late pleistocene sea level change in the bahamas from mass spectrometric u series dating of submerged speleothem abstract the dating of sea level events by a method independent of orbital tuning techniques and the marine foraminiferal isotopic record provides a test of both the dating of the marine record and the strength and nature of the relationship between climatic change sea level change and milankovitch cycles dating of fossil corals and submerged speleothem provides such independent information results of thermal ionization mass spectrometric u series dating on layers of flowstone speleothem from lucayan caverns grand bahama island indicate that multiple sea level events have passed through a depth of 10 to 15 m below modern sea level during the last 300 000 years these data are used to constrain the magnitude and timing of sea level fluctuations during the late pleistocene the new sea level curve constructed from these data correlates closely with the marine isotopic record sea level peaks occurred at around 233 215 125 and 100 ka bp there is no evidence at the depth from which this speleothem was retrieved of any high sea stands at 195 or 80 ka corresponding to the foraminiferal 7a and 5a oxygen isotope peaks there is also no evidence of a double rise around the 125 ka 5e peak although such an oscillation could have occurred above 10 m leaving no record in the lucayan caverns speleothem

plate generation in a simple model of lithosphere mantle flow with dynamic self lubrication one of the more enigmatic features of the earth s style of mantle convection is plate tectonics itself in particular the existence of strike slip or toroidal motion toroidal motion is uncharacteristic of basic thermal convection but necessarily forms through the interaction of convective flow and nonlinear rheological mechanisms recent studies have implied that the empirically determined power law rheologies of mantle silicates are not sufficient to generate the requisite toroidal motion a simple source sink model of mantle or lithospheric flow shows that dynamic self lubrication which arises through the coupling of viscous heating and temperature dependent viscosity is highly successful at generating strike slip motion in particular as the viscosity of the fluid system becomes more temperature dependent the toroidal flow field makes an abrupt transition from a state of weak unplate like motion to a state with intense and extremely focused structure in essence the fluid dynamical model develops strike slip faults

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

sea level during the penultimate interglacial period based on a submerged stalagmite from argentarola cave italy accurate constraints are presented for the absolute age duration and sea level for the highstand corresponding to marine isotope stage mis 7 1 our work is based on a stalagmite collected in a currently submerged cave from italy argentarola cave flooding of this coastal cave is clearly marked within the stalagmite by a thick marine overgrowth of serpulid worms precise uranium thorium u th ages measured by thermal ionization mass spectrometry suggest that the stalagmite growth varied in phase with climate during mis 7 2 and mis 6 the penultimate glacial period in addition the period of marine transgression is bracketed between 202 and 190 kyr bp defining a mis 7 1 duration of 11 2 kyr comparison with previous studies provides a new constraint on the sea level during mis 7 1 that has approximately remained between 318 and 39 m relative to present sea level the timing of the mis 7 1 highstand is in full agreement with the specmap chronology which contrasts with the growing body of evidence showing a significant phase difference for mis 5 5 a tempting conclusion is that second order sea level highstands such as mis 7 1 may be purely driven by astronomical changes in contrast with major terminations such as termination ii atmospheric co2 levels may partly explain this contrast since the two sea level transitions mis 7 2 7 1 and mis 6 5 5 are characterized by very different co2 rises 20 vs 100 ppm fl 2002 elsevier science b v all rights reserved

compressible convection with constant and variable viscosity the effect on slab formation geoid and topography two dimensional cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented density variations according to the adams williamson equation of state are included in the case of constant viscosity convection viscous and adiabatic heating damp the flow compressible density variations hardly influence geoid undulations however the lower thermal boundary layer becomes thinner and the mean cell temperature increases in the case of variable viscosity the nonlinear coupling between compression adiabatic and viscous heating and a temperature pressure and stress dependent rheology leads to important consequences the upwelling flow broadens and plumes are retarded the flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low viscosity region this mechanism seems to be important for the formation of subducting slabs extrapolated to mantle conditions two low viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle this process might be aided by an increase of negative buoyancy forces with depth as observed in the models further results are as follows increasing the dissipation number in variable viscosity convection may either damp or speed up convection depending on the rheology models with internal heating and a fixed bottom temperature show that the threshold to time dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended boussinesq approximation

speleothem palynology an undervalued tool in quaternary studies palynological analysis of cave sediments in particular the analysis of pollen from speleothems is not a widely adopted approach to palaeoenvironmental studies in the british isles this disregard of speleopalynology may be due to the lack of understanding of the cave pollen taphonomy and uncertainty and suspicion as to what cave pollen spectra actually represent however a number of studies of pollen from caves have been carried out particularly in belgium france and spain which have demonstrated speleothem pollen spectra to be representative of contemporary vegetation above the cave with pollen generally well preserved speleothems have the distinct advantage over other pollen sources that they can be dated by high precision tims u th dating up to an order of magnitude further back in time than 14c dating of organic sediments would allow this potentially allows previously unconstrained middle pleistocene palynological records to be precisely and accurately dated thus providing a geochronologically controlled terrestrial record of palaeoenvironmental change this paper presents a timely review of speleothem palynology aiming to eliminate biases and misconceptions and illustrate the enormous potential this unique source of palaeoenvironmental data holds particularly in providing accurately dated pollen records from previously understudied environments and time periods

intense local toroidal motion generated by variable viscosity compressible convection in 3 d spherical shell strong dynamical effects arising from lateral viscosity variations are found near the heads of fast upwelling plumes approaching the top boundary for unsteady 3 d compressible convection in a spherical geometry at an average rayleigh number of o 106 the temperatures at the center of fast upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature dependence of the viscosity although the laterally averaged ratio of toroidal to poloidal energy varies only around 10 the local toroidal velocities can reach about 40 of the poloidal velocities the strongest toroidal motions always take place right above or close to the fast upwelling plume heads and behave as vortical flows with time this finding may explain the origins of microplate rotation in regions with hotspot activity e g easter island which shows no direct relation to the transform faulting system near the mid ocean ridge

melting instabilities in the transition zone abstract melting in the transition zone could result from the rapid passage of hot upwelling through the perovskite spinel phase transition a large temperature increase could be produced above the perovskite to spinel transition by both viscous heating and a nonlinear positive feedback between temperature dependent viscosity and latent heat release the rise times for these instabilities have been estimated to be about 10 ma these instabilities were triggered by the episodic downwelling instabilities which preceded the flush events of cold material in layered mantle convection with phase transitions these results would suggest that melting in the transition zone would have had a greater tendency to occur in the past and might have caused the generation of komatiites in the archean

dynamical consequences on fast subducting slabs from a self regulating mechanism due to viscous heating in variable viscosity convection the authors have studied 2 d time dependent convection for a rheology which is both non newtonian and temperature dependent strong effects associated with viscous heating are found in the downwelling sheets which are heated on both sides with an intensity around o 10 sup 2 times the chondritic value the magnitude of viscous heating increases strongly with the subduction speed the slab interior is weakened by viscous heating and slab breakoff then takes place this process provides a self regulating mechanism for governing the speed of intact slabs able to reach the deep mantle timescales associated with viscous heating are quite short a few million years internal heating by radioactivity decreases the amount of shear heating 13 refs 5 figs

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

palaeoclimate determination from cave calcite deposits calcite deposits formed in limestone caves have been found to be an excellent repository of palaeoclimatic data for terrestrial environments the very presence of a relict deposit indicates non glacial conditions at the time of formation and both 14c and uranium series methods can be used to date the deposit and hence the age of these climatic conditions variations in 13c and 18o content of the calcite in 2h and 18o content of fluid inclusions in trace element concentrations and more recently in pollen assemblages trapped in the calcite are all potentially available as synchronous palaeoclimatic indicators previous work has tended to concentrate mainly on abundance of deposits as a palaeoclimatic indicator for the last 300 000 years this literature is briefly reviewed here together with the theory and methods of analysis of the u series and stable isotopic techniques the combined use of u series ages and 13c and 18o variations in cave calcites illustrates the potential for palaeoclimate determination previously unpublished results of stable isotopic variations in dated calcites from caves in northern england indicate the level of detail of stable isotopic variations and time resolution that can be obtained and the complexity of interpretation that may arise tentative palaeoclimatic signals for the periods 90 125 ka and 170 300 ka are presented more comprehensive studies are needed in future work especially in view of the difficulty in obtaining suitable deposits and the ethics of cave deposits conservation

thermomechanical modeling of pulsation tectonics and consequences on lithospheric dynamics pulsating phenomena can control continental breakup and the formation and closing of oceanic basins using a two dimensional time dependent convection model with a non linear temperature and depth dependent rheology we have obtained self consistently such types of pulsating behavior very hot mantle plumes ascend to the surface where they can quickly thin the lithosphere down to a few kilometers an episode of intense shear heating then ensues which is comparable to the length of the main episode for flood volcanism lasting a couple of myrs the highest plate velocities occur during this period when both the lithosphere and slab are profusely lubricated by the impingement of the hot plume

dislocation recovery in olivine under deep upper mantle conditions implications for creep and diffusion the kinetics of dislocation recovery in olivine have been studied up to 10 gpa and 1500 c using a multianvil apparatus dislocations in the 010 100 slip system were generated by high temperature deformation at room pressure and then the specimens were statically annealed at high pressures and temperatures the rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion a small activation volume v 6 1 cm3 mol was obtained which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport limiting species oxygen or silicon combined with the previous observation of a small activation volume for mg fe diffusion we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle we have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters such as stress exponents grain size exponents and activation volume for dislocation creep the results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

the role of recrystallization in the preferred orientation of olivine abstract the effects of recrystallization on the preferred orientation in olivine have been studied the recrystallization mechanisms studied include grain growth the deformation induced grain boundary migration digbm and subgrain rotation sgr grain boundary migration is involved in the first two but not in sgr significant preferred orientation was found only in the latter two where dislocations are involved the combined microstructural and fabric preferred orientation observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force the nature of preferred orientation due to dislocation related recrystallization digbm or sgr was found to depend on the mechanism of recrystallization the digbm produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate thereby significantly altering the preferred orientation formed by dislocation glide in contrast sgr does not essentially alter the deformation fabric and therefore the fabric associated with sgr recrystallization is related to the geometry of flow the present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism s of deformation and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the earth s interior where the dominant mechanism s of dynamic recrystallization can be different

dependence of creep in olivine on homologous temperature and its implications for flow in the mantle creep data for olivine when normalized by the melting temperature can be extrapolated directly to the pressure temperature and chemical environment of the upper mantle by using the appropriate solidus and geotherms the data presented here collected over a broad pressure range predict effective viscosities that are consistent with geophysical and tectonic constraints for a dry upper mantle these systematics suggest that the effective viscosity of the mantle has risen over the age of the earth as outgassing has progressively raised the solidus

modelling planetary dynamics by using the temperature at the core mantle boundary as a control variable effects of rheological layering on mantle heat transport abstract in planetary convection there has been a great emphasis laid on the usage of the rayleigh number as a control parameter for describing the vigor of convection however realistic mantle rheology not only depends on temperature pressure strain rate and composition but also on the nature of the dominant creep mechanism which varies with pressure and also with temperature it is difficult to study the effects of varying influences from the convective strength without also changing the mantle flow law in the process we have adopted the approach of using as the sole control parameter the temperature at the core mantle boundary t cmb in modelling planetary dynamics with a composite non newtonian and newtonian rheology which is temperature dependent in the upper mantle and both temperature and pressure dependent in the lower mantle increasing the t cmb strengthens convective vigor and leads to a non linear increase of averaged temperature heat flow and root mean squared velocity the interior viscosity decreases strongly with t cmb and internal heating due to radioactivity a viscosity maximum is found in the horizontally averaged viscosity profile at a depth around 2000 km this viscosity hill moves downward with diminishing amplitude in the face of increasing dissipation number and internal heating the bottom third of the lower mantle appears to be superadiabatic as a consequence of the stiff lower mantle rheology the scaling relationship between the nusselt nu number and t cmb shows a relatively insensitive increase of nu with t cmb in terms of an effective rayleigh number of the whole system ra e the power law exponent of the nu ra e relationship is very low around 0 12 strong pressure dependence of lower mantle rheology and its large volume relative to the entire mantle would induce a much lower cooling rate of the planet than previous models based on parameterized convection with a temperature dependent viscosity

high temperature creep of olivine single crystals 2 dislocation structures dislocation structures and densities in experimentally deformed single crystals of san carlos olivine were examined using the oxidation decoration technique the high temperature deformation experiments were conducted at various temperatures t differential stresses and oxygen fugacities o2 the samples were buffered against either orthopyroxene opx or magnesiowustite mw solid state powders and compressed along one of the three 45 orientations in samples that were deformed and subsequently quenched under load seven distinct dislocation structures were observed a for samples compressed parallel to 110 c at 16 50 mpa and o2 10 6atm two different dislocation arrangements were identified for opx buffered samples at 1400 t 1475 c the dislocation structure was composed of curved screw segments that had usually cross slipped and straight edge segments of comparable length at 1300 t 1350 c for opx buffered samples and at 1350 t 1500 c for mw buffered samples the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations b for both opx buffered and mw buffered samples compressed parallel to 101 c at 1340 t 1400 c and 30 60 mpa the dislocation configurations are characterized by zigzag near edge dislocations at o2 10 5atm and by straight cross slipped screw dislocations plus pinned edge dislocations at o2 10 9atm c for samples compressed parallel to 011 c at t 1400 c and 70 150mpa the primary dislocations change from long straight edge dislocations for o2 10 4atm and opx buffer to a combination of straight edge dislocations and polygon shaped half loops for o2 10 4atm and mw buffer to gently curved near edge dislocations for o2 10 9atm for both solid state buffers abundant long straight screw dislocations were present in each case for the 011 c samples all of the above variations in dislocation structure are paralleled by changes in the measured power law equation that describes the behavior of creep mechanisms of olivine thus the different dislocation structures are associated with different rate controlling creep mechanisms for all of the dislocation structures the dislocation density increases with differential stress according to the relation 1 4 dislocation structures in samples that were deformed and then statically annealed have also been studied for both 110 c and 011 c samples deformed over a wide range of experimental conditions the dislocation structures on the slip planes generally consist of short curved edge or mixed dislocation segments in 101 1 samples deformed at low o2 50 mpa and 1400 c the dislocation structure was much less altered by annealing than the dislocation structures in 110 c and 011 c samples the density of dislocations decreased much less for these 101 c samples than for the 110 c and 011 c samples due to climb of the edge segments the straight screw segments found in quenched samples became curved and many low angle 100 tilt boundaries connected by straight 100 screw dislocations developed during the annealing process

is the lower mantle rheology newtonian today the issue concerning the dominant creep mechanism in mantle convection has been studied numerically with a rheology incorporating both linear and non linear elements we have employed a rheology suggested by the recently obtained melting temperature of lower mantle constituents for an effective rayleigh number between 105 and 106 this type of strongly temperature and pressure dependent rheology induces a circulation in the earth s mantle which is characterized by a sluggish type of flow in the lower mantle with a prevailing newtonian character while in the upper mantle much more vigorous flows are developed in a predominantly non newtonian rheology

rapid conversion of elastic energy into plastic shear heating during incipient necking of the lithosphere an important and novel mechanism for ductile failure of the lithosphere is identified here which is intrinsic to the thermal mechanical feedback in a temperature dependent plastic body with coupled elastic fields both a temperature dependent power law visco elasto plastic rheology and a temperature dependent elasto plastic rheology are employed to study in a self consistent fashion the deformation of the lithosphere subject to extension by means of a two dimensional finite element code a structural perturbation initially localizes elasto plastic deformation only in its immediate vicinity however after 800 000 years have elapsed the localized zone of deformation takes off in a crack like fashion and travels to the bottom of the lithosphere in about 50 000 years time when the plate is severed thermal runaway is caused by mechanical heating triggered by the rapid energy transfer of the globally stored elastic energy into localized plastic dissipation in the ductile fault

the role of shear heating in lubricating mantle flow abstract the role of viscous heating in mantle flows has been studied within the framework of a steady state two dimensional cartesian model with a composite non newtonian and newtonian rheology there is temperature dependence in the upper mantle rheology and temperature and pressure dependent rheology in the lower mantle we have considered adiabatic and viscous heating using the extended boussinesq approximation we have selected as the only control variable the temperature at the core mantle boundary cmb we have compared systematically both the boussinesq and extended boussinesq solutions for the same rheology and same amount of internal heating by varying the temperature at the cmb from 2000 k to around 10 000 k viscosity contrasts in the system increase with the vigor of convection and reach up to five orders in magnitude both the vigor of convection and the global viscous heating increases systematically with the temperature at the cmb the rms velocity shows a power law type dependence on the temperature at the cmb with a power law exponent of 1 5 for high cmb temperatures greater then 3500 k a focused concentration of shear heating promotes the development of high strain rate and low viscosity zones next to descending flows which is primarily non newtonian in character in the high cmb temperature regime much more pronounced shear zones are developed along the downwelling for models with viscous heating present than for the boussinesq flows illustrating the important role played by mechanical heating on lubricating limbs in mantle circulation

geochronology and subsurface stratigraphy of pukapuka and rakahanga atolls cook islands late quaternary reef growth and sea level history eustatic sea level cycles superposed on thermal subsidence of an atoll produce layers of high sea level reefs separated by erosional unconformities coral samples from these reefs from cores drilled to 50 m beneath the lagoons of pukapuka and rakahanga atolls northern cook islands give electron spin resonance esr and u series ages ranging from the holocene to 600 000 yr b p subgroups of these ages and the stratigraphic position of their bounding unconformities define at least 5 periods of reef growth and high sea level 0 9000 yr b p 125 000 180 000 yr b p 180 000 230 000 yr b p 300 000 460 000 yr b p 460 000 650 000 yr b p only two ages fall within error of the last interglacial high sea level stand 125 000 135 000 yr b p this paucity of ages may result from extensive erosion of the last intergracial reef in addition post depositional isotope exchange may have altered the time ages of three coral samples to apparent ages that fall within glacial stage 6 for the record to be preserved vertical accretion during rising sea level must compensate for surface lowering from erosion during sea level lowstands and subsidence of the atoll erosion rates 6 63 cm 1000 yr can therefore be calculated from reef accretion rates 100 400 cm 1000 yr subsidence rates 2 6 cm 1000 yr and the duration of island submergence 8 15 of the last 600 000 yr the stratigraphy of coral ages indicates island subsidence rates of 4 5 2 8 cm 1000 yr for both islands a model of reef growth and erosion based on the stratigraphy of the cook islands atolls suggests average subsidence and erosion rates of between 3 6 and 15 20 cm 1000 yr respectively