geoid and topography for infinite prandtl number convection in a spherical shell geoid anomalies and surface and lower boundary topographies are calculated for numerically generated thermal convection in an infinite prandtl number boussinesq axisymmetric spherical fluid shell with constant gravity and viscosity calculations were done for heating entirely from below hfb and entirely from within hfw convective solutions were obtained for rayleigh numbers ra up to 20 times the critical ra in the hfb case and 27 times critical in the hfw case bifurcations that occur with varying ra are associated with different convective patterns boundary topography is positive at upwellings and negative at downwellings absolute topography is greater at the poles than at the equator due to poleward compression of the flow field lower boundary topography is greater than at the surface due to radial compression of the flow field the geoid is primarily dominated by the gravitational potential of surface masses due to surface boundary deformations hence the geoid parallels surface undulations in general boundary deformation increases with increasing cell wavelength this property may elucidate whether core mantle boundary cmb topography is due to mantle convection or mantle dregs chemically dense material which are only indirectly of dynamic origin and hence are unlikely to have the same wavelength dependence in its associated topography the ratios of boundary deformation and geoid to td coefficient of thermal expansion t characteristic temperature drop across the fluid shell and d shell thickness maximize with increasing ra at approximately 10 times ra critical and then steadily decrease out to the maximum ra calculated dimensionless geoid and topography in the hfb case are approximately 5 times greater than in the hfw case at the highest rayleigh numbers investigated dimensionless peak to trough amplitudes of geoid and topography are weak functions of ra which lends some justification to estimating the dimensional values of these quantities with earthlike parameters for parameter values characteristic of the earth s mantle the geoid and upper and lower boundary topograpies are approximately 1 km 10 km and 15 km for the hfb case and approximately 200 m 2 km and 3 km for the hfw case respectively the hfw values correlate with geophysical data more closely than the hfb values suggesting a predominance of internal heating in the mantle although this result is qualfied by the low rayleigh numbers and axisymmetry of the model calculations our study emphasizes that dynamically induced topography and geoid are sensitive to the mode of heating in the earth s mantle

banktop responses to quaternary fluctuations in sea level recorded in periplatform sediments periplatform sediment from a 12 m core recovered from northwest providence channel bahamas contains a geochemical and paleontological record of sea surface temperatures ice volumes and the response of banktops to highstands of sea level a comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported sr rich aragonite derived from banktop orgiuiisms superposed on a background of sr poor aragonite pteropods and calcite foraminifera and coccoliths derived from planktonic sources the pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes indicating decreased ice volume and rising sea level following these abrupt changes is a more gradual decline in bank derived sediment although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist we suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow water carbonate environments offbank transport is restricted as reefs sand shoals and islands reach sea level also green algal production may decline because progradation of tidal flats decreases lagoon area and the hydrologic and ecologic conditions change as lagoons are filled

compositional variations in calciturbidites due to sea level fluctuations late quaternary bahamas piston cores in tongue of the ocean bahamas revealed an alternating sequence of periplatform ooze and bankderived turbidites of the past 5 glacial and interglacial periods by using the point count method we have analyzed the composition of the turbidite sediment variations in the bank derived fraction are clearly linked to the glacial interglacial cycles nonskeletal components pellets ooids grapestones are most abundant in interglacial turbidites while skeletal components calcareous algae peneroplids and reef builders dominate in glacial turbidites we attribute this pattern to the exposure and flooding of the surrounding carbonate platforms during late quaternary sea level cycles because nonskeletal sediment is produced in the interior of the platforms its growth and subsequent export to the flanks are at a maximum when the banks are flooded i e during interglacial periods during glacials skeletal grains dominate because the banks are exposed and carbonate production is limited to a narrow belt of skeletal sands and fringing reefs the resulting compositional signal appeared to have a good correlation with aragonite variations in the periplatform ooze between the turbidites further compositional variation parallels the change in turbidite frequency described earlier highstand bundling frequency and composition of turbidites are both unaffected by diagenesis and therefore provide an excellent monitor of sealevel fluctuations

the carbonate signal and calcareous nannofossil distribution in an upper jurassic section balingen tieringen late oxfordian southern germany total abundance assemblage composition and the stratigraphic distribution of the six most abundant species of calcareous nannofossil were analysed in the upper oxfordian succession of balingen tieringen western swabian alb south germany this locality represents a deep shelf less than 200 m environment that is strongly influenced by its proximity to the jura shallow platform nannofossil abundance negatively correlates with carbonate content as well as with accumulation rate calcareous nannofossils thus did not produce the bulk of carbonates and their abundance was essentially controlled by the import of allochthonous carbonate mud gradual increase in carbonate content in the balingen tieringen succession can be related to an intensified carbonate mud export from the jura shallow platform to the swabian alb deep shelf during the late oxfordian changes in the nannofossil assemblage composition along the profile are interpreted as primary although a partial control by diagenesis on the assemblage can not be completely excluded the coccoliths lotharingius hauffii watznaueria britannica cyclagelosphaera margereli watznaueria barnesae watznaueria manivitae and the incertae sedis schizosphaerella spp display their highest relative abundance one after the other in a stratigraphic order this nannofossil succession parallels the gradual increase in allochthonous carbonate mud accumulation on the deep shelf as well as the gradual decrease in the intensity of siliciclastic input recorded on the jura shallow platform this suggests a climatic control by humidity on shallow platform carbonate production and export basinwards and on nutrient availability in deep shelf surface waters that itself controlled the nannofossil assemblage composition therefore the succession of highest relative abundance of l hauffii w britannica c margereli w barnesae w manivitae and schizosphaerella spp can represent a trophic preference continuum from more eutrophic to oligotrophic conditions

sediment flux across the great barrier reef shelf to the queensland trough over the last 300 ky the continental margin off northeast australia comprising the great barrier reef gbr platform and queensland trough is the largest tropical mixed siliciclastic carbonate depositional system in existence we describe a suite of 35 piston cores and two ocean drilling program odp sites from a 130 240 km rectangular area of the queensland trough the slope and basin setting east of the central gbr platform oxygen isotope records physical property magnetic susceptibility and greyscale logs analyses of bulk carbonate content and radiocarbon ages at these locations are used to construct a high resolution stratigraphy this information is used to quantify mass accumulation rates mars for siliciclastic and carbonate sediments accumulating in the queensland trough over the last 31 000 years for the slope highest mars of siliciclastic sediment occur during transgression 1 0 million tonnes per year mt yr 21 and lowest mars of siliciclastic 0 1 mt yr 21 and carbonate 0 2 mt yr 21 sediment occur during sea level lowstand carbonate mars are similar to siliciclastic mars for transgression and highstand 1 1 1 4 mt yr 21 in contrast for the basin mars of siliciclastic 0 0 1 mt yr 21 and carbonate sediment 0 2 0 4 mt yr 21 are continuously low and within a factor of two for lowstand transgression and highstand generic models for carbonate margins predict that maximum and minimum carbonate mars on the slope will occur during highstand and lowstand respectively conversely most models for siliciclastic margins suggest maximum and minimum siliciclastic mars will occur during lowstand and transgression respectively although carbonate mars in the queensland trough are similar to those predicted for carbonate depositional systems siliciclastic mars are the opposite given uniform siliciclastic mars in the basin through time we conclude that terrigenous material is stored on the shelf during sea level lowstand and released to the slope during transgression as wave driven currents transport shelf sediment offshore q 2000 elsevier science b v all rights reserved

an analysis of geophysical experiments to test newton s law of gravity signals reported as evidence for a non newtonian fifth force at a north carolina television tower and elsewhere can be explained in a conventional way by postulating small density variations underground the assumptions employed in earlier analyses which pointed to a failure of the inverse square law are examined and found to be difficult to justify

gravity estimation from stage a satellite to satellite tracking mission an analysis was performed to determine the accuracy of estimating gravity anomalies using satellite to satellite tracking between the space transportation system nasa s space shuttle and the constellation of gps satellites stage shuttle gps tracking for anomalous gravitation estimation second time derivatives of the observed ranges obtained from carrier phase measurements between the low and high satellites provide in situ line of sight components of gravitation as long as on board accelerometers are used to correct for the nongravitational signals absolute nondifferential single difference and double difference modes of tracking the gps satellites were considered in the analysis which showed that the last yields the highest gravity estimation accuracy with instrumentation already or soon to be available it is anticipated that the gravitational acceleration at altitude can be measured to an accuracy of about 0 3 mgal 1 mgal 10 5 m s2 1 g for an integration time of 60 s double difference acceleration data collected by the shuttle at 300 km altitude could depending on mission duration and data processing techniques yield estimates of 2 mean gravity anomalies on the earth s surface to an accuracy of about 5 mgal accuracy in 1 mean anomalies of about 4 5 mgal would be achievable using a free flyer at 160 km altitude and assuming a potentially feasible measurement accuracy of 0 1 mgal 30 s integration time

ammonoid and nannoplankton abundance in valanginian early cretaceous limestone marl successions from the southeast france basin carbonate dilution or productivity a quantification of calcareous nannofossil and ammonoid absolute abundance was carried out in the valanginian limestone marl alternations of the vergol section drome located in the epicontinental vocontian basin southeast france this section is characterised by four centimetric organic carbon rich layers barrande layers b1 b4 the aim of this work is to reconstruct changes in the distribution of nutrients in the water column on the base of the analysis of fossil organisms at different levels of the food chain calcareous nannofossil abundance correlates negatively with carbonate content thus calcareous nannoplankton probably did not produce the bulk of carbonates in the valanginian of the studied section nannofossil abundance was more likely controlled by carbonate mud produced and exported from shallow platform environments to the basin variations of carbonate export controlled by climate and hydrodynamics are responsible for the limestone marl alternations the levels of highest abundance of nannoplankton are interpreted as mainly linked to a reduced accumulation rate and to a minor extent to variations in primary production a dilution by allochthonous carbonate mud is further evidenced by the relationship between total organic carbon toc and calcium carbonate caco3 toc content decreases with increasing caco3 content except for one of the organic carbon rich layers b4 both taphonomy and dilution factors may have partly controlled the variations in macrofauna abundance these can be more likely interpreted as a response to palaeoenvironmental changes of the various taxa with different palaeoecological affinities the high abundance and dominance of bochianitids orthocone ammonoids in the layer b4 can be explained by their opportunistic behaviour and their deep nektonic possibly nektobenthic mode of life their great capacity for vertical mobility may have permitted brief feeding incursions to the bottom waters when the trophic resources were higher in depth than in the surface waters high trophic levels at the sea bottom may have also benefited the bivalves and gastropods which are very abundant in layer b4 the mode of life of different ammonoid families is also discussed

contrasting facies patterns in subtropical and temperate continental slope sediments inferences from east australian late quaternary records abstract studies of latest quaternary continental slope sediments at two localities on the east australian margin have revealed markedly different responses to late quaternary sea level fluctuations offshore of noosa in the sub tropics the sediment is predominantly a mixture of fine metastable carbonate siliciclastic material and pelagic carbonate important features of the stratigraphy include a siliciclastic dominated facies deposited relatively slowly during the last glacial lowstand sedimentation rate 8 cm ka and a calcareous facies rich in metastable carbonate deposited more rapidly during the late post glacial transgression sedimentation rates 15 24 cm ka highstand and transgressive sedimentation rates are greater than lowstand rates by a factor of 2 5 6 due to increased shelf carbonate productivity after flooding of the mid shelf off sydney in temperate latitudes continental slope sediment is largely a mixture of fine siliciclastic material and pelagic carbonate mean sedimentation rates range from 2 to 5 cm ka over the last four oxygen isotope stages with mean glacial interstadial rates higher than holocene rates by a factor of 1 36 this largely reflects the transfer of siliciclastic mud from the shelf to the slope during sea level regression in both localities facies changes on the slope are not related to specific sea level states e g lowstand facies transgressive facies etc but reflect instead the interaction of changing sea level with shelf morphology

strokes of upward illumination occurring within a few milliseconds after typical lightning return strokes 1 correlated data for natural cloud to ground lightning flashes were obtained with a high speed video camera 54000 frames per second five stations of electric field change antennas and a seven station vlf lf lightning network linet along with operational lightning and electric field networks at kennedy space center florida four observed flashes at 25 55 km distance had return stroke like upward illumination ui from the ground just after within 2 ms of a return stroke rs but in a separate channel connection to ground 0 7 to 2 5 km apart the ui events were visible for short duration 0 5 ms or less and short length to 1 7 km altitude or less and they had slow propagation speed 1 6 2 6 107 m s 1 and no apparent connection to the rs channel in one case the rs was no longer visible to the ground when the ui began 1 30 ms later although mid level channel was still distinct in another case the main rs had dimmed over the 0 52 ms interval before the ui but was still visible the remaining two cases had the entire length of the prior rs channel no longer visible for 40 and 240 s before the ui began 1 32 and 1 87 ms later electric field change data for the ui events resemble those of weak return strokes these events were not located by the operational ground strike and vhf detection systems all four ui events were detected by linet as negative ground strokes with small less than 7 ka peak current magnitude

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

geopotential orbit variations applications to error analysis the position and velocity errors of a near circular orbit due to geopotential uncertainty are developed in fourier series current satellite gravity models generate position uncertainties of up to 50 m alongtrack for close satellites 1000 km altitude in shallow resonance periods less than 5 days from coefficients of order 13 14 smaller radial errors are due to resonance and to first order terms poorly seen in current satellite geopotentials correlation of errors is strong by order and the same degree parity application of the development is made to 1 the prediction of errors for a future altimeter satellite 2 the capability of earth based tracking of a close mars orbiter in resolving that planet s gravity potential and 3 improvement of the resolution of the low degree geopotential from the geopotential research mission using tracking data from the global positioning system

quaternary slope development of the western leeward margin of the great bahama bank abstract to date work on the great bahama bank s western leeward margin has centred chiefly on seismic scale expressions of carbonate sequences and systems tracts however periplatform slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres it is these small scale high frequency cycles within the larger scale facies successions of the quaternary which form the main topic of this paper previous studies have shown that the small scale cycles correlate to the orbitally forced high frequency sea level changes therefore these cycles should indicate how sea level has affected the slope development and thus platform margin evolution during this period through detailed high resolution sequence stratigraphy of the great bahama bank s leeward margin obtained via 18o isotope and mineralogical xrd analyses confined by u th dating and nannofossil bioevents a greater understanding of the bedding geometries within the pleistocene holocene seismic sequences and clues as to the nature of the slope development has been achieved the high resolution seismic profiles indicate that since the plio pleistocene change in geometry in which the great bahama bank developed into a rimmed platform continued steepening and subsequent progradation of the leeward margin has typified slope development during the quaternary which is described as an accretionary slope however on the basis of our observations we conclude that only the early to lower middle pleistocene section isotope stages 45 20 and the holocene isotope stage 1 of the leeward margin is accretionary this indicates that a degree of erosion and or by passing has occurred on the leeward margin since the lower middle pleistocene isotope stage 19 during the first part of this period isotope stages 19 12 erosion and or by passing occurred in the middle to lower slope regions and toe of slope by the end of the upper middle to late pleistocene phase isotope stages 11 2 erosion also occurred on the upper slope this erosion by currents at the toe of slope and oversteepening of the upper and middle slopes have led to back cutting upslope and resulted in the progressive retreat of the toe of slope towards the platform to the east however the rise in sea level since the last glacial maximum to its present day level has allowed high productivity on the platform top during the holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks this has led to the redevelopment of an accretionary slope and continued westward progradation of the great bahama bank s western leeward margin

circulation from a joint gravity field solution determination of the general ocean with the development of satellite altimetry it is possible to infer the geostrophic velocity of the surface ocean currents if the geoid and the position of the satellite are known accurately errors in current geoid models and orbit computations both due primarily to errors in the earth s gravity field model have limited the use of altimeter data for this purpose the objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi stationary sea surface topography and the model for the gravity field satellite tracking data from twelve satellites were used along with seasat altimeter data for the solution the estimated model of compares well at long wavelengths with the hydrographic model of covariance analysis indicates that the geoid is separable from up to degree 9 at which point geoid error is comparable to the signal of

an estimate of equatorial gravity from terrestrial and satellite data equatorial gravity has been estimated from terrestrial gravity data and from satellite implied data the terrestrial estimate was based on the analysis of 43271 1 1 mean free air gravity anomalies the value estimated was 978032 35 0 5 mgals this value may be slightly 0 to 1 5 mgals too low because of the neglect of the downward continuation of surface free air anomalies to the geoid an alternate estimate based on the comparison of terrestrial anomalies and altimeter derived anomalies yields 978032 6 mgals equatorial gravity from space data depends on gm from lageos and lunar laser data and on a new estimate 6378136 2 0 5 m of the equatorial radius based on doppler tracking station positions and a new spherical harmonic expansion of the earth s gravity field to degree 360 the resultant equatorial gravity is 978032 78 0 2 mgals the difference between the two terrestrial estimates is 0 4 and 0 2 mgals or 0 4 ppm or 0 2 ppm the agreement is within the accuracy of the two separate estimates these comparisons can put constraints on parameters of non newtonian force models

the 1987 southeastern alaska gravity calibration range absolute and relative gravity measurements in june 1987 a gravimeter calibration range was set up in southeastern alaska and the yukon territory as part of a geophysical determination of the newtonian gravitational constant absolute gravity measurements were made between the range endpoints using the institute of geophysics and planetary physics absolute gravity meter the calibration range spans 171 841 0 014 mgal with a midpoint g value of 9 81746500 ms 2 relative gravity meters including a lacoste and romberg borehole gravity meter were read along this range a scale factor correction sfc for borehole meter 14 was found to be 8 1 1 5 10 4 and for meter g 349 the correction was 3 3 1 7 10 4 the sfc for meter d 85 has an upper bound of 1 0 10 4

an airborne gravity study of eastern north carolina the naval research laboratory nrl has developed a prototype airborne gravity measurement system the core of the system is a lacoste and romberg air sea gravity meter mounted on a three axis stable platform corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters the original prototype system was designed for use over oceanic areas we recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained the radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces vertical accelerations due to horizontal velocity over a curved rotating earth the eotvos correction and precise two dimensional positions are determined from a texas instrument p code global positioning system the updated system was tested

relationship between late pleistocene sea level variations carbonate platform morphology and aragonite production maldives indian ocean a piston core from the maldives carbonate platform was investigated for carbonate mineralogy grain size distributions calcium carbonate content and organic carbon the sedimentary record was linked to late pleistocene sea level variations using an age model based on oxygen isotopes obtained from planktonic foramanifera nannofossil biostratigraphy and 14c age determinations the correlation between the sedimentary record and late pleistocene sea level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the maldives carbonate platform platform flooding events were characterized by strongly increased deposition of aragonite and mud within the inner sea of the maldives exposure events in contrast can be recognized by rapid decreases in the values of both proxy records the results show that sediments on the maldives carbonate platform contain a continuous record of pleistocene sea level variations these sediments may therefore contribute to a better understanding of regional and even global sea level changes and yield new insights into the interplay between ocean currents and carbonate platform morphology

cool water carbonate sedimentology and eustasy pleistocene upper slope environments great australian bight site 1127 odp leg 182 the southern australian shelf is the largest area of the globe composed of cool water carbonates and provides the basis for the development of actualistic models of such sediments ocean drilling program leg 182 drilling of the sedimentary succession provides essential information to also decipher the dynamics of this depositional system sediment samples and geophysical logs from a core drilled on the eucla plateau off the coast of southern australia site 1127 have been used to study sedimentary cyclicity in an 800 m thick cool water carbonate pleistocene wedge this periplatform succession is dominated by bioclastic packstone and wackestone with minor intercalations of bioclastic grainstone fluctuations of stable oxygen isotope values in bulk carbonate samples parallel global climatic and related sea level fluctuations as predicted in the specmap curve the sea level controlled sedimentary cycles of the slope succession are several tens of meters thick lowstand deposits are relatively fine grained and include abundant sponge spicules whereas highstand deposits are coarser grained and contain abundant high mg calcite bioclasts aragonite content makes up to 36 of the cool water carbonates aragonite originates from tunicate spicules primary sea level controlled carbonate mineralogical and textural changes are likely to promote facies dependent differential diagenesis during later burial stages of such successions

correlated high speed video and radio interferometric observations of a cloud to ground lightning flash a six stroke cloud to ground lightning flash has been studied using observations from a high speed video camera 1000 frames s 1 and a vhf radio interferometer 1 s time resolution as well as additional electric magnetic and optical measurements the flash produced strokes along two channels to ground and a long 550 ms continuing current the video observations provided time resolved pictures of stepped and dart leaders short and long continuing currents and m components during the continuing currents and complemented and confirmed the interferometer observations of flash structure and development the m components were initiated by fast negative streamers inside the cloud which propagated into the conducting channel of the continuing current and subsequently brightened the channel to ground we call this sequence an m event dart leaders and the fast streamers of m and k events were found to be significantly brighter than stepped leaders and continuing currents to ground a number of streamers did not initiate m events that were identical to in cloud k streamers analysis of the m and k event occurrences indicated that the conducting channels of the continuing current both expanded and contracted with time an m type event was also observed during a dart leader it is proposed that the channel multiplicity within the flash resulted from cutoff of the channel to ground while charge continued to flow down the channel from the stroke source region stranding negative charge along the channel dart stepped leaders such as occurred during the third stroke are similarly explained because the stranded charge is observed to be greatest for initial strokes new channels to ground of stepped and dart stepped leaders are expected to follow initial strokes as is usually observed the video and electric field observations indicate that all return strokes have at least a short continuing current of the order of one or a few milliseconds the results also reinforce the well known observation that long continuing currents tend to follow relatively weak return strokes

source shifts to periplatform deposits during the early to middle miocene in response to climatic and oceanographic forcing maldives western indian ocean abstract periplatform carbonate ooze records a composite signal reflecting both the regional oceanographic state and the development of the adjacent carbonate platform deconvolving these signals offers the possibility to obtain a holistic perspective on the factors governing the evolution of the platform especially when the platform itself is not accessible here we examine a case study from periplatform deposition in site u1468 on the margins of the kardiva platform maldives this site has a well constrained record with seismic control of the geometric evolution of adjacent carbonate platforms here we integrate geochemistry sedimentology and external constraints to address the relationship between local platform evolution regional nutrient regime and global temperature sea level effects the lower to middle miocene succession at site u1468 is comprised of a wackestone to packstone sequence that hosts foraminifera and other bioclastic grains derived from neritic and pelagic sources as well as biogenic siliceous material such as sponge spicules the sequence records an increase in neritic supply to the periplatform from the late burdigalian to the early serravallian as recorded in sr ca and 13c values the increase in neritic material coincided with a stepwise intensification of the regional oxygen minimum zone omz recorded by mn ca ratios during these trends short lived oscillations in local carbonate production and omz intensity were superimposed on the larger trend the expansion of the omz was driven by periods of cooling while the increase in neritic production was driven by warming and sea level rise these results reinforce existing models of the evolution of the kardiva platform with respect to sea level and add to it a refined understanding of the relation to regional nutrient patterns

glacial interglacial differences in circulation and carbon cycling within the upper western north atlantic we investigated glacial interglacial changes in the circulation and carbon cycling in the western north atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of bahamian benthic foraminifera potential temperature salinity depth relations show that modern thermocline 200 1000 m and deep 1000 2000 m waters in the providence channels bahamas originate in the sargasso sea and are typical of the subtropical gyre gradients in the stable isotopic compositions of late holocene planulina and cibicidoides species from the bank margins 400 to 1500 m depth reflect temperature nutrient and isotopic gradients of modern subtropical gyre waters the difference between the 18o of glacial maximum and late holocene foraminifera is 2 1 for the upper 900 m of the water column and 1 6 for deeper waters indicating that these waters were 4 c and 2 c cooler respectively during glacial time the glacial temperature gradient dt dz was similar to today while the base of the thermocline was 100 m shallower these results differ significantly from our earlier results from multiple shell 18o analyses which implied upper thermocline waters were only 1 c cooler and dt dz was greater during the glacial maximum the difference occurs because bioturbation adversely affects multiple shell analyses of glacial aged samples from shallow water depths at all depths above 1500 m foraminiferal 13c are greater during the glacial maximum than the late holocene by at least 0 1 to 0 2 as much as 0 6 in the lower thermocline indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum this suggests greater more uniform ventilation of the thermocline results of single and multiple shell 13c analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial interglacial change of 13c was small cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop including large areas of the subtropical ocean a shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of ekman downwelling

radio interferometric observations of cloud to ground lightning phenomena in florida radio frequency observations of cloud to ground lightning in florida have been analyzed to document a number of features of the lightning the observations have been made using an interferometer system which determined the direction to the lightning radiation as a function of time during close lightning discharges observations are presented for about 50 radiating events during five multiple stroke normal polarity flashes to ground the results confirm and extend results of a similar study of new mexico lightning by rhodes et al 1994 dart leaders in cloud k events and attempted leaders are found to be the same phenomena namely a negative polarity streamer that propagates horizontally and or vertically to ground at estimated speeds of about 10 7 ms 1 down to 10 6 ms 1 the step or impulsive fast electric field change produced by some k events is found to be caused by positive breakdown back at the starting point of the negative streamer after the streamer has progressed over some or all of its distance this breakdown appears to initiate a forward stroke along the streamer channel which can renew the breakdown at the front end of the channel k breakdown which happens to occur during a continuing current discharge to ground produces an m event channel brightening when it connects with the conducting channel to ground the step fast field change of the m events is found to be produced at the time of connection inside the cloud no fast field change is detected when this breakdown reaches ground that would be associated with a return stroke m events can also be initiated by fast 10 7 ms 1 positive streamers which often propagate away from the leader source region within a few milliseconds of the arrival of a return stroke back in the source region these streamers often generate an even faster negative recoil event back along their extent which propagates down the channel to ground as the m event in all cases the current increase or channel brightening produced by k and m events is predicted to occur in a forward direction that is toward or down the channel to ground in the same direction as the initiating negative streamer positive breakdown is detected only immediately following return strokes or during some k streamers in both cases the electric stresses are expected to be very large suggesting that positive breakdown which produces a conducting channel is difficult to initiate results for one flash suggest that negative charge was left at low altitude by the initial stroke to ground this forced the subsequent leaders to develop a new path to ground first as series of attempted leaders and then as a dart stepped leader

electric fields preceding cloud to ground lightning flashes we analyzed in detail the electric field variations preceding the first return strokes of 80 cloud to ground lightning flashes in nine different storms observed at the nasa kennedy space center during the summers of 1976 and 1977 the electric field variations are best characterized as having two sections preliminary variations and stepped leader the stepped leader electric field change begins during a transition period of a few milliseconds duration marked by characteristic bipolar pulses the durations of stepped leaders lie most frequently in the range 6 20 milliseconds we infer from our measurements and critical review of the previous literature that there is only one type of stepped leader not the two types and often referred to in the literature

sequence stratigraphy and cyclic development of basal zechstein carbonate evaporite deposits with emphasis on zechstein 2 off platform carbonates upper permian northeast germany abstract although the basal zechstein is classically subdivided into two cycles z1 and z2 lower part it can alternatively be subdivided into three complete third order depositional sequences zs1 zs3 and the associated lowstand systems tract transgressive systems tract of a fourth sequence zs4 third and higher order sea level fluctuations influenced the depositional and diagenetic history of the carbonate evaporite alternations within the basal zechstein the main subject of the present study the zechstein 2 carbonate stassfurt carbonate or ca2 is interpreted to encompass both transgressive predominantly in slope and basinal settings and highstand systems tracts of zechstein sequence zs3 as well as part of the lowstand systems tract of the fourth zechstein sequence zs4 in northeastern germany hydrocarbon reservoirs of the ca2 developed along both northern and southern margins of the southern permian basin on top of the topography of the zechstein 1 cycle z1 this z1 topography resulted from deposition of the werra anhydrite a1 the highstand systems tract of the second zechstein sequence zs2 the northern ca2 carbonate platform is consistently narrow and fringed by coated grain and composite grain bars and shoals it is characterized by a steep fault controlled slope which precluded the formation of in situ shallow water grain dominated carbonates during relative sea level lowstands displaced shallow water coated grain and composite grain packstones wackestones may however occur as slope fan deposits turbidites in contrast the southern carbonate platform is wide tectonically dissected and fringed by grain dominated shallow water carbonates coated grain and composite grain bars and shoals zechstein 2 shallow water carbonates detectable by basal zechstein seismic anomalies also developed on pre existing topographical highs in off platform positions three depositional cycles ca2 cycles i ii and iii most probably reflecting high amplitude fourth order sea level fluctuations can be identified within the stassfurt carbonate our observations suggest that sea level variations possibly reflecting cyclicity in the milankovitch frequency band 0 1 0 4 ma and the pre zechstein topography pronounced during formation of the werra anhydrite were the most important factors in controlling the depositional as well as the diagenetic history of the zechstein 2 carbonate in off platform positions

snow load effect on the earth s rotation and gravitational field 1979 1985 a global monthly snow depth data set has been generated from the nimbus 7 satellite observations using passive microwave remote sensing techniques in this paper we analyze 7 years of data 1979 1985 to compute the snow load effects on the earth s rotation and low degree zonal gravitational field a uniform sea level decrease has been assumed in order to conserve water mass the resultant time series show dominant seasonal cycles the annual peak to peak variation in j2 is found to be 2 3 x 10 o that in j3 to be 1 1 x 10 o and believed to decrease rapidly for higher degrees the corresponding change in the length of day is 41 us the annual wobble excitation is 4 9 marc sec 109 o for the prograde motion component and 4 8 marc sec 28 o for the retrograde motion component the excitation power of the chandler wobble due to the snow load is estimated to be about 25 db less than the power needed to maintain the observed chandler wobble the superior quality of the satellite data over conventional data acquired by ground observations and modeling is demonstrated we also discuss the role of atmospheric water and the problems arising from the lack of snow load observations over the antarctic and greenland ice sheets

sea level related resedimentation processes on the northern slope of little bahama bank middle pleistocene to holocene middle pleistocene to holocene sediment variations observed in a 26 metre long core taken during a cruise of the rv marion dufresne are presented core md992202 was retrieved from the northern slope of little bahama bank and provides an excellent example for sedimentation processes in a mid slope depositional environment the sediment composition indicates sea level related deposition processes for the past 375 000 years marine isotope stages 1 to 11 the sediments consist of i periplatform ooze fine grained particles of shallow water and pelagic origin with moderate variations in carbonate content carbonate mineralogy and grain size and ii coarser intervals with cemented debris consisting of massive poorly sorted mud supported or clast supported deposits with an increased high magnesium calcite content during interglacial stages marine isotope stages 1 5 7 9 and 11 periplatform oozes i are characterized by higher aragonite contents finer grain size and higher organic contents whereas during glacial stages marine isotope stages 2 to 4 6 8 and 10 increased low magnesium and high magnesium calcite values coarser grain size and lower organic contents are recorded these glacial to interglacial differences in mineralogy grain size distribution and organic content clearly show the impact of climatically controlled sea level fluctuations on the sedimentation patterns of the northern slope of little bahama bank the coarser deposits ii occur mainly at the transitions from glacial to interglacial and interglacial to glacial stages and are interpreted as redeposition events indicating a direct link between sediment properties changes in mineralogy grain size distribution variations in organic contents and sea level fluctuations changes in hydrostatic pressure and the wave base position during sea level changes are proposed to have triggered these large scale sediment redepositions

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

holocene atlantic climate variations deduced from carbonate periplatform sediments leeward margin great bahama bank a marine sediment core from the leeward margin of great bahama bank gbb was subjected to a multiproxy study the aragonite dominated core md992201 comprises the past 7230 years in a decadal time resolution and shows sedimentation rates of up to 13 8 m kyr aragonite mass accumulation rates age differences between planktonic foraminifera and aragonite sediments and temperature distribution are used to deduce changes in aragonite production rates and paleocurrent strengths aragonite precipitation rates on gbb are controlled by exchange of carbonate ions and co2 loss due to temperature salinity conditions and biological activity and these are dependent on the current strength paleocurrent strengths on gbb show high current velocities during the periods 6000 5100 years bp 3500 2700 years bp and 1600 700 years bp lower current speeds existed during the time intervals 5100 3500 years bp 2700 1600 years bp and 700 100 years bp bahamian surface currents are directly linked to the north atlantic atmospheric circulation and thus periods with high low current speeds are proposed to be phases of strong weak atmospheric circulation

interpreting compositional variations of calciturbidites in relation to platform stratigraphy an example from the paleogene of se spain abstract in the sierra de aitana betic cordilleras southern spain a sequence of massive platform limestones outcrops study of these limestones indicates a late eocene age and has resulted in distinguishing four lithofacies associations several features suggest tilting uplift and partial exposure of this carbonate platform at the end of the eocene in a more basinward section this time interval is represented by calciturbidites the composition of which has been analysed microscopically the upper eocene turbidites are mainly composed of loose skeletal grains derived from the platform interior around the eocene oligocene boundary the composition changes abruptly and the lower oligocene turbidites mainly consist of platform derived limestone clasts and skeletal debris from the platform margin this compositional change is accompanied by a large decrease in volume of turbidite input it is concluded that both composition and amount of calciturbidites reflect the history of the adjacent carbonate platform this case study therefore provides new evidence for the mechanism of highstand shedding in carbonate depositional systems

an abrupt change in ridge axis gravity with spreading rate the global mid ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate fast spreading ridges have axial highs with little bathymetric relief and low amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium slow spreading ridges have large axial valleys bounded by rugged topography macdonald 1982 and large axial gravity troughs indicating that they are dynamically maintained while this variation in ridge axis morphology with spreading rate has been observed it has not been analyzed in a comprehensive manner moreover it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate such observations would provide important constraints on models of ridge axis dynamics vertical deflection profiles collected by the geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate in this study we have analyzed 44 geosat profiles over ridges with spreading rates ranging from 14 to 155 mm yr in agreement with previous studies we find that slow spreading ridges 70 mm yr are characterized by low amplitude ridge axis highs 15 rad unexpectedly we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60 70 mm yr ridge axis gravity signatures are highly variable for rates less than 65 mm yr and very uniform at higher rates the transition of the gravity signature appears to be more abrupt than the transition of the topographic signature suggesting an abrupt change in the style of isostatic compensation with spreading rate published models of ridge axis dynamics do not explain this sharp transition

displacements of the earth s surface due to atmospheric loading effects on gravity and baseline measurements atmospheric mass loads and deforms the earth s crust by performing a convolution sum between daily global barometric pressure data and mass loading green s functions we estimate the time dependent effects of atmospheric loading including those associated with short term synoptic storms on surface point positioning measurements and surface gravity observations we calculate the response for both an oceanless earth and an earth with an inverted barometer ocean load responses for near coastal stations are significantly affected by the inclusion of an inverted barometer ocean peak to peak vertical displacements are frequently 15 20 mm with accompanying gravity perturbations of 3 6 gal baseline changes can be as large as 20 mm or more the perturbations are largest at higher latitudes and during winter months these amplitudes are consistent with the results of rabbel and zschau 1985 who modeled synoptic pressure disturbances as gaussian functions of radius around a central point deformation can be adequately computed using real pressure data from points within about 1000 km of the station knowledge of local pressure alone is not sufficient rabbel and zschau s hypothesized corrections for these displacements which use local pressure and the regionally averaged pressure prove accurate at points well inland but are in general inadequate within a few hundred kilometers of the coast

punctuated environmental change along the holocene 6 ka bank margin of south west exuma sound bahamas characteristics of environmental change along the holocene 6 ka bp bank margin of south west exuma sound bahamas are defined by comparing mineral isotope and lithologic stratigraphic patterns in deep water 1081 m slope strata in odp hole 631a exuma sound with proximal 6 km shoal water and insular geology of the lee stocking island lsi region after platform submergence a bank top transition from non skeletal to skeletal dominated deposition occurred in the lsi region by 3 8 ka and is recorded in the adjacent slope sediments by a contemporaneous change in the abundance of transported allochems the transition at site 631 is punctuated by an anomalous shift in foraminiferal 13c 1 and 18o 3 which may represent a period of climate change that established a brackish coastal zone at least 6 km wide coincident 3 8 4 ka with bank top erosion and pedogenesis non skeletal allochems reappeared in hole 631a sediments as the dominant transported allochem type by 2 4 ka then again over the last several hundred years and are contemporaneous with periods of increased platform margin circulation in the lsi region such rejuvenation of platform circulation may have resulted from rapid relative rises in sea level superimposed on the net gradual post glacial rise at site 631 a prominent 25 decline in aragonite abundance occurred during the period from 2 4 ka to the last few hundred years and coincided with an increase in magnesian calcite and skeletal debris and a positive shift in foraminiferal 18o values these patterns lend support to the hypothesis that during this period the lsi margin was barred by skeletal shoals allowing bank top trapping of aragonite and hypersaline water with increased coastal salinity resulting from reflux of hypersaline waters through the shoals and or cooled coastal waters caused by reduced exchange of warmed bank derived water relative to wind generated upwelling along the exuma slope a diagenetic overprint may further enhance the recent decline in aragonite accumulation integration of holocene deep water slope and bank top stratigraphies along the bank margin of south west exuma sound reveals a pattern of punctuated environmental change superimposed on longer term development of interglacial conditions

eustatic and tectonic controls on deposition of hybrid siliciclastic carbonate basinal cycles discussion with examples changes in sea level represent one of the dominant controls on basinal sedimentation adjacent to mixed carbonate siliciclastic sediment source areas sedimentary responses to sea level change of the siliciclastic and carbonate components of these hybrid systems commonly result in deposition of alternating siliciclastic and shelf derived carbonate basinal deposits such deposition is particularly pronounced adjacent to carbonate platforms where alternate immersion exposure of the flat platform top results in carbonate highstand deposition and siliciclastic lowstand deposition in contrast small to moderate amplitude sea level drops in distally steepened ramp settings may expose only the inner part of the gently sloping shelf resulting in lowstand deposition of mixed sil ciclastic allodapic footnote 3 carbonate sediments along active margins tectonic uplift and subsidence of sediment source areas act as additional major controls on deposition of siliciclastic carbonate basinal cycles two of the most important tectonic effects are 1 uplift of siliciclastic sediment source areas resulting in increased erosion and swamping of carbonate shelf sources and 2 exposure or immersion of shelf carbonate sediment source areas due to vertical tectonic movements most published seismic stratigraphic studies of sea level control on deep sea sedimentation focus on lowstand deposition of siliciclastic sediment and commonly neglect the contribution of allodapic carbonate sediment deposited during sea level highstands however the examples discussed in this paper indicate that highstand carbonate deposits may make up a significant proportion of many hybrid carbonate siliciclastic basinal sequences

statistics and characteristics of cloud to ground lightning with multiple ground contacts 1 during the summer of 1997 386 cloud to ground cg lightning flashes were recorded on videotape near tucson arizona within this sample 136 flashes 35 struck the ground in two or more places that were separated by tens of meters or more within this subset of 136 which we term multiple channel flashes mcfs 88 exhibited two or more separate and distinct channels between the cloud base and the ground a type that we term a new channel flash ncf 37 forked or produced channels that struck the ground in two or more places and shared a common channel at higher altitudes a type that we term an altered channel flash acf and 11 exhibited both types of behavior the ratio of ncfs to acfs in arizona was 99 48 or 2 1 a value that is consistent with measurements in florida after allowing for differences in the cloud base heights altogether the 386 cg flashes produced 558 different strike points therefore the average number of strike points per cg flash was 1 45 67 of the new strike points were produced by the second stroke in the flash and 72 of the changes in channel geometry occurred after there had been just one stroke in the previous channel ten of the 48 acfs 21 developed along a visible branch of the previous channel and seven of the 136 mcfs 5 contained strokes that reilluminated the first channel after a second channel had formed the presence of a long continuing luminosity during the initial three or more consecutive video frames appeared to lower the probability of forming either a new or altered channel on the second stroke from 47 to 33 but the statistics are limited and our ability to identify individual strokes is biased by poor time resolution

controls and development of late quaternary periplatform carbonate stratigraphy in walton basin northeastern nicaragua rise caribbean sea the late quaternary sedimentary record in periplatform oozes and muds deposited on the northern nicaragua rise results from the interplay of four main controls 1 input of pelagic carbonates and bank derived fine aragonite and magnesian calcite 2 input of siliciclastic sediments 3 dispersal and removal of sediments by the caribbean current and 4 partial seafloor dissolution of metastable carbonates high accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages neritic carbonate productivity also peaked during interglacial stages when bank and shelf tops along the nicaragua rise were submerged within the photic zone because the bank and shelf tops remained mostly exposed during glacial stages the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of pedro bank and the southern shelf of jamaica a large volume of siliciclastic sediments transported from the coastal area of south america and the eastern and western regions of jamaica were deposited during glacial stages within the eastern and western deep extensions of walton basin this finding contrasts with the input pattern of siliciclastic sediments in walton basin itself where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima corresponding to intervals of maximum sea level transgression finally cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m this indicates that some bank derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m results from this study on the northeastern nicaragua rise indicate that not only do offbank transport and water column saturation state influence the late quaternary record but also that the variable strength of the caribbean current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late quaternary periplatform high resolution stratigraphy

genesis and dispersal of carbonate mud relative to late quaternary sea level change along a distally steepened carbonate ramp northwestern shelf western australia abstract accumulation of carbonate mud is minor on the northwest shelf western australia a broad 200 300 km wide distally steepened tropical carbonate ramp it is negligible along most of the inner ramp 0 to 50 m water depth owing to siliciclastic riverine input it is also negligible along the mid from 50 to 120 m ramp and where the entire ramp narrows to 100 km owing to constant reworking by storm waves and ocean swell in these areas carbonate mud is derived mostly from mechanical degradation of skeletal fragments but this does not include calcified remains of calcareous green algae e g halimeda although these plants grow locally only along the outer 120 to 200 m water depth ramp and slope is there any relatively abundant accumulation of carbonate mud here the sediment consists of modern pelagic foraminifer pteropod and nannoplantkon ooze and aragonitic needle rich 2 m micrite the latter having an age based on ams 14c of 19 ka distribution of aragonite micrite defines an extensive 5 0 104 km2 yet relatively narrow 40 km tract running 600 km along the platform margin crystal morphology mineralogy and isotope c o composition identify the micrite to be a seawater precipitate its production and seaward export having occurred during the last glacial maximum when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin now the modern distal slope today this sediment body lies stranded in deep water about 200 300 km seaward of the shallow inner ramp the volume of micrite produced appears equal to the present day production potential of the great bahama bank rapid postglacial rise in sea level and change in platform geometry climate and oceanography were very influential factors in addition to water depth and area of submergence in the production and distribution of aragonite micrite along the northwest shelf in general 1 epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats 2 not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments nor do they necessarily export large volumes of micrite to the periplatform realm and 3 some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems

carbonate mineral cycles generated by foraminiferal and pteropod response to pleistocene climate west florida ramp slope carbonate ramps are gently sloping depositional surfaces where shallow water coarse grained facies pass basinward into fine grained deep water sediments with no abrupt change in slope the objectives of this study are i to integrate the depositional processes recorded in the pleistocene stratigraphy of the west florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern gulf of mexico and ii to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations aragonitic pteropod rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials these cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera rather than preferential dissolution of either aragonitic or calcitic fractions species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the loop current and variations in water mass salinities as well as terrigenous dilution from the mississippi delta these cycles are the response to pleistocene glacial interglacial oscillations controlled by milankovitch orbital parameters although the organisms contributing to deep water carbonate environments have changed through geological time facies patterns as well as sedimentary textures and structures identified in the west florida sediments provide criteria for recognition of ancient ramps an understanding of the processes on a modern ramp slope such as west florida may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record

three dimensional gravity modeling of the geologic structure of long valley caldera a 48 mgal gravity low coincides with long valley caldera and is mainly attributed to low density caldera fill gravity measurements by unocal geothermal have been integrated with u s geological survey data vastly improving gravity station coverage throughout the caldera a strong regional gravity trend is mainly attributed to isostasy a best fitting based on regional control of basement densities airy heiskanen isostatic model was used for the regional correction a three dimensional multiple unit gravity modeling program with iterative capabilities was developed to model the residual gravity the density structure of long valley caldera and vicinity was modeled with 22 discrete density units most of which were based on geologic units information from drill hole lithologies surface geology and structural geology interpretations constrain the model some important points revealed by the three dimensional gravity modeling are that 1 the volume of ejected magma associated with the bishop tuff eruption is greater than previously thought 2 the caldera structure is strongly influenced by precaldera topography and the extensions of major active faults 3 the main west ring fracture is coincident with the inyo domes mono craters fracture system 4 a relatively low density region probably underlies the caldera and 5 a silicic magma chamber may underlie devils postpile

gravity gradiometer survey errors gradiometer system noise sampling effects downward continuation and limited data extent are the important contributors to moving base gravity gradiometer survey error we apply a two dimensional frequency domain approach in simulations of several sets of airborne survey conditions to assess the significance of the first two sources a special error allocation technique is used to account for the downward continuation and limited extent effects these two sources cannot be modeled adequately as measurement noise in a linear error estimation algorithm for a typical characterization of the earth s gravity field our modeling indicates that limited data extent generally contributes about one half of the total error variance associated with recovery of the gravity disturbance vector at the earth s surface gradiometer system noise typically contributes about one third however sampling effects are also very important and are controlled through the survey track spacing a 5 km track spacing provides a re

secular variation of calcium carbonate mineralogy an evaluation of ooid and micrite chemistries three ooid types are recognized from the lower tournaisian kohlenkalk shelf facies at velbert germany ooids from this unit have a predominantly concentric laminae fabric radial concentric and small radial fibrous ooids are minor components to this oolite the diagenetic response of kohlenkalk ooid chemistry is significantly different from that observed in contemporaneous crinoid and brachiopod material fabric evidence suggests that radial concentric and radial fibrous ooids were probably originally aragonite high mg calcite and high mg calcite respectively fabric and trace elemental chemistries of the concentric fabric ooids suggests that they were originally precipitated as aragonite and subsequently altered to low mg calcite

interferometric global positioning system three dimensional positioning source for airborne gravimetry the naval research laboratory nrl and the national geodetic survey recently collaborated on an experiment to test interferometric mode global positioning system gps tracking as a source of altitude and vertical acceleration for airborne gravity measurement reduction a texas instruments ti 4100 gps receiver and an extremely precise radar altimeter were installed in an nrl p 3 orion aircraft a second ti 4100 was located at a fixed base station of known location carrier phase data from four gps satellites were recorded from the fixed and airborne receivers and postprocessed to produce three dimensional displacements of the airborne station relative to the base station differences in gps and radar altimeter coordinate systems were modeled and removed the resulting gps and radar altitudes were compared the two unfiltered 1 hz data sets differed by an rms of 20 cm when aircraft dynamics were controlled by autopilot in straight and level flight during hand flown maneuvers such as turns climbs and descents rms differences varied from 40 to 80 cm a significant portion of the differences can be attributed to remaining coordinate system discrepancies and to uncompensated geometric effects due to aircraft motion we have also examined vertical acceleration time series produced by doubly differencing both data sets by applying a low pass filter which allows resolution of features with wavelengths of 20 km and greater at nominal aircraft speeds of 100 m s the rms difference between the radar and gps derived vertical accelerations was less than 2 mgal 2 10 5 m s2 during straight and level flight it should be possible to improve this accuracy and wavelength resolution with future improvements in measurement and processing techniques the results of this experiment show that the incorporation of interferometric mode gps tracking into the airborne gravity measurement system will make possible rapid and relatively inexpensive gravity surveys over any type of terrain while simultaneously providing accurate measurements of ground height profiles beneath the aircraft

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

a fractal interpretation of topography and geoid spectra on the earth moon venus and mars global spectra are available for topography and geoid on the earth venus mars and the moon if the spectral energy density has a power law dependence on wave number a fractal is defined the topography spectrum for the earth is a well defined fractal with d 1 5 this corresponds to brown noise with the amplitude proportional to the wave length although there is more scatter for the other planetary bodies the data for mars and the moon correlate well with the data for the earth venus topography also exhibits a brown noise behavior but with a smaller amplitude the power law dependence of the earth s geoid is known as kaula s law we show that uncompensated brown topography gives a geoid with a power law dependence that is in quite good agreement with kaula s law however the required amplitude is only 8 of the observed topography a similar result is found for the other bodies with the ratio of the amplitude of topography required to explain the geoid to the observed topography increasing to 72 for the moon

surface sediment carbonate mineralogy and water column chemistry nicaragua rise versus the bahamas abstract periplatform surface sediments were studied for carbonate mineralogy in conjunction with analyses of the water column for carbonate chemistry on the eastern northern nicaragua rise nnr in the caribbean sea the results show a strong correspondence between variations and disappearance with increasing water depth of metastable carbonate minerals fine aragonite and magnesian calcite and their respective saturation levels in the overlying waters similar correspondence between variations in sediment proportions of fine aragonite and magnesian calcite and their respective saturation levels has previously been established in the bahamas there are however significant differences between the two areas the sharp decrease in aragonite content and the measured aragonite saturation level occur at 4000 m in the bahamas compared to 1800 m on the eastern nnr in both areas magnesian calcite minima correspond to the in situ pco 2 maxima in the water column the magnesian calcite minimum however is at 950 m in the bahamas and 750 m on the eastern nnr magnesian calcite disappears in the bahamas at 3800 m and at 2000 m on the eastern nnr these results demonstrate the importance of the influence of overlying water chemistry on the preservation of metastable carbonate minerals in off bank periplatform sediments and they clearly demonstrate the difference in terms of carbonate preservation between the poorly ventilated waters of the caribbean sea and the well oxygenated waters of the adjacent atlantic ocean they also open the possibility of obtaining paleoceanographic information on the depth of the co 2 maximum o 2 minimum and its separation from the aragonite saturation depth in at least some areas

lightning subsequent stroke electric field peak greater than the first stroke peak and multiple ground terminations for 46 multiple stroke flashes in which each stroke ground termination was located using a tv camera network and thunder ranging 15 flashes 33 had one or more subsequent return strokes whose initial electric field peak normalized to 100 km was greater than the first stroke field peak of the flash in 9 of these 15 flashes the subsequent strokes with field peaks greater than the first stroke followed the same channel as the first stroke in five flashes the subsequent strokes with the greater peaks followed a different channel to ground and in one flash the subsequent strokes with the greater peaks occurred both in the first stroke channel and in a different channel the interstroke intervals immediately preceding the 13 larger subsequent strokes that followed the first stroke channel had a geometric mean gm duration of 98 ms 1 7 times greater than the gm of 57 ms for all 199 interstroke intervals 46 flashes without any selection eight of the 13 larger subsequent strokes for which leader durations were measurable had a gm leader duration of 0 55 ms 3 3 times smaller than the gm of 1 8 ms for 117 subsequent leaders with measurable duration in a previously formed channel of the 46 multiple stroke flashes for the six larger subsequent strokes that created a new channel to ground the preceding interstroke interval had a gm of 130 ms and the leader duration had a gm of 15 ms no subsequent stroke with peak field exceeding the first in any category had a preceding interstroke interval less than 35 ms analysis of direct current measurements from switzerland shows that subsequent stroke currents exhibit many features similar to those of florida subsequent stroke electric fields in 22 florida single stroke and multiple stroke ground flashes the distances between multiple channel terminations in a given flash 33 measurements ranged from 0 3 km to 7 3 km with a gm of 1 7 km

resolution and variance operators of gravity and gravity gradiometry gravity gradiometry represents a new potential field data set which may better constrain the density structure of the earth using singular value spectral decomposition of the gravity and gravity gradient kernels the model parameter resolution and model parameter variance of the two data types are compared using data from the defense mapping agency and a recently acquired collection of airborne gradient measurements from bell aerospace textron s gravity gradient survey system ggss the ggss was flown over a portion of southwestern oklahoma where the gravitational anomaly from the buried wichita basement rocks is over 60 mgal the corresponding maximum vertical gravity gradient was found to be 46 2 eotvos the determination of the subsurface density structure is cast as a linear inverse problem and for comparison a nonlinear inverse problem for both the linear and nonlinear inversions the gravity gradients improve the resolution and result in smaller variances than the vertical component of gravi

seismic sedimentologic interpretation of a carbonate slope north margin of little bahama bank abstract analyses of seismic profiles and descriptions of sediments from odp leg 101 cores were combined to investigate the evolution of the northern margin of little bahama bank which has been prograding northward since the early mio cene modern depositional systems on the midslope and lower slope are found to be uncharacteristic of interpreted an cient sedimentary environments large slump masses covered much of the lower slope during the middle miocene pos sibly triggered by a regional tectonic event throughout the late miocene and most of the pliocene a channel and levee system meandered across the sediment apron of the lower slope the modern lower slope contains no meandering chan nels although gullies incised in the midslope funnel sediments to the base of slope apron actively promoting sediment bypass on an accretionary margin these changes in sedimentation pattern on the lower slope indicate increasing strength of ocean bottom contour following currents from the pliocene pliocene to holocene gravitational creep has produced large scale rotational movement of unlithified sediments and a major detachment surface along the base of the middle miocene slump masses these creep lobes extend far into the lower slope where sediments are contorted by propagation of and movement along multiple minor detachment surfaces

gravity model studies of newberry volcano oregon newberry volcano a large quaternary volcano located about 60 km east of the axis of the high cascades volcanoes in central oregon has a coincident positive residual gravity anomaly of about 12 mgals model calculations of the gravity anomaly field suggest that the volcano is underlain by an intrusive complex of mafic composition of about 20 km diameter and 2 km thickness at depths above 4 km below sea level however uplifted basement in a northwest trending ridge may form part of the underlying excess mass thus reducing the volume of the subvolcanic intrusive a ring dike of mafic composition is inferred to intrude to near surface levels along the caldera ring fractures and low density fill of the caldera floor probably has a thickness of 0 7 0 9 km the gravity anomaly attributable to the volcano is reduced to the east across a north northwest trending gravity anomaly gradient through newberry caldera and suggests that normal perhaps extensional faulting has occurred subsequent to caldera formation and may have controlled the location of some late stage basaltic and rhyolitic eruptions significant amounts of felsic intrusive material may exist above the mafic intrusive zone but cannot be resolved by the gravity data

mixed carbonate siliciclastic sedimentation along the great barrier reef upper slope a challenge to the reciprocal sedimentation model abstract results of studies involving numerous cores and odp holes along the great barrier reef gbr margin and adjacent queensland trough and queensland plateau have challenged the use of a reciprocal sedimentation model to describe the sedimentary response of slope and basin settings to glacioeustatic sea level fluctuations upper slope sedimentation results from the relationships between sea level fluctuations antecedent topography and regional climate that play an important role in the type and amount of sediment deposited on the upper slope during glacial deglacial and interglacial times during the last glacial maximum lgm 20 ka ago upper slope sediments generally lacked siliciclastic material and are characterized by very low accumulation rates whereas early deglacial time termination i ti deposits are dominated by a siliciclastic and neritic carbonate pulse siliciclastic sedimentation was significantly reduced in the holocene while carbonate sedimentation remains elevated a new borehole iodp expedition 325 hole m0058a hole 58a recovered 82 of a 40 4 m hole on the upper slope east of noggin passage on the central gbr margin near cairns australia hole 58a provides a detailed sedimentary record during termination ii tii marine isotope stage 6 5e mis 6 5e deglacial transition and through most of interglacial mis 5 this hole along with two others odp leg 133 holes 820a and 819a from the upper slope east of grafton passage show carbonate siliciclastic cyclicity as the result of glacioeustatic change with the gbr shelf sedimentation at hole 58a is consistent with that of previous studies along the gbr margin focusing on the lgm to present and extends the upper slope sedimentary record back to tii and interglacial mis 5 a siliciclastic pulse similar to the one during ti occurred during the penultimate deglaciation tii however the maximum neritic aragonite export to the upper slope occurred not during peak mis 5e highstand when sea level was a few meters above modern position but subsequently during a time mis 5d to 5a when lowered sea level fluctuated between 30 and 50 m below present sea level siliciclastic sediments were reworked and exported to the upper slope when the lowstand fluvial plain was re flooded whereas neritic carbonate export to the slope reached a maximum when sea level fell and much of the mid to outer shelf re entered the photic zone subsequent to a drowning interval thus this analysis refines the mixed sedimentation models of upper slope sedimentation along the central gbr margin during the penultimate deglacial transgression and subsequent interglacial early and late highstand this study provides further evidence that mixed carbonate siliciclastic margins do not always behave in a predictable manner and that mixed margins both modern and ancient would benefit from detailed study of sediment transport in the context of sea level rise and fall

lightning activity following the return stroke natural lightning is both frequent and variable and thus a good subject for statistical studies a typical negative cloud to ground cg flash consists of multiple individual return strokes the spatial and temporal distributions of various lightning events throughout the discharge provide a surrogate look inside the cg flash and offer insight into the underlying physical processes in this study we combine 8years of national lightning detection networktm nldn and north alabama lightning mapping array nalma data to compute the spatial and temporal distributions of i subsequent nldn reported return strokes and ii lma reported sources around nldn reported cg strokes subsequent strokes are separated into those with the same contact point as the first stroke and those flowing along new lightning channels statistically the distribution of strokes along new channels evolves deterministically with 200 km s propagation speed from the original channel comparable to the speed of a stepped leader this suggests that the cg subsequent strokes forming new channels may be directly linked to the initial one by a propagating leader inside the cloud we present lma case studies and a multiyear analysis of nldn lma data that support this behavior our results are supported by ground truth measurements and video recordings from previous field studies

multistroke cloud to ground strike to the nasa f 106b airplane an analysis of electromagnetic waveform records and video images of a multistroke cloud to ground cg strike to the nasa f 106b instrumented airplane is presented the cg flash started as a lightning strike triggered by the airplane and later produced multiple return strokes to the ground the ground strike network registered six return strokes although there are some uncertainties in data interpretation resulting from lack of independent measurements with other than the airborne instruments recoil streamers and eight sequences of dart leaders followed by return strokes two more than were indicated by the ground network were identified in the airborne data at least three of the subsequent return strokes were attached to the airplane the analysis provides evidence that formation of recoil streamers and dart leaders is accompanied by a surge in continuous current this feature is similar to that observed in the bidirectional leader development during lightning initiation on the airplane

characteristics of a negative lightning with multiple ground terminations observed by a vhf lightning location system propagation characteristics of a negative cloud to ground lightning with four leader return stroke sequences exhibiting four ground terminations referred to as multiple termination flash are analyzed using a two dimensional mapping system of lightning radiation sources in vhf band along with broadband electric field change records the four stepped leader stroke sequences were produced from two different branches of the preliminary breakdown process inside the cloud and the time intervals between each adjacent two leader stroke sequences were 82 9 ms 81 4 ms and 152 2 ms respectively the first two stroke sequences were multiple termination strokes mtss and each mts showed two different ground terminations induced by two leader branches the corresponding electric field changes exhibited two fast rising waveforms separated by 1 5 ms and 2 7 ms respectively and the second peak was 3 4 times smaller than the first the second grounding branch in each mts occurred when its channel was in poor connection conditions to the main stroke channel the third and fourth leader stroke sequences connected to the ground with one of the different grounded channels of the second mts the dart leaders before these two strokes exhibited chaotic pulse trains cpts with significant slow electric field variations accompanying strong and continuous vhf radiations the cpts only occurred in the dart leader stage which propagated with a speed in an order of 107 m s different from dart leader attempted leader was associated with regular pulse trains and propagated at a speed order of 106 m s along the previous channel

direct u th dating of marine sediments from the two most recent interglacial periods a knowledge of the age of marine sediments is necessary to determine the timing of events and rates of processes in the marine realm and the relationships among marine and other climatically sensitive records the establishment of an accurate chronology for pleistocene marine sediments beyond the range of radiocarbon dating approximately the past 45 kyr has therefore been a goal of palaeoceanographers for decades early attempts1 2 based on measurements of the radionuclides 230th and 231pa were beset with problems and subsequent studies focused on tying fluctuations in marine sediment oxygen isotope records to events such as the formation of coral reef terraces and changes in the earth s magnetic polarity3 4 and tuning the resultant chronologies to the earth s orbitally driven insolation variations5 8 but these chronologies especially the age and duration of the last interglacial period have been challenged by several studies9 12 raising questions about the fundamental cause of pleistocene climate fluctuations here we report the direct u th dating of aragonite rich marine sediments from the bahamas and present an accurately dated marine oxygen isotope record for the last two interglacials we obtain dates of 120 127 kyr bp for the last interglacial and 189 190 kyr bp for the late stage 7 interglacial these dates are in accord with the general theory of orbitally forced climate fluctuations and demonstrate the potential of our direct dating approach for developing an absolute chronology for the pleistocene marine oxygen isotope record

a low frequency near field interferometric toa 3 d lightning mapping array we report on the development of an easily deployable lf near field interferometric time of arrival toa 3 d lightning mapping array applied to imaging of entire lightning flashes an interferometric cross correlation technique is applied in our system to compute windowed two sensor time differences with submicrosecond time resolution before toa is used for source location compared to previously reported lf lightning location systems our system captures many more lf sources this is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry toa processing method we show with five station measurements that the array detects and maps different lightning processes such as stepped and dart leaders during both in cloud and cloud to ground flashes lightning images mapped by our lf system are remarkably similar to those created by vhf mapping systems which may suggest some special links between lf and vhf emission during lightning processes

origin of sr rich magnesian calcite mud in a holocene pond basin lee stocking island bahamas abstract magnesian calcite represents about 90 of carbonate mud that has accumulated in a holocene 30 c hypersaline pond waters at times exceeding gypsum saturation elevated sr content is likely a product of changing sr cafluid ratios in pond water related to gypsum formation by incorporating carbon and oxygen isotope stratigraphies of indigenous biota the foraminifer ammonia beccarii and gastropod cerithidea sp with that of pond mud this sedimentary basin preserves a record of high order environmental changes over the last 1500 years that likely influenced mud precipitation and its accumulation these changes reflect variation in water residence time temperature likely bacterial productivity and bicarbonate sources

sedimentology and palynofacies of the zechstein 2 carbonate upper permian northwest germany implications for sequence stratigraphic subdivision abstract the zechstein 2 carbonate stassfurt carbonate or ca2 of the second zechstein cycle has been subdivided into third order and higher order sequences using sedimentology and palynofacies analysis as complementary tools this combination of tools provides a logical approach to the correlation of parasequences across different depositional settings in a platform to basin transect the ca2 reservoir overlies the anhydrites of the first zechstein cycle z12 the werra anhydrite a12 and is sealed by the anhydrites of the second zechstein cycle z2 the basal anhydrite a2 the ca2 can be subdivided into platform upper slope middle slope lower slope and basinal facies these facies can be further subdivided into a total of twenty six subfacies types the ca2 comprises a nearly complete third order sequence zs3 and part of the lowstand systems tract of the fourth zechstein sequence zs4 it can be further subdivided into seven parasequences identified by subfacies succession gamma ray log response and palynofacies interpretation indicating higher order intra ca2 relative sea level fluctuations sea level fluctuations at third order and higher order scale are reflected by cyclic changes in palynomaceral assemblages transgressive deposits including the maximum flooding surface are characterized by a high proportion of vitrinitic and inertinitic frequently blade shaped palynomacerals whereas various types of degraded palynomacerals occur most frequently during lowstand and highstand progradation phases intertidal carbonates at the base of the ca2 third order lowstand wedge are particularly distinct due to the abundance of palynomorphs resembling colonial green algae plaesiodictyon spp the proposed chronostratigraphic subdivision of the ca2 into third and high order sequences improves the prediction of lateral and vertical reservoir facies distribution

carbonate mineral cycles in ramp slope sediment eastern gulf of mexico carbonate mineral cycles controlled by pleistocene climatic rhythms have been discovered in a 11 6 m long piston core from the west florida carbonate ramp slope at 890 m of water depth in the eastern gulf of mexico relatively high concentrations of aragonite magnesian calcite and dolomite as well as insoluble residues total organics and mud sized sediment all correlate with glacial periods defined by oxygen isotope stratigraphy the aragonite is a low strontium variety derived from pteropods and there is no evidence that concentration variations are controlled by dissolution we propose that these mineral and textural cycles are the product of a combination of increased biological productivity and dilution by fine grained terrigenous mud during glacial episodes aragonite cycles in bahamian periplatform slope sediment have also been correlated with climatic change there the aragonite is a high strontium variety derived from shallow water calcareous algae with relatively high concentrations correlated with high sea levels during interglacial periods the carbonate mineral cycles along the west florida ramp slope are 180 out of phase with bahamian examples and also lack their marked asymmetry it is now apparent that for deepwater continental margin carbonates there are two types of carbonate mineral cyclicity one that is pelagic driven during glacial periods and one that is platform driven during interglacials these contrasts emphasize the differences in deep water cyclic sedimentation on carbonate ramps versus rimmed carbonate platforms that are controlled by physiography sediment source oceanographic setting and proximity to terrigenous input the productivity dilution model proposed here may be a valuable modern analog for ancient carbonate sequences deposited in epicontinental seas or on ramps attached to continental margins

glacial to holocene sedimentation on the western slope of great bahama bank abstract thick late quaternary sediment sections were recovered at several sites on the leeward slope of great bahama bank during leg 166 of the ocean drilling program these sections have paleoceanographic records with potentially high temporal resolution to make an initial assessment of the records corresponding to the holocene highstand of sea level we have identified and dated the sediments from the four upper slope sites 1004 1005 1008 and 1009 that were deposited during the period of time which spans the last glaciation through the holocene age identifications are based upon the abundances of the globorotalia menardii complex of planktonic foraminifera the stable oxygen isotopic ratios of bulk sediment and the planktonic foraminifera globogerinoides ruber and ams c 14 dating of bulk sediment comparison of these data with the sediment lithologic and geoacoustic properties shows that consistent stratigraphic relationships exist at each site the uppermost interval of aragonite rich sediments corresponds to the holocene highstand of sea level i e oxygen isotope stage 1 and these sediments are underlain by a relatively thin interval of aragonite poor partially lithified sediments which corresponds to the last glaciation when sea level was significantly lower than today i e oxygen isotope stages 2 4 the leg 166 upper slope sites possess carbonate accumulation and paleoceanographic proxy records with very high temporal resolution with sites 1004 1008 and 1009 appearing to have the greatest stratigraphic integrity comparison of core and high resolution seismic profile data establishes the holocene nature of the uppermost seismic unit in the stratigraphic package of the western slope of great bahama bank

the nature of the δ13c of periplatform sediments implications for stratigraphy and the global carbon cycle the carbon isotopic composition y 13 c of bulk carbonates obtained from a transect of sites drilled through platform and periplatform sediments of holocene to early miocene age has been compared to ascertain whether changes in the y 13 c can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the y 13 c of the dissolved inorganic carbon in the oceans over this time period five of the sites were drilled during leg 166 of the ocean drilling project 1003 1007 in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of holocene to oligocene age that are contained in 17 depositional sequences a q two shallow water sites clino and unda were situated on a extension of the same transect on great bahama bank in a water depth of 10 15 m with the exception of unda and clino the y 13 c of the carbonates ranges from 5x in the younger sequences to 1x in the early miocene in each of the sites the y 13 c is strongly positively correlated with the percentage of aragonite as a consequence the y 13 c of sequences a through f is strongly correlated reflecting the decreasing amount of aragonite with increasing depth in the two platform sites the y 13 c is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the pleistocene although the y 13 c of the individual sequences can be correlated in most instances between the odp holes the changes are not related to global changes in the y 13 c of the oceans which in contrast to the y 13 c of the platform sediments become isotopically lower towards the present day instead variations in the y 13 c appear to be related to varying mixtures of y 13 c rich banktop sediments and pelagic material

dry climate near the western pacific warm pool pleistocene caliches of the nansha islands south china sea abstract the nansha spratly islands are located in the middle of the south china sea at about 10 n near the northwestern margin of the western pacific warm pool wpwp a borehole was drilled at an atoll at nansha and cores were taken the upper 165 4 m of the borehole consists entirely of limestone of reef facies and lagoon facies which had been influenced by extensive meteoric diagenesis petrographic and geochemical studies have identified at least four subaerial exposure surfaces ses in the pleistocene carbonate sequence these ses are thought to have resulted from global sea level changes and characterized by caliche formation when developed in limestone caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm compared with 1800 2200 mm of annual rainfall of the present day nansha islands the nansha caliche therefore indicates the existence of several dry climate episodes during pleistocene sea level lowstands lower rainfall and higher evaporation during such dry conditions may explain the higher sea surface salinities reported elsewhere in the south china sea the nansha caliche may also indicate reduced extent of the wpwp and southerly shift of the intertropical convergence zone during major sea level lowstands

the decay of the spectrum of the gravitational potential and the topography for the earth summary the spectrum of the earth s gravitational potential and topography as represented by spherical harmonic expansions to degree 180 have been computed modelling the decay in the form of al values of a and for several degree l ranges were computed for degree range 5 180 was 2 54 for the potential and 2 16 for equivalent rock topography the potential decay was somewhat slower than that i e 3 implied by kaula s rule however at high degree ranges the values were larger 3 20 for degrees 101 180 agreeing better with recent determinations from terrestrial gravity data and geoid undulations implied by satellite altimetric data the values imply that the potential decays faster at higher l values the values of for topography were fairly uniform around 2 which agrees with a suggestion made by vening meinesz in 1951 we also found that the value for the earth s potential agrees well with the value implied by the topography with airy isostatic compensation with the depth of compensation equal to 30 km however the magnitude of the power implied by the topographic isostatic potential was approximately one third of the observed potential

the extension of lightning flashes from thunderstorms near cape canaveral florida the horizontal distance that intracloud ic and cloud to ground cg lightning flashes extend from thunderstorms is presented for warm season nonsevere storms near cape canaveral fl the warning decision support system integrated information wdss ii software allowed us to ingest display and make distance calculations using radar reflectivity data from nearby national weather service offices and lightning information from the national lightning detection network and the local lightning detection and ranging network the flash distances were determined manually using a measuring tool within wdss ii six categories of flashes were investigated a cg flashes measured from the edge of the convective core 30 dbz to the ground strike location that is beneath the attached anvil b cg bolt from the blue flashes measured from cloud edge 0 dbz to the ground strike point that contains only 0 dbz overhead c cg bolt from the blue flashes as in category b but measured from the edge of the precipitation 18 dbz d ic flashes that exit a portion of the cloud that is not an anvil measured from cloud edge 0 dbz e ic flashes within anvils measured from the edge of convective precipitation 30 dbz to the termination point within the anvil and f ic flashes exiting the edge 0 dbz of anvils results for each category are presented in the forms of frequency distributions of distances and their cumulative frequency distributions

sediment fluxes to marion plateau southern great barrier reef province over the last 130 ky new constraints on transgressive shedding off northeastern australia generic models for sediment accumulation predict that fluxes of siliciclastic and neritic carbonate material to continental slopes of tropical mixed systems will be out of phase over cycles of sea level change siliciclastic and carbonate fluxes should be highest during lowstands and highstands respectively and condensed sections should form during transgressions recent studies have documented the opposite response for slopes along portions of the largest modern tropical mixed system the northeast australian margin siliciclastic and carbonate fluxes are in phase and highest during transgression an outstanding issue however is the extent of this response because studies to date have mostly focused on sedimentation over the last 40 ky off the central gbr province an area flanked by a narrow shelf and a wet tropical hinterland here we investigate mixed sediment deposition in a core from marion plateau offshore a wide shelf and a dry tropical hinterland in the southern gbr province bulk carbonate content and carbonate mineralogy were determined downcore while foraminiferal oxygen isotopes and radiocarbon ages were used to constrain ages and accumulation carbonate content and mineralogy vary predictably with changes in sea level over the last 130 ky carbonate and aragonite are more abundant during highstands and less abundant during lowstands however component mass accumulation rates reveal that highest fluxes of all material siliciclastic low mg calcite aragonite and high mg calcite occurred during transgressions sedimentation consistently diverges from conventional models in all cores examined to date from slopes of the northeast australian margin regardless of core location and modern differences in climate and physiography the unexpected sediment response could result from changes in climate particularly increased precipitation during deglaciations although this does not explain elevated fluxes of low mg calcite alternatively fluvial sediments and diagenetically altered carbonate aggraded on the shelf during lowstands and were eroded and transported to the slope during transgressions

shelf edge delta and reef development on a mixed siliciclastic carbonate margin central great barrier reef the great barrier reef gbr is the world s largest extant mixed silicilastic carbonate margin previous research on the great barrier reef has suggested that the extensive barrier reef system may act as an impermeable barrier and limit the development of delta systems during lowstands but sufficient geophysical data to support this hypothesis are lacking we use dense sparker seismic and sub bottom profiler data to better understand the structure of a large lobe shaped feature 10 km 10 km on the shelf edge of the central gbr and the interactions between siliciclastic and carbonate sedimentary systems interpreted sparker seismic contains prograding clinoforms and suggest that the lobe shaped feature was a river dominated shelf edge delta a delta on the shelf edge implies that the presence of an exposed barrier reef was not a major impediment to deposition and that other adjacent lobe shaped features are also deltaic deposits the shelf edge deltas were deposited onto a broad upper slope terrace that allowed continued progradation and limited incision when sea level fell below the shelf edge delta foresets are commonly colonized by coral reefs but the spatial and temporal relationship between reefs and some deltaic units remains unclear the presence of multiple shelf edge deltas that link to previously mapped burdekin river paleo channels indicates a complex history of sedimentation with the burdekin river delta migrating up to 100 km along the gbr margin during the late quaternary regional bathymetric data suggest that large modern or recent shelf edge deltas are rare on the gbr and that there was a broad range of sedimentary processes operating along the margin of the gbr during periods of low sea level

late quaternary shedding of shallow marine carbonate along a tropical mixed siliciclastic carbonate shelf great barrier reef australia the north east australian margin is the largest modern example of a tropical mixed siliciclastic carbonate depositional system with an outer shelf hosting the great barrier reef gbr and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges the long term interplay between these sediment components and sea level is recorded in the queensland trough a 1 2 km deep n s elongate basin situated between the gbr platform and the queensland plateau in this paper 154 samples from 45 surface grabs and six well dated piston cores were analysed for total carbonate content carbonate mineralogy and sr concentration to establish spatial and temporal patterns of carbonate accumulation in the queensland trough over the last 300 kyr surface carbonate contents are lowest on the inner shelf 5 and in the trough axis 60 because of siliciclastic dilution carbonate on the shelf is mostly sr rich aragonite and high mg calcite hmc whereas that in the basin is mostly low mg calcite once normalized to remove the effects of siliciclastic dilution surface sr rich aragonite and hmc abundances decrease linearly to background levels 100 km seaward of the shelf edge core samples show that over time normalized aragonite and sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge this is consistent with changes in the production of coral and calcareous algae and the shedding of their debris from the shelf interestingly normalized hmc concentrations on the slope peak during periods of major transgression perhaps because of maximum off shelf transport from inter reef areas or intermediate water dissolution after accounting for siliciclastic dilution there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north east australian margin and those of pure carbonate margins such as the bahamas a limited set of basic processes including the formation and breakdown of carbonate on the shelf the transport of carbonate off the shelf and eustatic sea level probably controls carbonate accumulation in slope and basin settings of tropical environments irrespective of proximal siliciclastic sediment sources

triassic carbonate submarine fans along the arabian platform margin sumeini group oman the sumeini group formed along the passive continental margin slope that bounded the northeastern edge of the arabian carbonate platform with the initial development of this passive continental margin in oman during early to middle triassic time possibly permian small carbonate submarine fans of the c member of the maqam formation developed along a distally steepened slope the fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate calciclastic sediment within a thick interval of basinal lime mudstone and shale repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits sediment gravity flows primarily turbidites with lesser debris flows and grain flows transported the coarse sediments to the relatively deep submarine fans channel erosion was a major source of intraformational calcirudite two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources submarine canyons the northern fan system retrogrades and dies out upsection the southern fan system was apparently longer lived calciclastic sediments in it are more prevalent and occur throughout the section the proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner to mid fan channel complexes the distal portions include mostly lenticular unchannelized beds of calcarenite apparently mid to outer fan lobes carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment the carbonate submarine fans of the c member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and or transform tectonics the alternation of discrete sequences of calciclastic sediment with thick intervals of background sediment resulted from either sea level fluctuations or pulses of tectonic activity

spatial and temporal evolution of horizontally extensive lightning discharges associated with sprite producing positive cloud to ground flashes in northeastern spain 1 during the evening of 6 august 2008 a small mesoscale convective system mcs entered the area of radar and 2 d interferometric lightning detection system coverage in northeastern spain and produced 17 sprites recorded by a camera at only 95 180 km distance this study presents an analysis of the in cloud component of the sprite associated lightning flashes and those of other flashes the analysis focuses on the horizontal development of sprite producing lightning by discussing three examples divided into the periods before the positive cloud to ground flash cg between cg and the end of the sprite and the period after the sprite location and horizontal size of sprites appear to be well explained by the temporal and spatial development of the lightning path the majority of sprite producing discharges started directly at the rear side of developing and mature convective cores within the decaying mcs either with the cg or with preceding negative leaders the cg started a burst of vhf sources during which the sprite developed delayed carrot sprites developed after a secondary smaller burst and were well collocated with the burst toward the rear of the mcs the order of development of elements in a grouped sprite followed the direction of lightning propagation during the burst stage the second part of the analysis concentrates on the metrics of sequences of vhf sources and shows that sprites are indeed produced by the largest longest lasting discharges with particularly large line perpendicular dimensions 37 km median compared with 11 km for cg 25 ka

mineralogy and grain size variations along two carbonate margin to basin transects pedro bank northern nicaragua rise abstract the analysis of nine periplatform cores in the surrounding of the pedro bank carbonate platform showed the presence of characteristic depositional environments during the last 300 ky this subdivision is based on mineralogical and grain size variations in space and time along the leeward downcurrent transect the fine sediment fraction 40 km aragonite is still the most abundant mineral but the percentage decreases while the pelagic carbonate mineral content lmc increases within the subordinate coarse fraction classes 63 m the very fine sand fraction dominates at proximal sites 20 km show a more bimodal distribution pattern in the coarse grain sizes with maximum percentages within the very fine and medium sand fraction indicating a mixed neritic pelagic signal during glacial lowstands in sea level a twofold division in the spatial distribution of the periplatform sediments is evident a proximal environment 28 km showing a strong dominance of the fine fraction 90 the increased coarse fraction percentage during glacials at proximal sites results from various interacting processes 1 lower input of fine neritic sediments 2 increased current winnowing and 3 redepositional processes at the upper slope during lowered sea level and the export of this material to proximal basinal sites at upcurrent sites the mineralogy displays a similar spatial evolution as seen along the downcurrent margin but with overall reduced percentages this is the result of the lower export potential of the platform against the main direction of the caribbean current the most important factor for neritic sediment dispersal in the study area the coarse fraction content is slightly higher during interglacials compared to the downcurrent margin during glacials a similar reduction in the percentage of the coarse fraction is evident in a spatial context this also substantiates the reduced sediment export and redeposition potential at the upcurrent margin

effects of isostasy on large scale geoid signal ii geoid anomaly over the earth summary the geoid anomaly due to the oceanic and the continental lithosphere is a second order quantity which is sensitive to the definition of isostasy as the geoid and the compensation level 2200 km depth are both equipotential surfaces further potential changes are induced due to infilling of sea water above and mantle material below until equilibrium is maintained the final geoid is composed of seven second order effects in this work the surface of the earth is divided into 36 x 18 spherical trapezoids 10 x 10 each and age is assigned to each trapezoid containing oceanic lithosphere using the map of larson et al the geoid anomaly is calculated using the method of rings in which a set of 18 rings about an arbitrary point observation point covers the whole earth over most of the earth the direct dipole effect is dominant relative to the direct mass effect the final geoid anomaly after the self deformation effect can be up to twice as much as the initial mass and dipole signals the calculated geoid follows some major observed anomalies over the earth such as the geoid high in the west pacific and the low in the indian ocean but is poorly correlated with the observed geoid due to large and wide anomalies presumably caused by flow in the mantle such as the low over antarctica these results suggest although not statistically significant that the residual geoid variance reaches a minimum if the reference density under the oceanic lithosphere of 3 28 g is slightly higher than the 3 268 cm under the continental lithosphere

reef talus a popular misconception reef fronts have traditionally been regarded as comprising debris derived by contemporaneous erosion of the reef however evidence from wave transport indicates that on present day reefs the bulk of the debris generated in this way accumulates in the back reef area with only finer grained sediment carried off reef by retreating flows or by overwash nevertheless in contrast to this observation fore reef debris slopes are commonly considered characteristic of phanerozoic reefs this apparent error reflects the conflation of processes defining contemporary growth and accretion of the reef and the corresponding long term accretion of the carbonate platform on which it rests present day reefs are commonly although not exclusively additions to long lived carbonate platforms growth of the latter is intermittent and has been moderated by changes in sea level that for recent reefs have been on time scales of less than 100 ka during low sea level stands growth ceases or is translated downslope and earlier deposits are subject to lithification and subaerial erosion similar changes are applied on a larger scale to the aggrading growth of carbonate platforms but the bulk accretion of these includes quite different processes and reflects far longer timescales during low sea level stands the margins of platforms commonly become unstable with instability reflected in slope failure and in the shedding of blocks ranging from metres to kilometres in diameter associated with the generation of debris flows and turbidites it is argued that these are the materials that are commonly described as reef talus in ancient structures although their formation is largely independent of any contemporary reef growth difficulties arise where the reef and the platform are treated as a single functional entity it is important to recognize the conceptual distinction between them reef talus is a misleading description of the debris predominantly generated by platform erosion and slope failure

inversion of airborne gravity gradient data southwestern oklahoma the diagonal elements of the gravity gradient tensor as recorded by the bell airborne gravity gradient survey system ggss are used to determine the basement topography in southwestern oklahoma this determination is accomplished through a nonlinear inverse procedure based on the conjugate gradient algorithm the resulting model contains a ridge of shallow basement material or 3 0 km deep trending east southeast this ridge is bounded on the north and the south by basement troughs the northern trough extends as deep as 10 0 km the gradient field which results from this model fits most of the ggss observations within their estimated errors of 12 0 e 1 eotvos 10 super 6 mgal cm the model is in general agreement with a set of available oil well depths to the basement and with inferred faults in these igneous rocks to assess the derived solution the problem was linearized about the final model and the linear parameter resolution and parameter covariances were computed generally the basement depths are well resolved and the resolution matrix is diagonally dominant furthermore the parameter standard errors are small 72 parameters out of 98 have errors less than 1 0 km

global surface water induced seasonal variations in the earth s rotation and gravitational field summary the seasonal cycle in continental surface water storage is modelled first using a global meteorological data set of precipitation snow and rain and evapotranspiration compiled by willmott et af its effects on the annual wobble excitation y and the seaonal variations in the length of day lod and low degree zonal gravitational coefficients ji are then computed the resulting y agrees well with the widely referenced estimate by van hylckama however these results are questionable because an anatomy of the computation indicates a near complete east west longitudinal cancellation ia the snow contribution in contradiction to the satellite remote sensing data study by chao et af the problem arises from a chronic underestimation of asian snow mass by conventional methods in order to obtain an optimal estimate of y under present circumstances we combine the contribution of rainfall and evapotranspiration computed from willmott et af s data set with the snow load contribution computed by chao et al from satellite data the result indicates a great deal of cancellation between these two contributions especially in the prograde component because of their opposite seasonal cycles and the similar quantities of water mass involved the surface water contribution to y is thus far smaller than previously computed the seasonal variations in j2 j3 j4 and lod are similarly estimated because of the zonal nature of their weighting functions they suffer no longitudinal cancellation so that the chronic underestimation of asian snow is less of a problem furthermore their rainfall and snow load contributions largely reinforce each other quantitatively the surface water induced variation may prove to be a significant contributor to the overall seasonal non tidal variations in low degree ji coefficients and lod this study excludes greenland and antarctica where no pertinent hydrological data are available at present