carbon and strontium isotopes and global correlation of cambrian series 2 series 3 carbonate rocks in the keping area of the northwestern tarim basin nw china abstract one hundred and ten carbon and nineteen strontium isotopic values of outcropping cambrian series 2 and series 3 carbonate rocks in the penglaiba section of the keping area were analyzed effective isotopic data with little influence of diagenesis were used to address the global correlation the 13 c values exhibit two major positive excursions peaking at 3 1 and 3 3 and three major negative excursions peaking at 3 0 4 2 and 3 2 the carbon isotope excursions peaking at 3 0 and 4 2 across the cambrian series 2 series 3 boundary show good correlations with similar variations reported in siberia laurentia north china and south china in contrast the other three carbon isotope excursions peaking at 3 1 3 3 and 3 2 do not have a fairly good global correlation because of the lack of biostratigraphic data with respect to the 87 sr 86 sr ratios they show good correlation with those reported for laurentia and further support the global 13 c comparison on the basis of these new data it is showed that the combination of 13 c curves and 87 sr 86 sr variations serves as a powerful tool for correlation and subdivision of cambrian strata in the tarim basin of northwest china and provides new data for global correlation

climate change during the early paleogene in the bogota basin colombia inferred from paleosol carbon isotope stratigraphy major oxides and environmental magnetism abstract paleosols in the bogota basin colombia record an increase in chemical weathering across the paleocene eocene p e transition rock magnetic properties and major element geochemistry exhibit an abrupt change in an interval identified as including the p e boundary as established by previously published biostratigraphy and a u pb date on volcanic zircons 56 2 1 6 ma during the stratigraphic interval that contains the p e transition magnetic susceptibility increases significantly with an order of magnitude higher concentration of magnetite maghemite and hematite the preponderance of pure stoichiometric magnetite in the paleosols of this interval indicates that the increase in magnetic susceptibility is due to changes in the rate of pedogenesis rather than an increase in the erosion of nearby volcanic rocks which would contribute titanomagnetite pedogenic structures preserved within the paleosols a lack of iron oxides as cement friability of the sandstones and previously published thermochronologic data are not consistent with burial diagenesis as an explanation for the origin of the magnetic mineral assemblage these enhanced pedogenesis and related chemical weathering is also confirmed by increases in the concentration of fe 2 o 3 and al 2 o 3 and loss on ignition values an increase in mean precipitation across this interval is inferred by the lack of carbonate nodules and a decrease in sio 2 in paleosols thus we hypothesize that there was an intensification of chemical weathering during the interval that contains the p e transition

geochemistry of laterites stability of al goethite al hematite and fe super 3 kaolinite in bauxites and ferricretes an approach to the mechanism of concretion formation

a sulfate control on marine mid depth euxinia on the early cambrian ca 529 521 ma yangtze platform south china abstract given the high oxygen demands of complex metazoans a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early cambrian in this study we report new fe mo c s geochemical data from black shale rich strata of the early cambrian continental margin yangjiaping section in the yangtze platform south china integrated fe speciation and mo abundance data from the yangjiaping section in combination with previously reported data from shatan inner shelf songtao slope and longbizui basinal sections suggest mid depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters this redox structure is similar to conditions previously suggested for the ediacaran yangtze platform and other proterozoic marine basins indicating that proterozoic layered ocean redox structure in some basins persisted into the early cambrian furthermore the fe mo data demonstrate that the mid depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression excess highly reactive iron fe hr in deep water with a hydrothermal origin may have had a control on the mid depth euxinia given geochemical and geological evidence for basinal hydrothermal activities on the other hand a sulfate limitation on bacterial sulfate reduction bsr and sulfide generation was suggested by a weak correlation r 2 0 22 between pyrite iron versus fe hr and total organic carbon toc form anoxic deep water sections and high toc averagely 6 in the deepest ferruginous water longbizui section furthermore a spatial gradient was observed in sulfur isotopic composition of pyrite averagely from 0 2 to 21 0 consistent with a lateral gradient of sulfate in concentration or and in isotope from shore to distal waters on the transgressive yangtze platform we accordingly proposed that two distinct sources riverine input and ocean reservoir provided sulfate for bsr dominantly in near shore and distal areas respectively and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive bsr and pyrite formation during preceding transgression therefore our results support a sulfate control on the spatiotemporal distribution of mid depth marine euxinia on the yangtze platform in early cambrian when complex metazoans rapidly evolved

carbon isotope stratigraphy of the lower to middle cambrian on the eastern yangtze platform south china during the cambrian series 2 to 3 vast areas of carbonate strata were deposited on the yangtze platform south china providing excellent materials for carbon isotope stratigraphic studies previous studies have revealed the stratigraphic framework of the carbonate strata in the western yangtze platform yet little remains known about the carbon isotope signature of the strata in the eastern yangtze platform high resolution carbon isotope curves of the well k2 and well wn2 drilling sections that cover the traditional lower middle cambrian boundary in the eastern yangtze platform are reported in this study one positive m1 and two negative m2 and m3 13c excursions can be recognized in the mufushan formation of the well k2 section whereas three negative 13c excursions d1 d2 and d3 are found in the dachenling formation of the well wn2 section combined with palaeontological constraints the m1 excursion m2 and d1 excursions and m3 and d3 excursions are correlated with the mice mingxinsi carb on isotope excursion aece archaeocyathid extinction carbon isotope excursion and roece redlichiid olenellid extinction carbon isotope excursion on the generalized global 13c curve through the cambrian the m1 excursion is accompanied by high total organic carbon contents indicating that it was caused by high primary productivity in contrast the m2 d1 and d3 excursions all coincide with lithological changes implying increased seawater depth which supports a possible cause for the decreased 13c by transgression and the introduction of 13c depleted deep seawater into shallower seas evident differences in 13c values and amplitudes of the aece and roece can be recognized between the well k2 and well wn2 sections revealing the existence of a shallow to deep ocean 13c gradient during the carbon isotope excursion events to our knowledge this is the first report of high resolution 13c curves of the strata in the eastern yangtze platform and the 13c curve of the mufushan formation is currently the most complete 13c record of cambrian stage 4 reported for the yangtze platform our results complement the 13c dataset for the yangtze platform and provide a better understanding of the origin of the 13c excursions during this stage

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

liquidus phase relationships on the join anorthite forsterite quartz at 10 kbar with applications to basalt petrogenesis liquidus phase relationships determined on the join caal2si2o8 anorthite mg2sio4 forsterite sio2 quartz at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel enstatite and silica fields to expand the boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the sio2 apex as pressure increases therefore simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases as has been found in other studies also increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization although anorthite is unstable in simplified peridotite above 9 kbar in the system cao mgo al2o3 sio2 it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts

c o and sr isotope stratigraphy across the lower middle cambrian transition of the cantabrian zone spain and the montagne noire france west gondwana c o and sr isotopes have been analysed for three sections of the cantabrian zone northern spain and the southern montagne noire france that cross the lower middle cambrian transition or bilbilian leonian transition in its iberian sense the base of the leonian middle cambrian is regionally defined by the first appearance of the trilobite acadoparadoxides mureroensis which is well constrained in the cantabrian zone but absent in the montagne noire in the bilbilian parts of the three sections nearshore to peritidal strata are capped by erosive unconformities the succeeding leonian relative sea level rise is recorded in similar fashion in both environments leading to the deposition of aggradational bioclastic limestones and or transgressive limestone shale couplets the so called griotte facies overlain by offshore shales which relate to platform drowning and coastal smothering during the leonian caesaraugustan transition bilbilian leonian carbonates show a long term trend to higher 13c values that was punctuated by a succession of positive excursions in cremenes valdore cantabrian zone and ferrals les montagnes montagne noire two bilbilian 13c shifts an older uncertain peak at 0 1 0 01 and a younger certain shift at 0 6 1 7 and two leonian shifts peaking at 1 1 1 2 and 1 3 respectively are apparent at genestosa cantabrian zone a fifth early middle caesaraugustan shift is identified peaking at 2 1 these positive excursions and increasing trend are similar to published trends from the lower middle cambrian transition in siberia and australia although in those areas only two 13c excursions have been reported so far the marked negative 13c excursion reported from the lower middle cambrian boundary of laurentia and china is not recorded in our dataset oxygen isotope and trace element ca mn and sr data indicate that our 13c trends are unlikely to be diagenetic features while the lack of correlation with relative sea level fluctuations facies changes or benthic community dynamics means that they could be of potential for global stratigraphic correlation sr isotope compositions for the cantabrian zone succession exhibit consistent values upsection which suggests that seawater 87sr 86sr remained around 0 7089 0 7090 our more limited dataset for the montagne noire region confirms this value and both are consistent with published constraints from laurentia for seawater 87sr 86sr across the lower middle cambrian boundary the only other 87sr 86sr study to cover the boundary interval reported significantly lower 87sr 86sr values not vert similar0 7087 0 7088 indicating perhaps that the lower middle cambrian boundary in siberia lies stratigraphically below its equivalent levels in europe and laurentia however we consider it more likely that these siberian samples have been diagenetically altered to less radiogenic values

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

early cambrian chronostratigraphy and geochronology of south australia abstract the most successful chronostratigraphic correlation methods enlist multiple proxies such as biostratigraphy and chemostratigraphy to constrain the timing of globally important bio and geo events here we present the first regional high resolution shelly fossil biostratigraphy integrated with 13c chemostratigraphy and corresponding 18o data from the traditional lower cambrian terreneuvian and provisional cambrian series 2 of south australia the global zhuce shice positive excursions ii and iii and the care are captured in lower cambrian successions from the arrowie and stansbury basins the south australian shelly fossil biostratigraphy has a consistent relationship with the 13c results bolstering interpretation identification and correlation of the excursions positive excursion ii straddles the boundary between the kulparina rostrata and micrina etheridgei zones and the care straddles the boundary between the m etheridgei and dailyatia odyssei zones peaking in the lower parts of the latter zone new ca tims zircon dates from the upper hawker group and billy creek formation provide geochronologic calibration points for the upper d odyssei zone and corresponding chemostratigraphic curve embedding the lower cambrian successions from south australia into a global chronostratigraphic context this multi proxy investigation demonstrates the power of integrated methods for developing regional biostratigraphic schemes and facilitating robust global correlation of lower cambrian successions from south australia part of east gondwana with coeval terranes on other cambrian palaeocontinents including south and north china siberia laurentia avalonia and west gondwana

physical and chemical weathering rates and co2 consumption in a tropical lateritic environment the upper niger basin the chemical composition of niger river water measured bimonthly at bamako mali during the period 1990 1992 provides an estimate of present weathering rates in the upper niger basin the dominant weathering process is kaolinite formation monosiallitization however seasonal variations promote gibbsite formation in the rainy season september and smectite development in the dry season may the results show that lateritic profiles continue to develop even during very dry episodes the rate of profile development calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface is about 1 3 to 3 7 m myr a comparison between 43 river basins of the world shows that for similar runoff the co2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones

magnetic evidence for the origin of lateritic duricrusts in southern mali western africa abstract based on palaeomagnetic measurements a three stage model for the formation of lateritic duricrusts that postulates a para autochthonous origin of the duricrusts is proposed the directions of the natural remanent magnetization nrm which reside in lithogenic magnetite contained in the duricrust matrix have no preferential orientation the nrm directions of in situ formed hematite contained in the micronodules of the duricrusts are randomly distributed the random distribution of the hematite magnetization directions suggests a para autochthonous origin of the ferriferous micronodules which is explained by erosion and reworking prior to consolidation of the duricrusts these processes lead to a mechanical iron accumulation at the top of the lateritic profiles

combined dating of goethites and kaolinites from ferruginous duricrusts deciphering the late neogene erosion history of central amazonia abstract this study focuses on the dating of millimetric pisoliths from ferruginous duricrusts located in central amazonia brazil by coupling detailed mineralogy and two relevant dating methods i u th he analysis of goethites by mass spectrometry ii analysis of radiation induced defects of kaolinites embedded in duricrusts by electron paramagnetic resonance spectroscopy epr three samples collected at different elevations in the landscape were selected the goethite corrected ages range from 1 1 15 2 ma and consistently increase with elevation epr was performed on kaolinites after several cycles of a deferration procedure considering extreme geochemical scenarii the ages of kaolinites are 1 7 7 1 ma for a closed system and 4 0 16 7 ma for an open system with 100 rn loss ages of goethites and kaolinites are close in the limit of the uncertainty of the methods they show important periods of duricrust formation at middle miocene and at late miocene pliocene data allow the estimation of lowering rates between 3 and 8 m ma in agreement with independent data available in the literature for the central amazonia region or even for erosion of other cratons in the world the two dating methods can be compared on common duricrust samples by revealing single or multi step weathering erosion episodes related to relictual paleosurfaces they also bring promising contributions to an advanced knowledge of ancient and complex lateritic geosystems

triassic magnetic overprints related to albitization in granites from the morvan massif france the morvan massif located in the northern part of the french massif central shows widespread unusual deep alteration profiles in the palaeozoic basement rocks gneisses granites and rhyolites they are characterised by overall albitization chloritisation and haematisation albite and chlorite constitute their characteristic mineral paragenesis haematite pigmentation comes with this paragenesis and gives an overall pink coloration to the weathered rocks a paleomagnetic analysis was conducted after detailed petrographical and mineralogical studies of the alteration that lead to the albitized profiles special attention was devoted to the occurrence of haematite the main paleomagnetism carrier with the though to date this palaeoweathering event petrographical checking after heating treatments showed that deshydroxylation of chlorite also led to the formation of secondary haematite this haematite formed by recrystallization acquired remanence properties and thus superimposed and hided the original magnetic signal this problem has prevented the extraction of the original magnetic signal especially in the sections made up of rhyolites virtual geomagnetic poles vgps obtained from the characteristic directions carried by haematite have been compared to the european apparent polar wander path apwp an early triassic magnetic overprint has been recorded in the albitized basement rocks of the morvan massif the age obtained for this albitization phenomena strengthens the hypothesis that albitization developed in relation with palaeoweathering processes linked with the triassic palaeosurface therefore large scale continental weathering could be the main mechanism for late permian early triassic remagnetizations which are widespread throughout the palaeozoic formations of western europe as far as we know this is the first time that deep palaeoweathering phenomena affecting hard and non disaggregated weathered rocks of the basement have been directly dated palaeomagnetism appears as a precious tool to date alteration events developed in widely and long lasting emerged areas of the basement

high resolution chemostratigraphy of the cambrian ordovician gssp enhanced global correlation tool abstract the green point formation of the cow head group in western newfoundland canada represents the global stratotype section and point gssp for the cambrian ordovician boundary on laurentia the formation consists of the martin point lower and the broom point upper members which constitute a thick 170 m deep subtidal to slope marine carbonate sequence preservation of the micritic carbonates of the green point formation was evaluated by multiple petrographic and geochemical screening tools the 13 c and 18 o values of near primary micrites range from 4 7 to 1 7 vpdb and from 8 7 to 5 5 vpdb respectively with no significant correlation r 2 0 002 similarly the 13 c values show no significant correlation with mn sr ratios or total ree contents which suggest that the investigated carbonates retain their near primary 13 c signatures that can be utilized to construct a high resolution carbon isotope profile for the gssp the 13 c carb profile about the gssp trends generally positive into bed 22 of the green point formation the top of the c intermedius zone marks the onset of a negative 13 c carb shift to about 6 0 culminating in the upper part of the zone subsequently the 13 c carb profile trends positive with a double switch back about the boundary point and then continues on to the highest value in the uppermost part of bed 23 in addition the ce ce values approximately between 0 8 and 1 0 through the gssp suggest generally dysoxic waters before during and immediately after the boundary the high resolution chemostratigraphic results curve and trends covering the gssp should facilitate correlation of the cambrian ordovician boundary sections sequences from other locations

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

the paleoenvironmental evolution of the cambrian longwangmiao formation stage 4 toyonian on the yangtze platform south china petrographic and geochemical constrains abstract during the cambrian period stage 4 biodiversity and biomass declined rapidly and globally in an event known as the botomian toyonian crisis to gain a better understanding of the paleoenvironment in south china during this stage we present and interpret complex multi parameter carbonate analyses petrography carbon and oxygen isotopic composition and major trace and rare earth elements from the toyonian longwangmiao formation in the bandenggou section the longwangmiao formation comprises primarily carbonate deposits with massive detrital mineral occurrences in its lower middle sequence principal component analysis of the minerals and oxides leads to the definition of two intervals i and ii interval i is composed mainly of dolostones interbedded with sandstones and contains exceedingly high amounts of terrestrial original minerals and oxides as well as high ree y th and sc concentrations interval ii which is composed primarily of limestones has trace amounts of terrestrial original minerals and oxides and low ree y th and sc concentrations the 13c and 18o values show a significant positive shift from interval i to interval ii after eliminating the effects of diagenesis contributions through the analyses of diagenesis sensitive indexes 18o values mgo cao mn sr and fe sr ratios and the correlations of 13c 18o 18o eu eu eu eu ce ce and eu eu feo the sr and na concentrations of calcites in interval i are extremely high exceeding even the corresponding ranges of marine aragonites while calcites from interval ii have low sr and na concentrations that are within the range of marine low mg calcite however both show strong positive correlations between ca sr ca na and sr na the different calcite elemental behaviors in intervals i and ii suggest different carbonate precursors associated with seawater temperature and climate furthermore the sr ca ratios are negatively correlated with 13c and 18o values and the 13c values also negatively correlated with sio2 content these features indicate a significant change in the depositional environment during the cambrian toyonian stage from an early restricted marine environment under a warm and arid climate with massive terrestrial influxes to a later relatively open marine environment under a cool and humid climate with negligible terrestrial influxes this shift likely resulted in the observed changes in biomass and biodiversity

paleomagnetism applied to lateritic profiles to assess saprolite and duricrust formation processes the example of mont baduel profile french guiana abstract based on the fact that a chemical remanent magnetization crm can be acquired during the lateritization the paleomagnetic signal was used to interpret the succession of horizons from the weathering front to the top of the duricrust surface of a thick bauxitic profile in french guiana the 54 m thick profile is composed of a saprolite capped by a duricrust comprising successively a mottled zone a massive facies and a pisolitic facies at the top at the base of the saprolite the natural remanent magnetization nrm is composed of a superimposed magnetite and hematite contribution the magnetite showing an apparently inherited proterozoic direction within the profile the main magnetic carrier is hematite goethite does not provide any significant contribution five magnetic reversals have been identified within the saprolite and through comparing the reversal sequence with the reference geomagnetic polarity time scale gpts it has been possible to deduce the saprolitization rate thus the bruhnes matuyama 780 ka transition is observed at 8 8 m above the weathering front giving a 11 3 0 5 m ma saprolitization rate the duricrust is significantly divided into two levels within the massive facies the base of the duricrust shows paleomagnetic directions that fit with the present day situation whereas the upper part records directions of the south american apparent pole wander path apwp older than 10 ma the study shows that hematite precipitation was not uniform within the duricrust during lateritization and seems to preserve traces of successive and distinct lateritization phases thus the lower massive duricrust formed during a recent lateritization period whereas the upper duricrust has preserved a paleomagnetic signal of an older lateritization phase our duricrust paleomagnetic interpretation is supported by sem observations of changes in hematite morphology

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

a negative carbon isotope excursion defines the boundary from cambrian series 2 to cambrian series 3 on the yangtze platform south china a globally recorded negative carbon isotope excursion characterizes the transition from cambrian series 2 to cambrian series 3 this transition is also well exposed in sedimentary successions on the yangtze platform and the wuliu zengjiayan section guizhou province south china has been proposed as a potential global stratotype section and point gssp for this boundary here we report delta 13 c carb values for the jianshan and the wuliu zengjiayan sections both sections display a progressive decrease in delta 13 c from values around 3 parts per thousand upwards in stratigraphy to a pronounced delta 13 c minimum with values as low as 6 9 parts per thousand at the proposed boundary level and a return to delta 13 c values between 0 and 1 parts per thousand in the upper part of the sections the delta 13 c minimum is thought to be caused by a transgressive event flooding the shelf area with 13 c depleted basinal anoxic bottom water our delta 13 c data are in good agreement with carbon isotope profiles recorded elsewhere these define the so called roece event redlichiid oleneliid extinction carbon isotope excursion cf zhu et al 2006 2007 and may reflect the perturbation of the global carbon cycle during the cambrian series 2 to cambrian series 3 transition c 2009 elsevier b v all rights reserved

a vendian cambrian boundary succession from the northwestern margin of the siberian platform stratigraphy palaeontology chemostratigraphy and correlation siberia contains several key reference sections for studies of biological and environmental evolution across the proterozoic phanerozoic transition the platonovskaya formation exposed in the turukhansk region of western siberia is an uppermost proterozoic to cambrian succession whose trace and body fossils place broad limits on the age of deposition but do not permit detailed correlation with boundary successions elsewhere in contrast a striking negative carbon isotopic excursion in the lower part of the platonovskaya formation permits precise chemostratigraphic correlation with upper most yudomian successions in siberia and possibly worldwide in addition to providing a tool for correlation the isotopic excursion preserved in the platonovskaya and contemporaneous successions documents a major biogeochemical event likely involving the world ocean the excursion coincides with the palaeontological breakpoint between ediacaran and cambrian style assemblages suggesting a role for biogeochemical change in evolutionary events near the proterozoic cambrian boundary

sequence stratigraphy chemostratigraphy and facies analysis of cambrian series 2 series 3 boundary strata in northwestern scotland globally the series 2 series 3 boundary of the cambrian system coincides with a major carbon isotope excursion sea level changes and trilobite extinctions here we examine the sedimentology sequence stratigraphy and carbon isotope record of this interval in the cambrian strata durness group of nw scotland carbonate carbon isotope data from the lower part of the durness group ghrudaidh formation show that the shallow marine laurentian margin carbonates record two linked sea level and carbon isotopic events whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere correlation with global records sauk i sauk ii boundary and olenellus biostratigraphic constraint identifies them as representing the local expression of the roece and dice the upper part of the roece is recorded in the basal ghrudaidh formation whilst the dice is seen around 30m above the base of this unit both carbon isotope excursions co occur with surfaces interpreted to record regressive transgressive events that produced amalgamated sequence boundaries and ravinement flooding surfaces overlain by conglomerates of reworked intraclasts the roece has been linked with redlichiid and olenellid trilobite extinctions but in nw scotland olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

compaction of density stratified cumulates effect on trapped liquid distribution igneous rocks formed by the accumulation of crystals from a cooling magma body compose a significant fraction of both the terrestrial oceanic crust and the lunar crust and mantle here we report the results of numerical simulations of compaction of accumulating crystals for which the bulk density is not constant the simulations are designed to provide insight into the behavior of interstitial liquid during compaction when a sharp drop in the density of the cumulus minerals is encountered e g at the ultramafic mafic boundary in a layered intrusion the simulations provide an explanation for trapped liquid distributions associated with sharp mineral density transitions density transition in the mineral assemblage is a feature common to many layered intrusions and the oceanic crust this process does not require external mechanisms such as injections of new and different magmas and has important implications for the formation of pt ores found at ultramafic mafic transitions the model also has implic

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

first palaeomagnetic dating of ferricrete in new caledonia new insight on the morphogenesis and palaeoweathering of grande terre terra nova 24 77 85 2012 abstract comparing palaeolatitudes estimated from remanent magnetization data of ferricrete developed over laterite at two sites goro and tiebaghi against the apparent polar wandering path of australia provides new time constraints for the peridotite weathering in new caledonia the tiebaghi massif of the west coast klippes displays a single episode of ferricrete development during latest oligocene times by contrast the goro site in the southern part of the massif du sud reveals a more complex history with several episodes of ferricrete development from latest oligocene to pliocene quaternary times these data have made it possible to constrain the post obduction geodynamics and associated morphogenesis of new caledonia since related uplift probably prevented regolith development until latest oligocene times moreover the later uplift and deep erosion of the west coast klippes during the early miocene probably prevented further regolith preservation

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

gold predictivity mapping in french guiana using an expert guided data driven approach based on a regional scale gis the realistic estimation of gold mining in french guiana requires including the numerous illegal gold washing activities in predictivity mapping the combination of a classical approach based on the algebraic method of knox robinson and groves with innovative processing grid type geochemical and radiometric data as well as cluster analysis technique provides a better understanding of the structure of studied mineralized areas

palaeomagnetic dating of late cretaceous to early tertiary weathering in new england n s w australia abstract palaeomagnetic dating of australia s regolith is becoming more refined with advances in techniques and better definition of the late mesozoic and cenozoic apparent polar wander path of australia the suspected great antiquity of parts of the regolith is being confirmed through the application of palaeomagnetic dating to authigenic iron oxides which form during weathering earlier palaeomagnetic investigation tentatively suggested that deeply weathered volcanics in the new england area formed in the mesozoic implying that ideas regarding the palaeoenvironment of new england may need considerable revision published dates for other unweathered volcanics from this area are k ar ages of 21 8 ma 32 8 ma and 25 1 ma the earlier palaeomagnetic conclusions were based on results from only one the sugarloaf of the seven separate localities nine sites originally sampled these results being the only results that were internally consistent follow up work using a high sensitivity cryogenic magnetometer has now yielded consistent results from the six other localities which support those from the sugarloaf but a revised weathering age of about 60 ma seems more likely this age is virtually identical to that determined for the morney profile in southwestern queensland

role of late magmatic fluids in merensky type platinum deposits a discussion a number of features of platinum group element pge rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids the presence of pegmatitic textures intergrowth of sulfides with intercumulus volatile rich phases high cl content in volatile bearing phases and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of pge by a fluid phase of late magmatic derivation we argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive uniform pge concentrations were formed by magmatic cumulus processes the association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers which cause these layers to act as traps for fluids migrating through the crystal pile these fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in ci redissolution of these fluids into vapor undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites immiscible sulfide oxide liquid solidifies at near solidus temperatures in the interstitial pore space between early crystallizing silicate minerals and it is forced into areas occupied by late crystallizing ci bearing silicates this accounts for intergrowths of cu and pge rich sulfides with amphiboles and micas a ubiquitous feature of sulfide bearing gabbros further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration formation of fluid inclusions and possible local redistribution of sulfides and pge cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus graphite deposition may be catalyzed by sulfides and pge

palaeoecology of the archaeocyathan reefs from the lower cambrian harkless formation southern nevada western united states and carbon isotopic evidence for their demise abstract archaeocyath sponges the first group of reef building organisms have a globally distributed albeit restricted stratigraphic occurrence archaeocyathan reefs are well known from the western united states particularly in units like the montenegro poleta and harkless formations of the white inyo ranges and esmeralda county nevada while it is recognized that these organisms disappear at some time above the last occurrence of their reefs in the harkless formation the precise timing of their disappearance is unknown here we show that the last archaeocyathan reefs in the upper harkless formation near gold point nevada are overlain by 20 60 m of interbedded siltstone and carbonate that preserve a negative carbon isotopic excursion consistent with the aece event archaeocyath extinction carbon excursion point counts of reef thin sections and analysis of acid insoluble residues of reef and reef adjacent samples illustrate that these last reefs also harbor previously undescribed diversity and abundance of animals living in and among the reefs including an array of small shelly fossil taxa we confirm previous work that the reefs of the early cambrian were ecologically diverse and functioned in many ways like reefs of the later paleozoic even just prior to their global demise in the late early cambrian and that the extinction of archaeocyaths is recorded in strata of the harkless formation

reconstruction of early cambrian ocean chemistry from mo isotopes the neoproterozoic cambrian transition was a key time interval in the history of the earth especially for variations in oceanic and atmospheric chemical composition however two conflicting views exist concerning the nature of ocean chemistry across the precambrian cambrian boundary abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late ediacaran while other studies provide seemingly conflicting evidence for anoxic deep waters with ferruginous conditions fe ii enriched persisting into the cambrian here two early cambrian sedimentary platform and shelf slope sections in south china were investigated to trace early cambrian ocean chemistry from mo isotopes the results reveal that early cambrian sediments deposited under oxic to anoxic euxinic conditions have 98 95mo values ranging from 0 28 to 2 29 which suggests that early cambrian seawater may have had 98 95mo values of at least 2 29 similar to modern oceans the heaviest and relatively homogeneous 98 95mo values were recorded in siltstone samples formed under completely oxic conditions which is considered that mn oxide free shuttling was responsible for such heavy 98 95mo value further combined with fe species data and the accumulation extent of mo and u the variation of 98 95mo values in the two studied sections demonstrate a redox stratified ocean with completely oxic shallow water and predominantly anoxic even euxinic deeper water having developed early on which eventually became completely oxygenated this suggests that oceanic circulation at the time became reorganized and such changes in oceanic chemistry may have been responsible for triggering the cambrian explosion of biological diversity

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

morphology and microstructure of chromite crystals in chromitites from the merensky reef bushveld complex south africa the merensky reef of the bushveld complex consists of two chromitite layers separated by coarse grained melanorite microstructural analysis of the chromitite layers using electron backscatter diffraction analysis ebsd high resolution x ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals fine grained idiomorphic and large silicate inclusion bearing crystals the lower chromitite layer contains both populations whereas the upper contains only fine idiomorphic grains most of the inclusion bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre existing smaller idiomorphic crystals two possible mechanisms have been proposed for sintering of chromite crystals 1 amalgamation of a cluster of grains with the same original crystallographic orientation and 2 sintering of randomly orientated crystals followed by annealing into a single grain the ebsd data show no evidence for clusters of similarly oriented grains among the idiomorphic population nor for earlier presence of idiomorphic subgrains spatially related to inclusions and therefore are evidence against both of the proposed sintering mechanisms electron backscatter diffraction analysis maps show deformation related misorientations and curved subgrain boundaries within the large amoeboidal crystals and absence of such features in the fine grained population microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation an alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the merensky reef the upper chromite layer formed from a subsequent magma influx and hence lacked a mechanism to form dendritic chromite this accounts for the difference between the two layers

evolution of oceanic molybdenum and uranium reservoir size around the ediacaran cambrian transition evidence from western zhejiang south china abstract the cambrian explosion is one of the most fascinating episodes of diversification in the history of life however its relationship to the oxygenation of the oceans and atmosphere around the ediacaran cambrian transition is not fully understood marine inventories of redox sensitive trace elements reflect the relative balance of oxidative weathering on land and deposition in anoxic water masses and can be used to explore the evolution of oceanic and atmospheric redox conditions for this study we conducted a series of geochemical analyses on the upper lantian piyuancun and hetang formations in the chunye 1 well part of the lower yangtze block in western zhejiang iron speciation results indicate that the entire studied interval was deposited under anoxic conditions with three intervals of persistent euxinia occurring in the uppermost lantian fm the lower hetang formation fm and the upper hetang fm molybdenum mo and uranium u contents and mo toc and u toc ratios from the anoxic euxinic intervals of the chunye 1 well combined with published data from the sections in the middle and upper yangtze block suggest that the oceanic mo reservoir declined consistently from the ediacaran to cambrian stage 3 while the size of the oceanic u reservoir remained relatively constant both metals were depleted in the ocean in lower cambrian stage 4 before increasing markedly at the end of stage 4 the lack of an apparent increase in the size of the marine mo and u reservoir from the upper ediacaran to cambrian stage 3 suggests that oxic water masses did not expand until cambrian stage 4 the increase in marine mo and u availability in the upper hetang fm may have been due to the expansion of oxic water masses in the oceans associated with oxygenation of the atmosphere during cambrian stage 4 this expansion of oxic waters in the global ocean postdates the main phase of cambrian diversification suggesting that pervasive oxygenation of the ocean on a large scale was not the primary control on animal diversity following the ediacaran cambrian transition

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

pre tommotian age of the lower pestrotsvet formation in the selinde section on the siberian platform carbon isotopic evidence carbon isotopic data from the selinde section in the southeastern part of the siberian platform area are correlated with the reference isotopic profile from the lower cambrian stratotype sections of the lena aldan region but also show additional 13 c excursions unrecognized there the chemostratigraphic correlation suggests that the geological and fossil record of the lower pestrotsvet formation in the selinde section has a deeper history than the stratotype region this conclusion is important for both constraining the age of the earliest cambrian marine transgression on the siberian platform and providing a clearer understanding of the pace and order of early cambrian geochemical and biological events

palaeomagnetic dating of the borne de fer ferricrete ne france lower cretaceous continental weathering abstract a palaeomagnetic study was carried out on the borne de fer ferricrete in north eastern france close to the luxembourg border this ferricrete is overlain by kaolinitic red clay and caps a silto argilaceous saprolite 15 to 40 m thick showing weathering features evolution of the clay minerals from the base to the top and karstic solution pipes beneath this weathering profile lies above jurassic marl and limestone from around 425 to 450 m a s l and may correspond to a scarce remnant of an old weathering palaeosurface palaeomagnetic rockmagnetic and petrographic analyses reveal presence of a dominant hydroxide contribution with a magnetic remanence carried by goethite a small contribution of maghemite or hematite carries a more scattered but similar direction the virtual geomagnetic pole derived from the goethite component lies at lat 73 4 n lon 205 9 e dp 4 3 dm 6 2 this location close to the 120 140 ma reference poles and presence of normal and reversed polarities are consistent with a lower cretaceous age close to the long normal cretaceous superchron our results are consistent with the ones obtained recently in germany belgium and france and prepare the ground for reconstructing the lower cretaceous continental palaeosurface geometry and palaeogeography

oxygen isotope study of cayenne duricrust paleosurfaces implications for past climate and laterization processes over french guiana abstract oxygen isotopic compositions of weathering goethite hematite kaolinite and gibbsite in duricrust paleosurfaces of the cayenne region northeastern french guiana were investigated samples were microdrilled from different horizons and different pedogenetic microfacies of three well characterized duricrust units of eocene unit 1 and miocene units 2 and 3 age the results provided valuable information on duricrust formation processes and on climate change over french guiana the pristine original isotopic signal suited for reconstruction of past climates appears to be best preserved in the massive duricrust facies middle part of the duricrust in the overlying pisolotic duricrust and in the underlying mottled zone the isotopic signal is affected by subsequent re activation of weathering processes oxygen isotope data from massive duricrust indicate that cayenne duricrusts formed from meteoric waters increasingly depleted in 18o with age the youngest duricrust paleosurface unit 3 formed under humid climate similar to present day while the two older paleosurfaces formed from meteoric paleo water distinctly depleted in 18o reflecting a moonsonal contrasted ancient climate for unit 2 added to the 18o depleted nature of the pre miocene ocean for unit 1 the results further suggest that the major climatic change from tropical wet and dry to present day humid regime recorded in amazonia most likely occurred in the late miocene and that miocene laterization events may be responsible for the secondary bauxitization of the eocene duricrust paleosurface the oxygen isotopic composition of weathering minerals occurring in massive duricrust horizons and in particular in hematite nodules and matrix may be used as a tool to discriminate among the different duricrust paleosurfaces encountered in amazonia and constrain the evolution of past climates

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

timing of lateritization on the guiana shield synthesis of paleomagnetic results from french guiana and suriname abstract a paleomagnetic study was carried out on representative duricrust paleosurfaces of the guiana shield in french guiana cayenne region and kaw mountain and suriname moengo and bakhuis mountains the age of weathering of ferruginous and bauxitic duricrusts was estimated from comparison of paleomagnetic poles determined from these duricrust horizons with the south american apparent polar wander path apwp in most of the studied profiles hematite is the dominant magnetic carrier as indicated by unblocking temperatures that exceed 580 c for about 20 60 of the natural remanent magnetization nrm magnetite and goethite may also be present and contribute to the overall nrm but contribute little or nothing to the characteristic remanent magnetization which is estimated from principal component analysis of the medium to high unblocking temperature component paleomagnetic directions from the duricrust profiles generally cluster well dispersion parameters 30 and give paleomagnetic poles that fall between the cenozoic and the present day portion of the apwp recent paleomagnetic directions are usually detected in the uppermost part of the duricrust profiles which may have sustained some recent reworking in contrast the middle part of the duricrust of massive facies usually shows ancient directions the bauxitic surfaces of bakhuis mountains at 400 m elevation kaw mountain upper surface 300 m elevation and probably the cayenne unit 1 75 200 m elevation can be attributed to the sul americano lateritization cycle of paleocene eocene age with respectively average relative ages of 60 50 and 40 ma the bauxitic paleosurface of kaw mountain lower surface 220 m the coastal moengo deposit 40 m and the ferruginous surface of cayenne unit 2 20 50 m may be related to the late velhas lateritization cycle of miocene age with relative average ages of 13 and 10 ma the fe duricrust of cayenne unit 3 close to sea level and occupying most of the coastal plain is geomorphologically the youngest duricrust paleosurface but also seems related to a late miocene lateritization event 5 13 ma the oligocene appears to represent a major break in the development of weathering profiles with erosion as the predominant process which stripped 50 80 m from the landscape between the two main lateritization phases of paleocene eocene and miocene

structures at the base of the upper group 2 chromitite layer bushveld complex south africa on karee mine lonmin platinum abstract exposures in a now infilled pit mined for platiniferous ug2 chromitite in the bushveld complex south africa are described the layer of chromitite is underlain by anorthosite providing a dramatic colour contrast the interface between these two rock types shows evidence of various scales of irregularities in plan view small circular depressions less than 3 cm across and 5 mm deep occupy about 20 of the surface between them the contact is planar the anorthosite immediately underlying the chromitite has a planar fabric visible in thin sections that is not disturbed beneath these small depressions another set of depressions occurs about 40 cm in diameter and with variable depth several possible processes are reviewed for the origin of these irregularities especially the small scale structures but none explains all the features noted these processes include remelting diapirism impact generated dimpling gas escape and interference rippling we present a photographic record of these structures but present no definitive model for their interpretation

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

biogenic and abiogenic low mg calcite blmc and almc evaluation of seawater ree composition water masses and carbonate diagenesis the ree and shale normalized paas reesn values of modern brachiopods biogenic low mg calcite blmc represented by several species from high to low latitudes from shallow to deep waters and from warm and cold water environments define three distinct average seawater trends the warm and cold water brachiopods define two indistinguishable p 0 050 groups that mimic open ocean seawater ree chemistry exhibiting the typical lree enrichment with a slightly positive to negative ce anomaly followed by an otherwise invariant series other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ree and reesn trends from the previous two populations showing a slight enrichment in the mrees and an increasing trend in the hrees other groups of modern brachiopods are characterized by elevated reesn trends relative to the normal groups as well as by complexity of the series trends the most characteristic feature is the decrease in the hrees in these brachiopods from areas of unusual productivity i e such as upwelling currents fluvial input and aerosol dust deposition preserved brachiopods from the eocene and silurian exhibit reesn trends and ce anomalies similar to that of the open ocean modern brachiopods although their enriched ree concentrations suggest precipitation of blmc influenced by extrinsic environmental conditions preservation of the blmc was tested by comparing the ree and reesn trends of preserved eocene brachiopods to those of oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric water the altered blmc is enriched by approximately one order of magnitude in both ree and reesn trends relative to that in blmc of their preserved counterparts similarly the ree and reesn of preserved silurian brachiopod blmc were compared to those of their enclosing altered lime mudstone which exhibits features of partly closed system phreatic meteoric water diagenesis despite these differences in the diagenetic alteration systems and processes the rees and reesn trends of the blmc of altered brachiopods and of originally mixed mineralogy lime mudstones now diagenetic low mg calcite are enriched by about one order of magnitude relative to those observed in the coeval and preserved blmc in contrast to the changes in ree and reesn of carbonates exposed to phreatic meteoric water diagenesis are the ree compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids the ree and reesn trends of the burial cements mimic those of their host lime mudstone with all showing slight lree enrichment and slight hree depletion exhibiting a chevron pattern of the reesn trends the overall enrichment or depletion of the cement reesn trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system but also strong differences in the elemental and ree compositions of the burial fluids evaluation of the ce ce sn and la pr pr sn anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids

geochemical and isotopic characterization of the bodele depression dust source and implications for transatlantic dust transport to the amazon basin the bodele depression chad in the central sahara sahel region of northern africa is the most important source of mineral dust to the atmosphere globally the bodele depression is purportedly the largest source of saharan dust reaching the amazon basin by transatlantic transport here we have undertaken a comprehensive study of surface sediments from the bodele depression and dust deposits chad niger in order to characterize geochemically and isotopically sr nd and pb isotopes this dust source and evaluate its importance in present and past african dust records we similarly analyzed sedimentary deposits from the amazonian lowlands in order to assess postulated accumulation of african mineral dust in the amazon basin as well as its possible impact in fertilizing the amazon rainforest our results identify distinct sources of different ages and provenance in the bodele depression versus the amazon basin effectively ruling out an origin for the amazonian deposits such as the belterra clay layer by long term deposition of bodele depression material similarly no evidence for contributions from other potential source areas is provided by existing isotope data sr nd on saharan dusts instead the composition of these amazonian deposits is entirely consistent with derivation from in situ weathering and erosion of the precambrian amazonian craton with little if any andean contribution in the amazon basin the mass accumulation rate of eolian dust is only around one third of the vertical erosion rate in shield areas suggesting that saharan dust is consumed by tropical weathering contributing nutrients and stimulating plant growth but never accumulates as such in the amazon basin the chemical and isotope compositions found in the bodele depression are varied at the local scale and have contrasting signatures in the silica rich dry lake bed sediments and in the calcium rich mixed diatomites and surrounding sand material this unexpected finding implies that the bodele depression material is not pre mixed at the source to provide a homogeneous source of dust rather different isotope signatures can be emitted depending on subtle vagaries of dust producing events our characterization of the bodele depression components indicate that the bodele calcium rich component identified here is most likely released via eolian processes of sand grain saltation and abrasion and may be significant in the overall global budget of dusts carried out by the harmattan low level jet during the winter

trace element partitioning between apatite and silicate melts we present new experimental apatite melt trace element partition coefficients for a large number of trace elements cs rb ba la ce pr sm gd lu y sr zr hf nb ta u pb and th the experiments were conducted at pressures of 1 0 gpa and temperatures of 1250 c the rare earth elements la ce pr sm gd and lu y and sr are compatible in apatite whereas the larger lithophile elements cs rb and ba are strongly incompatible other trace elements such as u th and pb have partition coefficients close to unity

mass occurrence of oncoids at the cambrian series 2 series 3 transition implications for microbial resurgence following an early cambrian extinction abstract the traditional lower middle cambrian transition cambrian series 2 series 3 transition is marked by the first major biotic extinction of the phanerozoic eon this biotic crisis has been arguably linked to changes in ocean chemistry and or marine environments but their casual relationships remain controversial to better understand the microbial responses to paleoceanographic changes across this critical transition we have studied the environmental conditions for mass occurrence of oncoids in the western north china platform oncoids at the lower middle cambrian transition form 1 to 3 m thick massive beds show spherical subspherical morphology and contain 8 14 light dark cortical laminar couplets the light laminae are thicker and contain densely intertwined filamentous cyanobacteria that have calcified sheaths and a prostrate growth pattern the dark laminae are thinner and rich in organic matter relics pyrite framboids and heterotrophic bacteria in most oncoids cortical laminae show the same growth orientation for more than five light dark laminar couplets suggesting much less frequent grain overturning than generally thought both light and dark laminae contain well preserved organomineralization fabrics textures including extracellular polymeric substances epss nanoglobules polyhedrons and micropeloids suggesting oncoid formation in shallow marine environments with high alkalinity and active sulfate reduction the presence of pyrite framboids and heterotrophic bacterial relics implies anoxic dysoxic bottom water conditions stratigraphic correlation indicates that time equivalent oncolites and other microbialites are widespread not only in north china but also in other early cambrian successions globally the mass occurrence of oncoids coincides with the kalkarindji large igneous province of australia a prominent negative 13 c excursion roece or redlichiid olenellid extinction carbon isotope excursion a significant increase in 87 sr 86 sr and a large positive shift in 34 s the coincidence of these events suggests that the early cambrian biotic crisis may have been caused by an ocean anoxic event resulting from enhanced volcanic release of co 2 global warming and increased continental weathering preservation of massive oncolites is likely related to decreased metazoan grazing following an early cambrian biotic crisis during which archaeocyathids and many early cambrian trilobite taxa went to extinction the oncoid mass occurrence provides evidence for the resurgence of microbial life in anoxic dysoxic marine shelf environments concomitant with the early cambrian extinction event

episodic weathering in southwestern amazonia based on u th he dating of fe and mn lateritic duricrust abstract this study presents the results of mineralogical analysis and u th he dating of micrometric grains of hematite goethite cryptomelane and hollandite from six samples of bauxite fe mn mn al fe mn al and mn al p duricrusts located in southwestern amazonia brazil the samples were collected at different elevations to constrain temporal epochs of lateritization and to provide insights into the cratonic landscape evolution of proterozoic and paleozoic terrains in brazil three main phases of duricrust formation were recognized indicating a long period of lateritic weathering the first phase preserved as a fragment in fe duricrust on the paleozoic sedimentary rocks began in the permian 290 ma and lasted until the late cretaceous 65 ma the second and best preserved phase formed mn duricrusts on the paleoproterozoic apui lateritic surface 150 250 m a s l and on the aripuana lateritic surface

the δ13c excursions spanning the cambrian explosion to the canglangpuian mass extinction in the three gorges area south china abstract a remarkable increase of the animal genera and a subsequent mass extinction in the late early cambrian are well known as the cambrian explosion and the botomian toyonian crisis a composite global curve of the carbon isotope ratios for inorganic carbon 13ccarb shows multiple fluctuations during the evolution events and it indicates significant changes of the oceanic carbon cycle at that time this study reveals a new continuous isotopic chemostratigraphy for inorganic carbon 13ccarb from the bottom of the shipai to the base of the shilongdong formations in three gorges area south china this section covers the canglangpuian to the longwangmiaoian stages in the lower cambrian the 13ccarb variation exhibits three negative excursions a remarkably negative excursion down to ca 12 in the middle canglangpuian stage a negative excursion to ca 1 0 in the upper canglangpuian stage and a negative excursion to ca 1 0 in the longwangmiaoian stage respectively the largest negative 13ccarb excursion and a positive excursion before the excursion are definitely consistent with the 13ccarb negative shift aece during the mass extinction and the 13ccarb positive values mice during the increase of animal genera respectively however the minimum values of the negative shifts among south china siberia and canada sections are different from each other the positive 13ccarb excursion at the bottom of the canglangpuian stage indicates that primary productivities and organic carbon burial were enhanced a sea level rise in the qiongzhusian to bottom of the canglangpuian stages in south china corresponds to the sinsk transgression event in siberia and canada a eutrophication due to higher continental weathering during the transgression after the long term retrogression enhanced the high primary production and consequently promoted the significant increase of animal diversity on the other hand deposition of laminated black shales without bioturbation signatures and a decline of trilobite diversity are observed during the negative 13ccarb excursion in the canglangpuian stage indicating that the shallow water environment became anoxic at that time the negative 13ccarb shift indicates an influx of abundant 12co2 due to oxidation of organic carbons in seawater the difference of the minimum values among sections implies the local difference in size of the organic carbon reservoirs and extent of the degradation of the carbons the largest 13c anomaly in south china suggests the presence of the largest ocps due to higher activity of primary production and high degree of oxidation of the ocps because of higher activity of animals the coincidence of the timing of the negative 13c excursions in the canglangpuian stage among the sections indicates a global event and suggests that the onset was caused by increase of oxygen contents of seawater and atmosphere abundant oxygen yielded by the increased primary productivity in the atdabanian and the qiongzhusian stages caused onset of the oxidation of ocp and possibly led to the shallow water anoxia and the mass extinction of benthic animals in the botomian and the canglangpuian stage

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

stable isotope geochemistry of the lower cambrian sekwi formation northwest territories canada implications for ocean chemistry and secular curve generation abstract three curves delineating 13 c variations across the early cambrian sekwi formation carbonate ramp mackenzie mountains northwest territories canada are incorporated into a biostratigraphic and sequence stratigraphic framework that provides a higher resolution temporal and spatial framework for the sekwi formation the three 13 c curves were correlated across a range of depositional environments from tidal flat to deep subtidal on the sekwi carbonate ramp eight 13 c cycles in the curves a h were correlated across the basin complimenting the sequence stratigraphic framework the most complete 13 c curve from the sekwi formation correlates well with a composite early cambrian 13 c curve from the siberian platform however the differing magnitude and absolute timing of some of the correlative isotopic excursions records the influence of regional tectonism sea level changes and diagenesis on the sekwi ramp additionally the sekwi formation 13 c curve has more high frequency variations than the siberian curve due in part to the higher sample density used to construct the sekwi curve the early middle cambrian boundary was not sampled in this study but a large negative isotopic excursion in the very latest early cambrian coincides with a regional drowning that may be correlative throughout western laurentia but recognizing this event globally is problematic

multiple δ13c excursions spanning the cambrian explosion to the botomian crisis in siberia new high resolution 13 c data through the lower cambrian of siberia reveal multiple positive excursions coincident with phases in the explosion of invertebrate phyla nemakit daldynian to middle botomian stages comparison of the 18 o and trace element mg fe mu sr chemistry indicate that six new 13 c cycles are primary rather than diagenetic features with potential for global correlation positive 13 c excursions up to 3 indicate that fractional organic carbon burial rates were high but variable through the cambrian explosion values for 13 c dropped sharply from around 2 2 to 1 6 in botomian time coincident with mass extinction of the archaeocyathan reef biota the rapid fluctuation of the 13 c signal and the temporal coincidence of the pronounced negative shift with the extinction event hint that the 13 c record may record productivity variations during the early cambrian radiation

geologic and palaeomagnetic constraints on the formation of weathered profiles near inverell eastern australia abstract we use geologic and palaeomagnetic observations to constrain the timing and duration of weathering of the oakwood bauxite which outcrops in the new england region of new south wales australia the bauxite is developed on and overlain by mafic volcanic flows prior geochronologic results from surrounding regions indicate that age of these flows and thus the age of the bauxite is most likely between 18 and 26 ma though volcanics elsewhere in the central volcanic province have a much greater range of between 6 and 56 ma similarly comparison of a new palaeomagnetic pole computed from the bauxite pole latitude 75 4 n longitude 311 3 e n 58 a95 4 4 with the australian apparent polar wander path gives estimated ages of 21 27 ma individual samples of the bauxite have both normal and reversed polarity components of magnetization which beside providing a positive palaeomagnetic reversal test suggests that the duration of weathering exceeded 10 000 years and more likely lasted several million years our results suggest that the oakwood region had a palaeoclimate and palaeoenvironment that was conducive for deep weathering over a significant period in a the time interval from the mid oligocene to the early miocene

paleoproterozoic 2155 1970 ma evolution of the guiana shield transamazonian event in the light of new paleomagnetic data from french guiana abstract we present a comprehensive paleomagnetic study on paleoproterozoic 2173 2060 ma plutonic and metamorphic rocks from french guiana representative of the full range of the main transamazonian tectonothermal steps twenty seven groups of directions and poles were obtained from combination of 102 sites 613 samples based on age constraint similar lithology and or geographical proximity paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases this is also reinforced by positive field tests baked contact and reversal tests recent u pb and pb pb on zircon and complementary 40ar 39ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages estimated magnetic ages based on amphibole dating as a proxy enable us to construct a guiana shield apparent polar wander path for the 2155 1970 ma period it is also possible to present paleolatidudinal evolution and continental drift rates related to specific transamazonian tectonic regimes french guiana and probably the guiana shield were located at the equator from ca 2155 to 2130 ma during the meso rhyacian d1 magmatic accretion phase related to subduction of eorhyacian oceanic crust after closure of the eorhyacian ocean and collision of west african and amazonian plates the guiana shield moved the first evolution towards 60 latitude occurs after 2080 ma during the neorhyacian d2a post collisional sinistral transcurrent phase during the late rhyacian d2b phase up to 2050 ma the guiana shield reaches the pole and starts to move to lower latitudes on an opposite meridian by the orosirian d2c phase from ca 2050 to 1970 ma the guiana shield reaches the equator based on the amphibole 40ar 39ar dates we estimate the continental drift between 12 and 16 cm y for the meso to late rhyacian period followed by a lower rate between 9 and 14 cm y up to orosirian time this study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100 200 ma time period our results confirm the possibility of apwp construction on paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi method radiometric dating

lateritization and bauxitization events laterites and bauxites are produced in tropical soils by weathering which enriches iron of laterites and alu mina of bauxites as well as trace elements such as nickel gold phosphorus and niobium to ore grade la terites and bauxites can be redeposited into sedimentary sequences and remain as ores if not transported far and diluted with other materials the age of redeposited laterites and bauxites and of bauxitic and lateritic pa leosols can be established from the geologic age of overlying rocks an approach especially effective in pale osols within sequences of isotopically datable volcanic rocks lateritic profiles can also be dated by paleomag netic inclination in special cases in which land masses such as in australia and india drifted long distances northward during cenozoic time in addition cryptomelane and other k mn oxides can be dated by k ar and 40ar 39ar techniques to obtain multiple ages from different crystals in a single relict paleosol compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k y duration at 2 12 16 35 48 55 65 and 100 ma such events can also be inferred at times older than 100 ma from paleolatitudinal distribution of laterites and bauxites but these are poorly sampled laterite and bauxite peaks were coeval with times of global high warmth and pre cipitation elevated atmospheric carbon dioxide oceanic anoxia exceptional fossil preservation and mass ex tinction these co2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact flood basalt volcanism and methane outbursts concentration of bauxite and laterite resources in particular stratigraphic horizons formed during greenhouse crises suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

kaolinite dating from acrisol and ferralsol a new key to understanding the landscape evolution in nw amazonia brazil abstract ferralsols and acrisols are major types of soils of the amazon basin observed on various landform units comprising plateau surfaces incised hills at their edges and slopes the present study focuses on an acrisol developed on plateaus surfaces from northwest amazonia and a ferralsol from the convex hills of the incised plateau edges local geomorphologic models of weathering covers suggest that ferralsols are younger than plateau acrisols but the absolute chronology of their formation is still lacking this type of information is however critical to understand the evolution of northwest amazon basin landscapes and to identify its link with major climatic and geomorphologic events in this article kaolinite rich samples from soils and saprolites belonging to a transect in the sao gabriel da cachoeira region amazon state brasil are investigated based on their crystal chemical characters several types of kaolinites are identified using a previously developed methodology based on electron paramagnetic resonance epr spectroscopy crystallization ages are proposed for these different kaolinites saprolite kaolinites are dated from 6 to 3 6 ma in the acrisol profile and display significantly more recent ages

early permian pangea b to late permian pangea a the pre drift wegenerian model of pangea is almost universally accepted but debate exists on its pre jurassic configuration since tedirving introd ucedpangea b by placing gond wana farther to the east by v3000 km with respect to laurasia on the basis of paleomagnetic data new paleomagnetic data from radiometrically dated early permian volcanic rocks from parts of adria that are tectonically coherent with africa gondwana integrated with published coeval data from gondwana and laurasia again only from igneous rocks fully support a pangea b configuration in the early permian the use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for pangea b the ultimate option to reject pangea b is to abandon the geocentric axial dipole hypothesis by introducing a significant non dipole zonal octupole component in the late paleozoic time averaged geomagnetic field we demonstrate however by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from gondwana as well as laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for pangea b in the early permian we therefore regard the paleomagnetic evidence for an early permian pangea b as robust the transformation from pangea b to pangea a took place during the permian because late permian paleomagnetic data allow a pangea a configuration we therefore review geological evidence from the literature in support of an intra pangea dextral megashear system the transformation occurred after the cooling of the variscan mega suture andlasted v20 myr in this interval the neotethys ocean opened between india arabia and the cimmerian microcontinents in the east while widespread lithospheric wrenching and magmatism took place in the west around the adriatic promontory the general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right lateral shear couple between gondwana and laurasia during the permian transcurrent plate boundaries associated with the pangea transformation reactivated variscan shear zones andwere subsequently exploitedby the opening of western neotethyan seaways in the jurassic

magnetic ages of some indian laterites abstract the results of an extensive palaeomagnetic investigation of laterites from peninsular india are described from 62 samples 693 specimens were step wise thermally demagnetised to eliminate secondary magnetisations the ages of laterite formation were estimated after calculation of pole positions from site mean directions and comparison with the mesozoic tertiary apparent polar wander path apwp for india the high altitude laterites have undergone a complex magnetic history during the late cretaceous and early tertiary the low altitude laterites are mid to late tertiary in age a fundamental mechanism in laterite formation and preservation is suggested when the indian laterite results are compared with those from australia they are seen to be consistent with the relative timing of the northward drift of india and australia