mobility of rhenium platinum group elements and organic carbon during black shale weathering this study investigates the effects of black shale weathering on the re os isotope system platinum group element concentrations and the degradation of organic matter samples from a weathering profile in late devonian 365 myr ohio shale show a pronounced decrease 77 in organic carbon corg near the present soil surface relative to the interior portion of the outcrop a similar trend is observed for total n 67 loss conversely organic phosphorus porg concentrations increase by 59 near the soil surface the decrease in corg is accompanied by a pronounced decrease in re 99 and to a lesser extent os 39 palladium and pt do not appear to be significantly mobile the effects of re and os mobility on the re os isotope system are significant none of the samples plots on a 365 myr isochron rather the samples define a trend with a slope corresponding to an age of 18 myr with an initial 187os 188os of 6 1 this indicates recent disturbance of the re os system isotope mass balance calculations imply that the labile fraction of os is significantly more radiogenic 187os 188os of 7 8 than the average of the unweathered samples 187os 188os of 6 4 based on data from this study the molar ratio of labile re to corg in ohio shale is estimated at 7 10 8 we estimate the present day riverine black shale derived re flux to seawater using literature data on re burial in anoxic marine sediments and assuming steady state between re release during black shale weathering and re burial in anoxic marine sediments then the labile re corg observed in this study implies that 0 5 tmol of corg is released annually from weathering of black shales a trace lithology in the continental crust this flux corresponds to 12 of the estimated annual co2 flux from oxidative weathering of sedimentary rocks the labile molar re os of 270 indicates that black shale weathering releases 130 mol os per year which accounts for 7 of the riverine os input to seawater the data from this study support the notion that the crustal cycles of labile corg re and os are tightly coupled gray shales which are less corg re and os rich but much more abundant in the continental crust than black shales are likely to be even more important continental sources of re and os to seawater

molybdenum isotope constraints on the extent of late paleoproterozoic ocean euxinia abstract the expansion of anoxic and sulfidic euxinic deep oceans may explain the termination of global large iron formation if deposition at 1 85 ga but the areal extent of late paleoproterozoic ocean euxinia is poorly constrained here we use the molybdenum mo isotope composition 98 95 mo of black shales from the hr fe t typically 0 38 and ratios of pyrite fe to highly reactive fe fe py fe hr typically 2 rich because the fe py fe hr ratios do not imply sustained water column euxinia the conversion of molybdate moo 4 2 to tetrathiomolybdate mos 4 2 in bottom waters and sediments was probably not quantitative and thus the 98 95 mo 0 1 to 0 8 from these shales provides only a minimum estimate for coeval seawater by contrast black shales from the middle rove formation have high fe py fe hr 0 8 and fe hr fe t 0 38 which implies persistent water column euxinia concentrations of free sulfide may have been sufficient to result in near quantitative formation of tetrathiomolybdate and its removal from bottom waters the observed narrow range in 98 95 mo of 1 19 0 28 2sd defined by most euxinic shales is thus inferred to represent the mo isotope composition of late paleoproterozoic global seawater mass balance modeling suggests that organic rich sediments deposited beneath euxinic bottom waters represented a substantial fraction of the oceanic mo sink at 1 8 ga the modeling results suggest that euxinic marine environments were common along late paleoproterozoic ocean margins these environments could have served as efficient sinks for fe 2 aq upwelling from anoxic deeper waters hence the mo isotope data are consistent with fe speciation and other geochemical evidence that suggest an expansion of euxinic environments led to the cessation of global large if deposition after 1 85 ga

release of heavy metals during weathering of the lower cambrian black shales in western hunan china weathering of heavy metal enriched black shales may be one of the most important sources of environmental contamination in areas where black shales are distributed heavy metal release during weathering of the lower cambrian black shales lcbs in western hunan china was investigated using traditional geochemical methods and the icp ms analytical technique concentrations of 16 heavy metals 8 trace elements and p were measured for samples from selected weathering profiles at the taiping vanadium ore mine tp the matian phosphorous ore mine mt and taojiang stone coal mine tj the results show that the bedrock at these three profiles is enriched with sc v cr co ni cu zn pb th u mo cd sb tl and p based on mass balance calculation the percentages of heavy metals released in loss relative to immobile element nb were estimated the results show significant rates of release during weathering of v cr co ni cu zn u mo cd sn sb and tl for the tp profile sc cr mn co ni cu zn pb th cd and sn for the mt profile and sc mn co ni zn th cd sn and tl for the tj profile among these heavy metals co ni zn cd and sn show very similar features of release from each of the three weathering profiles the heavy metals released during weathering may affect the environment especially topsoil and surface waters and are possibly related to an observed high incidence of endemic diseases in the area

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

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

iron isotopes in the amazon river system weathering and transport signatures abstract trace metal clean plankton tows river samples aerosol leachates and porewater samples from the amazon river system were measured for their iron isotopic composition the overall isotopic range observed was 1 5 with fe isotopic variability observed between different types of tributaries dissolved fe and suspended load fe from two main channel sites were isotopically similar 0 1 to 0 3 in contrast the dissolved fe and suspended load fe from an organic rich tributary the negro river were isotopically distinct with the dissolved slightly isotopically heavy 0 3 and the suspended load isotopically light 0 9 the isotopically lightest sample from the amazon region was shelf porewater 1 3 in river water seawater mixing experiments the fe isotopic signal of dissolved fe of river water was modified by flocculation of isotopically heavy fe the variability of the fe isotopic composition of natural samples including biological and aqueous samples demonstrates that significant fractionation is associated with fe biogeochemistry in the environment

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

iron isotopic fractionation during continental weathering abstract the effect of continental weathering on the iron isotope compositions of natural materials is investigated unweathered igneous rocks pelagic clay and dust fall within the range 56 fe 0 0 3 rivers with large suspended loads also have 56 fe values near zero dilute streams have 56 fe values that trend towards lower 56 fe 1 suggesting that dissolved riverine iron is isotopically light relative to igneous rocks bulk soil and soil leaches display systematically different 56 fe profiles indicating that isotopically distinct fe pools are generated during pedogenesis nannofossil ooze which contains fe scavenged from the ocean water column has 56 fe 0 but is consistent with seawater dissolved fe having negative 56 fe it is inferred that continental weathering under modern oxidizing earth surface conditions preferentially releases dissolved fe with negative 56 fe which is transported in rivers to the ocean a preliminary analysis of the marine fe budget suggests that riverine fe has a substantial role in determining the 56 fe of both the modern and ancient oceans but other inputs particularly that from diagenesis of marine sediments may also be important since the chemical pathways of fe processing during weathering are dependent on oxidation state and biological activity fe isotopes may prove useful for detecting changes in these parameters in the geologic past

rhenium osmium and molybdenum isotope systematics of black shales from the lower cambrian niutitang formation sw china evidence of a well oxygenated ocean at ca 520 ma abstract the early cambrian was a key period in earth s history and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period new early cambrian data for re os and mo isotopic compositions and redox sensitive elements rse are reported here for the niutitang formation se chongqin sw china a re os age of 520 30 myr model 3 2 n 21 mswd 62 and a 187os 188os i ratio of 0 79 0 11 were obtained consistent with previously reported data for equivalent strata from other areas separate regression of samples with 187os 188os i of 0 72 0 79 and 0 81 0 89 produce more precise ages of 520 1 9 5 myr model1 2 n 7 mswd 1 0 and 513 10 myr model 1 2 n 8 mswd 0 96 respectively suggesting heterogeneity in the primary source of os as the dominant cause of scatter and uncertainty high initial 187os 188os of 0 79 0 11 may indicate a high os input from oxidative weathering of upper continental crust for early cambrian ocean based on rse data such as mo u v enrichment and ni co v cr v v ni moef uef re mo ratios two geochemically distinct zones upper and lower can be delineated in the sedimentary sequence as reflected in drillcore sections the upper section n 9 is characterized by low total organic carbon toc contents mean 1 8 1 5 wt low rse contents mo 0 008 0 006 10 3 g g v 0 22 0 15 10 3 g g u 0 008 0 004 10 3 g g and low v cr 2 0 1 7 ni co 5 9 3 5 moef uef 2 6 1 8 and mo toc 6 0 3 9 ratios in contrast the lower core section n 13 is characterized by high toc contents 4 4 2 2 wt high rse contents mo 0 10 0 08 10 3 g g v 1 0 1 0 10 3 g g u 0 05 0 04 10 3 g g high v cr 6 5 ni co 24 14 moef uef 6 3 2 2 and mo toc 21 9 ratios uncertainties are 1sd here significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic euxinic to oxic condition all samples display extremely low re mo ratios of 0 2 10 3 1 9 10 3 indicating the absence of intermediate reducing conditions possibly reflecting rapid ocean oxygenation the mo isotopic composition in the sedimentary succession of the yc9 core exhibits a large variation with 98 95mo values of 0 04 2 00 upper section 98 95mo values display an opposite trend to mn content and negative correlation with mo toc ratios indicating mo fractionation associated with adsorption on mn oxides or oxyhydroxides lower section 98 95mo values are positively correlated with mo toc ratios and negatively with re mo ratios suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions high 98 95mo of 2 00 and high average mo toc ratios of 21 10 6 g g wt were observed in euxinic shales in the niutitang formation suggesting extensive ocean oxygenation may have taken place at ca 520 ma

weathering of the new albany shale kentucky usa i weathering zones defined by mineralogy and major element composition abstract comprehensive understanding of chemical and mineralogical changes induced by weathering is valuable information when considering the supply of nutrients and toxic elements from rocks here minerals that release and fix major elements during progressive weathering of a bed of devonian new albany shale in eastern kentucky are documented samples were collected from unweathered core parent shale and across an outcrop excavated into a hillside 40 year prior to sampling quantitative x ray diffraction mineralogical data record progressive shale alteration across the outcrop mineral compositional changes reflect subtle alteration processes such as incongruent dissolution and cation exchange altered primary minerals include k feldspars plagioclase calcite pyrite and chlorite secondary minerals include jarosite gypsum goethite amorphous fe iii oxides and fe ii al sulfate salt efflorescence the mineralogy in weathered shale defines four weathered intervals on the outcrop zones a c and soil alteration of the weakly weathered shale zone a is attributed to the 40 a exposure of the shale in this zone pyrite oxidization produces acid that dissolves calcite and attacks chlorite forming gypsum jarosite and minor efflorescent salt the pre excavation active weathering front zone b is where complete pyrite oxidation and alteration of feldspar and organic matter result in increased permeability acidic weathering solutions seep through the permeable shale and evaporate on the surface forming abundant efflorescent salt jarosite and minor goethite intensely weathered shale zone c is depleted in feldspars chlorite gypsum jarosite and efflorescent salts but has retained much of its primary quartz illite and illite smectite goethite and amorphous fe iii oxides increase due to hydrolysis of jarosite enhanced permeability in this zone is due to a 14 loss of the original mass in parent shale denudation rates suggest that characteristics of zone c were acquired over 1 ma compositional differences between soil and zone c are largely attributed to illuvial processes formation of additional fe iii oxides and incorporation of modern organic matter

fe cycling in the shale hills critical zone observatory pennsylvania an analysis of biogeochemical weathering and fe isotope fractionation abstract during weathering fe in primary minerals is solubilized by ligands and or reduced by bacteria and released into soil porewaters such fe is then removed or reprecipitated in soils to understand these processes we analyzed fe chemistry and isotopic composition in regolith of the shale hills watershed a critical zone observatory in central pennsylvania overlying iron rich shale of the rose hill formation elemental concentrations were measured in soil from a well drained catena on a planar hillslope on the south side of the catchment based upon x ray diffraction and bulk elemental data loss of fe commences as clay begins to weather 15 cm below the depth of auger refusal more fe iii was present than fe ii in all soil samples from the ridge top to the valley floor both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock loss of ferrous fe is attributed mostly to abiotic or biotic oxidation loss of fe is most likely due to transport of micron sized particles that are not sampled by porous cup lysimeters but which are sampled in stream and ground waters the isotopic compositions 56 fe relative to irmm 014 of bulk fe and 0 5 n hcl extracted fe operationally designed to remove amorphous fe oxyhydr oxides range between 0 3 and 0 3 with 56 fe bulk extractable values between 0 2 and 0 4 throughout the soils along the catena 56 fe signatures of both bulk fe and hcl extracted fe become isotopically lighter as the extent of weathering proceeds the isotopic trends are attributed to one of two proposed mechanisms one mechanism involves fe fractionation during mobilization of fe from the parent material due to either fe reduction or ligand promoted dissolution the other mechanism involves fractionation during immobilization of fe oxyhydr oxides if the latter mechanism is true then shale which comprises one quarter of continental rocks could be an important source of isotopically heavy fe for rivers

pervasive oxygenation along late archaean ocean margins the photosynthetic production of oxygen in the ocean is thought to have begun at least 2 7 billion years ago the geochemistry of marine sediments deposited 2 6 billion years ago suggests that ocean margins were oxygenated at least 100 million years before the first significant increase in atmospheric oxygen concentrations

mineral weathering and elemental transport during hillslope evolution at the susquehanna shale hills critical zone observatory located in the uplands of the valley and ridge physiographic province of pennsylvania the susquehanna shale hills critical zone observatory ssho is a tectonically quiescent first order catchment developed on shales of the silurian rose hill formation we used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical chemical weathering fluxes about 25 m of bedrock was also drilled to estimate parent composition depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate weathering front overlying this extending from 6 m below the bedrock soil interface up into the soil is the feldspar dissolution front in the soils depletion profiles for k mg si fe and al relative to the bedrock define the illite and chlorite reaction fronts when combined with a cosmogenic nuclide derived erosion rate on watershed sediments these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite 1 5 10 17 mol m 2 s 1 and illite 2 9 10 17 mol m 2 s 1 than observed in the laboratory mineral reactions result in formation of vermiculite hydroxy interlayered vermiculite and minor kaolinite during weathering exchangeable divalent cations are replaced by al as soil ph decreases the losses of mg and k in the soils occur largely as solute fluxes in contrast losses of al and fe are mostly as downslope transport of fine particles physical erosion of bulk soils also occurs results from a steady state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions chemical weathering losses of mg na and k are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters chemical weathering slows down in the valley floor and al and si even show net accumulation the simplest model for the hillslope that is consistent with all observations is a steady state clay weathering limited system where soil production rates decrease with increasing soil thickness

climate dependence of feldspar weathering in shale soils along a latitudinal gradient although regolith the mantle of physically chemically and biologically altered material overlying bedrock covers much of earth s continents the rates and mechanisms of regolith formation are not well quantified without this knowledge predictions of the availability of soil to sustain earth s growing population are problematic to quantify the influence of climate on regolith formation a transect of study sites has been established on the same lithology silurian shale along a climatic gradient in the northern hemisphere as part of the susquehanna shale hills critical zone observatory pennsylvania usa the climate gradient is bounded by a cold wet end member in wales and a warm wet end member in puerto rico in between mean annual temperature mat and mean annual precipitation map increase to the south through new york pennsylvania virginia tennessee and alabama the site in puerto rico does not lie on the same shale formation as the appalachian sites but is similar in composition soils and rocks were sampled at geomorphologically similar ridgetop sites to compare and model shale weathering along the transect focusing on the low concentration non nutrient element na we observe that the extent and depth of na depletion is greater where mean annual temperature mat and precipitation map are higher na depletion a proxy for feldspar weathering is the deepest reaction documented in the augerable soil profiles this may therefore be the reaction that initiates the transformation of high bulk density bedrock to regolith of low bulk density based on the shale chemistry along the transect the time integrated na release rate qna increases exponentially as a function of mat and linearly with map ny the only site with shale till parent material is characterized by a qna that is 18 times faster than pa an observation which is attributed to the increased surface area of minerals due to grinding of the glacier and kinetically limited weathering in the north a calculated apparent arrhenius type temperature dependence across the transect excluding ny for the dissolution of feldspar na depletion is 99 15 kj mol 1 a value similar to field measured values of the activation energy 14 109 kj mol 1 or laboratory measured values of the enthalpy of the albite reaction 79 8 kj mol 1 observations from this transect document that weathering losses of na from silurian shale can be understood with models of regolith formation based on chemical and physical factors such that weathering progresses from kinetically limited sites wales to al to the transport limited site in puerto rico significant advances in our ability to predict regolith formation will be made as we apply more quantitative models to such transect studies on shales and other rocks types

processes that control mineral and element abundances in shales abstract a shale s composition can be used to infer physical chemical and biological processes that have acted on it over its life history and this knowledge is used to surmise conditions at the time when each process was active this review lists the most important processes that control shale mineral and element abundances during each of the five stages of its life history 1 during weathering transport and deposition the most important processes are chemical weathering hydraulic sorting and mixing adsorption reverse weathering and incorporation of organic matter and biominerals 2 during early diagenesis the most important processes are exchange across the water sediment interface compaction microbial respiration mineral transformations and concretion formation 3 during burial diagenesis the most important processes are compaction catagenesis and metagenesis and mineral transformations 4 during exhumation the most important processes are decompaction brine invasion groundwater penetration and microbial respiration 5 during weathering the most important processes are organic matter oxidation mineral oxidation and hydrolysis groundwater transport and disaggregation this classification of processes provides a way to envision how concurrent and consecutive processes that occurred over a shale s life history have affected its mineralogy and chemical composition

structure of volcanic glasses from the nmr epr perspective a preliminary application to the neapolitan yellow tuff glass samples from alkali trachytic pumice of the lower member lm level lm1 and upper member um of the neapolitan yellow tuff at campi flegrei caldera were studied by nuclear magnetic resonance nmr and electron paramagnetic resonance epr spectroscopy the epr data show that the relative occupancy of the different iron sites varies in samples from the lower member lm1 and upper member of the eruptive sequence the lm1 glasses are characterized by a more homogeneous distribution of the cationic site population of iron compared with um and by a different fe3 al3 substitution in the c2v symmetrical sites of the silicate framework 29si mas nmr spectra on glasses indicate that lm1 glasses are more depolymerized than um glasses indicating the existence of structurally distinct magma batches 27al mas nmr spectra indicate the occurrence in um glasses of aluminum in both tetrahedral and octahedral coordination the observed alvi is believed to be a primary feature of the glass since 27al cp mas nmr experiments do not reveal detectable esa hydrated al the al in sixfold coordination could be ascribed either to the more abundant alkali oh terminations in um in which alkalies would be extracted from their charge balance role for al thus altering its fourfold structural position or to permanent compaction during fast cooling and decompression of high pressure melts 23na cp mas nmr experiments indicate the existence of na oh groups in both lm1 and um glasses relatively more abundant in the latter although preliminary the results indicate the possible application of atomic scale studies to variables that determine the regimes of explosive volcanism

quantification of loss of calcite pyrite and organic matter due to weathering of toarcian black shales and effects on kerogen and bitumen characteristics abstract comparison of geochemical data on posidonia shale early toarcian from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering due to this process of weathering the bulk composition of organic matter especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles are changed estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution

century long record of mo isotopic composition in sediments of a seasonally anoxic estuary chesapeake bay abstract a double spike method was used to obtain mo isotope data for sediments and waters of the seasonally anoxic chesapeake bay and its primary tributary the susquehanna river the dissolved mo distribution in the estuary is non conservative reflecting minor mo loss to the sediments although removal of mo to the sediments does not have a large influence on the isotopic composition of the water column the 98mo of dissolved mo in most of the estuary is dominated by seawater six samples with salinity 15 have an average 98mo 2 17 0 12 which agrees well with a 98mo value for the cass 4 seawater standard of 2 23 a single sample of susquehanna river water has a 98mo of 1 02 consistent with recent findings of positive 98mo in rivers worldwide susquehanna river sediments in contrast have 98mo 0 1 the difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin the 98mo values for estuarine sediments are offset from values in the overlying water most samples deposited before 1925 have 98mo less than 0 similar to the susquehanna sediments subsequently there is an increase in the variability of 98mo with values ranging up to 0 8 the transition to increased variability coincides with the onset of authigenic mo deposition which was previously attributed to escalating summertime anoxia authigenic mo concentrations correlate poorly with 98mo in core samples suggesting that independent mechanisms influence the two parameters authigenic mo concentrations may be controlled by shifting pore water h2s levels while 98mo may be primarily affected by annual variations in mn refluxing

the impact of igneous bedrock weathering on the mo isotopic composition of stream waters natural samples and laboratory experiments abstract river waters have been shown to be systematically enriched in the heavy molybdenum mo isotopes when compared to typical granites and basalts which generally possess mo isotopic compositions 98 95 mo of around 0 this inconsistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine 98 95 mo signatures incongruent dissolution of primary bedrock however may be an important process by which the anomalous mo signatures of the river dissolved load are produced this study therefore investigates the effect of igneous crustal rock weathering on the aquatic 98 95 mo signal by comparing stream water and bedrock mo isotope data to results of bulk rock leach experiments for this purpose stream water and bedrock orthogneiss granite basalt as well as soil and vegetation samples were collected in a small catchment in the french massif central in accordance with the results of earlier studies on riverine mo both streams are isotopically heavier 98 95 mo 0 5 1 1 than the typical granites and basalts the excellent agreement of these data with those of mo released during experimental leaching of the basalt bedrock 0 6 1 0 identifies a predominance of basalt weathering over the stream water mo geochemistry while other processes i e soil formation secondary mineral precipitation and adsorption are subordinate in this catchment given that the basalt bulk rock 98 95 mo reflects a value typical for crustal magmatic rocks ca 0 1 mo isotope fractionation during the incongruent dissolution of basalt can explain the observed isotopically heavy aquatic mo signatures laser ablation analyses demonstrate that the volumetrically minor magmatic sulfides can be highly enriched in mo and mass balance calculations identify the sulfide melt inclusions as the principal mo source for the leach solutions these data suggest that the magmatic sulfides possess a distinctly heavier 98 95 mo signature than the coexisting silicate melt in this case mo would behave like fe by showing a detectable isotope fractionation at magmatic temperatures incongruent crustal bedrock weathering may thus cause a preferential release of heavy mo isotopes this effect however is highly dependent on the primary bedrock mineralogy consequently the average continental runoff may have been significantly affected by incongruent weathering during periods when the earth system was exceptionally far from steady state e g large glaciations with enhanced physical weathering or large subaerial basalt eruptions such as the deccan and the siberian plateau

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

microbial transformations of organic matter in black shales and implications for global biogeochemical cycles abstract the various roles that microorganisms play in transformation of organic matter in geologic environments are yet to be fully revealed many of these roles influence and perhaps control the composition of earth s atmosphere over geologic time by directly impacting global scale cycling of carbon dioxide oxygen and methane one example is the weathering of black shales exposure of organic matter rich sedimentary rocks to oxygenated earth surface conditions results in significant changes in outcrop scale rock geochemistry with implications for element cycling on a global scale this article reviews the progress of ongoing research of a black shale weathering profile exposed near clay city kentucky usa using tools of molecular biology microbial ecology isotope geochemistry and organic geochemistry this research explores the role of microorganisms in utilization and oxidation of sedimentary organic matter and sulfide mineral oxidation and examines the communities of microorganisms that may live in this environment from this and related studies we are developing greater awareness of the importance of microorganisms in transfer of organic materials among various reservoirs of the geosphere biosphere hydrosphere and atmosphere

unusually heavy stable mo isotope signatures of the ottawa river causes and implications for global riverine mo fluxes abstract the accurate use of mo isotope mass balance modelling of ancient oceans relies on the assumption that the 98mo of modern riverine inputs represents a reasonable estimate of the past a growing number of studies of global rivers have demonstrated significant variation in 98mo from the bedrock sources of mo the ottawa river canada was previously identified as having an anomalously heavy mo isotope composition with a 98mo signature close to seawater 2 3 for a seemingly pristine natural river to further explore this unusual signature we collected and filtered 29 water samples from the ottawa river tributaries and small lakes and analysed them for mo isotopes as well as major and trace elements here we fully document heavy 98mo signatures throughout the ottawa river and its absence in surrounding areas of the wider ottawa river basin orb our results reveal a progressive upstream increase in both mo concentration and 98mo signatures in the ottawa river towards values even heavier than seawater up to 3 13 the heaviest mo isotopic signature of river water measured to date in contrast the tributaries and lakes display far lighter and more consistent 98mo signatures within the range previously found in other rivers weathering of an isotopically heavy bedrock source fractionation during weathering and retention of light isotopes in soils have all been proposed as sources of heavy 98mo in rivers however none of these mechanisms can satisfactorily explain our new observations colloidal and particulate processes that remove elements downstream as inferred from some trace element proxies also cannot explain the decreasing 98mo since the preferential removal of light mo isotopes is predicted from these processes similarly the downstream trends show no apparent relationships with constructed dams or known potential industrial sources therefore our findings from the ottawa river are best explained as the dilution of a yet unidentified point source of heavy mo upstream of sampling or a significant permanent sink for light isotopes existing only in the upper reaches of the catchment in both cases anthropogenic contribution from a large mining district in the headwaters of the river must be considered and should be explored further fractionation of mo in waste rock storage facilities have been previously identified and may provide an unnatural sink for isotopically light mo through the rayleigh type fractionation of dissolved mo on oxyhydr oxide mineral surfaces the implied anthropogenic alteration of the natural mo cycle highlights the significant and wide reaching effects of unnatural point sources of mo on the cumulative 98mo signatures of the catchment and emphasises the necessity for detailed geochemical screening of anomalous river water isotope signatures before natural isotope compositions are inferred

isotopic evidence for fe cycling and repartitioning in ancient oxygen deficient settings examples from black shales of the mid to late devonian appalachian basin abstract we have measured iron fe isotope compositions of bulk samples and chemically extracted pyrite in two black shale units the oatka creek formation ocf and the younger geneseo formation gf deposited in the appalachian basin during the mid to late devonian the depositional redox conditions for these units are well established through multiple paleoproxies including degree of pyritization dop and ratios of total fe to al fe t al suggesting that both deposits reflect oxygen deficient environments but that euxinia anoxia with hydrogen sulfide in the bottom waters was more frequent and persistent during deposition of the ocf iron isotopes show systematic variations that are consistent with the inferred water column redox conditions samples from the ocf yield low and variable bulk fe isotope compositions 0 44 to 0 03 in 56 fe relative to average igneous rocks that are inversely correlated with fe t al whereas bulk 56 fe values 56 fe t of the gf fall in a narrower range 0 09 to 0 12 56 fe values of pyrite 56 fe py display good correlation with 56 fe t in the ocf but no such correlation is observed in the gf the fe isotope data and other paleo redox indicators when viewed collectively point to a benthic fe source on the shelf and shelf to basin transfer that operated during deposition of the ocf similar to what has been observed in the modern black sea for the first time this study confirms the strength of fe isotopes in delineating this fe enrichment mechanism in the ancient geological record and emphasizes the utility of the fe isotope proxy for fingerprinting and quantifying ancient biogeochemical cycling of fe

molybdenum sources and isotopic composition during early stages of pedogenesis along a basaltic climate transect abstract molybdenum mo is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes globally riverine 98mo values based on 98mo 95mo relative to nist srm 3134 are heavier relative to bedrock suggesting weathering processes preferentially retain light mo isotopes however the mechanisms governing this process in soils are poorly understood we investigated soil mo abundance and isotopic composition as a function of climate along a well constrained climosequence on a 10 kyr lava flow in hawaii receiving 660 to 2100 mm year 1 mean annual precipitation map we assessed mo abundance and isotopic composition as a function of soil organic matter om content short ranged ordered sro iron fe oxyhydr oxide abundance and mo loss gain we find net accumulation of mo across all soils up to 139 gain that is positively correlated with increasing map the highest mo gains are in surface soil horizons and are correlated with high om content the isotopic composition of soil mo deviates from underlying basaltic bedrock which ranges from 98mo values of 0 11 to 0 26 soil mo isotopic values are lightest at the dry sites 98mo 0 63 and become heavier with increasing precipitation up to 98mo 0 03 samples with the heaviest 98mo values are from horizons with the largest net gains of mo relative to bedrock and have higher om content in order to further constrain mo fluxes into and out of the soil system we measured mo isotope ratios in local rainwater average 98mo 1 11 groundwater average 98mo 0 14 and vegetation 98mo values between 0 18 and 0 64 based on these data we propose that mo in these soils is substantially augmented by additions from precipitation volcanic fog and potentially anthropogenic inputs of mo and that retention of these inputs within soils is likely related to mo om interactions the large atmospheric inputs of mo strongly modulate the 98mo values of dissolved mo fluxes from soil to values only slightly heavier than bedrock these patterns illustrate the potential for mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy 98mo values in the global riverine mo flux

fractionation of cu fe and zn isotopes during the oxidative weathering of sulfide rich rocks abstract we measured the fe cu and zn isotopic compositions of the fluids generated during leaching experiments with pyrite chalcopyrite and sphalerite rich rocks and with a sphalerite mineral separate our study demonstrates that the oxidative weathering of sulfide rich rocks can produce substantial variations in fe 1 75 to 1 0 56 fe solution pyrite rock and cu 0 0 to 2 0 65 cu solution chalcopyrite rock isotopes and small variations in zn isotopes 0 0 to 0 2 66 zn solution sphalerite in the fluid phase relative to the rock for the fe and cu systems we suggest that isotopic fractionation is caused by electron exchange driven e g fe ii fe iii and cu i cu ii redox reactions at the surfaces of the sulfide minerals that occur during air and aqueous chemical reactions under acidic conditions these reactions tend to enrich the fluid phase in the heavier fe and cu isotopes however under circumneutral ph conditions the fe isotopic composition in solution was controlled by the precipitation of fe iii oxide phases which enriched the solution in the lighter fe isotopes this investigation provides a preliminary framework for interpreting the impact of sulfide oxidation reactions on the distributions of stable fe cu and zn isotopes in natural waters

molybdenum isotope evidence for mild environmental oxygenation before the great oxidation event abstract we use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of o2 in the environment 50 ma before the great oxidation event goe in a high resolution profile from the 2 5 ga mt mcrae shale the molybdenum isotope compositions 98 95mo from samples throughout the sequence span a range from 0 99 to 1 86 all samples have heavier 98 95mo compared to average upper continental crust in addition the upper s1 and lower s2 black shale units within the mt mcrae shale exhibit systematic differences in average isotopic compositions and distinct patterns of 98 95mo variation heavier 98 95mo values occur in the s1 unit where 98 95mo correlates with mo enrichments in the s2 unit 98 95mo is not as heavy and is relatively invariant based on sedimentary fe proxies we infer that s1 sediments were deposited under euxinic conditions so that mo removal from bottom waters was likely quantitative thus 98 95mo in this interval likely records coeval seawater the lighter 98 95mo values in the s2 unit may indicate a less fractionated ocean mo inventory relative to the s1 unit however sedimentary fe proxies suggest that s2 sediments accumulated under a water column that was ferruginous rather than euxinic raising the possibility of non quantitative mo scavenging and hence an expressed 98 95mo fractionation relative to coeval seawater because any associated fractionations during this process would have favored the light isotope in sediments the lighter 98 95mo values in the s2 unit represent a lower limit on the value in contemporaneous seawater after evaluating a range of hypotheses we conclude that the isotopically heavy 98 95mo values seen throughout the mt mcrae shale likely reflect the effects of oxidative weathering and adsorption of mo to oxide mineral surfaces on land or in surface oceans the extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global mo isotope budget because of the small ocean mo inventory in the late archean documented by low concentrations of mo and low mo toc the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans however when juxtaposed against the record of 98 95mo through time our findings provide further evidence of the onset of environmental oxygenation before the goe

fingerprinting marcellus shale waste products from pb isotope and trace metal perspectives abstract drill cuttings generated during unconventional natural gas extraction from the marcellus shale appalachian basin u s a generally contain a very large component of organic rich black shale because of extensive lateral drilling into this target unit in this study element concentrations and pb isotope ratios obtained from leached drill cuttings spanning 600 m of stratigraphic section were used to assess the potential for short and long term environmental impacts from marcellus shale waste materials in comparison with material from surrounding formations leachates of the units above below and within the marcellus shale yielded cl br ratios of 100 150 similar to produced water values leachates from oxidized and unoxidized drill cuttings from the marcellus shale contain distinct suites of elevated trace metal concentrations including cd cu mo ni sb u v and zn the most elevated mo ni sb u and v concentrations are found in leachates from the lower portion of the marcellus shale the section typically exploited for natural gas production in addition lower 207 pb 206 pb ratios within the lower marcellus shale 0 661 0 733 provide a distinctive fingerprint from formations above 0 822 0 846 and below 0 796 0 810 reflecting 206 pb produced as a result of in situ 238 u decay within this organic rich black shale trace metal concentrations from the marcellus shale leachates are similar to total metal concentrations from other black shales these metal concentrations can exceed screening levels recommended by the epa and thus have the potential to impact soil and water quality depending on cuttings disposal methods

molybdenum isotope evidence for widespread anoxia in mid proterozoic oceans how much dissolved oxygen was present in the mid proterozoic oceans between 1 8 and 1 0 billion years ago is debated vigorously one model argues for oxygenation of the oceans soon after the initial rise of atmospheric oxygen 2 3 billion years ago recent evidence for h 2 s in some mid proterozoic marine basins suggests however that the deep ocean remained anoxic until much later new molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid proterozoic compared to the present day ocean consequently oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years

dissolution of arkose in dilute acetic acid solution under conditions relevant to burial diagenesis abstract the organic acid produced during diagenesis can cause mineral dissolution leading to the formation of secondary porosity zones arkose dissolution in organic acid solution is important in the evolution of reservoir quality and is influenced by various factors to investigate the effects of acetic acid on arkose dissolution under different burial diagenetic conditions seven batch experiments were conducted over a wide range of reaction conditions medium grained lithic arkose from songliao basin northeastern china was selected for the study four primary factors were considered temperature 80 c 120 c and 160 c ph 2 5 4 0 and 5 5 initial water rock mass ratio 10 1 and 15 1 and grain size 1 3 mm 3 5 mm each experiment was performed for 312 h and sampled six times in parallel results indicate that these four factors have significant effects on arkose dissolution in a certain temperature range feldspar dissolution increased but declined when the temperature exceeded a certain value different minerals elements were affected differently by temperature silicon was the element most sensitive to temperature under acidic conditions the dissolution rates of most minerals increased as the ph decreased grain size and water rock mass ratio affected the degree of reaction in the early period and then the form of mineral transformation in the late period the experimental results are generally consistent with previous field research outcomes understanding the factors that control sandstone dissolution under organic acid conditions will allow the easier detection of secondary porosity zones which may improve the precision of oil exploration

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

iron isotope systematics in estuaries the case of north river massachusetts usa abstract recent studies have suggested that rivers may present an isotopically light fe source to the oceans since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing it is important to investigate potential fractionation of fe isotopes during this process in this study we investigate the influence of the flocculation of fe rich colloids on the iron isotope composition of pristine estuarine waters and suspended particles the samples were collected along a salinity gradient from the fresh water to the ocean in the north river estuary ma usa estuarine samples were filtered at 0 22 m and the iron isotope composition of the two fractions dissolved and particles were analyzed using high resolution mc icp ms after chemical purification dissolved iron results show positive 56 fe values with an average of 0 43 0 04 relative to the irmm 14 standard and do not display any relationships with salinity or with percentage of colloid flocculation the iron isotopic composition of the particles suspended in fresh water is characterized by more negative 56 fe values than for dissolved fe and correlate with the percentage of fe flocculation particulate 56 fe values vary from 0 09 at no flocculation to 0 1 at the flocculation maximum which reflect mixing effects between river borne particles lithogenic particles derived from coastal seawaters and newly precipitated colloids since the process of flocculation produces minimal fe isotope fractionation in the dissolved fe pool we suggest that the pristine iron isotope composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be identified from the fresh water values however this study also suggests that 56 fe composition of rivers can also be characterized by more positive 56 fe values up to 0 3 relative to the crust than previously reported in order to improve our current understanding of the oceanic iron isotope cycling further work is now required to determine the processes controlling the fractionation of fe isotopes during continental run off

the isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans despite important biological and biogeochemical consequences of extensive ocean anoxic events their identification is controversial the marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of mo isotopes is known analysis of a set of rivers that account for 28 of global river runoff suggests more variable mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes suggesting near total anoxia in the proterozoic ocean and during mesozoic ocean anoxic events despite its important biological and biogeochemical consequences1 2 3 the identification of extensive oceanic anoxia in the geological record is controversial in particular global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean or in tectonically isolated basins such as the modern black sea the marine isotope geochemistry of molybdenum mo can help quantify the past oxygenation state of the ocean4 5 6 7 8 9 10 11 12 13 14 because to first approximation under oxic conditions lighter isotopes of mo are preferentially removed to sediments whereas in euxinic conditions quantitative removal leads to no fractionation however the isotopic composition of the mo input from rivers the main contributor of mo to the oceans is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4 5 6 7 8 9 10 11 12 13 14 here we present an isotopic analysis of mo in a set of rivers that together account for 22 of the global riverine water discharge we find a broad range of variability in the mo isotopic composition of these rivers with almost all samples enriched in the heavy isotopes compared with continental rocks our data remove key uncertainties associated with the marine mo isotope budget14 and strongly suggest near total anoxia in the mid proterozoic ocean8 as well as during mesozoic ocean anoxic events13

pyrite oxidation in shales implications for palaeo redox proxies based on geochemical and sem edx evidence abstract pyrite oxidation in marine shales during weathering has been widely studied however the effects of this process on geochemical proxies commonly used to reconstruct ocean redox conditions or the details of pyrite oxidation at the mineral grain scale have not received much scientific attention we conducted a four week laboratory experiment designed to stimulate pyrite oxidation at shale outcrops and to assess effects on the chemical phase of iron and sulphur in the samples both in bulk sediments useful for comparison to palaeo environmental geochemistry and at a finer scale to gain a better understanding of the nature of these changes geochemical and scanning electron microscopy sem techniques provide evidence for pyrite oxidation carbonate dissolution and iron oxyhydr oxide formation during the experiment the net effect of the experiment on the ratio of highly reactive iron phases fehr to total iron fet is minimal 0 03 difference suggesting that this redox proxy behaves relatively conservatively during weathering the effect of weathering on the ratio of pyrite bound iron to highly reactive iron fepy fehr used to investigate the availability of sulphur in contrast is pronounced up to 32 5 difference due to the oxidation of pyrite and the precipitation of iron oxyhydr oxides in the shale samples electron microscopy provides evidence that iron oxyhydr oxides precipitated in situ as rims around cores of pyrite particles passivating and protecting them from further oxidation the quantification of these partly oxidised pyrite particles is now possible using a novel automated particle analysis method coupled to chemical mapping developed in this study we conclude that this method can be of wide use both to quantify pyrite oxidation and assess the significance of fepy fehr at measured in shale outcrop samples

iron isotope fractionation between aqueous ferrous iron and goethite abstract the equilibrium fe isotope fractionation factor between aqueous fe ii and goethite has been experimentally measured to be 1 05 0 08 in 56 fe 54 fe 2 at 22 c using the three isotope method experiments were done using two sizes of goethite 81 11 nm and 590 42 nm and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface fe iii atoms have different isotopic properties than the bulk goethite these experiments indicate that the interaction of fe ii aq and goethite is dynamic and results in complete or near complete fe isotope exchange over 30 days involving at least four components fe ii aq goethite sorbed fe ii and fe iii surface the equilibrium fractionation factor between fe ii aq and fe ii sorb is the same for both sizes of goethite at 56 fe fe ii aq fe ii sorb 1 24 0 14 this fractionation factor is significantly different than the results of previous studies on fe ii sorption to goethite the proportion of the fe iii surface component is greatest in the experiments that used the smallest goethite and the fe iii surface fe ii aq fractionation is estimated to be at least 2 1 the high fe iii surface fe ii aq fractionation may exert a significant influence on the fe isotope compositions of aqueous fe ii in natural systems that contain nanoparticulate goethite including those involving bacterial iron reduction these results demonstrate that the isotopic properties of nano scale minerals may be distinct from micron scale or larger minerals as is the case for other thermodynamic properties of nanoparticles

influence of ph and dissolved si on fe isotope fractionation during dissimilatory microbial reduction of hematite microbial dissimilatory iron reduction dir has been identified as a mechanism for production of aqueous fe ii that has low 56 fe 54 fe ratios in modern and ancient suboxic environments that contain ferric oxides or hydroxides these studies suggest that dir could have played an important role in producing distinct fe isotope compositions in precambrian banded iron formations or other marine sedimentary rocks however the applicability of experimental studies of fe isotope fractionation produced by dir in geochemically simple systems to ancient marine environments remains unclear here we report fe isotope fractionations produced during dissimilatory microbial reduction of hematite by geobacter sulfurreducens in the presence and absence of dissolved si at neutral and alkaline ph hematite reduction was significantly decreased by si at alkaline but not neutral ph presumably due to si polymerization at the hematite surface the presence of si altered fe isotope fractionation factors between aqueous fe ii or sorbed fe ii and reactive fe iii reflecting changes in bonding environment of the reactive fe iii component at the oxide surface despite these changes in isotopic fractionations our results demonstrate that microbial fe iii oxide reduction produces fe ii with negative d 56 fe values under conditions of variable ph and dissolved si similar to the large inventory of negative d 56 fe in neoarchean and paleoproterozoic age marine sedimentary rocks

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

when do black shales tell molybdenum isotope tales molybdenum mo isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions organic rich black shales can be used to reconstruct ancient ocean mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater comparison of 98 95 mo in two devonian shale sequences of similar age the new york oatka creek and geneseo formations reveals that this assumption cannot be applied to all organic rich shales although both sequences contain laminated intervals elevated organic carbon and enrichments of redox sensitive metals the mean 98 95 mo differs systematically between the formations by 0 59 independent paleoredox indicators reveal that portions of the oatka creek formation were deposited under pervasively euxinic anoxic and sulfidic conditions whereas conditions during deposition of the geneseo formation were intermittently euxinic to suboxic oxygen deficient but not sulfidic in the water column we infer that reconstruction of ancient ocean 98 95 mo from organic rich shales requires independent verification of persistent local euxinia with these considerations in mind our data point to 98 95 mo in the devonian oceans 0 6 lighter than in today9s oceans consistent with expanded anoxia

fluid rock interactions during continuous diagenesis of sandstone reservoirs and their effects on reservoir porosity the cretaceous sandstone reservoir in the kuche depression tarim basin western china was investigated with reference to its reservoir property evolution during diagenesis six general diagenetic stages were recognised through petrographic mineralogical and geochemical analyses fluid rock interaction experiments were conducted under these six continuous diagenetic conditions that is with a simulation sequence of compaction early diagenesis organic acid incursion i elevated temperature pressure organic acid incursion ii late diagenesis corresponding to these six experimental stages a total of six models were constructed finally an extended model of fluid rock interaction during diagenesis at a geological timescale from 30 ma to present was constructed after various parameters had been validated results demonstrate the diagenetic stages from both the experimental and numerical simulations generally matched findings obtained from the petrographic and geochemical analyses i with compaction becoming weakened cementation by various minerals was gradually increased ii quartz overgrowth occurred because the contemporaneous sedimentary water was alkaline iii most minerals for example calcite and feldspar minerals displayed dissolution owing to the first organic acid incursion resulting in the visual porosity to increase to 29 26 iv increases of temperature and pressure caused a minor fluctuation of the porosity change v the cement that formed during earlier stages largely dissolved with the second organic acid incursion vi during the last stage the reservoir fluid was diluted by sedimentary alkaline water and most minerals precipitated under an alkalic environment the present porosity simulated is about 11 4 comparable with the actually measured data this study demonstrates that the combination of petrographic observations laboratory experiments and numerical simulations can not only reconstruct the diagenetic process but also provide a quantitative evaluation and prediction of reservoir petrophysical properties this article is protected by copyright all rights reserved

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

cu isotopes and concentrations during weathering of black shale of the marcellus formation huntingdon county pennsylvania usa abstract the complex weathering processes which govern the production of soil from bedrock have proven difficult to understand for many lithologies weathering of black shale is of particular interest because it releases organic carbon and heavy metals as solutes and therefore impacts the health of terrestrial and aquatic ecosystems to understand black shale weathering a geochemical survey was initiated for soils developed on shales of the marcellus formation at a zero order catchment at a satellite site of the susquehanna shale hills critical zone observatory located in jackson corner pennsylvania this formation is an organic and metal rich carbonaceous shale that underlies much of new york pennsylvania ohio and west virginia in this paper we focus on the effects of weathering on variations of cu isotopes in the shale cu concentration data for soil were normalized using ti concentrations to document the mobility of cu relative to bedrock at both the ridgetop and valley floor depletion profiles for cu are documented in the soils the cu in the soils is depleted in 65 cu average 65 cu 0 5 0 2 compared to the parent material average 65 cu 0 03 0 15 consistent with loss of cu from soils the pore waters contain 10 ppb cu on average and are enriched in the heavy isotope average value 65 cu 1 14 0 44 rayleigh fractionation models using the concentration and isotope data of the soils are consistent with pyrite weathering and loss of cu from the ridgetop but downslope transport and cu re precipitation at the valley floor

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

fractionation of fe isotopes by soil microbes and organic acids small natural variations in fe isotopes have been attributed to biological cycling however without understanding the mechanisms of fractionation it is impossible to interpret such variations here we show that the d 56 fe of fe dissolved from a silicate soil mineral by siderophore producing bacteria is as much as 0 8 lighter than bulk fe in the mineral a smaller isotopic shift is observed for fe released abiotically by two chelates and the magnitude of the shift increases with affinity of the ligand for fe consistent with a kinetic isotope effect during hydrolysis of fe at the mineral surface fe dissolved abiotically without chelates shows no isotopic shift the d 56 fe of the exchange fraction on soil grains is also lighter by 0 6 1 than fe from both hornblende and iron oxyhydroxides the kinetic isotope effect is therefore preserved in open systems such as soils when recorded in the rock record fe isotopic fractionation could document fe transport by organic molecules or by microbes where such entities were present in the geologic past

production of a molybdophore during metal targeted dissolution of silicates by soil bacteria although many bioessential metals are scarce in natural water and rock systems microbial secretion of high affinity ligands for metal extraction from solid phases has only been documented for fe however we have discovered that mo is extracted from a silicate by a high affinity ligand a possible molybdophore secreted by an n2 fixing soil bacterium the putative molybdophore aminochelin is secreted as a siderophore under fe depleted conditions but is also secreted under fe sufficient mo depleted conditions presumably molybdophore production facilitates uptake of mo for use in mo enzymes in contrast an fe requiring soil bacterium without a special mo requirement only enhances the release of fe from the silicate fractionation of mo stable isotopes during uptake to cells may provide a fingerprint for the importance of chelating ligands in such systems many such metal specific ligands secreted by prokaryotes for extraction of bioessential metals their effects on earth materials and their possible utility in the recovery of economic metals remain to be discovered

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

weathering of phosphorus in black shales 1 rock weathering is the ultimate source of phosphorus p to the oceans where p can be a limiting nutrient for biological production in this paper p weathering is examined in soil chronosequences formed in weathering profiles on the organic rich woodford shale new albany shale and green river shale at all sites organic p and inorganic p concentrations reveal that p weathering is far from complete prior to erosion carbon c p ratios decrease significantly from unweathered shale to the weathered shale at all sites which is driven by loss of total organic c with weathering here we characterize organic phosphorus across a weathering profile from the woodford shale using solid state cpmas 31p nmr spectroscopy techniques revealing that p esters are the dominant forms of p during all stages of weathering certain p esters appear to be resistant to chemical weathering during the millions of years between deposition uplift and erosion possibly representing a significant long term global sink for p

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day