authigenic vivianite in potomac river sediments control by ferric oxy hydroxides abstract sand size particles of vivianite fe3 po4 2 8h2o have been identified by x ray diffraction and scanning electron microscopy in surficial sediments of the tidal potomac river near a major sewage treatment plant vivianitc particles are concentrated in a surface layer of coarse sand dredge spoil which overlies much finer sediment similar to that found in adjacent areas although saturation indices indicate that the pore waters of virtually all the sediments in the study area are supersaturated with respect to vivianite it is found only in association with dredge spoil pore water profiles of fe p and eh as well as the size and morphology of individual grains indicate that the vivianite is authigenic and was not introduced with he dredge spoil it is suggested that the major control on the occurrence of the mineral is the presence or absence of amorphous ferric oxy hydroxides which apparently react with pore water phosphorus to form stable ferric hydroxy phosphates preferentially to the formation of vivianite

eastern mediterranean surface water nd during eemian sapropel s5 monitoring northerly mid latitude versus southerly sub tropical freshwater contributions abstract a well developed sapropel s5 was deposited in the eastern mediterranean during the last interglacial eemian 124 119 ka freshwater contributions to the basin at this time can be traced using the isotopic composition of nd in planktonic foraminifera this enables differentiation between radiogenic sources to the south under the influence of the african monsoon and unradiogenic sources to the north relating to the mid latitude westerlies here we compare new nd data from a core in the southeast aegean sea with published data from the ionian and levantine seas shifts towards more radiogenic nd in the lower and middle parts of sapropel s5 are most pronounced in the ionian sea record with nd and 18og ruber co varying more closely here than in the levantine and aegean seas this is consistent with a freshwater source proximal to the ionian sea site likely indicating a substantial reactivation of rivers flowing northward from the central saharan watershed the lack during s5 deposition of a noticeable shift towards more unradiogenic nd in the aegean record would exclude a large influx of water from the northern borders of the eastern mediterranean during sapropel deposition these findings support a scenario whereby the last interglacial eastern mediterranean was influenced strongly by the remote effects of an intensified african monsoon with more local precipitation in the northern borders contributing relatively little to the sea surface composition

a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

assessment of the impact of climate and anthropogenic factors on holocene mediterranean vegetation in europe on the basis of palaeohydrological records abstract correlations between lake level and river activity records from the western mediterranean and palaeohydrological records from northern africa and central europe suggest a distinction in the western mediterranean region between an early holocene period characterised by cooler and moister conditions than at present favourable to temperate deciduous trees and after 5000 yr bp a later holocene period with a warmer and drier climate the second phase was favourable to the extension of evergreen sclerophyllous trees possibly reinforced by human activities these two successive holocene periods reflect orbitally induced changes in summer insolation furthermore superimposed on this general climatic trend century scale climatic oscillations punctuated the whole holocene period decreases in river activity in the western mediterranean region occurred at ca 11 500 10 500 9000 7000 4000 3000 2000 and 800 cal yr bp these decreases coincided with lake level lowering in jura eastern france and glacier retreat in the northern alps and could be associated with a temporary expansion of sclerophyllous trees this general pattern could have resulted from an alternatingly southward northward displacement of the atlantic westerly jet

vivianite formation and its role in phosphorus retention in lake orn denmark abstract vivianite fe 3 po 4 2 8h 2 o may precipitate in anoxic lake sediments affecting the porewater orthophosphate concentration and thereby the trophic status of lakes we have investigated changes in lake diagenesis of fe and p 1969 2009 with particular attention focused on vivianite formation with sediment depth 0 20 cm in an iron silica carbon rich lake sediment ornso denmark porewaters were supersaturated for vivianite by two to five orders of magnitude upper 10 cm with porewater phosphate concentrations ranging between 0 69 and 10 mol l 1 in winter and summer concentrations ranging between 9 8 and 40 mol l 1 significant formation of vivianite was confirmed by x ray diffraction while scanning electron microscopy and electron dispersive x ray spectroscopy indicated an increase in vivianite crystal size with depth 20 to 70 m across variations in elemental composition of vivianite crystals in relation to at p and fe were especially seen going from 9 5 cm to 24 5 cm the total sediment fe pool was very large 3000 mol g 1 and total p increased from 200 mol g 1 to 400 mol g 1 descending down the sediment profile differential extraction experiments of p release at ph 3 estimated that vivianite amounts to between 3 and 5 of the total fe pool the total p burial fluxes estimate that 38 mol cm 2 yr 1 or 26 of sedimentary p in the lower sediments is sequestered as vivianite there are seasonal variations in the porewater composition with lower fe orthophosphate and higher sulfate concentrations during winter 5 c than during summer 15 c this suggests that temperature modulates the rate of organic matter degradation which in turns affects the rate of fe iii phase reduction release of phosphate and thereby the porewater fe 2 and orthophosphate concentrations and hence vivianite formation this work highlights the role vivianite can play for p retention in a si fe c rich lake sediment

physical controls associated with the distribution of sulfides in the voisey s bay ni cu co deposit labrador the voisey s bay ni cu co sulfide deposits occur within troctolites and olivine gabbros of the 1 34 ga voisey s bay intrusion formerly known as the reid brook intrusive complex the voisey s bay intrusion is the oldest known member of the predominantly anorthositic nain plutonic suite which straddles the ca 1 85 ga suture between archean gneisses of the nain province to the east and paleoproterozoic rocks of the churchill province to the west the reid brook discovery hill mini ovoid and ovoid zones are mineralized domains within a subvertical conduit dike system that appears to span between two large troctolite intrusions the eastern deeps chamber and the lower western deeps chamber the conduit trends east west and extends from near the top of the western deeps chamber at a depth of 700 m toward the north margin of the eastern deeps chamber close to surface all the mineralized zones in the voisey s bay deposits occur as magmatic textured sulfides associated with fragment bearing troctolites and olivine gabbros related to the conduit dikes rather than simply as basal accumulations at the floor of the chambers sulfides within the system are preferentially concentrated in traps where physical irregularities and changes in conduit morphology favor the precipitation capture and preservation of sulfides as a result of changes in the velocity and viscosity of a magma there is a consistent relationship between different sulfide textures and the physical environment existing in the conduits where each of the mineralized zones occurs this suggests that sulfides were deposited in their present sites by consistent magmatic processes within the conduits but were locally influenced by the physical parameters existing at these sites later remobilization or mass movement of sulfides would not generate the consistent textural relationships observed therefore sulfide deposition is ultimately related to a complex interplay of dike geometry i e changes in dike orientation and thickness and the fluid dynamics in the fragment bearing magma within a conduit system i e the ovoid mineralization appears to fill a bulge in the conduit and the reid brook zone is located close to the axis of inflection along its strike the eastern deeps zone although it does occur at the base of the eastern deeps chamber is more specifically associated with the line of entry for the feeder conduit close to a structural low at the base of the eastern deeps chamber the voisey s bay deposit does not conform to the traditional model for magmatic sulfide generation such as sudbury in which sulfides accumulate largely by gravitational settling within a magmatic chamber following attainment of sulfur saturation as a result of local assimilation in fact current evidence suggests in the eastern deeps mineralized zone that little gravitational settling took place because a large proportion of sulfides occur as high ni tenor minerals trapped interstitially in sulfides in coarse phases of the variable textured troctolite furthermore sulfide saturation and generation of magmas with varying sulfide contents clearly appear to have occurred in chambers at a depth of several kilometers rather than at the levels currently exposed in contrast to gravitational settling sulfide distribution within the voisey s bay deposit is controlled by magma emplacement through multiple braids of a dynamic channel like conduit dike system

reconstructing biomes from palaeoecological data a general method and its application to european pollen data at 0 and 6 ka biome models allow the results of experiments with atmospheric general circulation models to be translated into global maps of potential natural vegetation the use of biome models as a diagnostic tool for palaeoclimate simulations can yield maps that are directly comparable with palaeoecological pollen and plant macrofossil records provided these records are biomized i e assigned to biomes in a consistent way this article describes a method for the objective biomization of pollen samples based on fuzzy logic pollen types taxa are assigned to one or more plant functional types pfts then affinity scores are calculated for each biome in turn based on its list of characteristic pfts the pollen sample is assigned to the biome to which it has the highest affinity subject to a tie breaking rule modern pollen data from surface samples reflecting present vegetation across europe are used to validate the method pollen data from dated sediment cores are then used to reconstruct european vegetation patterns for 6 ka the reconstruction shows systematic differences from present that are consistent with previous interpretations the method has proved robust with respect to human impacts on vegetation and provides a rational way to interpret combinations of pollen types that do not have present day analogs the method demands minimal prior information and is therefore equally suitable for use in other regions with richer floras and or lower densities of available modern and fossil pollen samples than europe

feedbacks between climate and surface water in northern africa during the middle holocene observations indicate that the area of lakes and wetlands in northern africa was considerably greater during the middle holocene than at present simulations are designed to examine whether expanded surface waters may have had a significant impact on the strength of the summer monsoon of northern africa three experiments with the national center for atmospheric research community climate model ncar ccm3 are analyzed a modern and two middle holocene 6000 years before present simulations one with and one without prescribed expanded surface water there is a significant increase in the strength of the summer monsoon in the middle holocene simulation due to the enhanced seasonal insolation cycle the addition of surface waters result in a june july and august mean increase in the net surface radiation 5 an increase in the latent heat flux 30 a decrease in the sensible heat flux 10 and an increase in the near surface specific humidity 5 compared to the middle holocene simulation without surface water changes the changes in these simulated climate variables are comparable in scale to changes due to orbital forcing alone the expanded surface waters result in a cooling of the atmosphere and anticyclonic flow over the large water bodies in summer relative to the simulation without surface water changes the combination of increased atmospheric moisture and altered circulation results in significant changes to the precipitation distribution in northern africa including a small increase in the zonal mean july precipitation to the north of the lakes and a decrease to the south the geographic distribution of the precipitation with surface water changes is qualitatively in better agreement with observations than the distribution with orbital forcing alone but still does not fully match the expansion implied by observations nor the expansion required to produce the simulated middle holocene surface waters used in this study the results of this study suggest that surface waters were an important factor in the climate of northern africa during the middle and early holocene and that they must be included for accurate simulation of this climate

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

iron isotope differences between earth moon mars and vesta as possible records of contrasted accretion mechanisms the iron isotope compositions of shergotty nakhla chassigny snc meteorites thought to come from mars eucrites and diogenites assumed to sample asteroid 4 vesta and rocks from the moon and earth have been measured using high precision plasma source mass spectrometry the means of eight samples from mars and nine samples from vesta are within error identical despite a range of rock types they are lighter by 0 1 in 57fe 54fe compared to the average of 13 terrestrial mantle derived rocks the latter value is identical within uncertainty with a previously published mean of 46 igneous rocks from the earth the average for 14 lunar basalts and highland plutonic rocks covering a broad spectrum of major element composition is heavier by 0 1 in 57fe 54fe relative to our estimate for the earth s mantle and therefore 0 2 heavier than the eucrites diogenites and snc meteorites however the data scatter somewhat and the apollo 15 green glass and apollo 17 orange glass are identical to samples from mars and vesta there is no clear relationship between petrological characteristics and fe isotope composition despite a wide spectrum of samples instead contrasted planetary isotopic signatures are clearly resolved statistically after evaluating alternative scenario it appears that the most plausible explanation for the heavier fe in the earth and moon is that both objects grew via processes that involved partial vaporisation leading to kinetic iron isotope fractionation followed by minor loss this is consistent with the theory in which the moon is thought to have originated from a giant impact between the proto earth and another planet combined with numerical simulations fe isotope data can offer the potential to provide constraints on the processes that occurred in planetary accretion

sulfur isotope exchange and metal enrichment in the formation of magmatic cu ni pge deposits sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic cu ni pge deposits although low sulfur pge rich deposits e g j m reef of the stillwater complex merensky reef of the bushveld complex show little or no isotopic evidence for the involvement of crustally derived sulfur several high sulfur deposits e g noril sk duluth complex show strong evidence for contamination of mafic magmas by sulfur of crustal origin processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas for this reason 34s values are primarily dependent on sulfur source characteristics and variations can often be treated using a two component mixing approach however deposits such as those at noril sk and voisey s bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures these deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle derived melts that traversed the accumulated sulfide on their way to the surface our calculations illustrate the efficiency of reactions that involve the exchange of fe for cu ni or pge in producing increases in the metal tenor of the accumulated sulfide sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur sulfur isotope exchange is distinct from the addition of sulfur in the form of immiscible sulfide droplets from a sulfide saturated magma to the accumulated sulfide although both additive mixing and exchange can lead to similar isotopic effects stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems

in situ multiple sulfur isotope analysis by sims of pyrite chalcopyrite pyrrhotite and pentlandite to refine magmatic ore genetic models with growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic magmatic hydrothermal and hydrothermal ore systems these materials include sierra pyrite fes2 nifty b chalcopyrite cufes2 alexo pyrrhotite fe 1 x s and vmso pentlandite fe ni 9s8 that have been chemically characterized by electron microprobe analysis isotopically characterized for 33s 34s and 36s by fluorination gas source mass spectrometry and tested for homogeneity at the micro scale by secondary ion mass spectrometry beam sample interaction as a function of crystallographic orientation is determined to have no effect on 34s and 33s isotopic measurements of pentlandite these new findings provided the basis for a case study on the genesis of the long victor nickel sulfide deposit located in the world class kambalda nickel camp in the southern kalgoorlie terrane of western australia results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite pyrrhotite and chalcopyrite can better constrain genetic models related to ore forming processes data indicate that pentlandite pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar 33s values 0 2 this isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage the three sulfide phases show slightly variable 34s values 34schalcopyrite 2 9 0 3 34spentlandite 3 1 0 2 and 34spyrrhotite 3 9 0 5 which are indicative of natural fractionation careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage which may help resolve the nature of the ore forming process hence this sims based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic and alteration related sulfides which may not be resolved during bulk rock fluorination analysis the results indicate that unlike the giant dunite hosted komatiite systems that thermo mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative 33s values the kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales

sedimentary records of large holocene floods from the middle reaches of the yellow river china slackwater deposits have been used as evidence for the magnitude and frequency of palaeofloods which occurred prior to gauged and historical data sets e g holocene extreme flood reconstruction in north america and australia the slackwater deposits from xiaolangdi reach of the yellow river have been identified and related to floods according to both eyewitness accounts and the analysis of sediments the technique successfully delineated the peak flood level and estimated a discharge of the 1994 ad flood in the yellow river the discrepancy of the estimated discharge was less than 5 compared with the instrumental records when we applied the technique to palaeoflood analysis the highest slackwater deposits and flood indicators from the tributary mouths the low river terraces and the loess platforms and loess alcoves together with radiocarbon dating provided evidence of at least six floods with peak discharges greater than 27 300 m3 s 1 at xiaolangdi section over the last 8500 yr the largest flood in ca 7362 yr b p had an estimated discharge of 42 900 m3 s 1 which is the maximum flood event known in the yellow river the results provide a basis for a determining the floods which were comparable to the most cataclysmic flood during the last 2360 yr in 1843 ad and b estimating the flood magnitude for a 10 000 yr time scale relevant for building xiaolangdi dam one of the largest dams under construction in the world the reconstruction of palaeofloods also provides evidence for a phase between ca 6000 8000 yr b p which was much wetter than today as a result of increased summer precipitation and enhanced pacific monsoon conditions in eastern china

pollen based continental climate reconstructions at 6 and 21 ka a global synthesis subfossil pollen and plant macrofossil data derived from 14c dated sediment profiles can provide quantitative information on glacial and interglacial climates the data allow climate variables related to growing season warmth winter cold and plant available moisture to be reconstructed continental scale reconstructions have been made for the mid holocene mh around 6 ka and last glacial maximum lgm around 21 ka allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth assessment report ar5 of the intergovernmental panel on climate change the synthesis of the available mh and lgm climate reconstructions and their uncertainties obtained using modern analogue regression and model inversion techniques is presented for four temperature variables and two moisture variables reconstructions of the same variables based on surface pollen assemblages are shown to be accurate and unbiased reconstructed lgm and mh climate anomaly patterns are coherent consistent between variables and robust with respect to the choice of technique they support a conceptual model of the controls of late quaternary climate change whereby the first order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance

mid holocene east asian summer climate as simulated by the pmip2 models abstract in the present study datasets derived from twelve coupled ocean atmosphere general circulation models oagcms in the paleoclimate modelling intercomparison project phase two were used to analyze the east asian summer climate during the mid holocene about 6000 calendar years ago on the whole the oagcms reproduced warmer and wetter summer climate conditions in east asia during the mid holocene the multi model ensemble showed that in east asia the regionally averaged summer surface air temperature sat increased by 0 89 c summer precipitation was 5 8 higher and an obviously strengthened southerly wind corresponded to a strong summer monsoon in the mid holocene when compared to preindustrial levels the data model comparison in china reveals a good agreement between the oagcms results and the reconstructed changes in the summer sat in east china during the mid holocene in north china the simulated sat anomalies are 0 5 c lower overall than reconstruction in contrast the oagcms fail to capture the strongest warming in the southern qinghai tibetan plateau on the other hand the simulated precipitation agrees well with proxy data except for in the central parts of china where the simulated summer precipitation disagrees in sign with reconstruction in addition there is a large spread among the simulations particularly over and around the qinghai tibetan plateau and inter model discrepancies are larger for precipitation than for sat as a whole

paragneiss assimilation in the genesis of magmatic ni cu co sulfide mineralization at voisey s bay labrador δ34s δ13c and se s evidence the voisey s bay ni cu co deposit is located in ca 1 33 ga troctolitic rocks of the nain plutonic suite northern labrador it is widely regarded as a prime example of sulfide mineralization associated with a magmatic conduit system previous geological and geochemical studies have indicated that the voisey s bay intrusion was contaminated by interaction with both sulfide poor archean gneisses and with paleoproterozoic sulfide bearing tasiuyak metasedimentary gneiss limited sulfur isotope data previously indicated that only one of the four ore zones at voisey s bay is characterized by 34s values outside the range considered normal for uncontaminated mantle derived mafic magmas 0 2 new s isotope data presented here extends the previously reported range in 34s values for the tasiuyak gneiss from 17 0 to 18 3 per mil with an average of 1 4 per mil assimilation of this sulfur followed by isotope homogenization in the magmatic system is consistent with the isotope data from the ore zones and in line with the wealth of evidence that suggests that interaction with tasiuyak gneiss was an essential ingredient for ore formation in all the zones se s values of sulfide minerals in the ore zones at voisey s bay are also in the same range as those of whole rock samples from the tasiuyak gneiss 13c values of both mineralized and nonmineralized parts of the voisey s bay intrusion are anomalous 18 3 to 24 8 and similar to values of graphite in the tasiuyak gneiss both s and c contamination of the voisey s bay intrusion via diffusive transfer in a fluid phase derived from inclusions and wall rocks are considered unlikely because the area had been subjected to granulite grade metamorphism prior to emplacement of the nain plutonic suite either fractional melting or total fusion of xenoliths is considered a more likely mechanism for s addition to the mafic magma via pyrrhotite melting and for c addition in the form of co2 and ch4 produced by reaction of graphite with o and h bearing melt species

depositional setting of the late archean fe oxide and sulfide bearing chert and graphitic argillite in the shaw dome abitibi greenstone belt canada abstract interbedded chert rich exhalite and graphitic argillite are the only sedimentary rocks deposited in deep water settings during long lived hiatuses in mafic to ultramafic volcanism within the hart area of the shaw dome in the late archean abitibi greenstone belt in canada the fe oxide and sulfide bearing but predominantly cherty exhalite lithological unit in the hart area can be traced laterally to iron formation elsewhere in the shaw dome whole rock as well as fe and s isotope geochemistry suggest that the exhalite unit was formed as a result of direct precipitation from seawater distally from hydrothermal centres fractionation of fe isotopes through the precipitation of iron oxyhydroxides in a neutrally buoyant hydrothermal plume removed the heavier isotopes of fe resulting in the negative 56fe values observed in the exhalite in the hart area archean seawater is generally considered to be anoxic but moderate mn enrichments up to 1 87 wt mno in exhalite along with negative fe isotope values resulting from partial fe ii oxidation suggest the presence of oxygen in the upper part of the water column along the pathway of hydrothermal plumes from their source to the depositional site in the abitibi greenstone belt in contrast the graphitic argillite contains abundant pyrite nodules and bands that exhibit systematic negative fe isotope values but does not show mn enrichment this unit likely formed in a zone of upwelling of nutrient rich waters from deeper parts of the basin resulting in high organic productivity both exhalite and graphitic argillite have negative 33s values suggesting that sulfur was derived from seawater sulfate which is consistent with an anoxic atmosphere with sulfate aerosols produced by photochemical reactions combined our data indicates disequilibrium between anoxic atmosphere and partially oxygenated upper part of the water column during periods of volcanic quiescence in the 2 7 ga abitibi greenstone belt supporting the existence of oxidized oases within the archean ocean

vivianite is a key sink for phosphorus in sediments of the landsort deep an intermittently anoxic deep basin in the baltic sea phosphorus p is an essential nutrient for marine organisms its burial in hypoxic and anoxic marine basins is still incompletely understood recent studies suggest that p can be sequestered in sediments of such basins as reduced iron fe p but the exact phase and the underlying mechanisms that lead to its formation are unknown in this study we investigated sediments from the deepest basin in the baltic sea the landsort deep site m0063 that were retrieved during the integrated ocean drilling project iodp baltic sea paleoenvironment expedition 347 the record comprises the whole brackish marine littorina sea stage including past intervals of extensive hypoxia in the baltic sea that occurred during the holocene thermal maximum htmhi and the medieval climate anomaly mca1hi and mca2hi various redox proxies e g the presence of laminations and high mo contents suggest almost permanent bottom water hypoxia during the littorina sea stage in the landsort deep the bottom waters were likely even seasonally anoxic or sulfidic during the mca1hi and mca2hi and permanently sulfidic during the htmhi with the use of micro analysis of sieved minerals sem eds xrd and synchrotron based xas we show that mn and mg rich vivianite crystals are present at various depths in the littorina sea sediments we also have indications for vivianite in the mca1hi mca2hi and htmhi deposits the formation of vivianite thus likely explains the high fe bound p fraction throughout the whole littorina sea stage shuttling of fe and mn from the shelves into the basin and high inputs of p in settling organic matter are likely key drivers for vivianite formation our study shows that vivianite can likely form in near surface sediments under a broad range of bottom water redox conditions varying from hypoxic and anoxic to sulfidic

an overview of the upb geochronology of the paleoproterozoic torngat orogen northeastern canada abstract the paleoproterozoic torngat orogen northern labrador and northeastern quebec canada is the result of oblique collision between the nain north atlantic and rae cratons a comprehensive set of u pb ages allows the chronology of events to be closely bracketed subduction occurred below the nain craton from ca 1910 1870 ma and collision between the cratons may have occurred between 1869 and 1844 ma sinistral deformation continued along the intervening abloviak shear zone until ca 1824 ma and convergence was ultimately impeded by the partial subduction of the southern rae craton post collisional isostatic uplift of the southern rae craton initiated deformation in the komaktorvik zone ca 1798 ma a prominent bend in the abloviak shear zone may be the result of differential uplift of the western side of the orogen in the south sinistral strike slip and east as well as west side up deformation continued episodically in the komaktorvik zone until at least 1710 ma the tectonic development of the orogen can be explained in terms of modern plate tectonic processes

intensified mid holocene asian monsoon recorded in corals from kikai island subtropical northwestern pacific abstract coupled records of sr ca and oxygen isotope ratios 18 o of coral skeletons have been used to produce quantitative estimates of paleo sea surface temperature sst and 18 o of surface seawater that can in some cases be converted to sea surface salinity sss two fossil corals from kikai island in the subtropical northwestern pacific a location affected by east asian summer and winter monsoons were analyzed to investigate differences between mid holocene and present day sst and sss at 6180 cal yr bp ssts were roughly the same as today both in summer and winter 18 o seawater and sss values were higher both in summer 0 5 1 1 psu and in winter 0 2 0 6 psu than modern values at 7010 cal yr bp ssts were slightly cooler both in summer and winter 0 8 and 0 6 c whereas 18 o seawater and sss had higher values in summer 0 3 0 6 psu and in winter 0 8 1 9 psu than present day values these results are consistent with other marine records for the mid holocene of the low and midlatitudes in the northwestern pacific such regional conditions indicate that the east asian summer and winter monsoons were more intense in the mid holocene which was likely a function of the mid holocene insolation regime

the role of paragneiss assimilation in the origin of the voisey s bay ni cu sulfide deposit labrador multiple s and fe isotope evidence isotopic and geochemical studies conducted on the voisey s bay deposit labrador canada suggest crustal contamination of the primary magma as a trigger for sulfur saturation and formation of the deposit the use of multiple s isotopes has allowed for the identification of a bacterial sulfate reduction biosignature in the tasiuyak gneiss in the footwall to the voisey s bay deposit this putative biosignature is preserved in the deposit even at high silicate magma sulfide melt ratios r factor and links the s present in the voisey s bay deposit to the tasiuyak gneiss iron isotopes in the voisey s bay deposit have been reset to magmatic values at r factors 100 but s isotope data can be used to model higher r factors a contamination model results in calculated r factors of 433 177 the multiple s isotope data are a new proxy to directly link s from the deposit to crustal s sources even in deposits with high r factors where the equilibration with large amounts of silicate magma can make interpreting a link between the deposit and the sulfur source difficult

mid holocene land surface conditions in northern africa and the arabian peninsula a data set for the analysis of biogeophysical feedbacks in the climate system large changes in the extent of northern subtropical arid regions during the holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales models that omit biogeophysical feedback however are unable to account for the full magnitude of african monsoon amplification and extension during the early to middle holocene 9500 5000 years b p a data set describing land surface conditions 6000 years b p on a 1 1 grid across northern africa and the arabian peninsula has been prepared from published maps and other sources of palaeoenvironmental data with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the palaeoclimate modelling intercomparison project the data set includes information on the percentage of each grid cell occupied by specific vegetation types steppe savanna xerophytic woods scrub tropical deciduous forest and tropical montane evergreen forest open water lakes and wetlands plus information on the flow direction of major drainage channels for use in large scale palaeohydrological modeling the data set is available in digital form by anonymous ftp

the climate of north america and adjacent ocean waters ca 6 ka the climate of north america and the adjacent ocean at 6000 bp was estimated using five independent approaches using pollen data the terrestrial climate was estimated by the movement of ecozone boundaries and by the method of modern analogues both analyses indicate warmer temperatures in the western great lakes area and the northern great plains a model of sphagnum dominated peatland initiation when forced by canadian climate model 6 ka output projected a cooler and or wetter climate for continental western north america contrary to this a reconstruction of the distribution of sphagnum dominated peatlands in western canada indicates that they were located north of their modern distribution suggesting warmer and or drier conditions at 6000 bp this interpretation is strengthened by observations of lower lake levels at 6000 bp in western canada this drier climate may have been associated with warmer conditions as indicated by the quantitative climate reconstructions in general eastern north a

role of the biosphere in the mid holocene climate of west africa received 26 june 2001 revised 2 december 2001 accepted 5 march 2002 published 16 january 2003 1 in previous studies a zonally symmetric synchronously coupled biosphereatmosphere model zonalbam which includes explicit representation of ecosystem dynamics has been developed and validated based on current conditions over the region of west africa here we use zonalbam to study the response of the coupled biosphereatmosphere system to changes in the earth s orbital forcing during the middle holocene 6k yrs bp and the relative contribution of vegetation feedbacks simulations in which vegetation conditions were fixed to the current distribution show that an orbitally induced increased seasonality in insolation for the middle holocene by itself results in a 1 1 northward shift in the location of the southern margin of the sahara as compared to current solar forcings when vegetation is allowed to be dynamic a 2 4 northward shift is simulated however when dynamic vegetation is initialized to palaeovegetation a 5 1 northward shift is simulated bringing results more consistent with palaeoevidence based on previous studies on the role of the gradient of moist static energy on the dynamics of large scale tropical circulations a mechanism for the enhancement of the summer monsoon circulation has been developed our results suggest that multiple equilibria could have coexisted over the region of west africa during the middle holocene furthermore based on previous studies on the current climate over the region we hypothesize that transitions between the different equilibria could have taken place during the middle holocene causing the southern desert margin to migrate between 18 1n and 21 4n and shaping climate variability index terms 0315 atmospheric composition and structure biosphere atmosphere interactions 3322 meteorology and atmospheric dynamics land atmosphere interactions 3344 meteorology and atmospheric dynamics paleoclimatology 9305 information related to geographic region africa keywords west africa holocene vegetation dynamics monsoon climate change insolation

pyrite rb sr sm nd and fe isotopic constraints on the age and genesis of the qingchengzi pb zn deposits northeastern china abstract sr nd fe isotopes are reported for pyrites and whole rocks of ores and associated rocks for the qingchengzi pb zn deposits in the northeasern china to investigate the age sources and processes of the pb zn mineralization rb sr leaching isotopic dating of pyrites that crystallized coeval with galena and sphalerite from five subsamples of ores yields isochron ages of 143 12 ma to 159 12 ma with a weighted mean age of 151 8 5 2 ma this age can be interpreted to be the timing of pb zn mineralization coeval with the emplacement of late jurassic granites in the region the pyrites have negative end 152 ma values of 17 6 to 22 0 among the neoarchean ttg rocks paleoproterozoic meta sedimetary rocks i e wall rocks and granites and mesozoic mafic dykes and granites however the initial 87sr 86sr ratio of 0 7188 0 0004 is similar to those of late jurassic granites and paleoproterozoic rocks 0 7082 0 7183 distinct from those of mafic dykes 0 7069 0 7110 indicating the ore forming materials i e pb and zn were mainly derived from wall rocks and late jurassic granites combined with the previously published h o isotopes the new age and sr nd isotopic data indicate that the mineralizaing fluids were mainly derived from dehydration of granitic magmas interacted with meteoric water that extracted pb and zn from the wall rocks the 56fe values of pyrites in the pb zn ores and paleoproterozoic meta sedimentary rocks range from 0 23 to 1 09 and 0 49 to 0 17 respectively distinct from those of paleoproterozoic basement rocks and mesozoic rocks 0 03 to 0 28 combined with nd isotopes the large range of 56fe values for pyrites would be the result of rapid precipitation of pyrites in the wall rocks and contemperal deposition of pyrites in the residual fluids during interaction between wall rocks and ore forming fluids collectively our new sr nd fe isotopes of pyrites not only constrain the age and sources of pb and zn mineralization but also reveal the detailed mineralization processes providing a new method to constrain the genesis of hydrothermal pb zn deposits

on the late pleistocene holocene sapropel record of climatic and oceanographic variability in the eastern mediterranean sapropels and intercalated marls in a piston core from the eastern mediterranean are chemically and mineralogical distinct kaolinite smectite and total s contents fe a1 ba a1 co a1 mo a1 and v a1 are higher whereas quartz mg calcite illite contents si a1 ti a1 k a1 rb a1 and zr a1 are lower in the sapropels missing and ghost sapropels are identified by mineralogical and chemical properties that are not prone to diagenesis primary production was higher ba a1 and bottom water and or interstitial oxygenation was lower co a1 mo a1 ni a1 and v a1 during sapropel formation wind speeds quartz clay si a1 and zr a1 and bottom water salinities mg calcite calcite were higher during periods of marl formation sapropels represent a fundamentally different sedimentary facies whose formation is linked to changes in the hydrological balance in the basin driven by precessionally modulated changes in monsoon strength and subtropical precipitation changes which altered circulation production and sediment source areas

an evaluation of the simulated water balance of eurasia and northern africa at 6000 y bp using lake status data changes in the water balance of eurasia and northern africa in response to insolation forcing at 6000 y bp simulated by five atmospheric general circulation models have been compared with observations of changes in lake status all of the simulations show enhancement of the asian summer monsoon and of the high pressure cells over the pacific and central asia and the middle east causing wetter conditions in northern india and southern china and drier conditions along the chinese coast and west of the monsoon core all of the models show enhancement of the african monsoon causing wetter conditions in the zone between ca 10 20 n four of the models show conditions wetter than present in southern europe and drier than present in northern europe three of the models show conditions similar to present in the mid latitude continental interior while the remaining models show conditions somewhat drier than present the extent and location of each of the simulated changes varies between the models as does the mechanism producing these changes the lake data confirm some features of the simulations but indicate discrepancies between observed and simulated climates for example the data show 1 conditions wetter than present in central asia from india to northern china and mongolia indicating that the simulated asian monsoon expansion is too small 2 conditions wetter than present between ca 10 30 n in africa indicating that the simulated african monsoon expansion is too small 3 that northern europe was drier but the area of significantly drier conditions was more localized around the baltic than shown in the simulations 4 that southern europe was wetter than present apparently consistent with the simulations but pollen data suggest that this reflects an increase in summer rainfall whereas the models show winter precipitation and 5 that the mid latitude continental interior was generally wetter than present

oxygen isotope studies of the voisey s bay ni cu co deposit labrador canada oxygen isotope studies of the voisey s bay intrusion and surrounding country rocks were initiated to help assess the importance of magma contamination in the generation of ni cu co sulfide mineralization the 18o values of proterozoic tasiuyak paragneiss country rocks range from 8 3 to 16 1 per mil whereas those of enderbitic and mafic orthogneiss range from 5 7 to 8 7 per mil with most values less than 7 5 per mil the 18o values of the principal units of the voisey s bay intrusion fall in the range of 5 4 to 7 7 per mil with over 90 percent of the samples less than 7 per mil and not indicative of magma contamination strong evidence for country rock contamination of the mafic magma comes from inclusions of paragneiss found in the basal breccia sequence of the eastern deeps and the feeder breccia in the reid brook zone the gneiss inclusions range in 18o values from 4 7 to 10 6 per mil and are depleted in 18o relative to parental paragneiss country rocks loss of a siliceous 18o rich component has accompanied progressive conversion of the xenoliths to residual assemblages rich in plagioclase and hercynite elevated 18o values up to 9 3 are found in troctolitic and noritic matrix of the breccias but are restricted to distances less than 2 cm from the margins of inclusions these isotopic exchange profiles have been produced during subsolidus cooling and do not record the transport of the major high 18o component lost from the gneiss oxygen isotope evidence in the igneous rocks for assimilation of high 18o material derived from the gneiss is thought to have been destroyed via dilution as large volumes of magma moved from lower to higher chambers through a conduit system the introduction of the high 18o sio2 rich contaminant may have acted in concert with the introduction of country rock sulfide in promoting sulfide saturation of the voisey s bay magma

sulfur and oxygen isotopic evidence of country rock contamination in the voisey s bay ni cu co deposit labrador canada abstract the emplacement of basaltic magma into sulfide bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation induced changes in magma composition pelitic country rocks of the proterozoic tasiuyak gneiss in the area of the voisey s bay ni cu co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the voisey s bay intrusion sulfur isotopic compositions of the tasiuyak gneiss range from 0 9 to 17 0 values typical for sulfides produced via bacterial sulfate reduction in an open marine environment archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally derived sulfur sulfur isotopic compositions of the sulfide minerals from the voisey s bay deposit show consistent variations both spatially and with rock types disseminated and massive sulfides show a decrease in 34 s to the west with values typically between 0 and 2 in the eastern deeps ovoid and discovery hill zone and between 2 and 4 in the reid brook zone 34 s values of the mushua intrusion to the north and the normal troctolite in the eastern deeps are more positive ranging between 0 5 and 1 8 this range is taken to represent the isotopic composition of primary mantle derived sulfur in the area because the mushua intrusion and normal troctolite show the least geochemical evidence for contamination by country rocks sulfur isotopic data from the reid brook zone are consistent with up to a 50 sulfur contribution from the tasiuyak gneiss correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low sulfide enderbitic and quartzofeldspathic gneisses oxygen isotopic values of gneiss fragments in the basal breccia sequence and feeder breccia suggest that the assimilation process involved a greater proportion of high 18 o contaminant to the west 18 o values of the tasiuyak gneiss 5 9 to 14 0 enderbitic gneiss 6 4 to 8 7 and archean quartzofeldspathic gneiss 9 5 to 9 7 are consistent with an increased proportion of tasiuyak gneiss contaminant to the west isotopic data strongly indicate that sulfur from the tasiuyak gneiss has been involved in ore deposition at the voisey s bay deposit however sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization and that assimilation of both proterozoic and archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic physical mechanisms

geochemical study of a crater lake lake pavin puy de dome france constraints afforded by the particulate matter distribution in the element cycling within the lake abstract suspended particles concentrations and settling particle fluxes collected by sediment traps were analyzed along a vertical profile in lake pavin france at the same time as dissolved element concentrations which are reported elsewhere viollier et al 1995 results for iron manganese cobalt vanadium molybdenum uranium barium rubidium and cesium are presented here particulate concentrations of all elements increase significantly just above the sediment water interface manganese iron and barium particulate concentrations exhibit a peak just above the oxic anoxic boundary molybdenum and uranium particulate concentrations increase drastically below the oxic anoxic boundary manganese and cobalt fluxes are at a maximum at 55 m depth above the oxic anoxic boundary for all other elements fluxes are at a maximum at 85 m depth just above the bottom of the lake in all cases settling particles fluxes are greater than buried fluxes indicating that a dissolution reaction occurs at the bottom of the lake particulate iron is essentially present as fe iii oxides in the mixolimnion as fe ii phosphates in the monimolimnion and as pyrite in the sediment the change in the fe bearing mineral leads to either important remobilisation or immobilisation of trace element into or from solution comparison of fluxes in the sediment trap at 85 m depth with dissolved concentrations gradients above the bottom leads to the rejection of the previous hydrodynamic model of the lake the fluxes and gradient data are consistent with the tritium data only if the arrival of sublacustrine waters occurs in the mixolimnion residence times of waters in monimolimnion in the model derived from the present study are about 20 times greater than the previous values the dissolved element concentrations profiles are explained only by eddy diffusion with a coefficient varying from about 20 m2 a at the bottom of the lake to 0 2 m2 a 1 at the mixolimnion monimolimnion boundary transport parameters obtained in this way allow a quantitative description of the element cycles within the lake and an estimation of the elemental ratio sedimentation versus exportation by the outlets

roles of xenomelts xenoliths xenocrysts xenovolatiles residues and skarns in the genesis transport and localization of magmatic fe ni cu pge sulfides and chromite abstract most sulfide rich magmatic ni cu pge deposits form in dynamic magmatic systems by partial melting s bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components and generation of barren to weakly mineralized immiscible fe sulfide xenomelts into which ni cu co pge partition from the magma some exceptionally thick magmatic cr deposits may form by partial melting oxide bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components and generation of barren fe ti oxide xenocrysts into which cr mg v ti partition from the magma the products of these processes are variably preserved as skarns residues xenoliths xenocrysts xenomelts and xenovolatiles which play important to critical roles in ore genesis transport localization and or modification incorporation of barren xenoliths autoliths may induce small amounts of sulfide chromite to segregate but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors pge cu ni co and cr v ti respectively is required to make significant ore deposits silicate xenomelts are only rarely preserved but will be variably depleted in chalcophile and ferrous metals less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma but may aid downward sulfide transport by increasing the bulk density of the magma sulfide wets olivine so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets but other silicates and xenoliths may not be wetted by sulfides xenovolatiles may retard settling of or in some cases float dense sulfide droplets reactions of sulfide melts with felsic country rocks may generate fe rich skarns that may allow sulfide melts to fractionate to more extreme cu ni rich compositions xenoliths xenocrysts xenomelts and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas and are more likely to be preserved in less dynamic less turbulent systems domain phases than in more dynamic more turbulent systems domains phases massive to semi massive ni cu pge and cr mineralization and xenoliths are often localized within footwall embayments dilations jogs in dikes throats of magma conduits and the horizontal segments of dike chonolith and dike sill complexes which represent fluid dynamic traps for both ascending and descending sulfides oxides if skarns residues xenoliths xenocrysts xenomelts and or xenovolatiles are present they provide important constraints on ore genesis and they are valuable exploration indicators but they must be included in elemental and isotopic mass balance calculations

sulfide saturation mechanism of the poyi magmatic cu ni sulfide deposit in beishan xinjiang northwest china abstract the poyi cu ni deposit is hosted by the early permian pobei mafic ultramafic complex along the northern margin of the tarim plate this series of multiple intrusions in the poyi deposit can be divided into four lithologies gabbro dunite hornblende peridotite and wehrlite the ore body consists mainly of disseminated sulfides hosted by hornblende peridotite all the poyi deposit sulfides show positive 33 s values from 0 004 to 0 221 and negative 34 s values from 0 8 to 3 5 high ni contents occur in the hornblende peridotites which exhibit the highest 33 s value of 0 221 and the lowest 34 s value of 3 5 indicating contamination by sulfides from archean sedimentary rocks this contamination was important during sulfide saturation in the poyi intrusions and likely occurred at depth before the emplacement of the poyi intrusions the intrusions incorporated country rocks during their emplacement and consolidation and the degree of assimilation increases from the central lithofacies i e the hornblende peridotite to the marginal lithofacies i e the wehrlite dunite olivine gabbro and gabbro higher ni contents are correlated with lower degrees of contamination thus we infer that the contamination by the country paleoproterozoic rocks which contain significant amounts of gneiss and marble hindered sulfide saturation the whole rock ni content is negatively correlated with the mgo and fo contents in the olivine and positively correlated with the feo and mno contents in the olivine during crystallization olivine becomes gradually richer in feo but poorer in mgo and mn tends to be enriched in the late stages of the melt we infer that the fractional crystallization of olivine was an important factor during sulfide saturation

geochemical cycling of phosphorus in rivers abstract concentrations of contaminants in fresh waters are controlled not only by point and diffuse sources within the catchment but by a multitude of biogeochemical reactions riverine processes that may occur in the water column during transport as an example the results of mass balance studies using snapshot field sampling techniques for p transport in a large river in northern england r swale are summarised to identify the processes that contribute to internal cycling within the river and their relative size compared with the total export of p the interaction of p with sediments is an important factor controlling concentrations of soluble p in the water column during both low and high river flows with in river storage in some instances amounting to 30 of the load throughput antecedent weather conditions are identified as important in controlling the pool of fine sediment and associated p available for remobilisation during storm pulses examples of p reactions with sediments that have been identified from field studies are briefly reviewed firstly a mass balance approach to elucidate the net interaction of p with river bed sediments is presented and then two specific interactions of p with sediments are discussed a the coprecipitation of phosphate with calcite in lakes during phytoplankton blooms and in benthic algal biofilms on river sediments and b the formation of vivianite in the anoxic zone of bed sediment in eutrophic lakes and rivers phosphate coprecipitation with calcite is an important mechanism controlling p concentrations in some hardwater lakes and has been found to contribute as much as 30 of the dry mass associated with the colonisation and development of algal biofilms on hardwater river beds vivianite has been detected in anoxic river sediments up to 20 by mass receiving treated sewage although little is known about the conditions of formation and how widespread the occurrence of vivianite is in sediments

lake surface fluctuations since the late glaciation at lake daihai north central china a direct indicator of hydrological process response to east asian monsoon climate abstract sediment retrieved by coring from the center of enclosed lake daihai in north central china and lacustrine sediments obtained from lake terrace outcrops were employed to reconstruct the lake surface fluctuations and the relationships with monsoon climate since the late glaciation based on accelerator mass spectrometry ams radiocarbon chronology before ca 13 ka bp the surface of the lake was lower indicating a deficiency of effective precipitation and dry climate the result of relatively decreased monsoon precipitation during ca 13 11 ka bp the lake surface ascended to a relatively higher level marking a significant increase of effective precipitation the climate became humid indicating strengthened monsoon rainfall then the lake surface experienced a sharp decline ca 11 10 ka bp this could suggest a severe reduction of effective monsoon precipitation and drier climate ca 10 9 0 ka bp the lake surface area expanded temporarily as the level rose denoting the amelioration of monsoon related effective precipitation and a relatively humid climate subsequently the lake surface declined at ca 9 0 7 3 ka bp reflecting the prevalence of dry climate and relatively decreased monsoon precipitation higher lake level and larger lake depth indicated large scale expansion of the lake surface at ca 7 3 3 2 ka bp despite some secondary fluctuations this implied plentiful monsoon related effective precipitation and more humid climate in the lake basin since ca 3 2 ka bp the lake surface has experienced a severe fall except for a minor rise around 2 0 ka bp indicating dry climate as a whole from waning monsoon precipitation and relatively increased evaporation the effective precipitation variations revealed by lake surface fluctuations in lake daihai since the late glaciation are not only related to forcing factors such as insolation and temperature driven air masses from the continent and the ocean but are also influenced by regional feedback from topography vegetation coverage and hydrological status

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

shelf to basin iron shuttling enhances vivianite formation in deep baltic sea sediments coastal hypoxia is a growing and persistent problem largely attributable to enhanced terrestrial nutrient i e nitrogen and phosphorus loading recent studies suggest phosphorus removal through

comparison of north american pollen based temperature and global lake status with cccma agcm2 output at 6 ka abstract we present new paleoclimate reconstructions for the time period of 6000 yr bp 6 ka that permit a quantitative comparison to corresponding mapped patterns of the newest version of the canadian cccma agcm2 climate model july temperature reconstructions were estimated using the modern analog technique employing fossil pollen records from the north american pollen database we present a new method of optimizing critical choices within the application of the mat through modern calibration the pattern of 6 ka temperature anomalies generated by the cccma agcm2 generally agree with those of the pollen based reconstruction particularly in the presence of a band of positive temperature anomalies across the continent from the yukon through the great lakes region and along the st lawrence this agreement suggests that both the model and data are reconstructing a contraction in the polar vortex over north america at 6 ka major areas of disagreement between the pollen based reconstruction and model simulation are also areas where the data are sparse and edge effects produce less reliable reconstructions the cccma agcm2 simulates drier conditions in a west east band stretching from alaska through the canadian prairies and midwest united states in agreement with low lake level observations at 6 ka simulated and reconstructed precipitation evaporation are also in agreement over the southwest united states with the exception of north central africa the cccma agcm2 fails to capture the northward and eastern extension of the afro asian monsoon belt at 6 ka general agreement between cccma agcm2 p e anomalies and lake status change is found throughout europe at 6 ka

multiple sulfur isotopes and the evolution of earth s surface sulfur cycle abstract the distribution of sulfur isotopes in geological materials reveals information about earth history and biogeochemical processes research during the last several decades has used sulfur isotope geochemistry as a tool to better understand microbial processes harrison and thode 1958 kaplan 1975 monster et al 1979 peck 1959 peck 1962 rees 1973 and sediment diagenesis berner 1969 berner 1982 canfield et al 1993b earth historians also realized this potential as there exists a rich record of environmental change within the sedimentary records canfield and teske 1996 claypool et al 1980 goodwin et al 1976 habicht et al 2002 kah et al 2004 monster et al 1979 shen et al 2001 strauss 1993 thode and goodwin 1983 these applications have championed the use of the two most abundant sulfur isotopes 32s and 34s and provide a rich introduction to what the sulfur isotope record has to offer see canfield 2004 canfield and raiswell 1999 within the last decade this information has been supplemented by new data derived from the less abundant isotopes 33s and 36s the measurement of all four stable sulfur isotopes multiple sulfur isotope geochemistry has expanded our understanding of biological evolution and activity atmospheric chemistry and transport crustal recycling and many more fields related to earth surface processes see farquhar and wing 2003 here i present a review of recent works in multiple sulfur isotope geochemistry with a focus on results that inform our understanding of biogeochemical processes and earth surface evolution

the nain churchill boundary and the nain plutonic suite a regional perspective on the geologic setting of the voisey s bay ni cu co deposit northern labrador is the site of two major lithotectonic elements of eastern canadian shield geology first it encompasses a paleoproterozoic continental suture zone archean rocks of the nain province tectonically juxtaposed against paleoproterozoic and reworked archean rocks of the churchill province the nain churchill suture evolved between 1860 and 1740 ma impressing upon the archean and paleoproterozoic rocks a regional metamorphic and tectonic signature referred to as the torngat orogen an important component of the torngat orogen is the tasiuyak gneiss a paleoproterozoic metasedimentary unit that occupies most of the eastern margin of the churchill province second northern labrador is the site of a world renowned terrane of mesoproterozoic anorogenic igneous rocks the nain plutonic suite that sits astride the suture and acts as a stitching batholith between the nain and churchill provinces the nain plutonic suite comprising amalgamated granitic anorthositic dioritic and troctolitic intrusions largely obscures the suture for nearly half of its total length the suite evolved between 1350 and 1290 ma and is interpreted as a product of magmatism associated with limited intracontinental rifting of the conjoined nain churchill crust above a mantle plume the voisey s bay ni cu co deposit serendipitously discovered in 1993 resides within one of the oldest troctolitic intrusions of the nain plutonic suite a 1334 ma intrusion that transgresses a contact between archean and paleoproterozoic gneisses genetic controls on the setting of mineralization have been built upon the nain churchill boundary as a zone of weakness that was exploited by mantle magmas and have implicated the tasiuyak gneiss as a crustal sulfur source although the archean paleoproterozoic junction at voisey s bay is presently interpreted as representing the nain churchill boundary in reality the provincial affinity of the archean gneisses is equivocal this uncertainty dictates caution in assigning the nain churchill boundary as the convenient transcrustal conduit that allowed the voisey s bay mafic intrusions rapid transit from their mantle staging area modeling the sulfide deposit as being solely a result of sulfur saturation of mafic magma by proximal paleoproterozoic tasiuyak paragneisses is also questionable because other genetic processes may have been responsible for example desilication of gneiss is strongly indicated by aluminous refractory minerals in vestiges of fused tasiuyak gneiss the spatial distribution of the mineralized rock at voisey s bay apparently within the subchamber plumbing system feeding a larger intrusion and within remnants of the larger intrusion itself may serve as a paradigm for similar deposits in the nain area but the preerosional comparative morphology of that larger intrusion relative to the better preserved ones may never be known a thorough understanding of the setting and genesis of the voisey s bay deposit will better facilitate establishing exploration targets in other parts of the nain region in labrador as a whole and in other geologically similar regions of the world

authigenesis of vivianite as influenced by methane induced sulfidization in cold seep sediments off southwestern taiwan abstract authigenesis of iron rich phosphate nodules occurs in iron rich cold seep sediments md052911 core at yung an ridge offshore southwestern taiwan raman ftir and quantitative x ray energy dispersive spectroscopic analyses indicate that the phosphate mineral is vivianite or baricite and shows fe mg molar ratios spanning from ca 0 6 to 4 0 and a general down core trend of increasing fe mg ratios the formation of vivianite is limited to a depth interval of 13 17 mbsf meters below seafloor and is most prominent at 16 mbsf in association with high dissolved iron concentrations and depleted dissolved sulfide below a peak sulfidization zone enriched in mackinawite and greigite alternate growths of vivianite and iron monosulfides and compositional zoning with mg enriched towards the peripheries of individual nodules occur in the transition from the zone of vivianite mineralization to the sulfidization zone the crystallization of vivianite below the sulfidization front could have been favored by scavenging of downward diffusive dissolved sulfide from pore waters in the sulfidization zone alternate growths and overlapping of the zones of iron monosulfides and vivianite can be attributed to fluctuations of the sulfidization front and methane flux the discovery of vivianite in the yung an ridge sediments implies that authigenic vivianite can be an important sink for phosphorus burial in cold seep sediments that have high reactive iron contents and high sedimentation rates

effects of mid holocene river runoff on the arctic ocean sea ice system a numerical model study recent geological studies have revealed that the freshwater input to the arctic ocean was highly variable during the holocene in the present study the ine uence of mid holocene river runoff on large scale arctic ocean sea ice dynamics is examined using a general circulation model a palaeohydrological forcing for the time interval around 6 ka bp 1 4 c timescale is constructed by compiling data from the available literature keeping all other forcing e elds and bottom topography of the ocean model at present day values the effect of a changed freshwater input to the arctic ocean is isolated the model shows that freshwater supply is vitally important for the polar oceanic circulation in particular a close connection between siberian river runoff and the path of the transpolar drift tpd is found consistent with palaeoceanographic e ndings of driftwood delivery to the canadian arctic archipelago and bafe n bay the model results suggest that enhanced freshwater discharge during the mid holocene caused an eastward shift of the tpd with strengthened oute ow through fram strait

a new whitefish from the early quaternary of bluefish basin yukon territory canada and its paleoenvironmental implications a nearly complete fossil of a whitefish coregonus beringiaensis sp nov salmoniformes salmonidae coregoninae the oldest known record of the genus is described from ch ijee s bluff along the porcupine river bluefish basin yukon territory the new species is closest to but distinct from species within the coregonus clupeaformis complex especially in its low lateral line scale count quadrate with an extensive canal and pore complex y shaped lachrymal and a unique combination of counts and measurements of fins scales and body proportions the stratigraphic position of the source concretionary layer at the base of unit 3 a lacustrine unit is well below a magnetically reversed interval thought to represent at least the upper part of the matuyama chron the specimen was found above ice wedge pseudomorphs thought to be the first sign of the onset of cold temperatures in the early quaternary the fossil is older than 0 79 ma and probably younger than 2 58 ma palynomorphs in the concretion sugge

contrasting petrological and geochemical relationships in the voisey s bay and mushuau intrusions labrador canada implications for ore genesis new data on the voisey s bay and mushuau intrusions indicate that the two bodies which were formerly grouped as the reid brook complex differ markedly in age petrology geochemistry and the scale of known mineralization u pb dating of baddeleyite and zircon has revealed that the mushuau intrusion has an age of 1 313 ga which is 20 m y younger than the 1 333 ga voisey s bay intrusion the voisey s bay intrusion is host to the 124 million tons ni cu co sulfide deposit reserves resources mineralization occurs at the base of an upper subchamber the eastern deeps subchamber and in a conduit that connects the eastern deeps to a lower subchamber the reid brook subchamber the mineralization is associated with a basal breccia that contains abundant reacted inclusions of paragneiss within the eastern deeps the basal mineralization is overlain by a varied textured troctolite that contains variable amounts of disseminated sulfide and gneiss inclusions by comparison only minor amounts of sulfide mineralization have been observed within the mushuau intrusion the mushuau intrusion comprises a layered unit that has been intruded by a stellate textured troctolite along its contact with footwall orthogneiss the layered unit consists dominantly of melatroctolite at the margin with leucotroctolite toward the center and olivine gabbro at the core the cumulates of the mushuau intrusion are richer in olivine and thus have higher mgo contents than those of the voisey s bay intrusion notwithstanding this olivines in the melatroctolite inclusions of the voisey s bay intrusion which are thought to represent disrupted cumulus layers contain up to 81 mole percent forsterite whereas those in the melatroctolite of the mushuau intrusion contain no more than 70 mole percent forsterite nickel depletion in olivine is more pronounced in the voisey s bay intrusion than in the mushuau intrusion our data indicate that the parental magmas of both intrusions were broadly basaltic mgo 8 1 wt and that the magma of the mushuau intrusion was more evolved than that of the voisey s bay intrusion yet still capable of forming olivine rich cumulates the troctolites of the voisey s bay intrusion have ce yb 22 to 27 whereas most troctolites of the mushuau intrusion have ce yb 12 to 17 the rocks of the mushuau intrusion are simple mixtures of mantle derived magma and local orthogneiss whereas the rocks of the voisey s bay intrusion are formed by multiple stages of crustal contamination including an early stage of contamination at lower midcrustal level and a late stage of contamination by tasiuyak paragneiss at upper crustal level the assimilation of the sulfide bearing tasiuyak paragneiss by magma at voisey s bay may have promoted initial sulfide saturation and or increased sulfide oversaturation thus contributing to the segregation of large amounts of sulfide liquid from magma continuing flow of magma through a dynamic conduit system at voisey s bay has caused the local accumulation of sulfide of sufficient size and grade to constitute mineable reserves

a proposed mechanism for the formation of spherical vivianite crystal aggregates in sediments abstract vivianite fe 3 po 4 2 8h 2 o is often found in the form of nodules composed of spherical aggregates of crystals crystallization of vivianite in agar gels of various concentrations yield crystal aggregates nodules that have spherical morphology and a bimodal size distribution the aggregates were formed under both biotic and abiotic conditions when special redox cells fitted with electrodes were used more perfect spherical structures were formed when the electrodes were shorted than when they were on open circuit in nature vivianite nodules generally are found in sediments or clays that are gelatinous often caused by the presence of organic debris a model consistent with experimental observations and based on the dynamics of gels is proposed to explain a possible origin of nodular vivianite to maintain iron and phosphate concentrations in sedimentary pore spaces filled with gel like organic debris the electric field spanning the aerobic anerobic zones in the upper sediments may be an important driving force in addition to diffusion it is suggested that the combination of the gel medium in the pore spaces and the natural electric field in the upper sediments could be contributing causes to explain the spherical aggregates of vivianite crystals found in nature

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

silicon isotope fractionation during magmatic differentiation abstract the si isotopic composition of earth s mantle is thought to be homogeneous 30 si 0 29 0 08 2 s d and not greatly affected by partial melting and recycling previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle to understand this variation it is necessary to investigate the degree of si isotopic fractionation that takes place during magmatic differentiation here we report si isotopic compositions of lavas from hekla volcano iceland which has formed in a region devoid of old geochemically diverse crust we show that si isotopic composition varies linearly as a function of silica content with more differentiated rocks possessing heavier isotopic compositions data for samples from the afar rift zone as well as various igneous usgs standards are collinear with the hekla trend providing evidence of a fundamental relationship between magmatic differentiation and si isotopes the effect of fractionation has been tested by studying cumulates from the skaergaard complex which show that olivine and pyroxene are isotopically light and plagioclase heavy relative to the si isotopic composition of the earth s mantle therefore si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates at an average sio 2 content of 60 wt the predicted 30 si value of the continental crust that should result from magmatic fractionation alone is 0 23 0 05 2 s e barely heavier than the mantle this is at most a maximum estimate as this does not take into account weathered material whose formation drives the products toward lighter 30 si values mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the si isotopic composition of the mantle

quaternary environmental changes in the drylands of china a critical review abstract this paper reviews our current understanding of quaternary climate and landscape changes in the desert areas of northern china a key portion of the middle latitude drylands on earth combining earlier studies with our recent research and experience we offer a comprehensive picture of the state of chinese deserts during the quaternary and in the interest of enhancing future research identify knowledge gaps and areas of uncertainty lacustrine deposits found over an area ranging from china s western taklamakan desert to the eastern hunshandake sandy lands suggest that extensive lakes occurred in china s deserts during the pleistocene analysis of digital elevation models from srtm shuttle radar topography mission data supports this interpretation and shows the significant extent of these former lakes new estimates of mean annual evaporation of ca 1000 mm from lake surfaces and ca 100 mm from land surfaces confirms that local and regional rainfall is critical for maintenance of desert lakes in this temperate zone especially during intervals when the mean annual rainfall is more than 100 mm rapid shifts between sand seas and lakes in geologically and environmentally diverse settings suggest that the drylands of china are very sensitive ephemeral systems and not long lasting as previously thought available chronologies suggest that there were large lakes in the western taklamakan desert and the chadamu basin during mis marine isotope stage 3 at 30 ka probably related to a period of strong influence of northern hemispheric westerly winds channels and elevation models revealed by srtm data and remains of lacustrine sediments also indicate that there was a large lake in the hunshandake sandy lands in the eastern portion of the desert belt during the quaternary there is significant evidence that during the middle holocene strong summer monsoons led to a relatively large increase in moisture availability in the entire desert belt of northern china lacustrine records from the badain jaran desert in western inner mongolia suggest that it was generally dry before 10 cal ka becoming wetter from 10 to 4 cal ka and dry again afterwards study of palaeosols widely occurring in dune stratigraphy in the eastern portion of the desert belt suggests that there was a period of wet and warm climate in this region during the mid holocene optimum at a minimum between 6 ka and 4 ka but possibly lasting longer recent observations dealing with the generation and transport of dust from central asia indicate that the causal relationship between sand seas and loess sequences is not as close as previously assumed these results suggest that there is an urgent need to examine whether the frequency and amplitude of climatic variation in chinese deserts are somehow similar to that having occurred in the sahara desert of north africa deserts in northern china were also important focal regions for neolithic cultures during intervals when environmental conditions supported a denser vegetative cover there is evidence that some areas of woody vegetation to the west of badain jaran desert were deforested by humans by ca 4000 yr b p although it is still debatable whether humans have had a significant impact in other areas at that time opinions on the severity of desertification vary and are sometimes contradictory due to the lack of long term field based investigations

brunhes matuyama magnetic reversal dated at 790 000 yr b p by marine astronomical correlations abstract the presence of the brunhes matuyama magnetic reversal in deep sea core sediments makes possible an alternative to the usual k ar radioistope method of dating the reversal as found in rocks the alternative method uses correlations of northern hemisphere summer insolation with oxygen isotope ratios from tropical cores the latitude dependent insolation variations are calculated from planetary mechanics and thus provide a highly accurate astronomical time scale the insolation variations strongly influence glacial ice volume fluctuations that dominate the oxygenisotope ratio changes recorded in core sediments the summer half year insolation variations are identified with corresponding isotope ratio changes in cores from the present through glacial stage 20 misleading effects of discontinuities or major nonuniformities of sediment deposition are avoided by an analysis of the uniformity of v28 238 and v28 239 the principal cores studied and by comparisons with other cores the top section stages 1 to 10 of v28 238 is uniformly deposited and for this section an isotope ratio time scale is chosen that agrees with the thorium uranium date for the high sea stand of the last interglacial extreme over this interval major glacial extremes strong isotope ratio minima coincide consistently with major insolation minima at times of low orbital eccentricity in addition interstadials are directly associated with precessional insolation peaks and the envelope of isotope ratio peaks resembles the envelope of precessionally dominated insolation peaks the assumption that the glacial extremes depended similarly on insolation minima during stages 10 to 20 permits minor age shifts of strong isotope ratio minima in the two cores relative to ages based on uniform overal deposition to match the ages of low eccentricity insolation minima the age shifts reflect residual nonuniformities of deposition the validity of this matching procedure is supported by a resulting consistent identification of principal isotope ratio peaks with high and low latitude coincident insolation maxima the brunhes matuyama reversal is found intinterglacial stage 19 and is dated on the astronomical time scale at 790 000 5000 yr b p

how well can we simulate past climates evaluating the models using global palaeoenvironmental datasets abstract global syntheses of palaeoenvironmental data are required to test climate models under conditions different from the present data sets for this purpose contain data from spatially extensive networks of sites the data are either directly comparable to model output or readily interpretable in terms of modelled climate variables data sets must contain sufficient documentation to distinguish between raw primary and interpreted secondary tertiary data to evaluate the assumptions involved in interpretation of the data to exercise quality control and to select data appropriate for specific goals four data bases for the late quaternary documenting changes in lake levels since 30 14 c kyr bp the global lake status data base vegetation distribution at 18 14 c kyr and 6 14 c kyr bp biome 6000 aeolian accumulation rates during the last glacial interglacial cycle dirtmap and tropical terrestrial climates at the last glacial maximum the lgm tropical terrestrial data synthesis are summarised each has been used to evaluate simulations of last glacial maximum lgm 21 calendar kyr bp and or mid holocene 6 cal kyr bp environments comparisons have demonstrated that changes in radiative forcing and orography due to orbital and ice sheet variations explain the first order broad scale in space and time features of global climate change since the lgm however atmospheric models forced by 6 cal kyr bp orbital changes with unchanged surface conditions fail to capture quantitative aspects of the observed climate including the greatly increased magnitude and northward shift of the african monsoon during the early to mid holocene similarly comparisons with palaeoenvironmental datasets show that atmospheric models have underestimated the magnitude of cooling and drying of much of the land surface at the lgm the inclusion of feedbacks due to changes in ocean and land surface conditions at both times and atmospheric dust loading at the lgm appears to be required in order to produce a better simulation of these past climates the development of earth system models incorporating the dynamic interactions among ocean atmosphere and vegetation is therefore mandated by quaternary science results as well as climatological principles for greatest scientific benefit this development must be paralleled by continued advances in palaeodata analysis and synthesis which in turn will help to define questions that call for new focused data collection efforts

the experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt abstract a first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions experiments were performed in an internally heated gas pressure vessel at 500 mpa and temperatures between 840 and 1000 c for 120 168 h three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by mc icp ms i kinetic experiments using 57 fe enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h ii isotope exchange experiments using mixtures of hydrous peralkaline rhyolitic glass powder 4 wt h 2 o and natural pyrrhotites fe 1 x s as starting materials and iii crystallisation experiments in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt consistently showed that pyrrhotite preferentially incorporates light iron no temperature dependence of the fractionation factor was found between 840 and 1000 c within experimental and analytical precision an average fractionation factor of 56 fe 54 fe pyrrhotite melt 0 35 0 04 2se n 13 was determined for this temperature range predictions of fe isotope fractionation between fes and ferric iron dominated silicate minerals are consistent with our experimental results indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt consequently the fractionation factor determined in this study is representative for the specific fe 2 fe ratio of our peralkaline rhyolitic melt of 0 38 0 02 at higher fe 2 fe ratios a smaller fractionation factor is expected further investigation on fe isotope fractionation between other mineral phases and silicate melts is needed but the presented experimental results already suggest that even at high temperatures resolvable variations in the fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems

holocene glacier variability three case studies using an intermediate complexity climate model synthetic glacier length records are generated for the holocene epoch using a process based glacier model coupled to the intermediate complexity climate model ecbilt the glacier model consists of a massbalance component and an ice flow component the climate model is forced by the insolation change due to variations in the earth s orbital parameters we consider three glaciers ranging from maritime to continental at nigardsbreen southern norway the simulated long term trend in the annual mass balance is primarily determined by summer temperature with a smaller contribution from winter precipitation in the early holocene summers were warmer and winters wetter than today in the ecbilt simulation both signals seem consistent with proxy data the simulated glacier length shows a phase of rapid expansion during the mid holocene followed by more gradual growth at rhonegletscher the swiss alps ecbilt simulates warmer and wetter summers in the early holocene the temperature signal seemns realistic but proxy data and earlier modelling results are not conclusive with respect to the precipitation signal the implied glacier length shows a maximum extent at 3 5 kyr bp which seems unlikely this suggests that the simulated precipitation response is not realistic the simulated early holocene climate at abramov glacier kirghizia is characterized by high summer precipitation associated with a northward extension of the asian monsoon the precipitation signal reaches its maximum around 6 kyr bp which is consistent with the timing of the maximum in lake level data the simulated glacier length shows a pronounced postglacial maximum at the time of maximum monsoon intensity there is considerable centennial timescale variability in the simulated glacier length records these length variations are generated by internal climatic variability they are typically asynchronous among the three different glaciers ecbilt has reasonable skill in simulating the relative importance of temperature and precipitation as well as the seasonality of the forcing although the overall level of variability is underestimated length variations are shown to behave as a lagged moving average process with a glacier specific memory

lake evolution during the past 30 000 years in china and its implications for environmental change abstract taking china as a whole lake expansion occurred 500 b c 0 a d 650 950 a d and 1250 1650 a d over the past 3000 yr the expansions were mainly due to climatic fluctuations and correspond to frequent flood reports three regional characteristics of lake development have been recognized 1 fluctuations of near coastal lakes and those in other regions as well were out of phase during 500 b c 500 a d which possibly resulted from out of phase fluctuations of climate and sea level on 10 to 100 yr time scale 2 as the lower yellow river channel migrated southward the lakes in the south of the north china plain formed and reached their largest sizes much later than those in the north 3 for the lakes in the middle and lower yangtze river valley the higher a lake basin lies above a neighboring river and the further the lake is from the river s mouth in general the younger the lake is through land reclamation the ancient chinese have changed most of the lakes greatly however human activities were also constrained in certain ways by the natural changes of the lakes reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding

european lakes as palaeohydrological and palaeoclimatic indicators abstract lakes in temperate regions have fluctuated in size on annual to millenial time scales changes in water level or depth can be reconstructed by applying geomorphic sedimentological and biostratigraphic methods to transects of sediment cores this systematic field based approach has been used at a number of sites in europe it is illustrated by reconstructions of water level changes in lakes from southern sweden these lakes have shown broadly synchronous changes during the holocene such synchronous changes in water levels at several sites in a region reflect climatic changes inferences about the mechanism of the climatic change depend on identifying both synchronous changes within a region and differences in the direction magnitude and timing of lake level changes between regions continental compilations of lake level data are therefore necessary detailed studies are not available from enough sites but the interpretation of already existing stratigraphic and biostratigraphic data in terms of lake level changes has greatly increased the number of sites available for a continental scale synthesis in europe this eclectic literature based approach is illustrated with examples from southern europe lakes in the mediterranean region were high around the glacial maximum and again somtime during the first half of the holocene in the western mediterranean the peak of late glacial aridity occurred at 15 000 16 000 bp and the lakes were already high again by 12 000 bp in the east maximum late glacial aridity occurred between 13 000 and 11 000 bp and there was a gradual return to wetter climates during the early holocene such that 9000 bp was more arid than 6000 bp most lakes in the mediterranean region were high at 6000 bp the transition to arid conditions after 5000 bp occurred abruptly in the west and more gradually in the east the lakes of southern sweden indicate drier climates in the early holocene ca 9000 bp more effective moisture between 8000 and 6000 bp increased aridity between 5500 and 3500 bp and increased effective moisture again during the last 3000 years the history of lake level changes in southern sweden and the mediterranean region can be partially explained in terms of changes in the direct effects of insolation and in the strength and position of the westerlies and of the subtropical high pressure cell during the late quaternary

iron isotope fractionation during magmatic differentiation in kilauea iki lava lake magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks the extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements we report analyses of iron isotopes in basalts from kilauea iki lava lake hawaii the iron isotopic compositions 56fe 54fe of late stagemeltveins are 0 2 permil per thousand greater than values for olivine cumulates olivine phenocrysts are up to 1 2 per thousand lighter than those of whole rocks these results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole rock and crystal scales this characteristic of iron relative to the characteristics of magnesium and lithium for which no fractionation has been found may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

low latitude holocene hydroclimate derived from lake sediment flux and geochemistry this study investigates hydrological responses to climatic shifts using sediment flux data derived from two dated palaeolake records in south east arabia flux values are generally low during the early holocene humid period ehhp 9 0 6 4k cal a bp although several short lived pulses of increased detrital input are recorded the most prominent of which is dated between 8 3 and 7 9k cal a bp the ehhp is separated from the mid holocene humid period mhhp 5 0 4 3k cal a bp by a phase of increased sediment flux and aridity which began between 6 4 and 5 9k cal a bp and peaked between 5 2 and 5 0k cal a bp the termination of the mhhp is marked by a phase of high detrital sediment flux between 4 3 and 3 9k cal a bp while long term shifts in climate are most likely linked to changes in the summer position of the intertropical convergence zone and associated indian and african monsoon systems it is noted that the abrupt short term phases of aridity observed in both records are coeval with intervals of rapid climate change globally which triggered non linear widespread landscape reconfigurations throughout south east arabia

sensitivity of the african and asian monsoons to mid holocene insolation and data inferred surface changes abstract orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in africa shown by data during the mid holocene 6000 yr ago feedbacks associated with changes in ssts and land surface cover may be necessary to produce a sufficient increase in the monsoon a step toward a better understanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings inferred from observations in the first study ssts are lowered in the upwelling regions offshore of west africa and somalia and increased in the bay of bengal and south china sea in the second simulation the modern sahara desert is replaced by a combination of xerophytic woods scrub and grassland in both cases the amount of water vapor advected from oceanic sources is increased north of 10 n in africa in response to the increased land sea temperature contrast thereby enhancing rainfall but the magnitude of the simulated changes is much larger when land s

a review of the application of multiple s isotopes to magmatic ni cu pge deposits and the significance of spatially variable δ33s values the determination of 33 s values in conjunction with 34 s values provides an effective means of evaluating the importance of s derivation from archean country rocks in magmatic ni cu pge ore genesis particularly where 34 s values of potential s source rocks are near 0 and similar to most s of mantle derivation however 33 s values of sulfides in some archean rocks may also be near 0 care should be taken in the evaluation of s derivation from archean rocks based solely on this parameter although anomalous 33 s values are found primarily in archean rocks rare occurrences of nonzero 33 s values have been identified in sulfides from younger rocks and hence knowledge of the multiple s isotope composition of potential contaminants is essential for an accurate assessment of s contamination of mantle derived magmas restricted ranges in 33 s values may be characteristic of particular archean formations and when used in conjunction with mixing calculations may fingerprint a distinct s source rock spatial variability in 33 s values within different types of mineralized systems may be caused by the emplacement of isotopically distinct magma pulses by locally variable mixing proportions and the addition of secondary sulfide to the primary assemblage in the case of conduit type deposits in the midcontinent rift system three component mixing calculations show that both high and low 33 s values in some units that are similar to end member values in the archean country rocks must be a function of assimilation of primarily archean s even though proterozoic carbonaceous sedimentary country rocks contain in excess of 1 wt s and are particularly viable s source rocks

processes controlling phosphate adsorption by iron hydroxides in estuaries abstract phosphate adsorption by iron hydroxides is studied in the laboratory on p depleted and p saturated seston and on post glacial clays all samples from the saint lawrence estuary levels of adsorption and desorption are distinguished on the basis of significantly different ratios of the particulate iron hydroxides fecdb to inorganic non detrital phosphorus pcdb adsorption equilibrium is quickly reached for the desorbed and the sediment samples when agitated with po4 p enriched seawater this level is characterized by a mean fe p cdb value of 14 identical to the dominant one in the st lawrence p adsorption on p saturated particulate matter is found to be reversible under low reducing conditions because of the narrow ph range in the st lawrence estuary ph does not seem to influence the reaction rate the partial phosphate desorption corresponds to values of the fe p cdb ratio of 18 without any fe release these conditions are similar to those at the surface of estuarine marsh sediments the stirring experiments in the laboratory are adequately reproducing the natural behaviour of the particulate matter but the kinetics of the re adsorption reaction are much faster than in the estuary

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

s saturation history of nain plutonic suite mafic intrusions origin of the voisey s bay ni cu co sulfide deposit labrador canada the voisey s bay deposit is hosted in a 1 34 ga intrusion composed of troctolite olivine gabbro and ferrogabbro the sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites and also occurs as veins along structures in adjacent paragneiss a dyke is connected to the base of the north wall of the eastern deeps intrusion and the entry point of this dyke into the chamber is the locus of the eastern deeps nickel sulfide deposit a detailed exploration in the area between the eastern deeps and the ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke the eastern deeps deposit is surrounded by a halo of moderately to weakly mineralized variable textured troctolite vtt that reaches a maximum thickness above the axis of the eastern deeps deposit along the northern wall of the eastern deeps intrusion the massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the vtt and there is a marked break in ni tenor of sulfide between the two sulfides hosted in the dyke tend to have low metal tenors ni 100 2 5 3 5 sulfides in eastern deeps massive and breccia ores have intermediate ni tenors ni 100 3 5 4 and disseminated sulfides in overlying rocks have high ni tenors ni 100 4 8 four principal processes control the compositions of the voisey s bay sulfides coarse grained loop textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the pd ir ratio which is due to mineralogical variations where pentlandite is enriched in pd and ir is dispersed throughout the mineral assemblage a decrease in ir and rh from the margin of the ovoid toward cubanite rich parts at the central part of the ovoid is consistent with fractionation of the sulfide from the margins toward the center of the ovoid the ovoid ores have higher ni and pd tenor than the eastern deeps massive sulfides this is consistent with both a higher r factor and greater degree of silicate parental magma evolution in the ovoid than the eastern deeps the disseminated sulfides surrounding the eastern deeps deposit have some of the highest ni and pd tenors at voisey s bay which are indicative of not only more primitive magmas but also higher r factors than the ovoid or the eastern deeps vtt and normal textured troctolite of the eastern deeps that contain trace sulfide have 0 1 3 ppb pt and 0 1 3 ppb pd whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of pt and pd troctolites and olivine gabbros from other parts of the voisey s bay intrusion and other nain plutonic suite intrusions including the kiglapait newark bay barth island mushua and nain bay south intrusion also have low platinum group element abundances although it is possible that this is a signature of a widespread sulfide saturation event that pre dated ore formation at voisey s bay it is more likely that platinum group element pge depletion is a product of the source melting process where low degrees of melting resulted in the retention of pge in the mantle source if so this indicates that pge depletion should be used with caution as an exploration tool in the nain plutonic suite

climate in northern eurasia 6000 years ago reconstructed from pollen data using a climatic calibration based on the scores of the plant functional types pfts calculated for 1245 surface pollen spectra the climate at 6 ka bp has been reconstructed for a set of 116 pollen spectra from the former soviet union and mongolia the results are presented as maps of climatic anomalies and maps of probability classes showing the significance of these differences from the modern climate the reconstructed patterns are spatially coherent but have confidence levels that vary from region to region due to the often large error ranges at 6 ka the winters were more than 2oc warmer than today north of 50on with a high significance east of the urals summers were also more than 2oc warmer than today with a high level of confidence north of the polar circle and in central mongolia in the mid latitudes of siberia in northern kazakhstan and around the black and the caspian seas 6 ka summers were significantly cooler than today the reconstructed moisture availability ratio of actual to equilibrium evapotranspiration was more than 10 higher than today in the ukraine southern russia and northern mongolia and more than 10 lower than today in central mongolia this pattern corresponds partly with that of the water budget annual precipitation minus evaporation reconstructed from lake level records 1999 elsevier science b v all rights reserved

multiple sulfur isotopes and the evolution of the atmosphere interest in multiple sulfur isotope analyses has been fueled by recent reports of mass independent sulfur isotope signatures in the geologic record a non zero multiple isotopic signature of sulfur 33s and 36s is produced primarily through photochemical reactions and it is an almost perfect tracer of the source of sulfur once the signature is passed on to a given sulfur reservoir it will be preserved unless there is addition of sulfur with a different 33s or 36s in the geological record this signature has been used to study the evolution of the earth s atmosphere and to trace movement of sulfur through geological systems early in earth s history recently small but significant mass independent signatures have been reported for some younger samples raising the possibility of additional applications of multiple isotope studies the purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous but not uncommon isotopic signatures

evaluation of coupled ocean atmosphere simulations of the mid holocene using palaeovegetation data from the northern hemisphere extratropics we have used the biome4 biogeography biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean atmosphere general circulation models to mid holocene changes in orbital forcing in the mid to high latitudes of the northern hemisphere all the models produce a a northward shift of the northern limit of boreal forest in response to simulated summer warming in high latitudes the northward shift is markedly asymmetric with larger shifts in eurasia than in north america b an expansion of xerophytic vegetation in mid continental north america and eurasia in response to increased temperatures during the growing season c a northward expansion of temperate forests in eastern north america in response to simulated winter warming the northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in north america are supported by palaeovegetation data the expansion of xerophytic vegetation in mid continental north america is consistent with palaeodata although the extent may be over estimated the simulated expansion of xerophytic vegetation in eurasia is not supported by the data analysis of an asynchronous coupling of one model to an equilibrium vegetation model suggests vegetation feedback exacerbates this mid continental drying and produces conditions more unlike the observations not all features of the simulations are robust some models produce winter warming over europe while others produce winter cooling as a result some models show a northward shift of temperate forests consistent with though less marked than the expansion shown by data and others produce a reduction in temperate forests elucidation of the cause of such differences is a focus of the current phase of the palaeoclimate modelling intercomparison project

the occurrence identification and environmental relevance of vivianite in waterlogged soils and aquatic sediments abstract in this article we review the nature occurrence and environmental relevance of the authigenic ferrous iron phosphate mineral vivianite fe 3 po 4 2 8h 2 o in waterlogged soils and aquatic sediments we critically discuss existing work from freshwater and marine systems laboratory studies and microbial batch culture experiments aiming to deduce common characteristics of the mineral s occurrence and the processes governing its formation vivianite regularly occurs in close association with organic remains in iron rich sediments simultaneously it is a biogenic mineral product of metal reducing bacteria these findings suggest that vivianite nucleation in natural systems is directed by the activity of such bacteria and crystal growth is particularly favoured within protected microzones taking into account recent findings from coastal marine sediments where vivianite authigenesis has been shown to be coupled to the anaerobic oxidation of methane small scale microbially mediated reactions appear to be crucial for the formation of vivianite small scale heterogeneity within the sediment matrix may also explain why saturation calculations based upon bulk pore water constitutions often fail to accurately predict the occurrence of the mineral vivianite is not restricted to a specific trophic state of a system the mineral forms in oligotrophic as well as in eutrophic waters however depending on the iron inventory the production supply and degradation of organic matter determine the relative contribution of iron sulphide formation to the iron pool and the concentration of inorganic phosphate and fe 2 in pore waters thus vivianite authigenesis is also governed by bulk chemical conditions such as the rate of sulphide formation relative to that of fe 2 production this situation allows stimulation of vivianite formation by iron supplementation aimed at restoring eutrophic lakes recent results from coastal marine sediments suggest that vivianite authigenesis is of significance for p burial in the marine realm vivianite authigenesis is likely important at the global scale but has so far largely been ignored

comparing orthomagmatic and hydrothermal mineralization models for komatiite hosted nickel deposits in zimbabwe using multiple sulfur iron and nickel isotope data trojan and shangani mines are low grade 0 8 ni komatiite hosted nickel sulfide deposits associated with ca 2 7 ga volcano sedimentary sequences of the zimbabwe craton at both mines nickel sulfide mineralization is present in strongly deformed serpentinite bodies that are enveloped by a complex network of highly sheared silicified and sulfide bearing metasedimentary rocks strong polyphase structural metamorphic metasomatic overprints in both the trojan and shangani deposits make it difficult to ascertain if sulfide mineralization was derived from orthomagmatic or hydrothermal processes or by a combination of both multiple s fe and ni isotope analyses were applied to test these competing models massive ores at shangani mine show mass dependent fractionation of sulfur isotopes consistent with a mantle sulfur source whereas s isotope systematics of net textured ore and disseminated ore in talcose serpentinite indicates mixing of magmatic and sedimentary sulfur sources potentially via post magmatic hydrothermal processes a restricted range of strongly mass independent 33s values in ore samples from trojan mine likely reflects high temperature assimilation of sulfur from supracrustal rocks and later superimposed low temperature hydrothermal remobilization iron isotope values for most ni bearing sulfides show a narrow range suggesting that in contrast to sulfur nearly all of iron was derived from an igneous source negative ni isotope values also agree with derivation of ni from ultramafic melt and a significant high temperature fractionation of ni isotopes fe isotope values of some samples from shangani mine are more fractionated than expected to occur in high temperature magmatic systems further suggesting that hydrothermal processes were involved in either low grade ore formation liberation of ni from olivine by sulfur bearing hydrothermal fluids or remobilization of existing sulfides potentially inducing secondary ni sulfide mineralization

mass dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles abstract sulfur isotope studies of post archean terrestrial materials have focused on the ratio 34s 32s because additional isotopes 33s and 36s were thought to carry little information beyond the well known mass dependent relationship among multiple isotope ratios we report high precision analyses of 33s and 36s values defined as deviations of 33s and 36s from ideal mass dependent relationships for international reference materials and sedimentary sulfides of phanerozoic age by using a fluorination technique with a dual inlet isotope ratio mass spectrometer measured variations in 33s and 36s are explained as resulting from processes involve branching reactions two or more reservoirs formed or mixing irreversible processes in closed systems rayleigh distillation amplify the isotope effect we outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation the isotope systematics discussed here cannot explain the much larger variation of 33s and 36s observed in archean rock records furthermore phanerozoic samples we have studied show a characteristic 33s and 36s relationship that differs from those measured in archean rocks and laboratory photolysis experiments thus high precision analysis of 33s and 36s can be used to distinguish small non zero 33s and 36s produced by mass dependent processes from those produced by mass independent processes in archean rocks and extraterrestrial materials

the composition of mineralization at the voisey s bay ni cu sulfide deposit with special reference to platinum group elements the ni cu platinum group element pge and au contents of massive sulfide mineralization leopard textured mineralization sulfides in basal breccia sequence and sulfides in varied troctolite from four mineralized environments the reid brook zone the discovery hill zone the ovoid and mini ovoid and the eastern deeps at voisey s bay have been determined modeling of the compositional data indicates that the large zone of massive sulfide comprising the ovoid has crystallized from the base upward with a fractionated residual liquid becoming concentrated toward the top massive mineralization from the mini ovoid reid brook zone and eastern deeps has undergone postcrystalliztion modification possibly due to the influence of heat from new influxes of magma through the plumbing system this has resulted in a considerable redistribution of cu and to a lesser extent of ni and au leopard textured mineralization from the reid brook and discovery hill zones corresponds closely with the postulated sulfide liquid compositions responsible for the ovoid and mini ovoid but mineralization composition from the feeder to the eastern deeps is exceptional in having many of the characteristics of a monosulfide solid solution cumulate the composition of disseminated and blotchy sulfides in the varied troctolite of the eastern deeps requires a variable but generally higher r factor than that affecting most of the sulfides to have operated here the compositional variability of the voisey s bay mineralization is best explained in terms of a model calling for repeated influxes of magma through the plumbing system to the deposit