sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

relationships between sulphur organic carbon and iron in the modern sediments of the black sea abstract the ratio of sedimentary s to organic c has been used as a diagnostic criterion for recognizing anoxic environments in the sedimentary record here we examine the relationship between pyrite s and organic c in the modern unit 1 sediments of the black sea to re evaluate this suggested relationship in box cores from shallow oxic or near oxic water pyrite s contents ranging from 0 1 and 2 5 by weight do not correlate with organic c contents in cores from the deep anoxic basin pyrite s concentrations range between 1 0 and 2 5 and are linearly related to organic c contents with an intercept that is not significantly different from zero the s c ratio is around 0 31 in these sediments which is somewhat lower than the ratio of 0 36 considered to be characteristic of normal marine oxic sediments thus the black sea anoxic sediments appear to be depleted in s relative to organic c the degree of pyritization dop of the sediments may be a better measure of anoxic conditions of sedimentation the dop values measured here show that pyrite formation is fe limited the correlation between organic c and pyrite s probably being induced by a positive correlation between fine grained sediment components the original fe source for pyrite formation in the sediments and organic matter such fe limitation is probably characteristic of anoxic basins where pyrite formation can take place in the water column and fe oxyhydroxides do not accumulate in the surface sediments the results on the sample set examined here are at variance with earlier data and interpretations for the black sea in particular the suggested s enrichment of anoxic black sea sediments was not found because of fe limitation

late mesozoic magmatism in svalbard a review abstract late mesozoic mafic igneous rocks are widespread across the arctic region and are collectively referred to as the high arctic large igneous province halip in svalbard the halip is represented by the diabasodden suite an extensive system of predominantly basic intrusive doleritic rocks associated lava flows are exposed on the far east of the archipelago two main igneous centres have been proposed 1 central spitsbergen and 2 the eastern svalbard dolerite belt spanning hinlopenstretet and the islands of edgeoya and barentsoya offshore seismic and magnetic data suggest a link between the svalbard and franz joseph land igneous exposures intrusions occur predominantly as sills up to 100 m thick and laterally continuous for up to 30 km dykes are present locally and range in thickness from

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

mafic intrusions east of svalbard imaged by active source seismic tomography a seismic refraction and reflection tomography experiment was performed across the igneous province east of svalbard which is a part of the cretaceous high arctic large igneous province seismic travel times from 12 ocean bottom seismometers hydrophones deployed along a 170 km line are inverted to produce smooth 2d images of the crustal p wave velocity and geometry of the acoustic basement and moho the inversion of travel times was complemented by forward elastic wave propagation modeling integration with onshore geology as well as multichannel seismic magnetic and gravity data have provide additional constraints used in the geological interpretation the seismic p wave velocity increases rapidly with depth starting with 3 km s at the sea floor and reaching 5 5 km s at the bottom of the upper sedimentary layer the thickness of this layer increases eastward from 2 km to 3 5 km on average the p wave velocity in the crystalline crust increases with depth from 5 5 km s to 6 8 km s the crustal thickness is typical for continental shelf regions 30 34 km finger shaped high velocity anomalies one reaching 12 and two of 4 6 velocity perturbation are obtained these velocity anomalies are concomitant with lower cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs traced towards the northern barents sea passive continental margin we interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the arctic amerasia basin

the nature of mineralizing fluids of the kipushi zn cu deposit katanga democratic repubic of congo quantitative fluid inclusion analysis using laser ablation icp ms and bulk crush leach methods fluid inclusions from the epigenetic zn cu deposit of kipushi central african copper belt katanga democratic republic of congo have been subjected to an integrated microthermometric bulk crush leach and laser ablation inductively coupled plasma mass spectrometric la icp ms study to elucidate the physicochemical conditions of ore formation and the origin of the mineralizing fluids the kipushi deposit is hosted in carbonates and partly in siliciclastic rocks that have undergone regional low grade greenschist facies metamorphism and which were strongly deformed during the lufilian orogeny main deformation phase 550 ma investigation of host rock gangue and ore samples enabled reconstruction of the paragenesis of ore deposition the main zn cu ge pb sulfide mineralization was accompanied by three generations of hydrothermal dolomite and a generation of coarse grained quartz primary and pseudosecondary fluid inclusions in these minerals contain predominantly halite saturated aqueous brines 30 43 wt nacl equiv with locally unidentified trapped solids these inclusions mostly decrepitate upon heating before total homogenization which could suggest that they also trapped excess carbonic vapor they show petrographic and microthermometric evidence for postentrapment reequilibration prohibiting their use as accurate geothermometers and geobarometers previous studies on chlorite and sphalerite geothermometry framed the formation temperature of the kipushi cu zn deposit between 290 to 380 c younger fluids are present as secondary inclusions in the gangue minerals and as primary fluid inclusions in sphalerite from a minor secondary mineralization phase these two phase aqueous inclusions have a slightly lower salinity 23 31 wt nacl equiv and have homogenization temperatures ranging from 170 to 80 c in successive fluid inclusion assemblages sporadically some mixed aqueous carbonic and gaseous co2 ch4 inclusions are present but their genetic position is un clear and they are insignificant with regard to the mineralization process as a whole crush leach bulk analysis of the fluid inclusions show that the mineralizing fluids had a molar cl br ratio close to or somewhat higher than the seawater value which rules out a major contribution from bittern brines and suggests that the fluids gained their salinity primarily by the dissolution of evaporites younger fluids that circulated through the deposit after the main phase of ore formation have cl br and na br signatures indicative of a greater involvement of halite dissolution brines values much higher than that of seawater la icp ms analyses of the mineralizing fluids reveal that they have molar li na ca na mg na mn na and sr na in the range of natural high salinity fluids in metamorphic environments worldwide however they have relatively high molar fe na zn na pb na and especially very high ba na ratios their molar k na is also somewhat higher than for other low grade metamorphic fluids but is nevertheless in agreement with the values predicted by the k na geothermometer at the conditions of ore formation this suggests that the major element composition of the fluids was buffered by fluid mineral equilibria their relatively high metal contents ba zn fe and pb suggest that they interacted with and leached these elements from felsic basement rocks at higher temperatures whatever the origin of the fluids their salinity and the metals their uniformly high ba and ore metal content together with the absence of barite and the occurrence of ba silicates at kipushi indicates that sulfide mineralization occurred when the metal bearing fluids encountered a reducing h2s rich reservoir the ubiquitous presence of preexisting metamorphosed carbonic material shungite in the sulfides and host rock and a sulfur isotope signature of the ores in agreement with thermogenic sulfate reduction suggests that this condition was likely created due to the high initial reduced carbon content of the host rocks at kipushi which seems to be exceptional in the copper belt therefore we postulate that a critical factor in the formation of major vein type zn cu mineralization was the presence of a hydrocarbon rich facies of the lower nguba carbonates at kipushi

age of the barremian aptian boundary and onset of the cretaceous normal superchron abstract the age assigned to the boundary of the barremian and aptian stages remains one of the most poorly constrained post pangean stratigraphic boundaries the lack of a global boundary stratotype section and point gssp for the stage boundary has hampered efforts to calibrate the absolute age of the cretaceous period in the geological time scale the current definition of the barremian aptian boundary also approximates the onset of magnetic polarity chron m0r the end of this chron denotes the start of the cretaceous normal polarity superchron that is of fundamental importance for plate reconstructions currently there is up to 5 discrepancy in the age estimates of the barremian aptian boundary ca 126 121 ma and the start of the cretaceous normal superchron here we review available geochronological information from the late barremian and early aptian stages collected from the pacific ocean china california the ontong java nui large igneous province and the high arctic large igneous province by utilizing only robust geochronological data including u pb and recalibrated 40ar 39ar ages from sites with magnetic polarity information and or paleontological constraints we calculate a best estimate of between 123 8 and 121 8 ma for the barremian aptian boundary and the onset of chron m0r at 2 confidence using estimates of the duration of chron m0r 0 49 0 10 myr 2 we conservatively compute the start of the cretaceous normal superchron to between 123 4 and 121 2 ma 2 using an age of 83 07 0 15 ma 2 for the end of the cretaceous normal superchron the duration of the superchron is also constrained to between 38 0 and 40 5 myr 2 these age ranges for the barremian aptian boundary the onset of the cretaceous normal superchron and the duration of the superchron currently provide the best estimates until a gssp is formally ratified

jurassic to early cretaceous basin configuration s in the fingerdjupet subbasin sw barents sea abstract the fingerdjupet subbasin in the southwestern barents sea sits in a key tectonic location between deep rifts in the west and more stable platform areas in the east its evolution is characterized by extensional reactivation of n s and nne ssw faults with an older history of late permian and likely carboniferous activity superimposed on caledonian fabrics reactivations in the listric nne ssw terningen fault complex accommodated a semi regional rollover structure where the fingerdjupet subbasin developed in the hangingwall in parallel the randi fault set developed from outer arc extension and collapse of the rollover anticline n s to nne ssw faults and the presence of other fault trends indicate changes in the stress regime relating to tectonic activity in the north atlantic and arctic regions a latest triassic to middle jurassic extensional faulting event with e w striking faults is linked to activity in the hammerfest basin cessation of extensional tectonics before the late jurassic in the fingerdjupet subbasin however suggests rifting became localized to the hammerfest basin the late jurassic was a period of tectonic quiescence in the fingerdjupet subbasin before latest jurassic to hauterivian extensional faulting which reactivated n s and nne ssw faults barremian se prograding clinoforms filled the relief generated during this event before reaching the bjarmeland platform high angle nw prograding clinoforms on the western bjarmeland platform are linked to early barremian uplift of the loppa high the terningen fault complex and randi fault set were again reactivated in the aptian along with other major fault complexes in the sw barents sea leading to subaerial exposure of local highs this activity ceased by early albian post upper albian strata were removed by late cenozoic uplift and erosion but later tectonic activity has both reactivated e w and n s nne ssw faults and also established a nw se trend

sedimentary mercury enrichments as a marker for submarine large igneous province volcanism evidence from the mid cenomanian event and oceanic anoxic event 2 late cretaceous oceanic anoxic event 2 oae 2 during the cenomanian turonian transition 94 ma was the largest perturbation of the global carbon cycle in the mid cretaceous and can be recognized by a positive carbon isotope excursion in sedimentary strata although oae 2 has been linked to large scale volcanism several large igneous provinces lips were active at this time e g caribbean high arctic madagascan ontong java and little clear evidence links oae 2 to a specific lip the mid cenomanian event mce 96 ma identified by a small 1 positive carbon isotope excursion is often referred to as a prelude to oae 2 however no underlying cause has yet been demonstrated and its relationship to oae 2 is poorly constrained here we report sedimentary mercury hg concentration data from four sites three from the southern margin of the western interior seaway and one from demerara rise in the equatorial proto north atlantic ocean we find that in both areas increases in mercury concentrations and hg toc ratios coincide with the mce and the oae 2 however the increases found in these sites are of a lower magnitude than those found in records of many other mesozoic events possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events combined the new mercury data presented here are consistent with an initial magmatic pulse at the time of the mce with a second greater pulse at the onset of oae 2 possibly related to the emplacement of lips in the pacific ocean and or the high arctic

karelian shungite an indication of 2 0 ga old metamorphosed oil shale and generation of petroleum geology lithology and geochemistry abstract the ca 2 0 ga old 600 m thick upper zaonezhskaya formation near lake onega nw russia contains unusually high concentrations of corg up to 98 averaging around 25 the formation contains an estimated 25 1010 tonnes of organic carbon accumulated within an area of 9000 km2 organic material is represented by shungite which forms a black dense amorphous or nanocrystalline mass consisting of c with traces of n o s and h autochthonous shungite occurs as disseminated organic material 0 1 50 corg which when mixed with migrated bitumen now pyrobitumen appears as coal like seams and lenses of semilustrous and semimat layer shungite rocks oil shales 50 75 corg the migrated bitumen originally petroleum represented by the lustrous vein and layer shungite conformably fills interbedding spaces or cross cutting joints and usually contains 80 98 corg the shungite bearing rocks of the upper zaonezhskaya formation represent one of the most richest accumulations of organic material reported from the palaeoproterozoic and one of the geologically earliest stages of petroleum generation the sediments of the zaonezhskaya formation were initially deposited in brackish water in a non euxinic lagoonal environment the high c s ratio 8 1000 with a zero intercept on the c s cross plot indicates that deposition occurred in sulphur poor water intensive synchronous volcanism may have contributed to both the enhanced delivery of nutrients and elevated sedimentation rate and eventually to the high degree of preservation of organic material the integrated data suggest that the organic material has a biogenic origin most likely algal or bacterial the organic material suffered complex catagenetic and metamorphic alteration which is reflected in 1 the four modal distribution of corg content with maxima at 5 30 65 and 95 2 highly variable 13 c org 45 to 17 3 bimodal distribution of 13 c org with maxima at 28 and 39 and 4 low h c ratios 0 005 0 2 abundant diagenetic carbonates associated with shungite rocks 13 c carb 5 to 26 and the presence of pyrite 34 s 22 to 31 reflects substantial loss of organic matter via bacterial reduction of sulphate during diagenesis the shungite rocks are characterised by a further substantial loss 50 of biologically produced organic material in the course of thermal maturation and by a depletion in 12 c 10 the isotopic composition of carbonate concretions does not indicate the involvement of fermentative diagenesis conservative estimates give 13corg of 34 as the best value of the initial biomass lustrous vein and layer shungite containing more than 80 corg are considered to be allochthonous migrated bitumen originally petroleum the semilustrous and semimat layer shungite rocks containing 55 75 corg represent oil shales with both migrated bitumen originally petroleum and autochthonous kerogen residues the oil source rocks were apparently hosted in the zaonezhskaya formation the generated oil has migrated both vertically and laterally with the highest concentration in cupola structures the locality at shunga represents the most significant volume of trapped petroleum from the study area

some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins large volumes of carbon dioxide i e up to 90 v v have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the co2 including mantle degassing metamorphic reactions or magmatic processes the formation of large accumulations of large volumes of co2 in sedimentary basins is discussed with specific reference to published data from the pannonian basin hungary the cooper eromanga basin australia and the south viking graben north sea these basins have occurrences of large volumes i e over 15 v v of isotopically heavy carbon dioxide thought to be of inorganic origin and although they differ somewhat in terms of sedimentary fill all have major structural features close to the gas accumulations the available geochemical data are currently neither sufficient to confirm the specific origin s of the co2 e g mantle derived carbonate metamorphism nor unscramble co2 mixed from two or more sources ongoing studies especially those involving noble gas analyses should provide fuller answers

multiple palaeoproterozoic carbon burial episodes and excursions organic rich rocks averaging 2 5 total organic carbon and positive carbonate carbon isotope excursions view the mathml source and locally much higher i e the lomagundi jatuli event are hallmark features of palaeoproterozoic successions and are assumed to archive a global event of unique environmental conditions following the c 2 3 ga great oxidation event here we combine new and published geochronology that shows that the main palaeoproterozoic carbon burial episodes cbes preserved in russia gabon and australia were temporally discrete depositional events between c 2 10 and 1 85 ga in northwest russia we can also show that timing of the termination of the lomagundi jatuli event may have differed by up to 50 ma between localities and that ni mineralisation occurred at c 1920 ma further cbes have traits in common with mesozoic oceanic anoxic events oaes both are exceptionally organic rich relative to encasing strata associated with contemporaneous igneous activity and marked by organic carbon isotope profiles that exhibit a stepped decrease followed by a stabilisation period and recovery although cbe strata are thicker and of greater duration than oaes 100 s of metres versus metres 106 years versus 105 years their shared characteristics hint at a commonality of cause s and feedbacks this suggests that cbes represent processes that can be either basin specific or global in nature and a combination of circumstances that are not unique to the palaeoproterozoic our findings urge circumspection and re consideration of models that assume cbes are a deep time singularity

petrography carbonate mineralogy and geochemistry of thermally altered coal in permian coal measures hunter valley australia abstract carbonate minerals commonly occur in coals of many ages and from a utilisation viewpoint can be deleterious several studies have been undertaken of the carbonates in the permian coals of the hunter valley australia but few studies use a multi technique approach for this study a combined petrographic geochemical and mineralogical approach was used to determine the distribution and residence of carbonate minerals in coal that had been intruded by a dyke the dominant carbonate assemblages comprise primary siderite in inertinite rich microlithotypes and secondary calcite ankerite dolomite dawsonite in vitrinite rich microlithotypes the secondary carbonates were found in both the aureole of heated coal and also in an unheated mine face sample it is believed that the secondary carbonate minerals precipitated from magma derived fluids percolating through the coal following the emplacement of the intrusions the textures and distribution of the secondary carbonate minerals suggest that the temperature and pressure of the fluids may be just as important in developing fractures near dykes particularly those that have multiple phases of geometries cleat mineralogy and coal textures as direct heating from the intrusion the partitioning of primary siderite with inertinite and secondary carbonates with vitrinite indicates that it can be reasonably expected that there would be a partitioning of minerals in various density fractions derived from float sink tests and consequently a partitioning of elements with inertinite rich fractions containing elevated fe levels and vitrinite rich fractions containing elevated ca mg and al this partitioning has implications for the behaviour of the coal during washing and combustion and the composition of combustion products

geological controls on shallow gas distribution and seafloor seepage in an arctic fjord of spitsbergen norway abstract this study integrates high resolution shallow and deep geophysical datasets to study the seabed seepage systems linked to subsurface shallow gas and geological features influencing the fluid flow in nordfjorden central spitsbergen arctic norway this is one of the tributary fjords of the isfjorden fjord system in spitsbergen which has the highest density of pockmarks ranging up to 20 km2 the 535 pockmarks identified manually in the 300 km2 large study area have diameters ranging from 10 m to 212 m and depth varying between 1 m and 8 m sub surface seep related acoustic features such as enhanced reflections acoustic blanking and turbidity zones recorded on 55 km of new high resolution sub bottom acoustic data suggest shallow gas free and or dissolved occurrences in the marine sediments beneath the pockmarks structural features thrust faults and associated folds belonging to the palaeogene west spitsbergen fold and thrust belt and early cretaceous igneous sills have been interpreted on 2d multichannel seismic data in the triassic and permian successions some of these faults and igneous sills form seafloor ridges where pockmark density is high and they are typically aligned acoustic flares attributed to active gas seepage from the seafloor were imaged in the water column in the southern part of nordfjorden above the sub surface shallow gas occurrences and thrust faults the integrated analysis of these geophysical datasets suggests that the near seafloor tectonically deformed stratum plays an important role in the up dip propagation of fluids liquids and or gas distribution of shallow gas in marine sediments and seepage at the seafloor

effects of extreme heating on the elemental and isotopic compositions of an upper cretaceous coal abstract intrusion of igneous sills into coal seams in the maastrichtian vermejo formation of colorado new mexico rapidly and severely altered the organic matter content of the coal organic carbon concentrations drop from 65 in coal samples distant from sills to between 5 and 38 in coal samples close to a sill changes in organic matter c n ratios and 13 c values relative to distance from the sill indicate preferential losses of isotopically light forms of carbon from the coal the relatively high toc concentration elevated c n ratio and more negative 13 c value of coal closest to the sill suggest that rapid pyrolysis immobilized the thermal products as partially graphitized or mineral forms of carbon

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

new constraints on the age geochemistry and environmental impact of high arctic large igneous province magmatism tracing the extension of the alpha ridge onto ellesmere island canada

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

petroleum surface oil seeps from a palaeoproterozoic petrified giant oilfield early palaeoproterozoic rocks from the onega basin in russian fennoscandia contain evidence for substantial accumulation and preservation of organic matter up to 75 wt total organic carbon with an estimated original petroleum potential comparable to a modern supergiant oilfield the basin contains a uniquely preserved petrified oilfield including evidence of oil traps and oil migration pathways here we report the discovery of the surface expression of a migration pathway along which petroleum was flowing from the sub surface this surface oil seep the first occurrence ever reported from the palaeoproterozoic appears as original bitumen clasts redeposited in palaeoproterozoic lacustrine turbidites the 13corg of clastic pyrobitumen ranges between 35 4 and 36 0 n 14 which is within the range of interbed and vein trapped fossil oil 46 and 24 suggesting similar source biogenic organic matter whose isotopic composition was modified during thermal maturation is the likely source for the migrated hydrocarbon

geochronology of the tardree rhyolite complex northern ireland implications for zircon fission track studies the north atlantic igneous province and the age of the fish canyon sanidine standard abstract the british irish palaeogene igneous province bipip is part of the larger north atlantic igneous province and includes the lava fields of antrim mull and skye the tardree rhyolite complex trc in northern ireland forms an important stratigraphic unit between the lower and upper basalt formations of the antrim lava group alg previous zircon age determinations obtained from the trc have been used as a standard in zircon fission track studies but contradict several 40ar 39ar sanidine and u pb zircon results we provide new 40ar 39ar sanidine and u pb ca tims zircon ages which resolve this discrepancy two sanidine samples from the sandy braes vent and the columnar jointed dome forming rhyolites of tardree forest yield a weighted mean 40ar 39ar age of 61 13 0 42 ma 2 internal error ten u pb ca tims zircon analyses were undertaken eight of which employed the ca tims approach on both multi grain fractions and single grains six of the ca tims data yield a disequilibrium corrected weighted mean 206pb 238u age of 61 32 0 09 ma 2 the consistency of the 40ar 39ar ages with the ca tims u pb zircon age points to a closed system of both k and ar since eruption we propose that the crystallization age of the trc be taken as 61 32 0 09 ma and that the currently used age of the zircon fission track standard 58 4 0 7 ma be changed accordingly this also places the eruption of the trc in magnetochron c26r which is consistent with the reversed polarity magnetic remanence observed in the alg and supports the conclusion of ganerod et al 2010 that the lower basalt formation is older than the vaigat formation in western greenland no resolvable zircon inheritance has been detected by the tims analyses consistent with the fact that the temporal and geographic extent of rhyolitic magmatism within this sector of the bipip was very limited and hence was unlikely to provide inherited magmatic zircons from slightly older magmas antecrysts potentially older zircon xenocrysts would be derived from the underlying caledonian basement 400 ma or yet older rocks these should be easily detectable if the tardree zircon was to be employed as a u pb zircon standard the paired 40ar 39ar and 206pb 238u results from this study indicate an age of 28 393 0 194 ma for the widely used fish canyon sanidine standard and gives further support to the recent calibrations of kuiper et al 2008 and renne et al 2010

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

zircon dating ties ne atlantic sill emplacement to initial eocene global warming abstract the earth experienced rapid greenhouse gas induced global warming during the palaeocene eocene thermal maximum petm the source of the gas is however debated we have for the first time determined the ages of magmatic sills in the voring basin offshore norway zircon u pb ages of 55 6 0 3 and 56 3 0 4 ma demonstrate that sill emplacement was synchronous with the petm within small errors this discovery strengthens the hypothesis that global warming was triggered by rapid release of greenhouse gases generated by heating of organic rich sediments around intrusions in the ne atlantic rather than from dissociation of gas hydrates supplementary material u pb data are available at http www geolsoc org uk sup18392

2000 ma sulphide concretions from the productive formation of the pechenga greenstone belt nw russia genetic history based on morphological and isotopic evidence abstract two hundred and fifty nine sulphide samples from black shales of the productive formation belonging to the palaeoproterozoic 2000 ma petsamo supergroup in the kola region nw russia have been analysed for their sulphur isotope ratio the sulphide bearing black shales were formed in a non euxinic basin within an oceanic rift environment the rocks have undergone greenschist facies metamorphism the black shales are marked by very high s corg ratios and a strong positive correlation between corg and s r 0 70 with zero intercept on the sulphur axis different generations of sulphide concretions have been distinguished by means of concretional analysis individual isotope study of synsedimentary sulphide layers and sulphides from early middle and late diagenetic catagenetic sulphide concretions yields trends which suggest bacterial sulphate reduction as the concretion forming process the overall spread of 34 s from 8 3 to 24 9 indicates 34 s sulphate sulphide between 29 and 4 given a seawater sulphate at around 21 the first order trend is a progressive enrichment in 34 s with increase of burial two major populations of sulphides have been distinguished the first concretions and perhaps thin layers formed in an open system with respect to seawater sulphate and has a limited 34 s range of 2 7 to 3 8 this is coupled with extensive formation of diagenetic carbonate beds marked by 13 c of 8 7 to 12 5 the second population lenses thin layers concretions and microconcretions in carbonate nodules formed in a closed system and is characterized by a high fe2 s2 ratio and progressive development of a rayleigh distillation process which drove 34 s from 8 3 up to 24 9 the high fe2 s2 and possibly change in ph parameter resulted in pyrrhotite rather than in pyrite associated abundant carbonate nodules have 13 c in the range 8 2 to 14 1 overall low 34 s sulphate sulphide and the high s corg ratio are consistent with one stage bacterial sulphate reduction in a non euxinic basin the observed 34 s sulphate sulphide of 29 as maximum is within the pure sulphate reducing bacteria range of 4 46 advanced isotopic modification involving the oxidative part of the sulphur cycle has not been detected based on the data available one can not exclude the presence of a third population of sulphides thin massive layers with 34 s from 2 5 to 0 3 which might have inorganic origin and formed from an exhalative source

high δ13c paleoproterozoic carbonates from the transvaal supergroup south africa marbles from the transvaal supergroup kaapvaal craton south africa record two positive carbon isotope excursions to average 13 c carbonate values of 6 to 8 relative to v pdb separated by carbonates with normal marine 13 c values 0 2 these elevated values have been preserved through metamorphic devolatilization together with an already documented high 13 c excursion in overlying rocks three separate positive carbon isotope excursions occurred on the kaapvaal craton in the interval 2 43 to 1 93 ga it is not yet clear how many of these excursions reflect global rather than local processes

carbon sulfur plots to show diagenetic and epigenetic sulfidation in sediments abstract organic carbon vs sulfide sulfur plots are now being used regularly by many geochemists to help understand recent and ancient depositional environments and diagenetic processes usually these plots are useful to recognize nonmarine vs marine environments or oxic vs anoxic vs euxinic depositional environments however c vs s plots can also indicate diagenetic and epigenetic events that produce excess sulfide four new examples are presented and discussed

the early cretaceous barents sea sill complex distribution 40ar 39ar geochronology and implications for carbon gas formation abstract mafic igneous rocks of cretaceous age 80 130 ma scattered around the arctic ocean are commonly referred to as the high arctic large igneous province halip we have mapped out the distribution of halip igneous rocks in the barents sea region over the past decade based on integrated seismic gravity magnetic interpretation field work review of publications and analyses of new and vintage borehole and field samples the mapping reveals abundant igneous rocks in the northern and eastern barents sea covering an area of 900 000 km2 with a conservative volume estimate of 100 000 to 200 000 km3 of intrusions the igneous province is dominated by sheet intrusions injected into triassic and permian sedimentary rocks hydrothermal vent complexes are rare and only two potential vent complexes have been identified on seismic data in the eastern barents sea we have further done extensive radiometric dating of the igneous samples in the barents sea region new 40ar 39ar dating of thirteen samples from svalbard reveal ages of crystallization and alteration the large age span 60 140 ma for the raw ages is likely due to partial or complete overprint of the k ar system in plagioclase and the age of the magma emplacement is better represented by u pb tims ages only one of our 40ar 39ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published u pb tims ages of 122 125 ma the new data clearly document that relying on published data from the k ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40ar 39ar degassing data i e k ca we propose that the magmatism on svalbard and franz josef land represents a distinct magmatic event near the barremian aptian boundary 125 ma in the barents sea this early cretaceous barents sea magmatism resulted in the formation of the bssc barents sea sill complex bssc age rocks are also present in arctic canada sverdrup basin and on bennett island new siberia islands the massive injection of hot magma into potentially organic rich sediments in the eastern and northern barents basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles we estimate that up to 20 000 gt of carbon were potentially mobilized corresponding to 175 trillion barrels of oil equivalent the production rates and fate of the carbon gases are uncertain however we speculate that rapid release of aureole greenhouse gases methane may have triggered the oceanic anoxic event 1a oae1a and the associated negative 13c excursion in the early aptian some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east barents basin

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

petroleum geology and geochemistry of middle proterozoic mcarthur basin northern australia ii assessment of source rock potential five potential source rocks have been discovered in the middle proterozoic of the mcarthur basin the lacustrine barney creek formation mcarthur group and the marine velkerri formation roper group compare favorably in thickness and potential with demonstrated petroleum source rocks in the phanerozoic the former contains up to 7 total organic carbon toc and kerogen types i and ii toc values in the latter range up to 6 5 and the kerogen is type ii petrographic examination shows the organic matter is mostly lamalginite which on maturation yields nonfluorescent lamalginite and bitumen extractable hydrocarbon yields from mature samples indicate that good to excellent source rocks are present and there is ample evidence of migration having occurred tmax values derived from rock eval analysis may be used to define maturation levels in much the same way as in phanerozoic sediments at low maturation levels the reflectivities of lamalginite and bitumen are lower than vitrinite reflectance values calculated from methylphenanthrene indices but converge in the mature zone this convergence reflects both the inherent lower reflectivity of hydrogen rich macerals and the source control on methylphenanthrene indices at low maturity maturation levels in mcarthur group sediments vary from marginally mature to overmature with abrupt changes over short distances associated with faults hydrocarbon generation occurred prior to the deposition of the younger proterozoic sediments of the roper group in contrast the source rocks in the roper group re marginally mature to mature throughout the study area and are overmature only where they are affected by igneous intrusions considerations of burial history suggest that hydrocarbon generation may have occurred during deposition of the roper group although an early paleozoic timing cannot be excluded

late cretaceous bimodal magmatism northern ellesmere island isotopic age and origin in the yelverton bay region of northwestern ellesmere island bimodal intrusive and volcanic rocks are associated with a major fault in the proterozoic cambrian rocks of the pearya terrane the wootton intrusion consists mainly of gabbro with lesser amounts of granitic and hybrid rocks the hansen point volcanics are composed of felsic rocks and basalt plutonic zircons are very slightly discordant but volcanic zircons have unusually high degrees of inheritance interpreted u pb zircon ages of 92 0 1 0 ma for the wootton intrusion assuming a wide range of inheritance ages and of for the hansen point volcanics are close to the 93 ma average of hornblende k ar dates obtained earlier for a small quartz diorite pluton in central northernmost ellesmere island all fall into the early late cretaceous and indicate correlation with mafic volcanics of the cenomanian turonian strand fiord formation of eastern axel heiberg island the upper intercept age for the hansen point volcanics suggests that the felsi

discovery of lower cretaceous hydrothermal vent complexes in a late rifting setting southern north sea insights from 3d imaging we present data for 31 kilometre scale hydrothermal vent complexes and associated sand remobilization that have formed as a consequence of magmatic intrusions within salt strata southern north sea dutch sector this study uses 3d seismic data tied to three wells and artificial neural networks to describe conical and cylindrical vent structures located in the ne part of the broad fourteens basin the structures are composites of 58 subvertical pipes that form complexes and are 951 364 m in height and 487 426 m in diameter interactions of magma and fluids at depth triggered successive fluid pressure build ups in triassic sandstones resulting in upward propagated ruptures followed by the collapse of overlying deposits and the injection and extrusion of fluidized sand the relationship between vent tops and their overlying deposits indicates a mid hauterivian age unusual for volcanic activity in the north sea area this event could have a significant impact on hydrocarbon exploration as the hydrothermal vent complexes bypass a regional source rock and seal offering new migration routes and connectivity between potential reservoirs the intrusions also raise questions about the influence of transform tectonics on the focusing of volcanic activity during the late rifting phase of the north sea supplementary material tables of seismic attributes and the geometry of the vent complexes as well as a 3d visualisation of the vents are available at https doi org 10 6084 m9 figshare c 3571593

the precambrian supracrustal rocks of finland and their tectono exogenic evolution abstract the precambrian supracrustal rocks of finland were produced during four main evolutionary stages 1 late archaean pre svecokarelidic greenstone schist evolution on an unknown basement 2 at least four cycles of early proterozoic karelian continental epi pericontinental sedimentation and volcanism on or next to the cratonized late archaean basement 3 early proterozoic svecofennian eugeosynclinal evolution west of or in part on the late archaean basement and its cratonic cover and 4 middle to late proterozoic jotnia continental sedimentation on the stabilized early proterozoic post svecokarelidic basement

contact metamorphism and thermogenic gas generation in the voring and more basins offshore norway during the paleocene eocene thermal maximum voluminous volcanic intrusive activity took place in the voring and more basins at the paleocene eocene boundary at about 56 ma this event caused thermal maturation of cretaceous sedimentary rocks in the basins we have estimated the resulting thermogenic gas generation potential from contact metamorphism using numerical simulations calibrated using borehole data the borehole 6607 5 2 from the utgard sill complex in the voring basin contains two c 100 m thick sills and is used as a case study we present both new and compiled data showing that 1 the bulk organic content is reduced towards the sill intrusions 2 a c 1 km thick stratigraphic interval is thermally affected based on vitrinite reflectance data 3 relative emplacement timing can affect the gas yield by up to 25 and 4 some of the thermogenic methane is still present in the aureoles the numerical model is calibrated using data from 11 wells we estimate that the total gas generation potential for the two utgard sills equals that of the troll field c 10 gt ch 4 the largest producing gas field offshore norway we show that in the voring and more basins the total gas generation potential is up to 1500 gt ch 4 c 1100 gt c even from the relatively organic poor cretaceous source rocks with c 1 wt organic carbon with implications for the carbon cycle at the paleocene eocene boundary

unravelling the sources of carbon emissions at the onset of oceanic anoxic event oae 1a abstract the early aptian oceanic anoxic event oae 1a represents a major perturbation of the earth s climate system and in particular the carbon cycle as evidenced by widespread preservation of organic matter in marine settings and a characteristic negative carbon isotopic excursion cie at its onset followed by a broad positive cie the contemporaneous emplacement of a large igneous province lip is invoked as a trigger for oae 1a and oaes in general but this link and the ultimate source of the carbon perturbation at the onset of oaes is still debated in this study we simultaneously assimilate an atmospheric pco2 reconstruction along with a 13 c record from the spanish cau section in an earth system model to obtain a novel transient reconstruction of emission rates and identify the primary carbon emitting sources across the negative cie interval at the onset of oae 1a we reconstruct carbon emissions of 4 300 29 200 pg from a mixture of carbon sources this estimate is a lower bound as contemporaneous organic carbon burial is not accounted for carbon was first released at slow rates from a 13c depleted reservoir e g thermo and or biogenic methane from sill intrusions towards the end of the negative cie the rate of emissions increased and they became more 13c enriched likely from a dominantly volcanic source e g lips new osmium isotope 187os 188os measurements from the same section as the pco2 reconstruction and 13 c data reveal a shift to less radiogenic values coinciding with the change towards mantle derived carbon emissions as indicated by our modelling results lending further support to our interpretation these results highlight that geologically triggered carbon emissions were likely driving the oae onset

hydrocarbon shows and petroleum source rocks in sediments as old as 1 7 109 years we report the discovery of indigenous live oil in 1 4 gyr old rocks in the mcarthur basin of northern australia previously reported occurrences of indigenous precambrian oil are 1 gyr old1 5 potential petroleum source rocks in the mcarthur basin range in age from 1 4 to 1 7 gyr and were deposited in marine and lacustrine environments in parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons they span the period corresponding to the appearance of eukaryotic organisms and because of their low degree of thermal alteration they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers the hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin these results show that exploration of previously ignored mid proterozoic sediments may lead to the discovery of new reserves of oil

how contact metamorphism can trigger global climate changes modeling gas generation around igneous sills in sedimentary basins abstract large volumes of greenhouse gases such as ch4 and co2 form by contact metamorphism of organic rich sediments in aureoles around sill intrusions in sedimentary basins thermogenic gas generation and dehydration reactions in shale are treated numerically in order to quantify basin scale devolatilization we show that aureole thicknesses defined as the zone of elevated metamorphism relative to the background level vary within 30 250 of the sill thickness depending on the temperature of the host rock and intrusion besides the sill thickness in shales with total organic carbon content of 5 wt ch4 is the dominant volatile 85 135 kg m3 generated through organic cracking relative to h2o generation from dehydration reactions 30 110 kg m3 even using conservative estimates of melt volumes extrapolation of our results to the scale of sill complexes in a sedimentary basin indicates that devolatilization can have generated 2700 16200 gt ch4 in the karoo basin south africa and 600 3500 gt ch4 in the voring and more basins offshore norway the generation of volatiles is occurring on a time scale of 10 1000 years within an aureole of a single sill which makes the rate of sill emplacement the time constraining factor on a basin scale this study demonstrates that thousands of gigatons of potent greenhouse gases like methane can be generated during emplacement of large igneous provinces in sedimentary basins

application of stable carbon isotopes to early biochemical evolution on earth the emergence and early evolution of life on earth have long been intriguing subjects for scientific inquiry and a continuous source of intel lectual excitement current work at this frontier proceeds along a number of different avenues and comprises a variety of disciplinary approaches that together strongly suggest that the earth was inhabited by life as early as almost 4 gyr ago the present state of the field as inferred from the results of a broad interdisciplinary approach has recently been summarized by j w schopf and coworkers cf schopf 1 983 it is with the advantage of such a synoptic view that the present review addresses a single facet of the theme namely the potential isotopic infer ences of the presence of co2 fixing autotrophic organisms for the carbon cycle of the early earth with biologically derived organic carbon as well as carbonates limestone dolomite etc abundantly preserved in sediments relevant isotopic evidence should have been encoded in the geologic record from its onset about 3 8 gyr ago it has been known since the pioneering studies by nier gulbransen 1939 murphey nier 1941 and rankama 1948 that the trans formation of inorganic carbon into living matter entails a marked bias in favor of the light isotope c2c with the heavy species l3c retained in the inorganic reservoir subsequent work performed by many investigators e g craig 1953 park epstein 1960 has confirmed that all common

does large igneous province volcanism always perturb the mercury cycle comparing the records of oceanic anoxic event 2 and the end cretaceous to other mesozoic events mercury hg is increasingly being used as a sedimentary tracer of large igneous province lip volcanism and supports hypotheses of a coincidence between the formation of several lips and episodes of mass extinction and major environmental perturbation however numerous important questions remain to be answered before hg can be claimed as an unequivocal fingerprint of lip volcanism as well as an understanding of why some sedimentary records document clear hg enrichment signals whilst others do not of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle as well as the role played by depositional processes in recording global hg cycle perturbations here new mercury records of cretaceous oceanic anoxic event 2 oae 2 94 ma and the latest cretaceous 67 66 0 ma are presented oae 2 is associated with the emplacement of multiple predominantly submarine lips the latest cretaceous with subaerial volcanism of the deccan traps both of these connections are strongly supported by previously published trends towards unradiogenic osmium os isotope values in globally distributed sedimentary records hg data from both events show considerable variation between different locations attributed to the effectiveness of different sediment types in registering the hg signal with lithologically homogeneous records documenting more clear hg enrichments than sections with major changes in lithology such as limestones to claystones or organic rich shales crucially there is no geographically consistent signal of sedimentary hg enrichment in stratigraphic records of either oae 2 or the latest cretaceous that matches os isotope evidence for lip emplacement indicating that volcanism did not cause a global hg perturbation throughout the entire eruptive history of the lips formed at those times it is suggested that the discrepancy between os isotope and hg trends in records of oae 2 is caused by the limited dispersal range of hg emitted from submarine volcanoes compared to the global scale distribution of os a similar lack of correlation between these two proxies in uppermost cretaceous strata indicates that although subaerial volcanism can perturb the global hg cycle not all subaerial eruptions will do so these results highlight the variable impact of different volcanogenic processes on the efficiency of hg dispersal across the globe factors that could influence the impact of lip eruptions on the global mercury cycle include submarine versus subaerial volcanism volcanic intensity or explosivity and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic rich sediments

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

carbon and sulfur isotopic compositions of organic carbon and pyrite in sediments from the transvaal supergroup south africa abstract sedimentary organic matter and pyrite from parts of the lower transvaal supergroup and few samples from the olifantshoek group have been studied for their carbon and sulfur isotopic compositions respectively carbon isotopic compositions for minimally altered organic matter range from 43 to 31 sulfur isotope values for sedimentary pyrite display values between 1 3 to 23 6 results from this and previously published work are being interpreted with respect to past biochemical pathways carbon isotope data indicate autotrophic carbon fixation as the process of primary production in addition products of secondary biological reworking contributed to the total organic carbon content sulfur isotope data indicate bacterial sulfate reduction as the main process however changes in sulfate availability and or the rate of sulfate reduction may have caused the observed range in isotopic compositions

coal metamorphism by igneous intrusion in the raton basin co and nm implications for generation of volatiles abstract the raton basin contains a series of tertiary mafic dikes and sills that have altered millions of tons of coal to natural coke and may have played a minor role in generating some of the large coalbed methane resources currently being exploited in this region four outcrops within the raton basin were selected to investigate local coal rank elevation and volatile generation due to intrusive activity coal and organic shale samples were collected on traverses across meter scale dikes and sills and were analyzed for vitrinite reflectance total organic carbon bulk carbon isotopic ratios and coke petrography reflectance patterns in the contact aureoles of the dikes display very consistent concave up patterns with reflectance values returning to background within one intrusion width from the contact bulk coal samples collected across dikes display an increase in 13c of approximately 1 approaching all contacts reflectance patterns within the contact aureoles of sills do not decrease significantly until one half of an intrusion width away from the contact but then drop steeply to background values by one intrusion width away bulk coal samples display complex isotopic patterns approaching sills with an initial increase in 13c followed by a steep decrease of approximately 1 5 approaching the top contacts the distinct isotopic and reflectance patterns for dikes and sills may indicate that the coal in the contact aureoles of the sills experienced a longer heating duration than the coal near the dike contacts this longer heating duration was due to the low thermal conductivity of coal which acts as a thermal insulator to the sills that intrude coal beds this style of intrusion is commonly observed throughout the central portions of the raton basin by contrast dikes intrude a variety of country rock types and are not insulated by coal heat may escape the contact zone through nearby sandstones or shales based on reflectance patterns hydrothermal convection played a role in heat transfer away from dikes data from both the dike and sill contacts suggest that time plays a role in controlling peak vitrinite reflectance values at least over the short heating durations tens of years expected for meter scale intrusions decreasing total organic carbon values approaching all contacts suggests the generation and escape of volatile species such as methane isotopic values within the contacts of dikes are consistent with the loss of 12c enriched methane the isotopic pattern observed at the contacts of sills may be due to accumulation of 12c rich pyrolytic carbon close to the contacts which is supported by petrographic observations even though multiple thin sheet intrusions are insignificant for conductive heating on a basin scale methane generation by sheet intrusions was volumetrically important in the raton basin because sills preferentially intrude coal beds in this region

carbon sulfur iron systematics of the uppermost deep water sediments of the black sea box cores recovered during leg 4 of the 1988 r v knorr black sea oceanographic expedition from deep water regions of the basin were dominated by coccolith rich microlaminated unit 1 sediment and muddy gray turbidite layers both organic carbon oc and pyrite sulfur values for unit 1 display narrow ranges with mean concentrations of 5 3 1 1 1 wt and 1 3 0 3 wt respectively unit 1 is not enriched in pyrite s relative to sediments deposited under oxygenated bottom waters normal marine sediments with comparable oc concentrations carbon sulfur relationships evaluated on a calcium carbonate free basis to avoid spurious correlations resulting from dilution effects demonstrate that oc and pyrite s are essentially decoupled these observations combined with the persistence of elevated pore water sulfide to depth and a strong correlation between pyrite s and the detrital fe component argue strongly for limitation of pyrite formation in unit 1 by the availability of reactive fe unit 1 fe limitation is further indicated by degree of pyritization dop studies a measure of the extent to which the original potentially reactive fe has been transformed to pyrite these studies show sulfidation of reactive fe ranging from 57 to 78 with dop values independent of oc concentration unit 1 dop profiles suggest that the majority of the pyrite is formed in the sulfidic water column and or very close to the sediment water interface pyrite s concentrations of unit 1 when compared with the particulate reduced sulfur fluxes measured in time series sediment traps are compatible with predominantly water column pyrite formation because of the limitations in the supply of reactive fe associated with the comparatively high supply of oc the microlaminated sediment is characterized by c s ratios greater than those typical of holocene oxically deposited sediments the turbidite muds of the deep basin display high reduced s values relative to holocene normal marine sediments in samples with low oc concentrations low c s ratios this reflects pyrite formation under anoxic sulfidic bottom water conditions in a probable upper slope source region as well as during transport and final deposition intermediate dop values for the turbidites in part a product of their rapid rate of deposition reveal that fe limitation is ultimately not a factor and that further pyrite should form during burial however the very rapid rate at which unit 1 pyrite forms suggests fundamental differences between the overall reactivities of the fe phases associated with the microlaminated and turbiditic sediments the signature of water column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high dop values of unit 1 and the comparatively high levels of s associated with the low concentrations of oc of the turbidite muds this agrees with similar conclusions based on studies of ancient sedimentary rocks

contact metamorphic devolatilization of shales in the karoo basin south africa and the effects of multiple sill intrusions abstract quantification of fluid generation during contact metamorphism of shale is important for the understanding of metamorphic processes fluid flow in sedimentary basins and perturbations of the global carbon cycle in this study we provide geochemical and numerical analyses from the organic rich ecca group in the karoo basin south africa which was affected by contact metamorphism from multiple sill intrusions in the early jurassic organic matter was efficiently converted to hydrocarbons during contact metamorphism and complete loss of organic carbon in the innermost aureole is common mineral dehydration reactions are evident from the occurrence of metamorphic minerals like biotite and loss of the clay fraction towards the intrusive contact we have developed a numerical model in order to quantify fluid production from both inorganic and organic reactions during contact metamorphism the modelling results are constrained by data from two case studies in the karoo basin in order to obtain reliable estimates of the carbon loss from metamorphism of shale we show that single thin 15 m thick sills have a gas production potential of several gigatons of methane ch4 if emplaced over a 1000 km2 area furthermore the vertical spacing between simultaneously emplaced sills has an important influence on the gas generation potential when two sills are emplaced with a vertical spacing of 7 times the intrusion thickness the total ch4 generation is up to 35 more than for two separate sills data and modelling from five sills emplaced within the ecca group show hydrocarbon generation throughout the organic rich section with total carbon loss next to the sills this has implications for the fluid production and metamorphism in volcanic basins where multiple sills are common

an isotopic biogeochemical study of neoproterozoic and early cambrian sediments from the centralian superbasin australia abstract organic matter from neoproterozoic and early cambrian sediments of the amadeus and officer basins of the centralian superbasin australia has been studied for biomarker distributions and the carbon isotopic compositions of kerogen and individual hydrocarbons these sediments represent both shallow and deep water marine facies in the older sections and marine and saline lacustrine carbonate deposits in the cambrian hydrocarbon biomarker patterns were found to be quite consistent with the known sedimentary environments and provide valuable insights into the biogeochemical changes which accompanied the transition from a microbially dominated ocean to the early stages of metazoan radiation in particular carbon isotopic data for n alkyl and isoprenoid lipids presented here and in earlier studies showed a reversal in carbon isotopic ordering between the proterozoic and phanerozoic by comparison with the 13c of kerogen n alkyl lipids from deep water proterozoic sediments were enriched in 13c and appear to be derived mainly from heterotrophs whilst open marine phanerozoic counterparts are 13c depleted and evidently derived mainly from autotrophs data from the samples studied here are consistent with a model invoking a change in the redox structure of the ocean possibly aided by the innovation of faecal pellets

nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the karoo basin south africa abstract we have investigated the n geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes hvc in the karoo basin in south africa the hvc formed during phreatic eruptions associated with rapid devolatilization and pressure build up in contact aureoles around early jurassic sill intrusions by combining outcrop data from a hvc and core data from contact aureoles we investigate the relationship between light element release during metamorphism and vertical fluid migration sandstone and breccia from the hvc contain early diagenetic ammonium nh 4 bearing feldspar buddingtonite and illite ammonium occupies up to 95 of the a site in feldspar corresponding to concentrations up to 5 2 wt n bulk rock n isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups background samples have 15nair between 1 5 and 4 9 whereas minerals from the vent complex have 15n in the range 7 5 to 10 6 the n geochemistry of contact metamorphic shale from the lower stratigraphic units of the karoo basin shows that the vitrinite reflectance and 15n values are positively correlated shale with reflectivity values 4 ro are enriched in 15n with 15n values between 6 and 14 implying the release of isotopically light n into metamorphic fluids probably as n2 we suggest that the relatively high 15n values of the early diagenetic buddingtonite in the hvc reflect exchange of buddingtonite with n bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration we present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation focusing n bearing metamorphic fluids sourced in deeper levels of the basin the release of organic n from sediments at depth in volcanic basins could play a role in the geochemical cycle of n becoming particularly important during periods of intense volcanic activity

genesis of proterozoic iron formation sulphur isotope evidence seven units of carbonaceous shale or sulphide facies iron formation have been sampled they are associated with proterozoic iron formations that range in age from 1 9 to 2 5 ga sokoman and gunflint canada riverton united states penge south africa and brockman australia sulphur isotope ratios have been determined on the sulphides removed from these shales by both physical and chemical means the mean 34s composition of the seven units varies between 4 9 and 6 6 and the sample variance is low within each unit these distributions are more characteristic of hydrothermal sulphide than sulphide produced by biogenic reduction this hydrothermal sulphide is believed to have originated from high temperature reduction of seawater sulphate and from magmatic sulphide a model is suggested whereby this sulphide was exhaled into stratified anoxic oxic basins the sulphide and associated base metals were deposited in the reduced sediments beneath the anoxic waters while some iron and manganese was deposited on oxygenated shelves the data support but do not prove a hydrothermal exhalative origin for lower proterozoic iron formation

hydrothermal venting of greenhouse gases triggering early jurassic global warming abstract the climate change in the toarcian early jurassic was characterized by a major perturbation of the global carbon cycle the event lasted for approximately 200 000 years and was manifested by a global warming of 6 c anoxic conditions in the oceans and extinction of marine species the triggering mechanisms for the perturbation and environmental change are however strongly debated here we present evidence for a rapid formation and transport of greenhouse gases from the deep sedimentary reservoirs in the karoo basin south africa magmatic sills were emplaced during the initial stages of formation of the early jurassic karoo large igneous province and had a profound influence on the fate of light elements in the organic rich sedimentary host rocks total organic carbon contents and vitrinite reflectivity data from contact aureoles around the sills show that organic carbon was lost from the country rocks during heating we present data from a new type of geological structures termed breccia pipes rooted in the aureoles within the shale of the western karoo basin the breccia pipes are cylindrical structures up to 150 meters in diameter and are mainly comprised of brecciated and baked black shale thousands of breccia pipes were formed due to gas pressure build up during metamorphism of the shales resulting in venting of greenhouse gases to the toarcian atmosphere mass balance calculations constrained by new aureole data show that up to 1800 gt of co2 was formed from organic material in the western karoo basin about 15 times this amount of co2 27 400 gt may have formed in the entire basin during the intrusive event u pb dating of zircons from a sill related to many of the pipes demonstrates that the magma was emplaced 182 5 0 4 million years ago this supports a causal relationship between the intrusive volcanism the gas venting and the toarcian global warming

two billion year old isotopically heavy carbon evidence from the labrador trough canada forty eight samples of bulk carbonates were studied from the dunphy portage alder uve denault and abner formations and the undivided pistolet subgroup in the labrador trough quebec canada the carbonate units occur as a number of thick beds up to 300 m within 100 600 m thick formations they are interbedded with siliciclastic sediments and are composed of crystalline sparry micritic allochemical and stromatolitic dolostones the carbonate rocks are recrystallized and metamorphosed up to amphibolite grade they are chemically dolostones and include chemical or biochemical laminated and massive dolostones dolorudites dolarenites and dololitites which were deposited in shallow water dunphy denault and abner formations to relatively deep water uve formation marine environments reef facies are present in the denault and abner formations the dunphy portage alder and uve formations deposited between 2 17 and 2 14 ga yield isotopically heavy values of 13c pdb ranging from 5 3 to

geochemically driven exploration models in uplifted areas examples from the norwegian barents sea the norwegian barents sea with multiple source rock intervals represents a prime example of an overfilled petroleum system however several episodes of uplift and erosion from the paleocene until the pliocene pleistocene have caused the depletion of hydrocarbon accumulations in the region these uplift events were not only potentially catastrophic but have also caused the redistribution of the remaining oil and gas over laterally large distances in the barents sea region this redistribution directed petroleum to distal parts of the various hydrocarbon generating basins thus charging traps which otherwise would not have been reached therefore we expect that discoveries will be made in distal basin settings particularly in traps with partly leaking cap rocks which can bleed off gas and thereby retain oil many oil accumulations in the region represent various mixtures of oils from several different stratigraphic source intervals this suggests that triassic and paleozoic oils may be trapped below the presently drilled targets which are mostly jurassic in the hammerfest basin and older to the north and east deeper exploration targets also stand a higher chance of containing oil because the amount of gas being released from oil during uplift erosion and subsequent pressure release is lower uplift and erosion are followed by a reduction in temperature this is why hydrocarbon generation is believed to have ended in uplifted areas some discoveries in this data set suggest however a significant fresh gas charge

palaeomagnetic rock magnetic and mineralogical investigations of the lower triassic vardebukta formation from the southern part of the west spitsbergen fold and thrust beltk dudzisz and otherspalaeomagnetism of triassic rocks spitsbergen

non biogenic graphite in 3 8 ga metamorphic rocks from the isua district greenland abstract a 3 8 ga old metamorphic rock that contains up to 5 5 wt reduced carbon as graphite was found in the isua district greenland this rock exhibits a strong schistosity comprises mostly talc contains high mgo 28 0 wt ni 553 ppm and cr 267 ppm and low al2o3 2 13 wt and bears a high fe 2 o 3 feo ratio 0 94 the original rock was probably a submarine ultramafic tuff the carbon content and the 13c value of the rock vary on a microscopic scale a few mm3 scale from 0 7 to 5 5 wt and from 15 to 12 respectively the second rock a quartzite contains 0 13 wt c as reduced carbon with a 13c value of 11 25 these 13c values fall within the range 27 to 10 reported by previous investigators for graphite from the isua district the 13c values of the isua graphite have been interpreted by previous investigators e g schidlowski and aharon 1992 as metamorphic modification of organic matter with initial 13c values of 30 an important implication of this model has been that the 3 8 ga ocean already supported some biological activity however an examination of the carbon content vs 13c relationship of the graphite bearing rocks from isua has revealed that the 13c value generally increases with increasing carbon contents the rocks with 13c values less than 20 contain extremely small amounts of carbon the mineralogical and elemental characteristics of the rocks of this study a thermodynamic consideration of the temperature and composition of metamorphic fluids and carbon isotope mass calculations suggest that the major component of graphite with 13c values around 12 was formed by an inorganic rather than by a biological process the graphite appeared to have formed at temperatures between 700 and 400 c by one or both of the following two processes 1 reactions between co2 and ch4 co2 ch4 2c 2h2o during cooling of fluids with a co 2 ch 4 molar ratio of 1 and or 2 reactions between the feo component in the ultramafic rock with co2 rich fluids 4 feo co2 2 fe2o3 c the 13c c values for the fluids for both i and 2 were probably between 12 and 5 suggesting that the carbon bearing fluids could have been derived from the mantle

evolution of the marine stable carbon isotope record during the early cretaceous a focus on the late hauterivian and barremian in the tethyan realm abstract in order to improve our understanding of the relationships between the late hauterivian oceanic anoxic faraoni event contemporaneous platform drowning along the northern tethyan margin and global environmental change in general we established high resolution 13 c and 18 o curves for the late hauterivian and the entire barremian stage these data were obtained from whole rock carbonate samples from the veveyse de ch tel saint denis section switzerland the fiume bosso section and the nearby gorgo a cerbara section central italy and the angles section barremian stratotype france we observe an increase of 0 3 in mean 13 c values within sediments from the middle hauterivian subsaynella sayni ammonite zone to the hauterivian barremian boundary 13 c values remain essentially stable during the early barremian during the latest early barremian and most of the late barremian 13 c values increase slowly until the imerites giraudi zone and the latest barremian is characterized by a negative trend in 13 c values with minimal values at the barremian aptian boundary during the earliest aptian 13 c mean values start to rise again and attain 2 25 we interpret the evolution of the 13 c record as resulting from the interaction between changes in the carbon cycle in the tethyan basin and the adjacent platforms and continents in particular changes towards warmer and more humid conditions on the continent and coeval phases of platform drowning along the northern tethyan margin may have contributed to enhance the oceanic dissolved inorganic carbon dic reservoir which may have pushed the 13 c record towards more negative values and exerted a general attenuation on the 13 c record from this may have come the general change from a heterozoan to a photozoan carbonate platform community which influenced the evolution in 13 c values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins

large 18o and 13c depletions in greenschist facies carbonate rocks western arizona strongly deformed greenschist facies carbonate rocks in western arizona exhibit large 18 o and 13 c depletions relative to unmetamorphosed correlative rocks in northeastern arizona unmetamorphosed permian kaibab formation carbonates on the colorado plateau yield 13 c values of 1 to 5 and 18 o values of 28 to 37 in contrast greenschist facies kaibab formation in the little harquahala and granite wash mountains yields 13 c values of 6 to 4 and 18 o values of 9 to 24 the magnitude of the observed 13 c and 18 o depletions cannot be explained by decarbonation reactions and requires extensive exchange with low 13 c and 18 o fluids the 13 c depletions may reflect isotopic exchange with lower 13 c carbonate in the underlying paleozoic section and exchange with co 2 derived from oxidation of methane via reaction with hematite or sulfate in interbedded rocks these data demonstrate that large 13 c and 18 o depletions can occur in strongly deformed terranes during very low grade metamorphism depletions in 13 c and 18 o observed in some high grade metamorphic terranes may be inherited from much lower metamorphic grades

from field analogues to realistic seismic modelling a case study of an oil producing andesitic sill complex in the neuquen basin argentina interpretation of seismic data has played a major role in recent advances in the studies of igneous sill complexes seismic modelling studies based on field analogues represent a promising tool to close the scale gap between observations from outcrops and seismic data and support seismic interpretation virtual outcrop models are commonly used to include high resolution geological structures in models of seismic scale field analogues however realistic seismic modelling requires not only detailed structural input but also well constrained elastic properties and an adequate seismic modelling technique here we present a seismic modelling study of oil producing andesitic sills in the neuquen basin argentina which implements all modelling elements at high accuracy by combining virtual outcrop models well data and a 2 3 d filtering method our results indicate that the modelled seismic signatures of intrusive bodies observed in field analogues are characterized by frequency dependent interference and strong amplitude variations owing to highly variable elastic properties of both host rock and sills we demonstrate that detailed waveform patterns observed in real seismic data can be linked to intrusive bodies below the traditionally assumed limit of resolution via realistic seismic modelling this illustrates how an integrated modelling approach based on field analogues can aid seismic interpretation

effects of igneous intrusions on coalbed methane potential gunnedah basin australia abstract the gunnedah basin nsw australia contains more than 500 gt of coal and has been the subject of recent coalbed methane exploration large areas of the basin contain igneous intrusions and large areas of coal have been heat affected as a consequence a detailed study has been undertaken of coal seams intersected in a cored coalbed methane exploration drillhole in which two sill form igneous intrusions are present comparisons are made between coals that are unaltered and coals that have been heat affected using petrographic and chemical data coal seam gas desorption data and gas chemical analysis data results demonstrate that the two igneous intrusions have had a very positive effect on coalbed methane development the gas content in a number of heat affected coal seams within thermal aureoles above and below the sills is substantially higher than in adjacent unaffected coal seams in addition the intrusions have had little effect on gas quality the coals in the heat affected zone were found to contain gas with approximately 95 methane the coals in the thermal aureoles were found under the microscope to contain characteristic micropores and slits which collectively may serve to enhance gas adsorption capacity permeability and gas desorption gas contents below each of the sills is substantially higher than above the sill confirming earlier results that the sills appear to have acted as a reservoir seal during and for some time after intrusion the background coal rank in acm yannergee ddh 1 is in the high volatile bituminous range the igneous intrusions have resulted in an increase in rank such that large areas of coal have moved into the optimal thermogenic gas generation window this rank increase has affected a major part of the coal bearing sequence

impacts of palaeogeography and sea level changes on mid cretaceous climate climate is often depicted as globally warm during the middle part of the cretaceous however major tectonic events such as the break up of gondwana or sea level changes inducing the flooding of vast continental areas occurred from 120 to 95 ma resulting in drastic climatic re organisations in order to investigate the impact of plate tectonics and sea level change on climate we performed climatic simulations during the cenomanian 95 ma and aptian 120 ma using an atmospheric general circulation model problems with models lie in our knowledge of a few key factors for examples the middle cretaceous sea surface temperature gradient from equator to pole remains an open question and it is not clear yet how flat it was two sensitivity experiments for each period prescribing a flatter sst gradient based on oxygen isotopic data have been done in order to test its climatic consequences we discuss separately the effect of palaeogeography and meridional sst gradient impacts in the northern hemisphere the flooding of large continental areas due to sea level rise leads to a more equable climate in the mid latitudes however the impact of epicontinental seas varies seasonally from site to site due to local feedbacks in the southern hemisphere the breakup of gondwana deeply affects the climate of this region moderating the seasonal thermal variations in addition we test the impact of a flatter sst gradient on climate and we show that simulations with flat sst gradients compare well with field data we analyse the remaining model data discrepancies in terms of missing feedbacks co2 vegetation but also of new measurements that enable to infer a better meridional variation of sea surface temperature

geochemistry of precambrian carbonates vi aphebian albanel formations quebec canada abstract the 1 85 0 15 ga old albanel formations of northern quebec canada an example of paleoproterozoic carbonate buildups are lithologically composed of cherty dolostones subordinate limestones are encountered only near the base of the sequence coincident with maximal transgression the chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases such as chert quartz plagioclase illite k spar pyrite and fe mn oxides all abundant particularly in the basal part of the section the chemistry of the carbonate portion of limestones is strongly reset by post depositional phenomena but the relict high sr contents suggest that aragonite may have been an original precursor phase geochemically the carbonate portion of dolostones can be subdivided into two populations reflecting the stratigraphie bottom and top of the sequence trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures but in a more open system the latter reflects the greater lithological variability and permeability of the younger strata the 13c varies up section from about 3 to 1 perhaps a reflection of progressively more open marine circulation the associated four generations of post depositional pyrites have overlapping 34s from 7 1 to 54 with a mean of 24 a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and or replenishment the best measured 87 sr 86 sr of 0 70600 although possibly not a direct reflection of the coeval seawater may still be a near original value as indicated by its coherency with the roughly co stratigraphic paleo proterozoic sequences evaluation of the existing data indicates that strontium isotopic composition of the precambrian seawater was characterized by oscillations similar to those of the phanerozoic the interval of 0 8 1 5 ga ago being dominated by relatively nonradiogenic values followed by a radiogenic period at 1 6 2 2 ga ago near mantle values during the late archean and possibly another radiogenic peak at 3 5 ga ago

palaeoproterozoic 1 88 2 0 ga organic matter from the mugford kaumajet mountain group northern labrador abstract the palaeoproterozoic 1 88 2 0 ga mugford group of northern labrador is a relatively undeformed cover sequence developed on archaean crust of the nain craton the cod island formation occurs at the base of the group and consists of mixed shale chert dolostone and siltstone thin layers of highly matured organic material similar to anthracite have been found within a shale and chert sequence based on the nonmigrated and sedimentological context of the layers the material may represent some of the earliest intact in situ organic deposits known 13c isotope ratios of the organic matter in this deposit range from 31 79 to 32 22 and the h c atomic ratios from 0 05 to 0 08 the material represents the first truly biological remains known from the nain province of labrador and the earliest organic material in northeastern north america

testing early cretaceous africa south america fits with new palaeomagnetic data from the etendeka magmatic province namibia abstract we provide an independent test for the proposed pre breakup fits between the south american and african continental margins based on new palaeomagnetic data in the course of modelling the opening of the south atlantic ocean several reconstructions have been proposed which correspond to different estimates of the amount of pre drift extension in the south american and african continental margins and amounts of displacement along intracontinental accommodation zones the parana etendeka large igneous province erupted during a brief time interval between 136 and 131 ma just prior to opening of the south atlantic and has magmatic products on both the african etendeka lavas and south american parana lavas plates here we provide new palaeomagnetic constraints on the etendeka volcanic rocks these yield a declination of d dx 314 4 7 0 and inclination of i ix 42 7 8 3 plat 47 5 plon 88 9 k 26 1 a95 6 3 n 21 when our results are combined with another recently published dataset we obtain an etendeka palaeopole at 49 1 s 87 6 e d dx 315 9 3 8 i ix 43 9 4 4 k 23 4 a95 3 5 n 75 we compare three recent models for south atlantic ocean opening discuss which of the models best fits the palaeomagnetic data and present a revised reconstruction for west gondwana during the early cretaceous

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

toward age determination of the m0r barremian aptian boundary of the early cretaceous china most 973 program 2006cb701403 national natural science foundation of china 90714010 40573040 40634024 40221402 chinese academy of sciences kzcx3 sw 142

transient cooling episodes during cretaceous oceanic anoxic events with special reference to oae 1a early aptian the two major oceanic anoxic events of the cretaceous those of the early aptian oae 1a and the cenomanian turonian boundary oae 2 registered some of the highest temperatures reconstructed for the cretaceous period and are thought to be related to the input of volcanically derived carbon dioxide from one or more large igneous provinces widely distributed deposition of marine organic matter the hallmark of oaes and intensified silicate weathering in response to a globally accelerated hydrological cycle and or reaction of seawater with freshly extruded basalt are both potential mechanisms whereby the content of atmospheric carbon dioxide could have been drawn down to promote cooling on the assumption that this potential effect was not offset by increased addition of this volcanically derived greenhouse gas reduction in the supply of such carbon dioxide with deposition of organic matter and silicate weathering continuing at the same rate could also have produced cooling a transient fall in temperature and increase in marine dissolved oxygen levels is well documented for oae 2 in the form of the so called plenus cold event or benthic oxic event associated with southward invasion of certain boreal faunas and an increase in many redox sensitive and or chalcophilic elements in seawater caused by temporary loss of anoxic euxinic sinks as basalt seawater interaction continued apace high resolution studies of oae 1a show at least three cooling episodes of probable global distribution one of which recorded in the stratigraphy of the so called c4 segment is documented at high enough resolution to show correlation with an increase in carbon isotope values that was probably due to a rise in the quantities of organic matter being buried globally with consequent potential drawdown of atmospheric co2 and or reduction in volcanic input of this greenhouse gas both calcium and lithium isotope records suggest an increase in silicate weathering over the oae 1a interval but current relatively low resolution records cannot at present be tied to any one cooling episode although the lowest li isotope values do derive from the c4 segment evidence for reoxygenation of the ocean during the transient cooling episodes of oae 1a is meagre due to the lack of suitable sedimentary archives although a negative sulfur isotope excursion in a pacific shallow water carbonate section which can be interpreted as due to oxidation of pyrite and or sulfur rich organic matter in the global ocean suggests that this phenomenon may also have been a feature of the c4 segment further work is required to elucidate the similarities and differences between oae 2 and oae 1a but both phenomena are demonstrably global in reach and represent major disturbances to the carbon cycle with attendant effects on marine temperatures this article is part of a discussion meeting issue hyperthermals rapid and extreme global warming in our geological past

controls on organic carbon and molybdenum accumulation in cretaceous marine sediments from the cenomanian turonian interval including oceanic anoxic event 2 this study investigates the controls on organic carbon and molybdenum mo accumulation in sediments deposited within the western interior seaway across the cenomanian turonian boundary interval 94 34 93 04 ma including oceanic anoxic event 2 oae2 carbon fluxes to the sediment water interface reflecting changes in primary productivity and bottom water oxygen concentrations reflecting preservation effects are reconstructed from field data and used to constrain a benthic model that simulates the geochemistry of unconsolidated sediments as they were deposited the results show that increased availability of reactive iron prevents mo sequestration as thiomolybdate mos42 during oae2 o2 105 m by i inhibiting sulfate reduction and ii buffering any free sulfide that becomes available in the post oae2 period o2 50 m mo accumulation is favored by a large reduction in iron flux importantly this occurs in parallel with oxygenated bottom waters and high rates of aerobic carbon degradation in the surface sediments implying that elevated mo burial fluxes in ancient marine facies do not necessarily reflect euxinic or even anoxic conditions within the water column our findings suggest that both an increase in production and preservation lead to enrichment in organic carbon in the western interior seaway more generally the results demonstrate that a careful consideration of the coupling between iron carbon and oxygen cycles during the early stages of diagenesis is critical for interpreting geochemical proxies in modern and ancient settings

the aptian early cretaceous oceanic anoxic event oae1a in svalbard barents sea and the absolute age of the barremian aptian boundary abstract the age of the cretaceous barremian aptian boundary has been a long standing problem in the geological timescale age suggestions range between 126 ma and 117 ma making causal links between environmental crises and large scale processes such as large igneous provinces lips problematic new stratigraphic and radiometric data from three drillcores in svalbard show that barremian shelf siltstones and sandstones are overlain by aptian shallow marine mudstones the organic carbon content in the mudstones is higher than 1 0 wt with values ranging up to 70 wt in coal dominated intervals carbon isotope measurements of the toc show a negative 13 c excursion of 4 5 followed by a positive excursion of 6 7 within a 20 m interval identified as the early aptian oceanic anoxic event 1a oae1a this isotopic pattern of the excursion is in perfect agreement with excursions from lower latitude sections of the same age a 10 cm thick zircon bearing bentonite was discovered in the lower part of the barremian about 40 m below the oae1a interval u pb tims zircon analyses of two equivalent bentonites in adjacent boreholes gave a robust age of 123 1 0 3 ma based on the barremian depositional systems we estimate that a time interval of 1 2 myr separates the bentonite and the oae1a providing an age of about 121 122 ma for the barremian aptian boundary our results stress that the geological timescale needs revision and provide a benchmark test for the correlation between the oae1a and lips in general

a giant circumferential dyke swarm associated with the high arctic large igneous province halip abstract in this study we identify a giant circumferential mafic dyke swarm associated with the 135 75 ma high arctic large igneous province halip previously a halip giant radiating mafic dyke swarm with portions scattered across the canadian high arctic islands northern greenland svalbard and franz josef land was recognized in a pre drift plate tectonic reconstruction of the arctic region the radiating swarm has been interpreted to focus above a mantle plume responsible for halip magmatism the newly recognized halip giant circumferential swarm has a centre that is near the focus of the halip radiating system and hence is likely related to the halip plume elements of the circumferential swarm are located in each of the four regions where the radiating system is found the circumferential swarm has a quasi circular or slightly elliptical geometry an outer diameter of 1600 km and an arc of 220 it is one of the largest giant circumferential dyke swarms recognized on earth and could be linked to the outer edge of the flattening plume head it is also the first such swarm to have been identified by means of a plate tectonic reconstruction although giant circumferential dyke swarms appear to be relatively rare on earth possible analogues are common on venus and are also found on mars on venus giant circular or elliptical tectono magmatic features termed coronae are characterized by an annulus of graben or fissures and prominent topography some coronae include a radiating graben fissure system both radiating and circumferential graben may be underlain by dykes if so coronae could be analogues for terrestrial giant circumferential dyke swarms such as observed in the case of the halip

new perspective on aptian carbon isotope stratigraphy data from δ13c records of terrestrial organic matter carbon isotope analyses were performed on detrital woody materials in aptian mid cretaceous marine sediments of central hokkaido northern japan a positive 13cwood excursion from 25 4 to 21 8 following a remarkable negative isotope shift is recognized in the early aptian and a small positive anomaly is also found in the latest aptian this 13cwood profile is exactly in phase with the 13ccarbonate curve from a pacific guyot when the age of the guyot carbonates is revised using published sr isotope stratigraphies the highly conformable 13c profiles of pacific and hokkaido sections suggest that 13c compositions of aptian marine and terrestrial carbon reservoirs changed simultaneously by the same amplitude within the ocean atmosphere biosphere system fluctuation patterns of tethyan 13ccarbonate curves are slightly different from those of pacific and hokkaido sections such differences in 13c profiles may be attributed to the local paleoceanographic setting of the aptian mediterranean tethys

precambrian vein pyrobitumen evidence for petroleum generation and migration 2 ga ago abstract isotopic optical reflectance and chemical analyses of large pure samples of lower proterozoic vein pyrobitumens anthraxolite and thucholite of older reports collected from five widely separated localities in the great lakes region of the united states and canada suggest that in the lower proterozoic oil was generated from kerogen and migrated in much the same way as during the phanerozoic furthermore maturation and organic metamorphism followed the same successive stages that mark the generation and destruction of phanerozoic petroleum kerogens derived from algae and bacteria were deposited with subaqueous muds that ultimately became black shales or slates some of the kerogen however yielded petroleum which migrated into fractures and subsequently became pyrobitumen the 13c values average 32 6 pdb this value compares favorably with those from phanerozoic petroleum and asphaltites and is consistent with the proposition that the carbon isotopes were fractionated by biological activity reflectance values of the pyrobitumens range from 0 9 for elliot lake ontario to 9 5 for iron river michigan and h c ratios range from 0 6 for elliot lake to 0 1 for sudbury ontario when compared with the stages of petroleum maturation these values indicate maturation which ranges from the catagenesis stage through the metagenesis stage of petroleum generation and metamorphism

siberian gas venting and the end permian environmental crisis abstract the end of the permian period is marked by global warming and the biggest known mass extinction on earth the crisis is commonly attributed to the formation of the siberian traps large igneous province although the causal mechanisms remain disputed we show that heating of tunguska basin sediments by the ascending magma played a key role in triggering the crisis our conclusions are based on extensive field work in siberia in 2004 and 2006 heating of organic rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated 100 000 gt co2 the gases were released to the end permian atmosphere partly through spectacular pipe structures with kilometre sized craters dating of a sill intrusion by the u pb method shows that the gas release occurred at 252 0 0 4 million years ago overlapping in time with the end permian global warming and mass extinction heating experiments to 275 c on petroleum bearing rock salt from siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases the results indicate that global warming and ozone depletion were the two main drivers for the end permian environmental crisis we demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a large igneous provinces causes an environmental crisis or not we propose that a similar mechanism could have been responsible for the triassic jurassic 200 ma global warming and mass extinction based on the presence of thick sill intrusions in the evaporite deposits of the amazon basin in brazil

dinocyst stratigraphy of the valanginian aptian rurikfjellet and helvetiafjellet formations on spitsbergen arctic norway

release of methane from a volcanic basin as a mechanism for initial eocene global warming a 200 000 yr interval of extreme global warming marked the start of the eocene epoch about 55 million years ago negative carbon and oxygen isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid approximately 10 000 yr input of isotopically depleted carbon it has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change large scale hydrate melting however requires a hitherto unknown triggering mechanism here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the voring and more basins in the norwegian sea we propose that intrusion of voluminous mantle derived melts in carbon rich sedimentary strata in the northeast atlantic may have caused an explosive release of methane transported to the ocean or atmosphere through the vent complexes close to the palaeocene eocene boundary similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the siberian traps approximately 250 million years ago and the karoo igneous province approximately 183 million years ago

fracture characterization in upper permian carbonates in spitsbergen a workflow from digital outcrop to geo model abstract carbonates represent major hydrocarbon reservoirs but often exhibit highly heterogeneous reservoir properties outcrop analogues provide important insights into how parameters such as porosity permeability and natural fractures vary as such outcrops can bridge the scale gap between spatially extensive but poor resolution seismic data and 1d high resolution well data however traditional geological fieldwork typically gathers insufficient data to construct robust geological models in this study we have specifically set out to gather key data sets that enable the construction of a geology driven model we illustrate this workflow using the exceptionally well exposed carbonate dominated outcrops of the kapp starostin formation in central spitsbergen arctic norway we fully utilize emerging technologies notably geo referenced digital outcrop models doms to be able to gather quantitative sedimentological structural data from otherwise inaccessible cliffs doms generated from digital photos are used directly for automatic and manual mapping of fractures the digital data are complemented with traditional fieldwork sedimentological logging scanlines structural characterization in order to strengthen the dataset the geo modelling involves traditional facies and petrophysical modelling of the 12 identified facies along with outcrop based discrete fracture modelling finally the static geo model is upscaled and its applications are discussed the presented workflow uses carbonate outcrops of the kapp starostin formation as input but is highly applicable for other studies where outcrops can be utilized as direct input to constrain a geological model