satellite remote sensing of surface oceanic fronts in coastal waters off west central florida abstract two algorithms designed to detect deepwater oceanic features and arbitrary edge profiles were tuned to automatically delineate fronts in coastal waters off west central florida using satellite derived sea surface temperature sst chlorophyll a concentration chl normalized water leaving radiance nlw and fluorescence line height flh images during select periods in the spring and fall of 2004 and 2005 the dates correspond to recreational king mackerel scomberomorus cavalla tournaments a histogram based algorithm was useful to detect coastal surface sst nlw and flh fronts specifically a gradient based algorithm with a smaller kernel box of 3 3 pixels best identified nearshore

the lappajarvi meteorite crater finland petrography rb sr major and trace element geochemistry of the impact melt and basement rocks abstract impact melt lithologies of the 77 m y old finnish meteorite crater lappajarvi as well as the precambrian target rocks have been studied in detail to identify and characterize different impact melt types clast poor clast rich suevitic melt and to study their chemical major and trace elements and isotopic rb sr compositions in comparison to the composition of the target rocks the rb sr system of the whole melt body including the suevitic melt is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks chemical interactions exchange of alkali elements 87sr redistribution between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt exchange reactions between clasts and melt are determined by thermal activation but the degree of shock metamorphism in the clasts plays an important role too major and trace element distributions in impact melt and basement rocks indicate that the lappajarvi melt body chemically is extremely homogeneous even volatile elements such as zn and cu were not strongly fractionated comparison of the abundances of siderophile elements in the impact melt e g 118 177 ppm cr 195 340 ppm ni 6 12 ppb ir and calculated target rock mixture 79 mica schist 11 granite pegmatite 10 amphibolite e g 85 6 ppm cr 54 8 ppm ni 0 5 ppb ir revealed the chondritic nature c or h chondritic of the meteoritic projectile less than 2 of the meteorite can be detected in the coherent melt whereas the suevitic melt is uncontaminated by the projectile

mesoscale frontal structures in the canary upwelling system new front and filament detection algorithms applied to spatial and temporal patterns abstract an improved automatic detection of mesoscale frontal activity is proposed based on the edge detection algorithm initially presented by cayula and cornillon 1992 the performance and the sensitivity of their original method have been extensively tested on a very large modis sst data set at 1 km resolution over the canary upwelling system and compared to the results of a classic gradient based method its evaluation including the companion method using a contour following algorithm shows that a significant part of the fronts was not detected by the earlier method despite an overall robustness we propose here an improved implementation of the single image edge detection algorithm from the best combination of multiple detections based on sliding windows referred to as cmw the results show a very significant increase of the overall performance using a time series of 6 years 1 988 images we observed an average increase of 140 in the edge detection and 30 improvement in the average length of the segments additionally the sensitivity to the size of the detection window is lower and the necessity of using the following algorithm is greatly reduced we applied the cmw improvements to the canary current system and showed frontal activity developed along the whole coast with more intense fronts between cape ghir 30 30 n and cape beddouza 32 30 n and between cape juby 28 n and cape bojador 26 30 n the highest filament activity was associated with cape bojador followed by cape ghir and cape blanco 21 n considering the seasonality two main areas were identified north and south of cape juby with marked seasonality in the fronts and filaments no major interannual differences in frontal activity were observed in 2002 2007 except less seasonality during 2007

magnetic investigations of the west hawk deep bay and clearwater impact structures canada due to the effects of erosion tectonism and burial impact structures are often obscured or destroyed geophysical methods are increasingly being used in detecting the signatures of impact structures while gravity lows associated with impact structures are well understood associated magnetic anomaly lows are not in this study drill cores from three canadian impact structures were analyzed for rock magnetic properties and mineralogy in order to explain the magnetic anomaly lows associated with these structures samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility ams and natural remanent magnetization nrm parameters drill cores from the twin impact craters of the clearwater structure exhibited different nrm characteristics and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques the difference noted in their nrm characteristics was attributed to the acquisition of a viscous remanent magnetization vrm at depth in clearwater east at all three structures both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias melt rocks shocked crystalline rocks and in postimpact sedimentary infill the processes of brecciation alteration shock and infill by nonmagnetic sediments contribute to the development of the magnetic lows however a significant contribution to the observed magnetic anomalies was found by first order forward modelling to arise from basement rocks beneath the impact structures this zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact induced transient stress wave traveling away from the point of impact

geophysical research on the kardla impact structure hiiumaa island estonia the 4 km wide and 500 m deep circular kardla impact structure in hiiumaa island estonia of middle ordovician age 455 ma is buried under upper ordovician and quaternary sediments to constrain the geophysical models of the structure petrophysical properties such as magnetic susceptibility natural remanent magnetization nrm density electrical conductivity porosity and p wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area the results were used to interpret the central gravity anomaly of 3 mgal and the magnetic anomaly of 100 nt and also the surrounding weak positive anomalies revealed by high precision survey data the unshocked granitic rocks outside the structure have a mean density of 2630 kgm 3 their shocked counterparts have densities of 2400 kgm 3 at a depth of 500 m increasing up to 2550 kgm 3 at a depth of 850 m porosity and electrical conductivity decrease but p wave velocity increases as density increases away from the impact point thus the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for kardla as in many other impact structures the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks a new geophysical and geological model of the kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks together with structural data from numerous boreholes an important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores

geophysical and magneto structural study of the maâdna structure talemzane algeria insights on its age and origin the ma dna structure is located approximately 400 km south of algiers 33 19 n 4 19 e and emplaced in upper cretaceous to eocene limestones although accepted as an impact crater on the basis of alleged observations of shock diagnostic features such as planar deformation features pdfs in quartz grains previous works were limited and further studies are desirable to ascertain the structure formation process and its age for this purpose the crater was investigated using a multidisciplinary approach including field observations detailed cartography of the different geological and structural units geophysical surveys anisotropy of magnetic susceptibility paleomagnetism and petrography of the collected samples we found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies ones that might indicate the occurrence of a causative material which is at least in part identical geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface using soil magnetic susceptibility measurements the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite a primary magnetization is inferred from a stable remanence with both normal and reverse directions carried by these two minerals although this is supposed to be a chemical remagnetization its normal polarity nature is considered to be a pliocene component subsequent to the crater formation the pole falls onto the miocene pliocene part of the african apparent polar wander path apwp consequently we estimate then formation of the ma adna crater to have occurred during the time period extending from the late miocene to the early pliocene unfortunately our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones nor shocked minerals were found a dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis which can still be considered as plausible only a drilling may provide a definite answer

induced seismicity of the groningen gas field history and recent developments abstract induced seismicity of the groningen gas field is caused by the production of gas because of the large areal extent of the reservoir the long history of depletion and the available data sets which exist as a result of consequences and public unrest caused by induced seismicity the field presents a valuable case for studying the relationships among geologic flow dynamic geomechanical and seismological models gas production from the groningen field started in 1963 induced seismicity of the field first was recorded in 1991 ml 2 4 during the subsequent 10 years induced seismicity stayed at a rate of about five events ml 1 5 per year starting in 2003 the number of events and magnitudes started to increase in 2012 the largest event ml 3 6 occurred which caused the most damage to date as a consequence studies carried out in 2013 have fundamentally changed the way to look at the relationship between induced seismicity and gas depletion there appears to be a close link between

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

the lappajarvi impact structure finland age duration of crater cooling and implications for early life abstract an in depth approach of 40 ar 39 ar dating of the 23 km lappajarvi impact structure finland was performed using carefully selected single grain aliquots of optically fresh clast poor impact melt rock and recrystallized k feldspar melt particles separated from impact metamorphosed granite pegmatite step heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven k feldspar samples the melt rocks yielded fully concordant ages with a weighted mean of 76 37 0 46 ma dating of the k feldspar melt particles resulted in a series of younger plateau ages ranging from 75 11 0 36 to 76 11 0 35 ma the melt rock results combined with the oldest syn melt rock k feldspar age and including all sources of uncertainties yielded a weighted mean age of 76 20 0 29 ma 2 mswd 1 02 p 0 41 which is interpreted to represent the best estimate age of the lappajarvi impact the age spread of at least 1 1 0 5 ma between the crystallization of the impact melt rocks and the k feldspar melt particles warns against the use of single 40 ar 39 ar analysis to derive a meaningful impact age in turn the age offset monitors both rapid cooling of the lappajarvi melt sheet versus slow cooling and impact induced hydrothermal circulation within the crater basement most likely driven by long lasting heat flow from the hot central uplift of the impact structure this interpretation is in line with the grain size dependent argon diffusion parameters and apparent closure temperatures of 230 410 c determined for various domain sizes of k feldspars observed in this study our study shows that even in comparatively small impact craters post impact hydrothermal activity can be estimated to last between 600 ka and 1 6 ma and is technically resolved by means of the 40 ar 39 ar dating technique therefore the longevity of hydrothermal systems in medium sized impact craters might be an order of magnitude longer than previously estimated prolonged post impact heat flow in medium sized craters in analogy to lappajarvi might have played an important role in the emergence of life on early earth and possibly mars

a seismological model for earthquakes induced by fluid extraction from a subsurface reservoir a seismological model is developed for earthquakes induced by subsurface reservoir volume changes the approach is based on the work of kostrov 1974 and mcgarr 1976 linking total strain to the summed seismic moment in an earthquake catalog we refer to the fraction of the total strain expressed as seismic moment as the strain partitioning function a probability distribution for total seismic moment as a function of time is derived from an evolving earthquake catalog the moment distribution is taken to be a pareto sum distribution with confidence bounds estimated using approximations given by zaliapin et al 2005 in this way available seismic moment is expressed in terms of reservoir volume change and hence compaction in the case of a depleting reservoir the pareto sum distribution for moment and the pareto distribution underpinning the gutenberg richter law are sampled using monte carlo methods to simulate synthetic earthquake catalogs for subsequent estimation of seismic ground motion hazard we demonstrate the method by applying it to the groningen gas field a compaction model for the field calibrated using various geodetic data allows reservoir strain due to gas extraction to be expressed as a function of both spatial position and time since the start of production fitting with a generalized logistic function gives an empirical expression for the dependence of on reservoir compaction probability density maps for earthquake event locations can then be calculated from the compaction maps predicted seismic moment is shown to be strongly dependent on planned gas production

the subtropical front satellite observations during fronts 80 satellite derived sea surface temperature in situ hydrography data and satellite tracked drifter data collected during winter 1979 1980 as part of the fronts 80 experiment are used to describe the spatial structure of the north pacific subtropical front northeast of hawaii the satellite observations reveal a deformation scale meander 85 km 32 km in association with the front having an evolution time scale of 60 days the satellite measurements of the change in sea surface temperature across the front are typically 1 c within a distance of 10 km the correlation coefficients between the satellite data and the in situ hydrographic data were computed and found to be significantly nonzero above 800 m the high correlation at deep levels indicates that the satellite was viewing a feature with sizable vertical extent not just a surface phenomenon additionally the hydrographic data and drifter trajectories were combined with the satellite derived sea surface temperature data together they demonstrate that the feature observed by the satellite was nearly stationary during the experiment period in addition they also indicate that the flow along the front was nearly in geostrophic balance

paleomagnetism of impact breccias from the chicxulub crater implications for ejecta emplacement and hydrothermal processes abstract results of a paleomagnetic study of 89 samples from impact breccias from the chicxulub crater are presented and used to investigate on ejecta emplacement hydrothermal and alteration processes the impactite sequence sampled in cores from yaxcopoil 1 borehole is 100 m thick and formed by six subunits characterized by petrographic scan images geochemical and textural analyses which point to different emplacement modes impact occurred within reverse polarity chron c29r therefore magnetization acquired at crater formation and shortly thereafter will present reverse polarity which is supported by paleomagnetic measurements on melt samples from yucatan 6 borehole results after step wise thermal and alternating field demagnetization document a pattern of 23 upward and 29 downward inclinations at yaxcopoil 1 around 33 and 38 respectively measurements of magnetic hysteresis and variation of low field susceptibility with temperature suggest low ti titanomagnetites and magnetite as main magnetic carriers petrographic observations indicate the presence of fine grained magnetite hematite and fe oxyhydroxides related to hydrothermal alteration processes curie points are in the range of 520 580 c the wide range of unblocking temperature spectra points to variable grain sizes of a magnetic phase that unblocks around 580 c hysteresis ratio plots indicate most samples fall in the pseudo single domain field in some samples wasp waist constrained hysteresis loops suggest magnetite and hematite analyses of vector plots and coercivity and unblocking temperature spectra do not show apparent differences within and between subunits subunits are characterized by distinct textural and compositional differences in size type and relative abundance of clasts and melt rich or carbonate rich matrix types bulk properties vary with composition and clast contents particularly within the middle suevite and brecciated melt rock subunits that show higher values while low values characterize the lower suevite carbonate rich with rare basement clasts subunit magnetization acquisition mechanisms are discussed with reference to impactite characteristics and may relate to remagnetization from hydrothermal and post impact alteration processes

ion microprobe uranium lead dating of zircons from the lappajarvi impact crater western finland the lake lappajarvi impact crater lies in paleoproterozoic svecofennian metasedimentary rocks on the western side of the central finland granitoid complex 1 9 ga two conflicting ages have been reported for the meteorite impact an age of 77 3 0 4 ma on the basis of ar ar whole rock data from impact melt samples and a paleomagnetic age of 195 ma during studies on impact crater indicator minerals at lappajarvi zircons with an atypical appearance were found in suevite boulders these zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe u pb dating allowing a new and independent age estimate for the impact event at lappajarvi four spot analyses on two black coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1 8 ga for the source rock seventeen analyses were done on three dull zircon grains showing patchy impact related partial recrystallization most of these data fell fairly well on a single discordia line with intercept ages of 73 3 2 5 3 ma and 1854 2 5 1 ma however five of the data spots near the lower intercept end fell on the younger side of the line this was interpreted to indicate post impact loss of lead importantly the new ion microprobe u pb age of 73 3 f 5 3 ma is in a very good agreement with the previously reported ar ar age

detection of mesoscale thermal fronts from 4km data using smoothing techniques gradient based fronts classification and basin scale application abstract in order to optimize frontal detection in sea surface temperature fields at 4 km resolution a combined statistical and expert based approach is applied to test different spatial smoothing of the data prior to the detection process fronts are usually detected at 1 km resolution using the histogram based single image edge detection sied algorithm developed by cayula and cornillon in 1992 with a standard preliminary smoothing using a median filter and a 3 3 pixel kernel here detections are performed in three study regions off morocco the mozambique channel and north western australia and across the indian ocean basin using the combination of multiple windows cmw method developed by nieto demarcq and mcclatchie in 2012 which improves on the original cayula and cornillon algorithm detections at 4 km and 1 km resolution are compared fronts are divided into two intensity classes weak and strong according to their thermal gradient a preliminary smoothing is applied prior to the detection using different convolutions three type of filters median average and gaussian combined with four kernel sizes 3 3 5 5 7 7 and 9 9 pixels and three detection window sizes 16 16 24 24 and 32 32 pixels to test the effect of these smoothing combinations on reducing the background noise of the data and therefore on improving the frontal detection the performance of the combinations on 4 km data are evaluated using two criteria detection efficiency and front length we find that the optimal combination of preliminary smoothing parameters in enhancing detection efficiency and preserving front length includes a median filter a 16 16 pixel window size and a 5 5 pixel kernel for strong fronts and a 7 7 pixel kernel for weak fronts results show an improvement in detection performance from largest to smallest window size of 71 for strong fronts and 120 for weak fronts despite the small window used 16 16 pixels the length of the fronts has been preserved relative to that found with 1 km data this optimal preliminary smoothing and the cmw detection algorithm on 4 km sea surface temperature data are then used to describe the spatial distribution of the monthly frequencies of occurrence for both strong and weak fronts across the indian ocean basin in general strong fronts are observed in coastal areas whereas weak fronts with some seasonal exceptions are mainly located in the open ocean this study shows that adequate noise reduction achieved by a preliminary smoothing of the data considerably improves the frontal detection efficiency as well as the global quality of the results consequently the use of 4 km data enables frontal detections similar to 1 km data using a standard median 3 3 convolution in terms of detectability length and location this method is easily applicable to large regions or at the global scale with far less constraints of data manipulation and processing time using 4 km data relative to 1 km data

frequency of ring separations from the loop current in the gulf of mexico a revised estimate the most energetic events in the circulation of the gulf of mexico are the separation of large anticyclonic rings from the loop current building on previous work the authors examine all the apparent rings since july 1973 this new dataset includes the satellite altimetry since 1992 providing a set of 34 consecutive ring formations the primary advantage of altimetry is that the data remain available in the summer one finding is that the ambiguity of whether or not a ring has separated is reduced but not eliminated the uncertainty with which separation events can be specified remains approximately 4 weeks even with nearly continuous data primary peaks in the distribution of separation intervals are found at 6 and 11 months with a smaller peak at 9 months if the spectrum is smoothed heavily enough a peak in the distribution can be formed nearer 12 months but this near annual peak is a result more of the smoothing than of the data

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

shock experiments in range of 10 45 gpa with small multidomain magnetite in porous targets physical properties of multidomain magnetite bearing porous pellets shocked up to 45 gpa were measured the results show general magnetic softening as a result of shock however a relative magnetic hardening trend and slight magnetic susceptibility decrease is observed with increasing pressure among shocked samples initially the shock also seems to cause a slight decrease in porosity but at higher shock pressures macroscopic porosity increases progressively in our pellets the microscopic porosity remains almost unchanged since our samples have distinctly higher initial porosity compared with samples used in previous studies our results may be representative for impacts into highly porous magnetite bearing sedimentary or volcanic rocks and are relevant to impacts into such target rocks on earth and mars

paleomagnetic and rock magnetic study of the mistastin lake impact structure labrador canada implications for geomagnetic perturbation and shock effects abstract we carried out an integrated rock magnetic and paleomagnetic study of the 36 ma mistastin lake labrador canada meteorite impact structure in order to investigate whether energy from the collision influenced the geodynamo and to assess the effects of shock on the magnetic properties of the target basement rocks stepwise demagnetization of 114 specimens isolates a well defined magnetization component throughout the crater whose overall mean deviates slightly from the expected direction for north america at the time of impact paleointensity results from seven samples meeting stringent selection criteria show no significant difference with a global compilation from 40 to 30 ma the combined results including those from a 80 m thick profile of an impact melt unit discovery hill lend no support that the impact caused an aberration of the geodynamo within a few centuries of a bolide collision that created the 28 km diameter crater both titanium rich and titanium poor titanomagnetite carry the magnetic remanence in the impact melt rocks their relative proportions compositions and domain states are cooling rate dependent magnetic hysteresis parameters of the magnetite bearing anorthositic basement rocks reveal systematic changes as a function of distance from the crater s center with an increasing prevalence of single domain like grains toward the center changes with radial distance are also found in the character of the verwey transition in magnetite basement rocks were thermally overprinted when lying less than a meter from the impact melt rocks mesoproterozoic basement rocks more than a meter below the impact melt rocks hold similar magnetization directions to those expected from a 1500 ma result for laurentia no evidence exists that shock heating of the basement rocks exceeded 200 c at distances of 6 7 km from the crater s center

seasonal variability of sea surface temperature fronts on georges bank abstract sea surface temperature sst fronts extracted from archived 1985 1996 advanced very high resolution radiometer avhrr data are determined in the georges bank region resulting frontal locations clearly show the tidal mixing front on georges bank the probability of detecting an sst front at an individual pixel on georges bank is presented as a monthly climatology variations in the probability position and associated water depth of sst fronts in the georges bank region show the development of the tidal mixing front during the summer as well as the presence of a persistent front near the shelfbreak from the 12 yr time series of fronts 144 individual monthly frontal probability images are decomposed into the mean frontal position and empirical orthogonal functions eof using singular value decomposition svd spatial and temporal information from the first eof mode reveals the seasonal development of the tidal mixing front spatial patterns associated with the first mode also reveal the winter occurrence of fronts on the northeast peak area of georges bank which is most likely associated with scotian shelf water flow across the northeast channel the second mode also indicates the development of the hot spot often observed over georges bank during the late summer and early fall further evidence indicates that the mean position of detected sst fronts on southern georges bank changes approximately 10 km on bank from late spring to mid summer monthly mean positions are also compared to the distribution of simpson and hunter s 1974 tidal mixing parameter with resultant positions compared to historical stratification at those locations comparison of detected sst fronts to historical hydrographic data on georges bank points to the fronts being located in increasingly stratified water from may through august

the transition zone chlorophyll front a dynamic global feature defining migration and forage habitat for marine resources abstract pelagic ecosystem dynamics on all temporal scales may be driven by the dynamics of very specialized oceanic habitats one such habitat is the basin wide chlorophyll front located at the boundary between the low chlorophyll subtropical gyres and the high chlorophyll subarctic gyres global satellite maps of surface chlorophyll clearly show this feature in all oceans in the north pacific the front is over 8000 km long and seasonally migrates north and south about 1000 km in the winter this front is located at about 30 35 n latitude and in the summer at about 40 45 n it is a zone of surface convergence where cool vertically mixed high chlorophyll surface water on the north side sinks beneath warm stratified low chlorophyll water on the south side satellite telemetry data on movements of loggerhead turtles and detailed fisheries data for albacore tuna show that both apex predators travel along this front as they migrate across the north pacific the front is easily monitored with ocean color satellite remote sensing a change in the position of the tzcf between 1997 and 1998 appears to have altered the spatial distribution of loggerhead turtles the position and dynamics of the front varied substantially between the 1998 el nino and the 1999 la nina for example from may to july 1999 the transition zone chlorophyll front tzcf remained between about 35 n and 40 n latitude showing very little meandering whereas in 1998 during the same period the tzcf exhibited considerable meandering and greater monthly latitudinal movement catch rates for albacore were considerably higher in 1998 than in 1999 and we hypothesize that a meandering tzcf creates regions of convergence which enhances the foraging habitat for apex predators along the front

lonar lake palaeomagnetic evidence of shock origin summary palaeomagnetic and rock magnetic investigations were carried out on oriented rock samples collected from the inner walls of lonar lake which is believed to be a meteorite crater these studies show systematic variation in some magnetic parameters q jn k and declination whereas at surrounding localities these parameters show random variation a soft secondary shock component acquired in the present field of d 9 i 47 k 47 6 1 6 4 has been successfully isolated by vector difference analysis whereas the stable primary vector is similar to that of lower tertiary period reverse magnetization nrm and irm of the upper and lower flows respond differently to af demagnetization as a function of their ha it is shown that the nrm carried by the lower flows is similar to that of irm probably reset by the meteorite impact because of their low h carriers it is suggested that such magnetic property variations are characteristic of shock origin and hence we propose an impact origin for lonar lake

seasonal modulation of the west florida continental shelf circulation velocity data from 28 n 84 w on the west florida continental shelf are presented the data were sampled from october 1993 through january 1995 at 1 m intervals between 3 m and 42 m in a total water depth of 47 m their monthly means suggest an annual cycle hypothesized to be driven by a seasonally varying shelf wide baroclinic structure motions at semi diurnal diurnal and synoptic time scales are seasonally modulated both by wind forcing and stratification that decouples fluid motions from the damping effects of bottom friction these motions are presented in the form of progressive vector plots with 16 months of data probability density and distribution functions for along shore and across shore particle displacements over specific time intervals are constructed for daily intervals particles are equally likely to travel approximately 5 km depending upon confidence interval in any direction for monthly intervals particles may travel a few hundred km primarily along shore

petrography and shock related remagnetization of pyrrhotite in drill cores from the bosumtwi impact crater drilling project ghana rock magnetic and magnetic mineralogy data are presented from the international continental scientific drilling program icdp drill cores lb 07a and lb 08a of the bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact related remagnetization mechanism basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided magnetic susceptibility 150 500 10 6 si and natural remanent magnetization 10 3 10 1 a m are generally weak but locally higher values up to 10 6 10 3 si and 43 a m occur sixty three percent of the investigated rock specimens show q values above 1 indicating that remanence clearly dominates over induced magnetization which is a typical feature of impact structures ferrimagnetic pyrrhotite is the main magnetite phase which occurs besides minor magnetite and a magnetic phase with a curie temperature between 330 and 350 c interpreted as anomalous pyrrhotite coercive forces are between 20 and 40 mt brecciation and fracturing of pyrrhotite is a common feature confirming its pre impact origin grain sizes of pyrrhotite show a large variation but the numerous stress induced nanostructures observable by transmission electron microscopy tem are assumed to behave as single domain grains we suggest that the drilled rocks lost their pre shock remanence memory during the shock event and acquired a new stable remanence during shock induced grain size reduction the observed brittle microstructures indicate temperatures not higher than 250 c which is below the curie temperature of ferrimagnetic pyrrhotite 310 c

rock magnetic and paleomagnetic study of the keurusselka impact structure central finland abstract there are 31 proven impact structures in fennoscandia one of the most densely crater populated areas of the earth the recently discovered keurusselka impact structure 62 08 n 24 37 e is located within the central finland granitoid complex which formed 1890 1860 ma ago during the svecofennian orogeny it is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism e g planar deformation features in quartz new petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented the suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility the presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization four different remanent magnetization directions were isolated 1 a characteristic svecofennian target rock component a with a mean direction of d 334 8 i 45 6 95 14 9 yielding a pole plat 51 1 plon 241 9 a95 15 1 2 component b d 42 4 i 64 1 95 8 4 yielding a pole plat 61 0 plon 129 1 a95 10 6 3 component c d 159 5 i 65 4 95 10 7 yielding a pole plat 21 0 plon 39 3 a95 15 6 and 4 component e d 275 5 i 62 0 95 14 4 yielding a pole plat 39 7 plon 314 3 a95 19 7 components c and e are considered much younger possibly neoproterozoic overprints compared with the components a and b the pole of component b corresponds with the 1120 ma pole of salla diabase dyke and is in agreement with the 40ar 39ar age of 1140 ma from a pseudotachylitic breccia vein in a central part of the structure therefore component b could be related to the impact and thus represent the impact age

local and deep ocean forcing contributions to anomalous water properties on the west florida shelf 1 material property distributions on continental shelves result from the mixing and modifications of estuarine and deep ocean source waters how this occurs depends on the momentum and buoyancy that are input either locally on the shelf or from the deep ocean at the shelf break we address this question of local versus deep ocean forcing for the west florida shelf wfs using in situ data and a numerical circulation model the spring and summer seasons of 1998 and 1999 show distinctively different water properties on the shelf and at the shelf break we account for these differences by a combination of local forcing independent of the adjacent gulf of mexico loop current and interactions of the loop current with the shelf the primary role of the deep ocean is to set the height of material isopleths along the shelf slope whether or not these material isopleths broach the shelf break is then a consequence of local shelf wide wind and buoyancy forcing the subsequent along and across shelf distributions are accomplished through a combination of local and deep ocean effects with the bottom ekman layer being the major conduit for the across shelf transport of ecologically important materials

sea surface temperature fronts in the california current system from geostationary satellite observations 1 sea surface temperature sst fronts are determined for the 2001 2004 time period from geostationary operational environmental satellites goes data in the california current system ccs the probability of detecting a sst front at an individual pixel location in the ccs is presented as a bi monthly climatology fronts clearly indicate the seasonal evolution of coastal upwelling as well as meanders and filaments that are often linked with irregularities in coastline geometry winter is characterized by low frontal activity along the entire coast fronts first appear close to the coast during spring particularly south of cape blanco where upwelling favorable winds are already persistent the area of high frontal activity continues to increase during summer especially between monterey bay and cape blanco extending more than 300 km from the coast the region with high frontal activity widens at 2 6 km day 1 off northern baja california a band with persistent fronts is found close to the coast year round but there is no evidence of a seasonal widening of the area of higher activity during fall the weakening of upwelling favorable winds leads to a gradual decrease in frontal activity an empirical orthogonal function eof decomposition reveals the development of sst fronts associated with seasonal upwelling for locations north of monterey bay with less summer intensification to the south the first appearance of fronts close to the coast during spring and the occurrence of the fronts offshore later in the season are represented by additional statistically significant eof modes

geophysical signature of the tunnunik impact structure northwest territories canada in 2011 the discovery of shatter cones confirmed the 28 km diameter tunnunik complex impact structure northwest territories canada this study presents the first results of ground based electromagnetic gravimetric and magnetic surveys over this impact structure its central area is characterized by a 10 km wide negative gravity anomaly of about 3 mgal amplitude roughly corresponding to the area of shatter cones and associated with a positive magnetic field anomaly of 120 nt amplitude and 3 km wavelength the latter correlates well with the location of the deepest uplifted strata an impact tilted proterozoic dolomite layer of the shaler supergroup exposed near the center of the structure and intruded by dolerite dykes locally electromagnetic field data unveil a conductive superficial formation which corresponds to an 80 100 m thick sand layer covering the impact structure based on measurements of magnetic properties of rock samples we model the source of the magnetic anomaly as the magnetic sediments of the shaler supergroup combined with a core of uplifted crystalline basement with enhanced magnetization more classically the low gravity signature is attributed to a reduction in density measured on the brecciated target rocks and to the isolated sand formations however the present day fractured zone does not extend deeper than 1 km in our model indicating a possible 1 5 km of erosion since the time of impact about 430 ma ago

paleomagnetism and petrophysics of the janisjarvi impact structure russian karelia paleomagnetic rock magnetic and petrophysical results are presented for impactites and target rocks from the lake janisjarvi impact structure russian karelia the impactites tagamites suevites and lithic breccias are characterized by increased porosity and magnetization which is in agreement with observations performed at other impact structures thermomagnetic hysteresis and scanning electron microscope sem analysis document the presence of primary multidomain titanomagnetite with additional secondary titanomaghemite and ilmenohematite the characteristic impact related remanent magnetization chrm direction d 101 5 i 73 1 95 6 2 yields a pole lat 45 0 n long 76 9 e dp 9 9 dm 11 0 additionally the same component is observed as an overprint on some rocks located in the vicinity of the structure which provides proofs of its primary origin an attempt was made to determine the ancient geomagnetic field intensity seven reliable results were obtained yielding an ancient intensity of 68 7 7 6 t corresponding to vdm of 10 3 1 1 x 10 22 am 2 the intensity however appears to be biased toward high values mainly because of the concave shape of the arai diagrams the new paleomagnetic data and published isotopic ages for the structure are in disagreement according to well defined paleomagnetic data two possible ages for magnetization of jnisjrvi rocks exist 1 late sveconorwegian age 900 850 myr or 2 late cambrian age 500 myr however published isotopic ages are 718 5 myr k ar and 698 22 myr 39ar 40ar but such isotopic dating methods are often ambiguous for the impactites

intrusion of loop current waters onto the west florida continental shelf an intrusion of loop current water up desoto canyon and onto the west florida continental shelf to within 8 km of the shore occurred in february 1977 boat aircraft and satellite data collected in the area for another purpose were used to estimate the space and time scales of the intrusion and the ultimate fate of the intruded waters the duration of the event was 18 days oceanic waters advanced across the shelf at speeds of 20 cm s sup 1 at maximum intrusion 6650 km sup 2 of shelf were affected approximately half the intruded water receded off the shelf and half appears to have been modified in situ

the role of combined laser scanning and video techniques in monitoring wave by wave swash zone processes abstract simulating swash zone morphodynamics remains one of the major weaknesses of beach evolution models one of the reasons is the limited availability of data on morphological changes at the temporal scales of individual swash events this paper sets out to present a new hybrid system consisting of 2d 3d laser scanners and several video cameras which was designed to monitor swash zone topographic change on a wave by wave basis a methodology is proposed consisting of sensor calibration and several data processing steps allowing a fusion of different sensors such an approach can improve the performance of several field laboratory optical technique applications for nearshore hydro and morpho dynamic measurements digital elevation models from a 3d scanner were used in the extrinsic camera calibration procedure and reduced the geo rectification errors from 0 035 m dz 10 cm the sediment transport direction and intensity of the monitored swash events was relatively balanced and sediment transport rates ranged between 3 5 kg m 1 s 1 q t 3 5 kg m 1 s 1 extreme transport swash events became rarer as the morphology was reaching equilibrium

paleomagnetic and rock magnetic study of the yaxcopoil 1 impact breccia sequence chicxulub impact crater mexico results of a detailed paleomagnetic and rock magnetic study of samples of the impact breccia sequence cored in the yaxcopoil 1 yax 1 borehole between abou 800 m and 896 m are presented the yax 1 breccia sequence occurs from 794 63 m to 894 94 m and consists of rerdeposited melt rich clast size sorted fine grained suevites melt rich no clast size sorting medium grained suevites coarse suevitic melt agglomerates coarse melt rich heterogeneous suevites brecciated susevites and coarse carbonate and silicate melt suevites the low field susceptibility ranges from 0 3 to 40187 x 10 6 si and the nrm intensity ranges from 0 02 ma m up to 37510 ma m in general the nrm intensity and magnetic susceptibility present wide rangesand are positively correlated pointing to varying magnetic mineral contents and textures of the melt rich breccia sequence the vectorial composition and magnetic stability of nrm were investigated by both stepwise alternating field and thermal demagnetization in most cases characteristic single component magnetizations are observed both upward and downward inclinations are present through the sequence and we interpret the reverse magnetization as the primary component in the breccias both the clasts and matrix forming the breccia appear to have been subjected to a wide range of temperature pressure conditions and show distinct rock magnetic properties an extended interval of remanence acquisition and secondary partial or total remagnetization may explain the paleomagnetic results

the anomalous demagnetization of pyrrhotite summary the anomalous behaviour of pyrrhotite bearing rocks from the pennine orefield northern england on alternating field demagnetization is studied with particular reference to rotational remanent magnetization rrm using a heat treated sample it is demonstrated that a two axis tumbling demagnetizer is capable of producing a repeatable rrm along its inner axis of rotation which is comparable in strength to that produced in an instrument which uses single axis rotation and which is specifically designed to produce a rrm the unheated samples gave a scattered rrm which was of sufficient intensity to disturb the primary magnetization after heating the rrm become reproducible much larger and parallel to the rotation vector for rotation rates between 1 5 and 120 rev s 1 for an af frequency of 50 hz this behaviour is unlike all previously measured samples whether oxides or sulphides by other workers the source of the anomalous rrm and the change in rrm with heating are discussed with reference to the physical structure and the strong magnetocrystalline anisotropy in pyrrhotite

tides on the west florida shelf the principal semidiurnal m2 and s2 and diurnal k1 and o1 tidal constituents are described on the west florida continental shelf wfs using a combination of in situ measurements and a three dimensional primitive equation numerical model the measurements are of sea level and currents along the coastline and across the shelf respectively the model extends from west of the mississippi river to the florida keys with an open boundary arcing between it is along this open boundary that the regional model is forced by a global tide model standard barotropic tidal analyses are performed for both the data and the model and quantifiable metrics are provided for comparison based on these comparisons the authors present coamplitude and cophase charts for sea level and velocity hodographs for currents the semidiurnal constituents show marked spatial variability whereas the diurnal constituents are spatially more uniform apalachicola bay is a demarcation point for the semidiurnal tides that are well developed to the southeast along the wfs but are minimal to the west the largest semidiurnal tides are in the florida big bend and florida bay regions with a relative minimum in between just to the south of tampa bay these spatial distributions may be explained on the basis of local geometry a lagrangian stokes drift coherently directed toward the northwest is identified but is of relatively small magnitude when compared with the potential for particle transport by seasonal and synoptic scale forcing bottom stressinduced tidal mixing is examined and estimates are made of the bottom logarithmic layer height by the m2 tidal currents

variability in chlorophyll and sea surface temperature fronts in the long island sound outflow region from satellite observations 1 the spatial and temporal patterns of ocean color i e chlorophyll and sea surface temperature sst fronts in the coastal zone of the northwest atlantic were studied over a 2 year period chlorophyll images were derived from the sea viewing wide field of view sensor while sst data were obtained from the advanced very high resolution radiometer both flying onboard earth observing satellites an edge detection algorithm was applied to each of the image sets to identify chlorophyll and thermal fronts persistent chlorophyll and thermal fronts co occurred in the same vicinity of the inner middle shelf and were spatially congruent during second april may june and third july august september quarters and least coincident during the first january february march calculations of the frontogenesis function showed strongest frontogenic tendencies coincident with the chlorophyll and thermal fronts a major finding of this study was the congruence of both chlorophyll and thermal fronts at a time when the coastal current jet was the most robust strongly suggesting that it is the presence of this jet that may be a driving force in establishing these fronts except for a few months per year no clear chlorophyll enhancement was observed at thermal fronts

the influence of loop current perturbations on the formation and evolution of tortugas eddies in the southern straits of florida large cyclonic eddies on the northern edge of the florida current are the dominant mesoscale features within the southern straits of florida the most prominent of these features is a quasi stationary eddy that forms near the dry tortugas our observations compiled from 3 years of advanced very high resolution radiometer measurements in the straits of florida and gulf of mexico demonstrate a strong relationship between the generation of anticyclonic rings from the gulf of mexico loop current and the evolution of tortugas eddies within the southern straits of florida in six cases tortugas eddies evolve from cyclonic frontal eddies which form along the boundary of the loop current the eddies remain stationary near the dry tortugas until they are impacted by an approaching loop current frontal eddy the length of time an eddy spends near the dry tortugas is increased when the loop current sheds an anticyclonic ring the involvement of a loop current frontal eddy in the ring shedding process results in a delay in its and hence the tortugas eddy s downstream propagation results suggest that the lifetime of a tortugas eddy can be as long as 140 days when a ring shedding event occurs or as short as 50 days in the absence of any ring shedding events upon entering the straits of florida the tortugas eddies are deformed by the narrowing topography and shrink to approximately 55 of their original size as they propagated downstream the shrinking of these eddies is accompanied by an accelerated translation from 5 km d in the western straits of florida to 16 km d in the east

crustal evolution of fennoscandia palaeomagnetic constraints abstract palaeomagnetic poles from fennoscandia ranging in age from archaean to tertiary are compiled and graded using a modified briden duff classification scale an new filtering technique is applied to select only the most reliable poles for analysis the filtering takes into account the following information 1 1 source block of rock unit 2 2 age of rock 3 3 age of magnetization component 4 4 scatter of palaeomagnetic directions 5 5 information from multicomponent analysis of natural remanent magnetization nrm 6 6 whether the pole considered belongs to a cluster or subcluster of poles 7 7 magnetic polarity and 8 8 the author s original assignment of results data are still insufficient for the drawing of separate apparent polar wander paths apwp for different blocks or cratons of fennoscandia treating fennoscandia as a single plate a new apwp from archaean to permian is constructed from the five previously drawn apwp loops or hairpins only one the jatulian loop 2200 2000 ma disappears in filtering the loops during 1925 1700 ma and during 1100 800 ma ago are linked to svecofennian and sveconorwegian orogenies respectively palaeomagnetic data support the concept that these orogenies took place episodically three distinct orogenic pulses early middle and late can be distinguished in the cluster plots of palaeopoles the drift history of fennoscandia from archaean to permian is presented during most of geological history fennoscandia has occupied low to moderate latitudes and undergone considerable latitudinal shifts and rotations the svecofennian and sveconorwegian orogenies have different kinematic characteristics during the svecofennian orogeny fennoscandia drifted slowly while rotating a large amount in an anticlockwise sense during the sveconorwegian orogeny it drifted rapidly and rotated first clockwise and then anticlockwise the most striking feature in the drift velocity curves is however the pronounced maxima in the latitudinal drift and rotation rates 9 cm yr and 0 8 ma respectively during the late subjotnian jotnian anorogenic magmatism and rifting phase 1450 1250 ma ago possibly reflecting the passage of fennoscandia across a thermal upwelling hotspot at equatorial latitudes the use of palaeomagnetism in delineating and dating movements between blocks is demonstrated with three examples from the polar profile area the northernmost section of the european geotraverse

edge detection applied to satellite imagery of the oceans a computer edge detection algorithm for automatic delineation of mesoscale structure in digital satellite ir infrared images of the ocean is developed the popular derivative based edge operators are shown to be too sensitive to edge fine structure and to weak gradients to be useful in this application the edge detection algorithm is based on the gray level cooccurrence matrix glc which is commonly used in image texture analysis the cluster shade texture measure derived from the glc matrix is found to be an excellent edge detector that exhibits the characteristic of fine structure rejection while retaining edge sharpness this characteristic is highly desirable for analyzing oceanographic satellite images

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events