changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

chromium isotopes in carbonates a tracer for climate change and for reconstructing the redox state of ancient seawater abstract strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout earth s history mainly for the purpose of reconstructing ancient climatic changes we here present results of a new isotopic tracer system stable chromium isotopes applied to a late ediacaran marine carbonate sequence exposed in the calera de recalde syncline arroyo del soldado group uruguay the aim was to compare cr isotope signatures directly to 13 c 87 sr 86 sr and 143 nd 144 nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold warm periods accompanied by sea level changes in response to glaciation deglaciation at higher latitudes the studied section is characterized by a pronounced negative down to 3 3 13 c excursion in carbonates paralleled by a decrease of 87 sr 86 sr values chromium isotope signatures over this section also show a correlated decrease in 53 cr 53 cr 53 cr 52 cr sample 53 cr 52 cr srm979 1 1000 values from 0 29 to 0 17 which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high temperature magmatic sources linear correlations between 53 cr and end 570 ma 87 sr 86 sr and cr concentrations can be explained by mixing between two major input sources of cr nd and sr into the shallow seawater 1 a source characterized by negative 53 cr values of 0 2 low 87 sr 86 sr values of 0 707 and elevated 147 sm 144 nd values of 0 13 recognized as a subaqueous hydrothermal dominated input source and 2 a source characterized by positively fractionated 53 cr values of 0 2 higher 87 sr 86 sr values of 0 708 and lower 147 sm 144 nd values of 0 11 a source which is strongly affected by continentally derived input chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that at least locally neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and or that oxygen levels of the atmosphere were sufficient to transform cr iii into the more mobile hexavalent cr vi formed during weathering processes on land the fact that 87 sr 86 sr values despite 13 c fluctuations remain low indicative of a strong hydrothermal input into the basin at his time implies that co 2 limitation was the cause of negative 13 c and 53 cr excursions in otherwise nutrient rich late neoproterozoic basins and that glaciation is only one more consequence of a tectonically driven biologically mediated system in such a scenario glaciation acts as an amplifier of 53 cr signals these signals in marine carbonates are a sensitive tracer for redox processes in the ocean and or on land and have the potential to contribute significantly in combination with the other commonly used isotopic tracers to the reconstruction of climatic changes particularly those that are associated with major glaciation periods in earth s history

marine ferromanganese oxide a potentially important sink of light chromium isotopes abstract marine ferromanganese oxides are widely distributed in oxic marine sediments and are a sink for a range of metal elements derived from seawater their potential as a sink of chromium cr isotopes has yet to be investigated and may have important implications for our understanding of the cr isotope system in this study we reported cr isotope data of modern oceanic fe mn crusts for the first time the 53cr values of the surface scrapings of the fe mn crusts from the central north pacific seamounts range from 0 85 to 0 15 with an average of 0 42 0 34 2 n 11 which are lower than published data from pacific seawater 0 53 1 43 with an average of 0 79 these results reveal preferential removal of light cr isotopes from seawater into fe mn crusts with an isotopic fractionation 53cr 53crfe mn crust 53crseawater estimated from 2 3 to 0 7 averaging at ca 1 2 we suggest that such significant negative isotopic fractionation may have been reached through adsorption of dissolved cr vi oxyanions in the oxic water column onto the surface of mn oxide phases during the precipitation of fe mn crusts however our modelling results imply that the deposition of mn oxide sediments would make insignificant contribution to elevation of the global seawater average 53cr value

cr isotope systematics in the connecticut river estuary abstract stable chromium cr isotopes expressed in 53cr as the deviation of 53cr 52cr in samples from the international standard nist srm 979 are an emerging paleoredox proxy that has been used to track earth s redox evolution over a range of timescales there have been several major advances in our understanding of the cr isotope system but we are still developing a basic knowledge of the global cr isotope mass balance and there are key cr fluxes with poorly constrained isotopic compositions the cr isotope behavior in estuaries is one such poorly constrained aspect of the modern cr cycle therefore we collected water and suspended particulate samples from the estuary of the connecticut river and present the first cr isotope dataset from a salinity gradient as ph and salinity increases from brackish to salt water dissolved cr increases from 118 1 25 7 ng kg 1 n 6 to 225 1 26 0 ng kg 1 n 8 while particulate cr decreases from 341 3 319 8 ng kg 1 n 6 to 208 4 364 5 ng kg 1 n 8 dissolved 53cr decreases from 1 30 0 27 1 n 6 to 0 64 0 16 1 n 8 whereas particulate 53cr remained stable at 0 11 0 07 1 n 9 these data suggest that cr is lost from particles to solution when transitioning from brackish to salt water in the connecticut river estuary if these observations can be confirmed in other estuaries estuary systems can potentially enhance riverine dissolved cr fluxes and thus lower the cr oceanic residence time which has important implications for the cr isotope system as a paleoceanographic redox proxy there is a clear inverse correlation between dissolved cr and 53cr in the connecticut river estuary which yields a cr isotope fractionation factor of 0 89 that is indistinguishable from previous estimates based on open seawater samples large 53cr variation in oxygenated open and estuary seawater suggests that marine 53cr variations need not necessarily be exclusively tied to cr vi reduction in low oxygen zones therefore interpreting 53cr in the sedimentary record requires taking multiple processes into consideration

high temperature arc parallel normal faulting and transtension at the roots of an obliquely convergent orogen structural and kinematic data from northern coastal british columbia 54 5 n document intra arc deformation patterns at mid crustal levels during and after emplacement of the coast mountains batholith major arc parallel displacements occurred along high temperature 700 c ductile normal faults and steep sinistral transtensional shear zones within an obliquely convergent margin strain patterns and pluton emplacement were controlled by 1 structural anisotropies in host rocks that predate batholith emplacement and 2 kinematic compatibility requirements created by simultaneous motion on curved pluton bounding shear zones these controls superseded a partitioning of the arc parallel and arc normal components of oblique plate convergence onto strike slip and thrust faults respectively

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

oxygenation variations in the atmosphere and shallow seawaters of the yangtze platform during the ediacaran period clues from cr isotope and ce anomaly in carbonates abstract the late neoproterozoic era witnessed a significant oxygenation in earth s surface in line with the naissance and diversity of early animals however detailed variations of the oxygenation during this period remain controversial chromium isotope in ancient carbonates has shown its ability to trace the oxygenation history of the atmosphere hydrosphere system in this study we analyzed cr isotope and ce anomaly data of carbonates from the ediacaran doushantuo and dengying formations in the yangtze gorges area south china in order to reconstruct oxygenation variations in the atmosphere and shallow seawaters of the yangtze platform during the ediacaran period after evaluating and excluding detrital and diagenetic effects well preserved 53crauth signatures were obtained post marinoan carbonates record slightly positive 53crauth values 0 18 0 43 and slightly negative ce anomaly suggesting generally oxidizing conditions in the atmosphere with po2 over 1 pal and in shallow seawaters of the yangtze platform during the post marinoan phase which may have been related to the diversity of early ediacaran organisms shuram wonoka correlative carbonates are characterized by the fluctuations of 53crauth values 0 25 to 0 97 and a significant negative 13c excursion which may have resulted from partial oxidation of the large deep water doc reservoir under enhanced oxidative weathering conditions this process released 13c depleted carbon into dic pool and isotopically unfractionated cr iii into water columns the latter of which immediately precipitated by adsorption onto the detrital materials meanwhile an increase in local anoxia and even euxinia in the intrashelf basin of the yangtze platform driven by enhanced microbial sulfate reduction and the resultant h2s possibly limited regional biological evolution during this phase late ediacaran carbonates record elevated 53crauth values 0 40 0 96 together with the decreasing trend in ce ce values indicating a more oxygenated environment in the atmosphere and shallow seawaters of the yangtze platform during the late ediacaran phase which may have been beneficial to the diversification of late ediacaran animals

nitrogen isotope dynamics of the cariaco basin venezuela 1 we report 15n 14n measurements of water column nitrate and ammonium sinking particles and sediments from the cariaco basin an anoxic marine basin off the coast of venezuela water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic anoxic interface 275 m being free of a large denitrification signal the 15n of shallow thermocline nitrate is 3 5 significantly lower than the mean deep ocean nitrate 15n of 5 this may be due to the nitrification of newly fixed n whether it occurs within the basin or in open atlantic waters that flow into the cariaco over the sill the 15n 14n of the sinking flux in the deepest trap 1250 m is similar to that of thermocline nitrate as expected given the complete consumption of nitrate in the surface layer moreover the 15n 14n of the seafloor sediment is similar to that of the sinking flux as is common in environments of high export production low o2 and good organic matter preservation thus the modern cariaco basin records the 15n 14n of the thermocline nitrate which in turn may record the input of newly fixed n to the upper ocean be it local or more regional in origin

molybdenum accumulation in cariaco basin sediment over the past 24 k y a record of water column anoxia and climate molybdenum mo concentrations in a sediment core from the cariaco basin on the venezuelan continental shelf can be partitioned between a marine fraction and a terrigenous fraction the accumulation rate of the marine fraction of mo increased abruptly 15 000 calendar years ago 15 ka from 2 yr 1 to 4 g cm 2 yr 1 and then decreased abruptly at 9 ka the accumulation rate remained high throughout this 6 k y period but exhibited maxima at 15 14 and 12 5 ka corresponding in time to meltwater pulse ia into the gulf of mexico and the onset of the younger dryas cold event respectively the marine fraction of mo is interpreted in terms of redox conditions of bottom water as dictated by both the flux of settling organic matter and bottom water residence time correspondence between geochemical extremes in this core with changes in sea level and global climate demonstrates the high degree to which this ocean margin basin has responded to the paleoceanographic regime throughout the past 24 k y

mafic replenishments into floored silicic magma chambers commingling between contemporaneous mafic and felsic magmas is now widely recognized in a broad range of intrusions and intrusive complexes these interactions are important features for two main of reasons 1 the rapidly chilled margins of mafic magma against silicic magma commonly preserve the compositions of mafic liquids and 2 because the mafic magma solidifies rapidly the resulting final configurations of mafic and felsic magmas can provide insights into physical processes and changing viscosity contrasts and rheologies of magmas and felsic crystal mush during crystallization of the mafic magma mingling of contrasted magmas was first recognized in the 1950s wider recognition of interactions between mafic and silicic liquids led to concepts of net veining in the 1960s intramagmatic flows chilled basaltic layers separated by felsic cumulates in the 1970s and in the 1990s to mafic silicic layered intrusions masli which could be as much as a few kilometers thick and more than 100 km2 in area it was quickly appreciated that these masli preserved stratigraphic records of mafic replenishments into silicic magma chambers floored by felsic crystal mush volcanic studies had anticipated the occurrence of this last type of intrusion on the grounds that extensive ponding of basaltic magmas beneath silicic chambers was seen to be essential to keep large silicic systems like yellowstone active for millions of years this paper looks at the history of changing perceptions and interpretations of magma mingling and whether or not sill complexes are distinct from mafic silicic layered intrusions the stratigraphy of mafic silicic layered intrusions records changing magmatic compositions events and processes in a temporal framework comparable to that provided by coeval volcanic rocks as a result careful study of masli has great potential for linking plutonic and volcanic processes and events

granite magma migration and emplacement along thrusts this paper investigates the influence exerted by brittle tectonic structures in the emplacement of granite plutons in contractional settings we address both cases where contractional tectonics and magma intrusion are 1 coeval to study how active contractional tectonics controls the transport of magma and 2 diachronous to study the role of pre existing structures on the transport of magma in light of new experimental models we show that magma can rise along thrusts ramps and flats this phenomenon occurs for both low viscosity magma basalts to andesite and high viscosity magma dry granite the experimental results also allow the evaluation of the role played by magma viscosity in determining pluton geometries in addition a review of literature demonstrates a spatial and causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the pluton final shape the abundance of subhorizontal and tabular granitic intrusions indicates that the location of inflating granitic sills along thrust flats can be common we argue that active and pre existing flats and ramps thrusts provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway

tectonic setting of the sandia pluton an orogenic 1 4 ga granite in new mexico structural studies of the circa 1 42 ga sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the anorogenic label associated with this and other 1 4 ga granites in the southwestern united states the se margin of the pluton is a 1 to 2 km wide nw dipping ductile shear zone field and microstructural observations melt filled shear bands high temperature dynamic recrystallization of k feldspar megacrysts and crosscutting pegmatite dikes indicate that top to the nw normal movement in the shear zone took place in the presence of melt subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top to the nw shear sense suggesting that over large regions of the pluton magmatic flow mimicked solid state strain in the northern aureole contact metamorphic aluminosilicate porphyroblasts grew during the formation of a ne striking crenulation cleavage s3 and related folds of late stage pegmatite dikes these features document the synchroneity of magma emplacement shortening and metamorphism and indicate that the sandia pluton is syntectonic not anorogenic we interpret the kinematic consistency of structural elements from the base of the pluton the interior of the pluton and the northern aureole to reflect a regional larger than the pluton strain field and suggest that the orogeny recorded in and around the sandia pluton involved a three dimensional strain field with subhorizontal extension n s and contraction e w directions n s extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole east to se shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated s3 crenulation cleavage and east to se shortening or least extension directions in the pluton proper are documented by the intersections of orthogonal dikes thus emplacement of the sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin rather than local strains generated by emplacement

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

local and regional geochemical signatures of surface sediments from the cariaco basin and orinoco delta venezuela we have analyzed the chemical compositions of 87 samples from the cariaco basin and 20 samples from the orinoco delta venezuela in order to better characterize the fluvial sources of material to the cariaco basin we observe distinctive regional patterns in composition shelf sediments found near the tuy river manzanares river the araya margarita region and in the northern portion of the unare platform record reproducible compositional differences however linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin average trap values are best approximated by mixing upper crust 30 50 with local river sources 50 70 of either the tuy unare or neveri rivers and eolian dust

synchrotron based speciation of chromium in an oxisol from new caledonia importance of secondary fe oxyhydroxides in new caledonia the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by fe oxyhydroxides the speciation of trace elements such as cr ni and co in these oxisols remains a major subject of interest regarding mining and environmental issues we have assessed the speciation of chromium in the upper part of an oxisol by combining bulk and spatially resolved chemical analyses epma and sem eds with synchrotron based spectroscopic data exafs and xanes epma indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite enstatite and lizardite hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering however total analyses of major elements indicate only a slight depletion of cr together with an immobility of fe and al and drastic losses of si and mg after the weathering of the bedrock such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of fe 2 bearing primary silicates this efficiency of secondary fe oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of exafs data that indicates that these mineral species host between 67 and 75 wt of total cr compared to the 18 to 22 wt of total cr hosted by chromite in addition sem observation and sem eds analyses performed on the oxisol samples also show some evidence for chemical weathering of chromite chromite could then represent a past and or present source of chromium upon extended tropical weathering of the studied oxisol rather than a stable host these results emphasize the importance of secondary fe oxyhydroxides compared to cr spinels on chromium hosting in oxisols developed upon tropical weathering of ultramafic rocks although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite sorption reactions at the surface of these mineral species could also be involved in such a process in addition considering their potential oxidative reactivity that can generate cr 6 or enhance the chemical weathering of chromite the occurrence of mn oxides could significantly modify the behavior of chromium upon weathering these considerations indicate that further studies are needed to assess the actual potential of chromium release from oxisols developed upon weathering of ultramafic rocks under a tropical climate

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

tidally induced turbulent mixing at irving seamount modeling and measurements 1 the relationship of tidal currents and turbulence over irving seamount is examined by model and measurement surface tides are partially scattered into internal tides and particularly over the seamount summit current strength and shear greatly increase current shear causes the model s turbulent mixing at irving modeled turbulent mixing as represented by vertical diffusivity is largest o 10 3 m2 s 1 in the bottom boundary layer and in a turbulent column rising to the sea surface directly above the seamount pockets of less intense mixing occur within the turbulent column but their locations are highly variable field measurements confirm a turbulent mixing column atop irving of comparable intensity increased model diffusivity average of 7 10 5 m2 s 1 also occurs to a distance of several hundred kilometers from the seamount flanks in a band vertically centered at 800 m depth

pluton emplacement by wall rock thrusting hanging wall translation and extensional collapse latest devonian plutons of the cobequid fault zone nova scotia canada

fluctuations in late neoproterozoic atmospheric oxidation cr isotope chemostratigraphy and iron speciation of the late ediacaran lower arroyo del soldado group uruguay oxygenation of the earth s atmosphere occurred in two major steps near the beginning and near the end of the proterozoic eon 2500 to 542 ma ago but the details of this history are unclear chromium isotopes in iron rich chemical sediments offer a potential to highlight fine scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for earth s climate we observe strong positive fractionations in cr isotopes 53 cr up to 5 0 in iron rich cherts and banded iron formation horizons within the arroyo del soldado group ediacaran uruguay that can be explained by rapid effective oxidation of fe ii rich surface waters these fluctuations are correlated with variations in ratios of highly reactive iron fe hr to total iron fe tot which indicate a predominance of anoxic water columns fe hr fe tot 0 38 during the onset of oxidation pulses we favor the scenario by which isotopically heavy cr vi entered the basin after pulses of oxidative weathering on land and in which fe ii accumulated in the water column neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust derived detrital components of paleoproterozoic nd t dm model ages 2 1 2 2 ga provenance typical of the rio de la plata craton the association of positive 53 cr ferruginous fe hr fe tot 0 38 stratigraphic intervals with low diversity acritarch assemblages dominated by bavlinella faveolata strongly support models postulating a stratified eutrophic neoproterozoic ocean thus even within a few million years of the precambrian cambrian boundary paleoceanographic conditions resembled more those of paleoproterozoic oceans than phanerozoic and present oceans this highlights the sheer magnitude of ecological changes at the precambrian cambrian transition changes which ultimately led to the demise of the precambrian world and the birth of the metazoan dominated phanerozoic

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

redox condition in the nanhua basin during the waning of the sturtian glaciation a chromium isotope perspective abstract the neoproterozoic oxygenation event noe is often considered to be synchronously and possibly causally associated with the cryogenian glaciations reconstructing early cryogenian redox conditions is therefore essential for understanding the onset of the noe we analyzed cr isotope compositions 53cr as well as major and rare earth element ree concentrations of the banded iron formations bifs in the basal fulu formation of the xiajiajiang section nanhua basin which are thought to have been deposited during the waning stage of the sturtian glaciation strongly positively fractionated authigenic 53cr values were obtained after detrital correction ranging from 0 64 to 1 22 and averaging at 0 91 0 40 2 n 10 or after detrital evaluation 0 71 this suggests extensive oxidative cr cycling occurred at the earth s surface and implies that the po2 was probably higher than 1 pal during this period the lack of ce anomalies 0 90

modern climate forcing of terrigenous deposition in the tropics cariaco basin venezuela one approach to deciphering tropical quaternary paleoclimate records has been to study the composition of terrigenous material in order to infer past changes in hydrologic and atmospheric conditions here we present the inorganic geochemistry major and trace elements of modern sediment traps and shelf sediments in the tropical cariaco basin venezuela in order to characterize seasonal variation in deposition of the terrigenous component and to document linkages within the ocean atmosphere climate system we show that variation in the chemistry of terrigenous input to the basin is a sensitive monitor of the annual meridional migration of the intertropical convergence zone itcz additionally seasonal terrigenous relative abundances are decoupled from terrigenous absolute fluxes showing that abundances are not directly related to terrigenous delivery changes in mixing between local fluvial sediments fluvial and or eolian mafic material and saharan dust explains the observed seasonality the modern seasonal variations are significantly smaller than those observed in the glacial interglacial paleorecord indicating that climate sensitivity at the two time scales is very different a striking contrast in the behavior of ti al between the longer glacial interglacial time scale and the modern seasonality also suggests that long term terrigenous deposition is not linked simply to 100kyr shifting of the itcz the modern data can be reconciled with the glacial interglacial record by invoking generally drier conditions during glacial periods over tropical latitudes combined with intermittent wet conditions caused by precessional forcing 2007 elsevier b v all rights reserved

stable isotope records across the cretaceous paleogene transition stevns klint denmark new insights from the chromium isotope system abstract late cretaceous to early paleogene time represents a critical transitional period in earth history global events include short term instability in climate and ocean circulation large igneous province emplacement and catastrophic extinction of marine and terrestrial fauna due at least in part to a bolide impact event the response of the ocean system to cretaceous paleogene k pg global events has been the subject of much research yet fundamental questions remain regarding carbon cycling climate ocean mixing and redox conditions to help elucidate paleoceanographic changes across the k pg transition we revisit the classic maastrichtian to danian chalk limestone succession at stevns klint denmark with a suite of new geochemical data including insights from the redox sensitive chromium isotope system we use carbon and oxygen isotope trends to form a well constrained framework for the succession before presenting strontium isotope chromium isotope and rare earth element data for the units deposited both before and after the k pg event carbon isotopes show only minor perturbation across the k pg boundary and oxygen isotopes record a prominent positive excursion with both of these trends standing in contrast to many k pg boundary sections worldwide strontium isotopes record a spike towards more radiogenic values which is consistent with a globally observed short term weathering pulse the chromium isotope profile of maastrichtian chalk begins with strongly positive values followed by a sharp negative excursion associated with a facies shift and initiation of bottom currents in the chalk sea 53 cr values then recover before falling steadily across the k pg boundary and into the danian limestone chromium isotope trends do not correspond to changes in hydrothermal input or basin redox conditions instead we propose that mixing of water masses with distinct chromium isotope signatures controlled temporal chromium isotope variability the local mixing event inferred from our maastrichtian chromium isotope data could be related to a temporary inflow of tethyan bottom waters or the shift from end maastrichtian greenhouse warming associated with deccan volcanism to the worldwide cooling phase that immediately preceded the k pg boundary no immediate change in chromium isotopes is recorded across the k pg boundary indicating that the bolide impactor did not substantially perturb the marine chromium cycle at least at the scale resolvable by our data instead chromium isotope trends continued to be controlled by local oceanographic processes highlighting the potential of the chromium isotope system to record isotopic heterogeneity and mixing events in ancient oceans

evaluation of sediment trace metal records as paleoproductivity and paleoxygenation proxies in the upwelling center off concepcion chile 36 s a b s t r a c t this study analyzes the records of several trace metals sensitive to redox conditions in continental shelf sediments off concepcion chile 36 s the continental margin off concepcion 36 s 73 w lies beneath an important upwelling center characterized by high primary production rates and consequently high fluxes of organic matter in spring and summer this material settles to the seafloor where it decays pro ducing periods of very low oxygen content in bottom waters 1 ml l y 1 the goals of this study were to determine the input of trace metals to the sediment and to decipher how this information can be used to reveal variations in primary productivity or bottom oxygenation gravity cores collected at two stations vg06 2 over the mid shelf station 88 m water depth upper boundary of the oxygen minimum zone and vg06 3 over the outer shelf 120 m water depth within the oxygen minimum zone were sampled for high resolution profiles 1 cm of trace metals biogenic opal stable isotopes and total organic carbon the results suggest that the variability in the trace metal distribution on the continental shelf off concepcion is determined by redox conditions and the organic carbon flux to the bottom some sections of the sediment cores from the outer shelf showed appreciable authigenic enrichment of u cd and mo ef 5 10 2 5 and 10 16 respectively along with heavier values of d 15

the chromium isotopic composition of seawater and marine carbonates article history chromium isotopes are fractionated during redox reactions and have the potential to provide a record of changes in the oxygenation levels of the oceans in the geological past however cr is a trace metal in seawater and its low concentrations make isotopic measurements challenging here we report the first determinations of 53 cr for seawater from open ocean argentine basin and coastal southampton water settings using a double spike technique the total chromium concentration in seawater from southampton water is 1 85 nm whereas the cr content of argentine basin samples is 5 8 6 6 nm the 53cr value of seawater from the argentine basin is 0 491 0 556 in intermediate and deep waters and varies between 0 412 and 0 664 in surface waters 150 m the 53

chromium isotopes in marine hydrothermal sediments abstract hydrothermal chromium cr cycling contributes to marine cr inventories and their cr isotopic composition yet cr isotope effects associated with this cycling remain poorly documented here we determine the distribution isotopic composition and diagenetic mobility of cr in hydrothermal sediments from the distal flank of the south east pacific rise sepr dsdp site 598 we find that cr is primarily associated with the metalliferous iron oxyhydr oxide and detrital components of the sediment 0 4 3 6 mg kg 1 whereas cr concentrations are much lower in the dominant carbonate phase 80 cr from the sediment relative to fe we propose this loss is tied to oxidation of authigenic cr iii to cr vi followed by diagenetic remobilization and efflux from the sediment pile the bulk 53cr composition of the sepr sediments is isotopically light 0 24 to 0 57 0 05 and the authigenic 53cr is as light as 1 2 0 2 we argue that this light cr isotopic composition results from the partial reduction of oxic seawater bearing cr vi by hydrothermal vent fluids enriched in fe ii aq diagenetic oxidation of the reactive cr pool by mn oxides and loss of cr vi from the sediment may further deplete the sediment in 53cr during diagenesis the 53cr composition of the detrital cr fraction of the sediment average 53cr composition 0 05 0 04 falls within the igneous silicate earth ise range revealing that detrital cr delivered to this region of the pacific ocean is unfractionated and has carried a relatively constant 53cr composition over the last 5 7 million years together our results show that light 53cr compositions in hydrothermal sediments are imparted through a combination of processes previously overlooked in the marine cr biogeochemical cycle and that the 53cr composition of such sediments may provide a rich source of information on paleo marine redox conditions

magma emplacement during exhumation of the lower to mid crustal orogenic root the jihlava syenitoid pluton moldanubian unit bohemian massif in this study we present structural and ams data from the w335 ma ultrapotassic jihlava syenitoid pluton which intruded the lower to midcrustal orogenic root moldanubian unit in the bohemian massif central european variscides the emplacement of the pluton was accommodated by multiple processes such as ductile host rock shortening formation of sheeted zones by magma wedging magmatic stoping and possibly host rock displacement within a wide transtensional zone magmatic fabrics preserved in the pluton reflect both intrusive processes and regional strain margin parallel and weneewsw foliations which probably formed by strain during emplacement of inner magma pulses were overprinted by tectonic strain within a zone of distributed wrench dominated dextral transtension this zone probably accommodated exhumation of different segments in the eastern part of the moldanubian unit during pluton emplacement in contrast to existing models we emphasize that the jihlava pluton as well as other ultrapotassic plutons widespread in the moldanubian unit are structurally highly variable bodies emplaced by multiple intrusive processes our case study illustrates how careful documentation of structural relations around these ultrapotassic plutons may constrain the kinematic framework and local exhumation histories in different segments of the orogenic root during and shortly after the w340 ma mechanical event in the central european variscides 2006 elsevier ltd all rights reserved

the isotopic composition of authigenic chromium in anoxic marine sediments a case study from the cariaco basin chromium cr isotopes are an emerging proxy for tracking redox processes at the earth s surface however there has been limited exploration of the cr isotope record of modern and recent marine sediments the basic inorganic chemistry of cr suggests that anoxic marine basins should factor prominently in the global cr cycle and that sediments deposited within anoxic basins may offer a valuable cr isotope archive throughout earth s history here we present 53 cr data from sediments of the cariaco basin venezuela a type environment for large perennially anoxic basins with a relatively strong hydrological connection to the global oceans we document a marked positive shift in bulk 53 cr values following the termination of the last glacial maximum followed by relative stasis based on a suite of independent redox proxies this transition marks a switch from oxic to persistently anoxic and sulfidic euxinic depositional conditions within the basin we find good agreement between two independent approaches toward estimating the 53 cr composition of authigenic cr in euxinic cariaco basin sediments and that these estimates are very similar to the 53 cr composition of modern open atlantic ocean seawater these data together with considerations of reaction kinetics and mass balance within the cariaco basin are consistent with the hypothesis that anoxic marine settings can serve as a chemical archive of first order trends in seawater 53 cr composition additionally the cariaco basin data suggest that there has been secular stability in the average 53 cr value of atlantic seawater over the last 15 kyr

pluton accommodation at high strain rates in the upper continental crust the example of the central extremadura batholith spain abstract emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the central extremadura batholith spain the country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area the kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years fast strain rates 10 10 10 11 s 1 must concentrate in local structures e g initiation of kink folds even in zones deforming as a whole under typical strain rates 10 14 1 s 1 granite plutons might be used as strain rate gauges for syn plutonic structures

oxidative release of chromium from archean ultramafic rocks its transport and environmental impact a cr isotope perspective on the sukinda valley ore district orissa india abstract this study investigates cr isotope fractionation during soil formation from archean 3 1 3 3 ga ultramafic rocks in a chromite mining area in the southern singhbhum craton orissa india the cr isotope signatures of two studied weathering profiles range from non fractionated mantle values to negatively fractionated values as low as 53 cr 1 29 0 04 local surface waters are isotopically heavy relative to the soils this supports the hypothesis that during oxidative weathering isotopically heavy cr vi is leached from the soils to runoff the impact of mining pollution is observed downstream from the mine where surface water cr concentrations are significantly increased accompanied by a shift to less positive 53 cr values relative to upstream unpolluted surface water a microbial mat sample indicates that microbes have the potential to reduce and immobilize cr vi which could be a factor in controlling the hazardous impact of cr vi on health and environment the positive cr isotope signatures of the brahmani estuary and coastal seawater collected from the bay of bengal further indicate that the positively fractionated cr isotope signal from the catchment area is preserved during its transport to the sea isotopically lighter cr vi downstream from the mine is probably back reduced to cr iii during riverine transport leading to similar cr isotope values in the estuary as observed upstream from the mine

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

redox fluctuations in the early ordovician oceans an insight from chromium stable isotopes abstract chromium cr stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation recent co precipitation experiments have shown that cr vi is incorporated into the calcite lattice suggesting that carbonates are a potential sink for dissolved cr in the ocean and could effectively capture the cr isotope composition of ambient seawater thus the cr isotopic composition 53cr of carbonates has the potential to record changes in the oxygenation state of ancient oceans to test the potential of this system we measured 53cr values of limestones and dolostones from an early ordovician marine carbonate platform cerro la silla section of the precordillera san juan argentina an increasing number of studies support the hypothesis that the early ordovician oceans were stratified with respect to oxygen and that the global ocean experienced multiple episodes of anoxic expansion and contraction making this time period ideal for testing the usefulness of the cr isotope system the most prominent feature of the cerro la silla cr isotope profile is a positive excursion of 0 5 during the late tremadocian accompanied by exceptionally low cr concentrations

cr isotopic composition of the laobao cherts during the ediacaran cambrian transition in south china abstract in this study we present the cr isotopic compositions from the cherts in the laobao formation 551 522 ma that were deposited in deep water in south china the cr concentrations and isotopic compositions in the laobao cherts were dominated by the contemporaneous seawater and detrital phases the lower part of the laobao cherts show that the 53cr of contemporaneous deep seawater in the huanan basin was approximately 0 1 and the 53cr of detrital materials was approximately 0 2 these findings imply that surface 53cr enriched cr vi species did not affect the deep ocean until the late ediacaran because of the strong redox stratification in the ocean although the oxidative cr cycle in terrestrial settings had emerged before that time a slight rise in 53cr in the deep ocean to approximately 0 08 was estimated for the upper part of the laobao cherts if the 53cr rise and its amplitude can be further determined this will imply that shallow 53cr enriched cr vi species had an impact on the deep ocean and will suggest an oxidation of the ocean in the huanan basin around the ediacaran cambrian boundary 542 ma

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

modeling the thermal evolution of fault controlled magma emplacement models implications for the solidification of granitoid plutons a two dimensional finite diaerence model is used to simulate the conductive thermal regime attend ing construction and maintenance of a continental magma chamber by intrusion of granite dikes into grano diorite host rocks displaced at various spreading rates final intrusion shapes include tabular square and vertical rectangular bodies emplaced in the shallow crust 5 15 km and tabular bodies emplaced in the middle crust 15 20 km fed by dikes with widths of between 20 and 100 m the formation of a steady state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidifi cation of subsequently intruded material for spreading rates 10 mm year 1 construction of a steady state magma chamber in the shallow crust took 260 ka rectangular 360 ka square and 1 ma tabular whereas in the mid crust a steady state was reached in less than 30 ka tabular at faster spreading rates 25 and 50 mm year 1 ambient temperatures pass the solidus isotherm forming a steady state reservoir within 55 ka depending on intrusion depth and size for 10 25 mm year 1 spreading rates sheeted dikes make up from 10 to 100 of the intrusion the thermal modeling supports the following conclusions a episodic magma emplacement into a fault con trolled setting is a thermally viable means of constructing a steady state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus b isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction c the margins of most intrusions formed by this scen ario should contain sheeted dikes d the solidus isotherm and thus the solidification front that it tracks will become progressively curviplanar during the construction of the magma chamber and will not represent the in itial shape of the intrusions i e sheets e the steady state chamber will be smaller than the total intrusion dimensions and f magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front because it is unlikely that most large intrusions formed instan taneously the eaects of continued addition of heat on the migration of solidification fronts may have signifi cant implications for magmatic processes in many emplacement scenarios 1998 elsevier science ltd all rights reserved

climatically sensitive eolian and hemipelagic deposition in the cariaco basin venezuela over the past 578 000 years results from al ti and k al al ti and k al ratios in bulk sediment are used to interpret wind blown and hemipelagic sources of deposition to a 578 kyr record in the cariaco basin venezuela ocean drilling program site 1002 graphical and cross spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal 18o with both ratios being significantly higher during interglacials and lower during glacials k al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid venezuelan margin al ti decreases during glacials suggesting greater proportions of eolian rutile sourced from the northern sahara due to increased wind strength and or aridity this interpretation is consistent with previous studies of the chemistry and mineralogy of saharan derived eolian matter in the caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary al ti ratio

three dimensional shape and emplacement of the cardenchosa deformed pluton variscan orogen southwestern iberian massif the cardenchosa pluton is a lower carboniferous variscan granite located in the southwestern iberian massif it intruded along the contact between the sierra albarrana and azuaga tectonic units to the northwest the pluton connects with the left lateral azuaga fault the pluton appears in the footwall of the low angle normal casa del cafefault which crops out to the west of the granite gravimetric modelling shows the pluton to have a flat bottom at a depth of 2 km strain analysis of post emplacement deformation of the pluton indicates that a the deformation of the pluton accommodates the displacement of the azuaga fault and b the pluton prior to the solid state deformation was a lens shaped laccolith of approximately 10 km diameter and 2 km thickness the cardenchosa pluton was a single pulse of magma trapped in a rheological discontinuity of the upper crust the contact between the sierra albarrana and azuaga units the magma would ascend through dikes since no root has been detected the tectonic scenario during the intrusion was one of regional extension 2000 elsevier science ltd all rights reserved

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

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

authigenic chromium enrichments in proterozoic ironstones abstract chromium isotopes have recently emerged as a proxy for atmospheric oxygen levels in the precambrian chromium cr enrichments along with a suite of other redox sensitive metals have also been used as marine redox proxies for decades despite significant interest and growing use of this system which has been used to argue for low oxygen levels through the mid proterozoic leading up to the rise of animals there is a poor understanding of the host phases of cr in the sedimentary record without this context interpretation of measured isotopic signatures and concentrations of cr in various sedimentary archives can be ambiguous in order to shed light on this problem we have used electron microprobe elemental mapping to identify authigenic and detrital cr phases we have examined a range of ironstone samples that have been previously analyzed for cr isotopes and find that cr is predominantly associated with iron rich allochems instead of detrital cr mineral phases we did not find evidence for detrital chromite or other cr rich detrital minerals additionally we find evidence for authigenic ti enrichments and decoupling of al and ti elements traditionally used as detrital tracers similar to what has been reported in other slowly accumulating metal rich sediments e g iron manganese nodules lastly we have explored a range of possible sedimentary cr isotope signals that could result from changes in the percentage of detrital versus authigenic cr incorporated into a sample providing clear evidence for the ambiguity in interpretations of this isotope system without petrographic context

the acceleration of oceanic denitrification during deglacial warming the marine nitrogen cycle was altered during the transition from glacial to interglacial conditions an analysis of 15n records throughout the world s oceans suggests that rates of denitrification in the water column accelerated during the last deglaciation

a radiometric calibration of the specmap timescale the astronomical theory of climate change asserts that earth s climate is affected by changes in its orbit which vary the seasonal and latitudinal distribution of solar radiation this theory is the basis of the orbitally tuned specmap timescale a key constraint for this important chronology was the mid point of the penultimate deglaciation initially dated to 127 000 years ago recent work suggests this event may be considerably older casting doubt on the astronomical theory the specmap timescale and the accuracy of orbitally tuned chronologies difficulties with u th coral dating of sea level events have impeded progress on this problem because most corals are not closed systems here we use a new approach to u th dating that corrects for open system behavior and produces a sea level curve of sufficient resolution to confidently correlate with specmap over the last 240 000 years permitting a reassessment of both this critical chronology and a central tenet of climate change theory high precision ages for 24 oxygen isotope events provide a 240 000 year chronology for marine 18o records that is independent of orbital tuning assumptions although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the marine isotope stage 7 6 boundary a comparison of radiometric and specmap ages for identical isotope events suggest that the specmap timescale is quite accurate and that its errors were in general overestimated despite suborbital complexity orbital cyclicity is clearly evident in our record high amplitude sea level oscillations at periods greater than 20 000 years are very close in phase to summer insolation in the northern hemisphere although sea level changes cannot be uniquely tied to a specific season or latitude of insolation forcing the simplest explanation is that long period high amplitude sea level change is linked to northern hemisphere insolation forcing these results validate the principles of orbital tuning and suggest such timescales are generally robust

extensional pluton emplacement models do they work for large plutonic complexes shallow crustal to mid crustal magma emplacement models commonly rely on local or regional extension to solve the room problem we argue that existing extensional models explain the emplacement of sheeted dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons 1 rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall rock extension to be balanced by magma influx 2 rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma and 3 because of their sheetlike shapes and small thicknesses these small magma injections will cool rapidly and form sheeted plutons or dike swarms extension may however facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

weathering on land and transport of chromium to the ocean in a subtropical region misiones nw argentina a chromium stable isotope perspective abstract we have investigated the pathway of chromium from its mobilization on land and along its riverine transport in a subtropical region of south america misiones province argentina in an attempt to link cr stable isotope compositions recently measured in seawater with signals prevailing in rivers and ultimately with cr isotope effects observed during oxidative surface weathering in subtropical red soils cr concentrations and stable cr isotopic compositions expressed as 53cr values in two typical and representative surface profiles of weathered basalt show significant depletion of cr in the soils of up to 50 together with pronounced negatively fractionated 53cr values which are indicative of oxidative mobilization of heavy cr vi into the run off the behavior of cr in the studied weathering profiles is not correlated with that of other redox sensitive elements such as ce and u this is essentially due to the affinity of ree and u but not cr with secondary phosphates which form during weathering processes smaller tributaries in nw argentina to the parana river second largest river in south america carry dissolved cr in the order of 0 7 1 4 ppb 13 27 nm with 53cr values of 0 2 to 0 4 balancing the negatively fractionated weathering products the isotope composition and concentration of dissolved cr in the ca 1200 km long parana river from misiones to its estuary and discharge area into the south atlantic ocean remains relatively constant with an average cr concentration around 2 4 ppb 46 nm and an average 53cr value of 0 32 the cr concentration in the estuary itself drops by ca 50 but with only minor change in its cr isotope composition results from the parana estuary are identical with recently analyzed surface seawater from the argentine basin with cr contents of 0 3 ppb 6 nm and 53cr values 0 4 bonnand et al 2013 and indicate that there is only a minimal cr isotopic variability during riverine transport even during long transport distances as shown in our example of the parana river simple cr input flux calculations reveal that the parana river accounts for 5 of the total yearly cr flux to the world s oceans today and that its isotopic signature seems to be at least locally imparted to the surface seawater of the argentine basin whether or not this cr isotope signature is generally exhibited by the world s oceans needs further investigations particularly the characterization of seawater around the globe

mixing between granitic and dioritic crystal mushes guernsey channel islands uk abstract the contact zone between the cobo granite and bordeaux diorite complex of guernsey channel islands uk displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas initial interaction resulted in the formation of chilled mafic enclaves in granite magma as thermal equilibrium was approached some physical mixing took place to produce a heterogeneous marginal facies continued interaction resulted in incorporation of previously mixed magma into intruding magma the early mixed material is locally preserved as enclaves but more commonly underwent disaggregation promoted by its incompletely crystallised nature the mineral components becoming distributed as xenocrysts and often as microenclaves or glomeroxenocrysts into surrounding magma further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres these processes produced geochemical profiles which do not exhibit perfect mixing trends the petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition but also provide criteria by which such interaction may be recognised elsewhere many features in particular mineral scale disequilibria and small scale modal heterogeneity bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent as such it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

evaluation of multiple emplacement mechanisms the huichizi granite pluton qinling orogenic belt central china three end member granite emplacement mechanisms host rock ductile shortening hrds host rock rigid displacement hrrd and magmatic absorption ma are identified in this study of the huichizi pluton the internal fabrics are mainly flattening typesona 0 4 0 9u and strain is highoesa 1 0 0 6u near the boundaries and lowoesa 0 7 0 30u toward the central parts of the pluton in the 1 5 km wide contact aureole extension e is measured in syn emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1 14 km and 0 74 km respectively from this we conclude that the hrds produced only about 20 space for the present volume of the pluton the regional hrrd derived from the uplift by about 8 km of the qinling group in a flower structure contributed 19 032 space ms including stoping and material transfer processes yielded another 37 space of this stoping contributed a minimum of 1 space estimated from the surface area of 2 8 km 2 covered by xenoliths transfer processes provided about 36 of the needed space deduced from the average 64 36 ratio of hybridization of mantle derived to crustal qinling group derived magma the three mechanisms may act nearly simultaneously as a multiple emplacement mechanism this situation may apply to many other plutons as well 2000 elsevier science ltd all rights reserved

magma emplacement during arc perpendicular shortening an example from the cascades crystalline core washington it is increasingly inferred by others that active arcs at all crustal levels are undergoing arc perpendicular extension and that magma utilizes this extension or extension along arc parallel strike slip faults during emplacement however a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc perpendicular shortening without the aid of strike slip faulting this conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens evaluation of host rock material transfer processes that operated during emplacement of the 93 ma 10 to 14 km deep mount stuart batholith washington provides one example and includes the following 1 ductile downward flow in a narrow 0 2 body radii structural aureole 2 stoping and of lesser importance 3 regional folding thrusting and subhorizontal arc parallel extension we see no evidence that arc perpendicular extension or local extension along strike slip faults made space for this pluton nor for most other plutons in contractional arcs instead we argue that both localized and rapid around plutons and regional and slower downward flow during growth of crustal roots is a common process in such arcs

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

chromium isotope systematics in the connecticut river abstract limited constraints on cr isotope fractionation during weathering and river transport is currently a gap in our understanding of the chromium cr isotope system which is an emerging proxy in environmental and paleoenvironmental studies here we investigate cr mobility and isotope fractionation from the temperate connecticut river usa including cr concentrations and isotopic compositions of river water suspended particles riverbed sediments and weathering profiles the 53cr values of the connecticut river water range from 0 17 to 0 92 which are similar to or higher than the weathered rocks in the catchment 0 08 to 0 29 we find seasonal variations in dissolved 53cr values in some but not all tributaries suggesting that dissolved 53cr is not a simple function of seasonality but may also be influenced by sub catchment heterogeneity in lithology in contrast to dissolved cr we found consistent seasonal difference in suspended cr concentration and 53cr suspended 53cr is 0 1 higher than the unfractionated bse in the fall 0 01 0 13 but indistinguishable from the bse in the spring 0 11 to 0 00 the suspended cr concentration is also lower in the spring and with higher al mn fe concentrations the lower suspended cr concentration and 53cr in spring may be linked to increased silicate and oxide load with depleted cr due to stronger hydrological flux building from our dataset there is not a consistent correlation with climate zones in a compilation of 53cr and cr concentration data from river water and weathering profiles suggesting that climate is not a dominating factor controlling cr isotopic behavior during weathering suggesting that other factors e g local catchment conditions and dissolved organic matter may also be responsible for the observed river water 53cr variability

trace elements at the intersection of marine biological and geochemical evolution life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes these requirements differ between organisms and have evolved over geological time likely guided in some part by environmental conditions until recently most of what was understood regarding trace element concentrations in the precambrian oceans was inferred by extrapolation geochemical modeling and or genomic studies however in the past decade the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new and potentially more direct insights into secular changes in seawater composition and ultimately the evolution of the marine biosphere compiled records of many bioessential trace elements including ni mo p zn co cr se and i provide new insight into how trace element abundance in earth s ancient oceans may have been linked to biological evolution several of these trace elements display redox sensitive behavior while others are redox sensitive but not bioessential e g cr u their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere ocean redox conditions that are linked to biological evolution for example the activity of oxygen producing photosynthetic cyanobacteria in this review we summarize available precambrian trace element proxy data and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life we also examine several biologically relevant and or redox sensitive trace elements that have yet to be fully examined in the sedimentary rock record e g cu cd w and suggest several directions for future studies

ocean drilling program leg 112 peru continental margin part 2 sedimentary history and diagenesis in a coastal upwelling environment on the shelf and upper slope off peru the signal of coastal upwelling productivity and bottom water oxygen is well preserved in alternately laminated and bioturbated diatomaceous quaternary sediments global sea level fluctuations are the ultimate cause for these cyclic facies changes during late miocene time coastal upwelling was about 100 km west of the present centers along the edge of an emergent structure that subsequently subsided to form the modern slope the sediments are rich in organic carbon and intense microbially mediated decomposition of organic matter is evident in sulfate reduction and methanogenesis these processes are accompanied by the formation of diagenetic carbonates mostly ca rich dolomites and mg calcites the downhole isotopic signatures of these carbonate cements display distinct successions that reflect the vertical evolution of the pore fluid environment from the association of methane gas hydrates burial depth and low chloride interstitial fluids we suggest an additional process that could contribute to the characteristic chloride depletion in pore fluids of active margins release of interlayer water from clays without a mineral phase change the shelf sediments also contain a subsurface brine that stretches for more than 500 km from north to south over the area drilled the source of the brine remains uncertain although the composition of the oxygen isotopes suggests dissolution of evaporites by seawater

chromium isotope variations δ53 52cr in mantle derived sources and their weathering products implications for environmental studies and the evolution of δ53 52cr in the earth s mantle over geologic time here we report chromium isotope compositions expressed as delta cr 53 52 in per mil relative to nist 979 measured in selected cr rich minerals and rocks formed by the primary magmatic as well as the secondary metamorphic and weathering processes the main objectives of this study were i to further constrain the isotope composition of the earth s mantle cr inventory and its possible variation during geological history based on the analysis of globally distributed and stratigraphically constrained mantle derived chromites and ii to investigate the magnitude and systematics of cr isotope fractionation during oxidative weathering and secondary alteration i e hydration serpentinization of the magmatic cr sources specifically we analyzed delta cr 53 52 in a set of globally distributed mantle derived chromites femgcr2o4 n 30 collected from various locations in europe asia africa and south america and our results confirm that a chromite hosted earth s mantle cr inventory is uniform at 0 079 0 129 2sd which we named here as a canonical mantle d 53 52 cr signature furthermore our dataset of stratigraphically constrained chromites whose crystallization ages cover most of the earth s geological history indicate that the bulk cr isotope composition of the chromite hosted mantle inventory has remained uniform within about 0 100 since at least the early archean times similar to 3500 million years ago ma to investigate the systematics of cr isotope fractionation associated with alteration processes we analyzed a number of secondary cr rich minerals and variably altered ultramafic rocks i e serpentinized harzburgites lherzolites that revealed large positive delta cr 53 52 anomalies that are systematically shifted to higher values with an increasing degree of alteration and serpentinization the degree of aqueous alteration and serpentinization was quantified by the abundances of fluid mobile rb k elements and by the loss on ignition loi parameter which determines the amount of structurally bound water oh h2o present in secondary hydrated minerals like serpentine overall we observed that altered ultramafic rocks that yielded the highest loi values and the lowest amounts of fluid mobile elements also yielded the heaviest delta cr 53 52 signatures therefore we conclude that secondary alteration i e hydration serpentinization of ultramafic rocks in near surface oxidative environments tend to shift the bulk cr isotope composition of the weathered products to isotopically heavier values pointing to a dynamic redox cycling of cr in the earth s crustal and near surface environments hence if validated by future

the chromium isotope composition of reducing and oxic marine sediments the chromium cr isotope composition of marine sediments has the potential to provide new insights into the evolution of earth surface redox conditions there are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport the major source of seawater cr and with partial cr reduction during burial in marine sediments the major sink for seawater cr a more comprehensive understanding of these processes is needed to establish global cr isotope mass balance and to gauge the utility of cr isotopes as a paleoredox proxy for these purposes we investigated the cr isotope composition of reducing sediments from the upwelling zone of the peru margin and the deep cariaco basin chromium is present in marine sediments in both detrital and authigenic phases and to estimate the isotopic composition of the authigenic fraction we measured 53cr on a weakly acid leached fraction in addition to the bulk sediment in an effort to examine potential variability in the cr isotope composition of the detrital fraction we also measured 53cr on a variety of oxic marine sediments that contain minimal authigenic cr the average 53cr value of the oxic sediments examined here is 0 05 0 10 2 n 25 which is within the range of 53cr values characteristic of the bulk silicate earth this implies that uncertainty in estimates of authigenic 53cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of cr in detrital versus authigenic phases rather than to the cr isotopic composition of the detrital pool leaches of cariaco basin sediments have an average 53cr value of 0 38 0 10 2 n 7 which shows no dependency on sample location within the basin and is close to that of atlantic deepwater cr 0 5 this suggests that authigenic cr in anoxic sediments may reliably reflect the first order cr isotope composition of deepwaters for peru margin samples the average 53cr values of bulk sediments 0 59 0 06 2 and leach fractions 0 61 0 06 2 are also comparable with those of cariaco basin samples and modern deepwater values finally we found that the 53cr of peru margin samples correlates with 15n on glacial interglacial timescales which we attribute to secular variation in basinal or global ocean redox conditions thus the 53cr stratigraphic record of peru margin sediments indicates that cr isotopes may be suited to tracking geologically short term changes in ocean oxygenation

isotopic fractionation of oxygen and nitrogen in coastal marine sediments abstract the bulk and isotopic exchanges of o2 n2 no3 and nh4 between sediments and the overlying water benthic flux were measured at three shallow locations in puget sound using a benthic tripod oxygen consumption by the sediments averaged 4 4 mmol m 2 d 1 while n2 gas fluxes averaged 1 1 mmol m 2 d 1 the nitrogen gas flux out of the sediments was always greater than the corresponding no3 flux into the sediments indicating a large nitrification contribution to the n2 flux isotopic changes in overlying water 18o of o2 were small with an average apparent sediment respiration fractionation factor e of 3 this fractionation factor is much smaller than that measured in either the open ocean or in the laboratory the low degree of fractionation is hypothesized to occur because of the interaction of diffusive layers around reactive microsites within these sediments there was no apparent isotopic fractionation of downward diffusing no3 e 0 while the ammonium that diffused out of the sediments averaged 4 5 o heavier than both source organic matter within the sediments and overlying water no3 the large discrepancy in isotopic composition between the source organic material and nh4 can be attributed to significant isotopic fractionation of ammonium during nitrification in the oxic zone finally the 15n of n2 did not change significantly over the course of the incubations in agreement with the nitrogen isotopic budget for these sediments

oxygen climate and the chemical evolution of a 1400 million year old tropical marine setting the xiamaling formation is an exceptionally well preserved sedimentary succession deposited on a marine passive margin about 1400 million years ago we used a multi proxy approach including iron speciation trace metal dynamics and organic geochemistry to explore the evolution of ocean chemistry through most of the xiamaling formation this evolution is put in the context of the paleogeography and the sedimentological evolution of the xiamaling depositional system overall the xiamaling formation is informally divided into six units based on both sedimentological and geochemical criteria of the six units we fully explored four of them unit 4 the lowest unit we studied is comprised of deep water red muds periodically interrupted by green colored silt and sandy turbidites iron extraction results show that the red muds are enriched in highly reactive iron indicating a water column source for the iron however the low organic carbon contents low hydrogen index hi values and the oxidized nature of the reactive iron pool indicate deposition in oxygenated bottom waters we interpret unit 4 to represent a low productivity ferruginous oxygen minimum zone omz environment underlain by oxygenated bottom waters the transition to unit 3 reflects an increase in primary productivity and the development of a more biologically active omz that supported anoxygenic phototrophic bacteria still in this unit the bottom waters remained oxygenated the overlying unit 2 represents the transition to deep water deoxygen ation and anoxic waters at the sediment surface these waters were ferruginous in the bottom part of the unit and sulfidic euxinic towards the top in the uppermost unit 1 euxinic conditions continued punctuated by more frequent water column oxygenation towards the upper part of the unit we place the evolution of these chemical dynamics in the context of climate and climate change and in particular the placement of the xiamaling formation in relation to the intertropical convergence zone itcz and the resulting hadley cell dynamics also while our results demonstrate the persistence of anoxic water column conditions high in the water column during the deposition of the xiamaling formation they also demonstrate bottom water oxygenation near the seafloor during the deposition of three of the four units and over a time interval extending to 10 s of millions of years less

timing and nature of magmatic fabrics from structural relations around stoped blocks abstract structural relations around several large 100 m stoped blocks in the granodiorite of castle creek sierra nevada california suggest that magmatic fabrics formed as or after the space making phase of pluton emplacement had ended the stoped granite blocks are approximately 360 m below the exposed pluton roof magmatic foliations in granodiorite surrounding the blocks commonly follow km scale orientation trends to within 1 m of block contacts where they become deflected parallel to the block boundaries locally undeflected foliations occur within a few centimeters of block contacts mafic enclave fabric ellipsoid intensities 0 2 we draw the following conclusions concerning magmatic fabrics in the granodiorite of castle creek 1 magmatic fabrics formed during and or after the latest stages of pluton emplacement and thus contain little or no information about space making processes 2 magmatic fabrics had poor strain memory and 3 during magmatic fabric formation pluton strains were partially decoupled from host rock strains published data from other plutons indicate that these relationships may be common particularly in the brittle upper crust it follows from these conclusions that even in the best case magmatic fabrics contain only qualitative information about pluton emplacement mechanisms regional tectonics and internal magma chamber processes interpretations of magmatic fabrics must be corroborated by evaluating causes kinematics and magnitudes of syn emplacement host rock deformation in the worst case the causes of magmatic fabrics cannot be uniquely determined

redox fluctuations during the ediacaran cambrian transition nanhua basin south china insights from cr isotope and ree y data abstract reconstructing fluctuations in marine and atmospheric redox conditions during the late precambrian early cambrian is essential to better our understanding of the link between oxygen and biological evolution this study presents ree y and cr isotopic data together with toc data and redox sensitive trace metal concentrations of marine black shales and cherts from the cambrian jiumenchong formation and ediacaran liuchapo formation from south china in order to improve our understanding of the redox conditions in the nanhua basin during this critical time period the liuchapo formation records negative ce anomalies and highly fractionated 53cr values of up to 1 06 0 09 suggesting deposition under oxic conditions and implying high oxidative weathering rates during the end of the ediacaran the overlying jiumenchong formation is characterised by a stratigraphic upward decrease in 53cr values to typical igneous values as well as a decrease in negative ce anomalies and an increase in positive eu anomalies suggesting an increase in detrital content due to more aeolian weathering and deposition under anoxic conditions at the beginning of the cambrian the latter is also confirmed by the trends in v cr and th u ratios which further indicate an eventual return to dys oxic seawater conditions during the early cambrian

emplacement and deformation of the a type madeira granite amazonian craton brazil abstract the madeira granite is one of the paleoproterozoic 1 82 ga a type granite intrusions in the amazonian craton it is elongated in the ne sw direction and is composed of four facies classical structural techniques and the anisotropy of magnetic susceptibility ams method were applied to the study of its internal fabric magnetic susceptibility measurements thermomagnetic curves remanent coercivity spectra optical microscopy and sem scanning electron microscopy analyses were carried out on the earlier and later facies of the madeira granite the rapakivi granite rg and the albite granite ag respectively the last one is subdivided into the border albite granite bag and the core albite granite cag subfacies ams fabric pattern is controlled by pure magnetite in all facies despite significant amounts of hematite in the bag subfacies microstructural observations show that in almost all sites magnetic fabric correlates to magmatic state fabrics that are defined by a weak ne sw orientation of mafic and felsic silicates however strain mechanisms in both subfacies of ag also exhibit evidence for solid state deformation at high to moderate temperatures pegmatite dyke strike slip fault sf a b c hydrothermal vein normal fault f 1 2 and joint j structures were observed and their orientation and kinematics is consistent with the magmatic and solid state structures dykes sf a c and f 1 are usually orientated along the n70 e 40 n plane which is nearly parallel to the strike of ams and magmatic foliations in contrast veins sf b f 2 and some j are oriented perpendicular to the n70 e trend kinematic analysis in these structures shows evidence for a dextral sense of movement in the system in the brittle regime the coherent structural pattern for the three facies of madeira granite suggests that the different facies form a nested pluton the coherence in orientation and kinematics from magmatic to high temperature solid state and into the brittle regime indicates the continuity in the stress regime from the last magmatic stages until the complete cooling of the pluton likely along a ne sw dextral corridor related to the regional deformation in the uatuma anaua domain of the amazonian craton

batholith emplacement at mid crustal levels and its exhumation within an obliquely convergent margin abstract emplacement of the central part of the coast mountains batholith of northern coastal british columbia occurred within a regime characterized by oblique convergence between the farallon kula and north american plates we use new structural kinematic and u pb isotopic data to show that the locations geometry and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones these data also show that the most active period of pluton emplacement from 67 to 51 ma coincided with a change in style of deformation within the batholith prior to 67 ma plutons were emplaced within an arc dominated by regional scale contractional shear zones in contrast emplacement of 67 51 ma plutons occurred in an arc increasingly dominated by normal faults with arc parallel to oblique displacement and by sinistral transtensional shear zones we have identified and mapped the structure of three plutonic complexes composed of 67 to 51 ma plutons the khyex sill complex arden lake plutonic complex and quottoon plutonic complex shear zone controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes during and after this latest cretaceous paleogene period of intense plutonic activity and accompanying deformation the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 ma

the stable cr isotope inventory of solid earth reservoirs determined by double spike mc icp ms abstract we present the first comprehensive set of stable cr isotope data for the major igneous silicate earth reservoirs cr iii rich ores and minerals and hydrothermal chromates these were determined by mc icp ms using a double spike technique mantle xenoliths ultramafic rocks cumulates as well as oceanic and continental basalts share a common cr isotope composition with an average 53 52 cr value of 0 124 0 101 2 sd relative to the isotopically certified chromium standard nist srm 979 an isotopic difference between mantle xenoliths and basalts as was reported for iron was not observed for chromium thus the change in oxidation state that is observed when solid mantle rocks containing only trivalent chromium partially melt to form basaltic melts which predominantly contain bivalent chromium does not cause any measurable cr isotope fractionation chromite separates from major chromitite seams of the bushveld and great dyke layered igneous complexes are invariable in their cr isotope compositions and reproduce within uncertainties the average 53 52 cr srm 979 value of igneous silicate earth reservoirs this is important for environmental stable cr isotope studies because it labels the approximate isotope composition of industrial cr vi pollutants the cr isotope compositions of hydrothermal lead chromates crocoites pbcr vi o 4 from various localities yield 53 52 cr srm 979 values between 0 640 and 1 037 these cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached precipitation experiments revealed an isotope fractionation of 53 52 cr crocoite cr vi aq of only ca 0 10 0 05 at a temperature of 20 c thus the heavy cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes

global variability of chromium isotopes in seawater demonstrated by pacific atlantic and arctic ocean samples abstract seawater chromium cr isotope and concentration data are presented from multiple sites in the arctic ocean and three locations in the atlantic and pacific oceans a 2400 m profile illustrates the heterogeneity of 53cr in the arctic ocean with depth and water mass source pacific vs atlantic the highest 53cr values occur in pacific sourced waters which also have the lowest cr concentration chromium concentration and 53cr data from these locations in conjunction with published data for the south atlantic ocean yield a simple logarithmic function as predicted by rayleigh fractionation in a closed system the observed cr isotope signature is hypothesized to arise from fractionation during the reduction of cr vi in surface waters and oxygen minimum zones scavenging of isotopically light cr iii to deeper water and sediment and subsequent release of this seawater derived cr iii back into seawater either as organic complexes with cr iii or after oxidation to cr vi the isotopic fractionation factor e associated with cr cycling in seawater is estimated to be 0 80 0 03 2 samples from the sea ice affected surface mixed layer of the arctic ocean 10 m depth deviate from the general trend and samples proximal to rivers illustrate geographic variation in 53cr values for continental runoff but prompt loss of this signature away from the source

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength

theoretical estimates of equilibrium chromium isotope fractionations equilibrium chlorine isotope 37 cl 35 cl fractionations have been determined by using published vibrational spectra and force field modeling to calculate reduced partition function ratios for cl isotope exchange ab initio force fields calculated at the hf 6 31g d level are used to estimate unknown vibrational frequencies of 37 cl bearing molecules whereas crystalline phases are modeled by published lattice dynamics models calculated fractionations are principally controlled by the oxidation state of cl and its bond partners molecular mass or the absence of c h bonds also appears to play a role in determining relative fractionations among simple cl bearing organic species molecules and complexes with oxidized cl i e cl 0 cl etc will concentrate 37 cl relative to chlorides substances with cl at 298 k clo 2 containing cl 4 and clo 4 containing cl 7 will concentrate 37 cl relative to chlorides by as much as 27 and 73 respectively in rough agreement with earlier calculations among chlorides 37 cl will be concentrated in substances where cl is bonded to 2 cations i e fecl 2 mncl 2 micas and amphiboles relative to substances where cl is bonded to 1 cations such as nacl by 2 to 3 at 298 k organic molecules with c cl bonds will be even richer in 37 cl 5 to 9 at 298 k precipitation experiments in combination with our results provide an estimate for cl isotope partitioning in brines and suggest that silicates to the extent that their cl atoms are associated with nearest neighbor 2 cations analogous with fecl 2 and mncl 2 will have higher 37 cl 35 cl ratios than coexisting brine by 2 to 3 at room temperature calculated fractionations between hcl and cl 2 and between brines and such alteration minerals are in qualitative agreement with both experimental results and systematics observed in natural samples our results suggest that cl bearing organic molecules will have markedly higher 37 cl 35 cl ratios by 5 8 to 8 5 at 295 k than coexisting aqueous solutions at equilibrium predicted fractionations are consistent with the presence of an isotopically heavy reservoir of hcl that is in exchange equilibrium with cl aq in large marine aerosols

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr