numerical modeling of flow processes over gravelly surfaces using structured grids and a numerical porosity treatment this article describes the development and validation of a method for representing the complex surface topography of gravel bed rivers in high resolution three dimensional computational fluid dynamic models this is based on a regular structured grid and the application of a porosity modification to the mass conservation equation in which fully blocked cells are assigned a porosity of zero fully unblocked cells are assigned a porosity of one and partly blocked cells are assigned a porosity of between 0 and 1 according to the percentage of the cell volume that is blocked the model retains an equilibrium wall function and an rng type two equation turbulence model the model is combined with a 0 002 m resolution digital elevation model of a flume based water worked gravel bed surface acquired using two media digital photogrammetry and with surface elevations that are precise to 0 001 m the model is validated by comparison with velocity data measured using a three component acoustic doppler velocimeter adv model validation demonstrates a significantly improved level of agreement than in previous studies notably in relation to shear at the bed although the resolution of model predictions was significantly higher than the adv measurements making model assessment in the presence of strong shear especially difficult a series of simulations to assess model sensitivity to bed topographic and roughness representation were undertaken these demonstrated inherent limitations in the prediction of 3 d flow fields in gravel bed rivers without high resolution topographic representation they also showed that model predictions of downstream flux were more sensitive to topographic smoothing that to changes in the roughness parameterization reflecting the importance of both mass conservation i e blockage and momentum conservation effects at the grain and bed form scale model predictions allowed visualization of the structure of form flow interactions at high resolution in particular the most protruding bed particles exerted a critical control on the turbulent kinetic energy maxima typically observed at about 20 of the flow depth above the bed

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

trace metal signatures of eastern mediterranean sapropels abstract sapropel samples of pliocene to pleistocene age recovered during odp leg 160 from sites 964 ionian basin 969 mediterranean ridge 966 967 top and northern slope of the eratosthenes seamount south of cyprus and 968 cyprus margin were analysed for major and trace elements eastern mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox sensitive and or sulphide forming trace metals bi cd co cr cu mo ni re sb tl u v enrichment factors relative to average shale are comparable to those found in ctbe black shales the re content is extremely high in the sapropels studied with maximum values up to 1000 ng g re mo ratios averaging 2 7 10 3 are close to the seawater value high trace metal enrichments and re mo ratios close to the seawater value point to an anoxic water column during sapropel formation additionally enhanced ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes trace metal signatures of sapropels originating from different drill sites on a transect across the eastern mediterranean are not only influenced by water depth but also by their location in the eastern mediterranean metal enrichments seem to be higher at the western sites compared with the eastern sites this probably reflects a change in the circulation pattern during periods of sapropel formation pliocene sapropels exhibit elevated trace metal contents in comparison with pleistocene examples therefore different palaeoceanographic conditions may have developed during the pleistocene compared with the pliocene altering palaeoceanography was probably controlled by climatic change which may be related to the onset of glaciation of the northern hemisphere decreasing summer precipitation throughout the late pliocene and the pleistocene may also have influenced sapropel deposition

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

synchronous basin wide formation and redox controlled preservation of a mediterranean sapropel organic rich sedimentary units called sapropels have formed repeatedly in the eastern mediterranean sea in response to variations of solar radiation sapropel formation is due to a change either in the flux of organic matter to the sea floor from productivity changes or in preservation by bottom water oxygen levels however the relative importance of surface ocean productivity versus deep water preservation for the formation of these organic rich shale beds is still being debated and conflicting interpretations are often invoked1 2 3 4 5 6 7 here we analyse at high resolution the differences in the composition of the most recent sapropel s1 in a suite of cores covering the entire eastern mediterranean basin we demonstrate that during the 4 000 years of sapropel formation surface water salinity was reduced and the deep eastern mediterranean sea below 1 800 m depth was devoid of oxygen this resulted in the preferential basin wide preservation of sapropel s1 with different characteristics above and below 1 800 m depth as a result of different redox conditions we conclude that climate induced stratification of the ocean may therefore contribute to enhanced preservation of organic matter in sapropels and potentially also in black shales

cellular modelling of river catchments and reaches advantages limitations and prospects the last decade has witnessed the development of a series of cellular models that simulate the processes operating within river channels and drive their geomorphic evolution their proliferation can be partly attributed to the relative simplicity of cellular models and their ability to address some of the shortcomings of other numerical models by using relaxed interpretations of the equations determining fluid flow cellular models allow rapid solutions of water depths and velocities these can then be used to drive usually conventional sediment transport relations to determine erosion and deposition and alter the channel form the key advance of using these physically based yet simplified approaches is that they allow us to apply models to a range of spatial scales 1 100 km 2 and time periods 1 100 years that are especially relevant to contemporary management and fluvial studies however these approaches are not without their limitations and technical problems this paper reviews the findings of nearly 10 years of research into modelling fluvial systems with cellular techniques principally focusing on improvements in routing water and how fluvial erosion and deposition including lateral erosion are represented these ideas are illustrated using sample simulations of the river teifi wales a detailed case study is then presented demonstrating how cellular models can explore the interactions between vegetation and the morphological dynamics of the braided waitaki river new zealand finally difficulties associated with model validation and the problems prospects and future issues important to the further development and application of these cellular fluvial models are outlined 2007 elsevier b v all rights reserved

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

paleoclimate and paleoceanography over the past 20 000 yr in the mediterranean sea basins as indicated by sediment elemental proxies abstract marine sediments record paleoenvironmental changes over time through variations in major and trace element concentrations the main objective of this paper is to review such changes in the mediterranean sea basins over the last 20 ka using the inorganic chemistry and mineralogy of marine sediment records elemental ratio proxies that are mostly used are discussed and the uncertainties involved in using them for paleoclimate and paleoceanographic reconstructions are evaluated the focus on the mediterranean region is based on the sensitivity of this region to global climate changes due to its semi enclosed nature the elemental ratios that have been particularly useful for reconstructing terrigenous inputs into the mediterranean have been ti al and zr al ratios as proxies for eolian dust input and mg al k al and rb al ratios as proxies for fluvial input redox sensitive elements e g u mo v co ni cr have provided reliable reconstructions of oxygen conditions at the time of deposition some of these elements are also particularly susceptible to post depositional remobilization and record diagenetic processes instead of the original environmental signatures regarding productivity fluctuations most of the paleoproductivity reconstructions are based on the abundance of barite and ba excess algorithms the biogeochemistry of ba is however not fully understood and mechanisms for barite precipitation in the water column are not yet known two case studies are presented the eastern mediterranean sapropel s1 deposited between 10 8 and 6 1 cal ka bp and the westernmost mediterranean paleoclimate record over the last 20 ka

controls on phosphorus regeneration and burial during formation of eastern mediterranean sapropels the carbon c and phosphorus p geochemistry of sapropels from four sites in the eastern mediterranean sea was determined to obtain more insight into the role of differences in sediment accumulation rates and bottom water anoxia on p regeneration and burial in sediments sediment corg porg with org organic ratios above redfield indicate enhanced regeneration of p relative to c from organic matter during formation of the most recent sapropel s1 and a pliocene sapropel odp site 969e i 282c release of p from fe oxides was relatively unimportant increased burial of ca p authigenic carbonate fluorapatite and or biogenic hydroxyapatite occurred but was of minor importance compared to the enhanced p release from organic matter for sapropel s1 corg porg and corg preac with reac reactive defined as the sum of organic fe bound authigenic and biogenic ca p ratios decreased with increasing sedimentation rate and oxygen exposure and decreasing water depth whether the water column in the deep basin was dysoxic semi euxinic s1 or euxinic pliocene does not appear to affect the estimated burial efficiencies of porg and preac enhanced corg burial under the euxinic water column explains the much higher corg porg and corg preac ratios in the pliocene sapropel on average 2860 and 760 respectively compared to sapropel s1 on average 530 and 160 respectively

the sapropel record of the eastern mediterranean sea results of ocean drilling program leg 160 research on sediments recovered during ocean drilling leg 160 has concentrated on two issues the first concerned the stratigraphy of sapropel formation the second was oriented to clarify specific processes that explain sapropel origin progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites on the composites initial work has resulted in the establishment of high resolution and intermediate resolution stratigraphies for three sites 963 964 967 correlation of sedimentary cycles to astronomical insolation cycles extends the stratigraphies to sites 969 and 966 the sapropel occurrences in the marine and land sequences over the entire eastern mediterranean are correlated with the resolution that can be obtained from isotope studies groups of sapropels occurred simultaneously over the entire basin in detail however the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins the differences may be related to effects of water depth diagenesis and post depositional tectonic attenuation of sequences studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels caused the enrichment of trace metals associated with sapropels and helped to preserve primary sedimentary structures besides all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events

multiple sulfur isotopic evidence for a shallowly stratified ocean following the triassic jurassic boundary mass extinction the cause of the triassic jurassic tr j boundary biotic crisis one of the big five mass extinctions of the phanerozoic remains controversial in this study we analyzed multiple sulfur isotope compositions 33s 34s and 36s of pyrite and spy toc ratios in two tr j successions mariental mingolsheim from the european epicontinental seaway ees in order to better document ocean redox variations during the tr j transition our results show that upper rhaetian strata are characterized by 34s enriched pyrite low spy toc ratios and values of 33spy i e the deviation from the mass dependent array lower than that estimated for contemporaneous seawater sulfate suggesting an oxic suboxic depositional environment punctuated by brief anoxic events the overlying hettangian strata exhibit relatively 34s depleted pyrite high 33spy and spy toc values and the presence of green sulfur bacterial biomarkers indicate a shift toward to euxinic conditions the local development of intense marine anoxia thus postdated the tr j mass extinction which does not provide support for the hypothesis that euxinia was the main killing agent at the tr j transition sulfur and organic carbon isotopic records that reveal a water depth gradient i e more 34s 13c depleted with depth in combination with spy toc data suggest that the earliest jurassic ees was strongly stratified with a chemocline located at shallow depths just below storm wave base shallow oceanic stratification may have been a factor for widespread deposition of black shales a large positive shift in carbonate 13c values and a delay in the recovery of marine ecosystems following the tr j boundary crisis

trans mediterranean comparison of geochemical paleoproductivity proxies in a mid pleistocene interrupted sapropel abstract sapropel layers in the sediment record of the mediterranean sea reflect periods of climate induced elevated marine production associated with precessional minima some of these layers feature interruptions that represent centennial to millennial returns to the predominantly low productivity conditions of this sea we have measured multiple geochemical paleoproductivity proxies in the interrupted sapropel layer that corresponds to insolation cycle 90 955 ka at 1 cm intervals in a four site transect from the balearic basin to the levantine basin to assess similarities and differences in its expression at the four locations the interrupted sapropels are similarly divided by an organic carbon poor interval into an upper layer in which organic carbon concentrations reach 2 7 wt and a lower layer in which they peak between 1 and 3 wt the interruption was essentially synchronous in the balearic basin the tyrrhenian basin and the ionian basin but it was delayed and shorter in the levantine basin lighter nitrogen isotope compositions consistently accompany higher organic carbon accumulation rates in the sapropel layers which implies a shift from coccolith dominated organic matter production towards a mode of primary production in which cyanobacteria were important during times of sapropel deposition organic carbon isotopic compositions become heavier as organic carbon accumulation increases at three of the locations which is consistent with elevated productivity however they become lighter at the levantine basin site which suggests greater delivery of isotopically light fluvial waters to this part of the mediterranean sea the local paleoclimate conditions conducive to sapropel formation evidently were not completely uniform across the mediterranean region but their occurrences were synchronous moreover the co existence of an essentially same age interruption of the sapropels at the four locations shows that climate could change rapidly and concordantly across the region at much less than precessional intervals

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

continental margin molybdenum isotope signatures from the early eocene molybdenum mo isotope compositions of marine sedimentary deposits that span the paleocene eocene thermal maximum petm approximately 56 ma are presented from two tethys ocean sites guru fatima tajikistan and kheu river georgia local redox indicators suggest that both locations experienced anoxia and intervals of euxinia with hydrogen sulphide present in seawater during the early part of the petm however the mo isotope compositions expressed as 98 95mo for each site differ significantly local redox conditions were very stable at guru fatima where 98 95mo reached a maximum of 0 96 which is 0 7 lower than for early eocene seawater as recently inferred from euxinic arctic ocean deposits this observation supports the argument that a 0 7 difference between seawater and anoxic continental margin marine sediments documented at the present day might also be found in the paleo record in contrast local redox conditions at kheu river were not stable over the study interval and 98 95mo were much lower than at guru fatima the low 98 95mo values at kheu river are attributed to the influence of post depositional remobilization of mo by repeated adsorption and dissolution of fe mn oxides during brief intervals of bottom water oxidation the data highlight the importance of obtaining multi proxy constraints on both local redox and paleoceanographic setting before the mo isotope compositions of sedimentary deposits can be interpreted accurately

modes of sapropel formation in the eastern mediterranean some constraints based on pyrite properties abstract pyrite formation within and directly below sapropels in the eastern mediterranean was governed by the relative rates of sulphide production and fe liberation and supply to the organic rich layers at times of relatively high so2 4 reduction sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments the sources of fe for pyrite formation comprised detrital fe and diagenetically liberated fe ii from sapropel underlying sediments in organic rich sapropels input of fe from the water column via fe sulphide formation in the water may have been important as well rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred 34s values of pyrite are 33 to 50 below the sapropels 34s is lower than within the sapropels as a result of increased sulphide re oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment the percentage of initially formed sulphide that was re oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment it ranges from 34 to 80 varying significantly between sapropels increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels

multiple sulfur isotope constraints on sulfate driven anaerobic oxidation of methane evidence from authigenic pyrite in seepage areas of the south china sea abstract multiple sulfur isotope signatures and secondary ion mass spectroscopy sims sulfur isotope compositions of pyrite from two seafloor sites dh cl11 and hd109 in seepage areas of the south china sea were measured in order to study isotope effects of sulfate driven anaerobic oxidation of methane so 4 aom the multiple sulfur isotopes of pyrite reveal variable ranges for both sites 34 s between 44 1 and 2 9 for dh cl11 and between 43 8 and 1 6 for hd109 33 s between 0 02 and 0 17 for dh cl11 and between 0 03 and 0 14 for hd109 sims analysis reveals an extreme variability of 34 s values between 50 3 and 2 7 in dh cl11 between 50 1 and 52 4 in hd109 for three types of pyrite 1 framboids 2 zoned aggregates with radial overgrowth surrounding a framboidal core and 3 euhedral pyrite crystals the synchronous changes of geochemical proxies sulfate and methane concentrations 34 s sulfate and 18 o sulfate 34 s pyrite and pyrite content at the sulfate methane transition zone smtz at site dh cl11 are interpreted to be induced by so 4 aom under steady state conditions in contrast pyrite content and 34 s value fluctuations throughout core hd109 suggest that the sediment at this site was affected by multiple pyritization events during diagenesis multiple sulfur isotope signatures of early diagenetic pyrite i e with low and high 34 s values the latter above 315 cmbsf in dh cl11 above 70 cmbsf in hd109 in the upper sediment column suggest that organoclastic sulfate reduction osr and sulfur disproportionation generated the observed isotopic signatures in contrast to the early diagenetic 34 s depleted framboids the higher sims 34 s values of overgrowth and euhedral crystals suggest a late diagenetic 34 s enriched pool of dissolved sulfide derived from so 4 aom at the current and paleo smtzs interestingly pyrite resulting from so 4 aom in the smtz at site dh cl11 reveals a distinct pattern with higher 33 s values different from pyrite resulting from osr and sulfur disproportionation therefore paired 34 s and 33 s values may allow to differentiate osr and so 4 aom although a full understanding of the isotope effects associated with so 4 aom is hampered by uncertainties on the actual electron transfer mechanism in the syntrophic so 4 aom consortium

a compositional approach to the reconstruction of geochemical processes involved in the evolution of holocene marine flooded coastal karst basins mljet island croatia abstract geochemical patterns in the environment are always the result of certain processes therefore it is essential to decipher a process to properly evaluate the environmental role and potential of chemical elements compounds this allows the distinction between natural and anthropogenic influence on elemental concentrations however if the compositional nature of geochemical data is neglected erroneous or misleading conclusions regarding the processes involved are probable in this study the reconstruction of depositional environments and processes through the holocene in two sediment cores obtained from submerged sinkholes located on the island of mljet croatia was performed by taking into account the compositional nature of geochemical mineralogical and grainsize data problems involving compositional data are always multivariate for example the concentration of a single element does not carry any interpretative information as only the ratios between elements do this has led to the discovery of a large number of geochemical proxies based on elemental ratios which describe certain environmental conditions and processes involved nevertheless some proxies have been found to be restricted to only some specific environments thus preventing them from being used in general therefore some kind of relation between different proxies is necessary to obtain final conclusions however when using simple elemental ratios those correlations cannot be obtained due to the nature of compositional data with a sequential binary partition of a compositional vector orthonormal log ratio olr coordinates proxies can be constructed when based on expert knowledge those proxies fully acknowledge the geochemical properties of the chosen elements with one major difference that the correlation between newly obtained variables is mathematically well grounded as a result the final conclusion is more accurate in this research geochemical proxies obtained as a representation in olr coordinates of the elements that are enriched compared to the local soil were used to perform principal component analyses it helped to unravel the evolution of sedimentary environments mineralogical xrd and heavy mineral data and grain size analyses supported the conclusions obtained based solely on geochemical data furthermore data analysis suggests that the proxies for redox conditions described in the literature should be used with caution as their use is somewhat limited

impact of the circulation on sapropel formation in the eastern mediterranean the role of the thermohaline circulation in controlling export production oxygenation of deep waters and hence possible sapropel formation in the eastern mediterranean is examined using a simple nutrient cycling model the model is driven by velocity fields from a general circulation model and receives fluxes of nutrient from river run off and atmospheric deposition the model is used to study three scenarios a strong anti estuarine circulation a weakened anti estuarine circulation and a weak estuarine circulation nutrient transports ventilation of oxygen and deposition of organic matter are investigated in each case with a present day circulation the model provides reasonable agreement with observed phosphate and oxygen profiles and for export production with the weakened anti estuarine circulation consistent with surface salinity reconstructions for the most recent sapropel s1 there is a modest increase in export production and reduced ventilation leading to anoxia in intermediate and deep waters sapropel formation is possible near the coastal margins particularly if there is enhanced river run off with an estuarine circulation there is significant increase in export production in addition to anoxia below a shallow winter mixed layer while both the latter circulations allow sapropel formation in the model the estuarine case is distinguished by higher organic carbon deposition and anoxia in the near surface waters

glacial holocene salinity changes in the mediterranean sea hydrographic and depositional effects oxygen isotope changes as recorded by pleistocene foraminifera from open ocean sediments are primarily a function of changes in the isotopic composition of the global seawater reservoir caused by ice volume changes with a secondary temperature component1 oxygen isotope records from marginal basins such as the gulf of mexico2 the mediterranean sea3 5 and the red sea6 also exhibit the global signal but are far more complicated owing to the strong overprint of local climate conditions in particular glacial interglacial changes in continental aridity and river runoff produce significant salinity changes in these semi enclosed basins using available oxygen isotope records we estimate that glacial 18 000 yr bp salinities in the eastern and western mediterranean sea were 2 7 and 1 2 higher respectively than at present these elevated glacial salinities played an important part in preconditioning the eastern mediterranean for the eventual accumulation of organic carbon rich sediments sapropels at the end of deglaciation

tracing water column euxinia in eastern mediterranean sapropels s5 and s7 abstract sapropels s5 and s7 formed in the semi enclosed eastern mediterranean sea ems during peak interglacial periods mis5e and mis7a respectively are considered among the most strongly developed quaternary sapropels this study investigates the redox dynamics of the water column during their formation via fe isotope and fe speciation studies of cores taken at 2550 m depth at site odp 967 both sapropels show an inverse correlation between 56fe and fet al with slopes mostly matching that found for the black sea pointing to a benthic shelf to basin shuttle of fe and subsequent precipitation of fe sulphides in euxinic bottom waters an exception to these black sea type trends occurs during the later peak stages of s7 where the negative 56fe fet al slope shallows fe speciation studies reveal that the dominant highly reactive fe phase fehr in the sapropels is pyrite with fe oxyhydr oxides forming the second major mineral component correspondingly fehr fet plots show increased strengthening of anoxic water conditions during the passage from pre sapropel sediment into the sapropel nevertheless despite the evidence for euxinic conditions from both fe isotopes and high mo concentrations in the sapropel fepy fehr ratios remain below values commonly used to identify water column euxinia this apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline fe oxyhydr oxide minerals to the basin which resulted in a relatively low degree of sulphidation despite the presence of euxinic bottom waters thus the operationally defined ferruginous euxinic boundary for ems sapropels is better placed at fepy fehr 0 6 which is somewhat below the usually ascribed lower limit of 0 7 consistent with the significant presence of crystalline fe oxyhydr oxides the change in the 56fe fet al slope during peak s7 is ascribed to an enhanced monsoon driven flux of detrital fe iii oxides from the river nile into the ems basin and comcomitant diagenetic sulphidation euxinic water column conditions in sapropel s5 and s7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of fe oxyhydr oxide minerals both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

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

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

on the relationship between flow and suspended sediment transport over the crest of a sand dune rio parana argentina the links between large scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades with past studies illustrating the origin of such turbulence and its influence on flow resistance sediment transport and bedform morphology in this study of turbulence and sediment suspension over large sand dunes in the rio parana argentina time series of three dimensional velocity and at a point suspended sediment concentration and particle size were measured with an acoustic doppler current profiler and laser in situ scattering transmissometer respectively these time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment the analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections regions of high suspended sediment concentration are found to correlate well with such events the frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations such a finding suggests that a combination and interaction of vortex shedding and wake flapping changing length of the lee side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures including information on the topology of dune related turbulent flow structures at the flow stage investigated the turbulent flow events and their associated high suspended sediment concentrations are seen to grow with height above the bed until a threshold height ca 0 45 flow depth is reached above which they begin to decay and dissipate

comparative behavior of authigenic re u and mo during reoxidation and subsequent long term burial in marine sediments the trace element re is enriched in mildly reducing suboxic sediments close to or slightly later in the redox sequence than u and before reduction of mo in more reducing sulfidic sediments this work investigates the behavior of authigenic re during oxidation and long term burial in the sediments of several turbidites from the madeira abyssal plain exposure to bottom water o2 clearly remobilizes authigenic re as found in previous work but reimmobilization occurs in reducing sediments below the oxidation front over a depth range of 0 5 3 m the large spread of the re burndown peak must be kinetically controlled perhaps influenced by the corg content once reimmobilized however the authigenic re signal remains immobile for at least 3 4 myr authigenic u in contrast consistently forms peaks 0 5 m broad whereas authigenic mo is lost on oxidation and does not appear to be refixed unless authigenic pyrite is formed these results imply that any paleoenvironmental significance of authigenic re will be compromised in low sedimentation rate environments 10 cm kyr where there is significant exposure of authigenic re to bottom water o2 however authigenic re records from rapidly accumulating continental margin sediments with much less exposure to bottom water o2 still offer considerable potential as high fidelity recorders of past reducing conditions

development of photic zone euxinia in the eastern mediterranean basin during deposition of pliocene sapropels abstract carotenoids were analysed in ca 1 cm thick subsamples of three laterally time equivalent sapropels from a west east transect of the eastern mediterranean basin to study euxinic periods during pliocene sapropel formation the amount of intact isorenieratene summed all trans and cis isomers ranged from non detectable at the base and top of a sapropel up to 140 g g sediment in the central parts isorenieratene accumulation rates at the central and western site are remarkably similar and increase sharply to levels of up to 3 0 mg m 2 yr 1 in the central part of the sapropel and then drop to low levels this pattern indicates an expansion of euxinic conditions reaching into the photic zone followed by deepening of the chemocline during deposition of this pliocene sapropel the sapropel from the easternmost site of the basin which contains less organic carbon shows much lower isorenieratene accumulation rates and even absence of isorenieratene in the central part of the sapropel ba al ratios indicate enhanced palaeoproductivity during sapropel formation supporting previously proposed models according to which increased productivity is the driving force for the generation of euxinic conditions

isotope fractionation by sulfate reducing natural populations and the isotopic composition of sulfide in marine sediments isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid phase sulfides in the same sediments the measured fractionations during sulfate reduction could explain only between 41 and 85 of the 34s depletion in the sedimentary sulfides this result directly demonstrates that the depletion of 34s in solid sulfides is an expression of the combined activity of sulfate reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle the only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds elemental sulfur thiosulfate and sulfite in a simple model we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides the sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction by contrast a correlation was observed between sulfate reduction rate and the extent of 34s depletion into sedimentary sulfides the most 34s depleted sulfides found in sediments supporting the lowest rates of sulfate reduction thus the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate reduction rates the direct phototrophic oxidation of sulfide to sulfate with minimal fractionation may have been important in the sediments that had a high sulfate reduction rate

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

major and trace element characterization of oceanic anoxic event 1d oae 1d insight from the umbria marche basin central italy abstract the pialli level in the umbria marche basin central italy correlates with the lowermost part of the positive carbon isotopic excursion characterizing the late albian early cenomanian oceanic anoxic event 1d oae 1d high resolution litho bio and chemostratigraphic data from the monte petrano and le brecce sections allow for a bed by bed comparison of the two successions and discriminate local from basin scale signals we present new x ray fluorescence icp ms and toc data for both the limestones and the pialli level shales integrated with available carbonate carbon isotopes and nannofossil temperature and nutrient indices data indicate a homogenous background sedimentation dominated by pelagic carbonates biogenic silica with little contribution by clays the limited variation in lithogenic elements points to an essentially homogeneous detrital source area with limited fluvial terrigenous input higher mn concentrations coupled with low enrichments in redox sensitive elements such as u fe s re mo ag suggest that the pialli level shales represent temporary suboxia without reaching anoxia furthermore p authigenic ba cd and ni enrichments together with nannofossil nutrient index indicate generally low primary productivity conditions along the entire succession with only minor increases for some of the black to dark grey shales of the pialli level the nannofossil temperature index highlights a warm climate during the oae 1d with the warmest conditions experienced during the deposition of the pialli level shales during the late albian the warm and humid climate was interrupted by brief episodes of relatively warmer and less saline surface waters ensuring slower rates of bottom water renewal and producing temporary suboxic conditions such paleoceanographic dynamics would be the continuation of episodic warmer and humid pulses characterizing the late albian interval in the umbria marche basin as such the pialli level can be considered the result of a last episode closing a cycle before the establishment of a steadier climate during the early cenomanian

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

a model for iron deposition to euxinic black sea sediments sulfur isotope and c s fe elemental data from the lower devonian siegenian emsian hunsruck slate of germany allow the conditions that led to the pyritization of soft bodied fossils to be determined the slates preserving pyritized soft tissues generally contain low concentrations of organic matter 0 3 0 4 percent c and pyrite sulfur 0 0 2 percent s respectively but are unusually rich in total and hcl extractable iron concentrations of iron are significantly lower in samples of the hunsruck slate from areas where fossils lack pyritized soft tissues the fossil pyrite is very enriched in s 34 compared to fine grained disseminated pyrite in the adjacent slates delta fossil sediment 10 to 50 permil suggesting that pyritization of the fossils persisted into later stages of authigenic mineralization pyrite formation in the sediments was limited by the low concentrations of metabolisable organic matter leaving residual sulfate and iron available for continued fossil pyritization in organic rich microenvironments a diffusion with precipitation model indicates that a critical control on the initial preservation of readily metabolisable soft tissue in this way is the presence of high concentrations of sediment iron capable of being solubilized during shallow burial diagenesis in association with low concentrations of metabolisable organic matter in the matrix pore waters rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass

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

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

what regulates rhenium deposition in euxinic basins abstract rhenium is greatly enriched in black shales and euxinic basin sediments relative to its crustal abundance the enrichment process is generally assumed to involve reduction of reo 4 the principal re species in seawater to an insoluble re iv oxide or sulfide in suboxic and anoxic environments the specific biotic or abiotic mechanism of reduction has so far defeated efforts to discover it here an alternative mechanism of fixing re in sediments is examined experimentally thiolation of reo 4 leading to particle reactive thioperrhenates re vii o n s 4 n or to re vii sulfide precipitates or coprecipitates using optical absorption methodology we obtain the first measurements of equilibrium constants for formation of reo 3 s and res 4 as well as a solubility constant for a colloidal re 2 s 7 polymer from these constants we conclude that most euxinic environments contain insufficient sulfide to stabilize thioperrhenates as major species similarly re 2 s 7 polymers are too soluble to form in near neutral ph waters although they might form in acidic mine waters or during supergene hypogene enrichment processes to explain remarkable similarities between mo and re vertical profiles in the black sea we propose that both elements become thiolated at surfaces of a fe mo s colloidal phase and consequently coprecipitate coprecipitation implies that the triggering mechanism for re removal from euxinic basins is neither reduction nor thiolation of reo 4 but instead saturation of sulfidic waters with the fe mo s phase coprecipitation favors removal of mo more than re from the aqueous phase explaining increasing aqueous phase re mo ratios as these elements are depleted from sulfidic waters the raleigh fractionation law describes the process well similar partition coefficients describe the evolution of re mo ratios quantitatively both in sulfidic black sea waters and in sulfidic hingham bay pore waters

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

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

new neodymium isotope data quantify nile involvement in mediterranean anoxic episodes the development of widespread anoxic conditions in the deep oceans is evidenced by the accumulation and preservation of organic carbon rich sediments but its precise cause remains controversial the two most popular hypotheses involve 1 circulation induced increased stratification resulting in reduced oxygenation of deep waters or 2 enhanced productivity in the surface ocean increasing the raining down of organic matter and overwhelming the oxic remineralization potential of the deep ocean in the periodic development of deep water anoxia in the pliocene pleistocene mediterranean sea increased riverine runoff has been implicated both as a source for nutrients that fuel enhanced photic zone productivity and a source of a less dense freshwater cap leading to reduced circulation basin wide stagnation and deep water oxygen starvation monsoon driven increases in nile river discharge and increased regional precipitation due to enhanced westerly activity two mechanisms that represent fundamentally different climatic driving forces have both been suggested as causes of the altered freshwater balance here we present data that confirm a distinctive neodymium nd isotope signature for the nile river relative to the eastern mediterranean providing a new tracer of enhanced nile outflow into the mediterranean in the past we further present nd isotope data for planktonic foraminifera that suggest a clear increase in nile discharge during the central intense period of two recent anoxic events our data also suggest however that other regional freshwater sources were more important at the beginning and end of the anoxic events taken at face value the data appear to imply a temporal link between peaks in nile discharge and enhanced westerly activity

anomalous molybdenum isotope trends in upper pennsylvanian euxinic facies significance for use of δ98mo as a global marine redox proxy abstract the use of molybdenum isotope data 98mo from organic rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of 1 a residence time of mo in seawater much greater than the ocean mixing time and 2 quantitative removal of mo from a strongly euxinic h2s aq 11 m water column to the sediment thus preserving the seawater 98mo signature in this study we analyze mo isotopic variation in the hushpuckney shale a 73 cm thick unit representing the late transgressive to early regressive stages of a glacio eustatic cyclothem swope formation deposited in the late pennsylvanian midcontinent sea lpms of north america the hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character zones i and iii which accumulated under weakly euxinic conditions acquired relatively high 98mo values 0 9 to 1 1 whereas zone ii which accumulated under intensely euxinic conditions acquired lower 98mo values 0 6 zone iv which accumulated under suboxic conditions in the water column acquired the heaviest 98mo values 1 1 to 1 8 these results contrast with the pattern of redox 98mo covariation in modern marine environments in which the heaviest 98mo values are found in the most intensely euxinic facies we evaluate three different hypotheses to account for the mo isotopic patterns of the hushpuckney shale one hypothesis seawater freshwater mixing is rejected owing to isotopic mass balance considerations a second hypothesis is a local control on 98mo by water column redox cycling of mn with particulate mn oxyhydroxides adsorbing isotopically light mo and transferring it to the sediment a process that was most active during deposition of zone ii the significance of this scenario is that euxinic black shales may not reliably record global seawater 98mo in areas where a mn particulate shuttle is operative a third hypothesis is based on rapid secular variation of the mo isotope composition of late pennsylvanian global seawater in order to account for 98mo trends within the hushpuckney shale seawater 98mo must have varied by 1 2 at a 100 kyr timescale which would have been possible only if the residence time of mo in late pennsylvanian seawater was

precipitation of molybdenum from euxinic waters and the role of organic matter abstract molybdenum preserved in organic rich shales is a key archive employed in studying earth s past oxygenation and life s evolution in these shales positive correlations between mo and corg are common and have been attributed to mo scavenging from seawater by organic shuttles here we argue that known organic ligands are too weak or too scarce to capture more than minor amounts of mo from seawater especially in competition with sulfide alternatively we demonstrate that mo corg correlations can arise simply from microbially mediated oxidation of corg by so42 this process produces sulfide and lowers ph triggering precipitation of femos4 which soon transforms irreversibly to femos2 s2 in euxinic basins the extent of sulfate reduction progress increases with depth and can be modeled as an empirical function of biological productivity and water replacement time the response of dissolved mo to sulfate reduction progress occurs in three stages identified by a thermodynamic model in stage i progress is too small to precipitate mo from waters that are only mildly sulfidic in stage ii the flux of mo to sediments is promoted by sulfate reduction progress giving rise to positive mo corg correlations in sediments due to fluctuations in biological productivity in some cases precipitation in this stage begins before mos42 becomes the predominant dissolved mo species in which case femos4 with negative 98mo can be produced in stage iii the flux of mo becomes independent of further sulfate reduction progress and mo corg correlations degrade as reported in extremely corg rich shales even though the model omits a number of potentially important processes its predicted mo profiles in waters of several modern euxinic basins are qualitatively reasonable and are consistent with observed reduction of movi in water to moiv in sediments according to the model spikes in atmospheric pco2 will markedly increase mo accumulation and lower 98mo in euxinic basin sediments this appears to have occurred repeatedly during the phanerozoic and may be commencing now as a result of fossil fuel combustion minimal deposition of mo in organic rich shales during the archean has been attributed to low moo42 in seawater but seems more likely to be due to low so42

a dynamic concept for eastern mediterranean circulation and oxygenation during sapropel formation we propose that intermittent bottom water ventilation occurred throughout periods of sapropel deposition and that the recently reported sapropel interruptions represent centennial scale episodes of enhanced frequency intensity of that process in essence the modern high frequency variability in deep water formation affected by climatic variability over the northern basins on seasonal to longer time scales prevailed also at times of sapropel deposition although the overall ventilation state was much reduced this concept is supported by detailed multiple species isotope records for three aegean cores the presence of abundant globorotalia truncatulinoides within especially sapropels s7 and s8 in the western levantine basin observations of three rapid benthic repopulations within sapropel s6 in the deep western levantine basin a report of continuous benthic presence through sapropel s1 at intermediate deep locations offshore libya and further supporting information from the literature in the aegean records concomitant abundance of low oxygen tolerant benthic foraminifera and presence of the more oxyphilic benthic foraminifer uvigerina mediterranea with surface similar 13c values indicate repeated deep water re oxygenation events throughout the deposition of s1 the observations of a continuous benthic presence through s1 offshore from libya imply that no persistent anoxia developed at mid depth levels in that region which is far removed from direct deep ventilation influences the abundance of deep mesopelagic g truncatulinoides through several sapropels from the western levantine basin also suggests the presence of bio available oxygen at many hundreds of meters of depth moreover the rapid intermittent benthic repopulations within sapropels from the deep eastern mediterranean imply that bottom water anoxia was spatially restricted and or of a highly intermittent nature the short time scales of these repopulation events are incompatible with titration of an extensively anoxic water column and subsequent re establishment of water column anoxia we suggest that where anoxic azoic conditions were present they most likely were restricted to a veneer at the sediment water interface the extent of such an anoxic blanket depends on the balance between advective oxygen supply into the deep sea and biological and chemical oxygen demand the demand functions imply a decoupling of oxygenation from water mass advection allowing export production and corg posting rates to the sea floor to delimit the extent of the anoxic blanket in both space and time low productivity regions would develop no anoxic blanket allowing for the observed persistence of deep dwelling planktonic and bottom dwelling benthic faunas

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

comparison of fixed and moving vessel flow measurements with an adp in a large river a comparison of three dimensional flow velocity measurements made with an acoustic doppler profiler adp from fixed and moving vessels at cross sections of the parana river argentina was performed the purpose was to design a rapid and reliable procedure for quantifying the velocity field and related parameters such as bed shear velocity and the identification of secondary circulations in large rivers using an adp the fixed vessel measurements were performed over a period of 10 min at three vertical profiles along two of the sections these data were then compared with the results of ten moving vessel repeat transects made at each of the sections and which intersected the fixed vessel sampling locations using a number of different adp setup configurations from the velocity profiles obtained with both fixed and moving vessel measurements total bed shear velocity values were computed by applying the law of the wall the results indicate there can be significant differences between velocities obtained using the moving vessel method and fixed vessel measurements averaged over 10 min these differences in horizontal velocity can be significantly reduced by averaging five or more moving vessel transects with corresponding shear velocities calculated from five transect averages showing differences ranging between 10 and 15 dependent on the adp configuration location of the at a point vertical velocity profile in relation to large scale bed roughness may also be an influential factor and ideally the bed morphology should be quantified together with the adp derived velocities when using the adp to identify secondary flow cells it was found that although one cross section transect can provide a reasonable overall picture an average of five cross sections is necessary to resolve the finer details of flow the implications for applications that use moving vessel techniques for measurement and analysis of three dimensional flow structures including secondary flows are highlighted

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

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

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

patterns of organic carbon and nitrogen isotopic compositions of latest pliocene sapropels from six locations across the mediterranean sea abstract inter layered in the organic poor sediments that comprise most of the sedimentary record of the mediterranean sea are organic rich sapropels that represent periods of wetter climate increased primary productivity and suboxic to anoxic conditions at depth in order to explore processes important in the spatial and temporal variability of sapropel forming conditions we selected sedimentary sequences containing two latest pliocene sapropels at odp sites 974 and 975 in the western basin the vrica plio pleistocene type section and odp sites 964 967 and 969 in the eastern basin our main tools are nitrogen and organic carbon isotopes which are diagnostic of changes in nutrient origins and availability and in primary productivity extremely low 15n values to 5 1 indicate high rates of nitrogen fixation during sapropel formation stimulated by nearly complete consumption of nitrate in the water column due to exceptionally high rates of denitrification interruptions in sapropels represent brief intervals of low productivity and diminished nitrogen fixation the geographical distribution of the interruptions and the presence of spatial patterns in toc contents corg ntot ratios and 15n values suggest a central role of the eastern basin in initiating sapropel formation the eastern mediterranean evidently experienced higher rates of primary productivity during sapropel deposition and probably more intense dysoxic conditions in the water column organic carbon isotopes present an intriguing geographic trend across the eastern mediterranean that is probably related to non biological processes including variations in water temperature salinity and amounts of local fluvial runoff

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

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

simulation of braided river flow using a new cellular routing scheme abstract results are presented from a numerical simulation of two dimensional flow patterns in a braided river using a simple cellular routing scheme the results of the routing scheme are compared with field measurements of discharge per unit width obtained within the study reach at low flow and for higher flows with the predictions of a more sophisticated hydraulic model that solves the two dimensional shallow water form of the navier stokes equations an assessment is made of the sensitivity of the routing scheme to variations in the values of its main parameters and appropriate values are determined based on the physical characteristics of the study site and available flow measurements it is shown that despite the simple approach adopted by the cellular routing scheme to simulate processes of water redistribution it is able to replicate accurately both the field data and the results of the more sophisticated hydraulic model these results indicate that the routing scheme outlined here is able to overcome some of the limitations of previous simple cellular automata models and may be suitable for use in modelling bedload transport and channel change in complex fluvial environments as such this research represents a small and ongoing contribution to the field of numerical simulation of braided river processes

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

trace element geochemistry in brazilian cerrado soils the cerrado region on the central plateau of brazil contains some of the oldest soils on earth with some soils weathering in place since the late cretaceous the region is currently undergoing rapid agriculturalization and industrialization the objectives of this paper are 1 to inventory the native trace element contents of pristine cerrado soils and 2 to better understand which trace elements accumulate and which are depleted in well drained soils over extremely long periods of pedogenic weathering we sampled the 0 20 and 80 100 cm layers of 45 pedons located in three widely separated areas on three different geomorphic surfaces and determined total elemental contents by x ray fluorescence analysis in comparing the contents of cerrado soils to soils worldwide the contents of sc v cr ga zr nb ce and th are 1 2 times the world average y la and u are about the same and mn co ni cu zn rb sr ba and pb are 0 4 times the world average trace elements that accumulated during weathering are either pentavalent nb5 tetravalent ti4 zr4 u4 th4 or trivalent sc3 v3 cr3 ga3 y3 la3 ce3 at the acid ph of these soils while trace elements that were depleted are either divalent or monovalent rb mn2 co2 ni2 cu2 zn2 sr2 ba2 and pb2 at acid ph thus trace element geochemistry over long periods of pedogenic weathering at acid ph mirrors major element geochemistry in that mono and divalent cations are leached and depleted while tri tetra and pentavalent elements accumulate

crossing the divide representation of channels and processes in reduced complexity river models at reach and landscape scales abstract reduced complexity models have considerable potential as tools for elucidating river behaviour over periods of 10 0 10 4 years and consequently for addressing fundamental questions concerning the scale dependent nature of explanation in geomorphology this paper proposes a simple subdivision of reduced complexity models of river behaviour into two categories that mirror methodological developments in fluvial geomorphology over the past 50 years first high resolution cellular approaches that are implemented within a framework that resolves process form feedbacks at small time and space scales second models that incorporate section averaged representations of channel geometry and processes and that are typically underpinned by regime theory and equilibrium concepts examples of both model types are presented here in the form of a cellular representation of stream braiding and a combined lattice network model of alluvial fan evolution simulations conducted using these models demonstrate how small scale process form interactions determine the emergence of larger scale channel and fan morphology and in so doing regulate system response to external forcing in this sense both models demonstrate that internal feedbacks play a critical role in controlling river responses to environmental change over historic and holocene timescales however both classes of model are characterised by uncertainty in their parameterisation of geomorphic processes such that internal feedbacks and thresholds for channel response to external forcing may vary substantially between competing models methods of refining both approaches are considered and hybrid models based on lattice network structures and mechanistic representations of channel process form interactions are identified as a means of addressing the shortcomings of existing strategies

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

rhenium in the black sea comparison with molybdenum and uranium rhenium concentrations were measured in the black sea to determine the behavior of this redox sensitive element in an anoxic basin under reducing conditions re is removed from solution leading to its depletion from black sea deep waters compared with uranium water column re and mo profiles show evidence of removal from mid depths suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than u black sea surface waters are enriched in re reflecting re in rivers draining into the northern black sea that are enhanced up to 80 fold over other large world rivers this enrichment is most likely anthropogenic and may be due to coal burning in the region a simple two box model is used to demonstrate that re concentrations in the black sea are not in steady state with respect to current riverine inputs and authigenic fluxes riverine inputs of u and mo are not as significant perhaps reflecting the fractionation of re from the other elements during a high temperature combustion process re2o7 is volatile above 270 c

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

redistribution and geochemical behaviour of redox sensitive elements around s1 the most recent eastern mediterranean sapropel the youngest eastern mediterranean sapropel s1 is generally believed to have formed 7 9 14c ky b p the distribution of redox sensitive elements above in and below the sharply defined organic rich unit reveal that s 1 has already suffered heavy diagenetic alteration its present thickness based on its dark colour is only a fraction of its original thickness in one core visual evidence of s1 with an inferred initial thickness of 6 cm has been destroyed and a similar thickness appears to have been lost from the tops of another two examples this is caused by oxidation of the upper reaches of s 1 by a progressive oxidation front which commenced 5 2 5 4 14c ky ago and still appears to be active in the three cores studied diagenetic relocation of many redox sensitive elements has occurred most elements expected to be enriched in sapropels or sulphide rich sediments already have maximum concentrations above or below rather than in the present s 1 layers similarities are observed in the geochemical behaviour of as fe and p of co cu mn ni and zn and of cd mo sb ti u and v the behaviour of ba is distinctive and its immobility as barite makes it an indicator for palaeoproductivity and the original high s1 corg content the elements se and i exhibit particularly large sharp concentration peaks and together are proposed as useful indicators of the present boundary between oxic and post oxic conditions

decline in oceanic sulfate levels during the early mesoproterozoic abstract multiple sulfur isotope compositions 32 s 33 s 34 s and 36 s were analyzed for paired carbonate associated sulfate cas and disseminated pyrite py from the 1 6 ga gaoyuzhuang formation of the north china craton to reconstruct the history of sulfate levels in proterozoic oceans the 200 m thick study interval yielded relatively constant values for 34 s cas 13 0 1 8 34 s py 8 0 2 3 and 34 s cas py 5 as well as relatively constant 33 s 0 0 05 and 36 s 0 35 0 15 for both cas and pyrite limited variation in 34 s py and slightly lower 33 s of pyrite relative to cas suggest water column precipitation of pyrite limited fractionation of sulfur during microbial sulfate reduction as documented by 34 s cas py implies low seawater sulfate concentrations in the early mesoproterozoic ocean we quantitatively constrained paleo seawater so 4 2 using a novel modeling approach based on measured values of 34 s cas py and 34 s cas t max for the study unit 34 s cas py is 5 4 1 4 n 17 and 34 s cas t max is 6 8 34 myr 1 based on sedimentation rates of 30 150 m myr 1 these data indicate early mesoproterozoic seawater so 4 2 of p o 2 after great oxidation event i

evidence from molybdenum and iron isotopes and molybdenum uranium covariation for sulphidic bottom waters during eastern mediterranean sapropel s1 formation redox exerts a critical control on organic carbon rich sedimentation this is particularly true for eastern mediterranean sapropels where seawater stratification is regarded as a major driving force for oxygen depletion but in which sulphidic euxinic bottom waters occur only sporadically here we apply a powerful array of geochemical proxies fe and mo stable isotopes together with mo u ratios and redox sensitive trace elements rste to the determination of water redox evolution during the deposition of holocene s1 sapropel and its underlying and overlying sediments odp core 967d 2550 m depth rste are asymmetrically distributed within the sapropel with peak enrichments occurring in its lower early part negative correlations are found between fe57 and both fe al and s wt in the lower sapropel and are consistent with the benthic fe shuttle model fe enrichment in euxinic basins moef uef enrichment factor variations show well defined trends identical to those proposed for open marine settings in which sub oxic conditions in the background sediments give way to sulphidic waters at the rste peak in the lower sapropel the most notable features of the mo isotope profile are atypically light values mo98 95 0 7 in the lower sapropel such light mo isotope values relative to sea water mo98 95 2 3 have been related to oxic remobilisation however negative correlations between mo98 95 and fe al ba al mo al and s imply that the lowest mo isotopic compositions are associated with peak reducing conditions taken in conjunction with the evidence from the other proxies for a sulphidic water column the light mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters h2s aq 10 m the data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the holocene sapropel s1 molybdenum isotopes and ba al ratios identify a short lived sapropel re ventilation event timed to coincide with the 8 2 ka cold climatic event

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

water column dynamics during the last interglacial anoxic event in the mediterranean sapropel s5 1 marine organic rich sediments are important for long term carbon sequestration and as a source of fossil hydrocarbons but the processes underlying their formation remain elusive we present a multiproxy analysis of a relatively recent well preserved mediterranean organic rich deposit known as sapropel s5 which formed 124 119 thousand years ago ka b p where b p references year 1950 combining planktonic foraminiferal abundance records with stable isotope and organic biomarker data we reconcile the apparently contradicting coexistence of deep dwelling foraminiferal faunas with green sulphur bacteria that indicate photic zone euxinia s5 started with a rapid freshwater induced increase of density stratification that inhibited vertical mixing and deepwater ventilation through the first 900 years anoxic to euxinic deepwater conditions developed and extended up to roughly 200 m water depth then followed a 4100 year period of variability in the depth intensity of winter mixing with a roughly 1400 year periodicity coincident with fluctuations in water column stability s5 terminated with a marked decrease in density stratification and consequently renewed convective overturn

organic matter and trace element rich sapropels and black shales a geochemical comparison a distinct pliocene eastern mediterranean sapropel i 282 recovered from three ocean drilling program odp leg 160 sites has been investigated for its organic and inorganic composition this sapropel is characterized by high organic carbon corg and trace element contents and the presence of isorenieratene derivatives the latter suggests that the base of the photic zone was sulphidic during formation of the sapropel combined with evidence of bottom water anoxia preservation of laminae high redox sensitive trace element contents and the abundance and isotopic composition of pyrite this leads to the tentative conclusion that almost the entire water column may have been anoxic this anoxia resulted from high productivity and not from stagnation because an approximation of the trace element budget during sapropel formation shows that water exchange with the western mediterranean is needed entire water column anoxia has been suggested earlier for several black shales with regard to the depositional environment and the corg content however only the cenomanian turonian boundary event ctbe black shales appear to be comparable to this sapropel the proposed trace element removal mechanism of scavenging and co precipitation in an anoxic water column is thought to be similar for both types of deposits the ultimate trace element source for the sapropel however is seawater whereas it is hydrothermal and fluvial input for ctbe black shales because they have a larger temporal and spatial distribution nonetheless the corg rich eastern mediterranean pliocene sapropel discussed here may be considered to be a younger analogue of ctbe black shales

diagenetic pyritisation under eastern mediterranean sapropels caused by downward sulphide diffusion recurrent organic rich layers sapropels in eastern mediterranean sediments are enriched in corg fe and s sulphur and fe are enriched in a zone immediately below the sapropels whereas corg is not 34s values of bulk sediments and simple mass balance calculations indicate that so42 reduction has taken place in an open system with all hs formed at or close to the sediment surface formation of pyrite in the sapropel was fe limited and consequently excess hs was able to migrate downwards downward sulphidisation this resulted in the formation of pyrite below the sapropel by reaction of this hs with solid phase ferric iron and fe2 diffusing upwards from underlying sediments the fe2 source probably includes fe hydr oxide layers formed at former oxidation fronts above previously deposited and buried sapropels this downward sulphidisation mechanism allows accumulation of twice as much s in alternating organic rich anoxic organic poor suboxic sediments compared to what is preserved in organic rich anoxic sediments

sediment geochemistry of the thetis hypersaline anoxic basin eastern mediterranean sea abstract the thetis basin is a deep hypersaline anoxic basin in the western part of the mediterranean ridge eastern mediterranean sea it is characterised by highly sulphidic nacl saturated brine with one of the highest values of salinity reported from similar basins s 348 the strong pycnocline at the seawater brine interface induces permanent anoxic bottom conditions driving depositional processes and distribution patterns of redox sensitive metals in the sediments this work presents a high resolution dataset of sedimentological mineralogical and geochemical features and planktonic foraminifera distribution derived from the exploration of a 52 cm long box core sampled in the thetis sediments basing on direct comparisons with available data from other hypersaline records and ancient anoxic organic rich deep sea records sapropels and black shales we aim to explore the potential of these anoxic basins as natural laboratories to better understand the mechanisms driving geochemical patterns in recent ancient anoxic sediments

three dimensional investigation of volcanic textures by x ray microtomography and implications for conduit processes 1 synchrotron x ray computed microtomography ct was applied for the first time to clasts of pumice and scoria generated by active explosive volcanoes characterized by a range of eruptive styles mild strombolian to plinian and magmatic composition basaltic to trachytic the obtained two dimensional 2d tomographic images corresponding to sample views at different rotation angles were processed to reconstruct three dimensional 3d volumes and then used to make 3d measurements of vesicularity vesicle number density volume and connectivity for quantitative characterization of the investigated tephras the results indicate a positive correlation between vesicle number density and eruption intensity that is used to investigate modes of magma degassing in explosive eruptions in addition the vesicle geometry affecting the connected gas flow pathways in pyroclasts from strombolian eruptions is determined and related to the known permeabilities implications on the dynamics of explosive eruptions is discussed and ultimately used to quantitatively discriminate between different eruptive styles

anoxic development of sapropel s1 in the nile fan inferred from redox sensitive proxies fe speciation fe and mo isotopes abstract redox conditions and the mechanisms of redox development are a critical aspect of eastern mediterranean sapropels whose formation in oxygen depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima sapropels in the nile fan formed at relatively shallow water depths under the influence of the monsoon driven freshwater output from the river nile this work evaluates the redox evolution of holocene sapropel s1 in valpamed cruise core md9509 recovered at 880 mbsl in the ne nile fan using a combination of geochemical element proxies fe speciation fe and mo isotopes studies the productivity and redox proxies ba al mo al u al v al sb al show well defined enrichments in the sapropel but with a marked minimum at ca 8 2 ka indicative of reventilation corresponding to a well known global cooling event peak productivity and reducing signals occur close to the initiation of sapropel formation the proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel s1 recovered at the same depth but 380 km to the north md9501 supporting the notion suggested in previous studies of more reduced conditions in the nile fan the moef vs uef enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non sapropel sediments to anoxic non sulphidic water column conditions in the sapropel correspondingly the highly reactive fe pool fehr measured in fe speciation studies is dominated by fe oxyhydr oxide minerals in the background sediments whereas pyrite fepy becomes the dominant component of the fehr pool in the sapropel maximum fepy values in the sapropel coincide with peak productivity and reducing conditions implying a clear link between trace element uptake diagenetic bacterial sulphate reduction in anoxic porewater and fe mobilization in the sapropel iron isotope compositions 56fe in the sapropel do not show any departure from primary marine and detrital source sediment values and the absence of an fe al vs 56fe trend strongly argues against an fe shuttle molybdenum isotopes however show marked non conservative fractionation patterns background sediment 98 95mo values 0 2 to 0 7 are compatible with fractionation upon absorptive uptake by fe oxyhydr oxides and pyrite in contrast minimum 98 95mo values exhibited at peak sapropel reducing and pyrite producing conditions are most closely modeled by mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species the conservative fe isotope behavior mo isotope fractionation minima in the sapropel may be a characteristic of organic rich sediment diagenesis below an anoxic non sulphidic water body without the operation of a benthic fe shuttle

pyrite multiple sulfur isotope evidence for rapid expansion and contraction of the early paleoproterozoic seawater sulfate reservoir abstract earth s oxygenation is often described in terms of two unidirectional steps at the beginning and end of the proterozoic eon separated by a long lived intermediate redox state recent work defines a more complicated path to oxygenation exemplified by an apparent drop in oxidation state following the early paleoproterozoic lomagundi carbon isotope excursion the timing of this proposed drop in oxidation state is not well constrained and it is not clear how it relates to redox conditions during the remainder of the proterozoic here we present a study of pyrite multiple sulfur isotopes supported by fe speciation and organic carbon isotopes from early paleoproterozoic black shales we find evidence for the rapid expansion of the seawater sulfate reservoir during the great oxidation event at ca 2 3 ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca 2 05 ga this scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early paleoproterozoic comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early paleoproterozoic did not return until the late neoproterozoic

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single step microbial sulfate reduction coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380 m long sediment core show a large isotopic difference of up to 72 caused by in situ microbial sulfate reduction this is considerably larger than the assumed biological maximum of 46 derived from laboratory studies with pure cultures of sulfate reducing bacteria similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process involving sulfide reoxidation and disproportionation of sulfur intermediates our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single step fractionation this result indicates that the sulfate reducing communities and or their cellular metabolic activities in the deep biosphere may differ from those observed in near surface sediments or the water column

combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a pliocene sapropel from the eastern mediterranean sea layers of organic carbon rich sapropels in the sediment record of the mediterranean sea give evidence of repetitive changes in regional plio pleistocene climate results from biomarker molecule and major and trace element analyses of closely spaced samples are used to reconstruct the conditions leading to deposition of a pliocene sapropel at ocean drilling program odp site 969 on the mediterranean ridge organic carbon concentrations increase from 0 2 outside the sapropel and peak to more than 30 within it major and trace elemental composition and biomarker derived parameters indicate elevated productivity depletion of water column dissolved oxygen content and changes in sediment provenance in response to climatic changes budgets of rhenium thallium and other trace metals indicate that deep water exchange between the mediterranean subbasins and the atlantic ocean was not completely interrupted during sapropel formation enrichment factors of redox sensitive and sulfide forming trace metals as well as the presence of isorenieratene derivatives and high stanol sterol ratios point to an extended zone of anoxic water masses depth profiles of biomarker compositions sterols long chain alkenones alkandiols and ketols fatty acids indicate great floral diversity during deposition of a single sapropel and highlight the sensitive response of the marine community to variable environmental conditions changes in water mass circulation and eolian transport can be reconstructed by use of both lithogenic elements and average chain lengths of n alkanes acl index

the geochemical behavior and isotopic composition of hg in a mid pleistocene western mediterranean sapropel abstract the concentrations of mercury hg and other trace metals ni cu zn mo ba re u and the hg isotopic composition were examined across a dramatic redox and productivity transition in a mid pleistocene mediterranean sea sapropel sequence characteristic trace metal enrichment in organic rich layers was observed with organic rich sapropel layers ranging in hg concentration from 314 to 488 ng g avg 385 with an average enrichment in hg by a factor of 5 9 compared to organic poor background sediments which range from 39 to 94 ng g hg avg 66 comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers hg to the seafloor near complete scavenging of hg from the water column renders the sapropel hg isotopic composition representative of mid pleistocene mediterranean seawater sapropels have an average 202hg value of 0 91 0 15 n 5 1 sd and 199hg value of 0 11 0 03 n 5 1 sd background sediments have an average 202hg of 0 76 0 16 n 5 1 sd and 199hg of 0 05 0 01 n 5 1 sd which is indistinguishable from the sapropel values we suggest that the sapropel isotopic composition is most representative of the mid pleistocene tyrrhenian sea

high isotope fractionations during sulfate reduction in a low sulfate euxinic ocean analog a detailed record of the early earth sulfur s cycle is chronicled by the s isotope values of sulfide and sulfate preserved in the rock record interpretation of this record rests on our understanding of sulfur cycling in modern systems experiments and the resulting isotopic signatures very large fractionations in 34s of 70 are commonly measured between sulfide and sulfate in modern systems and in ancient sediments theoretical calculations suggest that sulfate reducing prokaryotes are capable of producing such large fractionations during the reduction of sulfate to sulfide although they have only been demonstrated to generate fractionations up to 48 here we report the first direct determination of 60 70 fractionations by natural populations of sulfate reducers these high fractionations occur under the relatively low sulfate conditions 1 1 2 mm of meromictic lago di cadagno in switzerland the major and minor isotopic composition of sulfide and sulfate in the lake water is consistent with sulfide produced by sulfate reduction with little evidence for modification by further oxidative sulfur cycling these observations help us to constrain the evolution of seawater sulfate concentrations

three dimensional cfd modeling of morphological bed changes in the danube river 1 the bed changes in a section of the river danube were computed using a 3 d computational fluid dynamics model a time series of discharges during the flood in 2002 was used the results compared reasonably well with regular bed level surveys before and after the flood the danube river section was 6 km long and located between vienna and the austrian slovakian border the fully three dimensional numerical model solved the navier stokes equations using the k epsilon turbulence closure nonuniform sediment transport was computed using the formulas of wu et al 2000b considering hiding exposure algorithms both bed deformation and sorting processes were calculated a number of parameter sensitivity tests were carried out on roughness values parameters in the sediment transport capacity formula parameters in the hiding exposure formulas critical shields number and variations in the formulas for the effect of a sloping bed additionally the sediment inflow to the section was varied together with investigations of physical parameters such as vegetation and groyne structures the results were most sensitive to the shields number for critical movement of the sediments an error analysis was performed to give a quantitative assessment of the parameter sensitivity tests

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels

sulphidic mediterranean surface waters during pliocene sapropel formation sapropels organic matter rich layers are common in neogene sediments of the eastern mediterranean sea the formation of these layers has been attributed to climate related increases in organic matter production1 2 3 and increased organic matter preservation due to oxygen depletion in more stagnant bottom waters2 3 here we report that eastern mediterranean pliocene sapropels4 contain molecular fossils of a compound isorenieratene known to be synthesized by photosynthetic green sulphur bacteria suggesting that sulphidic euxinic and therefore anoxic conditions prevailed in the photic zone of the water column these sapropels also have a high trace metal content which is probably due to the efficient scavenging of these metals by precipitating sulphides in a euxinic water column the abundance and sulphur isotope composition of pyrite are consistent with iron sulphide formation in the water column we conclude that basin wide water column euxinia occurred over substantial periods during pliocene sapropel formation in the eastern mediterranean sea and that the ultimate degradation of the increased organic matter production was strongly influential in generating and sustaining the euxinic conditions