did the massive magnetite lava flows of el laco chile form by magmatic or hydrothermal processes new constraints from magnetite composition by la icp ms the el laco magnetite deposits consist of more than 98 magnetite but show field textures remarkably similar to mafic lava flows therefore it has long been suggested that they represent a rare example of an effusive fe oxide liquid field and petrographic evidence however suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs we determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry la icp ms from various settings at el laco and compared them with magnetite from both igneous and hydrothermal environments this new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite lava flows formed the trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite including primary magnetite phenocrysts from the unaltered andesite host rocks at el laco instead the el laco magnetite is most similar in composition to hydrothermal magnetite from high temperature environments 500 c such as iron oxide copper gold iocg and porphyry cu deposits the magnetite trace elements from massive magnetite are characterised by 1 depletion in elements considered relatively immobile in hydrothermal fluids e g ti al cr zr hf and sc 2 enrichment in elements that are highly incompatible with magmatic magnetite rare earth elements ree si ca na and p and normally present in very low abundance in magmatic magnetite 3 high ni cr ratios which are typical of magnetite from hydrothermal environments and 4 oscillatory zoning of si ca mg ree and most high field strength elements and zoning truncations indicating dissolution similar to that formed in hydrothermal fe skarn deposits in addition secondary magnetite in altered brecciated host rock forming disseminations and veins has the same composition as magnetite from the massive lenses euhedral magnetite lining both open spaced veins in the brecciated host rock and along the walls of large hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and or physio chemical conditions of the fluid thus the chemical fingerprint of magnetite from the supposed el laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows by dissolution and precipitation of magnetite from high temperature fluids rather than a magmatic origin from an effusive fe oxide liquid

spatial fingerprint and magnitude of changes in the atlantic meridional overturning circulation during marine isotope stage 3 pronounced millennial scale climate variability during marine isotope stage 3 mis3 is considered to be linked to changes in the state of the atlantic meridional overturning circulation amoc i e a warm interstadial cold stadial state corresponds to a strong weak amoc based on a series of freshwater hosing extraction experiments with the state of the art community climate system model version 3 we construct a global spatial fingerprint of oceanic temperature anomalies in response to amoc changes under mis3 boundary conditions highest sensitivity to amoc changes especially in summer is found in northeastern north atlantic sea surface temperature but a characteristic temperature fingerprint is also found at subsurface levels after testing significance of the linear sea surface temperature sst amoc regressions the model results are combined with paleo sst records to estimate the magnitude of millennial scale dansgaard oeschger amoc variations during mis3 the results suggest a mean difference in amoc strength between interstadial and non heinrich stadial states of 9 2 1 2 sv 1

an abrupt weakening of the subpolar gyre as trigger of little ice age type episodes we investigate the mechanism of a decadal scale weakening shift in the strength of the subpolar gyre spg that is found in one among three last millennium simulations with a state of the art earth system model the spg shift triggers multicentennial anomalies in the north atlantic climate driven by long lasting internal feedbacks relating anomalous oceanic and atmospheric circulation sea ice extent and upper ocean salinity in the labrador sea yet changes throughout or after the shift are not associated with a persistent weakening of the atlantic meridional overturning circulation or shifts in the north atlantic oscillation the anomalous climate state of the north atlantic simulated after the shift agrees well with climate reconstructions from within the area which describe a transition between a stronger and weaker spg during the relatively warm medieval climate and the cold little ice age respectively however model and data differ in the timing of the onset the simulated spg shift is caused by a rapid increase in the freshwater export from the arctic and associated freshening in the upper labrador sea such freshwater anomaly relates to prominent thickening of the arctic sea ice following the cluster of relatively small magnitude volcanic eruptions by 1600 ce sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events a necessary causal link between the volcanic cluster and the spg shift can therefore be excluded instead preconditioning by internal variability explains discrepancies in the timing between the simulated spg shift and the reconstructed estimates for the little ice age onset

ocean circulation ice shelf and sea ice interactions explain dansgaard oeschger cycles the last glacial interval experienced abrupt climatic changes called dansgaard oeschger do events these events manifest themselves as rapid increases followed by slow decreases of oxygen isotope ratios in greenland ice core records despite promising advances a comprehensive theory of the do cycles with their repeated ups and downs of isotope ratios is still lacking here based on earlier hypotheses we introduce a dynamical model that explains the do variability by rapid retreat and slow regrowth of thick ice shelves and thin sea ice in conjunction with changing subsurface water temperatures due to insulation by the ice cover our model successfully reproduces observed features of the records such as the sawtooth shape of the do cycles waiting times between do events across the last glacial and the shifted antiphase relationship between greenland and antarctic ice cores our results show that these features can be obtained via internal feedbacks alone warming subsurface waters could have also contributed to the triggering of heinrich events our model thus offers a unified framework for explaining major features of multimillennial climate variability during glacial intervals

salt weathering of sandstone at the angkor monuments cambodia identification of the origins of salts using sulfur and strontium isotopes abstract the angkor monuments in northwestern cambodia which are primarily made of sandstone are suffering from deterioration due to salt weathering in order to elucidate the sources of the salts and salt weathering process this paper analyzed bulk chemical compositions and s sulfur and sr strontium isotopic ratios for the salts and surrounding environmental materials at places where bats inhabit the structures various kinds of sulfate and phosphate salts are formed on the surface of the sandstone the results of the analysis demonstrate that the s and p phosphorus components in the salts are primarily derived from bat guano moreover ca calcium which is the major element in the salts is suggested to be derived from both bat guano and the sandstone the isotopic data strongly suggests that bat guano is related to salt weathering removal of these animal excrements is essential for future conservation of these monuments on the other hand exfoliation of sandstone commonly occurs due to crystallization of calcite caco 3 on platform surfaces of pyramid type monuments a large amount of ca in calcite is suggested to be derived from calcite cement of the building sandstone

thermohaline instability and the formation of glacial north atlantic super polynyas at the onset of dansgaard oeschger warming events late quaternary rapid warming events inferred on the basis of oxygen isotopic data from greenland ice cores are the most prominent characteristic of millennial scale dansgaard oeschger oscillations in a coupled climate model simulation which has accurately reproduced this oscillatory behavior for the first time we show that formation of a glacial north atlantic super polynya characterizes the initial stage of transition from cold stadial to warm interstadial conditions the winter polynya forms within the otherwise sea ice covered north atlantic as a consequence of the onset of a thermohaline convective instability beneath an extensive stadial sea ice lid early in the stadial period the tendency for thermal convective instability of an extensive warm pool beneath the sea ice lid is strongly inhibited by a stabilizing vertical salinity gradient which gradually diminishes until a thermohaline convective instability occurs that leads to polynya formation and the rapid retreat of north atlantic sea ice cover

north atlantic ocean circulation and abrupt climate change during the last glaciation the most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations whose origin remains unresolved variations in oceanic meridional heat transport may contribute to these repeated climate changes which were most pronounced during marine isotope stage 3 the glacial interval 25 thousand to 60 thousand years ago we examined climate and ocean circulation proxies throughout this interval at high resolution in a deep north atlantic sediment core combining the kinematic tracer protactinium thorium pa th with the deep water mass tracer epibenthic 13c these indicators suggest reduced atlantic overturning circulation during every cool northern stadial with the greatest reductions during episodic hudson strait iceberg discharges while sharp northern warming followed reinvigorated overturning these results provide direct evidence for the ocean s persistent central role in abrupt glacial climate change

reversed north atlantic gyre dynamics in present and glacial climates the dynamics of the north atlantic subpolar gyre spg are assessed under present and glacial boundary conditions by investigating the spg sensitivity to surface wind stress changes in a coupled climate model to this end the gyre transport is decomposed in ekman thermohaline and bottom transports surface wind stress variations are found to play an important indirect role in spg dynamics through their effect on water mass densities our results suggest the existence of two dynamically distinct regimes of the spg depending on the absence or presence of deep water formation dwf in the nordic seas and a vigorous greenland scotland ridge gsr overflow in the first regime the gsr overflow is weak and the spg strength increases with wind stress as a result of enhanced outcropping of isopycnals in the centre of the spg as soon as a vigorous gsr overflow is established its associated positive density anomalies on the southern gsr slope reduce the spg strength this has implications for past glacial abrupt climate changes insofar as these can be explained through latitudinal shifts in north atlantic dwf sites and strengthening of the north atlantic current regardless of the ultimate trigger an abrupt shift of dwf into the nordic seas could result both in a drastic reduction of the spg strength and a sudden reversal in its sensitivity to wind stress variations our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes which have been largely neglected up to now in model studies

bistability of the atlantic subpolar gyre in a coarse resolution climate model 1 high resolution models indicate that the dynamics of the nordic seas and the subpolar gyre is crucial for deep water formation and overturning circulation in the atlantic during the last decades significant changes in ocean properties have been observed in this region we show that large scale dynamics as captured by coarse resolution climate models allows for a bistability of the atlantic subpolar gyre we suggest three positive feedbacks which yield the necessary nonlinearity for this behavior transitions between the two states can be triggered by small fluctuations in surface freshwater flux to the nordic seas only slightly larger than natural variability presently observed in the region

thermohaline oscillations induced by strong steady salinity forcing of ocean general circulation models abstract a series of numerical experiments is conducted with a three dimensional ocean general circulation model and a two dimensional counterpart both designed for efficient integration over diffusive millennial time scales with strong steady salinity fluxes salting at low latitudes and freshening at high basin mean temperature and several other diagnostics show a series of self sustaining oscillations the oscillations termed deep decoupling oscillations exhibit halocline catastrophes at regular intervals followed by warming deep decoupled phases when the deep overturning is weak cooling flushes and in the lower range of salinity forcing a coupled phase when the deep ocean advective diffusive heat balance is almost but not quite met it is suggested that oscillations arise when a steady overturning circulation encounters a contradiction the poleward salt and heat transport needed to maintain convection in the polar ocean requires more overturning than is consistent with the reduced therm

abrupt glacial climate shifts controlled by ice sheet changes the volume of the northern hemisphere ice sheet controlled abrupt millennial scale climate changes during the last glacial dansgaard oeschger events are millennial scale warm periods interrupting cold glacial climates although they are perhaps the most prominent example of abrupt change observed in the ice core record of the past 800 000 years their underlying physical controls remain debated xu zhang and colleagues now use a fully coupled climate model to show that dansgaard oeschger events tend to occur when ice sheets are at an intermediate height a condition in which small perturbations to the coupled atmosphere ocean sea ice system can lead to rapid nonlinear feedbacks and abrupt changes in climate during glacial periods of the late pleistocene an abundance of proxy data demonstrates the existence of large and repeated millennial scale warming episodes known as dansgaard oeschger do events1 this ubiquitous feature of rapid glacial climate change can be extended back as far as 800 000 years before present bp in the ice core record2 and has drawn broad attention within the science and policy making communities alike3 many studies have been dedicated to investigating the underlying causes of these changes but no coherent mechanism has yet been identified3 4 5 6 7 8 9 10 11 12 13 14 15 here we show by using a comprehensive fully coupled model16 that gradual changes in the height of the northern hemisphere ice sheets nhiss can alter the coupled atmosphere ocean system and cause rapid glacial climate shifts closely resembling do events the simulated global climate responses including abrupt warming in the north atlantic a northward shift of the tropical rainbelts and southern hemisphere cooling related to the bipolar seesaw are generally consistent with empirical evidence1 3 17 as a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets minor changes in the height of the nhiss and the amount of atmospheric co2 can trigger the rapid climate transitions via a local positive atmosphere ocean sea ice feedback in the north atlantic our results although based on a single model thus provide a coherent concept for understanding the recorded millennial scale variability and abrupt climate changes in the coupled atmosphere ocean system as well as their linkages to the volume of the intermediate ice sheets during glacials

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

the role of mixed boundary conditions in numerical models of the ocean s climate abstract several simple numerical experiments are conducted using both single and double hemisphere ocean basins under symmetric steady forcing to study de ocean s thermohaline circulation it is shown that a stable steady state obtained under a restoring surface boundary condition on salinity becomes unstable upon a switch to a flux boundary condition the polar halocline catastrope of f bryan occurs it is shown that further integration of this collapsed state ultimately yields a steady stable one cell circulation with the approach being essentially chaotic but with significant energy at decadal period the two hemisphere ocean passes through many stages in which violent overturning occurs o 80 101 m3 a 1 these flushes occurs in both hemispheres and are of one cell structure the time period between them bushes varies from seveal hundred to about one thousand years a single 12 vertical level hemispheric basin spun up from an initial state of rest under mixed boundary conditions restoring boun

the role of the north atlantic drift in the millennial timescale glacial climate fluctuations abstract the greenland ice core records show that the overall cold climate of the last glacial period was repeatedly interrupted by short rapid warmings the events known as dansgaard oeschger d o events are strongly imprinted in the north atlantic marine records suggesting that they were linked to north atlantic circulation changes however the causes of the d o events are poorly understood and they represent one of the most intriguing puzzles of the last glacial period in order to investigate a possible mechanism we have studied variations in the distribution of benthic and planktonic foraminifera oxygen isotopes and ice rafted debris during the last glacial period in eight cores from the north atlantic and the nordic seas taken at mid depth from 853 to 1760 m the parameters indicate in agreement with previous studies that the circulation system during the interstadials resembled the present system atlantic surface water flowed north into the nordic seas where most of it sank through convection and as cold deep water flowed back into the north atlantic during the stadials a halocline was established in the nordic seas and in the northernmost part of the north atlantic and the outflow from the nordic seas stopped however below the cold light surface layer the relatively warm water of the north atlantic drift continued to flow across the north atlantic and into the nordic seas here warming up the deep water masses we suggest that the warm water gradually made the water column unstable the pivotal abrupt climate warmings were caused by sudden upwelling of the deep warm water masses causing overturning of the entire water column and onset of convection

abrupt north atlantic circulation changes in response to gradual co2 forcing in a glacial climate state glacial climate is marked by abrupt millennial scale climate changes known as dansgaard oeschger cycles the most pronounced stadial coolings heinrich events are associated with massive iceberg discharges to the north atlantic these events have been linked to variations in the strength of the atlantic meridional overturning circulation however the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear here we show that in a fully coupled atmosphere ocean model gradual changes in atmospheric co2 concentrations can trigger abrupt climate changes associated with a regime of bi stability of the atlantic meridional overturning circulation under intermediate glacial conditions we find that changes in atmospheric co2 concentrations alter the transport of atmospheric moisture across central america which modulates the freshwater budget of the north atlantic and hence deep water formation in our simulations a change in atmospheric co2 levels of about 15 ppmv comparable to variations during dansgaard oeschger cycles containing heinrich events is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode because changes in the atlantic meridional overturning circulation are thought to alter atmospheric co2 levels we infer that atmospheric co2 may serve as a negative feedback to transitions between strong and weak circulation modes

topographic coupling of the atlantic overturning and gyre circulations abstractthe vorticity dynamics associated with the mean and time varying gyre and overturning circulations of the atlantic ocean are examined in a realistic ocean model hindcast simulation of the late twentieth century abyssal flow interaction with sloping bottom bathymetry gives rise to the bottom pressure torque bpt term of the vertically integrated vorticity equation the dominance of this term in the closure of the barotropic gyre circulation noted in previous studies is corroborated here for both non eddy resolving and eddy resolving versions of the parallel ocean program pop model this study shows that bpt is also a dominant term in the vorticity balance of the atlantic meridional overturning circulation amoc and therefore represents a key dynamical link between the overturning and gyre streamfunctions the interannual variability of the atlantic circulation over the last several decades viewed in terms of time varying integral vorticity balances demonstrates the fundamental role played by

sensitivity of the north atlantic ocean circulation to an abrupt change in the nordic sea overflow in a high resolution global coupled climate model 1 the sensitivity of the north atlantic ocean circulation to an abrupt change in the nordic sea overflow is investigated for the first time using a high resolution eddy permitting global coupled ocean atmosphere model gfdl cm2 5 the nordic sea overflow is perturbed through the change of the bathymetry in gfdl cm2 5 we analyze the atlantic meridional overturning circulation amoc adjustment process and the downstream oceanic response to the perturbation the results suggest that north of 34 n amoc changes induced by changes in the nordic sea overflow propagate on the slow tracer advection timescale instead of the fast kelvin wave timescale resulting in a time lead of several years between subpolar and subtropical amoc changes the results also show that a stronger and deeper penetrating nordic sea overflow leads to stronger and deeper amoc stronger northward ocean heat transport reduced labrador sea deep convection stronger cyclonic northern recirculation gyre nrg westward shift of the north atlantic current nac and southward shift of the gulf stream warmer sea surface temperature sst east of newfoundland and colder sst south of the grand banks stronger and deeper nac and gulf stream and stronger oceanic eddy activities along the nac and the gulf stream paths a stronger weaker nordic sea overflow also leads to a contracted expanded subpolar gyre spg this sensitivity study points to the important role of the nordic sea overflow in the large scale north atlantic ocean circulation and it is crucial for climate models to have a correct representation of the nordic sea overflow

dansgaard oeschger oscillations predicted in a comprehensive model of glacial climate a kicked salt oscillator in the atlantic during the period from 60 000 to 35 000 years ago summit greenland ice core records of the oxygen isotopic ratio 18o 16o exhibit intense millennium time scale oscillations these dansgaard oeschger oscillations have been interpreted to represent the variations in north atlantic air temperature caused by correlative changes in the strength of north atlantic deep water production we apply a comprehensive model of glacial climate to unambiguously identify the mechanism responsible for this phenomenon this is shown to involve a salt oscillation of relaxation oscillator form this nonlinear oscillation does not require the existence of feedback due to freshwater release from grounded ice on the continents during the warm phase of the cycle

anatomy of a dansgaard oeschger warming transition high resolution analysis of the north greenland ice core project ice core large and abrupt temperature oscillations during the last glacial period known as dansgaard oeschger do events are clearly observed in the greenland ice core record here we present a new high resolution chemical 2 mm and stable isotope 20 mm record from the north greenland ice core project ngrip ice core at the onset of one of the most prominent do events of the last glacial do 8 observed 38 000 years ago the unique subannual resolution ngrip record provides a true sequence of change during a do warming with detailed annual layer counting of very high depth resolution geochemical measurements used to determine the exact duration of the transition the continental ions indicative of long range atmospheric loading and dustiness from east asia are the first to change followed by the snow accumulation the moisture source conditions and finally the atmospheric temperature in greenland the sequence of events shows that atmospheric and oceanic source and circulation changes preceded the do warming by several years

gypsum a review of its role in the deterioration of building materials the deterioration of buildings and monuments by gypsum is the result of crystallization cycles of this salt although gypsum can dehydrate to a hemihydrate the mineral bassanite and to an anhydrate the mineral anhydrite this reaction occurs in nature on a geological time scale and therefore it is unlikely to occur when gypsum is found on and in building materials the caso4 h2o system appears deceptively simple however there are still discrepancies between the experimental and thermodynamically calculated data the reason for the latter can be attributed to the slow crystallization kinetics of anhydrite apart from this the large numbers of studies carried out on this system have focused on industrially important metastable phases such as the hemihydrate and soluble anhydrite the paper presents a review of the studies dealing with the phase equilibria of the caso4 h2o system as well as the influence of other salts on the solubility of gypsum it tries to glean out the relevant information that will serve to explain the deterioration observed on building materials by the crystallization of gypsum and thus allows developing improved conservation methods

defining early stages of iocg systems evidence from iron oxides in the outer shell of the olympic dam deposit south australia the iocg deposit at olympic dam south australia is hosted within the roxby downs granite which displays a weakly mineralised contact to the orebody hereafter outer shell in a mineralogical geochemical characterisation of fe oxides from the outer shell we show silician magnetite si magnetite and hfse bearing hematite define the early stages of alkali calcic alteration this association forms in the presence of hydrothermal k feldspar and calc silicates via overprinting of magmatic magnetite and ilmenite breakdown geochemical modelling at 400 c shows such reactions occur at ph fo2 conditions coinciding with shifts from k feldspar to sericite and ilmenite to rutile stability the subsequent si magnetite siderite association forms down t in the absence of k feldspar transition from granular to bladed morphologies in si magnetite is part of a series of fe oxide interconversions followed by formation of zoned u w sn mo bearing hematite enrichment in ree y and u in si magnetite and the prevalence of u w sn mo bearing hematite support a granite derived fluid combined petrographic and geochemical evidence show a transition among fe oxides from the outer shell to the orebody attributable to the evolution of the same fluid unusual massive magnetite intervals and fe oxide nodules in granite are considered due to either the presence of inherited lithologies metasomatic products or the result of magnetite rich crystal mush forming in the melt we propose a model corroborated by recently published data including high precision u pb dating of magmatic zircon and hydrothermal hematite in which an outer shell is initiated at the 6 8 km depth of granite emplacement during volatile release from fluids ponding at intrusion margins granite cupola collapse at shallower levels 2 3 km follows via uplift along faults facilitating intense brecciation and ore formation

weathering of the black limestone of historical monuments ljubljana slovenia oxygen and sulfur isotope composition of sulfate salts abstract the black limestone widely used in slovenian monuments particularly in the baroque architecture is deteriorating extensively due to salt crystallization samples of soluble salts from two important historical monuments in ljubljana slovenia were investigated in terms of their mineral and isotopic s and o compositions results revealed the presence of gypsum and soluble salts of the mgso 4 n h 2 o series such as starkeyite mgso 4 4h 2 o pentahydrite mgso 4 5h 2 o and hexahydrite mgso 4 6h 2 o whereas black crusts and subflorescences consisted of gypsum efflorescences appeared to be an assemblage of gypsum and mgso 4 hydrates sample 18 o sulfate values varied from 1 9 to 5 5 vs v smow and 34 s sulfate values from 19 8 to 3 2 vs v cdt the respective isotopic composition of analysed outdoor and indoor monument samples indicated different sources of contamination

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

dansgaard oeschger cycles interactions between ocean and sea ice intrinsic to the nordic seas dansgaard oeschger d o cycles are the most dramatic frequent and wide reaching abrupt climate changes in the geologic record on greenland d o cycles are characterized by an abrupt warming of 10 5 c from a cold stadial to a warm interstadial phase followed by gradual cooling before a rapid return to stadial conditions the mechanisms responsible for these millennial cycles are not fully understood but are widely thought to involve abrupt changes in atlantic meridional overturning circulation due to freshwater perturbations here we present a new high resolution multiproxy marine sediment core monitoring changes in the warm atlantic inflow to the nordic seas as well as in local sea ice cover and influx of ice rafted debris in contrast to previous studies the freshwater input is found to be coincident with warm interstadials on greenland and has a fennoscandian rather than laurentide source furthermore the data suggest a different thermohaline structure for the nordic seas during cold stadials in which relatively warm atlantic water circulates beneath a fresh surface layer and the presence of sea ice is inferred from benthic oxygen isotopes this implies a delicate balance between the warm subsurface atlantic water and fresh surface layer with the possibility of abrupt changes in sea ice cover and suggests a novel mechanism for the abrupt d o events observed in greenland ice cores

can north atlantic sea ice anomalies account for dansgaard oeschger climate signals abstract north atlantic sea ice anomalies are thought to play an important role in the abrupt dansgaard oeschger d o cycles of the last glacial period this model study investigates the impacts of changes in north atlantic sea ice extent in glacial climates to help provide geographical constraints on their involvement in d o cycles based on a coupled climate model simulation of the last glacial maximum 21 ka the nordic seas and western north atlantic broadly south of greenland are identified as two plausible regions for large and persistent displacements of the sea ice edge in the glacial north atlantic sea ice retreat scenarios targeting these regions are designed to represent ice cover changes associated with the cold to warm stadial to interstadial transitions of d o cycles the atmospheric responses to sea ice retreat in the nordic seas and in the western north atlantic are tested individually and together using an atmospheric general circulation model the nordic seas ice retreat causes 1

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

high resolution greenland ice core data show abrupt climate change happens in few years the last two abrupt warmings at the onset of our present warm interglacial period interrupted by the younger dryas cooling event were investigated at high temporal resolution from the north greenland ice core project ice core the deuterium excess a proxy of greenland precipitation moisture source switched mode within 1 to 3 years over these transitions and initiated a more gradual change over 50 years of the greenland air temperature as recorded by stable water isotopes the onsets of both abrupt greenland warmings were slightly preceded by decreasing greenland dust deposition reflecting the wetting of asian deserts a northern shift of the intertropical convergence zone could be the trigger of these abrupt shifts of northern hemisphere atmospheric circulation resulting in changes of 2 to 4 kelvin in greenland moisture source temperature from one year to the next

stochastic atmospheric forcing as a cause of greenland climate transitions abstractan unforced simulation of the community climate system model version 4 ccsm4 is found to have greenland warming and cooling events that resemble dansgaard oeschger cycles in pattern and magnitude with the caveat that only three transitions were available to be analyzed it is found that the transitions are triggered by stochastic atmospheric forcing the atmospheric anomalies change the strength of the subpolar gyre leading to a change in labrador sea sea ice concentration and meridional heat transport the changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the north atlantic indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western pacific warm pool are discussed the full evolution of the anomalous climate state depends crucially on the climatic background state

weathering of marbles and granites in marine environment petrophysical properties and special role of atmospheric salts the aim of this paper was to study the weathering mechanism of marbles and granites exposed to the marine environment at the delos archaeological site alterations as granular disintegration contour scaling and alveolus formations can be observed either at the base of blocks and columns or at the median high zones of monuments a white marble which originated from naxos island had a weak porosity 0 2 its porous network was organized into two subnetworks of rectilinear and sinuous cracks which limited capillary transfer to 10 cm the granite which originated from delos island had a relatively high porosity 2 15 and had a well connected system of cracks and microporous zones of weathered minerals this homogeneous network allowed good capillary transfer for long distances 1 2 m for the marble as for the granite evaporation occurred mainly at depth in the stone and encouraged its deterioration this damage depends on the dissolution and crystallization of salts which occur in cycles first by the capillary transfer of water and salt from the ground when the stones are located near the sea this processes can explain the deterioration of the marble and the granite bases and the decay of granite on all its surfaces second by fixation of water vapor by sea salt deposited on the stone by wind this phenomenon can explain the degradations observed on the median high parts of monuments in marble and granite even though for the granite the first mechanism was more active

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

origin of hydrothermal fluids at olympic dam preliminary results from fluid inclusions and stable isotopes fluid inclusions and stable isotope data provide evidence of the temperature composition and sources of hydrothermal fluids associated with formation of the fe rich breccias that host cu u au ag ore at olympic dam early magnetite associated with pyrite and siderite precipitated from fluids of high delta 18 o content approx 10ppm at temperatures near 400 degrees c and magnetite in k feldspar apatite veins at the nearby acropolis deposit probably formed from similar fluids in contrast hematite associated with ore bearing breccias formed at lower temperatures 200 degrees 400 degrees c from fluids of lower delta 18 o content 9ppm fluid inclusion homogenization temperatures in quartz fragments within hematite breccias overlap with and are lower than those indicated by isotope geothermometry two phase liquid plus vapor inclusions mainly homogenize at temperatures between 130 degrees and 280 degrees c with a mode at 170 degrees to 190 degrees c quartz fragments within rare laminated barite clasts display a larger range of homogenization temperatures from 110 degrees to 420 degrees c and may have preserved evidence of early high temperature hydrothermal events measurement of freezing point depressions in fluid inclusions provides evidence of widely varying salinities and complex solute chemistry indicated salinities vary from a minimum of 7 3 wt percent nacl equiv 1 3 m to a maximum of 23 7 wt percent nacl equiv 4 2 m in two phase fluid inclusions and up to 42 wt percent nacl equiv in halite saturated three phase inclusions all the fluid inclusions studied displayed evidence of additional solutes in addition to nacl all required a high degree of supercooling to induce freezing and displayed initial ice melting at temperatures below 23 degrees c and many formed distinctive brown granular ice in addition several inclusions also formed co 2 clathrates and other metastable solids qualitative assessment of these characteristics indicates fluids carrying multiple solutes probably including nacl kcl cacl 2 fecl 2 caf and co 2 these preliminary results suggest that at least two sources of fluids of contrasting temperature composition and oxygen isotope characteristics were involved in the formation of olympic dam early magnetite was deposited from hydrothermal fluids of deep seated possibly magmatic origin but magnetite bearing rocks are not significant hosts for copper uranium mineralization later hematite associated with ore bearing breccias was deposited by fluids of significantly lower temperature and lower delta 18 o values the geologic setting of the deposit a fault bounded depression at or near the palcosurface implies that waters of surficial origin seawater closed basin water or ground water may have been involved in the later stages of hydrothermal activity that formed the hematite breccias although our preliminary isotopic data do not require such waters they are consistent with this conclusion

isotopic composition of salt efflorescence from the sandstone castellated rocks of the bohemian cretaceous basin czech republic the origin of sulphates in sulphate rich efflorescences on quartz sandstones with a clay matrix exposed in rural areas of the czech republic is interpreted based upon an isotopic study of s and o sulphates such as gypsum and or alums exhibit 34s ranging from 1 3 to 6 1 and 18o from 5 3 to 8 8 the low variability of s and o isotopes indicates a common source of the sulphur and a similar mode of sulphate formation atmospheric sulphates with a similar isotopic signature occur in the area due to the combustion of sulphurous coal in power plants located a few tens of kilometres from the sampling points the sulphates crystallize from supersaturated pore waters that represent atmospheric precipitation rich in sulphates having percolated through the porous sandstone system the previously proposed model of efflorescence growth that it is due to the oxidation of pyrite can be excluded due to both the rare occurrence of pyrite and also to its different isotopic signature 34s about 22 although gypsum prevails in the central and eastern part of the studied area the north and north west of the bohemian cretaceous basin the most polluted region exhibits a significant presence of alums nh4 or k nh4 rich formation of alums can be explained by the partial dissolution of clay minerals or feldspars present in the sandstone matrix release of alumina from these phases is facilitated by the low ph of the precipitation ph 4 4 5 and also locally by organic acids traces of which were found in the studied efflorescences by the use of infrared spectroscopy

atmospheric nitrous oxide during the last 140 000 years article i nfo reconstructions of past atmospheric concentrations of greenhouse gases provide unique insight into the biogeochemical cycles and the past radiative forcing in the earth s climate system we present new measurements of atmospheric nitrous oxide along the ice cores of the north greenland ice core project and talos dome sites using records of several other ice cores we are now able to establish the first complete composite nitrous oxide record reaching back to the beginning of the previous interglacial about 140 000 yr ago on the basis of such composite ice core records we further calculate the radiative forcing of the three most important greenhouse gases carbon dioxide methane and nitrous oxide during more than a full glacial interglacial cycle nitrous oxide varies in line with climate reaching very low concentrations of about 200 parts per billion by volume during marine isotope stages 4 and 2 and showing substantial responses to millennial time scale climate variations during the last glacial a large part of these millennial time scale variations can be explained by parallel changes in the sources of methane and nitrous oxide however as revealed by high resolution measurements covering the dansgaard oeschger events 17 to 15 the evolution of these two greenhouse gases may be decoupled on the centennial time scale carbon dioxide and methane concentrations do not reach interglacial levels in the course of millennial time scale climate variations during the last glacial in contrast nitrous oxide often reaches interglacial concentrations in response to both glacial terminations and dansgaard oeschger events this indicates from a biogeochemical point of view similar drivers in both temporal cases while carbon dioxide and methane concentrations are more strongly controlled by climate changes in high latitudes nitrous oxide emissions changes may mainly stem from the ocean and or from soils located at low latitudes accordingly we speculate that high latitudes could play the leading role to trigger glacial terminations

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

silician magnetite from the dales gorge member of the brockman iron formation hamersley group western australia we report silician magnetite from banded iron formation bif in the dales gorge member of the brockman iron formation hamersley group western australia magnetite mesobands typically consisting of individual 100 m microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite silician magnetite overgrowths contain from 1 to 3 wt sio 2 whereas low si magnetite domains contain less than 1 wt sio 2 silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro x ray diffraction and high resolution transmission electron microscopy three textures are distinguished in magnetite mesobands 1 magnetite sub microlaminae with silician magnetite overgrowths 2 recrystallized magnetite fragments with silician magnetite overgrowths and 3 a complex intergrowth of magnetite and silician magnetite all three textures are found in magnetite mesobands from the bif4 5 and bif12 16 macrobands of the dales gorge type section drill core ddh 47a from wittenoom western australia magnetite domains contain numerous submicrometer to micrometer inclusions of quartz carbonate stilpnomelane and apatite whereas silician magnetite overgrowths are devoid of mineral inclusions the presence of mineral inclusions in magnetite indicates the bif oxide precipitate was not chemically pure iron oxyhydroxide oxide magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this bif silician magnetite is the dominant iron oxide in the dales gorge bif and is present in many other sub greenschist facies bifs worldwide we suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite thus silician magnetite is a potential biosignature in bifs

experimental evidence for non redox transformations between magnetite and hematite under h2 rich hydrothermal conditions abstract transformations of magnetite fe ii fe 2 iii o 4 to hematite fe 2 iii o 3 and vice versa have been thought by many scientists and engineers to require molecular o 2 and or h 2 thus the presence of magnetite and or hematite in rocks has been linked to a specific oxidation environment however the availability of reductants or oxidants in many geologic and industrial environments appears to have been too low to account for the transformations of iron oxides through redox reactions here we report the results of hydrothermal experiments in mildly acidic and h 2 rich aqueous solutions at 150 c which demonstrate that transformations of magnetite to hematite and hematite to magnetite occur rapidly without involving molecular o 2 or h 2 fe 3 o 4 mt 2h aq fe 2 o 3 hm fe aq 2 h 2 o the transformation products are chemically and structurally homogeneous and typically occur as euhedral single crystals much larger than the precursor minerals this suggests that in addition to the expected release of aqueous ferrous species to solution the transformations involve release of aqueous ferric species from the precursor oxides to the solution which reprecipitate without being reduced by h 2 these redox independent transformations may have been responsible for the formation of some iron oxides in natural systems such as high grade hematite ores that developed from banded iron formations bifs hematite rich deposits formed on mars corrosion products in power plants and other industrial systems

high latitude salinity effects and interhemispheric thermohaline circulations a general circulation model for the ocean is used to investigate the interaction between the global scale thermohaline circulation and the salinity distribution it is shown that an equatorially asymmetric circulation can be maintained even under equatorially symmetric basin geometry and surface forcing multiple equilibrium solutions are obtained for the same forcing by perturbing the high latitude salinity field in an otherwise equatorially symmetric initial condition the timescale of the transition from the symmetric circulation to an asymmetric circulation depends critically on the sign of the initial salinity perturbation

two stable equilibria of the atlantic subpolar gyre the cyclonic circulation of the atlantic subpolar gyre is a key mechanism for north atlantic climate variability on a wide range of time scales it is generally accepted that it is driven by both cyclonic winds and buoyancy forcing yet the individual importance and dynamical interactions of the two contributions remain unclear the authors propose a simplified four box model representing the convective basin of the labrador sea and its shallow and deep boundary current system the western subpolar gyre convective heat loss drives a baroclinicflow of relatively light water around the dense center eddy salt flux from the boundary current to the center increases with a stronger circulation favors the formation of dense waters and thereby sustains a strong baroclinic flow approximately 10 25 of the total in contrast when the baroclinic flow is not active surface waters may be too fresh to convect and a buoyancy driven circulation cannot develop this situation corresponds to a second stable circulation mode a hysteresis is found for variations in surface freshwater flux and the salinity of the near surface boundary current an analytical solution is presented and analyzed

evolution and architecture of a large felsic igneous province in western laurentia the 1 6 ga gawler range volcanics south australia the gawler range volcanics grv of south australia are an intracontinental subaerial large igneous province lip formed during supercontinent assembly in the mesoproterozoic like many lips the grv are intraplate were erupted over a geologically short time 2 million years and are dominated by voluminous lavas however the grv are overwhelmingly dominated by felsic units the grv are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved furthermore they are well exposed in very gently northward and eastward dipping sections up to 4 km thick allowing assessment of the facies architecture and evolution of this felsic volcanic lip the evolution of the grv can be clearly separated into two main stages initial eruptions at numerous volcanic centres produced small to moderate volume geochemically distinct felsic lavas and lava domes together with ignimbrites and minor mafic and intermediate lavas forming a sequence 0 5 to 3 km thick volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from phanerozoic felsic volcanic provinces however the second stage produced at least three voluminous felsic units each of which represents about 1000 3000 km3 of magma moreover the dominance of evenly porphyritic textures and lack of pyroclastic textures pumice shards broken crystal lithic clasts in these units suggests that they were erupted effusively and flowed as lavas each of these felsic lavas are generally dominated by a single uniform composition and commonly mingled with a subordinate and compositionally distinct lava

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes