crustal electrical conductivity in north central saskatchewan the north american central plains anomaly and its relation to a proterozoic plate margin seven recording magnetometers monitored time varying fields at points on a northwest southeast line 280 km long in north central saskatchewan during july 1981 the experiment was designed to test the hypothesis advanced in 1975 by alabi camfield and gough that the electrical conductivity anomaly in the north american central plains links with the wollaston domain in the exposed precambrian shield of saskatchewan from clear reversals in the phase of vertical variations it is evident that the conductor passes between two stations straddling the rottenstone la ronge magmatic belt to the immediate east of the wollaston domain enhanced horizontal variations transverse to the belt at a third intermediate station reinforce this interpretation vertical field response arrows obtained from daytime events in the period range 1 40 min clearly indicate the existence of a major conductor that extends to lower crustal depths beneath the belt to the northwest across the cree lake zone reversals in the directio

electrical conductivity of the continental crust geophysical measurements indicate that the earth s continental lower crust has a high electrical conductivity for which no simple cause has been found explanation usually relies on either saline fluids saturating the pores or interconnected highly conducting minerals such as graphite fe ti oxides and sulphides providing conducting pathways attempts in the laboratory to clarify the problem have hitherto been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270 c and 1000 c here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism s we have used the experimental data to construct electrical conductivity depth profiles for the continental crust which when compared with profiles obtained from magnetotelluric observations demonstrate that a mid to lower crust composed of amphibolite saturated with 0 5 m nacl shows electrical conductivities sufficient to explain conductivity depth profiles for the continental crust inferred from geophysical measurements

satellite and in situ observations of the bio optical signatures of two mesoscale eddies in the sargasso sea satellite ocean color imagery and field spectroradiometer observations are used to assess the bio optical signatures of two mesoscale features a cyclone c1 and an 181 water anticyclone a4 in the sargasso sea field determinations of upper layer bio optical properties such as the diffuse attenuation coefficient and remote sensing reflectance spectra show little statistically significant variations with distance to the eddy center for either eddy this contrasts field observations showing many fold higher phytoplankton pigment biomass at depth and for a4 higher primary production rates at depth than is typical for this region the cyclone c1 does show a significant decrease in the depth of the 1 photosynthetically available radiation par isolume with increasing distance from eddy center while the anticyclone a4 shows no coherent signal vs distance vertical profiles of bio optical properties show consistent patterns where subsurface maxima are displaced higher inside the core of the cyclone c1 than in the surrounding waters while the highest values of the diffuse attenuation coefficient at 443 nm are observed within the core of anticyclone a4 satellite observations of near surface bio optical properties show signals consistent with eddy physical characteristics although the magnitude of these variations is very small barely detectable by typical field measurement protocols mean values of bio optical properties are higher within the cyclone compared with its periphery but not for the anticyclone for both eddies significant inverse correlations are observed between time series of bio optical properties and eddy center sea level anomaly consistent response to wind speed is also noted following strong wind events bio optical parameters are elevated inside the anticyclone and are reduced inside the cyclone these observations demonstrate that a combination of physical processes including vertical eddy uplift eddy horizontal advection and eddy scale ekman pumping contribute to the bio optical imprint of mesoscale eddies the contributions of these forcing mechanisms change over the period of observation illustrating the limitations of inferring eddy bio optical dynamics from short term field observations the present analyses provide insights into the potential as well as the drawbacks of bio optical techniques for probing the biological and biogeochemical impacts of open ocean eddies

time scales of pattern evolution from cross spectrum analysis of advanced very high resolution radiometer and coastal zone color scanner imagery we have selected square subareas 110 km on a side from coastal zone color scanner czcs and advanced very high resolution radiometer avhrr images for 1981 in the california current region off northern california for which we could identify sequences of cloud free data over periods of days to weeks we applied a two dimensional fast fourier transform to images after median filtering x y plane removal and cosine tapering we formed autospectra and coherence spectra as functions of a scalar wavenumber coherence estimates between pairs of images were plotted against time separation between images for several wide wavenumber bands to provide a temporal lagged coherence function the temporal rate of loss of correlation decorrelation time scale in surface patterns provides a measure of the rate of pattern change or evolution as a function of spatial dimension we found that patterns evolved or lost correlation approximately twice as rapidly in upwelling jets as in the quieter regions between jets the rapid evolution of pigment patterns lifetime of about 1 week or less for scales of 50 100 km ought to hinder biomass transfer to zooplankton predators compared with phytoplankton patches that persist for longer times we found no significant differences between the statistics of czcs and avhrr images spectral shape or rate of decorrelation in addition in two of the three areas studied the peak correlation between avhrr and czcs images from the same area occurred at zero lag indicating that the patterns evolved simultaneously in the third area maximum coherence between thermal and pigment patterns occurred when pigment images lagged thermal images by 1 2 days mirroring the expected lag of high pigment behind low temperatures and high nutrients in recently upwelled water we conclude that in dynamic areas such as coastal upwelling systems the phytoplankton cells identified by pigment color patterns behave largely as passive scalars at the mesoscale and that growth death and sinking of phytoplankton collectively play at most a marginal role in determining the spectral statistics of the pigment patterns

electrical conductivity models for the continental crust based on laboratory measurements on high grade metamorphic rocks summary deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust however a lack of relevant laboratory based conductivity measurements makes interpretation of such soundings difficult laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0 4 gpa temperatures up to 300 c and with variable pore fluid pressure up to the confining pressure extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries the lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and or lower pore fluid pressures perhaps due to hydration reactions in areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure i e low to high with increasing depth seems more likely

geoelectromagnetic measurements across the southern senegal basin west africa abstract magnetotelluric and differential geomagnetic sounding surveys consisting of nine soundings were performed in 1984 along a 200 km profile across the southern senegal basin they were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents magnetotelluric data indicate that two dimensional resistivity models are appropriate for the region the zone above the basement is highly inhomogeneous in geoelectrical structure very conductive sediments 0 6 3 ohm m appear in the mesozoic cenozoic senegal basin these sediments lie at depths of up to 4500 m on the west end of the profile below this a modest resistivity material 10 30 ohm m extends to a maximum depth of about 3000 m the material at depth on the cast part of the traverse line is thought to be palaeozoic sediments of the bove basin the depth of the magnetotelluric basement lies between about 250 m in the east and 4800 m in the west the crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile considering the total section we observe a general trend towards lower resistivities at depths in excess of 100 km the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km an analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin it appears that the additional currents flowing in the striking direction of the senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well conducting sediments

electrical conductivity and carbon in metamorphic rocks of the yukon tanana terrane alaska electrical conductivity of a water saturated quartz mica garnet schist collected from a surface outcrop near the denali fault zone in the yukon tanana terrane of east central alaska increases slightly with pressure to about 200 mpa this behavior is unlike that exhibited by other yukon tanana samples or by most rocks from other locations detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 m thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation the stringer is probably responsible for the anomalous conductivity change with pressure making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature the carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast the deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism we interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer possibly during the main phase of metamorphism and deformation and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth i e 10 km brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer explaining in part why the rock does not exhibit anomalously high conductivity at 0 1 mpa 1 atm pressure the brittle deformation is interpreted to have been caused by unloading due to uplift the observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic throughgoing fractures

a magnetotelluric traverse across the mauritanides orogenic belt west africa summary magnetotelluric measurements with reference magnetometers for study of the anomalous geomagnetic variation field were made in the period range 10 10000 s at 10 sites crossing the main geological zones of eastern senegal west africa the results indicate that the electromagnetic fields are controlled by the dominant structural trends of the mauritanides orogenic belt apparent resistivities reflect the major structural units identified from surface geology data in the principal directions for most of the stations exhibit anisotropic behaviour and at certain sites the magnetotelluric responses are affected by surface distortion effects the study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton a lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust the quantitative interpretation of the magnetotelluric results is given in terms of 1 d and 2 d resistivity models it is shown that the data require the presence of a highly anomalous crustal structure 40 rm dipping to the east from a small depth beneath the outcropping margin of the west african craton in the mauritanides the crust is resistive 3000 rm t o a depth of 30 km and overlies a conducting layer of 10 km thick in the depth range 250 300 km resistivities decrease to less than 50 rm this may coincide with the top of the asthenosphere the geoelectric model is discussed in relation to the general geological background of the region considered

the methana volcano geothermal resource greece and its relationship to regional tectonics abstract geophysical methods of analysis were applied in order to investigate the deep structure and the geothermal potential of the methana volcano ne peloponnesus greece the study is based on a re evaluation and reinterpretation of legacy magnetotelluric mt data with modern analysis methods as well as 3 d inversion of aeromagnetic data constrained by in situ measurements of magnetic susceptibility magmatic systems are located in regions of active tectonic processes that often play a controlling role the mt method is effective in delineating low resistivity functional elements of volcanic systems such as magma chambers vents thermal fluid reservoirs and thermal fluid circulation conduits the latter two of which are typically associated with active faults the aeromagnetic data can assist in mapping the configuration hence emplacement modes of volcanic rocks at depth accordingly the joint interpretation of these lines of evidence together with structural and geochemical information is expected to allow insight into the influence of contemporary tectonics on the inception and evolution of the volcano the contemporary stress field is mainly extensional nne ssw oriented and overall homogeneous in the area of methana it allows for the formation of wnw ese north easterly dipping normal faults w e faults consistent with the synthetic dextral r shear direction of riedel s shear theory and nw se faults consistent with the antithetic sinistral r shear direction all such features have been mapped on methana peninsula the magnetotelluric data imaged a significant geothermal reservoir developing around an intersection of the three active fault zones normal r and r at depths of 1 1 5 km below the centre of the peninsula as well as elongate epiphenomenal conductivity anomalies associated with the circulation of thermal fluids along all three fault zones the 3 d magnetic susceptibility model strongly suggests that the intrusion and emplacement of magmas were guided by the same active fault zones with particular reference to the r and r shears whose influence is imprinted on the configuration of volcanic rocks at depth the joint interpretation of all lines of evidence indicates that magmatism and volcanism at methana are almost completely controlled by tectonic activity in a manner analogous to the situation of the large santorini volcanic complex it also indicates that the reservoir is replenished through the weak permeable zone created by the intersection of the r and r shears which is very probably collocated with the main vent of intrusive magmatic activity and may connect with a shallow magma chamber at depths greater than 4 5 km the apparently common origin and similarities differences in the circulation paths of thermal fluids may amply explain both the individual characteristics and similarities differences in the chemical composition of thermal spring discharges which have been reported by hitherto geochemical investigations

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

electrical resistivity of cajon pass cores at elevated pressures the electrical resistivity of rock cores recovered from the cajon pass hole ranged from 5 to 46 k m as determined at pressures to 200 mpa and frequencies of 10 30 000 hz the resistivities of cores recovered from depths less than 1 6 km are about one order of magnitude greater than the in situ resistivity as measured by the deep laterolog however at greater depths the laboratory resistivities and those from the deep laterolog are in fairly good agreement the results suggest that at these depths the fractures whether mineralized or fluid filled are no longer well interconnected

electrical imaging of narmada son lineament zone central india from magnetotellurics the narmada son lineament nsl zone is the second most important tectonic feature after himalayas in the indian geology magnetotelluric mt studies were carried out in the nsl zone along a 130 km long nne ssw trending profile the area of investigation extends from edlabad 20 46 16 7 5 59 05 in the south to khandwa 21 53 51 7 6 18 05 i n the north the data shows in general the validity of a two dimensional 2d approach besides providing details on the shallow crustal section the 2d modeling results resolved four high conductive zones extending from the middle to deep crust spatially coinciding with the major structural features in the area namely the gavligarh tapti barwani sukta and narmada south faults the model for the shallow section has brought out a moderately resistive layer 30 150 m representing the exposed deccan trap layer overlying a conductive layer 10 30 m inferred to be the subtrappean gondwana sediments the latter resting on a high resistive basement upper crust the deccan trap thickness varies from around a few hundred meters to as much as 1 5 km along the traverse a subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed the high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse the lower crustal segment is conductive over a major part of the profile considering the generally enhanced heat flow values in the nsl region coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from mt presence of zones of high density mafic bodies intrusives with fluids presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in nsl region are inferred 2004 elsevier b v all rights reserved

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

a catalogue of spanish archaeomagnetic data a total of 58 new archaeomagnetic directions has been determined from archaeological structures in spain together with five previous results they allow the compilation of the first archaeomagnetic catalogue for spain which includes 63 directions with ages ranging between the 2nd century bc and the 20th century ad characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization the hierarchical structure has been respected in the calculation of the mean site directions rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents the age estimate of the studied structures is generally well justified by archaeological constraints for six structures the proposed date is also supported by physical methods the data are in close agreement with the french secular variation sv curve this catalogue represents the first step in the construction of a sv curve for the iberian peninsula which will be of much use in archaeomagnetic dating and in modelling of the earth s magnetic field in western europe

electrical conductivity measurement of gneiss under mid to lower crustal p t conditions abstract we conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 k and a constant pressure of 1 gpa the analyzed gneisses were obtained from the higo metamorphic belt kyushu japan as the metamorphic conditions of these rocks have been well determined by previous studies we were able to select samples that were representative of the middle to lower crust prior to the conductivity measurements the samples were maintained at the maximum temperature for a long period until the electrical conductivity had stabilized our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 k conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window a plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations as observed by backscattered electron image bei and electron probe microanalysis epma we evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies our results are consistent with the conductivity values reported for other types of rocks we also compared the experiment results with data derived from electromagnetic em soundings electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central kyushu japan

electrical resistivity structure of the senegal basin as determined from magnetotelluric and differential geomagnetic soundings summary the methods of magnetotelluric mt and differential geomagnetic soundings dgs have been applied to study the electromagnetic response of the structure of the senegal sedimentary basin the measurements at 10 sites were carried out along a profile running perpendicularly across a zone of north south flexures and faults over the period range 10 1000 s variation of periods up to 10000 s were obtained at two sites the anomalous geomagnetic variation field across the major fault zone is characterized by an increase in the magnetic eastward component two dimensional modelling of the apparent resistivities and phases reveals a highly anomalous upper crustal structure involving a low resistivity zone in the central part of the basin less than 30 am to satisfy the additional long period data at two sites a deep conductor is also required at a depth below 300 km with a resistivity less than 10 am the crustal discontinuity located 200 km off the coast is possibly related to the opening of the atlantic ocean the difficulty in resolving the question of the nature of the crust in this region results in part from the limitations of the methods used at present for this determination

archaeomagnetic evidence for the age of a roman pottery kiln from calahorra spain summary the first report of a palaeomagnetic study of a roman pottery kiln is presented individual bricks from the walls of the heating chamber have been sampled and measured in order to determine their palaeomagnetic directions the measurements indicate that the innermost part of the structure was heated well above the curie point of haematite enabling the use of the palaeomagnetic declination and inclination after the axial dipole correction and fitting to the paris i t curve to infer the age of the last heating event

deep geothermal structure and mantle heat flow along the barents sea east alps geotraverse the deep geothermal structure of the continental crust along the barents sea eastern alps profile is investigated numerical geothermal models are constructed the mantle heat flows and the moho temperatures are computed for different tectonic blocks of the crust within ancient platform areas the mantle heat flow varies from 15 28 mw m 2 in the baltic shield to 6 14 mw m 2 in the stable russian plate in the baltic syncline the mantle heat flow is about 25 33 mw m 2 the maximum mantle heat flow value of 40 m w m 2 characterizes the central european platform mantle heat flow values vary from 15 to 35 mw m 2 in the hercynian part of central europe for the thermal model the seismic velocity data are used the moho temperature values vary from 400 c in the ancient platform area to 900 1000 c in the alpine zone in the northern baltic shield tm 600 c

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain

photosynthetic community responses to upwelling in mesoscale eddies in the subtropical north atlantic and pacific oceans in this paper we summarise the photo physiological responses of phytoplankton to upwelling of macronutrients in mesoscale eddies in the subtropical north atlantic eddies project sargasso sea and subtropical north pacific e flux project hawaii the observations obtained on two sets of cruises over 2 years occupied six cyclonic eddies and two mode water eddies the photosynthetic physiological parameters were measured using a bench top fluorescence induction and relaxation fire system and a submersible in situ fast repetition rate fluorometer frrf deployed on an undulating towed vehicle both of these instruments were used to provide highly sensitive and well resolved data on community responses the responses are dependent on both the type of eddy and its stage of development our results indicate that while cyclonic eddies in the atlantic and pacific can increase primary photosynthetic production early in their development mode water eddies in the subtropical north atlantic can support patchy blooms of large diatoms for long periods of time more than 3 months

a new contribution to the archaeomagnetic study of a roman pottery kiln from calahorra spain summary results from archaeomagnetic research on a roman pottery kiln are presented in a previous paper pares et al 1992 a collection of samples from the kiln s heating chamber was studied and palaeodirection results were obtained in the same paper dating was performed on the basis of the secular variation reference curve of the inclination at paris in the present study the palaeointensity was evaluated on the basis of the same collection of samples the mean value obtained is 60 9pt a comparison of this value with other available archaeointensity values obtained from neighbouring territories confirms the date estimated by pares et al 1992kthe middle of the 1st century ad

new chemical bio optical and physical observations of upper ocean response to the passage of a mesoscale eddy off bermuda a mesoscale eddy advected across the bermuda testbed mooring site over a 30 day period centered on july 14 1995 temperature and current measurements along with biogeochemical measurements were used to characterize the biological response of the upper ocean associated with the introduction of nitrate into the euphotic layer due to the doming of isotherms associated with the eddy complementary shipboard data showed an anomalous water mass which extended from a depth of 50 to 1000 m manifesting as a cold surface expression and warm anomaly at depth although mesoscale eddies are frequently observed in the sargasso sea the present observations are particularly unique because of the high temporal resolution measurements of the new instrumentation deployed on the mooring analyzers that measure nitrate plus nitrite were placed at depths of 80 and 200 m and bio optical sensors were located at depths of 20 35 45 71 and 86 m peak nitrate values of nearly 3 0 mm at 80 m and chlorophyll a values of 1 4 mg m 23 at 71 m were observed as well as a 25 to 30 meter shoaling of the 1 light level depth a doppler shift from the inertial period 22 8 hours to 25 2 hours was observed in several time series records due to the movement of the eddy across the mooring inertial pumping brought cold nutrient rich waters farther into the euphotic zone than would occur solely by isothermal lifting silicic acid was depleted to undetectable levels owing to the growth of diatoms within the eddy the chlorophyll a values associated with the eddy appear to be the largest recorded during the 8 years of the ongoing u s jgofs bermuda atlantic time series study bats program

the significance of terrestrial electrical conductivity variations the degree to which different materials of the earth can conduct electricity varies enormously from the insulating qualities of cold quartz rich crys talline rock to the metallic behavior of the liquid core a well recognized physical property the specific resistance or resistivity expresses quanti tatively the resistance to current flow and the fact that it can be sensed through measurements on the earth s surface makes it an important geophysical parameter the history of the investigation of resistivity is similar to that of other geophysical quantities such as seismic velocity when it was recognized that the earth as a whole was in some degree a conductor the initial aim was to determine a global average value of resistivity this was followed by the search for a function describing the radial variation while the latter effort is by no means completed a great deal of recent work has been concerned with departures from the average behavior and that aspect forms the basis of this paper lateral variations in electrical resistivity may provide information on the composition and water content of the crust and on the temperature and degree of partial melting in zones of the mantle the success of any investigation depends on the adequacy of electromagnetic measurements at the earth s surface the ability to mathematically infer a distribution of resistivities on the basis of induction theory and by no means least the knowledge through laboratory mea surements of electrical properties of earth materials under different phys ical conditions recent progress in each of these aspects is considered first a word as to units resistivity is conveniently expressed in ohm meters qm the resistance of a meter cube of the material as an indication

magnetotellurics along the fennoscandian long range profile summary estimates of magnetotelluric transfer functions along the fennoscandian long range fennolora profile are presented and discussed in relation to the major lithotectonic subdivisions of the swedish part of the baltic shield none of the transfer functions are found to be consistent with a one dimensional earth structure thereby making a quantitative interpretation difficult however the geological units are found to display characteristic differences in the obtained transfer functions and in some areas a quantitative interpretation can be justified the most interesting structure indicated is a low resistive 4 am crustal structure of lateral extension more than 150 km centred around the skellefte ore district a minimum thickness for this structure of 15 km is inferred

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

carbonaceous films in midcrustal rocks from the ktb borehole germany as characterized by time of flight secondary ion mass spectrometry 1 amphibolites from 4 6 and 9 1 km depth in the kontinentales tiefbohrprogramm der bundesrepublik deutschland ktb borehole have developed extensive interconnected networks of carbonaceous films on cracks and grain boundaries as determined by electron probe and time of flight secondary ion mass spectrometry in the sample from 9 1 km depth the carbon is dominantly to completely elemental in contrast the carbonaceous matter in the samples from 4 6 km depth comprises a mixture of elemental carbon and simple hydrocarbons such as alkanes and possibly c o h compounds the microcracks within the 4 6 km sample also contain a retrograde micro assemblage consisting of ferri oxy hydroxide calcite and possibly clay minerals suggesting that the carbonaceous matter and retrograde minerals formed together at relatively high crustal levels and at a time much later than the peak metamorphism because the carbon films likely influence the electrical conductivity of the rocks in situ it is proposed that production of hydrocarbons during retrograde metamorphism of grain boundary and microcrack carbon tends to increase the resistivity of the rocks this chemical destruction of the interconnectivity of electrical pathways may contribute to the observed diminished conductivity of the shallow crust relative in relation to thto the deep crust

the subtropical nutrient spiral 1 we present an extended series of observations and more comprehensive analysis of a tracer based measure of new production in the sargasso sea near bermuda using the 3 he flux gauge technique the estimated annually averaged nitrate flux of 0 84 0 26 mol m 2 yr 1 constitutes only that nitrate physically transported to the euphotic zone not nitrogen from biological sources e g nitrogen fixation or zooplankton migration we show that the flux estimate is quantitatively consistent with other observations including decade timescale evolution of the 3 h 3 he inventory in the main thermocline and export production estimates however we argue that the flux cannot be supplied in the long term by local diapycnal or isopycnal processes these considerations lead us to propose a three dimensional pathway whereby nutrients remineralized within the main thermocline are returned to the seasonally accessible layers within the subtropical gyre we describe this mechanism which we call the nutrient spiral as a sequence of steps where 1 nutrient rich thermocline waters are entrained into the gulf stream 2 enhanced diapycnal mixing moves nutrients upward onto lighter densities 3 detrainment and enhanced isopycnal mixing injects these waters into the seasonally accessible layer of the gyre recirculation region and 4 the nutrients become available to biota via eddy heaving and wintertime convection the spiral is closed when nutrients are utilized exported and then remineralized within the thermocline we present evidence regarding the characteristics of the spiral and discuss some implications of its operation within the biogeochemical cycle of the subtropical ocean index terms 4808 oceanography biological and chemical chemical tracers 4845 oceanography biological and chemical nutrients and nutrient cycling 4805 oceanography biological and chemical biogeochemical cycles 1615 4820 oceanography biological and chemical gases keywords nutrients productivity tritium helium 3

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

effect of water on the electrical conductivity of lower crustal clinopyroxene 1 the electrical conductivity of lower crustal clinopyroxene was measured at 6 12 kbar 250 1000 c and ni nio buffer conditions the dependence of electrical conductivity on water content was studied using both natural and preannealed samples separated from a fresh xenolith granulite with water contents from 0 to 375 ppm an end loaded piston cylinder apparatus and a solarton 1260 impedance gain phase analyzer were used in the study over a frequency range of 0 01 106 hz to obtain the complex impedance spectra the results show that the influence of pressure is very weak relative to temperature and water content and that two distinct mechanisms with different activation enthalpies dominate electrical conduction under dry and wet conditions for the dry sample the activation enthalpy is 102 kj mol and the main charge carriers are small polarons i e hopping of electron holes between ferrous and ferric irons for wet samples the electrical conductivity is significantly enhanced with an activation enthalpy of 70 kj mol and the charge carriers are likely to be protons under hydrous conditions the activation enthalpies are nearly independent of water content and the conductivity is a function of water content with an exponent of 1 as a major constituent of granulites with 60 modal volume in some regions clinopyroxene containing minor amounts of water may contribute significantly to the high electrical conductivity in the lower crust especially under stable continental regions

covariation of mesoscale ocean color and sea surface temperature patterns in the sargasso sea during the lifetime of the coastal zone color scanner there were 21 instances in which both satellite derived ocean color and sea surface temperature are simultaneously available over large areas of the sargasso sea these images reveal close correspondence between mesoscale structures observed in temperature and pigment fields in general higher lower pigment biomass occurs in mesoscale features consisting of cold warm temperature anomalies this relationship is consistent with the idea that upward displacement of isopycnals at the base of the euphotic zone by mesoscale eddies is an important mechanism of nutrient supply in the region

a census of eddies observed in north atlantic sofar float data abstract sofar floats that looped in discrete eddies were studied in order to map and describe the distribution and characteristics of eddies in the north atlantic one hundred eighteen individual looping float trajectories loopers were identified each consisting of two or more consecutive loops each looper was interpreted to be in a discrete eddy and its characteristics were estimated from the float trajectory the highest percentage of loopers occured at 700m in the newfoundland basin where roughly half of the float data were in loopers mostly cyclones in the gulf stream region approximately 20 of the float data recorded at 700m were in loopers again mostly cyclones overall 21 of 700m data and 6 of 2000m data were in loopers the fastest swirl speeds 40cm s 1 were in cyclones south of the gulf stream most of which were in gulf stream rings but numerous swift 35cm s 1 anticyclones were found there too swirl velocity decreased with depth to roughly half as swift at 2000m as at 700m for three eddies in the sargasso sea measured simultaneously with floats at these two depths in the western north atlantic the average swirl velocity of cyclones and anticyclones was the same translation velocity of eddies was generally westward to southwestward at a few cm s 1 the mean translation velocity of 39 eddies in the sargasso sea was u bar 2 8 0 4 cm s 1 v bar 0 4 0 4 cm s 1 many of these were located just south of the gulf stream in the region of its recirculation and were probably advected by a westward current there near the gulf stream and along its extension in the newfoundland basin eddies were often advected downstream with speeds up to 15 20cm s 1 and eddy trajectories were often complicated south of 30 n and near the western boundary 700m eddies were advected northwestward and 1300m and 2000m eddies southeastward by boundary currents there numerous energetic anticyclones were observed south of the gulf stream one was tracked for 430 days and its properties well measured the formation of these eddies has not been documented but they are inferred to have formed near and by the gulf stream and to consist of a thick layer of 18 c water lying above a depression in the thermocline analogous anticyclones were observed in the newfoundland basin seaward of the gulf stream extension there

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

lower crustal petrology from wide angle p and s wave measurements in the black forest abstract the lower crust of the black forest in southwest germany is strongly reflective for compressional waves but almost non reflective for shear waves if the reflectivity for p waves is caused by alternating high and low velocity rocks the s wave observations require that the low velocity rocks have a low poisson s ratio and vice versa a purely compositional petrological model for lower crustal rocks can fit the observed p and s wave velocity data provided a rather high quartz content 50 is acceptable for the lower crust

dynamical interpolation of mesoscale flows in the topex poseidon diamond surrounding the u s joint global ocean flux study bermuda atlantic time series study site an open boundary ocean model is configured in a domain bounded by the four topex poseidon t p ground tracks surrounding the u s joint global ocean flux study bermuda atlantic time series study bats site this implementation facilitates prescription of model boundary conditions directly from altimetric measurements both t p and ers 2 the expected error characteristics for a domain of this size with periodically updated boundary conditions are established with idealized numerical experiments using simulated data a hindcast simulation is then constructed using actual altimetric observations during the period october 1992 through september 1998 quantitative evaluation of the simulation suggests significant skill the correlation coefficient between predicted sea level anomaly and ers observations in the model interior is 0 89 that for predicted versus observed dynamic height anomaly based on hydrography at the bats site is 0 73 comparison with the idealized experiments suggests that the main source of error in the hindcast is temporal undersampling of the boundary conditions the hindcast simulation described herein provides a basis for retrospective analysis of bats observations in the context of the mesoscale eddy field

electrical conductivity measurement of granulite under mid to lower crustal pressure temperature conditions summary we have developed a technique to measure electrical conductivity of crustal rocks with relatively low conductivity and complicated mineral components in order to compare with results given by magneto telluric mt measurements a granulite from hidaka metamorphic belt hmb in hokkaido japan at high temperature and pressure conditions was obtained the granulite sample was ground and sintered under the conditions similar to those of mid to lower crust we have observed smooth and reversible change of conductivity with temperature up to about 900 k at 1 gpa the results were consistent with the electrical conductivity structures suggested by the mt data analysis considering pore fluid conduction mechanism or the role of accessory minerals in the rock the mechanisms of electrical conductivity paths in dry or basic rocks should be reconsidered

paleointensities of phonolitic obsidian influence of emplacement rotations and devitrification 1 a paleomagnetic study on phonolitic obsidian from six sites in tenerife spain is presented two sites are located at the 750 ka el pasajiron at the southern wall of las canadas caldera four sites correspond to the 115 17 bce montana blanca complex paleointensity determinations are performed with a modified thellier technique using checks for alteration and domain state additionally the anisotropy of the thermoremanence and the magnetic cooling rate dependency of each specimen are measured it was not possible to obtain good quality paleointensity data for el pasajiron thermal alteration was observed in most measurements systematic changes in rock magnetic properties and paleointensities within the profile of one site also suggest the presence of a chemical remanence crm which was probably acquired during devitrifiction of the upper part of the flow this crm cannot be seen in the arai plots themselves but only by comparison of data from different samples this underlines the importance of sampling only fresh glassy looking obsidians and not devitrified ones paleointensity determinations at montana blanca are of very good quality leading to an high success rate of 82 samples at one site show acquisition of trm during emplacement rotations however it could be shown that paleointensity data of rotated samples are reliable if the remanence carriers are in the single domain range an atrm anisotropy tensor of thermoremanent magnetization and cooling rate corrected paleointensity of 48 4 2 1 t and a vdm of 9 64 0 42 1022am2 were determined which are in very good agreement with other data from spain and morocco and to various geomagnetic field models