the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

on the characteristics and origin of closed depressions in loess derived soils in europe a case study from central belgium abstract a closed depression cd is a landform where the hillslopes encircle a common sediment depository and the sediment eroded from the surrounding hillslopes is trapped in the system despite the fact that closed depressions cds have been regularly reported in the literature little research on their origin exists therefore a case study was made of the morphological characteristics and genesis of cds in central belgium that are still visible in the landscape through field surveys 26 cds were found in a 43 km2 study area southwest of leuven most cds were situated on a plateau far from villages but close to important rural roads the morphological features of the cds i e diameter area maximum depth and steepest slope were measured the results show that there were significant differences between the morphology of the cds under forest and under cropland cds under cropland were larger shallower and had gentler slopes compared to those under forest based on these morphological features a hypothetical evolution scheme of cds was formulated through a detailed literature search eight out of 26 cds were identified as having an anthropogenic origin they were former quarries mines for calcareous sandstone or iron sandstone in four other cds augerings were made and detailed studies of their soil profiles within and in the vicinity of cds indicated that they are quarries for calcareous loess in one of the four cds with a catchment area of 0 588 ha the soil erosion rate was calculated using soil sequences phosphate content and charcoal analysis since 1950 ad the mean annual soil loss due to water and tillage erosion within the catchment of the cd ranged between 5 5 and 9 8 ton ha or 0 37 0 65 mm based on 14c dating it was calculated that mean annual soil loss since 430 ad amounted to 2 1 ton ha or 0 14 mm

survival of pyropic garnet in subducting plates abstract an experimental study has been conducted to clarify the conditions and kinetic aspects of the pyroxene garnet transformation by using synthetic polycrystals two pyroxenes garnet in the cao mgo al 2 o 3 sio 2 system the run products recovered from the in situ x ray diffraction experiments in a double stage multi anvil system at 15 5 20 4 gpa and 800 1600 c were carefully analyzed by scanning electron microscopy our results show that the pyroxene garnet transformation proceeds by overgrowth of garnet absorbing the surrounding pyroxene components above 1550 c at 18 4 gpa and 1320 c pyroxenes enstatite diopside directly decomposed to the assemblages wadsleyite stishovite ca perovskite without dissolving in garnet the pyroxene garnet transformation requires much higher temperatures than those needed for transformation of olivine to wadsleyite 1000 c and of pyroxenes to their high pressure phases 1300 c in laboratory time 10 3 s and grain 10 5 m scales therefore the pyroxene garnet transformation is likely to be kinetically inhibited at low temperatures in the subducting plate and the pyropic garnet could survive in the transition zone instead of equilibrium majoritic garnet

seismic evidence for a chemical heterogeneity in the midmantle a strong and slightly dipping seismic reflector beneath the mariana subduction zone 1 a clear later phase 80 s after the direct p wave is observed in most of individual seismograms recorded by a short period seismometer network in japan j array from a cluster of deep earthquakes that occurred at the northern mariana subduction zone this phase 1 shows a p wave particle motion 2 arrives later from earthquakes with shallower focal depths 3 has a steeper incident angle than that of p wave and 4 shows a deviation of a few degrees in the arrival azimuth from that of p wave we interpret it as an s to p converted wave which takes off downward from the source and is reflected at a velocity discontinuity reflector below the earthquakes travel time inversion shows that the seismic reflector dips southwest by 20 at 24 25 n 144 75 e and at a depth of 1115 km with a lateral extension at least 100 100 km the location corresponds to the lower edge of a high velocity anomaly in global tomographic models amplitude and waveform analyses suggest a decrease of s wave velocity by 2 6 and an increase of density by 2 9 within the reflector there is almost no difference in p wave velocity 1 between the reflector and the surrounding mantle the estimated thickness of the reflector is 12 km these observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly the seismic reflector might be a piece of subducted oceanic crust as suggested by a previous study it also could be related to the break down of the d phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies both scenarios imply that mechanical chemical segregation might occur within the subducted slab at midmantle condition

mineral assemblages of basalt in the lower mantle high pressure x ray diffraction and analytical transmission electron microscopy of laser heated diamond cell samples show temperature dependent phase assemblages in mid ocean ridge basalt morb at lower mantle pressures the assemblage observed below 2250 k consists of only stishovite plus an orthorhombic perovskite phase space group pbnm or cmcm having a composition close to that of morb but slightly depleted in sio2 with increasing temperature the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at 3250 k to orthorhombic and cubic perovskites plus stishovite and the cafe2o4 phase all of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle

intracrystalline nucleation during the post garnet transformation under large overpressure conditions in deep subducting slabs 1 the mechanism of the post garnet transformation in natural single crystalline pyropic garnet has been examined under large overpressure conditions of 31 43 gpa using the multianvil apparatus with sintered diamond anvils intracrystalline nucleation was found to be dominant above 35 38 gpa while only grain boundary nucleation is responsible at lower pressures in addition natural pyropic garnet transformed to a single phase of perovskite without decomposing to multiple phases above 38 gpa both intracrystalline nucleation and polymorphic growth of the post garnet transformation under large overpressure conditions would strongly enhance the transformation kinetics consequently the post garnet transformation may pwroceed rapidly in the subducting slab at depths near 950 km which would cause substantial rheological weakening of the subducting slab the metastable post garnet transformation may also cause the seismic discontinuities at a depth of 900 1080 km depth which is indeed observed beneath some subduction zones

evidence for a metastable olivine wedge inside the subducted mariana slab abstract an anomalous seismic velocity structure is revealed inside the subducted pacific plate at northern mariana near the deepest earthquake foci we observe anomalous later phases in the p wave coda for a small cluster of deep earthquakes and an anomalous differential p wave slowness between a few pairs of the earthquakes belonging to the cluster we interpret these observations with a 5 low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km the wedge is bounded by two steeply dipping 60 70 transition layers with the thickness of about 4 km the highly localized seismic anomaly inside the slab represents a wedge of metastable olivine embedded in dry ringwoodite deep earthquakes occur across only one of the boundaries of the metastable wedge with some of them outside the wedge the mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge

a pollen analytical record for hemp retting from dungeness foreland uk abstract this paper presents late holocene pollen analytical plant macrofossil and radiocarbon data from a sequence filling a natural pit developed in the gravel beaches of dungeness foreland southeast england pollen analysis of organic muds from the base of muddymore pit contain exceptionally high pollen frequencies for cannabis sativa l and indicate the past use of the site as a hemp retting pit the timing of the pollen record for cannabis ca ad 1000 1400 corresponds with a period during which the nearby town of lydd reached the height of its prosperity and importance as one of the lesser havens of the cinque ports confederation it is proposed that the hemp retting site at muddymore pit went a long way towards satisfying demand from the port for the products of hempen fibre such as rope for ships rigging and cloth for ships sails

prehistoric and roman gullying in the european oess belt a case study from central belgium in contrast with the understanding of present day soil erosion processes knowledge on past soil erosion phenomena is still rather limited although some studies report on severe gully erosion phases during the fourteenth and eighteenth centuries almost no evidence is available that documents earlier gully erosion phases this study investigates the development and age of two old permanent gullies that are conserved in the ancient meerdaal forest in central belgium the development history of both gullies is very similar in the first gully archaeological evidence was found indicating an erosion phase during roman times followed by a partial infilling of the gully in the second gully radiocarbon dating provided evidence of the same roman activity phase cal yr 46 bc ad 78 but also of an earlier incision phase during the middle bronze age cal yr 1743 1602 1568 1533 bc also here the erosion phase was followed by a partial infilling this limited infilling indicates that the catchment of the gullies was reforested quite rapidly hereby cutting off all runoff and sediment production this has led to a unique situation in the meerdaal forest with the conservation of about 43 similar large gullies in an area of about 17 km 2 this area has a high geovalue as the studied gullies are among the oldest and best conserved gullies in northwestern europe

regolith evolution on the millennial timescale from combined u th ra isotopes and in situ cosmogenic 10be analysis in a weathering profile strengbach catchment france abstract u th ra disequilibria cosmogenic in situ 10be concentrations and major and trace element concentrations have been analyzed in a 2 m deep weathering profile sampled at the summit of the granitic strengbach catchment france the data have been used to independently estimate both the long term regolith production and denudation rates and the weathering and erosion rates modeling of the 238u 234u 230th 226ra disequilibrium variations in the lower part of the profile yields a regolith production rate of 12 4 mm kyr 30 10 t km2 yr while modeling of the high resolution 10be concentration profile leads to an exposure age of 19 7 2 2 kyr an inherited concentration of 15 000 1 000 at g in quartz and a mean denudation rate of 22 10 mm kyr 37 15 t km2 yr the consistency between production and denudation rates suggests that on a millennial timescale the regolith mass balance at the summit of the catchment is close to a steady state even if the watershed may have been impacted by quaternary climatic changes and by recent anthropogenic perturbations e g 20th century acid rain and recent afforestation efforts the results also indicate that physical erosion is likely the dominant long term process of regolith denudation in the catchment furthermore the comparison of the long term production and denudation rates and of weathering and erosion rates determined from the depth profile analyses with the current weathering and erosion rates estimated at the outlet of the watershed based on monitoring of the water chemistry and sediment fluxes suggests that physical erosion may have varied more than the chemical weathering flux during the last 150 kyr although very few other sites with u series in situ 10be and stream monitoring data are available for comparison the current data suggest that 1 the mass balance steady state of regolith might be commonly achieved in soil mantled landscapes and 2 physical erosion has varied much more than chemical weathering in mid mountain catchments over the last 10 150 kyr these results highlight the importance of the combined analysis of u series nuclides and in situ 10be in the same weathering profile for the determination of key geomorphic parameters which are important to constraining landscape stability and the responses of landscapes to natural or anthropogenic forcing

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

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

oceanic plateau of the hawaiian mantle plume head subducted to the uppermost lower mantle the hawaiian emperor seamount chain that includes the hawaiian volcanoes was created by the hawaiian mantle plume although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the hawaiian hot spot the fate of this plateau is unclear we discovered a megameter scale portion of thickened oceanic crust in the uppermost lower mantle west of the sea of okhotsk by stacking seismic waveforms of ss precursors we propose that this thick crust represents a major part of the oceanic plateau that was created by the hawaiian plume head 100 million years ago and subducted 20 million to 30 million years ago our discovery provides temporal and spatial clues of the early history of the hawaiian plume for future plate reconstructions

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

in situ measurements of the phase transition boundary in mg3al2si3o12 implications for the nature of the seismic discontinuities in the earth s mantle abstract here we report the phase boundary of pyrope garnet mg3al2si3o12 to al bearing silicate perovskite plus corundum with the highest transition pressure determined by in situ measurements in a multi anvil apparatus at high temperature the consistency of the pressure scales by different standards of au nacl pt w and mo at high temperature was also evaluated by in situ x ray measurements our results together with recent in situ measurements of the post spinel transition in mg2sio4 irifune et al science 279 1998 1698 1700 and the ilmenite perovskite transition in mgsio3 ono et al geophys res lett 2000 submitted show that pressures determined in conventional quench experiments ito and takahashi j geophys res 94 1989 10637 10646 could have been overestimated by more than 2 gpa at pressures corresponding to the bottom of the transition zone on the basis of the in situ measurements the post spinel transition occurs at a depth 600 km that is too shallow to match with the 660 km seismic discontinuity in the earth s mantle therefore an olivine dominant mantle compositional model may be inconsistent with the seismic observations alternatively we propose a pyroxene garnet dominant transition zone with an appropriate al2o3 content ca 6 8 mol in which majorite garnet transforms to perovskite at the depth of the 660 km discontinuity any alternative models would have to consider chemical stratification in the mantle

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 and evolution of closed depressions in central belgium european loess belt closed depressions cds are lower lying areas where the sediment eroded from the surrounding soil surfaces draining towards the cd is trapped in the system cds have been reported in several regions of the european loess belt and are attributed either to natural processes e g dissolution of subsurface horizons or to human intervention e g quarrying previous studies focussed mainly on cropland areas where however only few and largely filled in cds remain the objectives of this study were to i assess the spatial distribution of cds under forest and cropland ii to determine and compare the morphology of cds under forest and under cropland and iii to determine the origin and age of these cds under forest in a study area located partly in ancient forest 13 km2 and partly in cropland 29 km2 a systematic survey revealed the presence of 71 cds under forest 5 3 cd km 2 and 30 cds under cropland 1 cd km 2 comparison of their morphology showed that cds under forest were significantly deeper with steeper sidewalls and a smaller surface area because of the erosion and deposition processes acting on the cds under cropland by comparing cds that had been under cropland for different time intervals the rate of this morphological evolution could be reconstructed analysis of the soil stratigraphy of two representative cds in the ancient forest area confirmed their origin as quarries most probably calcareous loess was excavated since this soil horizon about two to five meters thick was completely absent within the cds dating of the infilling of one cd by optically stimulated luminescence osl shows that the cd filled in between the first century bc and the fourth century ad this dating corresponds to the dating of sediment deposits in nearby human induced gullies that were attributed to an agricultural land use phase between the 18th century bc and the third century ad

land use changes and environmental dynamics in the upper rhone valley since neolithic times inferred from sediments in lac moras lac moras is a small lake located on a low elevation plateau in the upper rhone valley 304 m a s l the upper 5 m of accumulated sediment in the lake span 7500 years and offer a detailed record of environmental perturbations and land use history at a local scale a multiproxy analysis pollen charcoal and geochemical parameters led to the establishment of four periods of landscape dynamics the first evidence of human impacts was recorded during the neolithic and bronze age 6000 2700 cal bp these impacts were temporary and most likely affected small areas the second period in the iron age classical antiquity 2200 1900 cal bp appears to be a key period in which the intensification of anthropogenic pressure primarily grazing with localised areas of cultivation caused high intensity erosion events and deeply affected soil stability during the middle ages wheat rye and hemp cultures as well as tree farming walnut chestnut were intensively developed from 50 cal bp the 19th century on

density profiles of oceanic slabs and surrounding mantle integrated thermodynamic and thermal modeling and implications for the fate of slabs at the 660 km discontinuity abstract we have calculated the mineralogical properties of the earth s mantle and the lithological units constituting the subducting oceanic slabs within a wide range of p t conditions within the cao feo mgo al2o3 sio2 system except for the basalt top layer of a slab for which the system is extended to include na2o the mineralogical data are then converted using the appropriate p v t relations to bulk densities the calculated adiabatic density vs depth profile of the mantle between 200 and 725 km depths is in good agreement with geophysical and experimental data the density data of the different compositional units are combined with calculated thermal structures for a variety of slab mantle systems to construct equilibrium density profiles as a function of depth the mean equilibrium densities of the slabs within the transition zone 400 660 km depth are found to be 0 04 0 05 g cm3 greater than those of the ambient mantle within the same depth interval for the entire upper mantle density differences between slabs and ambient mantle are slightly less but the slabs still remain denser than the latter at 670 km depth slabs have lower density than the ambient lower mantle because of the commencement of perovskite forming reactions within the mantle and displacements of these reaction boundaries to higher pressures within the slabs as a consequence of their negative p t slopes if perovskite forming reactions within slabs are hindered for kinetic sluggishness then neutral buoyancy would be achieved when the slabs have penetrated 100 km into the lower mantle however using the available data on the kinetics of spinel to perovskite plus periclase reaction we conclude that the reaction would go to completion in a peru type young slab 41 myr and very likely also in a tonga type old slab 110 myr before these penetrated 100 km into the lower mantle thus slabs should always remain negatively buoyant and therefore continue to subduct through the lower mantle once it penetrates through the 660 km discontinuity despite a negative buoyancy force a slab could deflect at the top of the lower mantle 660 km because of factors resisting subduction namely viscosity jump low dip angle slab roll back and metastable persistence of olivine in cold slabs if published scale model experiments represent realistic approximations of the factors affecting plate subduction then according to our density data any slab with a dip angle of 40 50 would bend at the 660 km discontinuity if there is a viscosity jump of at least by a factor of 10 and trench migration the basalt top layer of a slab is denser than other slab components and the ambient mantle at all depths to 660 km and therefore should continue to sink into the lower mantle especially if a slab directly penetrates the 660 km barrier instead of peeling off in the transition zone to form a perched eclogite or piclogite layer as previously proposed the harzburgite layer which is sandwiched between denser basalt and lherzolite layers faces greater resistance to subduction especially in a young slab and thus could significantly contribute to the deformation of a slab near the 660 km discontinuity

in situ x ray diffraction study of the effect of water on the garnet perovskite transformation in morb and implications for the penetration of oceanic crust into the lower mantle abstract the phase boundary of the transformation of garnet to perovskite in hydrous mid ocean ridge basalt morb was determined by in situ high pressure and high temperature x ray diffraction xrd experiments the garnet bearing assemblage was formed in experiments at pressures below 21 gpa in contrast an assemblage of mg perovskite ca perovskite stishovite cf phase and or nal phase is stable at pressures above 21 gpa garnet and perovskite bearing assemblages were synthesized at a pressure range of 20 25 gpa and a temperature range of 1000 1600 c on the basis of our results we have located the garnet perovskite phase boundary and find that it can be described by the following equation p gpa 0 004t c 17 5 which positions it 2 gpa lower under dry conditions if the top 7 km of the slab is colder than 1200 c in the transition zone perovskite will appear at a depth shallower than that where ringwoodite decomposes this result indicates that the morb layer becomes denser than the peridotite portion of the slab throughout the transition zone and the lower mantle and thus no density crossover would exist between basalt and underlying peridotite along the cold subducting slab geotherm

human induced changes on wetlands a study case from nw iberia abstract wetlands are exceptional ecosystems that contribute to biodiversity and play a key role in the hydrological and carbon cycles knowledge of their long term ecology is essential for a proper understanding of these valuable ecosystems we present the application of multi proxy analyses to a 115 cm thick core from la molina mire alto de la espina located in nw iberia with a chronology spanning since 500 bc the mire is located in an area intensively mined for gold during the roman period and close to a water canalization system used for mining operations at that time our aim was to get insights into the development of the wetland by combining palynological records of hydro hygrophytes non pollen palynomorphs and geochemical analyses supported by 14c datings and multivariate statistics the results indicate a complex pattern of ecological succession during the local iron age the wetland was minerotrophic since 20 ad it was subjected to dramatic hydrological changes due to a rise of the water table fluctuating between the presence of open water and phases of drawdown finally by 745 ad the wetland experienced a rapid evolution towards ombrotrophic conditions high grazing pressure was detected for the last decades the significant change occurred during early roman empire seems to have been the consequence of the direct use of the wetland as a water reservoir of the canalisation system used for gold mining thus the current nature of the mire may be the result of human impact although multiple human and climate induced causes were potentially linked to the detected shifts however the system seems to have been resilient successfully buffering the changes without substantial alterations of its functioning our investigation shows that palaeoecological research is necessary to understand modifications in the structure of wetland ecosystems their long term ecology and the role of human induced changes

spatial analysis of factors controlling the presence of closed depressions and gullies under forest application of rare event logistic regression abstract although numerous studies have reported on past geomorphic change it is a common difficulty to identify the relative role of driving human or climatic forces to that end the possibilities of rare events logistic regression are explored in a 1300 hectare large ancient forest in central belgium here 43 permanent gullies and 71 closed depressions were observed that were probably caused by an anthropogenically induced land use change in the past this human origin was confirmed for a limited number of case study areas by recent geomorphic research with dating and soil profile analysis the objective of this study is to analyze the factors controlling the position of all gullies and closed depressions in the study area and investigate their mutual relation this will indicate whether or not the results obtained for the small case study areas can be extrapolated to the rest of the geomorphic features found in the whole study area the obtained logistic regression models show that both anthropogenic and physical factors are important in the spatial distribution of both geomorphic features for the prediction of the occurrence of closed depressions anthropogenic factors are most important however the probability of finding a depression is larger close to reported prehistoric and roman archaeological sites on well developed undisturbed luvisols and on northeast to southeast facing slopes the local slope gradient of the soil surface is the most important factor in the multivariate model for predicting the presence of the permanent gullies the occurrence probability of permanent gullies is also larger on northeast to southeast facing slopes the anthropogenic factor expressed by the distance to the closed depressions is significant as a univariate model this suggests that a critical slope gradient is a prerequisite for gully incision but gully formation still needs to be triggered by a human induced land use change it can therefore be concluded that both the gullies and the closed depressions in the meerdaal forest are related to past human activity either directly through mining of loess or indirectly through runoff production on cropland this study also showed the potential of rare events logistic regression for analyzing the spatial pattern of geomorphological features in relation to environmental factors

historical soil erosion and land use change during the last two millennia recorded in lake sediments of frickenhauser see northern bavaria central germany based on palaeolimnological investigations on a 1275 cm long lacustrine sediment record sediment yields sy and denudation rates dr for the last 2000 years were reconstructed from frickenhauser

searching for ancient forests a 2000 year history of land use in northeastern french forests deduced from the pollen compositions of closed depressions evidence of the agricultural use during roman or medieval times of forested areas formerly considered to be ancient as well as legacies of this former land use on plant biodiversity and soil pro

triassic to middle jurassic stratigraphy of the kerr mcgee 97 12 1 exploration well offshore southern england abstract the triassic through to the middle jurassic bathonian stratigraphy of the kerr mcgee 97 12 1 exploration well is described using petrophysical logs in association with lithological and palaeontological analyses over 6210 of sediment was penetrated with sixteen triassic and fourteen lower middle jurassic sedimentary units recognised with a total measured depth t d of 7310 within the sherwood sandstone group budleigh salterton pebble beds of early middle triassic olenekian anisian age as expected the sedimentary sequences are comparable to those described from the wessex basin especially the portland wight and dorset sub basins of particular note however is the very thick halite sequence 1074 within the carnian dunscombe mudstone formation and also an expanded paper shales 27 sequence within the earliest toarcian a normal fault is noted at 1260 with the late bajocian inferior oolite group in fault contact with the latest toarcian bridport sand formation down cliff clay member the late triassic rhaetian to middle jurassic bathonian yielded rich microfaunal and palynomorph assemblages all of which have been previously described throughout northwest europe four microfossils groups foraminifera ostracods dinocysts and miospores have been used to interpret stratigraphic ages and palaeoenvironments this study provides new lithological and microfossil data for the triassic to middle jurassic bathonian sequences preserved in the offshore portland wight basin and provides a key tie point reference section for future offshore drilling

kinetics of the post garnet transformation implications for density and rheology of subducting slabs abstract mechanisms and kinetics of the post garnet transformation in natural pyrope were examined at 28 1 31 3 gpa and 1320 2150 k by time resolved x ray diffraction measurements using synchrotron radiation combined with microstructural observations of the recovered samples the water content of samples recovered was estimated to be approximately 400 wt ppm h 2 o from infrared spectroscopic measurements the transformation occurs by the grain boundary reaction and reaction rims are formed along the grain boundaries of parental garnet grains decomposed post garnet assemblages consisting of mg and ca perovskites aluminous phase and stishovite show very fine grained symplectic texture analyses of kinetic data obtained have revealed that growth rates in the post garnet transformation are strongly time dependent extrapolations of the present kinetic data suggest that the post garnet transformation cannot complete at less than ca 1730 k even under geological timescales because of the slow growth kinetics due to the presence of the metastable garnet subducting slabs become buoyant in the lower mantle at less than ca 1730 k for the basaltic crust and at ca 1400 1600 k for the underlying peridotite layer rheological weakening of the basaltic crust is expected in the lower mantle through grain size reduction by the metastable post garnet transformation occurring under large overpressure conditions the kinetics of the post garnet transformation significantly affect both the density and rheology of subducting slabs in the lower mantle and possibly has important roles in the mixing and survival of chemically differentiated slab materials

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

pollen representation source area and basin size toward a unified theory of pollen analysis abstract the concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources the form of these functions is determined by physical processes standard formulas for elevated sources do not apply but the integrated form of sutton s equation for particle dispersal from a ground level source gives useful approximations applicable to pollen transport over a forest canopy simulations using this equation yielded source areas that increased realistically with basin size showed substantial differences between source areas for pollen grains with different deposition velocities and predicted that lighter pollen grains should become better represented with increasing basin size all of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments the model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities this extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern united states

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

molecular evidence for recent land use change from a swampy environment to a pond lorraine france evidence of a changing environment in the catchment area of a small pond lansquenet lorraine france during the last millennium was provided by coupling palynology data and organic ratios terrestrial to aquatic ratio tar hc the c29 vs c27 sterols c29 c27 st average chain length acl and aquatic macrophytes ratio paq the depositional and alteration conditions of organic matter om were provided by the 5 h stanols 5 sterols ratio this study shows that quite simple organic ratios clearly allow identification of the different evolution steps of a watershed from a swamp to the progressive settlement of a pond during the swampy period the c n at ca 11 1 tar hc ca 36 8 c29 c27 st ca 8 7 and paq 0 3 show a high contribution of allochthonous om associated with the presence of particular aquatic and mesohygrophilous species a woody level dated to the xiiith century reflects the period of the settlement of the pond in a more oxic environment 5 h stanol 5sterol ratio 0 2 then the progressive increase in aquatic input and the development of submerged and floating macrophytes are evidenced by low c n at 7 6 tar hc ca 2 7 c29 c27 st ca 2 and high paq ca 0 5 suggesting an increase in the water level and eutrophication of the pond a second oxic event 5 h stanols 5sterols ratio averaging 0 16 associated with extensive om degradation corresponds to the most recent dredging in 2002 and reflects an important disturbance of sediments due to pond management

depth variation of the mid mantle seismic discontinuity short period array seismograms of deep events that occurred in the indonesia japan and izu bonin arcs are stacked and beam formed to identify the near source s p converted waves that result from the mantle transition discontinuities most of the resulting images reveal the existence of a mid mantle seismic discontinuity 920 km discontinuity in these regions of the 15 events analyzed three that occurred at the western end of the indonesia arc show clear s p arrivals observable even in individual seismograms the mid mantle discontinuity is characterized by large depth variation 900 1080 km and velocity contrast variation in different subduction zones especially the depth variation of the mid mantle discontinuity beneath the indonesia arc where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end appears to be well correlated with the location of the high velocity anomalies in recent tomographic models however the mid mantle discontinuity cannot be simply coincided with the bottom of the high velocity anomalies because a velocity increase at the discontinuity is observed from the waveform analysis

holocene climate dynamics derived from pollen record of jiulongchi wetland in fanjing mountain southwest china abstract an alpine wetland core was obtained to a depth of 4 0 m from the fanjing mountain in southwest china sections of the core were dated with 14c data obtained with high resolution mass spectrometry ams and holocene climate changes were inferred by categorizing predominant tree and fern vegetation identified from pollen and spore counts at various core depths the temperature increased during 11 9 8 0 cal ka bp and the warmest period was during 8 0 4 6 cal ka bp then it gradually became cooler during the late holocene our temperature estimates were in good agreement with the previous studies of the holocene temperature in the asian monsoon areas there were two relatively wet periods of 10 7 8 0 cal ka bp and 4 6 2 6 cal ka bp there was an asynchronous precipitation trend in the south north and southwest china which was probably due to the different monsoon impacts in east asian summer monsoon easm area the monsoon precipitation gradually transgressed northward during the early to middle holocene and regressed southward during the late holocene our results will be useful to improve the understanding of the coherent spatio temporal patterns of climate evolution in easm region in china

absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity 1 interaction between subducting slabs and surrounding mantle near 660 km discontinuity is one of the key issues in the dynamic of deep earth distinguishing phase transformations occur in the both major parts of the slab basaltic crust and underlying peridotite post spinel transformation in peridotite takes place at 660 km whereas post garnet transformation in basalt is considered to occur at 720 800 km creating density crossover in this paper we determined the post garnet phase boundary in mid ocean ridge basalt representative for oceanic crust by in situ x ray diffraction studies the phase boundary can be expressed as p gpa 0 0041 t k 17 92 the present results imply that the basaltic component of the cold slab transformed to perovskite assembly at lower pressure than the surrounding mantle and provide direct evidence for penetration of the oceanic crust into the lower mantle without gravitational separation from the peridotite body of the slab

the origin of closed depressions in northeastern france a new assessment over 10 000 closed depressions cds are found in the silty plains of northeastern france these small wetlands support the growth of rare plant species although their origins which could be anthropogenic or geologic due to salt gypsum lens dissolution have been debated for 150 years they have not yet been the focus of an integrated study in 39 geological borings along a 15 km strip no salt gypsum lenses and more than 260 cds were recorded using lidar all of the investigated cds have a bathtub form with a flat bottom complete excavations clearly showed a cut contact between the sediment and the horizontal marl substratum at the bottom and a cut at the edges of the upper marl layers radiocarbon dating of sediment bottoms showed that sedimentation began between the second iron age and the roman period the frequencies of pollen and sporormiella type depict an open landscape with grassland pasture and cropland these convergent findings challenge the hypothesis that cds formed naturally and suggest that they area instead anthropogenic because no soil deposits were found around the cds digging may have been intended to marl the surrounding acidic silty soils the high density and small size of cds will allow the detailed reconstruction of landscape and biodiversity modifications in the region for the two last millennia

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

holocene vegetation history in the northern vosges mountains ne france palynological geochemical and sedimentological data palynological data from the northern vosges mountains nvm are very rare unlike for the southern and central vosges mountains where the past vegetation history is relatively well known as a con