sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

neoproterozoic iron formation an evaluation of its temporal environmental and tectonic significance article neoproterozoic iron formation nif provides evidence for the widespread return of anoxic and ferruginous ba sins during a time period associated with major changes in climate tectonics and biogeochemistry of the oceans here we summarize the stratigraphic contextof neoproterozoic iron formation and its geographic and temporal dis tribution it is evident that most nif is associated with the earlier cryogenian sturtian glacial epoch although it is possible that some nif may be ediacaran there is no incontrovertible evidence to support this age assignment the paleogeographic distribution of nif is consistent with anoxic and ferruginous conditions occurring in basins within rodinia or in rift basins developed on its margins consequently nif does not require whole ocean anoxia simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and or detrital sourcesofiron are capable ofsupplying sufficient ironto account forthe mass of the largernif occurrences this con clusion is reinforced by the available geochemical data that imply nif record is a mixture of hydrothermal and de trital components a common thread that appears to link most if not all nif is an association with mafi cv olcanics

seawater ph pco2 and co2 3 variations in the caribbean sea over the last 130 kyr a boron isotope and b ca study of planktic foraminifera abstract here a new analytical methodology is described for measuring the isotopic composition of boron in foraminifera using multicollector inductively coupled plasma mass spectrometry mc icpms this new approach is fast 10 samples analysed in duplicate per analytical session and accurate to better than 0 25 at 95 confidence with acceptable sample size requirements 1 3 mg of carbonate a core top calibration of several common planktic and two benthic species from geographically widespread localities shows a very close agreement between the isotopic composition measured by mc icpms and the isotopic composition of b oh 4 in seawater as predicted using the recently measured isotopic equilibrium factor of 1 0272 at the depth of habitat a down core and core top investigation of boron concentration b ca ratio shows that the partition coefficient is influenced by co2 3 complicating the application of this proxy nevertheless it is demonstrated that these two proxies can be used to fully constrain the carbonate system of surface water in the caribbean sea odp site 999a over the last 130 kyr this reconstruction shows that during much of the holocene and the last interglacial period surface water at site 999a was in equilibrium with the atmosphere with respect to co2 during the intervening colder periods although the surface water pco2 was lower than the holocene it was a minor to significant source of co2 to the atmosphere possibly due to either an expansion of the eastern equatorial atlantic upwelling zone or a more local expansion of coastal upwelling in the southern caribbean such reorganisation of the oceanic carbonate system in favour of a larger source of co2 to the atmosphere from the equatorial ocean may require mechanisms responsible for lowering atmospheric co2 during glacial periods to be more efficient than previously supposed

two cryogenian glacial successions compared aspects of the sturt and elatina sediment records of south australia abstract two cryogenian glacial sequences the sturt and elatina occur within the adelaide rift complex s australia and are part of an 8 km thick succession recording rodinia fragmentation and subsequent passive margin development we compare and contrast these glacial successions presenting new data including detailed logged sections for both glacial sequences facies analysis reveals a total of six facies associations recording a range of glacial and glacially related processes these are a diamictite facies association ice proximal debris flows a sheet sandstone facies association fluvial deposits a flaser and rhythmically bedded facies association tidal flat a hummocky cross stratified sandstone facies association shoreface to offshore transition zone deposits and an underflow facies association recording offshore deposition a sixth facies association a ferruginous lonestone bearing siltstone is restricted to the top of the elatina formation and records rapid environmental amelioration and emergence from glaciation in the central flinders ranges the sturt succession commences with a glacial erosion surface ges which in agreement with previous workers was probably enhanced by rifting this ges is overlain in ascending order by the pualco tillite holowilena ironstone and wilyerpa formation with the warcowie dolomite member at the base of this latter formation a second ges is preserved within this latter unit and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits significant base level falls within the wilyerpa formation are identified by fluvial deposits sitting disconformably upon offshore deposits in contrast within the elatina formation a single ges is proposed which defines the base of the formation the elatina formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation apart from a locally preserved basal diamictite the lower member consists of fluvial deposits whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits the boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others deglaciation left contrasting deposits the sturt deglaciation culminated in deposition of black shale of the tindelpina shale member of the tapley hill formation seemingly as a sheet like deposit over the adelaide rift complex and beyond into the amadeus basin of central australia the elatina deglaciation meanwhile began first with deposition of the sixth facies association ferruginous dropstone bearing siltstone then proceeded with deposition of cap dolostone of the nuccaleena formation

deep ocean early warning signals of an atlantic moc collapse a future collapse of the atlantic meridional overturning circulation moc has been identified as one of the most dangerous tipping points in the climate system it is therefore crucial to develop early warning indicators for such a potential collapse based on relatively short time series so far attempts to use indicators based on critical slowdown have been marginally successful based on complex climate network reconstruction we here present a promising new indicator for the moc collapse that efficiently monitors spatial changes in deep ocean circulation through our analysis of the performance of this indicator we formulate optimal locations of measurement of the moc to provide early warning signals of a collapse our results imply that an increase in spatial resolution of the atlantic moc observations i e at more sections can improve early detection because the spatial coherence in the deep ocean arising near the transition is better captured

iron rich metalliferous sediments on the east pacific rise prototype of undifferentiated metalliferous sediments on divergent plate boundaries abstract sediments in a zone on the east pacific rise with an especially high spreading rate were studied chemically mineralogically and microscopically they consist of a mixture of metalliferous sediments and plankton tests the metalliferous sediments were formed by an acidic hydrothermal leaching of tholeiitic basalt with seawater and subsequent precipitation in contact with cold near bottom seawater we assume the precipitation from hydrothermal solutions in this part of the east pacific rise to be undifferentiated due to the high spreading rate and the resulting rapid flow on the water through the basalt thus these metalliferous sediments are an initial stage type that have not undergone differentiation mn mo la cu v ni fe zn co and y all of which are leachable in acidic hydrothermal solutions are enriched in the metalliferous sediments in comparison to the tholeiitic basalts zr al and ti on the other hand which under the same conditions are not easily leached are reduced in their concentrations all components of the metalliferous sediments precipitated as hydroxides or as adsorbed ions on the hydroxides of other elements this is due to the oxidizing conditions in the near bottom seawater the sedimentation rate is high the almost 3 m long cores reach only to the late pleistocene the only distinctly observable diagenetic process for this period of time is the formation of goethite from amorphous iron oxides only for na k and rb does it seem possible that a distinct enrichment in the sediments by adsorption from the seawater could have taken place ca sr pb and perhaps sc are primarily bound to the planktonic carbonate part of the sediments

an indicator of the multiple equilibria regime of the atlantic meridional overturning circulation recent model results have suggested that there may be a scalar indicator s monitoring whether the atlantic meridional overturning circulation moc is in a multiple equilibrium regime the quantity s is based on the net freshwater transport by the moc into the atlantic basin it changes sign as soon as the steady atlantic moc enters the multiple equilibrium regime because of an increased freshwater input in the northern north atlantic this paper addresses the issue of why the sign of s is such a good indicator for the multiple equilibrium regime changes in the atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model the authors show that the net anomalous freshwater transport into or out of the atlantic resulting from the interactions of the velocity perturbations and salinity background field is coupled to the background steady state state freshwater budget and hence to s the sign of s precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the moc therefore it can indicate whether the moc is in a single or multiple equilibrium regime

large iron isotope fractionation at the oxic anoxic boundary in lake nyos abstract the degassing of volcanic lake nyos cameroon provides the opportunity to study the strong isotopic variation of dissolved fe ii in a well constrained redox cycle and to identify the governing processes by reaction transport modeling two depth profiles sampled in the lake in march 2004 and 2005 reveal an increase in iron concentrations and 57 fe from around 1 mg l 1 and 1 88 at 55 m depth up to 344 mg l 1 and 0 83 at the bottom of the lake respectively a steep increase in 57 fe was observed across the oxic anoxic boundary as many biological and geochemical processes are known to fractionate fe isotopes we used a calibrated reaction transport model to disentangle the processes governing the fe cycle the model combines the isotopic signatures of dissolved fe ii and settling fe iii particles with the concentration profiles and settling fluxes of the fe particles in the lake we show that the strong shift in 57 fe is caused by isotopic fractionation via dissimilatory fe reduction across the oxic anoxic boundary of lake nyos the shift towards more positive values below the oxic anoxic interface could be attributed to vertical mixing of a heavier component from the bottom of the lake

observed fingerprint of a weakening atlantic ocean overturning circulation the atlantic meridional overturning circulation amoc a system of ocean currents in the north atlantic has a major impact on climate yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements here we provide evidence for a weakening of the amoc by about 3 1 sverdrups around 15 per cent since the mid twentieth century this weakening is revealed by a characteristic spatial and seasonal sea surface temperature fingerprint consisting of a pattern of cooling in the subpolar atlantic ocean and warming in the gulf stream region and is calibrated through an ensemble of model simulations from the cmip5 project we find this fingerprint both in a high resolution climate model in response to increasing atmospheric carbon dioxide concentrations and in the temperature trends observed since the late nineteenth century the pattern can be explained by a slowdown in the amoc and reduced northward heat transport as well as an associated northward shift of the gulf stream comparisons with recent direct measurements from the rapid project and several other studies provide a consistent depiction of record low amoc values in recent years

the stability of the moc as diagnosed from model projections for pre industrial present and future climates the stability of the atlantic meridional overturning circulation moc is investigated for various climate scenario runs using data from the cmip3 archive of coupled atmosphere ocean models apart from atmospheric feedbacks the sign of the salt flux into the atlantic basin that is carried by the moc determines whether the moc is in the single or multiple equilibria regime this salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined atlantic and arctic basins consistent with the finding that almost all coupled climate models recover from hosing experiments it is found that most models feature a negative salt advection feedback in their pre industrial climate freshwater perturbations are damped by this feedback excluding the existence of a stable off state for the moc all models feature enhanced evaporation over the atlantic basin in future climates but for a moderate increase in radiative forcing b1 and 2 co2 scenarios there is a decrease of the fresh water flux carried by the moc into the atlantic the deficit is made up by increased fresh water transport by the gyre circulation in this forcing regime the salt advection feedback becomes less negative for three models from an ensemble of eight it is positive in a 2 co2 climate while two models feature a positive feedback in the pre industrial climate for even warmer climates a1b equilibrium and 4 co2 the salt feedback becomes more negative damping again it is shown that the decrease in northward fresh water transport at 34 s by the moc in b1 equilibrium and 2 co2 is due to a reduction of the inflow of intermediate waters relative to thermocline waters associated with a robust shoaling of the moc in future warmer climates in a1b and 4 co2 climates northward freshwater transport increases again the moc keeps shoaling but both intermediate and thermocline water masses freshen

geochemical effects of decoupled fractional crystallization and crustal assimilation most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization afc in some magmatic systems however assimilation and crystallization are not strictly related and the mass assimilated is decoupled from and therefore varies independently of the mass crystallized fca the geochemical consequences of fca are examined and compared to those of afc the behavior of incompatible elements is identical during afc and fca and ratios of these elements do not allow discrimination between the two processes major oxide least squares mass balance models do not discriminate between afc and fca at f 0 7 f fraction of melt remaining however fca yields magmas richer in compatible elements and with higher sr isotopic ratios than afc at a given value of f repeated cycles of fca and afc combined with magma mixing fam may result in unusual geochemical trends such as the evolution of a calc alkaline basaltic parent to a tholeiitic daughter magma or the evolution of low and medium k calc alkaline basalts to high k andesites dacites trachyandesites or trachydacites lavas erupted by the volcano micro profitis ilias on santorini hellenic arc greece provide an example of magmas which evolved by combined fractionation assimilation by fca and mixing

characterization of metalliferous sediment from a low temperature hydrothermal environment on the eastern flank of the east pacific rise abstract metalliferous deposits are described from the eastern flank of the east pacific rise epr offshore costa rica close to a basaltic seamount called dorado high based on heat flow data and porewater profiles the site is an area of active low temperature hydrothermal discharge we focus on the mineralogical and chemical analysis from a 124 cm long gravity core gc50 located on the northwestern slope of the 100 m high dorado in this core the sediments consist of detrital clay minerals as well as authigenic minerals such as zeolites apatites and fe mn rich oxyhydroxides in contrast the reference sediments from adjacent areas without hydrothermal activity are olive gray hemipelagic muds composed of volcanic glass particles clay minerals siliceous microfossils and some detrital quartz and feldspar bulk sediment chemistry and chemical enrichment factors calculated with respect to the reference sediment indicate that the most important chemical changes occurred at the base of the core from 100 to 124 cm bsf with strong enrichments in mno cao p 2 o 5 and fe 2 o 3 these enrichments are correlated with the occurrence of authigenic fe oxyhydroxide goethite and mn oxide todorokite and vernadite at 100 cm bsf and hydrothermal apatite 110 124 cm bsf in the upper section of the core from 12 to 70 cm the sediment is composed of abundant smectite and authigenic phillipsite and only minor chemical changes can be observed with respect to the reference sediments the ubiquitous presence of phillipsite suggests that the entire sedimentary column of core gc50 was first affected by diagenesis however below 70 cm bsf these phillipsites are partially dissolved and fe oxides occur from 110 to 124 cm followed upward by mn oxides at 100 cm this transition from fe to mn rich sediments can be interpreted in terms of an upward increasing redox potential paas normalized rey patterns of gc50 sediments present clearly negative ce and positive y anomalies inherited from seawater at the base of core gc50 these anomalies decrease upward which we interpret together with the transition from fe to mn rich sediments by an upward migrating low temperature hydrothermal fluid thus after a first stage of diagenesis the discharge of a low temperature hydrothermal fluid occurred through the sedimentary column leading to the precipitation of hydrothermal compounds that are lacking towards the surface

hydrothermal contribution to the oceanic dissolved iron inventory mineral dust and marine sediment resuspension are generally considered the primary sources of the nutrient iron to the oceans numerical model results suggest that iron released by hydrothermal activity is also an important source of dissolved iron particularly in the southern ocean iron limits phytoplankton growth and hence the biological carbon pump in the southern ocean1 models assessing the impacts of iron on the global carbon cycle generally rely on dust input and sediment resuspension as the predominant sources2 3 although it was previously thought that most iron from deep ocean hydrothermal activity was inaccessible to phytoplankton because of the formation of particulates4 it has been suggested that iron from hydrothermal activity5 6 7 may be an important source of oceanic dissolved iron8 9 10 11 12 13 here we use a global ocean model to assess the impacts of an annual dissolved iron flux of approximately 9 108 mol as estimated from regional observations of hydrothermal activity11 12 on the dissolved iron inventory of the world s oceans we find the response to the input of hydrothermal dissolved iron is greatest in the southern hemisphere oceans in particular observations of the distribution of dissolved iron in the southern ocean3 chever et al manuscript in preparation bowie et al manuscript in preparation can be replicated in our simulations only when our estimated iron flux from hydrothermal sources is included as the hydrothermal flux of iron is relatively constant over millennial timescales14 we propose that hydrothermal activity can buffer the oceanic dissolved iron inventory against shorter term fluctuations in dust deposition

late proterozoic outer shelf manganese and iron deposits at otjosondu namibia related to the damaran oceanic opening the otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late proterozoic damara orogen in spite of intense deformation and a high grade metamorphic overprint the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis at least two manganese silicate ore horizons manganese formations are associated with banded iron formations this sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high energy conditions manganese ore varieties and their country rocks have been classified using their texture mineralogy and geochemistry and have thus been interpreted in sedimentological and facies terms the evaluation of the vertical facies sequences their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible the development of facies from a near shore clastic regime to manganese bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf the facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf starting with the transgressive quartzarenites overlying an unconformity to the pre damara basement followed by chemical sediments of the ore bearing horizon and overlying carbonates and ending up with pelitic schists on a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation small scale sedimentary cycles are interpreted as due to glaciogene influenced fluctuations of the sea level the source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin initial spreading in the khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity these base metal sulfide deposits are associated with pyrite bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese upwelling along the shelf edge was involved and additional separation of mn and fe was due to a change in eh conditions associated with a redox interface intersecting with the outer shelf environment the evolution of the khomas sea provided the ore forming solutions and also initiated the subsidence of the passive shelf margin fluctuations in sea level were due to the basin development on one hand and glacial interglacial cycles on the other the latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

the behavior of rare earth elements in seawater precise determination of variations in the north pacific water column abstract we present new highly precise and accurate data for ten rare earth elements rees in two water column profiles and in bottom waters from three additional localities in the north pacific our results presented here and in an earlier publication indicate that the water column in the western north pacific is stratified with respect to the isotopic composition of nd as well as nd la sm nd and er la both the nd isotopic and the ree data show that ree transport is strongly influenced by horizontal advection even in the sluggish circulation of the pacific in general rees increase in concentration with depth at mid latitude they reach a maximum concentration at about 3000 m and then decrease to the bottom at high latitude heavy rees reach a maximum concentration near 3000 m whereas light rees continue to increase in concentration with depth the deep water maximum and bottom water depletion is interpreted as an advective feature resulting from the flow of a lower concentration of rees in antarctic bottom water under a higher concentration of rees in the north pacific concentrations of rees do not show a linear relationship with those of si indicating that the ree cycle is not directly coupled to that of the si cycle ree element ratios i e nd la correlate with hydrographie properties which suggests that major features of the ree patterns may be conserved during lateral transport comparison of our ree data to those in previously published reports on pacific seawater shows that the identification of ree distributions affected by advection is only made possible by the high precision analysis we developed for this study the ce concentration we observe is relatively constant over 200 4500 m depth at both sites where we have complete profiles this unique constancy of ce is in contrast to the contiguous rees la and nd which show substantial concentration variations with depth

observations inferences and mechanisms of the atlantic meridional overturning circulation a review this is a review about the atlantic meridional overturning circulation amoc its mean structure temporal variability controlling mechanisms and role in the coupled climate system the amoc plays a central role in climate through its heat and freshwater transports northward ocean heat transport achieved by the amoc is responsible for the relative warmth of the northern hemisphere compared to the southern hemisphere and is thought to play a role in setting the mean position of the intertropical convergence zone north of the equator the amoc is a key means by which heat anomalies are sequestered into the ocean s interior and thus modulates the trajectory of climate change fluctuations in the amoc have been linked to low frequency variability of atlantic sea surface temperatures with a host of implications for climate variability over surrounding landmasses on intra annual timescales variability in amoc is large and primarily reflects the response to local wind forcing meridional coherence of anomalies is limited to that of the wind field on interannual to decadal timescales amoc changes are primarily geostrophic and related to buoyancy anomalies on the western boundary a pacemaker region for decadal amoc changes is located in a western transition zone along the boundary between the subtropical and subpolar gyres decadal amoc anomalies are communicated meridionally from this region amoc observations as well as the expanded ocean observational network provided by the argo array and satellite altimetry are inspiring efforts to develop decadal predictability systems using coupled atmosphere ocean models initialized by ocean data

geochemical implications of rare earth element patterns in hydrothermal fluids from mid ocean ridges abstract rare earth element data are reported for vent fluids collected from eight deep sea hydrothermal areas including samples from sediment hosted and bare rock hydrothermal systems the morb normalized ree patterns from these diverse settings are remarkably similar to each other but distinctly different than seawater or basalt being lree enriched and possessing a strikingly positive eu anomaly the ree concentrations making up these patterns vary widely however both on a spatial and temporal basis when plotted against ionic radii these fluid trends are remarkably linear and show the same fractionation patterns exhibited by plagioclase during magma segregation this analysis suggests that the partitioning of the ree between fluid and rock during high temperature hydrothermal alteration of morb is dominated by crystochemical exchange during dissolution and recrystallization reactions at the exposed surfaces of plagioclase phenocrysts a comparison of ree ionic radii in eight coordination indicate that the partitioning of 3ree and 2eu are dominated by chemical substitution for ca 2 and sr 2 respectively these considerations predict that the total 3ree content of vent fluid varies in a systematic way during the transition from an rich morb plagioclase to ab rich hydrothermal plagioclase and that these parameters can be used as indicators of the average extent of alteration inside active hydrothermal systems

fingerprints for early detection of changes in the amoc abstractdifferent strategies have been proposed in previous studies for monitoring the atlantic meridional overturning circulation amoc as well as arrays to directly monitor the amoc strength v

sea ice free conditions during the sturtian glaciation early cryogenian south australia in the neoproterozoic snowball earth hypothesis shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations in the central flinders ranges south australia the yudnamutana subgroup of sturtian age includes diamictite sandstone and siltstone units of glaciomarine origin as much as 5000 m thick and is overlain by postglacial transgressive siltstone and shale of the tindelpina shale member tapley hill formation in the central flinders ranges the yudnamutana subgroup consists of 1 the pualco tillite gravity resedimented glacial deposits 2 the holowilena ironstone glacioturbidites 3 poorly stratified pebbly diamictite of the warcowie dolomite member lowermost wilyerpa formation gravity resedimented glacial deposits succeeded by 4 siltstones and sandstones with abundant hummocky cross stratification hcs storm deposits and 5 a lonestone bearing succession with cobble sized clasts in the upper wilyerpa formation ice rafted debris interpreted to record a glacial re advance in which hcs is absent because the action of oscillating waves is required to produce hcs on the seafloor its presence indicates an interval of significant meltback prior to glacial re advance given that the hcs occurs 2 km beneath the tindelpina shale member it signifies a major ice free interval during the sturtian glaciation

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

spatial analysis of early warning signals for a north atlantic climate transition in a coupled gcm the climate system can potentially switch from one stable state to another the closer a system is to a bifurcation point i e tipping point the more likely it is that even small perturbations can force the system to experience a state shift e g a collapsing atlantic meridional overturning circulation amoc and associated cooling in parts of the north atlantic here we present an abrupt state transition from a warm to a cold north atlantic climate state with expanded sea ice during an orbitally forced transient holocene simulation performed with the community climate system model version 3 the state transition is associated with a weakening of the amoc by about 33 in this simulation the changing background climate induced by slow external orbital forcing plays an important role for the abrupt climate shift the model allows the identification of regions and variables that play a key role for a potential climate transition and show early warning signals increase in autocorrelation and standard deviation as well as trends in skewness especially for sea surface salinity in the northern north atlantic are identified as robust early warning signals whereas no early warning signals are found in the time series of the amoc stream function

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

anomalously weak labrador sea convection and atlantic overturning during the past 150 years the atlantic meridional overturning circulation amoc is a system of ocean currents that has an essential role in earth s climate redistributing heat and influencing the carbon cycle1 2 the amoc has been shown to be weakening in recent years 1 this decline may reflect decadal scale variability in convection in the labrador sea but short observational datasets preclude a longer term perspective on the modern state and variability of labrador sea convection and the amoc1 3 5 here we provide several lines of palaeo oceanographic evidence that labrador sea deep convection and the amoc have been anomalously weak over the past 150 years or so since the end of the little ice age lia approximately ad 1850 compared with the preceding 1 500 years our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the lia or as a more gradual continued decline over the past 150 years this ambiguity probably arises from non amoc influences on the various proxies or from the different sensitivities of these proxies to individual components of the amoc we suggest that enhanced freshwater fluxes from the arctic and nordic seas towards the end of the lia sourced from melting glaciers and thickened sea ice that developed earlier in the lia weakened labrador sea convection and the amoc the lack of a subsequent recovery may have resulted from hysteresis or from twentieth century melting of the greenland ice sheet 6 our results suggest that recent decadal variability in labrador sea convection and the amoc has occurred during an atypical weak background state future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the amoc weakening described here

cryogenian glaciation and the onset of carbon isotope decoupling global carbon cycle perturbations throughout earth history are frequently linked to changing paleogeography glaciation ocean oxygenation and biological innovation a pronounced carbonate carbon isotope excursion during the ediacaran period 635 to 542 million years ago accompanied by invariant or decoupled organic carbon isotope values has been explained with a model that relies on a large oceanic reservoir of organic carbon we present carbonate and organic matter carbon isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the sturtian and marinoan glacial events of the cryogenian period approximately 720 to 635 million years ago growth of the organic carbon pool may be related to iron rich and sulfate poor deep ocean conditions facilitated by an increase in the fe s ratio of the riverine flux after sturtian glacial removal of a long lived continental regolith

meridional density gradients do not control the atlantic overturning circulation a wide body of modeling and theoretical scaling studies support the concept that changes to the atlantic meridional overturning circulation amoc whether forced by winds or buoyancy fluxes can be understood in terms of a simple causative relation between the amoc and an appropriately defined meridional density gradient mdg the mdg is supposed to translate directly into a meridional pressure gradient here two sets of experiments are performed using a modular ocean model coupled to an energy moisture balance model in which the positive amoc mdg relation breaks down in the first suite of seven model integrations it is found that increasing winds in the southern ocean cause an increase in overturning while the surface density difference between the equator and north atlantic drops in the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment dt that ranges from 238 to 98c an increase in dt results in increased sensitivity of density to temperature gradients the amoc in these model integrations drops as the mdg increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean traditional scaling analysis can only produce this weaker amoc if the scale depth decreases enough to compensate for the stronger mdg five estimates of the depth scale are evaluated and it is found that the changes in the amoc can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth these two depth scales are commonly assumed to be the same in theoretical models of the amoc it is suggested that the correlation between the mdg and amoc breaks down in these model integrations because the depth and strength of the amoc is influenced strongly by remote forcing such as southern ocean winds and antarctic bottom water formation

iron stable isotopes beyond biosignatures abstract the stable isotope geochemistry of fe has attracted intense interest in the past five years this interest was originally motivated by the possible use of fe isotopes in biosignature applications particularly in sediments from the ancient earth or mars this application is still being developed with particular attention to fractionation mechanisms understanding such mechanisms should also provide new insights into the environmental biogeochemistry of fe at the same time the fe isotope system holds promise for other exciting frontiers including applications in oceanography solid earth geochemistry and biomedicine such applications will be increasingly attractive as fe isotope analysis becomes routine

metalliferous sediments within lava sequences of the sumail ophiolite oman mineralogical and geochemical characterization origin and evolution abstract lens shaped siliceous pelagic sediments and metalliferous deposits sampled in the wadi salahi block of the sumail ophiolite are interbedded within the successive lava flows of the extrusive sequence petrological mineralogical and geochemical characteristics of these sediments allow their oceanic environment of deposition and their early diagenetic and later hydrothermal transformations to be defined the sediments are primary radiolarian oozes deposited in a highly productive and open equatorial pelagic realm the biogenic components are mixed with hydrothermal metallic compounds similar to those found in the axial zone of modern active ridges the deposits are subsequently transformed by various oceanic hydrothermal events which are expressed by specific secondary parageneses the sedimentary lenses from the lower lava sequence are epidotized whereas the uppermost lenses from the upper lava unit contain pumpellyite some peculiar lenses are not affected by these oceanic metamorphisms for instance the deposits closely associated with the zuhan orebody which are fe and silica rich with scarce sulfates or the manganiferous sediments located at the top of the lower lava sequences which were earlier lithified by the deposition of the overlying lava flows nowhere in the whole sedimentary section described were detrital or volcaniclastic remains found disproving the possibility that these sediments may have been deposited in a seamount or island arc environment

basalt geochemistry and tectonic discrimination within continental flood basalt provinces abstract continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings in one well studied classic flood basalt province the mesozoic karoo province of southern africa magmatism occurred in the following tectonic settings a continental rifting leading to ocean floor spreading in the south atlantic ocean etendeka suite of namibia b stretched continental lithosphere and rifting not leading directly to ocean floor formation lebombo suite of southeastern africa and c an a tectonic within plate continental setting characterized by an absence of faulting or warping lesotho highlands and karoo dolerites of south africa by means of spidergrams of the elements rb ba th nb k la ce sr nd p hf zr sm ti tb y v ni and cr uncontaminated tholeiites from c above i e the lesotho type continental flood basalts ltcfb are compared with mid ocean ridge basalts morb ocean island tholeiites oit and tholeiites and calc alkali basalts from subduction environments the comparison reveals the ltcfbs are geochemically distinct the differences are reflected in relative enrichments or depletions of the more incompatible elements rb ce to less incompatible elements ce y i e the overall slope of the spidergrams and in anomalous enrichments or depletions of one or more of the elements th k nb sr ti hf and zr the distinctive geochemical character of the lesotho ltcfbs is interpreted in terms of a lithospheric mantle source for the basalts this is supported by isotopic data there are no major geochemical differences between lesotho cfbs and basalts of the rift related etendeka and lebombo suites although the latter are somewhat enriched in rb ba and k however unlike the lesotho basalts the lebombo and etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late stage dolerites having morb oit i e asthenospheric geochemical characteristics the flood basalt silicic magmatism late stage dyke swarm association is characteristic of several rift or thinned lithosphere environments e g ethiopia skye eastern greenland but in many of these the flood basalts have ocean island basalt oit geochemical characteristics the lesotho type cfb geochemistry is exhibited by the grande ronde basalt of the columbia river group a possible subduction related flood basalt province and the basic rocks associated with mesozoic rifting in the north and south atlantic basalt geochemistry alone is unhelpful in determining the tectonic setting of cfbs although the rift related environments may be identified by the petrology and geochemistry of the whole igneous suite a two source model is proposed for the mantle derived basic rocks in rift related cfb provinces early enriched basalts are derived from the lithosphere and following pronounced lithospheric attenuation or rifting later morb like melts are emplaced from the rising asthenosphere the presence of both lesotho and oit type geochemical patterns in rift related cfbs suggests that the lithosphere exhibits different styles of enrichment

reconstructing monitoring and predicting multidecadal scale changes in the north atlantic thermohaline circulation with sea surface temperature sea surface temperature sst observations in the north atlantic indicate the existence of strong multidecadal variability with a unique spatial structure it is shown by means of a new global climate model which does not employ flux adjustments that the multidecadal sst variability is closely related to variations in the north atlantic thermohaline circulation thc the close correspondence between the north atlantic sst and thc variabilities allows in conjunction with the dynamical inertia of the thc for the prediction of the slowly varying component of the north atlantic climate system it is shown additionally that past variations of the north atlantic thc can be reconstructed from a simple north atlantic sst index and that future anthropogenically forced changes in the thc can be easily monitored by observing ssts the latter is confirmed by another state ofthe art global climate model finally the strong multidecadal variability may mask an anthropogenic signal in the north atlantic for some decades

iron and manganese deposits of the urucum district mato grosso do sul brazil the urucum district is located in the brazilian state of mato grosso do sul near the city of corumba on the brazilian bolivian border the district comprises an area of 200 km 2 where eight up to 1 000 m high tabular mountains rise out of the swampy lowlands of the pantanal the tabular mountains are partly built by an up to 350 m thick rock sequence of the santa cruz formation from bottom to top the santa cruz formation is composed of manganese ore horizon mn 1 an 80 m thick unit of ferruginous sandstones and manganese ore horizon mn 2 which is overlain by an up to 270 m thick monotonous sequence of hematite jaspilites forty meters above mn 2 is manganese ore horizon mn 3 another 40 m above mn 3 is manganese ore horizon mn 4 both are intercalated with the hematite jaspilites the iron ore reserves of the urucum district amount to approximately 36 billion tons the calculation of the manganese ore reserves in the four horizons results in 608 million tons comprised of 23 4 million tons of measured 34 5 million tons of indicated 465 1 million tons of inferred and 86 million tons of hypothetical and speculative reserves in the urucum district predominantly layered detritus rich and concretionary cryptomelane ores are found the ores consist mainly of micro to cryptocrystalline cryptomelane associated with hematite and quartz braunite pyrolusite and lithiophorite are minor constituents layered braunite ores mined are composed of braunite cryptomelane pyrolusite hematite and quartz the average mn contents of these four different types of manganese ores range from 25 63 percent in the detritus rich cryptomelane ores to 49 51 percent in the concretionary ones the fresh hematite jaspilitic iron ores contain about 50 percent fe whereas in the weathered and quartz depleted hematite jaspilites the iron content increases to 67 percent most of the iron and manganese ores were precipitated as chemical sediments in a partly ice covered fjordlike basin field observations rb sr isotope studies and leaching experiments show that large scale solution transport and reprecipitation processes led to supergene concentrations of iron and manganese in the underlying sandstones at the bottom of the santa cruz formation and in manganese ore horizon mn 1

on the evolution of atlantic meridional overturning circulation fingerprint and implications for decadal predictability in the north atlantic it has been suggested previously that the atlantic meridional overturning circulation amoc anomaly associated with changes in the north atlantic deep water formation propagates southward with an advection speed north of 34 n in this study using geophysical fluid dynamics laboratory coupled model version 2 1 gfdl cm2 1 we show that this slow southward propagation of the amoc anomaly is crucial for the evolution and the enhanced decadal predictability of the amoc fingerprint the leading mode of upper ocean heat content uohc in the extratropical north atlantic a positive amoc anomaly in northern high latitudes leads to a convergence divergence of the atlantic meridional heat transport mht anomaly in the subpolar gulf stream region thus warming in the subpolar gyre spg and cooling in the gulf stream region after several years recent decadal prediction studies successfully predicted the observed warm shift in the spg in the mid 1990s our results here provide the physical mechanism for the enhanced decadal prediction skills in the spg uohc

a multimodel study of sea surface temperature and subsurface density fingerprints of the atlantic meridional overturning circulation abstractthe atlantic meridional overturning circulation amoc is an important component of the north atlantic climate system here simulations from 10 coupled climate models are used to calculate patterns of sea surface temperature sst and subsurface density change associated with decadal amoc variability the models are evaluated using observational constraints and it is shown that all 10 models suffer from north atlantic deep water transports that are too shallow although the biases are least severe in the community climate system model version 4 ccsm4 in the models that best compare with observations positive amoc anomalies are associated with reduced labrador sea stratification and increased midocean 800 1800 m densities in the subpolar gyre maximum correlations occur when amoc anomalies lag labrador sea stratification and subsurface density anomalies by 2 6 yr and 0 3 yr respectively in all 10 models north atlantic warming follows positive amoc anomalies but the patterns and magnitud

iron formations related to mafic volcanism and ensialic rifting in the southern margin zone of the damara orogen namibia abstract iron formations occur within the metasedimentary metavolcanic sequence of the chuos formation in the southern margin zone of the upper proterozoic damara orogen of south west africa namibia the iron rich lithosome comprises magnetite quartz rock haematite quartz rock and magnetite quartz silicate carbonate rock on the basis of their host rock associations iron formation in diamictite sequences and in amphibolite are distinguished iron formation in amphibolite is developed both as a chemical sediment and as a composite rock consisting of chemical iron rich sediment with admixture of varying amounts of volcanic detritus iron formation in diamictite represents a pure chemical sediment both of the iron rich rocks contain abundant apatite and their manganese contents are low an exhalative origin for the iron formations of the southern margin zone related to mafic volcanism and ensialic rifting is suggested

density compensated overturning in the labrador sea the atlantic meridional overturning circulation a key constituent of the climate system is projected to slow down in the twenty first century due to a weakening of the labrador sea convection itself a response to greenhouse gas warming and or enhanced freshwater flux from the arctic however the first observations from the overturning in the subpolar north atlantic program reveal a minimal response of the meridional overturning circulation to the strong labrador sea convection during the winters of 2015 2016 from an analysis of the observational and reanalysis data we show here that this weak response can be explained by a strong density compensation in the labrador sea although convection induces important changes of temperature and salinity in the basin interior the export of the thermal and haline anomalies to the boundary current largely takes place along density surfaces as a result the transformation across density surfaces that is the imprint on the overturning circulation is relatively small this finding highlights the critical relationship between temperature and salinity in determining the overturning strength in the labrador sea and underlines the necessity of accurate freshwater flux estimates for improved meridional overturning circulation predictions compensation of the effects of salt and temperature changes on water density in the labrador sea can explain the minimal response of the atlantic overturning circulation to convection according to analyses of observations and reanalysis data

integrated correlation of the vendian to cambrian arroyo del soldado and corumba groups uruguay and brazil palaeogeographic palaeoclimatic and palaeobiologic implications abstract the corumba group of sw brazil and the arroyo del soldado group asg of uruguay are correlated on the basis of litho bio and chemostratigraphy both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf in the corumba basin sedimentation began in the varangerian represented by the glaciomarine puga formation a series of sea level fluctuations coupled with climatic changes are recorded up section while uppermost deposits of the asg are of lowermost cambrian age sedimentation ceased in the latest vendian in the corumba basin an assemblage of six species of organic walled microfossils dominated by bavlinella faveolata and soldadophycus bossii three species of vendotaenids and two species of skeletal fossils cloudina and titanotheca is described from the corumba group the vendotaenid eoholynia corumbensis sp nov is described from siltstones of the guaicurus formation an important diversity of skeletal fossils in the corumba arroyo del soldado and nama groups points to favourable vendian palaeoclimatic conditions in sw gondwana preliminary carbon isotopic data show a series of alternating positive and negative excursions corroborating the upper vendian age indicated by fossils for both units previously reported strontium isotopic data are also consistent with this age it is postulated that the corumba and asgs were deposited onto the same shelf which opened to the east the rio de la plata superterrane craton extends farther to the north than previously expected or it was already amalgamated with the amazonian craton by vendian times collision of the platform with the parana block caused closure of the basin during the cambrian early ordovician finally models of neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed

a sea change in our view of overturning in the subpolar north atlantic to provide an observational basis for the intergovernmental panel on climate change projections of a slowing atlantic meridional overturning circulation moc in the 21st century the overturning in the subpolar north atlantic program osnap observing system was launched in the summer of 2014 the first 21 month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the osnap line in a departure from the prevailing view that changes in deep water formation in the labrador sea dominate moc variability these results suggest that the conversion of warm salty shallow atlantic waters into colder fresher deep waters that move southward in the irminger and iceland basins is largely responsible for overturning and its variability in the subpolar basin

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

timely detection of changes in the meridional overturning circulation at 26 n in the atlantic it is investigated how changes in the north atlantic meridional overturning circulation moc might be reliably detected within a few decades using the observations provided by the rapid moc 26 n array previously detectability of moc changes had been investigated with a univariate moc time series exhibiting strong internal variability which would prohibit the detection of moc changes within a few decades here a modification of k hasselmann s fingerprint technique is used simulated observations are projected onto a time independent spatial pattern of natural variability to derive a time dependent detection variable the fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26 n against the strength of the moc at 26 n within the coupled echam5 max planck institute ocean model s mpi om control climate simulation this pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section onto this fixed spatial pattern of natural variability both the existing hydrographic observations and simulated observations mimicking the rapid moc 26 n array in three realizations of the intergovernmental panel on climate change ipcc scenario a1b are projected for a random observation error of 0 01 kg m 3 and only using zonal density gradients between 1700 and 3100 m depth statistically significant detection occurs with 95 reliability after about 30 yr in the model and climate change scenario analyzed here compared to using a single moc time series as the detection variable continuous observations of zonal density gradients reduce the detection time by 50 for the five hydrographic occupations of the 26 n transect none of the analyzed depth ranges shows a significant trend between 1957 and 2004 implying that there was no moc trend over the past 50 yr

a reversal of climatic trends in the north atlantic since 2005 the mid 1990s warming of the north atlantic subpolar gyre was probably related to strengthened overturning observations and numerical models suggest that a climate reversal to a cooling trend occurred around 2005

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

multidecadal north atlantic sea surface temperature and atlantic meridional overturning circulation variability in cmip5 historical simulations received 22 may 2013 revised 4 september 2013 accepted 8 september 2013 published 23 october 2013 1 in this paper simulated variability of the atlantic multidecadal oscillation amo and the atlantic meridional overturning circulation amoc and their relationship has been investigated for the first time climate models of the coupled model intercomparison project phase 5 cmip5 provided to the intergovernmental panel on climate change fifth assessment report ipcc ar5 in historical simulations have been used for this purpose the models show the most energetic variability on the multidecadal timescale band both with respect to the amo and amoc but with a large model spread in both amplitude and frequency the relationship between the amo and amoc in most of the models resembles the delayed advective oscillation proposed for the amoc on multidecadal timescales a speed up slow down of the amoc is in favor of generating a warm cold phase of the amo by the anomalous northward southward heat transport in the upper ocean which reversely leads to a weakening strengthening of the amoc through changes in the meridional density gradient after a delayed time of ocean adjustment this suggests that on multidecadal timescales the amo and amoc are related and interact with each other

atmosphere drives recent interannual variability of the atlantic meridional overturning circulation at 26 5 n the atlantic meridional overturning circulation amoc has been observed at 26 5 n since 2004 during 2009 2010 there was a transient 30 weakening of the amoc driven by anomalies in geostrophic and ekman transports here we use simulations based on the met office forecast ocean assimilation model foam to diagnose the relative importance of atmospheric forcings and internal ocean dynamics in driving the anomalous geostrophic circulation of 2009 2010 data assimilating experiments with foam accurately reproduce the mean strength and depth of the amoc at 26 5 n in addition agreement between simulated and observed stream functions in the deep ocean is improved when we calculate the amoc using a method that approximates the observing array at 26 5 n the main features of the geostrophic circulation anomaly are captured by an ensemble of simulations without data assimilation these model results suggest that the atmosphere played a dominant role in driving recent interannual variability of the amoc

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

the atlantic freshwater budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation 1 which physical processes effectively determine the stability regime of the atlantic meridional overturning circulation moc is not yet fully understood we investigate the role of the oceanic freshwater transport into the basin employing a coupled model of intermediate complexity by modifying the longitudinal variation of surface salinities near the southern border at 33 s the amount of salt flowing out of the atlantic via the brazil current can be regulated in turn this will influence whether the moc exports or imports salt water the latter is associated with a basin scale salinity overturning feedback which can be either positive or negative pulse experiments strongly suggest that its sign determines the existence of a monostable or bistable regime in our model

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

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

geochemistry of a hydrothermal sediment core from the obs vent field 21 n east pacific rise abstract we report a geochemical investigation of near vent hydrothermal sediment from the east pacific rise collected in a short 11 cm core by submersible from the obs vent site at 21 n the sediment has high concentrations of fe 23 25 cu 10 22 zn 4 13 and s 22 30 but low mn concentrations 108 279 ppm indicating a much fresher sulfidic input than in previously reported near vent sediments from the mid atlantic ridge further studies are required to fully characterise near field hydrothermal sediments but the data in this study allow the first comparisons with similar near vent material collected previously from the mid atlantic ridge as well as ridge flank data from the epr and juan de fuca ridge compared to ridge flank sediments the near field sediments exhibit high ratios of fe mn cu fe and zn fe unlike cu and zn pb is not so significantly enriched pb isotope compositions are indistinguishable from local morb indicating a dominant hydrothermal source for this element in these sediments this may not necessarily be the case for ridge flank sediments however where scavenging from seawater may also be important ree distributions throughout the core exhibit low concentrations and characteristics typical of vent fluids with lree enrichments and pronounced positive eu anomalies these patterns are similar to those of relatively fresh sulfides but there is also evidence for progressive oxidation downward into the core by ingressing seawater

signature of the atlantic meridional overturning circulation in sea level along the east coast of north america in this letter we examine the relationship between the north atlantic meridional overturning circulation moc and sea level sl along the east coast of north america in the eddy permitting ocean model occam we find a distinctive topography following pattern of sl variability in the western north atlantic that is closely linked with the changing strength of the moc with a 2 cm drop in sl along the us east coast corresponding to a 1sv increase in the moc we find a similar pattern of sl variability in the altimetry record and show that this meridionally coherent sl mode dominates interannual sl variability at tide gauges along the north american east coast between 40 50n hence we conclude that north american coastal sea level may indeed be a useful indicator of moc variability on interannual timescales allowing an observationally based estimate of the likely range of interannual moc fluctuations to be determined citation bingham r j and c w hughes 2009 signature of the atlantic meridional overturning circulation in sea level along the east coast of north america geophys res lett 36 l02603 doi 10 1029 2008gl036215

coherent surface subsurface fingerprint of the atlantic meridional overturning circulation 1 satellite altimeter data shows a weakening of the north atlantic subpolar gyre during the 1990s which is thought as an indicator of a slowdown of the atlantic meridional overturning circulation amoc however whether the recent slowing subpolar gyre is a decadal variation or a long term trend remains unclear here i show that altimeter data is highly correlated with instrumental subsurface ocean temperature data in the north atlantic and both show opposite signs between the subpolar gyre and the gulf stream path such a dipole pattern is a distinctive fingerprint of amoc variability as shown for the first time by a 1000 year coupled ocean atmosphere model simulation the results suggest that contrary to previous interpretations the recent slowdown of the subpolar gyre is a part of a multidecadal variation and suggests a strengthening of the amoc the ongoing satellite and subsurface temperature measurements could be used to monitor future amoc variations sensitively

how wet is wet precipitation constraints on late quaternary climate in the southern arabian peninsula it is generally recognized that the southern arabian peninsula has had two wet periods in the late quaternary to quantify wet a 28 000 year old capillary surface associated with a paleowater table was mapped and used as a surrogate for the water table in a ground water model analysis of this model suggests 1 4 mm year 1 of recharge is necessary to support the water table at the mapped elevations during the wet period climatic relations between rainfall and recharge in arid areas infer that annual rainfall during this wet period was approximately 200 50 mm year 1 or approximately five times the present rate

rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments abstract analyses have been made of ree contents of a well characterized suite of deep sea 4000 m principally todorokite bearing ferromanganese nodules and associated sediments from the pacific ocean ree in nodules and their sediments are closely related nodules with the largest positive ce anomalies are found on sediments with the smallest negative ce anomalies in contrast nodules with the highest contents of other rare earths 3 ree are found on sediments with the lowest 3 ree contents and vice versa 143 nd 144 nd ratios in the nodules 0 51244 point to an original seawater source but an identical ratio for sediments in combination with the ree patterns suggests that diagenetic reactions may transfer elements into the nodules analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to ree contents of nodules and sediments is negligible inter element relationships and leaching tests suggest that ree contents are controlled by a p rich phase with a ree pattern similar to that for biogenous apatite and an fe rich phase with a pattern the mirror image of that for sea water it is proposed that 3 ree concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate processes which favour the enrichment of transition metals in equatorial pacific nodules favour the depletion of 3 ree in nodules and enrichment of 3 ree in associated sediments in contrast ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions

reconstructing global overturning from meridional density gradients despite the complexity of the global ocean system numerous attempts have been made to scale the strength of the meridional overturning circulation moc principally in the north atlantic with large scale basin wide hydrographic properties in particular various approaches to scaling the moc with meridional density gradients have been proposed but the success of these has only been demonstrated under limited conditions here we present a scaling relationship linking overturning to twice vertically integrated meridional density gradients via the hydrostatic equation and a rotated form of the geostrophic equation this provides a meridional overturning streamfunction as a function of depth for each basin using a series of periodically forced experiments in a global coarse resolution configuration of the general circulation model nemo we explore the timescales over which this scaling is temporally valid we find that the scaling holds well in the upper atlantic cell at 1000 m for multi decadal and longer timescales accurately reconstructing the relative magnitude of the response for different frequencies and explaining over 85 of overturning variance on timescales of 64 2048 years despite the highly nonlinear response of the antarctic cell in the abyssal atlantic between 76 and 94 of the observed variability at 4000 m is reconstructed on timescales of 32 years and longer the scaling law is also applied in the indo pacific this analysis is extended to a higher resolution stochastically forced simulation for which correlations of between 0 79 and 0 99 are obtained with upper atlantic moc variability on timescales 25 years these results indicate that meridional density gradients and overturning are linked via meridional pressure gradients and that both the strength and structure of the moc can be reconstructed from hydrography on multi decadal and longer timescales provided that the link is made in this way

phanerozoic ironstones an introduction and review s u m m a r y ironstones particularly ooidal ironstones have long fascinated sedimentary geologists and have generated an enormous variety of interpretations but have remained poorly understood a recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences the first part of this introduction examines recent advances and attempts to produce a synthesis of them with particular reference to the genesis of marine ooidal ironstones current models for the formation of marine oolitic ironstone formations are discussed and a working model based on various lines of recent research is proposed this model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis the ooids may be mechanically modified and mineralogically transformed by reworking as well as by later diagenesis goethite ooids may be oxidized berthierine ooids reworked lateritic ooids or primary grains the formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply continued supply of iron and physical reworking many sedimentary environments may supply these parameters but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply in the second part of this introduction the terminology and classification are discussed and conventions proposed the term berthieroid is introduced as a non specific term for material of undetermined berthierine or chamosite composition the term ironstone is proposed as a petrological term and as an informal lithostratigraphic term a deposit composed of ironstones may be termed an ironstone formation it is recommended that a dunham style of classification is employed petrographically with ironstone replacing the stone of the original carbonate classification the nomenclature of allochems in ironstones is also discussed a glossary of ironstone terminology is provided

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

mineralogy and geochemistry of hydrothermal sediments from the serpentinite hosted saldanha hydrothermal field 36 34 n 33 26 w at mar abstract the saldanha hydrothermal field is located at the top of a serpentinized massif mount saldanha ms at a non transform offset nto5 along the mid atlantic ridge mar south of the azores it is one of the rare known sites on a worldwide basis where direct evidence of low temperature 7 9 c hydrothermal activity has been provided by direct observation of hydrothermal fluid venting through small orifices in the ocean floor sedimentary cover this study focuses on the mineralogy and geochemistry of 14 sediment cores collected at ms for comparison four samples collected at the rainbow site nto6 were also studied mount saldanha hydrothermal sediments are highly diluted within a dominant foraminiferal nanofossiliferous ooze with small fragments of underlying rocks the mineral assemblage of the hydrothermal component is characterized by sulphides nontronite smectites poorly crystallized mn oxyhydroxides and amorphous material cu zn and fe sulphides mn mg oxy hydroxides and putative manganobrucite were also identified in one sample collected at an orifice vent in this sample micro chimneys conduits composed of isocubanite and sphalerite were also identified mount saldanha sediments show a clear enrichment in elements such as mn mg fe cu p and v derived from hydrothermal fluids and ni cr and co derived from ultramafic rocks the geochemical data together with the observed mineral assemblage suggest that the hydrothermal fluids are at a higher temperature than those measured at the escape orifices 7 9 c and a strong enrichment in mg mainly at the top of the mount agrees with extensive mixing of the hydrothermal fluid with unmodified seawater nevertheless the mineral assemblage of ms sediments is consistent with the precipitation from hydrothermal fluids at much lower temperatures than at rainbow the presence of serpentinized and steatitized talcshist ultramafic rocks and the occurrence of a strong methane anomaly within the overlying water column collectively suggest that the hydrothermal circulation at ms is driven by exothermic reactions closely associated with the serpentinization process rainbow sediments have a higher concentration in transition metals and consequently an enrichment in sulphides these differences are likely to be a consequence of the higher temperature of hydrothermal fluids reflected in the composition of hydrothermal solutions and of a stronger hydrothermal flux at the rainbow site

a new age constraint on the onset of the neoproterozoic glaciations in the yangtze platform south china abstract a sample from the top of the banxi group early neoproterozoic in hunan province south china was dated by the shrimp zircon u pb method the resulting age of 725 10 ma probably solves the debate between the synglacial and the preglacial correlation of the liantuo formation and indicates that the liantuo formation most likely correlates with the preglacial banxi group since the banxi danzhou group is upwardly transitional to the glacial jiangkou group the age becomes a new maximum age constraint on the onset of the jiangkou glaciation which therefore most likely correlates with the sturtian glaciation

ocean atmosphere dynamical coupling fundamental to the atlantic multidecadal oscillation abstractthe north atlantic has shown large multidecadal temperature shifts during the twentieth century there is ongoing debate about whether this variability arises primarily through the influenc

cerium anomaly variations in ferromanganese nodules and crusts from the indian ocean fifty analyses of rare earth elements as well as mineralogical studies have been carried out on a suite of manganese nodules and crusts from the central indian basin and the western indian ocean the aim was to identify the processes controlling the ree patterns of the phases hosting the ree in the manganese nodules with an emphasis on an understanding of the ce anomaly this has involved separating the encrusting layers and nuclei physically as well as fe mn oxides from the aluminosilicate phase chemically using a 2 m hcl leach prior to analysis the presence of nodule nuclei seems to have little influence mostly 5 to a maximum of 30 on the overall magnitude of the ce anomalies in these nodules the ratios of concentrations of elements in the acid leachates and the corresponding bulk values yield flat ree patterns indicating that the aluminosilicate phase contributes very little to the ce anomalies interelement relations indicate that the ce anomalies are largely controlled by the amorphous mineral phase feooh xh2o the relationship of fe ce anomaly and mno2 further suggests that ce is chemisorbed onto iron oxyhydroxides which are epitaxially intergrown with mno2 the regional distribution of the ce anomaly values appears to depend on many of the factors responsible for the uptake of other minor metals in nodules and crusts

surface ocean ph response to variations in pco2 through two full glacial cycles abstract knowledge of past atmospheric pco 2 is important for evaluating the role of greenhouse gases in climate forcing ice core records show the tight correlation between climate change and pco 2 but records are limited to the past 900 kyr we present surface ocean ph and pco 2 data reconstructed from boron isotopes in planktonic foraminifera over two full glacial cycles 0 140 and 300 420 kyr the data co vary strongly with the vostok pco 2 record and demonstrate that the coupling between surface ocean chemistry and the atmosphere is recorded in marine archives allowing for quantitative estimation of atmospheric pco 2 beyond the reach of ice cores