thermal response of the mantle following the formation of a super plate 1 evidence indicating that the mantle below pangea was characterized by elevated temperatures supports the widely held view that a supercontinent insulates the underlying mantle implementing a 3d model of mantle convection featuring distinct oceanic and continental plates we explore different effects of supercontinent formation on mantle evolution we find that a halt in subduction along the margins of the site of the continental collision is sufficient to enable the formation of mantle plumes below a composite super plate and that the addition of continental properties that contribute to insulation have little effect on sub continental temperature our findings show that the mean temperature below a supercontinent surpasses that below the oceanic plates when the former is a perfect insulator but that continental thermal insulation plays only a minor role in the growth of sub supercontinent mantle plumes we suggest that the growth of a super oceanic plate can equally encourage the appearance of underlying upwellings

the influence of thermochemical convection on the fixity of mantle plumes abstract a general feature of both isochemical and thermochemical studies of mantle convection is that horizontal plume velocities tend to be smaller than typical convective velocities however it is not clear which system leads to a greater fixity of mantle plumes we perform two and three dimensional numerical calculations and compare both thermochemical and isochemical cases with similar convective vigor to determine whether presence of a dense component in the mantle can lead to smaller ratios of horizontal plume velocity to surface velocity we investigate different viscosity and density contrasts between chemical components in the thermochemical calculations and we perform isochemical calculations with both free slip and no slip bottom boundary conditions we then compare both visually and quantitatively the results of the thermochemical and isochemical calculations to determine which leads to greater plume fixity we find that horizontal plume velocities for thermochemical calculations are similar to those from isochemical calculations with no slip bottom boundary conditions in addition we find that plumes tend to be more fixed for isochemical cases with free slip bottom boundary conditions for two dimensional calculations however in three dimensions we find that plume fixity is similar to that observed in thermochemical calculations

the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

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

mantle convection flow reversals due to continental collisions rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness are included in a two dimensional numerical model of mantle convection model plates typically span six vertical increments of a finite difference mesh we model two identical continents being carried towards each other by a pair of underlying counter rotating mantle convection cells upon meeting at the model mid plane the model continents form a motionless rigid conducting supercontinent along a portion of the upper boundary while the model oceanic plates continue to move and recycle through the mantle the resulting changes in the mechanical boundary conditions at the upper surface prove to be important factors in facilitating flow reversal below the supercontinent leading to a subsequent dispersal of the individual continental blocks we find the following factors to favour the development of sustained flow reversal below our model supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and lower proportions of internal radiogenic heating within the mantle

mantle dynamics and the heat flow into the earth s continents reducedcontinental heat flow defined as the measured heat flow minus that due to radiogenic sources in the upper crust initially decreases with time after the last thermotectonic event in a region1 but then tends to stabilize at a roughly constant value which is higher than would be predicted by a conductive cooling model fig 1 here we argue that this behaviour results from the influence that continents have on mantle heat loss the finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale as is oceanic crust allows stable continental blocks to limit the local heat flux out of the mantle numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents causing lateral temperature variations at their base that parallel those in the mantle just below this tends to establish a constant flux of heat from the mantle into the continents which accounts for the trend in reduced heat flow

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

subduction zone metamorphism calc alkaline magmatism and convergent margin crustal evolution abstract at the dawn of the age of plate tectonics akiho miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages described contrasting geologic occurrences and inferred their characteristic ranges of physical conditions he advanced the paired metamorphic belt concept involving an oceanward narrow high p low t blueschist zone intimately intermixed with ophiolites and a landward broad low p high t realm associated with arc volcanic plutonic rocks more recent studies have illuminated the following 1 two main types of convergent plate junction can be distinguished pacific underflow of thousands of km of oceanic lithosphere and alpine closure of an intervening oceanic basin leading to continental collision 2 plate descent carries mafic felsic lithologies to depths of 35 120 km or more at relatively low temperatures producing high pressure hp pacific type and ultrahigh pressure uhp alpine type metamorphic terranes respectively 3 exhumed hp uhp complexes display low aggregate bulk densities reflecting chiefly buoyancy driven ascent of quartzofeldspathic allochthons 4 at a depth of 35 km in a warm lithosphere subduction zone sialic crust should melt if an aqueous fluid were abundant it does not hence the activity of h2o must be low at such depths 5 global observations indicate that volcanic plutonic arcs are sited 100 20 km above subducting oceanic plates devolatilization of sinking hydrated oceanic crust mantle promotes the generation of calc alkaline arc melts in this magmagenic zone 6 pacific type underflow of basaltic crust capped plates produces new and recycled continental crust whereas alpine type convergence reshuffles collided terranes but does not generate juvenile sialic crust

the ainsa fold and thrust oblique zone of the central pyrenees kinematics of a curved contractional system from paleomagnetic and structural data this work was funded by the project intectosal cgl2010 21968 c02 01 and by the grup de recerca de geodinamica i analisi de conques 2009sgr1198 secretaria d universitat i recerca del departament d economia i coneixement de la generalitat de catalunya josep poblet acknowledges financial support by research projects geociencias en iberia estudios integrados de topografia y evolucion 4d topo iberia csd2006 041 and desarrollo de fracturas y venas asociadas al plegamiento fraveple cgl2011 23628 funded by spanish ministries

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

creep and quakes on the northern transition zone of the san andreas fault from gps and insar data 1 the san juan bautista sjb segment forms the northern transition zone of the creeping section of the san andreas fault it is an area of moderate seismicity the largest instrumentally recorded earthquakes have been m5 5 however historic records suggest six m 6 earthquakes occurred near the sjb segment between 1840 and 1899 it is also an area that has experienced several slow earthquakes we perform a joint inversion of gps and insar data to determine the current rate and distribution of interseismic creep we find two low slip locked segments at mid seismogenic depths that may represent source regions for the 19th century earthquakes we find that the sjb segment is currently accumulating strain energy at the rate of one m6 3 6 7 earthquake per century at this rate the 1840 1899 sequence released 135 510 years of accumulated moment deficit

generation of long wavelength heterogeneity in the mantle by the dynamic interaction between plates and convection lateral variations in seismic velocity through its dependence on temperature can easily be generated at the gravest harmonics including degrees one and two by the dynamic interaction between plates and convection models of thermal convection with a single non subducting plate have been formulated in a cylindrical geometry plates of width one to four times times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath during rapid motion off of hot regions a large scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings both insulation and plume plume collisions pump energy into the lower wavenumber harmonics

effects of continental insulation and the partitioning of heat producing elements on the earth s heat loss 1 continental lithosphere influences heat loss by acting as a local insulator to the convecting mantle and by sequestering heat producing radioactive elements from the mantle continental heat production can have a two part effect since it decreases the amount of internal heat driving convection which lowers mantle temperature while also increasing the local insulating effect of continental lithosphere which raises mantle temperature we explored these competing effects using simulations that incorporated enriched continents within a mixed internal and bottom heated convecting mantle increasing continental surface area was found to enhance global heat loss for a range of heat production distributions and rayleigh numbers the effect of enriched continents was evident as a double peak in the continental surface area values that maximize global heat loss that the presence of continental lithosphere could increase average mantle temperature despite the mantle being depleted suggests that continents can significantly influence mantle potential temperature

generation of plumes under a localized high viscosity lid in 3 d spherical shell convection we conduct numerical simulations of incompressible infinite prandtl number convection in a spherical shell with a single localized high viscosity lid hvl on the top surface to understand the possible effects of the continental lithosphere on plume generation the temperature under the hvl increases rapidly after the emplacement of the hvl on constant viscosity convection with internal and bottom heatings later upwellings at the bottom merge into a large scale flow and a large plume emerges under the hvl this time scale depends on the existence of phase changes in our model whose rayleigh number is 10 6 the large plume is formed on a time scale of 1 gyr a simple scaling suggests that this time scale may be reduced by a several times if the rayleigh number is around 10 despite the complicated 3 d thermal structure it is dominated by an i 1 pattern controlled by the position of the hvl

mantle convection models of continental collision and breakup incorporating finite thickness plates abstract a two dimensional numerical model has been developed to study mantle convection during continental collision and breakup the model incorporates rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness we systematically investigate the influences of continental width diffusivity thickness and internal heating on continental collision and breakup in addition we consider the influence of different degrees of internal heating in the mantle for each case considered we model a pair of identical continents being carried towards a site of plate convergence by underlying counter rotating mantle convection cells the continents collide at the mid plane of the model to form a motionless rigid conducting supercontinent whereas oceanic plate material continues to recycle through the mantle we find that changes in the mechanical boundary conditions at the upper surface are important factors in initiating flow reversals below the supercontinent more generally our findings based on 144 models are that the following factors favour the initiation of flow reversals below the supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and continental crustal heating furthermore lower percentages of internal heating in the mantle are necessary to sustain and promote the subcontinental flow reversals in our models

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

structure of the deep crust beneath the central indian tectonic zone an integration of geophysical and xenolith data abstract xenoliths of garnet granulite and crustal eclogites in alkaline dykes from the northern margin of the dharwar craton suggest that the deep crust beneath this region is dominated by mafic granulite the granulites consist primarily of clinopyroxene orthopyroxene garnet and plagioclase clinopyroxene from most garnet bearing lithologies is eclogitic with jd ts 0 5 the jd component varies from 5 to 8 garnets from the granulites are similar to those in crustal eclogites whereas the ones from pyroxenites are akin to those in kimberlites composite xenoliths are indicative of interstratification of the lower crust by ultramafic rocks represented by pyroxenites and websterites that display prominent high density reflectors in deep seismic sounding profiles they represent cumulates in relict magma chambers and are expressed as conductors in magnetotelluric transects thermobarometric estimates for granulite xenoliths are 740 1094 c 0 71 1 3 gpa equivalent to depths of 20 40 km whereas those for pyroxenite xenoliths are 726 1042 c 0 57 1 05 gpa these provide for the first time a spatial context to these rock types suggesting derivation from lower crustal depths the lower crustal rocks beneath the central indian tectonic zone are near to their melting point as is evident from the anomalously high geothermal gradient due to which they show lower vps than those characteristic of typical garnet granulites the deep crust has undergone thickening both due to crustal inflation and under and intra plating beyond its original pre rift thickness overcompensating for the crustal thinning due to extension lateral flow of the lower crust has also contributed to crustal thickening in addition to under and intra plating transport of midcrustal rocks to the surface along ductile shear zones has caused additional crustal modification

relative timing of the storegga submarine slide methane release and climate change during the 8 2 ka cold event this paper highlights the similarity in the timing between the 8 2 ka cold event across the north atlantic region and one of the world s largest underwater slides the storegga submarine landslide that took place on the continental slope west of norway we argue on the basis of a reinterpretation of the age of tsunami deposits that date the slide as well as published ages of sediment directly resting upon the slide surface that the slide occurred between 8100 and 8200 cal yr near the end of the 8 2 ka cold event because sediment exposed at the base of the slide contained less methane gas hydrate c 8200 years ago than exists today and because greenland ice cores do not show an increase in methane at the time of the slide it is argued here that the slide did not release significant volumes of methane to the atmosphere and did not contribute to any change in temperature during or after the 8 2 ka cold event

organization of the tectonic plates in the last 200 myr the present tessellation of the earth s surface into tectonic plates displays a remarkably regular plate size distribution described by either one sornette and pisarenko 2003 or two bird 2003 statistically distinct groups characterised by large and small plate size a unique distribution implies a hierarchical structure from the largest to the smallest plate alternatively two distributions indicate distinct evolutionary laws for large and small plates the first tied to mantle flow the second determined by a hierarchical fragmentation process we analyse detailed reconstructions of plate boundaries during the last 200 myr and find that i large and small plates display distinct statistical distributions ii the small plates display little organisational change since 60 ma and iii the large plates oscillate between heterogeneous 200 170 myr and 65 50 ma and homogeneous 120 100 ma plate tessellations on a timescale of about 100 myr heterogeneous states are reached more rapidly while the plate configuration decays into homogeneous states following a slower asymptotic curve suggesting that heterogeneous configurations are excited states while homogeneous tessellations are equilibrium states we explain this evolution by proposing a model that alternates between bottom and top driven earth dynamics physically described by fluid dynamic analogies the rayleigh benard and benard marangoni convection respectively we discuss the implications for true polar wander tpw global kinematic reorganisations 50 and 100 ma and the earth s magnetic field inversion frequency

the timescales of plume generation caused by continental aggregation abstract to understand the thermal evolution of the mantle following the aggregation of non subductable thick continental lithosphere we study a numerical model in which a supercontinent simulated by high viscosity raft hvr covers a part of the top surface of a convection layer we model infinite prandtl number convection either in a three dimensional 3d spherical shell 3d rectangular box aspect ratios 8 and 4 or two dimensional 2d rectangular box aspect ratio 8 and except for the hvr we specify a constant viscosity the hvr which has a viscosity higher than that of its surrounding is instantaneously placed on the top surface of a well developed convection layer and its position is fixed our results from 3d spherical shell cases with and without phase transitions show the emergence of a large plume characterized by a long wavelength thermal anomaly a degree one pattern for a pangea like geometry we analyze the volume averaged temperature under the hvr t c the remaining oceanic area t o and total area t m to determine the timescale of plume generation the difference between t c and t o t co and t m show the existence of two characteristic timescales t co exhibits an initial rapid increase and may become constant or continue to gradually increase meanwhile t m shows a similar behavior but with a longer timescale we find that these timescales associated with the increase of t co and t m can be attributed to the formation of large scale flow i e plume and response of the whole system to the emplacement of the hvr respectively for 3d spherical cases we find that the timescale of plume generation is 1 2 gyr if the rayleigh number is 10 6 to determine the effects of the viscosity of the hvr 2d versus 3d modeling and the effects of the internal heating we have also studied 2d and 3d rectangular box cases a factor of about two variation exists in the timescale of plume generation it appears that the timescale becomes greater for a smaller amount of internal heating this may be attributed to the time dependent flow caused by the internal heating for 2d cases we find that the timescale of the high rayleigh number 10 7 case is shortened by a factor of three to five when compared to the ra 10 6 case which is consistent with the simple boundary layer theory this may imply that well developed plumes may arise with the timescale of 0 2 to 0 4 gyr for ra 10 7

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

supercontinent cycles disrupted by strong mantle plumes a theoretical basis for the regularity of supercontinent cycles is lacking here we show that periodic supercontinent cycles are unlikely if thermal instabilities originating at the core mantle boundary are of sufficient strength we couple multiple mobile continents with vigorous mantle convection in a spherical geometry regular supercontinent cycles lasting 400 50 m y occur in idealized models with three continents and a mantle heated purely from within by radioactive decay in a model incorporating six continents and strong mantle plumes this regularity is broken and supercontinents form only sporadically our results suggest that periodic supercontinent cycles are unlikely to occur in realistic earth models

the early geodynamic evolution of mars type planets abstract it is not clear whether mars once possessed active tectonics yet the question is critical for understanding the thermal evolution of mars and the origin and longevity of its early dynamo to address these issues we have coupled mantle flow simulations together with parameterized core evolution models to simulate the early evolution of mars like planets and constrain the influence of early mobile lid tectonics on core evolution we have explored a wide parameter suite encapsulating a range of uncertainties in initial conditions rheological parameters and surface strength we present successful models that experience early mobile lid behaviour with a later transition into a stagnant lid mode which reproduce core dynamo histories similar to the magnetic history of early mars

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

the continental drift convection cell continents on earth periodically assemble to form supercontinents and then break up again into smaller continental blocks the wilson cycle previous highly developed numerical models incorporate fixed continents while others indicate that continent movement modulates flow our simplified numerical model suggests that continental drift is fundamental a thermally insulating continent is anchored at its center to mantle flow on an otherwise stress free surface for infinite prandtl number cellular convection with constant material properties rayleigh numbers exceed 107 while continent widths and chamber lengths approach earth s values the wilson cycle is reproduced by a unique rugged monopolar continental drift convection cell subduction occurs at the cell s upstream end with cold slabs dipping at an angle beneath the moving continent as found in many continent subduction regions on earth drift enhances vertical heat transport up to 30 especially at the core mantle boundary and greatly decreases lateral mantle temperature differences

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

heterogeneity and time dependence in 3d spherical mantle convection models with continental drift abstract feedback between continents and large scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and wilson cycles paleomagnetism provides evidence for extensive continental displacements 10 000 km on time scales of 100 200 million years comparable to an intrinsic overturn in whole mantle convection here we model continental motions in vigorous 3d spherical convection models focusing on the effects of continent size mantle heating mode and a strong increase in lower mantle viscosity continents covering 30 10 and 3 of earth s surface representative of the former supercontinent pangea present day asia and antarctica respectively are introduced into simple end member mantle convection models characterized by pure core or internal heating and uniform or layered mantle viscosity supercontinents promote temperature anomalies on the largest scales spherical harmonic degrees 1 and 2 primarily through the organization of the long wavelength convective planform inherent in models with a high viscosity lower mantle bottom heating can promote long wavelength heterogeneity by clustering plumes beneath the continent however in isoviscous models small scale structure persists away from the continent regardless of the heating mode supercontinents respond to long wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years smaller continents are unable to promote long wavelength structure and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years continental velocities are roughly a factor of 3 smaller than those in oceanic regions an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates

dynamic feedback between a continentlike raft and thermal convection seismic observations of the mantle which include long wavelength structure a k 1 dependence of heterogeneity on harmonic k and a heterogeneous upper boundary layer and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection we have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry the azimuthal interconnectivity of this geometry is vital to resolve the two way dynamics between rafts and convection computations show that 1 raft motion is periodic 2 long wavelength thermal structure is significant within both thermal boundary layers and the fluid interior and 3 the large scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion these three results which are observed for a range of rayleigh numbers internal heating rates and raft sizes are a direct consequence of the dynamic interaction between the raft and convection the physical processes for a model with a rayleigh number of 10 5 are representative when the raft is stationary due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer heat accumulates beneath the raft and results in long wavelength thermal anomalies the long wavelength thermal anomalies enhance raft motion accompanying the enhanced raft movement the long wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest since convection models without rafts generate less long wavelength heterogeneity compared to the models with rafts or continental plates we suspect that continental plates may play a crucial role in mantle dynamics interestingly raft motion with a period of about 10 transit times is usually significant 10 transit times is about 600 m y if scaled to the earth this is close to the observed 300 500 m y period of supercontinent aggregation and dispersal

insulation and depletion due to thickened crust effects on melt production on mars and earth 1 the growth of thickened crustal units both depletes the mantle of heat producing elements and thermally insulates it these effects either cool or heat the mantle respectively and play an important role in subsequent crustal formation we employ mantle convection models including thickened crustal units of variable heat production to show that for increasing crustal extent mantle temperatures and melt production can either increase of decrease depending on the degree of enrichment of the crust with respect to the mantle the total heat production and the rayleigh number the formation of the continents on earth efficiently cooled the upper mantle resulting in lower subsequent rates of melt production and continental formation in contrast the growth of the martian highlands would have raised the temperatures of the martian mantle increasing rates of melt production and leading to runaway crustal growth this would have continued as long as the lithosphere of mars was mobile

a regional 8200 cal yr bp cooling event in northwest europe induced by final stages of the laurentide ice sheet deglaciation the most notable change in 18o in greenland ice cores during the holocene occurs at 8200 cal yr bp here we present a new high resolution marine record from the northern north sea along with tree ring data from germany which contain evidence of a pronounced temperature drop 2 c contemporaneous with that of the greenland ice core records the synchronous timing of the cooling event in the greenland ice cores marine record and tree ring data from northwest europe reflects a regional influence on the north atlantic ocean atmospheric system suggesting a prominent role of the north atlantic thermohaline circulation the operation of the north atlantic ocean circulation is sensitive to variation in the freshwater budget implying that any change in freshwater flux is capable of altering the north atlantic circulation system we hypothesise minor but long term freshwater fluxes in the final stages of the deglaciation of the laurentide ice sheet as a forcing mechanism 1998 john wiley sons ltd

effects of mantle heat source distribution on supercontinent stability a two dimensional cartesian geometry numerical model is used to study the influence of internal heating on the ability of mantle convection to rift and disperse an assembled supercontinent our wide aspect ratio models incorporate migrating plate boundaries and finite thickness rigid mobile oceanic and continental plates 12 000 km across we examine models characterized by both whole mantle and upper mantle convection parameters specifying different degrees of internal heating in the mantle we compare the patterns of convection that develop below stationary continental plates that form as a result of the aggregation of smaller mobile continents following continental collisions we find that a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere which are capable of initiating continental breakup our results suggest that continental basal stresses are largely controlled by the wavelength of the subcontinental convection pattern and that the inclusion of internal heating in the mantle generally increases this wavelength for models with 80 internal heating in which active upwellings are intrinsically absent basal stresses arise primarily from subduction at the continental margins and result in a geologically reasonable duration for our model supercontinent assemblages 200 myr for models with 40 or less internal heating continental breakup is controlled by mantle upwellings and our supercontinents remain intact much longer 600 700 myr

the impact of rayleigh number on assessing the significance of supercontinent insulation several processes unfold during the supercontinent cycle more than one of which might result in an elevation in subcontinental mantle temperatures thus multiple interpretations of the concept of continental insulation exist although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones there are differing views on what role continental insulation plays in the production of elevated mantle temperatures here we investigate how the heating mode of the mantle can change the influence of the thermal blanket effect we present 2 d and 3 d cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates the evolution of mantle thermal structure is examined after continental accretion at subduction zones e g the formation of pangea for a variety of different mantle heating modes our results show that in low rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases when compared to models with higher convective vigor broad hot upper mantle features generated in low rayleigh number models due in part to the thermal blanket effect are absent at higher rayleigh numbers we find that subcontinental heating in a high rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin rather than the influence of continental insulation in our models featuring earth like convective vigor we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation

influence of supercontinents on deep mantle flow abstract the assembly of supercontinents should impact mantle flow fields significantly affecting the distribution of subduction upwelling plumes lower mantle chemical heterogeneities and thus plausibly contributing to voluminous volcanism that is often associated with their breakup alternative explanations for this volcanism include insulation by the continent and thus elevated subcontinental mantle temperatures here we model the thermal and dynamic impact of supercontinents on earth like mobile lid convecting systems we confirm that insulating supercontinents over 3000 km extent can impact mantle temperatures but show the scale of temperature anomaly is significantly less for systems with strongly temperature dependent viscosities and mobile continents additionally for continents over 8000 km mantle temperatures are modulated by the development of small scale convecting systems under the continent which arise due to inefficient lateral convection of heat at these scales we demonstrate a statistically robust association between rising plumes supercontinental interiors for a variety of continental configurations driven largely by the tendency of subducting slabs to lock onto continental margins the distribution of slabs also affects the spatial positioning of deep mantle thermochemical anomalies which demonstrate stable configurations in either the sub supercontinent or intraoceanic domains we find externally forced rifting scenarios unable to generate significant melt rates and thus the ultimate cause of supercontinent breakup related volcanism is probably related to dynamic continental rifting in response to mantle reconfiguration events

interaction between the supercontinent cycle and the evolution of intrinsically dense provinces in the deep mantle shear wave travel times in the earth s deep mantle reveal broad steep sided seismologically distinct provinces lying on the core mantle boundary cmb the longevity and permanence of the two large principal provinces located below the sites of present day africa and the pacific ocean have become a matter of great interest examination of the flood basalt record and kimberlite eruption dating suggests the presence of these provinces may disclose a deep mantle component with a compositionally distinct origin that plays a role in the generation of mantle plumes at preferred locations by extension the presence of these provinces may affect the supercontinent cycle implementing a mantle convection model featuring distinct continental lithosphere and a compositionally anomalous and intrinsically dense caid component we study the distribution and mobility of naturally forming compositionally distinct provinces and their impact on model supercontinent assembly we employ 2d cartesian geometry calculations of thermo chemical convection with extremely low compositional diffusion to model earth like convective vigor on a global scale and find that an intrinsically dense mantle component generally aggregates into one or two broad provinces the positions of the provinces are time dependent but in many of our calculations the province locations are characterized by periods of fixity that reach several hundred million years eras of province and associated plume fixity are punctuated by periods of relatively rapid migration a correlation between supercontinent position and the locations of caid provinces is not supported by our findings however we find the frequency of supercontinent assemblies increases when caid provinces are present

influences on the positioning of mantle plumes following supercontinent formation several mantle convection studies analyzing the effects of supercontinent formation and dispersal show that the genesis of subcontinental plumes results from the formation of subduction zones at the edges of the supercontinent rather than from the effect of continental thermal insulation or thermochemical piles however the influence of subduction zone location on the position of subcontinental plumes has received little attention this study analyzes 2 d and 3 d numerical models of supercontinent formation in an isochemical mantle to assess the role of subduction and mantle viscosity contrast in the generation of subcontinental mantle plumes we find that once a critical supercontinent width is reached plumes do not form under the center of a supercontinent in studies featuring a low viscosity lower mantle the surface positions of the initial plumes arriving within 90 myr of supercontinent assembly become locked beneath the continent at a distance 2000 3000 km from the continental margin however the broad downwellings in simulations that feature a high viscosity lower mantle trigger plumes at a greater distance from the continental margin subduction for all mantle viscosity profiles subcontinental plumes show dependence on the location of supercontinent margin subduction as theories differ on the role of core mantle boundary chemical piles in plume formation it is significant that our isochemical models show that the formation of subduction zones at the margins of a supercontinent has a profound effect on subcontinental mantle dynamics our results may help to explain what determined the eruption sites of past and future large igneous provinces

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

methane gas release from the storegga submarine landslide linked to earlyholocene climate change a speculative hypothesis the 3000 km3 storegga submarine landslide occurred c 8150 years ago from an area rich in gas hydrates off the western coast of norway a synchronous increase of 80 100 ppb in atmospheric methane c

the storegga slides evidence from eastern scotland for a possible tsunami abstract the second storegga slide on the continental slope off western norway has been dated at between 8000 and 5000 yrs b p a prominent sand layer in flandrian holocene deposits along the coast of eastern scotland and dated at approximately 7000 yrs b p may have been deposited by a tsunami generated by the slide the altitude and stratigraphy of the layer allow estimates to be made of the magnitude of the earthquake which initiated the slide

assessing methane release from the colossal storegga submarine landslide 1 marine slope failure involving methane gas hydrate bearing sediments is one mechanism for releasing enormous quantities of methane to the ocean and atmosphere the storegga slide on the norwegian margin is the largest known holocene aged continental margin slope failure complex and is believed to have occurred in sediments that may have initially contained gas hydrate here we report pore water sulfate gradient measurements that are used as a proxy for the relative amounts of methane that exist in continental margin sediments associated with the colossal storegga slide these measurements suggest that a considerable inventory of methane occurs in sediments adjacent to and unaffected by the storegga slide events but indicate that methane is notably absent from sediments on the sole of the slide and distal deposits created by the slide events either methane was lost during previous pleistocene failure events or was never present in significant concentrations within the sediments that failed

a continuous record of temperature evolution over a sequence of dansgaard oeschger events during marine isotopic stage 4 76 to 62 kyr bp 1 our knowledge of the temperature evolution over greenland during dansgaard oeschger events do is currently qualitatively described through the water isotopic profile using two independent paleothermometry methods one based on air isotopic measurements and the other on the combined measurements of water isotopes dd and d 18 o we show a complete and quantitative reconstruction of temperature at the northgrip site over the period 76 to 62 kyr bp do 18 19 and 20 we confirm that the associated warmings are larger than those conventionally depicted by the water isotopes 11 c 16 c and 11 c for do 18 19 and 20 secondly we demonstrate that the relationship between temperature and d 18 o varies rapidly during the last glacial period even over a do finally our temperature reconstruction over do 19 agrees well with that predicted from simple climate models linking the do to iceberg discharges 2004 a continuous record of temperature evolution over a sequence of dansgaard oeschger events during marine isotopic stage 4 76 to 62 kyr bp geophys res lett 31 l22211 doi 10 1029 2004gl021193 2 the last glacial cycle was characterized by millenial scale climate fluctuations that have been documented in the north atlantic region through numerous marine and continental records bond et al 1997 genty et al 2003 the grip and gisp2 summit ice cores dansgaard et al 1993 grootes et al 1993 and the newly northgrip northgrip members 2004 ice core exhibit 25 do during the last glacial period these events are characterized by rapid e g in less than 100 years and large up to 16 c lang et al 1999 warmings over greenland 3 ice cores have already provided a wealth of information on do in greenland through the water isotopes for temperature changes chemical records for atmospheric circulation and analysis of air bubbles for changes in greenhouse gases concentration however temperature reconstruction from water isotopes is subject to large biases mainly due to i the seasonality of the precipitation i e periods without precipitation will not have their temperature recorded fawcett et al 1997 and ii changes in the oceanic source of greenland snow boyle 1997 whereas the latter can be estimated from the combined measurement of dd and d 18 o through the deuterium excess parameter d dd a 8 d 18 o we need additional information to account for the influence of seasonality and possible variation of the vertical atmospheric temperature profile the isotopic composition of the snow depends on the condensation temperature 4 one elegant way to overcome those difficulties is to use the isotopic composition of the air trapped in ice a method based on the thermal diffusion of gases nitrogen and argon which allows estimates of the amplitude of rapid temperature increases severinghaus and brook 1999 lang et al 1999 leuenberger et al 1999 this method gives directly access to the local mean surface temperature and is therefore not affected by seasonality the vertical temperature profile in the atmosphere nor the source temperature of the precipitation it confirms that the conventional temperature to water isotopes relationship based on the spatial slope a s the dd 18 o ice temperature slope calculated from present day surface data underestimates greenland temperature change cuffey et al 1995 the temporal slope a t dd 18 o ice dt at a given site between two different climates is lower than a s by up to a factor of 2 we have recently refined this approach using a sophisticated firnification and heat diffusion model goujon et al 2003 to quantify the temperature increase associated with large do 12 landais et al 2004 5 here we go beyond the simple quantification of the temperature increase a detailed set of d 15 n and d 40 ar measurements over do 18 19 and 20 provides us with strong constraints on the complete temperature scenario between 76 and 62 kyr bp this period roughly corresponds to marine isotope stage 4 with rapid ice sheet growth shackleton 1987 we have also measured a continuous dd profile in the ice that combined with the existing d 18 o record allows us to account for the source temperature effects this complete study makes possible the precise estimation of the temperature change over a sequence of do 6 the d 15 n and d 40 ar profiles figure 1 show a sharp peak corresponding to each warming rapid surface warm geophysical research letters vol 31 l22211