the effect of low temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites a case for acquisition of chemical remanent magnetization the effect of hydrothermal alteration on the thermoremanent magnetization trm of synthetic titanomagnetite tm40 fe2 6ti0 4o4 has been studied to simulate the alteration that occurs in the oceanic crust pseudo single domain titanomagnetite grains similar in size to those often found in oceanic basalts were dispersed in a permeable but rigid glass matrix this resulted in a trm in the sample which was subsequently oxidized in acidic solutions while a magnetic field 0 1 mt was applied perpendicular to the trm direction the experiments were conducted in a non magnetic stainless steel pressure vessel at 150 c in solutions of acidity varying from ph 2 to ph 7 in addition to being time and temperature dependent the acquired chemical remanent magnetization crm was also found to be very ph dependent the degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions long term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating it was found that during the alteration of tm40 to titanomaghemite the crm is along the ambient field direction from the onset and not partly or wholly along the trm direction as has been found in previous air oxidation experiments this has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples

model derived estimates of groundwater mean ages recharge rates effective porosities and storage in a limestone aquifer the edwards aquifer of south central texas u s a a highly fractured and faulted group of limestone formations is the major water supply for the san antonio area a discrete state compartment dsc model or mixing cell model based upon the conservation of environmental tritium within the aquifer was used to obtain estimates of groundwater mean ages recharge effective porosities and storage in the edwards aquifer in the vicinity of san antonio texas the model was calibrated and validated with the spatial and temporal 1953 1971 distributions of environmental 3h tritium in the groundwater the final model consisted of 34 cells eight of these cells represented the unconfined portion of the edwards aquifer in the vicinity of the balcones fault zone an area where recharge occurs via streamflow infiltration and direct infiltration of precipitation the model confirmed previous analyses of flow in the edwards system generally parallel to the balcones fault zone with restricted flow perpendicular to this zone groundwater mean ages ranged from 16 to over 130 yr the storage volume of the confined portion of the edwards aquifer is 30 9 km3 which corresponds to an average effective porosity of 4 8 range 1 9 8 the average annual recharge to the edwards aquifer during the period 1953 1971 was 0 614 km3 the study demonstrated that discrete state compartment models calibrated and validated with environmental tritium distributions can yield valuable hydrogeologic information that is difficult or expensive to obtain using traditional techniques the approach used in the study is particularly suited to limestone aquifers which are normally extremely difficult to analyze with traditional methods

flow dissolution and precipitation in dolomite 1 the evolution of hydraulic conductivity and porosity during the process of dedolomitization was examined in a series of laboratory experiments by analyzing the effects of concurrent dolomite dissolution and calcium carbonate precipitation linear flow experiments were performed in columns of crushed sucrosic dolomite by injecting different concentrations of hcl at various flow rates temporal changes in head gradient were used to calculate overall hydraulic conductivities of each column while chemical analyses of the effluent acid enabled estimation of porosity changes during the experiments after each experiment the rock samples were retrieved and sectioned in order to study the pore space geometry micromorphology and mineral concentrations a range of injected hcl concentrations and flow rates was identified which leads to oscillations in the effective hydraulic conductivity and porosity of the evolving rock samples in all cases however the porous medium ultimately clogged short term experiments were also used to study the formation of dissolution and precipitation bands along the columns under the experimental conditions dolomite dissolution is a reaction rate controlled process experiments indicated that as such the flow rate and the ph of the injected fluid affect dissolution only during the initial stages when calcium carbonate is dissolved on the other hand both the flow rate and the ph of the injected fluid strongly influence the precipitation process throughout the duration of the experiments because higher flow rates retard nucleation these findings are in qualitative accordance with field observations of dolomite formations

magnetic properties and ferromagnetic mineralogy of oceanic basalts summary rockmagnetic properties of a number of oceanic basalt samples collected by the dsdp odp drillings after leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples the electron probe micro analysis epma study indicated that chemical composition of the primary titanomagnetites has a variety particularly in minor constituent elements such as al2o3 mgo mno and v2o3 the variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid hi magma general crystallization trend is found that is the later crystallized titanomagnetites have higher ulvospinel component higher titanium oxide and lower iron oxide due to decrease in temperature and oxygen fugacity it is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements in the later stage al2o3 and mgo spinel component and v2o3 decrease whereas mno increases although there is a reversed crystallization trend with a few elements in several cases this reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence the primary variation of chemistry shows different trends of the secondary variation due to oxidation oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved epma technique being able to directly determine the absolote contents of oxygen in materials the result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice this is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts

withdrawn petrographic and origin of authigenic carbonate minerals and the associated fluid evolution in the lacustrine carbonate siliciclastic mudrocks a case from the low cretaceous of the erennaoer sag erlian basin ne china

the genesis of dedolomites a discusion based on reactive transport modeling abstract dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of ca rich water this process has been described in the literature as either increasing preserving or decreasing porosity of the initial dolostone according to textures observed in prades ne spain dedolomitization took place due to two simultaneous processes non pseudomorphic replacement of dolomite with calcite at the wall rock of fractures and pseudomorphic replacement in the bulk rock a multicomponent reactive transport model is used to analyze the two dedolomitization processes observed diffusion of ions from the fractures was the driving force for non pseudomorphic replacement in the wall rock the dissolution of dolomite and precipitation of calcite took place in accord with a 1 2 stoichiometry as a consequence of the mineral molar volume the porosity was sealed and replacement limited to a rim on both sides of the fractures according to calculations the development of a rim took less than 500 years advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock due to the absence of empty volume for expansion replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation according to calculations pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters as the infiltrating water was also undersaturated with respect to calcite one stage of calcite dissolution postdated dedolomitization this process was responsible for the widening of open fractures where the dissolution front followed the dedolomitization front into the dolomite matrix intensive calcite dissolution was also developed following the advective flux from the top of the bed downwards drusy calcite is observed to precipitate in the pore spaces which consist of intergrains fractures and dissolution cavities this process is not predicted in detail by reactive transport calculations although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization

a data study of the influence of the equatorial upper stratosphere on northern hemisphere stratospheric sudden warmings equatorial winds in the stratosphere are known to influence the frequency of stratospheric midwinter sudden warmings sudden warmings in turn influence the earth s climate both through their direct influence on polar temperatures and through the temperature dependence of ozone depletion in the lower stratosphere the conventional holton tan explanation for the equatorial influence on sudden warmings is in terms of the equatorial winds in the lower stratosphere 20 30 km acting as a waveguide for midlatitude planetary wave propagation this study employs stratospheric temperature analyses and equatorial rocketsonde wind data extending to 58 km to diagnose the relationship between the northern hemisphere polar temperatures and equatorial zonal winds at all height levels in the stratosphere in addition to the recognized holton tan relationship linking the polar temperatures to the quasi biennial oscillation in equatorial winds in the lower stratosphere a strong correlation of polar temperatures with equatorial winds in the upper stratosphere is found we suggest that this may be associated with the strength and vertical extent of the westerly phase of the semi annual oscillation in the upper stratosphere although the observations alone cannot provide a conclusive causal relationship the main diagnostic tools employed are correlation studies and composite analysis the results underline the need for continued high quality equatorial wind measurements at all stratospheric levels

geochemical evolution of groundwater in the carbonate aquifers of sierra de segura betic cordillera southern spain summary sierra de segura betic cordillera with a total area of over 3000 km 2 is the source of the two principal rivers in southern spain the guadalquivir and the segura due to the orographic effect of these mountains precipitations are considerably more abundant than in nearby lowland areas where the climate is semi arid sierra de segura is constituted of mesozoic and cenozoic sedimentary rocks among which there are thick limestone dolomitic formations which have given rise to extensive outcrops of permeable materials in geomorphological terms there is a large plateau intensively karstified that constitutes the main recharge area discharge takes place via a large number of springs of which the 50 most important add up to a mean spring flow of about 13 500 l s the active geochemical processes in aquifers of sierra de segura with their corresponding time sequence are dissolution of co 2 dissolution of calcite incongruent dissolution of dolomite dedolomitization exsolution of co 2 and precipitation of calcite more evolved water has higher temperature magnesium content and mg ca ratio therefore these parameters can be utilised as indicators of the degree of hydrochemical evolution in addition a good correlation between water temperature and magnesium concentration or mg ca ratio indicates that an increase in temperature accelerates the kinetics of the dissolution of dolomite finally the distribution of the temperatures in the vadose zone determined by atmospheric thermal gradient implies an apparent stratification of the predominant hydrochemical processes and of the groundwater physical and chemical characteristics

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

effects of an imposed quasi biennial oscillation in a comprehensive troposphere stratosphere mesosphere general circulation model a 48 yr integration was performed using the geophysical fluid dynamics laboratory skyhi troposphere stratosphere mesosphere gcm with an imposed zonal momentum forcing designed to produce a quasi biennial oscillation qbo in the tropical stratosphere in response to this forcing the model generates a qbo in the tropical circulation that includes some very realistic features notably the asymmetry between the strength of the descending easterly and westerly shear zones and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator the extratropical circulation in the northern hemisphere nh winter stratosphere is affected by the tropical qbo in a manner similar to that observed in particular the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30 mb winds are easterly again in rough agreement with observations despite this effect however the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical qbo the observed qbo dependence of the winter mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model the qbo was also found to have a profound influence on stratospheric stationary waves at low latitudes near and above 10 mb the nh stationary waves were found to penetrate across the equator during the westerly qbo phase but to be restricted to latitudes poleward of 108n during the easterly phase this means that the equatorial qbo in prevailing wind near and above 10 mb has a significant zonally asymmetric component if this is also true in the real atmosphere there are important implications for the adequacy of the current observational rawindsonde network near the equator analysis of the zonal mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the qbo cycle the exception appears to be the easterly acceleration phase below about 20 mb the implications of this for the generation of a self consistent qbo by gcms will be considered the effects of the imposed qbo on the troposphere were found to be very modest there does appear to be a statistically significant weakening by 1ms 21 of the high latitude winter vortex in the middle and upper troposphere given the very high predictability of the stratospheric qbo itself this effect could possibly be used to enhance the skill of seasonal weather forecasts no significant qbo influence was found in the model precipitation field

influence of climate and dolomite composition on dedolomitization insights from a multi proxy study in the central oman mountains abstract dedolomitization is an important diagenetic process that can yield information on the history of paleo fluids in a given aquifer or outcrop and by extension it offers a window into past hydrologic conditions dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates thus affecting reservoir quality despite considerable research the process of dedolomitization is still not entirely understood here new findings from the central oman mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization petrographic mineralogical and geochemical data from both altered and pristine dolomite hosted in jurassic carbonate rocks from the sahtan group and outcropping at wadi mistal in the jebel akhdar tectonic window reveal two dolomite bodies with different characteristics stratabound and fault related dolostone the ferroan dolomite crystals are larger in the fault related dolostone whereas the non ferroan dolomite crystals are smaller in the stratabound dolostone petrography reveals a complex suite of alteration textures including pristine dolomite recrystallized dolomite and calcitized dolomite dedolomite iron oxides are present pervasively in the altered rocks and different alteration textures co occur in the same sample relative to unaltered dolomite in this outcrop the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values based on field data and petrography two phases of alteration are evident an earlier dedolomitization event affecting the stratabound dolomite only and a more pervasive pleistocene holocene alteration phase associated with surface weathering that affected both the fault related dolomite and to a lesser extent the stratabound dolomite stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes dedolomitization was triggered by meteoric fluids that interacted with soil related carbon during humid climatic period s whereas recrystallization of ferroan fault related dolomite happened during more arid climatic period s with less developed soils and lower fluid rock interaction these results suggest that weathering of ferroan fault related dolomite can lead to formation of goethite and recrystallized non stoichiometric dolomite with a reset stable oxygen isotope signature and sometimes to dedolomitization

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

does the holton tan mechanism explain how the quasi biennial oscillation modulates the arctic polar vortex idealized experiments with the whole atmosphere community climate model waccm are used to explore the mechanism s whereby the stratospheric quasi biennial oscillation qbo modulates the northernhemispherewintertimestratosphericpolarvortex overall theeffectofthecriticallineemphasized in the holton tan mechanism is less important than the effect of the mean meridional circulation associated with qbo winds for the polar response to the qbo more specifically the introduction of easterly winds at the equator near 50 hpa 1 causes enhanced synoptic scale eliassen palm flux epf convergence in the subtropics from 150 to 50 hpa which leads to the subtropical critical line moving poleward in the lower stratosphere and2 createsabarriertoplanetarywavepropagationfromsubpolarlatitudestomidlatitudesin the middle andupper stratosphere e g less equatorwardepf near 508n whichleads to enhancedplanetary wave convergence in the polar vortex region these two effects are mechanistically distinct while the former is relatedtothesubtropicalcriticalline thelatterisduetothemeanmeridionalcirculationofthe qbo allof these effects are consistent with linear theory although the evolution of the entire wind distribution is only quasi linear because induced zonal wind changes cause the wave driving to shift and thereby positively feed back on the zonal wind changes finally downward propagation of the qbo in the equatorial stratosphere upper stratospheric equatorial zonal wind and changes in the tropospheric circulation appear to be less important than lower stratospheric easterlies for the polar stratospheric response overall an easterly qbo wind anomaly in the lower stratosphere leads to a weakened stratospheric polar vortex in agreement with previous studies although not because of changes in the subtropical critical line

oceanic magnetic anomaly amplitudes variation with sea floor spreading rate and possible implications abstract a qualitative examination of oceanic magnetic anomalies suggests that their amplitudes increase with the spreading rate of the ocean floor this suggestion has been investigated quantitatively by inversion of the anomalies a variance matching technique has been used to calculate the average magnetisation of the crust with crustal thickness held constant or to calculate the thickness with the magnetisation fixed in spite of variations in the computed intensities of magnetisation and thickness when compared to spreading rates the results show an over all increase in the intensities with the rate of spreading the mechanisms responsible for this could include greater irregularity in distribution of magnetic polarities within the crust at slow spreading ridges greater intensity of magnetisation in crust produced at fast spreading due to initial chemical properties or enhanced hydrothermal alteration and a dependence of crustal thickness on spreading rate

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

constraints on age and construction process of the foundation chain submarine volcanoes from magnetic modeling abstract the interaction between the foundation hotspot and the pacific antarctic ridge south pacific is one of the two known cases where a ridge is approaching a hotspot this ridge hotspot relative movement results in a change in the morphology of the volcanoes along the chain as the age of the lithosphere at the time of edifice formation progressively diminishes four hundred kilometers west of the pacific antarctic ridge axis volcanism is distributed along two sub parallel lines of volcanoes separated by distances diminishing from 100 km at the west to 50 km near the axis of the ridge the magnetic anomalies mapped on this part of the chain show a pattern clearly dominated by the magnetic signature of the seamounts the anomalies were forward modeled using a remanent magnetization of normal or reversed polarity within the volcano topography the resulting pattern of normal and reversed magnetization is consistent with the intervals of the geomagnetic polarity time scale for the last 5 ma the age of the oldest studied volcano these magnetic anomalies therefore represent an independent means of dating the volcanic edifices the ages deduced from the modeling are consistent with the hypothesis of volcanoes built by a hotspot on the fast moving pacific plate and are in good agreement with the published radiometric dates although these dates tend to correspond to the late constructional stages of the edifices the differences between magnetic and radiometric ages suggest that the average time span to build a volcano of the foundation chain is about 1 million years in agreement with other intraplate edifices the difference between the magnetic ages of the seamounts and of the underlying crust both deduced from the magnetic anomaly analysis show that the ridge has approached the hotspot at a rate of 40 km ma

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

sensitivity of gravity wave fluxes to interannual variations in tropical convection and zonal wind using an idealized model framework with high frequency tropical latent heating variability derived from global satellite observations of precipitation and clouds the authors examine the properties and effects of gravity waves in the lower stratosphere contrasting conditions in an el nino year and a la nina year the model generates a broad spectrum of tropical waves including planetary scale waves through mesoscale gravity waves the authors compare modeled monthly mean regional variations in wind and temperature with reanalyses and validate the modeled gravity waves using satellite and balloon based estimates of gravity wave momentum flux some interesting changes in the gravity spectrum of momentum flux are found in the model which are discussed in terms of the interannual variations in clouds precipitation and large scale winds while regional variations in clouds precipitation and winds are dramatic the mean gravity wave zonal momentum fluxes entering the stratosphere differ by only 11 the modeled intermittency in gravity wave momentum flux is shown to be very realistic compared to observations and the largest amplitude waves are related to significant gravity wave drag forces in the lowermost stratosphere this strong intermittency is generally absent or weak in climate models because of deficiencies in parameterizations of gravity wave intermittency these results suggest a way forward to improve model representations of the lowermost stratospheric quasi biennial oscillation winds and teleconnections

pseudospherulitic fibrous calcite in paleo groundwater unconformity related diagenetic carbonates paleocene of the ager basin and miocene of the madrid basin spain abstract massive decimeter thick intervals formed by mosaics of subspherical calcite crystals displaying an intracrystalline fibrous microfabric have been found in paleocene and miocene strata in spain the fibrous microfabric is marked by inclusion defined fibers that radiate from the central parts of the crystals which can be termed pseudospherulites and not spherulites because they are single crystals and not polycrystalline aggregates the horizons of pseudospherulitic calcite appear in meter scale laterally persistent levels of crystalline limestones that exhibit complex pseudospar fabrics they are interbedded with peritidal and or lacustrine deposits they grade downward into bedded dolomicrites and they can be linked to unconformities several meters higher in the section above intervening unaltered limestones the pseudospar zones are interpreted as early diagenetic paleo groundwater alteration products after dolomicrites the pseudospherulites are composed of low mg calcite with low contents of fe and mn and they have a stable isotope composition indicative of precipitation from meteoric water they form xenotopic mosaics in which goethite palygorskite and sepiolite may be present as inclusions and in the intercrystalline matrix cathodoluminescence microscopy reveals peripheral growth of the pseudospherulites which results from several phases of precipitation and even of dissolution the first phase of crystal growth may show rhombic habits and may contain corroded dolomite inclusions interpreted as an indication of growth at the expense of dolomicrite by a dissolution precipitation process the second phase of crystal growth is fibrous sensu stricto exhibits lobate growth laminae and shows indications of displacive growth the pseudospherulitic fabric is not relict and the pseudospherulites do not represent speleothems recrystallized microcodium or neomorphosed spherulitic carbonate instead they are interpreted as split crystals that grew within dolomicritic host sediments in part by dissolution precipitation and in part by displacement the hypothesis of a dolomicritic host is supported by the geological and diagenetic setting by the presence of corroded dolomite inclusions within the pseudospherulites and by the relationship between their growth and the authigenesis of sepiolite palygorskite and goethite the pseudospherulites grew in the shallow subsurface at or near the water table from predominantly oxidizing meteoric water and under periodically fluctuating geochemical conditions saturation redox caused by fluctuations of the water table during periods of subaerial exposure the presence of fossil bacteria within the pseudospherulites the anastomosing pattern of the inclusion defined fibers and the lobate growth forms of the fibrous calcite suggest some bacterial influence in the genesis of the pseudospherulitic fabric

structural framework of the edwards aquifer recharge zone in south central texas the edwards aquifer the major source of water for many communities in central texas is threatened by population growth and development over its recharge zone the location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the balcones fault system a major system of predominantly down to the southeast normal faults we investigate the geologic structure of the edwards aquifer to assess the large scale aquifer architecture analyze fault offset and stratigraphic juxtaposition relationships evaluate fault zone deformation and dissolution and fault system architecture and investigate fault block deformation and scaling of small scale intrablock normal faults characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault block communication and flow paths flow path constriction may be exacerbated by increased fault segment connectivity associated with large fault displacements also increased fault zone deformation associated with larger displacement faults is likely to further influence hydrologic properties overall faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault bedding intersections at all scales aquifer permeability is either unchanged or enhanced parallel to faults and in many cases decreased perpendicular to faults

seasonal evolution of the qbo induced wave forcing and circulation anomalies in the northern winter stratosphere diagnostics of wave mean flow and wave wave interactions in isentropic coordinates are analysed regarding the mechanisms that govern the holton tan effect hte whereby the tropical quasi biennial oscillation qbo modulates the northern winter stratospheric polar vortex we find a clear seasonal evolution of a qbo modulation of rossby waves and linear and nonlinear wave interactions along with multiple mechanisms at work in early winter when the lower stratospheric qbo is easterly there is enhanced upward stationary planetary wave activity in the subtropical to midlatitude lower stratosphere these upward propagating waves are first refracted towards the zero wind line approximately one month later poleward propagating wave anomalies appear at midlatitudes as a result of nonlinear wave interactions the enhanced upward wave propagation appears to be due to a combination of the qbo influence on the latitudinal location of the zero wind line and the increased midlatitude baroclinicity associated with the qbo induced meridional circulation this is further aided by a gradual poleward shift over the winter of the middle stratospheric waveguide refracting waves poleward the associated wave breaking weakens the polar vortex and drives a stronger meridional circulation throughout the lower to middle stratosphere contributing to the high latitude warming it is additionally found that in late winter the modified high latitude waveguide associated with the weaker polar vortex consequently reduces upward propagating transient planetary waves at high latitudes whilst simultaneously enhancing upward propagating synoptic waves at midlatitudes this highlights the need to view the hte and its mechanisms as an evolving phenomenon throughout the winter

using non conservative tracers to characterise karstification processes in the merinos colorado carrasco carbonate aquifer system southern spain the systematic sampling of the chemical composition of the groundwater from five karst springs including an overflow spring and one outflowing borehole have permitted to determine distinctive chemical changes in the waters that reflect the geochemical processes occurring in a carbonate aquifer system from southern spain the analysis of the dissolution parameters revealed that geochemical evolution of the karst waters basically depends on the availability of the minerals forming aquifer rocks and the residence time within the aquifers in the three proposed scenarios in the aquifers which include the preferential flow routines the more important geochemical processes taking place during the groundwater flow from the recharge to the discharge zones are co2 dissolution and exsolution outgassing calcite net dissolution calcite and dolomite sequential dissolution gypsum anhydrite and halite dissolution de dolomitization and calcite precipitation a detailed analysis of the hydrochemical data set saturation indices of the minerals and partial pressure of co2 in the waters joined to the application of geochemical modelling methods allowed the elaboration of a hydrogeochemical model of the studied aquifers the developed approach contributes to a better understanding of the karstification processes and the hydrogeological functioning of carbonate aquifers the latter being a crucial aspect for the suitable management of the water resources

modulation of northern hemisphere wintertime stationary planetary wave activity east asian climate relationships by the quasi biennial oscillation 1 the modulation of the relationship between the tropospheric stationary planetary wave activity and the east asian winter climate by the tropical quasi biennial oscillation qbo wind in the stratosphere is investigated in the qbo easterly phase a significant warming appears in northeastern asia in the presence of high wave activities this corresponds to a weakened east asian trough at 500 hpa which determines the extent to which cold waves penetrate into east asia however in the qbo westerly phase both the surface warming and the weakening of the east asian trough become insignificant in response to high wave activities the possible mechanism for this qbo modulation on the tropospheric wave activities may be attributed to the indirect influence of the qbo induced polar and extratropical stratospheric circulation changes under the qbo easterly phase conditions the tropospheric wave activity flux divergence in the higher latitude region is enhanced due to enhanced upward eliassen palm ep flux into the stratosphere while the wave flux convergence in the subtropical troposphere is increased due to the decrease in the equatorward flux this leads to an enhanced wave activity difference and thus the associated east asian climate anomalies become larger and robust an opposite effect i e reduced wave activity difference appears in the qbo westerly phase further analyses reveal that the qbo significantly modulates both the zonal wave number 1 and 2 in the sea level pressure field but not for wave number 3 the combination of wave number 1 and 2 patterns leads to the strongest surface temperature anomalies in northern asia

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

study of upper albian rudist buildups in the edwards formation using ground based hyperspectral imaging and terrestrial laser scanning abstract ground based hyperspectral imaging is used for development of digital outcrop models which can facilitate detailed qualitative and quantitative sedimentological analysis and augment the study of depositional environment diagenetic processes and hydrocarbon reservoir characterization in areas which are physically inaccessible for this investigation ground based hyperspectral imaging is combined with terrestrial laser scanning to produce mineralogical maps of late albian rudist buildups of the edwards formation in the lake georgetown spillway in williamson county texas the edwards formation consists of shallow water deposits of reef and associated interreef facies it is an aquifer in western texas and was investigated as a hydrocarbon play in south texas hyperspectral data were registered to a geometrically accurate laser point cloud generated mesh with sub pixel accuracy and were used to map compositional variation by distinguishing spectral properties unique to each material more calcitic flat topped toucasid rich bioherm facies were distinguished from overlying porous sucrosic dolostones and peloid wackestones and packstones of back reef facies ground truth was established by petrographic study of samples from this area this research integrates high resolution datasets to analyze geometrical and compositional properties of this carbonate formation at a finer scale than traditional methods have achieved and to model the geometry and composition of rudist buildups

contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres insights from the shape of the anomalies summary detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers including the lower crust and possibly the uppermost mantle is horizontally displaced with respect to that of the upper crust we examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper and lower crustal magnetization patterns thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the nusselt number and the depth of hydrothermal circulation in the crust and the temperature range of serpentinization the models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 dc successfully explain the anomalous skewness and the hook shape of observed sea level magnetic anomalies created at slow and intermediate spreading rates

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

a possible influence of the tropical quasi biennial oscillation on the variability of the extratropical circulation in the southern hemisphere 1 this study uses geopotential heights and zonal winds from the ncar ncep reanalysis and the naujokat tropical stratospheric zonal wind data set to evaluate the suggestion that the tropical quasi biennial oscillation qbo influenced the observed amplitude increase in zonal harmonic wave 1 in the southern hemisphere winter after 1975 fourier analysis was used to extract zonal wave 1 zw1 from the geopotential heights zw1 and the zonal winds were stratified by qbo phase and compared to those composited for years before and after 1975 the comparisons confirm that zw1 during the easterly phase of the qbo is larger than zw1 during the westerly phase the comparisons also suggest that the growth in zw1 seen after 1975 took place preferentially during winters with an easterly tropical qbo and weak stratospheric winds the change in zw1 is associated with surface level changes over the pacific and simultaneous upper level support by the easterly phase tropical qbo

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

modelling the baseline geochemistry of groundwater in a chalk aquifer considering solid solutions for carbonate phases the chalk aquifer of champagne france baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution the water rock interactions are modelled by the speciation code chess from field data and ca mg and sr aqueous concentrations in groundwater the stoichiometries of solid solutions are defined in each stratigraphic unit of the chalk aquifer from bulk geochemistry and chalk mineralogy of samples taken from boreholes the initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach

insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at odp iodp site 1256 from a cluster analysis of rock magnetic properties we analyze magnetic properties from ocean drilling program odp integrated odp iodp hole 1256d 6 44 1 n 91 56 1 w on the cocos plate in 15 2 ma oceanic crust generated by superfast seafloor spreading the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting fuzzy c means cluster analysis and nonlinear mapping are utilized to study down hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization the coercive force the ratio of the remanent coercive force and coercive force the low field magnetic susceptibility and the curie temperature to evaluate the effects of magmatic and hydrothermal processes on magnetic properties a statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas which differ from two distinct clusters mainly representing the dikes and gabbros extensive alteration can obliterate magnetic property differences between lavas dikes and gabbros the imprint of thermochemical alteration on the iron titanium oxides is only partially related to the porosity of the rocks thus the analysis complements interpretation based on electrofacies analysis all clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and or the presence of intergrowths the upper part of the extrusive sequence the granoblastic dikes and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization

pore fluid and diagenetic evolution of carbonate cements in lacustrine carbonate siliciclastic rocks a case from the lower cretaceous of the erennaoer sag erlian basin ne china lower cretaceous lacustrine carbonate siliciclastic rocks including siltstones and mudstones form a unique tight oil reservoir in the erennaoer sag in the erlian basin of northeastern china that is characterized by various carbonate cements and volcanic material based on integrated analysis of the petrology mineralogy and carbon and oxygen isotope geochemistry the origin compositions and evolution of carbonate cements and pore fluids are determined five types of carbonate cements are identified from oldest to youngest penecontemporaneous micritic crystalline ferroan dolomite 1 eogenetic micro crystalline ferroan dolomite 2 sparry micro crystalline ferroan calcite 1 mesogenetic micro crystalline non ferroan dolomite 3 and calcite 2 the evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods this may be advantageous to predict the distribution of high quality reservoirs the geochemistry of the paleo pore fluids in these mixed lithological reservoirs are more complicated thus the origin and distribution of pore fluids and their influences on carbonate cementation fluid rock interaction and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration in addition the evolution of the pore fluids of the arshan formation have experienced different salinity fluid environments under alkaline to weak acid conditions

dedolomitization and calcite cementation in the middle devonian winnipegosis formation in central saskatchewan canada limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the winnipegosis formation on the east central margin of the elk point basin where the overlying prairie evaporite deposits have been removed this type of crystalline limestone is interpreted as dedolomite based on petrographic observations the 18o and 13c values of the winnipegosis dedolomite vary from 12 8 to 11 9 vpdb vienna pee dee belemnite and from 0 5 to 1 7 vpdb respectively both values are significantly lower than those for the corresponding dolomite the 87sr 86sr ratios of the dedolomite are significantly higher between 0 7082 and 0 7087 the spatial distribution and geochemical data of the winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the prairie evaporite anhydrite during the latest mississippian to the early cretaceous when the basin was subjected to uplift and erosion the winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite including the occurrence of dedolomite restricted along fractures and adjacent areas dolomite patches floating in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts however the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the winnipegosis dedolomite coarsely to very coarsely crystalline subhedral to euhedral calcite cement is restricted in the dedolomite the petrographic features isotopic compositions and homogenization temperatures coupled with the burial history of the winnipegosis formation constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during late cretaceous to neogene time the more negative c isotopic signatures of the calcite cement 5 3 to 2 3 vpdb probably reflect a hydrocarbon derived carbon

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

dynamical response in the northern hemisphere midlatitude and high latitude winter to the qbo simulated by ccsr nies ccm 1 an analysis of the relationship between zonal wind in the equatorial stratosphere and zonal wind temperature and eliassen and palm e p flux in the northern hemisphere extratropical winter was performed using the ccsr nies chemistry climate model ccm and the japanese 25 year reanalysis jra 25 data the tropical zonal wind of the ccm was forced by observations including the observational equatorial quasi biennial oscillation qbo the influence of the qbo on the northern hemisphere winter was estimated by a composite analysis with statistical significance the analyses suggest that the difference in latitude of the critical line zero value line of zonal wind in the low latitudes around 10 hpa as well as 50 hpa between the easterly and westerly phases is related to the wave propagation and circulation over the whole depth of the stratosphere the circulation anomaly is further related to the temperature anomaly at the northern hemisphere midlatitudes and the zonal wind anomaly at the high latitudes these results suggest a mechanism through which 10 hpa qbo could influence the polar vortex while the mechanism of 50 hpa qbo influence is unclear but the possibility is not ruled out

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

problems concerning the evolution of oceanic lithosphere in the northern red sea abstract the pole of rotation for the opening of the red sea at 36 5 n 18 0 e arabia from africa implies the fastest spreading rates are in the south and are about 50 less in the north various studies show the spreading rate in the south to be about 1 cm yr over the last 4 5 ma problems arise as to the nature of the lithosphere beneath the northernmost red sea the problem is further complicated by the fact that the red sea may have evolved in three phases the first is the gulf of suez phase mainly oligocene the second corresponds to the first 62 km movement along the aqaba dead sea shear latest oligocene early miocene and the third to the second 45 km movement along the aqaba dead sea shear plio pleistocene these motions and their implications for the northernmost red sea are assessed the northern red sea differs from the central and southern red sea in having no clear magnetic lineations only a few isolated circular anomalies are observed there are two possible explanations 1 1 there is extremely thinned continental lithosphere occasionally pierced by strongly magnetised volcanic centres or 2 2 the oceanic lithosphere is present but without magnetic anomalies it is suggested that the first explanation applies to the first gulf of suez phase and the second explanation to the two later aqaba dead sea phases it is further suggested that the lack of magnetic anomalies may be due to a combination of large thicknesses of unstable sediments salt high temperatures and low sea floor spreading rates these lead to the evolution of oceanic lithosphere with slow cooling inhibiting the acquisition of strong magnetisation and giving subdued magnetic anomalies

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

model studies of the interannual variability of the northern hemisphere stratospheric winter circulation the role of the quasi biennial oscillation a series of experiments are described that examine the sensitivity of the northern hemisphere winter evolution to the equatorial quasi biennial oscillation qbo the prime tool for the experiments is a stratosphere mesosphere model the model is integrated over many years with the modelled equatorial winds relaxed towards observed values in order to simulate a realistic qbo in experiment a the equatorial winds are relaxed towards singapore radiosonde observations in the height region 16 32 km in contrast to previous modelling studies the holton tan relationship warm cold winters associated with easterly westerly qbo winds in the lower stratosphere is absent however in a second experiment run b in which the equatorial winds are relaxed towards rocketsonde data over the extended height range 16 58 km a realistic holton tan relationship is reproduced a series of further studies are described that explore in more detail the sensitivity to various equatorial height regions and to the bottom boundary forcing the experiments suggest that the evolution of the northern hemisphere winter circulation is sensitive to equatorial winds throughout the whole depth of the stratosphere and not just to the lower stratospheric wind direction as previously assumed

amplitude variation of marine magnetic anomalies to determine the large scale amplitude variation of marine magnetic anomalies 16 profiles have been analysed 8 across the mid atlantic ridge 5 across the east pacific rise and 3 across the indian ocean the profiles were normalized to the age of the anomalies and finally transformed to the same latitude of 45 both ridges show qualitatively the same trend with the central anomaly over the spreading axis clearly dominating the flanking anomalies gradually decrease in amplitude until a minimum is reached over an age range from 10 to 30 ma beyond this minimum the anomaly amplitudes progressively increase again to another maximum near the onset of the mesozoic magnetic anomalies there exists however a distinct difference in the absolute value of the magnetic anomalies for the two oceans analysed the observed amplitude variation can well be explained by a corresponding variation of the natural remanent magnetization of the upper 1000 meters of oceanic basalts as measured on individual samples from the deep sea drilling project model calculations lead us to propose a 3 layer model for the sources of marine magnetic anomalies an uppermost layer of 1000 m basalt corresponding more or less to seismic layer 2 with age dependent magnetization and two layers corresponding to seismic layers 3a and 3b with age independent magnetization the most likely cause for the age dependency of magnetization of the uppermost layer of oceanic basalts is gradual low temperature oxidation of the magnetic minerals contained in the rocks

chemical evolution of groundwater in triassic gypsum bearing carbonate aquifers las alpujarras southern spain a hydrochemical study employing modelling techniques was carried out using samples taken at 65 points springs and wells in triassic carbonate aquifers lujar gador unit alpujarride complex betic cordillera these aquifers are made up of limestones and dolomites with some gypsum scattered or interbedded though the area is semi arid recharge is relatively high because of their mountainous nature the carbonate rocks contain dense microfissuration the groundwater flow regime is predominantly diffuse the karstic forms are in general poorly developed two main hydrochemical processes have been identified in these aquifers one is incongruent dissolution of dolomite that determines the chemical composition of the less mineralised water the other is dedolomitisation dolomite dissolution together with calcite precipitation caused by dissolution of gypsum which becomes predominant when the flow encounters interbedded gypsum this reaction is also frequently associated with low temperature thermalism and can play a part in more intense local karstification cavities sinkholes high transmisivity in wells observed in the sectors of these aquifers where gypsum is more abundant a reaction path model has been used to simulate the geochemical processes through a hypothetical aquifer with similar lithology to the alpujarride carbonate aquifers successive stages of evolution through the carbonate sequence represented by different saturation states with respect to calcite dolomite gypsum and co2 have been modelled and then compared with the field data

modeling the quasi biennial oscillation s influence on isentropic tracer transport in the subtropics tracer transport in the lower stratosphere is investigated using an off line semi lagrangian transport model and winds from a mechanistic three dimensional middle atmosphere model interpolated to the 650 k isentropic surface over a winter season by comparing two simulations the effect of the quasi biennial oscillation s qbo s easterly or westerly wind phases on low latitude transport is examined especially regarding the formation of an apparent subtropical transport barrier at the equatorward edge of the surf zone the tracer field is diffused less than potential vorticity pv and shows sharper gradients and finer structures developing over the winter in both qbo phases the winter extratropical surf zone is bounded by large tracer gradients equatorward of about 15 latitude the inability of quasi stationary rossby waves to propagate into qbo easterlies leads to a much sharper edge forming between the rapidly stirred extratropical surf zone and the relatively unstirred tropics during the easterly qbo phase than occurs during the westerly qbo phase in the westerly phase some rossby wave activity can propagate across the equatorial westerlies and break on reaching the summer hemisphere easterly winds causing mixing there simulations using a tracer initially confined to the tropics show that more tropical air is mixed out to the winter extratropics by rossby wave stirring when the qbo phase is easterly despite the sharper band of stronger tracer or pv gradient which separates the tropics and the winter extratropics in this phase while air was mixed out of the tropics in both qbo phase simulations neither qbo phase simulation showed extratropical air being irreversibly mixed into the tropics these results are related to observations of the tropical reservoir of stratospheric aerosols especially the dynamics and transport of its winter hemisphere boundary

regional controls on the geochemical evolution of saline groundwaters in the edwards aquifer central texas geochemical and isotopic parameters are used to evaluate models for the evolution of saline groundwaters in the edwards aquifer which lies on the northwestern margin of the gulf of mexico sedimentary basin saline groundwaters termed badwaters range in salinity from 1000 to 12 500 mg l 1 total dissolved solids models for badwater evolution must account for complexities owing to 1 the range in compositions of saline basinal fluids that may migrate into the edwards aquifer from its down dip section and from underlying hydrostratigraphic units 2 the range of depositional diagenetic and mineralogic variations in the hist aquifer rocks 3 volcanic and intrusive igneous rocks that locally crosscut the aquifer 4 extensive faulting which in places juxtaposes edwards aquifer units with adjacent hydrostratigraphic units calcium mg na so4 cl and hco3 variations define six badwater hydrochemical facies strontium isotope values of freshwaters 87sr86sr 0 7077 0 7086 badwaters 0 7076 0 7094 and brines from aquifer rocks down dip 0 7078 0 7097 are nearly all higher than the range of most edwards aquifer rocks 0 7074 0 7077 and volcanic and intrusive igneous rocks that crosscut the aquifer 0 7032 0 7055 argillaceous carbonate rocks that underlie the edwards aquifer are likely sources of sr with high 87sr86sr values transitions between the hydrochemical facies in some locations correspond to changes in depositional environments in aquifer rocks and to changes in the intensity of faulting through the aquifer major element mineral saturation state and srca 87sr86sr variations in badwaters of the westernmost aquifer region indicate that incongruent dissolution of gypsum and recrystallization of calcite control the geochemical evolution of these badwaters srca na cl 87sr86sr variations in badwaters of extensively faulted areas to the east of the westernmost aquifer region are consistent with fluid mixing processes involving at least five endmember fluids including 1 saline groundwaters from two underlying hydrostratigraphic units 2 two endmember brines from down dip edwadrs aquifer equivalent units and 3 freashwaters that have interacted extensively with aquifer rocks the compositions of badwaters from the northeasternmost region reflect fluid mixing between freshwaters and saline groundwaters from underlying hydrostratigraphic units

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

the influence of the quasi biennial oscillation on global constituent distributions long lived stratospheric constituents observed by the upper atmosphere research satellite are compared between late northern winters 1991 1992 and 1992 1993 the phase of the quasi biennial oscillation qbo was such that the equatorial zonal wind was easterly westerly in the lower stratosphere during the first second winter in the tropics winter to winter differences in the latitude height distribution of zonally averaged n 2 o and h 2 o are associated with differences in large scale upwelling the qbo s mean meridional circulations and lateral mixing by incident extratropical rossby waves in the extratropics significant differences in zonally averaged n 2 o and h 2 o are also found which are believed to represent the extratropical constituent qbo the transport processes creating the extratropical constituent qbo are tentatively identified in the winter extratropics qbo modulation of the residual circulation appears to dominate in the upper stratosphere while qbo modulated rossby wave isentropic mixing dominates below about 10 mbar analysis of rossby waves at low latitudes as deduced from isobaric constituent distribution maps suggests that the meridional distribution of rossby wave breaking depends on the tropical zonal winds easterly qbo winds prevent rossby wave radiation into the tropics so that a strong constituent concentration gradient is generated in the winter subtropics westerly qbo winds allow some wave radiation into the tropics accompanied by isentropic mixing in the winter subtropics cross equatorial propagation and occasional mixing in the summer subtropics these effects depend on altitude and appear to vary in accord with the descending phases of the qbo

diagnosing the qbo s influence on circumpolar vortex variability using msu brightness temperatures and msu derived winds abstract using brightness temperatures from channels 3 and 4 of the microwave sounding unit msu as approximations to mean layer temperatures the geostrophic winds at 50 mb can be computed through a bottom up approach when this method is applied at high latitudes during austral winter and spring it is found that accurate descriptions of the seasonal evolution and interannual variability of the lower stratospheric circumpolar vortex are obtained variations in early spring vortex strength from year to year appear to relate well to variations in the timing of the first large late winter wavenumber one event in the lower stratosphere since wave forcing of the mean flow in the lower stratosphere is known to be weak the variability in vortex strength may result from variations in wave induced subsidence through the downward control principle previous studies have demonstrated a biennial harmonic in both extratropical wave forcing and the mean flow suggesting a link with the equatorially confined quas

hydrogeochemical modeling and dedolomitization processes in the patagonian boulders and patagonia formation in the eastern patagonia argentina the aim of this study was to contribute to the understanding of the hydrogeological system of the argentine patagonia extra andina region focused on the atlantic coastal area at 46 south latitude the hydrogeochemical aspects of this aquifer system were taken into consideration for this purpose the database includes 193 chemical analyses of major ions obtained through standard laboratory methods a statistical analysis of the data as well as the realization of hydrochemical models using phreeqc software was performed the regional groundwater flows from west to east with the recharge water being a sodium chloride bicarbonate type and the discharge of water is typified as a sodium sulfate chloride type the hydrogeochemical evolution of the aquifer shows a general augmentation of salinity and major ions in the groundwater flow path except for hco3 inverse hydrochemical modeling showed that gypsum dissolution with calcite precipitation ca2 na and mg2 na ionic exchange and dedolomitization are the main processes along the groundwater flow pathway

magnetism of al substituted magnetite reduced from al hematite this study was supported by national program on global change and air sea interaction gasi 04 01 02 the national natural science foundation of china grants 41504055 41430962 41374073 and 41025013 and chinese continental shelf deep drilling program gzh201100202 z x j further acknowledges support from the china postdoctoral science foundation a p r and d h acknowledge support from the australian research council grants dp110105419 and dp120103952

dolomite dissolution rates and possible holocene dedolomitization of water bearing units in the edwards aquifer south central texas abstract rates of dolomite dissolution can be used to test the concept based on geomorphologic evidence that a major part of the edwards aquifer could have formed within the holocene a timeframe of approximately 10 000 years during formation of the aquifer in the edwards limestone cretaceous albian of the balcones fault zone dolomite dissolution and porosity development were synchronous and the result of mixing zone dedolomitization initiation of the mixing zone in the early holocene 11 000 years before present is suggested by the maximum age of formation of major discharge sites that allowed the influx of meteoric water into brine filled dolomitic preaquifer units dedolomitization the dissolution of dolomite and net precipitation of calcite has left aquifer units that are calcitic and 40 vol interconnected pore space the mass of dolomite missing is obtained by comparison of stratigraphically equivalent altered and unaltered units one dissolution rate 1 76 10 4 mmol dolomite kg h 2 o 1 yr 1 is determined from this mass 10 4 yr reaction time and a log linear function describing the increase in mass discharge three orders of magnitude during aquifer formation the second estimated dissolution rate is obtained from the mass transfer of dolomite to solution calculated from the increase in magnesium in pore fluids selected from the modern aquifer to represent a typical flowpath during aquifer formation a reaction time of 10 4 yr for this mass transfer yields a rate of 0 56 10 4 mmol dolomite kg h 2 o 1 yr 1 both of these rates are comparable to modern rates of dolomite dissolution 0 3 to 4 5 10 4 mmol dolomite kg h 2 o 1 yr 1 calculated from measured reaction times in the tertiary floridan aquifer system in florida and the madison aquifer in the mississippian madison limestone of the northern great plains similarity of these rates to the estimated paleo rates of dolomite dissolution supports a 10 4 yr reaction timeframe the holocene reaction time also can be compared to a series of reaction times calculated by assuming that the mass of dolomite missing from the edwards was removed at rates observed in the floridan and madison aquifers these reaction times for complete removal of dolomite range from 2700 to 58 500 yr and span the pleistocene holocene boundary finally an estimated dolomite reaction rate during dedolomitization of the edwards aquifer based on surface area of exposed dolomite mmol cm 2 s 1 millimoles per square centimeter per second may be approximated from reaction times this rate is directly a function of the mass of dolomite removed and the surface area exposed per pore volume passing through the rock the surface area is available from the observed dolomite rhomb size in unaltered rock the rate of pore fluid movement is obtained from the averaged annual discharge rates during formation of the edwards aquifer calculated from all reaction times range from 10 13 to 10 14 mmol dolomite cm 2 s 1 these rates are faster than rates 10 18 mmol cm 2 s 1 measured in the pure laboratory system camg co 3 2 co 2 h 2 o but slower than rates determined in an alpine stream study 10 10 to 10 11 mmol cm 2 s 1 where cold glacial melt water flows over dolostone dolomite dissolution rates from both the edwards and other aquifers support the concept that a major part of the edwards aquifer could have formed within the holocene

magnetization of the oceanic crust thermoremanent magnetization of chemical remanent magnetization we propose a model in which chemical remanent magnetization crm acquired within the first 20 m y of crustal evolution may account for 80 of the bulk natural remanent magnetization of older basalts the crm of the crust is acquired as the original thermoremanent magnetization trm is lost through low temperature alteration the crm intensity and direction are controlled by the posternplacement polarity history this model explains several independent observations concerning the magnetization of the oceanic crust the model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern it also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the cretaceous quiet zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust the model also explains discrepancies in the anomaly skewness parameter observed for anomalies of cretaceous age further our study indicates varying rates of trm decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer

modification of the magnetic mineralogy in basalts due to fluid rock interactions in a high temperature geothermal system krafla iceland summary active high temperature 150 c geothermal areas like the krafla caldera ne iceland often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity the main alteration processes in such an environment are low temperature oxidation 350 c maghemitization and fluid rock interactions we investigated the rock magnetic properties natural remanent magnetization nrm magnetic susceptibility and their temperature and field variation and the mineralogy using x ray diffraction microscopic methods and electron microprobe analyses of two drill cores kh1 and kh3 from the rim of the krafla caldera the drill cores have distinctly lower nrm values average 3 a m 1 compared to younger surface basalts average 20 a m 1 along with a large variation in magnetic susceptibility 1 3 10 7 4 9 10 5 m3 kg 1 the secondary mineral assemblage sulphides sphene rutile and chlorite indicates an alteration within the chlorite smectite zone for both cores without depth zoning optical miscroscopy in combination with the bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite 1 titanomaghemite with intermediate titanium concentration and probably high vacancy concentration and 2 titanomaghemite with low titanium concentration and low vacancy concentration the mineral assemblages textures and magnetic properties deduced from the mentioned magnetic measurements indicate two stage transformation mechanism 1 dissolution of titanium at low ph under oxidizing conditions the ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene and fe2 migrates out of the spinel lattice forming titanomaghemite 2 formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions the latter mechanism produces ghost textures all titanomaghemite is transformed and only their former grain shapes are preserved with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved this mechanism could explain the significant magnetization loss which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like iceland or hawaii the production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of krafla

variations in oceanic crustal structure and implications for the fine scale magnetic anomaly signal marine magnetic anomalies are traditionally modelled using a constant thickness source layer usually defined as the extrusive lavas associated with seismic layer 2a recent seismic studies reveal that the thickness of layer 2a can vary by more than a factor of two an observation which has important implications for the magnetic anomaly source layer we model deep tow magnetometer data from the endeavour segment of the northern juan de fuca ridge in combination with nearby seismic data and find a positive correlation between the thickness of the seismically defined layer 2a and the magnetic anomaly source layer this suggests that short wavelength magnetic anomalies observed within individual magnetic polarity units can arise from variations in the thickness of the extrusives rather than geomagnetic field variations or secondary magnetization

recent large scale changes in the extratropical southern hemisphere atmospheric circulation abstract the zonal anomalies in the 500 hpa geopotential height field of the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalyses 1958 96 the zonal anomalies were constructed by removing the zonal mean from each grid point and principal components analysis was performed on the resulting spatial anomalies the principal components of the leading eof of the seasons march april may mam to september october november son indicate that a change has occurred in the extratropical large scale atmospheric circulation of the southern hemisphere it is estimated to have begun after 1975 and is limited to the southern late fall winter and early spring the elements of this change are a deeper pacific trough and higher indian ocean ridge at subtropical latitudes at higher latitudes the ridge over the pacific strengthened and shifted east while troughing occurred in the southwest atlantic weddell sea region consistent wi

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

kinetics of incongruent dissolution of carbonates in a chalk aquifer using reverse flow modelling s u m m a r y water rock interactions are major processes that control the solute transfer from natural or anthropo genic sources in aquifers in carbonate aquifers these water rock interactions are mainly characterised by the incongruent dissolutions of carbonates the in situ quantification of these reactions is few studied in the present paper we provide a new quantification of the kinetic rates of simultaneous incongruent dissolution of composite carbonate and dolomite in a chalk aquifer the approach is based on a mass balance model used to calculate the rates of mineral dissolution and precipitation along a 500 m to 4 km flow path starting at the groundwater divide line the groundwater transfer times between the groundwater divide line and the wells are calculated thanks to flow and transport modelling used in a backward particle tracking mode the uncertainties on porosity dispersivity and transmissivity are taken into account the rates are found ranging between 1 5 10 a5 and 1 5 10 a4 mol c5 chalk l a1 w yr a1 for the composite carbonate dissolution between 1 0 10 a7 and 3 0 10 a6 mol dolomite l a1 w yr a1 for the dolomite dissolution and between 1 0 10 a5 and 1 5 10 a4 mol calcite l a1 w yr a1 for the calcite precipitation the results highlight the importance of physical and geological properties of the aquifer

hydrogeochemical processes in clastic sedimentary rocks south korea a natural analogue study of the role of dedolomitization in geologic carbon storage abstract for long term mineral trapping of sequestered co 2 in deep saline aquifers base cations such as ca 2 and mg 2 are essential as a natural analogue study of geologic carbon storage in deep aquifers hosted in sedimentary formations we examined the hydrochemistry of sulfate rich up to 1140 mg l so 4 2 and moderately high p co 2 10 1 1 to 10 2 4 atm groundwater in a cretaceous non marine sedimentary basin in south korea with the objective to elucidate water rock interactions controlling the concentrations and behavior of base cations principal component analysis of the acquired hydrochemical data indicated that dissolution of carbonates calcite and dolomite and evaporite minerals halite and gypsum controls the chemical composition of groundwater resulting in substantial increases of the concentration s of ca 2 mg 2 na cl so 4 2 and total dissolved solids tds in deep groundwater na versus cl and ca 2 mg 2 versus hco 3 so 4 2 plots provided evidence for dissolution of halite and gypsum progressively increasing 34 s values of dissolved so 4 2 from 15 1 to 19 2 with increasing sulfate concentrations indicated gypsum dissolution ion ion plots esp mg 2 ca 2 ca 2 so 4 2 and mg 2 so 4 2 and saturation indices of calcite dolomite and gypsum suggest that the groundwater chemistry esp the concentrations of ca 2 and mg 2 is controlled by dedolomitization driven by gypsum dissolution groundwater in the study area does not reach complete equilibrium with respect to calcite and dolomite because of gypsum dissolution which controls the mg 2 ca 2 ratios of groundwater continued calcite precipitation triggered by an excess ca supply from gypsum dissolution reduces the concentrations of dissolved inorganic carbon dic in groundwater the increase of 13 c dic values from 11 1 to 6 5 concomitantly with increasing sulfate concentration was explained via geochemical modeling by dedolomitization under the rate constant ratio of about 0 038 between dolomite and calcite the model results agree well with the observed mg 2 ca 2 ratios and further suggest a potential increase of the void volume in the aquifer through dedolomitization by about 0 72 cm 3 l based on this analogue we suggest that dedolomitization in concert with dissolution of gypsum may constitute an important process releasing base cations for mineral trapping of injected co 2 in non marine clastic sedimentary strata containing carbonates and gypsum in south korea and elsewhere

seasonal variation of isentropic transport out of the tropical stratosphere the seasonal variation of the isentropic transport out of the tropical stratosphere is examined using contour advection calculations with winds from meteorological analyses for the period july 1991 to june 1994 calculations using winds from three different meteorological analyses are compared there is good qualitative agreement between the different calculations for extratropical structures but not for structures in the tropics there is also reasonable agreement between area diagnostics for transport out of the tropics derived from these calculations transport out of the tropics occurs in rossby wave breaking events in which filaments of tropical air are drawn into middle latitudes in all years these wave breaking events occur during the fall to spring period when there are westerlies throughout middle latitudes late september to early may in the northern hemisphere this includes periods before the formation of and after the breakup of the polar vortex in the northern hemisphere the rate of transport out of the tropics during fall to spring fluctuates around a constant value whereas in the southern hemisphere there is a relatively quiet midwinter period with maxima in the transport in early and late winter

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

modeling the quasi biennial oscillation s effect on the winter stratospheric circulation abstract the influence of the equatorial quasi biennial oscillation qbo on the winter middle atmosphere is modeled with a mechanistic global primitive equation model the model s polar vortex evolution is sensitive to the lower stratosphere s tropical winds with the polar vortex becoming more less disturbed as the lower stratospheric winds are more easterly westerly this agrees with the observed relationship between wintertime polar circulation strength and the phase of the qbo in the lower stratosphere in these experiments it is the extratropical planetary rossby waves that provide the tropical extratropical coupling mechanism more easterly tropical winds in the lower stratosphere act to confine the extratropical rossby waves farther north and closer to the vortex at the qbo altitudes weakening the vortex relative to the case of westerly qbo phase while the qbo winds occur in the lower stratosphere the anomaly in the polar vortex strength is strongest at higher levels