global p wave tomography on the effect of various mantle and core phases abstract in this work many global tomographic inversions and resolution tests are carried out to investigate the influence of various mantle and core phase data from the international seismological center isc data set on the determination of 3d velocity structure of the earth s interior our results show that when only the direct p data are used the resolution is good for most of the mantle except for the oceanic regions down to about 1000 km depth and for most of the d layer and pp rays can provide a better constraint on the structure down to the middle mantle in particular for the upper mantle under the oceans pcp can enhance the ray sampling of the middle and lower mantle around the pacific rim and europe while pdiff can help improve the spatial resolution in the lowermost mantle the outer core phases pkp pkikp and pkkp can improve the resolution in the lowermost mantle of the southern hemisphere and under oceanic regions when finer blocks or grid nodes are adopted to determine a high resolution model pp data are very useful for improving the upper mantle structure the resulting model inferred from all phases not only displays the general features contained in the previous global tomographic models but also reveals some new features for example the image of the hawaiian mantle plume is improved notably over the previous studies it is imaged as a continuous low velocity anomaly beneath the hawaiian hotspot from the core mantle boundary cmb to the surface implying that the hawaiian mantle plume indeed originates from the cmb low velocity anomalies along some mid oceanic ridges extend down to about 600 km depth our results suggested that later seismic phases are of great importance in better understanding the structure and dynamics of the earth s interior

the present day formation of apatite in mexican continental margin sediments abstract results of pore water and sediment analyses from the western mexican continental margin strongly suggest the present day formation of apatite the interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments apatite is identified within this maximum via x ray diffraction but is elsewhere undetectable in the core radioisotopic thorium uranium and radium data support the conclusion that this deposit is modern the present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

constraints on the core mantle boundary topography from p4kp pcp differential travel times 1 p4kp pcp differential travel times are examined to infer the core mantle boundary cmb topography a total of 362 p4kp pcp times are measured with a measurement error of 0 5 s the travel times are corrected for the earth s hydrostatic ellipticity and mantle heterogeneity using a p wave tomographic model spherical harmonic expansion up to degree 4 is adopted for model parameterization the p wave velocity heterogeneity is then estimated in the lowermost 150 km of the mantle to overcome the problem of underestimation of the velocity perturbation at the base of the mantle in the global 3 d p wave mantle model subsequently the cmb topography is inferred using the residues of the above processes since the odd degree components of the cmb topography are insensitive to the p4kp pcp times only the components of degrees 2 and 4 are solved for the resultant features indicate that the maximum amplitude of the cmb topography does not exceed 2 km with an uncertainty of less than 0 5 km a numerical test confirms that the pattern of degree 4 is more reliable with less amplitude recovery the obtained degree 4 pattern shows an amplitude of less than 1 km and indicates the presence of depressions under the circum pacific the central pacific and south africa

sea floor weathering of phosphate nodules off east australia its effect on uranium oxidation state and isotopic composition the relative abundances and 234u238u activity ratios of tetravalent and hexavalent u in 33 phosphorite nodules from the east australian continental margin have been determined u iv constitutes between 59 96 average 80 of the total u in non ferruginous quaternary nodules but between 11 86 average 63 in ferruginous tertiary nodules quaternary phosphorite nodules on the east australian continental margin form within well oxygenated low carbon flux environments on the upper continental slope generally at water depths between 350 450 m predictable relationships exist between the radiometric age and tetravalent u content u iv of the nodules and their p2o5 fe2o3 and labile organic carbon concentrations the nodules probably initially form as part of a more widespread phosphatic layer which contains 7 9 p2o5 has fe2o3k2o ratios between 3 4 hydrogen indices hi 80 and is relatively uranium rich u p2o5 10 4 15 20 virtually all the u is originally incorporated into the nodules as u iv the layer is subsequently broken up into discrete nodules which are periodically reworked and exposed at the sediment water interface during this exposure the nodules preferentially gain both apatite and iron hydroxide as a result their p2o5 concentrations increase to 16 18 within 80 kyr of their formation whereas the fe2o3k2o ratios increase to 8 10 little or no u apparently accompanies this apatite for the up2o5 ratios approximately halve as the p2o5 concentration doubles and the nodules have concordant 230th234u and 231pa235u ages contemporaneously exposure causes progressive oxidation of both the organic carbon hi decreases to 20 and tetravalent uranium u iv decreases to 60 70 contents the high u concentrations and high u iv of the east australian samples show that the presence of abundant organic matter is not necessary for the fixation of u in sedimentary apatite moreover the u iv in these phosphorites is not a direct function of the redox potential of the depositional environment but rather an indicator of the amount of sea floor weathering experienced by the nodules

topography at the core mantle boundary analysis of the earth s gravity field using ratio of gravity to geoid g n and cumulative degree contribution curve techniques improves resolution of the mass depth and dimensions of anomalous mass sources within the planet the source for four of the ten major geoid anomalies of the earth has large contributions from harmonic degrees 2 and 3 lie at the core mantle boundary and the greatest has a mass anomaly of 4 2 1022 grams that boundary has a density contrast 4 4 gm cm which is greater than at any other location including the contrast at the earth s surface with space thus relatively small vertical undulations of that boundary produce very large mass anomalies topographic relief of less than 3 km at the core mantle boundary occurs and accounts for the existence of the largest geoid anomalies occurring on the earth

growth history of phosphorite nodules inferred from their remanent magnetization abstract magnetization measurements of one phosphorite nodule from off new zealand and two nodules from off peru were performed on their thin slices the results reveal the presence of very weak and rather stable magnetizations and of both normal and reversed polarity intervals in two samples the generally localized distributions of the natural remanent magnetization directions imply that these nodules should have only slightly shaken centering a certain axis but cannot have frequently turned over during their growth the recording of magnetic reversals in two of three phosphorite nodules thus is not the result of diametrically turning over suggesting that these two nodules may have started to form in the matuyama reversed polarity chron the average long term accretion rates of the two nodules from off new zealand and off peru are estimated to be at least 0 01 mm and 0 036 mm per 1000 years respectively

palaeoenvironmental controls on the uranium distribution in an upper carboniferous black shale gastrioceras listeri marine band and associated strata england the mudrocks associated with the gastrioceras listeri g listeri marine band contain 0 to 200 ppm authigenic uranium several geochemical degree of pyritisation dop c s ratios and framboidal pyrite size distribution and palaeontological indicators oxygen restricted biofacies orb analysis suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen restricted biofacies orbs 2 and 3 the uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals these horizons may have been deposited when the oxygen minimum zone impinged on the seafloor brief oxygenation of an otherwise anoxic environment was conducive to francolite precipitation which then scavenged dissolved uranium slow sedimentation rates are also important in concentrating uranium and francolite thus enriched u values are only encountered under specific conditions of low but fluctuating oxygen regime and extremely slow sedimentation rates truly euxinic facies lacking any fossils and a uniformly small framboid population do not contain high concentrations of francolite and are not u rich

carbon and phosphorus cycling in anoxic sediments of the saanich inlet british columbia abstract uncertainty currently exists about the removal of carbon c and phosphorus p from the oceanic reservoir especially in low oxygen settings in this paper the cycling of c and p is examined in sediments from the anoxic saanich inlet cored by ocean drilling program odp leg 169s in 1996 at two sites although c org p org ratios are high and increase with depth in the saanich inlet this effect is due largely to a remobilization of p from an organic matter sink to an authigenic sink reducible sedimentary components act as temporary shuttles in this process even in this anoxic setting with the ultimate burial sink for the remobilized p being carbonate fluorapatite the effective c org p reactive molar ratio appears to be about 150 200 indicating some preferential loss of p compared to c during organic matter degradation but not approaching previously reported values of over 3000 in black shales reactive p accumulation rates in this basin range from 10 000 60 000 mol cm 2 kyr greatly exceeding the range of 500 8000 mol cm 2 kyr found in most continental margin settings including regions of modern phosphogenesis the initiation of marine sedimentation in the saanich inlet occurred after deglaciation and the high rates of p burial seen here may provide an end member example of the effects of sea level and margin sedimentation on the distribution of p within the marine p cycle

genesis of phosphorite crusts off peru abstract authigenic phosphorite crusts from the shelf off peru 9 40 s to 13 30 s consist of a facies with phosphatic coated grains covered by younger phosphatic laminite the crusts are composed of carbonate fluorapatite which probably formed via an amorphous precursor close to the sediment water interface as indicated by low f p2o5 ratios sr and ca isotopes as well as rare earth element patterns agreeing with seawater dominated fluids small negative ce anomalies and u enrichment in the laminite suggest suboxic conditions close to the sediment water interface during its formation increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and consequently point to episodical development of anoxic conditions during phosphogenesis the peruvian phosphorites formed episodically over an extended period of time lasting from middle miocene to pleistocene individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide rich phosphatic layers coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer phosphatic coated grains were later transported to the place of crust formation where subsequent laminite formation was favored under lower energy conditions a similar succession of phosphatic layers with varying contents of organic matter and sulfide rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains

subtidal stromatolites in monterey formation and other organic rich rocks as suggested source contributors to petroleum formation the miocene monterey formation contains thin organic rich laminations that mimic benthic mats forming today in coastal upwelling regimes of the eastern pacific modern mats are comprised dominantly of filamentous sulfur oxidizing bacteria of the family beggiatoaceae the characteristic spongy texture imparted to the sediments by these bacterial mats is most readily recognized using scanning electron microscopy sem the spongy network becomes progressively compressed with increasing compaction and diagenesis but is still distinguishable rock eval pyrolysis of modern mats reveals high s1 peaks and unusually high oxygen indices both parameters decrease gradually with diagenesis as evidenced by mat laminated monterey formation samples of different thermal grades in addition anomalously light nitrogen isotopic signatures of modern mats may be retained the dgrn15 values of mat laminated monterey formation samples although diagenetically enriched in 15n are still lighter than would be expected for algally dominated organic matter in marine low oxygen environments on the basis of organic richness and high hydrogen indices of mat laminated samples and coincidence of mat laminated rocks and oil in some fields in the san joaquin valley of california the organic matter assemblage contained within filamentous bacterial mats is suggested to be a significant contributor to petroleum formation additional evidence for this conclusion is provided by similar investigations of subtidal cyanobacterial mats in carbonate rocks such as the green river formation oil shales

oxygen isotope variations in phosphate of biogenic apatites ii phosphorite rocks abstract phosphorites from sedimentary sequences ranging in age from archaean to recent were analysed for 18 o in both the po 4 18 o p and co 3 18 o c in the apatite lattice the oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts the use of the longinelli and nuti 8 temperature equation yields temperatures for recent phosphorites that are in good agreement with those measured in the field the 18 o p values of ancient phosphorites decrease with increasing age these changes with time are not likely to be due to post depositional exchange changes in 18 o values of seawater and variations of temperature with time can account for the 18 o p time trend but the latter explanation is preferred in ancient phosphorites 18 o c in structurally bound carbonate in apatite is not a reliable geochemical indicator

early diagenesis of organic matter in peru continental margin sediments phosphorite precipitation abstract pore water chemistry total dissolved co 2 nh 4 no 3 no 2 po 4 si oh 4 ca mg fe mn so 4 h 2 s and f and titration alkalinity solid phase chemistry c org p org c tot n tot f si opal and s ii and sediment characteristics porosity dry bulk density and formation factors were determined on a centimeter scale spacing in the upper 20 40 cm of sediments under intense upwelling areas on the peru continental shelf these data demonstrate that carbonate fluorapatite cfa is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content up to 20 c org very high porosity 0 96 ml cm 3 and very low dry bulk density 3 dissolved phosphate concentrations at the sediment water interface range from 20 to 100 m orders of magnitude higher than bottom water concentrations and much higher than predicted from regeneration of organic matter the mechanism of this interfacial phosphate release is unclear but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments fluoride is incorporated into cfa by diffusion from the overlying seawater and carbonate ions are provided from pore water alkalinity magnesium concentrations in this reaction zone are not significantly different from those of seawater suggesting that magnesium depletion is not a necessary prerequisite for cfa precipitation the environment of precipitation is interface linked rather than driven by organic diagenesis of phosphorus deeper in the sediment most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region but almost all show the precipitation signature near the interface this interface linked early diagenetic porewater environment for the precipitation of cfa explains many of the geochemical characteristics of phosphorites and provides a testable model to compare the modern phosphogenic analog with ancient phosphorite deposits two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites

equivalent monopole source of the geomagnetic south atlantic anomaly the south atlantic magnetic anomaly saa is an important feature of the present geomagnetic field in this paper we model the space time evolution of this anomaly over the past 400 years in terms of the resultant between a decrease of a global axial dipole and an increase of a virtual local monopole source certain characteristics of this evolution are investigated and some considerations are made in the light of a possible special state of the global geomagnetic field dynamical regime among the possible speculations one is made regarding the topography of the core mantle boundary cmb and its possible aspect beneath the saa region in terms of simple sinusoidal undulations met by the monopole source during its centennial motion

a refined gravity model from lageos gem l2 a refined gravity field model goddard earth model gem l2 has been derived using the lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion this field also contributes to an improved understanding of long wavelength features such as the sea slope across broad ocean basins through its significant improvement of the long wavelength geoid through degree and order 4 the geoid for these terms has an accuracy estimated at 8 cm in gem l2 gem l2 as in all recent goddard earth models relies heavily on the precise near earth satellite laser ranging data in this case provided by nasa s crustal dynamics program two and a half years of lageos laser data acquired from over 20 well distributed stations were combined with the existing data from the best satellite derived model gem 9 to develop the new lageos model testing shows that the lageos gravity field error at long wavelengths is less than half that for gem 9 independent tests using well determined longitude accelerations of 24 hour satellites have verified the improved accuracy of the new model a comparison of global laser base stations from independent data sets of alternating 15 day data segments over the 2 1 2 years of lageos show total inter station positioning to 1 8 cm when using this new field the same comparison using the 1979 versus the 1980 lageos data yields 5 2 cm this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than 5 msec have been derived in the solution the adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in gem l2

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

organic carbon cycling and modern phosphorite formation on the east australian continental margin an overview during 1987 the australian bureau of mineral resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern australia between 28 and 32 s the objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements nitrogen phosphorus sulphur and iron in the sediments and to investigate the roles which these processes played in the formation of the modern phosphorites bacterial productivities sulphate reduction rates sedimentary oxygen and pore water concentrations of nitrate ammonia phosphate iron sulphate and fluoride were measured at sea the highest rates of microbial productivity were found in the surficial 0 20 mm sediments of the modern phosphorite zone in 350 460 m water depth these rates were about double those in shallower shelf 300 m sediments and 3 4 fold those rates in mid slope 600 1000 m sediments aerobic and anaerobic oxidation rates of organic matter calculated from sediment oxygen profiles and sulphate reduction rates were highest in the surface sediments in the modern phosphorite zone the recycling of sedimentary iron via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic anoxic boundary results in a near surface sedimentary trap for iron in the phosphorite zone sediments phosphate released from organic matter in the interfacial sediments and fluoride from seawater are scavenged by iron oxyhydroxides in the top few centimetres of sediment phosphorus in this way is decoupled from organic carbon in the near surface sediments and linked to the redox cycling of iron phosphate and fluoride scavenged onto iron oxyhydroxides and concentrated in the surficial sediments are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve the recycling process releases phosphate and fluoride for incorporation into apatite fluoride is depleted from pore waters at depths 18 cm phosphorite nodules form within anoxic sediments at depths 18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer combinations of rapid sediment mixing rates a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten fold that of the mid slope sediments long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules phosphorite nodules are not found in mid slope sediments probably because of combinations of relatively rapid sedimentation rates ineffective iron phosphorus and fluoride recycling and trapping mechanisms plus dilution and dissemination of any incipient apatite

the geological significance of the geoid data are being used to study terrestrial phenomena at scales from tens of thousands of kilometers down to hundreds or even tens of kilometers this paper focuses on the interpretation of global gravity in the form of the geoid and concentrates on what has already been learned from it about the structure of the earth at long to intermed iate wavelengths the longest wavelengths of the gravity field are dominated by density distributions that must be supported by very large scale convective processes i attempt here to document the claim that these density anomalies are both deep and old at slightly shorter length scales the geoidal anomalies give very useful constraints on possible subduction processes and lead to the conclusion that density contrasts in the subducting slabs must be regionally partially compensated and supported from below at yet shorter length scales satellite altimetry results give insight into the thermal and mechanical structure of the oceanic litho sphere seeming to favor plate over thermal boundary layer models of cooling at the shortest length scales the location and mode of compen sation of individual seamounts can be studied

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

the incorporation of uranium into diagenetic phosphorite abstract the behavior of u during the diagenetic formation of marine phosphorite has been modelled the model examines a dissolution reprecipitation replacement of skeletal hydroxyapatite calcium carbonate and earlier generated francolite by francolite the amount of organic matter consumed relative to the mass of francolite formed the replacement reaction progress and the concentration of u in the replaced phases are the important parameters which dictate the concentration of u in the phosphate rock a partition coefficient between apatite and interstitial solution was calculated and is u f 0 57 natural phosphorites have been examined and are discussed in the light of the proposed model the u mass balance in a recent phosphorite is in good accord with theoretical predictions differences in u concentrations between sea floor phosphorites are explained either by the original variation in the organic matter distribution in the corresponding sediments and or by mineralogical differences caco3 vs hydroxyapatite therein senonian phosphate rocks which were formed via the francolite francolite transformation demonstrate that during that process the organic matter content in the sediment was approximately 50 the model supports the idea that phosphorite rock formation is a multistage process

the mineralogy geochemistry and origin of pelletal phosphates in the english chalk abstract deposits of pelletal phosphates in the english chalk are made up of various phosphatised bioclasts mainly the faecal pellets of detrital feeding organisms such as worms and crustaceans xrd and infra red studies show that the clasts are composed of a poorly crystalline carbonate fluorapatite a finding that is substantiated by analytical data for major elements the trace element chemistry of the chalk phosphates is relative to an average shale highly anomalous high levels of biophilic elements such as i br and zn must be a reflection of the importance of biogenic agencies in dictating the formation of the phosphates low levels of ba rb zr and nb reflect a paucity of detrital minerals high levels of u sr f and certain rare earth elements are explicable in terms of inorganic adsorption processes that were operative after the formation of the initial phosphate phases in the chalk sea the formation of concentrated deposits of pelletal phosphate in the chalk of nw europe is hypothesised to have been related to the development of an upwelling regime over the epeiric chalk sea of the region and to the form ation of erosional cuvettes in the chalk ooze upwelling triggered an extensive growth of phytoplankton mainly coccolithophorids this being the first step in the enrichment of phosphorus and biophilic trace elements from seawater the phytoplankton were then passed onto higher trophic levels in the chalk sea probably copepods so leading to a further enrichment and fractionation of trace elements in the resulting biomass faecal pellets and the dead carpaces of the zooplankton plus other biogenic debris produced by nekton were then concentrated into the cuvette areas here a specific population of copraphageous worms and crustaceans converted the biogenic fall out into final generation faecal pellets consisting of carbonate mud and phosphorus bearing organic matter the latter was broken down by anaerobic bacterial activity so releasing inorganic phosphate into the sediment pore waters calcitic mud in the faecal pellets plus other calcitic components in the sediment such as foram tests was then replaced by intermediate phosphate phases such as structive due to uptake of f and co2 3 these metastable phases were then converted into the carbonate fluorapatite seen in the present deposits u sr and certain rare earth elements were taken up immediately after this change had been completed some of the original organic matter has been preserved as a characteristic nitrogen rich humic acid

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

the earth s gravity field and plate tectonics abstract a recent decomposition of the earth s gravity field has allowed mass anomaly sources spatially related to plate tectonic features to be distinguished from those mass anomaly sources that are not the earth s largest mass anomalies are not related the mass anomalies contributing to the degree 2 3 field are inferred to be the result of topography at the core mantle boundary produce the world s largest geoid anomalies and are unrelated to the plate tectonics occurring at our planet s surface the degree 4 10 field shows major narrow bands of positive geoid anomalies with marked coincidence everywhere with plate convergence zones the degree 4 10 mass anomaly appears to lie in the lower mantle beneath plate convergence zones below the teleseismically determined downgoing subducted slabs and may be an important driving force for plate tectonics contributing to the stabilization of the location of subduction zones through time subtracting the degree 10 field from observed data yields residual anomalies both the residual geoid and gravity anomalies result entirely from mass anomalies that lie in the outer part of the earth the residual geoid anomalies have much lower magnitude than those of the degree 2 3 and 4 10 fields axes of spreading in the indian ocean in the north atlantic and partially in the south atlantic lie along centers of positive residual geoid and gravity anomalies the east pacific rise is without expression presumably because of its fast spreading rate with broad topography fracture zones are commonly well defined because of their linearity although their geoid and gravity amplitudes are small amongst spreading centers without superimposed hot spots fracture zones and subduction zones the subduction zones have the largest residual gravity anomalies although their residual geoid anomalies are generally less than 20 m their residual gravity anomalies of 200 to 350 mgal show them to be the sites of the largest magnitude gravity anomalies of our planet residual geoid anomalies associated with the elevated topography of hot spot features reach 22 m

phosphate regeneration from sediments of the peru continental margin by dissolution of fish debris generally oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes stoichiometric modelling of such reactions takes into consideration varying proportions of decomposable organically bound p to account for the ratios among nutrients in depth concentration profiles of near surface sediments new results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound p dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10 of the nutrient pool in the waters of the peru undercurrent partitioning of p among the two sources reveals further that fish debris phosphate is about four times more important than organically bound p in nutrient generation from sediments of the peru continental margin not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area

deformation of internal boundaries in a viscoelastic earth and topographic coupling between the mantle and core summary in a two layer self gravitating viscoelastic maxwell earth is solved analytically and the behaviour of the internal interface which is non monotonic in time is shown to be due to the buoyancy effect across the interface deformation inside realistic earth models is calculated using the new normal mode method some modes with long relaxation times which do not contribute significantly to surface deformations are found to be significant for deformations inside the mantle flow patterns inside the mantle and the topograph at the core mantle boundary cmb due to surface glacial loads have been calculated glacial induced topography at the cmb unless suitably located is shown to be too small to significantly alter the total coupling mechanism and thus flow in the fluid core if there is any causal relationship between climatic changes and geomagnetic reversals then it is probably due to the transfer of rotational energy from the mantle to the core a revised estimate of 100 m drop in sea level is required to cause changes in the flow pattern in the core

carbon isotopic composition and lattice bound carbonate of peru chile margin phosphorites new light stable carbonate carbon isotope and lattice bound co2 data from quaternary peru chile margin phosphatic nodules crusts and pelletal grains and from associated dolomicritic concretions are presented which provide constraints on the timing and mechanisms of growth of these phases in organic carbon rich sediments comparison of 13c values from carbonate fluorapatite cfa nodules and pelletal grains 4 8 to 0 0 and 2 9 to 1 0 pdb respectively with pore water total dissolved 13c values from these sediments suggests early authigenic cfa precipitation from pore waters within a few centimeters of the sediment water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter in contrast the dolomicritic cores of nodules recovered from about 12 s display both strongly negative to positive 13c values 10 8 to 6 1 characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes the amount of structural carbonate in cfa suggests that carbonate substitution generally increases as 13c in cfa decreases a probable consequence of increasing carbonate and accompanying charge balancing substitutions in the cfa lattice in response to increasing pore water carbonate ion concentrations with depth below the sediment water interface in one buried upward growing nodule decreasing cfa 13c and increasing structural co2 also correspond to decreasing cfa growth rates these data suggest that in addition to other constraints such as pore water phosphorus and fluoride availability the lower limit of cfa precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations in organic rich peru chile margin sediments this depth threshold appears to be at approximately 5 10 cm below the sediment water interface where maximum cfa co2 contents of about 6 wt occur in less organic rich settings greater depths of precipitation of cfa may be anticipated below this relatively shallow depth of cfa precipitation on the peru shelf high pore water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

inference of core mantle boundary topography from isc pcp and pkp traveltimes we invert isc pcp and pkp absolute and differential traveltimes in an attempt to infer the long wavelength topography of the core mantle boundary cmb the data selection and processing methods are described and evaluated these travel time data are very noisy and the geographic distribution of the data is highly non uniform inhibiting reliable inference of cmb topography spatial averaging enhances the coherent component of the residual variance related to heterogeneity however the random component of the variance is much larger than the coherent component we show that for pcp data the coherent signal due to mantle heterogeneity overshadows that arising from the cmb and that the effects of mantle heterogeneity are mapped into our inferred cmb solutions the pcp data are not correlated across the spatial averaging bins and seem to have a strong bias due to small scale structure and or noise the non uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the cmb spatial patterns of cmb models inferred from different phases do not agree amplitudes of seismically inferred cmb undulations vary greatly the sensitivity of inferred cmb models to the processing spatial averaging procedure and inversion techniques are investigated topographic amplitudes increase strongly with increasing input residual variance the power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher based on this work we conclude that reliable inference of long wavelength cmb topography is not likely with the current isc data set or with a spherical harmonic expansion truncated to degree and order 6

depth estimates from ratios of gravity geoid and gravity gradient anomalies the utility of combining geoid gravity and vertical gravity gradient measurements for delineation of causative mass anomalies is explained and compared with spatial and spectral methods for depth estimation depth rules for various source geometries are reviewed and new rules developed for geoid gravity and vertical gravity gradient data both spatial and frequency domain methods are discussed simple ratios of single observations of different data types e g geoid gravity or vertical gravity gradient are shown to provide information comparable to the traditional spatial and frequency analyses of one data type alone

core mantle boundary topography from short period pcp pkp and pkkp data abstract we use a total of 839 369 pcp pkpab pkpbc pkpdf pkkpab and pkkpbc residual travel times from bull seism soc am 88 1998 722 grouped in 29 837 summary rays to constrain lateral variation in the depth to the core mantle boundary cmb we assumed a homogeneous outer core and the data were corrected for mantle structure and inner core anisotropy inversions of separate data sets yield amplitude variations of up to 5 km for pcp pkpab pkpbc and pkkp and 13 km for pkpdf this is larger than the cmb undulations inferred in geodetic studies and moreover the pcp results are not readily consistent with the inferences from pkp and pkkp although the source receiver ambiguity for the core refracted phases can explain some of it this discrepancy suggest that the travel time residuals cannot be explained by topography alone the wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and cmb topography in a second experiment we therefore re applied corrections for mantle structure outside a basal 290 km thick layer and inverted all data jointly for both cmb topography and volumetric heterogeneity within this layer the resultant cmb model can explain pcp pkp and pkkp residuals and has approximately 0 2 km excess core ellipticity which is in good agreement with inferences from free core nutation observations joint inversion yields a peak to peak amplitude of cmb topography of about 3 km and the inversion yields velocity variations of 5 in the basal layer the latter suggests a strong trade off between topography and volumetric heterogeneity but uncertainty analyses suggest that the variation in core radius can be resolved the spherical averages of all inverted topographic models suggest that the data are best fit if the actual cmb radius is 1 5 km less than in the earth reference model used i e the average outer core radius would be 3478 km

early diagenetic phosphate cements in the albian condensed glauconitic limestone of the tatra mountains western carpathians early diagenetic phosphate cements are described from the albian condensed glauconitic limestone of the tatra mountains western carpathians with regard to their macro and micromorphology distribution classification and genesis the cements occur within stratigraphically condensed semi pelagic foramini feral glauconitic layers and are associated with mature hardgrounds within the tatra albian limestone phosphate cement fabrics consist of crypto to microcrystalline carbonate fluorapatite and they occur as i rim envelopes ii infillings of intraparticle porosity iii rim cement iv multiple rim cement v palisade fabric and vi cluster cement micromorphological variability of the cement fabrics results from varying texture of the cemented sediment the nature of original porosity as well as from presence of associated microbial fabrics the microbial fabrics are interpreted as fossilized coccoid cyanobacteria phosphate cementation developed under peculiar early diagenetic conditions within semi closed microenvironments rich in organic matter in the marine phreatic environment the cementation contributed to the formation of phosphatic fossils and hardgrounds the accretion of the cements was due to concentration of biologically uptaken phosphorus near the sediment water interface enrichment of pore fluids with respect to phosphate and its precipitation within restricted microenvironments phosphate cementation post dated seafloor formation of pelletal glauconite but predated partial decomposition of organic matter as well as dissolution or neomorphism of aragonite and high mg calcite phosphate cementation occurred on a carbonate platform following the submersion of urgonian reefal build ups episodes of phosphate cementation were repeated during the sedimentation of the tatra albian limestone as a response to rapid relative sea level rises and increased influence of nutrient rich tethyan waters

226ra in phosphate nodules from the peru chile seafloor abstract 226ra and other uranium series radionuclides have been measured in a suite of marine phosphorite samples from the upwelling area off peru chile by gamma ray spectrometry and radiochemical techniques our results lead to the following conclusions 1 phosphorite nodules typically show unidirectional growth at rates of 1 to 10 mm kyr 2 very young samples less than a few thousand years contain slight excess amounts of 226ra probably derived from pore fluids and 3 slow but persistent leakage of 226ra out of phosphate nodules occurs resulting in systematically lower 226ra ages compared to 230th ages for samples older than about twenty thousand years radium fluxes from these phosphate nodules appear to be 1 to 3 orders of magnitude less than those calculated for deep sea sediments and ferromanganese nodules

uranium series isotopic studies of marine phosphorites and associated sediments from the east australian continental margin the uranium series disequilibria of phosphorites sediments and coexisting solitary corals from the outer continental shelf and upper slope off eastern australia have been determined in an attempt to relate the genesis of phosphorites in this area to an absolute time scale the234u 238u 230th 234u and231pa 235u activity ratios in the phosphorites are internally consistent and indicate that the apatite component of the phosphorites has remained a closed system with respect to u th and pa except for very minor losses of234u these losses do not affect the230th 234u derived ages and comprise only 1 2 of the total234u activity significantly less than the losses observed in phosphorite nodules from off peru chile a morphological subdivision of the phosphorites into either non ferruginous or ferruginous is supported by the isotopic data earthy friable non ferruginous phosphatic nodules range in age from 2 5 to 250 kyr holocene ages were obtained from five different locations in water depths ranging from 365 to 450 m in contrast the uranium series isotopes in well indurated ferruginous nodules occurring in the same general area but in shallower 350 m water are in radioactive equilibrium indicating that their ages exceed 800 kyr solitary corals caryophyllia planilamellata dennant 1906 are associated with non ferruginous nodules on the upper slope and commonly contain internal phosphatic molds several corals and their internal molds from one location were dated by uranium series methods the coral ages ranged from 17 to 20 kyr and were typically 10 15 kyr older than their respective phosphatic internal molds these results are consistent with the sedimentological observations and provide independent evidence that geologically meaningful ages can be obtained from marine phosphorites phosphate deposition off eastern australia appears to have been largely continuous throughout the late quaternary rather than being restricted to times of high sea level as reported for phosphorites off peru chile

amino acid biogeochemistry and dating of offshore peru chile phosphorites abstract amino acid biogeochemical results for a suite of indurated sea floor phosphate nodules from the peru chile margin confirm the youthful nature of these deposits as previously indicated by uranium series disequilibrium dating isoleucine epimerization is significant in the hydrolyzed acid soluble fraction of the nodules with a range in the d alloisoleucine l isoleucine ratio from 0 06 to 0 32 amino acid compositional patterns are uniform for all nodules and are similar to unlithified surface sediments from the region suggesting that the nodules and sediments constitute a single genetic series

uranium series and ams 14c studies of modern phosphatic pellets from peru shelf muds recent sediments and separated phosphate pellets 125 500 m in diameter from the peru shelf have been analyzed for uranium decay series isotopes and 14c in order to determine age relationships and mineralization rates uranium series ages of pellets separated from one box core are significantly higher than ams radiocarbon ages determined for the same pellets these differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages sediment radiocarbon and 210pb sediment results as well as some trends in the uranium series data suggest that high concentrations of phosphate pellets have accumulated in some peru shelf sediments without extensive reworking individual pellets apparently form very quickly on time scales of a few years estimated authigenic uptake rates of phosphorus into pellets ranges from 0 5 to 9 40 mol p cm 2 yr 1 somewhat higher than rates measured for nodules from the same area this is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits

lead isotope systematics of the giant massive sulphide deposits in the iberian pyrite belt lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the iberian pyrite belt as well as on overlying gossans and associated volcanic rocks all the massive sulphide deposits except for neves corvo and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18 183 206pb 204pb 15 622 207pb 204pb and 38 191 208pb 204pb indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks the isotopic compositions are representative of the average south iberian crust during the devonian to early carboniferous dinantian and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems this isotopic constancy is incompatible with multiple small independent hydrothermal cells of the east pacific rise type and fits much better with a model of hydrothermal convections driven by magmatic floor heating neves corvo is the only south iberian massive sulphide deposit to have a heterogeneous isotopic composition with in particular a highly radiogenic stanniferous ore 206pb 204pb of the cassiterite is 18 40 a model of lead mixing with three components is proposed to explain these variations 1 one derived from the devonian to early carboniferous dinantian continental crust that generated all the other massive ores 2 an eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides but independent of them and 3 a precambrian continental crust component the juxtaposition of three different sources places neves corvo in a specific paleogeographic situation that could also explain its mineralogical specificity the geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism the fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the iberian pyrite belt

major and trace element chemistry of gahnite as an exploration guide to broken hill type pb zn ag mineralization in the broken hill domain new south wales australia gahnite bearing rocks are common throughout the proterozoic broken hill domain new south wales australia where they are spatially associated with broken hill type pb zn ag mineralization including the supergiant broken hill deposit in the past such rocks have been utilized as exploration guides to ores of this type but their presence has had mixed success in discovering new occurrences of sulfide mineralization major element chemistry of gahnite has previously been used to define a compositional range associated with metamorphosed massive sulfides deposits including broken hill type deposits but it fails to distinguish sulfide rich from sulfide poor occurrences major and trace element data from la icp ms and electron microprobe analyses were obtained for gahnite from twelve broken hill type deposits to determine whether or not gahnite chemistry may be used to distinguish prospective exploration targets from nonprospective occurrences major and trace element data were discriminated using a principal component analysis and in a bivariate plot of zn fe versus ni cr v to distinguish gahnite associated with the broken hill deposit from that associated with sulfide poor lode pegmatite and sillimanite gneiss bivariate plots of zn fe versus trace element contents e g ga co mn co ni v cd suggest gahnite from the broken hill deposit has a relatively restricted compositional range that overlaps with some minor broken hill type occurrences based on the ore grade wt pb zn of rocks hosting gahnite at each locality gahnite in the highest grade mineralization from minor broken hill type deposits possess compositions that plot within the field for gahnite from the broken hill deposit which suggests that major and trace element chemistry e g zn fe 2 4 vs co 10 110 ppm ga 110 400 ppm mn 500 2 250 ppm and co 25 100 ppm vs ga 125 375 ppm may be used as an exploration guide to high grade ore

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

recognizing metasedimentary sequences potentially hosting concealed massive sulfide accumulations in the iberian pyrite belt using geochemical fingerprints abstract promising advances in the development of new greenfield and brownfield exploration methods and methodologies for massive sulfide ore systems in the iberian pyrite belt ipb have been made over the last two decades however the effects of lateral and vertical facies variations in metasedimentary piles forming the two main lithostratigraphic units that potentially host massive sulfide deposits the phyllite quartzite group pqg and the volcanic sedimentary complex vsc on mineral exploration remain unknown herein we report on results of a multi element geochemical study of 133 samples representing pqg and vsc metasedimentary rocks from the albernoa area 713 km2 this dataset is complemented by whole rock sr nd and pb isotopic data for a subset of 12 vsc metasedimentary rock samples the prevalent siliciclastic component in the analyzed rocks derives from an evolved felsic source as suggested by the zr sc th sc eu eu relationships that according to cia values should have been affected by significant weathering before they were deposited in the basin from middle devonian to visean time the siliciclastic component comprises clay and fine grained sand whose relative abundance can be distinguished on the basis of al2o3 tio2 sc tio2 ga tio2 zr tio2 and sio2 al2o3 ratios these terrigenous components are in places variably mixed with volcanic derived fractions leading to the formation of tuffaceous metapelites typical of the vsc sequences but also present in the upper stratigraphic sections of the pqg in general vsc samples display the following parameters 2 5 sio2 al2o3 6 5 0 02 tio2 al2o3 0 06 and 0 2 cao na2o k2o al2o3 0 25 whereas pqg samples display 2 5 sio2 al2o3 12 0 08 tio2 al2o3 0 58 and cao na2o k2o al2o3 0 3 fe2o3 tio2 ratios 10 and al2o3 al2o3 fe2o3 mno ratios 0 6 indicate the strong influence of an early developed prior to variscan metamorphism and deformation hydrothermal component on the prevalent siliciclastic component sometimes mixed with a variable volcanic derived fraction moreover values of as sb sc cu zn pb sc and 5 fe2o3 mgo mno al2o3 ratios between 1 and 10 indicate the influence of seafloor metasomatism processes and ratios 10 indicate close proximity to hydrothermal discharge potentially associated with massive sulfide mineralization the sr nd pb isotopic data show that vsc metasedimentary rocks are mostly derived from old reworked upper continental crust isotope ratios characterizing the main siliciclastic component are somewhat disturbed by the significant presence of volcanic derived components and or mineral changes due to early hydrothermal alteration mineralization processes which should have involved isotopically distinct fluids

long and short wavelengths of indian ocean geoid and gravity lows mid to upper mantle sources rapid drift and seismicity of kachchh and shillong plateau india abstract spectral analysis of the indian ocean geoid low provides depth to the large wavelength sources as 1300 700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels the three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine spinel transformation of successively increasing fe mg ratio as depth increases known as transition zones the first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field the low density rocks in this section appear to be related to the subducted indian tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments the relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere asthenosphere boundary lab under the indian continent and surrounding oceans respectively a low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows the subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid to upper mantle which is likely to set in baby plumes presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section they would make this region buoyant that appears to be responsible for the central indian ocean deformation zone with large scale folding faulting seismicity and high heat flow the same also appear to be responsible for the rapid drift of the indian plate short wavelength component of the indian ocean geoid low reflects most of the surface shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose kachchh and the shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries tectonics and residual geoid anomalies of the indian continent and adjoining regions suggest that the geoid highs of kachchh and the shillong plateau along the nw and the ne corners of india are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries geoid highs indicating high density mafic rocks suggest their connections extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries besides both these regions are affected by i prominent lithospheric flexure of the indian plate ii large scale mafic intrusions and iii interaction of proterozoic mobile belts with effects of present day himalayan orogeny that make them more vulnerable for seismic activity

further studies of the nature composition and ages of contemporary phosphorite from the namibian shelf major and minor element and radiochemical data are examined for a set of phosphorite concretions and other phosphatic material from the namibian shelf the concretions were separated on a morphological basis from grab samples taken in a transect of shallowing depth over the diatomaceous ooze belt compositional data indicate that the samples are fairly pure carbonate fluorapatite and trend towards a maximum 32 p2o5 content a range of compositional variability similar to other samples from the namibian shelf is observed radiochemical systematics confirm that pelletal phosphorite and glauconitized pelletal phosphorite from depths 170 m are at radioactive equilibrium and are therefore old and reworked the concretions collected from depths shallower than 78 m are entirely recent with maximum ages in the range 500 3800 years they may be much younger despite distinct textural and small compositional differences similar ages are found for separates from the same grab samples of fish scales and an unidentified high magnesian phosphate mineral are also shown to have incorporated uranium recently

trace metal records of regional paleoenvironmental variability in pennsylvanian upper carboniferous black shales abstract regional geochemical differences within a laterally continuous cyclic pennsylvanian upper carboniferous shale in midcontinent north america are interpreted in light of models of glacioeustatic forcing and new views on water column paleoredox stability and trace metal behavior in black shale environments specifically we characterize differences in transition metal fe mn mo v ni zn pb and u concentrations in black shales of the hushpuckney shale member of the swope limestone in iowa and equivalent black shale beds of the coffeyville formation in oklahoma although c s fe systematics and uniform 34 s depleted isotope ratios of pyrite indicate pervasive euxinic deposition anoxic and sulfidic bottom waters for these shales regional variations can be inferred for the efficiency of mo scavenging and for the rates of siliciclastic sedimentation as expressed in spatially varying fe al ratios black shales in iowa show mo enrichment roughly five times greater than that observed in coeval euxinic shales in oklahoma by contrast fe al ratios in oklahoma shales are as much as five times greater than the continental ratio of 0 5 observed in the over and underlying oxic facies and in the coeval black shales in iowa recent work in modern marine settings has shown that enrichments in fe commonly result from scavenging in a euxinic water column during syngenetic pyrite formation in contrast to fe the concentrations of other transition metals mo v ni pb zn u are typically more enriched in the black shales in iowa relative to oklahoma the transition metal trends in these paleozoic shales are reasonably interpreted in terms of early fixation in organic rich sediments due to euxinic water column conditions however regional variations in 1 rates of siliciclastic input 2 organic reservoirs including relative inputs of terrestrial versus marine organic matter and 3 additional inputs of metals to bottom waters from contemporaneous hydrothermal vents are additional key controls that lead to geographic variation in the extent of metal enrichments preserved in ancient organic rich sediments

mineralogy geochemistry and geological significance of tourmaline rich rocks from the paleozoic cinco villas massif western pyrenees spain tourmaline rich rocks associated with clastic metasedimentary rocks of carboniferous age occur in the cinco villas massif western pyrenees three types of tourmaline rich rocks were distinguished 1 fine grained stratiform tourmaline rich rocks which are associated with carbonaceous metapelites tr1 2 stratabound tourmaline rich rocks associated with metapelites in the contact aureole of the aya granitoid pluton tr2 3 stratabound to massive tourmaline rich rocks associated with psammopelites in contact with granites and pegmatites tr3 tourmalines belong to the schorl dravite solid solution series and have a wide compositional range from nearly end member dravite for tr1 tourmalines to schorl for tr3 tourmalines tr2 tourmalines have intermediate compositions the fe fe mg typically varies between 0 02 and 0 55 increasing from tr1 to tr3 the tr1 tourmalines commonly display a discontinuous chemical zoning with fe rich green cores 8 8 5 feo and mg rich colorless rims 10 11 mgo in contrast crystals that exhibit fine growth lamellae appear to lack significant chemical zoning oxygen and hydrogen isotope compositions also reveal major differences between tr1 and tr3 tourmalines the former displaying heavier 18o values 17 7 19 and d values 35 to 42 than tr3 tourmalines 11 to 13 and 47 to 76 respectively the tr2 tourmalines show intermediate values of 11 3 to 14 6 for 18o and 40 to 55 for d linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay rich and quartz rich end members indicative that some tourmaline rich rocks contain a significant detrital component chondrite normalized ree rare earth element patterns of tourmaline rich rocks are similar to those of surrounding unaltered clastic metasediments except for some tr1 rocks which are characterized by low contents of ree mass balance calculations show that tourmaline forming processes plus metamorphism led to mass and volume changes at mesoscopic scales 10 for the tr1 tourmalinites silicon fe mn and ree elements were partially lost from sedimentary rocks whereas mg and particularly b were added to pelitic sediments available data nevertheless do not allow an assessment of the boron source formation of the tr1 tourmaline rich rocks probably was the net result of several processes including direct precipitation from b rich hydrothermal fluids or colloids early diagenetic reactions of carbonaceous pelitic sediments with these fluids and subsequent recrystallization during regional metamorphism the tr2 tourmaline rich rocks mainly developed by metamorphic recrystallization of tr1 tourmaline rich rocks and veins adjacent to pegmatites and granitic rocks tr3 are the result of boron metasomatism the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism

comparison of radiocarbon and uranium series dating methods as applied to marine apatite abstract a comparison of uranium series geochronology and radiocarbon dating the first systematic test of its kind applied to authigenic marine apatite has been made for nine seafloor phosphate nodules sampled from the peruvian and chilean shelves radiocarbon ages agree well with the 230th ages for all six samples with 230th ages younger than the practical dating limit of the 14c technique for marine samples two of three old samples contain no significant 14c activities as expected however one sample with a high concentration of phosphate pellets contains more than 30 of the modern 14c activity the discrepancy between the 14c and u series age for this sample is thought to be due to post depositional adsorption of the particle reactive elements thorium and protactinium onto pellet surfaces

pleistocene phosphorites off the west coast of south africa we report here relatively modern phosphogenesis on the continental margin off south africa where palaeontologically old limestone middle eocene middle miocene has been phosphatized in late pleistocene times 62 76 kyr in an area of only moderate and seasonal present day upwelling1 2 previously modern phosphogenesis had been reported as occurring only off namibia3 15 peru chile16 21 and very recently east australia22 23 this report and those describing the late quaternary east australia phosphorite22 23 suggest that contrary to established opinion strongly enhanced upwelling may not be necessary for phosphorite formation and that oceanic phos phogenic models need not be based exclusively on localized oceanographie regimes such as exist off peru and namibia

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

excess protactinium in phosphate nodules from a coastal upwelling zone measurements of231pa 235u and230th 234u activity ratios have been made for a suite of quaternary age phosphate nodules from the upwelling zone off peru and chile many of these samples contain more231pa than would be predicted based on closed system230th 234u ages unlike samples from non upwelling environments which have been shown to have concordant231pa and230th ages peru chile phosphate nodules apparently pick up231pa preferentially to230th this is most likely a result of prior fractionation of protactinium and thorium in the open ocean highly productive coastal upwelling zones may be added to the growing list of localities where fractionation between thorium and protactinium isotopes has been observed in marine sediments

mantle layering from scs reverberations 4 the lower mantle and core mantle boundary this is the final installment in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms in this paper long period sh polarized multiple scs phases reflected once from a mantle discontinuity first order reverberations are used to search for abrupt shear wave impedance contrasts in the lower mantle beneath the geographic regions sampled the depth interval of 1000 2600 km bullen s region d appears free of any distinct radial layering in agreement with the majority of recent seismic models and the notion of near adiabatic compression of a compositionally homogeneous lower mantle to pass undetected discontinuities in d must be either small sh polarized reflection coefficient r0 1 0 highly intermittent or subject to lateral depth variations well in excess of 50 km at greater depths corresponding to the d region of the lowermost mantle we find evidence for a reflector of long period seismic energy situated an average of 325 km above the core mantle boundary cmb similar to the discontinuity proposed by t lay and coworkers our results indicate a 4 4 increase in shear wave impedance and a 1 7 increase in density if the 2 75 increase in shear velocity proposed by lay and helmberger 1983 holds true for our study area the discontinuity is observed beneath less than one third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle this plus an observed correlation of discontinuity depth and lower mantle velocity heterogeneity and the lack of experimental evidence for deep mantle phase transformations strongly favor a compositional origin mantle side layering nearer the cmb perhaps associated with a thin chemical boundary layer would mimic the effects of the surface crust producing a complex wave train of first and higher order reverberations superposed upon all other reverberative interval arrivals if present and unaccounted for cmb structure would drive estimates of crustal structure and qscs obtained by multiple scs waveform inversion away from their true values compared to independent estimates our results admit little bias monte carlo tests are used to assess the sensitivity of this technique to cmb layering which decreases with decreasing boundary layer thickness nonetheless the results limit cmb transition zones in excess of 20 km to impedance contrasts less than 12 8 for a 40 km transition beneath much of the western pacific australia melanesia and indonesia

phosphogenesis and active phosphorite formation in sediments from the arabian sea oxygen minimum zone in this study porewater chemistry solid phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the arabian sea three parameters namely a decrease of the dissolved phosphate and fluoride concentrations with depth saturation with respect to carbonate fluorapatite and the presence of a solid phase ca phosphate mineral all indicate that phosphogenesis is currently taking place at all three sites authigenic apatite precipitation rates vary between 0 076 and 1 04 molp cm 2 yr 1 and are of the same order of magnitude as reported for other high productivity areas precipitation of an intermediate precursor precedes francolite formation in the continental slope sediments on the karachi margin results of a diagenetic p model indicate that phosphogenesis is induced by high rates of organic matter degradation dissolution of fish debris is likely to provide a substantial additional source of phosphate redox iron cycling does not influence phosphogenesis in these environments model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis the highest rate of francolite formation was observed in a boxcore taken on the oman margin where it contributes to the formation of a holocene phosphorite deposit this observation contrasts with previous reports of only old phosphorites in this area phosphorites are presently forming on the oman margin as a result of a deposition of older reworked material from the continental shelf which has undergone an earlier phase of phosphogenesis b a high input of reactive p fish debris and degradable organic matter c a relatively low sediment accumulation rate and d the absence of winnowing on this location holocene phosphorite deposits may be less common on the oman margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope which limit the area where phosphorite formation may occur

uranium geochemistry and dating of pacific island apatite abstract uranium series disequilibrium dating of island phosphate deposits is evaluated in terms of known associated coral ages uranium geochemistry and stratigraphic sequences as well as the concordance between the geochronometers 234 u 238 u 230 th 234 u and 226 ra 238 u u vi is the predominant oxidation state of uranium in island phosphorites and by analogy to the youngest surficial deposits most of the uranium initially bound is in the form of u vi sorbed by surfaces from seawater insular deposits contain more organic matter than even very young ocean floor samples and this leads to a greater probability of reduction of available recoil uranium than occurs in marine deposits as a consequence r vi r t r vi where r represents the 234 u 238 u activity ratio this situation is completely opposite from that observed for marine origin phosphorites we determined that a fraction of u vi in ancient insular phosphorites is very labile and lost to alkaline carbonate solutions with a uranium activity ratio even more depleted in 234 u than the bulk r vi most younger samples appear to have no more discordance between 234 u 238 u 230 th 234 u and 226 ra 238 u than marine phosphorites of similar apparent age young surficial atoll rim apatite cements and unconsolidated phosphorites date in the range of 1500 8000 years b p consistent with the concept of partial submergence of low lying coral islands prior to 2000 years b p sub surficial samples in the same environment date older at about 20 000 years b p violation of the closed system assumption occurs in at least 6 out of 13 ancient 800 000 year samples uranium series disequilibrium dating of insular apatite shows some promise as a recorder of climatic sea level events but the assumptions necessary for valid ages must be carefully evaluated for each occurrence

ultra fine structures and genesis of the campanian negev high grade phosphorites southern israel ultra fine structures of the negev high grade phosphorites provide valuable clues to unravelling the genesis of these rocks the question of their differential areal distribution and the biosedimentary mechanisms involved in their considerable enrichment peculiar to these phosphorites is an intergranular phosphate matrix for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro organizations the phosphate particles fixed by this filamentous meshwork also consists of internally organized and non organized packed microbial remains of different types analogies between the fabrics of the matrix and the corpuscles lead to conception of a two stage depositional scheme for these phosphorites based on rhythmical repetition of two sedimentary mini events in a slightly oscillating very shallow marine system 1 a low energy event of microbial colonization of the mishash bottoms followed by early phosphatization of the organic structures mainly in marginal situations and 2 a higher energy event which broke up the phosphatic mats into debris redepositing them as clastic layers in nearby basinward sites while becoming bound by a new meshwork of filamentous microphytes the differential phosphatization of the intergranular microbial binder again occurring mostly in marginal localities produced highly enriched phosphorites minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks examining the structural and textural features the validity of the sedimentary mechanisms of recent phosphorite formation for the campanian negev rocks is discussed

sedimentology geochemistry and origin of phosphatic chalks the upper cretaceous deposits of nw europe santonian lower campanian and lower maastrichtian phosphatic chalks in northern france southern england and belgium are europe s largest sedimentary phosphatc deposits the stratigraphy sediment ology petrography mineralogy and geochemistry of the lithofacies are reviewed and new data presented depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high and low productivity phosphogenic systems it is concluded that phosphatic chalks were deposited in well oxygenated current swept environments phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates reduced bulk sediment accumulation rate and an enhanced rate of bioturbation precipitation of carbonate fluorapatite francolite accompanied the bacterially mediated decomposition of organic matter occurring within centimetres of the sediment seawater interface and taking place preferentially within microbial bodies and coatings in addition to the organically derived component pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction mineralization was probably a dominantly post oxic process but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation phosphogenesis occurred predominantly on the ne margins of the anglo paris basin where shallower sea floors and suitable palaeoceanographic conditions prevailed phosphogenic episodes were limited by sea level fluctuations which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity

geochemistry and occurrence of authigenetic phosphate nodules from the desmoinesian cyclic excello epeiric sea of the midcontinent usa abstract pennsylvanian cyclothemic black shales of the midcontinent which were deposited in glacial related epeiric seas containing high marine algal productivity due to the occurrence of swamps prior to maximum transgression coastal upwelling and terrigenous runoff are very rich in phosphate nodules the high nutrient content of the epeiric sea supported phytoplankton populations that created favourable environments for the accumulation of phosphorites in anoxic bottom waters this close association of organic matter and phosphate controlled nodule morphology spherical elongated bladed platy appear to be related to progressive decreases in the nutrient supply of the sea water from the ancient pacific ocean panthalassa in the south to the palaeoshorelines in the north the major and trace element contents of ten phosphorite nodules from representative exposures along the 400 mile long outcrop belt from the northern quachita to central missouri were determined major elements of the phosphorites are p 2 o 5 22 33 34 19 sio 2 3 56 25 15 fe 2 o 3 0 73 2 94 cao 30 77 43 22 na 2 o 0 30 4 51 k 2 o 0 35 2 27 al 2 o 3 4 31 8 02 and co 2 1 2 2 0 wt the distribution of mn zn cd and cu shows a sharp increase at locations 40 and 50 where terrigenous runoff waters were dominant the distribution of ni u co li cr v and mo does not show any significant change across the outcrop belt programmed pyrolysis indicates that the nodules are thermally immature with respect to hydrocarbon generation and kerogen types belong to type ii and type iii pyrolysis results are t max 418 432 c ol 17 168 hl 11 481 pl 0 04 0 4 and toc 0 65 7 07 wt direct precipitation in micropores 1 4 m in size is shown by petrographic and sem studies however the occurrence of nodules is explained by a calcite replacement model the ubiquitous association of organic matter and phosphorus is vitally important for growing of phytoplankton which extract phosphorus from sea water when phytoplankton die organic phosphorus oxidizes in the micropores to inorganic phosphorus species such as hpo 4 2 which replaces calcite to form apatite as a function of alkalinity ph eh and available ions concentric layers in nodules are due to progressive growth reaction with interstitial water the model proposed for the occurrence of phosphate nodules in the excello epeiric sea with bottom water anoxia is similar to that for the highest productive regions of the world such as walvis bay and off the coast of peru and chile

constraining the short scale core mantle boundary topography beneath kenai peninsula alaska with amplitudes of core reflected pcp wave previous analysis of short period pcp with seismic array reported rapid variation of pcp amplitude and suggested small scale variation of core mantle boundary cmb topography beneath kenai peninsula alaska in order to constrain the length scale l and topographic height variation h in the region we quantitatively compute synthetic pcp seismograms using the seismic representation theorem for a core mantle boundary cmb with topography bump h 0 or depression h 0 we model pcp amplitudes for l in the range of 60 500 km h in the range of 6 km depression to 5 km bump it is observed that cmb depressions do produce amplification and cmb bumps generate de amplification for short period pcp amplitudes after computation for various combinations of l and h amplification in the range of 2 4 5 is observed in the above mentioned parameter ranges the topography model of a cmb depression h 6 km l 6 predicts 1 hz pcp amplification consistent with observed pcp amplitude variation beneath kenai peninsula however pcp amplitude can also be affected by heterogeneities in the crust and mantle the estimated topography could be less pronounced

mass anomaly structure of the earth gravitational interaction is the weakest among the four known forces in the universe the particular gravity equipotential field that coincides with sea level is called the geoid and satellite data have revealed anomalies in its pattern that are puzzling to explain dynamic topography solutions have offered the preferred explanation for the past 15 years but problems remain an alternative explanation presented here contends that very large mass anomalies lie deep in the earth presumably produced by topography at the coremantle boundary cmb and only much smaller mass anomalies occur at shallower depths the principal distinction is whether one is willing to accept the possibility that processes within the earth s core may generate the cmb topography an analysis of the south american regional geoid high spherical harmonic degrees 2 10 indicates that only mass anomalies shallower than 1200 km are its cause this single positive source is contrary to dynamic topography solutions that would generate negative deflections of both the surface and the cmb in response to the positive subducted mass for the earth s fourth greatest geoid high our alternate explanation is tested utilizing a set of 9274 subduction 5 cube slab bloblet center points developed from reconstructed plate history that provide estimates of the locations of material subducted into the earth s mantle two global mass solutions are offered utilizing 1 only those bloblets in the outer 800 km and 2 only those bloblets in the outer 1400 km four point masses at 3000 km depth to simulate cmb topography produced by processes within the core unrelated to mantle dynamic topography complete the two mass models both models show reasonable agreement with patterns and magnitudes of the regional geoid and its component degree parts the model cmb mass anomalies are only a factor of 3 greater than those initially estimated by gravity to geoid ratios from observed values at geoid anomaly center locations difficulties with the dynamic topography solutions matters of the density of the subducted material the proper hydrostatic flattening value to use for analyses and possible causes for the cmb topography are discussed the subduction mass models are also used to estimate the magnitude of the driving force for plate tectonics 2 78 1020 to 3 23 1021 n approximately equivalent to 7 0 1012 to 8 1 1013 n m 1 these force values are comparable to published slab force estimates based on thermal considerations

stable isotopic characterisation of francolite formation stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits values of 13c and 34s in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions the exceptions are concretionary francolite from namibia which has an isotopic composition in keeping with its formation within organic rich sediments and that from offshore morocco which has an isotopic signature of the anoxic suboxic interface onshore deposits from jordan mexico south africa and possibly the permian phosphoria formation in the western u s a are substantially depleted in18o they appear to be too altered for deductions to be made about their environments of formation in other onshore deposits which are unaltered or minimally altered the isotopic composition suggests that some formed within sulphate reducing sediments sedhura morocco whilst francolite from the georgina basin of australia formed at the oxic anoxic boundary where oxidation of biogenic h2s decreases the 34s of pore water in general pelletal samples show non oxic isotopic signatures whilst non pelletal samples show oxic isotopic signatures but samples from namibia peru ica plateau and the californian and moroccan margins are exceptions to this rule morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature

selected palynomorphs from the lower to middle eocene of the south atlas border zone morocco and their environmental significance abstract results of palynological studies of early cenozoic carbonates in the south atlas border zone south morocco based on both pollen and microplankton permit reconstruction of the palaeo environment pollen found in a few samples of one profile near ifar west of boumalne could be assigned to two characteristic tropical coastal plant associations a mangrove swamp and a salt marsh as well as of a scarcely documented plant association of a tropical hinterland unaffected by salt water the microplankton assemblage dominated by one species carpatella cornuta suggested an unusual probably unfavourable shallow water environment the surprisingly low terrestrially derived input to this obviously nearshore tide stressed palaeo environment is supported by x ray diffraction analysis of the strata combining all the data two palaeoenvironmental models are possible 1 the south coast of a shallow tropical warm tidal atlantic gulf or 2 the coast of an island amid such a gulf in both cases this gulf must have been surrounded by presumably fairly dry scarcely vegetated hinterland

radiometric ages of phosphorites from the west coast of india abstract four algal carbonate nodules with 1 mm thick phosphorite coatings were dated by the 230 th 234 u and three of the carbonates by 14 c methods all ages 5 8 13 9 k y are in satisfactory agreement indicating that phosphorite deposition started soon after the carbonate nodule formation the u th concentrations and isotopic ratios as well as the u p weight ratios of the phosphorite coatings are in good agreement with those from other regions of the world

gravity topography and crustal evolution of venus abstract the gravity anomalies of venus although small by comparison with those on mars and the moon are still much larger than those on earth for large features on venus even the low degree spherical harmonic terms for venus gravity field indicate a close association of broad positive gravity anomalies with major topographic highs this is striking contrast to the situation on earth where the broad regional gravity anomalies show little correlation with continental masses or plate tectonic features but instead appear to be caused by deep mass anomalies a method for estimating radial gravity anomalies from line of sight acceleration data their interpolation and use of iteration for improved radial anomaly estimates is outlined a preliminary gravity anomaly map of venus at spacecraft altitude prepared using first estimate values is presented a profile across the western part of aphrodite along longitude 85 e was analyzed using time series techniques an elastic plate model would require a plate thickness of about 180 to 200 km to match the general amplitude of the observed gravity anomaly about 33 mgal a thickness much greater than that found for earth structures and because of high surface temperatures unlikely for venus an airy isostatic model convolved with the topography across aphrodite however provides a better match between the predicted and observed gravity anomalies if the nominal crustal thickness is about 70 to 80 km this thickness is over twice that for continental crust on the earth and considerably greater than that of the earth s basaltic ocean crust only 5 km a different differentiation history for venus than that of the earth thus is anticipated high gravity anomalies 110 mgal occur over beta regio and over the topographic high in eastern aphrodite both highs are associated with regions where detected lightning is clustered and thus the topographic features may be active volcanic constructs the large gravity anomalies at these two sites of volcanic activity require an explanation different than that indicated for western aphrodite

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

relationship between phosphogenesis and sequence architecture sequence stratigraphy and biostratigraphy in the middle ordovician of the armorican massif nw france abstract in the armorican massif nw france the middle ordovician formations contain abundant phosphatic p beds associated with terrigenous sediments laid down on a storm dominated shelf facies analysis and sequence stratigraphy carried out on sections located in the crozon peninsula w armorican massif allow us to distinguish four p facies 1 beds with p nodules and crusts contained within sediments laid down on the distal and median parts of the inner shelf 2 amalgamated beds with p nodules and crusts and 3 beds with lingulid brachiopods heavy minerals and p clasts associated with retrogradation phases 4 p clast bearing conglomerates associated with progradation phases the genesis of these facies and their location in the depositional sequences are controlled by the fluctuation of terrigenous inputs during sea level changes sediment volume partitioning we propose a genetic model integrating the superposition of very high frequency vhf and high frequency hf eustatic signals in this model the amalgamated beds and beds with lingulid brachiopods heavy minerals and p clasts result from the superposition of several sea level rise episodes of vhf cycles occurring within the retrogradation phase of a hf cycle the lateral extension of p facies was investigated in the eastern part of the armorican massif benioc domfront les tanneries and st aubin du cormier sections several early darriwilian transgressive events associated with phosphogenesis episodes are dated through high resolution chitinozoan studies some of these transgressive events are at least of regional scale e g ornensis and protocalix transgressive events the topmost calix transgressive event is clearly identified over the northern gondwanan and avalonian domains and may reflect a global eustatic signature

peruvian sediments as recorders of an evolving hiatus for the last 22 thousand years highlights review of sediment archives from the peruvian margin since the lgm focus on the evolutionary feature of the hiatus found in archives modern analogue for current dominated environments for paleo reconstructions new results for erosional potential of the non linear internal waves nliws abstract the peruvian continental margin is characterized by the presence of one of the strongest and most distinct oxygen minimum zones omzs in today s oceans therefore it has long been in the focus of oceanographic and geological investigations observations indicate that omzs are expanding in relation with currently changing climate to advance understanding of the temporal evolution of omzs and climate change complete paleoceanographic and palaeoclimatological reconstructions are needed however the development of paleoenvironmental scenarios for the period since the last glacial maximum at this region was hampered by a ubiquitous hiatus and short term interruptions of the stratigraphical record in the present study we combined the stratigraphical information from 31 sediment cores from the peruvian margin located between 3 and 18 s and water depths of 90 to 1300 m within and below today s omz in order to determine the extent of the hiatus and assess the responsible mechanisms a widespread unconformity and related erosional features omission surfaces and phosphorites were observed in sediment cores from the area south of 7 s depicting a prograding feature on the continental slope from south to north during the deglaciation combining recent oceanographic and sedimentological observations it is inferred that tide topography interaction and resulting non linear internal waves nliws shape the slope by erosion carry sediments upslope or downslope and leave widespread phosphoritic lag sediments while the peru chile undercurrent pcuc transports the resuspended sediments southward causing non deposition this exceptional sedimentary regime makes the peruvian margin a modern analogue for such environments overall our compilation of downcore records showed that enhanced bottom currents due to tide topography interaction were progressively evolving and affected a wider area with the onset of the last deglaciation elevated tidal amplitudes and variability of mid depth water masses i e density changes and hydrodynamics in relation with changing climate were potential reasons of this evolving feature of erosion and reworking additionally erosion and non deposition was observed widest and even was encountered on the continental shelf during the early holocene potentially indicating a strong phase of the pcuc mirroring today s el nino like conditions

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member

growth rates of phosphate nodules from the continental margin off peru modern sea floor phosphorite nodules sampled from off the coast of peru are shown to grow at rates of millimeters per thousand years in spite of the fact that the measured accumulation rates of the underlying sediment are two to four orders of magnitude faster phosphate nodules grow downward into soft sediment and the rates of phosphorus accumulation into nodules are approximately equal to the upward diffusive flux of dissolved phosphate inferred from pore water profiles these results demonstrate that phosphorus in nodules originates from regeneration in sediments rather than as a result of direct precipitation from bottom waters