geochemistry of major and trace elements and pb sr isotopes of a weathering profile developed on the lower cambrian black shales in central hunan china abstract this paper reports a geochemical study on the major and trace elements and pb sr isotopes of a weathering profile developed in the lower cambrian black shales in central hunan china six weathering horizons were identified and sampled vertically throughout the profile the chemical composition of the profile consists of variable concentrations of the major elements fe2o3 feo mno mgo cao na2o and p2o5 and of less variable concentrations of sio2 tio2 al2o3 and k2o the chemical change caused by weathering is estimated by mass balance calculations and the results show that the element mobility is characterised by substantial loss of sio2 feo cao k2o na2o loi cr v ba cs rb sr u and th and moderate loss of al2o3 mgo fe2o3 ni cu pb tl sn sc ge and ree y the high field strength elements tio2 sn sc u ga ge zr hf nb and ta were immobile during weathering the chemical changes and the pb sr isotopic data suggest that four types of chemical reactions occurred the oxidation of sulphide minerals e g pyrite and organic carbon os the dissolution of less resistant clinochlore ia calcite and p bearing minerals dl the dissolution of detrital albite and microcline da and the transformation of clay tc minerals e g muscovite and illite smectite these chemical reactions then led to two stages of geochemical processes an early stage of chemical differentiation and a later stage of chemical homogenisation the chemical differentiation dominated by the os dl and da reactions led to the leaching of mobile elements e g mgo na2o k2o p2o5 sr and ree and the redistribution of some less mobile elements e g sio2 and al2o3 in contrast the chemical homogenisation which was caused by tc reactions led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil soils derived from black shales in south china might result from the above two geochemical processes

coupled re os and u pb geochronology of the tonian chuar group grand canyon well preserved strata of the late tonian chuar group exposed in the grand canyon host fossil evidence for the development of eukaryotic predation the presence of unique biomarkers and large changes in c s and mo isotope chemostratigraphy despite the importance of this critical succession few radioisotopic age constraints are available to place these records into a global context here we couple high precision u pb chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims on zircon crystals with the rhenium osmium re os sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of earth history zircons recovered from a tuff within the uppermost walcott member of the kwagunt formation yield a weighted mean 206pb 238u age of 729 0 0 9 ma mean square of weighted deviates mswd 0 86 differing significantly from the previous air abrasion upper intercept age of 742 6 ma on zircons from this same horizon organic rich carbonates from the carbon canyon member of the galeros formation yield a model 1 re os age of 757 0 6 8 ma mswd 0 47 n 8 and an initial os isotope 187os 188os osi composition of 1 13 0 02 the radiogenic osi value from this horizon suggests that the basin was restricted from the open ocean during deposition of the carbon canyon member in agreement with sedimentological and stratigraphic data the re os geochronology of marcasite fes2 nodules from the awatubi member of the kwagunt formation yield a model 1 age of 751 0 7 6 ma mswd 0 37 n 5 with an osi of 0 44 0 01 this re os date is interpreted to constrain the growth of the marcasite nodules in the awatubi member during deposition the formation of sulfides and the less radiogenic osi value are consistent with an influx of sulfate laden seawater to the basin during deposition of the kwagunt formation attempts to apply the re os geochronometer to the walcott and tanner members of the chuar group failed to yield meaningful ages despite elevated re enrichments 20 ng g the re os data from these units yielded negative osi values which suggest disturbance to the re os system the low os abundances typically 100 pg g relative to the amount expected based on the elevated re abundances suggest leaching of os due to oxidative weathering on geologically recent time scales finally the carbon canyon member provides a useful case study for quantifying how input uncertainties in the re os geochronometer propagate into the resulting age uncertainty and we discuss the protocols that will yield the best improvement in age precision for future studies the u pb and re os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods our analysis suggests that for our data the most efficient way of reducing uncertainties in the presented re os dates is through improved precision of measured os values

germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

the geochemistry of rhenium and osmium in recent sediments from the black sea abstract the os isotopic composition of recent sediments from the black sea are reported with the concentrations of re os and major and minor elements osmium concentrations range from 0 23 to 0 69 ppb and re concentrations range from 21 to 85 ppb concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon 187 os 186 os ratios range from 5 7 to 7 1 all of these values are less radiogenic than the estimated 187 os 186 os ratio of seawater and average continental crust material variations in the os isotopic composition of these sediments are attributed to mixing of a hydrogenous component 187 os 186 os ratio between 7 and 9 and a less radiogenic detrital component 187 os 186 os ratio between 1 and 4 a mixing model is used to determine the fraction of os in the sediment associated with each component and the burial fluxes of re and os in the black sea are calculated in addition the data are used to estimate the magnitude of systematic error which such initial os isotopic heterogeneity may introduce into re os age determinations of ancient organic rich sediments

re os isotope systematics and weathering of precambrian crustal rocks implications for the marine osmium isotope record the effects of weathering of freshly exposed precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western wind river range in wyoming the os budget of the precambrian granitoid source rocks is dominated by os rich trace phases such as magnetite amongst the major silicates biotite or a trace phase in biotite carries most of the os and re and is also the most radiogenic mineral with 187os 186os of 113 and 187re 186os of 2 000 re os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes os with an isotopic composition similar to the isotopic composition of the bulk soils a very radiogenic fraction of os is mobilized through preferential weathering of biotite radiogenic runoff from precambrian shields inferred from osmium isotope analyses of freshwater fe mn nodules suggests that high latitude precambrian shields are important source areas of radiogenic os in seawater we propose that glacial scouring and weathering of glacial tills exposed after deglaciation of precambrian shields surrounding the north atlantic provides a mechanism for the slightly more radiogenic nature of north atlantic seawater compared to other seawater masses glacial interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial interglacial time scales in high latitude shield areas surrounding the north atlantic

petroleum source rock identification of united kingdom atlantic margin oil fields and the western canadian oil sands using platinum palladium osmium and rhenium implications for global petroleum systems this study demonstrates that petroleum and source rocks are enriched in pt and pd to the ppb level and that the 187os 188os composition coupled with the pt pd value permits the fingerprinting of petroleum to its source oils from the united kingdom atlantic margin sourced from the upper jurassic kimmeridge clay fm as well as source rock samples have been analysed for pt and pd when the pt pd value is compared with 187os 188os calculated at the time of oil generation osg the values from both the known source and the oils are similar demonstrating that they can be used as an oil to source fingerprinting tool this inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques e g biomarkers are severely hampered e g the world s largest oil sand deposit the west canadian oil sands wcos this has caused the source of the wcos to be hotly debated with no present day consensus between inputs from potential source units e g exshaw and gordondale fms 187os 188os and pt pd fingerprinting of the oil sands shows that the majority of the petroleum have similar 187os 188os and pt pd values supporting the hypothesis of one principal source analysis of the potential source rocks establishes that the principal source of the oil sands to be from the jurassic gordondale fm with a minor exshaw fm input thus the combination of previously pioneered re os petroleum geochronology with 187os 188os and pt pd values of petroleum permits both a temporal and spatial understanding of petroleum systems

effects of black shale weathering on the mobility of rhenium and platinum group elements radiogenic os from the continents dominates the late cenozoic marine os budget a variety of proposed radiogenic sources includes organic rich sediments precambrian shields and re rich sulfides identifying the dominant sources of radiogenic os to the oceans has a strong impact on past climate models because weathering of silicates acts as a net sink of co 2 whereas weathering of organic matter acts as a source here we investigate the effects of black shale weathering on the integrity of the re os isotope system and release of re and platinum group elements time correlative pairs of fresh drill core and weathered outcrop black shales of late ordovician age utica shale magnafacies quebec were analyzed for re os ir pt and pd as well as total s n and c as c carb and c org the results indicate that 25 64 of the initial re budget 45 90 of the os budget 14 65 of the ir budget 60 77 of the pt budget 69 86 of the pd budget and 44 96 of the c org budget are lost during weathering we use positive correlations of os pt and pd with c org and n to calculate the respective platinum group element concentrations in the unweathered organic matter end member as 3 ng g os 9 ng g pt and 11 24 ng g pd although black shales make up

the record of sea water 187os 186os variation through the cenozoic the variation of sea water 187os 186os during the cenozoic has been determined by analyzing the hydrogenous os released by acidic hydrogen peroxide leaching of layers in an abyssal north pacific pelagic clay core overall sea water 187os 186os has increased from 3 2 at 58 ma to the present day value of 8 6 this pattern is similar to the well known 87sr 86sr evolution of sea water the increase in the proportion of continental os 187os 186os 10 could be due to enhanced weathering rates of black shales associated with the uplift of the himalayas other elements associated with black shale weathering such as u and p should show effects in the sedimentary record

influence of palaeoweathering on trace metal concentrations and environmental proxies in black shales the mineralogical and chemical compositions of lower carboniferous tournaisian marine black shale from the kowala quarry the holy cross mountains poland were investigated this study focuses on disturbances in palaeoenvironmental proxies caused by palaeoweathering which progressively changed the major and trace element abundances palaeomagnetic investigations reveal that the devonian carboniferous succession was weathered during the permian triassic by the infiltration of oxidizing fluids related to karstification following post variscan exhumation the weathering process led to vermiculitization of chlorite partial dissolution of calcite and replacement of pyrite by hematite and goethite moreover the concentrations of some trace metals including co cu pb mo ni as and u significantly decreased consequently some elemental abundance ratios that are used as environmental proxies including u th ni co and v cr were altered elements that are bound to iron sulphides e g mo appear to be especially prone to mobilization by even a lightly weathered black shale the documented weathering including changes in elemental concentrations can potentially create misinterpretations of the original palaeoenvironmental conditions in addition the palaeoweathering of the studied samples appears to have substantially changed the carbon oxygen nitrogen and molybdenum stable isotope values the nitrogen and molybdenum stable isotope ratios in particular appear to be most sensitive to the effects of weathering and therefore are good indicators of palaeo weathering processes the major cause of these changes is the decay of organic matter and pyrite for the organic carbon stable isotopes ratios the main factor that controlls this process appears to be the preferential degradation of labile organic matter a combination of the total organic carbon toc total sulphur ts content mo concentration and stable isotope compositions seems to be the most useful for identify palaeo weathering our results suggest that reductions in ts and mo in tandem with diminished mo stable isotope values in the absence of obvious changes to the toc content provide the most compelling evidence of palaeo weathering

re os depositional ages and seawater os estimates for the frasnian famennian boundary implications for weathering rates land plant evolution and extinction mechanisms four toc rich shale intervals spanning the frasnian famennian f f boundary were recovered in a drillcore west valley nx 1 from western new york usa and radiometrically dated using re os two of the black shale intervals wvc785 from 2 9 m below and wvc754 from 6 4 m above the f f boundary respectively yielded statistically overlapping ages with uncertainties of 1 1 an interpolated age and associated graphically determined uncertainty of 372 4 3 8 ma provides new absolute age constraints on the f f boundary this date is 4 1 ma younger than the latest proposed f f boundary age of 376 1 ma obtained by interpolation of u pb dates from volcanic zircon kaufmann b 2006 calibrating the devonian time scale a synthesis of u pb id tims ages and conodont stratigraphy earth science reviews 76 175 190 and within uncertainty of the international commission on stratigraphy accepted date of 374 5 2 6 ma a third date from sample wvc802 8 2 m beneath the f f boundary yielded an imprecise age of 357 23 ma owing in part to a limited re os range the initial 187os 188os 0 45 to 0 47 reflecting contemporaneous seawater os values are low but similar to the value of 0 42 reported for the exshaw fm canada at the devonian mississippian boundary ca 361 ma selby d creaser r a 2005 direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer geology 33 545 548 this may suggest fairly constant and low global continental weathering rates during the late devonian although in view of the short residence time of os in seawater 1 4 104 yr further measurements are needed to assess potential short term variation in seawater os ratios owing to low os and re abundances at the f f boundary our data are inconsistent with long term volcanism and bolide impact as potential late devonian mass extinction mechanisms in addition the frasnian famennian ocean appears to have been depleted with respect to re possibly indicating an exhaustion of the re seawater reservoir owing to high burial rates of redox sensitive elements under dysoxic anoxic conditions leading up to the f f boundary

rhenium and osmium isotope and elemental behaviour accompanying laterite formation in the deccan region of india this study presents major trace element and re os isotope data for two laterite profiles from the deccan region in india the first profile from bidar developed upon deccan basalt itself the second from goa formed on an archean greywacke both profiles show the typical changes in mineralogy and chemistry accompanying laterite formation from little altered parent rock at depth to altered saprolite depleted in mobile elements to a highly altered si depleted fe rich laterite capped by an iron rich crust at the surface rhenium and os concentrations show a systematic enrichment with increasing degree of alteration where the highest concentrations are found in the fe rich laterite crust the level of os enrichment is greater than that of re and as a consequence 187re 188os ratios are lower in the laterite than the parent rock with time this leads to the development of 187os 188os ratios that are also less radiogenic than the parent rock and this relationship holds irrespective of rock type or age or the time of laterite formation the marked enrichment of both re and os and the patterns of change above and below the paleowatertable indicates that both elements are highly mobile high eh and low ph conditions facilitate re and os dissolution and redistribution occurs via groundwaters whereas with increasing ph higher in the profile both elements are scavenged out of solution and precipitate either as pge rich alloys or are strongly partitioned into oxides or oxyhydroxides nevertheless the isotope and elemental data for both profiles suggests that at least some of the re and os must be externally derived either from aeolian deposition or from the lateral movement of solute laden groundwaters given the high levels of os in laterites their extensive geographical coverage of the continents and their active role in the surface water cycle it seems likely that they will exert a significant influence on the continental flux of os to the oceans that will in turn be sensitive to tectonic or climatic change

geochemistry of soils derived from black shales in the ganziping mine area western hunan china the geochemistry of major and trace elements including heavy metals and rare earth elements of the fresh and weathered black shales and the soils derived from black shales in the ganziping mine area in western hunan province china were studied using the following techniques x ray fluorescence xrf inductively coupled plasma mass spectrometer icp ms and x ray diffraction xrd the results show that the black shale soils are significantly enriched with al2o3 and fe2o3 and depleted of mobile elements cao na2o and k2o the soils are also highly enriched with heavy metals u v ni ba cu zn and pb that may cause potential heavy metal contamination of the soils composition of the soils is homogeneous compared to the weathered black shales for which the concentrations of major elements except cao and na2o and trace elements except heavy metals u v ni ba cu zn and pb as well as the mobile sr show lower variations than in the weathered black shales ratios of zr hf ta nb y ho nd sm and ti ti zr of the soils are also less variable with values constantly similar to that of the fresh and weathered black shales correspondingly thus components of the soils are believed to be contributed from the parent black shales through weathering and pedogenesis it is concluded that the soils were formed by at least two stages of geochemical processes the early stage of chemical differentiation and the later stage of chemical homogenization the chemical differentiation that was taken during black shale weathering might have caused the depletion of cao and na2o and the enrichment of al2o3 and fe2o3 while the chemical homogenization that was taken during pedogenesis led to the depletion of sio2 and k2o and to the further enrichment of al2o3 and fe2o3 the heavy metal enrichment contamination of the soils was then genetically related to the enrichment of al2o3 and fe2o3 in the soils

north atlantic geoid high volcanism and glaciations 1 shallow topography geoid high and intense volcanism in the northern mid atlantic ridge are interpreted as enhanced by the loading on the adjacent continents by ice caps during upper cenozoic glaciations the load of ice packs on the continental lithospheres of north america and northern europe generated radial mantle flow at depth in our model these currents where flowing from west and east faced each other below the northern atlantic joining together and upwelling numerical modeling of this process supports the development of dynamic topography leading to uplift of the sea floor and inducing a regional geoid high the upper mantle being pumped from the deep mantle and rising to a few km shallower than average may have contributed to larger asthenospheric melting and to ridge centered excess magmatism as observed in the northern atlantic

the osmium isotopic composition of organic rich marine sediments abstract the osmium os concentration and 187 os 186 os ratio of several recent marine organic rich sediment samples from three widely separated sites have been measured os concentrations range from 0 095 to 0 212 ppb and 187 os 186 os ratios range from 8 2 to 8 9 the calculated fraction of hydrogenous os exceeds 78 in all samples thus the 187 os 186 os ratio of these samples reflects os isotopic composition of seawater the small range in measured 187 os 186 os ratio indicates that the os isotopic composition at these sites is fairly homogeneous the large magnitude of the os burial flux at these sites indicates the os burial in association with organic rich sediments is an important sink in the marine cycle of os these data also suggest that ancient organic rich sediments may provide a record of past variations in the os isotopic composition of seawater

extent and isotopic composition of fe and mo release from two pennsylvania shales in the presence of organic ligands and bacteria abstract isotopic fractionation of fe and mo during weathering could contribute toward the isotopic signatures of river and ocean waters to investigate weathering processes batch experiments were carried out at ph 6 under oxygenated conditions to investigate the influence of a nitrogen fixing soil bacterium and organic ligands on the extent and isotopic signature of fe and mo release from two pennsylvania shales with different mineralogies an olive to grey shale rose hill and a black shale marcellus results of these studies showed that fe and mo were mainly released from illite chlorite vermiculite and fe oxides in the rhs and from pyrite in the ms dissolution rates of the clays estimated from our batch experiments are broadly consistent with published estimates release of fe from both shales was only enhanced by the hydroxamate siderophore desferrioxamine b dfam in contrast none of the treatments enhanced release of mo furthermore release of this metal was measurable only for the black shale for both shales fe that was released in the presence of dfam was depleted in 56 fe as compared to the bulk rock in contrast leachate solutions were enriched in the heavier mo isotope as compared to the bulk marcellus in all experiments with and without organic ligands and the bacterium the isotopic fractionations in fe and mo are broadly consistent with previously reported fractionation effects during sorption onto precipitating fe mn oxides the observed mo isotope fractionation is also consistent with all published studies of rivers i e riverine mo is enriched in heavy mo isotopes the observed fe isotope fractionation is likewise consistent with published studies for some rivers i e riverine fe is sometimes depleted in 56 fe our experimental results are consistent with sorption of fe and mo onto fe oxyhydr oxides during weathering as a possible explanation for the isotopic signatures of these metals in many rivers

recristallisation des calcaires micritiques en fonction de la maturation thermique dans les basses terres du saint laurent du quebec typical sections of the ordovician carbonate platform sequence in the st lawrence lowlands beekmantown chazy black river trenton and utica groups were sampled at neuville joliette and montreal in the province of quebec the purpose of this work was to determine the level of thermal maturation of the carbonates using organic matter and clay studies and to correlate it with the crystal characteristics of the microcrystalline calcite the studied formations are supermature in the joliette and montreal areas whereas they are only mature at neuville the numerical analysis of the petrographic data shows that some microstructures observed in the rock samples and the size and morphology of the microcrystalline calcite crystals are partly related to the thermal maturation as determined by other methods

platinum group elements pge and rhenium in marine sediments across the cretaceous tertiary boundary constraints on re pge transport in the marine environment the nature of re platinum group element pge pt pd ir os ru transport in the marine environment was investigated by means of marine sediments at and across the cretaceous tertiary boundary ktb at two hemipelagic sites in europe and two pelagic sites in the north and south pacific a traverse across the ktb in the south pacific pelagic clay core found elevated levels of re pt ir os and ru each of which is approximately symmetrically distributed over a distance of 1 8 m across the ktb the re pge abundance patterns are fractionated from chondritic relative abundances ru pt pd and re contents are slightly subchondritic relative to ir and os is depleted by 95 relative to chondritic ir proportions a similar depletion in os 90 was found in a sample of the pelagic ktb in the north pacific but it is enriched in ru pt pd and re relative to ir the two hemipelagic ktb clays have near chondritic abundance patterns the 1 8 m wide re pge peak in the pelagic south pacific section cannot be reconciled with the fallout of a single impactor indicating that postdepositional redistribution has occurred the elemental profiles appear to fit diffusion profiles although bioturbation could have also played a role if diffusion had occurred over 65 ma the effective diffusivities are 10 13 cm 2 s much smaller than that of soluble cations in pore waters 10 6 cm 2 s the coupling of re and the pges during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances if redistribution was caused by diffusion then the effective diffusivities are the same fractionation of os from ir during the ktb interval must therefore have occurred during aqueous transport in the marine environment distinctly subchondritic os ir ratios throughout the cenozoic in the south pacific core further suggest that fractionation of os from ir in the marine environment is a general process throughout geologic time because most of the inputs of os and ir into the ocean have os ir ratios 1 mass balance calculations show that os and re burial fluxes in pelagic sediments account for only a small fraction of the riverine os 10 and re 0 1 inputs into the oceans in contrast burial of ir in pelagic sediments is similar to the riverine ir input indicating that pelagic sediments are a much larger repository for ir than for os and re if all of the missing os and re is assumed to reside in anoxic sediments in oceanic margins the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes however putting all of the missing os and re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of os at pelagic ktb sites where at most 25 of the k t impactor s os could have passed through estuaries if os is preferentially sequestered in anoxic marine environments it follows that the os ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition there is thus a clear fractionation of os and re from ir in precipitation out of sea water in pelagic sections accordingly it is inferred here that re and os are removed from sea water in anoxic marine depositional regimes

the isotopic composition of leachable osmium from river sediments the 187os186os ratios of acidic hydrogen peroxide leaches of river sediments are inferred to be representative of the isotopic composition of os delivered to sea water from the continents the 187os186os values for river sediment leaches from the eastern and central u s the ganges river and the rio maipo range from 10 1 to 21 5 these results suggest that the 187os186os of the soluble river load delivered from the major continental areas to the oceans is on average more radiogenic than average currently eroding continental crust 10 5 sediment leaches from three rivers from the western u s that drain ophiolitic peridotites have distinctly lower 187os186os values 1 4 7 1 such rivers may be significant contributors to sea water of the relatively unradiogenic osmium required to yield the contemporary oceanic 187os186os of 8 6

a reactive transport model for marcellus shale weathering abstract shale formations account for 25 of the land surface globally and contribute a large proportion of the natural gas used in the united states one of the most productive shale gas formations is the marcellus a black shale that is rich in organic matter and pyrite as a first step toward understanding how marcellus shale interacts with water in the surface or deep subsurface we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions constrained by soil and water chemistry data the simulation was carried out for 10 000 years since deglaciation assuming bedrock weathering and soil genesis began after the last glacial maximum results indicate weathering was initiated by pyrite dissolution for the first 1000 years leading to low ph and enhanced dissolution of chlorite and precipitation of iron hydroxides after pyrite depletion chlorite dissolved slowly primarily facilitated by the presence of co2 and organic acids forming vermiculite as a secondary mineral a sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases co2 and o2 specific surface area and flow velocity of infiltrating meteoric water the soil chemistry and mineralogy data could not be reproduced without including the reactive gases for example pyrite remained in the soil even after 10 000 years if o2 was not continuously present in the soil column likewise chlorite remained abundant and porosity remained small if co2 was not present in the soil gas the field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1 3 orders of magnitude smaller than surface area values measured for powdered minerals small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings in addition some mineral surface is likely interacting only with equilibrated pore fluids an increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far from equilibrium conditions we conclude from these observations that availability of reactive surface area and transport of h2o and gases are the most important factors affecting rates of marcellus shale weathering of the in the shallow subsurface this weathering study documents the utility of reactive transport modeling for complex subsurface processes such modelling could be extended to understand interactions between injected fluids and marcellus shale gas reservoirs at higher temperature pressure and salinity conditions

effect of oxygen co injected with carbon dioxide on gothic shale caprock co2 brine interaction during geologic carbon sequestration abstract co injection of oxygen a significant component in co 2 streams produced by the oxyfuel combustion process can cause a significant alteration of the redox state in deep geologic formations during geologic carbon sequestration the potential impact of co injected oxygen on the interaction between synthetic co 2 brine 0 1 m nacl and shale caprock gothic shale from the aneth unit in utah and mobilization of trace metals was investigated at 10 mpa and 75 c a range of relative volume percentages of o 2 to co 2 0 1 4 and 8 were used in these experiments to address the effect of oxygen on shale co 2 brine interaction under various conditions major mineral phases in gothic shale are quartz calcite dolomite montmorillonite and pyrite during gothic shale co 2 brine interaction in the presence of oxygen pyrite oxidation occurred extensively and caused enhanced dissolution of calcite and dolomite pyrite oxidation and calcite dissolution subsequently resulted in the precipitation of fe iii oxides and gypsum caso 4 2h 2 o in the presence of oxygen dissolved mn and ni were elevated because of oxidative dissolution of pyrite the mobility of dissolved ba was controlled by barite baso 4 precipitation in the presence of oxygen dissolved u in the experimental brines increased to 8 14 g l with concentrations being slightly higher in the absence of oxygen than in the presence of oxygen experimental and modeling results indicate the interaction between shale caprock and oxygen co injected with co 2 during geologic carbon sequestration can exert significant impacts on brine ph solubility of carbonate minerals stability of sulfide minerals and mobility of trace metals the major impact of oxygen is most likely to occur in the zone near co 2 injection wells where impurity gases can accumulate oxygen in co 2 brine migrating away from the injection well will be continually consumed through the reactions with sulfide minerals in deep geologic formations

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

re os isotope systematics in black shales from the lesser himalaya their chronology and role in the 187os 188os evolution of seawater abstract re and os abundances and os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines maldeota and durmala in the lesser himalaya the re and os concentrations in these samples exhibit wide range 0 2 to 264 ng g and 0 02 to 13 5 ng g respectively with a mean re os of 25 the 187 os 188 os of these samples also show a broad range 1 02 to 11 6 with many of them far more radiogenic than the currently eroding continental crust the black shales from the maldeota and durmala mines collected a few meters above the krol tal pc c boundary and occurring in the same stratigraphic horizon yield 187 re 187 os isochron ages of 554 16 and 552 22 ma respectively consistent with fossil evidences and those assigned for the pc c boundary at various other locations these results indicate closed system behaviour of re and os in these mine samples and reaffirm the validity of 187 re 187 os pair to date organic rich sediments the age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary re os age of 839 138 ma for black shales from theog collected from the inner belt these initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt however more analyses of black shales are needed to confirm this the role of weathering black shales from the himalaya in contributing to the os isotope evolution of seawater over the past 25 ma was assessed using a simple budget model for seawater os and assuming values for os fluxes and 187 os 188 os based on the available limited data on os concentration and its 187 os 188 os in rivers and oceans these calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187 os 188 os over the past 16 ma is difficult to satisfy by varying only the os flux from htp rivers 187 os 188 os kept constant at 1 324 or their 187 os 188 os 188 os flux constant at today s value 131 moles yr the requirements to balance os isotope budget in ocean through weathering of black shales however is easier to accommodate if both the os flux from htp himalayan tibetan plateau rivers and their 187 os 188 os are allowed to vary measurements of os abundance in htp rivers and its isotopic composition are needed to test the validity of the model results

osmium in marine sediments osmium is enriched in ferromanganese minerals and its concentrations follow those of other pt group metals except pd oxidative capture during solid formation is the probable mechanism for uptake in some sediments forming under reducing conditions phosphorites and anoxic coastal deposits there is also an enrichment this may relate to reductive capture although deposits also formed under reducing conditions from hydrothermal areas are depleted in osmium

himalayan uplift and osmium isotopes in oceans and rivers previous studies have shown that 187 os 188 os in seawater has become increasingly radiogenic over the last 40 ma in a manner analogous to strontium this rapid rise in the marine 187 os 188 os over the last 17 ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the himalayas and or selective weathering and erosion of highly radiogenic organic rich ancient sediments the key test of this hypothesis is the 187 os 188 os and the total osmium concentration of the himalayan rivers we report the concentration and isotopic composition of osmium in the ganges the brahmaputra and the indus rivers the 187 os 188 os of the ganges close to its source at kaudiyal 30 05 n 78 50 e is 2 65 and os 45 fm kg a second sample of the lower reaches of the ganges at patna 25 30 n 85 10 e gives 187 os 188 os 1 59 and os 171 fm kg the 187 os 188 os of the brahmaputra at guwahati 26 10 n 91 58 e is 1 07 and os 52 fm kg a sample of the indus besham 34 55 n 72 51 e has a 187 os 188 os of 1 2 and os 59 fm kg we infer that the himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans the overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the himalayas themselves appear not to be the dominant source

structural evolution of djebel debadib anticline a clue to the regional tectonic style of the tunisian atlas djebel debadib is a northeast trending anticline in the tunisian atlas near el kef tunisia the core of the anticline consists of a deformed triassic gypsum matrix breccia containing clasts of supratidal dolomite terrigeneous clastic rocks and ophites metabasites locally low grade metasedimentary rocks occurring as clasts in the evaporite may represent fragments of basement rock plucked from salt subcrop during deep seated flowage the multiply deformed triassic rocks are juxtaposed against cretaceous or younger rocks and jurassic and neocomian strata are not present in outcrop these characteristics have been cited in many studies to suggest a history of diapirism to explain both the internal structure of the triassic rocks and the contact relations with the younger rocks a previously unreported unconformity developed on the triassic rocks and overlain by aptian albian strata indicates that an important period of piercement occurred during the early cretaceous for the djebel debadib diapir from the mid cretaceous to the late eocene the continental margin of northern tunisia was a regional basin in which a thick sequence of carbonate rocks was deposited beginning in late oligocene time sandy clastic sediments accumulated on the older carbonate rocks above a regional unconformity and during the middle miocene atlas orogeny numerous northeast southwest folds deformed the rocks of the tunisian trough including the diapiric triassic masses these folds commonly boxlike in geometry were extensively faulted during the late miocene and pliocene a four phase tectonic model is recognized for the tunisian atlas 1 early mesozoic rifting of the north african continental margin 2 diapiric emplacement of the late triassic liassic evaporites into overlying strata beginning in the early cretaceous 3 folding of the cover strata in response to regional compression culminating in the middle miocene atlas orogeny 4 the development of transvers grabens during late orogenic normal faulting latest miocene to recent a major regional decollement is not considered necessary to account for the shortening of the sedimentary cover during the middle miocene deformation rather the principal driving mechanism of regional crustal shortening was the reactivation of early mesozoic normal faults as reverse faults that displaced basement rocks this model implies that the small amount of shortening recorded in the tunisian atlas involved active shortening and structural thickening of basement and the concurrent deformation of the sedimentary cover

behavior of re and os during low temperature alteration results from himalayan soils and altered black shales re os analyses were performed on five soil profiles developed on silicate lithologies in the narayani drainage basin of the himalayas as well as on several altered black shales and associated alteration phases the results indicate that all of these samples have lost large amounts of re this re loss approached 100 in the black shale derived soils which also most probably lost substantial quantities of os the near constancy of os and re concentrations with depth indicates that this loss occurred quite early in the alteration process rather than progressively during the course of soil development a slight decrease in 187os 188os was observed toward the surface of the two black shale soil profiles possibly indicating a minor preferential mobility of radiogenic 187os for this lithology in the black shale derived saprolite and soils the loss of re and os was related to the loss of organic carbon the importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the os contained in the whole rock analysis of alteration phases from the surface of a weathered black shale showed that os liberated during alteration was partially reprecipitated in fe oxides with os isotopic compositions very similar to that of the original rock whereas re was removed with the fluid phase these results demonstrate the disproportionately large role that black shale weathering may play in determining the re and os contents of continental runoff and thus ultimately of seawater but they also emphasize the possible complexity of this process the data suggest that a large fraction of the os contained in black shales is released during chemical alteration which is apparently not the case for most other silicate rocks therefore the potential contribution of black shales to the dissolved os budget of runoff and river water may be even greater than that expected from the high os concentrations of these rocks the extent to which this os will affect the seawater os composition depends on how efficiently it is recaptured by secondary phases such as fe mn oxides in situ or during transport these results also underscore the high mobility of re with respect to os at every stage in the erosional process thus explaining the order of magnitude increase in re os ratio from the black shale source rocks to seawater

rhenium and osmium isotopes in black shales and ni mo pge rich sulfide layers yukon territory canada and hunan and guizhou provinces china rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of devonian black shale and an associated ni enriched sulfide layer from the yukon territory canada the same composition information was also obtained for samples of early cambrian ni mo rich sulfide layers hosted in black shale in guizhou and hunan provinces china this study was undertaken to constrain the origin of the pge enrichment in the sulfide layers samples of the ni sulfide layer from the yukon territory are highly enriched in re os and other pge with distinctly higher re 192os but similar pt re compared to the black shale host re os isotopic data of the black shale and the sulfide layer are approximately isochronous and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales samples of the chinese sulfide layers are also highly enriched in re os and the other pge re 192os are lower than in the yukon sulfide layer re os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 ma similar to the depositional age of the black shale host the osmium isotopic data suggest that re and pge enrichment of the brecciated sulfide layers in both the yukon territory and in southern china may have occurred near the time of sediment deposition or during early diagenesis during the middle to late devonian and early cambrian respectively

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

the importance of organic rich shales to the geochemical cycles of rhenium and osmium abstract the role that marine organic rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high re os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust the high re os leads to rapidly increasing 187os 188os ratios we determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187re 188os and 187os 188os ratios of organic rich sediments we then utilize estimates of the abundance of organic rich sediments in the geologic record of different ages to estimate an average 187os 188os for these sediments we find that the calculated 187os 188os value of organic rich sediments varies between 2 22 and 2 27 depending on the volume age model used with error estimates of 2 07 to 2 42 and 2 12 to 2 48 respectively despite the fact that these sediments constitute a relatively small proportion of the upper continental crust they contribute a disproportionately large amount 50 to the upper continental crustal rhenium and to a lesser extent 5 10 osmium inventories owing to their high concentrations of these elements

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

a complete os excursion across a terrestrial cretaceous paleogene boundary at the west bijou site colorado with evidence for recent open system behavior abstract the few previously reported values of 187os 188os ratios from non marine k pg boundary sections are distinctly higher than the range of 187os 188os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust here re os data from the west bijou continental k pg boundary site located within the denver basin are used to better constrain the os isotopic composition of fallout from the chicxulub ejecta plume for the first time full vertical profiles of 187os 188os ratios and re and os concentrations across a continental k pg boundary section are reported within this section of lignite the lowest measured 187os 188os ratio 0 182 coincides with the k pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop however sediments with elevated os concentrations and measured 187os 188os below 0 23 extend over a 30 cm interval from 5 cm above the clay rich boundary interval 5 cm thick to 25 cm below maximum ir and os concentrations occur 3 cm and 10 cm below the k pg boundary respectively demonstrating greater diagenetic mobility of os relative to ir low 187os 188os above the k pg boundary suggests that accumulation of impact derived os at this site persisted after the impact event perhaps due to redistribution of impact debris in the surrounding area importantly calculated initial 187os 188os ratios throughout most of the section are impossibly low and require open system behavior likely in the form of re addition within the last 10 million years within the boundary interval re concentrations are among the lowest measured at bowring pit and calculated initial 187os 188os ratios are very similar to those previously reported from the western interior of the united states k pg boundary material 0 14 suggesting super chondritic 187os 188os is characteristic of the chicxulub ejecta plume beyond implications for our understanding of os as an impact tracer this study emphasizes the dramatic effect diagenesis can have on the re os system in organic rich environments especially through post depositional addition of re

silicate weathering and carbon cycle controls on the oligocene miocene transition glaciation changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient mi 1 glaciation event at the oligocene miocene transition omt 23 ma however detailed geochemical proxy data are required to test these hypotheses here we present records of li ca mg ca cd ca u ca 18o 13c and shell weight in planktonic foraminifera from marine sediments spanning the omt in the equatorial atlantic ocean li ca values increase by 1 mol mol across this interval we interpret this to indicate a 20 increase in silicate weathering rates which would have lowered atmospheric co2 potentially forcing the antarctic glaciation circa 23 ma 13c of thermocline dwelling planktonic foraminifera track the global increase in seawater 13c across the omt and during the mi 1 event hence supporting a hypothesized global increase in organic carbon burial rates high 13c previously measured in epipelagic planktonic foraminifera and high cd ca ratios during mi 1 are interpreted to represent locally enhanced primary productivity stimulated by increased nutrients supply to surface waters the fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation inferred from high foraminiferal u ca and enhanced respiratory dissolution of carbonates characterised by reduced foraminiferal shell weight replication of our results elsewhere would strengthen the case that weathering induced co2 sequestration preconditioned climate for antarctic ice sheet growth across the omt and increased burial of organic carbon acted as a feedback that intensified cooling at this time

the geochemical cycle of rhenium a reconnaissance rhenium re is one of a suite of elements including uranium and molybdenum that display conservative behavior in seawater and are enriched in anoxic sediments the decay of187re to187os provides a geochronometer in ancient sedimentary rocks and gives rise to os isotopic variations in nature in order to better characterize its sources to seawater re was measured in three major rivers amazon orinoco ganges brahmaputra and some of their tributaries re concentrations span four orders of magnitude from 0 02 to 400 pmol kg with the highest concentrations found in rivers draining black shales in the venezuelan andes mainstream re levels in the three rivers are between 1 and 10 pmol kg with a flux weighted average of 2 3 pmol kg the residence time for re in the oceans is estimated to be 750 000 yr with respect to river inputs re profiles from the atlantic and pacific oceans confirm that re behaves conservatively in seawater with no significant uptake onto particles and or recycling within the water column this is also true in the anoxic water column of the black sea re removal into anoxic sediments occurs at or below the sediment water interface as demonstrated in sediment pore waters from chesapeake bay in oxic sediments re is not cycled with manganese oxides and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component or in manganese nodules burial of re in anoxic sediments which accumulate on 0 3 of the ocean floor removes approximately 50 of the riverine re flux to the oceans hence oceanic re concentrations may be very sensitive to changes in the area of anoxic sedimentation

the glacial interglacial variation of seawater osmium isotopes as recorded in santa barbara basin abstract the pleistocene deposits of santa barbara basin sbb show laminations during interglacial periods typical of an anoxic or suboxic depositional environment and more sporadically laminated deposits during glacial times indicating a more oxic environment of deposition the 187os 188os value of sediment leaches varies as a function of climate consistent with a more radiogenic os signal during interglacial times and less radiogenic os values more typical of glacial times the signal is confounded by contributions from a high os detrital component with a variable 187os 188os value but generally agrees with prior result from deposits from other marine environments

a possible link between the seawater osmium isotope record and weathering of ancient sedimentary organic matter the os isotopic composition of leachable os from a north pacific pelagic clay sequence pegram et al 1992 and bulk sediment samples of metalliferous carbonates deposited near the east pacific rise ravizza 1993 display similar patterns of temporal variation fig 1 this similarity indicates that the composite record of t87os 186os ratio variations reflects changes in the os isotopic composition of seawater temporal changes in the sr isotopic composition of seawater record changes in the relative rates of continental weathering and oceanic crustal alteration the os isotopic composition of seawater should be influenced by similar processes in both the rb sr and re os systems the continental crust is characterized by large time integrated parent daughter ratios compared to the deep earth imparting relatively radiogenic sr and os signatures to continental material however the seawater sr and os records are decoupled from one another over the past 27 ma from 27 to 15 ma 87sr 86sr increased rapidly while the 18vos 186os ratio of seawater remained nearly constant at 15 ma the 87os ts6os ratio of seawater began to increase rapidly but the rate