ree mobility in groundwater proximate to the natural fission reactor at bangombe gabon the natural nuclear fission reactor at bangombe is located at a depth of approximately 12 m and has undergone supergene weathering and chemical exchange with groundwater under moderately oxidizing conditions this reactor has been studied as an analogue for the migration of radionuclides in a geologic repository five water samples were taken from a drill hole in the reactor zone and from drill holes that are situated along the direction of the groundwater flow dissolution of phosphates in the weathering profile provided an important mechanism for the mobilization of phosphorous and ree this phosphorous allowed the formation of secondary minerals e g phosphatian coffinites and fe uranyl phosphate hydroxide hydrates the filtered groundwaters dissolved phase obtained directly from the reactor zone had anomalous 143nd 146nd 145nd 146nd and 149sm 147sm isotope ratios of 0 7235 0 4933 and 0 843 respectively confirming mixing between a fissiogenic and normal ree component in this dissoved phase a small fraction of the nd 2 3 at is of fissiogenic origin the suspended loads filtered particulates of the same groundwater from within the reactor show similar isotopic anomalies with a fissiogenic nd contribution of 3 2 at similar fissiogenic nd contributions are observable for groundwater samples from outside the reactor zone 3 m distance both dissolved and suspended load samples from the more distant well 25 m have normal isotopic ratios thus the prevailing physico chemical conditions within this groundwater system allow the migration of the fissiogenic ree over at least 3 m but less than 25 m although ree phases are abundant in the reactor zone the ree concentrations of the water in contact with the sediments of this zone are very low 17 ppt the most important lanthanide and actinide bearing phases are uraninite coffinites and fe uranyl phosphate hydroxide hydrate the presence of all of these phases retards the mobility of the fissiogenic lanthanides and actinides

weathering process at the natural fission reactor of bangombe the uranium deposits in the basin of franceville gabon host the only natural fission reactors known in the world unique geological conditions favoured a natural fission reaction 2 ga ago this was detected by anomalous isotopic compositions of uranium and rare earth elements ree which are produced by the fission reaction in total 16 reactor zones were found most of them are mined out the reactor zone of bangombe is only 10 11 m below the surface this site has been influenced by surface weathering processes six drill cores have been sampled at the site of the reactor zone of bangombe during the course of the study and only one drill core bax 08 hit the core of the reactor from these data and previous drilling campaigns the reactor size is estimated to be 10 cm thick 2 3 m wide and 4 6 m long the migration of fission products can be traced by the anomalous isotope ratios of ree because of the fission process the 149sm 147sm ratio close to the reactor zone is only 0 28 normal 0 92 because of the intense neutron capture of 149sm and subsequent transmutation whereas 147sm is enriched by the fission reaction similar changes in isotopic patterns are detectable on other ree the isotope ratios of sm and nd of whole rock and fracture samples surrounding the reactor indicate that fission genic ree migrated only a few decimetres above and mainly below the reactor zone organic matter bitumen seems to act as a trap for fission genic ree additional ree patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model

isotopic evidence for trapped fissiogenic ree and nucleogenic pu in apatite and pb evolution at the oklo natural reactor abstract a part of the boundary layer of reactor zone 10 at the oklo natural reactor shows a unique petrologic texture which contains high grade uraninite and massive apatite concretions in order to study distribution behavior of fission products around the boundary between the reactor zone and the wall rock and to clarify the relation of migration mechanisms of fission products with geochemical factors in situ isotopic analyses of nd sm gd pb and u in uraninite and apatite from the sample were performed by sensitive high resolution ion microprobe shrimp sm and gd isotopic ratios of uraninite and apatite show evidence of neutron irradiation with fluence between 4 4 6 8 10 19 n cm 2 judging from the isotopic anomalies of nd and u the apatite coexisting with the uraninite plays an important role in trapping fissiogenic lree and nucleogenic 239 pu into the structure systematic pb isotopic data from apatite uraninite galena and minium suggest the following chronological interpretations 1 the apatite formed 1 92 0 01 ga ago and trapped fissiogenic light ree and nucleogenic 239 pu that migrated from the reactor during the criticality 2 the uraninite around the boundary between reactor and sandstone dissolved once 1 1 1 2 ga ago 3 galena grains were formed by u pb mobilization in association with the intrusion of dolerite dyke 0 45 0 83 ga ago 4 minium was derived from recent dissolution of galena under locally oxidizing conditions

instability in tropical pacific sea surface temperatures during the early aptian the cretaceous has long been recognized as a time when greenhouse conditions were fueled by elevated atmospheric co2 and accompanied by perturbations of the global carbon cycle described as oceanic anoxic events oaes yet the magnitude and frequency of temperature change during this interval of warm and equable climate are poorly constrained here we present a ighresolution record of sea surface temperatures ssts reconstructed using the tex86 paleothermometer for a sequence of early aptian organic rich sediments deposited during the first cretaceous oae oae1a at shatsky rise in the tropical pacific ssts range from 30 to 36 0c and include two prominent cooling episodes of 4 0c the cooler temperatures reflect significant temperature instability in the tropics likely triggered by changes in carbon cycling induced by enhanced burial of organic matter sst instability recorded during the early aptian in the pacific is comparable to that reported for the late albian early cenomanian in the atlantic suggesting that such climate perturbations may have recurred during the cretaceous with concomitant consequences for biota and the marine environment

high sea surface temperatures during the early aptian oceanic anoxic event 1a in the boreal realm the early aptian was characterized by the widespread occurrence of anoxia in the oceans known as oceanic anoxic event oae 1a intense degassing from submarine volcanic plateaus presumably resulted in high atmospheric co2 concentrations culminating in greenhouse conditions oae 1a can therefore be considered as a past natural experiment important to understand the future evolution of our climate paleotemperature estimates for oae 1a are however predominantly based on bulk oxygen isotopes which are susceptible to diagenetic overprinting while tex86 paleotemperature estimates are limited in number or derived from stratigraphically poorly constrained sections here we reconstructed for the first time sea surface water temperatures ssts based on the tex86 paleothermometer from an oae 1a section from the middle northern latitudes 39 n paleolatitude we find a sst rise starting prior to oae 1a and reaching a maximum during the event with ssts of 31 34 c 4 9 c higher than those of older hauterivian lower aptian sediments from the same sedimentary basin the end of oae 1a is marked by relatively lower ssts of 30 c these observations are supported by belemnite based oxygen isotope d18obel data and calcareous nannofossils our integrated data set clearly indicates that super greenhouse conditions prevailed during oae 1a at northern latitudes ssts are similar to those estimated for coeval low latitudinal sites suggesting that an equable warm climate with reduced latitudinal gradients characterized the early aptian

transient global cooling at the onset of early aptian oceanic anoxic event oae 1a the onset of the early aptian oceanic anoxic event oae 1a ca 120 ma coincided with a major perturbation of the carbon cycle which is reflected in the sedimentary carbon isotope record triggering mechanisms duration and climatic repercussions of this episode of accelerated organic matter burial remain poorly constrained here we present millennial scale bulk rock carbon and oxygen isotope data from a marly subtropical intrashelf basin la bedoule southeast france with unusually high sedimentation rates which track the onset of oae1a in unprecedented resolution our record reveals that the negative low amplitude delta 13 c excursion preceding oae1a lasted 100 k y implying that enhanced volcanic co 2 emission and or pulsed methane dissociation over a prolonged time span were instrumental in triggering oae1a the main positive carbon isotope shift at the onset of oae1a previously regarded as continuous occurred stepwise over an extended period of 300 k y transient climate cooling during the initial delta 13 c increase probably reflects ephemeral high latitude glaciation triggered by changes in radiative forcing and drawdown of atmospheric co 2

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

a novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids we propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high performance liquid chromatography mass spectrometry hplc ms analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol the characteristic membrane lipid of non thermophilic crenarchaeota which is especially abundant in the marine and lacustrine environment based on these findings an index was developed the so called branched and isoprenoid tetraether bit index based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol this bit index was applied to surface sediments from the angola basin where it was shown to trace the outflow of the congo river furthermore analyses of particulate organic matter from the north sea showed relatively higher bit indices in water column particulate organic matter near large river inputs a survey of globally distributed marine and lacustrine surface sediments shows that the bit index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable the new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 ma old

redistribution of ree pb and u by supergene weathering studied from in situ isotopic analyses of the bangombe natural reactor gabon abstract the isotopic analyses of rare earth elements ree pb and u in several kinds of minerals from the clay and black shale layers above the bangombe natural reactor gabon were performed using a sensitive high resolution microprobe shrimp to investigate the migration and retardation processes of fission products released from the reactor ree isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays which reveals that the clays play an important role in preventing fission products from spreading zircon crystals in the clays heterogeneously contain high u regions up to 28 3 wt with normal 235 u 238 u ratios 0 00725 and significant amounts of fissiogenic ree which suggest the occurrence of significant chemical fractionation between ree and u during the dissolution of reactor uraninite and the recrystallization of secondary u minerals the pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event and that a significant amount of pb was derived from 2 05 ga old original uraninite rather than reactor uraninite the u pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2 8 ga and geochemical evidence of the incomplete mixing of independent pb and u sources this result is consistent with previous chronological results in this area

evidence for a regional warm bias in the early cretaceous tex86 record abstract the cretaceous period 145 66 ma provides an opportunity to obtain insights into the adaptation of the climate system to increased atmospheric greenhouse gas concentrations the organic paleothermometer tex86 is one of the few proxies available for reconstructing quantitative estimates of upper ocean temperatures of this time period here we show that the sedimentary tex86 signal in the early cretaceous north and south atlantic shows systematic differences to other cretaceous samples in particular the relative increase in the fractional abundances of the crenarchaeol isomer compared to crenarchaeol exhibits similarities with surface sediments from the modern mediterranean and red sea dedicated climate model simulations suggest that the formation of warm and saline deep waters in the restricted north and south atlantic may have influenced tex86 export dynamics leading to a warm bias in reconstructed upper ocean temperatures applying a regional calibration from the modern mediterranean and red sea to corresponding tex86 data significantly improves the model data fit for the aptian oceanic anoxic event 1a and the overall comparison with other temperature proxies for the early cretaceous our results demonstrate the need to consider regional and temporal changes of the tex86 temperature relation for the reconstruction of deep time ocean temperatures

a volcanic co2 pulse triggered the cretaceous oceanic anoxic event 1a and a biocalcification crisis the aptian oceanic anoxic event 1a oae1a ca 120 ma ago is one of the most prominent of a series of geologically brief intervals in the cretaceous characterized by the deposition of organic carbon rich sediments oaes reflect major perturbations in the global carbon cycle evidenced by sedimentary carbon isotope records however the triggering mechanisms for oaes remain controversial here we present a bulk rock and molecular marine and terrestrial bio markers c isotope record at unprecedented time resolution from the cismon section of northern italy that shows that oae1a conditions were reached over a period of several thousands of years through a stepwise perturbation of the carbon cycle the documented sequence of events is most compatible with a trigger associated with increased co 2 emissions possibly leading to a doubling of p co 2 which in turn caused larger c isotope fractionation in marine and terrestrial organisms and a major biotic crisis in the calcareous nannoplankton our data also show that a release of isotopically light carbon from partial methane hydrate dissociation probably played a minor role in the oae1a carbon cycle perturbation

lead isotopic record of barremian aptian marine sediments implications for large igneous provinces and the aptian climatic crisis we present initial isotopic ratios of lead for early cretaceous barremian aptian sections from shatsky rise pacific and gorgo a cerbara italy our pb isotopic data track an interval representing oceanic anoxic event oae 1a which is characterized by quasi global deposition of organic carbon rich black shale pb isotopic compositions of sediments from shatsky rise decrease at the end of barremian time from radiogenic continental values to unradiogenic values and subsequently remained less radiogenic until the end of early aptian time we explain the isotopic shift by a significant increase in supply rate of unradiogenic pb most likely due to massive volcanism in contrast the pb isotopic compositions from the italian section which was situated at the western end of tethys are mostly identical to those of upper continental crust showing no significant change in supply rate of unradiogenic pb the discrepancy between two sites is attributed to quiescent deep submarine eruptions of pacific large igneous provinces lips such as the ontong java plateau ojp which severely limited dispersion of pb carrying particles out of the pacific ocean published os isotopic data from the italian section indicate two episodes of massive eruptions of ojp or contemporaneous manihiki and hikurangi plateaus starting from earliest aptian time slightly later than that indicated by the sedimentary pb isotopic record from shatsky rise differences in isotopic variations between pb and os likely reflect differences in their chemical behaviors in the oceans i e pb isotopic compositions would have varied in response to local or regional changes in sediment provenances whereas large scale changes in os inputs are required to explain variations in seawater os isotopic compositions our pb isotopic data together with the published os isotopic record provide new evidence for the eruptive history of ojp together with contemporaneous pacific plateaus and its environmental consequences starting from end barremian time and extending through early aptian time

shrimp in situ isotopic analyses of ree pb and u in micro minerals bearing fission products in the oklo and bangombe natural reactors a review of a natural analogue study for the migration of fission products abstract in the precambrian parts of the oklo okelobondo and bangombe uranium deposits of the republic of gabon central africa functioned as natural fission reactors many elements in the oklo and bangombe uranium deposits show variations in isotopic composition caused by a combination of nuclear fission neutron capture and radioactive decay isotopic studies provide useful information to understand the behavior of radionuclides in geological media in our recent work in situ ree pb and u isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a shrimp sensitive high resolution ion microprobe the isotopic results of the shrimp analyses on micro minerals found in and around the oklo and bangombe natural reactors are reviewed in this paper the data suggest the selective uptake behavior of 1 ra into illite and 2 pu into apatite 3 the formation process of secondary minerals bearing fissiogenic ree and depleted u 4 evidence of nuggets particles bearing fissiogenic platinum group elements pge and 5 from the u pb systematics of highly altered zircons the redistribution of u and pb

time temperature burial significance of devonian anthracite implies former great 6 5 km depth of burial of catskill mountains new york specimens of coalified plant debris in tully correlative strata of the gilboa formation uppermost middle devonian within the eastern catskill mountains of new york state have been converted to anthracite having a vitrinite reflectance of 2 5 this implies a level of organic metamorphism lom of 16 the specimens are about 350 m y old if 200 m y is taken as the duration of the time of exposure to the maximum geothermal temperature then the lom of 16 and other thermal indicators imply a maximum temperature of 190 sup 0 c using a geothermal gradient of 26 sup 0 c km sup 1 17 sup 0 f 1 000 ft sup 1 a former depth of burial of 6 5 km is implied such former deep burial is not usually inferred for the catskills but it is consistent with the idea that the thick about 6 4 km or 21 000 ft carboniferous strata of northeastern pennsylvania formerly extended northeast far enough to bury the catskills the lack of metamorphism of the paleozoic strata lying about 4 5 km beneath the tully correlative rocks and exposed in the adjacent hudson valley places low limits on the former geothermal gradient this supports the concept of great depth of former burial of the catskills for example 6 5 km of former burialmore and a geothermal gradient of 26 sup 0 c km sup 1 imply a temperature of 307 sup 0 c for the base of the paleozoic by contrast only 1 km of former burial requires a geothermal gradient of 170 sup 0 c km sup 1 which would have subjected the base of the paleozoic to a temperature of 955 sup 0 gamma which is far higher than the 600 to 650 sup 0 c recently inferred for the acadian age metamorphism of the taconic allochthon in southwestern massachusetts and adjoining areas less

geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at bangombe gabon to study geochemical processes for migration and fixation of fissiogenic rare earth elements ree in association with uranium dissolution in situ isotopic analyses using an ion microprobe were performed on u and ree bearing secondary minerals such as coffinite francoisite uraniferous goethite and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at bangombe gabon phosphate minerals such as phosphatian coffinite and francoisite with depleted 235u 235u 238u 0 00609 to 0 00638 contained large amount of fissiogenic light ree while micro sized uraninite grains in a solid bitumen aggregate have normal u isotopic values 235u 238u 0 00725 and small amount of fissiogenic ree components the proportions of fissiogenic and non fissiogenic ree components in four samples from the core of bax03 vary in depth ranging from 30 cm to 130 cm beneath the reactor which suggests mixing between fissiogenic isotopes from the reactor and non fissiogenic isotopes from original minerals in the sandstone significant chemical fractionation was observed between ce and the other ree in the secondary minerals which shows evidence of an oxidizing atmosphere during their formation pb isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of u pb decay system due to recent geologic alteration however systematic pb isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes pb and u from the reactor in association with dolerite dyke intrusion 0 798 ga ago and the formation of the secondary minerals by mixing event between 2 05 ga old original minerals and reactor materials due to recent alteration

a volcanically induced climate warming and floral change preceded the onset of oae1a early cretaceous the aptian oceanic anoxic event 1a oae1a is preceded by a prominent negative c isotope excursion ncie attributed to major volcanism on the ontong java plateau that is supposed to lead to a pco2 increase and a climate change lower aptian sporomorph assemblages in two pelagic sections from the southern tethys margin n italy were analysed in order to test if the postulated climate changes affected the terrestrial vegetation at the base of the ncie the sporomorph assemblages comprise abundant bisaccate pollen reflecting a warm temperate climate several tens of kiloyears ka after the start of the ncie decreasing bisaccate pollen and increasing classopollis spp and araucariacites spp suggest the beginning of a long term temperature rise palynomorphs indicate that maximum temperatures were reached several tens of ka after the end of the ncie and the onset of oae1a the highest temperatures coincide with arid conditions which could reflect a northward shift of the hot arid northern gondwana floral province as a result of an increasing pco2 over 200 ka after the onset of oae1a reduced volcanic activity and or increased black shale deposition allowed for a drawdown of most of the excess co2 and a southward shift of floral belts

mercury anomalies volcanic pulses and drowning episodes along the northern tethyan margin during the latest hauterivian earliest aptian abstract the latest hauterivian earliest aptian time interval includes three episodes of significant environmental change faraoni late early barremian and taxy episodes this time interval appears to partially overlap with large scale volcanic activity related to the tristan da cunha plume along the rio grande rise high arctic large igneous province volcanism and the early phase in the formation of the greater ontong java lip in the pacific the establishment of exact temporal relationships between volcanic activity and environmental change remains however a major challenge due to the scarcity of numerical ages for the early cretaceous we report mercury hg contents in uppermost hauterivian lowermost aptian marl limestone alternations from seven sections along a n s transect in the western tethys the hg contents in marl samples display rather scattered records which are generally well correlated with the total organic carbon toc records in associated limestone beds five distinct spikes in hg concentrations occur in three intervals latest hauterivian early barremian and latest barremian the small amounts of organic matter

biscutum constans coccolith size patterns across the mid cretaceous in the western tethys paleoecological implications abstract the morphometric record of the calcareous nannofossil species biscutum constans from the western tethys indicates that this species underwent marked size changes across the latest barremian cenomanian interval 28 myrs significant size reduction occurred during the core of the early aptian oceanic anoxic event oae 1a 120 ma and soon after oae 1a followed by a 9 myr long interval characterized by small specimens larger coccoliths marked the middle to late albian and particularly the lower part of the nannofossil subzone nc10a was characterized by abundant large specimens a minor size decrease coincided with the late albian oae 1d onset 100 ma but very small size was reached again during the latest cenomanian oae 2 94 4 ma the sequence of relatively long millions of years intervals marked by different average sizes is interpreted to reflect adaptation to changing paleoecological conditions temperature and fertility were not directly influencing the size of b constans on shorter time scale abrupt changes in ocean chemistry related to large igneous provinces submarine volcanism played a central role in secretion of small b constans coccoliths to face ocean acidification during oae 1a and oae 2 contrarily during the late early aptian early albian time interval smaller b constans size was related to cooler temperature which possibly also promoted co2 sequestration in cooler surface waters forcing b constans to produce small coccoliths to survive lowered ph in addition pulses of subaerial volcanism of the southern kerguelen plateau may have further affected the ocean chemistry the termination of intense volcanism and warmer temperature in the middle albian early cenomanian coincided with restoration of bigger b constans specimens

the barremian and aptian stepwise development of the oceanic anoxic event 1a oae 1a crisis integrated benthic and planktic high resolution palaeoecology along the gorgo a cerbara stratotype section umbria marche basin italy abstract the barremian and aptian were times of global plate reconfiguration and profound changes in the ocean climate system culminating in the worldwide deposition of lower aptian black shale layers oae 1a based on high resolution lithostratigraphic and micropaleontological analyses precursor conditions and timing of the anoxia are here reconstructed along a 33 06 m thick section in the proposed barremian aptian boundary gssp stratotype at gorgo a cerbara umbria marche basin italy a non uniform history of benthic foraminiferal diversification is interrupted by the selli level oae 1a and by three turnover points for both benthic and planktic organisms each highlighted by prominent breaks in lithology and outcrop morphology the first two points at 1 1 myr and 20 50 kyr prior to the onset of the oae 1a correspond respectively to the nannoconid bathypelagic calcareous nannofossils decline and crisis events and separate a pelagic limestone chert claystone lithological cycles of the uppermost maiolica formation showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic barremian benthic and planktic assemblage dominated by k selected nannoconids and rhizammina b greenish grey cherty marls of the lowermost marne a fucoidi formation associated to declining sedimentation rates and eutrophic early aptian assemblages dominated by radiolarians and lituolid benthic foraminifera and c anoxic radiolarites and shales of the carbonate free selli level high magnitude and frequency assemblage fluctuations occur between the nannoconid crisis 25 cm below the lowermost oae 1a black shale layer and the selli level base with suggestions of bottom water acidification at 800 kyr after the oae 1a end a third turnover point in the basal reddish member of the marne a fucoidi highlights middle aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera hedbergella spp and appearances of new benthic species marking a shift towards better oxygenation oae 1a oxygen and pelagic carbonate factory crises are here suggested to have been step wise and linked to global warming conditions

onset of the mid cretaceous greenhouse in the barremian aptian igneous events and the biological sedimentary and geochemical responses basalts and biostratigraphy dated at 125 120 ma from the ontong java and manihiki plateaus in the western pacific evidence the largest volcanic event in earth history in at least the past 160 m y the intervening nova canton trough rifted at about 121 118 ma and a number of guyots and seamounts formed concurrently or slightly later geological events that probably were responses to these volcanic tectonic events occurred in the following chronostratigraphic order biotic fluctuations began at about 122 5 ma at about 122 0 ma 87sr 86sr began to decline slowly metal concentrations of co mn pb yb and cu in sediments peaked at about 121 5 121 2 ma changes in planktonic communities and sedimentation culminated in a nannoconid crisis just prior to 120 5 ma and in the selli black shale oae la at about 120 5 119 5 ma a sharp drop in 13c occurred at the beginning of the selli event and rebounded into a longer positive excursion that reached a peak after the selli event at about 119 5 118 5 ma at 120 5 ma 87sr 86sr declined rapidly and reached a minimum at about 116 113 ma we speculate that the intensity of these latter responses suggests a corresponding peak in volcanic tectonic activity at about 121 119 ma

terrestrial record of methane hydrate dissociation in the early cretaceous reconstruction of changing c isotopic composition of early cretaceous atmospheric co2 from fossilized c3 vascular land plant tissue revealed a brief and striking negative excursion d 25 in atmospheric d 13 c followed by a rapid positive compensation d 15 during the aptian ca 117 ma mass balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion this would also result in an increased co2 o2 mixing ratio as o2 is consumed during ch4 oxidation to co2 spurring the exponential phase of angiosperm biogeographic expansion

calcareous nannoplankton response to surface water acidification around oceanic anoxic event 1a ocean acidification induced by atmospheric co2 may be a major threat to marine ecosystems particularly to calcareous nannoplankton we show that during the aptian 120 million years ago oceanic anoxic event 1a which resulted from a massive addition of volcanic co2 the morphological features of calcareous nannofossils traced the biological response to acidified surface waters we observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre anoxia calcification crisis ephemeral coccolith dwarfism and malformation represent species specific adjustments to survive lower ph whereas later abundance peaks indicate intermittent alkalinity recovery deepwater acidification occurred with a delay of 25 000 to 30 000 years after the dissolution climax nannoplankton and carbonate recovery developed over 160 000 years under persisting global dysoxia anoxia

reconstruction of vertical temperature gradients in past oceans proxy data from the hauterivian early barremian early cretaceous of the boreal realm abstract a total of 25 mudstone samples from two sections in northwest germany and northeast england palaeolatitude 40 n respectively of hauterivian and early barremian age were analysed using tex86 palaeothermometry in addition the stable isotope ratio 18o 13c and trace element content mg sr fe and mn of 138 belemnite guards from the same two outcrops were determined to reconstruct past sea water temperatures the tex86 based sea surface temperatures were constantly warm 24 26 c throughout the entire hauterivian for both sections and even warmer for the early barremian 27 30 c stable water temperatures prevailed over a period of 6 4 ma in the southernmost part of the boreal realm these findings clearly support the view of warm equable conditions for the hauterivian at least for mid latitudinal settings a constant off set between higher tex86 derived temperatures and lower 18obel derived temperatures was observed this may be explained by either increased sea water salinities a deep dwelling habitat of belemnites or a combination of both a by 3 higher salinity of the sea water below the thermocline which would at least partly explain the by 2 too positive 18obel values cannot be ruled out in these epicontinental settings a habitat of the belemnites below the thermocline in perhaps 50 100 m water depth may have contributed to the positive 18obel data as well the temperature difference between the tex86 and the 18obel findings might therefore reflect a vertical temperature gradient of the water column of 4 5 c and a salinity of the subsurface waters of 38

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

tex86 and stable δ18o paleothermometry of early cretaceous sediments implications for belemnite ecology and paleotemperature proxy application abstract recent studies have cast doubt on the unadjusted usage of jurassic and cretaceous 18 o paleotemperature data derived from belemnites since the latter data often reflect cooler paleotemperature estimates than would be expected in this study we address this problem by analysing rocks of barremian to early aptian age from two outcrops in northern germany using tex 86 paleothermometry along with 142 belemnite guards studied for their stable isotope 13 c 18 o and trace element composition magnesium strontium iron and manganese both tex 86 and 18 o bel indicate very warm water temperatures for a distinctive black shale sequence of late early barremian age hauptblatterton with temperatures of up to 29 c and 23 c respectively we observe a constant offset of tex 86 temperatures versus the 4 to 5 c cooler 18 o belemnite signal for this interval the late barremian sequence shows an increase of the 18 o bel values from 1 to 0 reflecting temperatures around 16 to 12 c while the contemporaneous tex 86 temperatures vary between 26 and 32 c the common occurrence of belemnites in the anoxic sediments of the early barremian implies however clearly a nektonic way of life similar to that of recent teuthids rather than a nektobenthic one like sepia this in turn suggests that the belemnites investigated here genera praeoxyteuthis aulacoteuthis oxyteuthis and neohibolites were active swimmers which inhabited a deeper habitat below the thermocline in an epicontinental sea of perhaps 100 to 250 m water depth the offset of the tex 86 and 18 o bel data is therefore interpreted to reflect temperature signals from two different depth habitats i e the tex 86 is selectively derived from warm sea surface waters and the belemnites likely occupied deeper and cooler waters with relative increasing salinities in the late barremian this study stresses the importance that the taxonomy paleobiology and ecology of the belemnite taxa genera species used for paleotemperature reconstructions must be considered before the 18 o bel signal can be further interpreted the variation of the 18 o signature gained from one belemnite population of 22 specimens by 1 1 suggests that the 18 o data of individual belemnites should be used with caution for reconstructing long termed paleotemperature trends

extremely high sea surface temperatures at low latitudes during the middle cretaceous as revealed by archaeal membrane lipids the middle cretaceous 125 88 ma greenhouse world was characterized by high atmospheric co 2 levels the general absence of polar ice caps and much higher global temperatures than at present both 1 8 o based and model based temperature reconstructions indicate extremely high sea surface temperatures ssts at high latitudes however there are a number of uncertainties with sst reconstructions based on 1 8 o isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the 1 8 o value of cretaceous seawater the paleoecology of middle cretaceous marine organisms and seawater ph here we applied a novel sst proxy i e text tetraether index of 86 carbon atoms based on the membrane lipids of marine crenarchaeota derived from middle cretaceous sedimentary rocks deposited at low latitudes the text proxy indicates that tropical ssts in the proto north atlantic were at 32 36 c during the early albian and late cenomanian early turonian this finding agrees with sst estimates based on 1 8 o paleothermometry of well preserved foraminifera as well as global circulation model calculations the text proxy indicates cooler ssts 27 32 c for the equatorial pacific during the early aptian which is in agreement with sst estimates based on 1 8 o paleothermometry

florencite la with fissiogenic rees from a natural fission reactor at bangombe gabon florecite la la ce 1 09 with fissiogenic rees and florecite ce la ce 0 62 have been identified in illite from the clay mantle surrounding a natural 2 ga fission reactor at bangombre and in sandstone beneath the reactor zone respectively florencite ce is apparently unrelated to nuclear processes and occurs with monazite ce apatite tio sub 2 probably anatase zircon and illite grains of florencite ce contain inclusions of thorite chalcopyrite and galena florencite la was found 5 cm from the open quotes core close quotes of the reactor and contains inclusions of galena and u ti bearing phases secondary uraninite and coffinite have precipitated on some of the florencite grains the chemical composition of florencite la as determined by electron microprobe analysis is la sub 0 38 ce sub 0 35 nd sub 0 06 sm sub 0 01 ca sub 0 03 sr sub 0 17 al sub 2 98 fe sub 0 02 sup 3 po sub 4 po sub 3 80 oh sub 0 20 oh sub 6 secondary ion mass spectrometry revealed that between 27 and 30 of nd and 67 and 71 of sm in florencite la is fissiogenic the presence of fissiogenic rees in open quotes florencite close quotes from the reactor zone in bangombe and their preferential concentration in florencite relative to the bulk sample of clay demonstrate that aluminous phosphates may have played a more significant role in the fixation of fissiogenic rees released from uraninite after the sustained fission reactions than sorption ontomore clays 30 refs 3 figs 2 tabs less

isotopic evidence for temperature variation during the early cretaceous late ryazanian mid hauterivian oxygen and carbon isotopic compositions have been determined from the belemnite genera acroteuthis and hibolites sampled from the early cretaceous ryazanian hauterivian interval of the speeton clay formation filey bay england the speeton clay formation consists of a series of claystones and calcareous mudrocks deposited in an epicontinental sea 18 o values from belemnites which met petrographic and chemical criteria for well preserved skeletal carbonate indicate warm marine palaeotemperatures c 12 15 c for much of the early valanginian whilst cool temperatures

episodes of high productivity and cooling in the early aptian alpine tethys a palynological and organic geochemical study of the early aptian selli unit oceanic anoxic event 1a of the western tethys provides new information about paleoceanography and documents pronounced changes in the regional climate pattern it also emphasizes the role of oxic degradation of organic matter even at horizons with the highest rates of organic carbon corg accumulation the absence of anoxia indicates that enhanced primary production is the main reason for these organic carbon accumulations and variations in preservation are related to pulses in primary production and nutrient recycling despite the high productivity character of the selli unit black shales the 13ccarb values are not marked by a positive shift but by an interval of unchanged values warm climatic conditions for deposition of the selli unit are inferred from the presence of southern provenance pollen and low 18o values for the interval above the selli unit changes in the pollen spectrum and an increase in 18o values reflect a hitherto undocumented late early aptian cooler and more humid climate and or a reorganization of the tethyan oceanographic circulation pattern

timing and magnitude of early aptian extreme warming unraveling primary δ18o variation in indurated pelagic carbonates at deep sea drilling project site 463 central pacific ocean abstract in order to elucidate early aptian marine paleotemperature evolution across the period of enhanced organic carbon corg burial oceanic anoxic event oae 1a stable isotope analyses were performed on pelagic limestones at deep sea drilling project site 463 central pacific ocean the 18o data exhibit a distinct anomaly by 2 spanning the oae 1a interval i e a 6 m thick phytoplanktonic corg rich unit constrained by magneto bio and 13c stratigraphy elucidation of paleotemperature significance of the 18o shift is made by taking account of recent sr ca evidence at the same section which revealed that geochemical signals in carbonate poor lithologies are relatively unaltered against burial diagenesis by discriminating 18o values from carbonate poor samples caco3 contents 5 30 wt it appears that an abrupt rise in sea surface temperatures ssts by 8 c 1 7 shift in 18o occurred immediately before oae 1a whereas a cooling mode likely prevailed during the peak corg burial in terms of its stratigraphic relationship as to the corg rich interval and to a pronounced negative 13c excursion as well as its timescale the observed sst rise resembles those associated with the paleocene eocene thermal maximum and more strikingly jurassic toarcian oae this observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism which was likely initiated by the greenhouse effect via massive release of ch4 or co2 from the isotopically light carbon reservoir and terminated by a negative productivity feedback