diurnal shear instability the descent of the surface shear layer and the deep cycle of equatorial turbulence a new theory of shear instability in a turbulent environment is applied to eight days of velocity and density profiles from the upper equatorial pacific this period featured a regular diurnal cycle of surface forcing together with a clear response in upper ocean mixing during the day a layer of stable stratification and shear forms at the surface during late afternoon and evening this stratified shear layer descends leaving the nocturnal mixing layer above it using high resolution current measurements the detailed structure of the descending shear layer is seen for the first time linear stability analysis is conducted using a new method that accountsfortheeffectsofpreexistingturbulenceoninstabilitygrowth shearinstabilityfollowsadiurnalcycle linked to the afternoon descent of the surface shear layer this cycle is revealed only when the effect of turbulence is accounted for in the stability analysis the cycle of instability leads the diurnal mixing cycle typically by 2 3h consistent with the time needed for instabilities to grow and break late at night the resulting turbulence suppresses further instabilities lending an asymmetry to the mixing cycle that has not been noticed in previous measurements deep cycle mixing is triggered by instabilities formed as the descending shear layer merges with the marginally unstable shear of the equatorial undercurrent in the morning turbulence decays and the upper ocean restratifies wind accelerates the near surface flow to form a new unstable shear layer and the cycle begins again

microstructure measurements along a quasi meridional transect in the northeastern atlantic ocean 1 this study presents vertical profiles of turbulence parameters obtained in the upper 100 m of the northeastern atlantic ocean along a transect from tropical permanently stratified waters to subpolar seasonally stratified waters in july august 2009 the focus is to fully characterize the vertical mixing along this transect for further studies related to phytoplankton and nutrient distributions derived values of temperature eddy diffusivity kt of temperature variance dissipation rate t and of turbulent kinetic energy dissipation rate e indicate a northward increase of turbulent mixing below the mixed layer in the northern stations where the wind stress is sufficiently high the vertical distributions of e in the mixed layer follow the wind stress scaling the related low wind scaling factors and the low values of e encountered at this cruise are in agreement with a strongly stratified upper water column at midday in summer

some theoretical predictions on the relationships among spreading rate mantle temperature and crustal thickness a series of numerical experiments on mantle flow and melting predict a positive relation between mantle temperature and crustal thickness the models also demonstrate that crust formed at slow spreading rates is more sensitive to variations in mantle temperature than crust formed at fast rates so that the range of calculated thicknesses is much greater for crust formed at slower rates an instantaneous mantle temperature increase results in a transient pulse of melt production that is also more pronounced at slower spreading rates the predicted behavior is caused by the interplay between mantle flow driven by plate separation and that driven by thermal compositional and melt related buoyancy a temperature increase results in a decrease in mantle viscosity and an increase in the depth at which melting begins a lower viscosity leads to stronger buoyancy driven flow that carries more mantle to shallow depths below the ridge thermal buoyancy effects which may result in cooling and mixing of depleted and undepleted material under the ridge appear to be of greater importance at slower spreading rates the steady state results are broadly consistent with global compilations of oceanic crustal thickness that show larger variations in crustal thickness at slower spreading rates than at faster rates thicknesses estimated from seismic refraction data from crust formed within a single segment of the mid atlantic ridge but at different spreading rates 1 0 to 1 9 cm yr are consistent with but do not prove the model results the transient pulse of melt production associated with a rapid increase in mantle temperature might occur when a ridge becomes proximal to a hot spot

efficiency of turbulent mixing in the abyssal ocean circulation turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation a key parameter used to characterize turbulent mixing is its efficiency defined here as the fraction of the energy available to turbulence that is invested in mixing efficiency is conventionally approximated by a constant value near one sixth here we show that efficiency varies significantly in the abyssal ocean and can be as large as approximately one third in density stratified regions near topographic features our results indicate that variations in efficiency exert a first order control over the rate of overturning of the lower branch of the meridional overturning circulation

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

transport and dynamics of the panay sill overflow in the philippine seas abstract observations of stratification and currents between june 2007 and march 2009 reveal a strong overflow between 400 and 570 m depth from the panay strait into the sulu sea the overflow water is derived from approximately 400 m deep in the south china sea temporal mean velocity is greater than 0 75 m s 1 at 50 m above the 570 m panay sill empirical orthogonal function analysis of a mooring time series shows that the flow is dominated by the bottom overflow current with little seasonal variance the overflow does not descend below 1250 m in the sulu sea but rather settles above high salinity deep water derived from the sulawesi sea the mean observed overflow transport at the sill is 0 32 106 m3 s 1 the observed transport was used to calculate a bulk diapycnal diffusivity of 4 4 10 4 m2 s 1 within the sulu sea slab 575 1250 m ventilated from panay strait analysis of froude number variation across the sill shows that the flow is hydraulically controlled a suitable hydraulic control model

structural and metamorphic evolution of the orocopia schist and related rocks southern california evidence for late movement on the orocopia fault the pelona orocopia and rand schists por schists of southern california and southwesternmost arizona are late mesozoic or early tertiary subduction complexes that underlie precambrian to mesozoic continental basement along the low angle vincent chocolate mountains vcm fault system the vcm faults are often considered to be remnants of the original subduction zone but recent work indicates that many have undergone substantial postsubduction reactivation in the orocopia mountains for example the orocopia schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the orocopia fault structures in the schist include isoclinal folds with axial planar schistosity open to tight folds that fold schistosity penetrative stretching lineations and crenulation lineations all of which show a nearly 360 range in trend folds and lineations that trend approximately ne sw occur throughout the schist and are thought to be part of an early phase of deformation related to subduction folds of this orientation show no consistent vergence folds and lineations that trend approximately nw se are concentrated near the orocopia fault and are interpreted to have formed during exhumation of the schist the nw se trending folds and shear indicators in late stage mylonite at the top of the schist consistently verge ne the exhumation event culminated in emplacement of the schist against brittlely deformed upper plate exhumation of the orocopia schist was accompanied by retrograde replacement of garnet biotite epidote and calcic amphibole by chlorite calcite and sericite matrix amphibole has a lower na al ratio than amphibole inclusions in albite consistent with a late stage decrease in pressure as ne vergence in the orocopia mountains is associated with exhumation of the schist the ne movement along other segments of the vcm fault may also be late and therefore have no bearing on the facing direction of the vcm subduction zone contrary to past interpretations

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

geochemistry and origin of mafic rocks from the pelona orocopia and rand schists southern california abstract metabasites from the pelona orocopia rand por schist a presumed subduction complex in southern california have been analyzed for major minor and trace element concentrations to investigate the nature origin and tectonic implications of these rock bodies element abundances indicate that these rocks are tholeiitic and alkalic basalts generated from several sources most samples group 1 resemble normal to transitional mid ocean ridge basalts n type to t type morb variation of morb like samples can be accounted for by low pressure crystal fractionation a second group group 2 which includes five samples from the rand mountains and one from the san gabriel mountains has elemental abundances similar to those of tholeiitic ocean island basalts or e type morb although phosphorus is unusually low several samples of group 2 have high mgo ni and cr characteristics which are commonly associated with crystal accumulation a final group group 3 consists of two samples from the orocopia mountains that resemble alkalic ocean island basalts the analyzed samples were collected from six different bodies of por schist there seem to be no major differences in basalt type which would imply a unique and separate source for any one area various sites of accumulation have been suggested for the protoliths of the por schist open ocean gulf of california type basin trapped back arc basin and rifted back arc basin the first three settings are entirely consistent with the basalt geochemistry the last option is less likely but not impossible

inversion of combined gravity and bathymetry data for crustal structure a prescription for downward continuation abstract we propose a new and improved recipe for inverting for crustal thickness variations from combined gravity and bathymetry data the new ingredients with respect to previous studies are the posing of this problem as an inverse problem and the use of a downward continuation filter which can be tailored to find the crustal thickness solution which satisfies a minimum slope or maximal smoothness criteria or a weighted combination of these desired solution features we explore several suggestions for determining the optimum smoothing parameters for a given crustal thickness inversion then we compare this new method to that used previously in marine gravity studies we find it to be an improved recipe for dealing with gravity data with observation errors of 3 5 mgal but not by much this recipe will be most useful in extracting more geological information from marine gravity data where apparent observation errors are 1 mgal e g from future global positioning satellite navigated studies

geochemistry of the shuksan greenschists and blueschists north cascades washington variably fractionated and altered metabasalts of oceanic affinity the shuksan schist comprises a structurally coherent metabasaltic member of the easton formation the uppermost allochthon shuksan thrust plate in the thrust system of the western north cascades of washington state late jurassic metamorphism at moderately high p t produced interlayering of actinolite bearing greenschist assemblages with blue amphibole bearing rocks major and trace element analyses of twelve greenschist and blueschist samples have been used to establish similarities between the basaltic protolith and moderately to strongly fractionated type i morb to distinguish the effects of seafloor alteration superimposed on the primary igneous chemistry and to evaluate the origin and nature of the chemical controls which produced the two mineral assemblages

boundary layer intrusions from a sloping bottom a mechanism for generating intermediate nepheloid layers 1 laboratory experiments were used to investigate the growth of intrusions due to internal wave reflection from a sloping boundary when normalized by the incident energy density flux the average intrusion spreading velocity was found to be a linear function of the frequency ratio c where is the frequency of the incident wave and c is the critical frequency at which the wave characteristic has the same angle as the bottom slope evenly spaced layers indicating thin perturbations in the background density gradient developed within the mixing region and spread into the tank interior the vertical spacing of these layers also bore a linear relationship to c a linear model of internal wave reflection suggests that these layers may be related to an isopycnal displacement or overturn scale intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions a balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux this balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal wave mixing at oceanic margins

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

amphibole compositions and metamorphic history of the rand schist and the greenschist unit of the catalina schist southern california the catalina schist and rand schist are two high p t terranes in southern california the catalina schist is correlated with the franciscan complex and occurs in the continental borderland it consists of a blueschist facies melange tectonically overlain by a greenschist unit which in turn is overthrust by an amphibolite unit the greenschist unit itself is inversely zoned from epidote amphibolite fades at the top through greenschist facies in the center to transitional blueschist greenschist facies at the base the rand schist is part of the eugeoclinal pelona orocopia schist terrane which lies interior to the present continental margin structurally beneath precambrian to mesozoic sialic basement the rand schist is inversely zoned from epidote amphibolite facies to transitional blueschist greenschist facies similar to the greenschist unit of the catalina schist

geochemistry and origin of volcanic rocks from tuzo wilson and bowie seamounts northeast pacific ocean the origin of the tuzo wilson seamounts 50 km south of the queen charlotte islands has been ascribed by various workers to either the pratt welker mantle plume which has formed the pratt welker seamount chain or the formation of a new segment of the explorer juan de fuca spreading ridge system abundances of major and trace elements in dredged alkali basalts from tuzo wilson and bowie seamounts 360 km northwest of tuzo wilson seamounts are typical of alkaline volcanism on ocean islands associated with mantle plumes but x7 r x6 r ratios 0 70252 0 70258 fall within the range of mid ocean ridge basalts morb from the explorer and juan de fuca ridges geochronological and chemical data from the pratt welker bowie and tuzo wilson seamounts suggest that the tuzo wilson seamounts are in an early stage of development as a result of activity of the pratt welker mantle plume but that contributions from both a depleted and an undepleted mantle source are necessary to reconcile trace element and sr isotope values modelling of rare earth behaviour during partial melting indicates that neither the tuzo wilson nor bowie basalts could be generated from a mantle source similar to that of the explorer or juan de fuca morb unless recent metasomatism has enriched the seamounts source region in incompatible elements

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

broadband acoustic quantification of stratified turbulence high frequency broadband acoustic scattering techniques have enabled the remote high resolution imaging and quantification of highly salt stratified turbulence in an estuary turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors the acoustic frequencies used span 120 600 khz which for the highly stratified and dynamic estuarine environment correspond to wavenumbers in the viscous convective subrange 500 2500 m 1 the acoustically measured spectral levels are in close agreement with spectral levels measured with closely co located micro conductivity probes the acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim bladdered fish the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure the direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt stratified estuary

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

structural geology of the pelona schist and vincent thrust san gabriel mountains california the vincent thrust of the san gabriel mountains southern california separates an upper plate that contains abundant granitic rocks from a lower plate composed of metagraywackes and metabasites pelona schist the vincent thrust is part of a regional thrust system of late cretaceous early tertiary age vincent chocolate mountain thrust several earlier workers have concluded that the upper plate was derived from the southwest implying the former presence of continental rocks separated from north america by a marginal oceanic basin other workers consider that regional correlations indicate thrusting of the opposite sense the pelona schist of the san gabriel mountains has undergone a complex structural history the earliest recognized structures are tight synmetamorphic isoclinal folds style 1 folds these folds are present throughout the entire 4 km of presently exposed section and do not become more appressed or more abundant near the thrust isoclinally refolded folds and sheath folds were observed at several localities textural observations indicate that the present schistosity of the pelona schist formed by the extreme flattening of crenulations developed in an earlier schistosity this is evidence that the isoclinally refolded folds although only infrequently noticed in the field are actually widespread the isoclinal folding was followed by the development of a macroscopic synform narrows synform and associated minor folds in the 700 m of pelona schist closest to the upper plate these folds style 2 folds are more open than the style 1 folds and do not have a well developed axial planar schistosity they are generally parallel in orientation to the style 1 folds both style 1 and style 2 folds are in turn parallel to tight folds in mylonitic rocks present at the base of the upper plate it is believed that the three groups of folds are cogenetic and possibly were produced in a subduction zone the sheath folds and other evidence of stretching parallel to fold axes might indicate that fold axes were rotated into the direction of thrusting on the other hand the parallelism of the early isoclinal folds and the later relatively open style 2 folds is evidence against rotation thus the orientation of fold axes relative to the thrusting direction is not clear subsequent to metamorphism the pelona schist and the mylonites were folded into a broad arch style 3 folding uncertainty as to the orientation of the axes of style 1 and 2 folds relative to the thrusting direction the presence of the style 3 folding and recently described paleomagnetic evidence of large scale rotation of crustal blocks in southern california make determination of thrusting direction impossible at the present time thus no evidence was found either to support or to invalidate the marginal basin model for the origin of the pelona schist

stratified turbulence and mixing efficiency in a salt wedge estuary abstracthigh resolution observations of velocity salinity and turbulence quantities were collected in a salt wedge estuary to quantify the efficiency of stratified mixing in a high energy environment during the ebb tide a midwater column layer of strong shear and stratification developed exhibiting near critical gradient richardson numbers and turbulent kinetic energy tke dissipation rates greater than 10 4 m2 s 3 based on inertial subrange spectra collocated estimates of scalar variance dissipation from microconductivity sensors were used to estimate buoyancy flux and the flux richardson number rif the majority of the samples were outside the boundary layer based on the ratio of ozmidov and boundary length scales and had a mean rif 0 23 0 01 dissipation flux coefficient 0 30 0 02 and a median gradient richardson number rig 0 25 the boundary influenced subset of the data had decreased efficiency with rif 0 17 0 02 0 20 0 03 and median rig 0 16 the relationship be

the upper ocean response to monsoonal forcing in the arabian sea seasonal and spatial variability abstract observations from four towed profiler surveys undertaken between december 1994 and october 1995 examine the seasonal and spatial variability of the upper ocean response to the monsoon cycle in the arabian sea although observed atmospheric forcing agrees well with modern climatologies cross basin patterns of mixed layer depth and water properties observed in 1994 1995 are not entirely consistent with an upper ocean response dominated by ekman pumping during the winter monsoon the mixed layer deepens dramatically with distance offshore surface cooling intensifies with offshore distance and a one dimensional response dominated by convective overturning could explain observed wintertime mixed layer depths except for waters associated with a filament extending offshore from the omani coast mixed layer depths and water properties show only modest cross basin contrasts during the southwest monsoon filament waters differ from surrounding mid basin waters having shallow mixed layers and water properties similar to those of waters upwelled near the omani coast in september following the southwest monsoon waters within 1000 km of the omani coast have cooled and freshened with marked changes in stratification extending well into the pycnocline estimates of ekman pumping and wind driven entrainment made using the southampton oceanographic center 1980 1995 surface flux and the levitus mixed layer climatologies indicate that during the southwest monsoon wind driven entrainment is considerably stronger than ekman pumping inshore of the windstress maximum ekman pumping partially counters wind driven entrainment while offshore the two processes act together to deepen the mixed layer as ekman pumping is too weak to counter wind driven mixed layer deepening inshore of the windstress maximum another mechanism must act to maintain the shallow mixed layers seen in our observations and in climatologies offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties ekman upwelling will modulate wind driven entrainment but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind driven mixing and horizontal advection

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

vertical heat transfer in the arctic ocean the role of double diffusive mixing 1 microstructure profiles collected in the central amundsen basin are analyzed in order to quantify the role of double diffusive mixing for vertical heat transfer from the atlantic layer in the profiles a persistent but laterally incoherent thermohaline staircase structure is identified in the 200 260 m depth range the staircase contains homogeneous layers with average height of 1 3 m and thin high gradient interfaces with average temperature and salinity jumps of about 0 065 c and 0 015 respectively when inferred from a commonly used diffusive convection parameterization the average vertical heat flux within the staircase is 0 6 w m 2 this is one order of magnitude larger than the molecular diffusion alone and of the same order as the overall heat loss from the atlantic layer in the deep basins of the arctic ocean the parameterization is evaluated using observed turbulent heat fluxes and is found to overestimate diffusive convective fluxes with up to an order of magnitude especially for weak thermal forcing small temperature jumps in staircase staircases coexist with thermohaline intrusions in the vertical temperature and salinity profiles lomonosov ridge is identified as a potential region for formation of intrusions it is found that salt fingering is the dominant process during intrusion growth whereas diffusive convection is the dominant process in maintaining the intrusions at steady state in the deep basins

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

a gravity and magnetic anomaly study of the extinct aegir ridge norwegian sea in 1990 the u s naval research laboratory mapped the extinct aegir ridge in the norwegian sea with seamarc ii side scan sonar hydrosweep multibeam seismics gravity and magnetics onboard r v maurice ewing we analyzed the 92 center beam bathymetry gravity and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 ma in bathymetric expression the rift valley deepens and narrows southward while the rift mountain summits become higher south of 67 n the western rift mountains and flanks stand higher than the eastern mountains and flanks free air anomalies along the ridge axis show undulating intermediate wavelength 100 km anomalies superimposed on a regional decrease of the rift valley negative from 0 mgal in the north to 55 mgal in the south after removal of sediment effects results from three dimensional gravity modeling imply a variation of crustal thickness a new rotation pole for the movement of the jan mayen microplate relative to the eurasia plate from 50 ma to 25 ma was found at 64 34 n 13 w based on magnetic lineations and inferred discontinuity traces a plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of 2 0 m y 36 ma followed by gradual deceleration to 0 at 25 ma full spreading rate between chrons c13n and c21n ranged from 13 mm yr in the north to 8 mm yr in the south

geochemistry of eclogites and associated rocks of the southeastern area of the french massif central origin of the protoliths abstract within the barrovian metamorphic series of the french massif central most of the eclogites are closely associated with metagabbros and meta ultrabasites the protoliths of the eclogites may represent basaltic or gabbroic dykes or sills of oceanic affinities erupted in either a continental domain or during early oceanization or opening up of an ensialic back arc basin both the nature and type of magma fractionation and of post magmatic evolution are consistent with a shallow crustal origin for the protoliths eclogitization postdates the low pressure magmatic differentiation and the volcano sedimentary supra crustal evolution

on the loss of wind induced near inertial energy to turbulent mixing in the upper ocean wind induced near inertial energy has been believed to be an important source for generating the ocean mixing required to maintain the global meridional overturning circulation in the present study the near inertial energy budget in a realistic 1 12 degrees model of the north atlantic ocean driven by synoptically varying wind forcing is examined the authors find that nearly 70 of the wind induced near inertial energy at the sea surface is lost to turbulent mixing within the top 200 m and hence is not available to generate diapycnal mixing at greater depth assuming this result can be extended to the global ocean it is estimated that the wind induced near inertial energy available for ocean mixing at depth is at most 0 1 tw this confirms a recent suggestion that the role of wind induced near inertial energy in sustaining the global overturning circulation might have been overemphasized

observations of turbulent mixing and hydrography in the marginal ice zone of the barents sea 1 measurements of hydrography currents microstructure shear and temperature were made at ice drift stations in the marginal ice zone miz of the northern barents sea highly variable mixing regimes were observed within and below the pycnocline elevated turbulent dissipation 5 15 10 7 w kg 1 was associated with strong vertical shear between the surface layer and the subsurface currents as well as strong tidal flow over shallow topography dissipation in the pycnocline was enhanced at stations with strong wind forcing during drifts under relatively calm wind and away from strong fronts and abrupt topography station mean dissipation values were up to a factor 50 lower and double diffusion contributed significantly to the vertical heat flux where hydrography favored diffusive layering independent measures of turbulent length scale from density overturns compared well with those inferred from the dissipation measurements the variability of dissipation was better captured using a scaling by shear suggested for shelves rather than shear variance models appropriate to the deep open ocean sufficiently resolved patches of enhanced temperature microstructure used in combination with dissipation measurements suggest mixing efficiency of rf 0 2 for patches stable to double diffusion comparable to the conventional upper bound of rf 0 17 mixing efficiency for double diffusive convection favorable cases is found to be significantly larger rf 0 36 water mass modification and fluxes of nutrients and dissolved carbon were found to have large local variability in accordance with the observed variability of vertical mixing in the miz

turbulent layering beneath the pycnocline in a strongly stratified pit lake temperature microstructure observations from a pit lake show alternate layers of high and low turbulence beneath a very strong pycnocline the layers were clearly defined around 0 3 0 5 m thick and were found both at the edge and in the center of the pit lake in the most energetic regions the turbulent energy dissipation rate reached 5 10 26 m 2 s 23 and the dissipation rate of thermal variance almost reached xu 5 10 24 8 c 2 s 21 the turbulent fluid in the middle of the lake had a higher dissipation ratio the thermal component of potential energy gain vs kinetic energy loss when compared with the sidewalls this is attributed to different forcing mechanisms for the turbulence we hypothesize that basin scale internal waves in conjunction with the topography create sidewall boundary layers in the interior regions of the pit lake the turbulent layers are probably generated by instability due to focusing of short wavelength internal waves a pit lake is a water body that forms in a mining void on the cessation of mining activity usually a lake only forms in regions where the accumulation of groundwater inflow and rainfall exceed evaporation alternately the pit lake can be actively filled as an aid to remediation or for storage some of the dissolved material that finds its way into a pit

petrology of basalts from loihi seamount hawaii abstract loihi seamount is the southeasternmost active volcano of the emperor hawaii linear volcanic chain it comprises a spectrum of basalt compositional varieties including basanite alkali basalt transitional basalt and tholeiite samples from four dredge collections made on scripps institution of oceanography benthic expedition in october 1982 are tholeiite the samples include highly vesicular olivine rich basalt and dense glass rich pillow fragments containing olivine and augite phenocrysts both quartz normative and olivine normative tholeiites are present minor and trace element data indicate relatively high abundances of low partition coefficient elements e g ti k p rb ba zr and suggest that the samples were derived by relatively small to moderate extent of partial melting of an undepleted mantle source olivine composition mgo cr and ni abundances and mg mg fe are typical of moderately fractionated to relatively unfractionated primary magmas the variations in chemistry between samples cannot be adequately explained by low pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated alternatively a heterogeneous source with variable abundances of certain trace elements or mixing of liquids may have been involved data for 3 he 4 he presented in a separate paper implies a mantle plume origin for the helium composition of the loihi samples there is little variation in the helium isotope ratio for samples having different compositions and textures the helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as loihi alkalic basalts and the whole source region for loihi may have a nearly uniform helium compositions even though other element abundances may be variable complex petrologic processes including variable melting fractional crystallization and magma mixing may have blurred original helium isotopic signatures

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

variations in apparent mixing efficiency in the north atlantic central water abstract microstructure data from the north atlantic tracer release experiment natre are presented providing detailed profiles of the thermal variance in the upper 360 m of the canary basin for the fall and spring seasons the osborn cox model is used to compute the diffusivity kt the diffusivity for the depth range 240 340 m is found to be 1 0 0 04 10 5 m2 s 1 in the fall and 2 2 0 1 10 5 m2 s 1 in the spring in good agreement with dye inferred diffusivities at similar depths measured turbulent kinetic energy tke dissipation rates were found to be contaminated by hydrodynamic noise so the osborn dissipation method was not used to compute k however data segments for which the tke dissipation rate e was large enough to be unaffected by noise were used to compute the apparent mixing efficiency d the computed d values are used to investigate variations in apparent mixing efficiency with respect to density ratio r and turbulence reynolds number e n2 in an attempt to eluci

mixing on a shallow shelf of the black sea abstract microstructure measurements were carried out on the shallow shelf of the black sea from the sea surface to the bottom using a free falling baklan s profiler released from an anchored ship a northeast to southwest transect consisting of eight measurement stations with several casts made at each station enabled the evaluation of microstructure statistics across the shelf the eddy and scalar diffusivities as well as the mixing efficiencies were evaluated for distinct layers that were identified based on mean stratification and the state of turbulence these include five main layers with persistent features upper and bottom boundary layers diurnal and main pycnoclines and a stratified weakly turbulent inner layer and several transient patchy features embedded within such layers quasi homogeneous active turbulent stratified dissipative and microstructure displacement patches the thorpe displacement scale lth measurements of this study together with those reported in dillon and gib

petrology of middle proterozoic diabases and picrites from lake nipigon canada mafic rocks at lake nipigon provide a record of rift related continental basaltic magmatism during the keweenawan event at 1109 ma the mafic rocks consist of an early volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes sheets and sills the diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units compositional variation in the sills indicates that they crystallized from several magma pulses

geochemistry and tectonic significance of the mafic volcanic blocks in the dunnage melange north central newfoundland abundant volcanic blocks are present in the dunnage melange these mafic volcanic rocks consist predominantly of pillow lava tuff breccia isolated pillow tuff breccia and minor amounts of ropy lava major and trace element compositions of the basalts reveal that these volcanics do not resemble calc alkaline or low potassium island arc suites rather the majority of the samples are enriched type ocean floor tholeiites whereas some show alkali basalt affinities discrimination diagrams suggest that these basalts may have been erupted as within plate basalts however the chemical composition of the volcanic blocks is most similar to that of basalts generated at bathymetric highs located astride or slightly off mid ocean ridges the geochemistry of the dunnage melange basalts is very similar to that of the mafic volcanic rocks from the nearby summerford group and the lawrence head formation this correlation is further supported by sedimentary and petrographic evidence and by partial age equivalency

shear and richardson number in a mode water eddy abstract measurements of stratification and shear were carried out as part of the eddies tracer release experiment in mode water eddy a4 during the summer of 2005 these measurements were accomplished using both shipboard instrumentation and a drifting mooring a strong relationship between shear intensity and distance from the center of the eddy a4 was observed with the shipboard adcp diapycnal diffusivity at the sf6 tracer isopycnal prior to and during the release was estimated from the drifting mooring to be 2 9 10 6 m2 s 1 diffusivity increased by an order of magnitude to 3 2 10 5 m2 s 1 during the period of the final tracer survey in early september which was similar to the value estimated from the tracer analysis for the whole experiment 3 5 10 5 m2 s 1 ledwell j r mcgillicuddy jr d j anderson l a 2008 nutrient flux into an intense deep chlorophyll layer in a mode water eddy deep sea research ii this issue doi 10 1016 j dsr2 2008 02 005

a basinwide estimate of vertical mixing in the upper pycnocline spreading of bomb tritium in the north pacific ocean abstract the vertical diffusivity kv in the upper half kilometer of the north pacific subtropical pycnocline is estimated from observations of the spreading rate of anthropogenic tritium the calculation is based on approximately 300 ocean tritium profiles made since tritium was introduced to the atmosphere via bomb testing in the 1960s the data coverage does not permit detailed mapping of tritium penetration especially in the sparsely sampled western pacific for this reason and to minimize advective effects the spreading rate is averaged within the closed streamlines of the subtropical gyre spanning 5 n to 40 n the result kv 1 5 0 7 10 5 m2 s 1 is consistent with inferences from microstructure and purposefully released tracer measurements in the north atlantic confirming that the spatially averaged rate of mixing in the upper pycnocline is substantially lower than the canonical munk estimate for the lower pycnocline

argon as a tracer of cross isopycnal mixing in the thermocline abstract noble gases such as argon are unaffected by chemical reactions in the ocean interior but a number of physical mechanisms can lead to measurable sea level atmospheric disequilibrium in subsurface waters of the ocean one such mechanism is the mixing of waters of different temperatures which can lead to supersaturation in the ocean interior the authors simulate the supersaturation mixing signature in the thermocline in a global ocean general circulation model parallel ocean program model version 1 4 pop 1 4 in contrast to existing mixing diagnostics such as dye tracers or microstructure measurements which yield the local recent rate of diabatic mixing argon disequilibrium traces an integrated lifetime history of subsurface mixing a theoretical model of the subtropical atlantic ocean gyre is built based on the competing time scales of horizontal and vertical mixing that agrees well with the full general circulation model argon supersaturation gradient in the thermocline these results