shear velocity structure of the northern california lithosphere the authors have determined the regional shear velocity structure of the lithosphere beneath the coast ranges and the great valley in northern california from inversion of three fundamental mode rayleigh wave phase velocity curves the dispersion measurements were made along three different paths crossing the coast ranges and great valley roughly perpendicular to the north american pacific plate boundary the three dispersion curves diverge at periods greater than about 20 s phase velocities are systematically higher from the northwest to the southeast along the transform margin inverting the phase velocities for crustal and upper mantle structure shows that this divergence is indicative of a 3 5 increase in the upper mantle shear velocity from the napa great valley region to the diablo range great valley region crustal shear velocities are consistent with the lithologies expected in the middle and lower crust the increase in mantle shear velocity from northwest to southeast is anticipated by a tectonic model for the development of the california transform margin in which asthenospheric material is emplaced at the base of the north american crust in the slab gap south of the mendocino triple junction adjacent to the plate boundary this process creates a subcrustal corridor of cooling asthenosphere whichmore is gradually incorporated in the lithosphere lid they suggest that this causes the observed increase in upper mantle shear velocity away from the triple junction finite difference simulations of rayleigh wave propagation across asthenosphere corridor models produce synthetic phase velocity curves which are similar to the field observations less

a new model for the origin of the anomalous radioactivity in niue island south pacific soils abstract the anomalous natural radioactivity of niue island is due to large excesses of both 230 th and 231 pa compared with their parent radionuclides it is suggested that material in niue island soils consisting mainly of goethite and gibbsite partially trapped within porous calcite was exposed to seawater a few hundred thousand years b p and the u content of the seawater adsorbed onto the soil minerals the island then rose and remained essentially above sea level until present the daughter products of the u isotopes grew into radioactive equilibrium with the u parents but then the u parents were leached away some time in the span 100 70 ka b p leaving large amounts of orphan daughters in the soils without supporting parents

effect of submarine groundwater discharge on the coastal ocean inorganic carbon cycle using radium ra isotopes we estimate that the average submarine groundwater discharge sgd flux marine plus terrestrial groundwater into the southwest florida shelf swfs was 20 6 10 3 107 and 18 6 8 3 107 m3 d21 in july and october 2009 respectively the terrestrial groundwater flux was the same order of magnitude as the local river discharge in july 2009 shelf water total alkalinity talk and dissolved inorganic carbon dic concentrations could not be explained by river inputs alone suggesting a groundwater source we estimated sgd fluxes of talk and dic using the sgd flux derived from a shelf water 226ra budget and talk and dic concentration differences between the groundwater and seawater these fluxes were also determined by the observed talk 226ra and dic 226ra relationships in the shelf water and the 226ra flux sustained by sgd these talk and dic fluxes were 11 71 times more than the combined input of local rivers suggesting that sgd was the dominant source of talk and dic to the swfs during 2009 sgd is an important component of the inorganic carbon budget for the coastal ocean

origin and characteristics of brackish groundwater in abu madi coastal area northern nile delta egypt abstract hydrogeochemical and geoelectrical resistivity investigations were carried out to assess the origin and characteristics of a brackish groundwater in abu madi coastal area twenty six surface water shallow and deep groundwater samples were collected and analyzed for various ionic concentrations as well as oxygen and hydrogen stable isotopic contents in addition a total of 20 vertical electrical sounding sites were conducted to investigate layers thicknesses resistivities and to detect the water bearing layers then 2 d electrical resistivity tomography ert along two profiles in the study area was conducted to get information about the surface water effect on recharge the stable isotopic composition and the chloride concentrations strongly suggest that the deep groundwater in abu madi area is dominated by inland freshwater with a minor seawater component this groundwater was recharged during the first and the third holocene humid climatic cycles where the sea surface was about 125 and 25 m below the current sea level respectively the brackish nature and higher piezometric surface of the groundwater as well as the occurrence of vertical low salinity seawater zone in front of the study area support the possibility of submarine groundwater discharge the geoelectrical resistivity surveying on the other hand revealed a number of geoelectrical groundwater bearing layers the main water bearing layer in the study area is represented by the sixth geoelectrical layer which has relatively high resistivity and a considerable thickness being consistent with the hydrogeochemical observations ert results point to the presence of shallow water bearing layers recharged from the surface water drains with low resistivity and surface rain water of moderate resistivity results from the hydrogeochemical analyses and the different hydrogeological data are consistent with the high resistivity values of this geoelectrical layer however the overall high specific conductance and high sodium hazards indicate that the groundwater in the study area should be carefully treated before irrigation and domestic uses through mixing it with surface irrigation water

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

advances in marine chemistry 1979 1982 these four years saw advances in many areas further developments in sampling and analytical techniques have opened new avenues of exploration for trace elements and particulate matter the impact of the geosecs expedition was further felt in the field of marine chemistry as the results were still being published a major field effort occurred as a follow on to the geosecs work tto and intensive sampling was performed in the newly discovered submarine hydrothermal vent fields an important and unresolved controversy has arisen regarding the ability to measure preformed co2 in the seawater system in many respects the field of marine chemistry appears to be gaining a momentum spurred by both improvements in analytic techniques and an increasing social and economic relevance

fault controlled dolomite bodies as palaeotectonic indicators and geofluid reservoirs new insights from gargano promontory outcrops the upper jurassic to lower cretaceous platform slope to basinal carbonate strata cropping out in gargano promontory southern italy are partly dolomitized fieldwork and laboratory analyses petrographic petrophysical and geochemical allowed the characterization of the dolomite bodies with respect to their distribution within the carbonate succession their dimensions geometries textural variability chemical stability age porosity genetic mechanisms and relation with tectonics the dolomite bodies range from metres to kilometres in size are fault related and fracture related and probably formed during the early cretaceous at 500 m burial depths and temperatures 50 c the proposed dolomitization model relies on mobilization of early cretaceous seawater that flowed downward and then upward along faults and fractures and was modified in its isotopic composition moving through triassic and jurassic strata that underlie the studied dolomitized succession despite the numerous cases reported in literature this study demonstrates that hydrothermal and or high temperature fluids are not necessarily required for fault controlled dolomitization distribution and geometries of dolomite bodies can be used for palaeotectonic reconstructions as they partly record the characteristics size attitude and kinematics of the palaeo faults even if not preserved that controlled dolomitization in gargano promontory dolomites record early cretaceous palaeo faults from metres to kilometres long striking north west south east to east west and characterized by normal to strike slip kinematics dolomitization increases the matrix porosity by up to 7 and therefore can improve the geofluid storage capacity of tight platform slope to basinal limestones the results have a great significance for characterization of geofluid for example hydrocarbons reservoirs hosted in similar dolomitized carbonate successions distribution size and shapes of reservoir rocks i e dolomite bodies could be broadly predictable if the characteristics of the palaeo fault system present at the time of dolomitization are known this article is protected by copyright all rights reserved

dating of sediments using accelerator mass spectrometry abstract an assessment is given of the current status and future prospects for the dating of recent sediments by the direct counting of radionuclides cosmogenic natural terrestrial and artificial using accelerator based mass spectrometers relevant isotopes and equipment are discussed the latter with particular reference to the 3 mv electrostatic accelerator system of the university of toronto inherent difficulties of the new method are outlined and possible applications are identified the ability to obtain radiocarbon dates from very small samples e g 100 g c illustrates the promise of the method

observation and inversion of surface wave group velocities across central india abstract surface wave group velocities across central india are measured from computer plots of frequency time analysis using multiple filter technique periods range from 5 to 56 sec for fundamental mode rayleigh wave 5 to 16 sec for first higher mode rayleigh wave and 5 to 49 sec for fundamental mode love wave the wave paths from koyna earthquakes to bokaro cross purely through the peninsular plateau the group velocities for these paths are found to be similar to those of surface waves recorded at poona from earthquake which lie to the northeast of bokaro in sub himalayan regions from this it is concluded that the shield structure of the peninsula extends to the sub himalayan regions to northeast of the peninsular plateu with a negligible part of sedimentary layer group velocities of fundamental mode rayleigh and love waves are also obtained for the paths from koyna earthquakes to shillong and are found to be similar to the above mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of bengal basin northern limit of this basin sedimentary layer has been identified mean and standard deviation of observed group velocity at each period are obtained for each of rayleigh fundamental rayleigh first higher and love fundamental modes the group velocities closely agree with those obtained earlier for the path between new delhi and kodaikanal and are also found to be similar to those observed in some shield regions monte carlo inversion method is applied and crust and upper mantle structure of the indian shield across central india is obtained crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study

crustal and upper mantle structures of the brazilian coast the rayleigh wave phase and group velocities in the period range of 24 39 sec obtained from two earthquakes which occurred in northeastern brazil and which were recorded by the brazilian seismological station rdj rio de janeiro have been used to study crustal and upper mantle structures of the brazilian coastal region three crustal and upper mantle models have been tried out to explain crustal and upper mantle structures of the region the upper crust has not been resolved due basically to the narrow period range of the phase and group velocities data the phase velocity inversions have exhibited good resolutions for both lower crust and upper mantle with shear wave velocities characteristic of these regions the group velocity data inversions for these models have showed good results only for the lower crust the shear wave velocities of the lower crust 3 86 and 3 89 km sec obtained with phase velocity inversions are similar to that 3 89 km sec found byhwang 1985 to the eastern south american region while group velocity inversions have presented shear velocity 3 75 km sec similar to that 3 78 km sec found bylazcano 1972 to the brazilian shield it was not possible to define sharply the crust mantle transition but an analysis of the phase and group velocity inversions results has indicated that the total thickness of the crust should be between 30 and 39 km the crustal and upper mantle model obtained with phase velocity inversion can be used as a preliminary model for the brazilian coast

three dimensional flow in the florida platform theoretical analysis of kohout convection at its type locality kohout convection is the name given to the circulation of saline groundwater deep within carbonate platforms first proposed by f a kohout in the 1960s for south florida it is now seen as an mg pump for dolomitization by seawater as proposed by kohout cold seawater is drawn into the florida platform from the deep straits of florida as part of a geothermally driven circulation in which the seawater then rises in the interior of the platform to mix and exit with the discharging meteoric water of the floridan aquifer system simulation of the asymmetrically emergent florida platform with the new three dimensional 3 d finite element groundwater flow and transport model sutra ms which couples salinity and temperature dependent density variations allows analysis of how much of the cyclic flow is due to geothermal heating free convection as opposed to mixing with meteoric water discharging to the shoreline forced convection simulation of the system with and without geothermal heating reveals that the inflow of seawater from the straits of florida would be similar without the heat flow but the distribution would differ significantly the addition of heat flow reduces the asymmetry of the circulation it decreases seawater inflows on the atlantic side by 8 and on the gulf of mexico side by half the study illustrates the complex interplay of freshwater saltwater mixing geothermal heat flow and projected dolomitization in complicated 3 d settings with asymmetric boundary conditions and realistic horizontal and vertical variations in hydraulic properties

magnetotelluric study of the teide tenerife and timanfaya tanzarote volcanic areas the application of a magnetotelluric surveying method developed specifically for detection of strong conductivity contrasts has permitted interpretations of the deep structure of two important volcanic formations in the canary archipelago the timanfaya lanzarote and teide tenerife volcanoes beneath the timanfaya volcano a highly conductive body p 1 m has been detected which is interpreted as a shallow magma chamber associated with the eruption which took place between the years 1730 and 1736 the core of this intrusion still largely molten would be located at approximately 4 km depth and would still maintain temperatures in excess of 900 c in the telde caldera the asymmetry of the apparent resistivity components indicates a double depression furthermore a highly conductive level encountered in the centre of the eastern part of the volcanic complex has been attributed to the persistance of relatively high temperatures in the root of the last salic dome extruded inside the caldera in both zones the deep limiting horizon is a highly conductive layer corresponding to the mantle 15 km in lanzarote and 13 km in tenerife whereas other more superficial discontinuities have been related to local structures

shallow subsurface diagenesis of pleistocene periplatform ooze northern bahamas previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic aragonite magnesian calcite calcite sediments are susceptible to early diagenesis only in current swept open seaways or where surficially exposed by erosion on the seafloor it has also been proposed that while in the shallow subsurface periplatform oozes retain their original mineralogy for at least 200 000 400 000 yr and remain unlithified for tens of millions of years evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late pleistocene age in two piston cores collected from water depths of 1 000 m north of little bahama bank it is shown that shallow 30 m subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440 000 yr and that cementation by calcite can produce chalk ooze sequences within the same time frame periplatform oozes that originally contain a high percentage of bank derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates and chalk ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m shallow subsurface diagenesis at water depths 1 000 m proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings exterior overgrowths and cement it is speculated that density driven kohout convection where seawaters under saturated with respect to magnesian calcite and aragonite and saturated supersaturated with respect to calcite flow through the margins of carbonate platforms is the primary driving mechanism for shallow subsurface diagenesis removal of mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the mg content of interstitial waters which is likely to increase the dolomitizing potential of kohout convection fluid flow

the application of the magnetotelluric impedance tensor to earthquake prediction research in greece continuous short period 0 1 3600 s measurements of the magnetotelluric field components were conducted simultaneously at two sites in greece for a period of 5 weeks the estimation of the impedance tensor elements from 8 hr windows of recorded data at each station is found to describe the local electromagnetic induction with great accuracy the computation of the residual electric field obtained as the difference between the observed and estimated inductive part of the electric fields clearly improves the detection of any local electric field anomaly this method was used in an attempt to detect precursory sci smic electrical signals ses that have been reported to precede earthquakes in greece the results indicate the success of this method in resolving ambiguities about the nature of the reported ses i e whether it has an external or internal source although during the recording period no large earthquake occurred five with a magnitude of between ml 4 0 and ml 4 3 occurred within a radius of 150 km from one of the stations the magnetotelluric recordings and the computed residual electric fields for the time intervals reported to contain the ses precursors to these events were analysed in some detail in two of the cases it was found that the sources could not be related to earthquake processes in the other three cases the nature of the source of the suspected ses although electric could not be established with certainty these results indicate that for earthquakes of small magnitude ml 4 3 or of large epicentral distance r 100 km the detection of a ses is extremely difficult

the occurrence and chemical implications of geothermal convection of seawater in continental shelves 1 geothermal convection is an important driving force for fluid flow and diagenesis in carbonate platforms and the similar shape of continental shelves in general suggests that geothermal convection could occur in continental shelves around the world numerical models based on seven transects of the shelf off the east coast of north america were developed to bracket possible fluxes simulations assuming relatively permeable sediments medium grained carbonates failed to produce significant thermal variations suggesting that chemical tracers will be required to detect this flow in the field fluxes calculated assuming lower permeabilities fine grained carbonates between cape fear and the savannah river were approximately 3 orders of magnitude smaller than river discharge for the area a difference similar to that between global river discharge and hydrothermal convection at mid ocean ridges sediment diagenesis during geothermal convection in continental shelves thus has the potential to affect ocean chemical budgets including potassium calcium and magnesium

self limiting geothermal convection in marine carbonate platforms large scale inward and upward density driven convective circulations occur in the porous structure of many marine carbonate platforms their geochemical implications are of prime importance for a variety of problems where a substantial transport of chemical species is required in the present study we show using simple thermal and hydraulic arguments that there is a negative feedback between convective flow and heat transfer this results in an upper limit on geothermally driven vertical fluxes irrespective of the hydraulic conductivity of the medium this concept is potentially applicable to a variety of problems taking the example of two on going debated questions in the reef scientific community i e diagenesis and nutrient cycling we show unambiguously that whereas convective circulations in coral reefs are compatible with dolomitization models they are too low by at least one to two orders of magnitude to be a significant process in the nutrient budget of coral reefs

group velocity tomography and lithospheric s velocity structure of the south american continent the lithosphere of the south american continent has been studied little especially in northern brazil the amazonian region a 3d lithospheric s velocity model of south america was obtained by first carrying out rayleigh and love wave groupvelocity tomography and then inverting the regionalized dispersion curves fundamental mode group velocities were measured using a multiple filtering technique more than 12 000 paths were examined and about 6000 rayleigh and 3500 love wave dispersion curves with good quality were retrieved checkerboard tests showed that our dataset permits the resolution of features 400 800 km across laterally in the central part of the continent from crustal to upper mantle depths our results confirm previous tomographic results and correlate well with the major geological provinces of south america the 3d s velocity model confirms both regional features of se brazil from p wave travel time tomography and continental scale features of central and western south america from waveform inversion e g lowest velocities in the andean upper mantle three parts of the nazca plate with flat subduction strong low velocity anomalies in the upper mantle depth beneath the chaco basin furthermore our 3d model revealed new features in the south american continent 1 high velocities in the lower crust were consistently found in regions with high bouguer or free air anomalies 2 the ne sw trending transbrasiliano shear zone was delineated by a ne sw low velocity belt at lithospheric depths 3 the eastern amazonian craton appears to have thicker lithosphere than the western

3 d thermoconvection in an anisotropic inclined sedimentary layer summary fan type sandstone deposits within a sedimentary basin generally present an anisotropic structure with maximum permeability in the paleo horizontal direction modelling of 3 d circulations in inclined isotropic porous layers suggests the following patterns of fluid motion for subcritical rayleigh numbers ra the fluid circulates along the steepest slope this is the well known basic flow at low inclinations the supercritical convection consists in polyhedric cells and beyond a critical slope of 32 longitudinal coils i e with axes parallel to the steepest slope are the preferential mode of convection we extend these results to anisotropic media with anisotropies up to 1000 by means of 3 d numerical modelling from a systematic linear study near the critical rayleigh number rac it turns out that the critical slope lowers to 12 7 4 2 1 4 for anisotropy ratios of 10 100 1000 respectively a complete non linear study at twice the critical rayleigh number and for a slope of 2 presents only longitudinal rolls their width is lower than four when the anisotropy does not exceed 1000 moreover we observe that the circulation of the fluid particles inside the coil follows a loop stretched in the slope direction with a length of six and 60 times the thickness of the layer for anisotropies of 100 and 1000 respectively we suggest that this offers a possibility of very far transport of minerals in the direction of the slope

226ra and 228ra in the mixing zones of the pee dee river winyah bay yangtze river and delaware bay estuaries 226ra and 228ra have non conservative excess concentrations in the mixing zones of the pee dee river winyah bay estuary the yangtze river estuary and the delaware bay estuary laboratory experiments using pee dee river sediment indicate desorption of 226ra to increase with increasing salinities up to 20 in winyah bay desorption from river borne sediments could contribute almost all of the increases for both isotopes desorption adds only a portion of the excess 228ra measured in the yangtse river and adjacent shelf waters and delaware bay in the yangtze river the mixing zone extends over a considerable portion of the continental shelf where 228ra is added to the water column by diffusion from bottom sediments while 226ra concentrations decrease from dilution diffusion of 228ra from bottom sediments in delaware bay primarily occurs in the upper part of the bay 22 water where fine grained sediments predominate a diffusive flux for 228ra of 0 33 dpm cm 2 year was determined for delaware bay

using multiple geochemical tracers to characterize the hydrogeology of the submarine spring off crescent beach florida abstract a spectacular submarine spring is located about 4 km east of crescent beach fl in the atlantic ocean the single vent feature of crescent beach spring provides a unique opportunity to examine onshore offshore hydrogeologic processes as well as point source submarine ground water discharge the floridan aquifer system in northeastern florida consists of tertiary interspersed limestone and dolomite strata impermeable beds confine the water bearing zones under artesian pressure miocene and younger confining strata have been eroded away at the vent feature enabling direct hydrologic communication of eocene ground water with coastal bottom waters the spring water had a salinity of 6 02 which was immediately diluted by ambient seawater during advection mixing the concentration of major solutes in spring water and onshore well waters confirm a generalized easterly flow direction of artesian ground water nutrient concentrations were generally low in the reducing vent samples and the majority of the total nitrogen species existed as nh 3 the submarine ground water tracers rn 222 1174 dpm l 1 dpm methane 232 nm and barium 294 5 nm were all highly enriched in the spring water relative to ambient seawater the concentrations of the reverse redox elements u v and mo were expectedly low in the submarine waters the strontium isotope ratio of the vent water 87 sr 86 sr 0 70798 suggests that the spring water contain an integrated signature indicative of floridan aquifer system ground water additional sr isotopic ratios from a series of surficial and lower floridan well samples suggest dynamic ground water mixing and do not provide clear evidence for a single hydrogeologic water source at the spring vent in this karst dominated aquifer such energetic mixing at the vent feature is expected and would be facilitated by conduit and fractured flow radium isotope activities were utilized to estimate flow path trajectories and to provide information on potential travel times between an onshore well and the spring using either 223 ra and 224 ra or 228 ra and qualifying this approach with several key assumptions estimates of water mass travel times from an upper floridan well in crescent beach to the submarine vent feature distance 4050 m are in the order of 0 01 0 1 m min 1

crust and upper mantle velocity structure beneath the northern and central indian ocean from the phase and group velocity of rayleigh and love waves abstract fundamental mode rayleigh and love waves generated by nine earthquakes which occurred in the central indian ocean with epicentres extending to 40 s and recorded at seven wwssn stations of central asia have been used to determine the phase and group velocity along various paths across the northern and central indian ocean the dispersion characteristics of rayleigh and love waves show a thick crust of 23 km thickness this anomalous crustal thickening having quasi continental oceanic structure may be explained by assuming the gradual transformation of top mantle material into material having either crustal like velocity or slightly lower than moho velocity p wave velocity 7 72 km s 1 s wave velocity 4 45 km s 1 the velocity structure down to a depth of 200 km has been obtained across the northern and central indian ocean a low velocity zone of 90 km with p wave velocity of 7 85 km s 1 and s wave velocity of 4 37 km s 1 is estimated to be centred at a depth of 78 km from the water surface this low shear velocity in the mantle may be caused by the partial melting and elevated temperature there the observed dispersion data for rayleigh and love waves also exhibit a strong anisotropy along different paths in the crust and upper mantle the observed dispersion data for rayleigh and love waves that cross the ninety east ridge are characterized by lower phase and group velocities as compared to waves having paths in a more western direction than this ridge axis however the phase velocities for rayleigh and love waves are characterized by lower values across the ninety east ridge as compared to the normal oceanic structure

a layered aquifer model of atoll island hydrology validation of a computer simulation abstract the finite element model femwater was used to simulate groundwater movement in the two dimensional vertical plane of an atoll island section the model island had dimensions consistent with enjebi island enewetak atoll a surficial holocene aquifer to a depth of 15m with hydraulic conductivity 6 9 10 4 ms 1 overlying a plio pleistocene aquifer with k 6 9 10 2 ms 1 a tidally varying water level was applied to the ocean and lagoon boundaries and the model was run until pseudo steady state conditions were achieved in the groundwater variations hydraulic head contours in the vertical section demonstrate that water motion under the supratidal portion of the island is almost exclusively vertical calculated water table tidal lags and efficiencies are in excellent agreement with measured values from shallow wells on enjebi island confirming the validity of the layered aquifer model and the approximate magnitudes of the hydraulic conductivities model results near island margins can be interpreted to explain other features of hydrologic field data from atoll islands the results emphasize the potential importance of tidal effects and vertical water flow on reef island groundwater systems and cast doubt on the utility of traditional models using the dupuit assumptions of essentially horizontal flow

estimating groundwater discharge into the northeastern gulf of mexico using radon 222 abstract submarine groundwater discharge sgd may provide important chemical constituents to the ocean but the dispersed nature of this process makes locating and quantifying its input extremely difficult since groundwater contains 3 4 orders of magnitude greater radon than seawater 222rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated we report development of a sgd tracing tool based on radon inventories in a coastal area of the northeastern gulf of mexico we evaluated factors that influence the concentration of radon in the water column i e production decay horizontal transport and loss across the pycnocline using a linked benthic exchange horizontal transport model total 222rn benthic fluxes 2420 dpm m 2 day 1 measured with in situ chambers are of the magnitude required to support measured sub pycnocline 222rn inventories while estimates of molecular diffusion show that this input is relatively small 230 dpm m 2 day 1 using this model approach together with measurements of the radon inventory we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 sec 1 into the 620 km2 study area this discharge equivalent to an upward advective velocity of approximately 2 10 cm day 1 dispersed over this entire study area is equivalent to approximately 20 first magnitude springs

dynamic hydrology and diagenesis of a submerged pleistocene fringing reef discovery bay jamaica abstract in the discovery bay area of north jamaica a pleistocene fringing reef grew and was subsequently planed off as sea level rose to and fell from a highstand approximately 5 to 10 m below modern sea level between 132 000 and 147 000 yrs bp dynamic movement of seawater through the reef today evidenced by flow of water in and out of open core holes has caused precipitation of cements which overprint previous diagenetic fabrics in certain zones in the reef flat and distal fore reef facies where porosity is diffuse modern water movement is slow and has had little effect on pre existing diagenetic fabrics which are dominantly vadose in texture in facies with conduit porosity as in the reef crest and proximal portions of the shallow fore reef seawater is channelized and high flow rates have resulted in the precipitation of substantial amounts of holocene sparry high magnesian calcite cements 15 18 mol mgco 3 up to 250 m thick resulting in incorporation of significant amounts of 14 c in the pleistocene rocks smaller mg calcite crystals having lower magnesium content are characteristic of zones with diffuse porosity the dynamic marine system apparently also favors the precipitation of dolomite in zones of diffuse porosity water motion within the reef results in the slow movement out of the man made conduits in zones of conduit porosity such as in facies having highly localized permeability water is sucked into open core holes in accordance with bernoulli s principle it is not known to what depths such channelized flow occurs data from a recording current meter demonstrate that the driving force responsible for the dynamic hydrology has a strong tidal component the tidal component is augmented by wind driven waves and is probably superimposed on another unidentified current which contributes to the flow of water in the reef at all tidal stages

upper mantle heterogeneities in africa deduced from rayleigh wave dispersion abstract rayleigh wave group velocities have been measured along 11 paths on the african continent north of the equator the results lead to a division of this part of the continent into three regions cratonic areas non cratonic areas mobile zones and a region located to the east of the meridian 31 e red sea neighbourhood ethiopia djibouti the highest velocities are found beneath the cratonic areas at 200 km depth 5 at greater depths the mobile zones exhibit higher velocities than the cratonic regions but the difference is not as marked 2 and the resolution is poor velocities in the eastern region are similar to those found under young oceanic regions

magnetotelluric regional strike a new method for determining regional strikes from the magnetotelluric impedance tensor z and tipper t is presented it involves the minimization of weighted sums of the squared magnitudes of elements of z or t over all frequencies and all stations of interest when applied to data from the mexicali valley baja california around the cerro prieto geothermal field for a particular weighting function the method yielded orientations that agree to within 2 9 degrees for three lines with a total of 16 stations the consistency of orientations can be attributed in part to the pronounced two dimensional 2 d geologic characteristics of the area but the techniques also proved stable at stations near three dimensional 3 d inhomogeneities

source dependent bias of sferics in magnetotelluric responses the predominant signals of audio frequency magnetotellurics amt are called sferics and they are generated by global lightning activity when sferic signals are small or infrequent measurement noise in electric and magnetic fields causes errors in estimated apparent resistivity and phase curves leading to great model uncertainty to reduce bias in apparent resistivity and phase we use a global propagation model to link sferic signals in time series amt data with commercially available lightning source information including strike time location and peak current we then investigate relationships between lightning strike location peak current and the quality of the estimated apparent resistivity and phase curves using the bounded influence remote reference processing code we use two empirical approaches to preprocessing time series amt data before estimation of apparent resistivity and phase stitching and stacking averaging we find that for single site amt data bias can be reduced by processing sferics from the closest and most powerful lightning strikes and omitting the lower amplitude signal deficient segments in between we hypothesized that bias can be further reduced by stacking sferics on the assumptions that lightning dipole moments are log normally distributed whereas the superposed noise is normally distributed due to interference between dissimilar sferic waveforms we tested a hybrid stitching stacking approached based on clustering sferics using a wavelet based waveform similarity algorithm our results indicate that the best approach to reduce bias was to stitch the closest and highest amplitude data

groundwater lens dynamics of atoll islands a variable density groundwater model is used to analyze the effects of various controls on the size of the freshwater lens the structure of the transition zone and the propagation of tidal fluctuations in a two layer atoll island groundwater system modeling results indicate that mixing of fresh water and saltwater occurs primarily as a result of oscillating vertical flow due to tidal fluctuations and depends to a lesser extent on transverse dispersion along the dominantly horizontal recharge discharge path of flow the controls on the amount of mixing are 1 the accumulated vertical distance which increases with tidal range and is restricted by vertical permeabilities and 2 vertical longitudinal dispersion comparison of cross sectional simulations of atoll islands using nontidal and tidal models shows that the nontidal model must use artificially high values of transverse dispersivity to compensate for the lack of tidally driven vertical mixing processes although the tidal model has high computational requirements it can be used to calibrate vertical permeabilities and is best suited for problems dealing with groundwater resource evaluations hydrologic events and hydrologic processes the limitations of the nontidal model are that it cannot be used for calibration of vertical permeabilities and will not realistically simulate those cases in which transition zones are thick or recharge low

density induced water circulations in atoll coral reefs a numerical study groundwater flow in an idealized atoll platform is simulated with castem 2000 a computer code developed at the cea commissariat a l energie atomique darcy s law and a coupled solute transport equation diffusion dispersion convection is solved by the mixed hybrid finite element method inward and upward circulations of oceanic water are shown to occur these steady state circulations result from temperature and to a lesser extent salinity gradients inclusion of a karstified dolomitic horizon observed at the base of the carbonate structure of certain atolls is necessary to account for the negative vertical temperature gradient indicated by field data the position of the platform in relation to the oceanic temperature and salinity distributions exerts a significant control on the interstitial hydraulic regime the effect of a secondary karstic layer closer to sea surface is also modeled in this case calculations show that the near surface carbonate edifice is deprived of a significant portion of the flow finally an upper limit on the possible flow rate through the platform is calculated this upper estimate is shown to be independent of the possible uncertainties on the hydraulic conductivity of the system corresponding upper limits on the nutrient fluxes of po and no are evaluated a comparison with 3 22 43 current import and export data for the surface ecosystem indicates that in every case nutrient fluxes hardly reach a few percent of the exports therefore the deep oceanic nutrient pool brought to the surface by thermally driven interstitial circulation is only a minor nutrient source for the reef ecosystem

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

crustal resistivity structure from magnetotelluric soundings in the colorado plateau basin and range provinces central and western arizona resistivity structure to about 25 km depth is defined from two dimensional modeling of 29 magnetotelluric mt soundings 0 002 5 hz that traverse 280 km of the southwestern colorado plateau transition zone and basin and range provinces in arizona from the surface to 5 km depth the mt model suggests structural relationships between low resistivity sedimentary and volcanic rocks 50 300 ohm m and high resistivity granitic and gneissic basement 500 9000 ohm m in the basin and range province the mt model and a seismic reflection section show a generally consistent distribution of supracrustal rocks that have relatively low to moderate resistivity mt and relatively strong locally coherent reflectivity the supracrustal zone defined by these physical properties is inferred to be composed of upper plate rocks above a middle tertiary detachment fault system which has been mapped in surrounding ranges some low angle fault zones inferred from seismic reflections to extend into high resistivity basement below the supracrustal rocks are not resolved by the mt model a low resistivity zone with a conductance of 500 s or more is modeled in the crust at an average depth of about 15 km in the basin and range province and transition zone and may deepen below the southwestern part of the colorado plateau in the basin and range province the top of the low resistivity may correspond to a reflective layer with a 6 s two way travel time this deep low resistivity zone might be caused by a small fraction of connected hydrous solutions or silicic melts

origin of dolomitization on the barbwire terrace canning basin western australia a thick areally extensive subsurface sequence of upper devonian carbonates occurs on the barbwire terrace in the canning basin of western australia it is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized slightly deeper water mark have remained as limestones the major regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite up to 600 m with no primary calcite a small volume of evaporitic supratidal dolomite is present at one location this dolomite is derived from highly saline fluids developed in an arid supratidal environment replacement dolomite of the regional dolomite type has a xenotopic form with undulose extinction and irregular crystal boundaries in addition saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite this suggests the regional dolomite type was precipitated at slightly elevated temperatures dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution geochemically the synsedimentary supratidal and regional dolomite types are quite distinctive supratidal dolomites have 6 o values which are significantly higher 6180 2 to i pdb than the regional dolomite type 6l80 9 to 2 pdb assuming the lowest 6 o values for the sabkha dolomite represent replacement in marine waters the oxygen isotopic composition for upper devonian canning basin marine dolomite would be around 6l80 2 pdb the petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin however sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the barbwire terrace therefore it is suggested that dolomitization may have taken place in a near surface environment with a major recrystallization event superimposed during burial diagenesis

remote reference magnetotellurics equipment and procedures during the past 3 years major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation the major factor in this improvement has been the introduction of the remote reference technique although the use of ultrasensitive magnetometers and minicomputers for in field data processing has also been important after a review of the remote reference technique this paper describes the equipment and procedures used for remote reference magnetotellurics by the authors magnetometers using d c superconducting quantum interference devices typically have a sensitivity of 10 14 t hz 1 2 a dynamic range of 10 7 in a 1 hz bandwidth and a slewing rate of 3 10 5 t s 1 at 10 khz the electric field measurements use conventional cu cuso 4 electrodes the remote magnetic reference signals are transmitted to the base station using fm analog telemetry the data are collected and processed by a minicomputer based on an lsi 11 microprocessor the essential results for example the apparent resistivities and the tipper components with their probable errors are available in the field practical details are given of the handling of superconducting devices low temperature cryostats and liquid helium in the field various spurious noise sources are mentioned and techniques for minimizing their effects are described

geochemistry of submarine warm springs in the limestone cavern of grotta azzurra capo palinuro italy evidence for mixing zone dolomitisation abstract subtidal springs in and around the submarine limestone cavern of grotta azzurra at capo palinuro italy discharge fluids which are warm 10 excess si alkalinity ca2 sr2 and mn relative to seawater are likely derived from the groundwater component or dissolution hydrothermal alteration of the host rocks magnesium has been removed from the seawater component in exchange for ca2 due to dolomitisation of the limestone and or hydrothermal alteration reactions saturation state calculations suggest that the vented fluids are near saturation with respect to calcite and supersaturated with respect to dolomite this and the presence of dolomite in the host rocks and cave floor sediments suggest that mixing zone dolomitisation of the limestones is occurring perhaps kinetically assisted by elevated temperature and or bacterial mediation in the reducing subseafloor zone one possible end member condition is considered for the thermal fluid zero mg which suggests an end member temperature of 50 5 c and a fluid composition derived from 38 seawater and 62 groundwater the heat source for the circulating fluids is uncertain but may involve warm underlying igneous rocks or heating via the geothermal gradient a continuous in situ record of vent fluid temperature salinity ph and o2 concentration collected within the cavern is consistent with our interpretation of the fluid origin and suggests that tidal forcing affects circulation and venting of the warm fluids

atoll island hydrogeology flow and freshwater occurrence in a tidally dominated system a layered aquifer model of groundwater occurrence in an atoll island was tested with a solute transport numerical model the computer model used sutra incorporates density dependent flow this can be significant in freshwater saltwater interactions associated with the freshwater lens of an atoll island boundary conditions for the model included ocean and lagoon tidal variations the model was calibrated to field data from enjebi island enewetak atoll and tested for sensitivity to a variety of parameters this resulted in a hydraulic conductivity of 10 m day 1 for the surficial aquifer and 1000 m day 1 for the deeper aquifer this combination of values gave an excellent reproduction of the tidal response data from test wells the average salinity distribution was closely reproduced using a dispersivity of 0 02m the computer simulation quantitatively supports the layered aquifer model including under conditions of density dependent flow and shows that tidal variations are the predominant driving force for flow beneath the island the oscillating vertical flow produced by the tidal variations creates an extensive mixing zone of brackish water the layered aquifer model with tidally driven flow is a significant improvement over the ghyben herzberg dupuit model as it is conventionally applied to groundwater studies for many pacific reef islands

structure of the crust and upper mantle beneath australia from rayleigh and love wave observations abstract fundamental and first higher modes of the rayleigh and love wave group velocities along seven paths in australia were jointly inverted by a controlled monte carlo procedure to obtain regional shear wave velocity structures of the crust and upper mantle our data support the results of gonez and cleary which show an s wave low velocity zone centred near 110 km depth in eastern australia however the thickness velocity contrast of the low velocity zone is significantly smaller the crustal models for eastern australia are characterized by upper crusts which are both thicker and have lower velocities than those in western australia and have a less sharp crust upper mantle boundary the s wave velocities for the upper mantle appear to be similar 4 55 km s 1 throughout the continent with no obvious dependence on the age of cratonization or crustal thickness

three dimensional inversion of ztem data summary z axis tipper electromagnetic technique ztem data are airborne electromagnetic data which record the vertical magnetic field that results from natural sources the data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground the transfer functions depend on frequency and provide information about the 3 d conductivity structure of the earth the practical frequency range is 30 720 hz and hence it is possible to see structures at depths of a kilometre or more if the earth is of moderate conductivity this depth of penetration is significantly greater than that obtained with controlled source em techniques and when coupled with rapid spatial acquisition with an airborne system means that ztem data can be used to map large scale structures that are difficult to survey with ground based surveys we present some fundamentals about understanding the signatures obtained with ztem transfer functions and then develop a gauss newton algorithm to invert ztem data the algorithm is applied to synthetic examples and to a field data set from the bingham canyon region in utah the field data set requires a workflow procedure to estimate appropriate noise levels in individual frequency components these noise levels can then be used to invert multiple frequencies simultaneously ztem data are insensitive to a 1 d conductivity structures and hence the background can be difficult to estimate we provide two methods to determine appropriate background models interestingly topography which is usually a hinderance in field data interpretation provides a first order signal in the ztem data and helps with this calibration

seasonal changes in groundwater input to a well mixed estuary estimated using radium isotopes and implications for coastal nutrient budgets 226ra half life 1 600 yr and 228ra half life 5 75 yr were used as tracers to determine seasonal changes in groundwater input to the pettaquamscutt estuary from june 1999 to june 2000 ra isotopes were observed to be in excess of activities due to input from streams rhode island sound water and particle desorption the source of excess 226ra and 228ra within the estuary is attributed to groundwater transport of ra derived from the weathering of bedrock within the pettaquamscutt watershed short lived 224ra half life 3 66 d was used to calculate an average residence time of the estuary of 8 4 d box models used to constrain seasonal changes in groundwater input to the estuary indicate highest groundwater input in the summer 6 4 20 l m 2 d 1 and lowest values in the winter 2 1 6 9 l m 2 d 1 these ra derived groundwater fluxes are similar to rates of aquifer recharge over the annual cycle using these ra derived groundwater fluxes the input of inorganic nitrogen and phosphorus from groundwater to the pettaquamscutt was estimated at 61 180 mmol m 2 yr 1 and 4 4 13 mmol m 2 yr 1 respectively

magnetotelluric observations across the juan de fuca subduction system in the emslab project a magnetotelluric mt transect has been obtained near latitude 45 n from the active juan de fuca spreading center across the subduction zone and cascades volcanic arc and into the back arc deschutes basin region this paper presents the mt data set and describes its major characteristics as they pertain to the resistivity of the subduction system in addition we discuss the measurement and processing procedures employed as well as important concerns in data interpretation broadband audiomagnetotelluric amt mt soundings approx 0 01 500 s period were collected on land with considerable redundancy in site location and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle fifteen long period mt recordings about 50 10 000 s on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers on the juan de fuca plate offshore 33 out of 39 sea floor instruments at 19 locations gave good results of these locations five magnetotelluric soundings plus two additional geomagnetic variation sites covering the period range 200 105 s approximately constitute the ocean bottom segment of our profile the feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30 50 s period this phase anomaly with a corresponding inflection in the apparent resistivity is continuous eastward from the seacoast and ends abruptly at the high cascades it signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle with the layer conductance decreasing eastward from the coast to a minimum under the coast range but increasing suddenly to the east of the central willamette basin the higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100 1000 s over the same distance the transverse electric mode apparent resistivity and phase on the land display a variety of three dimensional effects which make their interpretation difficult conversely both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge indicating that the measurements here are reflecting primarily the large scale tectonic structures of interest and are little disturbed by small near surface inhomogeneities the impedance data near the ridge are strongly suggestive of a low resistivity asthenosphere beneath resistive juan de fuca plate lithosphere approaching the coastline to the east both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

development of recent chronologies and evaluation of temporal variations in pb fluxes and sources in lake sediment and peat cores in a remote highly radiogenic environment cairngorm mountains scottish highlands the use of stable pb isotope analyses in conjunction with recent 210pb and anthropogenic radionuclide chronologies has become a well established method for evaluating historical trends in depositional fluxes and sources of atmospherically deposited pb using archival records in lake sediment or peat cores such studies rely upon i simple radioactive disequilibrium between unsupported 210pb and longer lived members of the 238u decay series and ii well defined values for the isotopic composition of contaminant pb and indigenous pb in the study area however areas of high natural radioactivity can present challenging environments for such studies with potential complications arising from more complex disequilibria in the 238u decay series and the occurrence at local or regional level of anomalous ill defined stable isotope ratios due to the presence of elevated levels of radiogenic pb results are presented here for a study of a sediment core from a freshwater lake loch einich in the high natural radioactivity area of the cairngorm mountains of scotland 238u decay series disequilibria revealed recent diagenetic re deposition of both u and 226ra the latter resulting in a requirement to use a modified calculation to derive a 210pb chronology for the core confidence in the chronology was provided by good agreement with the independent 241am chronology but the 137cs distribution was affected by significant post depositional mobility in the organic rich sediment the systematics of variations in 230th 232th and stable pb isotope ratio distributions were used to establish the indigenous pb characteristics of the sediment the relatively high radiogenic content of the indigenous pb resulted in complications in source apportionment in particular during the 20th century with multiple natural and anthropogenic sources precluding the use of a simple binary mixing model consequently 206pb 207pb ratios in scottish moss samples from an archive collection were used to provide the input term for atmospheric deposition in order to establish historical trends in indigenous and anthropogenic pb fluxes a test of the accuracy of the derived pb fluxes was provided by analysis of a core from a nearby blanket peat deposit great moss independent atmospheric and basal inputs gave a complex distribution of 210pb in the peat but this did not affect calculation of a 210pb chronology once again the 210pb chronology was supported by the 241am distribution temporal trends in anthropogenic pb deposition derived for the loch einich sediment core were in generally good agreement with those for the great moss peat core other peat cores and some other lake sediment cores from northern scotland providing confidence in the use of the archive moss data to characterise atmospheric deposition however sustained input of pb to loch einich sediment at relatively high levels in the late 20th century after the regional decline in atmospheric pb deposition suggested that catchment derived pb is now a significant component of the depositional flux for loch einich

chemical study of tectonically controlled hydrothermal dolomitization an example from the lessini mountains italy in the central southern alps feltre verona italy a 750 m thick interval of jurassic and cretaceous pelagic limestones shows post depositional partial brecciation and dolomitization the overlying 500 m thick tertiary sedimentary sequence is unaffected through the paleogene mostly submarine mafic volcanism has been documented in the area small scale extensional features were observed in the limestone near the contact with the dolomitized breccias their orientations measured in the field correspond to the tectonic framework of the area and give evidence for the contemporaneousness of the volcanic activity and the brecciation the distribution and petrologic characteristics of the basalts combined with the orientation of the extensional features observed in the field allow a paleotectonic reconstruction this tectonic scenario can be viewed as a back arc extension an eastward prolongation of the spreading that divided southern france from the sardinia corsica calabria block generating the ligurian balearic basin in the late oligocene

dolomitization of atolls by sea water convection flow test of a hypothesis at niue south pacific the core of the former atoll on niue island south pacific is extensively dolomitized a detailed stable isotope study reveals that the dolomites are uniformly enriched in 18o and 13c 18o 2 8 0 5 13c 2 0 0 3 relative to their carbonate precursor the sr isotope ratios are equally constant with depth yielding 87sr 86sr values between 0 70912 and 0 70916 the observed isotopic compositions point to sea water as the dominant dolomitizing fluid the sr age of the dolomites is younger than the biostratigraphic age mid to late miocene placing the dolomitization event at plio pleistocene time a model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water sea water is drawn through the atoll margin and transferred upward by convective flow delivering mg to the sites of dolomite precipitation transported with the sea water are volcano derived metals fe cu zn mn as evidenced by the chemical gradient in