sulfur geochemical constraints on mesoproterozoic restricted marine deposition lower belt supergroup northwestern united states abstract despite nearly a century of study by a diverse group of precambrian workers competing interpretations have persisted that favor both marine and nonmarine depositional settings for the mesoproterozoic belt supergroup of the northwestern united states our data for sedimentary pyrite formation in unmetamorphosed organic rich shales of the newland formation of the lower belt supergroup western montana argue that at least a portion of the belt basin during its early history was characterized by restricted marine conditions with only episodic incursions of seawater this model is supported by ratios of organic carbon corg to pyrite sulfur that once corrected for selective diagenetic and thermal loss of corg and the inferred high reactivity of the corg reservoir in the absence of vascular land plant remains suggest degrees of sulfate limitation consistent with a restricted marine setting the isotopically heavy disseminated pyrite that dominates the black shales shows a broad range of 34s values of bacterial origin 34s ranges from 8 7 to 36 3 mean 7 6 n 41 the pyrite data overlap with the 34s range of coeval barite sulfate within the basin and are similar to independent estimates for the sulfate of mesoproterozoic seawater these relationships have allowed us to delineate a record of bacteriogenic pyrite that formed in a modified marine reservoir where the supply of sulfate was limited a bacterial mechanism is further suggested by the morphological character of this early formed pyrite including the ubiquitous framboids after comparing our results with those from present day localities such as the black sea and sites of lacustrine deposition we conclude that the abundant 34s enriched bacteriogenic disseminated pyrite observed in black shales of the newland formation must record a style of sulfate limitation possible only within an isolated marine basin receiving an intermittent flux of seawater although the entire belt basin likely remained a restricted marine setting throughout deposition of the lower belt supergroup sulfur isotope data from the newland formation at two localities separated by more than 150 km in the eastern belt basin reveal stratigraphic trends in the strength of the marine connection specifically the connection evolved such that inputs of seawater sulfate increased progressively in frequency and possibly strength during deposition of the newland formation overall the hypothesized restricted marine setting when viewed in light of independent tectonic sedimentologic and geochronologic evidence suggests syndepositional and likely episodic continental rifting which helps constrain the proterozoic paleotectonic history of western laurentia

understanding of elastic anisotropy of shale under triaxial loading porosity deformation approach abstracttheoretical models for the description of stress dependencies for anisotropic rocks are important for building velocity models and the interpretation of seismic data with respect to stress lithology and pore content especially important is the accurate description of the change of anisotropy due to applied anisotropic stress by means of a quantitative model to aid the interpretation the porosity deformation approach pda originally introduced as the piezosensitivity theory describes stress induced changes of elastic properties without any knowledge about the geometry and orientation of the microcracks we have developed a formulation of the theory for a transverse isotropic rock which was loaded parallel to the bedding plane and we found that it can be described theoretically as an orthorhombic rock we also found out how properties that described the velocity change due to hydrostatic loading can be used to predict elastic anisotropy under additional axial loading the number of free para

shallow stratification prevailed for 1700 to 1300 ma ocean evidence from organic carbon isotopes in the north china craton abstract the late paleoproterozoic to early mesoproterozoic from 1700 ma to 1300 ma was highlighted by the assembly of the nuna supercontinent expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes the isotopic composition of carbonate carbon 13ccarb was surprisingly constant during this interval but little is known about the secular variation in the organic carbon isotopic composition 13corg here we report 13corg data from the latest paleoproterozoic 1650 ma to early mesoproterozoic 1300 ma succession in north china the 13corg values range from 25 to 34 and are dependent on sedimentary facies in subtidal and deeper environments 13corg values are low and constant ca 32 but relatively enriched and more variable in shallower intertidal and supratidal environments we attribute the facies dependent variation in 13corg to the presence of a shallow chemocline a probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and or anaerobic photoautotrophic microbes in relatively deep environments from the latest paleoproterozoic to early mesoproterozoic continental shelf of north china

shallow water anoxia in the mesoproterozoic ocean evidence from the bashkir meganticlinorium southern urals abstract the apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the proterozoic eon 2 5 0 542 ga it is generally accepted that from 1 85 ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous fe ii rich with episodic euxinia confined to productive continental margins and intracontinental basins the geochemical record that informs this picture however is currently sparse and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time here we report fe s c systematics alongside major al mn and trace metal mo u data for 1 6 0 8 ga marine sediments from the bashkir meganticlinorium bma southern urals russia our fe speciation data reveal a water column dominated by ferruginous conditions no evidence for euxinia and oxygenated waters confined to only the shallowest near coastal settings trace metal data support these findings with mo and u enrichment factors in anoxic sediments implying the operation of a particulate fe oxyhydr oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic mo enrichments sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the bma succession which in combination with a low organic carbon loading promoted the development of ferruginous rather than euxinic water column conditions the restriction of oxic conditions to the shallowest near coastal depositional settings within the bashkir basin contrasts to other redox studies from this period such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the mesoproterozoic era and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period

molybdenum drawdown during cretaceous oceanic anoxic event 2 during the cretaceous greenhouse episodes of widespread ocean deoxygenation were associated with globally occurring events of black shale deposition possibly the most pronounced of these oceanic anoxic events oae s was the cenomanian turonian oae2 94 ma however although certain redox sensitive trace metals tend to be preferentially sequestered in sediments deposited under anoxic conditions with mo drawdown being specifically prone to euxinic settings these elements are generally somewhat depleted in sediments deposited during oae2 to understand the driving factors responsible for this depleted trace metal drawdown we have studied a low latitude section from the proto north atlantic ocean tarfaya s57 where existing biomarker and iron sulphur data point to a dominantly euxinic water column with periodic transitions to ferruginous fe rich water column conditions we utilise a variety of redox proxies fe speciation redox sensitive trace metals and mo isotopes which in combination allows us to evaluate the detailed nature of ocean redox conditions and hence controls on trace metal drawdown the results suggest that seawater 98mo values may have ranged between 0 6 and 1 1 during oae2 likely connected to changes in the local mo reservoir as a consequence of low and probably heterogeneous concentrations of mo in the ocean the very low mo toc ratios at tarfaya and elsewhere in the proto north atlantic may support a model in which deep water circulation was partially restricted within and between the north atlantic and other ocean basins we propose that the combination of a low and possibly heterogeneous 98mo of seawater mo together with low mo toc ratios points to a large decrease in the global oceanic mo reservoir during oae2 reflecting a major global scale increase in mo drawdown under persistent euxinic conditions 2016 elsevier b v

an indicator of water column anoxia resolution of biofacies variations in the kimmeridge clay upper jurassic u k abstract an indicator of anoxicity ia defined as the ratio pyrite fe oxide fe total fe can be used to recognize sediments deposited under an anoxic water column modern marine sediments deposited under oxygenated bottom waters mainly have ia values 0 5 ia values in the central kimmeridge clay upper jurassic uk for fissile organic rich shales indicate that deposition occurred principally from anoxic bottom waters with infrequent oxygenation events

assessing the utility of fe al and fe speciation to record water column redox conditions in carbonate rich sediments abstract geochemical proxies based on fe abundance fe al and fe speciation have been widely applied to marine sediments in order to unravel paleo depositional redox conditions though geological time to date however these fe proxies have only been calibrated in relation to modern and ancient fine grained siliciclastic marine sediments this clearly limits their use particularly in relation to carbonate rich sediments and rocks to address this we here explore the applicability of fe based redox proxies in carbonates through three approaches first we have compiled fe al data for modern marine sediments to investigate variability in fe enrichments as a function of carbonate content and depositional setting second we have expanded this approach with a compilation of new and existing fe speciation data for modern and ancient marine sediments deposited under oxic and euxinic anoxic sulfidic water column conditions finally we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the fe speciation redox proxy modern marine sediments deposited under oxic conditions show no relationship between fe al and carbonate content these sediments have an average fe al ratio of 0 55 0 11 with some higher values potentially being attributable to steady state early diagenetic remobilization of fe towards the sediment water interface in contrast significant fe al enrichments occur as a consequence of water column fe mineral formation and deposition either under anoxic conditions or due to input of anoxic hydrothermal fluids into oxic seawater iron speciation data also show no direct correlation with carbonate content and instead three groups can be distinguished based on total fe fe t and organic c contents sediments deposited under oxic water column conditions with fe t 0 5 wt generally plot below the lower fe hr fe t siliciclastic reference threshold 0 38 for distinguishing anoxic environments regardless of carbonate content also consistent with siliciclastic calibrations carbonate rich sediments deposited under anoxic water column conditions tend to have fe hr fe t ratios 0 38 in contrast oxic carbonate rich sediments with low fe t hr fe t ratios suggesting that the use of fe speciation for such samples is not appropriate for evaluating water column redox conditions taken together our data suggest that an fe t content of 0 5 wt rather than carbonate content represents the most suitable basis for identifying whether carbonate rich sediments are appropriate for fe based redox proxy analyses analysis of burial dolostones suggests that the fe speciation proxy may also be compromised by deep burial dolomitization where there has been a source of mobile fe to enrich rocks during recrystallization however pre screening can identify such samples this new assessment expands the utility of fe based redox proxies to also incorporate appropriate carbonate rich rocks provided that care is taken to assess the possible impact of deep burial dolomitization

oceanic molybdenum drawdown by epeiric sea expansion in the mesoproterozoic abstract molybdenum is a bioessential micronutrient whose abundance in the global oceans may have played a primary role in evolution of the earth s nitrogen cycle and ultimately in the timing of the ecologic expansion of early eukaryotes because molybdenum mo is delivered to the ocean under conditions of oxic weathering and is removed in suboxic and euxinic environments the concentration of mo in the oceans reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation recent compilations of mo within euxinic shales scott et al 2008 och and shields zhou 2012 sahoo et al 2012 indicate that the oceanic reservoir of mo did not rise substantially above crustal values until the great oxidation event 2 3 ga and a modern mo cycle did not develop until the contraction of ocean euxinia in the latest proterozoic at present a paucity of data from euxinic shales of the late mesoproterozoic 1 3 to 1 0 ga limits our ability to relate marine redox evolution to biological innovation during a critical interval of eukaryotic development here we present data from marine shales of the 1 1 ga atar and el mreiti groups mauritania which were deposited during sea level highstand when shallow epeiric seas covered much of the west african craton epicratonic strata of the el mreiti group were deposited under fluctuating redox conditions near a shallow chemocline as evidenced by iron speciation and record generally low mo concentrations

a shale hosted cr isotope record of low atmospheric oxygen during the proterozoic the emergence and expansion of animal life on earth represents a dramatic shift in the structure and complexity of the biosphere a lack of firm constraints on surface oxygen levels during the mid proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or instead simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger here we present chromium cr isotope data from marine black shales that provide evidence for minimal cr oxidation throughout the mid proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late neoproterozoic this observation requires very low background oxygen levels 1 of present atmospheric levels accepting previously proposed estimates of p o2 levels needed to induce cr isotope fractionation our data provide support for the persistence of an earth system in which baseline atmospheric p o2 would have been low enough to inhibit the diversification of animals until ca 800 ma more generally evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on earth

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

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

structural framework of the bashkirian anticlinorium sw urals the bashkirian anticlinorium of the southwestern urals shows a much more complex structural architecture and tectonic evolution than previously known pre uralian proterozoic extensional and compressional structures controlled significantly the uralian tectonic convergence a long lasting proterozoic rift process created extensional basement structures and a riphean basin topography which influenced the formation of the western fold and thrust belt with inversion structures during the uralian deformation a complete orogenic cycle during cadomian times including terrane accretion at the eastern margin of the east european platform resulted in a high level cadomian basement complex which controlled the onset of uralian deformation and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the main uralian fault the uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes tectonic convergence started in the late devonian with ophiolite obduction tectonic accretion of basin and slope units and early flysch deposits zilair flysch the accretionary complex prograded from the se to the nw continuous nw se directed convergence resulted finally in the formation of an early orogenic wedge thrusting the cadomian basement complex onto the east european platform the main tectonic shortening was connected with these two stages and although not well constrained appears to be of late devonian to carboniferous age in the permian a final stage of e w compression is observed throughout the sw urals in the west the fold and thrust belt prograded to the west with reactivation of former extensional structures and minor shortening in the east this phase was related to intense back thrusting the east european platform was subducted beneath the magnitogorsk magmatic arc during the late paleozoic collision the thick and cold east european platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening tectonic stacking and high strain at its eastern margin whereas the first orogenic wedge is of thick skinned type with the involvement of crystalline basement even the later west directed wedge is not typically thin skinned as the depth of the basal detachment appears below 15 km and the involvement of archean basement can be assumed

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

sources and mechanisms for the enrichment of highly reactive iron in euxinic black sea sediments the deep basinal euxinic sediments of the black sea are enriched in iron that is highly susceptible to sulfidization compared to oxic suboxic continental margin sediments in the black sea and oxic dysoxic continental margin and deep sea sediments worldwide a mass balance treatment of iron speciation data from three deep basin sediment cores shows that this enrichment is due to a combination of 1 highly reactive iron bearing phases sulfides and oxides whose ultimate source was by diagenetic mobilization from the shelf wijsman and others 2001 and 2 enhanced iron reactivity in the lithogenous component of deep basinal sediments the cause of the enhanced reactivity of lithogenous iron is problematical possible mechanisms include microbial oxidation of non reactive iron silicates and the preferential deposi tion of a fine grained reactive iron enriched lithogenous component in the deep basin by the fractionation of the lithogenous flux during transport across the shelf the application of paleoredox indicators based on iron reactivity degree of pyritization indicator of anoxicity should take into account that the availability of highly reactive iron and hence the concentration of reactive iron phases in the sediment is controlled by a variety of chemical physical and biological factors some of which are not related directly to redox conditions in the water column

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

compressional wave dispersion due to rock matrix stiffening by clay squirt flow the standard biot gassmann theory of poroelasticity fails to explain strong compressional wave velocity dispersion experimentally observed in 12 tight siltstone with clay filled pores in order to analyze and understand the results we developed a new double porosity model of clay squirt flow where wave induced local fluid flow occurs between the micropores in clay aggregates and intergranular macropores the model is validated based on the combined study of ultrasonic experiments on specimens at different saturation conditions and theoretical predictions the presence of a sub pore scale structure of clay micropores contained in intergranular macropores where the fluid does not have enough time to achieve mechanical equilibrium at ultrasonic frequencies and thus stiffens the rock matrix provides a suitable explanation of the experimental data moreover the model provides a new bound for estimating the compressional wave velocity of tight rocks saturated with two immiscible liquids the theoretical predictions indicate that the velocity variation between gas and liquid saturated specimens is predominantly induced by the clay squirt stiffening effect on the rock matrix and not by fluid substitution the effect contributes more than 90 to the variation in the porosity range of 0 5 thus clay squirt flow dominates the relationships between compressional wave velocity and pore fluid in tight rocks

poro acoustoelasticity of fluid saturated rocks we generalize the classical theory of acoustoelasticity to the porous case one fluid and a solid frame and finite deformations a unified treatment of non linear acous toelasticity of finite strains in fluid saturated porous rocks is developed on the basis of biot s theory a strain energy function formed with eleven terms combined with biot s kinetic and dissipation energies yields lagrange s equations and consequently the wave equation of the medium the velocities and dissipation factors of the p and s waves are obtained as a function of the 2nd and 3rd order elastic constants for hydrostatic and uniaxial loading the theory yields the limit to the classical theory if the fluid is replaced with a solid with the same properties of the frame we con sider sandstone and obtain results for open pore jacketed and closed pore jacketed gedanken experiments finally we compare the theoretical results with experimental data

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

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

extremely low oxygen concentration in mid proterozoic shallow seawaters abstract the mid proterozoic 1 8 0 8 ga witnessed the first appearance but unusually low diversification of eukaryotes the stagnant biotic evolution during this billion year long period commonly referred to as the boring billion was arguably ascribed to low oxygen levels in atmosphere and ocean however evidence supporting low oxygen in shallow marine environments where early eukaryotes first evolved is generally lacking or insufficient here we report rare earth element and yttrium ree y data particularly cerium ce anomalies from a suite of mid proterozoic sedimentary rocks of the north china platform the new data from north china in combination with available ce anomaly data from other proterozoic successions demonstrate that during mid proterozoic negative ce anomalies did not occur until 1 54 ga and after 1 54 ga only episodic negative ce anomalies were present in shallow water carbonates trace element enrichments u ef v ef and mo ef remained at the average continental crust level before 1 54 ga but showed a sudden increase at 1 54 ga the data suggest that oxygen concentration in shallow marine environments of the mid proterozoic ocean was extremely low probably

the main tectonic events depositional history and the palaeogeography of the southern urals during the riphean early palaeozoic abstract investigation of former and new sections in the classic riphean region of the bashkirian anticlinorium ba revealed that this palaeozoic segment of the passive margin of the east european platform eep during its proterozoic history was initially formed as an intracratonic aulacogen basin which developed as an inter cratonic half structure only after the early ordovician opening of the uralian palaeo ocean the originally opposite half structure of this riphean basin may have been located either on the afro european part of gondwana or along the margins of the siberian craton or elsewhere but definitely not along the conjugate permian collision suture with the kazakh block or the mugodzhary microcontinent the ca 1150 ma 1635 to 488 ma depositional history of the bashkirian basin was punctuated several times due to major tectonic events folding downfaulting uplift magmatic intrusions metamorphism which may have caused extended periods of erosion and non deposition furthermore numerous cyclothems of unknown regional extent are incorporated into the enormous time span involved in the evolution of the ca 12 15 km thick riphean sequence the riphean vendian sedimentary assemblages reflect alternating shallow marine to peritidal and continental fluviatile environments the regional distribution of which is illustrated in sets of palaeogeographic maps angular unconformities are seen at the base of the lower riphean ai formation dated at 1635 ma at the base of the middle riphean mashak formation dated at 1340 ma at the base of the upper riphean zilmerdak formation approx 1000 ma and below the middle ordovician quartzites containing shelly fossils which overlie steeply folded and slightly metamorphosed proterozoic suites along the southeastern margin of the bashkirian anticlinorium during the vendian to ordovician interval major eastern parts of the bashkirian anticlinorium as well as regions further north along the european margin of the urals were subjected to strong diastrophism probably related to the cadomian or baikalian orogeny later on during the ordovician the western half part of the bashkirian basin turned into a marginal basin of the east european platform and thus was modified by the early stages of the uralian oceanic development

coastal river plumes collisions and coalescence abstract plumes of buoyant river water spread in the ocean from river mouths and these plumes influence water quality sediment dispersal primary productivity and circulation along the world s coasts most investigations of river plumes have focused on large rivers in a coastal region for which the physical spreading of the plume is assumed to be independent from the influence of other buoyant plumes here we provide new understanding of the spreading patterns of multiple plumes interacting along simplified coastal settings by investigating i the relative likelihood of plume to plume interactions at different settings using geophysical scaling ii the diversity of plume frontal collision types and the effects of these collisions on spreading patterns of plume waters using a two dimensional hydrodynamic model and iii the fundamental differences in plume spreading patterns between coasts with single and multiple rivers using a three dimensional hydrodynamic model geophysical scaling suggests that coastal margins with numerous small rivers watershed areas 2 such as found along most active geologic coastal margins were much more likely to have river plumes that collide and interact than coastal settings with large rivers watershed areas 100 000 km 2 when two plume fronts meet several types of collision attributes were found including refection subduction and occlusion we found that the relative differences in pre collision plume densities and thicknesses strongly influenced the resulting collision types the three dimensional spreading of buoyant plumes was found to be influenced by the presence of additional rivers for all modeled scenarios including those with and without coriolis and wind combined these results suggest that plume to plume interactions are common phenomena for coastal regions offshore of the world s smaller rivers and for coastal settings with multiple river mouths in close proximity and that the spreading and fate of river waters in these settings will be strongly influenced by these interactions we conclude that new investigations are needed to characterize how plumes interact offshore of river mouths to better understand the transport and fate of terrestrial sources of pollution nutrients and other materials in the ocean

a global transition to ferruginous conditions in the early neoproterozoic oceans eukaryotic life expanded during the proterozoic eon1 2 5 to 0 542 billion years ago against a background of fluctuating ocean chemistry2 3 4 after about 1 8 billion years ago the global ocean is thought to have been characterized by oxygenated surface waters with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron containing ferruginous deeper waters5 6 7 the spatial extent of sulphidic waters probably varied through time5 6 but this surface to deep redox structure is suggested to have persisted until the first neoproterozoic glaciation about 717 million years ago8 9 10 11 here we report an analysis of ocean redox conditions throughout the proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops we find a global transition from sulphidic to ferruginous mid depth waters in the earliest neoproterozoic coincident with the amalgamation of the supercontinent rodinia at low latitudes we suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on rodinia we propose that this transition essentially detoxified ocean margin settings allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

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

the low temperature geochemical cycle of iron from continental fluxes to marine sediment deposition suspended sediments from 34 major rivers geographically wide spread and 36 glacial meltwater streams have been examined for their variations in different operationally defined iron fractions fehr iron oxides soluble in dithionite fepr iron soluble in boiling hcl but not in dithionite and feu total iron less that soluble in boiling hcl river particulates show a close association between fehr and total iron fet reflecting the effects of chemical weathering which derive oxide iron from and retain it in close association with total iron consistent with this continental scale average fehr fet ratios vary with runoff ratios average river runoff per unit area average precipitation per unit area by contrast the diminished effects of chemical weathering produce no recognizable association of fehr with fet in glacial particulates and instead both fepr and feu are closely correlated with fet reflecting essentially pristine mineralogy a comparison of the globally averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in fehr fepr and fet but enriched in feu the river and glacial particulate data are combined with estimates of authigenic hydrothermal atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for fehr f e pr f e u and fet this budget suggests that the differences between riverine particulates and marine sediments can be explained by i preferentially removing fehr from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains salt marshes and estuaries and ii mixing the resulting riverine particulates with fehr depleted glacial particulates preliminary measurements of inner shore sediments are consistent with i above phanerozoic and modern normal marine sediments have similar iron speciation characteristics which implies the existence of a long term steady state for the iron cycle this steady state could be maintained by a glacioeustatic feedback where fehr enriched riverine particulates are either more effectively trapped when sealevel is high small ice masses diminished glacial erosion or are mixed with greater masses of fehr depleted glacial particulates when sealevel is low large ice masses enhanced glacial erosion further important controls on the steady state for fehr operate through the formation of euxinic sediments and ironstones which also provide sealevel dependent sinks for fehr enriched sediment

the metamorphic complex of beloretzk sw urals russia â a terrane with a polyphase meso to neoproterozoic thermo dynamic evolution abstract an integrated geological study of the tectono metamorphic evolution of the metamorphic complex of beloretzk mcb which is part of the eastern bashkirian mega anticlinorium bma sw urals russia shows that the main lithological units are neoproterozoic riphean and vendian age siliciclastic to carbonate successions granitic syenitic and mafic intrusions together with subaerial equivalents comprise the neo and mesoproterozoic magmatic rocks the metamorphic grade ranges from diagenetic and very low grade in the western bma to high grade in the mcb the n s trending zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern bma structural data pb pb single zircon ages 40 ar 39 ar cooling ages and the provenance signature of riphean and vendian siliciclastic rocks in the western bma give evidence of mesoproterozoic grenvillian rifting deformation and eclogite facies metamorphism in the mcb and a neoproterozoic cadomian orogenic event in the sw urals three pre ordovician deformation phases can be identified in the mcb the first sse vergent isoclinal folding phase d 1 is younger than the intrusion of mafic dykes pb pb single zircon 1350 ma and older than the eclogite facies metamorphism high p low t eclogite facies metamorphism is bracketed by d 1 and the intrusion of the achmerovo granite pb pb single zircon 970 ma an extensional sinistral top down to nw directed shearing d 2 is correlated with the first exhumation of the mcb e vergent folding and thrusting d 3 occurred at retrograde greenschist facies metamorphic conditions the tremolite 40 ar 39 ar cooling age 718 5 ma of amphibolitic eclogite and muscovite 40 ar 39 ar cooling ages about 550 ma of mica schists indicate that a maximum temperature of 500 50 c was not reached during the neoproterozoic orogeny the style and timing of the neoproterozoic orogeny show similarities to the cadomian aged timan range nw of the polar urals geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the zuratkul fault suggest that the mcb is exotic with respect to the se margin of the east european platform thus the mcb is named the beloretzk terrane recognition of the beloretzk terrane and the neoproterozoic orogeny at the eastern margin of baltica has important implications for neoproterozoic plate reconstruction and suggests that the eastern margin of baltica might have lain close to the avalonian cadomian belt

marine redox conditions in the middle proterozoic ocean and isotopic constraints on authigenic carbonate formation insights from the chuanlinggou formation yanshan basin north china abstract to improve our understanding of ocean chemistry and biogeochemical cycling following the termination of large scale paleoproterozoic iron formation if deposition 1 85 billion years ago ga we conducted a fe s c mo geochemical study of the 1 65 ga chuanlinggou formation yanshan basin north china despite the cessation of if deposition our results suggest the presence of anoxic but non euxinic ferruginous conditions persisted below the surface mixed layer for the deepest portion of the continental rifting basin and that this pattern is apparently independent of the local organic carbon content however our paired s isotope data of carbonate associated sulfate and pyrite suggest presence of sulfate in pore fluids which is not consistent with insufficient sulfate for bacterial sulfate reduction in the water column despite evidence for deposition under anoxic conditions sedimentary molybdenum mo concentrations are mostly not enriched relative to average continental crust this relationship is consistent with the notion that sulfide dominated conditions in the water column and or the sediments are required for mo enrichment and validates past assertions that mo enrichment patterns in ancient shales track both the local presence and global distribution of euxinia specifically in addition we identified extensive diagenetic carbonate precipitation in the upper chuanlinggou formation with only moderately negative 13 c values 3 4 1 4 we propose with support from a numerical model that these diagenetic carbon isotope values were most likely derived from precipitation of carbonates dominantly in the methanic zone within the sediments diagenetic carbonate precipitation in the methanic zone is likely to have been more extensive in the proterozoic than the phanerozoic due to porewater oxidant limitation

chemostratigraphy of the shaler supergroup victoria island nw canada a record of ocean composition prior to the cryogenian glaciations a b s t r a c t a new 13 ccarb curve combined with 13 corg values is presented for the upper shaler supergroup 900 to 720 ma amundsen basin northwestern canada the dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust there is a pronounced negative 13 c excursion in the wynniatt formation that can be correlated with a putative worldwide negative car bon isotope excursion namely the bitter springs stage however in the amundsen basin the 13 ccarb excursion drops to anomalously negative values 14 which we attribute to local overprints wherein isotopically light carbon in pore waters released by oxidation of methane and organic matter during sulphate and iron reduction was incorporated into authigenic carbonate cement we document basin euxinia and anoxia during the same time interval using a multi proxy approach specifically fe speciation and redox sensitive trace metal data patterns of pronounced enrichment in mo v and u concentrations in euxinic black shales suggest that the bitter springs stage was a transitional period in earth s redox evo lution from the more reduced global oceans during the mid proterozoic to the more oxygenated oceans during the phanerozoic 2015 elsevier b v all rights reserved

neoproterozoic metamorphism and deformation at the southeastern margin of the east european craton uralides russia the eastern margin of the east european craton eec has a long lasting geological record of precambrian age archaean and proterozoic strata are exposed in the western fold and thrust belt of the uralides and are known from drill cores and geophysical data below the palaeozoic cover in the uralides and its western foredeep in the southern uralides sedimentary metamorphic and magmatic rocks of riphean and vendian age occur in the bashkirian mega anticlinorium bma and the beloretzk terrane in the eastern part of the bma yamantau anticlinorium and the beloretzk terrane k ar ages of the 2 m size fraction of phyllites potassic white mica and slates illite give evidence for a complex pre uralian metamorphic and deformational history of the precambrian basement at the southeastern margin of the eec interpretation of the k ar ages considered the variation of secondary foliation and the diagenetic to metamorphic grade in the yamantau anticlinorium the greenschist facies metamorphism of the mesoproterozoic siliciclastic rocks is of early neoproterozoic origin about 970 ma and the s1 cleavage formation of late neoproterozoic about 550 ma the second wide spaced cleavage is of uralian origin in the central and western part of the bma the diagenetic to incipient metamorphic grade developed in late neoproterozoic time in post uralian time proterozoic siliciclastic rocks with a cleavage of uralian age have not been exhumed to the surface of the bma late neoproterozoic thrusts and faults within the eastern margin of the eec are reactivated during the uralian deformation

turbidite depositional influences on the diagenesis of beecher s trilobite bed and the hunsruck slate sites of soft tissue pyritization chemical signatures of enrichment of highly reactive iron and framboid size distributions are reported in turbidite sediments that host soft tissue pyritization beecher9s trilobite bed upper ordovician and the hunsruck slate lower devonian these signatures demonstrate that the sediment of beecher9s trilobite bed was enriched in highly reactive iron prior to turbidite transport but that no enrichment was present in the hunsruck slate turbidite transport and re sedimentation altered framboid size distributions small diameter framboids

modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans the increase in atmospheric oxygen ca 2 4 ga had a significant impact on the geochemical cycling of fe the history of environmental oxygenation may be recorded in the fe isotope composition of archean and proterozoic sediments but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations here we present fe isotope data and iron aluminum ratios from the black sea oxic shelf and euxinic basin the isotope data demonstrate that shelf fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin we propose that there is net transport of isotopically light fe from sediments of the shelf to those of the distal anoxic basin consistent with enrichments in reactive fe seen in the deep basin the low 56 fe benthic fe flux is generated during the coupling of microbial fe iii reduction or sulfidization with fe 2 aq oxidation low 56 fe values reported previously from late archean sedimentary pyrites may be an isotopic fingerprint of analogous fe redox cycling in the late archean oceans this interpretation implies shallow water fe redox recycling in the late archean we predict that the light isotopic compositions of the late archean will prove to be distinct from those of the early archean before fe redox cycling became an important process and we infer that this difference may be related to the presence of oxygen in the surface ocean

nonlinear acoustoelastic constants of dry and saturated rocks 1 conventional descriptions of the stress dependence of acoustic velocities in rock are often based on microcrack models a more general model independent description that ignores microstructure is third order elasticity theory this theory has had much success in metals and crystals and has recently been applied to rocks as well we measured third order elastic constants in several rocks both dry and water saturated a five velocity measurement technique was used where a rock sample is stressed either hydrostatically or uniaxially and compressional and shear velocities are measured as functions of stress in dry rocks we find that third order elasticity theory provides a good description of velocity versus stress however in water saturated rocks agreement between theoretical predictions and measurements is significantly worse than in dry rocks this indicates that third order elasticity theory does not fully describe the stress dependence of velocities in water saturated rock under our experimental conditions detailed analysis of the velocity data suggests that rock dilatancy may be responsible for the discrepancies between theory and experiment

thick sulfate evaporite accumulations marking a mid neoproterozoic oxygenation event ten stone formation northwest territories canada thick sulfate evaporite accumulations are absent from proterozoic strata between ca 2000 and ca 1000 ma and detailed sedimentologic stratigraphic and geochemical data for the oldest neoproterozoic thick marine sulfate evaporite successions are largely lacking the middle neoproterozoic ten stone formation little dal group northwest territories canada consists of 500 m of pelagic lagoonal gypsite and anhydritite rocks consisting of the minerals gypsum and anhydrite deposited shortly before the ca 811 ma bitter springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of rodinia the thickness of regional stratigraphic subdivisions of this formation defined by subtle silt and carbonate bearing intervals indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest deposition of the ten stone formation began with abrupt tectonically triggered subsidence and restriction and ended equally abruptly as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply with no evidence for hiatus or erosion stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom hugging dense lagoonal brine from overlying waters deposition of the ten stone formation in a basin that experienced intermittent basin scale tectonic adjustments as recorded by details of its stratigraphy supports the interpretation that the mackenzie mountains supergroup accumulated in an extensional tectonically active intracratonic basin whose structure resembled a lower plate extensional system the absence of halite from the ten stone formation contrasts with its abundance in the stratigraphically lower gypsum free dodo creek formation suggesting that deposition of the lower to middle little dal group spanned a major oxygenation event during which the sequence of evaporite mineral precipitation from seawater changed from halite first to sulfate first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir the limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid neoproterozoic atmosphere an abundance of sulfate in seawater and marine oxygenation extending below storm wave base the mineralogy sedimentology stratigraphy and geochemistry of the ten stone formation are virtually indistinguishable from those of thick phanerozoic deep water below wave base evaporite successions and indicate that the tectonic climatic and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca 811 ma thick sulfate evaporite successions in equivalent stratigraphic positions just below the bitter springs carbon isotope excursion elsewhere in laurentia as well as on the congo craton and in south australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to rodinia s breakup high relative burial rates of organic matter prevailed before the breakup of rodinia and led to oxygenation of the atmosphere ocean system growth of the seawater sulfate reservoir and in association with a warm and arid climate deposition for the first time in earth s history of thick sulfate evaporites in the middle neoproterozoic 100 m y before the first cryogenian glacial episode the neoproterozoic oxygenation event may have taken place in several steps the first of which preceded the bitter springs anomaly

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

rates of reaction between silicate iron and dissolved sulfide in peru margin sediments porewaters in site 680 peru margin sediments contain dissolved sulfide over a depth of approximately 70 m which at a sedimentation rate of 0 005 cm yr 1 gives a sediment exposure time to dissolved sulfide of about 1 4 106 yr reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly reactive silicate iron fraction defined as the difference between iron extracted by dithionite fed at room temperature and that extracted by boiling concentrated hcl feh normalised to the total iron content fet straight line plots are obtained for 1n feh fed fet against time of burial from which a first order rate constant of 0 29 10 6 yr 1 equivalent to a half life of 2 4 106 yr can be derived for the sulfidation of this silicate iron comparable half lives are also found for the same poorly reactive iron fraction in the nearby site 681 and 684 sediments this silicate fe fraction comprises 0 8 1 0 fe only 30 60 of which reacts even with 1 5 3 million years exposure to dissolved sulfide diagenetic models based on porewater concentrations of sulfate and sulfide and solid phase iron contents at site 680 are consistent in indicating that this poorly reactive iron fraction is only sulfidized on a million year time scale silicate iron not extracted by hcl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments

comprehensive observation and modeling of earthquake and temperature related seismic velocity changes in northern chile with passive image interferometry we report on earthquake and temperature related velocity changes in high frequency autocorrelations of ambient noise data from seismic stations of the integrated plate boundary observatory chile project in northern chile daily autocorrelation functions are analyzed over a period of 5 years with passive image interferometry a short term velocity drop recovering after several days to weeks is observed for the mw 7 7 tocopilla earthquake at most stations at the two stations pb05 and patcx we observe a long term velocity decrease recovering over the course of around 2 years while station pb05 is located in the rupture area of the tocopilla earthquake this is not the case for station patcx station patcx is situated in an area influenced by salt sediment in the vicinity of salar grande and presents a superior sensitivity to ground acceleration and periodic surface induced changes due to this high sensitivity we observe a velocity response of several regional earthquakes at patcx and we can show for the first time a linear relationship between the amplitude of velocity drops and peak ground acceleration for data from a single station this relationship does not hold true when comparing different stations due to the different sensitivity of the station environments furthermore we observe periodic annual velocity changes at patcx analyzing data at a temporal resolution below 1 day we are able to identify changes with a period of 24 h too the characteristics of the seismic velocity with annual and daily periods indicate an atmospheric origin of the velocity changes that we confirm with a model based on thermally induced stress this comprehensive model explains the lag time dependence of the temperature related seismic velocity changes involving the distribution of temperature fluctuations the relationship between temperature stress and velocity change plus autocorrelation sensitivity kernels

heterogeneous redox conditions and a shallow chemocline in the mesoproterozoic ocean evidence from carbon sulfur iron relationships abstract oxygenation of earth s oceans in the proterozoic was a protracted process that began prior to the great oxidation event goe and culminated in deep ocean ventilation following neoproterozoic glacial episodes with both of these major thresholds accompanied by profound changes in the evolution of earth s biosphere our understanding of the redox structure of mesoproterozoic oceans however is considerably less certain limited proxy investigations of oceanic redox in the mesoproterozoic have focused largely on early mesoproterozoic shelf to basinal settings with comparatively little attention paid to epeiric sea environments that dominate the late mesoproterozoic sedimentary record here we report multi proxy shale redox data from epicratonic and pericratonic environments of the 1 1 ga atar and el mreiti groups taoudeni basin mauritania these strata were deposited across the west african craton during global sea level highstand and provide insight into the redox structure of mesoproterozoic epeiric seas iron speciation pyrite sulfur isotope and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments we suggest that a protracted increase in biospheric oxygenation in the late mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow nearshore areas contributing to the fragmentary pattern of early eukaryotic evolution and diversification

re os age constraints and new observations of proterozoic glacial deposits in the vazante group brazil abstract a new re os radiometric age date for an organic rich shale horizon from the vazante group in brazil coupled with geological observations provide evidence for late mesoproterozoic glacial episodes conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time field observations of a reverse fault juxtaposing older mesoproterozoic sedimentary rocks above younger neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of u pb detrital zircon ages through the succession combined the geochronological geochemical and stratigraphic evidence suggest that the vazante group sediments accumulated along a passive margin of the sao francisco craton and are correlative with the neighboring paranoa group biomarker sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post glacial transgression and deposition of one of the bituminous shale horizons the relationship of the glaciogenic vazante group to other late mesoproterozoic successions such as the non glacial atar group in west africa and bylot supergroup in arctic canada however remains enigmatic

anoxia in the terrestrial environment during the late mesoproterozoic a significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the precambrian in contrast the oxygenation history of terrestrial aquatic environments has received little attention despite the significance of such settings for early eukaryote evolution to address this we provide here a geochemical and isotopic assessment of sediments from the late mesoproterozoic nonesuch formation of central north america we utilize rhenium osmium re os geochronology to yield a depositional age of 1078 24 ma while os isotope compositions support existing evidence for a lacustrine setting fe s c systematics suggest that the nonesuch formation was deposited from an anoxic fe rich ferruginous water column thus similar to the marine realm anoxia persisted in terrestrial aquatic environments in the middle to late proterozoic but sulfidic water column conditions were not ubiquitous our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution