magnesium isotopic systematics of mafic rocks during continental subduction abstract magnesium isotopic compositions of a set of prograde metamorphosed rocks greenschists amphibolites and eclogites from east china were measured in order to understand the behavior of mg isotopes during continental subduction the 26mg values of the greenschists vary from 0 269 0 057 to 0 133 0 042 2sd and correlate negatively with the mgo contents and nb u ratios possibly due to the assimilation of isotopicallly heavy felsic schists 26mg up to 0 099 during the genesis of morb like protoliths the two subgroups of amphibolites have slightly different mg isotopic compositions the group i amphibolites with cumulate origin have 26mg values ranging from 0 243 0 061 to 0 192 0 050 which reflects clinopyroxene accumulation nature of the protolith the group ii amphibolites have relatively light 26mg values varying from 0 358 0 061 to 0 224 0 056 which may result from either source contamination or crustal assimilation of isotopically light components e g carbonates eclogitic garnet and clinopyroxene display large inter mineral mg isotope fractionations 26mgcpx grt 1 097 0 056 to 1 645 0 081 equilibrated at peak metamorphic temperatures the 26mg values of the eclogites vary from 0 348 0 041 to 0 137 0 063 overlapping the ranges of greenschists and amphibolites such mg isotopic variations are not related to unrepresentative sampling nor host eclogite chemical interactions instead they reflect the protolith heterogeneities collectively the similarity of mg isotopic compositions among the greenschists 0 196 0 044 amphibolites 0 271 0 042 and eclogites 0 226 0 044 suggests that mg isotope fractionation during continental subduction if any is limited

living coccolithophore communities in the norwegian greenland sea and their record in sediments abstract investigation of epipelagic coccolithophore communities along a transect from the voring plateau off norway to scoresbysund east greenland and comparison with coccolith assemblages in underlying surface sediments reveal that sedimentation of coccoliths is dependent on the following factors 1 composition of the coccolithophore communities and their bloom phases 2 grazing and selective destruction by zooplanktonic organisms 3 type of vertical flux in fecal pellets and 4 varying carbonate dissolution in different water masses dissolution increases towards the boundaries of carbonate sedimentation as a result the original composition of the living communities becomes increasingly obscured in the impoverished taphocoenoses however differential resistance to dissolution of the two primary calcareous nannoplankton species emiliana huxleyi and coccolithus pelagicus is such that occurrence and ratio of the two species in surface sediments reflect the various water masses in the norwegian greenland sea

enamel diagenesis at south african australopith sites implications for paleoecological reconstruction with trace elements abstract elemental ratio data from archaeological and paleontological bone have often been used for paleoecological reconstruction but recent studies have shown that even when solubility profiling techniques are employed in an attempt to recover biogenic signals bone is an unreliable material as a result there has been renewed interest in using enamel for such studies as it is known to be less susceptible to diagenesis nevertheless enamel is not immune from diagenetic processes and several studies have suggested that paleoecologically relevant elements may be altered in fossil enamel here we investigate sr ba zn and pb compositions of enamel from south african karstic cave sites in an effort to ascertain whether or not this material provides reliable paleoecological information we compared enamel data for mammals from three fossil sites aged 1 8 3 0 ma all of which are on dolomites with data from modern mammals living on dolomitic and granitic substrates sr ca and ba ca are about three times higher in enamel from modern mammals on granites than those living on dolomites stressing the need for geologically appropriate modern fossil comparisons after pretreatment with dilute acid we found no evidence of increased sr ca ba ca or pb ca in fossil enamel in contrast zn ca increased by over five times at one site makapansgat but much more subtly elsewhere ecological patterning in sr ca ba ca and sr ba ratios was also retained in fossil enamel this study suggests that sr ca ba ca and pb ca data likely preserve paleoecological information from these sites but also demonstrates that geologically similar sites can differ in the degree to which they impart certain elements zn in this case to fossils thus screening is probably necessary on a site by site basis lastly further investigation of elemental distributions in modern foodwebs is necessary before elemental ratio analysis can become a common tool for paleoecological reconstruction

an investigation of barium calcium and strontium as palaeodietary indicators in the southwestern cape south africa samples of modern indigenous foods and archaeological human skeletons from the southwestern cape of south africa were analyzed for barium strontium and calcium barium strontium and barium calcium successfully differentiate marine and terrestrial foods measurement of all three elements improves the resolution of the marine terrestrial separation and enhances the trophic level differentiation analysis of the archaeological human skeletons indicates that removal of diagenetic contamination is critical in order to obtain useful elemental analyses where such removal is possible barium strontium and barium calcium techniques can be particularly valuable in distinguishing marine and terrestrial inputs into prehistoric human diets in areas where stable carbon analyses are not applicable e g in regions with terrestrial c 4 flora

massive magnesium depletion and isotope fractionation in weathered basalts abstract magnesium isotopes have been shown to fractionate significantly during continental weathering however the degree and direction of fractionation varies from one study to the next and the main factors that control mg isotope fractionation during weathering have yet to be delineated here we report mg contents and isotopic compositions for two 10 m deep drill cores through bauxites developed on columbia river basalts crbs samples from these two drill cores have very low mgo contents 0 12 0 25 wt and variable but high 26mg values from 0 1 to up to 1 8 the heaviest isotopic composition ever reported for mg relative to the average values of fresh crbs 26mg 0 23 0 07 and mgo 5 9 wt these features reflect the near complete loss of mg in the isotopically heavy regolith the most highly weathered bauxites at the tops of the profiles have the lowest 26mg values reflecting the addition of isotopically light eolian dust excluding these samples 26mg in bauxites displays a weak positive correlation with gibbsite abundance suggesting that gibbsite preferentially retains 26mg in the bauxites the integrated mg isotopic fractionation factors between bauxite and fluid 26mgbauxite fluid inferred from rayleigh fractionation calculations vary from 0 05 to 0 4 which is similar to the range inferred from previous studies of igneous rock weathering profiles though none of these previous studies found the extremely heavy 26mg and massive mg depletion seen here collectively studies of weathering profiles suggest loss of isotopically light mg from the continents which must contribute to the very light 26mg seen in river water and seawater

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

positive correlation between li and mg isotope ratios in the river waters of the mackenzie basin challenges the interpretation of apparent isotopic fractionation during weathering during chemical weathering magnesium mg is released by the dissolution of both carbonate and silicate sources the degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs or cycling by a series of processes associated with weathering is poorly constrained in the river waters of the mackenzie basin canada the mg content is high and mg isotope ratios 26mg 24mg expressed as mg 26 show in excess of one per mil variability part of this variability is attributed to the 3 range in the carbonate and silicate rocks drained despite this inherent lithological control on river water mg 26 values there is also evidence for a fractionation control a linear positive covariation between lithium 7li 6li expressed as li 7 and mg isotope ratios in the river waters of the mackenzie basin is reported this covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with mg isotope ratios

boron and magnesium isotopic composition of seawater the isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes the isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard given the long residence times of both elements 10 7 years it is commonly assumed that seawater is isotopically homogenous for these systems yet no systematic studies currently exist here we present the b and mg isotopic composition of 26 28 seawater samples from a number of ocean basins that encompass a significant range in salinity 32 to 38 psu temperature 0 3 to 25 9c and water depth 0 to 1240 m we find no significant or systematic variation for either system in accordance with their long residence times we recommend that the mean values we report 11 b 39 61 0 04 2 s e n 28 25 mg 0 43 0 01 2 s e n 26 26 mg 0 82 0 01 2 s e n 26 be used in future studies involving mg and b isotopes copyright 2010 by the american geophysical union

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

dinoflagellate cyst distribution in high latitude marine environments and quantitative reconstruction of sea surface salinity temperature and seasonality a data base of 179 reference sites documents the relations between the assemblages of organic walled dinoflagellate cysts and sea surface temperature salinity and seasonality throughout the north atlantic adjacent subpolar basins labrador sea baffin bay irminger and iceland basins and epicontinental environments off eastern canada estuary and gulf of st lawrence hudson bay principal component analyses show close relationships between dinoflagellate cyst data and sea surface conditions the first component 71 1 of the variance correlates with the winter temperature salinity and seasonal duration of sea ice cover whereas the second component 11 3 of the variance appears mainly related to summer temperature transfer functions using the best analogue method were tested by reconstructing modern sea surface conditions on the basis of the reference dinoflagellate cyst assemblages the correlation coefficient between instrumental averages and reconstructed values ranges from 0 87 august tem

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle to investigate whether magnesium isotopes are fractionated during basalt differentiation we have performed high precision mg isotopic analyses by multi collector inductively coupled plasma mass spectrometry mc icp ms on a set of well characterized samples from kilauea iki lava lake hawaii usa samples from the kilauea iki lava lake produced by closed system crystal melt fractionation range from olivine rich cumulates to highly differentiated basalts with mgo content ranging from 2 37 to 26 87 wt our results demonstrate that although these basalts have diverse chemical compositions mineralogies crystallization temperatures and degrees of differentiation their mg isotopic compositions display no measurable variation within the limits of our external precision average 26 mg 0 36 0 10 and 25 mg 0 20 0 07 uncertainties are 2sd this indicates that mg isotopic fractionation during crystal melt fractionation at temperatures of 1055 c is undetectable at the level of precision of the current investigation calculations based on our data suggest that at near magmatic temperatures the maximum fractionation in the 26 mg 24 mg ratio between olivine and melt is 0 07 two additional oceanic basalts two continental basalts bcr 1 and bcr 2 and two primitive carbonaceous chondrites allende and murchison analyzed in this study have mg isotopic compositions similar to the kilauea iki lava lake samples in contrast to a recent report u wiechert a n halliday non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 2007 360 371 the results presented here suggest that the bulk silicate earth has a chondritic mg isotopic composition 2007 published by elsevier b v

origins of two types of archean potassic granite constrained by mg isotopes and statistical geochemistry implications for continental crustal evolution abstract the chemical composition of the upper continental crust ucc dramatically changed from predominantly sodic to more potassic during the late archean 3000 2500 ma although how this change occurred remains poorly constrained the origins of neoarchean potassic granites may provide clues for this crucial change as their formation dominated the compositional transition of the ucc in this study we conducted high precision mg isotopic analyses of neoarchean potassic granites 2558 2520 ma and tonalite trondhjemite granodiorites ttgs 2595 2574 ma from the north china craton and statistical geochemistry in k2o na2o ratios of global archean granitoids results show that these ttgs with k2o na2o ratios of 0 61 and 26mg values of 0 39 to 0 22 were generated from heterogeneous basaltic sources in contrast potassic granites yield two distinctive groups of 26mg values and can be subdivided into i and s types the i type granite has mantle like 26mg values of 0 28 to 0 22 and weak enrichment in k2o k2o na2o 1 0 1 5 and was likely derived by partial melting of igneous sources the s type is characterized by much higher 26mg values of 0 60 to 0 91 and strong k2o enrichment k2o na2o 1 57 23 26 with geochemical characteristics suggesting derivation by partial melting of sediments based on the geochemical characteristics of the s type potassic granites in this study we sort the s type granites out of 4066 archean felsic igneous rocks worldwide their temporal distribution suggests that s type potassic granites have appeared in the middle archean 3500 3000 ma and became widespread in the late archean 3000 2500 ma but were scarce in the early archean 4000 3500 ma the onset of s type granites indicates reworking of supracrustal rocks may have started since 3500 ma and widespread s type potassic granites during 3000 2500 ma play an important role for the high k2o na2o ratio of ucc at the end of archean a mass balance calculation shows that 25 88 wt i type 4 12 wt s type potassic granite and 70 wt ttgs could increase the k2o na2o ratio of the ucc to 0 83 which is the global average value for felsic rocks at the end archean to increase k2o na2o of the archean ucc contributions made by i and s type potassic granites could probably reach 3 2 respectively consequently the evolved crustal composition from predominantly sodic to potassic results from the synergy of re working of igneous unaltered and sedimentary weathered rocks by which the former contributed more quantitatively while the latter contributed more efficiently

mg isotope fractionation during partial melting of garnet bearing sources an adakite perspective abstract garnet that has a distinctly light mg isotopic composition compared to other mafic minerals is commonly present in deep crustal and mantle sources whether partial melting of these garnet bearing sources can lead to significant mg isotope fractionation or not remains poorly understood in order to investigate mg isotope fractionation during partial melting of garnet bearing sources we report mg isotopic data of 17 well characterized low mg adakitic rocks lma from the dabie orogen central china these lma are known for derivation from mafic sources leaving eclogitic residua with variable abundance of garnet our data together with previously published results show that fresh dabie lma have homogeneous mg isotopic compositions with an average 26mg of 0 22 0 08 2sd n 18 except that two samples display higher 26mg 0 05 to 0 01 due to alteration during sericitization and chloritization of biotite the lack of correlation between 26mg and dy yb n of the dabie lma indicates an unresolvable residual garnet effect given that mg isotopic compositions of the dabie lma are identical with local mafic basement within the current precision 26mgmelt cpx is estimated to be around 0 02 limited isotope fractionation of the dabie lma could be attributed to the dilution effect of residual clinopyroxene co existing with garnet the estimated 26mgmelt cpx is then applied to predict the possible isotope fractionation that may occur during hydrous and dehydration melting of subducted oceanic crust with a lower melting temperature and more abundant garnet in the residua slab derived melts may have 26mg higher than their sources by up to 0 36 which however can be easily erased by later melt mantle interaction in this regard mg isotopes can serve as a powerful tracer of recycled supracrustal materials

svalbard ice sheet decay after the last glacial maximum new insights from micropalaeontological and organic biomarker paleoceanographical reconstructions abstract a marine sediment core retrieved from the middle continental slope of the northwestern barents sea was analyzed for its geochemical alkenones and micropalaeontological diatoms coccolithophores and dinocyst content in order to reconstruct the evolution of upper ocean conditions and ice sheet dynamics during the last 25 kyr additionally quantitative reconstructions of sea surface conditions temperature salinity and sea ice cover extent were conducted based on the best analogue technique applied to dinocyst assemblages and on the alkenone unsaturation index the sediment core contains a post last glacial maximum depositional sequence unaffected by stratigraphic discontinuities low salinity and laminated sediments after 20 cal kyr bp indicate a massive settling of meltwater sediment laden plumes from the initial melting of the svalbard barents sea ice sheet on western svalbard first record of measurable alkenones together with a drop of the number of months of sea ice cover and increase in ssts suggests an intensification of the influx of atlantic waters into the study area at 15 cal kyr bp representing the termination of the last glacial period and onset of the bolling interstadial the first occurrence of diatoms and increase in the abundance of all microfossils marked the onset of the holocene at 11 2 cal kyr bp when modern type sea surface conditions were rapidly established in western svalbard reconstructions based on dinocyst data and alkenone unsaturation index suggest relatively warm and stable temperatures between 9 9 and 8 9 kyr bp and a decrease of ssts from 4 2 cal kyr bp to present coinciding with the holocene thermal maximum and the decrease of summer insolation in the high latitude northern hemisphere respectively

coastal upwelling and eddy development off nova scotia satellite photos during the summer of 1984 show the development of a band of cool water along the seaward coast of nova scotia during a month long period of upwelling favorable winds the timing of the appearance of the cool water band the rate at which it broadened and the bottom temperature evolution at several representative points along the coast are shown to be in good agreement with an idealized two layer model of the wind driven shelf circulation the presence of the cool water is clearly attributable to upwelling after the temperature front moves offshore wavelike features develop rapidly with alongshore wavelengths of 50 75 km and similar maximum cross shore displacement amplitudes the associated alongshore propagation speed is at most 0 02 m s 1 to the northeast the length and time scales of these features are shown to be consistent with results from simple linear instability models which allow for both potential and kinetic energy release from the mean state by perturbations results suggest that the wavelike features are primarily due to baroclinic instability of the upwelling associated base state rough estimates suggest that upwelling and subsequent eddy development can play an important role in the supply of nutrients to the surface waters of the shelf

using mg isotope ratios to trace cenozoic weathering changes a case study from the chinese loess plateau abstract the magnesium isotope system is a useful tracer of weathering processes due to its sensitivity to secondary mineral formation we present elemental and isotopic data in bulk loess and paleosol sediments from the luochuan profile in the chinese loess plateau clp due to a relatively constant pre weathering mineralogy changes in the sediment chemistry with depth can be interpreted to reflect changes in weathering processes that are ultimately controlled by climate we find that the luochuan sediments are all enriched in light mg relative to the upper continental crust ucc 0 22 the isotopically light signature is a result of three way mixing between mg sourced from silicate calcite and dolomite changes in 26 mg between loess and paleosol reflect the higher weathering intensities during pedogenesis and corresponding loss of easily leachable calcite enriched in light mg isotopes from the loess layers this is confirmed by the results of a simple leaching procedure in which the leachate phase contains high ca concentrations and isotopically light 26 mg values of up to 2 any isotopic fractionation of mg imparted by isotopically heavy clay mineral formation appears to be insignificant and is obscured by the presence of detrital dolomite that is resistant to acid leaching and is isotopically light the isotopic composition of mg at luochuan broadly follows the magnetic susceptibility record in that loess layers are relatively isotopically light and have lower magnetic susceptibilities than their adjacent paleosol as such the changes in 26 mg values with depth do correspond to changes in chemical weathering however the driving factor behind these changing 26 mg values is carbonate preservation which in itself is not a highly sensitive paleoclimate proxy from a global perspective the 26 mg value of luochuan loess is similar to other loess sediments which are often isotopically light relative to the ucc we have recalculated the average 26 mg value of global loess sediments to be 0 58 1 18 this reflects the fact that 26 mg values in loess are highly heterogeneous and are generally enriched in 24 mg compared to typical ucc rocks due to preservation of high amounts of isotopically light carbonate minerals for this reason loess sediments cannot be used to approximate the isotopic composition of mg in the earth s crust

magnesium isotopic systematics of the makran arc magmas iran implications for crust mantle mg isotopic balance abstract continental crust loses mg by lower crustal foundering and chemical weathering to become its intermediate silicic composition both processes should preferentially retain heavy mg isotopes in the crust yet the estimated mg isotopic ratio for the bulk continental crust is indistinguishable from that for unmodified mantle this can be elucidated by two notions that are not mutually exclusive i the basaltic parent to the crust has mg isotopes lighter than the mantle and has since become mantle like as a result of mg loss by igneous and weathering fractionation and ii heavy mg isotopes in the continental crust are constantly balanced by a hidden input of light mg isotopes arc magmas are arguably building blocks of the continental crust and their mg isotopic signature could be used to place limits on the above views here we conducted a mg isotopic study of the makran arc a rare continental arc in iran within the alpine himalayan orogenic belt the measured mg isotopic ratios for most mafic and intermediate samples are mantle like with 26mg values ranging from 0 32 to 0 20 after excluding dubious samples affected by alteration the silicic samples span a broad range of isotopic ratios with 26mg values ranging from 0 32 to 0 32 beyond that of unmodified mantle 26mg 0 25 0 04 the isotopic data form a trend from mantle like 26mg at relatively high mgo towards elevated 26mg at low mgo requiring the isotopic variability be related to differentiation in an open system involving assimilation of high 26mg crustal rocks our results are generally consistent with mg isotopic data compiled for global arcs where magmas have mg isotopic ratios ranging from mantle like to 0 7 per mil above the mantle range and compositions substantially below that range are rare thus heavy mg isotopes in the continents predicted by igneous and weathering fractionation might have been compensated by a hidden input of light mg isotopes which is presumably in the form of carbonates leading to a bulk crustal mg isotopic composition that is mantle like

distribution of dinoflagellate cysts in surface sediments of the northern north atlantic in relation to nutrient content and productivity in surface waters abstract analyses of dinoflagellate cyst assemblages from the surface sediments of 371 sites from the north atlantic ocean were performed in order to define relationships with nitrate phosphate silica and productivity in the upper water layer statistical analyses reveal close links between dinocyst assemblages and nitrate content in february that provide a measurement of nitrate availability amongst the taxa in the assemblages nematosphaeropsis labyrinthus which positively correlates with nutrients shows an eutrophic relationship whereas impagidinium aculeatum impagidinium patulum spiniferites ramosus spiniferites mirabilis and lingulodinium machaerophorum have a rather oligotrophic relationship on these grounds transfer functions were tested using multiple regressions and the best analogue technique to reconstruct nitrate content both methods yielded reasonable estimates and allow the reconstruction of the nitrate concentrations in february with an accuracy better than 1 45 mol m 3 at the scale of the study area there are also tenuous links between the dinocyst assemblages and productivity transfer functions were tentatively tested for the reconstruction of annual productivity and yield estimates with an accuracy of about 26 5 gc m 2 year 1 these results demonstrate that dinocyst assemblages can be used for the reconstruction of productivity and nitrates

a cold and fresh ocean surface in the nordic seas during mis 11 significance for the future ocean paleoceanographical studies of marine isotope stage mis 11 have revealed higher than present sea surface temperatures ssts in the north atlantic and in parts of the arctic but lower than present ssts in the nordic seas the main throughflow area of warm water into the arctic ocean we resolve this contradiction by complementing sst data based on planktic foraminiferal abundances with surface salinity changes using hydrogen isotopic compositions of alkenones in a core from the central nordic seas the data indicate the prevalence of a relatively cold low salinity surface water layer in the nordic seas during most of mis 11 in spite of the low density surface layer which was kept buoyant by continuous melting of surrounding glaciers warmer atlantic water was still propagating northward at the subsurface thus maintaining meridional overturning circulation this study can help to better constrain the impact of continuous melting of greenland and arctic ice on high latitude ocean circulation and climate

magnesium isotopic variations in loess origins and implications abstract loess deposits serve as important continental archives for studying quaternary climatic variations and for estimating the average chemical composition of the upper continental crust here we report high precision mg isotopic data for 19 loess samples from china argentina and europe which were previously used to estimate the composition of the upper continental crust the results show that global loess samples have heterogeneous mg isotopic compositions with 26mg per mil deviation of the 26mg 24mg ratio from the dsm3 standard values ranging from 1 64 to 0 25 and a weighted average of 0 89 which is lighter than both crust and mantle silicates mgo content and 26mg of loess positively correlate with cao al2o3 ratio suggesting a two component mixing between carbonates and secondary silicate minerals the large variation in mg isotopic composition of loess results from a combination of factors including source heterogeneity eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit our results suggest that mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change but may not represent the average mg isotopic composition of the upper continental crust due to mixing sorting of isotopically distinct components and isotope fractionation during loess deposit formation

calcareous nannofossil changes during the mid pleistocene revolution paleoecologic and paleoceanographic evidence from north atlantic site 980 981 abstract new quantitative data on calcareous nannofossil assemblages through 583 1138 thousand years ago kyr have been acquired in high sample resolution 2 3 ky at northern atlantic odp ocean drilling program site 980 981 assemblage composition reflects a temperate paleobioprovince suggesting significant influx of warm north atlantic current at the site location during the mid pleistocene transition between marine isotope stage mis 35 and 15 three intervals have been described based on changes in nannofossil composition and abundance fluctuations of taxa reticulofenestrids are abundant just in the first interval mis 35 22 which is also characterised by the absence of gephyrocapsa 4 m and c pelagicus s l the latter occurring in upper mis 24 only the second interval corresponds to the expanded warm mis 21 that is accompanied by high carbonate content absence of both nannofossil reworking and ice rafted debris ird higher abundance of c leptoporus 5 8 m and h carteri and absence of c pelagicus s l and medium gephyrocapsids the third interval mis 20 15 records the occurrence of c pelagicus s l and medium gephyrocapsa generally higher abundance and higher amplitude fluctuations of helicoliths and absence of reticulofenestrids the major changes in nannofossil assemblages seem to be the result of combined paleoenvironmental factors related to sea surface water features temperature and nutrient availability sea ice cover and possibly to deep and intermediate water production of both northern and southern hemisphere origin during glacial and interglacial phases complex inter relationships between environmental parameters and evolutionary ecological factors are inferred to explain the patterns of coccolithus pelagicus subspecies whose distribution and abundance fluctuations are compared with the available patterns of turborotalia quinqueloba and neogloboquadrina pachyderma left coiled at site 980 and with the c pelagicus subspecies patterns from north atlantic dsdp deep sea drilling project site 607 and south atlantic odp site 1090 data collected provide additional information on stratigraphic distribution of selected taxa the first occurrence fo of gephyrocapsa omega is confirmed as a diachronous event even in the atlantic ocean while the first and last common occurrence fco lco of r asanoi appear as diachronous and synchronous events respectively on a global scale

comparison of coccolith and dinocyst assemblages in the northern north atlantic how well do they relate with surface hydrography abstract coccolith and dinoflagellate cyst or dinocyst population counts were compiled from existing surface sediment databases as well as new counts in order to establish an 87 sample database for which assemblages of both microfossil groups are known this database allowed a direct comparison of the distribution of coccolith and dinocyst assemblages in the subtropical to subpolar north atlantic in addition the relationship between these assemblages and sea surface environmental parameters was addressed in order to identify possible differences in the ecology of the two plankton groups the comparison highlights an excellent correspondence between dinocyst assemblages coccolith assemblages and the distribution of the surface water masses represented in our database notably in the subtropical and temperate domains in the subpolar domain coccolith assemblages are much less diversified than dinocyst assemblages in terms of species as a result the discrimination between the subpolar water masses based on coccolith assemblages is not as clear as in the subtropical temperate regions whereas dinocyst assemblages show a distribution pattern closely related with surface hydrography canonical correspondence analyses performed on coccolith and dinocyst assemblages show that sea surface temperature is the primary environmental parameter influencing the distribution of both groups dinocyst assemblages also seem to respond to the distance to the coast and may therefore bring additional information compared with coccolith assemblages other significant environmental factors include sea surface salinity and productivity but their relative importance changes depending on the inclusion of samples from extreme environmental settings in the database results suggest that the complexity of the interrelationships between the various environmental parameters makes it difficult to adequately bring to light all the different environments and their associated coccolith dinocyst assemblages in multivariate analyses however each surface water mass represented in our database is characterized by a unique combination of environmental parameters as well as by distinct associations of coccolith and dinocyst assemblages thus showing that these microfossil groups closely relate to sea surface conditions including temperature salinity and productivity

intensified chemical weathering during early triassic revealed by magnesium isotopes abstract marine ecosystem recovery after the latest permian mass extinction lpme was a protracted process during the early triassic 252 247 ma owing to repeated climatic and environmental perturbations chemical weathering can supply nutrients to the ocean and may have played an important role in the early triassic carbon cycle and biological recovery however only limited geochemical records of chemical weathering during the early triassic have been presented to date and the relationship between changes in weathering intensity and the slow recovery of marine ecosystems is effectively unknown here we report magnesium mg isotopic compositions of the siliciclastic components from shallow marine carbonates in two well studied upper permian lower triassic sections in iran and south china to track changes in chemical weathering intensity after the lpme both sections display a wide range of 26mg values 2 09 to 1 10 at zal 2 30 to 0 33 at zuodeng we identified two distinct stages i and ii in each section based on 26mg values and major elemental ratios variations of 26mg values in stage i changhsingian to mid dienerian are mainly controlled by mineralogical composition that can obscure weathering signals by contrast 26mg variations in stage ii upper dienerian to upper spathian are independent of lithology and inferred to reflect control by chemical weathering intensity the trends in chemical weathering intensity within stage ii correspond to first order variations in climate and carbon cycling marine ecosystem recovery during the early triassic may have been linked to recurrent episodes of intense chemical weathering caused by co2 degassing and climate warming this study demonstrates the potential utility of the mg isotopic compositions of the silicate fraction in marine carbonates as a proxy for chemical weathering intensity laying the groundwork for general applications of this method to deep time earth systems

co variation of mg and c isotopes in late precambrian carbonates of the siberian platform a new tool for tracing the change in weathering regime abstract this study reports significant mg isotopic variations in 32 samples of late precambrian dolomites from various parts of the siberian platform exhibiting a range of 13 c from 10 to 6 v pdb in the ediacaran dolomites from the n and ne part of the patom paleobasin a systematic increase of 26 mg from 2 3 0 2 to 0 8 0 3 dsm3 along with a 13 c decrease from 6 to 10 26 mg 0 072 13 c 1 83 with a correlation coefficient of r 2 0 73 n 20 are observed within the stratigraphic sequence of ediacaran and ediacaran lower cambrian transition successions all positive c isotope excursions co vary with negative mg isotope shifts and vice versa other samples from the siberian platform lay on this regression with r 2 0 66 n 28 except for 4 samples probably diagenetic dolostones that exhibit a positive 26 mg 13 c correlation the enrichment of sedimentary dolomite in heavy carbon and light mg isotopes may be interpreted as a result of a complex interplay of contrasting sedimentation conditions sea regression versus sea transgression regime yielding different weathering regimes and relative contributions of carbonate versus silicate rocks the enrichment of both oceanic water and carbonate sediments by 26 mg could occur during transgression and would coincide with sedimentation of low 13 c carbonates whereas sedimentation of the depleted in 26 mg and enriched in 13 c carbonates could happen under regression regime when exposed carbonate platforms and coastal planes are subjected to intense weathering

apparent long term cooling of the sea surface in the northeast atlantic and mediterranean during the holocene reconstructions of upper ocean temperature t during the holocene 10 0 ka b p were established using the alkenone method from seven high accumulation sediment cores raised from the northeast atlantic and the mediterranean sea 361n 751n all these paleo t records document an apparent long term cooling during the last 10 kyr in records with indication of a constant trend the apparent cooling ranges from 0 27 to 0 151c kyr 1 records with indication of time variable trend show peak to peak amplitudes in apparent temperatures of 1 2 2 91c a principal component analysis shows that there is one factor which accounts for a very large fraction 67 of the total variance in the biomarker paleo t records and which dominates these records over other potential secondary influences two possible contributions are 1 a widespread surface cooling which may be associated with the transition fromthe hypsithermal interval b9 5 7 ka b p to the neoglaciation b5 7 0 ka b p and 2 a change in the seasonal timing and or duration of the growth period of alkenone producers prymnesiophyte algae the first contribution is consistent with many climate proxy records from the northeast atlantic area and with climate model simulations including milankovitch forcing the second contribution is consistent with the divergence between biomarker and summer faunal paleo t fromearly to late holocene observed in two cores further work is necessary and in particular the apparent discordance between biomarker and faunal t records for the relative stable holocene period must be understood to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records r 2002 elsevier science ltd all rights reserved

sea surface conditions in the southern nordic seas during the holocene based on dinoflagellate cyst assemblages dinoflagellate cyst dinocyst records from the southern nordic seas were compiled in order to evaluate the evolution of upper ocean conditions on a millennial timescale and supported by a highly resolved record from the voring plateau after the transitional phase from the last deglaciation three main phases define the holocene the early holocene 7 5 ka bp features important numbers of cool temperate species dominated by nematosphaeropsis labyrinthus and impagidinium pallidum in the west the assemblage composition changes drastically at the transition from the early to the mid holocene from when on operculodinium centrocarpum dominates the changeover is dated between 6 1 and 7 5 ka bp perhaps earlier closer to the iceland scotland ridge and appears to be linked to the onset of a modern type of surface circulation warmest assemblages occur at the voring plateau shortly after the transition when atlantic waters also appear to have spread farthest westward the recurrence of colder elements can be linked to cooling from 2 4 ka bp at the voring plateau and presumably earlier in the west but is difficult to date there because of the low sedimentation rates this is a general issue in many areas of the nordic seas and appears to have an important effect on cyst concentrations and assemblage composition with the possible loss of oxygenation sensitive cysts in the older parts of the cores comparing dinocyst based quantitative reconstructions with those retrieved from other plankton reveals a significantly different trend between proxies linked to a differing autecological response to seasonal changes at their respective depth habitats

near surface circulation of the nordic seas as measured by lagrangian drifters in the period june 1991 to august 1993 107 argos tracked drifters drogued to 15 m depth were released in the nordic seas or greenland iceland and norwegian seas the drifter movements revealed the strong and spatially confined current systems along the surface salinity fronts of the iceland faroe frontal zone and of the norwegian coast and along the continental margins and their extensions to the barents sea and spitsbergen the norwegian atlantic current is composed of three distinct streams two continental margin and one coastal branches which join into one single swift mean current west of the lofoten and vesteralen islands where the strongest measured currents are in excess of 110 cm s 1 in addition to the general cyclonic gyre circulation in the nordic seas the drifters indicate smaller cyclonic circulation patterns in all the major subbasins i e the iceland plateau the norwegian the lofoten and the greenland basins no surface signature of the east icelandic current is disclosed by the drifters interpolated and low pass filtered position data were used to construct maps of 15 m depth ensemble mean velocity velocity variability and residence time vigorous eddy fields are dominant in the strong currents and in the lofoten basin eulerian correlations indicate that they tend to propagate to the west in contrast the iceland plateau appears quiescent both in the mean and eddy velocities single particle diffusivities are computed and are found to be in the range 1 7 107 cm2 s 1 the corresponding lagrangian timescale and space scale are 1 3 days and 10 40 km respectively these lagrangian drifter measurements compose the first basin scale accurate near surface velocity data set of the nordic seas

preparation and characterization of bone and tooth collagen for isotopic analysis abstract criteria are presented for the identification of diagenetic alteration of carbon and nitrogen isotope ratios of bone and tooth collagen prepared by a widely used method measurements of collagen concentrations in tooth and bone atomic c n ratios and carbon and nitrogen concentrations in collagen of 359 historic and prehistoric african humans and modern and prehistoric east african non human mammals are described carbon isotope ratios of collagen lipids from four bones are also presented compared to bone whole teeth have significantly lower collagen concentrations lower carbon and nitrogen concentrations in collagen and similar c n ratios carbon and nitrogen concentrations and c n ratios are relatively constant over a wide range of collagen concentrations however prehistoric specimens with very low collagen concentrations have highly variable c n ratios very low carbon and nitrogen concentrations in collagen and stable carbon and nitrogen isotope ratios unlike collagen at the transition from well preserved to poorly preserved collagen the most reliable indicator of collagen preservation is the concentration of carbon and nitrogen in collagen concentrations of c and n drop abruptly by an order of magnitude at this transition point these attributes provide simple criteria for assessing sample quality since collagen preservation can vary greatly within prehistoric sites these attributes should be reported for each specimen use of purification procedures that remove acid and base soluble contaminants and particulate matter carbonates fulvic acids lipids humic acids sediments and rootlets are recommended wider adoption of these procedures would insure comparability of results between laboratories and permit independent and objective evaluation of sample preservation and more precise dietary climatic and habitat interpretations of collagen isotopic analyses

surface water dynamics in the reykjanes ridge area during the holocene as revealed by coccolith assemblages abstract the calcareous nannofossil assemblages from sediment core ds97 2p from the reykjanes ridge have been investigated to document oceanographic changes in surface water during the holocene the recorded variations in coccolithophore species assemblages and accumulation rates indicate that the region was subjected to rapid changes of surface water masses throughout the entire holocene coccolithophore assemblages generally are of low species diversity and consist mainly of emiliania huxleyi and coccolithus pelagicus ssp pelagicus two major events occurred at 8 5 7 ka and at 4 5 3 5 ka showing higher coccolith accumulation rates suggesting that the influence of relatively warm atlantic waters via the irminger current was strong in the investigated area the coccolithophore assemblages have been compared with diatom foraminifer and sedimentological records within the same core these data supported by a comparison with previously published proxy records add credit to the hypothesis that holocene changes did not occur uniformly across the north atlantic the results have highlighted the holocene pattern in the north atlantic as a period influenced by strong regionalism with discrepancies in the hydrographical trends and in the distribution of the planktonic proxies

comment on environmental setting micro morphologies and stable c o isotope composition of cold climate carbonate precipitates a review and evaluation of their potential as paleoclimatic proxies by denis lacelle quaternary science reviews 26 1670 1689

magnesium isotope fractionation during granite weathering abstract the mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate mg isotope behavior during the weathering of felsic rocks in a continental environment 26mg generally exhibits a small increase from the bedrock values which range from 0 17 0 06 to 0 07 0 06 until the upper two meters in which a decrease is observed producing near surface values between 0 14 0 06 and 0 09 0 06 the small observed increase in isotopic composition below 2 m is likely due to mg loss to the hydrosphere during biotite weathering and in situ illite production while the decrease at depths

north atlantic ocean surface currents 1 close to 1800 surface drifters are used to investigate the 15 m circulation of the north atlantic ocean the data are used to describe structures of the average eulerian circulation and of the associated eddy variability the data resolve scales on the order of 50 km which have hitherto not been systematically described in particular near shelf breaks and near the most intense currents the gulf stream the north atlantic current nac and the frontal currents of the subpolar gyre this reveals a complex series of quasi permanent eddies meanders and recirculation patterns gulf stream intensity is portrayed as changing abruptly near 54 w east of which it is identified as two current branches centered near 39 and 41 5 n the northern one connecting more directly with the nac and the southern one with the recirculation gyre and the azores current ac many features of the currents are controlled by topography in particular currents are often intensified near shelf breaks or parallel to ridges in the subpolar gyre however the largest northward branch of the nac in the icelandic basin is located near the deepest bathymetry not near steep bathymetry other currents in particular in the subtropical gyre are less clearly related to topography for instance the ac is featured as a zonal eastward current extending far west of the mid atlantic ridge mar to at least 55 w and possibly 63 w in the interior and away from topographic features the eddy kinetic energy eke is the largest where the mean currents are the largest in the subpolar gyre there are striking differences in eke between southward flowing currents the labrador and east greenland current and the northward flowing currents west greenland current and branches of the nac which have higher eke the areas of weakest variability are located in the southwest part of the subpolar gyre northeast of iceland and in the eastern atlantic south of 45 n the ac eddies and the mesoscales south of the canary islands transect this eastern eddy desert drifter trajectories are used as realizations of lagrangian particles in the vicinity of current cores these illustrate the variety of paths or connections between different current systems and demonstrate cross stream dispersions

structure and transports of the east greenland current at 75 n from moored current meters the east greenland current runs from 80 n to 60 n from the fram strait to the denmark strait via the nordic seas it transports waters from the arctic and the nordic seas into the atlantic and also acts as a western intensified southward return flow for waters recirculating within the greenland sea gyre itself an area important for deep water formation data from current meters moored across the current at 75 n in 1994 1995 show a large seasonal variation in the current the annual mean transport is 21 3 sv taking 9 w as the eastern boundary varying from 11 sv in summer to 37 sv in winter errors approximately 5 sv no significant seasonal signal has been observed in the fram or denmark straits suggesting that the seasonal transport is confined within the greenland sea using temperature and velocity data we split the flow at 75 n into two parts a mainly wind driven circulation annual mean of order 19 sv which is trapped within the greenland sea gyre and exhibits a large seasonal cycle transporting predominantly the waters of the greenland sea and a steadier throughflow probably thermohaline driven of order 8 sv in the annual mean with very little seasonal variation data from previous years 1987 1994 indicate the interannual variability of the current is low assuming a spatially coherent structure to the current we extend the time series of the transport back to 1991 and suggest it may be possible to monitor the total transport with one suitably placed mooring

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

the magnesium isotope budget of the modern ocean constraints from riverine magnesium isotope ratios we have measured the magnesium mg isotope ratios in 45 rivers including 16 of the largest rivers in the world covering a range of geologic tectonic and climatic drainage basin environments the range in riverine 26mg is 2 5 half the variation in terrestrial rock although the mg isotopic composition of the source rock is important for small rivers at a global scale lithology is of limited significance for mg isotope ratios in rivers part of the variability at a global scale may be attributed to fractionation in the weathering environment the rivers analysed constitute 30 of the global mg riverine flux to the oceans and a flux weighted mg isotope composition of global runoff has been estimated at 1 09 even taking into account uncertainty this is distinct from seawater at 0 82 this difference arises either from the fractionation of mg isotope ratios in the ocean and or a mg budget which is not at steady state the difference is consistent with fractionation by carbonate precipitation in the simplest steady state scenario where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output mg isotope ratios imply a minimum dolomite mg flux of 9 of the total output mg flux the implications of the distinct isotopic composition of the riverine flux from modern seawater is that the mg isotope ratio of seawater has probably varied outside of current analytical uncertainty mg isotope ratios may therefore provide a valuable new tracer in palaeo oceanography

water masses and circulation pathways through the iceland basin during vivaldi 1996 quasi synoptic data from late 1996 spanning the subpolar north atlantic have been used to determine the major pathways of the north atlantic current nac at that time high spatial resolution allows fronts to be accurately positioned on each seasoar section a clearly defined front of the nac the southern branch turns north at around 25 w and continues through the middle of the iceland basin as far as 60 n 20 w a second branch the northern branch or subarctic front turns north around 30 w and retroflects westward north of 54 n to re enter the irminger basin and become part of the irminger current up the western side of the reykjanes ridge a third eastward branch turns sharply northwest at the mouth of the rockall trough to skirt the southwestern margin of hatton bank this branch carries a tongue of saline eastern north atlantic water enaw over hatton bank and in consequence enaw covers the whole of the hatton and rockall banks as well as the rockall trough bounded in the west by the southern branch the most saline water found in rockall trough spills out into the northern iceland basin between rockall and lousy banks this saline weakly stratified tongue can be traced westward to the south of iceland continuing southwestward along the eastern flank of the reykjanes ridge subarctic intermediate water is carried into the iceland basin creating a fresh tongue bounded east and west by the more saline enaw over hatton bank and the eastern flank of the reykjanes ridge respectively

dinoflagellate cysts as indicators of water quality and productivity in british columbia estuarine environments abstract palynological analyses of 60 surface sediment samples from estuarine environments near vancouver island including the georgia strait gs the effingham eff and the seymour belize sb inlets were performed in order to document the distribution of dinoflagellate cyst assemblages and their relationship to hydrographic conditions productivity and nutrient concentrations we tested transfer functions using the analogue method and suggest that dinoflagellate cyst assemblages can be used to reconstruct primary productivity temperature and salinity the eff and sb inlets are characterized by a dominance of autotrophic taxa particularly operculodinium centrocarpum whereas the protoperidinioid and gymnodinial heterotrophic taxa such as quinquecuspis concreta and brigantedinium spp dominate the assemblages of the gs multivariate analysis shows that this distribution is closely linked to primary productivity sea surface temperature sst and spring silica concentration the abundance of autotrophic taxa in the eff and sb inlets is associated with high primary productivity and low summer sst indicating summer upwelling of coastal british columbia whereas the heterotrophic taxa that characterizes the gs assemblages are related to low productivity high summer sst and high silica concentration during spring multivariate analysis shows that the most important environmental parameters related to dinocyst distribution in the restricted embayment of the gs are distance to the shore distance to vancouver harbor spring sea surface salinity sss spring phosphate concentration and spring productivity the autotrophic taxa are generally more common in coastal and shallow waters but spiniferites ramosus and pentapharsodinium dalei show an opposite correlation to spring productivity and salinity p dalei is particularly abundant around vancouver harbor near highly urbanized shores and within the fraser river plume where salinity is low and spring productivity and continental runoff are high s ramosus shows its highest abundance on the western coast of gs protoperidinioid and gymnodinial cysts characterize distal zones within the central and southern gs that are associated with a mixture of brackish waters coming from the fraser river and deep upwelling waters entering the gs via juan de fuca strait the relationship between dinoflagellate cyst assemblages and primary productivity in these estuarine systems differs from that in oceanic and outer neritic zones where the abundance of heterotrophic taxa is commonly associated with upwelling and high productivity

surface circulation in the eastern north atlantic from drifters and altimetry a description of the near surface circulation and its properties is the result of the analysis of a drifting buoy data set in the eastern north atlantic between the iberian peninsula the azores and the canary islands world ocean circulation experiment tropical ocean global atmosphere experiment drifters equipped with holey sock drogues centered at 15 m depth collected a total of 14 4 years of data the drifters sampled a rather inhomogeneous velocity field with a weak mean flow regime and eddies of different scales they meandered southward everywhere in the study region except in the iberian coastal transition zone north of 41degreesn where they headed northward the near surface mean velocity field obtained from the drifter data set shows all important mean currents including the poleward portugal coastal countercurrent during the fall winter and early spring off western and northern iberia the southward portugal coastal current the slow offshore southward flow of the portugal current during the whole year the southwestward canary current and the eastward azores current which extends to the vicinity of the african coast near the gulf of cadiz maps of the eddy kinetic energy field were obtained from the drifters and from satellite altimetry it provides the largest part of the total kinetic energy the rate of dispersion is estimated from the lagrangian statistics of the drifting buoys the dispersion of the drifters in the study region is well modeled by a simple description of eddy diffusion assuming homogeneous turbulence ensemble mean diffusivities k and the langrangian integral length scales and timescales l and t were obtained for the zonal and meridional directions the sea surface temperature measured along the drifter trajectories is used to produce estimates of the eddy diffusivity which is compared with the diffusivity estimates obtained from the theory of taylor the eddy diffusivity is found to be approximately proportional to the eddy kinetic energy discrete eddies and meanders were observed using drifters and altimetry in order to map and describe their geographical distribution and characteristics in the eastern north atlantic

magnesium isotope systematics of the lithologically varied moselle river basin france abstract magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the moselle basin northeastern france this small basin is remarkable for its well constrained and varied lithology on a small distance scale and this is reflected in river water sr isotope compositions upstream where the moselle river drains silicate rocks of the vosges mountains waters are characterized by relatively high 87sr 86sr ratios 0 7128 0 7174 in contrast downstream of the city of epinal where the moselle river flows through carbonates and evaporites of the lorraine plateau 87sr 86sr ratios are lower down to 0 70824 magnesium in river waters draining silicates is systematically depleted in heavy isotopes 26mg values range from 1 2 to 0 7 relative to the value presently estimated for the continental crust and a local diorite 0 5 in comparison 26mg values measured in soil samples are higher 0 0 this suggests that mg isotope fractionation occurs during mineral leaching and or formation of secondary clay minerals on the lorraine plateau tributaries draining marls carbonates and evaporites are characterized by low ca mg 1 5 3 2 and low ca sr 80 400 when compared to local carbonate rocks ca mg 29 59 ca sr 370 2200 similar to other rivers draining carbonates the most likely cause of the mg and sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer 26mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year from 1 4 to 1 6 and from 1 2 to 1 4 respectively within the range defined for the underlying rocks downstream of epinal the compositions of the moselle river samples in a 26mg vs 87sr 86sr diagram can be explained by mixing curves between silicate carbonate and evaporite waters with a significant contribution from the vosgian silicate lithologies 70 temporal co variation between 26mg and 87sr 86sr for the moselle river throughout year is also observed and is consistent with a higher contribution from the vosges mountains in winter in terms of runoff and dissolved element flux overall this study shows that mg isotopes measured in waters rocks and soils coupled with other tracers such as sr isotopes could be used to better constrain riverine mg sources particularly if analytical uncertainties in mg isotope measurements can be improved in order to perform more precise quantifications

reorganization of the upper ocean circulation in the mid holocene in the northeastern atlantic a micropaleontological investigation was conducted on two sediment cores from the reykjanes ridge rr core lo09 14 59 12 30 n 31 05 94 w and the faroe shetland channel fsc core hm03 133 25 60 06 55 n 06 04 18 w to document hydrographical changes of the north atlantic current nac during the holocene dinocyst and coccolith assemblages were analyzed and quantitative reconstructions of sea surface temperatures ssts and sea surface salinities ssss were conducted based on dinocyst assemblages both proxies suggest a major reorganization of surface circulation patterns in the northeastern north atlantic between 7 and 5 4 ka bp at both sites ssss before 6 5 7 ka bp were lower than during the mid late holocene suggesting dispersal of meltwater through the nac long term trends of ssts however show higher than present summer ssts on the rr from 9 3 to 6 ka bp and lower than present ssts in the fsc until ca 5 4 ka bp the contrasted sst trends at the two sites suggest that decreasing summer

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

regional differences in bone collagen δ13c and δ15n of pleistocene mammoths implications for paleoecology of the mammoth steppe in this study we present bone collagen 13c and 15n values from a large set of pleistocene woolly mammoths mammuthus primigenius from siberia alaska and yukon overall results for mammoth specimens from eastern beringia alaska and yukon significantly differ for both 13c and 15n values from those from western beringia northeastern siberia in agreement with palynological entomological and physiographic data from the same regions these isotopic differences strongly imply that the mammoth steppe the extensive ice free region spanning northern eurasia and northwestern north america was ecologically variable along its east west axis to a significant degree prior to the last glacial maximum lgm the high latitude portions of siberia and the russian far east appear to have been colder and more arid than central alaska and yukon which were ecologically more diverse during the lgm itself however isotopic signatures of mammoths from eastern beringia support the argument that this region also experienced an extremely cold and arid climate in terms of overall temporal trend beringia thus went from a condition prior to the lgm of greater ecological variability in the east to one of uniformly cold and dry conditions during the lgm

millennial scale variability in atlantic water advection to the nordic seas derived from holocene coccolith concentration records abstract abundance patterns of coccolith species in two holocene marine cores retrieved off norway and northern iceland are indicative of millennial scale modulations in the flow of the main norwegian atlantic current and secondary north iceland irminger current branches of the north atlantic drift to the nordic seas long term trends in coccolith abundance changes reflect major holocene steps in atlantic water transfer to the nordic seas at orbital scale with important constraints on the convective activity of the nordic seas that leads to the formation of the precursor water mass of north atlantic deep water millennial scale holocene episodes of increased advection of atlantic waters off norway are associated with enhanced winter precipitation over scandinavia increased sea salt fluxes over greenland and strengthened wind over iceland thereby suggesting a common atmospheric forcing the location and intensity of the westerlies and the associated changes in mid to high latitude pressure gradients our biotic data indicate an opposite pattern of atlantic water inflow at suborbital scale between the western denmark and eastern iceland scotland straits of the northern atlantic throughout the holocene this as supported by present observational and simulated data further highlights the role of atmospheric oscillations in the recent history of the north atlantic nordic seas water mass exchanges across the greenland scotland ridge such atmospheric processes are thought to explain the observed coupling between periods of excess export of arctic sea ice to the nordic seas and intervals of maximum inflow of atlantic water to the norwegian sea throughout the last 11 000 years

adsorption and desorption controlled magnesium isotope fractionation during extreme weathering of basalt in hainan island china magnesium isotopic compositions of a set of clay rich saprolites developed on the neogene tholeiitic basalt from hainan island in southern china have been measured in order to document the behavior of mg isotopes during continental weathering compared with unaltered basalts 26mg 0 36 the overlying saprolites are strongly depleted in mg i e th mg 99 1 to 92 9 and display highly variable 26mg ranging from 0 49 to 0 40 magnesium concentration and 26mg value of the saprolites display a general increasing trend upwards in the lower part of the profile but a decreasing trend towards the surface in the upper part the variations of mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes 1 adsorption of mg to kaolin minerals kaolinite and halloysite with preferential uptake of heavy mg isotopes onto kaolin minerals and 2 desorption of mg through cation exchange of mg with the relatively lower hydration energy cations in the upper profile evidence for adsorption is supported by the positive correlation between 26mg and the modal abundance of kaolin minerals in saprolite of the lower profile while negative correlations between 26mg and concentrations of lower hydration energy cations e g sr and cs in the upper profile support the desorption process our results highlight that adsorption and desorption of mg on clay minerals play an important role in behavior of mg isotopes during extreme weathering which may help to explain the large variation in mg isotopic composition of river waters

trophic structure and climatic information from isotopic signatures in pleistocene cave fauna of southern england the preservation of trophic structure and climatic information in kent s cavern upper pleistocene mammal bones and teeth was assessed by comparing the isotopic composition of modern and fossil equivalents yields of collagen from both bone n 19 and tooth n 49 were extremely variable with values relative to modern bone ranging from 0 to 100 no evidence of preferential preservation of tooth collagen was detected the carbon and nitrogen isotopic differences in the herbivore versus carnivore collagen from kent s cavern fauna were consistent with those observed in modern faunas moreover an enrichment of 0 4 1 7 in15n was observed in tooth collagen of both deer and hyena as compared to bone collagen this enrichment presumably reflects a trophic shift from the consumption of milk during infancy herbivores and carnivores had distinct differences in the carbon isotopic composition of enamel carbonate hydroxylapatite similar to those measured in modern specimens from similar climatic environments the spacing between the 13c values difference between isotopic composition in collagen and carbonate hydroxylapatite of kent s cavern herbivores and carnivores is similar to that measured in modern mammals from a single locality the preservation of primary oxygen isotopic composition of enamel carbonate hydroxylapatite was more difficult to assess however oxygen isotopic compositions of kent s cavern enamel are systematically lower than those of contemporaneous faunas from southern france which is consistent with a latitudinal effect on rainfall oxygen isotopic compositions although the kent s cavern specimens have been subjected to extensive diagenetic alteration the biological isotopic signals seem to have utility for paleoecological reconstructions

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

nursing weaning and tooth development in woolly mammoths from old crow yukon canada implications for pleistocene extinctions abstract this study investigates differences in the 13ccol 15n 13csc 18osc sr ca and ba ca values of juvenile and adult woolly mammoths mammuthus primigenius from old crow yukon canada the data indicate that nursing in woolly mammoths lasted at least three years and was associated with minimal decreases in 13ccol 0 2 large decreases in 13csc 1 5 and large increases in 15n 2 and 18osc 2 values sr ca and ba ca ratios suggest that woolly mammoth juveniles began consuming plant foods between 2 and 3 african elephant years of age much later than the initiation of weaning in modern elephants we hypothesize that delayed weaning was an adaptation to increased predation risk and decreased food quality quantity during the extended hours of darkness that occur in winter at high latitudes prolonged nursing and delayed weaning may have made mammoths particularly vulnerable to climatic stressors or human hunting

the effect of sea surface properties on shell morphology and size of the planktonic foraminifer neogloboquadrina pachyderma in the north atlantic abstract the variability in size and shape of shells of the polar planktonic foraminifer neogloboquadrina pachyderma have been quantified in 33 recent surface sediment samples throughout the northern atlantic ocean and correlated with the properties of the ambient surface waters the aim of the study was to determine whether any of the morphological features could be used to reconstruct sea surface properties in the polar realm of the north atlantic where most paleotemperature proxies appear to fail the analyses revealed that shell morphology is only weakly controlled by habitat properties whereas shell size showed a strong correlation with sea surface temperature the regression of mean shell size on sea surface temperature revealed the presence of two trends among the sinistrally coiled shells a continuous increase in shell size with decreasing sst in sediments deposited under polar water masses and a continuous increase in shell size with increasing sst in samples from transitional waters the second trend mirrors the trend observed for dextrally coiled shells which are frequent in the same samples and signal the presence of n incompta the identical mean shell size trends among the sinistral and dextral specimens in the temperate samples confirms the results of earlier genetic studies which indicated the existence of a small but distinct proportion of opposite coiling in n incompta to which the sinistral shells in the temperate samples could be attributed the linear correlation between mean shell size and sea surface temperature in the polar domain summer sst

the major ion δ44 40ca δ44 42ca and δ26 24mg geochemistry of granite weathering at ph 1 and t 25 c power law processes and the relative reactivity of minerals abstract we dissolved boulder creek granodiorite in a plug flow reactor for 5794 h at ph 1 and t 25 c the primary purpose of the experiment was to identify controls on dissolved 44 40ca 44 42ca and 26 24mg values during granite weathering herein we also examine the origin of ca and mg isotopic variability among minerals composing the boulder creek granodiorite and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output nine ca bearing minerals display an 8 80 range of 44 40ca values and a 0 51 range of 44 42ca values three mg bearing minerals display a 1 53 range of 26 24mg values these ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks most of the 44 40ca variability reflects 40ca enrichment in k feldspar and to a lesser extent biotite due to the radioactive decay of 40k over the 1 7 ga age of the rock whereas the entire range of 44 42ca values reflects mass dependent isotope fractionation during igneous differentiation and crystallization the range of 26 24mg values may represent either fractionation during the chloritization of biotite or interaction of the boulder creek granodiorite with mg rich metamorphic fluids having low 26 24mg values the elemental and isotopic composition of the reactor output varied substantially during the experiment we synthesize the mineralogical and fluid data using coupled mass conservation equations solved at non steady state model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations while surface area normalized dissolution rate constants were time invariant specific surface area increased as a power law function of time through positive feedbacks between mechanical disaggregation chemical dissolution and mineral depletion variations in dissolved 44 40ca 44 42ca and 26 24mg values reflect conservative mixing rather than fractionation apatite and calcite initially control 44 40ca and 44 42ca values followed by biotite titanite epidote hornblende and plagioclase the release of radiogenic 40ca clearly defines the period where biotite dissolution dominates the brucite layer of chlorite initially controls 26 24mg values followed by biotite the tot layer of chlorite and hornblende through direct isotopic tracking these results demonstrate that trace minerals such as apatite and calcite in the case of ca and brucite in the case of mg dominate elemental release during the incipient stages of granite weathering the results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages the ca and mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies

multiproxy fossil comparison reveals contrasting surface ocean conditions in the western iceland sea for the last two interglacials abstract dinoflagellate cyst dinocyst coccolith and planktic foraminiferal assemblages from a core in the western iceland sea were used to reconstruct and compare the surface ocean developments of the holocene and the last interglacial marine isotopic stage or mis 5e while increasing subpolar planktic foraminifers from 10 ka indicate subsurface warming peaking around 7 5 ka the dinocyst data suggest that the uppermost ocean remained dominated by cold waters until 6 5 ka a reduced advection of cold polar waters through the east greenland east icelandic current thereafter entailed warmest and most saline holocene conditions between 6 5 and 5 ka in turn followed by a general cooling trend by contrast both planktic foraminifers and dinocysts show an increased presence of atlantic type waters from 122 ka onward resulting in a mis 5e thermal optimum around 120 5 ka nonetheless occasional freshwater input from melting drift ice created stratified but also seasonally variable conditions during this first half of mis 5e this stratification signature disappeared at 120 ka when a marked repositioning of the oceanographic fronts occurred slightly colder conditions are indicated by both phyto and zooplankton from there on until the end of mis 5e around 117 ka a late mis 5e cooling at the iceland plateau is opposite to a late mis 5e optimum observed in the eastern nordic seas this regional difference is likely explained by various feedback mechanisms following the major reorganisation of the oceanic fronts at 120 ka the holocene and mis 5e interglacial variability is not reflected in the quasi monospecific coccolith assemblages and illustrates the low sensitivity of living coccolithophore communities to subtle temperature changes in the low temperature regions of the nordic seas overall quite different surface water properties appear to have characterised both interglacial intervals with a higher share of warm atlantic elements in the planktic communities during mis 5e this suggests a higher contribution of atlantic waters in the southwestern nordic seas probably due to a more northward expansion of the irminger current under weakened polar east greenland east icelandic currents such a reduced influence of polar waters in the southwestern nordic seas may thus explain other evidence for relatively warm conditions in mis 5e all around southern greenland

lithium magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in iceland this study presents lithium magnesium and silicon isotope ratios from pore waters and soils from a well characterised histic andosol in south west iceland the soil 7li composition ranges between values slightly lighter than basalt to those that are much heavier 1 1 to 26 8 and are possibly influenced by sea salt in contrast precipitation corrected dissolved pore water 7li values 1 8 10 0 appear to reflect preferential adsorption of 6li onto secondary minerals where allophane supersaturation results in high 7li values conversely low 7li together with high li are probably due to destabilisation of allophane at low ph and thus desorption of li when compared to icelandic river values it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters precipitation corrected pore water mg isotope ratios 26mg range between 0 46 and 0 12 and correlate with the amount of heavy mg adsorbed onto the soil exchange complex silicon isotopes in the soils are isotopically lighter 30si 0 91 to 0 53 than basalt 0 29 whereas pore waters are heavier 0 13 to 1 03 soil 30si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals dissolved si isotopes also respond to chemical weathering processes and show that isotopically heavy 30si corresponds to high cation fluxes and high secondary mineral formation however comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions this indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent in turn this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation and thus on the chemical weathering processes

magnesium isotope fractionation during shale weathering in the shale hills critical zone observatory accumulation of light mg isotopes in soils by clay mineral transformation abstract magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments but few have focused on the mechanisms that control mg isotope fractionation during shale weathering in this study we focus on understanding mg isotope fractionation in the shale hills catchment in central pennsylvania mg isotope ratios were measured systematically in weathering products along geochemical pathways of mg during shale weathering from bedrock to soils and soil pore water on a planar hillslope and to sediments stream water and groundwater on a valley floor significant variations of mg isotopic values were observed 26mg values 0 6 to 0 1 of stream and soil pore waters are about 0 5 to 1 lighter than the shale bedrock 26mg values of 0 4 consistent with previous observations that lighter mg isotopes are preferentially released to water during silicate weathering dissolution of the carbonate mineral ankerite depleted in the shallow soils but present in bedrock at greater depths produced higher mg2 concentrations but lower 26mg values 1 1 in groundwater 1 5 lighter than the bedrock 26mg values 0 2 to 0 4 of soil samples on the planar hillslope are either similar or up to 0 2 lighter than the bedrock hence a heavy mg isotope reservoir complementary to the lighter mg isotopes in soil pore water and stream water is missing from the residual soils on the hillslope in addition soil samples show a slight but systematic decreasing trend in 26mg values with increasing weathering duration towards the surface we suggest that the accumulation of light mg isotopes in surface soils at shale hills is due to a combined effect of i sequestration of isotopically light mg from soil water during clay dissolution precipitation reactions and ii loss of isotopically heavy particulate mg in micron sized particles from the hillslope as suspended sediments this latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation stream sediments 26mg values of 0 3 to 0 5 accumulated on the valley floor are 0 2 heavier than the bedrock and are thus consistent with that mobile particulates are the heavy mg isotope reservoir our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter mg isotopes in residual bulk soils this example also demonstrates that transport of isotopically distinct fine particles from clay rich systems could be a new and important mechanism to drive the mg isotope compositions of silicate weathering residuals this mechanism drives fractionation in an opposite direction as might be expected from previous studies i e residual soils are driven to lighter mg values and sediments become isotopically heavier

magnesium retention on the soil exchange complex controlling mg isotope variations in soils soil solutions and vegetation in volcanic soils iceland understanding the biogeochemical cycle of magnesium mg is not only crucial for terrestrial ecology as this element is a key nutrient for plants but also for quantifying chemical weathering fluxes of mg and associated atmospheric co2 consumption requiring distinction of biotic from abiotic contributions to mg fluxes exported to the hydrosphere here mg isotope compositions are reported for parent basalt bulk soils clay fractions exchangeable mg seasonal soil solutions and vegetation for five types of volcanic soils in iceland in order to improve the understanding of sources and processes controlling mg supply to vegetation and export to the hydrosphere bulk soils 26mg 0 40 0 11 are isotopically similar to the parent basalt 26mg 0 31 whereas clay fractions 26mg 0 62 0 12 exchangeable mg 26mg 0 75 0 14 and soil solutions 26mg 0 89 0 16 are all isotopically lighter than the basalt these compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex successive adsorption desorption of heavy mg isotopes leads to the preferential loss of heavy mg from the soil profile leaving soils with light mg isotope compositions relative to the parent basalt additionally external contributions from sea spray and organic matter decomposition result in a mixture of mg sources on the soil exchange complex vegetation preferentially takes up heavy mg from the soil exchange complex 26mgplant exch 0 50 0 09 and changes in 26mg in vegetation reflect changes in bioavailable mg sources in soils this study highlights the major role of mg retention on the soil exchange complex amongst the factors controlling mg isotope variations in soils and soil solutions and demonstrates that mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of mg that is bioavailable for plants and is exported to the hydrosphere

investigation of magnesium isotope fractionation during granite differentiation implication for mg isotopic composition of the continental crust abstract high precision mg isotopic analysis was performed on a suite of well characterized i type granitoids and associated hornblende and biotite minerals from the dabie orogen in central china to address the behavior of mg isotopes during granite differentiation although these granitoids formed through different degrees of partial melting and fractional crystallization with large variations in elemental and mineral compositions their 26 mg values vary from 0 26 to 0 14 and are indistinguishable within our analytical precision 0 07 2 sd coexisting hornblendes and biotites in these granitoids display similar mg isotopic composition with 26 mg ranging from 0 31 to 0 14 in hornblendes and 0 23 to 0 12 in biotites the inter mineral fractionation factors 26 mg hbl bt 26 mg hbl 26 mg bt vary from 0 10 to 0 02 with an average 0 06 0 08 2 sd the limited inter mineral fractionation agrees with the theoretic prediction that mg cations in both hornblende and biotite are octahedrally coordinated with oxygen which restricts the magnitude of equilibrium isotope fractionation overall data from both bulk granitoids and associated mineral separates suggest that mg isotope fractionation during i type granite differentiation is limited collectively granitoids studied here have mg isotopic composition similar to that of terrestrial basalts and peridotites 26 mg 0 21 0 07 vs 0 25 0 07 2 sd confirming that magmatic processes do not significantly fractionate mg isotopes the continental crust in the dabie orogen as sampled by these i type granitoids has a mantle like mg isotopic composition given that significant mg isotope fractionation occurs during chemical weathering processes mg isotopes may potentially be used for tracing granite genesis in particular if sedimentary materials are involved in granite sources

plankton in the norwegian greenland sea from living communities to sediment assemblages an actualistic approach a synoptic study is carried out to reconstruct the development of the plankton community in the late quaternary in the norwegian greenland sea it comprises quantitative analyses of coccolithophores dinoflagellate cysts diatoms and radiolarians an actualistic approach is applied to evaluate the different fossil records of these plankton groups the preliminary results of the current investigation are reported here

distribution patterns of dinoflagellate cysts and other organic walled microfossils in recent norwegian greenland sea sediments dinoflagellate cysts and other organic walled microfossils have been studied in recent surface sediments from the entire norwegian greenland sea more than 30 taxa have been recognized of which only few show a distinct distribution pattern and allow description of four assemblages the occurrence of most taxa is related to the relatively warmer waters of the norwegian sea algidaspaeridium minutum s 1 brigantedinium simplex and impagidinium pallidum are the only species showing a preference for colder water masses two species i pallidum and nematosphaeropsis labyrinthus are mainly restricted to the oceanic environment whereas the other species have also been reported from neritic environments in previous studies due to the limited knowledge of the ecological and sedimentological factors influencing the occurrence of dinoflagellate cysts in oceanic environments their distribution in recent sediments can be only related to surface water masses in a broad sense although the distribution of assemblages correlates with specific surface water masses comparison with assemblages recovered from sediment traps deployed basinwide in the norwegian greenland sea dale and dale 1992 revealed some major discrepancies in species composition and percentage abundances the differences cannot be explained with certainty at the moment although there is some evidence that transport of dinoflagellate cysts and other fossilizable microplankton in water masses by currents in sea ice and sediments may modify the assemblages found in recent oceanic surface sediments from the norwegian greenland sea

high temperature inter mineral magnesium isotope fractionation in eclogite from the dabie orogen china abstract to investigate the magnitude and mechanism of inter mineral mg isotope fractionation at high temperatures we report high precision analyses of mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from bixiling in the dabie orogen china magnesium isotopic compositions of whole rocks 26mg of 0 44 to 0 26 are similar to the estimated 26mg values of the mantle suggesting mg isotopic inheritance from a gabbroic protolith with limited mg isotope fractionation during eclogite facies metamorphism by contrast mineral separates are highly heterogeneous with 26mg values ranging from 0 30 to 0 60 in phengite from 0 16 to 0 40 in omphacite and from 0 95 to 0 74 in garnet phengite and omphacite are 1 heavier in 26mg than coexisting garnet indicating large high temperature inter mineral mg isotope fractionations the constant 26mgomphacite garnet 26mgomphacite 26mggarnet value 1 14 0 04 together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet suggests an equilibrium mg isotope fractionation controlled by the difference in coordination number of mg between omphacite six and garnet eight the 1 14 fractionation is the largest high temperature equilibrium inter mineral mg isotope fractionation observed so far and makes the omphacite garnet mg isotope fractionation a potential geothermometer by contrast mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1 25 to 1 47 and from 0 14 to 0 32 respectively this implies mg isotopic disequilibria between phengite and garnet omphacite which might result from the mg isotopic variation in phengites due to mg isotope exchange between phengites and retrograde fluids in bixiling eclogites

mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences guadeloupe magnesium mg stable isotopes are increasingly used as a weathering proxy in soils and rivers but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure a better understanding of the behaviour of mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone here we investigate isotopic variations in 26mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from guadeloupe formed under contrasting climatic conditions soils formed in drier conditions low precipitation contain smectite whereas soils formed under wet conditions high rainfall are characterized by halloysite and fe oxides or kaolinite all clay fractions have mg isotopic compositions 26mg 0 41 to 0 10 similar to or heavier than their parent andesite 26mg 0 47 supporting the preferential incorporation of heavy mg isotopes in secondary mg bearing clay minerals with the first direct measurements on clay fractions soils with lighter mg isotope compositions have greater quantities of exchangeable mg the data support a contribution from sea spray to the exchangeable mg pool correlated to the soilweathering degree this study highlights for the first time that the soil 26mg not only depend on 26mg of the parent rock and on any fractionation that might occur but also on the mg retention on the exchange complex which could in turn be controlled by external inputs such as sea spray

late quaternary sea surface conditions in the laurentian fan evidence from coccolith and dinocyst assemblages abstract the analyses of coccoliths and dinocysts from the images core md95 2033 collected on the laurentian fan 44 39 87n 55 37 21w water depth 1412 m in the northwest north atlantic provide a new dataset to assess the magnitude of climatic changes during the holocene with a millennial time resolution coccolith and dinocyst data were combined to reconstruct changes in past sea surface conditions in addition the dinocyst assemblages were used to reconstruct the sea surface temperature sst and salinity sss based on the modern analog technique the distribution of the coccolith and dinocyst assemblages suggests a succession of climatic events during the late quaternary and early holocene linked to change in the influence of the labrador current and or the st lawrence river discharge the dinocyst assemblages recorded a thermal optimum during the mid holocene in the late holocene coccolith concentrations show two peaks at 2 3 and 1 1 ky which coincide with the presence of species with different ecological properties likely due to the localized influence of the slope water jet swj current the presence of this current has played a role in the development of the coccolith and dinocyst assemblages and sst fluctuations especially in the last 2 ky overall the interaction between the st lawrence river the swj the lc and the nac has all controlled the sss ssts and the development of the coccolith and dinocyst assemblages confirming a significant regionalism of the entire area

variations in surface water mass conditions in the norwegian sea evidence from holocene coccolith and dinoflagellate cyst assemblages coccolith and dinoflagellate cyst assemblages have been investigated in five sediment cores from the norwegian sea and fram strait both fossil groups are characterized by similar patterns of composition the assemblages contain high proportions of single species the coccolith flora is of low diversity and consists almost entirely of coccolithus pelagicus and emiliania huxleyi the dinoflagellate cysts are generally dominated by operculodinium centrocarpum and nematosphaeropsis labyrinthus other species especially bitectatodinium tepikiense peridinium faeroense and impagidinium pallidum sometimes contribute considerably to the assemblages based on the abundance of the assemblages and the ratio change between the dominating species it has been possible to establish three intervals of distinct major changes in surface water mass conditions sparse occurrences of coccoliths and dinoflagellate cysts have been observed before 10 000 yrs b p indicating harsh environmental conditions with a distinct influence of meltwater and temporarily very slight inflow of atlantic water the modern surface water circulation pattern was reinitiated during termination ib the assemblages suggest slightly cooler and probably less saline surface water conditions than are present today until 7500 yrs b p solar insolation may have caused a first temperature peak which is responsible for the early holocene productivity maximum a considerable change in the composition of dinocyst and coccolith assemblages occurs corresponding approximately to the onset of the holocene climatic optimum this change was most probably linked to an almost synchronous reorganization of the hydrographic properties in the entire north atlantic realm after the ice sheets had vanished since 6000 yrs b p the norwegian current with its modern oceanographic and ecological properties has been fully established

distribution of coccoliths in surface sediments in the norwegian greenland sea abstract the distribution of coccoliths has been examined in 20 surface sediment samples along an east west transect in the norwegian sea based on the abundance and species richness of coccoliths three biogeographic zones have been established along this transect the distribution of coccolith assemblages in the surface sediments appears to match closely with the locations of the different surface water masses in the norwegian greenland sea