spatiotemporal variability of west african rainfall part ii associated surface and airmass characteristics abstract in the second part of this study the variability of surface and airmass characteristics associated with west african rainfall fields is investigated ship observations over the tropical atlantic and rawinsonde records over west africa are used for the months of july august and september from 1948 to 1978 a composite analysis based on the sign of monthly rainfall departures north of 10 n indicates that the sahelian drought may result from a more southern than normal position of the intertropical convergence zone itcz over the tropical atlantic a composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10 n it shows two types of sahelian drought the first is linked to an enhanced surface atmospheric circulation over the northern tropical atlantic higher sea surface temperatures in the gulf of guinea a convergent moisture flux anomaly south of 10 n along the western african coast and a lower larger water content in the monsoon fl

characterization of model spread in pmip2 mid holocene simulations of the african monsoon abstractsimulations of the west african monsoon wam for the present day climate 0 ka and the mid holocene 6 ka using the coupled models from the paleoclimate modelling intercomparison project phase 2 pmip2 are assessed in this study the authors first compare the ensemble simulations with modern observations and proxy estimates of past precipitation showing that the pmip2 model median captures the basic features of the wam for 0 ka and the changes at 6 ka despite systematic biases in the preindustrial pi simulations and underestimates of the northward extent and intensity of precipitation changes the model spread is then discussed based on a classification of the monsoonal convective regimes for a subset of seven coupled models two major categories of model are defined based on their differences in simulating deep and moderate convective regimes in the pi simulations changes in precipitation at 6 ka are dominated by changes in the large scale dynamics for most of the pmip2 models and are cha

a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

creep strength of ringwoodite measured at pressure temperature conditions of the lower part of the mantle transition zone using a deformation dia apparatus abstract creep strength of ringwoodite is important for understanding complicated patterns of the mantle convection in and around the mantle transition zone to determine the creep strength of ringwoodite we expanded pressure temperature conditions of in situ stress strain measurements in a deformation dia apparatus combined with synchrotron x ray to those of the lower part of the mantle transition zone the expansion of the pressure temperature conditions was made by shrinking anvil truncation to 2 0 mm and the development of a cell assembly for in situ deformation experiments up to 1700 k utilizing the developed technique creep strength measurements on polycrystalline ringwoodite were performed at 16 9 18 0 gpa and 1300 1700 k during axial deformation with strain rates of 1 48 3 59 10 5 s 1 to strains of 13 2 24 9 based on mechanical and microstructural observations we infer that ringwoodite deformed by exponential dislocation creep through the peierls mechanism at 1300 1400 k and power law dislocation creep at 1500 1700 k the creep strength of ringwoodite is apparently lower than that of bridgmanite wadsleyite and olivine the present result implies the possibility that the lower mantle transition zone is a low viscosity layer further creep strength data of these minerals are necessary to be determined above 13 5 gpa and high temperatures to determine viscosity structure in and around the lower mantle transition zone at strain rates relevant to the mantle convection

variations in the zn si record over the last interglacial glacial transition we examined the zinc content of diatom frustules as an indicator of past changes in surface seawater zn2 concentration zn si data of samples from three cores located in the south atlantic sector of the southern ocean spanning the last interglacial glacial transition are presented changes in the zn si record are linked to changes in the surface water zn2 concentration the source of variation in zn2 concentration appears to be via changes in deep water upwelling and circulation we rule out changes in phytoplankton productivity and aeolian dust input as a source of variation in the zn si record likewise the zn si data are not linked to shifts in the diatom species composition of the sediment or sediment preservation effects the zn si results presented do not support the zinc hypothesis there is no link between the uptake of co2 by phytoplankton as inferred from the 13c record and the zn si record

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

quantitative reconstructions of annual rainfall in africa 6000 years ago model data comparison precipitation are based on 85 pollen sites 14 c dated at 6000 500 years b p and distributed over the whole of africa to improve the reliability of the pollen based climate reconstruction two methods are used the modern analogues technique mat and the plant functional types pft methods we then conduct a model data comparison for five distinct regions allowing an evaluation of model outputs the sahara sahel the eastern sahara western equatorial africa east africa and madagascar the pollen inferred reconstructions are compared with 21 mid holocene simulations yielded by atmospheric general circulation models agcms and coupled ocean atmosphere vegetation models oavgcms the large scale feature of the hydrological changes is shown to be well captured by most of the models data show that during the mid holocene the sahara was considerably wetter than today 200 to 700 mm yr the results reinforce the conclusion that the agcms significantly underestimate this precipitation increase in the sahara whereas the oavgcm simulations are in accordance with the data our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in east africa and madagascar the model data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen based reconstruction shows no change or only slight drying there

hydrothermal origin of syndepositional chert bands and nodules in the mesoproterozoic wumishan formation implications for the evolution of mesoproterozoic cratonic basin north china abstract abundant chert bands and nodules are discovered throughout the mesoproterozoic wumishan formation 1 5 ga in the north china block the middle part of the wumishan formation consists of cyclic deposition of calcareous shale massive microbial dolostone thrombolites and stromatolites and laminated dolostone chert bands occur most abundantly in the laminated dolostone although they are also found with lower abundance in other lithologies chert bands are normally parallel to the bedding plane and show soft deformation structures suggesting early silicification at or near the water sediment interface the wumishan chert is composed of two types of silica fabric the microcrystalline and cryptocrystalline quartz that derives from the replacement of host dolomitic rock and the mosaic chalcedonic quartz representing typical void filling precipitation we measured the germanium silica ratios ge si and the ree compositions of the chert bands nodules collected from a 60 m interval in the middle part of the wumishan formation in the mopanyu section ge si ratios of the buck samples measured by icp ms range from 13 6 to 35 6 mol mol additional la icp ms analyses allow the measurements of individual silica fabric providing more detailed information about the geochemistry of chert nodules and bands ge si ratios of the replacive chert vary between 10 and 20 mol mol while the mosaic chalcedonic quartz has wider range of variation 1 1 93 2 mol mol these values are 1 order of magnitude higher than the modern seawater value 0 72 mol mol but is within the range of hydrothermal fluids 3 4 51 3 mol mol average 13 mol mol n 101 suggesting hydrothermal fluid as an important si source this conclusion is consistent with the positive eu anomaly in the ree pattern a mass balance calculation indicates that hydrothermal fluid would account for 10 15 20 25 40 55 and 75 of si for the fluid temperature of 100 c 150 c 200 c and 250 c respectively although large igneous provinces are not discovered in the north china block during the deposition of the wumishan formation the studied wumishan cherts have significant contribution from hydrothermal fluid which might be related to the early rifting activities in the north china block before the breakup of supercontinent nuna

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

transfer of germanium to marine sediments insights from its accumulation in radiolarites and authigenic capture under reducing conditions some examples through geological ages in the geosphere germanium ge has a chemical behavior close to that of silicon si and ge commonly substitutes for si in small proportions in silicates studying the evolution of the respective proportions of ge and si through time allows us to better constrain the global si cycle the marine inventory of ge present as dissolved germanic acid is facing two main sinks known through the study of present sediments 1 incorporation into diatom frustules and transfer to sediments by these shuttles 2 capture of ge released to pore water through frustule dissolution by authigenic mineral phases forming within reducing sediments our goals are to determine whether such a bio induced transfer of ge is also achieved by radiolarian and whether ge could be trapped directly from seawater into authigenic phases with no intervention of opal secreting organisms shuttles to this end we studied two paleozoic radiolarite formations and geological formations dated of devonian jurassic and cretaceous deposited under more or less drastic redox conditions our results show that the ge si values observed for these radiolarites are close to slightly above those measured from modern diatoms and sponges in addition our results confirm what is observed with some present day reducing sediments the ancient sediments that underwent reducing depositional conditions are authigenically enriched in ge furthermore it is probable that at least a part of the authigenic ge came directly from seawater the recurrence and extent through time and space of anoxic conditions affecting sea bottoms have been quite important through the geological times consequently the capture of ge by reducing sediments must have impacted ge distribution and in turn the evolution of the seawater ge si ratio

pollen based continental climate reconstructions at 6 and 21 ka a global synthesis subfossil pollen and plant macrofossil data derived from 14c dated sediment profiles can provide quantitative information on glacial and interglacial climates the data allow climate variables related to growing season warmth winter cold and plant available moisture to be reconstructed continental scale reconstructions have been made for the mid holocene mh around 6 ka and last glacial maximum lgm around 21 ka allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth assessment report ar5 of the intergovernmental panel on climate change the synthesis of the available mh and lgm climate reconstructions and their uncertainties obtained using modern analogue regression and model inversion techniques is presented for four temperature variables and two moisture variables reconstructions of the same variables based on surface pollen assemblages are shown to be accurate and unbiased reconstructed lgm and mh climate anomaly patterns are coherent consistent between variables and robust with respect to the choice of technique they support a conceptual model of the controls of late quaternary climate change whereby the first order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance

shear localization in transformation induced faulting first order similarities to brittle shear failure abstract transformation induced faulting a leading candidate for the mechanism of deep focus earthquakes leads to catastrophic shear failure in laboratory specimens by the growth and coalescence of microscopic mode i anticracks or packets of denser microcrystalline transformed material we have studied this phenomenon in mg 2 geo 4 olivine undergoing a polymorphic phase transformation to its denser spinel phase at 2 7 gpa and 1200 k under applied stress in order to better understand the details of anticrack growth and coalescence which leads to localized shear zones and macroscopic failure here we report observations that address the early stages of shear localization preserved in one experiment of this series are microstructures typical of anticrack nucleation and growth as well as those involved in the self organization of these features during the shear localization process we have analyzed sem and optical images of individual anticrack reaction interfaces and coalesced structures from this experiment and another experiment conducted under similar conditions but with a different strain history we have isolated the reaction boundaries in these images and have performed fractal analysis of these boundary lines we find that the fractal dimension of isolated uncoalesced anticrack structures is consistently higher than that of structures formed from coalescence and coarsening of fine grained reaction products this result is consistent with the varying effects of stress and thermal feedback at these interfaces and also represents an interesting parallel with the fractal nature of brittle cracks and fault surfaces we also measured the length of the long axis of uncoalesced anticrack structures found in the experiment arrested at failure and find a roughly log normal distribution of sizes very similar to the distribution of microcrack lengths reported in experiments involving brittle shear failure microcracks and microanticracks both also commonly nucleate on grain boundaries intracrystalline flaws and phase boundaries shear zones faults formed by either of these mechanisms also regularly form at 30 to maximum compression from this set of observations we conclude that the general phenomenon of shear failure creates similar features in both systems perhaps in any system which generates a shear instability despite the major differences in the detailed microphysics because of the detailed and unambiguous measurements that are possible with images of transformation induced faulting experiments further studies of this faulting mechanism may provide new insights into brittle failure and shear localization processes

a revised picture of the structure of the monsoon and land itcz over west africa this article presents an overview of the land itcz intertropical convergence zone over west africa based on analysis of ncar ncep reanalysis data the picture that emerges is much different than the classic one the most important feature is that the itcz is effectively independent of the system that produces most of the rainfall rainfall linked directly to this zone of surface convergence generally affects only the southern sahara and the northern most sahel and only in abnormally wet years in the region a second feature is that the rainbelt normally assumed to represent the itcz is instead produced by a large core of ascent lying between the african easterly jet and the tropical easterly jet this region corresponds to the southern track of african easterly waves which distribute the rainfall this finding underscores the need to distinguish between the itcz and the feature better termed the tropical rainbelt the latter is conventionally but improperly used in remote sensing studies to denote the surface itcz over west africa the new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift which in turn is controlled by the characteristics of the african easterly jet and tropical easterly jet rather than by moisture convergence this new picture also includes a circulation feature not generally considered in most analyses of the region this feature a low level westerly jet termed the african westerly jet plays a significant role in interannual and multidecadal variability in the sahel region of west africa included are discussions of the how this new view relates to other aspects of west africa meteorology such as moisture sources rainfall production and forecasting desertification climate monitoring hurricanes and interannual variability the west african monsoon is also related to a new paradigm for examining the interannual variability of rainfall over west africa one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north south displacements of this feature the new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the african easterly waves the disturbances that are linked to rainfall over west africa and spawn hurricanes over the atlantic and with our knowledge of the prevailing synoptic and dynamic features this article demonstrate a new aspect of the west africa monsoon a bimodal state with one mode linked to dry conditions in the sahel and the other linked to wet conditions the switch between modes appears to be linked to an inertial instability mechanism with the cross equatorial pressure gradient being a critical factor the biomodal state has been shown for the month of august only but this month contributes most of the interannual variability this new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability but also to the multidecadal variability evidenced in the region between the 1950s and 1980s

plastic deformation experiments to high strain on mantle transition zone minerals wadsleyite and ringwoodite in the rotational drickamer apparatus we report the results of plastic deformation experiments on polycrystalline wadsleyite and ringwoodite performed at 15 23 gpa and 1300 2100 k conducted using the rotational drickamer apparatus rda wadsleyite was synthesized from fine grained 2 mm san carlos olivine in a kawai type multianvil apparatus the average grain size of the resulting wadsleyite was 1 2 mm the initial water content of the undeformed wadsleyite was 24 000 26 000 h 10 6 si but the final water content is variable and less than the initial water content ringwoodite was synthesized from wadsleyite in situ in the rda both strain and stress were measured in situ using a synchrotron x ray facility determinations of strains and strain rates were made from x ray radiographs of the sample using a mo foil strain marker in the sample assembly the state of stress was determined from the observed d spacing of multiple lattice planes as a function of azimuth angle samples were deformed at various strain rates at around 10 4 10 5 s 1 deformation mechanisms were inferred from the stress exponent and the microstructures we determined the stress exponent n for wadsleyite to be 673 suggesting dislocation creep was the dominant deformation mechanism in wadsleyite at grain sizes of 1 mm our samples were still deforming primarily by dislocation creep however small dislocation free grains are also observed suggesting that diffusion creep may operate in some parts of our samples 2012 published by elsevier b v

flow law for ringwoodite at subduction zone conditions the plastic properties of ringwoodite a high pressure modification of olivine are experimentally determined at the pressure of 20 gpa and temperatures up to 1350 c stress relaxation experiments have been carried out on fine grained samples using a two stage t cup multiple anvil high pressure apparatus with synchrotron generated x rays we observe the presence of a significant weakening of the shear strength with a decrease from 2 5 to 0 2 gpa between 600 and 1000 c in this region the activation energy u is about 275 kj mol and it decreases with increasing stress reaching a value of about 200 kj mol when the shear stress attains 2 5 gpa we propose that the plasticity is associated with vigorous dislocation glide controlled by short range lattice resistance forces at lower temperatures the shear strength is mildly dependent on temperature reaching 3 gpa at room temperature while above 1000 c the flow may be governed by a power law relation with n about 3 5 2003 elsevier science b v all rights reserved

a test of ge si opal as a paleorecorder of ge si seawater late pleistocene variations of germanium to silicon ratios in marine diatom shells from sediment cores ge si opal are coherent with the global isotope record of glacial to interglacial climate change these variations are thought to reflect changes in ge si seawater driven by climate modulated alterations in oceanic ge si sources and sinks however an important criterion for interpreting ge si opal as a monitor of whole ocean ge si seawater is that the opal burial ratio be insensitive both to local diatom production and surface ocean silica concentrations so called biological fractionation effects and to differential dissolution artifacts so called diagenesis offsets here we test these assumptions by comparing model ocean sediment ge si opal distributions with data from holocene and glacial sediments across the high latitude indian antarctic ocean siliceous ooze belt in contrast to the model the data show no gradients in either holocene or glacial ge si opal values across productivity zones displaying dramatic changes in biosiliceous production opal burial and dissolution this evidence supports the contention that fractionation effects are small and that observed down core variations in ge si opal faithfully record secular changes in ge si seawater

diagenetic ge si fractionation in continental margin environments further evidence for a nonopal ge sink we present data for dissolved germanic and silicic acids from several settings sediment pore water profiles collected from the peru chile continental margin fluxes measured with in situ benthic flux chambers and shipboard whole core incubations and water column profiles from the california continental margin collectively these data show that ge and si are fractionated in these continental margin sedimentary environments during diagenesis with 50 of the ge released by opal dissolution being sequestered within the sediments the areal extent of this diagenetic fractionation covers station depths from 100 m to 1000 m sediments from these sites typically have high pore water fe2 present in the upper 2 cm at sites with low fe2 concentrations in the upper pore waters the ge si benthic regeneration ratio indicates little or no fractionation during diatom dissolution consistent with the sedimentary fractionation water column dissolved ge si ratios along the continental margin are on average lower 0 66 mol mol than the global average ratio 0 72 mol mol mortlock and froelich 1996 this lower average ratio is driven by two distinct ge si data trends having similar slopes but different intercepts data from the upper 150 m has a ge si slope of 0 74 0 04 mol mol 2 and an intercept of 0 5 0 4 whereas below 550 m the slope is 0 70 0 06 mol mol but the intercept is 5 0 8 0 assuming that the sediments sampled here are representative of all reducing marine environments an assumption requiring further testing our calculations indicate that sequestration of ge into a nonopal phase throughout the global ocean in the depth range 0 2 1 km is sufficient to balance the ge budget thus we tentatively conclude that sequestering of ge in reducing continental margin sediments is the missing ge sink

regional deformation and offshore crustal local faulting as combined processes to explain uplift through time constrained by investigating differentially uplifted late quaternary palaeoshorelines the foreland hyblean plateau se sicily

germanium incorporation into sponge spicules development of a proxy for reconstructing inorganic germanium and silicon concentrations in seawater abstract measurements of germanium ge in deep sea sponge skeletons are presented for sponges collected by dredge and for spicules isolated from a range of deep sea sediment cores germanium to silicon si ratios ge si sp for sponge silica ranged between 0 075 and 0 380 mol mol which are much lower than the present day seawater ge si ratio of 0 7 mol mol a plot of ge si sp versus estimated seawater ge and si concentration produced a linear relationship with the ge content of spicules increasing with seawater ge concentration plots of ge si sp versus depth for both dredged sponges and sediment bound spicules produced oceanic profiles similar to those of dissolved ge and si concentration to explain the fractionation seen in the sponge ge si sp data two models are presented the first model used to interpret the data assumes that sponges only respond to the ge content of the ambient seawater implying that ge incorporation into sponge silica is independent of the seawater ge si ratio up to a si concentration of about 100 mol l this model is consistent with the 68 ge uptake results of davie et al e i davie t l simpson r garonne experimental germanium incorporation into siliceous sponge spicules biol cell 48 1983 191 202 for cultured spicules their results showed that the incorporation of ge in sponge silica is only dependent on the ge concentration of the water in which a sponge is growing the second model used to explain the data assumes that ge si sp fractionation results from subtle differences in the uptake kinetics of ge and si while the assumptions used by each model to describe the data are different it is possible to use sponge ge si sp data to reconstruct palaeo ge concentrations using model i and to reconstruct palaeo si concentrations using both models palaeo si concentrations estimated using both models are in good agreement

what have we learnt from palaeoclimate simulations there has been a gradual evolution in the way that palaeoclimate modelling and palaeoenvironmental data are used together to understand how the earth system works from an initial and largely descriptive phase through explicit hypothesis testing to diagnosis of underlying mechanisms analyses of past climate states are now regarded as integral to the evaluation of climate models and have become part of the toolkit used to assess the likely realism of future projections palaeoclimate assessment has demonstrated that changes in large scale features of climate that are governed by the energy and water balance show consistent responses to changes in forcing in different climate states and these consistent responses are reproduced by climate models however state of the art models are still largely unable to reproduce observed changes in climate at a regional scale reliably while palaeoclimate analyses of state of the art climate models suggest an urgent need for model improvement much work is also needed on extending and improving palaeoclimate reconstructions and quantifying and reducing both numerical and interpretative uncertainties

instabilities from phase transitions and the timescales of mantle thermal evolution we have investigated the effects of mantle phase transitions on the earth s thermal evolution by numerical modelling our results show that the tendency of layering grows with increasing vigor of convection a self consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed for a temperature dependent effective viscosity we found that the thermal history of the earth is characterized by two timescales one is associated with layered convection in the early period which is followed by a transition period with dramatic overturns as the rayleigh number decreases sufficiently low whole mantle convection prevails and a faster cooling timescale ensues

experimental investigation of eclogite rheology and its fabrics at high temperature and pressure eclogite plays an important role in mantle convection and geodynamics in subduction zones an improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology however the deformation processes and associated fabrics in eclogite are not well understood incompatible views of deformation mechanism have been proposed for both garnet and omphacite we present here deformation behaviour of eclogite at temperatures of 1027 1427 c confining pressures of 2 5 3 5 gpa and strain rates of 1 10 5 s 1 to 5 10 4 s 1 we obtained a power law creep for the high temperature and pressure deformation of a dry eclogite 50 vol garnet 40 omphacite and 10 quartz with a 103 3 1 0 n 3 5 0 4 e 403 30 kj mol 1 and v 27 2 cm3 mol 1 the two principal minerals of eclogite have greatly different strengths progressive increase of garnet results in a smooth increase in strength analysis by electron back scattered diffraction shows that 1 garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions 2 omphacite shows pronounced lattice preferred orientations lpos suggesting a dominant dislocation creep mechanism further investigation into the water effects on eclogite show 3 water content does not influence the style of omphacite fabric but increases slightly the fabric strength 4 grain boundary processes dominate the deformation of garnet under high water fugacity or high shear strain conditions yielding a random lpo similar to that of non deforming garnet despite the strong shape preferred orientation spo observed 110 001 slip may dominate the deformation of rutile quartz displays complicated and inconsistent lpos in eclogite these results are remarkably similar to observations from deformed eclogites in nature

paleozoic and mesozoic silica rich seawater evidence from hematitic chert jasper deposits laterally extensive beds of highly siliceous hematitic chert jasper are associated with many volcanogenic massive sulfide vms deposits of late cambrian to early cretaceous age yet are unknown in analogous younger including modern settings textural studies suggest that vms related jaspers in the ordovician lokken ophiolite of norway were originally deposited as si and fe rich gels that precipitated from hydrothermal plumes as colloidal silica and iron oxyhydroxide particles rare earth element patterns and ge si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 10 3 to 10 4 similar to modern examples we propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids as suggested by previous workers but instead was deposited from silica rich seawater flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume a model involving amorphous silica opal a precursors to the jaspers suggests that silica contents of cambrian early cretaceous oceans were at least 110 mg l sio 2 compared to values of 40 60 mg l sio 2 estimated in other studies the evolution of ancient silica rich to modern fe rich precipitates in submarine hydrothermal plumes reflects a changeover from silica saturated to silica depleted seawater through phanerozoic time due mainly to ocean wide emergence of diatoms in the cretaceous

germanium silicon fractionation in a river influenced continental margin the northern gulf of mexico abstract in this study we have sampled the water column and sediments of the gulf of mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of ge and si water column ge si ratios across the gulf of mexico continental shelf range from 1 9 to 25 mol mol which is elevated compared to the global ocean value of 0 7 mol mol the ge enrichment in the gulf of mexico seawater is primarily due to anthropogenic contamination of the mississippi river which is the main ge and si source to the area and to a smaller extent due to discrimination against ge during biogenic silica bsi production ge si 1 2 1 8 mol mol especially by radiolarians and siliceous sponges ge si 0 6 1 1 mol mol most sediment pore waters ge si 0 3 4 5 mol mol and sediment incubation experiments benthic flux ge si 0 9 1 2 mol mol indicate precipitation of authigenic phases that sequester ge from pore waters non opal sink this process appears to be independent of oxidation reduction reactions and suggests that authigenic aluminosilicate formation reverse weathering may be the dominant ge sink in marine sediments compilation of previously published data shows that in continental margins non opal ge burial flux is controlled by bsi supply while in open ocean sediments it is 10 100 times lower and most likely limited by the supply of lithogenic material we provide a measurement based estimate of the global non opal ge burial flux as 4 32 mmol yr 1 encompassing the 2 16 mmol yr 1 needed to keep the global marine ge cycle at steady state

early diagenesis of germanium in sediments of the antarctic south atlantic in search of the missing ge sink pore water and solid phase geochemistry profiles were obtained from several cores between 41 degree s and 53 degree s in the atlantic sector of the southern ocean pore water nitrate manganese and iron profiles delineate standard redox zones in these sediments and help characterize those with classic vs burn down behaviors pore water si and ge profiles demonstrate that ge released during opal dissolution is removed pervasively throughout the uppermost interval of silicate release and also downwards into the suboxic zone by as yet unidentified precipitation mechanisms these results indicate that early diagenesis of ge is uncoupled from that of opal solid phase extractions fe mn u mo ge cu ni co v and cd in a few cores suggest that anthigenic ge removal in the suboxic zone is not associated with peaks in authigenic mn cycling mno sub 2 and related metals but rather with processes deeper in the sediments perhaps fe or u diagenesis below the interval of ge removal pre water ge increases linearly with depth by over two orders of magnitude indicating a deep below recovery source of large magnitude the fraction of opal derived ge precipitated authigenically in these sediments ranges from approximately 1 to 96 and correlates strongly with the detrital fraction asmore well as the detrital to opal ratio both of which generally decrease from north to south the ge sink observed in these sediments would need to be globally representative to account for the entire missing ge sink in today s oceanic ge balance which seems unlikely benthic fluxes of ge and si estimated from these pore water profiles and from measurements in three benthic flux chamber experiments at high carbon rain continental margin sites demonstrate that the ge si rate released from the seafloor in locations with high benthic silicate and carbon fluxes is congruent with holocene opal dissolution ge si approximately 0 7 x 10 sup minus 6 in contrast ge si flux ratios in areas with low silicate fluxes are ge depleted ge si 0 5 x 10 sup minus 6 the authors speculate that glacial interglacial changes in oceanic ge si as recorded in diatoms may be due in part to variations in this authigenic ge sink perhaps related to shifts in siliceous productivity from open ocean antarctic siliceous oozes during interglacials to areas of higher detrital input sub antarctic continental margins and possibly also to generally more reducing conditions in sediments during glacials less

terrestrial paleorecords of ge si cycling derived from lake diatoms abstract modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium silicon ge si ratios compared to regions with less extensive chemical weathering thought to be the result of ge fractionation in 2 1 clays temporal variations in ge si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering however this relationship has not been adequately ground truthed by a terrestrial based paleoclimate study utilizing ge si ratios in lake diatoms to this end a sediment core was extracted from dry lake el 2763 m in the headwaters of the santa ana river of southern california in july 1996 the dry lake drainage basin is comprised of biotite muscovite gneiss and granite with sparse pine forests and relatively high relief the core had a basal ams 14c age of 8 350 60 ybp we successfully separated diatom samples large enough for cleaning dissolution and chemical analysis without contamination by detrital materials confirmed by trace metal analyses two dissolved opal samples were analyzed for ge and si concentrations along with modern water samples collected from waters within the drainage basin diatoms obtained from 8000 year old sediments near the bottom of the core had an opal ge si of 0 79 10 6 mol mol a composite sample of diatoms deposited within the past 100 years yielded a significantly lower ge si of 0 34 10 6 analysis of ge si was also performed from a variety of other materials in the dry lake basin including stream and lake water unweathered bedrock soils and mineral separates together with sedimentologic records from the lake sediments and ge si recorded in other basin materials it appears that the high ge si values recorded in the 8000 year opal sample were the result of preferential early weathering of high ge si biotite and muscovite minerals from the slopes thus the detailed examination of ge si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive ge si paleorecords in some settings

last glacial maximum and holocene climate in ccsm3 abstract the climate sensitivity of the community climate system model version 3 ccsm3 is studied for two past climate forcings the last glacial maximum lgm and the mid holocene the lgm approximately 21 000 yr ago is a glacial period with large changes in the greenhouse gases sea level and ice sheets the mid holocene approximately 6000 yr ago occurred during the current interglacial with primary changes in the seasonal solar irradiance the lgm ccsm3 simulation has a global cooling of 4 5 c compared to preindustrial pi conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at lgm tropical sea surface temperature sst cools by 1 7 c and tropical land temperature cools by 2 6 c on average simulations with the ccsm3 slab ocean model suggest that about half of the global cooling is explained by the reduced lgm concentration of atmospheric co2 50 of present day concentrations there is an increase in the antarctic circumpolar current an

plastic anisotropy and slip systems in ringwoodite deformed to high shear strain in the rotational drickamer apparatus abstract high shear strain deformation experiments up to equivalent strains of 180 were performed on ringwoodite at conditions of 23 gpa and 1800 k at very large shear strains deviation in the strengths of the 3 1 1 and 4 0 0 lattice planes is observed indicating plastic anisotropy lattice strain theory was applied to calculate effective viscosities of the available slip systems from the observed plastic anisotropy we find that the effective viscosities unitless are proportional to 0 43 0 46 and 1 54 for the 1 1 1 1 1 0 1 1 0 1 1 0 and 0 0 1 1 1 0 slip systems respectively this indicates that 1 1 1 1 1 0 slip is slightly softer than 1 1 0 1 1 0 at these conditions additionally the 1 1 1 1 1 0 slip system has significantly more symmetric variants than the slip systems involving other lattice planes thus it is expected to dominate deformation this is confirmed by polycrystal plasticity modeling using the viscoplastic self consistent code vpsc which indicates that simulations with dominant 1 1 1 1 1 0 slip provide the best match to experimental textures

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

germanium silica ratios in diagenetic chert nodules from the ediacaran doushantuo formation south china abstract germanium silica ge si ratios of dolostone and mudstone hosted chert nodules from the ediacaran 635 542 ma doushantuo formation in the yangtze gorges area south china are reported these chert nodules typically have a calcite rim a pyrite rim and a silica core the latter sometimes containing disseminated pyrite the silica core was precipitated by early diagenetic replacement of carbonate and silty muddy sediments two types of chert nodules are identified based on their mineralogy and geochemistry type 1 chert nodules are poor in disseminated pyrite in the silica core they also have low al and show a strong positive correlation between al contents and ge si with a near zero or negative intercept in contrast type 2 chert nodules contain abundant disseminated pyrite in the silica core and show a weakly positive correlation between ge si ratios and al contents with a large positive intercept on the ge si axis the ge si of doushantuo nodules are greater than those of cretaceous deep sea cherts suggesting that the ge si ratio of ediacaran seawater porewater was greater than the cretaceous due to the more effective discrimination against ge by inorganic opal precipitation relative to biogenic opal precipitation the positive correlation between ge si and al can be interpreted using a mixing model with a pure chert characterized by a low ge si ratio and an al rich endmember characterized by a high ge si ratio the latter is most likely represented by a clay component but the model based estimate of the ge si ratio inferred for the al rich clay endmember is much higher than that of phanerozoic clay minerals these high ge si ratios for the clay endmember may be related to the generally high ge si ratio of ediacaran seawater but could also be related to clay organic matter interactions organic matter absorbed to clays could provide an additional source of ge because certain organic molecules are known to have a high affinity for ge due to their strong metal ion chelating properties the high ge si ratio of the al rich endmember in type 1 chert nodules suggests that ge in porewaters from which these cherts precipitated may have been dominated by ge organic complexes the low ge si ratio inferred for the al rich endmember in type 2 chert nodules is therefore taken to indicate that ge was released from organic matter perhaps due to anaerobic degradation of organic matter accompanied by the formation of pyrite and was redistributed between clay organic endmembers and pure chert resulting in a non zero intercept in the ge si vs al 2 o 3 plots these observations suggest that a strong terrestrial influence in a restricted sedimentary basin or a high content of dissolved organic carbon in ediacaran seawater and porewater may have contributed to the dominance of ge organic complexes in the doushantuo basin in the yangtze gorges area

a late pliocene diatom ge si record from the southeast atlantic abstract as a result of both culture and sediment core studies the ratio of germanium ge to silicon si in diatom shells has been proposed as a proxy for monitoring whole ocean changes in seawater ge si a ratio affected by changes in continental weathering however because of the difficulties of extracting and cleaning diatom frustules from deep sea sediments only samples from highly pure diatom oozes in the antarctic region have been previously analyzed here we present data on diatom ge si ratios ge si opal for the time interval between 3 1 and 1 9 ma from a mid latitude coastal upwelling area where significant terrigenous sediment input complicated the sample processing and analyses in general our ge si opal values show the same decreasing trend after 2 6 ma than previously measured in antarctic sediments shemesh et al 1989 paleoceanography 4 221 231 but with a noisier background that may reflect the local imprint of proximal continental input superimposed upon global changes in the ocean reservoir the time of initiation of large scale north hemisphere glaciation at 2 6 ma is characterized by a declining pattern of diatom ge si ratios which could have resulted from a global increase in the input of riverine si due to enhanced silica weathering and or equatorward northward intrusions of subantarctic waters enriched in silica high ge si opal ratios are associated with high opal contents from the same sediment samples and with warm climate as indicated by depleted benthic foraminiferal 18 o values from the north and equatorial atlantic cold periods signified by enriched benthic 18 o values on the contrary are associated with lower ge si opal ratios we interpret diatom ge si values to reflect the prevailing weathering state on the continents with greater chemical weathering during warm and wet periods of the pliocene and less during cooler and drier intervals

deformation of olivine spinel aggregates in the system mg ni 2geo4 deformed to high strain in torsion implications for upper mantle anisotropy 1 deformation of mantle phases to high strain is fundamental for understanding mantle dynamics however it is technically challenging to perform deformation experiments yielding accurate mechanical results at the high pressures required for certain mantle phases such as wadsleyite and ringwoodite deformation experiments on analog materials of such high pressure phases can provide insight into the deformation properties of the mantle the mg ni 2geo4 system undergoes the olivine spinel transformation at much lower pressures than the silicate counterpart hence the high strain deformation properties of an olivine spinel rock can be studied at experimentally tractable pressures and temperatures experiments were conducted in a gas medium deformation apparatus at constant angular velocity a temperature of 1473 k and 300 mpa confining pressure at these conditions with a mg mg ni ratio of 0 9 the resulting aggregate comprised 70 olivine 30 spinel and 1 orthopyroxene the applied strain rate ranged from 10 4 to 10 5 s 1 yielding shear stresses supported by the samples of between 100 and 250 mpa samples were deformed to a range of shear strains between 1 and 7 in order to study the microstructural development during high strain deformation in these experiments the spinel phase did not deform to produce a crystallographic preferred orientation cpo the olivine phase produced a cpo which is consistent with slip on the 001 hk0 slip system if this slip system is dominant in olivine below 250 300 km in the upper mantle the observed seismic anisotropy can be explained without resorting to changes in flow regime or deformation mechanism at this depth

a new class of microstructures which lead to transformation induced faulting in magnesium germanate 1 transformation induced faulting has been studied extensively in natural olivine ice and a variety of other olivine analog systems over the last 15 years and remains a likely candidate mechanism for initiating deep focus earthquakes the mechanism involves the formation of microscopic packets of fine grained denser materials from a metastable host which then behave as mode i features that coalesce to allow bulk mode ii shear failure we present evidence for a new class of transformation microstructures that also allows shear failure at high pressures but which does not require the formation of widely distributed mode i features prior to shear failure we have observed thin planar zones of transformation in coarse grained 150 m mg2geo4 olivine that form along crystallographic planes in deformed grains at temperatures cooler than those that allow significant development of anticracks the development of these zones is rapid and only occurs after 25 bulk strain increasing strain leads to progressive development of these zones in turn leading to bulk shear failure once a continuous pathway of fine grained spinel is formed that traverses the specimen transmission electron microscopy analysis shows that slip bands in olivine along 011 and 010 formed from dislocation pileups on these slip planes transform preferentially to a nanocrystalline aggregate of the denser spinel phase aided by the high strain energy of these bands this new partially strain driven mechanism compliments shear failure by anticracks as described in previous studies and adds new possibilities for the triggering of bulk shear failure by rapid mineralogical transformations

diatom inferred salinity and carbonate oxygen isotopes in holocene waterbodies of the western sahara and sahel africa abstract thirteen holocene palaeolakes in the western sahara and sahel have provided diatom records with carbonate oxygen isotope profiles available from eight of them most of these palaeolakes were groundwater fed lake water chemistry is reconstructed using diatom transfer functions lake water salinity and 18 o records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15 000 yr data are in general agreement with climate simulations using coupled atmosphere ocean vegetation models which show a mid holocene wetting over the whole of northwest africa and a rapid drying by 6 4 ka the lake record also shows that at many sites the major lake infilling lags the end of the younger dryas by 1 2 ka regional differences also appear in the timing of the lake hydrological optimum ca 10 5 8 5 and 7 5 4 5 ka in the northern sahara 10 8 5 ka in the air tenere 10 5 7 or 4 5 in the sahel and 7 5 ka in lake chad the whole of the holocene is punctuated by short term drying events changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity during the wet holocene period however the very low values in the southern sahara also imply changes in the moisture transport pattern or rainfall mechanisms data suggest an apparent decrease in 18 o content of precipitation along the monsoon flow in contrast with modern patterns changes in water availability and quality have driven population migrations in and out of the sahara sahel but relationships between climate and cultures are complex short term dry events might have driven inventive adaptations in the sahara drying at 5 4 5 ka coincides with both the collapse of the classical neolithic civilization and the settlement of new cultures

maintenance of the african easterly jet the maintenance of the african easterly jet aej has been examined using a zonally symmetric general circulation model with simple parametrizations it is shown that the aej is maintained in association with two diabatically forced meridional circulations one associated with surface fluxes and dry convection in the saharan heat low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this the heat low heating which reaches the height of the aej around 700 mb is particularly important in maintaining the aej and its associated meridional gradients in potential vorticity it is concluded that the mean observed aej results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it

phase transformations in a harzburgite composition to 26 gpa implications for dynamical behaviour of the subducting slab abstract the mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5 26 gpa at temperatures of 1300 1400 c the pyroxene garnet component of the harzburgite composition harzburgite minus 82 wt olivine transforms to majorite garnet by 18 19 gpa and further disproportionates to the assemblage of garnet stishovite mg 2 sio 4 spinel above 20 gpa at still higher pressures first ilmenite 22 24 gpa and then perovskite mgsio 3 24 26 gpa are found to coexist with garnet garnet disappears at 26 gpa and almost complete transition to perovskite is achieved at this pressure the mineral proportions and density profiles in the subducting oceanic lithosphere modelled by a combination of 80 harzburgite 20 primitive morb compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km under isothermal conditions the slab has a density similar to surrounding mantle to a depth of 600 km however between 600 and 700 km the slab is up to 0 08 g cm 3 denser than surrounding mantle this is caused primarily by the higher alumina content in pyrolite as compared to harzburgite which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite the presence of alumina also smears out the garnet perovskite transition in pyrolite over a depth interval of 50 km whereas this transformation is much sharper in the harzburgite composition calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser by 0 1 0 3 g cm 3 than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth these results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

possible flush instability in mantle convection at the archaean proterozoic transition the late archaean eon 3 to 2 5 billion years ago seems to have been a time of profound geological changes1 5 rapid growth of the continents of the volume of cratonic sediments and of the area of the continental shelves was accompanied by widespread emplacement of potassium rich granitic rocks leading to the cratonization of the continental crust3 6 there is also evidence for climate change7 8 at this time possibly reflecting changes in the composition of the atmosphere and the volume of the oceans the period ends with the huronian glaciation the first well documented glacial episode9 and this transition appears to be marked by a strong increase in the intensity of the geomagnetic field10 here we present simulations of mantle convection at the end of the late archaean and show that a breakdown of two layer convection a flush instability11 occurring at this time could have led to a period of volcanic and tectonic activity that might account for the reported geological and palaeoenvironmental changes

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

rheology of mg2geo4 olivine and spinel harzburgite implications for earth s mantle transition zone geophysical modeling requires an upper mantle low viscosity zone lvz recent results suggest that the lvz could be within the mantle transition zone mtz or above it depending on boundary conditions the rheology of the olivine dominated uppermost mantle is well constrained experimentally but not the mtz here we report first time measurement of the rheology of mg2geo4 spinel analogue of ringwoodite the dominant mineral of the lower mtz in the dislocation creep regime and compare it with that of mg2geo4 olivine in both cases we incorporate 15 20 pyroxene the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies under similar conditions spinel dominated material is about twice as strong as olivine dominated material our results suggest that unless h2o weakening of ringwoodite or phase transformation induced grain size reduction occurs in the lower mtz it will be more viscous than above the 410 km discontinuity

shearing instabilities accompanying high pressure phase transformations and the mechanics of deep earthquakes deep earthquakes have been a paradox since their discovery in the 1920s the combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers yet earthquakes occur continually in subduction zones to 700 km the expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self organized failure analogous to but different from brittle failure the only high pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material this fluid spontaneously forms mode i microcracks or microanticracks that self organize via the elastic strain fields at their tips leading to shear failure growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher pressure polymorphs in either case fault propagation could be enhanced by shear heating just as is sometimes the case with frictional sliding in the crust extensive seismological interrogation of the region of the tonga subduction zone in the southwest pacific ocean provides evidence suggesting significant metastable olivine with implication for its presence in other regions of deep seismicity if metastable olivine is confirmed either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast

elasticity and strength of hydrous ringwoodite at high pressure oh bearing hydrous ringwoodite compressed non hydrostatically in a diamond anvil cell supports a differential stress that increases from 2 9 to 4 5 gpa over the pressure range of 6 7 13 2 gpa at room temperature this result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart kaver and duffy geophys res lett 28 2001 2691 2694 the elastic anisotropy 2c44 c11 c12 of hydrous ringwoodite is measured to be 0 87 7 throughout this pressure range similar to measured values for anhydrous ringwoodite kaver and duffy geophys res lett 28 2001 2691 2694 this lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure these results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature resulting in stronger preferred lattice orientation this in turn may be seismically detectable which opens the possibility of using seismic anisotropy as a marker for local volatile containing areas within the upper mantle and transition zone

anisotropic growth in the olivine spinel transformation of mg2geo4 under nonhydrostatic stress abstract the olivine spinel phase transformation in mg 2 geo 4 does not occur by a martensitic mechanism the evidence from samples transformed in a griggs type solid medium deformation apparatus are 1 1 lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism 2 2 the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations 3 3 application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress a velocity ratio of 1 7 0 7 5 4 has been determined for the growth rate of the spinel from measurements on residual olivine grains the interphase grain boundary in samples transformed under stress has cusp shaped fingers of spinel with a blunt end separated by thin spikes of olivine samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel the preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress the predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end the discrepancy may be due to surface energy which has not been considered here or to local deviations of the applied macroscopic stress

sensitivity of the african and asian monsoons to mid holocene insolation and data inferred surface changes abstract orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in africa shown by data during the mid holocene 6000 yr ago feedbacks associated with changes in ssts and land surface cover may be necessary to produce a sufficient increase in the monsoon a step toward a better understanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings inferred from observations in the first study ssts are lowered in the upwelling regions offshore of west africa and somalia and increased in the bay of bengal and south china sea in the second simulation the modern sahara desert is replaced by a combination of xerophytic woods scrub and grassland in both cases the amount of water vapor advected from oceanic sources is increased north of 10 n in africa in response to the increased land sea temperature contrast thereby enhancing rainfall but the magnitude of the simulated changes is much larger when land s

germanium silica ratio and rare earth element composition of silica filling in sheet cracks of the doushantuo cap carbonates south china constraining hydrothermal activity during the marinoan snowball earth glaciation abstract the rapid diversification of eukaryotes within 3 million years after the marinoan global glaciation suggests the potential linkage between the snowball earth event and biological evolution to understand how the global glaciation and the biological evolution were linked it is necessary to reconstruct the sequence of events with high temporal resolution that occurred during the deglaciation in this study we focus on silica filling within sheet cracks sheet cracks abundantly occur in the basal part of the marinoan cap carbonate 635 ma and are characterized by the interconnected horizontal and vertical cracks that are sequentially cemented by dolospars silica and calcite as the second stage of crack filling the origin of silica filling remains controversial if the seawater origin is confirmed the silica filling might archive the signal of ocean chemistry in the aftermath of snowball earth event in this study we measured the germanium silica ratios ge si and rare earth elements ree compositions of silica filling from the doushantuo cap carbonate in the yangtze platform south china the silica filling has low ge si ratios 1 mol mol and displays a light ree lree depleted ree pattern normalized to paas with positive eu anomalies eu eu these results indicate the predominant seawater origin of silica with some contribution from hydrothermal fluid suggesting the penecontemporaneous precipitation of silica filling with respect to sheet crack formation in order to quantify the contribution from hydrothermal fluid we developed a binary mixing model by using ge si ratio and eu eu as two independent proxies the modeling results indicate mixing of 5 vol hydrothermal fluid with contemporaneous seawater or 20 wt of si in silica filling from hydrothermal fluid the modeling results also show that the synglacial hydrothermal flux might be one order of magnitude lower than the modern level suggesting extremely weak hydrothermal activities during the marinoan snowball glaciation

strong inheritance of texture between perovskite and post perovskite in the d layer flow in the deep mantle is thought to create textures in the high pressure mineral post perovskite laboratory simulations of the transformation between lower pressure perovskite and post perovskite show that post perovskite can also inherit textures from the perovskite phase and vice versa

theoretical prediction for several important equilibrium ge isotope fractionation factors and geological implications article i nfo this study estimates equilibrium fractionation factors in the ge isotope system including the dominant aqueous ge oh 4 and geo oh 3 species in seawater ge bearing organic complexes e g ge catechol ge oxalic acid and ge citric acid and ge in quartz or opal albite k feldspar olivine and sphalerite like structures estimations are based on urey model or bigeleisen mayer equation and high level quantum

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

creep mechanism in mg2geo4 effects of a phase transition we deformed polycrystalline mg2geo4 olivine and spinel in a griggs type solid medium apparatus flow of mg2geo4 olivine can be represented by 6 5 l07 3 5 exp 105 kcal mol rt where kbar is the differential stress and s 1 is the natural strain rate for mg2geo4 spinel the flow law is 2 6 104 2exp 73 kcal mol rt the low stress exponent and activation enthalpy coupled with fine grain size 3 m suggest that mg2geo4 spinel deformed by a superplastic mechanism flow parameters for the olivine phase suggest a dislocation creep mechanism comparison of theoretical superplastic flow laws for mg2geo4 olivine with the spinel phase data suggests that strain rates in mg2geo4 spinel are only about a factor of 3 lower than for mg2geo4 olivine of the same grain size a similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion transformation from mg2geo4 olivine to spinel reduced grain size to approximately 3 m thus we might expect a similar reduction in grain size in the earth s transition zone which could result in superplastic deformation of the transformed phase and cause a weak decoupling zone at the transition boundary in the mantle superplasticity brought about by transformation induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle

coupled ge si and ge isotope ratios as geochemical tracers of seafloor hydrothermal systems case studies at loihi seamount and east pacific rise 9 50 n germanium ge and silicon si exhibit similar geochemical behavior in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater in this study ge isotope and ge si ratio systematics were investigated in low temperature hydrothermal vents from loihi seamount pacific ocean 18 54 n 155 15 w and results were compared to high temperature vents from the east pacific rise epr at 9 50 n loihi offers the opportunity to understand contrasting ge and si behavior in low temperature seafloor hydrothermal systems characterized by abundant fe oxyhydroxide deposition at the seafloor the results show that both ge si and 74 70ge in hydrothermal fluids are fractionated relative to the basaltic host rocks the enrichment in ge vs si relative to fresh basalts together with ge isotope fractionation 74 70gefluid basalt up to 1 15 at epr 9 50 n and 1 64 at loihi are best explained by the precipitation of minerals e g quartz and fe sulfides during higher temperature seawater rock reactions in the subsurface the study of fe rich hydrothermal deposits at loihi largely composed of fe oxyhydroxides shows that ge isotopes are also fractionated upon mineral precipitation at the seafloor we obtained an average ge isotope fractionation factor between fe oxyhydroxide ferrihydrite and dissolved ge in the fluid of 2 0 0 6 2sd and a maximum value of 3 6 0 6 2sd which is consistent with recent theoretical and experimental studies the study of a hydrothermal chimney at bio 9 vent at epr 9 50 n also demonstrates that ge isotopes are fractionated by approximately 5 6 0 6 2sd during precipitation of metal sulfides under hydrothermal conditions using combined ge si and estimated ge isotope signatures of ge sinks and sources in seawater we propose a preliminary oceanic budget of ge which reveals that an important sink referred as the missing ge sink may correspond to ge sequestration into authigenic fe oxyhydroxides in marine sediments this study shows that combining ge si and 74 70ge systematics provides a useful tool to trace hydrothermal ge and si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits

generation of the african easterly jet and its role in determining west african precipitation abstract an examination of analyses and model simulations is used to show that the african easterly jet forms over west africa in summer as a result of strong meridional soil moisture gradients in a series of gcm experiments the imposition of realistic surface wetness contrasts between the sahara and equatorial africa leads to strong positive meridional temperature gradients at the surface and in the lower troposphere the associated easterly shear in the atmosphere is strong enough to establish easterly flow the african easterly jet above the monsoon westerlies at the surface positive temperature gradients associated with the summertime distributions of solar radiation ssts or clouds are not large enough to produce the easterly jet in the absence of soil moisture gradients a thermally direct ageostrophic circulation is identified that can accelerate the largely geostrophic zonal flow and maintain the jet while moisture converges throughout the lower troposphere over east africa moisture divergenc

oceanic germanium silicon ratios evaluation of the potential overprint of temperature on weathering signals 1 diatom remains indicate the oceanic ge si ratio mmol mol has varied temporally ranging from miocene values of 0 9 to last glacial maximum lgm values of 0 55 with the present value of 0 69 these ratios lie between those for the primary sources for these elements continental weathering ge si 0 5 and hydrothermal fluids ge si 11 previous explanations for temporal variation have focused on variations in the relative strength of the primary inputs alternatively the cause may be temporal variation in the relative strength of the two sinks for ge opal and a nonopal sink found in margin sediments the importance of the latter depends on the rate of opal rain to the seafloor in margin environments as temperature decreases opal dissolution rates in the water column decrease permitting a larger fraction of the opal rain to reach the seafloor before dissolving this increases the rate of ge uptake into nonopal phases a model calculation predicts that a decrease of 3 5 c in lgm ocean temperature should increase opal rain to the margin seafloor by a factor of 1 5 an increase of 3 5 ci n miocene temperature should decrease rain by a factor of 0 6 the predicted changes for ge uptake in the nonopal sink can account for essentially all of the temporal ge si variation recorded in diatoms the absence of larger changes constrains silicate weathering rates relative to the present as 106 16 during the lgm and 88 12 during the mid miocene 15 20 ma however additional uncertainty exists because some factors of potential importance remain unconstrained index terms 0330 atmospheric composition and structure geochemical cycles 1050 geochemistry marine geochemistry 4835 4850 1886 hydrology weathering 1625 4845 oceanography biological and chemical nutrients and nutrient cycling 4267 oceanography general paleoceanography keywords germanium silicon weathering