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

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

controls on mo isotope fractionations in a mn rich anoxic marine sediment gullmar fjord sweden abstract molybdenum isotopes have emerged as an important tool for determining the spatial extent of past ocean redox conditions however our understanding of mo cycling and associated isotopic fractionations in modern and ancient continental margin sediments is limited restricting attempts to fully utilize the potential of mo isotopes this is particularly the case with regard to the relative significance of mo isotope fractionations associated with biogeochemical mn and fe oxide cycling under conditions of low dissolved sulfide to rectify this we have investigated sediments from gullmar fjord sweden which contain high concentrations of mn oxides and exhibit a strong redox zonation between mn and fe reduction a combination of dissolved and particulate mo isotope measurements with porewater and solid phase mn and fe characterization reveals that the main influence on porewater 98 mo compositions is exerted by adsorption and desorption processes during reductive dissolution of both mn and fe oxyhydr oxide minerals in areas enriched in mn oxides distinctively low solid phase 98 mo values are observed 1 0 and 0 4 which occur due to the large mo isotope fractionation associated with mo adsorption to mn oxides however where mn oxide concentrations are lower authigenic 98 mo is largely controlled by fe oxyhydr oxide minerals resulting in more positive 98 mo isotopic compositions 0 5 and 2 our results suggest that for modern anoxic marine sediments where deposition occurs beneath an oxic to suboxic water column 98 mo is dominantly controlled by the relative availability of mn and fe oxides and the production of dissolved sulfide

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

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

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

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

extreme change in sulfide concentrations in the black sea during the little ice age reconstructed using molybdenum isotopes the black sea is the largest and most studied anoxic basin in the modern world much of this research has focused on the redox structure of the water column specifically on the driving forces behind variations in the position stability and structure of the oxic anoxic interface chemocline however none of these studies has been able to quantify the historical sulfide concentrations associated with the changes in chemocline depth using the isotopic composition of molybdenum in sediments as a proxy we show for the first time that varying concentrations of dissolved sulfide can be fingerprinted in historical systems our molybdenum isotope data indicate that in the region of the bosporus inlet the chemocline rose more than 65 m reaching concentrations over 100 m sulfide in the bottom water ca 300 yr b p this historical shoaling of the chemocline and extreme change in bottom water sulfide concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic influences on the black sea chemistry hydrology the first cold interval of the little ice age when temperature and circulation changes occurred in the black sea basins may have provided the natural trigger for this extreme rise in bottom water sulfide concentrations

an early permian coastal flora dominated by germaropteris martinsii from basinal sediments in the midland basin west texas abstract fossils found in cores from wells in the midland basin of west texas include several kinds of terrestrial plants and a variety of marine animal remains depositional settings ranged from basin slope to deep water basin floor hence the presence of land plants was unexpected the fossil plant assemblage is depauperate dominated by germaropteris martinsii a permian age peltasperm other specimens include the peltasperm supaia sphenopteris germanica axes of uncertain affinity and incertae sedis remains presumed to be terrestrial plants fossil plants are found predominantly in fine grained siliceous mudrocks between coarser grained calcareous floatstones and wackestones packstones interpreted as debrites and turbidites suggesting that the plants were carried from land by surface currents before sinking to the basin floor and being buried by slowly accumulating hemipelagic sediment specimens were examined from drillcores in 14 wells spanning an interval from the lower wolfcamp through the lower leonard this record of g martinsii in lower permian wolfcamp rocks is among the earliest occurrences of these plants which have been found most abundantly in upper permian strata of western europe

the diagenetic origin and depositional history of the cherry valley member middle devonian marcellus formation abstract we report 13c 18o 34s records coupled with elemental pyrite morphology and fluid inclusion data spanning the cherry valley member in the middle devonian marcellus shale from two cores located across a depth gradient in the appalachian basin the goal of the study was to evaluate the diagenetic and redox history of this carbonate interbedded layer within a prominent shale gas system three lithological units were identified in the cherry valley member a nodular clay rich marlstone lower unit a fossiliferous limestone middle unit and a calcareous mudstone upper unit low 13c and 18o values of the carbonate matrix display a mixing trend among marine carbonate authigenic carbonate and late stage diagenetic cementation the petrology of barite nodules and associated 34sbarite values suggest that a shoaling sulfate methane transition zone created by methane advection contributed to the nodule growth and pyritization along with other early and late diagenetic processes this mechanism provided the main carbonate source for the lower unit due to elevated rates of aerobic methane oxidation the 34spyrite values u and mo contents and pyrite framboid size distributions together constrain the redox history of the cherry valley member an anoxic euxinic marine environment rather than a fully oxygenated environment continued through the lower unit in areas with deeper paleo bathymetry until it was interrupted by an episode of dysoxia during the deposition of the middle unit basin scale anoxia was then renewed during a subsequent 3rd order sea level rise in the upper unit the entire cherry valley member was ultimately altered by warm burial fluid interaction through vertically distributed veins the combined body of evidence indicates that the duration and spatial distribution of oxygenated marine conditions during the deposition of cherry valley member within the marcellus formation may have been overestimated

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

arctic black shale formation during cretaceous oceanic anoxic event 2 the late cretaceous oceanic anoxic event 2 oae2 represents a major perturbation of the global carbon cycle caused by the widespread deposition of organic rich black shales although the paleoceanographic response and the spatial extent of bottom water anoxia in low and mid paleolatitudes are reasonably well constrained for oae2 similar data from high paleolatitudes are lacking here we present palynostratigraphy and organic carbon isotope stratigraphy from the sverdrup basin axel heiberg island canada it is shown that episodes of high marine organic carbon burial at paleolatitudes of 70 n is contemporaneous with the widely observed occurrence of black shale deposition during oae2 paleontological lithological and geochemical data indicate normal marine conditions with persistent anoxic bottom waters during oae2 the results imply that the high marine primary productivity pulse during oae2 may have caused massive organic carbon burial on the arctic shelf in general with important implications for hydrocarbon source rock distribution in the arctic region

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

the manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins a meaningful application of mo as a paleo redox proxy requires an understanding of mo cycling in modern reducing environments stagnant euxinic basins such as the black sea are generally regarded as model systems for understanding euxinic systems during early earth history however drawing direct parallels between the black sea and open marine euxinic margins is somewhat complicated by differences in the seawater residence time between these two environments we report sediment and pore water mo u mn and fe data for a euxinic basin with a short seawater residence time the weakly restricted gotland deep in the baltic sea here prolonged periods of euxinia alternate with brief inflow events during which well oxygenated saline water penetrates into the basin during these inflow events dissolved mn and fe that has accumulated within the euxinic deep water can be oxidized and precipitated co variations of mo and u within the sediment suggest that these inflow and oxygenation events may favor mo accumulation in the sediment through adsorption to freshly oxidized mn and fe solid phases once mo is sequestered within the deeper euxinic water and sediments mo retention can be further facilitated by conversion to thiomolybdate species and interactions with organic matter and metal sulfides by comparing our data with those from previous studies where a mn and fe shuttle for mo has been demonstrated we identify two prerequisites for the occurrence of this mechanism first there must be a water column oxic anoxic redox boundary this provides a solubility contrast for mn and fe second the residence time of seawater in the system has to be short weeks to a few years the latter criterion can be met through regular inflow in weakly restricted basins or upwelling in oxygen minimum zones at open marine continental margins based on prior work we suggest that similar conditions to those currently represented by the gotland deep may have prevailed at euxinic ocean margins during the proterozoic a boundary between euxinic and oxic water masses overlying the continental shelf may have resulted in accelerated mo transport through the water column with mn and fe oxyhydr oxides we propose that this mechanism along with mo isotope fractionation during adsorption could contribute to the light mo isotope composition observed in open marine euxinic sediment facies of the proterozoic

molybdenum evidence for expansive sulfidic water masses in 750 ma oceans abstract the ediacaran appearance of large animals including motile bilaterians is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances po 2 to date direct evidence for low oxygen in pre ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time because the abundance and isotopic composition of molybdenum mo in organic rich euxinic sediments can vary in response to changes in global redox conditions mo geochemistry provides independent constraints on the global evolution of well oxygenated environments here we establish a theoretical framework to access global marine mo cycle in the past from the abundance and isotope composition of ancient seawater further we investigate the 750 ma walcott member of the chuar group grand canyon which accumulated in a rift basin with open connection to the ocean iron speciation data from upper walcott shales indicate that local bottom waters were anoxic and sulfidic consistent with their high organic content up to 20 wt similar facies in phanerozoic successions contain high concentrations of redox sensitive metals but in the walcott member abundances of mo and u as well as mo toc 0 5 ppm wt are low 98 mo values also fall well below modern equivalents 0 99 0 13 versus 2 35 today these signatures are consistent with model predictions where sulfidic waters cover 1 4 of the global seafloor corresponding to a 20 80 fold increase compared to the modern ocean therefore our results suggest globally expansive sulfidic water masses in mid neoproterozoic oceans bridging a nearly 700 million year gap in previous mo data we propose that anoxic and sulfidic euxinic conditions governed mo cycling in the oceans even as ferruginous subsurface waters re appeared 800 750 ma and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic o 2 level consistent with the hypothesis that po 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the ediacaran period

the behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic lake cadagno switzerland abstract molybdenum mo isotope studies in black shales can provide information about the redox evolution of the earth s oceans provided the isotopic consequences of mo burial into its major sinks are well understood previous applications of the mo isotope paleo ocean redox proxy assumed quantitative scavenging of mo when buried into sulfidic sediments this paper contains the first complete suite of mo isotope fractionation observations in a sulfidic water column and sediment system the meromictic lake cadagno switzerland a small alpine lake with a pronounced oxygen sulfide transition reaching up to h 2 s 200 m in the bottom waters or about 300 m total sulfide s 2 h 2 s hs s 2 we find that mo behaves conservatively in the oxic zone and non conservatively in the sulfidic zone where dissolved mo concentrations decrease from 14 nm to 2 8 nm across this transition dissolved mo in the upper oxic waters has a 98 mo oxic 0 9 0 1 which matches that of the riverine input 98 mo river 0 9 0 1 in the deeper sulfidic waters a subaquatic source delivers mo at 1 55 0 1 but the dissolved mo is even heavier at 98 mo sulfidic 1 8 sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in mo with depth with 98 mo values significantly fractionated at 0 8 to 1 2 both near the chemocline and in the deepest trap suspended particulates in the sulfidic waters carry lighter mo than the ambient dissolved mo pool by 0 3 1 5 sedimentary mo concentrations correlate with total organic carbon and yield mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area solid phase mo in the sediment shows a slightly positive 98 mo trend with depth from 98 mo 1 2 to 1 4 while the pore waters show dramatic enrichments of mo 2000 nm with a relatively light isotope signature of 98 mo 0 9 1 0 these data are explained if mo is converted to particle reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles isotope fractionation is expressed in the water column despite the high sulfide concentrations because the rate of mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation however elemental and isotopic mass balances show that mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water this efficient mo drawdown is expected to occur in settings where h 2 s is very much in excess over mo or in a restricted setting where the water renewal rate is slow compared to the mo burial rate we present a model for the mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems

molybdenum isotope fractionation in pelagic euxinia evidence from the modern black and baltic seas here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the black sea and two deeps of the baltic sea dissolved molybdenum in all water samples is depleted in salinity normalized concentration and enriched in the heavy isotope 98mo values up to 2 9 compared to previously published isotope data of sedimentary molybdenum from the same range of water depths furthermore 98mo values of all water samples from the black sea and anoxic deeps of the baltic sea are heavier than open ocean water the observed isotope fractionation between sediments and the anoxic water column of the black sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions an extrapolation to a theoretical pure mos42 solution indicates a fractionation constant between mos42 and authigenic solid mo of 0 5 0 3 measured waters with all thiomolybdates coexisting in various proportions show larger but non linear fractionation the best explanation for our field observations is mo scavenging by the thiomolybdates dominantly but not exclusively present in the form of mos42 the mo isotopic compositions of samples from the sediments and anoxic water column of the baltic sea are in overall agreement with those of the black sea at intermediate depth and corresponding sulphide concentrations the more dynamic changes of redox conditions in the baltic deeps complicate the black sea derived relationship between thiomolybdates and mo isotopic composition in particular the occasional flushing mixing of the deep waters affects the corresponding water column and sedimentary data 98mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations the results can be explained by non conservative behaviour of mo under suboxic to anoxic conditions in the shallow bottom parts of the basin most pronounced on the nw shelf of the black sea

characterization of organic matter and its implications for pyrite hosted refractory gold mineralization along the south purulia shear zone eastern india abstract refractory gold bearing sulphide mineralization is spatially associated with the carbonaceous matter rich meta sedimentary rocks of the proterozoic south purulia shear zone spsz the carbon rich layers occurring as black continuous laminae are the metamorphosed derivatives of carbonaceous matter which were originally present in the sedimentary rocks and bear the characteristics of initial sedimentary environment of the refractory gold bearing formations of spsz the carbonaceous laminae aligned along the bedding foliation planes got remobilized into the sulfide bearing quartz carbonate veins during the metamorphic and deformational events the typical disordered bands as well as graphitic band in the raman spectra indicate that the carbonaceous matter is partially graphitized from the precursor carbon rich layers the light and homogeneous carbon isotopic values 13c 26 7 to 28 2 of the graphitized carbon suggests its origin due to the evolution from an organic precursor the 13c values fall within the range typical for organic carbon deposited during the paleo to meso proterozoic period the organic matter is considered to have played an essential role in buffering the oxygen fugacity of the initial sedimentary basin the dissolved sulfate ions in seawater were reduced resulting in sulfidation and adsorption of au in sedimentary pyrites to form refractory gold later during the deformational stages the hydrothermal fluids interacted with the graphitized matter which provided suitable redox conditions for the precipitation of auriferous sulfides such as pyrite pyrrhotite chalcopyrite and arsenopyrite within the quartz carbonate veins

diagenesis of oxyanions v u re and mo in pore waters and sediments from a continental margin this research tests the hypothesis that trace metals respond to the extent of reducing conditions in a predictable way we describe pore water and sediment measurements of iron fe manganese mn vanadium v uranium u rhenium re and molybdenum mo along a transect off washington state usa sediments become less reducing away from the continent and the stations have a range of oxygen penetration depths depth to unmeasurable o2 concentration varying from a few millimeters to five centi meters when oxygen penetrates 1 cm or less fe is reduced in the pore waters but reoxidized near the sediment water interface preventing a flux of fe 2 to overlying waters whereas mn oxides are reduced and mn 2 diffuses to overlying waters both re and u authigenically accumulate in sediments only at the most reducing location where the oxygen penetrates 0 3 cm below the sediment water interface does the surface 30 cm of sediments become reducing enough to authigenically accumulate mo stations in close proximity to the juan de fuca ridge crest are enriched in mn and fe from hydrothermal plume processes both v and mo clearly associate with mn cycling whereas u may be associating with either mn oxides and or fe oxyhydroxides rhenium is uncomplicated by adsorption to mn oxides and or fe oxyhydroxides and re accumulation in sediments appears to be due solely to the extent of reducing conditions therefore authigenic sediment re enrichment appears to be the best indicator for intermediate reducing conditions where oxygen penetrates less than 1 cm below the sediment water interface when coupled with negligible authigenic mo enrichment copyright 2005 elsevier ltd 1 introduction oceanic and atmospheric carbon cycles are strongly coupled with the ocean reservoir of dissolved inorganic carbon being 60 times larger than that of the atmosphere broecker and peng 1987 models that couple both ocean and atmosphere have been used to investigate why atmospheric carbon dioxide pco2 atm was reduced during the last glacial age and thus may be useful in predicting the environmental impact of future changes in pco2 atm some of these models require a large increase in organic carbon export through the more efficient use of nutrients to predict the lower pco2 atm observed during

contrasting molybdenum cycling and isotopic properties in euxinic versus non euxinic sediments and sedimentary rocks refining the paleoproxies abstract the redox sensitive transition element molybdenum is present in the earth s crust at trace concentrations but is abundant in seawater marine sediments and ancient sedimentary rocks these occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of mo its isotopic composition and the covariation of mo with total organic carbon toc in ancient black shales however these proxies have almost exclusively targeted the identification and interpretation of euxinic environments where bottom waters are both anoxic and sulfidic here we present a discussion of the mo geochemistry of non euxinic sediments where hydrogen sulfide is sometimes present yet always restricted to pore waters we propose a new paleoredox application that uses mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters under ideal conditions it is also possible to infer the presence of mn oxides recycling in non euxinic paleoenvironments based on the mo isotope composition of ancient black shales providing a critical constraint on bottom water oxygenation

conodont biostratigraphy and astronomical tuning of the lower middle ordovician liangjiashan north china and huanghuachang south china marine sections abstract in this study systematic conodont biostratigraphy and cyclostratigraphy are carried out on ordovician sections from the north china and south china blocks the liangjiashan section situated on the north china block margin accumulated shallow marine carbonate during the early ordovician the huanghuachang section gssp for the dapingian stage situated on the south china block margin accumulated thinly bedded carbonates and minor clastics in a deep shelf environment conodont biostratigraphy and 405 kyr long eccentricity cycles interpreted from the liangjiashan and huanghuachang sections are used to establish an astronomical time scale ats for the early middle ordovician period the ats indicates that the duration of the floian stage is 7 08 0 405 myr the temporal ranges of lower middle ordovician conodont biozones are also estimated the durations of the serratognathus bilobatus serratognathus extensus oepikodus communis oepikodus evae and baltoniodus triangularis zones are 1 0 myr 5 1 myr 0 2 myr 0 8 myr and 0 2 myr respectively the evolution of the ratio of obliquity to total power and short eccentricity power to total power depicts periodicities of 1 myr for the s4 s3 term and 1 9 myr shifting to 1 myr for the g4 g3 term the shift in the g4 g3 term may indicate a chaotic transition in the mars and earth orbital motions from a s4 s3 2 g4 g3 to s4 s3 g4 g3 resonance state during the early ordovician

redox conditions and nitrogen cycling during the triassic jurassic transition a new perspective from the mid panthalassa abstract we determined redox sensitive element concentrations along with 15ntn values from the triassic jurassic shales interbedded within deep sea cherts at the katsuyama section sw japan to clarify the redox and related nitrogen cycle conditions in mid panthalassa during the tjb biotic crisis focusing particularly on the linkages with camp volcanism slight enrichments of mn and mo across the tjb suggest a vertically expanded mid panthalassic omz above a well oxygenated deep ocean moreover high resolution 15ntn records during the triassic jurassic transition exhibit a negative nitrogen isotope excursion nnie just above the tjb relative to the previously reported negative carbon isotope excursion ncie across the tjb the nnie can be divided into two intervals negative interval 1 ni 1 during the ncie and ni 2 after the ncie the nnie can be explained as an interval of nitrate rich conditions reflecting an oxic panthalassic ocean during the triassic jurassic transition our geochemical dataset provides new insights into environmental perturbations in mid panthalassia during the tjb biotic crisis 1 a vertically expanded omz and decreased primary productivity owing to global warming caused by camp volcanism across the tjb resulted in an increased nitrate concentration during the ni 1 and 2 eutrophic nitrate rich conditions developed in mid panthalassa during the ni 2 due to the enhanced continental weathering and deep water upwelling significantly our results are the first to demonstrate that the omzs were expanded not only in the shallow marine regions but also in pelagic sites across the tjb suggesting that the globally expanded omzs can be regarded as a crucial driver for the tjb biotic crisis

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

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

triassic jurassic organic carbon isotope stratigraphy of key sections in the western tethys realm austria the late triassic period is recognized as one of the five major mass extinctions in the fossil record all these important intervals in earth history are associated with excursions in c isotope records thought to have been caused by perturbations in the global carbon cycle the nature and causes of c isotopic events across the triassic jurassic t j transition however are poorly understood we present several new high resolution organic c isotope records from the eiberg basin austria including the proposed global boundary stratotype section and point gssp for the base of the jurassic the triassic jurassic boundary interval in these records is characterized by the initial and main negative organic carbon isotope excursions cie of up to 8 the initial and main cies are biostratigraphically constrained by first and last occurrences of boundary defining macro and microfossils e g ammonites high resolution c isotope records appear to be an excellent correlation proxy for this period in the eiberg basin pyrolysis analysis demonstrates increased hydrogen index hi values for organic matter coinciding with the initial cie terrestrial organic matter influx and mass occurrences of green algae remains may have influenced the c isotope composition of the sedimentary organic matter this may have contributed to the extreme amplitude of the initial cie in the eiberg basin

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

cyclic late katian through hirnantian glacioeustasy and its control of the development of the organic rich wufeng and longmaxi shales south china abstract deposition of the organic rich shales of the wufeng and lowermost longmaxi formations late katian through hirnantian occurred in a relatively stable tectonic setting and commenced at approximately 4 5 myr prior to the end ordovician glacial maximum these organic rich shales are important because they provide a continuous sedimentary record of high frequency eustatic changes associated with ice sheet expansion and shrinkage here we carried out a high resolution cyclostratigraphic study of fe3 series and associated geochemical analysis of the wufeng and lowermost longmaxi shales in order to better understand sea level cyclicity in the late katian through hirnantian ocean the organic rich shales deposited during this interval in the ehd1 drill core yichang china record four major myr scale eustatic cycles which were associated with third order sea level fluctuations within each of these major cycles 3 4 minor eustatic cycles corresponding to 405 kyr long eccentricity astronomical cycles were recognized and representing fourth order eustatic changes the cyclostratigraphic age of this study is generally in agreement with the radiometric dates in gts2012 although the duration of the hirnantian stage is 0 5 myr longer than previously thought this study also infers glacial controls on the depositional environment of the organic rich shales as enhanced cooling triggered ice sheet expansion and concurrent sea level fall leading to higher thermohaline circulation intensity and enhanced paleoproductivity through upwelling and terrestrial influx in contrast warming intervals led to ice sheet shrinkage and concurrent deepening characterized by water column stratification and reduced nutrient supply the results provide evidence in support of continuous continental glaciation in the late katian through hirnantian

chemostratigraphic resolution of widespread reducing conditions in the southwest pacific ocean during the late paleocene abstract paleogene strata throughout much of the southwest pacific is comprised of thick successions of fine grained siliceous to calcareous sediments however during the late paleocene the south pacific experienced widespread deposition of organic rich and 13corg enriched sediments that are recorded in the waipawa formation and the correlative waipawa organofacies the mechanisms responsible for the elevated input and preservation of organic matter are still poorly understood using a multi proxy chemostratigraphic approach we assess paleoceanographic conditions including paleo redox states and paleo productivity of the waipawa formation eight key sections from the east coast basin new zealand were sampled and analysed for multi element geochemistry and bulk pyrolysis sedimentological stratigraphic and chemostratigraphic evidence indicates the south pacific underwent an abrupt regression event during the late paleocene resulting in on shelf erosion and or bypassing of continental sourced sediments from the shelf into deeper waters the change in sediment delivery further increased nutrients and terrestrial organic matter loading to the site of deposition coeval excursions in ti zr and rb sr indicate a shift to an aluminosilicate dominated sediment supply resulting in a modest increase in marine productivity that cannot account for the anomalous organic matter enrichment changes in the hydrogen index show a transition from terrestrial to mixed terrestrial marine organic matter occurred during the late paleocene in which high concentration of type ii iii kerogens were buried and preserved in association with enhanced water column reducing conditions in deep marine settings elevated concentrations of redox sensitive trace elements e g u mo fe s are consistent with alternating phases of dysoxic to oxic conditions suggesting an expansion of an oxygen minimum zone enhancing preservation and delivery of organic matter to the seafloor leading to the subsequent burial and preservation of organic matter within anoxic sediments these new data provide insight into the depositional conditions and the petroleum potential of prospective source rocks in this basin using an integrated organic and inorganic geochemical approach to refine our knowledge of the depositional environment and paleoceanographic changes in shelfal to bathyal marine sediments of the late paleocene

bottom water oxygenation history in southeastern arabian sea during the past 140 ka results from redox sensitive elements the concentrations of multiple redox sensitive elements such as re u mo cd v sb and tl were determined in sediments from the southeastern arabian sea 9 o 21 n 71 o 59 e to understand the bottom water oxygenation history throughout the past 140 ka the enrichment of redox sensitive elements re u cd and sb above average crustal abundances suggests that the last glacial maxima 17 48 ka stadials of marine isotope stage mis 5 5b and 5d and glacial termination gt ii 133 ka were associated with suboxic bottom water conditions sediments deposited during these suboxic conditions show the highest re content up to 54 ppb normalized to a carbonate free basis which is highly enriched over average continental crust 0 4ppb and these sediments appear to be the major sink for the global mass balance estimation marine isotope stages 1 3 4 and interstadials of mis 5 5a 5c 5e were all associated with near oxic conditions overall the lack of enrichment of mo and v above crustal abundance and a high re mo ppm ppm ratio avg 18 2x10 3 suggest that sediments of the southeastern arabian sea never contained free h2s during the last 140 ka these changes in the bottom water oxygen content can be related to the oceanic circulation pattern during this time and in part are reflected in relationships between the timing of redox changes and paleoproductivity proxies

biostratigraphy and carbon and nitrogen geochemistry of the spice event in cambrian low grade metamorphic black shale southern norway abstract we present new chemo and biostratigraphical data from 90 m of alum shale formation stratigraphy from the middle cambrian series 3 to the lowermost ordovician tremadocian at krekling southern norway the alum shale was deposited during a transgression covering parts of the baltic craton and contains up to 13 5 wt total organic carbon toc increasing up section through the furongian pyrolysis confirms that the alum shale is affected by low grade metamorphism either from a nearby pluton or during deep burial where the remaining organic matter has lost most of the hydrocarbon productivity s2 0 0 9 mghc g rock compared to equivalent formations in sweden toc 8 22 wt s2 20 80 still the carbon isotopic trend 13 c org shows that the positive carbon isotope excursion known as the spice event furongian is preserved and that the carbon isotopes did not fractionate significantly while undergoing metamorphism the 13 c org increases steadily from the base of the section from 30 8 to a maximum of 27 3 likely due to increasing atmospheric oxygen concentrations and cyanobacteria productivity the heaviest values are recorded in an interval with abundant pyrrhotite nodules suggesting that the organic productivity led to euxinic conditions as shown from several other spice sections worldwide the evolution of bulk rock nitrogen isotopes 15 n shows positive correlations with 13 c org and toc with an increase from 2 to 1 5 before the onset of the spice negative 15 n values indicate that atmospheric nitrogen was fixed in the shallow ocean during the transition from low to high toc shale at about 18 m in the section both the nitrogen content and the 15 n increase but the 15 n is not high enough to imply water mass nitrogen reduction denitrification as a dominant process instead we suggest that the krekling section was affected by sediment denitrification during spice a process resulting in a ca 1 5 post depositional increase in 15 n at the same time in the upper parts of the water column atmospheric nitrogen was still fixed by cyanobacteria we conclude that the cambrian ocean transgressed the baltic craton and inhibited deep water upwelling leaving geochemical signals characteristic of other greenhouse climates spanning the phanerozoic

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

elevated pco2 leading to late triassic extinction persistent photic zone euxinia and rising sea levels the late triassic mass extinction event is the most severe global warming related crisis to have affected important extant marine groups such as scleractinian corals and offers potential insights into climate change scenarios here we present evidence from chlorobi derived biomarkers of episodic and persistent photic zone euxinia from biomarkers and stable carbon isotopes we present evidence of rapid mixing of atmospheric and oceanic carbon reservoirs global versus regional trends are resolved in kerogen organic matter type carbonate 13c and bulk and pyrite 34s this suite of data demonstrates for the first time a comprehensive organic and stable isotope geochemical reconstruction of events leading up to the late triassic extinction event and its aftermath the cascade of events prior to during and after the extinction is remarkably similar to those reported for the late permian extinction the largest extinction event of the phanerozoic we predict that similar conditions will have occurred during all past episodes of rapid global warming and biotic crisis that are associated with similar rises in p co2

trace element and ree composition of a low temperature ridge flank hydrothermal spring abstract warm 25 c hydrothermal springs have been sampled on baby bare a basaltic outcrop on 3 5 ma old crust 100 km east of the endeavor segment of the juan de fuca ridge the source for these springs is a 62 to 64 c formation water that has cooled conductively as it ascends to feed the springs this water originated as bottom seawater that probably descended into basement 52 km to the southwest at another much larger outcrop called grizzly bare as this seawater flows towards baby bare it is heated and altered by reactions within basaltic basement and by diffusive fluxes to and from the overlying sediment concentrations of mn co ni zn cd and mo in the spring waters are greater than in bottom seawater indicating that the oceanic crust is a source for these elements to the oceans at least a portion of this increase probably results from the redox cycling of mn in sedimentary sources near the basement interface that produces a diffusive flux to basement formation waters additional removal of mo and inputs of the other five elements to two of the three springs are observed locally near sites of venting where density gradients can form shallow circulation cells within the sediment and diffusive exchange occurs concentrations of cu u v y and the rare earth elements rees excluding ce in these samples are less than in bottom seawater indicating that the oceanic crust is a net sink for these elements in this environment copper is probably removed into newly formed carbonate and or sulfide phases removal of the oxyanions u and v is consistent with a net removal of phosphate demonstrated previously for ridge flank hydrothermal systems similarly removal of y and the rees is associated with carbonate phosphate rich and oxide phases calculated maximum global chemical fluxes from warm ridge flank hydrothermal systems such as baby bare are insignificant relative to riverine fluxes for these elements except possibly for mn and mo the impact on global geochemical budgets for these elements from lower temperature

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

controls on organic matter accumulation on the early cambrian western yangtze platform south china abstract the origin of widespread organic rich shales on the early cambrian yangtze platform of nanhua basin south china has been attributed to high primary productivity to test this hypothesis we generated multiple paleoproductivity ocar and par paleoredox corg p ratios upwelling restriction coef mnef co ppm mn mo toc and cd mo ratios and terrigenous flux proxies al and unreactive fe feu for the lower cambrian niutitang formation in the outer shelf jinsha section which records a typical transition from the organic rich black shales to overlying organic lean mudstones or siltstones these new data were integrated with published paleoredox proxies iron speciation and redox sensitive trace metals for the same section our results suggest that the organic rich toc 4 6 1 8 lower member lm 0 37 m of the niutitang formation records ferruginous conditions high productivity levels and low siliciclastic fluxes and that the organic poor toc 0 1 0 1 upper member um 37 329 m was characterized by dominantly oxic conditions declining productivity levels and high siliciclastic fluxes our results suggest a redox shift from ferruginous to oxic conditions and enhanced terrigenous fluxes in response to falling sea level from the lm to the um secular variations in primary productivity and redox conditions are strongly correlated which is consistent with a positive feedback high primary productivity led to increasing water column anoxia which enhanced organic p recycling stimulating further productivity variations in primary productivity may reflect variable upwelling intensity in a semi restricted shelf setting linked to sea level changes taken together these findings suggest that a combination of primary productivity redox conditions and terrigenous fluxes driven by sea level changes played a key role in organic matter accumulation on the early cambrian western yangtze platform

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

the fit between amazonia baltica and laurentia during the mesoproterozoic assemblage of the supercontinent rodinia abstract during the mesoproterozoic western amazonia and southern baltica both show progressive outgrowth by orogenic accretion to older cratons correlation of these orogenic belts indicates that amazonia and baltica were joined and evolved together throughout the mesoproterozoic similar correlations also suggest a close relationship of amazonia with the labrador part of laurentia at the same time

holocene and late glacial palaeoceanography and palaeolimnology of the black sea changing sediment provenance and basin hydrography over the past 20 000 years abstract the elemental geochemistry of late pleistocene and holocene sediments of the black sea recovered in box cores from the basin margins and a 5 m gravity core from the central abyssal region of the basin identifies two terrigenous sediment sources over the last 20 kyrs one source region includes anatolia and the southern caucasus the second region is the area drained by rivers entering the black sea from eastern europe alkali metal al and heavy light rare earth element ratios reveal that the relative contribution of the two sources shifted abruptly every few thousand years during the late glacial and early holocene lacustrine phase of the basin the shifts in source were coeval with changes in the lake level as determined from the distribution of quartz and the heavy mineral hosted trace elements ti and zr the geochemistry of the abyssal sediments further recorded a sequence of changes to the geochemistry of the water column following the lacustrine phase when high salinity mediterranean water entered the basin beginning 9 3 kyrs bp bottom water that had been oxic throughout the lake phase became anoxic at approximately 8 4 kyrs bp as recorded by the accumulation from the water column of several redox sensitive trace metals mo re u the accumulation of organic carbon and several trace nutrients cd cu ni zn increased sharply ca 0 4 kyrs later at 8 0 kyrs bp reflecting an increase of primary productivity its increase was coeval with a shift in the dinoflagellate ecology from stenohaline to euryhaline assemblages during this profound environmental change from the lacustrine to the marine phase the accumulation rate of the lithogenous sediment fraction decreased as much as 10 fold in response to the rise of the water level in the basin from a low stand ca 9 3 ka to its current level

molybdenum isotope fractionation and speciation in a euxinic lake testing ways to discern isotope fractionation processes in a sulfidic setting abstract the molybdenum mo isotope composition in euxinic shales has been used as a proxy for the global distribution of anoxic conditions in ancient oceans and since more recently also as a proxy for sulfide concentrations in depositional environments however there is currently no way to distinguish isotope fractionation at low bottom water sulfide concentrations in local basins from global secular isotope variations associated with changing seawater composition this uncertainty is challenging the use of mo isotopes for paleoceanographic reconstructions to explore this further we present new data from sediments deposited over the past 9800 years in one of the best studied euxinic localities in the world lake cadagno in switzerland the sample set allows us to test ways to discern isotope fractionation processes at play in a highly restricted euxinic basin most of our drill core samples n 18 show high 98mo values similar to previously studied shallow sediments indicative of quantitative mo removal from the water column dahl et al 2010a however a few samples n 3 deposited between about 1200 and 3400 years ago carry low 98mo values and have been isotopically fractionated in the lake sedimentological and geochemical characterizations show that these 98mo fractionated sediments formed during times of frequent injection of o2 and sediment rich river water into the deep sulfidic water column a positive correlation between 98mo and sedimentary mo contents suggests that isotope fractionation occurred during times of non quantitative mo removal although mn oxide cycling at the chemocline might also contribute a subordinate proportion of 98mo depleted molybdenum into the sulfidic zone sedimentary mo u enrichments relative to oxic lake water further supports the hypothesis that a particulate mo shuttle was most efficient during times of quantitative mo removal therefore periods with inefficient mo capture are ascribed to incomplete conversion of molybdate to particle reactive mo species when bottom water h2s levels were low or less stable than today using xafs spectroscopy we found that the two distinct mo compounds predominating in the sediments moiv s and movi os are not diagnostic for isotope fractionation that has occurred in lake cadagno instead we infer that 98mo fractionated products forming via a low sulfide mo pathway can be subsequently altered with little or no isotopic imprint during remobilization and re precipitation e g at higher sulfide levels in the sediments as well as during post depositional oxidation future work could investigate local 98mo fractionation processes expressed in other euxinic settings and explore other sedimentary metrics to constrain the steps involved in the euxinic burial pathway s one tantalizing prospect of this is to distinguish between local bottomwater sulfide levels and variations in the fraction of global seafloor anoxia from the mo isotope composition in ancient euxinic mudrocks

on the onset of central atlantic magmatic province camp volcanism and environmental and carbon cycle change at the triassic jurassic transition neuquen basin argentina abstract the triassic jurassic transition is characterized by the end triassic mass extinction approximately synchronous with the onset of emplacement of the central atlantic magmatic province camp and associated with a major negative carbon isotope excursion cie affecting the ocean atmosphere system here we present new data total organic carbon pyrolysis analysis carbon isotopes from bulk organic matter elemental mercury and other elemental contents from a southern hemisphere triassic jurassic boundary succession in the neuquen basin argentina the end triassic mass extinction there coincides with a relatively small 2 3 negative cie in bulk organic matter and we present a model that suggests that extreme aridity across the western pangaean landmass may have resulted in rather limited terrestrial organic matter flux to the sedimentary realm in eastern panthalassic marine basins hypothetically reducing the magnitude of the observed negative cie in 13ctoc increased deposition of sedimentary hg and hg toc and hg zr in the marine neuquen basin began stratigraphically before the negative cie associated with the end triassic mass extinction and thus before the commencement in north america and africa of camp related basaltic volcanism but possibly coinciding with the early emplacement of camp associated intrusives dykes and sills this relative chronology suggests thermal alteration of intruded country rocks and or intrusive magmatic degassing of hg as potential major sources of elevated hg fluxes to the atmosphere at this time the neuquen basin experienced the development of dysoxic anoxic marine conditions across the triassic jurassic transition enabling increased preservation of organic matter simple mass balance calculations show that enhanced carbon burial rates can explain the inferred evolution of the global exogenic carbon cycle across this time interval

a sulfidic driver for the end ordovician mass extinction the end ordovician extinction consisted of two discrete pulses both linked in various ways to glaciation at the south pole the first phase starting just below the normalograptus extraordinarius zone particularly affected nektonic and planktonic species while the second pulse associated with the normalograptus persculptus zone was less selective glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end ordovician extinction but a general consensus is lacking we have used geochemical redox indicators such as iron speciation molybdenum concentrations pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key hirnantian sections these indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end ordovician extinction and that these conditions expanded towards the second pulse even though the n extraordinarius zone appears to have been a time of oxygenated deposition pyrite is significantly enriched in 34s in our sections as well as in sections reported from south china this suggests a widespread reduction in marine sulfate concentrations which we attribute to an increase in pyrite burial during the early hirnantian the s isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well we argue that euxinic conditions prevailed and intensified in the early hirnantian oceans and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites in the n persculptus zone an interval associated with a major sea level rise our redox indicators suggests that euxinic conditions and ferruginous in some places encroached onto the continental shelves in our model the expansion of euxinic conditions during the n extraordinarius zone was generated by a reorganization of nutrient cycling during sea level fall and we argue overall that these dynamics in ocean chemistry played an important role for the end ordovician mass extinction during the first pulse of the extinction euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep water benthic and nektonic organisms during the second pulse the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats

combining sedimentological trace metal mn mo and molecular evidence for reconstructing past water column redox conditions the example of meromictic lake cadagno swiss alps here we present sedimentological trace metal and molecular evidence for tracking bottom water redox state conditions during the past 12 500 years in nowadays sulfidic and meromictic lake cadagno switzerland a 10 5 m long sediment core from the lake covering the holocene period was investigated for concentration variations of the trace metals mn and mo xrf core scanning and icp ms measurements and for the presence of anoxygenic phototrophic sulfur bacteria carotenoid pigment analysis and 16s rdna real time pcr our trace metal analysis documents an oxic intermediate sulfidic redox transition period beginning shortly after the lake formation similar to 12 5 kyr ago the oxic period is characterized by low sedimentary mn and mo concentrations as well as by the absence of any remnants of anoxygenic phototrophic sulfur bacteria enhanced accumulation preservation of mn up to 5 6 wt in the sediments indicates an intermediate mn enriched oxygenation state with fluctuating redox conditions during a similar to 2300 year long transition interval between similar to 12 1 and 9 8 kyr bp we propose that the high mn concentrations are the result of enhanced mn2 leaching from the sediments during reducing conditions and subsequent rapid precipitation of mn oxyhydr oxide minerals during episodic and short term water column mixing events mainly due to flood induced underflows at 9800 130 cal yr bp a rapid transition to fully sulfidic conditions is indicated by the marked enrichment of mo in the sediments up to 490 ppm accompanied by an abrupt drop in mn concentrations and the increase of molecular biomarkers that indicate the presence of anoxygenic photosynthetic bacteria in the water column persistently high mo concentrations 80 ppm provide evidence that sulfidic conditions prevailed thereafter until modern times without any lasting hypolimnetic ventilation and reoxygenation hence lake cadagno with its persistently stable chemocline offers a framework to study in great temporal detail over similar to 12 kyr the development of phototrophic sulfur bacteria communities and redox processes in a sulfidic environment possibly depicting analogous conditions in an ancient ocean our study underscores the value of combining sedimentological geochemical and microbiological approaches to characterize paleo environmental and redox conditions in lacustrine and marine settings

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

mid holocene enso issues in quantitative model proxy data comparisons 1 evaluation of climate model simulations using observed data contributes to the assessment of confidence in model predictions of future climate change the mid holocene represents an opportunity to evaluate model simulations of el nino southern oscillation enso in comparison with coral proxy evidence of reduced enso amplitude quantitative comparisons between coral records and model output have been limited by 1 the use of different measures of enso amplitude 2 possible sampling of natural variability in short records and 3 uncertainty about the stationarity of the relationship between central pacific sea surface temperature sst variability and enso signals at the coral site we examine these issues using modern and fossil coral records from the western pacific and model simulations of preindustrial and mid holocene climate as a measure of enso amplitude the standard deviation is found to be preferable to event frequency or size as event based measures are highly dependent on the choice of threshold and may be unreliable for a small number of events model enso amplitude is found to be strongly dependent on the choice of averaging period with calendar year averages smoothing the seasonal enso signal a relatively robust relationship between sst variability in the nino3 4 region and the enso sst and precipitation anomalies archived in corals is demonstrated for the instrumental period and for a set of model simulations remaining uncertainty about changes in enso teleconnections under paleoclimate conditions implies the need for additional proxy records from enso sensitive regions before quantitative reconstructions of enso amplitude can be used to evaluate model sensitivity

the quantification and application of handheld energy dispersive x ray fluorescence ed xrf in mudrock chemostratigraphy and geochemistry abstract traditionally analytical techniques such as wavelength dispersive x ray fluorescence wd xrf inductively coupled plasma icp and mass spectrometry icp ms instrumental neutron activation analysis inaa or stationary bench top energy dispersive x ray fluorescence ed xrf have been used to generate quantitative geochemical results however a more efficient means of data collection using portable ed xrf instrumentation now allows the investigator to acquire rapid non destructive quantitative measurements on drill core and clean flat rock surfaces in addition to pressed powder pellets typically used in wd xrf analysis similar to traditional xrf methods quantification using the handheld ed xrf requires a matrix specific calibration unfortunately very few internationally accepted mudrock or shale reference materials exist and their elemental ranges provide inadequate coverage for the geochemical diversity of mudrocks in order to return reliable calibrated results a unique set of reference materials has been developed that incorporates a wide range of mudrock elemental compositions the current method provides elemental calibrations for major elements heavier than sodium and the following trace elements ba v cr ni cu zn rb sr y zr nb mo th and u a comparison of handheld energy dispersive and wavelength dispersive x ray fluorescence ed xrf and wd xrf results from pressed powder pellets of mississippian age barnett shale of north central texas usa is presented in order to evaluate the reliability of the reference calibration and the quantification of unknown samples using two different instrument platforms it will be demonstrated that calibrated results from the handheld ed xrf effectively define chemostratigraphic changes in real time as a consequence quantified results can be used immediately to assess changes in bulk mineralogy paleo redox conditions and to link down core geochemical changes to stratigraphic sedimentological and paleoenvironmental observations

graphite mo w s 2 intergrowth as a palaeoenvironmental proxy in metasedimentary rocks abstract molybdenum enrichment in pristine organic c rich sedimentary rocks forms the basis for inferring the presence of dissolved oxygen in seawater organic matter removes dissolved hexavalent mo from seawater where anoxic and euxinic conditions are attained however it is unknown whether this mo based proxy is retained under metamorphic conditions where organic c is no longer preserved here we describe aggregates of graphite and molybdenite mos2 containing up to 21 mass per cent of w as tungstenite ws2 in solid solution these aggregates are disseminated in a sulfide rich mn silicate carbonate rock queluzite metamorphosed under amphibolite facies conditions within the archaean barbacena greenstone belt in minas gerais brazil our finding indicates that i w is like mo a palaeoenvironmental proxy ii the w proxy is sensitive to high fs2 fo2 environments iii both mo and w proxies survive amphibolite facies overprint as mo w s2 intergrown with graphite archaean greenstones are potential candidates for storing palaeoenvironmental information as mo w s2 graphite intergrowths

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

permo triassic boundary superanoxia and stratified superocean records from lost deep sea pelagic cherts of japan and british columbia canada recorded a long term and worldwide deep sea anoxic oxygen depleted event across the permo triassic or paleozoic and mesozoic boundary 251 2 million years ago the symmetry in lithostratigraphy and redox condition of the boundary sections suggest that the superocean panthalassa became totally stratified for nearly 20 million years across the boundary the timing of onset climax and termination of the oceanic stratification correspond to global biotic events including the end guadalupian decline the end permian extinction and mid triassic recovery

algal blooms and marine snow mechanisms that enhance preservation of organic carbon in ancient fine grained sediments abstract combined petrographic and geochemical methods are used to investigate the microfabrics present in thin sections prepared from representative organic carbon rich mudstones collected from three successions the kimmeridge clay formation the jet rock member of the whitby mudstone formation and the pebble shale and hue shale this study was initiated to determine how organic carbon rich materials were being delivered to the sediment water interface and what happened to them after deposition prior to deep burial analyses of the fabrics present shows that they exhibit many common attributes in particular they are all 1 highly heterogeneous on the scale of a thin section 2 organized into thin beds the organomineralic aggregates present in these mudstones are interpreted to be ancient examples of marine snow this marine snow likely formed in the water column particularly during phytoplankton blooms and was then transported rapidly to the seafloor the existence of the thin beds with homogenized tops and an in situ infauna indicates that between blooms there was sufficient oxygen and time for a mixed layer to develop as a result of sediment colonization by diminutive organisms using either aerobic or dysaerobic metabolic pathways these textures suggest that the constituents of these mudstones were delivered neither as a continuous rain of sediment nor were the bottom waters persistently anoxic in addition the presence of thin lags and sharp based beds suggests that the seafloor was being episodically reworked during deposition these fabrics indicate that conditions in the water columns and at the seafloors while these rocks were being deposited were very dynamic and episodic fluxes of high concentrations of organic carbon to the seafloor during phytoplankton blooms likely enhanced preservation of organic carbon

basinal restriction black shales re os dating and the early toarcian jurassic oceanic anoxic event 1 profiles of mo total organic carbon toc through the lower toarcian black shales of the cleveland basin yorkshire united kingdom and the posidonia shale of germany and switzerland reveal water mass restriction during the interval from late tenuicostatum zone times to early bifrons zone times times which include that of the putative early toarcian oceanic anoxic event the degree of restriction is revealed by crossplots of mo and toc concentrations for the cleveland basin which define two linear arrays with regression slopes ppm of 0 5 and 17 the slope of 0 5 applies to sediment from the upper semicelatum and exaratum subzones this value which is one tenth of that for modern sediments from the black sea mo toc regression slope 4 5 reveals that water mass restriction during this interval was around 10 times more severe than in the modern black sea the renewal frequency of the water mass was between 4 and 40 ka the mo toc regression slope of 17 applies to the overlying falciferum and commune subzones the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years the more restricted of the two intervals has been termed the early toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to nw europe crossplots of re os and mo against toc show similar trends of increasing element concentration with increase in toc but with differing slopes together with modeling of 187 os 188 os and d 98 mo the element toc trends show that drawdown of re os and mo was essentially complete during upper semicelatum and exaratum subzone times mo toc regression slope of 0 5 drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of re os and mo via a low salinity surface layer created isotopic excursions of up to 1 3 in d 98 mo and up to 0 6 for 187 os 188 os restriction thereby compromises attempts to date toarcian black shales and possibly all black shales using re os chronology and introduces a confounding influence in the attempts to use d 98 mo and initial 187 os 188 os for palaeo oceanographic interpretation