molybdenum isotope signatures in continental margin marine sediments abstract we present molybdenum isotope data for four sediment profiles from continental margin settings each profile has a distinctive average isotope composition ranging from 98 95 mo 0 5 to 1 3 relative to j and m laboratory standard this range lies between the modern ocean water value 2 3 and the values typical of mo adsorbed onto mn oxides 0 7 0 1 an important finding of this study is the apparent co variation between the mo isotope composition and the accumulation rate of authigenic mo under reducing conditions this relationship suggests that the chemical processes responsible for mo accumulation under reducing conditions produce an isotope signature in marine sediments in addition to the relationship between mo accumulation and the mo isotope signature there is also a relationship between these parameters and the rate of organic carbon oxidation and burial these relationships suggest that the mo isotope signature of reducing sediments may serve as a tracer for the cycling of organic carbon in continental margin sediments however additional data will be required to refine any such relationships

a tale of shales the relative roles of production decomposition and dilution in the accumulation of organic rich strata middle upper devonian appalachian basin abstract a new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged more firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry this consensus recognizes the interdependent roles of sedimentation primary production and microbial metabolism in favor of earlier end member models e g production vs preservation in this study one of the classic stratigraphic sequences upon which the preservation end member was based is re interpreted in light of this new consensus the study employs an extensive new sedimentological biogeochemical database from cores drilled in western new york the database spans over 500 m and 15 my of devonian deposition in the appalachian basin and provides a framework for comparative study of organic matter burial the major conclusions are 1 few organic rich units were deposited under pervasive anoxic sulfidic water columns 2 establishment and breakdown of seasonal thermoclines on annual or longer timescales were the predominant mode of stratification and 3 under such conditions remineralization of bio limiting nutrients may have played a key role in organic matter burial by creating a eutrophication pump this pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments and episodically in bottom waters a final conclusion asserts that the master variable for organic matter accumulation was relative sea level change which exerted influence on clastic dilution preservation and production processes sea level rise events led to sediment starvation and organic carbon concentration in distal basin sediments as well as to decreased effectiveness of seasonal mixing and thus longer build up intervals for remineralized nutrients episodic mixing of nutrient enriched bottom waters led to enhanced production ultimately increased clastic sediment delivery and water column mixing during relative sea level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply thus terminating deposition of strata enriched in organic carbon

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

black shale deposition and faunal overturn in the devonian appalachian basin clastic starvation seasonal water column mixing and efficient biolimiting nutrient recycling integrated geochemical data suggest that black shale deposition in the devonian geneseo formation of western new york was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing once established however black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced n and p regeneration from sediments sustained high primary productivity by returning nutrients to the photic zone during mixing and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification this feedback was ultimately disrupted by rising siliciclastic influx which diluted organic matter and restored benthic redox stability the abrupt overturn of diverse long standing appalachian basin marine communities may have been the result of trophic resource destabilization during geneseo deposition

the 23 500 y 14 c bp white pumice plinian eruption and associated debris avalanche and tochimilco lava flow of popocatepetl volcano mexico abstract the white pumice wp is one of the thickest and most voluminous plinian fallouts produced by popocatepetl volcano in central mexico during the late pleistocene holocene its eruption 23 500 14 c y bp 27 800 cal bp was triggered by the catastrophic failure of the sw flank of the volcano the resulting debris avalanche was highly mobile reaching 72 km from the cone with an apparent coefficient of friction l h of 0 06 the deposit covers an area of 1200 km 2 and has a volume of 10 4 km 3 this gigantic landslide characterized by exceptionally large proximal hummocks 400 m provoked the sudden decompression of the hydrothermal and magmatic systems which produced an initial blast followed by the rise of a plinian column that reached an altitude of 33 km the isopach map allows the recognition of a dispersal axis pointing toward the south where an area of 2490 km 2 was covered by 10 cm of pumice and ash the total volume of the pumice fallout was estimated at 1 9 km 3 dre dense rock equivalent pumice clasts are dacitic 62 66 wt sio 2 anhydrous basis highly vesicular 55 88 vol and display a seriate texture with phenocrysts of plagioclase hornblende augite hypersthene oxides ti magnetite and ilmenite apatite as the eruption advanced discharge rates became more intermittent and the height of the column fluctuated and finally collapsed generating pumice and ash flows that were emplaced around the volcano this short but intense activity was followed during subsequent years by rain induced lahars that reached great distances from the volcano at the same time more degassed andesitic dacitic 61 65 wt sio 2 magma was erupted effusively 4 4 km 3 dre in the new horseshoe shaped 5 km wide crater from which the tochimilco lava flow descended toward the sse where it inundated an area of 68 km 2 and reached as far as 22 km from its source since then multiple eruptions have reconstructed the summit cone almost completely obliterating the horseshoe shaped crater during the course of this catastrophic eruption vei 5 a total volume of 6 3 km 3 dre of juvenile magma pumice and lava were emitted and at least an additional 10 km 3 of pre existing rocks debris avalanche mobilized it surpasses in magnitude most other known plinian eruptions from popocatepetl and can be envisaged as an example of a worst case scenario for hazard evaluation purposes it dramatically changed the morphology of the volcano and had profound and far reaching effects beyond its immediate vicinity along rivers draining surrounding plains as well as on the lacustrine basins e g chalco to the ne a repeat of such an eruption in this densely populated area would certainly cause a major calamity of unprecedented dimensions

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

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

sulfur isotopic trends and pathways of iron sulfide formation in upper holocene sediments of the anoxic black sea down core trends for extents of iron sulfidation in upper holocene microlaminated deposits of the black sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and or very close to the sediment water interface sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core with a mean 34scdt value of 37 2 0 9 1 n 18 these isotopic trends support the hypothesis of early pyrite formation isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and to 34s values for water column dissolved sulfide within a narrow zone immediately below the o2 h2s interface these three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column 175 m with 34s values for the deep dissolved sulfide reservoir being consistently lower by 2 to 4 these isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow shallow region at the top of the sulfidic water mass isotopic data from a fine grained turbidite collected in the deep basin and from rapidly accumulating iron monosulfide rich muds on the anoxic upper slope are enriched in34s relative to the microlaminated deposits the enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial a broad suite of geochemical data including sulfur isotopic results are consistent with an upper slope sediment source for the ubiquitous muddy turbidites of the deep basin the collective effects of reworking transport and redeposition result in a complete conversion of avs to pyrite with little additional iron sulfidation this conclusion is supported by a detailed isotopic mass balance the present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin sediments of the upper slope and the genetically linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep basin floor suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin importantly isotopic patterns from the modern black sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record

an integrated assessment of a type euxinic deposit evidence for multiple controls on black shale deposition in the middle devonian oatka creek formation an integrated lithologic paleontologic and multi proxy geochemical study of the middle devonian oatka creek formation a black shale in the northern appalachian basin indicates that a number of different factors contributed to organic carbon rich black shale deposition conditions leading to this organic rich sedimentary deposit were ultimately controlled by a relative sealevel rise dominantly eustatic but with possible contributions from local tectonics which cut off the supply of carbonate to the basin geochemical proxy evidence such as mo ti fe ti corg spy and 34spy suggests that as sealevel continued to rise after the carbonate supply was cut off a threshold was crossed at which point conditions in the basin shifted from dominantly anoxic to dominantly euxinic anoxic sulfidic bottom waters concurrent with the shift to dominantly euxinic conditions the supply of siliciclastic sediments was cut off resulting in a condensed horizon as evidenced by the elemental ratios of si al and k fe mg and the relative concentration of eolian silt as determined petrographically and from scanning electron microscopy sediment starvation in the basin appears to have facilitated the biogeochemical re cycling of c n and p specifically the elemental ratios of c n and p and the stable carbon isotope composition of organic matter suggest that the preferential regeneration of p under anoxic conditions and of n during the oxic phase of oxic anoxic oscillation led to enhanced primary production in surface waters thereby maintaining euxinic conditions in the bottom waters through respiration of settling organic matter finally it is observed that though conditions in the basin seem to have remained consistently anoxic sulfidic for some time after the initial shift to euxinic conditions a progressive increase in siliciclastic sedimentation led to a corresponding decrease in the enrichment of various redox related elements illustrating the overriding control that sedimentation can have on geochemical proxy records

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

the late devonian frasnian famennian event in south china patterns and causes of extinctions sea level changes and isotope variations abstract the frasnian famennian f f event may be recognized in various facies areas in south china in the pelagic cherty basin facies both lower and upper kellwasser events can seemingly be recognized in the deeper water carbonate facies the f f event level is well controlled in light of conodont biostratigraphy in the shallow water carbonate and mixed carbonate shale facies the f f boundary may be defined by clear taxonomic distinction in benthic fossils articulate brachiopods and corals as well as evidence from minor pelagic fossils post extinction recovery rate of benthic organisms differs in different facies settings and different taxonomic groups in open shallow water platform to inter platform depression areas brachiopods seem to recover quickly probably in the middle pa triangularis zone benthic ostracodes seem to recover at a later stage recovery of rugose corals did not happen until the uppermost famennian three steps of the f f mass extinction are postulated 1 extinction of diverse brachiopods including most atrypids 2 extinction of pelagic conodonts 3 extinction of both benthic faunas very abundant and diverse rugose corals and ostracodes and pelagic conodonts evidence of an end frasnian regression in south china is clear particularly in shallow water settings however in deeper water settings the picture is complicated with evidence of both sea level rise and fall in the latest frasnian it may be assumed from overall data so far known that crustal evolution itself and associated multiple volcanic hydrothermal activities may have mainly caused frequent and rapid climatically warming cooling alterations and sea level changes as well as marine ecologic collapse eutrophication microbial blooming seawater acidification and anoxia which may explain the f f extinction pattern in south china

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

authigenic molybdenum isotope signatures in marine sediments we present new mo isotope data from the mexican continen tal margin that in conjunction with previous data allow us to propose a mechanistic description of the mo isotope system in ma rine sediments we hypothesize that there are unique environmen tally dependent mo isotope signatures recorded in marine sedi ments that reflect the mechanisms responsible for authigenic mo accumulation open ocean anoxic sites defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water exhibit a 98 95 mo isotope signature of 1 6 we believe this val ue reflects mo sulfide formation via diagenetic processes within sediments quantitative formation of mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value 2 3 as typified by samples from the highly sulfidic black sea in contrast to these reducing settings manganese oxide rich sediments have measured mo isotope values that are more negative relative to seawater than any other sediment samples analyzed to date similar to fe mn crusts 0 7 most measured mo isotope compositions of marine sediments from open ocean settings appear to reflect a mix ture of lithogenic mo 0 0 and the mo signature of a specific authigenic mo accumulation mechanism we therefore suggest that mo isotopes may record unique signatures that reflect the domi nant chemical mechanism for mo sequestration into sediments

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

trace metal records of regional paleoenvironmental variability in pennsylvanian upper carboniferous black shales abstract regional geochemical differences within a laterally continuous cyclic pennsylvanian upper carboniferous shale in midcontinent north america are interpreted in light of models of glacioeustatic forcing and new views on water column paleoredox stability and trace metal behavior in black shale environments specifically we characterize differences in transition metal fe mn mo v ni zn pb and u concentrations in black shales of the hushpuckney shale member of the swope limestone in iowa and equivalent black shale beds of the coffeyville formation in oklahoma although c s fe systematics and uniform 34 s depleted isotope ratios of pyrite indicate pervasive euxinic deposition anoxic and sulfidic bottom waters for these shales regional variations can be inferred for the efficiency of mo scavenging and for the rates of siliciclastic sedimentation as expressed in spatially varying fe al ratios black shales in iowa show mo enrichment roughly five times greater than that observed in coeval euxinic shales in oklahoma by contrast fe al ratios in oklahoma shales are as much as five times greater than the continental ratio of 0 5 observed in the over and underlying oxic facies and in the coeval black shales in iowa recent work in modern marine settings has shown that enrichments in fe commonly result from scavenging in a euxinic water column during syngenetic pyrite formation in contrast to fe the concentrations of other transition metals mo v ni pb zn u are typically more enriched in the black shales in iowa relative to oklahoma the transition metal trends in these paleozoic shales are reasonably interpreted in terms of early fixation in organic rich sediments due to euxinic water column conditions however regional variations in 1 rates of siliciclastic input 2 organic reservoirs including relative inputs of terrestrial versus marine organic matter and 3 additional inputs of metals to bottom waters from contemporaneous hydrothermal vents are additional key controls that lead to geographic variation in the extent of metal enrichments preserved in ancient organic rich sediments

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

pge re os and mo isotope systematics in archean and early proterozoic sedimentary systems as proxies for redox conditions of the early earth abstract re os data and pge concentrations as well as mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the barberton greenstone belt south africa fig tree and moodies groups 3 25 3 15 ga the belingwe greenstone belt zimbabwe manjeri formation ca 2 7 ga and shales from the witwatersrand ventersdorp and transvaal supergroups south africa ranging from 2 95 to 2 2 ga moderately oxidizing conditions are required to mobilize re and mo in the environment mo fractionation only occurs in solution and these parameters thus have potential use as paleoredox proxies for the early earth pge re abundance patterns of barberton greenstone belt sediments are uniform and very similar in shape to those of komatiites this indicates 1 that the pge came from a source of predominantly ultramafic composition and 2 that pge were transported and deposited essentially in particulate form sediments from the younger belingwe greenstone belt show more fractionated pge re patterns and have re os ratios 10 to 100 higher than those of barberton sediments their pge abundance patterns and re os ratios are intermediate between those of the mid archean shales and neoproterozoic to recent black shales they reflect scavenging of re from solution in the sedimentary environment 98 95 mo values of black shales of all ages correlate with their concentrations the barberton greenstone belt samples have 1 3 ppm mo similar to a granitoid basaltic source this mo has 98 95 mo between 1 9 and 2 4 relative to present day mean ocean water molybdenum momo and is thus not isotopically fractionated relative to such a source similar to the pge this indicates transport in solid form sediments from the belingwe greenstone belt show in part enhanced mo concentrations up to 6 ppm and mo isotope fractionation 98 95 mo up to 1 4 relative to momo the combined pge re and mo data show mainly reducing conditions in the mid archean and suggest that by 2 7 ga the atmosphere and oceans had become more oxidizing substantially younger samples from the transvaal supergroup to ca 2 2 ga surprisingly have mainly low mo concentrations around 1 ppm and show no significant mo isotope fractionation relative to the continental source among possible explanations for this are a return to reducing atmospheric conditions after 2 7 ga reservoir effects or mo removal by sulfide precipitation following sulfate reduction in early proterozoic oceans

mineralogy and cooling history of the calcium aluminum chromium enriched ureilite lewis cliff 88774 the lew 88774 ureilite is extraordinarily rich in ca al and cr and mineralogically quite different from other ureilites in that it consists mainly of exsolved pyroxene olivine cr rich spinel and c the presence of coarse exsolved pyroxene in lew 88774 is unique because pyroxene in most other ureilites is not exsolved the pyroxene has bulk wo contents of 15 20 mol and has coarse exsolution lamellae of augite and low ca pyroxene 50 m in width the compositions of the exsolved augite ca33 7mg52 8fe13 5 and host low ca pyroxene ca4 4mg75fe20 6 show that these exsolution lamellae were equilibrated at 1280 c a computer simulation of the cooling rate obtained by solving the diffusion equation for reproducing the diffusion profile of cao across the lamellae suggests that the pyroxene was cooled at 0 01 c year until the temperature reached 1160 c this cooling rate corresponds to a depth of at least 1 km in the parent body assuming it was covered by a rock like material therefore lew 88774 was held at this high temperature for 1 2 104years the proposed cooling history is consistent with that of other ureilites with coarsegrained unexsolved pigeonites lewis cliff 88774 includes abundant cr rich spinel in comparison with other ureilites the range of feo content of spinels in lew 88774 is from 1 3 wt to 21 wt fe fe mg 0 04 0 6 the cr rich and fe poor spinel in lew 88774 has less fe feo 1 3 wt than spinels in other achondrites we classify this spinel as an fe al bearing picrochromite most ureilites are depleted in ca and al but this meteorite has high ca and al concentrations in this respect as well as mineral assemblage and the presence of coarse exsolution lamellae in pyroxene lew 88774 is a unique ureilite most differentiated meteorites are poor in volatile elements such as zn but the lew 88774 spinels contain abundant zn up to 0 6 wt we note that such a high zn concentration in spinel has been observed in the carbonaceous chondrites and recrystallized chondrites this unusual ureilite has more primitive characteristics than most other ureilites

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

transient deep water oxygenation in the early cambrian nanhua basin south china abstract many late neoproterozoic to early cambrian fossils of multicellular eukaryotes including those of benthic animals are found preserved under anoxic and even euxinic bottom water conditions which is contradictory to the consensus that oxygen is essential to eukaryotes to investigate this conundrum we conducted an integrated study of iron speciation redox sensitive trace elements and mo isotopes 98 mo on the black shale interval of the lower cambrian hetang formation 535 521 ma at lantian south china in which benthic sponge fossils are abundant in the lower member lm but absent in the upper member um iron speciation data point to uniformly anoxic ferruginous conditions in the lm and euxinic conditions in the um whereas the trace element and 98 mo data show greater secular variation in redox conditions the lm shows higher mean trace element concentrations mo 108 ppm u 36 ppm v 791 ppm and lower and more variable 98 mo 0 13 to 1 76 relative to the um mo 45 ppm u 18 ppm v 265 ppm 98 mo 1 59 to 1 67 and ratios of redox sensitive trace element concentrations to total organic carbon are significantly more variable and higher on average in the lm relative to the um the appearance of sponge fossils and lower 98 mo values correlate strongly with gray i e lighter colored layers in the lm these patterns can best be interpreted as recording mainly euxinic conditions throughout deposition of the study units with more intense background euxinia in the lm relative to the um but also with frequent transient oxygenation events in the lm that do not appear in the um the transient oxygenation events of the lm led to the initial colonization of the deep nanhua basin by sponges and the termination of these events in the um caused sponge faunas to disappear until a general rise in o 2 levels later in the cambrian permitted their return to deeper water habitats our study also illustrates that multiple geochemical and paleobiological proxies exhibit different responses in poikiloredox environments i e characterized by small scale spatial and high frequency temporal variations which can lead to apparent contradictions between metazoan fossil occurrences and their inferred watermass redox conditions

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

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

geochemical and multi isotopic 87sr 86sr 143nd 144nd 238u 235u perspectives of sediment sources depositional conditions and diagenesis of the marcellus shale appalachian basin usa abstract we investigate sediment sources depositional conditions and diagenetic processes affecting the middle devonian marcellus shale in the appalachian basin eastern usa a major target of natural gas exploration multiple proxies including trace metal contents rare earth elements ree the sm nd and rb sr isotope systems and u isotopes were applied to whole rock digestions and sequentially extracted fractions of the marcellus shale and adjacent units from two locations in the appalachian basin the narrow range of end values from 7 8 to 6 4 at 390 ma is consistent with derivation of the clastic sedimentary component of the marcellus shale from a well mixed source of fluvial and eolian material of the grenville orogenic belt and indicate minimal post depositional alteration of the sm nd system while silicate minerals host 80 of the ree in the shale data from sequentially extracted fractions reflect post depositional modifications at the mineralogical scale which is not observed in whole rock ree patterns limestone units thought to have formed under open ocean oxic conditions have 238u values and ree patterns consistent with modern seawater the 238u values in whole rock shale and authigenic phases are greater than those of modern seawater and the upper crust the 238u values of reduced phases the oxidizable fraction consisting of organics and sulfide minerals are 0 6 greater than that of modern seawater bulk shale and carbonate cement extracted from the shale have similar 238u values and are greater than 238u values of adjacent limestone units we suggest these trends are due to the accumulation of chemically and more likely biologically reduced u from anoxic to euxinic bottom water as well as the influence of diagenetic reactions between pore fluids and surrounding sediment and organic matter during diagenesis and catagenesis

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

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