exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

atlantic meridional overturning circulation during the last glacial maximum the circulation of the deep atlantic ocean during the height of the last ice age appears to have been quite different from today we review observations implying that atlantic meridional overturning circulation during the last glacial maximum was neither extremely sluggish nor an enhanced version of present day circulation the distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the atlantic only slightly greater than today however evidence from multiple water mass tracers supports a different distribution of deep water properties including density which is dynamically linked to circulation

interdecadal variability of the thermohaline circulation in box ocean models forced by fixed surface fluxes intrinsic modes of decadal variability are analyzed using box geometry ocean models forced by constant surface fluxes an extensive parameter sensitivity analysis of the oscillatory behavior is carried out with respect to the spherical cartesian geometry the b effect the coriolis parameter the parameterization of momentum dissipation and associated boundary conditions the vertical and horizontal diffusivities the convective adjustment algorithm the horizontal and vertical model resolution the forcing amplitude and the basin width the oscillations stand out as a robust geostrophic feature whose amplitude is mainly controlled by the horizontal diffusivity unsuccessful attempts to reproduce the variability in zonally averaged 2d models suggest that the 3d adjustment processes are necessary however the b effect is not necessary for decadal variability to occur and therefore classical rossby waves play no fundamental role in the mechanism various experiments with different geometry and forcing are conducted and do not support the necessity of viscous numerical boundary waves or any boundary in sustaining the oscillations the models show two types of oscillatory behavior 1 temperature anomalies propagating geostrophically westward in the eastward jet northern part of the basin and inducing an opposite anomaly in their wake and 2 stationary temperature anomalies in the northwest quarter that respond to the western boundary current transport changes and reinforce geostrophically this change until the opposite temperature anomaly built on the east finally reverses the meridional overturning anomaly the analysis of the transition from steady to oscillatory states using heat fluxes diagnosed at the equilibrium under restoring boundary conditions and the comparison of the variability under various forcing fields suggest that the oscillations are triggered in the regions of strongest surface cooling finally a simple box model analogy is proposed that captures the crucial phase shift between meridional overturning and north south density gradient anomalies on these decadal timescales

rapid subtropical north atlantic salinity oscillations across dansgaard oeschger cycles geochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age the dansgaard oeschger cycles were coupled to fluctuations in north atlantic meridional overturning circulation through its regulation of poleward heat flux the balance between cold meltwater from the north and warm salty subtropical gyre waters from the south influenced the strength and location of north atlantic overturning circulation during this period of highly variable climate here we investigate how rapid reorganizations of the ocean atmosphere system across these cycles are linked to salinity changes in the subtropical north atlantic gyre we combine mg ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four dansgaard oeschger cycles spanning 45 9 59 2 kyr ago to generate a seawater salinity proxy record from a subtropical gyre deep sea sediment core we show that north atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials covarying with inferred shifts in the tropical atlantic hydrologic cycle and north atlantic overturning circulation these salinity oscillations suggest a reduction in precipitation into the north atlantic and or reduced export of deep salty thermohaline waters during stadials we hypothesize that increased stadial salinities preconditioned the north atlantic ocean for a rapid return to deep overturning circulation and high latitude warming by contributing to increased north atlantic surface water density on interstadial transitions

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

the thermohaline circulation and the control of ice ages abstract it is found that the global salinity variations associated with the thermohaline circulation may have a tendency to make the circulation increasingly asymmetric with respect to the equator as a consequence the salinity difference between the pacific and the atlantic ocean may be slowly increasing such a process could have a time scale long enough to be comparable with the time span between major glaciations a speculative glaciation cycle is proposed which involves the above mentioned property of the thermohaline circulation in this cycle the role of a northern hemisphere glaciation is to bring excess freshwater from the pacific to the atlantic

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

rain driven by receding ice sheets as a cause of past climate change 1 the younger dryas cold period which interrupted the transition from the last ice age to modern conditions in greenland is one of the most dramatic incidents of abrupt climate change reconstructed from paleoclimate proxy records changes in the atlantic ocean overturning circulation in response to freshwater fluxes from melting ice are frequently invoked to explain this and other past climate changes here we propose an alternative mechanism in which the receding glacial ice sheets cause the atmospheric circulation to enter a regime with greater net precipitation in the north atlantic region this leads to a significant reduction in ocean overturning circulation causing an increase in sea ice extent and hence colder temperatures positive feedbacks associated with sea ice amplify the cooling we support the proposed mechanism with the results of a state of the art global climate model our results suggest that the atmospheric precipitation response to receding glacial ice sheets could have contributed to the younger dryas cooling as well as to other past climate changes involving the ocean overturning circulation

a wind induced thermohaline circulation hysteresis and millennial variability regimes abstract the multiple equilibria of the thermohaline circulation thc used here in the sense of the meridional overturning circulation as function of the surface freshwater flux has been studied intensively following a stommel paper from 1961 it is shown here that multistability and hysteresis of the thc also exist when the wind stress amplitude is varied as a control parameter both the massachusetts institute of technology ocean general circulation model mitgcm and a simple three box model are used to study and explain different dynamical regimes of the thc and thc variability as a function of the wind stress amplitude starting with active winds and a thermally dominant thermohaline circulation state the wind stress amplitude is slowly reduced to zero over a time period of 40 000 yr 40 kyr and then increased again to its initial value over another 40 kyr it is found that during the decreasing wind stress phase the thc remains thermally dominant until very low wind stress amplitude at which

rapid fluctuations in the deep north atlantic heat budget during the last glacial period 1 our understanding of past ocean climate interactions has been hampered by a paucity of proxy reconstructions of physical ocean circulation parameters here we present the first detailed record of deepwater temperature variability that resolves dansgaard oeschger climate fluctuations during the last glaciation results indicate a positive coupling between abrupt temperature changes over greenland and in the deep north atlantic that was mediated by perturbations to the atlantic overturning circulation the occurrence of sharp warming events at the center of the heinrich stadials 4 and 5 further indicates that the response of the ocean interior was less static across these major climate events than has been previously apparent

chemical overprinting of magmatism by weathering a practical method for evaluating the degree of chemical weathering of granitoids abstract quantitative determination of the degree of chemical weathering of rocks is a fundamental task in environmental and engineering geology and many weathering indices based on whole rock chemistry have been proposed however most classical indices are of limited application to granitoids in a wide area because these lithotypes generally exhibit wide chemical variation arising from their petrogenesis the chemical evolution produced during rock weathering therefore overprints pre existing magmatic chemical variation this problem can cause many classical weathering indices to yield misleading results this study proposes a method that compensates for the influence of petrogenesis on calculation of the weathering index the method is based on a bivariate plot of the magmatic chemical variation mcv in granitoids and the degree of chemical weathering dcw the mcv axis must be based on an element that reflects magmatic processes and is also relatively immobile during rock weathering in this study tio 2 contents are utilized for the mcv the dcw axis is fundamentally defined by the ratios of more mobile to less mobile elements during weathering and hence many classical indices can be applied the improved value of the degree of chemical weathering dcw i for a weathered rock is derived by dcw i s mcv cv mcv 1 dcw 1 where mcv 1 is the measured composition e g tio 2 content of the weathered rock dcw 1 denotes the ratios of more mobile to less mobile elements of the weathered rock the s parameter is the slope of the least square linear regression for fresh granitoids in the mcv dcw relationship mcv cv is a correction factor which is given by the average point on the mcv axis e g average tio 2 of the fresh rocks this method is useful for evaluating the degree of weathering of various granitoids and enhances the practical application of many weathering indices

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

experimentally determined stability of clinopyroxene garnet corundum in the system cao mgo al 2 o 3 sio 2

erosion by rivers and transport pathways in the ocean a provenance tool using 40ar 39ar incremental heating on fine grained sediment measured from 39 ar and 37 ar respectively are consistent with typical values for k and ca bearing minerals calculations show that the bulk mineralogy is reflected in the outgassing k ca spectra and identify plagioclase as the dominant mineral contributing to the plateau defining portion of the age spectra a second model predicts bulk sediment ages from integrated bedrock cooling age area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for overall this technique shows considerable promise not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems

weak oceanic heat transport as a cause of the instability of glacial climates the stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean atmosphere sea ice coupled model it turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates the large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further south the nature of the instability is of oceanic origin identical to that found in ocean models under mixed boundary conditions with similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters the deep water temperatures being regulated by the temperature of freezing weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation within the unstable regime periodic millennial oscillations occur spontaneously the climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients both states are unstable the atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content during the abrupt warming events that punctuate the course of a millennial oscillation sea ice variations are shown respectively to damp amplify the amplitude of the oceanic atmospheric response this sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period the so called dansgaard oeschger events may be internal instabilities of the climate system

source and tectonic implications of tonalite trondhjemite magmatism in the klamath mountains in the klamath mountains voluminous tonalite trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 ma early cretaceous it occurred about 5 to 10 m y after the 165 to 159 ma josephine ophiolite was thrust beneath older parts of the province during the nevadan orogeny thrusting from 155 to 148 ma the magmatism also corresponds to a period of slow or no subduction most of the plutons crop out in the south central klamath mountains in california but one occurs in oregon at the northern end of the province compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later more voluminous tonalitic and trondhjemitic intrusions most plutons consist almost entirely of tonalite and trondhjemite poorly defined concentric zoning is common tonalitic rocks are typically of the low al type but trondhjemites are generally of the high al type even those that occur in the same pluton as low al tonalite the suite is characterized by low abundances of k2o rb zr and heavy rare earth elements sr contents are generally moderate 450 ppm by comparison with sr rich arc lavas interpreted to be slab melts up to 2000 ppm initial 87sr 86sr 18o and nd are typical of mantle derived magmas or of crustally derived magmas with a metabasic source compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source transitional mid ocean ridge to island arc basalt but not by fractional crystallyzation of a basaltic parent melting models require a residual assemblage of clinopyroxene garnet plagioclase amphibole residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab such models are consistent with the metabasic part of the josephine ophiolite as the source because the josephine ophiolite was at low t during nevadan thrusting an external heat source was probably necessary to achieve significant degrees of melting heat was probably extracted from mantle derived basaltic melts which were parental to the mafic precursors of the tonalite trondhjemite suite thus under appropriate tectonic and thermal conditions heterogeneous mafic crustal rocks can melt to form both low and high al tonalitic and trondhjemitic magmas slab melting is not necessary

instability of glacial climate in a model of the ocean atmosphere cryosphere system in contrast to the relatively stable climate of the past 10 000 years during glacial times the north atlantic region experienced large amplitude transitions between cold stadial and warm interstadial states in this modeling study we demonstrate that hydrological interactions between the atlantic thermohaline circulation thc and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode these interactions have the potential to destabilize the thc which is already more sensitive for glacial conditions than for the present day climate thus providing an explanation for the increased variability of glacial climate

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

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

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor 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

roots of an archean volcanic arc complex the lac des iles area in ontario canada abstract the lac des iles area is located in the wabigoon subprovince an archean granite greenstone terrain in the superior province in ontario canada the area consists of several unmetamorphosed plutons of biotite and hornblende tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss the mafic ultramafic bodies are concentric in shape and consist of periodotite pyroxenite gabbro and hornblende gabbro cumulates and diorite the largest intrusion the lac des iles complex consists of several mafic and ultramafic centers an internal sm nd isochron from the southern ultramafic complex gives an intrusion age of 2736 28 ma mswd 1 5 nd t 1 2 this age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons the tonalitic gneiss represents older continental crust and had an nd value of 1 at this time the nd t values of the felsic and mafic ultramafic suites are positive ranging from 0 to 2 this range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust such as the tonalite gneiss during their ascent this implies that the tonalite magmas in the lac des iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere the morphology and the rock assemblages of the mafic and ultramafic intrusions in the lac des iles area resemble alaskan type intrusions which are common components of phanerozoic magmatic arcs associated with subduction zones in addition the crystallization sequence in the mafic ultramafic intrusions suggests that they are derived from a parental magma with high al basalt affinities the mafic ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements and the tonalites belong to calc alkaline rock series displaying fractionation trends towards trondhjemitic compositions thus the archean lac des iles area and the root zones of phanerozoic active continental margins share many geological geochemical and petrological features which would suggest that the lac des iles area formed in an andean type continental margin

the fast climate fluctuations during the stadial and interstadial climate states rapid climate changes during the last glacial period were first observed in ice core records dansgaard and others 1982 these shifts between interstadials called dansgaard oeschger d o events and stadials or deep glaciation were later seen in atlantic sediment records bond and others 1993 pointing to the ocean circulation as a strong component in the dynamics of these shifts wright and stocker 1991 the interstadial states are observed to have a characteristic sawtooth shape indicating a gradual drift of the stable interstadial state toward the stable stadial state in order to contrast the two climate states we have separated the 18o signal from the greenland icecore project ice core into periods corresponding to the two states the climate variability in the two different climatic states is different johnsen and others 1997 we find that the standard deviation is significantly larger in the stadial than in the interstadial state both states are found to have a larger standard deviation than the holocene part of the record the correlation times in the different states are difficult to obtain because of limited data resolution and diffusion of the isotopic signal however using a statistical technique we have estimated the correlation times we do not find significant differences in the correlation times which are of the order of months in the different climatic states these findings are interpreted in the context of a simple linear stochastic model which provides information about the relative roles of the climatic forcing and the stability of the climate state governing the climate variability

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

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

simulating the ice thickness distribution in a coupled climate model climate simulations in a global coupled model are investigated using a dynamic thermodynamic sea ice and snow model with sophisticated thermodynamics and a subgrid scale parameterization for multiple ice thicknesses in addition to the sea ice component the model includes a full primitive equation ocean and a simple energy moisture balance atmosphere we introduce a formulation of the ice thickness distribution that is lagrangian in thickness space the method is designed to use fewer thickness categories because it adjusts to place resolution where it is needed most and it is free of diffusive effects that tend to smooth eulerian distributions experiments demonstrate that the model does reasonably well in simulating the mean arctic climate we find the climate of the arctic and northern north atlantic is sensitive to resolving the ice thickness distribution when comparing the model results to a simulation with a two level sea ice model the ice thickness distribution causes ice export through fram strait to be more variable and more strongly linked to meridional overturning in the north atlantic ocean the lagrangian formulation of the ice thickness distribution allows for the inclusion of a vertical temperature profile with relative ease compared to an eulerian method we find ice growth rates and ocean surface salinity differ in our model with a well resolved vertical temperature profile in the ice and snow and an explicit brine pocket parameterization compared to a simulation with semtner zero layer thermodynamics although these differences are important for the climate of the arctic the effects of an ice thickness distribution are more dramatic and extend into the northern north atlantic sensitivity experiments indicate that five ice thickness categories with 50 cm vertical temperature resolution capture the effects of the ice thickness distribution on the heat and freshwater exchange across the surface in the presence of sea ice in these simulations

a numerical study of mineral zoning formed during the cooling of finite volume grains a numerical model is used to study compositional profiles formed by cation exchange and diffusion at constant cooling rates in spherical mineral grains fe mg zoning in garnet olivine pairs is used as an example zoned concentration profiles can be described by an exponential function changes in the parameters of the function can be used to evaluate the effects of changes in inclusion size cooling history diffusion parameters and exchange enthalpy the composition of an olivine inclusion within garnet depends strongly on inclusion size due to finite volume effects as does zoning in the garnet surrounding the inclusion thus estimates of equilibration temperatures obtained from small inclusions of one phase within another require a size correction however for a spherical garnet within an olivine matrix the effect is small for garnet radii greater than 1000 m the shapes of zoned concentration profiles are useful for bounding both the rate of cooling and the final temperature in addition the concentration ratios fe garnet fe olivine may be used to estimate the cooling rate once the final temperature is bounded

the role of oceanic heat transport and wind stress forcing in abrupt millennial scale climate transitions the last glacial period was punctuated by rapid climate shifts known as dansgaard oeschger events with strong imprint in the north atlantic sector suggesting that they were linked with the atlantic meridional overturning circulation here an idealized single hemisphere three dimensional ocean atmosphere sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the holocene focusing on the physics of noise free millennial oscillations under steady external solar forcing it was shown that cold climates become unstable that is exhibit abrupt millennial scale transitions for significantly lower freshwater fluxes than warm climates in agreement with previous studies making use of zonally averaged coupled models this fundamental difference is a direct consequence of the weaker stratification of the glacial ocean mainly caused by upper ocean cooling using a two hemisphere configuration of a coupled climate model of intermediate complexity it is shown that this result is robust to the added presence of a bottom water mass of southern origin the analysis reveals that under particular conditions a pronounced interdecadal variability develops during warm interstadials while the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions these interstadial interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes the wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing in particular it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of dansgaard oeschger cycles

coherent resonant millennial scale climate oscillations triggered by massive meltwater pulses the role of mean and stochastic freshwater forcing on the generation of millennial scale climate variability in the north atlantic is studied using a low order coupled atmosphere ocean sea ice model it is shown that millennial scale oscillations can be excited stochastically when the north atlantic ocean is fresh enough this finding is used in order to interpret the aftermath of massive iceberg surges heinrich events in the glacial north atlantic which are characterized by an excitation of dansgaard oeschger events based on model results it is hypothesized that heinrich events trigger dansgaard oeschger cycles and that furthermore the occurrence of heinrich events is dependent on the accumulated climatic effect of a series of dansgaard oeschger events this scenario leads to a coupled ocean ice sheet oscillation that shares many similarities with the bond cycle further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic linear and nonlinear deterministic components a schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data

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

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

glacial abrupt climate changes and dansgaard oeschger oscillations in a coupled climate model 1 there are three fundamental features which characterize large glacial millennial dansgaard oeschger oscillations 1 the climatic transitions were abrupt and large 2 the lengths of both interstadials and stadials and the period of dansgaard oeschger oscillations were not uniform and 3 there were no large millennial oscillations during an early stage of a glacial period and a peak glacial period in this modeling study we offer a consistent explanation for these three features by employing an earth system model of intermediate complexity we demonstrate that a moderate global cooling forces the atlantic meridional overturning circulation moc into an unstable state and hence causes the flip flop of the atlantic moc between a strong mode and a weak mode the durations of both interstadials and stadials associated with these millennial oscillations are modulated by the changing background climate in qualitative agreement with the observations in a warm climate the atlantic moc is strong and stable with the deep water formed mainly by intense heat loss to the atmosphere in a cold climate the atlantic moc is weak and stable and this mode is largely maintained by the process of sea ice brine rejection since the dansgaard oeschger oscillations result from an alternation between these two moc states during an intermediate phase climate we conclude that brine rejection plays a necessary role in the oscillations confirming a hypothesis suggested in some proxy data studies

wally was right predictive ability of the north atlantic conveyor belt hypothesis for abrupt climate change linked abrupt changes of north atlantic deep water formation north atlantic sea ice extent and widespread climate occurred repeatedly during the last ice age cycle and beyond in response to changing freshwater fluxes and perhaps other causes this paradigm developed and championed especially by w s broecker has repeatedly proven to be successfully predictive as well as explanatory with high confidence much work remains to fully understand what happened and to assess possible implications for the future but the foundations for this work are remarkably solid

new analytical and numerical geospeedometers tested on garnet pyroxenites from braganca nappe complex ne portugal abstract two new geospeedometric approaches an analytical one phys earth planet inter 110 1999 95 and a numerical one am j sci 29 1998 30 are tested to evaluate cooling rate on upper mantle derived pyroxenites from the braganca massif ne portugal these estimations coupled with standard geobarometric techniques allow theoretically the determination of exhumation rates responsible for incorporation of these rocks within the lower crust in the present paper we discuss the most significant sources of uncertainties in geospeedometry whether numerical or analytical precision of temperature estimates and input diffusion data none of the available geospeedometrical tools when used independently is able to provide definitive results with less than a factor 10 in precision cross calibration is thus required in order to provide reliable cooling rate estimates we obtain for pyroxenites from the braganca massif a minimum cooling rate from 1000 to 750 c of the order of 250 c ma for a depth interval of 10 15 km

submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc sawyers bar area central klamath mountains california usa new bulk rock oxygen isotope data indicate a complicated history of fluid rock interactions in the upper few kilometers of a basaltic arc attending its mesozoic accretion to the western margin of north america folded multiply recrystallized weakly metasomatized mafic volcanics and interstratified sediments of the sawyers bar terrane an eastern segment of the western triassic and paleozoic belt were investigated the following scenario can now be reconstructed 1 island arc tholeiites iats ocean island basalts oibs and distal turbidites were deposited in a subsea environment during permian and early mesozoic time 170 245 ma basaltic rocks underwent low temperature alteration by seawater recrystallization occurred at 100 200 c and 1 kbar alkali exchange and variable mg enrichment were accompanied by increases in bulk rock 18o values of the greenstones from 6 to approximately 10 o preceeding initial stages of island arc formation 2 middle jurassic 165 170 ma suturing of the seaward oceanic arc structurally beneath a landward 227 ma blueschist terrane resulted in regional deformation and greenschist facies metamorphism pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300 425 c 3 1 kbar 3 granitoid plutons emplaced during late middle jurassic time 160 165 ma heated adjacent wallrocks to 500 600 c at pressures of approximately 2 3 kbar thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high 18o fluids with intercalated greenstones 18o values in iat and oib metavolcanics increased from 9 to 10 along axial portions of ns trending folds to more than 15 o where metabasalts are intimately interlayered with the metasediments construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid rock isotopic exchanges reflecting the p t time evolution of the complex

16 c rapid temperature variation in central greenland 70 000 years ago variations in the29n2 28n2 ratio of air bubbles trapped in polar ice cores and their relation to variations of the 18o 16o of the ice allow past surface temperature variations and ice age gas age differences to be determined high resolution measurements of29n2 28n2 in dansgaard oeschger event 19 around 70 000 years before the present in ice from central greenland show that at the beginning of the event the ice age gas age difference was 1090 100 years with the use of a combined firn densification temperature and gas diffusion model the 18oice temperature coefficient was determined to be 0 42 0 05 per mil per kelvin this coefficient implies a mean surface temperature change of 16 0 kelvin between 14 3 and 18 1 kelvin which differs substantially from values derived from borehole temperatures and modern spatial 18oice surface temperature correlations

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

rapid transitions and ultra low frequency behaviour in a 40 kyr integration with a coupled climate model of intermediate complexity a 40 kyr integration with the coupled atmosphere ocean sea ice model of intermediate complexity ecbilt for present boundary conditions has been performed the climate of ecbilt displays quasi periodical behaviour with a period of approximately 13 kyr the quasi periodical behaviour is characterized by large changes in the overturning cell in the southern ocean the southern cell fluctuates between two quasi stationary states with accompaning changes in the atmospheric circulation in the southern hemisphere the transition between these states is rapid and resembles the polar halocline catastrophes and flushes as observed in ocean general circulation models under mixed boundary conditions the sea ice influence on both the surface heat and fresh water flux appears to be crucial for the existence and the prolongation of the quasi stationary states the atmospheric circulation of those two quasi stationary states displays large regional differences over antarctica resulting in even opposite surface air temperature trends for certain locations during the transition from one state to another

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

the different nature of the interdecadal variability of the thermohaline circulation under mixed and flux boundary conditions the differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse resolution ocean model in an idealized flat bottom single hemisphere basin objective features are determined that allow one type of oscillation to be distinguished versus the other first by performing a linear stability analysis of the steady state obtained under restoring boundary conditions it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises respectively from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state based on the budgets of density variance it is shown next that the two types of oscillations have different energy sources under the constant flux boundary condition the thermal mode the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability whereas under mixed boundary conditions the salinity mode a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation furthermore the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance in addition the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure the authors propose that these differences will allow one to identify which type of oscillation if any is at play in the more exhaustive climate models

last glacial maximum ocean thermohaline circulation pmip2 model intercomparisons and data constraints the ocean thermohaline circulation is important for transports of heat and the carbon cycle we present results from pmip2 coupled atmosphere ocean simulations with four climate models that are also being used for future assessments these models give very different glacial thermohaline circulations even with comparable circulations for present an integrated approach using results from these simulations for last glacial maximum lgm with proxies of the state of the glacial surface and deep atlantic supports the interpretation from nutrient tracers that the boundary between north atlantic deep water and antarctic bottom water was much shallower during this period there is less constraint from this integrated reconstruction regarding the strength of the lgm north atlantic overturning circulation although together they suggest that it was neither appreciably stronger nor weaker than modern two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing

geospeedometry of metamorphic rocks examples in the rantasalmi sulkava and kiuruvesi areas eastern finland biotite garnet diffusion couples compositional zoning in biotite garnet pairs in metamorphic rocks from eastern finland has been studied the mg profiles in the garnet side of biotite garnet crystal pairs have been interpreted by means of lasaga s theory geospeedometry however the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference this frame of reference gives the fluxes directly by means of the numbers of diffusing ions which helps to check the 1 dimensionality of the analysis assumed in lasaga s theory it is also shown that the recently argued effect of the third cation ca is negligible in our samples we were able to investigate satisfactory profiles in three samples from different areas the values for the cooling rate are a few degrees per million years if the diffusion data obtained by freer are adopted the cooling rates are in agreement with recent estimates based on the k ar ages on biotite in the same areas

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

sea ice switches and abrupt climate change we propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea ice cover we explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism we emphasize that because of the significant influence of sea ice on the climate system it seems that high priority should be given to developing ways for reconstructing high resolution in space and time sea ice extent for past climate change events if proxy data can confirm that sea ice was indeed the major player in past abrupt climate change events it seems less likely that such dramatic abrupt changes will occur due to global warming when extensive sea ice cover will not be present

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

is the wind stress forcing essential for the meridional overturning circulation 1 we use a global couple d atm osphere ocean sea ice model of interm ediate complexity to demonstrate that wind forcing i s a crucial element to sustain meridional overturning flow i n t he atlantic neglecting wind stress in our multi century lon g simulations leads to a complete shutdown of the conveyor belt ci r culation this r esult may have tremendous impacts f or an assessment of the sensitivity of 2 d clima t e models which typicall y do not capture wind driven gyres it is argued that wind effects may be a key element in determining the f ate and length of a collapsed thc state possible paleo implications will be discussed

polyphase variscan emplacement of exotic terranes morais and braganca massifs onto iberian successions evidence from 40ar 39ar mineral ages abstract the braganca and morais nappe complexes comprise internally imbricated continental tectonic units including schists deformed metagranite granulite and local eclogite and a structurally underlying ophiolite nappe high pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint by contrast rocks within the ophiolite nappe display prograde amphibolite facies assemblages hornblende concentrates from both tectonic units exposed in the braganca nappe complex record 40 ar 39 ar plateau and isotope correlation ages of c 385 390 ma a similar date is recorded by hornblende within mylonitic amphibolite ophiolite protolith along a ductile thrust which separates the two major nappe units muscovite within blastomylonitic metagranite lagoa augen gneiss in upper structural levels of the morais massif records 370 375 ma 40 ar 39 ar plateau ages muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 ma age together the 40 ar 39 ar data suggest that a regionally significant early variscan amphibolite facies metamorphism occurred prior to c 385 390 ma early devonian this was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex the composite nappe complex must have been maintained at relatively high structural levels throughout the remaining variscan evolution because there is no evidence for post 370 ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined this is in marked contrast to a significant late devonian c 330 ma locally high pressure metamorphism which is recorded in structurally underlying elements of the lower allochthon part of a foreland thrust complex composed of metasedimentary units with iberian protolith affinities together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c 330 ma with resultant deformation and metamorphism of structurally underlying components of the iberian thrust complex

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

a simulation of the last glacial maximum climate using the ncar ccsm the national center for atmospheric research community climate system model ncar ccsm is used in a coupled atmosphere ocean sea ice simulation of the last glacial maximum lgm around 21 000 years ago climate in the tropics the simulation shows a moderate cooling of 3 c over land and 2 c in the ocean in zonal average this cooling is about 1 c cooler than the climap sea surface temperatures ssts but consistent with recent estimates of both land and sea surface temperature changes subtropical waters are cooled by 2 2 5 c also in agreement with recent estimates the simulated oceanic thermohaline circulation at the lgm is not only shallower but also weaker than the modern with a migration of deep water formation site in the north atlantic as suggested by the paleoceanographic evidences the simulated northward flow of antarctic bottom water aabw is enhanced these deep circulation changes are attributable to the increased surface density flux in the southern ocean caused by sea ice expansion at the lgm both the gulf stream and the kuroshio are intensified due to the overall increase of wind stress over the subtropical oceans the intensified zonal wind stress and southward shift of its maximum in the southern ocean effectively enhances the transport of the antarctic circumpolar current acc by more than 50 simulated ssts are lowered by up to 8 c in the midlatitudes simulated conditions in the north atlantic are warmer and with less sea ice than indicated by climap again in agreement with more recent estimates the increased meridional sst gradient at the lgm results in an enhanced hadley circulation and increased midlatitude storm track precipitation the increased baroclinic storm activity also intensifies the meridional atmospheric heat transport a sensitivity experiment shows that about half of the simulated tropical cooling at the lgm originates from reduced atmospheric concentrations of greenhouse gases

on the robustness of the interdecadal modes of the thermohaline circulation ocean models in box geometry forced by constant surface fluxes of density have been found to spontaneously generate interdecadal oscillations of the thermohaline circulation this paper analyzes the sensitivity of these oscillations to various physical effects including the presence of mesoscale turbulence various thermal surface boundary conditions and the presence of wind forcing or bottom topography the role of unstable long baroclinic waves is also reexamined in an attempt to understand the oscillation period in idealized geometry it is found that the low frequency variability of the thermohaline circulation under quasi constant surface fluxes is a robust feature of the large scale circulation it is not strongly affected by energetic mesoscale turbulence the oscillation period is relatively invariant with respect to varying resolution and momentum and tracer horizontal mixing coefficients although it loses some regularity as shorter and longer periods of variability emerge when the mesoscale activity increases in strength with smaller mixing coefficients the oscillations are also retained as the ocean model is coupled to an interactive atmospheric energy balance model the thermohaline modes are robust to a range of exchange coefficients that widens with the amplitude of the mean circulation the presence of an additional wind forced component generally weakens the oscillation and depending on the relative strength of thermodynamic and dynamic forcings the oscillation may be completely killed a simple interpretation is given highlighting the role of upward ekman pumping in damping density anomalies finally the interaction of these baroclinic modes with bottom topography depends strongly on the relative directions of the mean topographic features and the mean currents and baroclinic waves but usually results in a damping influence