tracing euxinia by molybdenum concentrations in sediments using handheld x ray fluorescence spectroscopy hhxrf abstract elevated molybdenum mo contents in organic rich sediments are indicative of deposition from an anoxic and sulfide rich euxinic water column this can be used for tracing past euxinic conditions in ancient oceans from sedimentary archives conventional analytical detection of elevated molybdenum levels is however expensive and cannot be directly performed in the field here we show that handheld x ray absorption spectroscopy hhxrf on both rock powders and rock surfaces provides a powerful tool for laboratory or field based geochemical characterization of mo content and the tracing of euxinic depositional conditions we further present a sedimentary mo database compiled from over 300 globally distributed modern marine sites to show that modern euxinic marine basins are characterized by molybdenum enrichment 25 ppm relative to average crust 1 5 ppm and river suspended matter 3 ppm only mn oxide rich deposits from oxygenated deep sea settings match such high mo enrichments but these are easily distinguished from mo rich sediments from the continental shelves hence past and present organic rich sediments enriched in mo are indicative of at least intermittent water column euxinia we analyzed the molybdenum content of powdered and homogenized samples of holocene miocene triassic jurassic ordovician and cambrian age by hhxrf and inductively coupled plasma mass spectrometry icpms we further analyzed 2 certified nist standard references nist 2781 with mo 47 ppm and nist 2702 with mo 10 ppm for analytical control analytical precision 1 after 30 120 and 300 seconds of measuring time was 4 2 and 1 ppm with a respective detection limit of 11 5 3 ppm 3 noise level the data were accurate to within the given precision 1 after a daily calibration to samples covering a range of mo concentrations from 0 to 30 ppm hand held xrf equipment also allows mo measurements directly on fresh rock surfaces both in the field and under laboratory conditions rock samples from a cambrian drill core closely match icpms obtained mo concentrations from homogenous powders of the same rocks collectively our data show that hhxrf as a quick and reliable method for the precise and accurate quantification of molybdenum concentrations in both rock powders and at fresh rock surfaces the non destructive nature of the hhxrf measurements is combined with low per sample analytical costs minor sample preparation and easy applicability in the field

continuously increasing δ98mo values in neoarchean black shales and iron formations from the hamersley basin abstract we present mo c and o isotope data from black shales carbonate and oxide facies iron formations from the hamersley group western australia that range in age from 2 6 to 2 5 billion years the data show a continuous increase from near crustal 98 mo values of around 0 50 for the oldest marra mamba and wittenoom formations towards higher values of up to 1 51 for the youngest sample of the brockman iron formation thereby the trend in increasing 98 mo values is portrayed by both carbonate facies iron formations and black shales considering the positive correlation between mo concentration and total organic carbon we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales a temporal increase in the seawater 98 mo over the period 2 6 2 5 ga is observed assuming an overall low mo isotope fractionation during both mo removal processes oxide facies iron formations show lowest mo concentrations lowest total organic carbon and slightly lower 98 mo compared to nearly contemporaneous black shales this may indicate that in iron formation settings with very low organic matter burial rates the preferential adsorption of light mo isotopes onto fe oxyhydr oxides becomes more relevant a similar mo isotope pattern was previously found in contemporaneous black shales and carbonates of the griqualand west basin south africa the consistent and concomitant increase in 98 mo after 2 54 billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the hamersley basin and the griqualand west basin the modeling of the oceanic mo inventory in relation to the mo in and outflux suggests that the long term build up of an isotopically heavy seawater mo reservoir requires a sedimentary sink for isotopically light mo the search for this sink i e adsorption onto mn oxides in well oxygenated surface oceans and or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings remains debated but its relevance becomes more important closer to the great oxidation event and is probably related to already weakly oxidizing conditions even prior to the 2 5 ga whiff of oxygen

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

pinch and swell structure and shear zones in viscoplastic layers abstract two dimensional finite element simulations are presented for the extension of stiff viscoplastic layers embedded in a weaker viscous matrix layers and matrix exhibit power law flow laws and the layers exhibit additionally a von mises yield stress the power law flow law applies to rock deformation in the diffusion and dislocation creep regime and the von mises plasticity to the low temperature plasticity regime e g dislocation glide simulations show that pinch and swell structure forms for small viscosity ratio i e 10 20 and typical power law stress exponents i e 1 5 the pinches in layers with initial random geometrical perturbation form consecutively i e not simultaneous in multilayers pinches on both the single layer and the multilayer scale develop furthermore shear zones develop due to the oblique linkage of pinches across the multilayer these shear zones have a stable position cause a normal drag geometry and exhibit significant displacement the numerical results and the importance of low temperature plasticity are supported by field observations microstructural observations and ebsd orientation maps for pinch and swell structure in calcite veins the presented models can explain strain localization by shear zone formation during bulk pure shear extension of viscoplastic multilayers without any material softening or feed back mechanism e g shear heating

geochemistry of peruvian near surface sediments sixteen short sediment cores were recovered from the upper edge ueo within wo and below bo the oxygen minimum zone omz off peru during cruise 147 of r v sonne solids were analyzed for major trace elements total organic carbon total inorganic carbon total sulfur the stable sulfur isotope composition 34 s of pyrite and sulfate reduction rates srr pore waters were analyzed for dissolved sulfate sulfide and 34 s of sulfate in all cores highest srr were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and or diffusion of sulfate from bottom water 34 s of dissolved sulfate showed only minor downcore increases strong 32 s enrichments in sedimentary pyrite to 48 vs v cdt are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction manganese and co are significantly depleted in peruvian upwelling sediments most likely due to mobilization from particles settling through the omz whereas release of both elements from reducing sediments only seems to occur in near coastal sites cadmium mo and re are exceptionally enriched in wo sediments 600 m water depth high re and moderate cd and mo enrichments are seen in bo sediments 600 m water depth re mo ratios indicate anoxic and suboxic conditions for wo and bo sediments respectively cadmium and mo downcore profiles suggest considerable contribution to ueo wo sediments by a biodetrital phase whereas re presumably accumulates via diffusion across the sediment water interface to precipitation depth uranium is distinctly enriched in wo sediments due to sulfidic conditions and in some bo sediments due to phosphorites silver transfer to suboxic bo sediments is likely governed by diatomaceous matter input whereas in anoxic wo sediments ag is presumably trapped due to sulfide precipitation cadmium cu zn ni cr ag and t1 predominantly accumulate via biogenic pre concentration in plankton remains rhenium sb as v u and mo are enriched in accordance with seawater te availability lead and bi enrichment in ueo surface sediments is likely contributed by anthropogenic activity mining accumulation rates of toc cd mo u and v from peruvian and namibian sediments exceed those from the oman margin and gulf of california due to enhanced preservation off peru and namibia copyright 2004 elsevier ltd

geochemical and anthropogenic enrichments of mo in sediments from perennially oxic and seasonally anoxic lakes in eastern canada abstract we measured the vertical distributions of mo fe mn sulfide sulfate organic carbon major ions and ph in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in eastern canada as well as those of mo acid volatile sulfide fe mn al organic c 210pb and 137cs in sediment cores from the same sites the only input of anthropogenic mo to these lakes comes from atmospheric deposition the relatively monotonous distribution of mo in the porewater of three seasonally anoxic basins suggests that mo is not redistributed in the sedimentary column during periods of anoxia in contrast porewater mo profiles obtained at three sampling dates in a perennially oxic basin display sharp mo peaks below the sediment water interface indicating redistribution subsequent to deposition modeling of these latter porewater mo profiles with a diagenetic reaction transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that mo is released at 1 2 cm depth as a consequence of the reductive dissolution of fe oxyhydroxides and scavenged both at the vicinity of the sediment water interface by re adsorption onto authigenic fe oxyhydroxides and deeper in the sediments where dissolved sulfide concentrations are higher the estimated rate constant for the adsorption of mo onto fe oxyhydroxides is 36 45 cm3 mol 1 s 1 diagenetic modeling indicates that authigenic mo in sediments of the perennially oxic basin represents about one third of the total solid phase mo in the first cm below the sediment water interface and only one tenth below this horizon if we assume that no authigenic mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment mo concentrations which are up to 3 16 times higher than the average lithogenic composition depending on the lake are mainly due to atmospheric deposition of anthropogenic mo and not to the formation of authigenic mo phases reconstructed historical records of the atmospheric mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then emissions to the atmosphere associated with the smelting of non ferrous ores and coal combustion appear to be the most important sources of anthropogenic mo

mississippian early carboniferous stromatolite mounds in a fore reef slope setting laibin guangxi south china the mississippian early carboniferous is generally a period of scarce carbonate buildups in south china this study documents outcrops of stromatolite mounds at mengcun and helv villages in laibin city guangxi province south china the stromatolite mounds contain various stromatolite morphologies including laminar wavy laminar domal or hemispheroidal bulbous and flabellate growth columns intramound rocks are brachiopod floatstone and dark thin bedded laminated micrite limestone individual stromatolites at mengcun village are generally 3 6 cm thick and morphologically represent relatively shallow water laminar planar and wavy undulated stromatolites and deeper water domal bulbous and columnar forms where mounds were formed the stromatolites continued growing upward up to 60 cm thick thrombolitic fabrics also occur but are not common stromatolite microscopic structure shows the bulk of the lamination to consist of wavy microbialite and discrete thin micritic laminae these mounds are intercalated in deep water fore reef talus breccia packstone formed as a bioclastic debris flow and thin bedded limestone containing common chert layers of the tatang formation late visean further evidence supporting the deep water setting of the stromatolite mounds are 1 a laterally thinning horizon of brachiopod floatstone containing deep water small thin shelled brachiopods peloidal micritic sediments and low diversity mixed fauna e g thin shelled brachiopods tube like worms and algae that have been interpreted as storm deposits 2 common fore reef talus breccias 3 lack of sedimentary structures indicating current action 4 preservation of lamination with sponge spicules and 5 lack of bioturbation suggesting that the stromatolites grew in a relatively low energy deep water setting the stromatolite mounds are the first described stromatolite mounds in mississippian strata of south china and contain evidence that supports interpretations of 1 growth history of mississippian microbial buildups and 2 environmental controls on stromatolite growth and lithification

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

anaerobic diagenesis of silica and carbon in continental margin sediments discrete zones of tco2 production pore water profiles of dissolved si ca2 so42 ch4 and tco2 dissolved inorganic carbon dic were determined from multicores and gravity cores collected at nine sites off southern california the west coast of mexico and within the gulf of california these sites were located within the eastern north pacific oxygen minimum zone at depths of 400 to 900 m and in settings where bottom water oxygen concentrations were 3 m and sediments were laminated pore water profiles were defined at a resolution of millimeters whole core squeezing centimeters sectioning and squeezing and meters gravity core sectioning and squeezing and diffusive fluxes were calculated for different zones within the sediment column the flux of dissolved silica across the sediment water interface swi ranged from 0 3 to 3 4 mmol si m 2d 1 and tco2 fluxes ranged from 0 8 to 4 6 mmol c m 2d 1 a positive correlation r 0 74 existed between these fluxes yet these two constituents exhibited significantly different diagenetic behavior downcore dissolved si generally reached a constant concentration between 450 and 900 m in the upper few cm whereas tco2 concentrations increased monotonically with depth methane was detected at micromolar levels in sediment intervals between 0 and 60 cm and at five sites increased to millimolar levels at depths of 80 to 170 cm at the horizon marking the appearance of millimolar levels of methane there was a distinct change in slope of the sulfate and tco2 gradients a flux budget for this horizon was determined by using linear fits to pore water profiles these budgets indicate that the upward tco2 flux away from this horizon is 40 to 50 greater than the downward sulfate flux to this horizon given that the tco2 flux to this horizon from below was quite small this imbalance suggests that anaerobic oxidation of methane by sulfate is not the only process producing tco2 within this horizon a budget for tco2 at this horizon is balanced when 40 to 80 of the sulfate flux is attributed to organic carbon remineralization of the dic that diffuses across the swi 20 to 40 is generated by reactions occurring within or below this deep reaction horizon

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

molybdenum isotopic evidence for oxic marine conditions during the latest permian extinction the latest permian extinction lpe ca 252 ma represents the most severe extinction event in earth s history the cause is still debated but widespread marine anoxic to euxinic h2s rich conditions from deep to shallow water environments are commonly suggested as a proxy for marine oxygen levels we analyzed 98 95mo of two lpe sections that represent a gradient in water depth on the northwest margin of pangea results from deep water slope environments show a large shift in 98 95mo values from 2 02 to 2 23 at the extinction horizon consistent with onset of euxinic conditions in contrast sub storm wave base shelf environments show little change in the molybdenum isotopic composition 1 34 to 0 05 indicating ongoing oxic conditions across the lpe these results indicate that areas of the continental shelf of northwest pangea underwent mass extinction under oxic conditions throughout the lpe event and that shallow water anoxia was therefore not a global phenomenon

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

an expanded shale δ98mo record permits recurrent shallow marine oxygenation during the neoarchean abstract multiple attempts have been made using the ancient shale record to track the molybdenum isotope composition 98mo of seawater during the final two hundred million years of the archean eon 2 7 to 2 5 billion years ago or ga this seawater 98mo value is important because it should have scaled with levels of ocean oxygenation during the runup to the great oxidation event goe unfortunately however it is difficult to tell if the majority of the existing late archean shale 98mo record tracks an ancient seawater value here we further attempt to track pre goe seawater 98mo using an expanded and well characterized shale sample set from western australia jeerinah wittenoom and mt sylvia formations and south africa nauga and klein naute formations most importantly and in contrast to most previous mo isotope studies of similarly aged shales local redox conditions for our shales have been independently constrained using the iron fe speciation proxy and bottom water mo contents at the time of deposition have been qualitatively estimated using mo toc ratios local redox conditions and bottom water mo availability are important parameters because transfer of the seawater 98mo to sediments today is shown to be dependent on these conditions according to our updated sedimentary 98mo record seawater 98mo commonly exceeded 1 0 between 2 69 ga and 2 50 ga in order to drive such a heavy seawater 98mo there must have been a marine sink with a strong preference for lighter mass mo isotopes frequently present over this timeframe the operation of some anaerobic processes in late archean marine settings could theoretically explain the heavier seawater 98mo such processes are known to promote the preferential retention of lighter mass mo isotopes in marine sediments today e g interactions between mo and organic matter or the formation of thio complexes alternatively or in addition adsorption of lighter mass mo isotopes to fe and mn oxide minerals formed in oxygenated marine environments can explain the heavier seawater 98mo a compilation of previous work suggests that oxygenated shallow marine environments were fairly common during the late archean and thus mo adsorption to fe mn oxides formed in these settings probably played an important role in driving heavy seawater 98mo over this timeframe

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

diagenesis in anoxic sediments from the california continental borderland and its influence on iron sulfur and magnetite behavior solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical magnetic and x ray diffraction techniques in three california continental borderland basins the observed profiles have been fit by simple reaction diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur more than 95 of the solid phase reduced sulfur is pyrite and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent depth integrated sulfate reduction rates for modern sediments in san pedro santa catalina and san nicolas basins are 11 4 6 3 and 6 3 8 8 mol cm 2yr 1 respectively measurements of solid phase iron species indicate that surficial sediments are enriched in easily reducible ferric oxyhydroxides the enrichment is maintained by a combination of oxidation of fe2 diffusing upward from underlying anoxic sediments as well as input of fresh sediment enriched in ferric oxyhydroxides the three primary sources for iron converted to pyrite and the sequence in which they are utilized are ferric oxyhydroxides magnetite and other crystalline oxides and exchangeable iron in phyllosilicates the majority 50 80 of the iron converted to pyrite is from the silicates and budgetary calculations indicate the amount of iron released from san pedro basin silicates agrees within 35 with the amount of magnesium removed from pore water to solid phases fe2 is enriched in near surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation at depth pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates sulfide produced at depth diffuses upward until it reaches sediments with available iron causing a step like increase in solid phase sulfur concentration over 90 of the magnetite present in surficial sediments is dissolved at depth due to reaction with h2s a model is developed to predict the depth at which magnetite dissolution should occur based on sulfate reduction rates and the flux of ferric oxyhydroxides the results of this model predict the onset of dissolution at depths of 5 40 cm in different basins and agree well with the observed depths of magnetite dissolution

preservation of particulate non lithogenic uranium in marine sediments particulate non lithogenic uranium pnu excess u above detrital background levels found in marine particulate matter is formed in surface waters throughout the ocean previous studies have shown that pnu is regenerated completely prior to burial of particles in sediments within well oxygenated open ocean regions however the fate of pnu has never been examined in ocean margin regions or in anoxic basins here we evaluate the preservation of pnu in ocean margin sediments and within semi enclosed basins using samples from sediment traps deployed at multiple depths and surface sediments organic carbon fluxes at the sediment trap locations ranged from 0 1 to 4 3 g cm 2 kyr while the dissolved oxygen concentration in the water column ranged from 3 mt o 270 m preservation of pnu increases with decreasing dissolved oxygen concentration approaching 100 preservation at oxygen concentration 25 m pnu contributes as much as 40 to 70 of the total authigenic u in sediments in the santa barbara basin and seasonally anoxic saanich inlet and some 10 to 50 of the total authigenic u in sediments off the central california margin copyright 2002 elsevier science ltd

intense reducing conditions during the last deglaciation and heinrich events h1 h2 h3 in sediments from the oxygen minimum zone off goa eastern arabian sea abstract two sediment cores abp 32 gc 01r and abp 32 gc 03 were collected at a water depth of 642 m and 1086 m off goa from the present day oxygen minimum zone omz of eastern arabian sea eas cover time span of last 18 ka and 32 ka respectively were analysed for multi proxy redox sensitive elements to understand the variation in the redox conditions and factors responsible for its development redox sensitive elements concentration and their normalized ratios mn al u th mo al and ce ce suggest that sediment core abp 32 gc 01r is under more reducing conditions due to its location within the centre of omz compared to core abp 32 gc 03 which is at the base of omz sediments from the eas are of non euxinic environments where dissolved sulfide is present but restricted to the sediment pore waters lack of significant correlation r n yb n ratio is 1 during the last 10 ka holocene 14 10 ka includes younger dryas and b lling aller d 18 14 ka last deglaciation and heinrich events suggesting less reducing terrigenous dominated and intense reducing condition respectively

controls on trace metal authigenic enrichment in reducing sediments insights from modern oxygen deficient settings any effort to reconstruct earth history using variations in authigenic enrichments of redox sensitive and biogeochemically important trace metals must rest on a fundamental understanding of their modern oceanic and sedimentary geochemistry further unravelling the multiple controls on sedimentary enrichments requires a multi element approach of the range of metals studied most is known about the behavior of fe mn and mo in this study we compare the authigenic enrichment patterns of these elements with a group whose behavior is not as well defined cd cu zn and ni in three oxygen poor settings the black sea the cariaco basin venezuela and the peru margin these three settings span a range of biogeochemical environments allowing us to isolate the different controls on sedimentary enrichment our approach relying on the covariation of elemental enrichment factors ef defined for element x as efx x al sample x al lithogenic has previously been applied to mo and u to elucidate paleoenvironmental information on for example benthic redox conditions the particulate shuttle and the evolution of water mass chemistry we find two key controls on trace metal enrichment first the concentration of an element in the lithogenic background sediment used in calculating efx controls the magnitude of potential enrichment maximum enrichment factors of 376 and 800 are calculated for mo 1 ppm in detrital sediments and cd 0 3 ppm respectively compared to values not greater than 17 in any setting for the other five metals 45 ppm to 4 5 wt in detrital sediments second there is a relationship between the aqueous concentration of the element in overlying seawater and its degree of enrichment in the sediment we further identify four important processes for delivery of trace metals to the sediment these are 1 cellular uptake especially important for zn and cd 2 interaction co precipitation with sulfide mo cu and cd 3 passive scavenging via the traditional particulate shuttle mo ni and cu and 4 an association with the benthic fe redox shuttle mn ni finally we summarize the oceanic mass balance of cd and mo and place the first constraints on the contribution of reducing sediments to the oceanic mass balance of cu zn and ni we show that reducing sediments are the ultimate repository for up to half the total output flux of these elements from the oceanic dissolved pool

transport and release of transition elements during early diagenesis sequential leaching of sediments from manop sites m and h part i ph 5 acetic acid leach sediments from manop sites m and h in the eastern tropical pacific ocean can be partitioned into operationally defined phases by means of a sequential leach procedure this paper reports the results of the first leach in the sequence an acetic acid solution buffered as p h 5 with sodium acetate this leach is designed to remove carbonate bound and sorbed cations the only cation bound in a consistent ratio to calcium in the carbonate is strontium the molar ratio is 2 10 3 in contrast transition metals are sorbed onto the surfaces of other sedimentary particles the proportions sorbed range from 2 to 10 of the total manganese about 10 of the total nickel and copper and less than 1 of the total iron the pool of sorbed metals in surface sediments is sufficiently large and the rate of biological stirring is sufficiently rapid for this metal reservoir potentially to dominate the porewater reservoir in supplying metals to ferromanganese nodules a simple model for nodule growth based upon transfer of metals through this reservoir suggests that only 1 of the sedimentary sorbed metals within a radius of 2 to 9 cm is required to support typical nodule growth rates

molybdenum in the northeast pacific ocean1 the concentration of dissolved molybdenum has been determined in several profiles from the northeast pacific ocean the salinity normalized concentration is essentially constant in all open ocean samples 107 nm 1 2 5 nm the dissolved distribution is not significantly affected by biological cycling

molybdenum isotope multiple sulfur isotope and redox sensitive element behavior in early pleistocene mediterranean sapropels abstract organic rich sediments sapropels deposited in the mediterranean are presumed to have formed during periods of increased productivity and or deep water oxygen depletion possibly including the development of sulfidic conditions euxinia geochemical redox proxies re mo mo isotopes v fe al and multiple s isotopes in 8 sapropels from the pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition these same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel forming episodes in one intensively sampled sapropel deposited between 1 450 and 1 458 ma changing concentrations of organic carbon ba re mo v and fe al track one another closely reflecting coupling between water column euxinia and biological productivity multiple s isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation but moved into the water column during sapropel deposition molybdenum isotopes of these 8 sapropels encompass a range of values 98mo 0 2 to 1 7 but are all 98mo depleted relative to seawater 98mo 2 3 suggesting that quantitative removal of mo did not occur this finding contrasts with modern black sea sediments in general re mo ratios in sapropels are greater than in modern seawater implying that the water column was not sufficiently sulfidic during sapropel forming episodes to induce complete removal of both these elements surprisingly the heaviest 98mo values are found within hemipelagic sediments very few of the hemipelagic samples preserve the negative 98mo values commonly associated with modern oxic marine sediments many of the hemipelagic samples also contained higher concentrations of re and mo than are common in oxic sediments these features may be attributable to diffusion from the sapropels of a 98mo enriched component into the hemipelagic sediments

atypical mo isotope signatures in eastern mediterranean sediments suboxic sapropel s1 sediments of the eastern mediterranean carry an untypically light 98 95mo signal being even lighter than oxic mn enriched sediments and mn crusts nodules which were previously thought to carry the lightest mo isotope values the evaluation and comparison of oxic s1 diagenetically overprinted oxic s1 and suboxic s1 sediments indicates that this light isotope signal is primarily an oxic signal that has been overprinted by secondary diagenetic processes occurring after sediment burial such secondary processes bear the potential of additional mo isotope fractionation in particular in non steady state diagenetic environments that involve the discontinuous re location of the redox boundary

methane along the western mexican margin we investigated the processes controlling the water column and sediment methane distributions at 14 stations along the western mexican margin in and around the gulf of california stations were grouped into two categories coastal basins and open margins diffusive methane fluxes from the sediment at all sites as estimated from sediment methane gradients were 0 24 5 5 mmol m 22 d 21 with the highest fluxes observed on the pacific margin of baja california at both basin and open margin sites these high rates occur despite the lack of significant terrestrial input to these sediments reflecting the importance of upwelling induced productivity methane concentrations in the upper water column were supersaturated with respect to the present atmosphere at all sites with sea air fluxes of methane of 0 5 5 9 mmol m 22 d 21 four of the open margin sites had seafloor depths extending below the oxygen minimum zone 400 800 m and contained low methane concentrations below the subsurface methane maximum the remaining margin site was shallow 593 m with a seafloor that intersected the oxygen minimum zone and had elevated methane concentrations throughout the water column this indicates that such sediments may be a significant source of methane to the eastern tropical north pacific etnp the seawater within silled basin sites also had supersaturated methane concentrations reflecting the anoxic conditions within the basins however methane levels were low at the sill depth indicating that the silled basins were unlikely to be significant sources of methane to the etnp we observed an inverse relationship between methane concentration and d 13 c ch4 value in the basin waters consistent with biological aerobic oxidation of methane being released from the sediment an apparent kinetic isotopic fractionation factor of 1 0100 0 0038 was calculated for this process isotopically heavy methane resulting from similar oxidation of seafloor derived methane may be the source of large pools of heavy methane previously

growth rates of manganese rich nodules at manop site h eastern north pacific abstract estimates of the growth rates of ferromanganese nodules at manop site h 6 33 n 92 49 w lie between 7 and 1200 mm my the most probable range estimated from nodule size distributions nodule chemistry accumulation rates of authigenic elements and u series modeling is 30 300 mm my such rates are much faster than measured values for most pelagic nodules the distributions of u series and stable elements suggest that a typical nodule consists of two compositionally distinct non detrital components one rich in mn u and sb and the other in fe co ree ni cu and th when the uncorrected u series data which do not show simple decay patterns with depth are normalized to the relative proportion of the fe rich component the data yield well constrained growth rates of 55 mm my for a nodule top and 200 mm my for its bottom the component enriched in mn which is most abundant on nodule bottoms appears to accrete much faster than the component enriched in fe

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

multiproxy reconstruction of oceanographic conditions in the southern epeiric kupferschiefer sea late permian based on redox sensitive trace elements molybdenum isotopes and biomarkers abstract the key drivers controlling the redox state of seawater and sediment pore waters in low energy environments can be inferred from redox sensitive trace elements rste molecular biomarkers and trace metal isotopes here we apply a combination of these tools to the upper permian kupferschiefer t1 from the thuringian basin deposited in the southern part of the semi enclosed kupferschiefer sea enrichment patterns of the rstes molybdenum mo and uranium u as well as biomarker data attest to the rapid development of euxinic conditions in basin settings during early t1 times which became progressively less extreme during t1 deposition the evolution of redox conditions in basinal settings and the associated delay in the onset of euxinia at more shallow marginal sites can be attributed to the interaction of sea level change with basin paleogeography euxinia in the southern kupferschiefer sea did not lead to near quantitative depletion of aqueous mo possibly due to short deepwater renewal times in the thuringian basin low aqueous h 2 s concentrations the continuous resupply of rste during transgression and declining burial rates of rstes throughout t1 times drawdown of rste is however indicated for euxinic lagoon environments moreover admixture of freshwater supplied to these lagoons by rivers strongly impacted local seawater chemistry the highest mo isotope compositions of 1 70 in basin sediments allow a minimum kupferschiefer sea seawater composition of 2 40 to be estimated this composition is similar to the 2 30 estimate for the late permian open ocean and confirms a strong hydrographic connection between the epeiric kupferschiefer sea and the global ocean the substantial variation in mo isotope signatures is paralleled by diagnostic shifts in biomarkers responding to oxygenation in different parts of the water column water column chemistry has been affected by variation in sea level hydrodynamic restriction riverine freshwater influx and evaporitic conditions in shallow lagoons elucidation of the relative role of each driving factor by a single geochemical proxy is not feasible but the complex scenario can be disentangled by a multiproxy approach

hydrogen sulphide release to surface waters at the precambrian cambrian boundary changes in environmental conditions at the precambrian cambrian transition around 542 million years ago have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the cambrian explosion the nature of the environmental changes is still a matter of debate however wille et al now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years with the help of a box model of the oceanic molybdenum cycle they find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal this suggests that the early cambrian animal radiation may have been triggered by a major change in ocean circulation following a long period during which the ocean was stratified with sulphidic deep water this paper reports molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 million years the findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water animal like multicellular fossils appeared towards the end of the precambrian followed by a rapid increase in the abundance and diversity of fossils during the early cambrian period an event also known as the cambrian explosion 1 2 3 changes in the environmental conditions at the precambrian cambrian transition about 542 myr ago have been suggested as a possible explanation for this event but are still a matter of debate1 2 3 here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 myr we find a transient molybdenum isotope signal immediately after the precambrian cambrian transition using a box model of the oceanic molybdenum cycle we find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal our findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water

records of trace metals in sediments from the oregon shelf and slope investigating the occurrence of hypoxia over the past several thousand years article i nfo hypoxic b62 mol kg or 1 43 ml l o2 to anoxic conditions have been repeatedly observed over the last 10 years on the oregon shelf while similar conditions are absent in historical records from 1950 to 1999 this study seeks to identify whether similar instances of decadal length hypoxia anoxia have occurred in the oregon coastal zone prior to recorded history and to shed light on potential causes for these events we have measured redox sensitive metals uranium vanadium andmolybdenumconcentrationprofilesin7coresacrossthecoastal affected region and in 3 cores from deeper water sites results indicate regional variability in redox conditions through time the northern sites show no metal enrichment throughout the cores while the southern sites show strong metal enrichment at the base of the cores indicative of previous hypoxic anoxic conditions the southernsitesindicateprogressionintimetowardlesshypoxic anoxicburial inconflictwithrecenthydrograph

inverse correlation between the molybdenum and uranium isotope compositions of upper devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations abstract coupled mo u isotope data from the upper devonian kettle point formation ontario canada provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales in the gore of chatham core the kettle point black shales have high mo concentrations 48 473 g g and consistently high mo u ratios 3 times the mo u ratio of modern seawater suggesting a euxinic depositional environment these shales yield an inverse correlation r 0 89 p

molybdenum isotope variations in molybdenite vapor transport and rayleigh fractionation of mo molybdenum isotopes in 20 molybdenite samples dated by the re os method and representing a range of geologic settings show mass dependent fractionation spanning 0 63 per atomic mass unit amu previous mo isotope data for molybdenite reveal variations in fractionation of 0 5 amu interpretation of these data is hampered however by limited sample numbers in each study lack of a common standard for interlaboratory comparison and limited range of geologic settings here we show that mo isotope compositions of molybdenites do not correlate with crystallization temperature age geographic distribution or geologic conditions rather rayleigh distillation may explain variations of as much as 0 34 amu in a single molybdenite occurrence exceeding the proposed variability in average continental crust vapor transport and rapid precipitation of mo in propagating fractures may account for isotope fractionation of mo and perhaps other metals at very small scales if so the average isotopic composition of mo at each molybdenite occurrence may be representative of bulk crust our results suggest that the isotopic composition of mo delivered to the oceans is uniform geographically and through geologic time

variations in denitrification and ventilation within the arabian sea oxygen minimum zone during the holocene the continental slope of india is exposed to an intense perennial oxygen minimum zone omz supporting pelagic denitrification sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of omz during the past 9 500 years the 15n of sediment suggests that the omz waters were less ventilated during the early holocene between 9 5 and 8 5 ka bp resulting in intensified denitrifying conditions with an average 15n value of 7 8 while at the same time stable mo isotope composition average 98mo of 0 02 indicates that the bottom waters that were in contact with the sediments were better oxygenated by the mid holocene omz became more oxygenated suppressing denitrification average 15n of 6 2 while bottom waters gradually became less oxygenated average 98mo of 1 7 the mid holocene reduction in denitrification coincided with a global decrease in atmospheric n2o as inferred from ice core records which is consistent with a decreased contribution from the arabian sea since 5 5 ka bp omz waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

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

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

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

sulfidity controls molybdenum isotope fractionation into euxinic sediments evidence from the modern black sea molybdenum mo isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry it is therefore fundamental to understand the processes controlling mo partitioning into modern marine environments this study identifies the availability of dissolved sulfide as the dominant control on overall mo removal from the water column in euxinic systems mo isotopic composition of surface sediments from different localities of the black sea demonstrates complete fixation of mo only below 400 m water depth above a critical concentration of 11 mol l 1 aqueous hydrogen sulfide in the bottom water the mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2 3 in contrast significant mo isotope fractionation into less euxinic sediments is evident at shallower depths in the black sea as well as in temporarily euxinic deeps of the baltic sea consistent with the observed lower maximum sulfide concentrations in the respective water columns therefore mo isotope signatures in the modern black sea constrain the processes responsible for global mo removal from the ocean by euxinic sediments furthermore models of past ocean anoxia reconstruction have to consider that the seawater mo isotopic composition is not per se archived in euxinic 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

tracking the evolution of seawater mo isotopes through the ediacaran cambrian transition abstract the rapid increase in animal diversity and abundance over the ediacaran cambrian boundary has been attributed to among other parameters pronounced earth surface oxygenation at that time however whether atmospheric oxygen elevated to modern levels or whether the extent was smaller and oxygen remained relatively muted until the late paleozoic is currently debated here we present new mo isotope datasets of drill core material for two key black shale units the ediacaran doushantuo formation and the lower cambrian jiumenchong formation located on the yangtze block these data capture a 0 7 isotope fractionation between sediments experiencing quantitative and non quantitative mo removal by identifying this offset we provide integrated estimates for global seawater mo isotope compositions during the deposition of these two formations 1 2 1 4 and 1 8 2 respectively when paired with a mass balance model our results indicate that a large proportion of the early cambrian ocean was bathed under oxic waters we also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans

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

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

sedimentary molybdenum cycling in the aftermath of seawater inflow to the intermittently euxinic gotland deep central baltic sea abstract molybdenum mo concentrations and isotope compositions in sediments and shales are commonly used as proxies for anoxic and sulfidic i e euxinic conditions in the water column of paleo marine systems a basic assumption underlying this practice is that the proxy signal extracted from the geological record is controlled by long term order of decades to millennia mo scavenging in the euxinic water column rather than mo deposition during brief episodes or events order of weeks to months to test whether this assumption is viable we studied the biogeochemical cycling of mo and its isotopes in sediments of the intermittently euxinic gotland deep in the central baltic sea here multiannual to decadal periods of euxinia are occasionally interrupted by inflow events during which well oxygenated water from the north sea penetrates into the basin during these events manganese mn oxyhydr oxide minerals are precipitated in the water column which are known to scavenge mo we present sediment and pore water mo and mo isotope data for sediment cores which were taken before and after a series of inflow events between 2014 and 2016 after seawater inflow pore water mo concentrations in anoxic surface sediments exceed the salinity normalized concentration by more than two orders of magnitude and coincide with transient peaks of dissolved mn a fraction of the mo liberated into the pore water is transported by diffusion in a downward direction and sequestered by organic matter within the sulfidic zone of the sediment diffusive flux calculations as well as a mass balance that is based on the sedimentary mo isotope composition suggest that about equal proportions of the mo accumulating in the basin are delivered by mn oxyhydr oxide minerals during inflow events and mo scavenging with hydrogen sulfide during euxinic periods since the anoxic surface sediment where mo is released from mn oxyhydr oxides are separated by several centimeters from the deeper sulfidic layers where mo is removed the solid phase record of mo concentration and isotope composition would be misinterpreted if steady state mo accumulation was assumed based on our observations in the gotland deep we argue that short term redox fluctuations need to be considered when interpreting mo based paleo records

role of a strong oxygen deficient zone in the preservation and degradation of organic matter a carbon budget for the continental margins of northwest mexico and washington state rates of organic carbon oxidation in marine sediments were determined for the continental margins of northwest mexico and washington state with the goal of assessing the role of oxygen in the preservation of organic matter on a margin with a strong oxygen deficient zone and on a typical western continental margin total carbon oxidation rates including rates for individual electron acceptors o2 no3 and so4 were determined at depths ranging from 100 to 3000 m on both margins carbon oxidation rates were generally higher on the washington margin than on the mexican margin the relative importance of the different electron acceptors varied across the two margins and was related primarily to the availability of o2 and no3 from the overlying water the relative contribution of o2 consumption increased in deeper sediments 2000 m as aerobic processes began to dominate the total carbon oxidation rate denitrification rates were highest in washington sediments however denitrification represented a larger fraction of the total carbon oxidation rate in the mexican sediments 40 for mexico vs 30 for washington sulfate reduction accounted for as much as 79 of the total carbon oxidation rate in shallow sediments and less than 20 in deep sediments on both margins the offshore trends in carbon oxidation rate appeared to be related to the organic carbon input rate pore water o2 and no3 penetration depths were shallowest in nearshore stations and increased offshore regeneration ratios of c n p reveal non redfield behavior on both margins carbon budgets for the two margins demonstrate that off mexico a much greater percentage of the organic matter produced in the surface ocean reached the sediments 15 vs 8 for mexico and washington respectively on the mexican margin 8 of the primary production escaped oxidation in the surface sediments to be permanently buried as compared with only 1 2 of the primary production on the washington margin this suggests that oxygen deficient conditions on mexican margin are linked to enhanced carbon preservation

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

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

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

heterogeneous oxygenation states in the atlantic and tethys oceans during oceanic anoxic event 2 abstract the cenomanian turonian boundary ca 93 5 ma is marked by an episode of profound environmental change including a major perturbation of the carbon cycle and an oceanic anoxic event oae 2 here we present molybdenum mo isotope variations within the oae 2 interval for four sections from the western tethys furlo and la contessa and the north atlantic odp site 1276 and dsdp site 367 the main target of this study is to investigate the extent of reducing conditions truly global in extent or restricted to poorly ventilated restricted deep basins with particular reference to the relationship between the change in the oxygenation state of the ocean and the link to global perturbations of the carbon cycle recorded in carbon isotopes all four sections show fluctuations in the redox sensitive trace metal rste distribution suggesting rapid variations in local redox conditions ranging from anoxic to euxinic the rste enrichment factors efs also suggest different depositional conditions and paleoceanographic processes in the western tethys versus the north atlantic whereas the north atlantic sites show evidence of weak watermass restriction associated with the action of a particulate shuttle within the water column the efs of the tethyan sections are characteristic of unrestricted marine systems mo isotopes show surprisingly negative values through the tethyan sections at the onset of oae 2 an increasing trend in mo 98 is observed with values ranging from 0 6 to 0 6 during the second half of oae 2 the mo 98 curve shows a progressive shift towards more negative values in the north atlantic mo 98 signatures from odp site 1276 show a similar behaviour as observed in the western tethys at dsdp site 367 mo isotopes are generally heavier during oae 2 fluctuating around an average value of 1 1 this is consistent with fully euxinic conditions and the black shales deposited may have recorded the seawater signature during oae 2 the mo isotope signature is lighter than the modern ocean value 2 3 and suggests a higher proportion of bottom water euxinia 10 although this estimate is greater than the euxinic fraction in the modern ocean our results challenge the suggestion of a globally euxinic deep waters during oae 2 the variations in mo toc ratios within the north atlantic and tethys oceans are in good agreement with a general drawdown of mo within the oae interval suggesting a general increase in anoxic conditions during oae 2 our results highlight the role of local parameters in the development of euxinia even during period of widespread anoxia such as oaes

a molybdenum isotope record of eocene thermal maximum 2 implications for global ocean redox during the early eocene during the early eocene a series of short term global warming events hyperthermals occurred in response to the rapid release of carbon into the oceans and atmosphere in order to investigate the response of ocean redox to global warming we have determined the molybdenum isotope compositions 98 95 mo of samples spanning one such hyperthermal eocene thermal maximum 2 etm 2 54 1ma from integrated ocean drilling program expedition 302 site m0004a in the arctic ocean the highest 98 95 mo in our sample set 2 00 0 11 corresponds to the development of local euxinia at site m0004a during the peak of etm 2 which we interpret as recording the global seawater 98 95 mo at that time the etm 2 seawater 98 95 mo is indistinguishable from a recent estimate of seawater 98 95 mo from an earlier hyperthermal paleocene eocene thermal maximum petm 55 9ma 98 95 mo 2 08 0 11 it is argued that the similarity in seawater 98 95 mo during etm 2 and the petm was caused by the development of transient euxinia in the arctic ocean during each hyperthermal that allowed sediments accumulating in this basin to capture the long term 98 95 mo of early eocene seawater our new data therefore place a minimum constraint on the magnitude of transient global seafloor deoxygenation during early eocene hyperthermals

links between seawater paleoredox and the formation of sediment hosted massive sulphide shms deposits fe speciation and mo isotope constraints from late devonian mudstones abstract many models of sediment hosted massive sulphide shms deposit formation invoke basin restriction events that resulted in long term stagnation and anoxia in which sulphidic euxinic conditions ultimately prevailed euxinic conditions are then thought to provide a chemical trap for hydrothermally exhaled base metals here we present fe speciation and mo isotope data for organic rich mudstones from two drill holes intersecting upper devonian strata deposited along the passive margin of ancestral north america one drill hole intersects a 35 m thick sequence of shms mineralisation while the other intersects correlative un mineralised strata all samples have high fehr fet values 0 38 indicating water column anoxia for the majority of samples in the un mineralised drill hole the levels of pyritisation fall below the threshold typically used to define euxinic conditions fepy fehr 0 70 in contrast higher levels of pyritisation in the mineralised drill hole median fepy fehr 0 86 likely developed via diagenetic pyrite enrichment whereas pb and zn are negatively correlated with mo mo u co variation is consistent with fe oxyhydr oxide particulate shuttling in the water column in addition a weak correlation between toc p and mo provides further evidence that mo was sourced via authigenic rather than hydrothermal processes the 98mo values 0 66 to 1 02 are uniform between both drill holes and substantially lower than constraints for late devonian seawater 1 5 to 2 0 consistent with mo adsorption to fe oxyhydr oxides collectively the data provide evidence that local seawater was dominantly ferruginous anoxic non sulphidic at macmillan pass regional variability in the extent of ferruginous low toc p and euxinic high toc p conditions likely contributed to a balance between p regeneration and p enrichment that maintained nutrient availability and productivity in the selwyn basin during the late devonian we argue that high primary productivity and enhanced organic carbon burial are key variables for promoting sulphate reduction in the sub surface moreover how such conditions are maintained over long periods of basin evolution is more important for producing effective metal traps in shms systems rather than a specific localised redox condition of seawater i e euxinia

variations in the isotopic composition of molybdenum in freshwater lake systems variations in molybdenum isotopic composition spanning the range of similar to 2 3 parts per thousand in the terms of mo 97 mo 95 ratio have been measured in sediment cores from three lakes in northern sweden and north western russia these variations have been produced by both isotopically variable input of mo into the lakes due to mo isotopic heterogeneity of bedrock in the drainage basins and fractionation in the lake systems due to temporal variations in limnological conditions mo isotope abundances of bedrock in the lake drainage basins have been documented by analysis of mo isotope ratios of a suite of molybdenite occurrences collected in the studied area and of detrital fractions of the lake sediment cores the median delta mo 97 value of the investigated molybdenites is 0 26 parts per thousand with standard deviation of 0 43 parts per thousand n 19 whereas the median delta mo 97 value of detrital sediment fractions from two lakes is 0 40 parts per thousand with standard deviation of 0 36 parts per thousand n 15 the isotopic compositiori of mo in the sediment cores has been found to be dependent on redox conditions of the water columns and the dominant type of scavenging phases hydrous fe oxides have been shown to be an efficient scavenger of mo from porewater under oxic conditions oxidative precipitation of fe ii in the sediments resulted in co precipitation of mo and significant authigenic enrichment at the redox boundary in spite of a pronounced increase in mo concentration associated with fe oxides at the redox boundary the isotopic composition of mo in this zone varies insignificantly suggesting little or no isotope fractionation during scavenging of mo by hydrous fe oxides in a lake with anoxic bottom water a chironomid inferred reconstruction of o 2 conditions in the bottom water through the holocene indicates that increased o 2 concentrations are generally associated with low delta mo 97 mo 95 values of the sediments whereas lowered o 2 contents of the bottom water are accompanied by relatively high delta mo 97 mo 95 values thus confirming the potential of mo isotope data to be a proxy for redox conditions of overlying waters however it is pointed out that other processes including input of isotopically heterogeneous mo and mn cycling in the redox stratified water column can be a primary cause of variations in mo isotopic compositions of lake sediments less

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

tungsten molybdenum fractionation in estuarine environments abstract dissolved tungsten w and molybdenum mo concentrations were measured in surface waters and sediment pore waters of terrebonne bay a shallow estuary in the mississippi river delta to investigate the biogeochemical processes that fractionate these group 6 elements relative to one another during transit from weathering to sedimentary environments although many of the chemical properties of w and mo are similar the two elements behave autonomously and the fractionation mechanisms are only partly understood in sulfidic pore waters dissolved mo is depleted relative to river water seawater mixtures whereas dissolved w is 10 fold enriched reductive dissolution of poorly crystalline phases like ferrihydrite which is a preferential host of w relative to mo in grain coatings on river borne particles can explain the dissolved w enrichment dissolved w becomes increasingly enriched as h 2 s aq rises above about 60 m due to transformation of wo 4 2 to thiotungstates as well as to additional reductive dissolution of phases that host w in contrast as rising sulfide transforms moo 4 2 to thiomolybdates in pore waters dissolved mo is suppressed probably owing to equilibration with an fe mo s phase this putative phase appears to control the aqueous ion product q fe 2 mos 4 2 0 6 h 2 s 0 0 4 h 0 8 at a value of 10 7 78 concentrations of dissolved w and mo in pore waters bear no relation to concentrations in surface waters of the same salinity in surface waters dissolved mo is nearly conserved in the estuarine mixing zone dissolved w appears also to be conserved except for several cases where w may have been enhanced by exchange with underlying w rich pore waters with increasing salinity the molar mo w ratio rises from about 10 to about 1000 in surface waters whereas it is mostly vi is more susceptible to reduction than w vi but the latter is more prone to adopting octahedral coordination we propose that the first difference facilitates mo but not w precipitation in sulfidic sediments whereas the second explains tungsten s proportionately greater sequestration on river borne particles and its subsequent release to sulfidic pore waters after the particles are deposited in the delta and become subject to reductive dissolution during diagenesis

early diagenesis of organic matter in peru continental margin sediments phosphorite precipitation abstract pore water chemistry total dissolved co 2 nh 4 no 3 no 2 po 4 si oh 4 ca mg fe mn so 4 h 2 s and f and titration alkalinity solid phase chemistry c org p org c tot n tot f si opal and s ii and sediment characteristics porosity dry bulk density and formation factors were determined on a centimeter scale spacing in the upper 20 40 cm of sediments under intense upwelling areas on the peru continental shelf these data demonstrate that carbonate fluorapatite cfa is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content up to 20 c org very high porosity 0 96 ml cm 3 and very low dry bulk density 3 dissolved phosphate concentrations at the sediment water interface range from 20 to 100 m orders of magnitude higher than bottom water concentrations and much higher than predicted from regeneration of organic matter the mechanism of this interfacial phosphate release is unclear but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments fluoride is incorporated into cfa by diffusion from the overlying seawater and carbonate ions are provided from pore water alkalinity magnesium concentrations in this reaction zone are not significantly different from those of seawater suggesting that magnesium depletion is not a necessary prerequisite for cfa precipitation the environment of precipitation is interface linked rather than driven by organic diagenesis of phosphorus deeper in the sediment most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region but almost all show the precipitation signature near the interface this interface linked early diagenetic porewater environment for the precipitation of cfa explains many of the geochemical characteristics of phosphorites and provides a testable model to compare the modern phosphogenic analog with ancient phosphorite deposits two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites