zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

experimental tests for particle size dependent bioturbation in the deep ocean the potential for particle size dependent bioturbation rates was experimentally tested at 1 240 m in the santa catalina basin eastern pacific spherical glass bead tracers in five size classes 816 17 31 32 62 63 125 and 126 420 m were spread over the sediment surface and tube cored 997 d later downcore concentrations of glass beads were enumerated in each of the five size categories and page s l test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer in all cores the null hypothesis was rejected finer tracers penetrated deeper into the sediment in two of the three cores vertical biodiffusivities were computed from concentration profiles of downcore tracers these also showed size dependence with biodiffusivities ranging from 1 cm2 yr for the 8 1 6 pm fraction to 0 1 cm2 yr for the 1 25 420 pm size class these data demonstrate that vertical bioturbation rates are particle size dependent in santa catalina basin the likely cause is preferential ingestion and downward transport of fine particles by deposit feeders

a new appraisal of depositional cyclicity in the neoarchean paleoproterozoic dales gorge member brockman iron formation hamersley basin australia abstract the dales gorge member dgm of the brockman iron formation hamersley basin western australia is a classic banded iron formation bif that was deposited during the archean proterozoic transition just prior to the great oxidation event the goal of this study is to apply modern cyclostratigraphic principles to investigate the possible origins of the characteristic stratigraphic banding in this bif which indicates the influence of quasi regular cyclic processes in the deposition of iron rich and chert rich lithologies a grayscale scan series of the dgm type section 47a drillcore photographs from three cores drilled in the wittenoon gorge vicinity is examined using well established methods of time series analysis complex banding patterns suggest that milankovitch forcing may have taken place with orbital rotational parameters that were significantly modified from the present day due to continuous planetary tidal dissipation over the past 2 5 billion years the banding patterns change systematically and dramatically within the member in response to slowly varying environmental conditions which are interpreted to be related to low frequency sea level changes and or tectonic evolution of the hamersley basin

dynamics of an assembly of rigid ice floes in this paper we present a model describing the dynamics of a population of ice floes with arbitrary shapes and sizes which are exposed to atmospheric and oceanic skin drag the granular model presented is based on simplified momentum equations for ice floe motion between collisions and on the resolution of linear complementarity problems to deal with ice floe collisions between collisions the motion of an individual ice floe satisfies the linear and angular momentum conservation equations with classical formula applied to account for atmospheric and oceanic skin drag to deal with collisions before they lead to interpenetration we included a linear complementarity problem based on the signorini condition and coulombs law the nature of the contact is described through a constant coefficient of friction as well as a coefficient of restitution 0 e 1 describing the loss of kinetic energy during the collision in the present version of our model this coefficient is fixed the model was validated using data obtained from the motion of interacting artificial wood floes in a test basin the results of simulations comprising few hundreds of ice floes of various shapes and sizes exposed to different forcing scenarios and under different configurations are also discussed they show that the progressive clustering of ice floes as the result of kinetic energy dissipation during collisions is well captured and suggest a collisional regimes of floe dispersion at small scales different from a large scale regime essentially driven by wind forcing

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

mean mixed depth of sediments the wherefore and the why the activities and consequently the bioturbational effects of deposit feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide environmentally invariant mean of 9 8 cm with a standard deviation of 4 5 cm currently available theories of infaunal behavior cannot predict quantitatively this observation a new simple model that accounts for the feedback between resource food abundance its reactivity and the intensity of bioturbation leads to a quantitative estimate of 9 7 cm this model constitutes a fundamental advance in our understanding bioturbation

authigenic uranium in atlantic sediments of the last glacial stage a diagenetic phenomenon cores from three different atlantic localities equatorial cape verde rise and porcupine abyssal plain are shown to have anomalous high u contents 5 8 ppm total in sediments laid down during the last glacial stage 12 24 ky bp radiocarbon data demonstrate that the sediments hosting the peak u levels were accumulated at rates similar to those immediately above and below all the cores exhibit maximum mn levels characteristic colour changes and maximum u levels in the same sequence with increasing depth in core on the evidence of the similarities between the cores and pore water u data from a porcupine abyssal plain site it is proposed that the authigenic u enrichments are syndiagenetic and possibly active no correlation is observed between sediment authigenic u and corg contents the source of enrichment is bottom water u which has diffused downwards into the sediments to be sorbed at a particular redox level located 10 30 cm below the oxic post oxic boundary marked by the colour change the magnitude of the enrichments is caused by the persistence of this boundary at a particular level as a result of the decrease in mean sediment accumulation rate between the last glacial stage 5 2 up to at least 19 1 cm ky 1 and the holocene 2 2 4 1 cm ky 1 similar accumulation rate contrasts are expected to be widespread in the atlantic and the implications for previous reported work particularly from the mid atlantic ridge are discussed

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

trace metals in metalliferous sediments manop site m interfacial pore water profiles we present data showing enrichments of metals mn cu v cr cd and ni and dissolved organic matter dom in surficial pore waters depths 1 cm at a metalliferous sediment site in the pacific we also present a model of metal release during particulate organic carbon poc oxidation and opal dissolution at the sediment seawater interface a comparison of measured interfacial metal concentrations with those predicted from the model suggest that cd ni and cr are released during oxidation of poc at the sediment seawater interface however for v cu and mn additional source s and or processes are required to account for the measured interfacial concentrations for cu recycling via interfacial remineralization and release to bottom waters with subsequent scavenging out of bottom waters by particulates probably provides the major additional source for v we suggest that in addition to some contribution from a benthic flux bottom water scavenging scenario complexation with dom at the interface serves as a trap to accumulate v released from particulates in the zone of oxygen reduction for manganese we suggest that most of the interfacial pore water content is produced from reduction of sedimentary mn iv by reactions with dom at the sediment seawater interface the calculated benthic flux of manganese as a result of this reaction may account for a large fraction of the particulate manganese rain that is apparently not preserved in the sediment at this site

size distribution and shape properties of relatively small sea ice floes in the antarctic marginal ice zone in late winter in the marginal sea ice zone miz where relatively small ice floes are dominant the floe size distribution is an important parameter affecting melt processes given the larger cumulative perimeter of multiple small floes compared with a single ice floe of the same area smaller ice floes are therefore subject to increased lateral melt however the available data have been very limited so far analysis of sea ice in the sea of okhotsk revealed that while floe size distribution is basically scale invariant a regime shift occurs at a size of about 40 m in order to extend this preliminary result to the antarctic miz and further examine the controlling factors the first concurrent ice floe size and ice thickness measurements were conducted in the northwestern weddell sea and off wilkes land around 64 s 117 e with a heli borne digital video camera in the late winter of 2006 and 2007 respectively the floe sizes ranged from 2 to 100 m our analysis shows 1 the scale invariance and regime shift are confirmed in both regions 2 the floe size at which regime shift occurs slightly increases from 20 to 40 m with ice thickness consistent with the theory of the flexural failure of sea ice and 3 the aspect ratio is 1 6 1 9 on average close to the previous results based on these results the processes affecting the floe size distribution and the subsequent implications on melt processes are discussed by applying a renormalization group method to interpret the scale invariance in floe size distribution the fractal dimension is related to the fragility of sea ice these results indicate the importance of wave ice interaction in determining the floe size distribution

bioturbation short lived radioisotopes and the tracer dependence of biodiffusion coefficients bioturbation refers to the mixing of sediment particles resulting from benthic faunal activity it is the dominant particle mixing process in most marine sediments and exerts an important control on diagenetic processes in models bioturbation is usually treated as a diffusive process where the biodiffusion coefficient db characterizes the biological mixing intensity biodiffusion coefficients are classically computed by fitting a diffusive model to vertical profiles of particle bound radioisotopes one peculiar observation is tracer dependence db values from short lived tracers tend to be larger than those obtained from long lived tracers from the same site recent theoretical work based on random walk theory and lattice automaton bioturbation simulations labs has suggested that this tracer dependence is simply a model artifact and has concluded that the biodiffusion model is not applicable to the short observational time scales associated with short lived radioisotopes here we have compiled a global dataset of db values obtained from different radiotracers to assess tracer dependence from a data perspective tracer dependence is significant in low mixing environments like slope and deep sea sediments but is not present in intensely mixed coastal areas tracer dependence is absent when the number of mixing events is larger than 20 or the potential length scale is greater than 0 5 cm roughly this comes down to tracer derived db values greater than 2 cm2 yr 1 this condition is met for 68 50 and 8 of published db values obtained from coastal continental slope and abyssal environments respectively these results show that short lived radioisotopes are suitable to quantify biodiffusion mixing in sedimentary environments featuring intense bioturbation

the behaviour of manganese in atlantic carbonate sediments abstract manganese porewater and solid phase data are considered from two different atlantic carbonate sediments both with good geochemical characterisation the first case is a continuously accumulated alternating pelagic marl carbonate ooze where there is a correlation in post oxic conditions between mn ii porewater levels and an acetate leachable mn fraction of the sediment this leachable mn appears to have an association with calcite and in this example the leached residue mn content approximates to that of shale in the second case a very recent compositionally homogeneous calcareous turbidite is underlain by mn oxyhydroxide at the former sediment surface which is now acting as a source of mn ii by reduction sorption of mn by the turbidite sediment and shale like leached residue mn levels in anoxic conditions are again observed as mn ii diffuses towards present oxic conditions although no unique equilibrium is achieved similar older buried turbidites have concentration decreases in solid phase mn content preserved from bottom to top as relict evidence that this process has occurred in these turbidites 54 64 of the total mn is leachable by the acetate method and therefore presumed to be sorbed by calcite the oldest example has persisted for 125 kyr it is suggested that the slow breakdown of metastable mn oxyhydroxide and sorption of mn ii to calcite surfaces are important controls on the behaviour of mn in pelagic carbonates in mildly reducing conditions where accumulation fluctuations occur these processes act together with accumulation to limit overall mn migration and to prevent the large mn enrichments seen near the sediment surface in more reducing environments

reconstruction of the last deglaciation deconvolved records of δ18o profiles micropaleontological variations and accelerator mass spectrometric14c dating bioturbation acts as a low pass filter in displacing and reducing the amplitudes of stratigraphic signals this often leads to a loss of high frequency events in the stratigraphic record in addition when considering an isotopic signal 18o 14c measured in stratigraphic carriers such as foraminifera bioturbation and carrier abundance changes can create artifacts which may be falsely interpreted as leads or lags in the paleoclimatic record

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

redox zonation of elements at an oxic post oxic boundary in deep sea sediments abstract deep sea sediments generally provide limited evidence of redox control on the post depositional redistributions of elements this is because only a fraction of any element is reactive and observed elemental distributions are the result of a number of competing processes which are simultaneously active so that redox effects are not dominant a detailed investigation is reported of a core which is unusual in showing unequivocal evidence of direct redox control operating in a homogeneous environment that is relatively free from competing processes a progressive oxidation front has migrated slowly downwards into a uniform turbidite over a long period ca 250 ky the reactive fractions of several redox sensitive elements now show a remarkable degree of organisation around the redox boundary of this front with the different elements exhibiting individual responses solid phase enrichments are not coincident but are arranged in a clear succession of concentration peaks along the redox gradient from oxic to postoxic conditions the succession is mn i fe se and pt cu and pb v zn and sb and u the movement directions of most elements can be inferred from the peak shapes against the homogeneous turbidite background composition the system seems to have been essentially isolated within the sediment for most elements so that no loss or gain can be ascribed to redox processes exceptions are u and v which have been augmented by uptake from ocean bottom waters into the sediments by an efficient redox trap mechanism

remobilization of transition metals in surficial pelagic sediments from the eastern pacific abstract the top thirty centimeters of sediment at two sites in the eastern equatorial pacific contain evidence of post depositional remobilization of mn fe co ni cu and zn remobilization takes place as mn and fe oxyhydroxides are released to the pore water during the microbially mediated decomposition of organic matter precipitation of the dissolved metals in near surface more oxic strata controls the solid phase distribution of mn fe and zn the solid phase redistribution of co and ni requires only suitable material for readsorption comparison of pore water fluxes with solid phase metal distributions in the solid sediment indicates no loss of dissolved metal to the overlying water column at the present time loss of mn during the quaternary is indicated by the composition of the sediments however leaching experiments suggest that portions of the mobile mn fe co ni and cu are fixed by incorporation in authigenic smectite in the surficial sediments

copper and manganese in hemipelagic sediments at 21 n east pacific rise diagenetic contrasts abstract relatively rapidly accumulating 7 cm kyr and organic rich 1 5 corg epr sediments near 21 n are characterized by pronounced nitrate reduction in the upper decimetre and by an enrichment of mn and increased fe al ratio in surficial sediments joint consideration of solid phase and pore water data indicates that the distributions of both mn and fe in the sediments are in disequilibrium with their respective pore water profiles precipitation of upward diffusing mn and fe is occurring at present below the enriched horizons producing perched enrichments indicative of non steady state diagenesis consideration of a high resolution 14c age vs depth curve and a decrease toward the interface from 1 51 to 1 37 wt over the top 7 cm of the corg profile suggests that the diagenetic disequilibrium evident for mn and fe may be due to a geologically recent decrease in productivity such a decrease would be manifest in the sediments by a downwardly migrating oxidation front which would progressively erode the mn and fe pore water profiles producing the existing decoupled surface enrichments in contrast to mn the distribution of cu has apparently not been perturbed by non steady state phenomena a first order decrease with depth of the dissolved copper concentration is accompanied by a first order increase in solid phase cu application of a diffusion reaction model to these data predicts a sedimentation rate of 8 cm kyr 1 which agrees well with the 14c determined rate of 7 3 cm kyr 1 implying that the pore water and solid phase distributions are in diagenetic equilibrium available evidence suggests that the major host for non detrital cu in hemipelagic sediments is authigenic nontronite because this phase appears to form slowly there may be a kinetic constraint on the amount of copper which can be incorporated authigenically in rapidly accumulating hemipelagic deposits support for this hypothesis is given by the observed inverse proportionality between the sedimentation rate and the concentration of non detrital cu in surface sediments from at least seven widespread locations in the eastern pacific it is suggested that the variation observed in the salt and caco3 free cu concentration in hemipelagic sediments is a consequence largely of the residence time of sediments at the surface where high dissolved cu concentrations prevail

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

the archaean and early proterozoic banded iron formations of north china their characteristics geotectonic relations chemistry and implications for crustal growth abstract bif occurs extensively throughout the archaean and early proterozoic terrains of northern china there are four main iron metallogenic provinces anshan in n e china e hebei and wutai in n china and qingling dengfeng taihua on the south margin of the north china platform all bifs have undergone varying degrees of regional metamorphism from subgreenschist to granulite facies and deformation the chinese bifs have three marked characteristics 1 most bif are archaean in age their formation reaching a peak during the late archaean 2 bif in china occur in both high grade regions and greenstone belts the former are more significant their reserves accounting for more than three quarters of those of the precambrian bif in china 3 oxide iron formation is predominant in nearly all archaean bif silicate and carbonate iron formations are rare the few carbonate iron formations occur in lower proterozoic sediments there is no difference in chemical composition between bif in high grade regions and in greenstone belts however there are geochemical and petrological differences between their associated supracrustal sequences the presence of such large amounts of bif within high grade regions is unusual from their regional geology rock association and from the geochemistry of associated metavolcanics and orthogneisses we suggest that the high grade type of bif formed in arcs or back arc basins which were intruded by voluminous tonalites and granodiorites later converted by deformation and high grade metamorphism into orthogneisses such processes of accretion and crustal growth are different in kind but not in principle from those operating today

active diagenetic formation of metal rich layers in n e atlantic sediments sediment cores from the porcupine abyssal plain exhibit an indurated layer 0 5 3 cm thick at depths of approximately 50 cm this is some 15 20 cm below the glacial holocene transition as interpreted by radiocarbon dating and the palaeontological criteria of ruddiman and mcintyre 1981 the layer is forming currently at the oxic post oxic boundary in the sediments as revealed by pore water data o2 and no 3 are present in solution above the layer while fe2 mn2 po3 4 and nh 4 are present in solution below and all these species show concentration gradients indicating fluxes into the layer these data are consistent with the hypothesis for the initiation and sustained formation of such layers proposed by wilsonet al 1986a b the elements mn ni co fe p v cu zn and u are all enriched to varying degrees in the vicinity of the layer some differential stratification of these elements in the vertical consistent with a redox control is observed at one site with a 0 5 cm layer with mn ni and co above fe p v and cu in the layer and u below at another site the metal rich layer has higher fe and p concentrations and is more indurated here all enrichments except co are contained within a single layer sample 3 cm thick

phosphate regeneration from sediments of the peru continental margin by dissolution of fish debris generally oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes stoichiometric modelling of such reactions takes into consideration varying proportions of decomposable organically bound p to account for the ratios among nutrients in depth concentration profiles of near surface sediments new results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound p dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10 of the nutrient pool in the waters of the peru undercurrent partitioning of p among the two sources reveals further that fish debris phosphate is about four times more important than organically bound p in nutrient generation from sediments of the peru continental margin not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area

is burial velocity a master parameter for bioturbation abstract the intensity of biological mixing in a marine sediment is widely believed to be related to the flux of labile organic matter at the same time this flux has been shown to be strongly correlated to the burial velocity one would then expect a significant correlation between biological mixing coefficient d b and burial velocity a nonlinear regression of existing data with a power law eqns 2 and 3 of the text produces a statistically significant correlation the statistical significance of the regression in no way implies causation and in fact it accounts for only about 22 of the total variance in the dataset thus reducing its physical significance a number of additional processes contribute to the large observed variance including timing of mixing events temporal and spatial heterogeneity differences in temporal scales sampling artifacts and specific particle associations with due care the d b relationship may prove useful in defining the likely range of d b values in a particular environment a similar analysis of the tracer identified surface mixed depth shows that it is essentially independent of burial velocity with a worldwide mean of 9 8 4 5 cm depth of mixing is probably more closely related to the energetic costs of deep burrowing

mixing sedimentation rates and age dating for sediment cores abstract analytic solutions to the advection diffusion equation useful as a guide for the interpretation of mixing sedimentation rate and age dating effects in sediment cores have been derived various simplified mixing models appropriate to 14 c and 210 pb measurements have been considered the emphasis has been on a discussion of the various interrelations and dependencies among the mixing coefficient sedimentation rate and radioactive decay constant parameters in determining the 14 c and 210 pb concentrations as a function of depth porosity variations within the sediment column are also considered and the transformed advection diffusion equation including porosity effects is derived

non steady state diagenetic record in eastern equatorial atlantic sediments abstract the radiocarbon age profile of a sediment core from the eastern equatorial atlantic shows a marked decrease in both clay and carbonate accumulation rates at the end of the last glacial period with overall rates of 3 2 and 8 1 cm kyr for the holocene and late glacial respectively the transition from glacial to postglacial conditions recorded in the sediments at 39 cm is above the visual fe ii fe iii redox boundary at 48 cm which has been used in the past to identify the glacial section and even further above the increase in preserved organic carbon content at 55 cm it is concluded from the present organic carbon profile and recent observations in this area and elsewhere that organic diagenesis in these sediments is not steady state instead it is suggested that the uppermost glacial sediments lost carbon through oxic remineralisation at the change to postglacial conditions and that this process is now continued with nitrate as the electron acceptor these observations suggest that the double mno x solid phase peak which occurs in these and similar sediments is a direct consequence of an adjustment of the oxic layer depth following the decrease in carbon flux at the onset of the holocene

arctic sea ice velocity field general circulation and turbulent like fluctuations 1 using buoy trajectories of the iabp data set we analyze the arctic sea ice velocity field as the superposition of a mean field and fluctuations we study how the mean field can be objectively defined using appropriate spatial and temporal averaging scales depending on the season considered 400 km and 5 months for winter i e approximately all the polar winter duration and 200 km and 2 months for summer i e approximately all the polar summer duration the mean velocity field shows a strong intra annual between winter and the following summer as well as interannual variability the fluctuations i e the remaining part of the velocity field after subtracting the mean field are analyzed in terms of diffusion properties although the arctic sea ice cover is a solid we show that the fluctuations follow the same diffusion regimes as the ones predicted for turbulent flows as observed in geophysical fluids like the ocean or the atmosphere we found that the integral time and the diffusivity of sea ice are in the same ranges as those estimated for the ocean i e 1 5 days in winter and 1 3 days in summer and 0 44 103 m2 s for winter and 0 45 103 m2 s in summer respectively however the statistics of the sea ice fluctuating velocity deviate from classical turbulence theory as they show exponential instead of gaussian distributions sea ice velocity and acceleration are intermittent and both are characterized by a multifractal scaling the oceanic and atmospheric dynamic forcing cannot explain solely the statistical properties of sea ice kinematics and dynamics we argue that sea ice dynamic is significantly influenced by the interplay of multiple fractures that are activated intermittently within the ice pack

ironstone deposits hosted in eocene carbonates from bahariya egypt new perspective on cherty ironstone occurrences abstract this paper gives new insight into the genesis of cherty ironstone deposits the research was centered on well exposed unique cherty ironstone mineralization associated with eocene carbonates from the northern part of the bahariya depression egypt the economically important ironstones occur in the naqb formation early eocene which is mainly formed of shallow marine carbonate deposits periods of lowstand sea level caused extensive early dissolution karstification of the depositional carbonates and dolomitization associated with mixing zones of fresh and marine pore water in faulted areas the eocene carbonate deposits were transformed into cherty ironstone with preservation of the precursor carbonate sedimentary features i e skeletal and non skeletal grain types thickness bedding lateral and vertical sequential arrangement and karst profiles the ore deposits are composed of iron oxyhydroxides mainly hematite and goethite chert in the form of micro to macro quartz and chalcedony various manganese minerals barite and a number of subordinate sulfate and clay minerals detailed petrographic analysis shows that quartz and iron oxides were coetaneous and selectively replaced carbonates the coarse dolomite crystals having been preferentially transformed into quartz whereas the micro crystalline carbonates were replaced by the iron oxyhydroxides a number of petrographic sedimentological and structural features including the presence of hydrothermal mediated minerals e g jacobsite the geochemistry of the ore minerals as well as the structure controlled location of the mineralization suggest a hydrothermal source for the ore bearing fluids circulating through major faults and reflect their proximity to centers of magmatism the proposed formation model can contribute to better understanding of the genetic mechanisms of formation of banded iron formations bifs that were abundant during the precambrian

a review of discrete element simulation of ice structure interaction sea ice loads on marine structures are caused by the failure process of ice against the structure the failure process is affected by both the structure and the ice thus is called icestructure int

the diagenesis of phosphorus in a nearshore marine sediment abstract chemical analyses have been made for total inorganic phosphorus total organic phosphorus and organic carbon in plankton and in anoxic sediments from long island sound u s a when combined with laboratory experiments the measurements of other studies on the same sediments and mathematical modelling the results indicate that 1 1 a large proportion of dissolved phosphate in the upper 10 cm of sediment zone of bioturbation is provided by the release of adsorbed phosphate during the reduction of ferric oxyhydroxides 2 2 below the zone of bioturbation phosphate is liberated to solution solely via organic matter decomposition and the release is stoichiometrically coupled to bacterial sulfate reduction and ammonia formation 3 3 a steady state diagenetic model can be used to predict the profile of organic phosphorus from that of dissolved phosphate at depths below the zone of bioturbation 4 4 the flux of dissolved phosphate out of the sediment must arise largely from phosphate liberation occurring very close 5 5 there is a large preferential loss of phosphorus relative to carbon from organic matter at the time of or prior to burial whereas after burial phosphorus is not lost preferentially at this location

paleoproterozoic positive δ13ccarb excursion in the northeastern sino korean craton evidence of the lomagundi event abstract the 2 33 2 06 ga positive 13 c carb excursion associated with environmental change and the breakup of the kenorland or superia supercontinent is called the lomagundi or jatulian event or great oxidation event and has been reported in many early precambrian cratons but not yet in the sino korean craton the guanmenshan formation of the liaohe group occurs in the northeastern part of the sino korean craton 13 c carb and 18 o values in 42 samples from this formation range from 3 5 5 9 v pdb and 15 4 24 8 v smow respectively showing a clear positive 13 c carb excursion that characterizes the lomagundi event thirty five of the 42 samples with less hydrothermal alteration have higher 13 c carb and 18 o carb values than the other 7 samples obviously affected by fluid flow confirming that it was fluid flow that reduced the 13 c carb and 18 o carb values this positive 13 c carb excursion places deposition of the guanmenshan formation within the age range of 2 33 2 06 ga

banded iron formations through much of precambrian time similarities between proterozoic 1 8 2 5 gyr and archean 2 5 gyr banded iron formations are probably more significant than their differences the contrasts largely reflect differences in the tectonic settings of proterozoic and archean terrains archean banded iron formations are not as thick nor laterally as extensive as the major proterozoic iron formations nevertheless some archean iron formations have strike lengths of over 150 200 km and may have been quite extensive prior to the deformation that has affected most archean terrains stratigraphic sequences in which iron formations occur are highly variable and indicate that iron formations formed in many depositional environments sedimentary textures in the iron formations are dominated either by granules and oolites or laminations including microbanding reflecting differences in their physical conditions of deposition granular and oolitic textures are abundant in only three proterozoic depositional basins and most precambrian iron forma

increased subaerial volcanism and the rise of atmospheric oxygen 2 5 billion years ago the hypothesis that the establishment of a permanently oxygenated atmosphere at the archaean proterozoic transition approximately 2 5 billion years ago occurred when oxygen producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older to sustain vanishingly low oxygen levels despite near modern rates of oxygen production from approximately 2 7 2 5 billion years ago thus requires that oxygen sinks must have been much larger than they are now here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the archaean era enhanced submarine volcanism was abruptly and permanently diminished during the archaean proterozoic transition observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the proterozoic submarine volcanoes are more reducing than subaerial volcanoes so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

effect of shear rupture on aggregate scale formation in sea ice a discrete element model is used to study shear rupture of sea ice under convergent wind stresses the model includes compressive tensile and shear rupture of viscous elastic joints connecting floes that move under the action of the wind stresses the adopted shear rupture is governed by coulomb s criterion the ice pack is a 400 km long square domain consisting of 4 km size floes in the standard case with tensile strength 10 times smaller than the compressive strength under uniaxial compression the failure regime is mainly shear rupture with the most probable scenario corresponding to that with the minimum failure work the orientation of cracks delineating formed aggregates is bimodal with the peaks around the angles given by the wing crack theory determining diamond shaped blocks the ice block floe aggregate size decreases as the wind stress gradient increases since the elastic strain energy grows faster leading to a higher speed of crack propagation as the tensile strength grows shear rupture becomes harder to attain and compressive failure becomes equally important leading to elongation of blocks perpendicular to the compression direction and the blocks grow larger in the standard case as the wind stress confinement ratio increases the failure mode changes at a confinement ratio within 0 2 0 4 which corresponds to the analytical critical confinement ratio of 0 32 below this value the cracks are bimodal delineating diamond shape aggregates while above this value failure becomes isotropic and is determined by small scale stress anomalies due to irregularities in floe shape

formation of an aggregate scale in arctic sea ice 1 the ice pack covering northern seas is a mixture of thick ridged and rafted ice undeformed ice and open water conventional eulerian arctic sea ice models use a plastic yield surface to characterize the constitutive behavior of the pack an alternative is to adopt a discontinuous lagrangian approach and explicitly model the formation of leads and pressure ridges we use a lagrangian ice model that consists of thousands of discrete polygonal floes 1 4 km in width at the beginning of a simulation the ice floes are frozen together in a square domain we apply a linearly varying wind stress that deforms the pack by stretching the viscous elastic joints between adjacent floes fractures propagate along joints forming a crack pattern in the model ice pack the crack pattern defines a system of large plates 10 100 km in width that are aggregates of many individual floes the average size of the plates is determined by a competition between the rate of crack creation and the speed of the relaxation wave that travels outward from a newly broken joint and reduces stresses in the surrounding pack simulation results are used to characterize the formation of the aggregate structure and to determine how the rate of crack creation and the average area of the aggregate plates depends on tensile strength the wind stress gradient and the size of the individual floes after the formation of the aggregate scale plate structure subsequent deformation occurs at the plate boundaries since the usual state of the ice pack is a state of failure an interesting situation is created in which the initial wind driven deformation creates the material conditions or aggregate structure under which subsequent deformation occurs

bias in the paleoceanographic time series tests with a numerical model of u corg and al burial 1 we have constructed a nonsteady state numerical model of organic carbon aluminum and uranium burial to examine the dynamic response of sediments to variations on thousand years scale the three components are defined by distinct behaviors in the sedimentary system the aluminum is a conservative component the organic carbon is a nonconservative component degraded in superficial sediment and the uranium is a component affected by redox conditions by introducing a simultaneous variation of incoming fluxes of corg and al geochemical profiles of al corg and u clearly show separate distributions after burial each component record the synchronous variation of the incoming fluxes but the position in the sedimentary pile of the recording differs from a component to another we show that a minimum of u could be linked to a rapid ascent of the oxic front during an increase in corg flux

major precambrian events and mineralization in the north china craton preface abstract this special issue of ore geology reviews assembles a set of thirteen papers which provide new insights on the major precambrian events and mineralization in the north china craton ncc these contributions cover a detailed evaluation of the major metallogenic systems including the bif cu ree mo and graphite deposits in the ncc

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

concentration oxidation state and particulate flux of uranium in the black sea abstract in contrast to its unreactive behavior in the open ocean uranium is removed from seawater to sediments of anoxic marine basins four biogeochemical processes have been proposed to account for this removal active uptake by organisms whose remains are preserved by the anoxic conditions of the sediments complexation of u vi by particulate organic matter chemical reduction of soluble u vi to insoluble u iv which is scavenged to the sediments by settling particles and precipitation of uranium within the sediments themselves the black sea is the largest anoxic marine basin in the world today waters below 150 200 m are anoxic and h2s levels build up to about 400 mol 1 1 in the bottom water maximum depth 2200 m concentrations of total u and the oxidation state of dissolved u were measured throughout the water column while particulate u fluxes were determined from time series sediment trap samples collected in the deep anoxic zone bottom waters were depleted in u by 40 relative to the initial u content of the water two independent methods gave residence times of dissolved u in the deep water column of about 103 years uranium occurred throughout the water column as soluble chemically labile reducible u vi rather than the thermodynamically favored u iv uranium is neither reduced to u iv by the high levels of h2s nor is it rapidly scavenged from the water column as would be expected for strongly hydrolyzed tetravalent actinides like u iv fluxes of particulate authigenic u measured with sediment traps are at least two orders of magnitude less than the rate of authigenic u burial in black sea sediments therefore the first three processes listed above contribute negligibly to the deposition of u in the sediments of anoxic marine basins such as the black sea the dominant removal process must involve precipitation of u in the sediments rather than scavenging from the water column

early diagenetic processes in the muddy sediments of the bay of biscay abstract in order to understand the early diagenesis processes occurring in continental margin environment modern sediments collected in six different sites from the bay of biscay have been studied these sites can be separated into two groups in the shallowest stations where sediments are highly bioturbated organic carbon levels are higher than 2 in the deepest stations sediments are much less bioturbated and organic carbon levels are lower in all sites the vertical distribution of redox sensitive species can be explained by the well established depth sequence of redox reactions based on the bacterially mediated oxidation of organic matter we have considered some alternative reaction pathways to explain the profiles of fe mn and n species these reactions deal with the ammonia oxidation by manganese oxide the aerobic denitrification and the oxidation of dissolved iron ii by nitrate or mn oxides vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the mn oxides and the oxidation of all the upward diffusing fe ii they may also be responsible for a significant part of the total dinitrogen production the relative importance of these pathways on early diagenetic processes and benthic fluxes has not been determined and must be examined with additional experimental works our study suggests however that the coupling between the benthic cycles of iron manganese and nitrogen could strongly influence the carbon cycling at the ocean floor

on the mesoscale interaction of lead ice and floes the plasticity of the arctic ice pack depends on its granular nature in particular on the size and distribution of areas of thin ice and open water surrounding multiyear ice floes the paper begins with construction of a mesoscale 10 100 km granular model of the central arctic ice pack the mesoscale model is based on a dynamic particle simulation in which individual multiyear ice floes and surrounding parcels of first year ice are explicitly modeled as discrete convex polygons in a two dimensional domain deformation of the domain produces areas of localized failure and areas of open water the areas of localized failure are modeled as pressure ridging events using the results of numerical experiments performed with a computer simulation of the ridging process the paper focuses on the results of numerical experiments performed with the mesoscale model in the experiments the model ice pack is biaxially deformed at constant strain rates the principal strain rates are varied to create deformation states ranging from pure shear to uniform compression the results define the shape and magnitude of the plastic yield surface the strain rate vectors associated with points on the yield surface the partition of energy dissipation between ridging and in plane sliding and the changes in the ice thickness distribution associated with various deformation states

diagenetic separation of cadmium and manganese in suboxic continental margin sediments abstract the vertical distribution of cd differs from that of mn in sediment cores taken from the continental margins of the arctic and atlantic oceans the oxic surface sediment is depleted in cd and enriched in mn whereas the subsurface sediment is enriched in cd and depleted in mn in some cores the distributions of solid phase and dissolved mn are offset the porewater profiles indicate that mn is being precipitated relatively deep in the sediment whereas the solid phase distributions indicate that mn has been precipitated very close to the sediment surface in the recent past at one deep site located near the base of the continental slope 3000 m the offset may be caused by a relatively recent decrease in ocean productivity a corresponding decrease in the organic carbon flux to the sediment and a subsequent migration of the redox boundary deeper into the sediment however at a site where the upper sediment layers have accumulated during the last few decades this explanation cannot account for the offset distributions we propose that a fluctuating input of organic matter to the sediment on seasonal or decadal timescales causes the position of the redox boundary to fluctuate the fluctuating redox boundary pumps dissolved mn upward and concentrates it as mn oxide in a distinct layer at the upper limit of the redox excursion at the same time it pumps cd downward and concentrates it in a distinct layer at the lower limit of the redox excursion the net result of a fluctuating redox boundary is to increase the spatial separation of cd and mn during the early diagenesis of suboxic sediments

when deep diagenesis in arctic ocean sediments compromises manganese based geochronology abstract we used a diagenetic model to test the hypothesis that manganese rich layers in gas hydrate bearing arctic ocean sediments are reliable time markers for interglacial periods in the model diagenesis is fuelled by two sources of reactive carbon particulate organic carbon settling to the sediment surface and methane diffusing up from deep gas hydrate deposits the model includes oxidation of organic carbon and soluble reduced manganese by oxygen supplied continuously from an invariant bottom water oxygen reservoir reduction of particulate manganese by hydrogen sulfide generated through anaerobic methane oxidation transport of dissolved oxygen and manganese by diffusion and advective transport of particulate components by burial particulate organic matter and particulate manganese are only supplied to the sediment during interglacials sulfate reduction is not modeled explicitly instead the effect of anaerobic methane oxidation on mn reduction is simulated at the lower boundary of the model by prescribing that particulate manganese is reduced there to soluble mn ii the soluble reduced mn then diffuses upward and is oxidatively precipitated to mn iv by downward diffusing oxygen the upward flux of soluble mn ii is thus a function of the rate at which particulate manganese is advected into the mn reduction layer at the bottom of the model it is not synchronous with events at the sediment water interface model runs reveal that under idealized but realistic conditions for the arctic ocean oxidation of upward diffusing mn ii generates post depositional manganese enrichments that cannot readily be distinguished from the manganese rich sediment layers that accumulate during interglacials this compromises the use of manganese rich layers as proxies for interglacial periods in contrast manganese rich layers may be used as first order markers of interglacial periods in sediments where gas hydrates or other forms of reactive carbon are absent

a new modeling framework for sea ice mechanics based on elasto brittle rheology we present a new modeling framework for sea ice mechanics based on elasto brittle eb behavior the eb framework considers sea ice as a continuous elastic plate encountering progressive damage simulating the opening of cracks and leads as a result of long range elastic interactions the stress relaxation following a damage event can induce an avalanche of damage damage propagates in narrow linear features resulting in a very heterogeneous strain field idealized simulations of the arctic sea ice cover are analyzed in terms of ice strain rates and contrasted to observations and simulations performed with the classical viscous plastic vp rheology the statistical and scaling properties of ice strain rates are used as the evaluation metric we show that eb simulations give a good representation of the shear faulting mechanism that accommodates most sea ice deformation the distributions of strain rates and the scaling laws of ice deformation are well captured by the eb framework which is not the case for vp simulations these results suggest that the properties of ice deformation emerge from elasto brittle ice mechanical behavior and motivate the implementation of the eb framework in a global sea ice model

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

scaling properties of sea ice deformation from buoy dispersion analysis 1 a temporal and spatial scaling analysis of arctic sea ice deformation is performed over timescales from 3 h to 3 months and over spatial scales from 300 m to 300 km the deformation is derived from the dispersion of pairs of drifting buoys using the iabp international arctic buoy program buoy data sets this study characterizes the deformation of a very large solid plate the arctic sea ice cover stressed by heterogeneous forcing terms like winds and ocean currents it shows that the sea ice deformation rate depends on the scales of observation following specific space and time scaling laws these scaling properties share similarities with those observed for turbulent fluids especially for the ocean and the atmosphere however in our case the time scaling exponent depends on the spatial scale and the spatial exponent on the temporal scale which implies a time space coupling an analysis of the exponent values shows that arctic sea ice deformation is very heterogeneous and intermittent whatever the scales i e it cannot be considered as viscous like even at very large time and or spatial scales instead it suggests a deformation accommodated by a multiscale fracturing faulting processes

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

formation of sea ice bridges in narrow straits in response to wind and water stresses ice bridges are rigid structures composed of sea ice that form seasonally in the many straits and channels of the canadian arctic archipelago driven primarily by atmospheric stresses these ice bridges are formed when sufficiently thick ice jams during the course of its flow between land masses resulting in a region of stationary compacted ice that is separated from a region of flowing open water a polynya by a static arch using a continuum description of sea ice that is widely used in climate modeling we present an asymptotic theory of the process of formation of such bridges in slender channels when the motion of the ice is driven by external wind and water stresses we show that for an arbitrary channel shape ice bridges can only form within a range of ice properties that is determined by the channel geometry and the external stress we then compare the results of our theory with direct numerical simulations and observational evidence finally we provide simple analytical expressions for the mean velocity of the ice flow as a function of the channel shape the properties of the ice and the wind and water stresses along the channel

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

geochronology and geochemistry of the nanfen iron deposit in the anshan benxi area north china craton implications for 2 55 ga crustal growth and the genesis of high grade iron ores abstract the nanfen iron deposit situated in the anshan benxi area of the north china craton ncc hosts scattered banded iron formations bifs and bif hosted high grade iron orebodies defined by feot 80 wt the associated roof and bottom rocks which occur in parallel to orebodies are mica quartz schist and chlorite hornblende schist locally interlayers respectively the paas normalized rare earth element ree patterns indicate that quartz magnetite banded iron formations bifs are enriched in heavy rees with positive la eu and y anomalies indicating that the nanfen bif precipitated from a mixture of ambient seawater and submarine high t hydrothermal fluids 0 1 in addition the bif exhibits low concentrations of al2o3 and tio2 with depletion in hfses high field strength elements e g nb ta zr hf and ti suggesting little contamination with continental detritus excepting lower eu eu values the high grade iron ores display similar geochemical features to the bif likely implying that they have a same iron source but did not form synchronously electron microprobe analyses empa and in situ la icp ms analyses reveal that in relative to the magnetite grains in high grade ores the magnetite grains in the bif contain higher mno mgo and zn concentrations and lower al2o3 and ree abundances implying different formation conditions thus we argue that high grade ores are likely to be alteration products of primary bifs chlorite hornblende schist meta basalt samples exhibit low sio2 contents 43 78 50 30 wt with flat chondrite normalized ree and n morb normalized trace element patterns which are similar to those of babb back arc basin basalt the zircon u pb dating results indicate that magmatic and metamorphic zircons from chlorite hornblende schist formed at approximately 2552 ma and 2482 ma respectively the former volcanic age could constrain the deposition age of the nanfen bif and the latter reflects a subsequent metamorphic event which is also reflected by the 2480 ma hydrothermal zircons selected from the bif moreover most zircons from chlorite hornblende schist have positive hf t values 0 22 7 96 close to the depleted mantle evolutionary curve implying that the source magma were derived from a depleted mantle with some contamination from ancient crust hence we propose that the nanfen bif is a neoarchean algoma type bif and its deposition was most likely benefited due to crustal growth

genesis of banded iron formations a series of experimental simulations abstract a series of experimental simulations of the genetic mechanism of banded iron formations has been conducted using a suite of self designed glassware to model the hypergene circulation of surface water the simulation process involved two parts in the first part the weathering and leaching of basalt by strongly acidic meteoric water during early precambrian volcanism would have resulted in amorphous silica precipitation and strongly acidic ore bearing solutions ph 2 saturated meteoric water without contamination of other volcanic gases would increase the ph value of the solution when the ph increased to 1 25 5 50 ferric hydroxides and amorphous silica would alternately precipitate owing to periodic heating and continuous addition of high ph basalt leaching solutions this is a possible explanation of the genetic mechanism of the algoma type banded iron formations

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

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

a 36 kyr geochemical record from the sea of japan of organic matter flux variations and changes in intermediate water oxygen concentrations intervals of organic c and carbonate rich laminated sediments occur in the sea of japan with roughly the same frequency as temperature changes observed in greenland ice cores providing clear evidence of rapid oceanographic change during the past 36 kyr planktonic foraminiferal 18o data suggest that only the laminated sediments deposited during the last glacial maximum lgm and perhaps one other interval formed during a period of increased water column stratification sedimentary re and mo data are consistent with bottom waters that were sulfidic during the lgm and suboxic during other laminated intervals results of a numerical model of corg and re burial are consistent with a mechanism whereby an increased corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and or more saline surface waters or by stronger northeasterly monsoonal winds

four stages of pressure ridging the pressure ridging process is simulated using a two dimensional particle model blocks are broken from an intact sheet of relatively thin lead ice pushed against a thick multiyear floe at a constant speed the blocks of ice rubble accumulate to form the ridge sail and keel during the simulations the energy consumed in ridge growth including dissipation is explicitly calculated on the basis of the results of simulations performed with the model the ridging process can be divided into four distinct stages the first stage begins with an intact sheet of lead ice impacting a floe and ends when the sail reaches its maximum height in the second stage the ridge keel deepens and widens the stage ends when the maximum keel draft is reached in the third stage the direction of growth is leadward creating a rubble field of more or less uniform thickness the third stage ends when the supply of thin ice is exhausted in the fourth stage the rubble field is compressed between converging floes the results of simulations establish the dependence of ridging energetics in the first and second stages on the thickness of the ice sheet and the amount of ice pushed into the ridge the average profiles of the simulated ridges delineate the growth process in the first second and third stages the energetics and profiles of the fourth stage were described by hopkins et al 1991 lead ice extents of up to 1300 m are pushed into ridges to determine maximum sail heights keel drafts and ridging forces