abrupt global ocean anoxia during the late ordovician early silurian detected using uranium isotopes of marine carbonates widespread marine anoxia is hypothesized as the trigger for the second pulse of the late ordovician hirnantian mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters we test the anoxia hypothesis using 238u values of marine limestones as a global seawater redox proxy the 238u trends at anticosti island canada document an abrupt late hirnantian 0 3 negative shift continuing through the early silurian indicating more reducing seawater conditions the lack of observed anoxic facies and no covariance among 238u values and other local redox proxies suggests that the 238u trends represent a global ocean redox record the hirnantian ocean anoxic event hoae onset is coincident with the extinction pulse indicating its importance in triggering it anoxia initiated during high sea levels before peak hirnantian glaciation and continued into the subsequent lowstand and early silurian deglacial eustatic rise implying that major climatic and eustatic changes had little effect on global ocean redox conditions the hoae occurred during a global 13c positive excursion but lasted longer indicating that controls on the c budget were partially decoupled from global ocean redox trends u cycle modeling suggests that there was a 15 increase in anoxic seafloor area and 80 of seawater u was sequestered into anoxic sediments during the hoae unlike other ocean anoxic events oae the hoae occurred during peak and waning icehouse conditions rather than during greenhouse climates we interpret that anoxia was driven by global cooling which reorganized thermohaline circulation decreased deep ocean ventilation enhanced nutrient fluxes stimulated productivity which lead to expanded oxygen minimum zones

interferometer studies of equatorial f region irregularities and drifts a radar interferometer technique developed at jicamarca peru and first used to study electrojet irregularities has now been used successfully to study plasma turbulence in the equatorial f region our first results have shown that the most turbulent echoes appear to come from a region that extends for tens of kilometers in altitude but for only a kilometer or less in the east west direction this slab may very well be the wall of a depleted region a plasma bubble sometimes the irregularities can be tracked as they move eastward or westward velocity profiles for the evening period obtained in this way show a strong shear with westward velocities at the lowest altitudes observed and eastward velocities above a plausible explanation for this shear is that the westward drifts are driven by electric fields produced by westward e region winds and mapped up along magnetic field lines while at higher heights where the electron density is greater the drifts are controlled by the f region dynamo driven by eastward winds

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

nd isotope systematics of coarse and fine grained sediments examples from the middle proterozoic belt purcell supergroup sm nd isotopic compositions are reported for 20 fine and 5 coarse grained samples of the belt purcell supergroup of western north america varepsilon nd and sm nd ratios for fine grained belt purcell sediments are remarkably uniform indicating crustal residence ages ranging only from 1 6 to 2 1 ga varepsilon nd and sm nd ratios for coarse grained sediments however are much more varied and contain evidence of sm nd fractionation during sedimentary recycling assuming that this ree fractionation occurred between the time source areas were eroded and when sediment was deposited a new method of calculating crustal residence age intervals is derived for the coarse grained samples which serves to place constraints on the average time since source materials were extracted from the mantle the nd data from both sample groups provide an average age estimate of 2 0 pm 0 3 ga for the middle proterozoic crust from which most of the belt purcell supergroup was derived the isotopic data toge

relations between subduction slab heating slab dehydration and continental growth abstract the hydrated altered basalts of the oceanic crust if subducted could release large volumes of water during their phase transformations and partial melting reactions if heat from shear strain recrystallizations largely heats the altered basaltic layer only a 2 km thick layer could be heated significantly before the slab reaches 80 125 km depth most of the released waters should be driven into the deeper but cooler regions at the slab and andesitic tonalitic partial melts should form in the heated altered basalt layer large volumes of water initially at the surface could be added to the mantle and tonalitic partial melts could be lost from the oceanic crust and added to the continental plate and could account for continental growth

water column euxinia and wildfire evidence during deposition of the upper famennian hangenberg event horizon from the holy cross mountains central poland a palynological study of the uppermost famennian section from kowala quarry holy cross mountains central poland allowed recognition of two miospore zones lv retispora lepidophyta apiculiretusispora verrucosa and ln retispora lepidophyta verrucosisporites nitidus based on palynology and sedimentology the black shale within the upper part of the section is identified as equivalent to the hangenberg black shale which is known globally this black shale contains compounds characteristic of photic zone euxinia including isorenieratane and its derivatives such compounds are absent in the organic poor marls and shales occurring below the ln zone and are present only as traces in the layers just above the black shale indicating fluctuations in the oxygen minimum zone during uppermost famennian sedimentation palynofacies show high amounts of amorphous organic matter and prasinophyte concentrations in the black shale and a subsequent significant decrease of amorphous organic matter concomitant with a rapid increase of terrestrial input mainly miospores with common tetrads in the layers above the black shale this supports the relatively rapid change in the taxonomic composition of phytoplankton caused by fluctuations of the chemocline the whole succession corresponds to one sea level rise and fall the presence of high concentrations of peri condensed polycyclic aromatic hydrocarbons and large amounts of small charcoal particles at the hangenberg event horizon indicate the occurrence of wildfires such observations suggest that atmospheric o2 levels had exceeded the critical threshold of 13 above which wildfires may occur by latest famennian time

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

preliminary paleomagnetic results from the upper carboniferous of uliastai block inner mongolia china paleomagnetic samples from five sites from the upper carboniferous baoligemiao formation of the uliastai block 117 10 e 45 30 n preserve stable normal and reversed magnetizations with mean d 138 3 i 47 8 a95 9 7 k 9 9 irm acquisition and thermal demagnetization of irm indicate that the magnetization in these pyroclastic sediments is predominantly carried by magnetite however higher coercivity minerals contribute to the remnant magnetization as well the pole position 53 4 n 14 8 e derived from these results is near those of sino korea and central mongolia but significantly different from that of siberia suggesting that the uliastai was not a siberian related fragment as traditionally proposed instead the block along with the mongolia and sonid blocks may have moved as a series of north facing arc terranes frontal and remnant arcs of the sino korean continent during the late paleozoic possibly an ancient analogue of the southeast asian archipelago today

demise of a devonian carboniferous carbonate ramp by eutrophication the occurrence of organic rich mudrocks overlying carbonate deposits in the rock record has typically been ascribed to changes in relative sea level we propose alternatively that progressive eutrophication of the euphotic zone caused the ecological demise of a late devonian marine carbonate ramp system in alberta and its subsequent burial in organic rich mudrocks similar lithologic transitions elsewhere in the stratigraphic record may also reflect eutrophication rather than major shifts in sea level the transition from upper devonian carbonates to devonian carboniferous black mudrocks in alberta is characterized by a gradual decrease in net carbonate accumulation as indicated by 1 an upward increase in mud phosphate and glauconite 2 the existence of firmground surfaces and 3 a change from a dominantly phototrophic to heterotrophic benthic assemblage an upward trend to lower bulk 13 c organic and 15 n values reflects the eutrophication of surface waters by upwelling of nutrient laden deeper waters resultant primary production propelled higher organic matter flux to the sea floor caused the demise of carbonate producing benthic organisms and promoted burial of carbonate deposits by organic rich mudrocks

local overprints on the global carbonate δ13c signal in devonian carboniferous boundary successions of south china abstract an integrated study of the litho bio and chemostratigraphy of the devonian carboniferous boundary at four sections qilinzhai malanbian gedongguan and long an in south china was undertaken in order to better understand paleoenvironmental changes and controls on 13 c carb variation during the hangenberg crisis sedimentological data record a major regression in the middle siphonodella praesulcata zone which coincided with the hangenberg extinction he in south china our new 13 c carb data document a negative 13 c carb shift near the base of the middle s praesulcata zone which may have been related to the he prior to and during the he respiration of organic matter contributed abundant 12 c enriched dissolved inorganic carbon dic to the restricted nanning carbonate platform resulting in a negative vertical 13 c dic gradient in the study area in the upper s praesulcata zone all four sections exhibit a positive 13 c carb shift suggesting that a vigorous biological pump existed in the aftermath of the latest devonian glaciation however peak 13 c carb values differ markedly among the study sections suggesting that local carbon cycling processes played an important role during the initial post glacial transgression

geochemical evidence for euxinia during the late devonian extinction events in the michigan basin u s a abstract several mass extinction events occurred in the late devonian but the trigger for these events remains elusive in this study geochemical evidence in the late devonian antrim shale michigan basin u s a records episodic euxinia contemporaneous with these extinction events diagnostic changes in iron proxy data and elevated trace metal enrichments correspond to the kellwasser crisis in this study carbon sulfur iron and trace metal geochemistry preserved in the antrim formation validates the establishment and expansion of euxinic conditions associated with the kellwasser crisis and the frasnian famennian boundary the strength of the sequential extraction iron mineral data presented here in concert with trace metal and sulfur isotope proxies provides definitive signatures of euxinia when other data may be more ambiguous in regard to paleoredox conditions during the time of the frasnian famennian boundary extensive sulfide oxidation at the chemocline the result of fe limiting conditions within the basin provides an alternative explanation for the oceanic decline in 34 s so 4 during and following the frasnian famennian event our geochemical evidence indicating the presence of anoxia in the michigan basin is consistent with data from other globally distributed locations euxinia should be considered a key driver for these global extinction events and possibly others such as the hangenberg event in the late devonian

changes in marine nitrogen fixation and denitrification rates during the end devonian mass extinction abstract the end devonian was a critical interval in earth history recording the transition from the early paleozoic greenhouse climate to the late paleozoic icehouse climate a mass extinction at this time the hangenberg extinction eliminated 21 of genera and 16 of families of marine invertebrates although its causes remain poorly understood the marine nitrogen cycle is intimately related to the nutrient status of seawater microbial community composition and the redox condition of the oceans and thus it may provide insights into the mechanism s of the end devonian mass extinction and climate change here we analyzed high stratigraphic resolution bulk sediment nitrogen isotope variation 15 n bulk in three sections from south china 1 long an an isolated carbonate platform section 2 qilinzhai a shallow water carbonate platform section between the yangtze oldland and the youjiang trough and 3 malanbian a shallow water carbonate platform section between the yangtze and cathaysia oldlands and distant from deep water areas our 15 n bulk records show a major perturbation to the marine nitrogen cycle in the middle siphonodella praesulcata zone which is marked by a decline in 15 n bullk from 4 to 0 at long an and from 4 5 to 2 5 at malanbian this perturbation coincided with the main end devonian mass extinction as well as with a negative shift in carbonate carbon isotopes 13 c carb the shift to lower 15 n values suggests locally enhanced nitrogen fixation possibly as a consequence of increased denitrification in response to globally expanded marine anoxia thus these 15 n bulk data are consistent with the hypothesis that intense anoxia was the immediate cause of the end devonian mass extinction the 15 n bulk records generated in this study suggest that there was significant spatial and temporal variation in the nitrogen isotope composition of seawater nitrate in the yangtze marginal sea during the latest devonian this variation may have been related to the complex paleogeography of the south china craton which contained numerous isolated and continent attached platforms and deep water basins that influenced watermass circulation nutrient inventories and primary productivity levels and watermass ventilation and redox conditions

an iodine record of paleoproterozoic surface ocean oxygenation constraining oxygen levels in the early precambrian surface ocean has been a longstanding goal but efforts have been challenged by the availability of suitable proxies here we present a novel approach iodine geochemistry which broadens our perspective by providing constraints on shallow carbonate dominated marine settings iodate io 3 persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals allowing iodine to calcium ratios i ca in shallow carbonates to be used as a paleoredox indicator our data from a series of mesoarchean through paleoproterozoic carbonates deposited under shallow marine conditions reveal a progressive surface ocean oxygenation in the early paleoproterozoic these data seem to indicate that a largely anoxic surface ocean extended throughout the archean until the great oxidation event goe at ca 2 4 ga implying that previous inferences of pre goe oxygen production may reflect oxygen oases transient oxidation events or oxygen levels below those required for io 3 accumulation the data suggest formation and persistence of io 3 and consequently surface ocean oxygen concentrations of at least 1 m during the goe following the initial rise of oxygen carbonate associated iodine in globally extensive carbonate units deposited during the lomagundi positive carbon isotope excursion at ca 2 22 2 1 ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions

precambrian ophiolite belts of southern siberia russia and their metallogeny abstract greenstone and ophiolite belts of different ages in southern siberia are compared the archaean olondo greenstone belt 3 0 2 9 ga within the aldan shield is different from younger ophiolite belts the early proterozoic baikal muya ophiolite belt 1 9 ga containing both komatiites and a complete ophiolitic sequence of the tholeiite series and the late proterozoic early cambrian 0 6 0 52 ga kurtushibin and ilchir ophiolite belts in western and eastern sayan are fairly comparable with young ophiolites and modern crust of oceanic type in the olondo greenstone belt there are komatiite and tholeiite series that are represented by both volcanic rocks and differentiated sills but no dyke complex is present in the baikal muya ophiolite belt there are metavolcanic rocks including komatiites and komatiitic basalts a dyke complex upper cumulate gabbros and lower metaperidotites ophiolites of the kurtushibin belt in the western sayan and ilchir belt in eastern sayan are examples of complete ophiolitic sequences with long and complicated evolution histories including development of a marianite boninite series of primitive island arc type asbestos chromite fe ti oxide polymetallic and gold deposits occurring precambrian ophiolites are briefly described when ophiolites appeared about 2 0 1 9 ga ago cr ni cu and asbestos were concentrated within them the composition of ophiolites and associated ores changed significantly with time a change in upper mantle convection may be the key to the magmatic and metallogenic evolution

upper ocean oxygenation dynamics from i ca ratios during the cenomanian turonian oae 2 global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans leading to increases in organic production the degradation of which causes a further decrease in dissolved oxygen in extreme cases in the geological past this trajectory has led to catastrophic marine oxygen depletion during the so called oceanic anoxic events oaes how the water column oscillated between generally oxic conditions and local global anoxia remains a challenging question exacerbated by a lack of sensitive redox proxies especially for the suboxic window to address this problem we use bulk carbonate i ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the cretaceous oae 2 in general i ca ratios were relatively low preceding and during the oae interval indicating deep suboxic or anoxic waters exchanging directly with near surface waters however individual sites display a wide range of initial values and excursions in i ca through the oae interval reflecting the importance of local controls and suggesting a high spatial variability in redox state both i ca and an earth system model suggest that the northeast proto atlantic had notably higher oxygen levels in the upper water column than the rest of the north atlantic indicating that anoxia was not global during oae 2 and that important regional differences in redox conditions existed a lack of correlation with calcium lithium and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the i ca proxy during oae 2

frequency domain interferometry a high resolution radar technique for studies of atmospheric turbulence a new radar technique suitable for high resolution studies of narrow scattering layers in the atmosphere is introduced the positions of the layers within the radar scattering volume are deduced from the phase differences between radar returns at two distinct transmitter frequencies initial mesospheric observations demonstrate the applicability of the technique and show that the scattering layers are often a few hundred meters thick and widely separated in the mesosphere their vertical motions are generally consistent with vertical doppler velocity estimates

greenhouse to icehouse a biostratigraphic review of latest devonian mississippian glaciations and their global effects the latest devonian mississippian interval records the long term transition from devonian greenhouse conditions into the late palaeozoic ice age lpia this transition was punctuated by three short glaciation events in the latest famennian mid tournaisian and visean stages respectively primary evidence for glaciation is based on diamictite deposits and striated pavements in south america appalachia and africa the aim of this review is to assess the primary biostratigraphic and sedimentological data constraining diamictite deposits through this transition these data are then compared to the wider record of eustasy mass extinction and isotope stratigraphy in the lower palaeolatitudes precise age determinations are vital to integrate high and low palaeolatitude datasets and to understand the glacial control on wider global changes palynological techniques currently provide the best biostratigraphic tool to date these glacial deposits and to correlate the effects of glaciation globally this review highlights a high degree of uncertainty in the known history of early lpia glaciation as much of the primary stratigraphic data are limited and or unpublished future high resolution stratigraphic studies are needed to constrain the history of glaciation both spatially and temporally through the latest devonian and mississippian

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

devonian climate sea level and evolutionary events an introduction the face of planet earth has changed significantly through geological time dynamic processes active today such as plate tectonics and climate change have shaped the earth s surface and impacted biodiversity patterns from the beginning organisms on the other hand have the capacity to significantly alter earth s hydrological and geochemical cycles its atmosphere and climate sediments and even hard rocks deep down under the surface abiotic biotic interactions characterize earth s system history and together with biotic competition and food webs were the main trigger of evolutionary change innovations and biodiversity fluctuations within the palaeozoic the devonian was an especially interesting time interval as it was characterized by the mid paleozoic predator revolution signor brett 1984 brett 2003 and the related nekton revolution klug et al 2010 characterized by the blooms of free swimming cephalopods including the oldest ammonoids and fish groups e g toothed sharks and giant placoderms the rise of more advanced vertebrates including the oldest tetrapods e g blieck et al 2007 2010 niedzwiedzki et al 2010 the most extensive reef complexes of the phanerozoic e g kiessling 2008 and the greening of land by the diversification and spread of land plants including the oldest forests e g stein et al 2012 giesen berry 2013 which resulted in new soil types and changing weathering these major evolutionary trends did not unfold in a long interval of environmental stability but in times of numerous and repeated geologically brief global events that punctuated prolonged periods up to several million years in duration of relative stability termed ecological evolutionary subunits ee subunits boucot 1990 brett baird 1995 brett et al 2009 the bounding events even those of lesser intensity produced major re structuring in local to global ecosystems and are seen as critical drivers of long term evolutionary patterns brett 2012 these linked abiotic and biotic events and extinctions of different magnitude have been summarized by house 1983 1985 2002 walliser 1984 1996 and more recently by becker et al 2012 the devonian event succession is summarized in figure 1 two first order mass extinctions at the frasnian famennian boundary kellwasser crisis and at the end of the devonian hangenberg crisis characterized by the loss of major fossil groups classes and orders and complete ecosystems e g metazoan reefs early forests have to be viewed in the context of a complex global event sequence there are important similarities between discrete pulses phases of the major biotic crises and individual smaller scale events in our understanding second order global events are characterized by sudden extinctions in many groups and ecosystems including the complete disappearance of several widespread and diverse organism groups orders and families examples are the basal emsian atopus event where the planktonic graptolites finally died out the taghanic crisis frasnes events and lower kellwasser event third order global events show globally elevated extinction rates often at lower taxonomic level genera and species but within many clades and in several ecosystems examples are the silurian devonian boundary klonk event and the daleje chot c kacak condroz and annulata events fourth order global extinctions refer to the sudden disappearance of relatively fewer but widespread groups which implies a global not regional trigger this category may include the lochkovian pragian boundary

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

ophiolites as indicators of the geodynamic evolution of the tethyan ocean abstract reviewing the ophiolites of the alpine chain from the western mediterranean to the indian ocean we subdivide them into ten groups of outcrops the geological environment through time the internal composition and history are given for each emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean like crust to their incorporation through accretion to a continent one can distinguish low versus high tholeiitic partial melting fairly or poorly established ridges simple ridge type crust versus complex ridge ensimatic arc and even more complex type long lived versus short lived oceanic crust stages abrupt large scale pure obduction versus progressive collision related obduction or simple progressive juxtaposition to continent the major events through time in the tethyan ophiolitic belts can be listed as follows spreading ridge activity during the jurassic liguria dinaro hellenides lesser caucasus and the cretaceous peri arabic nain sabzewar with a jump of ridge in between repeated change from ridge to ensimatic arc dinaro hellenic lesser caucasus peri arabic west indian during the latest jurassic and cretaceous upper cretaceous tertiary collision related obductions pontic lesser caucasus liguria squeezing out the elevated portions of these basins lack of oceanic magmatism during the cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining mesozoic crust van nain sabzewar zahedan mekran

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

intra oceanic arcs of the paleo asian ocean abstract the paper reviews and integrates geological geochronological geochemical and isotope data from 21 intra oceanic arcs ioa of the paleo asian ocean pao which have been identified in the central asian orogenic belt the world largest accretionary orogeny the data we discuss structural position of intra oceanic arc volcanic rocks in association with back arc terranes and accretionary complexes major periods of intra oceanic arc magmatism and related juvenile crustal growth lithologies of island arc terranes geochemical features and typical ranges of nd isotope values of volcanic rocks four groups of ioas have been recognized neoproterozoic early cambrian early paleozoic middle paleozoic and late paleozoic the neoproterozoic early cambrian or siberian group includes eleven intra oceanic arcs of eastern and western tuva sayan southern siberia russia northern and southwestern mongolia and russian altai the early paleozoic or kazakhstan group includes selety urumbai bozshakol chingiz and baydaulet aqastau arc terranes of the kazakh orocline the middle paleozoic or southern group includes six arc terranes in the tienshan orogen chinese altai east kazakhstan west junggar and southern mongoia only one late paleozoic intra oceanic arc has been reliably identified in the caob bogda in the chinese tienshan probably due to pao shrinking and termination the lithologies of the modern and fossil arcs are similar although the fossil arcs contain more calc alkaline varieties suggesting either their more evolved character or different conditions of magma generation of special importance is identification of back arc basins in old accretionary orogens because boninites may be absent in both modern and fossil ioas the three typical scenarios of back arc formation active margin rifting intra oceanic arc rifting and fore arc rifting were reconstructed in fossil intra oceanic arcs some arcs might be tectonically eroded and or directly subducted into the deep mantle therefore the structural and compositional records of fossil intra oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length life span and crust thickness

the 2 7 2 0 ga volcano sedimentary record of africa india and australia evidence for global and local changes in sea level and continental freeboard abstract the 2 7 2 0 ga volcano sedimentary records of the african indian and australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas concomitant with lowered freeboard carbonate banded iron formation bif platforms characterised the first such event at ca 2 6 2 4 ga africa and australia to 2 7 ga india these earlier globally enhanced sea levels are ascribed to increased mid ocean ridge activity possibly related to breakup of a postulated late archaean southern supercontinent alternatively a transition from global scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the late archaean nelson 1998 earth planet sci lett 158 109 119 both explanations of increased mid ocean ridge activity are compatible with significant early to middle archaean crustal growth armstrong 1981 phil trans r soc london a 301 443 472 with the emergent high freeboard cratons being subjected to aggressive weathering and erosion enhanced continental crustal growth near the archaean proterozoic boundary mclennan and taylor 1982 j geol 90 347 361 related to the development of significant island arc complexes would have resulted in common lowered freeboard enhanced sea level conditions at the passive margins of the southern cratons the diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and or the thermal state of the cratons from ca 2 4 2 2 ga cratons that make up the present day continents of india africa and australia had relatively high continental freeboard and lowered sea levels glacigenic deposits are preserved on the kaapvaal africa singhbhum india and pilbara australia cratons the second broadly defined drowning event at ca 2 15 ga was probably due to post glacial climatic amelioration freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned in india carbonates were more prominent than the siliciclastic sediments including prominent black shales seen in africa and australia

carbon oxygen and strontium isotopes in paleozoic carbonate components an evaluation of original seawater chemistry proxies abstract brachiopods trilobites cements and whole rock matrix material were evaluated for their reliability as proxies of original seawater carbon oxygen and strontium isotope chemistry in this process coeval material was evaluated from individual horizons of formations spanning the permian to cambrian unaltered shells of low mg calcite articulated brachiopods retain original seawater oxygen carbon and strontium isotope compositions as old as late ordovician assessments of older specimens are hampered by a lack of suitable material coeval brachiopod whole rock sets a definitive assessment of cement marine chemistry is difficult due to the paucity of material but despite this caveat cements hold some promise for retaining original carbon and possibly strontium isotope seawater values in contrast the potential of whole rock material as a proxy of original seawater chemistry is quite complex in some but not all instances the carbon isotopic composition of whole rock after detailed scrutiny and evaluation appears to represent an original seawater chemistry signal in a few instances their oxygen isotope compositions reflect original seawater values unlike the stable isotope compositions the strontium isotopes of all studied whole rock material permian to upper cambrian appear not to reflect original seawater chemistry values thus the potential for retaining original seawater isotope chemistry and serving as specific proxies in order of decreasing reliability are 1 unaltered low mg calcite brachiopods c o and sr isotopes 2 pristine marine cements c and sr isotopes and 3 whole rock material c isotopes carbon oxygen and strontium isotope compositions of ordovician trilobites mirror those of coeval unaltered brachiopods cambrian trilobites hold significant promise as an important proxy of original seawater isotope chemistry but further studies are needed to ascertain their full potential the survey of isotopes in some ordovician and cambrian intermediate low mg calcite trilobites demonstrates their potential as an important proxy of original seawater chemistry

a speculation on the structure of the d layer the growth of anti crust at the core mantle boundary through the subduction history of the earth abstract the growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the earth history are missing in order to test a hypothesis that the former crust was subducted to the deep mantle we performed phase assemblage analysis in the systems of mid oceanic ridge basalt morb anorthosite and tonalite trondhjemite granite ttg down to the core mantle boundary cmb conditions results show that all these materials can be subducted to the cmb leading to the development of a compositional layering in the d layer we speculate that there could be five layers of feo enriched melt from partial melting of morb morb crust anorthosite ttg and slab or mantle peridotite in ascending order although the polymorphic transformation of perovskite to post perovskite in mg fe sio3 may explain the seismic discontinuity at the top of the d layer d discontinuity the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the d discontinuity the d layer can be an anti crust made up mostly of ttg anorthosite derived from the former continental crust tectonic style of the anti crust at the cmb is similar to that at the surface at both places chemically distinct layers are density stratified and are also characterized by the processes of accretion magmatism and metasomatism

a new ecological severity ranking of major phanerozoic biodiversity crises a new ecological severity ranking of the major phanerozoic biodiversity crises is proposed in which the capitanian crisis is ranked lesser than the frasnian late devonian but greater than the serpukhovian end mississippian and the famennian end devonian crisis is ranked as equal in ecological impact to the hirnantian end ordovician two new decouplings between taxonomic severity and ecological severity are revealed in these analyses the capitanian and famennian crises in which the ecological impact of the biodiversity loss was markedly different from the magnitude of the biodiversity loss these analyses also reveal that the great devonian interchange gdi invasive species event in the givetian biodiversity crisis may provide an important palaeoecological analog for the study of present day extinction and homogenization in ecosystems produced by modern invasive species

juvenile versus recycled crust in the central asian orogenic belt implications from ocean plate stratigraphy blueschist belts and intra oceanic arcs abstract new or juvenile crust forms and grows mainly through mafic to andesitic magmatism at pacific type or accretionary type convergent margins as well as via tectonic accretion of oceanic and island arc terranes and translation of continental terranes during the last decades the juvenile or recycled nature of crust has been commonly evaluated using whole rock isotope and hf in zircon isotope methods however evidence for the accretionary or pacific type nature of an orogenic belt comes from geological data for example from the presence of accretionary complexes ac intra oceanic arcs ioa oceanic plate stratigraphy units ops and morb oib derived blueschist belts bsb the central asian orogenic belt caob represents the world s largest province of phanerozoic juvenile crustal growth during ca 800 m y between the east european siberian north china and tarim cratons from geological point of view the caob is a typical pacific type belt as it hosts numerous occurrences of accretionary complexes intra oceanic arcs ops units and morb oib derived blueschist belts in spite of its accretionary nature supported by positive whole rock nd isotope characteristics in caob granitoids the hf in zircon isotope data reveal a big portion of recycled crust such a controversy can be explained by presence of accreted microcontinents isotopically mixed igneous reservoirs and by the tectonic erosion of juvenile crust the most probable localities of tectonic erosion in the caob are the middle and southern tienshan and southern transbaikalia because these regions comprise a predominantly recycled crust based on isotope data but the geological data show the presence of intra oceanic arcs blueschist belts and accreted ops with oceanic island basalts oib and tectonically juxtaposed coeval arc granitoids and accretionary units this warrants combination of detailed geological studies with isotopic results as on their own they may not reflect such processes as tectonic erosion of juvenile crust and or arc subduction

organically bound iodine as a bottom water redox proxy preliminary validation and application abstract carbonate associated iodine i ca has been used as a proxy of local upper ocean redox conditions and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of earth history to further explore the utility of iodine as a paleo environmental proxy we present here a new method of extracting organically bound iodine iorg from shale using volumes of samples on the order of tens of milligrams thus offering the potential for high resolution work across thin shale beds the ratio of iorg to total organic carbon i toc in modern surface and subsurface sediments decreases with decreasing bottom water oxygen which may be used to reconstruct paleo redox changes as a proof of concept we evaluate the i toc proxy in holocene sediments from the baltic sea landsort deep iodp 347 and discuss those data within a framework of additional independent redox proxies e g iron speciation and mo the results imply that i toc may be sensitive to hypoxic suboxic conditions complementary to proxies sensitive to more reducing anoxic euxinic conditions then we test the usage of i toc in sediments deposited during late cretaceous cenomanian turonian oceanic anoxic event oae 2 from 94 million years ago ma we generated i toc and iorg records from six oae 2 sections tarfaya morocco furlo central italy demerara rise western equatorial atlantic cape verde basin eastern equatorial atlantic south ferriby uk and kerguelen plateau southern indian ocean which provide a broad spatial coverage generally i toc decreases over the interval recorded by the positive carbon isotope excursion the global signature of oae 2 suggesting an expansion of more reducing bottom water conditions and consistent with independent constraints from iron speciation and redox sensitive trace metals e g mo relatively higher i toc values thus more oxic conditions are recorded at two high latitude sites for oae 2 supporting previous model simulations cgenie that indicated higher bottom water oxygen concentrations in these regions our results also indicate that organic rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory

deciphering the upper famennian hangenberg black shale depositional environments based on multi proxy record abstract presented for the first time in this paper are the results of a detailed multi proxy investigation conducted on the hangenberg black shale hbs from the polish part of the laurussian shelf which provide details about the environmental conditions in existence during deposition of the shale and the role played by anoxic conditions in the mass extinction events that occurred at the end of the devonian times inorganic and organic redox indicators indicate that bottom water redox conditions changed periodically from being mainly anoxic euxinic to oxic or being partially depleted in oxygen u th values above 1 25 ni co values above 7 and v v ni values above 0 8 recorded from the lower part of the hbs all point to anoxic euxinic conditions being present as do high total organic carbon contents toc above 10 and degree of pyritisation dop values around 0 75 however the presence of benthic fauna over the lower part of the hbs attests to opportunistic colonisation of the seafloor during oxic episodes evidence for similar episodes has also been recognised over the middle part of the hbs e g u th values below 1 25 ni co values below 4 v v ni values below 0 8 toc values of 3 to 5 5 and dop values of 0 4 to 0 75 plus the common occurrence of benthic fauna anoxic conditions returned during the deposition of the upper part of the hsb though they were not as well developed as when the lower part of the shale was laid down of interest is the presence of small pyrite framboids and isorenieratene biomarkers in all the analysed samples which suggest that euxinic conditions persisted in the photic zone of the water column a twenty centimetre thick layer of volcanogenic deposits tuffites is present in the middle part of the hbs the geochemical characteristics of which are typical of ocean floor basalts the occurrence of volcanogenic material below and within the hbs together with an absence of calcium carbonate over its upper part the presence of abundant tetrads just above the black shale and a drastic decrease in faunal frequency in the upper part of the hbs imply that volcanism may have caused oceanic acidification or hypercapnia that in turn potentially may have influenced the hangenberg mass extinction event

neoproterozoic 800 ma orogeny in the tuva mongolia massif siberia island arc continent collision at the northeast rodinia margin abstract the tuva mongolia massif is a composite precambrian terrane incorporated into the palaeozoic sayany baikalian belt its neoproterozoic amalgamation history involves early 800 ma and late baikalian 600 550 ma orogenic phases two palaeogeographic elements are identified in the early baikalian stage the gargan microcontinent and the dunzhugur oceanic arc they are represented by the gargan glyba block and the island arc ophiolites overthrusting it the gargan glyba is a two layer platform comprising an early precambrian crystalline basement and a neoproterozoic passive margin sedimentary cover the upper part comprises olistostromes deposited in a foreland basin during the early baikalian orogeny the dunzhugur arc ophiolite form klippen fringing the gargan glyba and show a comprehensive oceanic arc ophiolite succession the dunzhugur arc faced the microcontinent as shown by the occurrence of forearc complexes the arc continent collision followed a pattern similar to phanerozoic collisions when the marginal basin lithosphere had been completely subducted the microcontinental edge partially underthrust the arc and the forearc ophiolite overrode it continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin owing to the lithospheric break a new subduction zone inclined beneath the gargan microcontinent emerged initial melts of the newly formed continental arc are represented by tonalites intruded into the gargan microcontinent basement and its cover and into the ophiolite nappe the tonalite rb sr mineral isochron age is 812 18 ma which is similar to a u pb zircon age of 785 11 ma a period of tonalite magmatism in meso cenozoic orogenic belts is recognized some 1 10 m y after the collision accordingly the dunzhugur island arc gargan microcontinent collision is conventionally dated at around 800 ma it is highly probable that in the early neoproterozoic the gargan continental block was part of the southern in modern coordinates margin of the siberia craton it is suggested that a chain of precambrian massifs represents an elongate block separated from siberia in the late neoproterozoic the tuva mongolia massif is situated in the northwest part of this chain these events occurred on the ne neoproterozoic margin of rodinia facing the world ocean

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

geochemical evolution of archean shales from south africa i the swaziland and pongola supergroups abstract the fig tree group and moodies group part of the swaziland supergroup are exposed in the barberton mountain land and are 3 3 3 5 ga old shales from the fig tree group are characterized by variable rare earth element ree patterns with lan ybn 5 5 1 8 95 confidence and eu eu 0 97 0 18 shales from the moodies group have more fractionated ree patterns lan ybn 8 7 2 0 and less variable eu anomalies eu eu 0 92 0 02 both groups are characterized by very high abundances of cr and ni modelling of those trace elements which are relatively unfractionated during sedimentary processes ree th sc is consistent with the derivation of these sedimentary rocks from a mixture of the archean bimodal igneous suite mafic volcanics tonalites trondhjemites felsic volcanics with the moodies group requiring a larger percentage of the felsic end member the data for shales from the swaziland supergroup support models suggesting that the archean exposed crust was of more mafic composition than the post archean upper crust and that intracrustal melting was relatively unimportant in differentiating the archean crust the pongola supergroup is younger than the swaziland supergroup but older than 2 9 ga shales and mudstones from the nsuze and mozaan groups show ree characteristics intermediate between the older shales of the swaziland supergroup and typical post archean shales one quartzite sample from the witwatersrand supergroup ca 2 6 ga has a ree pattern virtually parallel to average post archean shales but with much lower abundances the change from typical archean to post archean sedimentary trace element patterns has been documented in early proterozoic successions from canada and australia this change has been related to a short interval of intense growth and chemical differentiation of the continental crust during the late archean 2 7 2 5 ga in southern africa this change appears to have taken place much earlier 3 2 2 9 ga but can be related to similar processes thus the period of crustal evolution related to the archean proterozoic boundary while virtually episodic regionally occurred over a protracted period world wide during the period ca 3 2 2 5 ga

effects of interaction between ultramafic tectonite and mafic magma on nd pb sr isotopic systems in the neoproterozoic chaya massif baikal muya ophiolite belt sm nd rb sr and u pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the chaya massif baikal muya ophiolite belt eastern siberia in order to determine the relationship between mantle ultramafic and crustal mafic sections geological relations in the chaya massif indicate that the mafic magmas were emplaced into and interacted with older solid peridotite hand picked acid leached primary rock forming and accessory minerals olivine orthopyroxene clinopyroxene and plagioclase from the two harzburgite samples show coherent behavior and yield147sm 144nd 143nd 144nd and238u 204pb 206pb 204pb mineral isochrons corresponding to ages of 640 58 ma 95 confidence level and 620 71 ma respectively these values are indistinguishable from the crystallization age of the chaya mafic units of 627 25 ma a weighted average of internal isochron sm nd ages of four mafic cumulates the rb sr and sm nd isotopic systems in the harzburgite whole rock samples were disturbed by hydrothermal alteration these alteration related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution on the basis of initial sr and nd values ultramafic and mafic rocks of the chaya massif can be divided into two groups 1 harzburgites and the lower mafic unit gabbronorites withend 6 6 to 7 1 andesr 11 to 16 and 2 websterite of the lower unit and gabbronorites of the upper mafic unit end 4 6 to 6 1 andesr 8 to 9 initial pb isotopic ratios are identical in all rocks studied with mean values of206pb 204pb 16 994 0 023 and207pb 204pb 15 363 0 015 the similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence the isotopic data agree with a hypothesized formation of the chaya massif in a suprasubduction zone environment

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

shallow water redox conditions from the permian triassic boundary microbialite the rare earth element and iodine geochemistry of carbonates from turkey and south china redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments but their interpretation may not always be straightforward to evaluate the inherent complexities rare earths and yttrium rey iodine and major element concentrations are determined in two carbonate sections spanning the permian triassic p tr transition in demirtas turkey and cili south china we use major oxides to identify non seawater rey sources such as siliciclastics fe oxides phosphates and diagenetic fluids additionally we employ y ho ratio la anomaly and light rare earth element depletion to identify which samples preserve a seawater like reysn distribution in contrast to past interpretations we find that the p tr boundary microbialites in both sections contain rey signatures indicative of deposition in an oxic environment these boundary microbialites have their base at the extinction horizon and are widespread within the tethyan region in the cili section the underlying permian limestone also preserves a seawater like rey signature with a negative ce anomaly this indicates that the water column was oxygenated both before and after the extinction event the permian limestone in the demirtas section does not preserve a seawater like reysn distribution so the absence of a ce anomaly cannot be used to distinguish prevailing redox conditions during deposition however in these samples we find the presence of a diverse permian benthic community sufficient to identify deposition in an oxic environment the geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in south china the microbialite has been proposed as a disaster facies in part resulting from the exclusion of grazers although anoxia is one of the suggested mechanisms for the exclusion of grazers we find that it is not supported by geochemical and biotic evidence

fine scale la icp ms study of redox oscillations and reey cycling during the latest devonian hangenberg crisis moravian karst czech republic abstract trace elementsand reey distribution of thinly laminated limestone laminite and underlying pre event calciturbidite siphonodella praesulcata conodont zone deposited in association with the hangenberg black shale event were analyzed using laser ablation inductively coupled plasma mass spectrometry these deposits were also analyzed by carbonate petrography and scanning electron microscopy our study documents the origin of authigenic microbial limestones during the latest devonian hangenberg extinction event when unusual oceanic conditions appear to have enhanced precipitation of calcite within microbial carbonates detrital contamination of the calciturbidite and laminated limestone is very low as demonstrated by low contents of al zr ti and th these deposits display only limited degree of early diagenetic overprint of limestones and some of them fall within or very close to the seawater field elevated u and mo enrichments and covariation patterns reflect the episodic establishment of oxic to euxinic bottom condition during deposition of the laminite the oxic conditions that existed during accumulation of the underlying calciturbidite were likely the result of turbidity current introduction of oxygen into the bottom environment ree contents of both the laminite and calciturbidite are nearly ten times higher than those of most modern carbonates and similar to devonian microbialites from the canning basin calciturbidite display a prominent mreen bulge shale normalized ree patterns of the overlying laminite are similar to those of modern seawater and devonian microbialites from the canning basin lree depletion and progressive enrichments in heavier ree reflecting the dominance of authigenic carbonate phases minor contamination is bound to apatites siliciclastic minerals pyrites and organic matter the element content of authigenic phases benefited from fe and mn oxyhydroxide redox cycling negative ce and positive eu anomalies low y ho ratios and very low al al fe mn ratios suggest an influence of diluted high temperature hydrothermal fluids perhaps related to latest devonian rift magmatism in the eastern part of the rhenohercynian zone the proliferation of anachronistic facies following the devonian carboniferous d c extinction event recognized in moravia and traced worldwide seems to share some features in common with anachronistic carbonate sequences documented in the aftermath of the end permian mass extinction

geochemical and nd isotopic systematics of granites from the arunta inlier central australia implications for proterozoic crustal evolution geochemical and sm nd isotopic results are reported for granites from the proterozoic arunta inlier of central australia these combined with new geochronological data for the granites zhao and bennett 1995 allow important constraints to be placed on proterozoic tectonic and crustal evolution in central australia granites from the arunta inlier can be divided into three major geochemical groups a calcalkaline trondhjemitic group cat a high heat production group hhp and a volumetrically significant main group the cat group which occurs only in the southern margin of the inlier is characterised by high na2o nak sr krb and sry and relatively low k2o rb rbsr th u ree nb and y analogous to calc alkaline suites occurring in modern convergent plate margins the hhp group which occurs mainly in the interior of the arunta inlier and is spatially associated with the main group is characterised by high k rb th u rbsr and rbzr and relatively low sr ba nak krb barb mgo cr and ni the main group which occurs throughout the arunta inlier is geochemically intermediate between the cat and hhp groups it is geochemically analogous to the 1880 1850 ma old barramundi igneous association recognised in other proterozoic terrains of central australia and can be further subdivided into four age subgroups 1820 1780 1750 1650 and 1615 1600 ma respectively all three groups show similar negative nb anomalies on the trace element normalized diagrams despite the geochemical diversity of the arunta granites no correlations between the nd isotopic and geochemical signatures are observed the ranges of initial nd values and nd depleted mantle model ages tnddm for the three groups of granites overlap with each other overall there are two groups of tnddm ages the most common ranging from 2 33 to 1 96 ga and the other from 1 83 to 1 72 ga although initial nd values of the granites show large variations there is a general trend for the nd values to increase i e become less negative with decreasing crystallization ages the large geochemical and isotopic variations of the arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform 2 3 2 1 ga old barramundi type underplate source for the origin of the granites the combined geochemical and nd isotopic data suggest the sources of the granites contain at least two main components an palaeoproterozoic mantle derived component and an older crustal component the older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism the increasing nd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island arc materials being incorporated in the subducted sediments alternatively a juvenile mantle derived component may have been added to the source region of the younger granites it is apparent that the nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two stage protolith models can as yet satisfactorily explain the observations the petrogenesis of the three geochemical groups of granites can be interpreted in a plate tectonics scenario it is considered that the cat group was formed by fractionation of arc type magmas and or partial melting of arc related intrusions or underplates in a subduction related continental margin setting whilst the formation of the main group may involve partial melting of fractionated and or modified arc related underplates but not necessarily in a subduction environment the younger hhp group was probably generated by remelting of the coexisting main group during subsequent tectonothermal events

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

iodine to calcium ratios in marine carbonate as a paleo redox proxy during oceanic anoxic events periods of globally distributed extreme oxygen depletion so called oceanic anoxic events oaes have been recognized in the mesozoic geological record and appear to be characteristic of episodes of extreme warmth here we explore the application of iodine calcium ratios i ca in marine carbonate as a new geochemical proxy to constrain seawater redox change and provide additional insights into the response of ocean chemistry to ancient climatic warming iodine has long been known as a redox sensitive and biophilic element mainly present as iodate and iodide in seawater iodate converting to iodide under anoxic conditions laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure likely substituting for the co 3 2 ion a fall in the i ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early toarcian and cenomanian turonian oaes is interpreted both as a response to a decrease in the iodate iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic matter burial the results suggest that i ca ratios in carbonates may be used to monitor seawater oxidation levels throughout earth history

enhanced nitrogen fixation in the immediate aftermath of the latest permian marine mass extinction the carbon 13 c org and nitrogen 15 n isotopic compositions of bulk organic matter were analyzed in two high resolution permian triassic transitional sections containing microbialite in south china the results from these shallow marine sections show that an abrupt negative shift in 15 n from 3 to 0 occurred immediately after the latest permian mass extinction lpe in both sections concurrent with a distinct negative shift in 13 c org the persistently low values of 15 n suggest that following the lpe microbial nitrogen fixation became the main source of biologically available nitrogen in the nanpanjiang basin and perhaps over a broader region of the eastern paleotethys ocean enhanced n fixation is probably indicative of the prevalence of stratified anoxic water masses characterized by intense denitrification and or anaerobic ammonium oxidation at the time perturbation of the marine nitrogen cycle might have contributed to high temperatures following the main marine mass extinction through the release of the greenhouse gas n 2 o the sharp declines in 15 n and 13 c org may be ascribed to an abrupt change in shallow water microbial communities which differed in composition from contemporaneous deep water communities

climate controlled mass extinctions facies and sea level changes around the devonian carboniferous boundary in the eastern anti atlas se morocco abstract detailed sedimentological and biostratigraphical studies of successions in the eastern anti atlas se morocco focused on changing depositional environments and reconstructions of sea level changes during the late famennian to middle tournaisian the investigations cover a transect from the central maider to the easternmost and southern tafilalt regions near the moroccan algerian border the moroccan data are compared with the standard succession of the german rhenish massif and with the regional and global stratigraphic record the results testify to high frequency sea level changes including two global transgressive and anoxic events the main hangenberg event hangenberg black shale level at the top of the wocklumeria zone upper devonian vi e and the lower alum shale event at the lower middle tournaisian lower carboniferous i ii boundary regionally both anoxic intervals coincided with mass extinctions followed by rapid radiation especially of pelagic biota a significant shallowing and regression occurred just below the devonian carboniferous boundary at the northern margin of gondwana and caused the sudden n nw progradation of a large deltaic complex over 300 500 km thick siliciclastic successions filled fast subsiding synsedimentary troughs of the maider and amessoui syncline southern tafilalt correlation with the glacial phases of southern gondwana south america south africa indicates a glacial eustatic origin of sea level changes and a major sea level fall in the scale of 100 m or more coinciding with the main regressive phase of the extended hangenberg crisis interval the new results are in accord with the hypothesis of an episode of cool humid climate at the end of the devonian that allowed contemporaneous glacial buildup in high latitudes and extensive clastic discharge in the middle latitudes of nw africa

evidence for local and global redox conditions at an early ordovician tremadocian mass extinction abstract profound changes in environmental conditions particularly atmospheric oxygen levels are thought to be important drivers of several major biotic events e g mass extinctions and diversifications the early paleozoic represents a key interval in the oxygenation of the ocean atmosphere system and evolution of the biosphere global proxies e g carbon 13c and sulfur 34s isotopes are used to diagnose potential changes in oxygenation and infer causes of environmental change and biotic turnover the cambrian ordovician contains several trilobite extinctions some are apparently local but others are globally correlative that are attributed to anoxia based on coeval positive 13c and 34s excursions these extinction and excursion events have yet to be coupled with more recently developed proxies thought to be more reflective of local redox conditions in the water column e g i ca to confirm whether these extinctions were associated with oxygen crises over a regional or global scale here we examine an early ordovician tremadocian stage extinction event previously interpreted to reflect a continuation of recurrent early paleozoic anoxic events that expanded into nearshore environments 13c 34s and i ca trends were measured from three sections in the great basin region to test whether i ca trends support the notion that anoxia was locally present in the water column along the laurentian margin evidence for anoxia is based on coincident but not always synchronous positive 13c and 34s excursions mainly from carbonate associated sulfate and less so from pyrite data a 30 extinction of standing generic diversity and near zero i ca values although evidence for local water column anoxia from the i ca proxy broadly agrees with intervals of global anoxia inferred from 13c and 34s trends a more complex picture is evident where spatially and temporally variable local trends are superimposed on time averaged global trends stratigraphic sections from the distal and deeper part of the basin shingle pass and meiklejohn peak preserve synchronous global 13c and 34s and water column i ca evidence for anoxia but not at the more proximal section ibex ut although geochemical and paleontological evidence point toward anoxia as the driver of this early ordovician extinction event differences between i ca and 13c 34s signals suggest regional variation in the timing extent and persistence of anoxia

palaeostress reconstructions and geodynamics of the baikal region central asia part i palaeozoic and mesozoic pre rift evolution abstract this paper presents the first palaeostress results obtained for the basement of the baikal rift system in southern siberia russia large scale structural analysis and palaeostress reconstructions show that the palaeozoic mesozoic kinematic history precursor of the baikal cenozoic rifting is characterized by the succession of six regional palaeostress stages stress inversion of fault slip data and earthquake focal mechanisms is performed using an improved right dieder method followed by rotational optimization d delvaux tensor program the results are interpreted in the light of recent developments in the investigation of regional intraplate stress field and used as additional constraints for palaeogeodynamic reconstruction of central asia after the final palaeozoic closure of the palaeo asian ocean on the southern margin of the siberian platform the marginal suture with the sayan baikal caledonian belt was repeatedly and preferentially reactivated during the subsequent palaeozoic and mesozoic history this suture zone also controlled the opening of the baikal rift system in the cenozoic the progressive closure of the palaeo asian and mongol okhotsk oceans generated successive continental collisions which were recorded in the baikal area by brittle ductile and brittle deformations the first two palaeostress stages correspond to the successive collage of precambrian microcontinents and caledonian terranes along the southern margin of the siberian platform 1 late cambrian early ordovician n s compression and 2 late silurian early devonian nw se compression the next two stages are related to the remote effects of the complex evolution of the western palaeo asian ocean southwest of the siberian continent 3 late devonian early carboniferous n s compression recorded only in the altai region and 4 late carboniferous early permian e w compression recorded both in the altai and baikal regions the last stages are the consequences of the mongol okhotsk oceanic closure 5 late permian triassic nw se extension with development of cordilleran type metamorphic core complexes and volcanism along the active margin of the mongol okhotsk ocean 6 initial development of early middle jurassic 10 15 km wide molassic basins in trans baikal and large foredeeps along the southern margin of the siberian platform in probable extensional context but for which no palaeostress data are available and 7 final closure of the mongol okhotsk ocean in the cretaceous this last event results from the collision between the mongol china and the siberian plates and is evidenced by the inversion of the middle mesozoic basins and by late jurassic early cretaceous coal bearing sedimentation in trans baikal this long tectonic history yielded a highly heterogeneous basement in the baikal area precursor of the cenozoic rifting

spread f and the structure of equatorial ionization depletions in the southern anomaly region combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator in chile as part of the miseta campaign in fall 1994 a ccd enhanced all sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11 s concurrently a digisonde collocated with the photometer monitored the appearance of spread f in between the ionograms the sounder operated as a doppler interferometer identifying the locations of f layer irregularities associated with the spread f they were found to lie inside the emission depletion regions the hf sounder requiring orthogonality with the field aligned f layer irregularities to generate the spread structure tracked these irregularities inside the emission depletion bands as they drifted eastward ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site model calculations indicate that the sounder rays encounter orthogonality with the earth s magnetic field within the depletion bubble southward from the site consistent with the local dip angle the combination of optical images with hf radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a doppler interferometer the digisonde measurements and ray tracing show for the first time that the spread f signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion

perturbation of the marine nitrogen cycle during the late ordovician glaciation and mass extinction abstract the late ordovician was a critical interval in geologic history during which both the biosphere and marine environments underwent severe perturbations including one of the big five phanerozoic mass extinctions and the massive but short term 0 5 myr hirnantian glaciation the onset and termination of the hirnantian glaciation have been widely accepted as the triggers for the two extinction pulses that comprise the late ordovician biocrisis but the mechanisms that caused the hirnantian glaciation itself remain poorly known here we analyze the nitrogen isotope composition 15 n of two sections in south china wangjiawan and nanbazi in order to better understand nitrogen cycle perturbations in the late ordovician ocean and their relationship to contemporaneous climatic and biogeochemical changes low 15 n 1 in the upper katian and lower rhuddanian of both sections suggests intensive i e near quantitative denitrification and thus nitrogen fixation as the main source of biologically available nitrogen for primary producers a positive 15 n excursion in both sections during the hirnantian indicates weaker i e non quantitative denitrification possibly as a result of more vigorous thermohaline circulation and improved ocean ventilation weaker denitrification would have reduced the flux of n 2 o an intermediate product of denitrification to the atmosphere n 2 o is a potent greenhouse gas and a major decline in its production would have led to cooler climatic conditions and ultimately the hirnantian glaciation a global survey of published nitrogen isotope records suggests that similar processes operated broadly within the late ordovician global ocean

biogeochemical controls on black shale deposition during the frasnian famennian biotic crisis in the illinois and appalachian basins usa inferred from stable isotopes of nitrogen and carbon abstract the frasnian famennian biotic crisis is marked by two distinct intervals known as the lower and upper kellwasser events kwes that in many locations are associated with deposition of organic rich shales sedimentary nitrogen and carbon isotopes offer insight into the biogeochemical processing of nutrients production of organic matter and palaeoceanographic conditions during the kwes here we present new bulk nitrogen 15nbulk and organic carbon 13corg isotope data from the late devonian appalachian and illinois basins ab and ib with a focus on intervals encompassing the kwes black shales from the ib and ab including the kwes are 15n depleted 1 0 2 0 and have significantly lower 15nbulk than interbedded grey shales 0 5 4 0 a trend consistent with many instances of black shale deposition in the phanerozoic organic carbon isotopes exhibit the broad positive excursions 3 5 from background that are typical of the kwes globally superimposed over these positive excursions in 13corg are sharp decreases of up to 3 0 within the black shale beds to as low as 30 5 the pattern of 15nbulk and 13corg values suggests that the depth of the chemocline and the degree of water column stratification exert a primary control on both 15nbulk and 13corg during black shale deposition in the context of the frasnian famennian biotic crisis the oscillating redox state and changing temperatures would have likely placed extreme stress on organisms within the marine environment of the ab and ib and may potentially have been a contributing factor to diversity loss over this time period

experimental observations of marine iodide oxidation using a novel sparge interface mc icp ms technique abstract iodine speciation is used as a redox indicator and water mass tracer however the kinetics and mechanisms of oxidation of the reduced iodine species iodide i in seawater remain largely unresolved here we provide constraints on i oxidation rates from seawater incubation experiments using a sensitive radiotracer incubation approach with high precision mass spectrometry samples are inoculated with iodine 129 as i incubated under controlled conditions then oxidized and reduced iodine species are isolated using liquid chromatographic separation samples are subsequently aspirated using a novel sparging interface and the 129i 127i ratio of both iodine species is determined via multi collector inductively coupled plasma mass spectrometry this highly sensitive analytical approach is optimized for estimating the predicted slow rates of iodine redox transformations in small volume seawater samples at marine relevant total iodine concentrations application of this method to coastal samples from martha s vineyard sound usa evidences the heretofore elusive oxidation of i under marine analogue conditions our results demonstrate an increase in the 129i 127i of aqueous oxidized iodine species in unamended seawater most likely due to formation of hypoiodous acid and then dissolved organic iodine at rates of 118 189 nm yr these rates are comparable to those inferred from previous open ocean mass balance approaches but have the benefit of directly linking iodine speciation changes to in situ i oxidation though our treatment experiments do not definitively isolate the oxidation mechanism they provide preliminary insight into important roles for both abiotic and biotic factors as well as iodine transformations between dissolved particulate and gaseous forms ultimately the derived rates and novel experimental technique have potential to improve marine iodine cycling models and provide constraints on the importance of in situ processes in setting patterns of iodine speciation

episodic ages of greenstones a key to mantle dynamics two greenstone age minima at 2 5 2 2 ga and 1 65 1 35 ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction from the age distributions of greenstones and juvenile continental crust earth history can be divided into three stages i 2 8 ga when greenstones and microcontinents formed and collided continuously although probably not forming a supercontinent until about 3 ga ii 2 8 1 3 ga where a clear episodisity is apparent in greenstone eruption and collision ages and where two greenstone age minima coincide in age with supercontinents and iii 1 3 ga when greenstones and continents appear to have formed and collided continuously perhaps in response to significant overlap in the times of dispersal and assembly of supercontinents greenstone age peaks 2 7 1 9 and 1 3 ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs through the 660 km discontinuity to the d layer initiating an episode of major mantle plume activity continuous production and preservation of greenstones in the last 1 3 gy is consistent with seismic tomography data which suggest a steady sinking of slabs into the lower mantle

crustal and lithospheric structure of inactive volcanic arc terrains in fiji abstract the crustal and lithospheric velocity structure of the fiji area may place important constraints on the evolution of island arcs in the region here we obtain receiver functions from one permanent seismic station and three temporary networks on the fiji platform the lau ridge and other small islands to develop point estimates of crustal thickness under the networks we obtain preliminary estimates of crustal thickness and poisson s ratio from h stacking of the receiver functions we then perform an inversion for shear wave velocity structure using a joint inversion of receiver functions and rayleigh wave dispersion curves from ambient seismic noise and teleseismic events the joint inversion results show an average crustal thickness of 30 km beneath viti levu the largest fiji island suggesting an extensive period of arc crustal formation and thickening in contrast we find shallower moho depths averaging 22 km beneath the smaller fiji islands and the lau ridge the limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal fiji islands which may be due to the early rifting event forming the lau basin together with the crustal thinning of this area as well and to thermal lithospheric erosion processes associated with adjacent back arc spreading a relatively slow velocity middle crustal layer roughly of 11 16 km thickness exists throughout fiji suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation

tonga ridge and lau basin crustal structure from seismic refraction data 1 the crustal structure across the tonga lau arc back arc system from the lau ridge to the pacific plate 178 170 w 18 19 s is modeled using data from an 840 km long air gun refraction line over 19 ocean bottom seismometers and one land station the data reveal that the pacific plate crust is 5 5 km thick with a velocity structure similar to that found at the present day east pacific rise epr beneath tonga ridge an intermediate velocity layer 6 7 km s is up to 7 5 km thick and has a velocity depth distribution similar to andesitic rocks found in continental crust the crust is abnormally thin 4 km at the boundary between the tonga ridge and the lau basin at the east end of lau basin the crust is 5 5 6 5 km thick and resembles crust formed at the epr except for a thicker sheeted dike section 2 3 km and thinner lower crust 2 km the lau basin crust thickens to 7 8 km near the central lau spreading center clsc mostly through thickening of the lower crust the crust thickens again to 8 5 9 5 km at 50 km west of the clsc mostly through thickening of the midcrust in the thick westernmost section the crustal structure is uniform even though one part of this section formed through extension of arc type crust while the rest was created at an oceanic spreading center the relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center

nitrogen cycling during the cretaceous cenomanian turonian oceanic anoxic event ii low 15n values ranging from 1 2 to 3 9 and atomic carbon nitrogen c n ratios of 25 50 are characteristic of black shales deposited during the cenomanian turonian boundary interval and oceanic anoxic event ii oae ii an observed antithetic relationship between c n ratios and 15n could suggest a predominance of terrestrially derived organic matter or a diagenetic control on 15n variability shifting bulk 15n values lower however hydrogen indices hi generally 450 and a positive correlation of hi with c n mitigates against a significant terrestrial organic matter fraction high c n values are likely the result preferential degradation of labile n rich compounds during early diagenesis and loss of n as ammonium from sediments through time a hypothetical model that considers the degradation of a 15n enriched labile protein fraction yields only small 1 2 negative shifts in 15n however 15n depletion during diagenesis is contrary to normal isotope kinetics which should result in 15n enrichment of bulk organic matter therefore we conclude that the bulk 15n values in this study reflect primary changes in the nitrogen cycle the 15n data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient nitrogen poor phosphorus replete waters during oae ii and from the mid cenomanian to santonian at demerara rise the low 15n values which are significantly lower than bulk 15n values in modern regions where nitrogen fixation is known to be important probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation during the peak of oae ii a marked shift to lower 15n values is observed this shift possibly reflects greater utilization of 15n depleted ammonium during a chemocline upward excursion cue dominance of low 15n values from other periods of more widespread marine anoxia is likely the result of similar processes

i ca evidence for upper ocean deoxygenation during the petm anthropogenic global warming affects marine ecosystems in complex ways and declining ocean oxygenation is a growing concern forecasting the geographical and bathymetric extent rate and intensity of future deoxygenation and its effects on oceanic biota however remains highly challenging because of the complex feedbacks in the earth ocean biota system information on past global warming events such as the paleocene eocene thermal maximum petm 55 5 ma a potential analog for present and future global warming may help in such forecasting documenting past ocean deoxygenation however is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column as yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones omzs we apply a novel proxy for paleoredox conditions the iodine to calcium ratio i ca in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans and compared our reconstruction with modeled oxygen levels the reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the atlantic indian oceans and possibly the pacific ocean as well during the petm due to vertical and potentially lateral expansion of omzs

local and global perspectives on carbon and nitrogen cycling during the hirnantian glaciation abstract the impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the late ordovician hirnantian glaciation three sections were studied two from nevada one proximal and one distal and one from the yukon the opportunity to compare 13 c carb 13 c org and 13 c grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater 13 c values at each stratigraphic section using the best preserved sedimentary components from the two nevada sections a lowstand sedimentary gradient of 4 was found between platform and ocean settings during the hirnantian glaciation gradients of similar magnitude have been reported from hirnantian platforms in the canadian arctic and baltic basins taking the outboard sections as the most representative of the ocean carbon pool a mean positive 13 c shift of just 2 7 0 4 is calculated which is significantly smaller than the 5 7 positive shifts recorded in epeiric platform settings the discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas it is hypothesized that increased carbonate weathering caused 13 c values of local carbon weathering fluxes to increase which in turn caused seawater 13 c values to increase in basin proximal settings in addition nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings driving sedimentary 13 c values even higher much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive a 15 n profile reconstructed from deposits of the distal vinini creek section records a 1 5 positive shift through the hirnantian glacial interval interpreted as a major late ordovician upwelling zone high organic carbon 1 wt and high hydrogen indices 400 600 make the vinini creek section a suitable candidate for the preservation of original sedimentary 15 n records to explain the results a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers it is hypothesized that in greenhouse climates the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean s expansive oxygen minimum zones this caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone yielding sedimentary 15 n values between 0 and 2 by contrast the cooler icehouse climate of the hirnantian led to increased ocean ventilation greater partitioning of atmospheric oxygen into downwelling surface waters and oxygen minimum zone shrinkage as a result fixed nitrogen levels climbed in the ocean s abyssal depths due to the lower denitrification rates this in turn allowed nitrogen with high 15 n values to recycle back into the photic zone driving sedimentary 15 n values higher during the hirnantian glaciation

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

first frequency domain interferometry observations of large scale vertical motion in the atmosphere the spatiotemporal distribution of the vertical velocity at synoptic and subsynoptic scales is key to the patterns of weather and climate on earth on these scales the vertical velocity is on the order of one to a few centimeters per second typically about three orders of magnitude smaller than typical horizontal wind velocities because of the smallness of large scale vertical velocities relative to typical horizontal velocities a direct observation of the large scale vertical air velocity is extremely difficult in a case study on observational material obtained during a 68 h experiment using the sousy very high frequency vhf radar in the harz mountains in germany the authors present the first intercomparison between three different sources of physical information that can provide large scale vertical wind velocities i the doppler shifts observed with a vertically pointing vhf radar ii the rates of change of the altitudes of refractiveindex discontinuities as identified with frequency domain interferometry fdi which is still a relatively unexplored technique in meteorology and iii the output of a regional numerical weather prediction model nwpm which has been set up to model the meteorological situation during the observational period there are several phenomena that have been known to possibly cause significant biases in mean vertical velocities retrieved from the doppler shifts measured with vertically pointing clear air vhf radars i stationary or nonstationary gravity waves with vertical velocity amplitudes up to the order o f1ms 21 ii stationary or horizontally advected tilted refractive index discontinuities that are aspect sensitive in the vhf regime and iii a correlation between the radar reflectivity fluctuations and the vertical velocity fluctuations within a vertically propagating gravity wave on the basis of an intercomparison between the vertical velocities retrieved from i standard doppler vhf radar observations ii vhf fdi observations and iii the nwpm output the authors present first evidence that under ideal conditions vhf fdi can be used to directly monitor large scale vertical motion

extensive marine anoxia associated with the late devonian hangenberg crisis abstract the global hangenberg crisis near the devonian carboniferous boundary dcb represents one of the major phanerozoic mass extinction events which shaped the roots of modern vertebrate biodiversity marine anoxia has been cited as the proximate kill mechanism for this event however the detailed timing duration and extent of global marine redox chemistry changes across this critical interval remain controversial because most of the studies to date only constrain changes in local or regional redox chemistry thus opinions on the significance of anoxia as a kill mechanism are variable from anoxia being a primary driver to being relatively unimportant in this study we explore the evolution of global marine redox chemistry using u isotopes of marine limestones the 238 u trends at long an section in south china document systematic oscillations with three negative shifts punctuated by two positive events in between the magnitude of the 238 u oscillations implies that the sediments do not record contemporaneous seawater with a constant offset at all times the lack of covariation between 238 u data and diagenetic indicators e g mn and sr contents mn sr ratio 18 o suggests that the 238 u trends are not produced by the same post depositional diagenetic processes instead trace metal enrichments suggest that more reducing conditions prevailed during the deposition of the two positive events we present plausible model scenarios that fit the observed 238 u trends in the context of redox sensitive trace metal data suggesting marine anoxia expanded in the latest devonian oceans to cover 5 of the continental shelf seafloor area the rapid expansion of marine anoxia coincident with the onset of the hangenberg crisis supports marine anoxia as an important kill mechanism biogeochemical modeling of the coupled c p u cycles suggests that intensified continental weathering for example assisted by the spread of seed plants with deeper root systems at this time could have triggered expansion of marine anoxia and other global changes e g positive excursion in 13 ccarb and decrease in sea surface temperature in the latest devonian the anoxic event is inferred to have been transient as climatic cooling would have reduced weathering fluxes

the evolution of pre mesozoic ophiolites in northern eurasia a comparative review abstract in this paper we will compare the riphean late proterozoic ophiolites of the north baikalian region and polar urals of the bohemian and saxonian granulite massifs the paleozoic ophiolites of west and east sayan urals stara planina and polish sudetes etc in these examples the similarity of the main stages of the tectonic evolution of the paleoasiatic and iapetus oceans is shown it is clear that the majority of them had a multistage evolution in oceanic back arc or ensimatic arc condition further the general scheme of this evolution is shown

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

a high resolution multiproxy stratigraphic analysis of the devonian carboniferous boundary sections in the moravian karst czech republic and a correlation with the carnic alps austria a multidisciplinary correlation of the devonian carboniferous d c boundary sections from the moravian karst czech republic and the carnic alps austria based on conodont and foraminifer biostratigraphy microfacies analysis field gamma ray spectroscopy grs carbon isotopes and element geochemistry is presented in this paper the study is focused on the interval from the middle palmatolepis gracilis expansa zone late famennian to the siphonodella sandbergi zone early tournaisian in lesni lom moravian karst a positive 13 c excursion in the bisphatodus costatus protognathodus kockeli interregnum from a distinct laminated carbonate horizon is correlated with a carbon isotope excursion from the grune schneid section of the carnic alps and is interpreted as the equivalent of the hangenberg black shales and a local expression of the global hangenberg event sensu stricto higher up at both sections a significant increase in the terrigenous input which is inferred from the grs signal and elevated concentrations of terrigenous elements si ti zr rb al etc provides another correlation tieline and is interpreted as the equivalent of the hangenberg sandstone both horizons are discussed in terms of relative sea level fluctuations and palaeoceanographic changes recent studies show that conodont biostratigraphy is facing serious problems associated with the taxonomy of the first siphonodellids their dependence on facies and discontinuous occurrences of protognathodids at the d c boundary therefore the correlative potential of geochemical and petrophysical signatures is high and offers an alternative for the refining of the problematic biostratigraphic division of the d c boundary

radar interferometer observations of mesospheric echoing layers at jicamarca vhf radar observations at jicamarca show that the most intense mesospheric echoes at the equator are returned from thin horizontal layers with lifetimes ranging from tens of minutes to many hours a few such layers may be present at a time typically separated in altitude by a few kilometers interferometric studies of radar returns from these layers indicate that the characteristic widths of the corresponding doppler spectra are usually determined by the spatial integration effect due to finite size scattering volumes radar returns from lower mesospheric layers appear to be due to randomly distributed scatterers convected through the observation volume by nonturbulent wind fields the scatterers in question may possibly be specularity points or glints on corrugated refractive index surfaces higher mesospheric layer echoes however imply the existence of turbulent flow fields in the vertical plane with dominant eddy motions at scale sizes comparable with the scattering volume dimensions of about a kilometer

discovery of agglutinated benthic foraminifera in devonian black shales and their relevance for the redox state of ancient seas abstract agglutinated foraminifera are benthic organisms that occur in marginal marine to bathyal environments though some taxa can live in oxygen deficient environments they require at least some oxygen in order to persist at the seafloor the discovery that they occur widely in late devonian black shales has a bearing on the boundary conditions required for episodes of extensive carbon sequestration in marine sediments and their connection to atmospheric composition and global climate devonian black shales of the eastern us have been studied extensively to determine the fundamental controls on carbon burial and a range of mechanisms has been proposed finding agglutinated benthic foraminifera in these black shales refocuses the debate about their origin and points to limitations of earlier models

subduction and atmospheric escape of earth s seawater constrained by hydrogen isotopes abstract the hydrogen isotopic d h ratio reflects the global cycling and evolution of water on earth as it fractionates through planetary processes we model the water cycle taking seafloor hydrothermal alteration chemical alteration of continental crust slab subduction hydrogen escape from the early earth and degassing at mid ocean ridges hot spots and arcs into account the differences in d h ratios between present day oceans oceanic and continental crust and mantle are thought to reflect isotopic fractionation through seafloor alteration chemical alteration and slab dehydration however if the speed of plate tectonics has been nearly constant throughout earth s history the degassing and regassing rates are too small to reach the present day d h ratios we show that a hydrogen escape from reduced early atmosphere b secular net regassing or c faster plate tectonics on early earth is needed to reproduce the present day d h ratios of the water reservoirs the low d h ratio of archean seawater at 3 8 ga has previously been interpreted as a signature of a hydrogen escape but we find it can also be explained either by b secular net degassing or by c faster plate tectonics on early earth the rates of hydrogen escape from early earth and secular regassing on present day earth are constrained to be lower than 2 1 10 11 kg yr and 3 9 10 11 kg yr consequently the volume of water in the present day mantle could result entirely from the regassing through earth s history in that case the volume of initial oceans could be 2 to 3 times larger than that of current earth we suggest that in addition to the d h ratio of archean seawater identifying the d h ratios of both seawater and mantle throughout earth s history would allow to distinguish these evolutionary scenarios

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

cooling driven oceanic anoxia across the smithian spathian boundary mid early triassic abstract the smithian spathian boundary ssb represents a major climatic oceanic biotic event within the 5 myr long recovery interval of the early triassic following the end permian mass extinction the ssb was associated with pronounced cooling following the middle smithian hyper greenhouse a 2 to 8 positive carbon isotope excursion and a second order mass extinction yet its underlying cause s remain poorly understood here we review oceanic environmental changes during the middle smithian to early spathian interval and their potential mechanisms in addition we undertook an analysis of the carbon sulfur iron c s fe systematics of the south majiashan section which was located on the paleo western margin of the south china craton during the early triassic to better understand the causation of these environmental changes this analysis revealed low 13ccarb i e the n3 minimum in the middle smithian but rapidly rising 13ccarb i e to the p3 maximum and 34scas within the ssb interval i e late smithian earliest spathian both the middle smithian and ssb intervals are marked by development of marine euxinia which we hypothesize had fundamentally different causes the middle smithian was associated with hyperwarming the smithian thermal maximum which was probably triggered by a peak in magmatic activity of the siberian traps lip in contrast the ssb interval was associated with global climatic cooling which may have stimulated oceanic overturning circulation and upwelling on continental margins leading to transient local increases in marine productivity and expansion of oceanic oxygen minimum zones omzs the concurrent positive excursions in 13ccarb and 34scas at south majiashan and other sections worldwide are evidence of enhanced burial of organic carbon and pyrite on a regional or global scale as well as of a concurrent decline in atmospheric pco2 that may have positively re enforced the ssb cooling event

is the ordos basin floored by a trapped oceanic plateau abstract the ordos basin in china has about 10 km of neoarchean to quaternary sediments covering an enigmatic basement of uncertain origin the basement is tectonically stable has a thick mantle root low heat flow few earthquakes and has been slowly subsiding for billions of years the basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition and is similar to some other cratons world wide and also to several major oceanic plateaus it was accreted to the amalgamated eastern block and central orogenic belt of the north china craton ncc in the paleoproterozoic then involved in several proterozoic tectonic events including being over thrust by an accretionary orogen and intruded by andean arc related magmas and then involved in a continent continent collision during amalgamation with the columbia supercontinent thus the basement rocks are deformed metamorphosed to granulite facies and determining their initial origin is difficult we suggest that the data is consistent with an origin as an oceanic plateau that accreted to the ncc and later experienced different episodes of differentiation associated with later subduction and collisions formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases but also re activated and de cratonized in some cases in asia where they have been affected by younger subduction hydration slab roll back and melt peridotite reactions this suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons and de cratonization is not only influenced by younger tectonic activity

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

geochemical evidence for expansion of marine euxinia during an early silurian llandovery wenlock boundary mass extinction abstract repeated biotic crises have become the hallmark for the silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations causal mechanisms linking these marine extinction events with positive carbon isotope 13 c excursions paleoceanographic change and climate remain poorly constrained here we examine the positive 13 c excursion across the llandovery wenlock boundary and the associated ireviken extinction event this positive 13 c excursion has been interpreted to reflect a major change in global oceanographic climate state and enhanced organic carbon burial new geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon and sulfur isotopes 34 s and iodine i ca mg proxies the high resolution 13 c and 34 s data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles with minimal temporal offsets between the two systems numerical box modeling of 13 c and 34 s data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite which are most likely due to enhanced burial under euxinic anoxic and sulfidic conditions independently i ca mg values point to locally anoxic bottom waters in the distal and deeper basinal setting in nevada before during and after the ireviken positive 13 c excursion i ca mg values in the proximal shelf setting in tennessee show relatively oxic waters during the onset of peak 13 c values after which bottom water oxygen concentrations dropped throughout the remainder of the excursion this multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the silurian biotic event and positive 13 c excursion integration of these geochemical data for local and global scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea level rise points toward a shoaling of anoxic and or euxinic waters onto the shelf as a driver for the ireviken extinction event

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

tracing paleoredox conditions across the devonian carboniferous boundary event a case study from carbonate dominated settings of belgium the czech republic and northern france the devonian carboniferous boundary was coeval with the hangenberg crisis which is regarded as a first order mass extinction event related with large sea level changes and widespread anoxia this study aims to trace the geochemical paleoproductivity and paleoredox proxies across the devonian carboniferous carbonate dominated successions of the namur dinant basin belgium northern france and the moravo silesian basin czech republic located in the variscan rhenohercynian zone the research was focused on the distribution of oxides sio2 al2o3 fe2o3 cao na2o k2o tio2 mno and trace elements th u v zr mo zn pb ni cu measured by icp oes and icp ms respectively the enrichment factors of mo and u and their ratios were used as the main paleoredox proxies whereas the enrichment factors of zn cu pb and ni were employed to trace the paleoproductivity changes high values of zr al2o3 0 001 tio2 al2o3 0 06 and fe ti 20 and low values of al al fe mn 035 coinciding with an increase of paleoredox proxies suggest a possible volcanic and hydrothermal source of nutrients related to late devonian and early carboniferous extensional magmatism in the rhenohercynian domain although a relationship between higher paleoproductivity and enhanced continental runoff of volcanic material cannot be excluded the studied devonian carboniferous boundary sections from the namur dinant basin gendron celles ardennes quarry reveal three levels with similar vectors of mo and u enrichments the pre crisis famennian pattern is typical for weakly restricted basins with fe mn redox cycling accompanying vertical fluctuations of the oxic anoxic chemocline close to the sediment water interface with the influence of particulate shuttle the mo ef and u ef patterns corresponding to the unrestricted or weakly restricted basin with alternating suboxic to anoxic conditions are considered to represent the transgressive hangenberg black shale event the lower tournaisian mo ef and u ef patterns fall along a vector in the direction of the strongly restricted marine conditions with prevailing suboxic conditions in the moravo silesian basin lesni lom section the pre crisis famennian mo ef and u ef vector indicates an unrestricted marine trend which converges with that of restricted systems redox conditions range from suboxic to euxinic in the tournaisian mo ef and u ef oscillate between oxic and anoxic conditions and fall along a vector of strongly restricted marine conditions increased isolation of both studied basins in the early tournaisian is regarded as a reflection of a significant eustatic fall during the hangenberg crisis linked to a severe episode of the gondwana glaciation these paleohydrographic patterns have correlative potential that should be considered for a revised devonian carboniferous boundary definition

biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end permian mass extinction event article i nfo the permian triassic boundary event at 252 2 ma marks the largest extinction of marine fauna in the phanerozoic and there is a wide consensus that the extinction coincided with an intense oceanic anoxic event thestratotypeofthechanghsingianstage preciselyconstrainedbytheptbglobalstratotypesectionandpoint gssp and the gssp for the wuchiapingian changhsingian boundary both at meishan in southern china is well documented in respect to geochronology and the pattern of extinction here we report secular trends in bulk isotopic parameters and lipid biomarkers in a core spanning 214 m of stratigraphic section across the ptb and through the entire changhsingian interval our analysis of these data viewed in the context of relative sea level change and strontium isotopes reveals distinct shifts in paleoenvironmental conditions and profound changes in plankton ecology well before and following the biological extinction event specifically patterns of steroids and triterpenoids indicate a marine plankton community that was heavily dominated by bacteria during the late wuchiapingian middle changhsingian and early griesbachian stages secular trends in aromatic hydrocarbons diagnostic for anoxygenic green sulphur bacteria chlorobiaceae identify periods when euxinic conditions extended into the photic zone during the entire changhsingian stage here also the 15

geochemical constraints on the growth of the continental crust several lines of evidence indicate that the archean upper crust was considerably more mafic than the present day upper crust there has been no significant change in ree and th abundances in post archean clastic sedimentary rocks suggesting that there has been no change in the composition of the upper crust during the post archean this indicates that if there have been any additions to the post archean upper crust they must have had similar composition to the upper crust itself geochemical modelling of ree and th abundances in sedimentary rocks suggests that the minimum ratio of post archean to archean upper crustal composition required to eliminate the archean upper crustal trace element signature within analytical uncertainty is about 4 1 such a model also is supported by isotopic data using plausible assumptions regarding the volume of archean crust isostatic relations and extreme models of the earth s degassing history it is proposed that approximately 65 75 of the continental crust formed