climatic impact of a greenland deglaciation and its possible irreversibility warmer climate conditions persisting for a period of many centuries could lead to the disappearance of the greenland ice sheet with a related 7 m rise in sea level the question is addressed of whether the ice sheet could be regenerated if preindustrial climate conditions were reestablished after its melting the hadcm3 coupled atmosphere ocean gcm is used to simulate the global and regional climate with preindustrial atmospheric greenhouse gas composition and with the greenland ice sheet removed two separate cases are considered in one the surface topography of greenland is given by that of the bedrock currently buried under the ice sheet in the other a readjustment to isostatic equilibrium of the unloaded orography is taken into account giving higher elevations in both cases there is greater summer melting than in the current climate leading to partially snow free summers with much higher temperatures on the long term average there is no accumulation of snow the implication of this result is that the removal of the greenland ice sheet due to a prolonged climatic warming would be irreversible

faster seafloor spreading and lithosphere production during the mid cenozoic concurrent changes in seawater chemistry sea level and climate since the mid cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges the present day area distribution of seafloor ages however is most easily explained if lithosphere production rates were nearly constant during the past 180 m y we examined spatial gradients of present day seafloor ages and inferred ages for the subducted farallon plate to construct a history of spreading rates in each major ocean basin since ca 140 ma revealing dramatic cenozoic events globally seafloor spreading rates increased by similar to 20 during the early cenozoic due to an increase in plate speeds in the pacific basin since then subduction of the fast spreading pacific farallon ridge system has led to a 12 decrease in average global spreading rate and an 18 or more decrease in the total rate of lithosphere production by the most conservative estimates these rapid changes during the cenozoic defy models of steady state seafloor formation and demonstrate the time dependent and evolving nature of plate tectonics on earth

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

multiple s isotopic constraints on paleo redox and sulfate concentrations across the ediacaran cambrian transition in south china abstract changes in oceanic chemical conditions are frequently invoked as a primary driver of biodiversification and mass extinction events in earth history during the ediacaran cambrian transition two major biological events including the extinction of the ediacaran biota and the cambrian explosion occurred approximately coincident with perturbations of the carbon and sulfur cycle these dramatic biological changes and coeval geochemical events are commonly thought to be linked but their causal relationships remain controversial here we present multiple s isotopic compositions of pyrite 34spy and 33spy from the xiaotan section spanning the ediacaran cambrian transition interval in south china our isotopic results show that negative 33spy values together with positive 34spy values occurred across the ediacaran cambrian boundary the negative 33spy data suggest mixing of sulfide from deep waters during the upwelling of sulfidic waters into oxygenated shallow waters the coincidence between the negative 33spy and the approximate timing of the extinction of the ediacaran biota implies that paleo redox perturbations may have contributed to the disappearance of the ediacaran biota negative 33spy values coupled with positive 34spy values were also observed in the cambrian zhongyicun member when the phosphorites were deposited suggesting a strong linkage between the upwelling of nutrient rich sulfidic deep waters and the deposition of the phosphorites in oxygenated shelf areas in addition using box models and stoichiometry calculations we quantitatively estimated elevated burial rates of organic matter and pyrite based on the parallel positive 13ccarb 34spy and 33spy excursions observed at xiaotan within cambrian stage 2 our mass balance modelling results provide an independent constraint on sulfate concentrations which were 4 mm or lower in the early cambrian oceans the probable increase in oxygen levels resulting from the enhanced burial of pyrite and organic matter could have provided a more habitable environment for early animals and thus paved the way for the radiation of early cambrian animals

convective heat transfer as a function of wavelength implications for the cooling of the earth 1 attempting to reconstruct the thermal history of the earth from a geophysical point of view has for a long time been in disagreement with geochemical data the geophysical approach uses parameterized models of mantle cooling the rate of cooling of the earth at the beginning of its history obtained in these models is generally too rapid to allow a sufficient present day secular cooling rate geochemical estimates of radioactive element concentrations in the mantle then appear too low to explain the observed present mantle heat loss cooling models use scaling laws for the mean heat flux out of the mantle as a function of its rayleigh number of the form q ra b recent studies have introduced very low values of the exponent b which can help reduce the cooling rate of the mantle the present study instead focuses on the coefficient c in the relation q cr a b and in particular on its variation with the wavelength of convection the heat transfer strongly depends on the wavelength of convection the length scale of convection in earth s mantle is that of plate tectonics implying convective cells of wide aspect ratio taking into account the long wavelength of convection in earth s mantle can significantly reduce the efficiency of heat transfer the likely variations of this wavelength with the wilson cycle thus imply important variations of the heat flow out of the earth on a intermediate timescale of 100 ma which renders parameterized models of thermal evolution inaccurate for quantitative predictions

modeling the influence of greenland ice sheet melting on the atlantic meridional overturning circulation during the next millennia a three dimensional earth system model of intermediate complexity including a dynamic ice sheet component has been used to investigate the long term evolution of the greenland ice sheet and its effects on the atlantic meridional overturning circulation amoc in response to a range of stabilized anthropogenic forcings our results suggest that the greenland ice sheet volume should experience a significant decrease in the future for a radiative forcing exceeding 7 5 w m 2 the modeled ice sheet melts away within 3000 years a number of feedbacks operate during this deglaciation implying a strong nonlinear relationship between the radiative forcing and the melting rate only in the most extreme scenarios considered the freshwater flux from greenland into the surrounding oceans of ca 0 1 sv during a few centuries induces a noticeable weakening of the amoc in the model

freshwater content variability in the arctic ocean 1 arctic ocean model simulations have revealed that the arctic ocean has a basin wide oscillation with cyclonic and anticyclonic circulation anomalies arctic ocean oscillation aoo that has a prominent decadal variability proshutinsky and johnson 1997 this study explores how the simulated aoo affects the arctic ocean stratification and its relationship to the sea ice cover variations the simulation uses the princeton ocean model coupled to sea ice hakkinen and mellor 1992 hakkinen 1999 the surface forcing is based on national centers for environmental prediction national center for atmospheric research reanalysis and its climatology of which the latter is used to force the model spin up phase our focus is to investigate the competition between ocean dynamics and ice formation melt on the arctic basin wide freshwater balance we find that changes in the atlantic water inflow can explain almost all of the simulated freshwater anomalies in the main arctic basin the atlantic water inflow anomalies are an essential part of aoo which is the wind driven barotropic response to the arctic oscillation ao the baroclinic response to ao such as ekman pumping in the beaufort gyre and ice melt freeze anomalies in response to ao are less significant considering the whole arctic freshwater balance

influence of continental growth on mid ocean ridge depth the interconnectedness of life water and plate tectonics is strikingly apparent along mid ocean ridges mor where communities of organisms flourish off the disequilibrium of chemical potentials created by circulation of hydrothermal fluids driven by earth s heat nisbet and sleep 2001 staudigel et al 2004 moreover submarine hydrothermal environments may be critical for the emergence of life on earth nisbet and sleep 2001 oceans were likely present in the hadean valley et al 2002 harrison 2009 but questions remain about early earth s global tectonics van hunen and moyen 2012 including when seafloor spreading began and whether mid oceanic ridges were deep enough for maximum hydrothermal activities kasting et al 2006 for example plate tectonics influences global sea level by driving secular variations in the volume of ocean basins due to continental growth flament et al 2008 similarly variations in the distribution of seafloor age and associated subsidence flament et al 2008 due to assembly and dispersal of supercontinents coltice et al 2012 explains the largest sea level variation over the past 140 myr muller et al 2008 using synthetic plate configurations derived from numerical models of mantle convection coltice et al 2012 2014 appropriate for early earth we show that mor has remained submerged and its depths potentially constant over geologic time thus conditions in the early earth existed for hydrothermal vents at similar depths as today providing environments conducive for the development of life and allowing for processes such as hydrothermal alteration of oceanic crust to influence the mantle s geochemical evolution this article is protected by copyright all rights reserved

the impact of greenland on cyclone evolution in the north atlantic in order to understand better the role of greenland s orography in determining the position and strength of the icelandic low we have carried out a number of experiments using a numerical weather prediction model in those experiments greenland s orography was modified and the impact on cyclone developments over the north atlantic was investigated we have focused on one fairly characteristic winter case from january 1995 but another one from january 1993 gave qualitatively similar results we have found evidence that the deepening of baroclinic cyclones near iceland is hampered by the presence of greenland s orography this has been shown to be related to the inability of cold air from the north to cross over greenland leading to a distortion of the thermal field associated with the disturbance and a halting of the progression of the cold front in the january 1995 case a characteristic secondary cyclone forming between greenland and iceland was shown to be entirely orographic in that it was absent in runs where greenland s orography was removed the results are considered in the context of recent theoretical studies of flow splitting in stratified flow impinging on high mountains

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

mechanisms for decadal scale variability in a simulated atlantic meridional overturning circulation variability in the atlantic meridional over turning circulation amoc has been analysed using a 600 year pre industrial control simulation with the bergen climate model the typical amoc variability has ampli tudes of 1 sverdrup 1 sv 10 6 m 3 s 1 and time scales of 40 70 years the model is reproducing the observed dense water formation regions and has very realistic ocean transports and water mass distributions the dense water produced in the labrador sea 1 3 and in the nordic seas including the water entrained into the dense overflows across the greenland scotland ridge gsr 2 3 are the sources of north atlantic deep water nadw forming the lower limb of the amoc s northern overturning the variability in the labrador sea and the nordic seas con vection is driven by decadal scale air sea fluxes in the convective region that can be related to opposite phases of the north atlantic oscillation the labrador sea convec tion is directly linked to the variability in amoc linkages between convection and water mass transformation in the nordic seas are more indirect the scandinavian pattern the third mode of atmospheric variability in the north atlantic is a driver of the ocean s poleward heat transport pht the overall constraint on northern water mass transformation increased pht is both associated with an increased water mass exchange across the gsr and a stronger amoc

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

time variability in cenozoic reconstructions of mantle heat flow plate tectonic cycles and implications for earth s thermal evolution the thermal evolution of earth is governed by the rate of secular cooling and the amount of radiogenic heating if mantle heat sources are known surface heat flow at different times may be used to deduce the efficiency of convective cooling and ultimately the temporal character of plate tectonics we estimate global heat flow from 65 ma to the present using seafloor age reconstructions and a modified half space cooling model and we find that heat flow has decreased by 0 15 every million years during the cenozoic by examining geometric trends in plate reconstructions since 120 ma we show that the reduction in heat flow is due to a decrease in the area of ridge proximal oceanic crust even accounting for uncertainties in plate reconstructions the rate of heat flow decrease is an order of magnitude faster than estimates based on smooth parameterized cooling models this implies that heat flow experiences short term fluctuations associated with plate tectonic cyclicity continental separation does not appear to directly control convective wavelengths but rather indirectly affects how oceanic plate systems adjust to accommodate global heat transport given that today s heat flow may be unusually low secular cooling rates estimated from present day values will tend to underestimate the average cooling rate thus a mechanism that causes less efficient tectonic heat transport at higher temperatures may be required to prevent an unreasonably hot mantle in the recent past

dynamics of the atlantic meridional overturning circulation part 2 forcing by winds and buoyancy abstract recently schloesser et al 2012 explored the dynamics of the descending branch of meridional overturning circulations mocs by obtaining analytic solutions to a variable density 2 layer model vlom forced only by a surface buoyancy flux key processes involved are the poleward thickening of the upper layer along the eastern boundary due to kelvin wave adjustments the westward propagation of that coastal structure by rossby waves and their damping by mixing the resulting zonal pressure gradient causes the surface moc branch to converge into the northern basin near the eastern boundary in this paper we extend the schloesser et al 2012 study to include forcing by a zonal wind stress x y much of the paper is devoted to the derivation and analysis of analytic solutions to vlom for validation we also report corresponding numerical solutions to an ocean general circulation model ogcm solutions are obtained in a flat bottom rectangular basin confined to the northern hemisphere the buoyancy forcing relaxes upper ocean density to a prescribed profile y that increases polewards until it becomes as large as the deep ocean density at latitude y2 north of y2 then the ocean is homogeneous a 1 layer system the wind stress x drives subtropical and subpolar gyres and in our standard solution the latter extends north of y2 vertical diffusion is not included in vlom minimized in the ogcm consequently the moc is not closed by upwelling associated with interior diffusion but rather by flow through the southern boundary of the basin into a southern boundary sponge layer in the ogcm and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary solutions forced by x and differ markedly from those forced only by because water flows across y2 throughout the interior of the subpolar gyre not just near the eastern boundary in some of our solutions the strength of the moc s descending branch is determined entirely by this wind driven mechanism whereas in others it is also affected by rossby wave damping near the eastern boundary upwelling can occur in the interior of the subpolar gyre and in the western boundary layer providing shortcuts for the overturning circulation consequently there are different rates for the convergence of upper layer water near y 2 m n and the export of deep water south of the subpolar gyre m the latter being a better measure of large scale moc strength when western boundary upwelling occurs in our solutions m is independent of the diapycnal processes in the subpolar ocean

expanding greenland ice sheet enhances sensitivity of plio pleistocene climate to obliquity forcing in the kiel climate model proxy data suggest the onset of northern hemisphere glaciation during the plio pleistocene transition from 3 2 to 2 5 ma resulted in enhanced climate variability at the obliquity 41 kyr frequency here we investigate the influence of the expanding greenland ice sheet gris on the mean climate and obliquity related variability in a series of climate model simulations these suggest that an expanding gris weakens the atlantic meridional overturning circulation amoc by 1 sv mainly due to reduced heat loss in the greenland iceland norwegian sea moreover the growing gris amplifies the hadley circulation response to obliquity forcing driving variations in freshwater export from the tropical atlantic and in turn variations of the amoc the stronger amoc response to obliquity forcing by about a factor of two results in a stronger global mean near surface temperature response we conclude that the amoc response to obliquity forcing is important to understand the enhanced climate variability at the obliquity frequency during the plio pleistocene transition

a world without greenland impacts on the northern hemisphere winter circulation in low and high resolution models to investigate the effect of greenland s orography on the northern hemisphere winter circulation experiments with an atmospheric gcm are conducted a perturbed integration where standard orography is reduced to sea level in the greenland area is compared to a standard orography control integration the outcome of these experiments suggests that the existence of high mountains at greenland causes a reinforcement of the stationary wave field in the atlantic sector colder temperatures to the west of greenland and warmer temperatures to the east and south over the north atlantic the impact on the flow field cannot be understood in the framework of standing rossby waves but it indicates a resonance between remotely forced stationary waves and local thermo dynamics the pattern of the north atlantic oscillation nao in particular the northern centre lies further to the east in the flat greenland experiment compared to the control run and the observations together with the fact that the climatological low pressure system around iceland hardly shifts this suggests that the location of the nao is not necessarily tied to the time mean pressure distributions considering the model resolution as a parameter experiments with a high resolution t106 suggest that the near field changes are represented sufficiently by a t42 resolution a standard resolution used in state of the art coupled climate models in contrast far field changes depend critically on model resolution hemispheric circulation and temperature changes differ substantially from low to high resolution and generalized statements about the impact of greenland s orography cannot be made

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

the impact of greenland s deglaciation on the arctic circulation 2 the greenland ice sheet is the largest orographic feature of the arctic mountains of such a scale together with surface heating or cooling anomalies exert a strong influence on the atmospheric circulation of the northern hemisphere the impact of greenland s ice sheet on the atmospheric circulation is of importance due to its position in vicinity to the north atlantic storm tracks the impact of greenland s deglaciation on cyclone tracks has been investigated in a regional numerical weather prediction model

mantle convection with strong subduction zones because mantle viscosity is temperature dependent cold subducting lithosphere should be strong which implies that the rapid localized deformation associated with subduction should strongly resist plate motions due to computational constraints the deformation of a subducting plate cannot be accurately resolved in mantle scale convection models so its effect on convection is difficult to investigate we have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid by imposing velocity boundary conditions in the vicinity of the subduction zone we enforce a geometry for subduction producing a slab with a realistic thermal structure to make the model dynamically consistent we specify a rate for subduction that balances the energy budget for convection which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts this expression is determined here from a local model of bending for a strong viscous lithosphere by implementing subduction in this way we have demonstrated convection with plates and slabs that resemble those observed on earth but in which up to 40 per cent of the mantle s total convective resistance is associated with deformation occurring within the subduction zone this additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab for sufficiently strong lithosphere the bending deformation slows surface plates sufficiently that they no longer actively participate in global scale convection which occurs instead beneath a sluggish lid by introducing a low viscosity asthenosphere beneath the oceanic plate we demonstrate that small scale convection at the base of oceanic lithosphere may limit plate thickness and thus the resistance to bending and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle for a cooling earth the effective lithosphere viscosity should be nearly constant but the mantle viscosity should increase with time thus subduction resisted convection should produce nearly constant plate velocities and heat flow over time which has implications for the earth s thermal evolution we estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 10 23 pa s but only if some mechanism such as small scale convection prevents the bending resistance from stopping plates altogether such a mechanism could be fundamental to plate tectonics and earth s thermal history

a multimodel study of sea surface temperature and subsurface density fingerprints of the atlantic meridional overturning circulation abstractthe atlantic meridional overturning circulation amoc is an important component of the north atlantic climate system here simulations from 10 coupled climate models are used to calculate patterns of sea surface temperature sst and subsurface density change associated with decadal amoc variability the models are evaluated using observational constraints and it is shown that all 10 models suffer from north atlantic deep water transports that are too shallow although the biases are least severe in the community climate system model version 4 ccsm4 in the models that best compare with observations positive amoc anomalies are associated with reduced labrador sea stratification and increased midocean 800 1800 m densities in the subpolar gyre maximum correlations occur when amoc anomalies lag labrador sea stratification and subsurface density anomalies by 2 6 yr and 0 3 yr respectively in all 10 models north atlantic warming follows positive amoc anomalies but the patterns and magnitud

seismic tomography surface uplift and the breakup of gondwanaland integrating mantle convection backwards in time mantle density heterogeneities imaged using seismic tomography contain information about time dependent mantle flow and mantle structures that existed in the past we model the history of mantle flow using a tomographic image of the mantle beneath southern africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition if the resulting backwards integrated model for structures is used as a starting point for a forwards convection model today s mantle can be adequately reconstructed if we do not integrate backwards more than than about 50 75 ma flow can also be reliably reversed through the mesozoic but only if instability of the lower boundary layer can be suppressed our model predicts that the large seismically slow and presumably hot structure beneath southern africa produced 500 700 m of dynamic topography throughout the cenozoic since 30 ma uplift has moved from eastern to southern africa where uplift rates are 10 m myr consistent with observations during the mesozoic the modeled topographic high is situated near gondwanaland rifting raising the possibility that this buoyant structure may have been involved with this breakup

impact of greenland orography on the atlantic meridional overturning circulation we show that the absence of the greenland ice sheet would have important consequences on the north atlantic ocean circulation even without taking into account the effect of the freshwater input to the ocean from ice melting these effects are investigated in a 600year long coupled ocean atmosphere simulation with the high resolution global climate model ec earth 3 0 1 once a new equilibrium is established a cooling of eurasia and of the north atlantic and a poleward shift of the subtropical jet are observed these hemispheric changes are ascribed to a weakening of the atlantic meridional overturning circulation amoc by about 12 we attribute this slowdown to a reduction in salinity of the arctic basin and to the related change of the mass and salt transport through the fram strait a consequence of the new surface wind pattern over the lower orography this idealized experiment illustrates the sensitivity of the amoc to local surface winds

modeling seafloor spreading rates through time a steady state model of crust production and destruction for the past 180 m y was proposed by b parsons and advocated by d rowley such a model has serious implications for models of secular variations in e g global sea level global climate and seawater chemistry this paper presents an analysis of the steady state model and then offers alternative extensions of that model that allow for non steady state production of ocean crust with time results suggest that the observed linear decrease in area versus age of ocean floor does not force a steady state view of seafloor spreading

mantle control of the geodynamo consequences of top down regulation the mantle global circulation including deep subduction and lower mantle superplumes exerts first order controls on the evolution of the core the history of the geodynamo and the structure of the geomagnetic field mantle global circulation models that include realistic plate motions deep subduction and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present day global average heat flux at the core mantle boundary cmb exceeds 85 mw m 2 this is sufficient to drive the present day geodynamo by thermochemical convection and implies a very young inner core with inner core nucleation between 400 and 1100 ma the mantle global circulation also generates spatially heterogeneous heat flux at the cmb with peak to peak lateral variations exceeding 100 mw m 2 such extreme lateral variability in cmb heat flux in conjunction with the high thermal conductivity of the core implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core producing localized circulation patterns beneath the cmb tied to the mantle heterogeneity and long lived deviations from axial symmetry in the geomagnetic field

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

circum antarctic palaeobathymetry illustrated examples from cenozoic to recent times abstract we use a revised set of seafloor spreading isochrons and updated plate tectonic reconstructions to produce palaeo age and basement depth grids for the southern ocean the palaeo depth maps illustrate the evolution of the ocean basins widening and deepening and oceanic gateway development surrounding antarctica we provide examples of palaeo basement depth profiles at selected times between 45 ma and the present across the tasman gateway and drake passage the digital grids provide improved boundary conditions for palaeoceanographic circulation models as well as for backtracking sediments from odp drilling

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

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

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

global reconstructions of cenozoic seafloor ages implications for bathymetry and sea level abstract although accurate estimates of cenozoic seafloor ages will serve to further our understanding of the relationship between mantle dynamics plate tectonics and a variety of surficial geological processes it is difficult to estimate ages of subducted seafloor however given the near constancy of surface velocities within a tectonic stage we can estimate cenozoic plate ages even for subducted lithosphere we reconstruct seafloor ages based on the cenozoic plate reconstructions and absolute rotation poles of gordon and jurdy r g gordon and d m jurdy cenozoic global plate motions j geophys res 91 1986 12389 12406 for the western pacific we explore alternative models based on the reconstructions of hall r hall cenozoic geological and plate tectonic evolution of se asia and the sw pacific computer based reconstructions model and animations j asian earth sci 20 2002 353 434 both reconstructions indicate an increase in average seafloor age since the early cenozoic resulting in an increase in the volume of ocean basins and a decreased sea level since the early cenozoic these trends are more pronounced for the gordon and jurdy r g gordon and d m jurdy cenozoic global plate motions j geophys res 91 1986 12 389 12 406 reconstruction because the hall r hall cenozoic geological and plate tectonic evolution of se asia and the sw pacific computer based reconstructions model and animations j asian earth sci 20 2002 353 434 reconstruction retains older seafloor in the western pacific which approximately halves the predicted sea level decrease since the early cenozoic 250 vs 125 m compared to geologic estimates of 150 m these changes in sea level occur despite decreases in oceanic lithosphere production rates of only about 20 in both models thus the changing distribution of seafloor age has a larger effect on sea level than changes in spreading rates or ridge lengths these reconstructions can also be used to estimate past heat flow the volume of subducted buoyancy and changes in the bathymetry of the cenozoic ocean basins

past and present seafloor age distributions and the temporal evolution of plate tectonic heat transport abstract variations in earth s rates of seafloor generation and recycling have far reaching consequences for sea level ocean chemistry and climate however there is little agreement on the correct parameterization for the time dependent evolution of plate motions a strong constraint is given by seafloor age distributions which are affected by variations in average spreading rate ridge length and the age distribution of seafloor being removed by subduction using a simplified evolution model we explore which physical parameterizations of these quantities are compatible with broad trends in the area per seafloor age statistics for the present day and back to 140 ma from paleo age reconstructions we show that a probability of subduction based on plate buoyancy slab pull or sqrt age and a time varying spreading rate fits the observed age distributions as well as or better than a subduction probability consistent with an unvarying triangular age distribution and age independent destruction of ocean floor instead we interpret the present near triangular distribution of ages as a snapshot of a transient state of the evolving oceanic plate system current seafloor ages still contain hints of a 60 myr periodicity in seafloor production and using paleoages we find that a 250 myr period variation is consistent with geologically based reconstructions of production rate variations these long period variations also imply a decrease of oceanic heat flow by 0 25 ma during the last 140 ma caused by a 25 50 decrease in the rate of seafloor production our study offers an improved understanding of the non uniformitarian evolution of plate tectonics and the interplay between continental cycles and the self organization of the oceanic plates

temporal variability of the atlantic meridional overturning circulation at 26 5 n the vigor of atlantic meridional overturning circulation moc is thought to be vulnerable to global warming but its short term temporal variability is unknown so changes inferred from sparse observations on the decadal time scale of recent climate change are uncertain we combine continuous measurements of the moc beginning in 2004 using the purposefully designed transatlantic rapid climate change array of moored instruments deployed along 26 5 n with time series of gulf stream transport and surface layer ekman transport to quantify its intra annual variability the year long average overturning is 18 7 5 6 sverdrups sv range 4 0 to 34 9 sv where 1 sv a flow of ocean water of 106 cubic meters per second interannual changes in the overturning can be monitored with a resolution of 1 5 sv

reheating of old oceanic lithosphere deductions from observations deep wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy when the depths of these basins are corrected for the effect of sediment accumulation and variation in crustal thickness the principal non thermal factors have been eliminated we collect the most precise and reliable values of heat flow for the same basins from multi penetration measurements with in situ thermal conductivity or deep sea drilling thermal gradients backed up by surface surveys the 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere when considered without regard to age all the points fall at greater depths than predicted by the plate models with constant temperature lower boundaries and remarkably close to boundary layer cooling with parameters determined from the pre 80 ma depth and heat flow history of the ocean floor they are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age modeling of discrete reheating events shows that near boundary layer conditions are re established after about 40 myr but corresponding to a younger than real age the data therefore favor discrete reheating events rather than a continuously hot basal boundary as implicitly assumed by the plate model lithospheric reheating appears to start only on ocean floor 100 ma the data alone cannot discriminate between a few discrete reheating events due to convective peel off at the base of the lithosphere or one or more catastrophic events however the distribution of points from the blake bahama basin is more consistent with distal reheating associated with the bermuda hotspot than local convective peel off

response of the north atlantic subpolar gyre to persistent north atlantic oscillation like forcing the response of the north atlantic subpolar gyre spg to a persistent positive or negative phase of the north atlantic oscillation nao is investigated using an ocean general circulation model forced with idealized atmospheric reanalysis fields the integrations are analyzed with reference to a base line integration for which the model is forced with idealized fields representing a neutral state of the nao in the positive nao case the results suggest that the well known cooling and strengthening of the spg are after about 10 years replaced by a warming and subsequent weakening of the spg the latter changes are caused by the advection of warm water from the subtropical gyre stg region driven by a spin up of the atlantic meridional overturning circulation amoc and the effect of an anomalous wind stress curl in the northeastern north atlantic which counteracts the local buoyancy forcing of the spg in the negative nao case however the spg response does not involve a sign reversal but rather shows a gradual weakening throughout the integration the asymmetric spg response to the sign of persistent nao like forcing and the different time scales involved demonstrate strong non linearity in the north atlantic ocean circulation response to atmospheric forcing the latter finding indicates that analysis based on the arithmetic difference between the two nao states e g nao minus nao may hide important aspects of the ocean response to atmospheric forcing

causes of decadal changes of the freshwater content in the arctic ocean abstractdecadal changes of the liquid freshwater content in the arctic ocean are studied with a suite of forward and adjoint model simulations adjoint sensitivities show that freshwater volume changes in the norwegian atlantic current north of the lofoten basin and a salinity maximum in the fram strait and in the canadian archipelago lead to an enhanced northward transport of freshwater the dynamical sensitivities indicate that stronger freshwater export from the arctic is related to an enhanced cyclonic circulation around greenland with an enhanced export through the canadian archipelago and a stronger circulation within the fram strait associated with this circulation around greenland is a large scale cyclonic circulation in the arctic cyclonic wind stress anomalies in the arctic ocean as well as over the nordic seas and parts of the subpolar atlantic are optimal to force the freshwater transport changes results from a simulation over the period 1948 2010 corroborate the result that arctic freshwat