamount suggests the edifice has a low average density of 2 38g cm3 and contains a dense body within its western flank these results are consistent with seismic magnetic and petrologic studies of jasper seamount

overturning circulation driven by breaking internal waves in the deep ocean a global estimate of the water mass transformation by internal wave driven mixing in the deep ocean is presented the estimate is based on the energy conversion from tidal and geostrophic motions into internal waves combined with a turbulent mixing parameterization we show that internal wave driven mixing in the deep ocean can sustain 20 30 sv of water mass transformation one third or more of this transformation is attributed to lee waves generated by geostrophic motions flowing over rough topography primarily in the southern ocean while these results are uncertain due to many assumptions poorly constrained parameters and data noise that enter in the calculation the result that lee wave driven mixing plays an important role in the abyssal ocean circulation is likely robust the implication is that lee wave driven mixing should be represented in ocean and climate models but currently it is not citation nikurashin m and r ferrari 2013 overturning circulation driven by breaking internal waves in the deep ocean geophys res lett 40 doi 10 1002 grl 50542

opaque minerals in geothermal well no 7 krafla northern iceland abstract study of the opaque minerals from well no 7 krafla indicates two mineral assemblages 1 hydrothermally altered igneous minerals and 2 secondary minerals that have precipitated from the geothermal fluid at depths down to 2140 m and at temperatures up to more than 340 c chief amongst the chemically precipitated minerals are pyrite pyrrhotite and goethite which is described here for the first time in an icelandic geothermal drill hole the geothermal system at krafla has been periodically disturbed by the influx of volcanic emanations this article attempts to interpret by use of thermochemical calculations the processes affecting the precipitated mineral assemblage

nonuniform magnetization of jasper seamount paleopoles derived from seamounts have been used to reconstruct the tectonic history of ocean basins however the interpretation of seamount magnetization models and the validity of seamount paleopoles may be affected by inhomogeneous magnetization multibeam bathymetric data sea surface and deep tow magnetic field data and paleomagnetic analyses of dredged samples were used to examine the origin of nonuniform magnetization within jasper seamount 30 27 n 122 44 w models indicate that the seamount is predominantly reversely magnetized with local zones of normal polarity as corroborated by deep tow measurements lithologies likely to be volumetrically important in a seamount edifice show highly variable magnetic properties basalts have high intensities 0 5 27 0 a m high koenigsberger ratios q and low viscous remanence vrm acquisition low q ratios and high vrm acquisition coefficients of coarse grained material and volcaniclastics suggest that they may have substantial viscous and induced components models for jasper are characterized by low uniform intensities and far sided paleopoles the shallow model inclinations may be attributed to nondipolar components in the time averaged geomagnetic field the low intensities of the uniform models and the large nonuniform component in the seminorm solutions imply a complex distribution of magnetization sources within jasper this nonuniformity may result from either lithological variability or construction of the seamount spanning two or more polarity intervals

test for bias in paleomagnetically determined paleolatitudes from pacific plate deep sea drilling project sediments paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the pacific plate have been previously used in determining reference poles for the pacific plate apparent polar wander apw path from discordant paleoinclinations and corresponding paleolatitudes determined from these data some workers have inferred tectonic tilting or large horizontal motion of some deep sea drilling sites relative to other parts of the pacific plate here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a pacific plate reference apw path determined from seamount poles skewness and amplitudes of magnetic profiles across magnetically lineated seafloor and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies the reference apw path includes a new determination of the early cretaceous pole from several types of paleomagnetic data which agree well with one another sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference apw path a discrepancy that corresponds to inclinations shallower than expected thus most deep sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference apw paths inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinations

observation of a large lee wave in the drake passage abstractlee waves are thought to play a prominent role in southern ocean dynamics facilitating a transfer of energy from the jets of the antarctic circumpolar current to microscale turbulent motions important in water mass transformations two em apex profiling floats deployed in the drake passage during the diapycnal and isopycnal mixing experiment dimes independently measured a 120 20 m vertical amplitude lee wave over the shackleton fracture zone a model for steady em apex motion is developed to calculate absolute vertical water velocity augmenting the horizontal velocity measurements made by the floats the wave exhibits fluctuations in all three velocity components of over 15 cm s 1 and an intrinsic frequency close to the local buoyancy frequency the wave is observed to transport energy and horizontal momentum vertically at respective peak rates of 1 3 0 2 w m 2 and 8 1 n m 2 the rate of turbulent kinetic energy dissipation is estimated using both thorpe scales and a method that isolate

tidally forced internal waves and overturns observed on a slope results from home abstract tidal mixing over a slope was explored using moored time series observations on kaena ridge extending northwest from oahu hawaii during the survey component of the hawaii ocean mixing experiment home a mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453 m water depth for 2 months beginning in november 2000 the semidiurnal barotropic tidal currents at this site have a significant cross ridge component favorable for exciting an internal tidal response a large amplitude response is expected given that the slope of the topography 4 5 is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency density overturns were inferred from temperature profiles measured every 2 min the number and strength of the overturns are greater in the 200 m nearest the bottom with overturns exceeding 24 m present at any depth nearly

single plume model for asynchronous formation of the lamont seamounts and adjacent eeast pacific rise terrains sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial pacific the reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava the extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs the relative ages indicate that the locus of volcanic eruptions has migrated toward the east pacific rise axis zero age activity lies just to the west of the east pacific rise south of the clipperton transform and is probably the cause of an anomalously shallow neovolcanic zone the evidence fits a scenario in which eruptive centers appear evolve and become extinct in succession with little or no overlap in time the eruptive centers are fed in sequence through conduits from buoyant tightly focused magma plumes the near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits if the plume volume is small and the hydraulic head is not reached the seamount remains small with only a small crater the plumes ascend asynchronously from a common region of nucleation in the asthenosphere plumes that follow in close succession build a composite seamount with a string of summit calderas plumes that rise less often result in a line of discrete circular volcanoes

geomagnetic reversal frequency since the late cretaceous models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the heirtzler et al 1 magnetic polarity time scale which has subsequently been superseded examination of newer time scales shows that the mean reversal frequency is dominated in the cenozoic and late cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed subdivision into two periods of stationary behavior is no longer warranted the distribution of polarity intervals is visibly not poissonian but lacks short intervals the labrecque et al 2 polarity time scale shows the positions of 57 small wavelength marine magnetic anomalies which may represent short polarity chrons after adding these short events the distribution of all polarity intervals in the age range 0 40 myr is stationary and does not differ significantly from a poisson distribution a strong asymmetry develops in which normal polarity chrons are poisson distributed but reversed polarity chrons are gamma distributed with indexk 2 this asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the late cenozoic if short wavelength anomalies arise from polarity chrons the geomagnetic field may be more stable in one polarity than the other alternative explanations of the origin of short wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons however the observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long term memory

a simple parameterization of turbulent tidal mixing near supercritical topography abstract a simple parameterization for tidal dissipation near supercritical topography designed to be applied at deep midocean ridges is presented in this parameterization radiation of internal tides is quantified using a linear knife edge model vertical internal wave modes that have nonrotating phase speeds slower than the tidal advection speed are assumed to dissipate locally primarily because of hydraulic effects near the ridge crest evidence for high modes being dissipated is given in idealized numerical models of tidal flow over a gaussian ridge these idealized models also give guidance for where in the water column the predicted dissipation should be placed the dissipation recipe holds if the coriolis frequency f is varied as long as hn w f where n is the stratification h is the topographic height and w is a width scale this parameterization is not applicable to shallower topography which has significantly more dissipation because near critical processes dominate the observed turbul

geothermal model of the salazie cirque reunion island volcanic and structural implications abstract a programme of geothermal exploration on reunion island resulted in the siting of an exploration geothermal borehole in salazie cirque the borehole attained a depth of 2108 m and encountered temperatures of 192 8 c no aquifer was found the geological information obtained made it possible to propose a stratigraphy for the deep underlying rocks beneath the salazie cirque on the piton des neiges and thus to interpret the events that followed the emergence of the island a large gabbroic mass was intersected and found to comprise several successive intrusions the principal one of which is continuously zoned from melagabbro to monzonite modeling of geophysical data on the drillhole results enabled the distribution of the different geological units to be interpreted a geothermal model is proposed taking into account the dynamic evolution of the hydrothermal processes that preceded and accompanied emplacement of the gabbro responsible for the geothermal anomaly underlying the three cirques that sculpt the flanks of the piton des neiges

the generation and trapping of solitary waves over topography observations of tidal flow over a sill show that upstream solitary wave generation occurred in conjunction with the downstream movement of a streamline bifurcation over the obstacle crest solitary waves appeared upstream of the bifurcation riding on the surface stratification shear flow instabilities on the plunging interface were also evident and may contribute to wave generation through subharmonic interaction nonlinear waves generated downstream of an internal control became trapped in the supercritical flow

seamount magnetic polarity and cretaceous volcanism of the pacific basin in this paper we examine the magnetic polarity for pacific basin seamounts exclusive of the hawaiian ridge on pacific crust older than 65 ma seamounts of normal polarity predominate on crust younger than 65 ma seamounts exhibit normal and reversed polarity as well as mixed polarity we show that viscous components do not dominate seamount magnetization and we suggest that a large number of normal seamounts were formed during a major magmatic event postulated for the period 110 70 ma this period overlaps with the long cretaceous normal period 118 84 ma and corresponds to 85 normal polarity for earth9s magnetic field the correspondence of a period of high magmatic activity with a long period of normal polarity supports the possible correlation between those processes responsible for magma production and for magnetic field generation

the generation of large amplitude unsteady lee waves by subinertial k1 tidal flow a possible vertical mixing mechanism in the kuril straits numerical experiments with a two dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the kuril straits and their effect on vertical mixing the results show that sill scale internal waves at the k1 tidal frequency are confined to the sill slopes because the k1 tide is subinertial in the kuril straits in contrast to previous theories the authors show that intense short internal waves generated at the sill breaks by the subinertial k1 tidal current can propagate upstream as the tidal current slackens theoretical considerations identify these short waves as unsteady lee waves which tend to be trapped at the generation region and grow into large amplitude waves eventually inducing vigorous mixing along their ray paths in particular superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of 103 cm2 s21 this quite intense mixing reaches down to the density layer of the north pacific intermediate water npiw in contrast the m2 tidal current does not cause such strong vertical mixing because most of generated internal waves propagate away as first mode internal tides and because the barotropic flow amplitude is small the authors therefore suggest the possibility that generation of lee waves through interactions between the k1 current and the bottom topography of the kuril straits contributes to the observed modification of the okhotsk sea water required in the formation of the npiw

jasper seamount seven million years of volcanism jasper seamount is a young mid sized 690 km sup 3 oceanic intraplate volcano located about 500 km west southwest of san diego california reliable sup 40 ar sup 39 ar age data were obtained for several milligram sized samples of 4 to 10 ma plagioclase by using a defocused laser beam to clean the samples before fusion gee and staudigel suggested that jasper seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas overlain by flank alkalic series lavas and summit alkalic series lavas twenty nine individual sup 40 ar sup 39 ar laser fusion analyses on nine samples confirm the stratigraphy 10 3 10 0 ma for the flank transitonal series 8 7 7 5 ma for the flank alkalic series and 4 8 4 1 ma for the summit alkalic series the alkalinity of the lavas clearly increases with time and there appear to be 1 to 3 m y hiatuses between each series the age data are consistent with the complex magnetic anomaly of jasper however the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca 11 ma prior to the onset of chron c5n the duration of volcanism of jasper seamount is slightly longer than the duration of volcanism at hawaiian volcanoes suggesting thatmore individual age data from seamounts may constrain the age of a seamount only to within about 7 m y unless the stage of volcanism can be unambiguously determined extrapolating from the results of our study similar precision in age determinations should be possible on 50 mg of 1 ma plagioclase from mid ocean ridge basalt opening new possibilities in the geochronology of young low potassium volcanic rocks less

model estimates of m2 internal tide energetics at the hawaiian ridge 1 a primitive equation model is used to examine the structure and energetics of m 2 internal tides generated at the hawaiian ridge recent estimates based on altimeter data suggest that 20 gw of barotropic tidal dissipation occurs at the ridge with conversion to internal tides believed to be the dominant dissipation mechanism the model simulates an internal tide that accounts for 9 7 gw of radiated energy away from the ridge in the northeast and southwest directions the strongest generation occurs at three sites where enhanced barotropic currents flow across elongated topographic features the depth integrated baroclinic energy flux and energy densities at these sites are on the order of 10 4 w m 1 and 10 4 j m 2 respectively a modal decomposition indicates that 62 of the outgoing energy flux is accounted for by the first internal mode 15 is accounted for by the second mode and less than 5 is accounted for by each subsequent higher mode the tidal dissipation due to bottom friction along the ridge is estimated to be 0 1 gw the level of turbulent dissipation near the ridge owing to tidal energy conversion remains to be determined to assess fully the barotropic baroclinic energy budget

internal hydraulic flows on the continental shelf high drag states over a small bank observations of currents hydrography and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three dimensional topographic feature on oregon s continental shelf the flow becomes critical at the crest of the bank forming a strong supercritical downslope flow in the lower layer farther downstream internal hydraulic jumps form as the bottom flow becomes subcritical as a consequence turbulence is greatly enhanced in the bottom boundary layer in the sheared interface above the swiftly flowing bottom current and in the internal hydraulic jump the dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two layer flow this enhanced turbulence constitutes a high drag state of the flow in which the form drag introduced by the topography exerts significant influence on the flow around it and mixing is increased 102 103 times the background values

internal wave breaking at concave and convex continental slopes abstract internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic massachusetts institute of technology general circulation model mitgcm overturning shear instability and resultant mixing are examined results are compared with simulations of wave reflection from planar slopes with angles greater than less than and equal to the critical angle in contrast to the analytic predictions no reduction in mixing is found for the concave slope as compared with the other slopes in all cases stratification is eroded in a band above the slope bounded at its outer

turbulent mixing and hydraulic control of abyssal water in the samoan passage 1 we report the first direct turbulence observations in the samoan passage sp a 40 km wide notch in the south pacific bathymetry through which flows most of the water supplying the north pacific abyssal circulation the observed turbulence is 1000 to 10 000 times typical abyssal levels strong enough to completely mix away the densest water entering the passage confirming inferences from previous coarser temperature and salinity sections accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence specifically the flow accelerates substantially at the primary sill within the passage reaching speeds as great as 0 55 m s 1 a strong hydraulic response is seen with layers first rising to clear the sill and then plunging hundreds of meters downward turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill in addition to the primary sill other locations along the multiple interconnected channels through the samoan passage also have an effect on the mixing of the dense water in fact quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers suggesting that abyssal mixing depends sensitively on bathymetric details on small scales

the formation and fate of internal waves in the south china sea internal oceanic waves are subsurface gravity waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study here observations of such waves in the south china sea reveal their formation mechanism extreme turbulence relationship to the kuroshio current and energy budget internal waves are the underwater version of more familiar surface waves they can be enormous and travel thousands of kilometres before breaking the south china sea is known to be home to the largest internal waves in the world s oceans but their size generation mechanisms and role in the regional energy budget are unknown matthew alford and colleagues now present the results from the iwise observational campaign and reveal that internal waves more than 200 metres high break in the south china sea and create turbulence that is orders of magnitude larger than in the open ocean and that wave formation is influenced by the kuroshio current these results now allow for a complete energy budget of the south china sea and for a more accurate incorporation of internal waves into climate models internal gravity waves the subsurface analogue of the familiar surface gravity waves that break on beaches are ubiquitous in the ocean because of their strong vertical and horizontal currents and the turbulent mixing caused by their breaking they affect a panoply of ocean processes such as the supply of nutrients for photosynthesis1 sediment and pollutant transport2 and acoustic transmission3 they also pose hazards for man made structures in the ocean4 generated primarily by the wind and the tides internal waves can travel thousands of kilometres from their sources before breaking5 making it challenging to observe them and to include them in numerical climate models which are sensitive to their effects6 7 for over a decade studies8 9 10 11 have targeted the south china sea where the oceans most powerful known internal waves are generated in the luzon strait and steepen dramatically as they propagate west confusion has persisted regarding their mechanism of generation variability and energy budget however owing to the lack of in situ data from the luzon strait where extreme flow conditions make measurements difficult here we use new observations and numerical models to 1 show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena 2 reveal the existence of 200 metre high breaking internal waves in the region of generation that give rise to turbulence levels 10 000 times that in the open ocean 3 determine that the kuroshio western boundary current noticeably refracts the internal wave field emanating from the luzon strait and 4 demonstrate a factor of two agreement between modelled and observed energy fluxes which allows us to produce an observationally supported energy budget of the region together these findings give a cradle to grave picture of internal waves on a basin scale which will support further improvements of their representation in numerical climate predictions

a cautionary tale for palaeomagnetists a spurious apparent single component remanence due to overlap of blocking temperature spectra of two components the magnetic properties of the upper cretaceous red limestones from the montsec thrust sheet in the pyrenees reveal an unusual arrangement of the unblocking temperatures and coercivities of the magnetic mineral phases which leads to an apparently single component remanence being isolated on thermal demagnetization that this is an artefact of the almost complete overlap of the blocking temperature spectra of the two real components can be deduced from alternating field and thermal demagnetization of a composite isothermal remanent magnetization irm when two irm components are imparted to the sample at right angles to each other and each affecting grains with different coercivities thermal demagnetizations of the composite irm does not isolate the components but alternating field demagnetization does illustrating the superiority of the latter treatment in this case

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

depth trends in magnetic properties in an area of prolonged cold seawater drawdown in uppermost troodos type oceanic crust study of the oxide minerals and measurements of the magnetic properties of about 100 samples distributed over a 475 m continuous drill core section from the uppermost part of an extrusive sequence of the troodos cyprus ophiolite close to 35 02 n 33 11 e in the vicinity of malounda yields the following principal results 1 low temperature alteration is intense to a depth of 260 m below which it is relatively much subdued 2 saturation magnetization natural remanence intensity and initial susceptibility all increase continuously with depth throughout the section 3 curie temperature decreases with depth to 260 m below which a change in the style of the thermal decay of magnetization behaviour takes place 4 pillowed and massive flows differ in their magnetic properties the results are consistent with the downwards decrease in low temperature alteration also noted by k gillis from the distribution of secondary minerals the downwards variations in magnetic properties closely follow in both

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

an estimate of tidal energy lost to turbulence at the hawaiian ridge abstract an integrated analysis of turbulence observations from four unique instrument platforms obtained over the hawaiian ridge leads to an assessment of the vertical cross ridge and along ridge structure of turbulence dissipation rate and diffusivity the diffusivity near the seafloor was on average 15 times that in the midwater column at 1000 m depth the diffusivity atop the ridge was 30 times that 10 km off the ridge decreasing to background oceanic values by 60 km a weak factor of 2 spring neap variation in dissipation was observed the observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide e and the depth integrated dissipation d such that d e1 0 5 at sites along the ridge this kinematic relationship is supported by combining a simple knife edge model to estimate internal tide generation with wave wave interaction time scales to estimate dissipation the along ridge kinematic relationship and the observed vertical and cross ridge st

structure propagation and mixing of energetic baroclinic tides in mamala bay oahu hawaii abstract large semidiurnal vertical displacements 100 m and strong baroclinic currents 0 5 m s 1 several times as large as barotropic currents dominate motions in mamala bay outside the mouth of pearl harbor hawaii during september 2002 the authors sought to characterize them with a 2 month mclane moored profiler deployment and a 4 day intensive survey with a towed ctd adcp and the research vessel r v revelle hydrographic sonar spatial maps and time series of turbulent dissipation rate diapycnal diffusivity k isopycnal displacement velocity u energy e and energy flux f are presented dissipation rate peaks in the lower 150 m during rising isopycnals and high strain and shows a factor of 50 spring neap modulation the largest k values in the western bay near a submarine ridge exceed 10 3 m2 s 1 the m2 phases of and u increase toward the west implying a westward phase velocity cp 1 m s 1 and horizontal wavelength 60 km consistent with theoretical mode 1 values these phase

energy flux and dissipation in luzon strait two tales of two ridges abstractinternal tide generation propagation and dissipation are investigated in luzon strait a system of two quasi parallel ridges situated between taiwan and the philippines two profiling moorings deployed for about 20 days and a set of nineteen 36 h lowered adcp ctd time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals measurements were concentrated on a northern line where the ridge spacing was approximately equal to the mode 1 wavelength for semidiurnal motions and a southern line where the spacing was approximately two thirds that the authors contrast the two sites to emphasize the potential importance of resonance between generation sites throughout luzon strait baroclinic energy energy fluxes and turbulent dissipation were some of the strongest ever measured peak to peak baroclinic velocity and vertical displacements often exceeded 2 m s 1 and 300 m respectively energy fluxes exceeding 60 kw m 1 were measured at spring tide at the wester

observations of tidal internal wave beams at kauai channel hawaii abstract to observe the across ridge structure of internal tides density and velocity were measured using seasoar and a doppler sonar over the upper 400 600 m of the ocean extending 152 km on each side of the hawaiian ridge at kauai channel eighteen sections were completed in about 18 days with sampling intentionally detuned from the lunar semidiurnal m2 tide so that averaging over all sections was equivalent to phase averaging the m2 tide velocity and displacement variance and several covariances involving velocity and displacement showed one m2 internal wave beam on each side of the ridge and reflection of the beams off of the surface theoretical ray slopes aligned with the observed beams and originated from the sides of the ridge energy flux was in agreement with internal wave generation at the ridge inferred turbulent dissipation was elevated relative to open ocean values near tidal beams energy flux was larger than total dissipation almost everywhere across the ridge internal wave energy fl

topographic form drag on tides and low frequency flow observations of nonlinear lee waves over a tall submarine ridge near palau abstracttowed shipboard and moored observations show internal gravity waves over a tall supercritical submarine ridge that reaches to 1000 m below the ocean surface in the tropical western pacific

internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge abstract recent observations from the hawaiian ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide and then propagate up the slope as internal bores when the flow reverses a series of numerical simulations explores the parameter space of topographic slope barotropic velocity stratification and forcing frequency to identify the parameter regime in which these internal jumps are possible theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large which is shown to imply steep topographic slopes such that dh dxn 1 the vertical length scale of the jumps is predicted to depend on the flow speed such that the jump froude number is of order unity the numerical results agree with the theoreti

the characteristics and significance of secondary magnetite in a profile through the dike component of the troodos cyprus ophiolite secondary magnetite sm is the dominant fe ti oxide over a 3 4 km thick vertical interval in a profile through the troodos cyprus ophiolite sm is important in dikes and flows where dike density exceeds about 30 within the transition from extrusives to sheeted dikes and remains important until dike density falls below about 30 in the transition from sheeted dikes to plutonics the association of sm with high dike density suggests that it is a product of the thermal metamorphism of hydrothermally metamorphosed dikes and flows the most common occurrence of sm is at the site of primary titanomagnetites that had been altered earlier through deuteric low temperature and regional hydrothermal alteration processes sm often closely pseudomorphs the form of primary magnetite pm grains but it can be distinguished from pm using a series of criteria sm is low in tio2 and other impurities estimated and observed curie temperatures are undistinguishable which combined with optical evidence indicates tha

the history of intrusive activity on the island of la palma canary islands abstract the island of la palma located in the northwest of the canary archipelago consists of two main geological units 1 the subaerial extrusive coberta series covers most of the island and consists of alkali basaltic lavas phonolitic plugs and pyroclastic rocks erupted between 1 61 ma and the recent 2 the basement complex is exposed in the barranco de las angustias and the caldera de taburiente and consists of the submarine intrusive and extrusive seamount series and a basal plutonic complex alkali basaltic dikes of different ages and orientations intruded in both units three groups of dikes and sills can be distinguished based on their field relationships petrography geochemistry and ages the oldest group i represent the feeder dikes to the seamount series intruded between 2 9 and 4 0 ma b p they were originally steep with a n05 w strike and perpendicular to the originally nearly flat bedding of the layered seamount series subsequent to emplacement group i dikes and the extrusives of the seamount series were rotated into their present orientation group ii sills intruded parallel to the bedding of the seamount series 45 230 are coeval with or younger than group i but older than the erosional unconformity separating the coberta and the basement complex i e 1 6 ma 50 vol of the volcanic edifice of the la palma seamount and highlight the importance of intrusive processes for island growth intrusions into central oceanic volcanoes like la palma may result in an uplift and rotation of more than 30 km 3 by up to 45

measurement of tidal form drag using seafloor pressure sensors abstractas currents flow over rough topography the pressure difference between the up and downstream sides results in form drag a force that opposes the flow measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography an array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1 km headland at the surface in puget sound washington the form drag per unit length of the ridge reached 1 104 n m 1 during peak flood tides the tidally averaged power removed from the tidal currents by form drag was 0 2 w m 2 which is 30 times larger than power losses to friction form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography maximum turbulent kinetic energy dissipation rates of 5 10 5 w kg 1 were measured with a microstructure pr

spatially broad observations of internal waves in the upper ocean at the hawaiian ridge abstract the density and current structure at the hawaiian ridge was observed using seasoar and doppler sonar during a survey extending from oahu to brooks banks across and along ridge changes in internal wave statistics in the upper ocean within 200 km of the ridge are investigated internal waves with trough to crest amplitude as large as 60 m and horizontal wavelength of about 50 km are observed repeatedly in across ridge sections of potential density within 150 km of the ridge kinetic and potential energy density exceed open ocean values with maxima about 10 times garrett munk levels in the kauai channel kc the kinetic energy density is largest along an m2 internal tide ray the ray originates at the northern edge of the ridge peak at a large across ridge change in topographic slope and terminates at the ocean surface about 30 40 km south of the ridge peak kinetic and potential energy density are larger on the south side of the ridge at kc the side with larger topographic slope energy densi

the role of upstream waves and a downstream density pool in the growth of lee waves stratified flow over the knight inlet sill abstract observations and modeling simulations are presented that illustrate the importance of a density contrast and the upstream response to the time dependence of stratified flow over the knight inlet sill repeated sections of velocity and density show that the flow during ebb and flood tides is quite different a large lee wave develops early in flood tide whereas lee wave growth is suppressed until the second half of ebb tide there is a large upstream response that displaces as much water as accumulates in the lee wave one that is large enough to also block the flow at a depth roughly consistent with simple dynamics there is a large density contrast between the seaward and landward sides of the sill and a salty pool of water is found in the seaward basin that is not found landward the interface with this salty pool demarks the point of flow separation during ebb initially suppressing the lee wave and then acting as its lower boundary a simple two dimensional numerical model of the inlet wa

a statistical theory of seamount magnetism the standard least squares estimation of the average magnetization of a seamount is generally regarded as unsatisfactory the reason for its poor performance is i assert due to the strong correlation and uneven magnitudes of the unfitted noise component of the magnetic anomaly least squares estimation depends for its efficiency and freedom from bias upon the statistical independence and common distribution of the random contamination the primary contributor to the unfitted field is the part due to nonuniformity of the magnetic material within the seamount if the magnetic heterogeneities are modeled as a random stationary vector field of known functional form it is possible to calculate the covariance matrix of the magnetic field noise then to the extent that the statistical description of the magnetic sources is valid the correlation can be properly accounted for in the least squares procedure by weighting the observations to form an equilibrated set with random components that are independent and identically distributed the main use of the average magnetization vector is in the determination of the associated paleopole position i show how the least squares estimates of the magnetization vector and its uncertainty are mapped into a confidence region for the pole position application of this theory to a young seamount in the south pacific is highly satisfactory the model stationary vector field chosen for the magnetization is one with isotropic directional behavior and a very short length scale of correlation it is found that the equilibrated residuals are far less correlated than those of the conventional procedure and the misfits are uniformly distributed throughout the equilibrated data the estimated paleopole position is within a few degrees of the north geographic pole which is the expected location because the seamount is geologically young traditional least squares fitting gave a pole at latitude 60 n the 95 confidence zone associated with the paleopole position has semiaxes measuring 16 by 13 and thus the uncertainty is not much larger than those of standard paleomagnetic work

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

internal lee waves and baroclinic bores over a tropical seamount shark hot spot abstract oceanographic observations were made with a subsurface oceanographic mooring over the summit and flanks of two neighbouring seamounts in the tropical indian ocean to identify processes that may be responsible for the aggregation of silvertip sharks carcharhinus albimarginatus in the deep water drop off surrounding the summits the seamounts which are in the chagos archipelago in the british indian ocean territories are narrow in horizontal extent 15 sides that rise from depths of 600 m and flat summits at a depth of 70 m they are subjected to forcing at subinertial basin scales and local scales that include a mixed tidal regime and storm generated near inertial waves at the drop off at a depth of between 70 and 100 m isotherms oscillate at both diurnal and semidiurnal frequencies with amplitudes of 20 30 m the waves of tidal origin are accompanied by short period 5 min internal waves with amplitudes o 10 m and frequencies close to the local buoyancy frequency n within the thermocline which is the maximum frequency possible for freely propagating internal waves the tidal oscillations result from internal lee waves with 30 m vertical wavelength generated by the prevailing currents over the supercritical seamount flanks whereby the bottom slope is greater than the internal tide wave slope the near n waves are due to enhanced shear associated with the hydraulic jumps that form from the lee waves due to the abrupt transition from steeply sloping sides to a relatively flat summit the jumps manifest themselves as bottom trapped bores that propagate up the slope towards the summit further observations over the summit reveal that the bores subsequently flush the summits with cold water with tidal periodicity the bores which have long wave phase speeds more than double that of the bore particle velocities are characterised by intense vertical velocities 0 1 m s 1 and inferred local resuspension but relatively little turbulence based on temperature overturns our results strongly implicate lee waves as the dynamic mechanism of leading order importance to the previously observed accumulation of biomass adjacent to the supercritical slopes that are commonplace throughout the archipelago we propose that further investigation should identify the spatiotemporal correlation between internal wave activity and fish schooling around the summit and whether such schooling attracts predators

tidal mixing processes amid small scale deep ocean topography the nature of tide topography interaction reflects the topographic scales experienced by water parcels during their tidal excursions in the deep ocean these scales are typically subkilometer yet direct observations of tidal processes on such scales are lacking at one site a saddle amid steep and complex mid atlantic ridge topography observations reveal tidally pulsed bottom trapped fronts overflows and lee waves in response to a tide combined with a mean flow of similar amplitude the tidal pulsing of the fronts and overflows was only evident locally and their phase became unpredictable over scales of hundreds of meters enhanced turbulence in a 100 200 m thick bottom boundary layer had an estimated dissipation rate of 2 6 10 2 w m 2 exceeding the large scale average of tidal dissipation in mid ocean ridge environments but by less than an order of magnitude this site was not a dissipation hotspot and the processes observed could provide widely distributed mixing to the meridional overturning circulation

direct breaking of the internal tide near topography kaena ridge hawaii abstract barotropic to baroclinic conversion and attendant phenomena were recently examined at the kaena ridge as an aspect of the hawaii ocean mixing experiment two distinct mixing processes appear to be at work in the waters above the 1100 m deep ridge crest at middepths above 400 m mixing events resemble their open ocean counterparts there is no apparent modulation of mixing rates with the fortnightly cycle and they are well modeled by standard open ocean parameterizations nearer to the topography there is quasi deterministic breaking associated with each baroclinic crest passage large amplitude small scale internal waves are triggered by tidal forcing consistent with lee wave formation at the ridge break these waves have vertical wavelengths on the order of 400 m during spring tides the waves are nonlinear and exhibit convective instabilities on their leading edge dissipation rates exceed those predicted by the open ocean parameterizations by up to a factor of 100 with the disparity in

seismic tomography of jasper seamount a vertical section of the interior structure of jasper seamount was modeled using a spectral tomographic inversion of p wave travel times an array of ocean bottom seismographs obss deployed over the seamount detected the arrivals from a series of ocean bottom shots a reference velocity model reveals that average compressional velocities within the seamount are similar to those found within kilauea and are consistently slower than velocities at equivalent depths in typical oceanic crust this suggests jasper seamount has a high average porosity perturbations from the reference model were imaged by tomographic inversion a high velocity zone within the northwest flank of the seamount may result from dikes associated with a radial rift or from a shallow solidified magma reservoir a low velocity summit may result from shallow explosive eruptions the tomographic model is consistent with the results of gravity magnetic and dredging analyses

diapycnal mixing associated with an overflow in a deep submarine canyon abstract in order to close the global overturning circulation the buoyancy loss to the atmosphere associated with the formation of deep and bottom water at high latitudes must be balanced by buoyancy gain elsewhere in case of water that is not in contact with the atmosphere except in its formation region the required buoyancy gain is accomplished primarily by diapycnal mixing recent observations suggest that a significant portion of the diapycnal buoyancy fluxes in the abyss is associated with overflows in submarine valleys or canyons especially on the flanks of slow spreading mid ocean ridges in the present study hydrographic and velocity data from an overflow across a sill rising 1000 m above the floor of the rift valley of the mid atlantic ridge near 36 n were used to infer the associated diapycnal mixing the cross sill density gradients are characterized by a vertical dipole with an upper layer of streamwise density increase overlying a lower layer where the density decreases along the flow this is qualitatively consistent with the effects of diapycnal mixing transferring buoyancy from the upper into the lower layer hydrographic and velocity profiles provide evidence for strong mixing associated with the overflow across the sill in addition to thick layers that are susceptible to shear instability there are numerous density overturns indicating a spatially averaged diffusivity of order 10 2 m 2 s 1 inversions of the density equation reveal that the cross sill gradients can be accounted for either by adiabatic vertical advection or by vertical eddy diffusion although the adiabatic solutions are considered energetically implausible because they require upwelling in the layer of cross sill density increase eddy diffusive solutions both with and without adiabatic downwelling are characterized by diffusivity profiles attaining local maxima of order 10 2 m 2 s 1 250 m above the sill depth the vertical structure of the inversion derived diffusivities is consistent with the thorpe scale based estimates and with observations from overflows in major ocean passages which also are often characterized by local diffusivity maxima of order 10 2 m 2 s 1 a few 100 m above sill depth combining the diffusivity profiles near the sill with budget derived bulk estimates for 150 km of rift valley implies that 50 of the mixing in the rift valley is associated with overflows while the remainder is caused by other processes including the breaking of tidally forced internal waves

internal tide radiation from mendocino escarpment abstract strong semidiurnal internal tides are observed near mendocino escarpment in full depth profile time series of velocity temperature and salinity velocity and density profiles are combined to estimate the internal tide energy flux divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment a baroclinic energy flux of 7 kw m 1 radiates from the escarpment corresponding to 3 of the 220 kw m 1 fluxing poleward in the surface tide energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site 90 km north of the escarpment coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection beyond the first surface reflection north of the escarpment the energy flux drops by 2 kw m 1 and beams are no longer discernible turbulence as inferred from finescale parameterizations is elevated by over two orders of magnitude relative to th