topographic and boundary effects on steady and unsteady flow through straits an overview of topographic and boundary effects in flows through straits is presented the emphasis is on the various types of interaction of uniform and stratified flow with the topography of straits especially the interaction with sills the interaction of a single layer flow in a channel with a sill obstacle depends on the froude number of the flow and the relative height of the sill two classification schemes one steady and one pseudo steady are given for single layer flow in straits where the effects of rotation are important a further parameter the ratio of the channel width to a rossby radius is needed and a similar classification scheme as for the non rotating flows can be obtained unsteady flows often result in bores for single layer flows and internal bores in stratified flows and the behaviour of these is discussed the flow of stratified flow past sills may generate lee waves while the increased flow speeds at the sill may allow deep water to be drawn up and over the sill aspiration features of the strait topography away from the sill including slopes at the side of the channel and also downstream of the sill channel curvature and channel widening have important effects on the character of the flow and on mixing

tides in the strait of gibraltar the strait of gibraltar is where the transition occurs between two distinct tidal regimes the north atlantic where tidal ranges are in excess of 2 m and the western mediterranean where tidal ranges are less than 1 m within the strait the tide is principally semidiurnal observations indicate that on average 96 of the bottom pressure and 74 of the current variability are contained in the semidiurnal band 2 cpd the structure of the local cotidal chart of the m2 tidal constituent is complicated but can be explained in terms of the along and across strait momentum balances along the strait the pressure gradient is mainly balanced by the acceleration of the flow although friction can be of appreciable magnitude corresponding to a decay time scale of 3 16 hours across the strait the momentum balance appears mostly geostrophic but better observations would be required to explain residual terms available observations indicate that the m2 tide has no net energy flux through the strait at the sill the tidal transport in the upper layer exceeds that in the lower layer by nearly a factor of 2 at the eastern end of the strait between algeciras and ceuta the tidal transport is larger in the lower layer this apparent shift in the location of maximum tidal transport is explained in terms of the kinematics of the interface between atlantic and mediterranean waters a mean transport of 0 21 sv due to subinertial and tidal flows carries atlantic water through the strait compensated by an approximately equal return in the lower layer

water exchange between algeciras bay and the strait of gibraltar a study based on hf coastal radar abstract this study analyses the water mass exchanges at subinertial scale between algeciras bay and the adjacent strait of gibraltar the mechanisms triggering this exchange process is investigated with the aid of recently acquired data on surface currents obtained using a system of hf coastal radars deployed on the eastern side of the strait and remotely sensed images of sea surface temperature sst and chlorophyll from the modis sensor of the aqua satellite hf radar data on surface currents are analyzed by the application of real empirical orthogonal function eof decomposition which produces three eof modes explaining more than 70 of the variance of the surface currents at the mouth of the bay modes 2 3 and 6 mode 2 is related to the fluctuations of the atlantic jet in the central zone of the strait mainly due to a combined effect of the atmospheric pressure fluctuations in the western mediterranean sea and local wind in the eastern side of the strait mode 3 is related to the coastal currents induced by zonal wind forcing on the north western coast of the strait and alboran sea and mode 6 seems to be related to water transport induced by winds blowing with a significant north component into and out of the bay

transport estimates at the western section of the strait of gibraltar a combined experimental and numerical modeling study 1 three yearlong time series of acoustic doppler current profiler adcp observations at a single station in espartel sill strait of gibraltar were used to compute an outflow of q2 0 82 sv through the main channel the cross strait structure of the velocity field or the outflow through a secondary channel north of the submarine ridge of majuan in espartel section is not captured by observations so that an improved version of a numerical model cepom has been used to fill the observational gap previously the model performance has been checked against historical data sets by comparing harmonic constants of the main diurnal and semidiurnal constituents from observed and modeled data at different sites of the strait considering the great complexity of tidal dynamics in the area the comparison is quite satisfactory and validates the model to infer the exchange at longer timescales using a climatological april in the simulation extracting a single station from the model at the same position as the monitoring station and processing the data similarly the model gives an outflow through the southern channel 13 higher than observations the inclusion of the cross strait structure of velocity reduces the computed outflow through the southern channel whereas the contribution of the northern channel brings the total outflow close to that computed using a single station 5 smaller if the same correction is applied to observations the total outflow would reduce to q2 0 78 sv the paper also assesses the importance of eddy fluxes to the total outflow their contribution being negligible 5

propagation regimes and populations of internal waves in the mediterranean sea basin abstract the geographical and seasonal distributions of kinematic and nonlinear parameters of long internal waves are derived from the generalized digital environmental model gdem climatology for the mediterranean sea region including the black sea the considered parameters are phase speed of long internal waves and the coefficients at the dispersion quadratic and cubic terms of the weakly nonlinear korteweg de vries type models in particular the gardner model these parameters govern the possible polarities and shapes of solitary internal waves their limiting amplitudes and propagation speeds the key outcome is an express estimate of the expected parameters of internal waves for different regions of the mediterranean basin

sea level variations in the western mediterranean studied by a numerical tidal model of the strait of gibraltar aspects of the sea level changes in the western mediterranean sea are investigated using a numerical tidal model of the strait of gibraltar as a prerequisite the performance of this model that is a two dimensional nonlinear two layer boundary fitted coordinate numerical model based on the hydrostatic approximation on an f plane is assessed in the simulation of mean and tidal circulation of the strait of gibraltar the model is forced by imposing mean interface and surface displacements as well as m2 s2 o1 and k1 tidal components along the atlantic and mediterranean model open boundaries model results are compared with observations and with results obtained from a tidal inverse model for the eastern entrance of the strait of gibraltar in general good agreement is found a sensitivity study performed by varying different model parameters shows that the model behaves reasonably well in the simulation of the averaged circulation the model is then used to investigate the climatological sensitivity of the simulated dynamics in the strait of gibraltar to changes in the density difference between atlantic and mediterranean waters for this purpose given a certain density difference between atlantic and mediterranean waters the authors iteratively searched for that sea level drop between the atlantic and the mediterranean that fulfills the mass balance of the mediterranean it is found that an increase of the density difference leads to an increase of the exchange flow and to an increase of the sea level drop between the two basins a trend in the sea level drop of o 1 cm yr21 such as the one observed between 1994 and 1997 is explained by the model as the result of a trend of o 10 24 yr21 in the relative density difference between the mediterranean and atlantic waters the observed north south asymmetry in this trend is also captured by the model and it is found to arise from changes in the along strait velocity results suggest that the dynamics within the strait of gibraltar cannot be neglected when sea level changes in the western mediterranean basin are investigated

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

morphostructure of an incipient subduction zone along a transform plate boundary puysegur ridge and trench multibeam bathymetric and geophysical data reveal a major strike slip fault that extends along the summit of the puysegur ridge east of the puysegur trench the northward structural development of this ridge trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m y at the southern end of the trench where subduction has not yet started the puysegur ridge has a narrow 50 km steep sided cross section and the axial strike slip fault separates a shallow 125 625 m flat topped eastern crest from a deeper 400 1600 m western crest these characteristics indicate differential uplift during the initial stage of shortening on the lower plate an incipient 5 2 km deep trench developed in conjunction with normal and reverse faults suggesting strong interplate coupling across the trench northward the ridge broadens linearly to 80 km wide its western flank has locally collapsed and the ridge summit has subsided possibly by 1 5 km suggesting that the interplate coupling decreases and that a benioff zone is being formed concomitant to the northward ridge evolution the trench deepens to 6 2 km and normal fault throws increase along its outer wall indicating greater flexure of the downgoing plate

control variability and internal bore evolution in the strait of gibraltar a 2 d two layer model study a two dimensional high resolution non linear two layer free surface boundary fitted co ordinate hydrostatic model was applied to study the time space variability of hydraulic controls and the development of internal bores in the strait of gibraltar the model predicts the occurrence of four averaged over a tropical month controls located to the west of the spartel sill at the spartel and camarinal sills and in the tarifa narrows the last of these controls is apparent in the sense that it consists of discrete fragments alternating with subcritical flow regions the only control which extends over the whole width of the strait is the control at the camarinal sill but it breaks down during neap tide too this control exists concurrently with the control in the tarifa narrows for short periods while for much of the tropical month there is either just one or neither of the controls the model predicts the development of a hydraulic jump and a jump drop pair near the camarinal sill the appearance of bulges of mediterranean water to the east and west of the sill the large amplitude and small amplitude internal bores released from the camarinal sill which travel respectively eastward and westward and their transformation due to radial spreading and dissipative effects also presented here are the results illustrating the effects of earth s rotation on the internal bores in the strait of gibraltar

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

improving the analysis of biogeochemical patterns associated with internal waves in the strait of gibraltar using remote sensing images abstract high amplitude internal waves haiws are physical processes observed in the strait of gibraltar the narrow channel between the atlantic ocean and the mediterranean sea these internal waves are generated over the camarinal sill western side of the strait during the tidal outflow toward the atlantic ocean when critical hydraulic conditions are established haiws remain over the sill for up to 4 h until the outflow slackens being then released mostly towards the mediterranean sea these have been previously observed using synthetic aperture radar sar which captures variations in surface water roughness however in this work we use high resolution optical remote sensing with the aim of examining the influence of haiws on biogeochemical processes we used hyperspectral images from the hyperspectral imager for the coastal ocean hico and high spatial resolution 10 m images from the multispectral instrument msi onboard the sentinel 2a satellite this work represents the first attempt to examine the relation between internal wave generation and the water constituents of the camarinal sill using hyperspectral and high spatial resolution remote sensing images this enhanced spatial and spectral resolution revealed the detailed biogeochemical patterns associated with the internal waves and suggests local enhancements of productivity associated with internal waves trains

submesoscale tidally induced biogeochemical patterns in the strait of gibraltar abstract tidal forcing and its fortnightly variation are known to be one of the main regulating agents of physical and biogeochemical signatures in the strait of gibraltar and surrounding areas samples obtained during spring and neap tides in the region were analyzed to determine the influence of this tidal variation on the submesoscale distribution of water masses and biological elements during spring tides strong and intermittent mixing processes between mediterranean and atlantic waters occur in the vicinity of the camarinal sill together with an eastward advection of those mixed waters into the alboran sea furthermore the intense suction of surface coastal waters into the main channel of the strait was detected as chlorophyll patches in the alboran sea during spring tides in contrast the most characteristic phenomenon during neap tides was the arrival of pulses of relatively nutrient rich north atlantic central waters to the surface regions of the alboran sea in addition traces of the suction of coastal waters were observed for the first time during neap tides therefore our results show that submesoscale processes are crucial in the dynamics of the strait of gibraltar and they must be considered for the correct description of the biogeochemical features of alboran sea especially during an inactive period of the coastal upwelling

joint canada u s ocean wave investigation project an overview of the georgia strait experiment an overview of the georgia strait experiment joint canada u s ocean wave investigation project or jowip 1983 is presented containing a list of participants objectives procedures and main results jowip was designed to provide quantitative comparisons between surface measurements and remote sar imagery of internal waves and narrow v surface ship wakes first order bragg modeling gives factor of 3 accuracy for l band internal wave modulation the composite model and possibly nonlinear surface wave internal wave interaction theory are required at x band narrow v wakes were imaged in dabob bay washington under low wind conditions at l band surface slopes in the kelvin wakes are not large enough to produce specular reflections even at a radar incidence angle of 23 modulations along the v arms are strongly evident and have the same wavelength as the transverse kelvin wake component these results and others are described in detail in other papers in this same issue

baroclinic m2 tidal circulation in algeciras bay and its implications for the water exchange with the strait of gibraltar observational and 3 d model results 1 the m2 tidal circulation in algeciras bay strait of gibraltar is analyzed using a 3 d nonlinear baroclinic hydrodynamic model in conjunction with observed data series results show the influence of the density stratification on the vertical structure of the m2 currents in algeciras bay although its tidal dynamics shows major differences with respect to the strait of gibraltar whereas the m2 currents in the strait present mainly barotropic behavior the baroclinic effects prevail in algeciras bay a notable finding is the presence of a tidal m2 counter current system between the upper atlantic and the lower mediterranean water layers within the bay with amplitudes of up to 25 cm s 1 the interface between the two layers oscillates in antiphase relation with respect to the free surface elevation with amplitudes of almost 20 m the presence of the submarine algeciras canyon was found to be determinant in the three dimensional structure of tidal currents within the bay strengthening the baroclinic tidal regime of currents this situation has quantitative consequences for the flow exchange processes between algeciras bay and the outer strait with rates 20 times higher than those obtained when considering only the barotropic behavior as well as inflow outflow lateral recirculation volumes during half a tidal cycle that account for more than 20 of the net accumulated volume this flow exchange system was found to be affected by the nonlinear interaction processes between the first baroclinic period of resonance of algeciras bay and the m2 tide

late quaternary multi genetic processes and products on the northern gulf of cadiz upper continental slope sw iberian peninsula abstract on continental margins the upper slope to shelf break environment forms a critical region where sediment supply hydrographic activity and gravitational processes determine how and when sediments are partitioned between the shallow and deep marine realm on the sw iberian margin relatively few studies have addressed the dynamics of this region although it holds key information regarding the link between the sedimentary evolution of the continental shelf and the contourite depositional system on the middle slope this work therefore presents a high resolution analysis of the morphological and stratigraphic expressions of late quaternary dominantly last glacial and present day interglacial sedimentary processes on the upper slope and shelf margin sector between 7 and 7 30 w the integration of seismic bathymetric and hydrographic data reveals the presence of alongslope processes and products a bottom current related plastered drift moat erosional surface and terrace an internal wave tide controlled sediment wave field downslope gravitational processes and products an upper slope shelf margin valley system slumps debrites gullies neotectonic elements diapirs and fluid flow features pockmarks bright spots the spatial distribution of these features indicates that the study area becomes increasingly alongslope dominated towards the w and oppositely more downslope dominated towards the e because sediment supply to the latter area is enhanced under the dominant eastward dispersal of fluvially supplied sediments on the shelf in addition glacial interglacial variations in the amount of sediments supplied to the shelf edge and the intensity of oceanographic processes in the study area also generate a distinct temporal variability with glacial and interglacial intervals respectively recording principally downslope and alongslope controlled morphological elements finally regardless of these overall spatial and temporal patterns diapirism and fluid flow are inferred to locally destabilize sediments and induce small scale mass wasting in the study area these findings are not only relevant to the northern gulf of cadiz but also to the general understanding of sedimentary dynamics and controls in mixed downslope and alongslope controlled upper slope to shelf margin settings worldwide

analysis of short period internal waves using wave induced surface displacement a three dimensional model approach in algeciras bay and the strait of gibraltar 1 a three dimensional nonlinear high resolution sigma coordinate hydrodynamic model was applied to study the sea surface manifestation of short period internal waves measured in algeciras bay and the strait of gibraltar model results reproduce the tidally induced generation of the internal bore over the camarinal sill and its disintegration into wave trains as it moves eastward while propagating along the strait of gibraltar toward the mediterranean sea the wave trains partly penetrate into algeciras bay with typical oscillation periods of 20 and 40 min the modeled wave induced surface train structures are compared with satellite images and in situ observational data obtained from two pressure sensors located inside the bay results demonstrate that wave induced sea surface displacements are indicators of the presence of internal waves and may be used in the context of the internal wave analysis when surface oscillations are captured with sufficient precision

burial evolution of evaporites with implications for sublacustrine fan reservoir quality a case study from the eocene es4x interval dongying depression bohai bay basin china abstract eocene sublacustrine fan sandstones that developed in a rift basin are important tight gas reservoirs in the dongying depression bohai bay basin northeastern china two units of evaporites developed at the top and bottom of the lower unit of the es4 interval es4x consist predominantly of anhydrite with subordinate gypsum evaporite and related diagenetic processes greatly influenced reservoir quality 13 c values for micritic dolomite cements in es4x are depleted 7 45 to 2 57 due to microbial sulfate reduction msr under shallow burial conditions and this interpretation is supported by large 34 s fractionation between anhydrite and framboidal pyrite precipitation temperatures for micritic dolomite are calculated as 57 5 72 8 c anhydritization of gypsum probably occurred at 100 5 145 2 c during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements this process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores thermochemical sulfate reduction tsr probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in es4x ankerite cements likely were derived from tsr as reflected in negative 13 c values 7 12 to 3 70 and high calculated temperatures 121 3 185 1 c dissolution by products e g saddle dolomite ankerite nodular pyrite related to tsr precipitated in adjacent pores a lack of significant 34 s fractionation between parent sulfate and nodular pyrite indicates that tsr occurred in a relatively closed system therefore dissolution of gypsum and anhydrite related to tsr contributed little to reservoir quality middle fan lithofacies with better sorting porosity and permeability than inner and outer fan lithofacies constitute high quality reservoirs in es4x

fortnightly and monthly variability of the exchange through the strait of gibraltar the fortnightly and monthly variability of the exchange through the strait of gibraltar has been studied from two simultaneous five month long moored datasets at camarinal sill and the east section the study focuses on the msf and mm tidal components and their role for the subinertial exchange a significant monthly signal is observed in the upper layer transport also a significant fortnightly signal is observed in the lower layer transport which minimum maximum flow toward the atlantic takes place approximately on spring tides in consequence the net transport has both signals with maximum taking place during neap tides and a small monthly inequality fortnightly and monthly variability in the interface depth is also observed at camarinal sill the interface being deeper on neap and shallower on spring tides at the east section the interface depth signals are not significant the subinertial variability of the transports is separated in two contributions the first one is called quasistatic transport and arises from the subinertial fluctuations of currents the second contribution called tidally rectified transports arise from the non linear correlation of currents and interface depth at tidal frequencies the tidally rectified transports are important at camarinal but not at the east section an apparent contradiction between the fortnightly signals of the subinertial currents and subinertial transports is resolved when the fortnightly signal of the tidally rectified transports are considered the fortnightly signal of the quasistatic and tidally rectified transports mutually cancel in the upper layer but not in the lower layer where the rectified transports dominate a simple model for the spring tide mixing forcing accounts for the fortnightly variability of the lower layer quasistatic transports but underestimates it for the upper layer finally the observed lower layer transport is compatible with the hydraulic control condition at camarinal sill except for certain periods during intense spring tides 2006 elsevier ltd all rights reserved

transformations of mg and ca sulfate hydrates in mars regolith salt hydrates have an active role in regolith development on mars the mg sulfate system with highly variable values of n in the formula mgso 4 n h 2 o is particularly subject to transformations among several crystalline and amorphous forms the ca sulfate system caso 4 n h 2 o is likely to be associated with the mg sulfates in most occurrences but is less susceptible to transformations in n desiccation of mgso 4 n h 2 o occurs in exposed soils at the martian equator in summer where higher daytime temperatures at low relative humidity prevail against sluggish nighttime rehydration at high relative humidity desiccation and rehydration are both accelerated in the finest size fractions particularly in silt size aeolian particles subject to global redistribution by dust storms this redistribution and periodic excursions into long term episodes of high obliquity work to rehydrate desiccated mgso 4 n h 2 o to form epsomite mgso 4 7h 2 o at higher latitudes in the first case and more globally in the latter kieserite a monohydrate form of mgso 4 n h 2 o resistant to desiccation can survive equatorial summer conditions but not protracted high relative humidity preservation of kieserite at the surface may place limits on the equatorial distribution of ice during past episodes of high obliquity deeper horizons in equatorial regolith may preserve hydrated phases through repeated obliquity episodes raising the possibility of an ancient regolith archive of past hydration at shallower depths in the regolith in situ determination of the hydration states of the mg sulfates and possibly the ca sulfates may be used to constrain regolith dynamics if rates and modes of transitions in n can be fully characterized

a tide generated internal waveform in the western approaches to the strait of gibraltar aircraft expendable bathythermograph radar infrared scanner and visual data collected in october 1982 1984 and 1985 as well as space shuttle photographs indicate that an internal waveform depending on flow conditions either an internal hydraulic jump or a lee wave is a semipermanent feature over the camarinal sill in the western approaches to the strait of gibraltar comparison of these data with both historic and recent current meter mooring data indicates that the waveform generated at the interface between the atlantic and mediterranean waters undergoes periodic changes wrought by interaction of the tidal current and the flow of the atlantic and mediterranean waters we believe that the waveform is a major mixing mechanism between the two water masses

experimental vnir reflectance spectroscopy of gypsum dehydration investigating the gypsum to bassanite transition the spectral behavior of gypsum dehydration in the visible to near infrared 350 2500 nm wavelength range was investigated by partially dehydrating the four main habits of gypsum alabaster satin spar selenite and massive to form bassanite powdered samples of gypsum dehydrated at 100 115 c and hand samples dehydrated at 115 130 c coinciding with a peak in mass water loss in the samples as gypsum dehydrates its characteristic h2o absorption bands at 1443 and 1945 nm shift to shorter wavelengths band depths and widths of absorptions at 1200 1400 1600 1750 1945 2100 2200 and 2400 nm all decrease with increasing temperature the samples also underwent visible changes upon dehydration both at the macroscopic and microscopic level becoming very friable with an increase in fine grains no consistent relationship was observed between dehydration temperature and grain size or habit the samples were monitored for up to 20 months after dehydration during which time none rehydrated to form gypsum while the transition from gypsum to bassanite is very abrupt bassanite does not readily rehydrate to form gypsum again in ambient conditions and therefore may not be as unstable as previously thought by terrestrial occurrences of bassanite being predominantly restricted to hyperarid climates

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

analyzing the tidal related origin of subinertial flows through the strait of gibraltar 1 the effects of tidal dynamics on subinertial flows through the strait of gibraltar are analyzed as found in previous studies an empirical orthogonal function analysis of subinertial currents at the camarinal sill yields two dominant oscillation modes the first mode presents a barotropic character and rather irregular fluctuations and it has been related to meteorological forcing the second mode is baroclinic and presents a clear deterministic behavior with time that seems to be related to tidal forcing against the hypothesis proposed in previous studies stating that tidal mixing cycles explain the second mode we show by using a one dimensional numerical model of two layer immiscible shallow water that the origin of this mode may basically be related to nonlinear interactions among the main semidiurnal tidal constituents through the advective terms in the momentum balance and other nonlinear terms in the volume conservation equations that mode is also crucial to understanding the vertical shear time variations of the horizontal currents in particular it minimizes the differences in the maximum shear between neap and spring tides

generation mechanism of internal waves by a vertically sheared tidal flow over a sill the internal response over a sill to strong tidal flow superimposed on a vertically sheared steady flow is examined with a simple two layer analytical model as in the case where the vertical shear is absent the upstream propagating internal wave is gradually formed through interference of infinitesimal amplitude internal waves elementary waves emanating from the sill at each instant of time hibiya 1986 with the composite internal froude number exceeding falling below unity in the accelerating decelerating stage of the two layer flow the upstream propagating elementary waves come close together over the sill in this case however the amplitude of the elementary wave is increased when the lower layer velocity is larger than the upper layer velocity whereas it is reduced when the upper layer velocity is larger than the lower layer velocity as a result the internal wave field over the sill differs markedly between the ebb and flood tidal phases it follows from this analytical result that in the strait of gibraltar the internal wave propagating mediterraneanward is significantly amplified during the ebb westward phase of the tidal flow whereas such amplification does not take place throughout the tidal cycle for the internal wave propagating atlanticward this is consistent with the result from satellite observation that the sea surface signatures caused by underlying large amplitude internal wave trains appear only on the mediterranean side and not on the atlantic side

incipient subduction at the contact with stretched continental crust the puysegur trench a seismic benioff zone and plate kinematics show puysegur trench south of new zealand transitioning to subduction because the local structure and its influence on subduction initiation is poorly understood we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles our early results show that the overriding pacific plate beneath the solander basin is composed of block faulted and thinned continental crust and the inner trench wall of northern puysegur ridge is composed of folded and faulted sediment the megathrust interface has been imaged and shows 500 m of downgoing undisturbed sediments combining plate kinematic history with seismic velocity inferred density we show that the density difference across the plate boundary changed as oblique strike slip plate motion juxtaposed dense oceanic crust with thinned continental crust the density difference rapidly increased 18 to 15 ma coincident with subduction initiation suggesting that compositional differences have a large influence on subduction initiation

numerical modeling of the semidiurnal tidal exchange through the strait of gibraltar 1 a three dimensional sigma coordinate free surface model is used to investigate the semidiurnal tidal exchange through the strait of gibraltar the model makes use of a coastal following curvilinear orthogonal grid that includes the gulf of cadiz and the alboran sea with very high resolution in the strait 500 m a lock exchange initial condition is used the western part of the model domain is filled with atlantic water whereas the eastern part is filled with mediterranean water the model is forced at the open boundaries through the specification of the semidiurnal m2 and s2 tidal surface elevation the model is run over a spring neap cycle fortnightly period and the results are compared with available observed data simulated cotidal maps of the m2 and s2 tidal elevation components are in quantitative and qualitative good agreement with observed data as well as with the simulated major and minor axis of tidal ellipse the model reproduces the generation and the subsequent propagation of internal bores both eastward and westward showing that they are always generated during the fortnightly period however the principal aim of this work is to quantify the effects of tidal forcing on mean quantities entrainment and transport of atlantic and mediterranean water along the strait model results reveal that the contribution of the semidiurnal tidal component m2 to the transport is relevant over camarinal sill whereas it is negligible at the eastern end of the strait model results indicate also that the effect of the semidiurnal tide is to increment the mean transport by about 30 both for the inflow and the outflow

transition from strike slip faulting to oblique subduction active tectonics at the puysegur margin south new zealand abstract south of new zealand the pacific australia pac aus plate boundary runs along the intracontinental alpine fault the puysegur subduction front and the intraoceanic puysegur fault the puysegur fault is located along puysegur ridge which terminates at ca 47 s against the continental puysegur bank in a complex zone of deformation called the snares zone at puysegur trench the australian plate subducts beneath puysegur bank and the fiordland massif east of fiordland and puysegur bank the moonlight fault system mfs represents the eocene strike slip plate boundary interpretation of seafloor morphology and seismic reflection profiles acquired over puysegur bank and the snares zone allows study of the transition from intraoceanic strike slip faulting along the puysegur ridge to oblique subduction at the puysegur trench and to better understand the genetic link between the puysegur fault and the mfs seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 geodynz survey and single beam echo soundings acquired by the nz royal navy the snares zone is the key transition zone from strike slip faulting to subduction it divides into three sectors namely east nw and sw sectors a conspicuous 3600 m deep trough the snares trough separates the nw and east sectors the east sector is characterised by the ne termination of puysegur ridge into right stepping en echelon ridges that accommodate a change of strike from the puysegur fault to the mfs between 48 s and 47 s in the nw sector and the snares trough a series of transpressional faults splay northwards from the puysegur fault between 49 50 s and 48 s thrusts develop progressively at puysegur trench into a decollement north of 48 s the snares trough develops between two splays of the puysegur fault indicating superficial extension associated with the subsidence of puysegur ridge seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the snares fault and terminate at the top of the puysegur trench slope between ca 48 s and 46 30 s the relative plate motion appears to be distributed over the puysegur subduction zone and the strike slip faults located on the edge of the upper plate conversely north of ca 46 s a lack of active strike slip faulting along the mfs and across most of puysegur bank indicates that the subduction in the northern part of puysegur trench accounts for most of the oblique convergence hence active transpression in the snares fault zone indicates that the relative pac aus plate motion is transferred from strike slip faulting along the puysegur fault to subduction at puysegur trench the progressive transition from thrusts at puysegur trench and strike slip faulting at the puysegur fault to oblique subduction at puysegur trench suggests that the subduction interface progressively developed from a western shallow splay of the puysegur fault it implies that the transfer fault links the subduction interface at depth a tectonic sliver is identified between puysegur trench and the puysegur fault its northwards motion relative to the pacific plate implies that is might collide with puysegur bank

electron backscatter diffraction ebsd analysis of bassanite transformation textures and crystal structure produced from experimentally deformed and dehydrated gypsum dehydration reactions have important effects on fluid flow porepressure and brittle failure in the earth the rates and mechanismsof reaction are relevant to understanding those effects the micro structures ofa set ofsamples containing bassanite and gypsum fromdehydration experiments on volterra gypsum have been analysed byconventional transmitted light microscopy and electron backscatterdiffraction ebsd these techniques have been used to character ize the varying crystal morphology and to measure the crystallo graphic preferred orientation cpo of the samples in somesamples bassanite grains show two distinct morphologies large 0 5 2 mm euhedral crystals and small 550mm acicular crys tals these represent the two different bassanite crystal habits bassanite was indexed using ebsd for thefirsttime and problemsrelated tosignificant mis indexing resulting from the pseudotrigonalsymmetry of bassanite are resolved the original starting materialhas a strong local and measurable cpo a strong cpo inpost experiment bassanite is found to mimic the original gypsumc axis orientation bassanite grain size is related to the degree ofreaction overstep spatial heterogeneity in bassanite distributionmay indicate the development of fluid pathways within the samples

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

dynamics of subduction initiation with different evolutionary pathways changes of plate motion may have induced subduction initiation si but the tectonic history of si is different from one subduction zone to another izu bonin mariana ibm si accompanied by strong backarc spreading and voluminous eruption of boninites contrasts with the aleutians which shows neither using finite element models we explore visco elasto plastic parameters and driving boundary conditions for si evolution with an imposed velocity we find three different evolutionary modes of si continuous without backarc spreading continuous with backarc spreading and a segmented mode with an increase in the coefficient of friction and a decrease in the rate of plastic weakening the amount of convergence needed for si increases from 20 to 220 km while the mode changes from segmented to continuous with backarc spreading and eventually to continuous without backarc spreading if the imposed velocity boundary condition is replaced with an imposed stress the amount of convergence needed for si is reduced and backarc spreading does not occur these geodynamic models provide a basis for understanding the divergent geological pathways of si first ibm evolution is consistent with subduction of an old strong plate with an imposed velocity which founders causing intense backarc spreading and boninitic volcanism second the new hebrides si is in the segmented mode due to its weak plate strength third the puysegur si is in the continuous without backarc spreading mode with no associated volcanic activities fourth the aleutians si has neither trench rollback nor backarc spreading because the slab is regulated by constant ridge push forces

coastal altimetry products in the strait of gibraltar this paper analyzes the availability and accuracy of coastal altimetry sea level products in the strait of gibraltar all possible repeats of two sections of the envisat and altika ground tracks were used in the eastern and western portions of the strait for envisat along track sea level anomalies slas at 18 hz posting rate were computed using ranges from two sources namely the official sensor geophysical data records sgdrs and the outputs of a coastal waveform retracker the adaptive leading edge subwaveform ales retracker in addition slas at 1 hz were obtained from the centre for topographic studies of the ocean and hydrosphere ctoh for altika along track sla at 40 hz was also computed both from sgdr and ales ranges the sea state bias correction was recomputed for the ales retracked envisat sla the quality of these altimeter products was validated using two tide gauges located on the southern coast of spain for envisat the availability of data close to the coast depends crucially on the strategy followed for data screening most of the rejected data were due to the radar instrument operating in a low precision nonocean mode we observed an improvement of about 20 in the accuracy of the envisat slas from ales compared to the standard sgdr and the reprocessed ctoh data sets altika shows higher accuracy with no significant differences between sgdr and ales the use of products from both missions allows longer times series leading to a better understanding of the hydrodynamic processes in the study area

exchange through the strait of gibraltar to measure the exchange between the atlantic and mediterranean through the strait of gibraltar an array of current meter moorings was deployed for a year in the strait during 1985 1986 a novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously these salinity measurements determine the water mass characteristics of the flows crossing the sill they allow definition of the 37 psu isohaline as the interface between inflowing fresher atlantic water and outflowing saltier mediterranean water and they enable time series to be developed for the depth of this interface for the upper layer inflow and for the lower layer outflow from these measurements the time averaged outflow of mediterranean water is estimated to be 0 68sv 1sv 1 106m3s 1 and the outflow salinity transport defined to be the outflow times the salinity excess above a basic atlantic water salinity of 36 1 psu is estimated to be 1 50 103m3s 1 1sv 1psu 1 103m3s 1 equivalent to a net evaporation over the mediterranean basin of 52cm y 1 extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time averaged inflow of 0 93sv which is believed to be unrealistically high in view of the better measured outflow and net evaporation thus a more realistic estimate of the inflow is 0 72sv equal to the sum of the outflow and net evaporation as required by the mass budget for the mediterranean sea such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by lacombe and richez 1982 the exchange across the gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations the mean currents are smaller than had been expected reaching a peak value of only about 60 cm s 1 in the deep outflow over the sill the tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents for the m2 tide at the sill the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1 2m s 1 furthermore the inflow and interface depth have similar phases as a consequence the upper layer is deep on the inflowing tide so that a large slug of atlantic water crosses the sill into the mediterranean on the outflowing tide the interface is shallow so that a large slug of mediterranean water crosses the sill into the atlantic similar processes occur for the s2 o1 and k1 tides though the amplitude are smaller in this manner tidal oscillations lead to a time averaged exchange of water masses across the gibraltar sill the inflow and outflow defined to be the instantaneous transports above and below the 37psu isohaline interface exhibit m2 tidal amplitudes of 2 3 sv and 1 3sv respectively thus the tides are large enough to reverse the mean upper layer inflow and lower layer outflow daily averaged inflow and outflow transports exhibit low frequency fluctuations with standard deviations of 0 37sv and 0 22sv respectively such low frequency fluctuations have been shown previously to be associated with barotropic flows through the strait of gibraltar compensating for sea level variations over the mediterranean due to atmospheric pressure fluctuations candela winant and bryden 1989 finally from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the strait of gibraltar

generation mechanism of internal waves by tidal flow over a sill the generation mechanism of internal waves by relatively strong tidal flow over a sill is clarified analytically special attention is directed to the role of the tidal advection effect which is examined by use of characteristics an internal wave which propagates upstream is gradually formed over the sill through the interference among infinitesimal amplitude internal waves elementary waves emanated from the sill at each instant of time in the accelerating decelerating stage of tidal flow the effective amplification of an internal wave takes place as the froude number exceeds falls below unity because during this period the internal wave slowly travels downstream upstream crossing over the sill where elementary waves are efficiently superimposed when the strength of the tidal advection effect is appropriate the internal wave formed in the accelerating stage ac wave and that formed in the decelerating stage dc wave overlap so that the resultant wave height becomes very large since the relative position of ac and dc waves varies depending on the strength of the tidal advection effect the resultant internal wave form is strongly affected by it when the tidal advection effect is strong the ac wave is carried too far downstream to interfere with the dc wave and hence the resultant wave form consists of two crests and two troughs when the tidal advection effect is moderate the optimum interference between ac and dc waves occurs and the resultant wave form consists of one crest and one trough with its horizontal scale approaching that of the sill when the tidal advection effect is weak the internal wave height becomes very small since elementary waves propagate almost freely in the tidal flow without coming close together and the resultant wave form approaches a sinusoidal wave of tidal frequency

time spatial variability observed in velocity of propagation of the internal bore in the strait of gibraltar 1 some aspects of the time spatial variability of the phase speed of the internal bore generated almost every tidal cycle in camarinal sill are revised using a set of high resolution experimental data collected in two different positions of the strait during may 2003 this variability is mainly driven by the intense tidal currents comparable with the intrinsic propagation velocity of the first mode baroclinic bore it is shown that the importance of the diurnal tide in the strait of gibraltar induces a considerable diurnal inequality on the bore velocity with an observed maximum difference of 0 7 ms 1 between the speed of two consecutives bores propagating along the eastern part of the strait a regularly spatial pattern has been also found the internal bore reaches its maximum velocity in tarifa narrows a theoretical estimation predicts an extreme phase speed of 2 6 ms 1 during our period of study

microbially induced dolomite precipitates in eocene lacustrine siliciclastic sequences in the dongying depression bohai bay basin china evidence from petrology geochemistry and numerical modeling this study investigates the origin of dolomite by examining thick lacustrine sequences of evaporites mudstones and deep water sublacustrine fan sandstones in the lower unit of the fourth member of the eocene shahejie formation es4x in bohai bay basin china this petrographic geochemical and numerical modeling study recognizes evidence for microbially mediated dolomite precipitates in the es4x sandstones including the following 1 spherical or elliptical dolomite aggregates consisting of submicrometer sized crystals 2 negative 13cvpdb vpdb refers to vienna peedee belemnite values of the dolomite cements 6 56 to 3 35 3 relatively low precipitation temperatures of dolomite cements 39 c 65 c 102 f 149 f based on 18ovpdb values from 8 76 to 4 65 and 4 negative 34svcdt vcdt refers to vienna canon diablo troilite values of associated framboidal pyrite 3 9 to 5 7 with respect to an anhydrite precursor 21 2 to 37 8 one dimensional reactive transport modeling incorporating biodegradation kinetics was conducted to evaluate the link between biodegradation of organic matter and geochemical reactions of inorganic minerals during microbial sulfate reduction msr microbial metabolism in the model creates favorable geochemical conditions for dolomite precipitation by increasing ph carbonate alkalinity and mg2 ca2 ratio the modeling results reproduce petrographic observations associated with the msr process and indicate that the kinetics of microbial growth can significantly modify the geochemistry of simulated waters and enhance microbially mediated dolomite precipitation therefore this study illustrates the effect of msr in dolomite precipitation in a semiclosed and reducing continental lacustrine basin

the age and origin of miocene pliocene fault reactivations in the upper plate of an incipient subduction zone puysegur margin new zealand

subduction initiation with vertical lithospheric heterogeneities and new fault formation how subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied we investigate this with a geodynamic model for the puysegur incipient subduction zone pisz south of new zealand the model incorporates a true free surface elasto visco plastic rheology and phase changes our predictions fit the morphology of the puysegur trench and ridge and the deformation history on the overriding plate we show how a new thrust fault forms and evolves into a smooth subduction interface and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation consistent with two fault system at pisz the model suggests that the pisz may not yet be self sustaining we propose that the snares zone or snares trough is caused by plate coupling differences between shallower and deeper parts the tectonic sliver between two faults experiences strong rotation and low density material accumulates beneath the snares zone

internal waves in the strait of gibraltar and their role in the vertical mixing processes within the bay of algeciras abstract this article presents some of the results of an oceanographic survey carried out in the bay of algeciras strait of gibraltar as part of a research project intended to assess the environmental quality of the bay one of the most interesting findings was the step like patterns presented by density profiles within the bay which were indicative of notable vertical mixing activity there the analysis of the observations indicates that those mixing processes may be explained by the interaction between the local internal tide dynamics and the large amplitude internal waves entering the bay which seem to originate from the camarinal sill region

airborne synthetic aperture radar tracking of internal waves in the strait of gibraltar abstract as part of the international gibraltar experiment we realized on june 22 and 24 1986 two surveys of the strait of gibraltar on board an aircraft equipped with a synthetic aperture radar our objective was to observe at spring tides and during two twelve hour tidal cycles at 24 h interval the generation of internal wave trains linked to the hydraulic jump formed west of the sill during the westward phase of the tidal current and their eastward propagation in the strait the speed of propagation of these waves and the effect of the diurnal component of the tide on their generation and propagation could then be determined our results suggest that two solitary waves of equal amplitude propagated eastwards in the strait on june 22 the tidal coefficient being equal to 92 with a speed relative to the ground of 2 1 to 2 6 m s 1 24 h later during the second survey on june 24 tidal coefficient 90 we observed the propagation of a train of non linear waves the speed of the leading wave of which being about 1 9 ms 1 our data show that other waves pass over the camarinal sill after the release of the bore and secondary internal wave trains are shown to propagate eastwards from there although our sar data show the appearance of waves west of the northern sill at about 4 h after high water hw the mechanism leading to their generation is not clear these waves could propagate eastwards all along the strait and or northwestwards along the western spanish coast they could be responsible for the solitary type events observed at the eastern entrance of the strait at about 7 h after hw by ziegenbein 1969 1970 these events are noticeable in the hydrological parameters time series of armi and farmer 1988 and in the high rate current data 2 min apart from their april 1986 cruise besides these alongstrait waves our sar data show the existence of cross strait waves and give an idea of their wavelength and speed of propagation their presence leads to perturbations in the current revealed by simultaneous current data at the camarinal sill and north of cape cires

evidence of multimodal structure of the baroclinic tide in the strait of gibraltar 1 the multimodal structure of the baroclinic tides in the strait of gibraltar is studied using observations and numerical simulations observational data and model results revealed the presence of two types of tidal internal waves generated over camarinal sill cs one propagates toward the mediterranean and disintegrates into series of nonlinear short internal waves with amplitudes of 50 m and more the second type behind the first propagates slower and has a longer wavelength the vertical structure with both upward and downward displacements of isopycnals can be identified as a manifestation of higher baroclinic modes analysis of the empirical orthogonal functions of the adcp measurements performed over cs and model time series has shown that the second baroclinic mode predominates in the second type of internal waves its amplitude can reach one third that of the first baroclinic mode of the leading waves of depression

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

nonlinear features of internal waves off baja california as observed from the seasat imaging radar the synthetic aperture radar sar on seasat has yielded well defined images of quasi periodic internal waves in the waters on the west side of baja california these waves occur in groupings separated by 15 23 km each group having 2 20 striations with wavelengths of the order of 300 m or longer they appear in water approximately 200 m deep shoreward of several banks at the continental shelf edge whose depths are as shallow as 15 m the wave surface signatures exhibit clear nonlinear features higher than linear group velocities and decreases in wavelength crest length and amplitude toward the rear of the packet environmental data for the area have been examined and these show a well developed mixed layer low winds and vigorous tidal action thus stratification wind speed tidal current and bottom topography apparently combine to establish proper conditions for tidal generation of internal waves the images have been analyzed from the standpoint of nonlinear wave theory and it is shown that the vertical displacement of an isopycnal surface can be estimated from a combination of sar imagery vertical density profiles and simple theoretical expressions numerical examples are given

the importance of sub mesoscale processes for the exchange of properties through the strait of gibraltar abstract this article presents a detailed analysis of the sub mesoscale transport processes in the strait of gibraltar the interest is focussed on the camarinal sill region and special attention is paid to the across strait transport processes the divergences and convergences in the central zone and the small scale circulation patterns along the northern coastal margin the analysis is based on high resolution 7 m sst images acquired by an air borne hyper spectral scanner and has been complemented with a rhodamine release experiment continuous thermo salinograph records acoustic doppler current adcp profiles from both moorings and vessel mounted experiments and numerical modelling it is deduced from the analysis that the coupling between the upwelling processes induced by the internal tide and the generation of large amplitude internal waves and the cyclonic eddies formed on the coastal margin seems to be the mechanism that explains the chlorophyll maxima frequently found on the coastal margin of the studied area further as a consequence of the small scale patterns of circulation induced by the internal waves the suspended substances are displaced from the coastal margins toward the central zones and later are carried by the westward current toward the convergence zones created by the internal waves where they may be retained and accumulate then in the eastward phase of the tidal current over the camarinal sill these nuclei of concentrated substances nutrients chlorophyll and plankton are transported toward the alboran sea where they must contribute in part to the primary productivity there

a numerical model of internal wave refraction in the strait of gibraltar abstract the relative importance of depth cross strait interface till and horizontal current shear upon the wave front shapes of internal waves in the strait of gibraltar are examined by means of a numerical refraction model results from the model are also compared with observations of wave front shape made in the eastern strait using a shore based marine radar which imaged the surface effects of the waves the model is linear and assumes a time independent two layer fluid in which the depth of each layer may be any function of position the effect of current on the propagation is modeled by the addition of a term in the dispersion relation that increments the phase speed by an amount equal to the current component in the propagation direction at the depth of maximum internal wave displacement this current may also be any function of position the results suggest that in the strait of gibraltar interface tilt and horizontal shear both have a strong influence on the refraction whereas total depth is

a nonlinear model of the shoaling and refraction of interfacial solitary waves in the ocean part i development of the model and investigations of the shoaling effect abstract oceanic internal waves are frequently observed to be of large amplitude and to exhibit significant wave front curvature as seen in synthetic aperture radar images the aim of this paper is to present a refraction model in two horizontal dimensions including moderate amplitude effects to model these common properties of internal waves the model incorporates second order nonlinearity and uses a ray method to allow arbitrary refraction for these preliminary investigations with the model a two layer ocean is assumed with an upper layer thickness of 50 m and a density difference across the layers of 0 5 kg m 3 an important aspect affecting the refraction of nonlinear waves is shoaling as the phase speed of a nonlinear wave depends on its amplitude in part i of this paper the development of the model is described and the shoaling phenomenon is investigated in detail the shoaling of interfacial internal solitary waves over both slowly varying and rapidly varying bathymetry is studied a wedg

spectral properties of ca sulfates gypsum bassanite and anhydrite this study of the spectral properties of ca sulfates was initiated to support remote detection of these minerals on mars gypsum bassanite and anhydrite are the currently known forms of ca sulfates they are typically found in sedimentary evaporites on earth but can also form via reaction of acidic fluids associated with volcanic activity reflectance emission transmittance and raman spectra are discussed here for various sample forms gypsum and bassanite spectra exhibit characteristic and distinct triplet bands near 1 4 1 5 m a strong band near 1 93 1 94 m and multiple features near 2 1 2 3 m attributed to h2o anhydrite bassanite and gypsum all have so4 combination and overtone features from 4 2 5 m that are present in reflectance spectra the mid ir region spectra exhibit strong so4 3 and 4 vibrational bands near 1150 1200 and 600 680 cm 1 8 5 and 16 m respectively additional weaker features are observed near 1005 1015 cm 1 10 m for 1 and near 470 510 cm 1 20 m for 2 the mid ir h2o bending vibration occurs near 1623 1630 cm 1 6 2 m the visible near infrared region spectra are brighter for the finer grained samples in reflectance and emission spectra of the mid ir region the 4 bands begin to invert for the finer grained samples and the 1 vibration occurs as a band instead of a peak and has the strongest intensity for the finer grained samples the 2 vibration is a sharp band for anhydrite and a broad peak for gypsum the band center of the 1 vibration follows a trend of decreasing frequency increasing wavelength with increasing hydration of the sample in the transmittance raman and reflectance spectra anhydrite forms at elevated temperatures compared to gypsum and at lower temperature salt concentration and ph than bassanite the relative humidity controls whether bassanite or gypsum is stable thus distinguishing among gypsum bassanite and anhydrite via remote sensing can provide constraints on the geochemical environment

the boiling water phenomena at camarinal sill the strait of gibraltar abstract a detailed description of high amplitude steady topographic internal waves recorded at camarinal sill during a survey on the r v investigador is presented these internal waves are generated during the maximum outflow westward stage of the tidal current and remain over the sill for more than 4 h until the outflow slackens then being released towards the mediterranean their amplitudes are comparable to the well known internal bore of camarinal sill occurring during maximum outflow during spring tides however they respond to a different physical origin and their spatial features are also quite different in fact the favourable hydraulic conditions for the generation of steady waves over the sill inhibit the internal bore generation and vice versa analysis of the observations suggests that steady internal waves are the result of a resonant response of the stratified fluid over the sill to the forcing of the flow on the across sill topography an important consequence of the steady internal waves with clear biological implications is the significant mixing phenomena that are induced mixing is enabled by an enhancement of the shear at the trough together with a significant induced vertical advection

analysis of nonlinear internal waves in the new york bight this paper presents the results on internal wave analysis of the current meter mooring data in the new york bight from the sar internal wave signature experiment sarsex selective sets of current and temperature measurements are analyzed for a parametric study of internal wave characteristics and shear effects on wave parameters are studied based on the limited data from moorings the solitary wave theory has been extended for the wave evolution on a continental shelf by including dissipation and shoaling effects a series of numerical simulations are performed to demonstrate the relative importance of dissipation and shoaling effects from an observed initial wave packet at the upstream mooring the numerical evolution simulation compares reasonably well with the measurements at the distant mooring for the leading two large solitons

a study of internal wave propagation in the strait of gibraltar using shore based marine radar images abstract an x band marine radar on windmill hill gibraltar was used to monitor the propagation of internal waves in the strait of gibraltar during march april and june 1986 surface roughness features of the waves were observed out to a range of 15 km from photographic images of the radar screen the positions of image features were measured manually using a digitizing table and these data were then computer processed to give wave arrival times and phase speeds during most tidal cycles an internal undular bore generated at camarinal sill was observed to pass eastward a waves these occurred throughout the experiments although less frequently near neap tides the time of travel to gibraltar varied significantly so that the bore arrived between four and nine hours after high water arrival times were found to be closely related to the amplitude of the upper layer tidal current this current had a strong diurnal component which was manifested as an inequality between the arrival times of success

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

hydraulic criticality of the exchange flow through the strait of gibraltar abstract the hydraulic state of the exchange circulation through the strait of gibraltar is defined using a recently developed critical condition that accounts for cross channel variations in layer thickness and velocity applied to the output of a high resolution three dimensional numerical model simulating the tidal exchange the numerical model uses a coastal following curvilinear orthogonal grid which includes in addition to the strait of gibraltar the gulf of cadiz and the alboran sea the model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics m2 s2 o1 and k1 the simulation covers an entire tropical month the hydraulic analysis is carried out approximating the continuous vertical stratification first as a two layer system and then as a three layer system in the latter the transition zone generated by entrainment and mixing between the atlantic and mediterranean flows

petrography and genesis of quaternary coastal gypcrete in north kuwait arabian gulf abstract gypcrete crusts cover late quaternary playa deposits within the northern coastal flat of kuwait bay arabian gulf this study provides insight into the genesis of the gypcrete crust associated with the fluvial playa in an arid coastal area the playa sediments are mostly muddy sand rich in diagenetic gypsum the latter is concentrated at the top part of this playa where the gypcretic crust is developed field investigation megascopic microscopic and nanoscopic examination and xrd analysis revealed the occurrence of several types of gypcretes namely massive hard crust hard clustered gypsum roses gypseous rhyzocrete and powdery crust with the exception of the last type all gypcretes are formed largely of gypsum with subordinate amounts of bassanite two genetic types of bassanite are recognised these are replacive and precipitative replacive bassanite is the most common and is mainly represented by topotactic alteration of gypsum whereas the precipitative mode is mostly restricted to the powdery crumbled crust and root cavity fill the micromorphology of both types of bassanite is discussed it is suggested that the stability of bassanite is attributed to the elevated ambient temperatures in the presence of highly saline brine and organic acids

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

wind induced cross strait sea level variability in the strait of gibraltar from coastal altimetry and in situ measurements abstract coastal altimetry products are available and are being extensively validated their accuracy has been assessed in many coastal zones around the world and they are ready for exploitation near the shore this opens a variety of applications of the sea level data obtained from the specific reprocessing of radar altimeter signals in the coastal strip in this work we retracked altimeter waveforms of the european space agency satellites ers 2 ra and envisat ra 2 from descending track 0360 over the eastern side of the strait of gibraltar using the adaptive leading edge sub waveform ales retracker we estimated along track sea level anomaly at sla profiles ra 2 at high posting rate 18 hz using improved range and geophysical corrections tides were removed with a global model dtu10 that displays a good performance in the study area the mean root square sum rss of the main constituents obtained with dtu10 and 11 tide gauge stations was 4 3 cm in agreement with the rss using a high resolution local hydrodynamic model uca2 5d 4 2 cm we also estimated a local mean sea surface by reprocessing ers 2 envisat waveforms track 0360 with ales the use of this local model gave more realistic at sla than the values obtained with the global model dtu15mss finally the along track absolute dynamic topography at adt was estimated using a local mean dynamic topography obtained with the local hydrodynamic model uca2 5d we analysed the cross strait variability of the sea level difference between the african spanish coasts along the selected track segment this was compared to the sea level cross strait difference from the records of two tide gauges located in the african ceuta and spanish tarifa coasts the sea level differences from altimetry and tide gauges were linked to the zonal component of the wind we found a positive and significant 95 c l correlation between easterlies westerlies and positive negative cross strait sea level differences between the southern and northern coasts of the strait in both datasets altimetry r 0 54 and in situ r 0 82

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

study of the generation and propagation of internal waves in the strait of gibraltar using a numerical model and synthetic aperture radar images of the european ers 1 satellite a weakly nonhydrostatic two layer numerical model based on the boussinesq equations is presented which is capable of describing among others the generation and propagation of nonlinear weakly dispersive internal waves in the strait of gibraltar the model depends on one space coordinate only but it retains several features of a fully three dimensional model by including a realistic bottom profile a variable channel width and a trapezoidal channel cross section the nonlinear primitive boussinesq equations include horizontal diffusion bottom friction and friction between the two water layers the model is driven by a height difference of the mean interface depth between the atlantic and the mediterranean boundaries and by semidiurnal tidal oscillations of the barotropic transport the model presented in this paper describes 1 the mean and tidal flow in the strait of gibraltar 2 the variation of the depth of the interface during a tidal cycle 3 the generation of strong depressions of the interface at the western sides of the spartel sill and the camarinal sill 4 the generation of strong eastward propagating internal bores and 5 their disintegration into trains of internal solitary waves the surface convergence patterns associated with depressions of the interface at the camarinal sill internal bores and internal solitary waves are calculated and compared with roughness patterns visible on synthetic aperture radar sar images of the first european remote sensing satellite ers 1 in total 155 ers 1 sar scenes from 94 satellite overflights over the strait of gibraltar which were acquired in the period from january 1992 to march 1995 have been analyzed it is shown that the proposed model is capable of explaining the observed temporal and spatial evolution of surface roughness patterns associated with eastward propagating internal waves inside the strait of gibraltar as well as the observed east west asymmetry of the internal wave field

diverse aqueous environments on ancient mars revealed in the southern highlands high resolution spectral images reveal many new exposures of hydrated minerals across the noachian southern highlands of mars several different phyllosilicates are observed with inferred accompanying phases ranging from zeolites to sulfates to chlorides each in distinct geologic settings hydrated sulfates are observed in several locations at mid latitudes and likely formed independently from the equatorial sulfates these assemblages suggest a greater diversity of aqueous environments on ancient mars than has previously been recognized and their distribution implies that alteration was widespread during the noachian period not restricted to a few locations

tracking metamorphic dehydration reactions in real time with transmission small and wide angle synchrotron x ray scattering the case of gypsum dehydration we present a review of a unique non destructive method for the real time monitoring of phase transformations and nano pore evolution in dehydrating rocks transmission small and wide angle synchrotron x ray scattering saxs waxs it is shown how saxs waxs can be applied to investigating rock samples dehydrated in a purpose built loading cell that allows the coeval application of high temperature axial confinement and fluid pressure or flow to the specimen because synchrotron sources deliver extremely bright monochromatic x rays across a wide energy spectrum they enable the in situ examination of confined rock samples with thicknesses of 1 mm at a time resolution of order seconds hence fast kinetics with reaction completion times of about hundreds of seconds can be tracked with beam sizes of order tens to hundreds of micrometres it is possible to monitor multiple interrogation points in a sample with a lateral extent of a few centimetres thus resolving potential lateral spatial effects during dehydration and enlarging sample statistics significantly therefore the saxs waxs method offers the opportunity to acquire data on a striking range of length scales for rock samples with thicknesses of 10 3 m and widths of 10 2 m a lateral interrogation point spacing of 10 5 m can be achieved within each irradiated interrogation point volume information concerning pores with sizes between 10 9 and 10 7 m and the crystal lattice on the scale of 10 10 m is acquired in real time this article presents a summary of the physical principles underpinning transmission x ray scattering with the aim of providing a guide for the design and interpretation of time resolved saxs waxs experiments it is elucidated 1 when and how saxs data can be used to analyse total porosity internal surface area and pore size distributions in rocks on length scales from 1 to 300 nm 2 how waxs can be employed to track lattice transformations in situ and 3 which limitations and complicating factors should be considered during experimental design data analysis and interpretation to illustrate the key capabilities of the saxs waxs method we present a series of dehydration experiments on a well studied natural gypsum rock volterra alabaster our results demonstrate that saxs waxs is excellently suited for the in situ tracking of dehydration kinetics and the associated evolution of nano pores the phase transformation from gypsum to bassanite is correlated directly with nano void growth on length scales between 1 and 11 nm for the first time a comparison of the saxs waxs kinetic results with literature data emphasises the need for future dehydration experiments on rock specimens because of the impact of rock fabric and the generally heterogeneous and transient nature of dehydration reactions in nature it is anticipated that the saxs waxs method combined with in situ loading cells will constitute an invaluable tool in the ongoing quest for understanding dehydration and other mineral replacement reactions in rocks quantitatively

influence of geometry and eclogitization on oceanic plateau subduction abstract subduction is driven by the negative buoyancy of the slab which depends on both the temperature and composition of the lithospheric plate in the case of subduction of an oceanic plateau larger thickness of the lower density crust and harzburgite layers can locally decrease the negative buoyancy available to drive subduction however the ability of a plateau to have an impact on subduction depends on the total buoyancy of the slab and therefore also depends on size of the plateau and the basalt to eclogite transition which substantially increases the density of the crust using 2 d numerical finite element models of subduction we investigate the role of eclogitization and oceanic plateau size thickness and length in the process of oceanic plateau subduction model results show that eclogitization of the crust substantially increases the chances that a plateau will be subducted because the extra buoyancy of larger plateaus is lost through the phase transition for kinematically driven models all plateaus are subducted regardless of the thickness or width extrapolation of these results to 3 d geometry shows that sufficient slab buoyancy is available to sustain subduction for narrow plateaus with slab widths 6 7 times the plateau width depending on plateau thickness and length in dynamically driven models subduction of oceanic plateaus of various sizes can substantially slow subduction and large plateaus 200 km long and 25 km thick can lead to break off of the slab and underplating of the plateau

the sulu sea internal soliton experiment abstract we present the results of a comprehensive study of large amplitude internal solitons generated in the sulu sea in the philippines by intense tidal flow over a sharp bathymetric feature surface signatures of these waves have been observed in images from the dmsp landsat and nimbus 7 satellites and the space shuttle sir a synthetic aperture radar a two week experiment was conducted in the sulu sea in may 1980 from r v oceanograher during which a three mooring phased array of current meters and thermistors was implanted near the wave source and at 82 km and 200 km from the source seventeen soliton packets were observed whose properties underwent significant change as they propagated across the sea vertical profiles of density temperature and acoustic backscatter as well as radar and photographic surface signatures were obtained from the ship data from satellites moorings and the ship give unprecedented information on the spatial and temporal characteristics of solitary waves which ha

an improved composite surface model for the radar backscattering cross section of the ocean surface 2 model response to surface roughness variations and the radar imaging of underwater bottom topography in the companion paper we have presented an improved composite surface model for the calculation of normalized radar backscattering cross sections nrcs of the ocean surface the proposed model accounts for the impact of the full two dimensional ocean wave spectrum on the radar backscatter and was shown to reproduce measured absolute nrcs values for a variety of radar configurations and wind speeds satisfactorily after some reasonable tuning of the input ocean wave spectrum this paper focuses on the modulation of the nrcs in the presence of spatially varying surface currents first the sensitivity of the nrcs to intensity variations of different ocean wave spectral components is investigated then the hydrodynamic modulation of the wave spectrum over underwater bottom topography in tidal waters is computed in different ways and the resulting radar signatures are discussed the composite surface model yields comparable radar signatures at high 10 ghz x band and low 1 ghz l band radar frequencies which is in much better agreement with experimental results than the predictions of a first order bragg scattering model on the other hand measured variations of the nrcs at high radar frequencies appear to be still underestimated in some cases which may be due to shortcomings of our description of the wave current interaction by conventional weak hydrodynamic interaction theory possible improvements of the theory are discussed and requirements for future experiments are formulated

theory of radar imaging of internal waves radar images taken over ocean areas in particular those obtained by the synthetic aperture radar sar onboard the american seasat satellite in 19781 2 sometimes show features that seem to be surface manifestations of oceanic internal waves3 11 here i present a theory explaining the large radar signatures of internal waves in which the imaging is attributed to variations in the short scale surface roughness induced by current variations associated with internal waves

three layer view of transports and hydraulics in the strait of gibraltar a three dimensional model study 1 the ability of a three dimensional sigma coordinate free surface model to reproduce the interfacial layer between the atlantic and mediterranean waters in the strait of gibraltar is verified the model uses a coastal following curvilinear orthogonal grid that includes the gulf of cadiz and the alboran sea with very high resolution in the strait less than 500 m it is forced by imposing the two major semidiurnal tidal components m2 and s2 along the atlantic and the mediterranean open boundaries the main experiment covers an entire spring neap cycle fortnight period results show that the model is able to reproduce a thick interfacial layer that carries a substantial fraction of the total transport the interfacial thickness is strongly modulated by tidal forcing that enhances the vertical exchanges between layers in order to take into account the thick interfacial layer a three layer approximation is used to study the hydraulics results show that the exchange regime is intermittently maximal due to cyclically controlled regions over camarinal sill and within tarifa narrows the analysis has been repeated in a two layer framework to evaluate the impact of the introduction of an interfacial layer on the calculation of hydraulics in the two layer approximation there is not any controlled region extending on the whole cross section so the exchange is always submaximal concluding the two layer approximation underestimates hydraulic controls in the strait respect to the three layer approach

from passive continental margin to mountain belt insights from analytical and numerical models and application to taiwan abstract the physics of deformation at plate boundaries is insufficiently understood taiwan is particularly interesting in this context since geological and geophysical arguments indicate that passive margin sediments were exhumed into an active mountain belt over a relatively short period of time classically taiwan has been described as a critical wedge orogeny in which deformation is entirely restricted to the crust more recent data however indicate that deeper parts of the lithosphere also participate in deformation in order to study the physical feasibility of various endmember hypothesis we have performed two dimensional 2 d thermo mechanical numerical experiments in which both mantle lithospheric and surface processes are taken into account and in which crustal deformation evolves as a function of mantle flow our setup consists of ocean ocean subduction followed by passive continental margin deformation results indicate that in most cases the subducting slab becomes close to vertical upon arrival of continental material to the trench whereas crustal rheology deformation and surface processes do not modify mantle flow significantly they are important for crustal scale processes relatively little crustal exhumation occurs if the crust is homogeneous and erosion rates are small significant exhumation in a dome like structure is obtained if a weak lower crust and large erosion rates are present the presence of an oceanic arc in the overriding oceanic lithosphere modifies but does not control crustal deformation synthetic metamorphic facies maps and thermochronological data computed from the numerical models are in reasonable agreement with observations in southern taiwan if exhumation occurs in a dome like structure further physical insight in the doming mechanisms is obtained by a combination of analytical techniques and crustal scale numerical models this analysis indicates that the dome like exhumation mechanism can be attributed to a compressional mechanical instability facilitated by both a weak lower crust and large rates of erosion both conditions are likely to be satisfied in taiwan

calcium sulfate veins characterized by chemcam curiosity at gale crater mars the curiosity rover has analyzed abundant light toned fracture fill material within the yellowknife bay sedimentary deposits the chemcam instrument coupled with mastcam and chemcam remote micro imager images was able to demonstrate that these fracture fills consist of calcium sulfate veins many of which appear to be hydrated at a level expected for gypsum and bassanite anhydrite is locally present and is found in a location characterized by a nodular texture an intricate assemblage of veins crosses the sediments which were likely formed by precipitation from fluids circulating through fractures the presence of veins throughout the entire similar to 5 m thick yellowknife bay sediments suggests that this process occurred well after sedimentation and cementation lithification of those sediments the sulfur rich fluids may have originated in previously precipitated sulfate rich layers either before the deposition of the sheepbed mudstones or from unrelated units such as the sulfates at the base of mount sharp the occurrence of these veins after the episodes of deposition of fluvial sediments at the surface suggests persistent aqueous activity in relatively nonacidic conditions

fracture zone subduction and reactivation across the puysegur ridge trench system southern new zealand a two dimentional kinematic model for lithospheric thrusting that considers the flexural interaction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench parallel fracture zones on the topography and free air gravity anomalies of the puysegur ridge trench system fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported the implication of reactivating fracture zones on the outer trench slope is to suppress the flexural bulge creating topographic relief between the fracture zone and the trench and reducing the trench gravity anomaly in the studied region the trench subparallel l atalante fracture zone accounts for the lack of a flexural bulge strong outer trench slope and gravity gradients the southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly the effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate creating a trough located above the underthrust fracture zone the vertical offset and gravity modifications are functions of the fracture zone position relative to the trench this model explains the 3500 m deep troughs and the 1700 m subsidence observed at the puysegur ridge we speculate that reactivating underthrust fracture zones can facilitate strike slip faults development within the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins this process can account for basal tectonic erosion

scylla and charybdis observed from space scylla and charybdis are in greek mythology two immortal and irresistible monsters who beset the narrow waters separating the italian peninsula from sicily they give poetic expressions to exceptional oceanographic features encountered in the strait of messina recently the seasat satellite has added new information on the oceanography of this strait for the first time experimental evidence is presented that internal waves are generated in the strait of messina the generation mechanism is linked to tidal currents washing over the shallow sill within the strait the experimental data originate from a synthetic aperture radar sar image obtained from the seasat satellite on sept 15 1978 and from subsequent in situ measurements carried out from the italian research vessel bannock in nov 1980 the circular wave pattern visible on the seasat sar image is interpreted in terms of internal solitons furthermore the seasat sar image also reveals tagli special types of tidal bores encountered in the strait of messina

water masses and circulation in the western alboran sea and in the straits of gibraltar abstract some new elements of alboran sea circulation and water masses in close connection with flows through the straits of gibraltar are presented a general description is given of water masses found in 1981 during mediprod iv cruise from the gulf of cadiz to the gulf of lions followed by a brief review of the conditions found in the straits of gibraltar based on 1981 and 1961 observations the hydrological structure vertical and horizontal and surface and bottom currents typical of the western alboran sea are analysed we present some detailed observations about vertical currents in the deep waters near the straits and finally we analyse the characteristics of the mediterranean overflow through the straits our conclusions in part confirm those of bryden and stommel 1982 and in part shed light on the general pattern of the mediterranean currents entering the alboran sea from the east however considering the suggestion of stommel bryden and mangelsdorf 1973 that the deep currents in the interior of the western mediterranean basin are basically controlled by a bernoulli flow induced by the intense currents in the straits and the weak vertical stratification in the mediterranean we suggest that an interaction of a large scale circulation pattern and the topography of the alboran basin which is much shallower than the western mediterranean basin may play a major role in controlling hydraulically the deep mediterranean waters overflow a detailed water mass analysis of 1981 data and comparison with previous data revealed a time and space variability in the mediterranean waters the scales of this variability are unknown but the s variations make it difficult to make assumptions as to the depth of origin of the deep waters found in the alboran sea which flow into the atlantic ocean over the gibraltar sill

internal solitons on the pycnocline generation propagation and shoaling and breaking over a slope in part 1 a study is made of the internal solitary wave on the pycnocline of a continuously stratified fluid a korteweg de vries kdv equation for the interfacial displacement is developed following benney s method for long nonlinear waves experiments were conducted in a long wave tank with the pycnocline at several different depths below the free surface while keeping the total depth approximately constant a step like pool of light water trapped behind a sliding gate served as the initial disturbance condition the number of solitons generated was verified to satisfy the prediction of inverse scattering theory the fully developed soliton was found to satisfy the kdv theory for all ratios of upper layer thickness to total depth in part 2 of this study we investigate experimentally the evolution and breaking of an internal solitary wave as it shoals on a sloping bottom connecting the deeper region where the waves were generated to a shallower shelf region it is found through quantitative measurements that the onset of wave breaking was governed by shear instability which was initiated when the local gradient richardson number became less than the internal solitary wave of depression was found to steepen at the back of the wave before breaking in contrast with waves of elevation two slopes were used with ratios 1 16 and 1 9 and the fluid was a boussinesq fluid with weak stratification using brine solutions

visible and near infrared 0 4 2 5 μm reflectance spectra of playa evaporite minerals visible and near infrared vnir 0 4 2 4 m reflectance spectra were recorded for 35 saline minerals that represent the wide range of mineral and brine chemical compositions found in playa evaporite settings the spectra show that many of the saline minerals exhibit diagnostic near infrared absorption bands chiefly attributable to vibrations of hydrogen bonded structural water molecules vnir reflectance spectra can be used to detect minor hydrate phases present in mixtures dominated by anhydrous halite or thenardite and therefore will be useful in combination with x ray diffraction data for characterizing natural saline mineral assemblages in addition vnir reflectance spectra are sensitive to differences in sample hydration state and should facilitate in situ studies of minerals that occur as fragile transitory dehydration products in natural salt crusts the use of spectral reflectance measurements in playa studies should aid in mapping evaporite mineral distributions and may provide insight into the geochemical and hydrological controls on playa mineral and brine development

vertical variations in bassanite distribution patterns in near surface sediments southern egypt abstract nile valley sediments of the el adaima area in southern egypt contain bassanite caso 4 0 5h 2 o associated with gypsum caso 4 2h 2 o in the upper 1 m of the deposits the presence of bassanite at this site formed by dehydration of gypsum is marked by variations in mode of occurrence with depth in the lowest bassanite bearing interval bassanite occurs mainly along the contact between gypsum crystals in pores which is related to the protection of bassanite from rehydration in those parts of the gypsum aggregates at a higher level bassanite occurs along the sides of lenticular gypsum crystals in the sediment matrix near the top of the studied sequence bassanite mainly appears as inclusions in gypsum crystals which represents a texture that developed by repeated dehydration and rehydration elsewhere in the upper part of the deposits bassanite occurs as aggregates of small crystals in pores without associated gypsum which was entirely dehydrated to bassanite in these parts these vertical variations in bassanite distribution patterns record differences in the diagenetic history of the deposits between depth intervals

small scale processes in straits small scale studies in straits reveal much about their internal dynamics highlights of recent work are described to provide an overview of the rapid progress being made in understanding processes within straits and how they affect outside regions owing to the early state of work on small scale processes within straits the emphasis is on identifying and understanding the dominant processes and mechanisms leading to mixing and the mixing itself theoretical numerical and observational approaches are included and some suggestions are made about future directions

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

an early theory of ocean circulation j s von waitz and his explanation of the currents in the strait of gibraltar the task of unravelling the factors governing the circulation of the oceans is one of the foremost in physical oceanography yet the origin of the idea of the internal circulation of the ocean is obscure only since the time of the challenger expedition which sailed in 1872 has it been generally accepted that density differences within the ocean due to variations in salinity and temperature can set great water masses in motion it was w b carpenter s conviction based on the earlier work of h m s s lightning and porcupine between 1868 and 1870 that cold dense water flowed from polar regions towards the equator in the ocean depths while a return flow of warmer lighter water took place at the surface which led him to promote the expedition deacon 1971 prestwlch 1875 traced back similar ideas to the early 1800s in the intervening years they had fallen out of favour when confusion arose over results obtained with six self registering thermometers insufficiently protected against the effects of pressure mcconnell 1978 and the widespread belief that sea water s maximum density occurred at the same temperature as in fresh water temperature differences were generally held to be responsible for circulation but some writers such as maury 1855 in the physical geography of the sea thought that salinity was more important before the end of the 18th century it would not have been at all obvious from studying observations of subsurface temperature that such a situation existed measurements in the mediterranean such as those made by marsigli 1725 indicated a mean temperature below the level affected by seasonal changes there was no reason to think of this as exceptional and it fitted in well with the ideas of those who believed the centre of the earth to be warm and its heat to be transmitted through the sea floor if it were not for this and the vertical mixing which ensued said dortous de mairan 1749 the water in the depths of the sea would turn to ice measurements in the southern ocean made on captain cook s second voyage appeared to confirm this view by showing the existence of warmer water beneath the cold surface layer deacon 1971 proof of the presence of polar water in the depths of tropical seas was not obtained until after 1800 but thompson 1798 had already explained examples of deep sea temperature below the atmospheric mean prevailing in the latitudes where they were taken by the spread of cold water due to its greater density however his was not the first attempt to outline a theory of ocean circulation a similar idea but relying on salinity instead of temperature differences had been put forward in a remarkable paper nearly half a century earlier in 1755 the royal swedish academy of sciences published in its transactions a paper by a german author herrwaitz privy councillor to the landgrave of hesse cassel it was entitled an investigation into the reasons why the water of the atlantic ocean always flows into the mediterranean sea through the straits of gibraltar waitz 1755 argued that the difference

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

sulfates in the north polar region of mars detected by omega mars express the observatoire pour la mineralogie l eau les glaces et l activite omega imaging spectrometer observed the northern circumpolar regions of mars at a resolution of a few kilometers an extended region at 240 e 85 n with an area of 60 kilometers by 200 kilometers exhibits absorptions at wavelengths of 1 45 1 75 1 94 2 22 2 26 and 2 48 micrometers these signatures can be unambiguously attributed to calcium rich sulfates most likely gypsum this region corresponds to the dark longitudinal dunes of olympia planitia these observations reveal that water alteration played a major role in the formation of the constituting minerals of northern circumpolar terrains

on the determination of characteristics of the interior ocean dynamics from radar signatures of internal solitary waves in this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar sar images the first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section nrcs associated with internal solitary waves for this purpose we consider a data set composed of sar and in situ measurements carried out from 1991 to 1997 in the region of the strait of messina the recognition and the interpretation of sea surface patterns of internal solitary waves in the strait of messina can be used to study characteristics of the density distribution in the area the internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations i e variations induced by the larger scale circulation in the horizontal density distribution in order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the nrcs associated with internal solitary waves several numerical simulations are carried out using a radar imaging model these simulations are performed by assuming different wind conditions and internal wave parameters it is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured nrcs associated with internal solitary waves

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow