a hydrological continuum in permafrost environments the morphological signatures of melt driven hydrology on earth and mars abstract this paper examines relationships between the morphology of cold desert hydrological features and the water sources and it relates the formation of diagnostic deposits and erosion patterns to environmentally controlled patterns of discharge in particular three sets of landforms found in the mcmurdo dry valleys of antarctica are compared to similar features on mars antarctic streams rivers to amazonian valleys terrestrial gullies to martian gullies and water tracks to recurring slope lineae similarities in landform morphology are highlighted for each set hydrological characteristics are aggregated or calculated and they are shown to scale comparably across the landforms for the terrestrial and martian groups accordingly a hydrological continuum is proposed linking the magnitude and the persistence of water sources in cold desert environments to flow response and the formation of diagnostic landforms this concept provides a framework for examining the relationships between climate hydrology and geomorphology in the cold desert environments of these two worlds

stable isotopic biogeochemistry of carbon and nitrogen in a perennially ice covered antarctic lake lake hoare 77 degrees 38 s 162 degrees 53 e is an amictic oligotrophic 34 m deep closed basin lake in taylor valley antarctica its perennial ice cover minimizes wind generated currents and reduces light penetration as well as restricts sediment deposition into the lake and the exchange of atmospheric gases between the water column and the atmosphere the biological community of lake hoare consists solely of microorganisms both planktonic populations and benthic microbial mats lake hoare is one of several perennially ice covered lakes in the mcmurdo dry valleys that represent the end member conditions of cold desert and saline lakes the dry valley lakes provide a unique opportunity to examine lacustrine processes that operate at all latitudes but under an extreme set of environmental conditions the dry valley lakes may also offer a valuable record of catchment and global changes in the past and present furthermore these lakes are modern day equivalents of periglacial lakes that are likely to have been common during periods of glacial maxima at temperate latitudes we have analyzed the dissolved inorganic carbon dic of lake hoare for delta 13c and the organic matter of the sediments and sediment trap material for delta 13c and delta 15n the delta 13c of the dic indicates that 12c is differentially removed in the shallow oxic portions of the lake via photosynthesis in the anoxic portions of the lake 27 34 m a net addition of 12c to the dic pool occurs via organic matter decomposition the dissolution of caco3 at depth also contributes to the dic pool except near the canada glacier where a substantial amount of allochthonous organic matter enters the lake the organic carbon being deposited on the lake bottom at different sites is isotopically similar suggesting an autochthonous source for the organic carbon preliminary inorganic carbon flux calculations suggest that a high percentage of the organic carbon fixed in the water column is remineralized as it falls through the water column at nearby lake fryxell the substantial relative to lake hoare glacial meltstream input overprints fryxell s shallow water biological delta 13c signal with delta 13c depleted dic in contrast lake hoare is not significantly affected by surface water input and mixing and therefore the delta 13c patterns observed arise primarily from biological dynamics within the lake organic matter in lake hoare is depleted in 15n which we suggest is partially the result of the addition of relatively light inorganic nitrogen into the lake system from terrestrial sources

lithium in waters of a polar desert abstract lithium li concentrations were measured in a series of stream and lake samples from a cold dry desert the mcmurdo dry valleys mdv in southern victoria land antarctica li geochemistry has been of great interest and utility in part because of its solubility and mobility during rockwater interactions in general the values determined for the streams and the dilute lake waters in this polar desert environment are similar to those found in temperate streams and lakes the source of li to these waters is a combination of marine aerosol and crustal weathering within the stream channels the hypersaline hypolimnion of lake bonney has extremely high concentrations of li which has been concentrated via a previous drawdown of the lake and its subsequent evapoconcentration the different distributions of li within the lakes is also due to past climatic changes and their effects on the chemical evolution of the lakes the high relative mobility of li controls its distribution within the aquatic systems of this cold desert environment

effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes abstract thermal and thermo kinetic models of subduction zones have been based in general on a constant value of thermal conductivity e g 3 138 w m k for solving the conservation of energy equation in order to make such models more realistic we have used variable thermal conductivities k that depend on pressure temperature and mineralogy olivine wadsleyite or ringwoodite as determined recently by xu et al xu y shankland t j linhardt s rubie d c langenhorst f klasinski k 2004 thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k phys earth planet inter 143 144 321 336 we have used these variable conductivities to model a variety of slabs with thermal parameters vertical subduction rate age of lithosphere at the trench ranging between 3500 and 17 000 km our models show that when radiative transfer is included k could plausibly be increased by as much as 2 w m k relative to a standard constant lithospheric value 3 138 w m k stein c a stein s 1992 a model for the global variation in oceanic depth and heat flow with lithospheric age nature 359 123 129 causing temperatures in the interiors of the slabs to increase by 50 100 c in turn the volume of metastable olivine in the slabs is reduced by as much as 20 30 with the maximum depth of olivine metastability decreasing by 30 50 km in general the maximum depths of olivine metastability are 160 230 km less than the depths of the deepest seismicity even in the extreme case of tonga metastable olivine falls short of the deepest earthquakes by 140 km in our model even taking the various uncertainties into account these results indicate that deep focus earthquakes occur in regions already transformed to wadsleyite or ringwoodite and must therefore be caused by a mechanism other than transformational faulting

boron isotopic geochemistry of the mcmurdo dry valley lakes antarctica abstract the geochemistry of boron was investigated in the ice covered lakes and glacier meltwater streams within taylor and wright valley of the mcmurdo dry valleys mcm antarctica in order to achieve a greater understanding of the source of boron to these aquatic systems and how in lake processes control boron concentration selected lake depths surface and bottom water and streams were analyzed for boron geochemistry boron stable isotope values in these waters span the range of 12 3 to 51 4 which corresponds to the variations from glacier meltwater streams to the hypolimnion of a highly evaporated hypersaline lake the data demonstrate that the major sources of b to the aquatic system are via terrestrial chemical weathering of aluminosilicates within the stream channels and a marine source either currently being introduced by marine derived aerosols or in the form of ancient seawater lakes fryxell hoare and upper waters of lake joyce which experience more terrestrial influence of aluminosilicate chemical weathering via glacial meltwater streams display a mixture of these two major sources while the source of b in the bottom waters of lake joyce appears to be primarily of marine origin lakes bonney and vanda and the blood falls brine have a marine like source whose 11b values have become more positive by mineral precipitation and or adsorption don juan pond displays a terrestrial aluminosilicate influence of a marine like source these hypersaline lake waters from antarctica are similar in 11b to other hypersaline lake waters globally suggesting that similar processes control their b geochemistry

ringwoodite growth rates from olivine with 75ppmw h2o metastable olivine must be nearly anhydrous to exist in the mantle transition zone it has been previously demonstrated that as little as 300 ppmw h 2o increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis to further test this hypothesis we present new ringwoodite growth rate measurements from olivine with 75 ppmw h 2o at 18 gpa and 700 900 and 1100 c these growth rates are nearly identical to those from olivine with 300 ppmw h 2o and significantly higher than those from nominally anhydrous olivine we infer that transformation of olivine with 75 300 ppmw h 2o is primarily enhanced by hydrolytic weakening of reaction rims which reduces the elastic strain energy barrier to growth we present a new method for fitting non linear nominally anhydrous data to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine based on previous thermokinetic modeling these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone metastable persistence of olivine into the mantle transition zone would therefore require 75 ppmw h 2o

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

the inference of mantle viscosity from an inversion of the fennoscandian relaxation spectrum summary a formal inverse theory for mantle viscosity is here applied to a relaxation spectrum derived from the post glacial uplift of fennoscandia the spectrum represents the set of eigenfrequencies or inverse decay times for the fundamental mode of viscous gravitational relaxation between the spherical harmonic degrees 14 to 45 and 65 to 80 theoretical predictions of the eigenfrequencies are based upon the determination of the zeroes of the secular determinant function derived for a spherically symmetric self gravitating visco elastic planet differential kernels relating shifts in the eigenfrequencies to arbitrary perturbations in the radial viscosity profile i e frechet kernels are computed using the variational principle derived by peltier 1976 the inversions are performed within the framework of non linear bayesian inference and the problem has been parameterized in terms of the logarithm of viscosity the inversions have yielded a set of robust constraints which all models for the radial viscosity profile below fennoscandia must satisfy the a posteriori estimates and variance reduction are found to be insensitive to the a priori variance ascribed to the model layers the constraints have furthermore been summarized into a set of a posteriori estimates of the average model viscosity value in radial regions consistent with the resolving power of the data which decreases from a radial length scale of approximately 120km at the base of the lithosphere to 1200km at 1000km depth the data provide essentially no information regarding the mantle rheology below 1200km depth for example for earth models with a lithospheric thickness lt of 100 km the volumetric average logarithm of viscosity in regions in the depth ranges 1040 400 km 670 210 km and 235 100 km is constrained to be respectively 21 03 0 09 20 70 0 08 and 20 37 0 19 we have repeated the inversions for a number of assumed lithospheric thicknesses and have found that a relatively low viscosity layer in the sublithospheric region with respect to the underlying upper mantle is required for lt 120km in this respect we have quantified the previously described trade off between a decrease in the viscosity of this region and a decrease in lt cathles 1975 in forward analyses of the glacial isostatic adjustment data set it is common to use earth models with isoviscous upper and lower mantle regions to investigate this two layer case we have also performed inversions which assume perfect correlation amongst the model layers in the upper and separately the lower mantle under this strict model space limitation the inversions yield models with upper and lower mantle viscosities in the range 3 7 1020 4 5 1020 pa s and 2 2 1021 1 9 1021 pa s respectively the ranges are obtained from a suite of inversions using lithospheric thickness from 70 km to 145 km the a posteriori constraints generated from the bayesian inversions are used together with a statistic based on the computed misfit to the fennoscandian relaxation spectrum to rule out a number of previously published viscosity models

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

evidence for a large scale remnant of subducted lithosphere beneath fiji we combine spatial variations of p and s wave speeds 1000 fault plane solutions and 6600 well determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the tonga back arc we show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep dipping wadati benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere the detached slab may be from a previous episode of subduction along the fossil vitiaz trench about 5 to 8 million years ago the juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle such a configuration if common in the past would allow the preservation of a primordial component in the lower mantle

solute and isotope geochemistry of subsurface ice melt seeps in taylor valley antarctica the mcmurdo dry valleys of antarctica are a polar desert region with watersheds dominated by glacial melt recent ground exploration reveals unusual surface fl ow seep features not directly supplied by glacial melt much of this seep water is potentially derived from permafrost snow patches refrozen precipitation accumulated in the subsurface buried glacier ice or even groundwater from the deep subsurface flow features that lack obvious glacier melt sources were identifi ed in archived aerial photographs of taylor valley this valley was surveyed for extant and extinct seeps and the locations of geomorphic features in fi active seeps were documented water samples from seeps were analyzed for major ions and stable isotopes of hydrogen and oxygen solute chemistry and isotopic signatures of seeps are distinct from those of nearby streams and glaciers with the seeps having elevated solute concentrations all but one seep had water isotopically heavier than water from nearby glaciers and streams suggesting that seep waters have been substantially modifi ed if they had been derived originally from the same meteoric sources that supply local glaciers and streams the seeps are important because they compose a previously overlooked component of the desert hydrological cycle seep features in the dry valleys are potential terrestrial analogs for the geologically young gullies observed on mars which are thought to be evidence of groundwater seepage and surface runoff

free energy minimization by simulated annealing with applications to lithospheric slabs and mantle plumes an optimization algorithm based upon the method of simulated annealing is of utility in calculating equilibrium phase assemblages as functions of pressure temperature and chemical composition operating by analogy to the statistical mechanics of the chemical system it is applicable both to problems of strict chemical equilibrium and to problems involving metastability the method reproduces known phase diagrams and illustrates the expected thermal deflection of phase transitions in thermal models of subducting lithospheric slabs and buoyant mantle plumes it reveals temperature induced changes in phase transition sharpness and the stability of fe rich phase within an field in cold slab thermal models and it suggests that transitions such as the possible breakdown of silicate perovskite to mixed oxides can amplify velocity anomalies

effects of slab mineralogy on subduction rates although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan cies their mineralogy can also have signicant eects we explore this by using thermo kinetic modeling to predict mineralogy and compare the resultant buoyant driving force to the opposing viscous drag phase transitions of mg fe 2sio4 in subducting slabs depend on thermal struc ture in two ways first equilibrium phase boundaries should be deflected causing local buoyancy anomalies whose sign depends upon that of the clapeyron slope as slabs rst enter the transition zone negative anomalies should acceler ate them but positive anomalies that arise when they fully penetrate the transition zone should slow them such ef fects may induce geologically abrupt changes in plate mo tions second olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities the coldest and fastest slabs should be slowed more greatly thus narrowing the range of feasible subduc tion rates decreased descent rates however allow slabs to warm and metastable wedges to thermally erode such neg ative feedback mechanisms may serve to regulate subduction rates

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

origin of crystalline cold desert salts in the mcmurdo region antarctica abstract the distribution of crystalline salt minerals in deposits in the mcmurdo region of antarctica has been examined to study the origin of these salts sulphate chloride sodium and calcium salts are most frequently encountered salts containing chloride and sodium ions become less common away from the coast sulphate salts are more regularly distributed but tend to be related isotopically to sea water sulphate salts containing magnesium ion tend to exist mainly on substrates composed of basic igneous rocks whereas calcium and carbonate salts are present on all rock types in the region these distributions show that salts of marine origin are regionally and quantitatively most important but that chemical weathering of mafic materials in rocks and soils is also significant however biological volcanic and hydrothermal processes are or have been active contributing to salts in local areas that include penguin rookeries and eastern taylor valley the summit area of erebus volcano and subsurface rocks respectively

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

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

the geochemistry of antarctic streams and their role in the evolution of four lakes of the mcmurdo dry valleys abstract major ion concentrations are reported for nineteen meltwater streams in the mcmurdo dry valleys antarctica and for the four lakes fed by these streams the most important solute acquisition processes for these flowing waters are dissolution of marine derived salts dissolution of calcite coatings derived from chemical weathering of parent rock and direct weathering of silicates although lakes joyce hoare and fryxell have undergone significant evaporative concentration the only simple salt which is predicted to form at present is calcite the shallow waters of all four lakes were supersaturated with respect to this mineral but were undersaturated with respect to other simple salts geochemical budgets however indicate that so4 k and mg in addition to ca and hco3 have been removed from some of these lakes over recent geological time bacterial sulfate reduction followed by precipitation of metal sulfides is likely to be a sink for sulfate sinks for mg and k are more problematic but may include formation of mgcalcites and in the case of k reverse weathering reactions the ages of the contemporary water columns as indicated by chloride budgets are relatively young and these lakes could have evolved their chemical compositions over a few thousand years of present stream flows it is not necessary to invoke trapped seawater or other salt sources to explain their evolution

normal mode coupling due to azimuthal anisotropy in the transition zone an example from taiwan island summary we investigate the effect of azimuthal anisotropy on coupling between spheroidal and toroidal modes at frequencies below 4 mhz after large earthquakes of mw 7 8 based on the seismic records from broad band array in taiwan for seismology stations strong coupling of 0s25 0t25 is widely observed at taiwan island but earth s rotation cannot cause coupling between 0s25 and 0t25 according to coupling selection rules different from the coriolis coupling the anisotropic coupling of 0s25 0t25 becomes notable only when stations close to nodal lines of 0s25 and the coupling attenuates so rapidly that it is notable on the spectra of 18 24 hr vertical component records modelling indicates that local azimuthal anisotropy is the most possible cause for the strong coupling the abnormally strong coupling of 0s20 0t21 observed at taiwan island also shows similar characteristic of nodal nature and fast attenuation suggesting local azimuthal anisotropy as the dominant factor for the coupling rather than the earth s rotation isotropy and transverse isotropy our estimation of sensitivity kernels indicates that the coupled modes 0s20 0t21 and 0s25 0t25 show peak sensitivity to azimuthal anisotropy at depth of 400 650 km suggesting that the strong coupling may be explained by the presence of azimuthal anisotropy in the upper mantle transition zone beneath taiwan island the east dipping eurasian slab and the north dipping philippine sea plate beneath the taiwan orogen are responsible for the formation of the anisotropy

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

effects of glacial meltwater inflows and moat freezing on mixing in an ice covered antarctic lake as interpreted from stable isotope and tritium distributions perennially ice covered lakes in the mcmurdo dry valleys have risen several meters over the past two decades due to climatic warming and increased glacial meltwater inflow to elucidate the hydrologic responses to changing climate and the effects on lake mixing processes we measured the stable isotope al80 and 6d and tritium concentrations of water and ice samples collected in the lake fryxell watershed from 1987 through 1990 stable isotope enrichment resulted from evaporation in stream and moat samples and from sublimation in surface lake ice samples tritium enrichment resulted from exchange with the postnuclear atmosphere in stream and moat samples rapid injection of tritiated water into the upper water column of the lake and incorporation of this water into the ice cover resulted in uniformly elevated tritium contents 3 0 tu in these reservoirs tritium was also present in deep water suggesting that a component of bottom water was recently at the surface during summer melted lake ice and stream water forms the moat water excluded from ice formation during fall moat freezing enriched in solutes and tritium and depleted in iso and 2h relative to water below 15 m depth may sink as density currents to the bottom of the lake seasonal lake circulation in response to climate driven surface inflow is therefore responsible for the distribution of both water isotopes and dissolved solutes in lake fryxell

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

non lithostatic pressure in subduction zones abstract the pressure at depth is not directly observable and no one knows precisely to which extent the pressure conditions in subduction zones recorded by high pressure metamorphic rocks deviate from mantle lithostatic pressure as an alternative to large scale complex numerical models of subduction zones the analytical subduction channel model can give us some insight on the physical processes that control the development of non lithostatic pressure as well as some estimation of its amplitude we propose a new approach coupling the flow of crust within the channel to the deformation of the mantle bounding the channel occurring as the pressure within the channel deviates from mantle lithostatic values while for very weak crust within the subduction channel the channel walls are rigid and channel geometry does not vary for stronger crust our coupled approach unravels a new domain of behaviour where the mantle is no longer completely rigid and the deformation of the channel walls prevents arbitrarily large non lithostatic pressure to develop this new regime poses an upper bound on the amplitude of non lithostatic pressure within the channel that depends only on the mantle viscosity the transition from one regime to another is dependent on an adimensional parameter m c h 0 3 l 3 incorporating not only mantle and crust viscosity but also the geometry of the channel the development of larger non lithostatic pressure in thinner channels than in larger ones predicted in the rigid channel model is partly inhibited in the fully coupled model as thinner channels more easily induce channel wall deformation the lengthscale of the channel width perturbations influences the amplitude of non lithostatic pressure as small scale ones inducing a more rigid response of the mantle potentially trigger larger non lithostatic pressure

evidence of deep circulation in two perennially ice covered antarctic lakes the perennial ice covers found on many of the lakes in the mcmurdo dry valley region of the antarctic have been postulated to severely limit mixing and convective turnover of these unique lakes in this work we utilize chlorofluorocarbon cfc concentration profiles from lakes hoare and fryxell in the mcmurdo dry valley to determine the extent of deep vertical mixing occurring over the last 50 years near the ice water interface cfc concentrations in both lakes were well above saturation in accordance with atmospheric gas supersaturations resulting from freezing under the perennial ice covers evidence of mixing throughout the water column at lake hoare was confirmed by the presence of cfcs throughout the water column and suggests vertical mixing times of 20 30 years in lake fryxell cfc 11 cfc 12 and cfc 113 were found in the upper water column however degradation of cfc 11 and cfc 12 in the anoxic bottom waters appears to be occurring with cfc 113 only present in these bottom waters the presence of cfc 113 in the bottom waters in conjunction with previous work detecting tritium in these waters strongly argues for the presence of convective mixing in lake fryxell the evidence for deep mixing in these lakes may be an important yet overlooked phenomenon in the limnology of perennially icecovered lakes

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

small 660 km seismic discontinuity beneath tibet implies resting ground for detached lithosphere 1 using seismic profiles comprising of high resolution triplicate waveforms across apertures of over 1 000 km we show that because of high p wave speed vp near the bottom of the mantle transition zone the contrast in vp across the 660 km discontinuity beneath central tibet is small only about 70 of that beneath the northern indian shield this subhorizontal anomaly of high vp is most likely a remnant of detached mantle lithosphere that recently sank to depth thus providing key evidence for a direct connection between continental collision near the surface and deep seated dynamics in the mantle

geochemistry of the onyx river wright valley antarctica and its role in the chemical evolution of lake vanda the onyx river wright valley antarctica is a dilute meltwater stream originating in the vicinity of the wright lower glacier it acquires a significant fraction of its salt content when glacial meltwaters contact wright valley soils at lake brownworth and the concentrations of all ions increase with distance along the 28 km channel down to lake vanda average millimolar concentrations of major ions at the vanda weir during the 1980 1981 flow season were ca 0 119 mg 0 061 na 0 212 k 0 033 q 0 212 so4 0 045 hco3 0 295 and sio2 0 049 based on the flow measurements of chinn 1982 this amounts to an annual flux in moles to lake vanda of ca 0 238 106 mg 0 122 106 na 0 424 106 k 0 066 106 cl 0 424 106 so4 0 09 106 hco3 0 59 106 sio2 0 098 106 in spite of the large salt input from this source equilibrium evaporation of onyx river water would have resulted in early calcite deposition and in the formation of a na mg cl hco3 brine rather than in the ca na mg cl waters observed in lake vanda the river alone could not have produced a brine having the qualitative geochemical features of the lower saline waters of lake vanda it is proposed that the vanda brine is instead the result of past 1200 yrs bp mixing events between onyx river inflows and calcium chloride rich deep groundwaters derived from the don juan basin the mixing model presented here shows that the onyx river is the major contributor of k hco3 so4 and possibly mg found in the lake and a significant contributor approximately one half of the observed na calcium and cl on the other hand came largely from deep groundwater sources in the don juan basin all concentrations except mg are well predicted by this model the chemical composition of the geologically recent upper lake is explained in terms of ionic diffusion from the pre formed brine coupled with onyx river inflow ionic ratios calculated from this latter model are in very good agreement with those observed in the lake at 35 meters

aqueous corrosion of olivine in the mars meteorite miller range mil 03346 during antarctic weathering implications for water on mars abstract several nakhlites clinopyroxenite meteorites from mars contain olivine phenocrysts with corrosion features identical in size shape and distribution to the smaller etch pits of well characterized weathered terrestrial olivine miller range mil 03346 is an antarctic nakhlite find recovered after long exposure to antarctic conditions the distribution of discrete olivine etch pits almost exclusively within a few hundred microns of allocation mil 03346 171 s documentably exposed surface suggests that they formed by terrestrial weathering in antarctica the small size of olivine etch pits in mil 03346 171 relative to commonly much larger etch pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short or the weathering conditions to which it was exposed did not favor olivine corrosion in the form of etch pit formation or both time scales for the formation of etch pits estimated from experimentally determined dissolution rates of olivine over a range of phs are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on mars etch pits of the observed size on mil 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic antarctic conditions but the terrestrial age of mil 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline antarctic water the paucity of similar etch pits in olivine from the interior of mil 03346 suggests that olivine in this mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1 3 billion years on mars than its exterior was subjected to during its pleistocene holocene exposure to antarctic weathering conditions

the effect of metastable pyroxene on the slab dynamics seismic studies show that some subducting slabs penetrate straight into the lower mantle whereas others seem to flatten near the base of the mantle transition zone slab stagnation is often attributed to an increase in viscosity and phase transformations in the olivine system however recent mineral physics studies showed that due to extremely low transformational diffusion rates low density metastable pyroxene may persist into the transition zone in cool slabs here we use a dynamically fully self consistent subduction model to investigate the influence of metastable pyroxene on the dynamics of subducting oceanic lithosphere our results show that metastable pyroxene affects slab buoyancy at least as much as olivine metastability however unlike metastable olivine which can inhibit slab penetration in the lower mantle only for cold old and fast slabs metastable pyroxene is likely to also affect sinking of relatively young and slow slabs

geodynamic models of deep subduction abstract numerical and laboratory models that highlight the mechanisms leading to a complex morphology of subducted lithospheric slabs in the mantle transition zone are reviewed an increase of intrinsic density with depth an increase of viscosity or phase transitions with negative clapeyron slope have an inhibiting influence on deep subduction the impingement of slabs on a viscosity and density interface has been studied in laboratory tanks using corn syrup slab interaction with equilibrium and non equilibrium phase transitions has been modelled numerically in two dimensions both the laboratory and the numerical experiments can reproduce the variety of slab behaviour that is found in tomographic images of subduction zones including cases of straight penetration into the lower mantle flattening at the 660 km discontinuity folding and thickening of slabs and sinking of slabs into the lower mantle at the endpoint of a flat lying segment aside from the material and phase transition properties the tectonic conditions play an important role in particular the retrograde motion of the point of subduction trench rollback has an influence on slab penetration into the lower mantle a question that still needs to be clarified is the mutual interaction between plate kinematics and the subduction process through the transition zone

deep earthquakes in real slabs deep earthquakes shouldn9t happen yet they do at depths greater than about 50 kilometers rock begins to flow rather than fracture and fracture is the phenomenon associated with seismic activity in their perspective stein and rubie discusses the highlights of a recent conference on subduction the geological process whereby one lithospheric plate is pushed under another deep earthquakes can take place in subducting slabs owing to the complex phase transformations that occur as minerals are heated and put under pressure as they are pushed deeper conference participants debated current models of deep seismic activity and their limitations

variable conductivity effects on the thermal structure of subducting slabs understanding the thermal structure of the lithosphere in subduction zone environments is a key element in the realization of the source of deep focus earthquakes and other geodynamic processes model predictions of the thermal structure of subducting slabs are dependent upon a priori knowledge of how thermal conductivity varies within the slab using recent results for the variation of thermal conductivity as a function of temperature pressure and gross mineralogy we investigate how models that utilize a realistic model for variable conductivity compare with canonical models for constant conductivity results suggest that jumps in thermal conductivity across the phase boundaries that arise due to transformations to and spinel phases lead to old fast steeply dipping slabs being considerably warmer than previously thought although these results do not rule out the possibility of a metastable wedge source of deep focus earthquakes they do imply the likelihood of metastable olivine being found at depth is less than once thought

earthquakes and strain in subhorizontal slabs 1 using an extensive database of fault plane solutions and precise locations of hypocenters we show that the classic patterns of downdip extension dde or downdip compression ddc in subduction zones deteriorate when the dip of the slab is less than about 20 this result is depth independent demonstrated by both intermediate focus depths from 70 to 300 km and deep focus depths greater than 300 km earthquakes the absence of pattern in seismic strain in subhorizontal slabs also occurs locally over scales of about 10 km as evident from a detailed analysis of a large mw 7 1 earthquake sequence beneath fiji following the paradigm that a uniform strain of dde ddc results from sinking of the cold dense slab as it encounters resistance from the highly viscous mantle at depth breakdown of dde ddc in subhorizontal slabs reflects waning negative buoyancy slab pull in the downdip direction our results place a constraint on the magnitude of slab pull that is required to dominate over localized sources of stress and to align seismic strain release in dipping slabs under the condition of a vanishing slab pull eliminating the only obvious source of regional stress the abundance of earthquakes in subhorizontal slabs indicates that a locally variable source of stress is both necessary and sufficient to sustain the accumulation of elastic strain required to generate intermediate and deep focus seismicity evidence is growing that the process of seismogenesis under high pressures including localized sources of stress is tied to the presence of petrologic anomalies

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

a petrologic anomaly accompanying outboard earthquakes beneath fiji tonga corresponding evidence from broadband p and s waveforms 1 using high resolution triplicate body waveforms we investigate strong lateral variations in p and sh wave speeds vp and vs beneath the tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr 1 under these conditions the lack of prominent anomalies of high vp and vs in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle this inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the wadati benioff zone beneath the back arc we find a sharp east west trending demarcation north of which outboard earthquakes occur and vp and vs abruptly drop by 3 thus the effect of cold temperature to raise vp and vs as indicated by the presence of outboard earthquakes must be counteracted by petrologic variations we interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes considering the density of these candidates and the strong effect of partial melt on vs volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in vp and vs meanwhile metastable olivine remains a viable candidate as a cold slab passes below the 410 km discontinuity olivine becomes metastable and makes the tip of the slab buoyant such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

time dependent interaction between subduction dynamics and phase transition kinetics summary numerical subduction models are presented that account for the kinetics of the olivine spinel transformation and the equilibrium transformation of spinel into perovskite and magnesiowustite temperature depth and stress dependent rheology is used buoyancy due to temperature and the olivine spinel and magnesiowustite perovskite phases leads to free subduction into the transition zone and retarded penetration into the lower mantle a dynamic feed back mechanism is found between the different buoyancy forces the kinetics of the phase transition and the release of latent heat this mechanism modulates the parachute effect of the metastable olivine mo in a subducting slab and leads to characteristic temporal variations of the depth of mo and with a time delay variations of the subduction velocity time periods are of the order of 3 4 myr this mechanism allows to explain variations of the depths of the deepest earthquakes in otherwise similar subduction zones further conclusions are as follows the maximum depth of mo increases roughly linearly with lithospheric thickness and the maximum depth may reach as deep as 720 750 km different subduction zones may represent different stages of velocity depth phase loops of mo

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

diatom response to climatic and tectonic forcing of a palaeolake at the southeastern margin of the tibetan plateau during the late pleistocene between 140 and 35 ka bp abstract the heqing basin is located in yunnan province south western china near the southeastern edge of the tibetan plateau a region that is strongly influenced by the asian monsoon its long sedimentary record represents an archive of global importance for palaeoclimate research this study presents the results of diatom analysis of the last 140 35 ka of the record dated using ams 14 c and magnetostratigraphy diatoms are generally well preserved apart at the top of the sequence after 35 ka bp in which diatoms were too few to be counted the diatom data provide a detailed history of palaeoenvironmental changes during the late pleistocene and demonstrate the sensitivity of diatoms to changes in climate and hydrology in particular they provide proxy evidence for water depth and indirect evidence for temperature changes that are characterized by glacial interglacial shifts between planktonic and benthic diatoms and variations in the percentage of the warm water and eutrophic species aulacoseira granulata respectively in the heqing sequence remarkably high proportions of benthic diatoms coincide with warm and wet conditions suggesting that at this site the diatom signal is mediated by climate catchment vegetation processes such as the concentration of nutrient and runoff from the catchment from 132 ka bp however the tectonic uplift of the basin caused a change in the hydrological setting of the lake pollen and geochemical data are then used to help interpreting the diatom sequence and disentangle the direct effects of climate from those mediated by the conditions on the catchment such as the influence of glacier melt water

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

the effect of variable thermal diffusivity on kinematic models of subduction 1 we examine the influence of variable thermal properties on the thermal state of a subducting slab in the upper mantle and the transition zone by combining a kinematic slab model with models of thermal conductivity thermal diffusivity and conductivity models for major mantle minerals in the mgo sio2 system are developed based on experimental measurements on these minerals at high pressure and temperature the models show significantly higher thermal conductivity for stishovite and clinopyroxene compared to the mg2sio4 polymorphs and majorite garnet in our subduction model we consider scenarios with a differentiated slab basalt harzburgite pyrolite in a pyrolite mantle and uniform composition for both the slab and the mantle pyrolite or a pure mg2sio4 based system the role of highly conductive pyroxene is examined by taking it into account in some models and replacing it in others with majorite garnet this choice has a strong influence on the thermal state of the slab shifting the depth of the 1000 k temperature anomaly by as much as 100 km this is caused by faster cooling of the plate at the surface and thermal insulating effects once subducted temperature differences between models with variable thermal diffusivity and those with constant parameters can reach 125 k i e 10 of total thermal anomalies of the slab relative to an average geotherm taking into account the stable mineral phases we evaluate density variations between different models and find that variable thermal diffusivity results in a modest increase of negative buoyancy of the slab

hydrochemical analysis of spatial and temporal variations of solute composition in surface and subsurface waters of a high arctic catchment abstract spatial and seasonal variations of solute composition were studied in different compartments of a partly glaciated arctic catchment at liefdefjorden nw spitsbergen main emphasis was put on the snowmelt period from june till end of july water samples were taken periodically from soils along longitudinal streamwater profiles and from surface runoff concentrations of total monomeric al c altot in soil solutions were generally low and were below the detection limit of a modified hydroxychinolin method in surface waters comparing these concentrations withc altot in pristine and strong acid influenced areas of temperate latitudes no signs of an exhaustion of geochemical buffer systems could be assessed in this arctic catchment spatial variations in bulk ion composition of soil solutions exceeded seasonal variations during snow melt spatial variations could not be attributed to hillslope position of soil sampling sites thus spatial heterogeneity of rock mineralogy and soil substrates determined soil solution composition bulk ion composition strongly pointed to calcite dissolution as the dominant geochemical process thereby dissolution is enhanced by biologically produced co2 in areas with continuous vegetation cover in soil solutions co2 oversaturation reached maximum values of 20 fold the mean atmospheric partial pressure of co2 ion composition along longitudinal stream water profiles revealed seasonal as well as spatial trends solute enriched subsurface flow contributed significantly to total discharge in the biologically active part of the catchment during mid melting season precipitation events at late summer displaced solute enriched subsurface waters from the active layer resulting in maximum solute concentrations in stream water

high pressure γ camgsi2o6 does penta coordinated silicon exist in the earth s mantle in situ x ray diffraction experiments with natural fe and al bearing diopside single crystals and density functional theory dft calculations on diopside end member composition indicate the existence of a new high pressure diopside polymorph with rare penta coordinated silicon on compression diopside transforms to the phase at 50 gpa which in turn on decompression is observed to convert to the known phase below 47 gpa the new diopside polymorph constitutes another recent example of penta coordinated silicon vsi in over compressed metastable crystalline silicates suggesting that vsi may exist in the transition zone and the uppermost lower mantle in appreciable quantities not only in silicate glass and melts but also in crystalline phases contained in the coldest parts of subducted stagnant slabs vsi may have significant influences on buoyancy wave velocity anomalies deformation mechanisms chemical reactivity of silicate rocks and seismicity within the slab

dating quaternary lacustrine sediments in the mcmurdo dry valleys antarctica abstract reports of erroneously old 14c dates for modern antarctic materials have thrown doubt into 14c chronologies the carbon reservoir effect purported to exist in quaternary lacustrine sediments of the mcmurdo dry valleys was investigated by studying 14c distribution and testing alternate dating techniques our results show that the carbon reservoir effect is not pervasive stream and near shore microbial mats and dissolved inorganic carbon dic in the surface waters of lake fryxell are in equilibrium with modern 14co2 the surface waters of lake hoare and lake bonney however have dic 14c ages of 1650 and 2080 yr b p respectively these older age estimates are suggested to be due to the direct input of large amounts of glacial melt with relict dic on the other hand lake fryxell receives only a minor component of its inflow directly from a glacier while a large component must travel long distances in numerous shallow ephemeral streams after leaving local valley glaciers this mode of melt water input allows the water to equilibrate with modern co2 before entering the lake bottom water 14c ages for lake hoare closely match surface sediment ages supporting the widely published period 1200 yr b p after a 1650 yr reservoir correction when most dry valley lakes apparently evaporated to small brine ponds and or disappeared completely lake bonney bottom water is 8000 yr b p carbon dating is shown to be a viable technique for lake edge deposits and possibly lake bottom deposits where a correction to the sediment surface age can be obtained however we conclude that deep water paleolake deposits can not be reliably dated using 14c alone because of an inability to determine the age of the reservoir correction i e accounting for the initial carbon reservoir plus the age of the bottom water a suite of alternative and complimentary dating techniques were tested on modern and late holocene lacustrine deposits these include thermoluminescence tl 210pb 137cs and paleomagnetism of these techniques tl or the more sensitive optically stimulated luminescence holds the most promise for correcting lake sediment 14c ages tl dating of the modern lake hoare sediments showed an 1000 yr relict signal this signal is unaffected by the age of the lake water both 210pb and 137cs occurred in very low levels in the sediments and do not appear to be viable dating techniques for the perennially ice covered lakes of the mcmurdo dry valleys paleomagnetism was not suited to the coarse grained nature of lake hoare bottom sediments but could be a useful alternative given a finer sediment type

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

the metastable olivine wedge in fast subducting slabs constraints from thermo kinetic coupling abstract the effects of a strong non linear thermo kinetic coupling in the olivine spinel transition are investigated with a 2d model for fast subducting slabs the thermo kinetic coupling problem is solved in the lagrangian framework by the differential algebraic method and by using experimental input parameters narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity while the thermo kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist in contrast to recent kinetic models both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge both processes trigger the ongoing phase transformation in the cold slab interior a thermal instability over a short timescale 10 4 10 5 yr can occur due to a positive thermo kinetic feedback this effect is enhanced at greater depths the depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature slab width and the activation energy for diffusion variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge hence predictions about the exact position of the metastable olivine wedge are rather difficult we propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating while the thermal instability is enhanced by the latent heat release associated with the olivine spinel phase transition

seismic constraints on mechanisms of deep earthquake rupture 1 observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine cumulative seismic moment release increases sharply at 530 km depth in both cold and warm slabs implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change which would be temperature dependent this together with the observation that deep seismicity predominates near the garnet rich upper boundary of the subducting plate former oceanic crust suggests that the transformation of garnet to perovskite which may release considerable water is integral in the cause of earthquakes deeper than 530 km where most deep earthquakes occur

from subduction to collision control of deep processes on the evolution of convergent plate boundary 1 using laboratory experiments we investigate the dynamics of the collisional process that follows the closure of an oceanic basin the evolution of these experiments systematically shows four successive episodes of deformation which correspond to 1 the initiation of oceanic subduction 2 a mature period of oceanic subduction 3 an episode of continental subduction during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates and 4 continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds we observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere the slab pull force in turn depends on the amount of subducted oceanic material on the thickness of the convective mantle and also on the rheology of the slab our experiments indeed suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle decreasing the slab pull level and triggering the rapid onset of collision we observe two possible modes of slab deformation slab break off and development of viscous instabilities we define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation in all cases oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km prior to the onset of continental collision

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

geochemistry of great salt lake utah i hydrochemistry since 1850 abstract the hydrochemistry of great salt lake utah has been defined for the historic period 1850 through 1982 from published data combined with new observations the water balance depends largely on river inflow atmospheric precipitation onto the lake surface and evaporation input of the major solutes can best be accounted for by mixing dilute calcium bicarbonate type river waters with nacl dominated hydrothermal springs prior to 1930 lake concentrations fluctuated inversely with lake volume in response to small climatic variations since then salt precipitation and dissolution have significantly modified lake brine compositions and have led to density stratification and the formation of brine pockets of differing composition brine mixing has become an important component of brine evolution we have used calculated evaporation curves with mineral precipitation and dissolution to clarify these processes pore fluids represent important storage for solutes solute profiles can be modeled by simple one dimensional diffusion calculations short term historic variations in lake composition affect shallow pore fluids in the upper 2 metres of sediment

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins